functionalized perylene bisimides

Photoinduced processes of

functionalized perylene bisimides

Photoinduced processes of

functionalized perylene bisimides

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 15 januari 2004 om 16.00 uur

door

Edda Elizabeth Neuteboom

geboren te Bunnik

Dit proefschrift is goedgekeurd door de promotoren: prof.dr.ir. R.A.J. Janssen en prof.dr. E.W. Meijer This research has been financially supported by the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (NWO-CW) and the Eindhoven University of Technology in the PIONIER program (98400). Omslagontwerp: Erik Hoogendorp, www.178aardigeontwerpers.nl Druk: Universiteitsdrukkerij, Technische Universiteit Eindhoven CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Neuteboom, Edda E. Photoinduced processes of functionalized perylene bisimides / by Edda E. Neuteboom. – Eindhoven : Technische Universiteit Eindhoven, 2004. Proefschrift. – ISBN 90-386-2815-3 NUR 913 Trefwoorden: fysisch-organische chemie / supramoleculaire chemie / fotogeïnduceerde energieoverdracht en elektronenoverdracht / organische halfgeleiders / geleidende polymeren; polyimiden Subject headings: physical organic chemistry / supramolecular chemistry / photoinduced energy transfer and electrontransfer / organic semiconductors / conducting polymers; polyimides

Table of contents Chapter 1 General introduction 1 Chapter 2 Photophysical properties of functionalized perylene imides 5

2.1 Introduction 5

2.2 Synthesis and properties of perylene imides 6 2.2.1 Synthesis 6 2.2.2 Optical and electrochemical properties 8

2.3 Photoinduced energy and electron transfer 9 2.3.1 Electron transfer 10 2.3.2 Energy transfer 12 2.3.3 Spectroscopy of energy and charge transfer 13

2.4 Porphyrin – perylene imide arrays 14 2.4.1 Bichromophoric dyads and extended structures 14

2.4.1.1 Porphyrin and oxochlorin dyads 14 2.4.1.2 Perylene monoimide-porphyrin wires 17 2.4.1.3 Porphyrins bearing multiple perylene imides 18

2.4.2 Perylene-porphyrin switches 20 2.4.2.1 Light intensity-dependent optical switch 20 2.4.2.2 Switching based on influence of photogenerated electric field 21 2.4.2.3 Photoswitched decay rates 22

2.5 Conjugated oligomer/polymer – perylene imides 24 2.5.1 Oligo- and polyfluorenes 24

2.5.1.1 Perylene bisimides incorporated in the polymer chain 24

2.5.1.2 Polyfluorenes with perylene monoimide endcappers 25

2.5.1.3 Perylene monoimide as side chain 27

2.5.2 Oligo(p-phenylene vinylene)s – perylene bisimides 27

2.5.2.1 Covalently linked perylene bisimides and OPVs 28

2.5.2.2 Hydrogen-bonded architectures 29

2.6 Dendritic structures with perylene imides 29 2.6.1 Perylene bisimide as dendritic core 30

2.6.2 Perylene monoimide substituted triphenylamine core dendrimer 31

2.6.3 Dendrimers with terrylene as energy acceptor 32

2.6.3.1 Terrylene in the rim 32

2.6.3.2 Terrylene as luminescent dendrimer core 34

2.7 Miscellaneous systems 36 2.7.1 Tetrathiafulvalene as electron donor 36

2.7.2 Calix[4]arene as electron donor 37

2.7.3 Pyrene and ferrocene as donors in self-assembled squares 38

2.7.4 Anthraquinone as energy donor 39

2.8 Perylene imide as an electron donor 39 2.8.1 Perylene-perylene dyads 40

2.8.1.1 Perylene bisimide as electron donor 40

2.8.1.2 Perylene trisimide as energy donor 42

2.8.2 Heterodimers 43

2.9 Application of energy and charge transfer in devices 44

2.10 Conclusion 46

2.11 References 46 Chapter 3 Photoluminescence of self-organized perylene bisimide polymers 53

3.1 Introduction 54

3.2 Synthesis 55

3.3 Absorption and emission 56 3.3.1 Temperature dependent absorption 57

3.3.2 Temperature dependent fluorescence and excitation spectra 58

3.4 Aggregate fluorescence 60

3.5 Analysis of fluorescence lifetime experiments 63

3.6 Conclusion 64

3.7 Experimental section 65

3.8 References 66 Chapter 4 Alternating oligo(p-phenylene vinylene) – perylene bisimide copolymers 69

4.1 Introduction 70

4.2 Synthesis and characterization 73 4.2.1 Synthesis of the polymers and model compounds 73

4.2.2 Characterization of the polymers 75

4.3 Absorption and circular dichroism spectra 75

4.4 Charge separation 77 4.4.1 Electrochemistry 77 4.4.2 Energy for charge separation 77

4.4.3 Fluorescence spectra and fluorescence quenching 79

4.4.4 Photoinduced absorption in thin films 82

4.4.5 Photoinduced absorption in solution 84

4.5 Photovoltaic devices 86

4.6 Kinetics of electron and energy transfer 87

4.7 Theoretical modeling of charge separation and recombination 90

4.8 Conclusion 92


4.10 References and notes 99

Chapter 5 Conjugated oligo(p-phenylene vinylene) polymers with dangling perylene

bisimides 103

5.1 Introduction 104

5.2 Synthesis 105

5.3 Absorption spectra in solution 108

5.4 Circular dichroism spectra in solution 110

5.5 Charge separation 111 5.5.1 Fluorescence spectra and fluorescence quenching in solution 112

5.5.2 Photoinduced absorption in solution 115

5.5.3 Photoinduced absorption in solid state 116

5.6 Photovoltaic devices 118

5.7 Conclusion 119


5.9 References 124 Chapter 6 Singlet-energy transfer in quadruple hydrogen-bonded

oligo(p-phenylenevinylene)-perylene bisimide dyads 125


6.2 Synthesis 127

6.3 Keto-enol equilibrium 127

6.4 Photophysical measurements 128 6.4.1 Optical properties 128

6.4.2 Fluorescence quenching 129

6.4.3 Transient photoinduced absorption 131

6.5 Discussion 132 6.5.1 Förster energy transfer 132

6.5.2 Electron transfer 133

6.6 Hydrogen bonding by the bisurea motif 135 6.6.1 Synthesis and infrared characterization 135

6.6.2 STM experiments 136

6.7 Conclusion 138


6.9 References and notes 140 Chapter 7 Multiple-perylene bisimide compounds 143


7.2 Synthesis 145

7.3 Photophysical properties of PERY3 and PERY4 148

7.4 Photophysical processes in PERY2 149 7.4.1 UV/Visible absorption spectroscopy 149

7.4.2 Fluorescence experiments 150

7.4.3 Fluorescence lifetime experiments 152

7.4.4 Transient photoinduced absorption spectroscopy 153

7.5 Discussion 154

7.6 Conclusion 156


7.8 References 158

Summary 161

Samenvatting 164

Curriculum Vitae 167

List of publications 168

Dankwoord 169

1

1

General introduction

Inspired by intriguing applications in molecular electronics, light energy conversion,

photocatalysis, and artificial photosynthesis, the photoinduced energy and electron transfer in

donor-acceptor systems attract extensive attention. The photosynthetic reaction center is by

far the most elegant example known that illustrates how carefully aligned molecular arrays of

photo- and redox-active components can work in concert to convert and store solar energy,

utilizing photoinduced energy and electron transfer reactions.1 Although artificial systems are

still far from Nature’s success in assembling, positioning, and organizing such intricate

functional architectures, the exciting prospect that polymers might contribute to cheap ‘green

energy’ makes this subject a highly active area of fascinating contemporary interdisciplinary

research.

Photoinduced charge transfer at the interface of an electron donating (p-type) and

electron accepting (n-type) material is perhaps the most crucial event in organic solar cells. In

1986 Tang demonstrated the first organic photovoltaic cell consisting of a double-layer of

two molecular materials, copper phthalocyanine (p-type) and a perylene tetracarboxylic

derivative (n-type), sandwiched between a transparent front and a metal back electrode.2 It

took almost a decade before the bulk-heterojunction architecture was introduced, in which the

donor and acceptor molecules or polymers are randomly mixed to create a composite material

exhibiting phase separation on a nanoscale.3,4 The main advantage of this approach, that bears

similarities to dye-sensitized nanocrystalline titanium dioxide solar cells,5 is that the

interfacial area between the p- and n-type materials is increased enormously, resulting in a

more efficient charge generation and improved use of the solar radiation. At present, the best

bulk-heterojunction organic photovoltaic devices have demonstrated efficiencies for solar

energy conversion in the range of 2.5-3.5 %,6-8 in excess of the performance of the

photosynthetic reaction center and approaching that of inorganic semiconductors.

In bulk-heterojunction cells, the photoactive layer is generally obtained by spin casting

from a common solution. Since the phase separation is a spontaneous process, it is

Chapter 1

2

intrinsically difficult to control the dimensions of the micromorphology and, hence, there is a

risk that photogenerated charges become trapped in isolated domains that are not in contact

with the appropriate electrode or that recombination of charges can occur when the donor and

acceptor are too well mixed.

To circumvent these problems, a new strategy was defined, by which it might be possible

to obtain a better and a predefined control over the organization of the two chromophores. In

this approach the p and n chromophores are united in a single macromolecular structure, e.g.

in an alternating repetition of donors and acceptors or as pedant groups dangling from the

polymer backbone. By controlling the distance and length between the two segments via

covalent bonds, it should be possible to achieve control over the characteristic distance

between phase boundaries in composite materials, while their relative volumes predefine the

micromorphology. This principle is well established for non-conjugated block copolymers

and has recently been introduced in the field of π-conjugated materials.9,10 In block

copolymers, the intrinsic tendency of each block to aggregate in an individual phase often

leads to long-range ordering on a three dimensional level. In the optimal morphology for

solar cells, the donor and acceptor materials have a large interface, while the characteristic

dimension of the microstructure is at the scale of the exciton diffusion length (~10-20 nm),

and the two phases provide a direct pathway for charge transport to the electrodes (Figure

1.1)

GlassITO

Metal

light

GlassITO

Metal

light

+

-

GlassITO

Metal

light

GlassITO

Metal

light

++

-

n -ty

pe

p -ty

pe

23

4--

++ 4

1

Metal

ITOGlass

light

GlassITO

Metal

light

GlassITO

Metal

light

++

--

GlassITO

Metal

light

GlassITO

Metal

light

+

-

n -ty

pe

p -ty

pe

23

4-

+ 4

1

Glass

light

1

Glass

light

GlassITO

Metal

light

GlassITO

Metal

light

+

-

GlassITO

Metal

light

GlassITO

Metal

light

++

-

n -ty

pe

p -ty

pe

23

4--

++ 4

1

Metal

ITOGlass

light

GlassITO

Metal

light

GlassITO

Metal

light

++

--

GlassITO

Metal

light

GlassITO

Metal

light

+

-

n -ty

pe

p -ty

pe

23

4-

+ 4

1

Glass

light

1

Glass

light

Figure 1.1. Optimum configuration of an organic solar cell. (1) creation of an exciton; (2)

migration of the exciton to the p-n interface; (3) charge transfer; (4) transport of positive and

negative charges to the electrodes.

General introduction

3

The aim of this thesis is to synthesize and study a variety of new donor-acceptor

polymers, which are based on perylene bisimide as an electron acceptor and oligo(p-

phenylene vinylene) as a donor. The first motivation for this research line is to explore the

accessibility of these materials in terms of synthesis and solubility of the materials, as a first

step to polymers that consist of large acceptor blocks and large donor blocks. A second

motivation is to study, using photophysical techniques, the interactions between the different

chromophores in these unique architectures in solution and solid state.

The remainder of the thesis is organized as follows. Chapter 2 provides an overview on

the existing literature on photoinduced energy and electron transfer reactions in molecular

arrays consisting of perylene monoimides or bisimides as an acceptor, in combination with

molecular or polymeric donors. The attention is focused on the parameters such as redox

potentials, excited state energies, and relative orientation and distance in controlling and

discriminating between photoinduced energy and charge transfer.

In Chapter 3 the synthesis and aggregation in solution of three alternating perylene

bisimide – polytetrahydrofuran copolymers are presented.11 The polymers are studied in

detail with absorption and (time-resolved) fluorescence spectroscopy to show that the length

of the nonconjugated segments has a substantial influence on the aggregation in solution and

the fluorescent properties. Clear evidence for excimer-like emission was obtained. The first

example of an alternating donor-acceptor copolymer is described in Chapter 4.12 In these

novel materials perylene bisimides are connected along the polymer chain with oligo(p-

phenylene vinylene)s via two different spacers. The photophysical properties of these

materials are studied in detail with femtosecond transient pump-probe spectroscopy and

compared to a cyclic model compound. The rates for photoinduced charge separation and

recombination are related to conformational freedom of the donor and acceptor units in the

polymer chains. The same chromophores are used in Chapter 5, to create a conjugated and a

cross-conjugated donor polymer backbone bearing pendant perylene bisimides. Such a

configuration is also referred to as a ‘double cable’, as it potentially provides transport of

electrons and holes along one chain in an antiparallel fashion. The rates of charge separation

and recombination in these polymers differ significantly from the rates obtained for the

alternating copolymers in Chapter 4. This is a consequence of the shorter distance between

the chromophores.

An alternative way to connect perylene bisimide to oligo(p-phenylene vinylene) is

presented in Chapter 6.13 Here, quadruple hydrogen bonds are utilized to construct a

supramolecular donor-acceptor dyad. The high binding constant of the quadruple hydrogen-

Chapter 1

4

bonding motif allows, for the first time, to time-resolve the photoinduced energy transfer

process in a dyad in dilute solution. Furthermore, an outline is given of how urea

functionalities, incorporated into multichromophoric systems, might be able to direct the

packing of donor and acceptors away from forming alternating stacks.

Finally, Chapter 7 describes the synthesis and fluorescent properties of three

multichromophoric perylene bisimides, connected in pseudo-linear, trigonal, and tetrahedral

configurations. An unexpected charge transfer reaction was found for the linear system, but

not for the two larger systems.

References

(1) Electron Transfer in Chemistry Vol. 3: Biological and Artificial Supramolecular Systems, V. Balzani (Ed.), Wiley-VCH Weinheim 2001.

(2) C. W. Tang, Appl. Phys. Lett. 1986, 48, 183.

(3) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789.

(4) J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Maratti, A. B. Holmes, Nature 1995, 376, 498.

(5) B. O’Regan, M. Grätzel, Nature 1991, 353, 737.

(6) S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz, J. C. Hummelen, Appl. Phys. Lett. 2001, 78, 841.

(7) P. Schilinsky, C. Waldauf, C. J. Brabec, Appl. Phys. Lett. 2002, 81, 3885.

(8) M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen, P. A. van Hal, R. A. J. Janssen, Angew. Chem. Int. Ed. 2003, 42, 3371.

(9) G. Widawski, M. Rawiso, B. François, Nature 1994, 369, 387.

(10) M. A. Hempenius, B. M. W. Langeveld-Voss, J. A. E. H. van Haare, R. A. J. Janssen, S. S. Sheiko, J. P. Spatz, M. Möller, E. W. Meijer, J. Am. Chem. Soc. 1998, 120, 2798.

(11) (a) E. E. Neuteboom, R. A. J. Janssen, E. W. Meijer, Synth. Met. 2001, 121, 1283; (b) E. E. Neuteboom, S. C. J. Meskers, E. W. Meijer, R. A. J. Janssen, Macromol. Chem. Phys., in press.

(12) E. E. Neuteboom, S. C. J. Meskers, P. A. van Hal, J. K. J. van Duren, E. W. Meijer, R. A. J. Janssen, H. Dupin, G. Pourtois, J. Cornil, R. Lazzaroni, J.-L. Brédas, D. Beljonne, J. Am. Chem. Soc. 2003, 125, 8625.

(13) E. E. Neuteboom, E. H. A. Beckers, S. C. J. Meskers, E. W. Meijer, R. A. J. Janssen, Org. Biomol. Chem. 2003, 1, 198.

5

2

Photophysical properties of functionalized perylene

imides

2.1 Introduction The chemistry of the perylene bisimides started with the work of Kardos in 1913.1,2 He

described the reaction of naphthalene-1,8-dicarboximide in molten alkali to perylene

bisimides. These products found their use as vat dyes and various derivatives are still

produced today for red dyes and pigments. Perylene bisimides combine a strong absorption in

the visible region with a fluorescence quantum yields near unity and a high stability towards

photooxidation. Furthermore, perylene bisimides feature a relatively low reduction potential,

which enables their use as an n-type semiconductor and as an electron acceptor in

photoinduced charge transfer reactions. Because of their appealing properties, perylene

monoimide and perylene bisimide derivatives have been utilized in various electronic and

optical applications such as field-effect transistors,3 fluorescent solar collectors,4,5

electrophotographic devices,6 dye lasers,7,8 photovoltaic cells,9 and light-emitting diodes

(LEDs).10

Since the pioneering work of Wasielewski et al. in 1992,11 the research on perylene

imides took a new and exciting direction. Perylene bisimides were covalently connected to

organic chromophores to create multifunctional photo- and electro-active systems.

Photoexcitation of these compounds results in intramolecular energy or charge transfer with

the perylene imide as the electron-accepting moiety. Potential application of these molecules

in molecular and supramolecular electronics, photocatalysis and materials for LEDs and solar

cells has resulted in a high activity in this field in recent years.

In this chapter an overview is given of perylene imides incorporated in functional

molecular architectures, focusing on their photophysical properties. Section 2 briefly

introduces the synthesis and intrinsic properties of perylene imides, while section 3 lays out

Chapter 2

6

basic concepts of photoinduced energy and electron transfer. The remaining sections discuss

perylene imides linked to porphyrins (section 4), π-conjugated oligomers and polymers

(section 5), dendrimers (section 6), and miscellaneous electro-active groups (section 7).

Section 8 gives an overview of systems in which perylene imides act as electron donors,

rather than as electron acceptors. Finally, section 9 discusses the use energy and electron

transfer reactions involving perylene imides in LEDs and photovoltaic cells.

2.2 Synthesis and properties of perylene imides

2.2.1 Synthesis

The most common procedure to synthesize symmetrical perylene bisimides 2 is the

condensation reaction of perylene-3,4:9,10-tetracarboxylic dianhydride 1 with a primary

amine (Figure 2.1).12,13 The choice of the solvent for this reaction depends on the reactivity of

the amine. For reactive amines, e.g. alkyl amines, water or DMF14 can be used at 100 – 160

°C. Solvents as quinoline or molten imidazole are necessary for less reactive amines such as

aromatic amines with reaction temperatures of 160 – 180 °C. Zinc salts like zinc acetate are

catalysts for the reaction; it has been suggested that they have an ability to solubilize the

anhydride.

O

O OO

O O N

N OO

O O

R1

R1N

N OO

O O

R1

R2

O

N OO

O O

R1

CO2K

N OOR1

CO2H

1 2 3 4 5

R1-NH2 KOH acid R2-NH2

Figure 2.1. Synthesis of symmetrical and non-symmetrical perylene bisimides.

Formation of non-symmetrically substituted perylene bisimides 5 by performing the

reaction with two different amines usually does not take place because of differences in

reactivity of the two amines. Although it is possible to partially condensate the dianhydride 1

with a primary amine to form 4,15 this method is not widely applied. Non-symmetrically

substituted perylene bisimides 5 are generally obtained in a multistep procedure (Figure

2.1).14,16 First, symmetrically substituted perylene bisimides 2 are partially saponificated into

Photophysical properties of functionalized perylene imides

7

the potassium salt 3 by reaction with KOH in tert-butanol, and subsequent treatment with

acid. The obtained monoimide monoanhydride 4 can be reacted with an amine to form the

non-symmetrically substituted bisimide 5.

O

O OO

O O N

N OO

O OH

R1

N

N OO

O O

R1

R2

N

O OO

O OH

CO2K

O OO

CO2H

N

O OO

O OR1

1 876 5

R1-NH2KOH NH3 R2-Br

R1-NH2 R2-NH2

4

Figure 2.2. Alternative route to non-symmetrically substituted perylene bisimides.

An alternative route to non-symmetrically substituted perylene bisimides that does not

require perylene bisimide as a starting material, is shown in Figure 2.2.14,16 The

monopotassium salt 6 of dianhydride 1 reacts in a condensation reaction with ammonia to

imide 7.17 After reaction with a primary amine to 8, a nucleophilic substitution reaction with

an alkylbromide in presence of a base leads to bismide 5, where R2 = alkyl. Another option is

to react the monopotassium salt 6 with an amine to monoimide 4. This compound can react

with another amine to bismide 5 (Figure 2.2).

N

N OO

O OR1

R1

R2R2

R2R2

Figure 2.3. Common substitution positions of perylene bisimides.

Chapter 2

8

The solubility of the perylene bisimide strongly depends on the substitution (Figure 2.3).

It is found that high solubility in organic solvents is obtained when the two N-terminal groups

R1 are secondary alkyl residues having two long chains18 or ortho-substituted phenyl

groups.19 These substituents are forced out of the plane of the chromophore and thereby

hamper the face-to-face π-π stacking of the perylene bisimides, which has a positive effect on

the solubility of the molecules. Another possibility to increase the solubility of the perylene

bisimide is by substituting the bay-positions R2. Chlorination of 1 can lead to 1,6,7,12-

tetrachloro-3,4:9,10-perylenedianhydride.20 It is possible to replace the chlorines by phenoxy

derivatives by reaction with phenols in presence of a base. Because of their bulkiness, the

substituents in the bay-region force the aromatic core to bend, which increases the solubility.

Although it is possible to substitute the bay-positions, this approach is not always utilized

because the purification of the tetrachloro compound is very laborious.12 Utilizing another

procedure, 1,7-substituted perylene bisimides can be synthesized, although in this

bromination reaction other dibromo isomers are also formed.21

2.2.2 Optical and electrochemical properties

An appealing property of perylene bisimides is that they are very stable under ambient

conditions and high light intensities.22 Perylene imides are characterized by a vibronically

structured strong absorption in the visible region of the spectrum and exhibit a similarly

strong fluorescence (Figure 2.4). The fluorescence quantum yield is near one and the lifetime

of the singlet-excited state is approximately 4 ns.

300 350 400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Inte

nsity

Wavelength (nm)

Figure 2.4. Normalized absorption and fluorescence spectra of N,N´-bis(1-ethylpropyl)-

perylene-3,4:9,10-tertacarboxylic-bisimide


9

Perylene imides have a rather low reduction potential and are reversibly reduced and

oxidized. The optical characteristic λabs and λPL, the fluorescence quantum yield φPL, the

fluorescence lifetime τ, and the first oxidation and reduction potentials Eox and Ered of some

perylene derivatives are collected in Table 2.1.

Table 2.1. Basic electrochemical (vs SCE in acetonitrile) and optical properties (in toluene)

of perylene and perylene imides.

Compound Eox

(V)

Ered

(V)

λabs

(nm)

λPL

(nm)

φPL τ

(ns)

Reference

Perylene 1.04 –1.73 440 446 0.89 4.6 23,24,25

Perylene monoimide

R1=2,5-di-

tertbutylphenyl

1.05a

–1.24a

505 528

0.99

4.8

26

Perylene bisimide

R1=2,5-di-

tertbutylphenyl

1.36a –0.81a 528

530 0.97

3.6

27

Perylene bisimide

R1 = 1-hexylheptyl

1.61 –0.59 524b 534b 0.99b 4.0b 28,29,30

a Versus Ag/Ag+, in butyronitrile Fc/Fc+ = +0.19 V; b in dichloromethane.

2.3 Photoinduced energy and electron transfer Perylene imides can engage in a variety of excited state energy and electron transfer

reactions, either as donor or as acceptor. Specific examples will be outlined in the remainder

of this chapter. This section discusses some of the mechanisms and theoretical concepts for

energy and electron transfer.

Figure 2.5 schematically illustrates the photoinduced energy and electron transfer

processes that can occur between an electron rich donor D and an electron deficient acceptor

A in a molecular D–A dyad. As shown in Figure 2.5, electron transfer occurs from donor to

acceptor, producing D+ + A–, but can originate by photoexcitation of either the donor (D*) or

acceptor (A*). With excitation of the acceptor, the process is also referred to a hole transfer.

In contrast, energy transfer is only possible when the component with the highest optical

band gap, Eg, is excited. In principle these processes can occur irrespective of the spin

multiplicity of the excited states as long as the spin is conserved in the transfer, even though

the transfer mechanism may be different. The energy and electron transfer reactions compete

Chapter 2

10

with the intrinsic decay processes of the photoexcited donor or acceptor such as thermal

deactivation, internal conversion, intersystem crossing, fluorescence and phosphorescence.

EgD

EgA

D A* D+ A- D* A

a b

EgD > EgA

EgD < EgA

c

Figure 2.5. Photoinduced energy and electron transfer between donor D and acceptor A: (a)

electron (hole) transfer with excitation of the acceptor (A*); (b) electron transfer with

excitation of the donor (D*); (c) energy transfer.

2.3.1 Electron transfer

For electron transfer to occur in the photoexcited state of a donor-acceptor system, two

different conditions must be met. First, the electron transfer reaction must be exergonic and

second, the rate of the reaction must be sufficient to compete with other decay processes. For

donor-acceptor molecules the energetics and kinetics of electron transfer reactions have been

extensively studied and many of their features can be described by the Weller equation for

the change in Gibbs free energy for charge separation and by Marcus´ theory for electron

transfer.

The Weller equation provides an estimate for the change in free energy for charge

separation ∆GCS in molecular D-A systems, by summing three terms describing the energy of

excited state redox reaction, the Coulomb term for the non-infinite distance between positive

and negative charges, and a term for the solvation of the ions formed:31

( ) ( )( )

−

+−−−−=∆ −+

sref0

2

ccs0

2

00redoxCS1111

84AD

εεπεεπε rr

e

R

eEEEeG (2.1)


11

In this equation Eox(D) and Ered(A) are the oxidation and reduction potentials of the donor

and acceptor moieties measured in a solvent with a relative permittivity εref and E00 is the

energy of the excited state from which the electron transfer occurs. Rcc is the center-to-center

distance of the positive and negative charges in the charge-separated state and εs is the

relative permittivity of the solvent of interest. The radii of the positive and negative ions are

represented by r+ and r–. Furthermore, –e is the electron charge and ε0 is the vacuum

permittivity. Equation 2.1 shows that electron transfer over longer distances is energetically

less favored and that for a given donor-acceptor combination the free energy for charge

separation becomes more negative when the polarity (εs) of the solvent is increased.

Marcus´ theory describes the kinetics of nonadiabatic electron transfer reactions in terms

of the free energy barrier ∆G‡ for electron transfer, the reorganization energy λ, and the

electronic coupling V between donor and acceptor in the excited state according to:32-34

∆−

=

Tk

GV

Tkhk

B

‡2

21

B2

3

CS exp4

λπ

(2.2)

Marcus showed that the barrier ∆G‡ is determined by the change in free energy (∆GCS)

and the reorganization energy (λ) via:

( )

λλ

4

2CS‡ +∆=∆ G

G (2.3)

The reorganization energy consists of an internal contribution (λi) and a solvent term (λs),

which can be approximated via the Born-Hush approach to give after summation:35,36

−

−

++=+= −+

s2

cc0

2

isi11111

2

1

4 επελλλλ

nRrr

e (2.4)

Marcus´ theory, in particular Equation 2.2, foresees that when the driving force for

electron transfer (–∆GCS) increases and the free energy becomes more negative, the activation

energy barrier is lowered and, hence, the electron transfer rate rises until the reorganization

energy equalizes the change in free energy for charge separation (λ = –∆GCS). At this point

the maximum rate for electron transfer is obtained. However, a further increase in the free

Chapter 2

12

energy implies that the activation energy barrier reappears and, hence, the electron transfer is

slowed down. The prediction implicates three regions for electron transfer: the ‘normal’

region, where the change in free energy for electron transfer is less than the solvent

reorganization energy (–∆GCS < λ), the optimal region (–∆GCS = λ), and the ‘inverted’ region,

where the change in free energy is larger (–∆GCS > λ).

The distance and orientation of the donor and acceptor molecules are of major

importance for the rate of electron transfer. The most important effect of these parameters is

the electronic coupling V2, which decreases exponentially with increasing center-to-center

distance:

[ ])(exp)()( 0cc02

cc2 RRRVRV −−= β (2.5)

Here R0 is the contact distance, and β is the parameter that measures the sensitivity of the

electronic coupling matrix element to the distance (the so-called damping factor). For many

systems β is in the range of 0.85-2.5 Å–1.37

2.3.2 Energy transfer

According to Förster,38 energy transfer from a singlet-excited donor to an acceptor may

occur via coupling of transition dipole moments. The Förster expression for the rate of energy

transfer kET is:

6

cET

1

=

d

Rk

τ (2.6)

In this equation τ is the lifetime of the excited state of the donor chromophore in absence

of transfer, Rc is the critical transfer radius (Förster radius), and d the center-to-center

distance between the two chromophoric centers. In Förster’s theory, the critical transfer

radius is given by:

F4A

5PL

26

c128

10ln9000J

nNR

πφκ= (2.7)


13

with NA Avogadro’s number, n the refractive index of the medium, φPL the fluorescence

quantum yield of the donor in the absence of transfer, and κ is the orientation factor, which

describes the dependence of the energy transfer rate on the mutual orientation of the

transition dipole moments of donor and acceptor chromophores. JF represents the overlap

between the absorption ( )(νε ) of the acceptor and the normalized fluorescence ( )(νF ) of

the donor on an energy scale (cm-1) defined as:

∫

∫=νν

νννεν

dF

dFJ

)(

])()([ 4

F (2.8)

The Förster mechanism is by far not the only process by which energy transfer may

occur. Because of a lack of transition dipole moments, triplet-triplet energy transfer cannot be

described with Förster’s mechanism. Instead, here the Dexter mechanism is often considered

in which a direct exchange of electrons in the excited state occurs. In strong contrast with

Förster’s mechanism, Dexter energy transfer requires an overlap of wave functions on donor

and acceptor and therefore decreases exponentially with increasing distance.

2.3.3 Spectroscopy of energy and charge transfer

Many donor-acceptor compounds that are discussed in the remainder of this chapter have

been studied with respect to photoinduced energy or charge transfer reactions. These

processes can be studied with time-resolved pump-probe absorption spectroscopy.39 In charge

transfer, the photogenerated ions generally give rise to electronic absorption spectra that

differ from the neutral molecules, and monitoring the changes as function of the time delay

after pulsed excitation gives insight into the kinetics of the forward and backward process.

Likewise, recording the excited state absorption (Sn←S1) or stimulated emission (S1→S0) can

give insights into the decay or formation of an excited state. A variety of setups exist,

operating on timescales from a few femtoseconds up to milliseconds. Besides spectral

information at certain time delays or time windows after photoexcitation, a characteristic

absorption, bleaching, or stimulated emission band can be followed in time to determine the

temporal evolution of the photoexcitation.

In addition to these pump-probe experiments, fluorescence spectroscopy is often used to

determine the extent and rate of quenching processes such as energy and electron transfer.

Fluorescence spectroscopy is generally much more sensitive than absorption spectroscopy,

Chapter 2

14

but often only gives indirect information.39 The fluorescence quenching (I0/I) of the donor or

acceptor, e.g., provides an estimate of the rate for energy or electron transfer that competes

with intrinsic decay of the excited state via kET = (I0/I–1)/τ or kCS = (I0/I–1)/τ, in which τ is the

lifetime of the excited state of the donor in absence of transfer. It is important to realize

however that fluorescence quenching can also be the result of other processes such as

aggregation.

Fluorescence lifetimes measured with time-correlated single-photon counting (TCSPC)

or fluorescence upconversion can also be used to determine kinetics of tranfers. If energy or

charge transfer takes place, the fluorescence lifetime is reduced compared to the intrinsic

lifetime by the competing process via:

( ) intrinsicquenchingobserved1

observed kkk +==−τ (2.9)

2.4 Porphyrin – perylene imide arrays Metalloporphyrins exhibit several characteristic absorption bands: two Q bands (~500-

600 nm), an intense B band (also called Soret band, ~380-420 nm), and weak N, L, and M

bands (~215-325 nm).40 The Q bands of the free-base porphyrins (i.e., two hydrogens in the

center) consist of four bands as a result of the different symmetry. After photoexcitation of

porphyrins luminescence can occur via different mechanisms, i.e. fluorescence, delayed

fluorescence, and phosphorescence. The delayed fluorescence is caused by repopulation of

the S1 state from the T1 state, which requires an input of energy to T1 that can occur by

thermal repopulation or triplet-triplet collisions. The presence of transition metals increases

intersystem crossing and, hence, decreases the fluorescence quantum yield. Likewise, the

presence and nature of a central metal atom influences the oxidation potential. E.g., for

tetraphenylporphyrin (TPP) derivatives, the first half-wave oxidation potentials of free-base

TTP, ZnTPP, and MgTTP are +1.11, +0.86, and +0.72 V (vs SCE, Fc/Fc+ = +0.48 V, Pt),

respectively.41

2.4.1 Bichromophoric dyads and extended structures

2.4.1.1 Porphyrin and oxochlorin dyads

Lindsey et al. have synthesized and investigated a large variety of perylene (bis)imide-

porphyrin26,27,42-44 and perylene (bis)imide-oxochlorin dyads (9-11, Figure 2.6).45,46 These

molecules consist of zinc Zn, magnesium Mg, and free-base Fb complexes of porphyrin or


15

oxochlorin covalently bound to perylene mono- or bisimides. The compounds have been

studied in apolar and polar solvents to elucidate the different photophysical processes that

take place after photoexcitation.

NN

O

O

O

O

R N

O

OR

N N

NN

NN

N N

O

9-Zn: R = por, M = Zn2+

9-Mg: R = por, M = Mg2+

9-Fb: R = por, M = 2H+

por oxo

10-Zn: R = por, M = Zn2+

10-Mg: R = por, M = Mg2+

10-Fb: R = por, M = 2H+

11-Zn: R = oxo, M = Zn2+

11-Mg: R = oxo, M = Mg2+

11-Fb: R = oxo, M = 2H+

M M

Figure 2.6. Perylene bisimide and monoimde dyads with free-base and metalloporphyrins

and oxochlorins.26,27,46

The comparison of results of perylene bisimide – porphyrin dyad 9-Fb with 9-Zn and 9-

Mg in toluene (εs = 2.4) and acetonitrile (εs = 37.4) reveals that in both solvents the

photoexcited perylene bisimide decays by energy transfer (kET ~1.9-3.2 × 1011 s-1) to the

porphyrin singlet state and by hole transfer to the porphyrin (kCS ~0.8-2.3 × 1011 s-1).27,42 The

extent of energy transfer is 70-85 % for dyads 9-Fb and 9-Zn, and around 45-50 % for 9-Mg

in both solvents. For all molecules the energy of the charge-separated state is lower in

acetonitrile than in toluene, consistent with Equation 2.1. The relative position of the charge-

separated state with respect to the porphyrin S1 state depends on the actual complexes of the

porphyrin. In both solvents the charge-separated state of 9-Fb is higher in energy than the

excited porphyrin, whilst for 9-Zn and 9-Mg it is lower in energy as a result of their lower

oxidation potentials. This has consequences for the decay pathways of the charge-separated

state and of the porphyrin singlet-excited state. In acetonitrile and toluene, the charge-

separated state of 9-Fb decays with more than 90 % efficiency to the singlet-excited state of

the porphyrin, which subsequently decays to the ground state with a fluorescence quantum

yield similar to that of an isolated Fb porphyrin. In contrast, for 9-Zn and 9-Mg, the

Chapter 2

16

porphyrin singlet-excited state decays via electron transfer to the charge-separated state,

albeit with a rate (kCS = 2.0-7.1 × 109 s-1) that is lower than the charge separation from the

perylene excited state. The electron transfer and the nonemissive recombination, result in a

negligible fluorescence of the two metalloporphyrin-containing dyads. The rate for charge

recombination in the charge-separated state depends on the polarity of the solvent and was

found to be at least a factor ten faster in acetonitrile than in toluene. The differences in the

rate constants for charge separation and charge recombination are in qualitative agreement

with Marcus´ theory for nonadiabatic electron transfer.

Upon replacement of the perylene bisimide in 9 with a perylene monoimide (10), the

reduction potential becomes more negative (Table 2.2), the linker shortens, and the site of

attachment changes from the imide nitrogen to the C9 position of the perylene core, which

has a higher electron density. In these perylene monoimide – porphyrin dyads 10, no charge

transfer takes place in toluene after photoexcitation of the perylene.26 Instead, almost

exclusively, an ultrafast energy transfer occurs with kET ≥ 2 × 1012 s-1. The energy transfer

product, the singlet-excited porphyrin, subsequently decays to ground state with the quantum

yield and lifetime equivalent to the corresponding isolated porphyrins. In acetonitrile, the

singlet-excited state of the perylene decays rapidly by a combination of energy transfer to the

porphyrin excited state and hole transfer to a charge-separated state (kCS + kET ≥ 2.5 × 1012

s-1).43 For the metalloporphyrins of 10-Zn and 10-Mg, the porphyrin-excited state undergoes a

quantitative electron transfer with a high rate (kCS > 1012 s-1). Electron transfer also occurs for

10-Fb in acetonitrile, but the rate of charge separation is quite slow (kCS = 1.4 × 109 s-1),

because the energy level of the charge-separated state is probably either slightly above or

equal to the level of the excited free-base porphyrin. In acetonitrile, the charge-separated state

decays within a few picoseconds by recombination to the ground state.

Table 2.2. Oxidation and reduction potentials (V) of dyads in butyronitrile containing 0.1 M

TBAH. E½ vs Ag/Ag+; E½ of Fc/Fc+ = 0.19 V. compound Eox

perylene

Eox (1)

oxo/por

Eox (2)

oxo/por

Ered (1)

perylene

Ered (2)

perylene

Ered (1)

oxo/por

Ered (2)

oxo/por

9-Zn +1.40 +0.52 +0.90 –0.77 –1.06 –1.71 –2.12

10-Zn +1.10 +0.50 +0.88 –1.14 –1.60 –1.75 –2.18

11-Zn +1.40 +0.51 +0.75 –0.76 –1.04 –1.56 –1.94


17

Another variation of 9 was studied, where the porphyrin was replaced by an oxochlorin

moiety (11).46 The oxidation potential of the electron donor in 11 is similar to that of 9 (Table

2.2), but the oxochlorin has an enhanced absorption in the red compared to the porphyrin.

Photoexcitation of the perylene bisimide chromophores in 11-Mg results in energy transfer

and hole transfer to oxochlorin in both toluene and benzonitrile (εs = 25.2). The photoexcited

oxochlorin undergoes an electron transfer to provide the charge-separated state. For the 11-Fb

and 11-Zn dyads, a similar decay of the perylene excited state via energy and hole transfer

occurs in both solvents, but the electron transfer from the oxochlorin excited state is inhibited

because it is less exergonic. For a related perylene monoimide Zn oxochlorin dyad, joined via

a diphenylethyne linker (not shown), a similarly efficient photoinduced energy transfer was

observed to the oxochlorin ground state, but without subsequent quenching of the excited

oxochlorin because the level of the charge-separated state is expected to be above the

perylene monoimide and oxochlorin singlet states in both toluene and benzonitrile.45 The

quantum yield for fluorescence of the singlet-excited oxochlorin (φPL < 0.05) is low because

this state primarily decays via intersystem crossing to the triplet state.

2.4.1.2 Perylene monoimide-porphyrin wires

Tri- and multi-chromophoric wires have been studied by Lindsey et al. to explore the

energy transfer processes in synthetic analogues of the photosynthetic light-harvesting arrays.

Linear arrays 12 and 13 each comprise three different chromophoric units A, B, and C (Figure

2.7).47,48 Wire 12 consists of perylene monoimide (A), a zinc porphyrin (B), and a free-base

porphyrin (C), while perylene monoimide (A), a bis(free-base porphyrin) (B) and a free base

phthalocyanine (C) are connected in array 13. In both arrays, excitation of the perylene

monoimide (A) in toluene around 490 nm, results in a rapid sequential energy transfer via B

to C, which in turn fluoresces at wavelengths of 650 and 720 nm (Q bands of C in 12) and

700 nm (13). For 12 the rapid, efficient through bond energy transfer occurs with an overall

rate of kET = 3.8 × 1010 s-1 and efficiency >99 %. For 13, the excited state of the

phthalocyanine is formed with two time constants of 2 ps (90 %) and 13 ps (10 %). Related

energy transfer cascades containing a perylene N-substituted monoimide, up to three zinc

porphyrins, and a terminal free base porphyrin, linked via diphenylethyne segments, have

been studied and showed similarly fast and efficient energy transfer reactions.48 In the longest

wire the overall rate (kET = 5.3 × 1010 s-1) and efficiency (81 %) remain high. It would

certainly be interesting to study these perylene monoimide-porphyrin wires also in more polar

solvents, to investigate if charge separation would occur.

Chapter 2

18

NN

NN

HH

N N

NNH

HN N

NNH

H

N

O

O

N N

NNH

HN N

NNN

O

O

12

13

A B C

Zn

Figure 2.7. Molecular photonic wires that exhibit efficient excited-state energy transfer from

A to C.47,48

2.4.1.3 Porphyrins bearing multiple perylene imides

Nanoparticles of tetrakis(perylene bisimide) substituted zinc tetraphenylporphyrin 14

(Figure 2.8), exhibit interesting properties with respect to photoinduced charge separation.

Self-assembly of 14 driven by van der Waals stacking into ordered 150 nm nanoparticles

takes place in solution and solid state,49 as evidenced from UV/Visible, 1H NMR, and AFM

experiments. The proposed structure of these nanoparticles is shown in Figure 2.8.

Photoinduced charge transfer in the nanoparticles occurs in toluene with near unit efficiency

with a rate of kCS = 3.1 × 1011 s-1 and the charges recombine with kCR = 1.4 × 108 s-1. The

charge transfer occurs after excitation at 420 nm (mainly porphyrin) or at 515 nm (selectively

perylene bisimide). The charge transfer is faster than for model compounds (not shown)

consisting of one zinc porphyrin containing only one or two perylene bisimides. In addition,

the recombination is somewhat slower than in the model compounds. Charge recombination

within the (14)n nanoparticle produces not only the ground state, but also the perylene

bisimide triplet state. This is a consequence of radical ion pair intersystem crossing within the

initially formed singlet radical ion pair. The average radical ion pair distance was estimated

to be 21 Å, while the chromophore center-to-center distance in the 14 is only 14.4 Å. This


19

implies that the electron on the perylene bisimide is five layers (Figure 2.8) removed form the

layer with the hole on the porphyrin.

N

N

O

O

O

O O

O

N

N

O

O

O

OO

O

N

N

O

O

O

O

O

O

N

N

N

N

N

N

O

O

O

O

O

O

14

Zn

Figure 2.8. Structure of 14 and cartoon of the self-assembled (14)12 nanoparticles.49 The

cartoon is reproduced with permission from reference 49. Copyright 2002 American

Chemical Society.

A large variety of perylene monoimide – porphyrin molecules and rods have been

synthesized and studied by Lindsey et al.50-52 Up to eight perylene monoimides have been

coupled to one porphyrin unit. Among these are the oligomers shown in Figure 2.9 in which

the porphyrin moieties are substituted with two or four perylene monoimides. Oligomers 15

and 16 were obtained via the Glaser polymerization, 17 via the Sonogashira, and 18 via the

Suzuki coupling.51 The Suzuki polymerization was less efficient, such that only short

oligomers and decomposition products of 18 were formed. Fluorescence studies of 15, 16,

and 17 revealed that efficient energy transfer to the porphyrin takes place after

Chapter 2

20

photoexcitation of the perylene monoimide at 490 nm. The excited state of the porphyrin is

almost unaffected by the presence of multiple perylenes, consistent with results obtained for

non-oligomeric dyads that resemble some of the monomers.50

N

O

O

O

O

ON N

NN

R1 R1

R2 R2

R3

R3 R3

n

N N

NN

R1 R1

n

N N

NN

R1 R1

R1 R1

n

pery

18: R1 = pery

15: R1 = pery, R2 = H, R3 = Me16: R1 = R2 = pery, R3 =H

17: R1 = pery

Zn

Zn Zn

Figure 2.9. Rod-like perylene monoimide – porphyrin oligomers.50-52

2.4.2 Perylene-porphyrin switches

Wasielewski et al. have shown that perylene imide-porphyrin systems can have

intriguing applications as photoswitches, utilizing a variety of concepts. For instance,

switching can be driven by the light intensity of the excitation or by different excitation

wavelengths. In this section some elegant examples will be discussed.

2.4.2.1 Light intensity-dependent optical switch

The first donor-acceptor-donor compound containing perylene bisimide 19 (Figure 2.10)

was reported by Wasielewski et al. in 1992.11 The perylene bisimide in 19 has reduction

potentials of Ered(1) = –0.50 V and Ered(2) = –0.73 V, while the porphyrin oxidizes at +0.92 V

(in pyridine, vs SCE). In pyridine solution, excitation of 19 at 585 nm with low light intensity

(1 photon per molecule) results in the formation of a D+–A––D charge-separated state with a

rate of kCS,1 = 1.1 × 1011 s-1, which subsequently recombines with a rate of kCR,1 = 9.1× 109

s-1. At high light intensity (20 photons per molecule) both porphyrins are simultaneously


21

excited and the rate for charge separation is twice as high (kCS,1 = 2.0 × 1011 s-1) in

consequence of the twofold increase in concentration of the excited porphyrins in the donor-

acceptor-donor. Twofold excitation (D*–A–D*) ultimately results in the formation of a D+–

A2––D+ doubly charge separated state, via D+–A––D*. Because the singly and doubly reduced

perylene bisimides have different wavelengths for maximum absorption (713 nm and 546

nm), the molecule represents a light intensity-dependent optical switch. At low light intensity

photoinduced electron transfer in 19 results in the absorption at 713 nm (D+–A––D), while at

high light intensity the absorption is at 546 nm (D+–A2––D+). The formation of the dianion

goes via the monoanion, with a rate constant of kCS,2 = 5.6 × 109 s-1, while the recombination

of the dianion is in the nanosecond regime (kCR,2 = 2.0× 108 s-1).

N

N N

N

C5H11

C5H11

N

NN

N

C5H11

NN

O

O

O

O

HH H

HH11C5 C5H11

C5H1119

Figure 2.10. Donor-acceptor-donor light intensity-dependent optical switch.11

2.4.2.2 Switching based on influence of photogenerated electric field

Another design of a photoswitch is 20 (Figure 2.11), consisting of two donor-acceptor

dyads that are linked together by a dimethylbenzene group between the acceptors to give a

D1–A1–A2–D2 configuration.53 The two donor-acceptor dyads are a perylene monoimide –

naphthalene bisimide (D1–A1) and a zinc porphyrin-pyromellitimide (D2–A2), respectively.

D1 can be selectively excited with 645 nm light, resulting in the formation of the D1+–A1

– ion

pair. Excitation of D2 at 420 nm results in the selective creation of the D2+–A2

– ion pair.

However, by using sequential excitation pulses at 645 nm and 420 nm (or vice versa),

formation of either D2+–A2

– (or D1+–A1

–) is completely inhibited by the presence of the

adjacent ion pair in the same molecule. This is caused by the electric field created by the

adjacent ion pair. Calculations showed that the electric field raises the energy of the other ion

pair by about 0.2 eV, leading to values of ∆GCS = 0 eV for D1+–A1

– and ∆GCS = +0.05 eV for

D2+–A2

–, in the presence of the other ion pair. Since the rate of formation of the electric field

is equal to the rate of charge separation (kCS(D1+–A1

–) = 1.6 × 1010 s-1, kCS(D2+–A2

–) = 3.7 ×

1010 s-1), this molecule can be switched in the picosecond timescale.

Chapter 2

22

N

O

O

N

O

O

N N

O

O

O

O

N H11C5

NN

N N

C5H11

C5H11

N

O

O

O

O

D1 A1 A2 D2

20

Zn

Figure 2.11. Picosecond molecular switch based on a photogenerated electric field.11

Also a triad molecule consisting of a zinc porphyrin – pyromellitimide – perylene

monoimide (D–A–C) has been reported that uses the influence of a photogenerated electric

field to create a picosecond molecular switch.54 Here the formation of an intense electric field

by a photoinduced charge separation that creates the D+–A––C ion pair, causes a 15 nm

electrochromic red shift of the charge-transfer absorption of C, whose absorption at 530 nm is

significantly diminished by the presence of D+–A–.

2.4.2.3 Photoswitched decay rates

Another elegant example of a perylene monoimide-porphyrin switch is triad 21 (Figure

2.12) that contains a gradient of oxidation and reduction potentials: zinc porphyrin (Eox =

+0.65 V), perylene monoimide (Ered = –0.90 and Eox = +1.40 V), and naphthalene bisimide

(Ered = –0.53 V) (vs SCE in butyronitrile).55 Therefore, excitation of 21 dissolved in THF

with 400 nm light results within 1 ps, either by direct excitation or by energy transfer, in the

formation of the singlet-excited state of the zinc porphyrin, followed by an initial electron

transfer (kCS = 1.7 × 1010 s-1) to the perylene monoimide (D+–B––A). Subsequently a shift of

the negative charge occurs to the naphthalene bisimide with a rate of kCS = 7.1 × 109 s-1.

Charge recombination of the zinc porphyrin-naphthalene bisimide charge-separated state

(D+–B–A–) occurs with rate of kCR = 1.4 × 106 s-1. Interestingly, the authors demonstrated that

it is possible to switch the reaction pathway and rate of charge recombination. This was

accomplished by selective photoexcitation of the perylene monoimide that bridges the

charged units. The positive charge located on the zinc porphyrin shifts to the perylene

monoimide after the excitation of the perylene at it absorption at 520 nm, resulting in the

perylene – naphthalene (D–B+–A–) charge-separated state. The decay of this charge-separated


23

state is 7000 times faster (kCR = 1 × 1010 s-1) than the decay of the porphyrin-naphthalene

(D+–B–A–) charge-separated state.

NN

O

ON

O

O

H11C5

N N

NN

C5H11

C5H11

O

O

C8H17

O

O

Zn

21

D B A

Figure 2.12. Donor-bridge-acceptor exhibiting ultrafast photoswitched charge transmission

through the bridge.55

Recently a tetramethylphenylenediamine – perylene monoimide – zinc porphyrin –

pyromellitimide D1–A1–D2–A2 tetrad was reported (Figure 2.13).56 Selective excitation at

either 540 or 420 nm leads to the formation of D1+–A1

––D2–A2 and D1–A1–D2+–A2

–

respectively. However, sequential formation of both ion pairs on a time scale as short as 50

ps, yields D1+–A1

––D2+–A2

–, irrespective of which chromophore is excited first. In

consequence, the decay of the charge-separated state can be modulated between charge

recombination from D1+–A1

––D2–A2 or D1–A1–D2+–A2

– (fast, kCR = 3.1 × 109 and 1.8 × 1010

s-1, respectively) and the decay of D1+–A1–D2–A2

– (slow, kCR = 1.5 × 106 s-1).

N

NN

O

O

O

O

N N

NN

C5H11

C5H11

NN

O

O

O

O

C8H17N

Zn

22D1 A1 D2 A2

Figure 2.13. Molecular tetrad that produces a long-lived charge-separated state only when

two excitation pulses are applied.56

Chapter 2

24

2.5 Conjugated oligomer/polymer – perylene imides Several strategies for incorporating perylene monoimides and bisimides in non-

conjugated polymers have been reported, e.g. condensation polymerization of perylene

dianhydride with diamines57 and ATRP reaction of styrene with a perylene bisimide

initiator.58 Perylene imide grafted polymers have been obtained by copolymerization of a

perylene bisimide monomer with other monomers59 and by reaction of a perylene imide

derivative with functional groups of a polymer.60 In this section, however, π-conjugated

oligomers and polymers linked to perylene imides are discussed that not only serve in

offering an architectural scaffold, but also represent electroactive chromophores.

2.5.1 Oligo- and polyfluorenes

Polyfluorene and polyindenofluorene are frequently used in polymer LEDs as blue-

emitters.61,62 Compared to poly(p-phenylene) these polymers have the same backbone, but

they have the advantage that solubilizing side groups can be attached without causing an out-

of plane twisting of the phenyl rings.63 The planarization of the backbone increases the

conjugation along the backbone. This effect is more pronounced for the ladder-type

polyindenofluorene than for the polyfluorene, which has only half the number of bridges.

Several well-defined oligomers of dialkylfluorene have been isolated and studied.64,65 In

solution their maximum absorption shifts from 329 nm (dimer), via 384 nm (decamer), to 388

nm (polymer). In films the absorption is slightly red-shifted and becomes broader. The

photoluminescence in solution shifts accordingly from 392 nm (dimer) to 445 nm (polymer).

The fluorescence maximum of the hexamer is already equal to that of the polymer. The

emission of the oligomers in solid state is quite similar to that in solution. Hence,

polyfluorenes are good candidates to transfer singlet-excited state energy to perylene imides,

while their relatively high oxidation potential inhibits the competing electron transfer.

2.5.1.1 Perylene bisimides incorporated in the polymer chain

An interesting model compound for perylene bisimides in a polymer main chain is

compound 23 (Figure 2.14), reported by Belfield et al.66,67 It consists of a perylene bisimide

substituted on the imide positions with a 7-benzothiazole-2-yl-9,9-didecylflouren-2-yl group.

In THF, 23 has two strong absorption bands between 270 and 385 nm and from 410 to 545

nm. The first band mainly originates from the fluorenyl groups and the latter from the

perylene bisimide. Excitation at either 350 nm or at 510 nm results exclusively in an emission

with bands at 540, 585 and 625 nm corresponding to emission from the perylene bisimide,


25

indicating an intramolecular energy transfer process from the excited fluorenyl to the

perylene bisimide. Remarkably, the fluorescence quantum yield of 23 in dichloromethane

was reported to be only φPL = 0.003, but the origin of the quenching was not discussed.67

NN

O

OO

ON

S S

NC10H21C10H21H21C10 H21C10

23

Figure 2.14. Perylene bisimide substituted fluorene.67

Müllen et al. reacted small percentages (1-5 %) of perylene bismide monomers with

fluorene monomers in a (random) Yamamoto copolymerization.68 The resulting perylene

bisimide-fluorene main chain polymers exhibit efficient energy transfer to the perylene

bisimide in the solid state. In solution however, this process is virtually absent and

fluorescence spectra show predominantly the emission of the fluorenes. Tests of these

polymers demonstrated that they have considerable potential as orange-red emissive materials

in LEDs.

Polyimides containing diphenylfluorene and 4,4´-(hexafluoroisopropylidene) diphthalic

imide segments with various percentages of perylene bisimide (24, Figure 2.15) show very

efficient energy transfer in chloroform and in the solid state.69,70 Efficient energy transfer

already occurs for a polymer with 0.33 wt-% of perylene bisimide. The emission in polymer

films with high perylene bisimide content in the main chain (6.76 wt-%), originates from

perylene bisimide excimers, exhibiting a characteristic emission at ~650 nm.

NN

O

O

O

On

x

x N N

O

O

O

O

F3C CF3

-1

24

Figure 2.15. Polyimide containing fluorene and perylene bisimide units in the backbone.69,70

2.5.1.2 Polyfluorenes with perylene monoimide endcappers

The synthesis and photophysical properties of various oligo- and polyfluorenes with

perylene monoimides endcappers (Figure 2.16) have been reported by Müllen et al. Upon

excitation, perylene monoimide endcapped terfluorene 25 exhibits an intramolecular energy

Chapter 2

26

transfer between the fluorene and the perylenes and between the perylenes in solution.71

Modulated single molecule fluorescence intensity traces on 25 allowed a conformational

analysis of 25 in a polymer matrix, revealing a predominant banana-shape-like chain

conformation of the endcapped terfluorene.71

Yamamoto polymerization of dibromofluorene and monobrominated perylenemonoimide

resulted in the formation of polymer 26.68 This polymer has an Mn of 21000 g/mol and a

polydispersity of 2.1. Approximately 40 % of the chains bear two endcappers, with the

remainder having only one perylene monoimide chromophore. Energy transfer in 26 was not

observed in solution, but is almost complete in the solid state, giving rise to a narrow red

emission with a maximum at 613 nm and a high photoluminescence quantum yield of >60 %.

Another example of endcapped polymers is 27, where a polyindenofluorene is terminated

at both ends by perylene monomide.72,73 The actual length of the polyindenofluorene is in the

range of 13-29 repeat units, but the effective conjugation length is restricted to approximately

6 units. Polymer 27 has been studied in p-xylene solution and in solid state. In solution, the

energy transfer from the polymer to the perylene monoimide is in competition with radiative

and nonradiative decay on the polymer, because both the polyindenofluorene and the

perylene monoimde moiety fluoresce, upon excitation of the fluorene. The decay of the

polyindenofluorene excited state occurs on a time scale of ~500 ps. The rather slow energy

transfer in solution was ascribed to being dominated by intrachain energy migration. In a

film, however, excitation of the polyindenofluorene at 400 nm results in a fluorescence

spectrum that is dominated by the fluorescence from the perylene monoimide moieties in the

range of 540 to 730 nm. This luminescence most likely occurs from interchain energy

transfer from the polymer to the endcaps. Time-resolved photoluminescence experiments

revealed that this transfer is complete within 30-40 ps after excitation and, hence, that energy

transfer is much faster in films than in solution. From time-resolved photoluminescence

experiments a Förster radius Rc = 1.8 ± 0.3 nm was obtained. Quantum chemical calculations

on 27 indicate a two-step mechanism for intrachain energy transfer with hopping along the

chains as rate-limiting step in solution, while the interchain transfer in the solid state is much

more efficient, mainly due to larger electronic coupling matrix elements between closely

lying chains.


27

DONOR

R R

n

R R

R R

n

N

O

OO

O

N

O

OO

O

25: R = C8H17, n = 3 26: R = C6H13

27: R = 2-ethylhexyl

DONOR

Figure 2.16. Perylene monoimide endcapped polyfluorene and polyindenofluorene.68,71-73

2.5.1.3 Perylene monoimide as side chain

An elegant example of side-chain substituted polyfluorenes is the statistical copolymer

28, containing 33 mol % of fluorene units with pendant perylene monoimides (Figure 2.17).68

Because of the high perylene monoimide concentration a very efficient energy transfer occurs

in this polymer, both in solution and in the solid state. A blend of 28 with an alkoxy-

substituted poly(p-phenylene vinylene) (MDMO-PPV) has also been investigated for its use

in solar cells.74 This blend gave photoinduced charge transfer upon excitation at 390 nm with

a rate kCS = 5.1 × 109 s-1 and an efficiency of 78 %. In photovoltaic devices with the blend as

an active layer, external quantum efficiencies (EQE) approaching 7 % were obtained.

H17C8 C8H17

m

NO O

OO

H17C8

n

28

Figure 2.17. Perylene monoimide side chains on a polyfluorene.68

2.5.2 Oligo(p-phenylene vinylene)s – perylene bisimides

Dialkoxy substituted oligo(p-phenylene vinylene)s (OPVns, n being the number of

phenylene rings) have been studied extensively75-77 and incorporated as the active material in

Chapter 2

28

LEDs78 and photovoltaic cells.79 The maximum absorption wavelengths range from 354 nm

(OPV2) to 481 nm (OPV12) in chloroform solution, the latter being similar to that of the

polymer, which indicates that 12 is the effective conjugation length. A concomitant increase

of the maximum fluorescence wavelength from 413 nm (OPV2) to 552 nm (OPV12) occurs.

OPVns can act as energy and electron donors towards fullerenes and have been incorporated

with fullerene C60 acceptors in covalent donor-acceptor dyads for solar cell applications.80,81

2.5.2.1 Covalently linked perylene bisimides and OPVs

A unique property of triad 29, in which two OPV4s are directly linked to the imide

nitrogens of a central perylene bisimide (Figure 2.18),82 is that it possesses a liquid-

crystalline mesophase between 215 and 310 °C. Upon annealing of thin films of 29, the donor

and acceptor functionalities self-assemble into an ordered material. The liquid-crystalline

behavior is caused by the combination of the flexible dodecyloxy chains at the termini and

the rigid perylene core. Photoinduced electron transfer was observed for 29 in THF solution

and in the solid state after excitation of the OPV4 donor at 450 nm with a rate kCS ≥ 1012 s-1.

Interestingly, the rates for charge recombination were found to be different in solution (kCR =

9.1 × 1010 s-1), in an amorphous film (kCR = 2.2 × 1010 s-1), and in an annealed more ordered

film (kCR = 4.1 × 1010 s-1 and kCR = 5.9 × 109 s-1, bi-exponential). The longer lifetime in the

solid state was explained by the migration of the photogeneric charges to neighboring

molecules. A higher mobility of the charges in the more ordered film ensures that the charges

can escape from geminate recombination and explains the increased lifetime under these

conditions.

In a related example 30, the OPV4s and perylene bisimide have been linked by via

saturated carbamate functionality between the imides of the perylene and the first aromatic

ring of the OPV4 (Figure 2.18).83 Triad 30 forms intermolecular hydrogen bonds in

chloroform and tetrachloroethane, but not in the more polar THF. Efficient photoinduced

electron transfer from OPV4 to perylene bisimide was inferred from fluorescence quenching

in various solvents, irrespective of which chromophore was excited. In the solid state the

characteristic absorption bands of the OPV4+• radical cation and perylene bisimide radical

anion were detected by photoinduced absorption spectroscopy.

Various novel polymeric architectures comprising covalently bound OPVns and perylene

bisimides and their photophysical properties are described in Chapters 4 and 5 of this thesis.


29

RNN

O

O

O

O

R

N

NN

O

O

O

OO O

O

N

O

H HRR

NN

O

O

O

O

H H

O

O O

O

NN

NN

N

HH

HH

RRN

NN

N

N

HH

HH

OO

OO

OC12H25

OC12H25

OC12H25

29

30 31

R =

Figure 2.18. Covalently and hydrogen-bonded oligo(p-phenylene vinylene) – perylene

bisimide donor-acceptor-donor triads.82-84

2.5.2.2 Hydrogen-bonded architectures

The supramolecular donor-acceptor-donor triad 31 is formed via two triple hydrogen

bonds of a perylene bisimide with two diaminotriazine functionalized OPV4s (Figure 2.18).84

In methylcyclohexane at room temperature these hydrogen-bonded triads aggregate into

helical fibers as inferred from circular dichroism (CD), UV/Visible spectroscopy, and AFM

imaging. Disassembly of these stacks occurs at 60 °C. Preliminary photophysical experiments

indicated that a fast photoinduced electron transfer (kCS = ~1012 s-1) occurs in this

supramolecular system upon excitation of the OPV donor.

In Chapter 6 of this thesis a quadruple hydrogen-bonded OPV4 perylene bisimide dyad is

presented that exhibits a fast intramolecular energy transfer reaction.

2.6 Dendritic structures with perylene imides Dendrimers are well-defined globular macromolecules that consist of a central core from

which branches (dendrons) diverge.85-87 They can have a high number of (functional) end

groups. The extent of branching of the dendrons classifies the generation of the dendrimer.

The number of end groups increases exponentially with each generation. Dendrimers are

appealing in the field of π-conjugated systems, for instance in the area of organic LEDs.88-91

Their globular organization of π-conjugated segments often prevents crystallization of the

chromophores, which otherwise may lead to reduction of the fluorescence quantum yield.

Moreover, different chromophores can be incorporated in one dendrimer in order to combine

Chapter 2

30

control over the emission wavelength with processing properties. Müllen and De Schryver et

al. have largely developed the chemistry and photophyiscs of dendrimers based on perylene

imides in the core or at the periphery and their work is discussed below.

O

OO

O

N OO

NO O

O

N N

O

O

OR R

RRO

R =

32

33

Figure 2.19. Second-generation polyphenylene dendrimers with a perylene bisimide cores.92

2.6.1 Perylene bisimide as dendritic core

Various zero-, first-, second-, and third-generation polyphenylene dendrimers containing

a perylene bisimide core have been studied,92 of which dendrimers 3293 and 33 with second-


31

generation polyphenylene dendrons are two examples (Figure 2.19). In THF solution, both

dendrimers have absorption bands in the UV region (280-350 nm) and in the visible region

(400-600 nm), originating from polyphenylene and perylene bisimide, respectively.

Excitation at 310 nm leads to an intense emission with a maximum around 600 nm,

corresponding to the emission of the perylene bisimide. Furthermore, a negligible or weak

fluorescence of the polyphenylene dendrons around 365 nm was detected. This result

indicates that an almost complete intramolecular energy transfer occurs. With increasing

generations, the energy transfer efficiency decreases slightly to 86 % for the third-generation

dendrimer of 32, most likely as a result of the increased distance between donor and acceptor.

In the second series, of which 33 is an example, the energy transfer is more efficient, given

the negligible residual fluorescence of the polyphenylene dendrons.

2.6.2 Perylene monoimide substituted triphenylamine core dendrimer

The synthetic route to dendrimer 34 affords two constitutional isomers that differ by the

position at the rim where the perylene monoimide is linked to the triphenylamine core

dendrimer.94,95 Figure 2.20 shows one isomer. In the second isomer the perylene is connected

at the position indicated with the asterisk (Figure 2.20). Triphenylamine has a relatively low

oxidation potential (Eox = 0.92 V, vs SCE in acetonitrile96) and is a good candidate for

donating an electron to the electron deficient perylene monoimide. A mixture of the two

isomeric dendrimers, studied in toluene and diethyl ether, exhibited intriguing photophysical

properties. In toluene no electron transfer occurs and the quantum yield of fluorescence is

close to 1. In diethyl ether, a moderately polar solvent, photoexcitation results in a fast long-

range through-space intramolecular electron transfer, leading to a long-lived ion pair state.

Two distinct sets of forward and backward electron-transfer rates were obtained,

corresponding to the two isomers. Interestingly, the back electron transfer is a thermally

activated process that yields the locally excited state of the perylene imide to produce delayed

fluorescence. Oxygen was found to influence the photophysical kinetics of the system very

strongly, probably because it interacts with the nearly degenerated triplet and singlet ion-pair

state. At low temperatures (77 K) the thermally activated delayed local fluorescence is

replaced by charge recombination luminescence from the long-lived charge separated state.

Chapter 2

32

N

O

O

N

* 34

Figure 2.20. One isomer of a triphenyl amine core dendrimer substituted with a perylene

monoimide.94,95

2.6.3 Dendrimers with terrylene as energy acceptor

Terrylene imides have a more extended π-system than the corresponding perylene

imides. In this section dendritic structures containing benzoylterrylene imide and terrylene

bisimide are discussed in which energy transfer takes place from perylene monoimide to

terrylene imide. Terrylene bisimide exhibits a half-wave oxidation potential Eox = +1.12 V

and reduction potential of Ered = –0.63 V (vs SCE).97 Benzoylterrylene imide is slightly more

difficult to reduce with a Ered = –0.83 V.98 The optical properties of terrylene bisimides

depend on their substituents, but in general they have strong absorption in the region of 500

to 700-750 nm with λmax at 650-700 nm, which is shifted about 100 nm bathochromically

compared to perylene bisimides. The terrylene imides fluoresce with quantum yields φPL of

~0.6-0.9 in the region of 600-800 nm. The fact that the fluorescence spectrum of perylene

imides has a good overlap with the absorption spectra of terrylene imides makes the latter

species very suitable as energy acceptor of the perylene excited state.

2.6.3.1 Terrylene in the rim

Figure 2.21 shows a first generation polyphenylene dendrimer 35 that bears three

perylene monoimides and one benzoylterrylene monoimide at the para-substituted

periphery.99,100 The absorption spectrum of this compound in chloroform has three well-

separated absorption envelopes in the region up to 350 nm (polyphenyl), 400-600 nm

(perylene monoimide) and 600-770 nm (benzoylterrylene monoimide). In 35, the

fluorescence quantum yield of the perylene monoimide is decreased by 96 %. Instead, 35

exhibits the emission of the terrylene monoimde chromophore as a result of an efficient


33

energy transfer. Because of the well-defined molecular core structure and high shape

persistence, energy hopping between the identical perylene chromophores and trapping at the

terrylene monoimide could be quantitatively studied and described using Förster theory. It

was found that energy hopping between all perylene imides takes place with a rate khop = 4.6

× 109 s-1. Trapping occurred at two different rates, kET = 5.7 × 109 s-1 and kET = 1.9 × 1010 s-1.

It was found that on average one of the three perylene monoimides takes a much better

through-space orientation or smaller interchromophoric distance toward the terrylene

acceptor than the other two, explaining the two different rates for the energy transfer.

For related multichromophoric compounds containing two, three, or four perylene

monoimides on the meta-positions of the phenyl rings in the rim, and lacking the terrylene

imide (not shown), a similar Förster type excitation energy transfer process was found with a

rate of kET = 5-10 × 109 s-1.101 In these compounds intramolecular singlet-singlet annihilation

takes place between excited perylene monoimides at high excitation densities with a rate kSS

= 1011 s-1.102 A similar singlet-singlet annihilation process also occurs, and is even more

important, in the corresponding para-substituted polyphenylene dendrimers with multiple

perylene monoimides.103 Ensemble and single molecule spectroscopic measurements on a

bichromophoric first generation polyphenylene dendrimer with para substituted perylenes

gave evidence for energy hopping, singlet-singlet annihilation, and singlet-triplet

annihilation.104 In a comparative study between first- and second-generation polyphenylene

dendrimers with para substituted perylene monoimides in the rim, the hopping rate of

second-generation dendrimers (khop = 0.85 × 109 s-1)105 was more than five times smaller than

in the corresponding first generation dendrimers.100

Chapter 2

34

N

O

O OO

O

N

O

O

N OO

N

O

O

35

Figure 2.21. Polyphenylene dendrimers with multiple perylene monoimide chromophores

and a terrylene monoimde acceptor trap. 99,100

2.6.3.2 Terrylene as luminescent dendrimer core

Polyphenyl dendrimers 36-1, 36-2, and 36-3 (Figure 2.22) contain a terrylene bisimide

core with perylene monoimides or naphthalene monoimides (for 36-3a) at the periphery and

have been studied with respect to intramolecular energy-transfer processes.106-109 Besides the

various dendrons at the bay-position of the central terrylene bisimide, two different

substituents on the imide functionalities have been described: two 2,6-diisopropyl phenyls

(a), or one cyclohexyl and one 2,6-diisopropyl phenyl (b). Dendrimer 36-1a is substituted

with first generation dendrons and contains a total of four perylene monoimides. Excitation of

the perylene monoimides of 36-1a in toluene at 480 nm, resulted in fluorescence of the

terrylene bisimide at ~700 nm with an almost complete (93 %) disappearance of the

fluorescence of the perylene monoimide at ~550 nm, indicating a very efficient energy

transfer.106 The energy transfer from perylene monoimide to terrylene bisimide in the related


35

donor-acceptor system 36-1b has two rate constants, kET = 2.5 × 1011 s-1 and kET = 4.0 × 1010

s-1.108 This phenomenon was also found for dendrimer 35, and was explained in terms of

different interchromophoric distances, caused by conformational variations. The two rate

constants obtained for 36-1b, were attributed to two independent energy transfer processes

going on in two different structural isomers with different in interchromophoric orientations

or distances. More detailed information on the intramolecular energy transfer in 36-1b was

obtained from single-molecule spectroscopy experiments.107 The authors demonstrated that

the excitation energy from the perylene monoimide could be transferred via a Förster

mechanism to the ground state as well as to the triplet state of the terrylene bisimide. In

addition, a stepwise bleaching of the terrylene emission was observed and was explained in

terms of a successive deterioration of the perylene chromophores. Of course, bleaching of the

five chromophores may occur in any order, and when the terrylene is bleached first, perylene

bisimides start to emit. Excitation energy dependent studies indicated that in dendrimer 36-

1b, energy transfer competes with singlet-singlet annihilation.109 With exception to the

single-molecule spectroscopy experiments, similar results were obtained for dendrimers 36-

2a and 36-2b.106,108,109 The two energy transfer processes in the case of 36-2b have rates of

kET = 4.5 × 1010 s-1 and kET = 1.5 × 1010 s-1 and are slower than for 36-1b because of a larger

center-to-center distance of the donor and acceptor, which is 2.3 nm for 36-1b and 3.1 nm for

36-2b. Dendrimer 36-3a contains three different chromophores and its absorption spectrum in

toluene consists of three regions at 350-400 nm, 440-560 nm and 590-730 nm, originating

from naphthalene imide, perylene imide and terrylene bisimide respectively, covering the

whole spectrum of visible light.106 Emission of the naphthalene imide (λmax = 341 nm) does

not overlap with the terrylene bisimide absorption (λmax = 665 nm). Hence, excitation of 36-

3a at 370 nm leads to a stepwise energy transfer from the naphthalene imide via the perylene

imide to the terrylene bisimide acceptor. This is demonstrated by a strong emission at 700 nm

(terrylene bisimide), and only weak emissions at 431 nm and 555 nm (perylene imide).

Chapter 2

36

NN R1

O

OO

O

R2

R2

R2

R2

NO

O

N

O

O

N

O

OO

N OO

N

O

O

O

N

O

OO

G1

G2 G3

36-1a, R1 = a, R2 = G136-2a, R1 = a, R2 = G236-3a, R1 = a, R2 = G3

a

b36-1b, R1 = b, R2 = G136-2b, R1 = b, R2 = G2

Figure 2.22. Polyphenylene dendrimers with terrylene as a luminescent core.106-109

2.7 Miscellaneous systems In this section several intriguing photoactive donor-acceptor architectures will be

discussed in which perylene imide is linked to various electro-active chromophores.

2.7.1 Tetrathiafulvalene as electron donor

Three triads 37, 38, and 39 (Figure 2.23), consisting of a perylene bisimide substituted

with tetrathiafulvalene (TTF) derivatives have been studied in chloroform.110 The triads 37-

39 have reduction potentials of ca. –0.67 and –0.85 V (perylene bisimide) and oxidation

potentials of ca. +0.47 V and +0.85 V (TTF) (vs Ag/AgCl in chloroform). The molecules

exhibit the characteristic absorptions of the TTF (around 320 nm) and perylene bisimide

(420-550 nm), indicating the absence of a strong charge-transfer interaction in the ground

state. However, when the molecules were excited at 480 nm, the fluorescence intensity at 540


37

and 574 nm was reduced for all triads in comparison to the fluorescence of a perylene

bisimide model compound. Similarly, the fluorescence lifetimes decreased, yielding 1.01 ns

for 37, 195 ps for 38, and 178 ps for 39 compared to 3.66 ns for the perylene bisimide

chromophore. These observations were tentatively ascribed to a photoinduced electron

transfer from the TTF units to the perylene bisimide. The different distances between the TTF

and the perylene bisimides, which are 25 Å for 37 and 12 Å for 38 and 39, assuming

extended conformations, rationalize the different behavior of the triads.

S

SS

S

S S

S

C6H13

C6H13

S

S

S S

S

S

S

O

O

O

O

O

O

SS

S

S S

S

NN

O

O

O

O

R R

37: R = a38: R = b39: R = c

a

bc

Figure 2.23. Donor-acceptor-donor triads based on TTF and perylene bisimide.110

2.7.2 Calix[4]arene as electron donor

A perylene bisimide with phenoxy substituents on the bay-position is less prone to

reduction than perylene bisimide itself. Würthner et al. elegantly demonstrated the difference

by studying two perylene bisimides 40 and 41, having hydrogen and phenoxy substituents on

the bay positions, respectively.111 Both 40 and 41 are substituted with electron-rich

calix[4]arenes at the imide positions (Figure 2.24). Fluorescence experiments indicated that

the fluorescence of 40 is completely quenched, whereas 41 emits with a relatively high

quantum yield φPL = 0.63. The quenching of 40 was ascribed to photoinduced electron

transfer.

Chapter 2

38

Figure 2.24. Calix[4]arene functionalized perylene bisimide.111 Reproduced with permission

from reference 111. Copyright 2002 American Chemical Society.

2.7.3 Pyrene and ferrocene as donors in self-assembled squares

Macrocyclic metal complexes were first introduced by Fujita et al.112 and this concept

was adopted by Würthner et al. to incorporate perylene bisimide ligands (42, Figure 2.25).113

Macrocycle 43 is a square consisting of four perylene bisimide ligands complexed with Pt

and comprising in total sixteen pyrene moieties.114 The formation of square 43 was achieved

by equimolar mixing of a di-4-pyridine substituted perylene bisimide and [Pt(dppp)][(OTf)2]

in dichloromethane. In the UV/Visible absorption spectrum, pyrene gives rise to transitions at

314, 328, and 344 nm and the perylene bisimide absorbs mainly from 400 to 600 nm, with

three bands at 450, 525, and 575 nm. Selective excitation of the pyrenes at 344 nm results in

an almost exclusive emission originating from the perylene bisimide with a maximum around

610 nm. This result points to energy transfer from the excited pyrene to the perylene

bisimide. The excitation spectrum of the fluorescence at 630 nm reveals that about 50 % of

the excited pyrene chromophores participate in the energy transfer process. Recently, similar

perylene bisimide squares (44 and 45) were reported that bear 16 ferrocene units on the

periphery.115 These squares and also their ferrocene-substituted perylene bisimide ligands are

hardly fluorescent in dichloromethane solution (φPL < 0.002), while squares and ligands with

only perylene bisimide have very high fluorescence quantum yields (φPL = 0.86-0.94).116 It

was presumed that electron transfer from ferrocene to perylene bisimide causes this

quenching.

40 41


39

NNN N

O

O

O

O

N

R R

R R

Pt

PP Pt

PP

N

N

N

OO

OO

N

RR

RR

NN N

O

O

O

O

N

R R

R R

Pt

PP Pt

PP

N

N

OO

OO

N

RR

RR

OO

O8 CF3SO3

-

O

O

O n

O

Ph2

Ph2Ph2

Ph2

Ph2

Ph2 Ph2

Ph2

8+

42 R = a43 R = b44 R = c45 R = d

a

c : n = 3d : n = 4

b

Fe

Figure 2.25. Electro-active molecular squares containing perylene bisimdes.113-116

2.7.4 Anthraquinone as energy donor

Energy transfer has been studied on various perylene bisimide-anthraquinone dyads.117

9,10-Anthraquinone absorbs between 300 and 350 nm and exhibits only a weak luminescence

itself, however it can act as an energy donor for perylene bisimide, provided the energy

transfer is fast enough. It was found that energy transfer in these dyads strongly depends on

the relative orientation of the chromophores. This is illustrated by dyads 46 and 47 (Figure

2.26). Upon excitation between 300 and 350 nm dyad 46 undergoes energy transfer, whereas

isomeric dyad 47 does not.

NN

OO

O O

R

O

O

O

O

a

b

46 R = a47 R = b

Figure 2.26. Perylene bisimides with non-fluorescent anthtaquinones.117

2.8 Perylene imide as an electron donor

Interestingly, perylene imides not always take the role of an electron-accepting moiety in

donor-acceptor systems, but in some cases they act as the electron donating species. This has

Chapter 2

40

been described for homo dimers and hetero dimers. For instance, in several perylene

bisimide-perylene bisimide dyads charge transfer occurs from one perylene bisimide to the

other. Also in heterodimers of perylene imides with more electron deficient chromophores,

perylene imide can act as the electron donor.

2.8.1 Perylene-perylene dyads

In perylene bisimides substituted with cyclic amines such as N-pyrrolidinyl, N-

piperidinyl, and N-morpholinyl in the 1- and 7-position, the maximum absorption wavelength

has shifted from 525 nm (unsubstituted) to 650–700 nm.118 These substituents also have a

major effect on the oxidation potential, which reduces to +0.72, +0.80, and +0.87 V

respectively (vs SCE in butyronitrile, Fc/Fc+ = +0.52 V). The reduction potential is less

affected and shifts negative by only 0.3 V. The effects are caused by the electron donating

effect of the amines into the perylene core, leading to the formation of a charge-transfer

transition.119 Substitution of a perylene monoimide with a N-pyrrolidinyl or N-piperidinyl

unit on the 9-position results in oxidation potentials of +0.61 and +0.81 V respectively (vs

SCE in butyronitrile).120 In tetrad 20, where the perylene is substituted with a N-pyrrolidinyl

on the 9-position, the perylene monoimide functioned as an electron donor. In the following

several examples of electron donating perylene imides will be discussed.

2.8.1.1 Perylene bisimide as electron donor

Several molecules have been reported that contain two or more perylene bisimides.

Langhals et al. synthesized chains of two and three perylene bisimides and a

tetraphenylmethane, which is substituted with a perylene bisimide on each para-

position.121,122 These multiple perylene bisimides have high fluorescence quantum yields

approaching φPL = 1. This section will focus on multiple-perylene bisimide compounds in

which electron or energy transfer occurs after excitation.

An interesting example was provided by Wasielewski et al. Dimers 48 and 49 have one

perylene bisimide that acts as an electron donor and an identical adjacent perylene bisimide

that acts as electron acceptor (Figure 2.27).123 Both perylene bisimides are substituted with N-

pyrrolidinyl on the 1- and 7-positions. For the cofacial dimer 49 a quantitative ultrafast

charge separation occurs in toluene (kCS = 1.9 × 1012 s-1, kCR = 4.5 × 109 s-1) and in 2-

methyltetrahydrofuran (2-MeTHF) (kCS = 3.0 × 1012 s-1, kCR = 2.6 × 1010 s-1). Excitation of

linear dimer 48 only results in charge separation in the more polar 2-MeTHF (kCS = 1.8 × 1010

s-1, kCR = 1.0 × 1010 s-1). It is likely that in the higher polarity solvent, solvent dipole


41

fluctuations lead to symmetry breaking in the excited state of 48, producing the Zwitterionic

state.

N

NO O

O O

N

N

N

NO O

O O

N

N

O

N N

O

O

O

O

N

N

N N

O

O

O

O

N

N

48

49

Figure 2.27. Linear and cofacial dimers of a perylene bisimide.123

In heterodimer 50 a 1,7-bis(pyrrolidin-1-yl)-perylene bisimide is linked to a 1,7-bis(3,5-

di-tert-butylphenoxy)-perylene bisimide (Figure 2.28).119 The absorption spectrum of 50

exhibits two distinct absorption bands at 550 and 695 nm, corresponding to the two

chromophores. Selective excitation of the pyrrolidinyl-substituted chromophore at 680 nm

results in charge separation reaction in both toluene (kCS = 2.0 × 1010 s-1, kCR = 5.0 × 108 s-1)

and 2-MeTHF (kCS = 7.7 × 1010 s-1, kCR = 3.3 × 108 s-1). Excitation of the phenoxy-substituted

chromophore with either 420 or 530 nm, gave identical rate constants for the charge

separation, indicating that energy transfer occurs faster than the time resolution set by the

instrument response (~180 fs).

N N

OO

O O

O

O

N N

OO

O ON

N

50

Figure 2.28. Asymmetric donor-acceptor dyad based on perylene bisimides.119

Recently intramolecular electron transfer between two perylene bisimides was reported

for dimers 51-n with n = 0 to 3 (Figure 2.29).124 The occurrence of intramolecular electron

Chapter 2

42

transfer has been inferred from fluorescence quenching studies in solution and using single

molecule spectroscopy. Remarkably, the extent of electron transfer does not simply decrease

with increasing chromophore separation, but follows an oscillating pattern with n the number

of phenyl rings. A stronger fluorescence quenching (more electron transfer) was observed for

n = 0 and n = 2, than for n = 1 and n = 3. This behavior has been explained in terms of the

relative orientation of the chromophores. Dimers 51-0 and 51-2 have a twisted conformation

of the perylene bisimides with a low overlap between the orbitals of the electron donor and

acceptor. According to the authors this is necessary to obtain a more stable charge-separated

state. Interestingly, this explanation contrasts with Equation 2.1 that indicates that a charge-

separated state is energetically less favorable for larger Rcc. Moreover, this result is at strong

variance with Equation 2.5, which shows that there is an exponential decrease of the

electronic coupling with increasing distance. Values for β are often in the range of 0.85-2.5

Å-1, 37 and hence Equation 2.5 suggests that the rate for electron transfer decreases by at least

6 (and possibly up to 10) orders of magnitude going from n = 0 to 3.

NNNN

O

O

O

O

O

O

O

O

R Rn

n = 0, R = 1-hexylheptyln = 1-3, R = 1-nonyldecyl51-n

Figure 2.29. Oligophenylene bridged symmetric perylene bisimides.124

2.8.1.2 Perylene trisimide as energy donor

In dyad 52, the perylene trisimide moiety can transfer the singlet-excited state energy to

the perylene bisimide.125 The perylene trisimide absorbs in the regions 300-380 nm and 390-

490 nm, while perylene bisimide absorbs mainly in the region of 400-550 nm. After

excitation with wavelengths ranging from 370 to 527 nm no fluorescence of the trisimide at

450-600 nm was observed in dyad 52 (Figure 2.30). Instead, only emission from the bisimide

at 525-650 nm was detected in these experiments, with a quantum yield of 1, independent of

the excitation wavelength. This points to efficient energy transfer from the trisimide to the

bisimide, which was confirmed by excitation spectra. The rate was determined as kET ≈ 3.6 ×

1012 s-1, which is faster than the rate of intramolecular non-radiative relaxation within the

donor kNR = 8.8 × 107 s-1.


43

NN

O

O

O

O

N

N

N OO

OO

O

O

52

Figure 2.30. Bichromophoric perylene trisimide-bisimide.125

2.8.2 Heterodimers

Several naphthalene bisimide – perylene imide dyads have been reported by Wasielewski

et al. Naphthalene bismide has been linked to perylene bisimide (53)119 and perylene

monoimide (54),120 both bearing the N-pyrrolidinyl group (Figure 2.31). Upon exciting the

perylene bisimide at 420 nm, charge separation occurs in toluene and 2-MeTHF. Similar to

50, the charge separation and recombination rates are higher in 2-MeTHF than in toluene

solution (Table 2.3), consistent with Marcus´ theory.

Table 2.3. Free energies and time constants for charge separation and charge recombination

solvent compound ∆GCS (eV) kCS (ns-1) ∆GCR (eV) kCR (ns-1)

toluene 50 –0.29 20 –1.45 0.5

53 –0.32 25 –1.41 1.8

54 –0.36 130 –1.54 6.8

2-MeTHF 50 –0.49 77 –1.24 3.3

53 –0.56 167 –1.17 77

54 –0.42 420 –1.44 330

Excitation at 400 nm of dyad 54 also leads to charge separation, exhibiting solvent

dependence, but with much higher rates. Electroabsorption spectra indicated that there is a

significant difference between the ground and excited-state dipole moments of the two

perylenes, with values for ∆µ of 3.5 and 15.4 D for 53 and 54, respectively. This result can be

rationalized by the fact that the perylene units in 53 and 54 have two and one imides,

Chapter 2

44

respectively, which makes that excitation of these chromophores results in a greater

redistribution of the negative charge toward the single imide group in 54 than toward the two

imides of 53. Because of this asymmetry of the charge distribution in the perylene of 54, the

effective charge transfer distance is shorter than in 53, leading to an increase in electronic

coupling and an increase in kCS. Studies on 54 demonstrated that this compound exhibits

photorefractive effects in nematic liquid crystalline composites.126

N N

O

O

NN

O

O

O

O

N

N

O

O

H17C8 N N

O

O

N

O

O

O

O

H17C8

O

O

N

53 54

Figure 2.31. Perylene bisimides as electron donors in dyads with naphthalene bisimide.119,120

2.9 Application of energy and charge transfer in devices

The use of energy and electron transfer reactions involving perylene imides in opto-

electronic devices is an area of increasing interest.

So far a limited number of reports exist describing the use of perylene bisimides in

organic LEDs. The most recent example has been presented by Müllen and Friend et al.,

showing a series of as statistically distributed main chain copolymers of fluorenes and

perylene bisimides that were used to make efficient LEDs with emission colors covering the

whole visible spectrum.68 Earlier applications of perylene bisimides in LEDs comprise

incorporation main chain polymers,127 perylene bisimide as dopant in hybrid sol-gel

matrices,128 as dye dopant for color-tuning,10,129 or for improving electron transport properties

of a PPV polymer.130

In 1986, Tang reported an early example of a double-layer organic solar cell.9 The device

consisted of a layer of copper phthalocyanine (CuPc) as donor and p-type semiconductor on

top of a layer of perylene bisbenzimidazole as acceptor and n-type semiconductor and gave a

power conversion efficiency of about 1 % and a relative spectral response exceeding 50 %.

Since then, several photovoltaic devices incorporating perylene bisimide derivatives together

with p-type molecular organic donors have been reported.131,132 Besides a variety of

phthalocyanines,133-142 other p-type molecules such as N,N´-diphenyl-N,N´-ditolyl

benzidine143 and pentacene,144 have been used in the double layer devices. Attempts have


45

been made to improve the charge transport in the cells by introducing liquid crystalline

phthalocyanine donors in double-layer devices.145 The best devices so far are based on

multistacks of bilayers of CuPc and perylene bisbenzimidazole with Ag interlayers. These

devices reach power conversion efficiencies of about 2.5 %.146 Apart from small p-type

organic molecules, conjugated polymers like poly(p-phenylene vinylene)147,148 and

polythiophene149 have been used as donor in bilayer devices with perylene bisimides. So far

these cells have provided power conversion efficiencies up to 0.71 % with a monochromatic

external quantum efficiency of 18 %.148

An alternative and possibly promising approach to photovoltaic cells is the bulk

heterojunction cell in which donor and acceptor materials are mixed on a nanoscopic scale,

rather than in a bilayer, to create large interface that enhances the likelihood of excitons being

dissociated into free charge carriers because the distance to interface is less than the exciton

diffusion length. Following this approach perylene bisimides have been mixed with poly(p-

phenylene vinylene)s150-152 and poly(3-hexylthiophene).153 The latter devices showed an

external quantum efficiency of 11 % after annealing to improve the morphology.153 In a

similar approach perylene bisimides have been incorporated into bulk heterojunction devices

with liquid crystalline hexabenzocoronene to give external quantum efficiencies of more than

34 %. Here the self organization of the materials was important in creating an optimized

structure for charge generation and transport.154

Perylene bisimide derivatives have also been considered as dyes for dye-sensitized solar

cells. Willig et al. reported the subpicosecond electron injection from a phosphonic acid

perylene derivative into TiO2.155 Gregg et al. showed that perylene-3,4-dicarboxylic acid-

9,10-(5-phenanthroline)-carboximide strongly absorbs on SnO2. In combination with a liquid

electrolyte this dye-sensitized nanoporous SnO2 gave photovoltaic cells with a power

conversion efficiency of 0.89 % and external quantum efficiencies close to 30 %.156 More

recently, multichromophoric soluble perylene derivatives on nanoporous SnO2157 were tested

and new perylene photosensitizers were described based upon perylene bisimides with more

favorable energetics for electron injection into TiO2.158

Although blends of perylene bisimides and other optoactive materials have shown to be

applicable in photovoltaic devices and organic LEDs, there are some drawbacks to this

approach. Even when the optimal phase separation for the functional properties can been

obtained, the stability of the morphology in time can be low because of the phase separation

into larger clusters can originate. An approach to achieve an intimate mixing of the different

components and sustaining this morphology in time is connecting the different functional

Chapter 2

46

moiety covalently or in a supramolecular fashion. This thesis describes a number of such

novel materials.

2.10 Conclusion The previous sections have illustrated the present state-of-the-art on the photophysical

properties of donor-acceptor systems in which perylene imides are incorporated as donors or

acceptors in energy and electron transfer reactions. The versatility in changing the optical and

electrochemical properties by tuning the substitution pattern using in electron-withdrawing or

-donating substituents has allowed utilizing the perylene chromophore in a multitude of

functions. In combination with their electron transport properties and extraordinary stability,

perylene bisimides are likely to fulfill an important role in future electro-optical molecular

materials and devices.

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Chapter 2

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53

3

Photoluminescence of self-organized perylene bisimide

polymers*

Abstract

Three polymers consisting of alternating perylene bisimide chromophores and

flexible polytetrahydrofuran segments of different length have been synthesized

and studied using absorption and (time-resolved) photoluminescence

spectroscopy. In ortho-dichlorobenzene, the chromophores self organize to form

H-aggregates. The extent of aggregation decreases with increasing temperature.

The photoluminescence spectra of the self-organized polymers consist of

vibronically resolved monomeric perylene bisimide fluorescence (λmax = 538 nm,

τ = 3.9 ns) and unstructured excimer-type emission (λmax = 635 nm, τ = 17 ns).

An additional short-lived (τ ≈ 2 ns) luminescence component is observed and is

ascribed to the dynamic deactivation of the monomeric photoexcited state via

excimer formation or energy transfer.

*This work has been published: (a) E. E. Neuteboom, R. A. J. Janssen, E. W.

Meijer, Synth. Met. 2001, 121, 1283. (b) E. E. Neuteboom, S. C. J. Meskers, E.

W. Meijer, R. A. J. Janssen, Macromol. Chem. Phys., in press.

Chapter 3

54

3.1 Introduction Substituted perylene bisimides1 are thermally and photochemically stable organic

semiconductors. As molecules, they have been incorporated in electronic and optical devices

such as field-effect transistors,2 electrophotographic applications,3 and photovoltaic devices.4

For certain applications, it is desirable to organize these chromophores spatially. This can be

achieved by non-covalent interactions such as self-assembly via liquid crystals5 or hydrogen

bonding,6 in combination with π-π stacking.7 Alternatively, perylene bisimides have been

incorporated in large structures such as dendrimers8 or polymers9 via covalent bonding.

In general, interactions between two or more chromophores affect the luminescence

properties.10 One option is excimer formation by the collision of an electronically excited

molecule with a molecule in the ground state. The excimer is stabilized by excitonic

interactions and charge-transfer,11 and the electronic properties differ considerably from that

of the monomer. Excimers often exhibit a characteristic broad and unstructured, red-shifted

emission with a decay time and quantum yield that differ considerably from those for the

monomer. The large average intermolecular distance in dilute solution makes that excimers

are mainly observed in concentrated solution. However, when chromophores are brought in

close proximity, e.g. by a flexible scaffold, the probability for excimer formation is greatly

enhanced. Excimer-type emission has also been observed in the solid state. As an example,

ultrathin submonolayer films of perylene bisimides show monomeric emission, while the

luminescence of thicker samples is dominated by a red-shifted ‘excimer-type’ transition.12,13

In the solid state, and in general in pre-organized or aggregated chromophores, coupling of

transition dipole moments may occur, resulting in a splitting of electronic levels in the

excited state. The photoluminescence properties of these exciton-coupled chromophores,

strongly depend on their relative orientation and may show the characteristics of an excimer.

In this respect it is of interest to study the photoluminescence properties of

polychromophores, i.e. polymers in which several chromophores are linked by flexible

segments.14 Interestingly, polymers offer a means to construct ordered nanoscale domains

through self-organization on the basis of competing interactions. Perhaps the most studied

example is provided by block copolymers, where the combination of complementary and

antagonistic interactions between chemically connected blocks leads to self-organization with

length scales on the order of 10 to 100 nm.15,16

In this chapter the synthesis of copolymers P1, P2, and P3 (Figure 3.1), consisting of

alternating segments of perylene bisimide chromophores and polytetrahydrofuran (polyTHF)

of different average length (3, 14, and 33 THF units, respectively) is reported. In these

Photoluminescence of self-organized perylene bisimide polymers

55

copolymers π-conjugated (hard) blocks are incorporated together with non-π-conjugated

(soft) blocks of different length in an attempt to obtain ordered conjugated polymers on a

nanoscopic scale and assess the formation of perylene bisimide aggregates in these

copolymers in solution. This chapter presents a detailed study of the UV/Visible absorption

and photoluminescence properties of these polymers in solution ortho-dichlorobenzene

(ODCB) and identifies, in addition to the monomeric emission, a longer-lived excimer-type

emission and a shorter-lived component caused by a dynamic quenching process.

NN

O

O

O

O

OO

n

m

NN

O

O

O

O

OO

O

O

O

O

NH2 OO NH2m

P1: m~3P2: m~14P3: m~33

PERY

+

m-cresolisoquinoline200 °C

Figure 3.1. Synthesis of alternating perylene bisimide – polyTHF copolymers and the

reference compound.

3.2 Synthesis The synthesis is performed following the reaction scheme outlined in Figure 3.1. Starting

material perylene-3,4:9,10-tetracarboxylic dianhydride and three different bis(3-aminopropyl)

terminated polytetrahydrofurans were allowed to react to yield the copolymers P1, P2 and

P3. For the reaction a 15 wt-% mixture of equimolar amounts of dianhydride and amino

terminated polytetrahydrofuran was heated in m-cresol and isoquinoline to 200 °C for 16

hours.17 After cooling to room temperature the reaction mixture was precipitated in diethyl

ether and the product was washed several times with diethyl ether and water. The polymers

were dried overnight under vacuum at 150 °C and obtained in moderate to good yields as

dark red to black solids. The 1H NMR spectra confirm the expected structures. In the FT-IR

ATR-spectrum of the products peaks at 1695 and 1652 cm-1 (C=O of 6-membered imides)

were detected, while the characteristic vibrations of the starting products are absent. The size

exclusion chromatogram of each of the products with chloroform as eluent gives values of Mn

Chapter 3

56

of 4.4 kg/mol (P1), 13.4 kg/mol (P2) and 17.6 kg/mol (P3) with polystyrene standards. From

these number the mean value of n, which is the number of repeat units in the polymers, was

calculated as 4.8, 8.4 and 5.9 for P1, P2 and P3 respectively. TGA measurements show that

the polymers start to decompose around 250 °C.

3.3 Absorption and emission The normalized UV/Visible absorption spectra of the three polymers (P1, P2, and P3)

and of N,N´-bis(1-ethylpropyl)-3,4:9,10-perylenebiscarboximide (PERY)18 as a reference

compound, all dissolved in ODCB (~10-6 M perylene bisimide chromophore), reveal distinct

differences (Figure 3.2a). The reference compound is highly soluble as a result of the

secondary carbon atoms next to the nitrogen atoms, which force the alkyl chains out of the

plane of the molecule and thereby hamper the face-to-face π-π stacking of the perylene

bisimides. Consequently, the absorption spectrum of PERY corresponds to that of a

molecularly dissolved chromophore and exhibits a strong 0-0 vibronic transition at 530 nm.

The perylene bisimides in polymers P1, P2, and P3 lack these secondary carbon atoms and,

as a consequence, they exhibit a stronger tendency to self organize via π-π stacking in

solution. This aggregation results in a change of the relative absorption intensities in the

vibronic bands and, additionally, in a new low-intensity absorption at wavelengths higher

than the 0-0 transition of the molecularly dissolved chromophore. Figure 3.2a shows that

each polymer is aggregated to a different extent. The absorption band at 493 nm is most

pronounced for the polymers with the shortest polyTHF segments and indicates more

aggregation. In a first approximation, this can be rationalized by considering that long spacers

enhance the solubility of the chromophores. The changes in the optical spectra are

reminiscent of H-aggregates for which the coupling of transition dipole moments causes a

splitting in the excited state. For parallel orientations of the chromophores the low-energy

state of an H-aggregate corresponds to a dipole-forbidden transition and the high-energy state

to a dipole-allowed absorption. The blue-shifted absorption maximum and the red shift of the

absorption onset observed for P1, P2, and P3 are consistent with the formation of H-type

aggregates (largely face-to-face) of perylene bisimides.

Aggregation of chromophores not only affects the absorption, but also the emission

spectra. The fluorescence spectra of the polymers and the reference compound in ODCB

recorded with excitation at 530 nm (Figure 3.2b) show that shorter polyTHF segments (i.e.

more aggregation) result in more quenching of the photoluminescence. Apart from the

change in intensity, the fluorescence spectra of P1, P2, P3, and PERY are rather similar.


57

Compared to the fluorescence quantum yield of PERY, which is close to unity, the emission

at 538 nm of the polymers P1, P2, and P3 is quenched by a factor of 3.3, 2.5, and 1.5,

respectively. The decrease of fluorescence intensity is consistent with the formation of H-like

aggregates in which the low-energy excited state is not radiatively coupled to the ground

state.

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

1.2

550 600 650 7000.0

0.5

1.0

1.5

2.0

2.5 ba

Nor

mal

ized

Abs

orpt

ion

(O.D

.)

Wavelength (nm)

PL

Inte

nsity

(co

unts

x 1

0-6)

Wavelength (nm)

Figure 3.2. Optical spectra of P1 (dot-dashed line), P2 (dotted line), P3 (dashed line), and

PERY (solid line) in ODCB (~10-6 M chromophore). (a) Normalized UV/Visible absorption.

(b) Fluorescence with excitation at 530 nm, corrected for optical density.

3.3.1 Temperature dependent absorption

The copolymers show interesting thermochromic behavior in ODCB solution (Figure

3.3). While all three solutions show a similar blue shift of the three principal vibronic

transitions with heating, marked differences occur for their relative intensities, depending on

the polyTHF block length (Figure 3.4a). The ratio of the intensities of the 0←0 vibronic

transition (532 nm at 20 °C) and the 1←0 vibronic transition (494 nm at 20 °C), represented

by I01, increases for P1 from 0.87 at 20 °C to 1.08 at 110 °C. The spectra of the P2 solution

(Figure 3.3b) show also an increase of I01, although the increase is less strong. At 20 °C I01

has a value of 1.12 and at 110 °C a value of 1.29. The spectra of the P3 solution (Figure 3.3c)

display a different behavior. Although the spectra decrease in intensity by heating (due to

expansion of the solvent), I01 is almost constant with an average value around 1.41. This

points to a situation where the perylene bisimide blocks in the polymers P1 and P2 are clearly

more aggregated than in P3 and that the aggregates can be partially dissolved upon heating.

Chapter 3

58

0.0

0.1

0.2

0.3 a T=20oC T=50oC T=80oC T=110oC

0.0

0.1

0.2

0.3 b

Inte

nsity

(a.

u.)

400 450 500 550 6000.0

0.1

0.2

0.3 c

Wavelength (nm)

Figure 3.3. Temperature dependent UV/Visible spectra of block-copolymers in ODCB. (a)

10.2 µg/ml P1, (b) 12.8 µg/ml P2, (c) 17.5 µg/ml P3.

20 40 60 80 100

0.9

1.0

1.1

1.2

1.3

1.4

1.5

I 01 a

bsor

ptio

n sp

ectr

a (-

) a

P1 P2 P3

Temperature (oC)

20 40 60 80 100

2.0

2.1

2.2

2.3

2.4

I 01 e

xcita

tion

spec

tra

(-)

b

Temperature (oC)

20 40 60 80 1001.3

1.4

1.5

1.6c

I 10 e

mis

sion

spe

ctra

(-)

Temperature (oC)

Figure 3.4. Temperature dependence of (a) I01 in absorption, (b) I10 in emission and (c) I01 in

excitation for polymers P1, P2 and P3 in ODCB (see text).

3.3.2 Temperature dependent fluorescence and excitation spectra

Also the fluorescence and excitation spectra were studied at different temperatures. The

photoluminescence spectra of the copolymers show two principal vibronic transitions (Figure

3.5a-c). At 20 °C the 0→0 vibronic transition is at 539 nm and the 1→0 vibronic transition is

at 582 nm for P1. Similar to the UV/Visible spectra, the emission spectra of the three

solutions show a blue shift upon heating. The I01 intensity ratio now decreases from 20 to 110


59

°C (Figure 3.4b). For P1, I01 is reduced from 2.35 to 2.01; for P2 I01 value drops from 2.39 to

2.15, while the P3 solution exhibits the smallest decrease from 2.39 to 2.28.

The excitation spectra of the perylene bisimide luminescence (Figure 3.5d-f) again exhibit

the same blue shift upon heating. All three solutions show a decreasing I01 intensity ratio with

increasing temperature (Figure 3.4c). In the temperature range from 20 to 110 °C, I01

decreases from 1.58 to 1.40 for P1, from 1.50 to 1.37 for P2, and from 1.42 to 1.33 for P3.

The values at 110 °C approach the value of 1.41 obtained from the UV/Visible data of the P3

solution.

The I01 values allow to compare the shape of the absorption and the excitation spectra.

For the solution with P1, these shapes are quite different at low temperatures, but they

become more similar at 110 °C. This is also the case for the P2 solution. For the P3 solution

this effect is less expressed, because its shape is more constant with temperature.

These results are consistent with the view that the emission originates only from

molecularly dissolved perylene bisimide blocks in the polymer chains and not from the

aggregated perylene bisimide blocks.

a T=20oC T=50oC T=80oC T=110oC

b

PL

Inte

nsity

(a.

u.)

550 600 650

Wavelength (nm)

c

d

Inte

nsity

(a.

u.)

e

400 450 500 550

Wavelength (nm)

f

Figure 3.5. Fluorescence (left) and excitation (right) spectra of block-copolymers in ODCB.

(a, d) 3.0 µg/ml P1; (b, e) 2.8 µg/ml P2; (c, f) 4.7 µg/ml P3.

Chapter 3

60

3.4 Aggregate fluorescence Of course it is of interest to see what happens when the aggregates are selectively

excited. Photoluminescence spectra recorded with excitation at 580 nm, i.e. below the optical

band gap of the perylene bisimide molecule, reveal that the aggregates exhibit a distinctly

different emission (Figure 3.6a). P1, which has the shortest polyTHF segments and hence the

highest tendency to aggregate, shows a pronounced, unstructured, fluorescence band around

635 nm. With increasing length of the polyTHF segment, this aggregate emission becomes

more difficult to observe and it is virtually absent for the PERY reference compound.

600 650 700 7500

5

10

500 550 600 6500.0

0.5

1.0

Wavelength (nm)

a

PL

Inte

nsity

(co

unts

x 1

0-3) b

Nor

mal

ized

PL

Inte

nsity

Wavelength (nm)

Figure 3.6. (a) Photoluminescence of P1 (dot-dashed line), P2 (dotted line), P3 (dashed

line), and PERY (solid line) in ODCB (~10-6 M chromophore) after excitation at 580 nm,

(arbitrarily) scaled to the absorption at 530 nm. (b) Normalized photoluminescence of P2 in

ODCB recorded at t = 6 ns (solid squares) and t = 251 ns (open squares) after excitation at

400 nm.

The photoluminescence at different wavelengths was studied in more detail with time-

correlated single photon counting (TCSPC). Fluorescence spectra were recorded at different

intervals after excitation at 400 nm with a 60 ps pulse. The normalized emission spectra of P2

at 6 and 251 ns after the pulse are shown Figure 3.6b. At 6 ns, the spectrum shows peaks at

538 and 580 nm, similar to those of the steady-state fluorescence with excitation at 530 nm

(Figure 3.2b). After 251 ns, the relative intensity of the peak at 538 nm has decreased

dramatically and the maximum emission is now around 630 nm, comparable to the steady-

state photoluminescence with excitation at 580 nm. This shows that the fluorescence of

photoexcited aggregates is much longer-lived than that of molecularly dissolved

chromophores.


61

Table 3.1. Lifetimes (τ), relative pre-exponential factors (A), and χ2-parameter for the fits of

the fluorescence decays of P1, P2, P3, and PERY in ODCB at different emission wavelengths

(λ) with excitation at 400 nm.

λ (nm)

537.5 580 605 630 650

τ

(ns)

A

(%)

χ2

τ

(ns)

A

(%)

χ2

τ

(ns)

A

(%)

χ2

τ

(ns)

A

(%)

χ2

τ

(ns)

A

(%)

χ2

2.95 20 2.91 17 1.25 5

3.9 80 3.9 81 3.9 78 3.9 67 3.9 54 P1

17 0

1.47

17 2

1.35

17 17

1.05

17 33

1.02

17 46

1.01

2.31 32 1.94 24 1.54 15 1.21 13

3.9 68 3.9 74 3.9 69 3.9 57 3.9 56 P2

17 0

1.38

17 1

1.08

17 17

1.03

17 30

1.06

17 44

1.12

2.48 26 2.28 23 2.56 22 2.15 18

3.9 74 3.9 77 3.9 74 3.9 73 3.9 86 P3

17 0

1.68

17 0

1.19

17 5

1.20

17 9

0.96

17 14

1.01

PERY 3.94 100 1.22 3.99 100 0.92

To investigate the differences in excited state lifetime in more detail temporal profiles of

the fluorescence at different wavelengths (537.5, 580, 605, 630 and 650 nm) have been

recorded. The fluorescence decay of the PERY reference compound can accurately be

described with a single lifetime of about 3.9 ns at all wavelengths. The value of 3.9 ns is in

agreement with reports on another perylene bisimide derivative.19,20 In contrast, the

fluorescence decay of the polymers cannot be described with a single exponential decay. At

650 nm, the fluorescence decay of each of the three polymers can be accurately be described

using time constants of 3.9 ns and 17 ns, the latter corresponding to that of the aggregate

emission.19 The relative pre-exponential factors A (Table 3.1) indicate that the extent of

aggregation decreases going from P1 to P3. On the other hand, bi-exponential fits based on

decay constants of 3.9 and 17 ns did not give satisfying results at wavelengths below 650 nm,

and an additional time constant is required to obtain a good fit. As an example, the decay at

580 nm for P1 and the residuals of the bi-exponential (χ2 = 2.27) and tri-exponential fits (χ2 =

1.35) are shown in Figure 3.7.

Chapter 3

62

0 10 20 30 40

100

1000

10000

c

b

a

Cou

nts

Time (ns)

-505

Res

idua

ls

-505

Res

idua

ls

Figure 3.7. (a) Photoluminescence decay (solid line) of P1 in ODCB at 580 nm after

excitation at 400 nm, fitted with bi-exponential (solid circles) and tri-exponential (open

circles) decay functions (for time constants see text and Table 3.1). (b) Residual counts for

bi-exponential fit. (c) Residual counts for tri-exponential fit.

For the latter fit fixed decay constants of 3.9 and 17 ns were used, while leaving the third

lifetime and the relative contributions as adjustable parameters. The results of the fits for the

fluorescence at 537.5, 580, 605, 630 and 650 nm are collected in Table 3.1 and shown in

Figure 3.8 for the polymers and PERY, together with the reduced χ2-parameter, which is a

measure for the quality of the fit. The tri-exponential fits resulted in χ2 values close to unity.

The third ‘free’ time constant converged to a value of 2 (±1) ns in all cases (Table 3.1). For

all polymers, the 17 ns aggregate emission is more pronounced at higher wavelengths.

Polymers P1 and P2 behave rather similarly, while P3 shows a stronger contribution of the

two shorter decay constants.


63

550 600 6500

20

40

17 ns

2 ns

3.9 ns

Rel

ativ

e pr

e-ex

pone

ntia

l fac

tor

(%)

Wavelength (nm)

550 600 650

60

80

550 600 650

0

20

Figure 3.8. Relative pre-exponential factors of the fast decay (~2 ns, top), 3.9 ns lifetime

(middle) and 17 ns lifetime (bottom) of the photoluminescence decay of P1 (squares), P2

(circles) and P3 (triangles) in ODCB at different wavelengths after excitation at 400 nm.

3.5 Analysis of fluorescence lifetime experiments It is of interest to analyze the origin of the decay constants in more detail. The lifetime of

3.9 ns is readily ascribed to the ‘monomeric’ perylene bisimide chromophore, while the

lifetime of 17 ns is attributed to an aggregate or ‘excimer-type’ emission. The term ‘excimer-

type’ is used because the emission exhibits three characteristic features of an excimer (i.e. an

unstructured, longer-lived, red-shifted emission that is not present in the monomer), while on

the other hand one cannot at all exclude that the emitting species is also bound in the ground

state, because a distinct aggregate absorption is observed. A further complication arises

because one could also argue that the aggregation pre-organizes the polymer into a low-

structured system in which the chromophores, as result of their proximity, have a strong

tendency to form an excimer after photoexcitation.

Interestingly, the short ~2 ns lifetime cannot be ascribed to an isolated perylene bisimide

moiety but must be due to a perylene bisimide chromophore involved in a dynamic process

that reduces the lifetime of the excited state. One possible explanation is the collision,

involving a dynamic motion of the polymer backbone, of a photoexcited perylene bisimide

with another ground state chromophore or aggregate, resulting in the formation of an

excimer. Another explanation is a singlet-energy transfer from a molecular dissolved

photoexcited perylene bisimide in one part of the chain to an aggregate of perylene bisimides

Chapter 3

64

in another part. Since fluorescence and absorption spectra of molecularly dissolved and

aggregated chromophores have a good spectral overlap, such energy transfer might involve

the Förster energy-transfer mechanism.

a cb

energy transfer

a cb

energy transfer

a cb

energy transfer

a cb

energy transfer

a cb

energy transfer

Figure 3.9. Proposed sates of self-organization of the alternating copolymers. Perylene

bisimide chromophores are represented by the gray rectangles; polyTHF by the solid lines.

(a) self-organized aggregate state; (b) semi-aggregate conformation; (c) random-coil

conformation. Although the cartoon represents intrachain aggregation of chromophores, it is

likely that more chains participate in the aggregate.

In Figure 3.9, the conformational states of the polymers are shown schematically. The

rectangles represent the chromophores and the black lines the flexible polyTHF segments.

Polymers in conformation a with π-π stacked aggregated chromophores will mainly give

‘excimer-type’ emission, while those in conformation c will predominantly show molecular

fluorescence. Chain b, which is partly aggregated, will show both types of emission and has

the additional possibility that a photoexcitation on the isolated chromophore is transferred to

the aggregate or that it undergoes a conformational change to structure a. These dynamic

processes may explain the ~2 ns lifetime.

3.6 Conclusion The absorption and fluorescent properties of three alternating copolymers, consisting of

polyTHF segments of different length and perylene bisimides, have been investigated. The

perylene bisimide chromophores form H-like aggregates in ODCB solution. The tendency to

form aggregates increases for shorter polyTHF segments. The aggregates can be partially

broken upon heating. Molecular dissolved parts of the chains exhibit a vibronically

structured, short-lived (3.9 ns), ‘monomeric’ perylene bisimide fluorescence with a 0-0

transition at 538 nm. The self-organized, polymeric perylene chromophores exhibit an

unstructured, long-lived (17 ns), red-shifted (630-650 nm) ‘excimer-type’ fluorescence,


65

characteristic of perylene bisimide (micro)aggregates. Apart from the ‘monomeric’ and

‘excimer-type’ fluorescent lifetimes, an additional lifetime of around 2 ns has been observed.

The 2 ns lifetime is tentatively ascribed to the deactivation of the monomeric photoexcited

state via dynamic excimer formation or energy transfer.

3.7 Experimental section General methods

All reagents and solvents were used as received or purified using standard procedures. The synthesis of the

bis(3-aminopropyl)-poly(tetrahydrofuran) for the synthesis of P2 and P3 is described in reference 21. NMR

spectra were recorded at room temperature on a Bruker NMR Spectrometer at 400 MHz for 1H nuclei. Chemical

shifts are given in ppm (δ) relative to tetramethylsilane. Infrared (FT-IR) spectra were recorded on a Perkin-

Elmer Spectrum One UATR FT-IR. A Shimadzu LC-Chemstation 3D (HP 1100 Series) with a Polymer

Laboratories MIXED-D column (Particle size 5 µm; Length/I.D. (mm): 300 × 7.5) and UV detection was

employed for size exclusion chromatography (SEC), using CHCl3 as an eluent (1 mL/min). UV/Visible

absorption spectra were recorded on a Perkin Elmer Lambda 900 and a Lambda 40 spectrophotometer.

Fluorescence spectra were recorded on an Edinburgh Instruments FS920 double-monochromator spectrometer

with a Peltier-cooled red-sensitive photomultiplier. Temperature dependent fluorescence spectra were recorded

on a Perkin Elmer LS50B luminescence spectrometer. Time-correlated single photon counting fluorescence

studies were performed using an Edinburgh Instruments LifeSpec-PS spectrometer, consisting of a 400 nm

picosecond laser (PicoQuant PDL 800B) operated at 2.5 MHz and a Peltier-cooled Hamamatsu micro-channel

plate photomultiplier (R3809U-50). Lifetimes were determined from the data using the Edinburgh Instruments

software package by fitting the decay in a 1.8- 37 ns time window after the excitation.

P1. Perylene-3,4:9,10-tetracarboxylic dianhydride (0.585 g, 1.49 mmol) and bis(3-aminopropyl)-

poly(tetrahydrofuran), Mn = 370 g/mol, PDI = 1.8 (0.545 g, 1.49 mmol) were stirred in m-cresol (6 mL) and

isoquinoline (0.6 mL) at 200 °C under argon flux for 16 h. After cooling the solution was filtered and

subsequently poured in diethyl ether (600 mL). After decanting, the residue was washed with diethyl ether,

water, and diethyl ether. After drying at 150 °C under vacuum overnight 0.841 g (78 %) of a black solid was

obtained. 1H NMR (400 MHz, CDCl3): δ 8.60-7.60 (br signal, 8H), 4.19 (br signal, 4H), 3.58 (br signal, 4H),

3.41 (br signal, 16 H), 2.06 (br signal, 4H), 1.66 (br signal, 16 H); FT-IR (ATR): ν 2856, 1695, 1649, 1594,

1433, 1403, 1355, 1245, 1176, 1108, 809, 745 cm-1; SEC (CHCl3, versus polystyrene): Mw = 5.5 kg/mol, Mn =

4.4 kg/mol.



isoquinoline (3 mL) at 200 °C under argon flux for 23 h. Solvents were almost, but not completely removed

upon vacuum distillation. The liquid residue was added to diethyl ether (700 mL) and was stirred for 1 h. The

solvents were filtered off and the solid washed with diethyl ether, water, and again diethyl ether. After the

product was dried overnight at 150 °C under vacuum 5.75 g (92 %) of black solid was obtained. 1H NMR (400

MHz, CDCl3): δ 8.70-7.80 (br signal, 8H), 4.29 (br signal, 4H), 3.58 (br t, 4H), 3.41 (br t, 76 H), 2.06 (t, J = 6.6

Hz, 4H), 1.62 (br t, 76 H); FT-IR (ATR): ν 2851, 1695, 1651, 1593, 1444, 1355, 1243, 1103, 809, 744 cm-1;

SEC (CHCl3, versus polystyrene): Mw = 29.8 kg/mol, Mn = 13.4 kg/mol.

Chapter 3

66



isoquinoline (0.6 mL) at 200 °C under argon flux for 17 h. The reaction mixture was poured into diethyl ether

(750 mL) and the solid was filtered off. The solid was washed with diethyl ether, water, and diethyl ether. The

solid was subsequently dissolved in CH2Cl2 and was filtered through a paper filter. The solid was dried at 150

°C under vacuum overnight. 0.245 g (24 %) of a black solid was obtained. 1H NMR (400 MHz, CDCl3): δ 8.80-

8.20 (br signal, 8H), 4.31 (br signal, 4H), 3.58 (br t, 4H), 3.41 (br signal, 166H), 2.06 (br t, 4H), 1.62 (br signal,

166H); FT-IR (ATR): ν 2938, 2853, 1696, 1656, 1595, 1446, 1358, 1104, 810, 745 cm-1; SEC (CHCl3, versus

polystyrene): Mw = 53.5 kg/mol, Mn = 17.6 kg/mol.

3.8 References

(1) H. Langhals, Heterocycles 1995, 40, 477.

(2) (a) G. Horowitz, F. Kouki, P. Spearman, D. Fichou, C. Nogues, X. Pan, F. Garnier, Adv. Mater. 1996, 8, 242; (b) P. R. L. Malenfant, C. D. Dimitrakopoulos, J. D. Gelorme, L. L. Kosbar, T. O. Graham, A. Curioni, W. Andreoni, Appl. Phys. Lett. 2002, 80, 2517.

(3) K. Y. Law, Chem. Rev. 1993, 93, 449.

(4) (a) C. W. Tang, Appl. Phys. Lett. 1986, 48, 183; (b) B. A. Bregg, Appl. Phys. Lett. 1995, 67, 1271; (c) S. Ferrere, A. Zaban, B. A. Gregg, J. Phys. Chem. B 1997, 101, 4490; (d) L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, 1119.

(5) (a) R. A. Cormier, B. A. Gregg, J. Phys. Chem. 1997, 101, 11004; (b) R. A. Cormier, B. A. Gregg, Chem. Mater. 1998, 10, 1309; (c) C. W. Struijk, A. B. Sieval, J. E. J. Dakhorst, M. van Dijk, P. Kimkes, R. B. M. Koehorst, H. Donker, T. J. Schaafsma, S. J. Picken, A. M. van de Craats, J. M. Warman, H. Zuilhof, E. J. R. Sudhölter, J. Am. Chem. Soc. 2000, 122, 11057; (d) F. Würthner, C. Thalacker, S. Diele, C. Tschierske, Chem. Eur. J. 2001, 7, 2245.

(6) F. Würthner, C. Thalacker, A. Sautter, W. Schärtl, W. Imbach, O. Hollricher, Chem. Eur. J. 2000, 6, 3871.

(7) (a) W. Wang, J. J. Han, L.-Q. Wang, L.-S. Li, W. J. Shaw, A. D. Q. Li, Nano Lett. 2003, 3, 455; (b) W. Wang, L.-S. Li, G. Helms, H.-H. Zhou, A. D. Q. Li, J. Am. Chem. Soc. 2003, 125, 1120.

(8) (a) P. Tinnefeld, K. D. Weston, T. Vosch, M. Cotlet, T. Weil, J. Hofkens, K. Müllen, F. C. De Schryver, M. Sauer, J. Am. Chem. Soc. 2002, 124, 14310; (b) G. Schweitzer, R. Gronheid, S. Jordens, M. Lor, G. De Belder, T. Weil, E. Reuther, K. Müllen, F. C. De Schryver, J. Phys. Chem. A 2003, 107, 3199; (c) H. Langhals, M. Speckbacher, Eur. J. Org. Chem. 2001, 13, 2481; (d) T. Weil, E. Reuther, K. Müllen, Angew. Chem. Int. Ed. 2002, 41, 1900.

(9) (a) U. Anton, K. Müllen, Macromolecules 1993, 26, 1248; (b) D. Dotcheva, M. Klapper, K. Müllen, Macromol. Chem. Phys. 1994, 195, 1905; (c) H. Quante, P. Sclichtling, U. Rohr, Y. Geerts, K. Müllen, Macromol. Chem. Phys. 1996, 197, 4029; (d) Z. Y. Wang, Y. Qi, J. P. Gao, G. G. Sacrioante, P. R. Sundaradajan, J. D. Duff, Macromolecules 1998, 31, 2075; (e) M. Moffitt, J. P. S. Farinha, M. A. Winnik, U. Rohr, K. Müllen, Macromolecules 1999, 32, 4895.


67

(10) M. Pope, C. E. Swenberg, “Electronic Processes in Organic Crystals”, 1st edition, Oxford University Press, New York 1982.

(11) (a) J. B. Birks, “Photophysics of Aromatic Molecules”, Wiley, New York, 1970; (b) N. J. Turro, “Modern Molecular Photochemistry”, University Science Books, Sausalito 1991.

(12) U. Gómez, M. Leonhardt, H. Port, H. C. Wolf, Chem. Phys. Lett. 1997, 268, 1.

(13) K. Puech, H. Fröb, K. Leo, J. Lumin. 1997, 72-74, 524.

(14) (a) C. Spies, R. Gehrke J. Phys Chem. A 2002, 106, 5348; (b) T. Hasegawa , K. Horie Progr. Polym. Sci. 2001, 26, 259; (c) C. Spies, A. Lorenc , R. Gehrke, H. R. Kricheldorf Macromol. Chem. Phys. 2003, 204, 813. (d) J. Duhamel, A. S. Jones, T. J. Dickson Macromolecules 2000, 33, 6344.

(15) F. S. Bates, Science 1991, 251, 898.

(16) E. L. Thomas, Science 1999, 286, 1307.

(17) S. Icli, H. Icil, Spec. Lett. 1996, 29, 1253.


(19) W. E. Ford, P. V. Kamat, J. Phys. Chem. 1987, 91, 6373.

(20) P. B. Bisht, K. Fukuda, S. Hirayama, Chem. Phys. Lett. 1996, 258, 71.

(21) R. M. Versteegen, Well-defined Thermoplastic Elastomers, Ph. D. Thesis, Eindhoven University of Technology, 2003, ISBN 90-386-2834-X.

68

69

4

Alternating oligo(p-phenylene vinylene) – perylene

bisimide copolymers*

Abstract

A Suzuki polycondensation reaction has been used to synthesize two copolymers

consisting of alternating oligo(p-phenylene vinylene) (OPV) donor and perylene

bisimide (PERY) acceptor chromophores. The copolymers differ by the length of

the saturated spacer that connects the OPV and PERY units. Photoinduced

singlet-energy transfer and photoinduced charge separation in these

polychromophores have been studied in solution and in the solid state via

photoluminescence and femtosecond pump-probe spectroscopy. In both polymers

a photoinduced electron transfer occurs within a few picoseconds after excitation

of the OPV or the PERY chromophore. The electron transfer from the OPV

excited state competes with a singlet-energy transfer state to the PERY

chromophore. The differences in rate constant for the electron and energy

transfer processes are discussed on the basis of correlated quantum-chemical

calculations and in terms of conformational preferences and folding of the two

polymers. In solution, the lifetime of the charge-separated state is longer than in

the films where geminate recombination is much faster. However, in the films

some charges are able to escape from geminate recombination, diffuse away, and

can be collected at the electrodes when the polymers are incorporated in a

photovoltaic device.

*This work has been published: E. E. Neuteboom, S. C. J. Meskers, P. A. van

Hal, J. K. J. van Duren, E. W. Meijer, R. A. J. Janssen, H. Dupin, G. Pourtois, J.

Cornil, R. Lazzaroni, J.-L. Brédas, D. Beljonne, J. Am. Chem. Soc. 2003, 125,

8625.

Chapter 4

70

4.1 Introduction Photoinduced energy and electron transfer between photo-active and electro-active donor

(D) and acceptor (A) moieties continues to be an extremely active area of research.1-6

Numerous photophysical studies of organic D-A molecules have been performed to elucidate

and control resonant energy transfer, charge separation and recombination, and to assess the

role of intermediates in the deactivation of the initially excited states. The intriguing

photophysical and photochemical processes that take place in the natural photosynthetic

reaction center have stimulated efforts to design and create artificial donor-acceptor

architectures in an attempt to mimic the conversion of light in chemical energy or to create

electrical power directly.

Photoinduced electron transfer between donor and acceptor materials is also the initial

step in organic and polymer solar cells. In these cells, excitations, created by the absorption

of light, must be able to diffuse to the interface between the two materials where charge

generation can occur. However, the exciton diffusion length in organic and polymer materials

is often limited to about 10 nm. This implies that an intimate, nanoscopic, mixing of donor

and acceptor is favorable for charge creation. Following this principle, the bulk-

heterojunction cell, in which a spontaneous phase separation of the two components into a

disordered blend is used to create a large interface, has become one of the most promising

concepts in the field of polymer solar cells and resulted in external quantum efficiencies of

more than 50 % at the absorption maximum.7-9 A further increase of performance is expected

by designing materials that have an improved overlap of their absorption spectrum with the

terrestrial solar radiation, especially in the near-infrared region.

A general drawback of the bulk-heterojunction is the fact that the transport and collection

of charges in a disordered nanoscale blend can be hindered by phase boundaries and

discontinuities such as spherical objects and cul-de-sacs. Therefore, the extent, the

characteristic dimension, and the contours of the phase separation in polymer-polymer or

polymer-molecule blends are essential parameters of bulk-heterojunction solar cells. The

actual morphology depends, amongst others, on details of the preparation procedure (e.g.

solvent, temperature, drying speed, substrate). The as-prepared morphology is likely to be

kinetically determined, rather than thermodynamically stable and subject to further

reorganization in time or with temperature.

One way to overcome these drawbacks is by covalently linking the donor and acceptor in

a single polymer chain. The covalent bond enables a predefined control over the

characteristic distance between donor and acceptor and thereby the extent of phase

Alternating oligo(p-phenylene vinylene) – perylene bisimide copolymers

71

separation. Two types of covalently linked donor-acceptor polymers have recently come to

the attention (Figure 4.1). The first consists of semiconducting polymers as a donor with

pendant acceptor groups10-15 while the second has extended donor and acceptor units arranged

in a diblock copolymer.16,17 The latter strategy may have important advantages because the

intrinsic tendency of each segment in block copolymers to aggregate in an individual phase

provides a means to create a well-ordered nanoscale morphology (e.g. spheres, cylinders,

lamellae), governed by the relative volume fractions. While this principle has been utilized in

various engineering materials to create fascinating architectures, the use of block copolymers

in functional, conjugated polymers has received limited attention.18-22

O

O

OO

O

O

O

OO

O

O

O

OO

O

O

O

OO

O

OC8H17OC8H17

H17C8O

OC8H17

H17C8O

OC8H17

H17C8O

H17C8O n

m

Figure 4.1. Examples of donor-acceptor polymers. Top: PPV-like donor with pendant C60-

acceptors.10 Bottom: diblock copolymer with a PPV block and a C60 substituted polystyrene

block.16,17

Both oligo(p-phenylene vinylene) (OPV)10,16,23-26 and perylene bisimide (PERY)27-31

chromophores have been utilized in bulk-heterojunction-like solar cell configurations as

donor and as acceptor materials, but their combination has received little attention. In this

chapter the synthesis and properties of a new class of donor-acceptor polymers are presented.

The polymers are represented by P1 and P2 (Figure 4.2) and consist of alternating OPV and

PERY segments connected via saturated spacers. The photophysics of these two alternating

copolymers has been studied in detail both experimentally and theoretically. On the one hand,

Chapter 4

72

fluorescence and femtosecond pump-probe spectroscopy in solution and in the solid state

have been used to elucidate the temporal evolution of the photoexcited state and determine

the rates for charge separation and recombination; the photophysical properties are compared

to those of a cyclic model compound, M1 (Figure 4.2), in which the two groups are held in a

face-to-face orientation. On the other hand, correlated quantum-chemical calculations have

been performed to determine the different molecular parameters that are involved in the

charge separation and charge recombination rates in P2 and M1. The photoinduced electron

transfer reactions in the polychromophoric alternating D-A arrays occur for P1 and P2 in the

picosecond regime, both in solution and in the solid state. To explain the influence of the

spacer length, it is proposed that there is a significant effect of the conformation of the

polymer chain on the rate for photoinduced energy and electron transfer.

O

O

O

O

O

O

O

O

OO N

O

O

O

O

O

O

N

O

O

O

OO

O

n

O

O

O

O

O

O

N N

O

O O

OO O

n

NN

O

O

O

O

OO

OO

O

O

O

O

O

OO

O

N NO O

P1

P2

M1

M2

M3

Figure 4.2. Structures of oligo(p-phenylene vinylene) – perylene bisimide alternating

copolymers P1 and P2, and their model compounds M1, M2, and M3.


73

4.2 Synthesis and characterization

4.2.1 Synthesis of the polymers and model compounds

The synthesis of polymers P1 and P2, and the cyclic model compound M1 was

accomplished using a palladium-catalyzed aryl-aryl coupling of appropriately functionalized

perylene bisimide and oligo(p-phenylene vinylene) monomers 6, 10, and 13. The synthesis of

the perylene bisimide monomer 6 (Scheme 4.1) started with a convergent approach to the

amine 5. On one side, a Williamson etherification of 6-bromo-1-hexanol and 4-bromophenol

afforded alcohol 1, which was tosylated to give 2. On the other side, the amine of (S)-(+)-

leucinol was protected with a phthalimide group to imide 3. A second Williamson

etherification of alcohol 3 with tosylate 2 gave diether 4, which was deprotected with

hydrazine to the free amine 5. Finally reaction of two equivalents of 5 with perylene-

3,4:9,10-tetracarboxylic dianhydride (PTCDA) resulted in perylene bisimide 6.

OOBr N N

O

O

O

OO

O Br

OH BrOHBr O Br

OH

O BrSO

O

O

NH2OH

N

O

OOH

N

O

OO

O Br

NH2O

O Br

1

2

3 4

5

6

a

b

c d

e f

+

Scheme 4.1. Synthesis of the perylene bisimide monomer 6; (a) K2CO3, TBAB, acetone,

reflux, 91 %; (b) TosCl, pyridine, 4 °C, 75 %; (c) phthalic anhydride, toluene, reflux, 94 %;

(d) 2, potassium tert-butoxide, DMF, 70 °C, 44 %; (e) N2H4·H2O, methanol, reflux, 95 %; (f)

perylene-3,4:9,10-tetracarboxylic dianhydride, Zn(OAc)2, imidazole, 180 °C, 55 %.

Chapter 4

74

The second perylene bisimide monomer 10 was obtained following a slightly different

strategy (Scheme 4.2). Reaction of (S)-(+)-leucinol with di-tert-butyl dicarbonate afforded

BOC protected amine 7. A Mitsunobu reaction of 7 with 4-bromophenol was carried out to

yield ether 8. Deprotection of 8 by reaction with trifluoroacetic acid (TFA) and NaHCO3

afforded amine 9, which was reacted with PTCDA to perylene bisimide 10.

O NOH

O

H

O NO

OBr

H

NH2OH

NH2O Br OBr

NO Br

N

O

O

O

O

7 8

9 10

g h

i j

Scheme 4.2. Synthesis of the perylene bisimide monomer 10; (g) BOC2O, THF, 97 %; (h) 4-

bromophenol, PPh3, DEAD, toluene, 23 %; (i) 1. TFA, CH2Cl2; 2. NaHCO3, 76 %; (j)

perylene-3,4:9,10-tetracarboxylic dianhydride, Zn(OAc)2, imidazole, 180 °C, 65 %.

The synthesis of the oligo(p-phenylene vinylene) (OPV) monomer 13 (Scheme 4.3)

started with treatment of (E,E)-1,4-bis{4-bromo-2,5,-bis[(S)-2-methylbutoxy]styryl}-2,5-

bis[(S)-2-metylbutoxy]benzene32 11 with butyllithium and subsequent reaction with trimethyl

borate and water to yield bisboronic acid 12. Condensation of bisboronic acid 12 with pinacol

resulted in the formation of the bisboralane 13.

Br

Br

OR*

R*O

OR*

R*O

OR*

R*O

B

B

OR*

R*O

OR*

R*O

OR*

R*O

OH

OH

OH

OH

O

OB

O

OB

OR*

R*O

OR*

R*O

OR*

R*O

11 12

13

k

l

R* =

Scheme 4.3. Synthesis of OPV monomer 13; (k) 1. n-BuLi, THF, –78 °C; 2. B(OMe)3, –78

°C; 3. H2O, 51 %; (l) pinacol, CH2Cl2, reflux, 58 %.


75

All monomers were fully characterized using 1H and 13C NMR spectroscopy, elemental

analysis, and MALDI-TOF mass spectrometry.

Polymers P1 and P2 were synthesized by a Suzuki-polymerization using Pd(PPh3)4 as a

catalyst from equimolar amounts of bisboralane oligo(p-phenylene vinylene) monomer 13

and the perylene bisimide monomers 6 and 10, respectively. Polymerization of 6 with 13,

afforded apart from P1, cyclic compound M1 as a by-product, which could be isolated and

purified by extraction with acetone, column chromatography, and preparative size exclusion

chromatography (SEC).

Bis(4-methoxphenyl)-OPV3, M2, was synthesized by Suzuki coupling of 13 with 4-

bromoanisol for comparison with the polymers in the optical experiments.

4.2.2 Characterization of the polymers

The polymers were characterized by 1H and 13C NMR spectroscopy. In the 1H NMR

spectra of the polymers, the signals in the aromatic region have shifted compared to the

spectra of the monomers. For P1, the signals of the protons of the bromophenyl ring at 7.14

and 6.46 ppm in monomer 6 shift to 7.47 and 6.81 ppm in the polymer. In the 1H NMR

spectrum of P2 these signals appear at 7.48 and 6.91 ppm, compared to 7.29 and 6.76 ppm

for monomer 10. The 1H NMR signals of the starting monomers could no longer be detected

in the polymers. The molecular weight of the polymers determined by size exclusion

chromatography (SEC) versus polystyrene standards is Mn = 12.6 kg/mol for P1 and Mn =

10.8 kg/mol for P2 with polydispersities of 2.1 and 2.4, respectively. MALDI-TOF mass

spectrometry on P1 and P2 was not successful, but confirmed the cyclic structure of M1 by

reproducing the exact molar mass in a high-resolution spectrum (MW = 1738.034, m/z =

1738.021 [M•]+)

4.3 Absorption and circular dichroism spectra UV/Visible absorption spectra of polymers P1 and P2 in toluene solution are shown in

Figure 4.3, together with the absorption spectra of the model compounds M2 and M3.33

Absorption bands of the PERY chromophore appear at 460, 490, and 525 nm, while the OPV

absorption peaks at ~425 nm. As can be seen in Figure 4.3, the spectra of the two polymers

are almost identical and close to a linear superposition of the spectra of M2 and M3. This

demonstrates that in the two polymers the two chromophores are indeed present in a 1:1 ratio.

Chapter 4

76

300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0 P1 P2 M2 M3 M2 + M3

Nor

mal

ized

Abs

orba

nce

Wavelength (nm)

Figure 4.3. Absorption spectra of polymers P1 and P2, and compounds M2 and M3,

dissolved in toluene. The spectra of P1, P2, and M3 have been normalized at 527 nm. The

spectrum of M2 is normalized with respect to that of M3 to represent similar concentrations.

The open circles represent the spectrum of a 1:1 molar mixture of M2 and M3.

The absorption spectra of P1 and P2 give no indication of a major electronic interaction

between the OPV and PERY chromophores in the ground state. The absorption spectra

further reveal that the singlet-excited state of the PERY unit lies below that of the OPV

chromophore. As a result, excitation of the OPV chromophore may result in singlet-energy

transfer to the PERY unit. Such excitation transfer has recently been described in perylene-

end-capped polyindenofluorene.34,35

For M1 such an electronic interaction is much more likely because the two chromophores

are in close proximity. Moreover, the enantiomerically pure (S)-2-methylbutoxy side chains

of the OPV fragment and the two (S) stereocenters of the leucinol group could give rise to

preferential helical twisting of the OPV and PERY chromophores in the face-to-face

orientation present in M1. The UV/Visible absorption spectrum of M1 (Figure 4.4) shows

that, compared to P1, there is a small but distinct red shift of the OPV band from 428 to 439

nm. The circular dichroism (CD) spectrum of M1 (Figure 4.4) reveals a Cotton effect, with a

negative part at 438 nm (OPV) and a positive part at 486 and 524 nm (PERY). Following the

exciton-coupling model,36 a bisignated Cotton effect is consistent with two interacting

chromophores in a helical orientation. Exciton theory of chiral bichromophoric systems

suggests that the positive Cotton effect of M1 at the higher wavelengths indicates that

transition dipole moments of the OPV and PERY chromophores in M1 are in a right-handed

twisted conformation.36 From these results M1 emerges as a new chiral macrocycle,


77

consisting of two electronically different interacting conjugated chromophores. Chiral

macrocycles with two identical chromophores have been described recently.37,38

350 400 450 500 550

-2

-1

0

1

2

3

CD

(mde

g)

Wavelength (nm)

0.0

0.5

1.0

Abs

orba

nce

(a.u

.)

Figure 4.4. Absorption (top) and circular dichroism (bottom) spectra of compound M1 (solid

lines) in toluene. For comparison the absorption spectrum of P1 is shown (dashed line).

4.4 Charge separation

4.4.1 Electrochemistry

The oxidation and reduction potentials of the oligo(p-phenylene vinylene) and perylene

bisimide segments were determined by cyclic voltammetry in dichloromethane (vs SCE).

Two reversible oxidation waves were found for M2 at half-wave potentials of Eox,1 = 0.80 V

and Eox,2 = 0.94 V. For M3 the reduction potentials were determined at Ered,1 = –0.65 V and

Ered,2 = –0.85 V.

4.4.2 Energy for charge separation

The redox potentials can be used to estimate the Gibbs free energy of the intramolecular

charge-separated state (GCS) of the donor-acceptor polymers P1 and P2, and of the cyclic

dyad M1 with respect to the electronic ground state in a solvent with polarity εs via the

Weller equation:39

Chapter 4

78

−

+−−−= −+

sref0

2

ccs0

2

redoxCS1111

84))A()D((

εεπεεπε rr

e

R

eEEeG (4.1)

To calculate GCS, the oxidation potential of the donor Eox(D) and the reduction potential

of the acceptor Ered(A) of M2 and M3, determined in dichloromethane (εref = 8.93), were

used. The radius of the PERY anion (r– = 4.7 Å) was estimated from the density (ρ = 1.59 g

cm-3) of N,N´-dimethylperylene-3,4:9,10-tetracarboxylic-bisimide from the X-ray

crystallographic data via r– = [3M/(4πρNA)]1/3.40 The radius of the positive ion of OPV

segment was estimated to be r+ = 5.1 Å.25 The distance between the centers of donor and

acceptor moieties, Rcc, was determined from geometries of M1, P1, and P2 optimized at the

Molecular Mechanics level with the Dreiding and Universal force fields; these values are 4,

31, and 22 Å for M1, P1, and P2, respectively. Because P1 has a flexible spacer between the

OPV and PERY moieties, the distance of 31 Å for P1 is an upper limit corresponding to the

fully extended structure. It is assumed that the value of 4 Å is the lower limit for Rcc in P1 in

analogy with the situation in M1. With these parameters, it was found that the intramolecular

charge-separated state in M1, P1, and P2 lies at lower energy than those of the OPV or

PERY singlet-excited states (Table 4.1, Figure 4.5) in toluene (εs = 2.38). Hence,

photoinduced electron transfer is expected to be an exergonic reaction for these systems.

Table 4.1. Free energy of intramolecular and intermolecular charge-separated states

calculated from Equation 4.1 in toluene and relative to the ground state and the singlet-

excited states of OPV and PERY.a

∆GCS = GCS – E00 (eV)

Rcc (Å) GCS (eV) S1 OPV S1 PERY

M1 4 0.85 –1.74 –1.46

P1 31b 2.17 –0.42 –0.15

P2 22b 2.09 –0.50 –0.23

M2 + M3 ∞ 2.36 –0.23 0.05 a The following parameters were used Eox(D) = 0.80 V, Ered(A) = –0.65 V, r+ = 5.1 Å, r– = 4.7

Å, εs = 2.38, εref = 8.93. For OPV(S1): E00 = 2.59 eV, for PERY(S1): E00 = 2.31 eV. b

Assuming a fully extended structure of the spacer.


79

0.5

1.0

1.5

2.0

2.5

M2 + M3

P2

M1

P1S

1 PERY

S1 OPV

En

erg

y (e

V)

Figure 4.5. Energies of the singlet-excited states of the OPV and PERY chromophores; the

intramolecular charge-separated states of M1, P1, and P2; and the intermolecular charge-

separated state between M2 and M3. The energy levels of the charge-separated states were

calculated from Equation 4.1 using the parameters described in the text and assuming

toluene (εs = 2.38) as the solvent. The range of values for P1, depending on Rcc, is given by

the arrow.

4.4.3 Fluorescence spectra and fluorescence quenching

To investigate the photophysics and especially to address the possibility of photoinduced

energy or electron transfer reactions between the donor and acceptor moieties of the polymers

P1 and P2, their fluorescence was studied in toluene solution. The fluorescence spectra of

M2 and of the two polymers recorded with (preferential) excitation of the OPV chromophore

at 410 nm reveal that in P1 and P2 the OPV fluorescence at 482 nm is quenched (Figure 4.6).

Compared to M2, the OPV fluorescence is reduced by a factor of Q = 650 for P1 and by Q =

700 for P2 (Table 4.2). Comparison with fluorescence spectrum of perylene bisimide model

M3, recorded with excitation at 525 nm, shows that both polymers give some emission that

originates from the PERY chromophore. When the perylene bisimide segments of P1 and P2

were selectively excited at 525 nm, it became evident, however, that also the PERY emission

in the polymers is strongly quenched. Quenching factors of 30 and 630 (Table 4.2) were

found for the perylene emission of P1 and P2, respectively, relative to that of M3. For the

cyclic model compound M1, photoluminescence of the OPV chromophore could no longer

be identified (Q > 15000), indicating a very efficient deactivation of its singlet excited state.

The quenching of the PERY emission in M1 was substantial (Q = 270, Table 4.2), but less

than for P2.

Chapter 4

80

450 500 550 600 650

0

100

200

300

400

500

x20

x20

P1 P2 M2 M3

PL

Inte

nsity

Wavelength (nm)

Figure 4.6. Fluorescence spectra of polymers P1 and P2 and OPV-model M2 upon

(preferential) excitation of the OPV chromophore at 410 nm and of PERY-model M3 upon

excitation at 525 nm. The spectra are corrected for the optical density at the excitation

wavelength. The spectra of the polymers are multiplied by 20 for clarity.

Table 4.2. Rate constants for energy transfer, direct and indirect charge separation, and

charge recombination as obtained from photoluminescence quenching and pump-probe PIA

spectroscopy for P1, P2, and M1 in toluene solution and solid films.

PL Quenching PIA OPVa PIA PERYb

QOPVc QPERY

c (kET + kCS

d) d

(ns-1)

kCSi e

(ns-1)

kCSd

(ns-1)

kCR

(ns-1)

kCSi

(ns-1)

kCR

(ns-1)

P1 toluene 650 30 540 7 630 1.9 1400 1.4

film ≥2300 9.5

P2 toluene 700 630 580 160 350 0.6 270 0.5

film ≥2300 9.5

M1 toluene ≥15000 270 ≥12500 70 1700 6.9 1200 5.9 a Excitation of the OPV chormophore at 450 nm. b Excitation of the PERY chromophore at

520 nm. c Excitation of OPV at 410 nm and PERY at 525 nm. d From QOPV and Equation 4.2. e From QPERY and Equation 4.3.

These results can be rationalized by considering the diagram shown in Figure 4.7. After

photoexcitation of the OPV chromophore, the intrinsic radiative (kR) and nonradiative decay


81

(kNR) of the donor can be quenched by either a singlet-energy transfer reaction (kET) to the

PERY chromophore or a charge separation (kCS) to a charge-separated state (CSS) in which

an electron is transferred from the OPV to the PERY unit. Photoexcitation of the PERY unit,

however, cannot give singlet-energy transfer because it is endergonic, but charge separation

can still occur, resulting in quenching of the PERY fluorescence.

kRD

kET

kCSikCS

d

kRA

S0 S0

S1

S1

CSS

OPV PERY

kNRD

kNRA

kRD

kET

kCSikCS

d

kRA

S0 S0

S1

S1

CSS

OPV PERY

kNRD

kNRA

Figure 4.7. Jablonski diagram representing the different photophysical events that can take

place in the donor-acceptor (D-A) polymers P1 and P2 and model M1 upon excitation of the

OPV chromophore (open arrow). The singlet energy transfer (kET), the direct (kCSd) and

indirect (kCSi) electron transfer, the radiative emission (kR) and non-radiative emission (kNR)

are indicated.

From the quenching of the PERY emission in P1, P2, and M1 it is concluded that in

these systems electron transfer takes place when the acceptor unit is photoexcited. To

examine whether singlet-energy transfer occurs, the excitation spectra of the PERY emission

at 579 nm were recorded in toluene.41 Figure 4.8 shows that in the excitation spectrum of the

PERY emission of P1 and P2 both chromophores contribute to an equal extent. This indicates

that the rate for singlet-energy transfer from the OPV singlet-state (kET, Figure 4.7) in P1 and

P2 is at least of the same order of magnitude as the rate for direct electron transfer (kCSd,

Figure 4.7). In contrast, the excitation spectrum of M1 differs from its absorption spectrum in

the 400-475 nm region (Figure 4.8). This suggests that direct electron transfer from the S1

state of OPV is more important here.

Chapter 4

82

0.0

0.5

1.0

0.0

0.5

1.0

350 400 450 500 5500.0

0.5

1.0

No

rma

lize

d in

ten

sity

Wavelength (nm)

Figure 4.8. Normalized absorption (dashed lines) and excitation (solid lines) spectra of

polymer P1 (top), polymer P2 (middle), and model M1 recorded in toluene (λem = 579 nm).

4.4.4 Photoinduced absorption in thin films

In order to investigate the photoinduced electron transfer processes in more detail

photoinduced absorption (PIA) spectroscopy on long and short time scales was performed.

The near steady state PIA spectrum of a thin film of P1 on quartz, recorded with excitation at

528 nm at 80 K (Figure 4.9) shows the formation of PERY–• radical anions and OPV+•

radical cations. The peaks at 1.28, 1.54, and 1.72 eV are readily attributed to the PERY–•

radical anion bands.42 The absorption band at 0.72 eV is assigned to the OPV+• radical

cations; its position is in between the absorption bands of OPV3+• and OPV4+• radical

cations.43 Comparison with spectra of OPV3+• and OPV4+• indicates that a second absorption

of the OPV+• radical cation in P1 is expected to overlap with the PERY–• radical anion bands

in the 1.5-2.0 eV region. An almost identical PIA spectrum was recorded for polymer P2

(Figure 4.9). Moreover, the PIA spectrum does not depend on whether the sample is

irradiated at 458 nm (preferential excitation of OPV) or at 528 nm (selective excitation of

PERY). This demonstrates that both hole and electron transfer processes take place in the

solid state.


83

0.5 1.0 1.5 2.0

-0.1

0.0

0.1

0.2

0.3

0.4

OPV+•

PERY-•PERY-•

PERY-•

OPV+•

-∆T

/T x

103

Energy (eV)

Figure 4.9. Near steady state photoinduced absorption spectra of P1 (solid line) and P2

(dashed line) as thin films on quartz recorded at 80 K. Excitation wavelength is 528 nm,

modulation frequency 275 Hz.

The PIA signals of both P1 and P2 were found to increase in a non-linear fashion with

the intensity of the excitation beam following a power-law behavior with an exponent in the

order of 0.6-0.7, indicating that bimolecular decay processes, such as non-geminate charge-

recombination, contribute to the decay. All PIA bands exhibited an almost identical

dependence on changing the modulation frequency, which could be fitted to a combined

expression of a power-law dependence and bimolecular decay,44 giving exponents of –0.40 to

–0.45 and lifetimes in the range from 1-2 ms.

Films of P1 and P2 were also investigated with femtosecond pump-probe spectroscopy.

In Figure 4.10 the normalized change in transmission ∆T/T at 1450 nm (0.86 eV) for both

polymers is plotted versus the time delay after excitation with a ~150 fs pulse at 450 nm. At

this probe wavelength a negative differential transmission is observed, consistent with the

fact that at 1450 nm exclusively the formation of OPV+• radical cations is probed, and hence

the event of photoinduced charge separation. Figure 4.10 shows that the rate for charge

separation (~0.4 ps) is extremely fast for both P1 and P2 in the film. The recombination

process contains several components. At short times after photoexcitation the recombination

is dominated by a process occurring with a time constant of ~105 ps, however, after 600 ps,

around 20 % of the charges is still present in both polymer films. These residual charges

decay only very slowly. The 105 ps decay is interpreted as geminate recombination and the

Chapter 4

84

longer lived components are attributed to those charges that diffuse away from their original

position and eventually may become trapped.

0 100 200 300 400 500 600

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

∆T/T

Time delay (ps)-2 0 2 4 6 8

-1.0

-0.8

-0.6

-0.4

-0.2

0.0ba

τ = 105 ps

τ = 430 fs

Time delay (ps)

∆T

/T

Figure 4.10. Normalized differential transmission dynamics of films of polymers P1 (●) and

P2 (○) at room temperature, recorded at 1450 nm (low-energy absorption of OPV radical

cations) after excitation at 450 nm. Solid lines are fits to exponential growth (a) and decay

(b).

4.4.5 Photoinduced absorption in solution

Femtosecond pump-probe spectroscopy was also used to study polymers P1 and P2 in

toluene solution (Figure 4.11). Again the OPV chromophore was excited at 450 nm and the

change in transmission of the OPV+• radical cation band at 1450 nm was probed in time.

While in the solid state P1 and P2 behave very similarly, significant differences are observed

in solution. In toluene, charge separation in P1 (~1.6 ps) is faster than in polymer P2 (~2.9

ps). Both are somewhat slower than in the solid state. Also the rate for charge recombination

in solution is lower than in the films. Fitting a mono-exponential decay to the data for P1 and

P2 (Figure 4.11) provides time constants for recombination of positive and negative charges

of 530 and 1600 ps, respectively. As a result of the difference in rate constants, ~75 % of the

charges are still present after 600 ps in P2 and ~40 % in P1. The rates of charge separation

and charge recombination are summarized in Table 4.2.


85

0 100 200 300 400 500 600

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4 b

∆T/T

Time delay (ps)0 2 4 6 8 10

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4 a

τ = 1600 ps

τ = 530 psτ = 2.9 ps

τ = 1.6 ps

∆T

/T

Time delay (ps)

Figure 4.11. Normalized differential transmission dynamics of polymers P1 (●) and P2 (○) in

toluene solution at room temperature, recorded at 1450 nm (low-energy absorption of OPV

radical cations) after excitation at 450 nm. Solid lines are fits to monoexponential growth (a)

and decay (b).

Pump-probe experiments have also been performed with excitation at 520 nm, where the

PERY chromophore is directly excited, again with probing at 1450 nm to observe OPV+•

radical cations. The results (Table 4.2) are similar to those obtained with excitation at 450 nm

and demonstrate that kCSd and kCS

i are of the same order of magnitude.

Transient pump-probe experiments on M1 in toluene solution (Figure 4.12) have been

performed for comparison to the data of P1 and P2 in solution and film. The forward charge

transfer upon photoexcitation of M1 in toluene is significantly faster than for P1 and P2 in

the same solvent. Actually, the time constant of ~0.6 ps for the forward reaction is close to

that observed in films of P1 and P2. The back electron transfer (recombination) in M1 is

mono-exponential with a time constant of ~144 ps. The recombination rate of M1 is therefore

higher than that of P1 and P2 in solution, but close to values observed in the polymer films.

Similar to P1 and P2, photoexcitation of the PERY chromophore at 520 nm instead of OPV,

provided comparable values for the rate for charge separation. The close correspondence in

charge formation and recombination between M1 in toluene and the P1 and P2 films, can be

rationalized by considering that the face-to-face orientation of the OPV and PERY photo-

active groups in M1, is also present in the solid state films of P1 and P2.

Chapter 4

86

0 50 100 150 200 250

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-2 0 2 4 6

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 b

τ = 144 ps

∆T/T

Time delay (ps)

a

τ = 0.6 ps

∆T/T

Time delay (ps)

Figure 4.12. Normalized differential transmission dynamics of M1 in toluene solution at

room temperature, recorded at 1450 nm (low-energy absorption of OPV radical cations)

after excitation at 450 nm. Solid lines are fits to monoexponential growth (a) and decay (b).

4.5 Photovoltaic devices Photovoltaic devices were prepared by spin casting solutions of P1 and P2 in chloroform

on indium tin oxide covered with a layer of polyethylenedioxythiophene polystyrenesulfonate

(PEDOT:PSS) as the transparent front electrode. A LiF/Al back electrode was deposited in

vacuum. The J-V characteristics of the ITO/PEDOT:PSS/polymer/LiF/Al devices were

measured in the dark and under simulated AM1.5 conditions (Figure 4.13). Both polymers

gave similar results. Although the open circuit voltage in both cells is relatively high (VOC =

1.20 and 0.97 V, for P1 and P2, respectively), the short circuit current density (JSC = 0.008

mA cm-2 for P1 and JSC = 0.012 mA cm-2 for P2), and the fill factor (FF = 0.25-0.26) are low.

The low currents are attributed to poor transport characteristics, because the initial charge

generation is efficient as inferred from fluorescence quenching in the films.


87

0.0 0.5 1.0 1.5 2.0

-0.01

0.00

0.01

0.02a

J (m

A c

m-2)

Bias (V)

0.0 0.5 1.0 1.5 2.0

-0.01

0.00

0.01

0.02b

J (m

A c

m-2)

Bias (V) Figure 4.13. Current density – voltage characteristics of photovoltaic devices

(ITO/PEDOT:PSS/polymer/LiF/Al) of polymers P1 (a) and P2 (b) in the dark (●) and under

illumination (100 mW/cm2) (○).

4.6 Kinetics of electron and energy transfer Fluorescence quenching and pump-probe experiments can be used to estimate the rate

constants for energy transfer (kET) and for direct and indirect charge separation (kCSd

and kCSi,

respectively) (Figure 4.7). The sum of kET and kCSd can be related to the quenching of the

OPV fluorescence (QOPV) using the fluorescence lifetime of the donor segment (τOPV ) via:

OPV

OPVdCSET

1

τ−=+ Q

kk (4.2)

In a similar fashion it is possible to relate the quenching of the PERY fluorescence to the

rate for the indirect charge separation:

PERY

PERYiCS

1

τ−= Q

k (4.3)

The fluorescence lifetimes of M2 and M3 in toluene solution were determined by time-

correlated single photon counting. The time resolved fluorescence signal followed a mono-

exponential decay with time constants of 1.2 and 4.0 ns in toluene for M2 and M3,

respectively. The use of Equations 4.2 and 4.3 is not without difficulty because small

concentrations of fluorescent impurities, such as monomers, can easily lead to an

underestimation of the actual quenching. Pump-probe experiments generally give more

reliable numbers because they probe the bulk of the sample.

Chapter 4

88

The results are collected in Table 4.2. The OPV fluorescence quenching in P1 and P2 in

toluene indicates that in the polymers kET + kCSd

= 5.4 - 5.8 × 1011 s-1; while the rate is much

higher in the cyclic molecule M1 (kET + kCSd

≥ 1.2 × 1013 s-1). The data are in satisfactory

agreement with the rates for direct charge formation derived from pump-probe spectroscopy:

kCSd = 6.3 × 1011 s-1 for P1, 3.5 × 1011 s-1 for P2, and ≥ 1.7 × 1012 for M1 (Table 4.2). The

significantly lower value for M1 in the latter experiment is limited by the time resolution of

the pump-probe spectroscopy (0.4-0.5 ps), and hence, the value obtained from the OPV

quenching is probably more accurate here. Although these results cannot be used to unravel

the individual contributions of kET and kCSd in a reliable fashion, the fluorescence excitation

experiments have shown that these processes are competitive.

Comparison of the pump-probe PIA data shows that the rate for direct charge separation

in toluene solution decreases in the sequence M1, P1, and P2 (Table 4.2). The high rate for

M1 is consistent with a face-to-face orientation of the two chromophores, which ensures an

efficient electronic coupling in the excited state, vide infra. The higher rate for P1 than for P2

is at first sight surprising, because of the longer spacer between the OPV and PERY groups in

P1. This difference may be rationalized by the fact that the conformation of P1 is

heterogeneous on the picosecond time scale. The origin of this heterogeneity is the

conformational flexibility of the polymer chain, which implies that the OPV and PERY

segments can be in a range of conformations from fully extended (~31 Å) to face-to-face (~4

Å). The face-to-face orientation, which likely results in a high rate (kCSd), is not possible for

P2, because the spacer is more rigid. These differences are schematically shown in Figure

4.14. The rate for charge recombination (kCR) decreases in the sequence M1, P1, P2 and,

hence, follows the same trend as observed for charge separation. Likely, similar

conformational differences are the cause for this result. At present it is unknown whether

migration of photogenerated charges along the polymer chain occurs. The most likely

mechanism for such a process would certainly involve folding to circumvent that positive (or

negative) charges must reside on or hop over acceptor (or donor) units.

The rates for indirect charge separation, i.e. originating from the excited state of the

PERY acceptor, in M1, P1 and P2 as determined by pump-probe spectroscopy are similar to

those for direct charge transfer (Table 4.2). As can be seen in Table 4.2, the values obtained

from pump-probe spectroscopy differ significantly from those determined from the PERY

fluorescence quenching, especially for P1 and M1. The presence of a small (less than 0.4 %)

impurity containing the PERY chromophore can explain this discrepancy for M1, but for P1

the rather low PL quenching (QPERY = 30) is considered, at least in part, intrinsic. In a fully


89

extended conformation of the linker (~31 Å) of P1 the electronic coupling of the OPV and

PERY chromophores is negligible in the excited state and, hence, a low rate for electron

transfer, causing a decreased quenching of the PERY emission. Because singlet-energy

transfer is less dependent on the distance than electron transfer, the OPV fluorescence in P1

is quenched to the same extent as in P2. The conformational freedom of P1 can thus be used

to rationalize the widely different values obtained for kCSi using PL quenching and PIA

spectroscopy.

P1

P2

M1

solution solid state

Figure 4.14. Cartoon representation of the conformation of alternating donor-acceptor

copolymers in solution. Donor (dark gray) and acceptor (light gray) units are linked by long

(P1) or short (P2) spacers. Face-to-face contacts of the two chromophores in P1 enhance the

rates for photoinduced energy and electron transfer compared to the end-to-end orientation

in P2. In the solid state face-to-face interactions, like the one enforced in M1, are

omnipresent for both P1 and P2 via interchain interactions.

The rate for charge separation in the films of P1 and P2, kCSd ≥ 2.3 × 1012 s-1, is at the

limit of the time resolution of the pump-probe set-up. The recombination, kCR = 9.5 × 109 s-1,

is much slower. Both processes, however, are significantly faster than the corresponding

reactions in toluene solution. The increased rate for charge separation can be understood by

assuming that in the films face-to-face orientations of the OPV and PERY chromophores

(like the one in M1) are present (Figure 4.14). Although there is no evidence from

morphological studies, several polymer materials45-48 exist in which electron rich (donor) and

electron deficient (acceptor) segments form alternating stacks in solution and in the solid

state. The driving force for such orientations is the π-stacking in combination with

electrostatic or weak charge transfer interaction.49,50 Also in D-A molecular crystals the usual

case is to have stacks with alternating D-A molecules.51 A preference for face-to-face D-A

interactions as schematically drawn in Figure 4.14, contrast with the microphase separation

Chapter 4

90

that often occurs in block copolymers. Apparently, the tendency for the OPV and PERY

segments to give alternating stacks in thin films of P1 and P2 is stronger than the antagonistic

interactions that direct the microscopic phase separation. One likely explanation for this

result is the limited length of the D and A segments in P1 and P2. The proposition of the

presence of alternating stacks of OPV and PERY in films of P1 and P2 is supported by the

similar values of kCSd obtained for M1 and the polymer films (Table 4.2). In full agreement

with this view, the initial recombination in the films is fast, and the rate constant (kCR = 9.5 ×

109 s-1) is again similar to the one observed for M1 (kCR = 6.9 × 109 s-1). The long-lived

charge carriers in the films result from those charges that escape from geminate

recombination and diffuse to different sites in the films. The relatively low number of charges

(<20 %, Figure 4.10) that live up to 1 ns explains, at least in part, the low currents observed

in photovoltaic cells made from P1 and P2. If the face-to-face orientations of OPV and PERY

are predominant in the films, this could be a more important reason for the low currents,

because such orientation would severely limit charge transport.

4.7 Theoretical modeling of charge separation and recombination At the University of Mons-Hainaut, correlated semiempirical quantum-chemical

calculations have been performed by Dr. H. Dupin and Dr. D. Beljonne on the model

compound M1 and a monomer unit of P2 to assess the various molecular parameters relevant

in the charge separation and recombination processes.52 For the former process, the cases

were considered where the charge transfer is induced by excitation of either the OPV donor

(D*A → D+A– process; that is the direct transfer, corresponding to electron transfer from

donor D to acceptor A) or the acceptor (DA* → D+A– process; that is the indirect transfer,

corresponding to hole transfer from A to D). The internal and external reorganization

energies λi and λs, the electronic couplings VRP, the change in free Gibbs energy ∆G0 and the

rate of the processes kRP were calculated, going from reactant R to product P. They were

calculated for the extended structure of P2 and a cofacial arrangement (0° to 40° angle) of the

chromophores in M1. The obtained results are listed in Table 4.3.

The driving force, ∆G0, is calculated to be negative in both systems (confirming that the

charge separation is an exergonic process) and significantly smaller for P2 than M1, as a

result of the strong interaction between the positive and negative charges in the cyclic

structure (which stabilizes the charge-separated state). Quantitatively, the ∆G0 values are

markedly different from those obtained from the Weller equation (Equation 4.1), Table 4.1.

The reason for this discrepancy lies mainly in the second term of Equation 4.1, i.e., the


91

Coulomb attraction between the positive and negative charges. While a point charge model is

assumed in Equation 4.1, a more reliable estimate of this Coulomb interaction is obtained

using the quantum-chemical calculations, which takes into account the actual charge

distributions along the donor/acceptor chromophores. When correcting Equation 4.1 for these

effects, the changes in Gibbs free energy for direct charge separation (induced by exciting the

OPV segment) are ~ –0.9 and –0.4 eV for M1 and P2, respectively, in excellent agreement

with the values in Table 4.3 (–1.1 and –0.6 eV, respectively).

Table 4.3. Molecular parameters involved in the calculation of the charge separation and

charge recombination rates in toluene solution. λi and λs are the internal and external

reorganization energies, respectively and ∆G0 is the change in free Gibbs energy. The

numbers between parentheses have been obtained on the basis of the DA ground-state

geometry.

Process λi (eV) λs (eV) VRP (cm-1) ∆G0 (eV) kRP (ns-1)

M1

D*A→ D+A– 0.22 0.05 435 (570) –1.1 451 (770)

DA*→ D+A– 0.31 0.04 65 (71) –1.0 84 (100)

D+A–→ DA 0.45 0.04 177 (190) –1.6 43 (50)

P2

D*A→ D+A– 0.25 0.08 9.2 –0.6 10

DA*→ D+A– 0.31 0.08 8.0 –0.6 7

D+A–→ DA 0.45 0.08 11.6 –2.1 0.007

Furthermore, the parameters in Table 4.3 indicate that the charge recombination process

in P2 and, to a lesser extent in M1, is in the ‘inverted’ regime in Marcus theory (for |∆G0| >>

λ, the rate decreases with increasing driving force). For charge separation, |∆G0| in P2 is close

to the total reorganization energy λ (= λi + λs), while in M1 |∆G0| is significantly larger than

λ. Therefore, the activation barrier for photoinduced charge transfer is smaller in P2 than in

M1. The calculated rates predict in the case of M1 a more efficient electron transfer process

in comparison to the hole transfer process, while they display similar rates in P2.

The trends in Table 4.3 are fully consistent with the experimental results in Table 4.2;

however, there are significant deviations between the measured and calculated rates as a

result of the high sensitivity of the rates on the electronic couplings and mostly the various

thermodynamical parameters involved in the calculations.

Chapter 4

92

4.8 Conclusion Two new copolymers (P1 and P2, Figure 4.2) consisting of alternating OPV donor and

PERY acceptor segments have been synthesized using a palladium catalyzed Suzuki reaction.

Additionally, a cyclic compound M1 in which the two chromophores are in a face-to-face

orientation could be isolated from the polymerization of P1. Excitation of either of the

chromophores in P1, P2, or M1 results in photoinduced energy- and electron-transfer

reactions. Photoluminescence quenching, photoluminescence lifetime, and transient pump-

probe spectroscopy have been used to assess rates for energy transfer (kET), direct and indirect

charge separation (kCSd and kCS

i), and charge recombination (kCR) of these materials in

solution and in thin films (Figure 4.7, Table 4.2). The direct forward electron transfer, i.e.

from OPV(S1), is extremely fast (kCSd ≥ 3.5 × 1011 s-1) for all systems, but especially high in

the case of M1. The rate for singlet-energy transfer from OPV(S1) to PERY(S1) is

competitive with the fast direct charge separation. The recombination rates vary more

strongly with the sample (Table 4.2). The longest lifetimes (0.5-1.6 ns) are observed for P1

and P2 in toluene. Quantum-chemical calculations confirm that higher rates are obtained in

M1 with respect to P2; this mostly comes from a larger electronic coupling due to the strong

donor-acceptor interactions in the face-to-face arrangement of the macrocyclic compound.

The calculations point to the very strong dependence of the electronic couplings between

donor and acceptor, on molecular orientations (geometries), orbital symmetries and energies,

and the nature of the initial excited state.

In the films, geminate recombination is much faster, although some charges are able to

diffuse away and are longer lived. Polymers P1 and P2 have been tested in photovoltaic

devices. Working cells with satisfactory open circuit voltages (VOC = 1-1.2 V) were prepared,

but the short circuit current densities (JSC = 0.008 - 0.012 mA cm-2) under AM1.5 conditions

were extremely low. The low currents likely result from fast geminate recombination (more

than 80 % recombination in the first ns) and poor transport characteristics due to face-to-face

orientations of OPV and PERY segments in alternating stacks in the polymer films.

The photophysical results on P1 and P2 provide an important guideline for design of new

alternating copolymers that could be more effective in polymer photovoltaic cells. To

overcome the intrinsic tendency of donor and acceptor segments to give alternating stacks

(Figure 4.14), stronger antagonistic interactions that direct the microscopic morphology

should be introduced. This might be accomplished by using segments that differ more

strongly in size, or by utilizing the low entropy of mixing (e.g. in block copolymers), or by

introducing anchoring points at the polymer chains (e.g. via interchain hydrogen bonds) that


93

secure the relative positions of donor and acceptor. Because charge separation is slower for

an end-to-end substitution of the two chromophores (as in P2) than for a face-to-face

orientation (as in M1), care must be taken that the rate of electron transfer remains high

enough to guarantee efficient charge generation in these desired isolated neighboring stacks

of donors and acceptors.

4.9 Experimental section General methods

For general information about experimental procedures see also section 3.7. Tetrahydrofuran was freshly

distilled over potassium/sodium. NMR spectra were recorded at room temperature on a Varian Gemini

spectrometer at frequencies of 300 and 75 MHz for 1H and 13C nuclei or a Bruker spectrometer at frequencies of

400 and 100 MHz for 1H and 13C nuclei. Cyclic voltammograms were measured in 0.1 M tetrabutylammonium

hexafluorophosphate (TBAPF6) as a supporting electrolyte in dichloromethane using a Potentioscan Wenking

POS73 potentiostat. The working electrode was a Pt disk (0.2 cm2), the counter electrode was a Pt plate (0.5

cm2), and a saturated calomel electrode (SCE) was used as reference electrode, calibrated against Fc/Fc+ (+0.43

V). Matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry was conducted

on a Perseptive Biosystems Voyager DE-Pro MALDI-TOF mass spectrometer. GC-MS measurements were

performed on a Shimadzu GC/MS-QP5000. Elemental analysis was carried out on a Perkin-Elmer 2400 series II

CHN analyzer.

Photoinduced absorption spectroscopy

Near steady-state photoinduced absorption (PIA) spectra were recorded between 0.25 and 3 eV by exciting

a thin drop-cast film on quartz in an Oxford Optistat continuous flow cryostat at 80 K with a mechanically

modulated (typically 275 Hz) cw Ar-ion laser (Spectra Physics 2025) pump beam tuned to 458 or 528 nm (25

mW, beam diameter of 2 mm) and monitoring the resulting change in transmission (∆T) of a tungsten-halogen

white-light probe beam after dispersion by a triple grating monochromator, using Si, InGaAs, and (cooled) InSb

detectors.

The femtosecond laser system used for pump-probe experiments consists of an amplified Ti-sapphire laser

(Spectra Physics Hurricane), providing 150 fs pulses at 800 nm with an energy of 750 µJ at 1 kHz. Pump (450

nm or 520 nm, fluence 0.5 µJ/mm2) and probe (1450 nm) pulses were created by optical parametric

amplification and twofold frequency doubling using two OPAs (Spectra Physics OPA-C). The pump beam was

linearly polarized at the magic angle (54.7°) with respect to the probe beam. The temporal evolution was

recorded using an InGaAs detector and standard lock-in detection at 500 Hz.

Photovoltaic cells

For fabrication of devices polyethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS, Bayer AG) was

spin coated from an aqueous dispersion under ambient conditions on pre-cleaned (washing, UV ozone

treatment) ITO covered glass substrates and the layer was dried by annealing the substrate on a hot plate. The

resulting PEDOT:PSS film thickness is about 100 nm. The polymers P1 and P2 were spin cast from chloroform

Chapter 4

94

to give an active layer of ~100 nm thickness as determined with a Tencor P10 surface profiler. Finally a LiF(1

nm)/Al(100 nm) back electrode was deposited by thermal evaporation under vacuum (5×10-6 mbar, 1 ppm O2

and < 1 ppm H2O). The active area of the device is 0.1 cm2. Illumination was performed with a Steurnagel

SolarConstant 1200 solar simulator, set to 100 mW cm-2.

6-(4-Bromophenoxy)hexan-1-ol (1). 6-Bromo-1-hexanol (3.0 mL, 21.9 mmol), 4-bromophenol (3.60 g,

20.8 mmol), K2CO3 (12.95 g, 93.7 mmol) and tetrabutylammonium bromide (0.34 g, 1.1 mmol) were stirred in

acetone (100 mL) under reflux for 22 h. After cooling to room temperature the reaction mixture was filtered and

the residue was washed two times with acetone. The organic fractions were combined and acetone was removed

in vacuo. The residue was dissolved in dichloromethane and subsequently washed with 1N HCl, water (dist.),

and brine. The solution was dried over Na2SO4 and the solvent was removed in vacuo to yield 5.18 g (91 %) of 1

as a light pink oil. The product was used without any further purification. 1H NMR (CDCl3): δ 7.36 (d, J = 8.6

Hz, 2H), 6.77 (d, J = 8.6 Hz, 2H), 3.92 (t, J = 6.4 Hz, 2H), 3.66 (dt, J = 6.3, 5.5 Hz, 2H), 1.79 (m, 2H), 1.61 (m,

2H), 1.49 (m, 4H), 13C NMR (CDCl3): δ 158.17, 132.19, 116.27, 112.60, 68.08, 62.87, 32.64, 29.12, 25.83,

25.50. GC-MS (MW for C12H1779BrO2 = 272.04) m/z = 272 [M]+.

Toluene-4-sulfonic acid 6-(4-bromophenoxy)hexyl ester (2). A solution of 1 (1.25 g, 4.6 mmol) in

pyridine (2 mL) was stirred at 0 °C. Pyridine (1 mL) was added together with p-toluenesulfonyl chloride (1.13

g, 6.0 mmol) to the solution. The mixture was stirred for 15 min. under argon while a precipitate began to form.

Stirring was continued overnight at 4 °C. Subsequently the flask was warmed to room temperature and after 15

min the mixture was poured into ice-water (50 mL). The aqueous phase was extracted twice with diethyl ether

(50 mL) and once with ethyl acetate (50 mL). Subsequently the organic phases were combined and washed with

2N HCl (3×, 100 mL), distilled water (4×, 100 mL), dried on Na2SO4 the solution was filtered and the solvent

was removed in vacuo. The product was purified by column chromatography (silica gel, CH2Cl2) to yield 1.47 g

(75 %) 2 as a clear oil. 1H NMR (CDCl3): δ 7.79 (d, J = 8.5 Hz, 2H), 7.37-7.32 (m, 4H) 6.75 (d, J = 9.1 Hz, 2H),

4.04 (t, J = 6.5 Hz, 2H), 3.88 (t, J = 6.3 Hz, 2H), 2.44 (s, 3H), 1.70 (m, 4H), 1.40 (m, 4H). 13C NMR (CDCl3): δ

158.13, 144.70, 133.22, 132.24, 129.84, 127.91, 116.27, 112.71, 70.44, 67.90, 28.94, 28.78, 25.45, 25.17, 21.65.

GC-MS (MW for C19H2381BrO4S = 428.04) m/z = 428 [M+]. Anal. Calcd. for C19H23BrO4S: C 53.40, H 5.42.

Found: C 53.46, H 5.43.

(S)-2-(1-Hydroxymethyl-3-methylbutyl)isoindole-1,3-dione (3). A solution of phthalic anhydride (3.37

g, 22.8 mmol) and (S)-(+)-leucinol (3.0 mL, 23.5 mmol) in toluene (20 mL) (dried on molsieves) was stirred at

80 °C. After 15 min. the temperature was raised to 130 °C. During the reflux the water was removed by a Dean-

Stark distillation setup. After refluxing for a few hours the reaction mixture was cooled to room temperature and

toluene was removed in vacuo. The residue was dissolved in dichloromethane and was subsequently washed

with 1N HCl, water (dist.), and brine. The solution was dried over Na2SO4 and after filtration the solvent was

removed in vacuo to yield 5.31 g (94 %) of 3 as a light yellow oil. 1H NMR (CDCl3): δ 7.84 (m, 2H), 7.72 (m,

2H), 4.47 (m, 1H), 4.03 (m, 1H), 3.86 (dd, J = 11.8, 3.6 Hz, 1H), 2.48 (br s, 1H), 2.00 (m, 1H), 1.55 (m, 2H),

0.93 (dd, J = 7.7, 6.3 Hz, 6H). 13C NMR (CDCl3): δ 169.19, 134.10, 131.84, 123.35, 63.79, 52.07, 37.23, 25.01,

23.01, 22.00. GC-MS (MW = 247) m/z = 247 [M]+. Anal. Calcd. for C14H17NO3: C 68.00, H 6.93, N 5.66.

Found: C 67.48, H 6.98, N 5.52.


95

(S)-2-{1-[6-(4-Bromophenoxy)hexyloxy]methyl-3-methylbutyl}isoindole-1,3-dione (4). A mixture

of 3 (1.33 g, 5.4 mmol), 2 (2.30 g, 5.4 mmol) and potassium tert-butoxide (0.70 g, 6.2 mmol) in DMF (15 mL)

was stirred at 70 °C for 42 h. After cooling to room temperature the mixture was added to 1N HCl and ethyl

acetate. The organic layer was separated and was washed with 1N HCl, water (dist.), and brine. After drying

over Na2SO4 the solution was filtered and the solvent was removed in vacuo. The product was purified by

column chromatography (silica gel, CH2Cl2/acetone 9:1) and 1.18 g (44 %) of 4 was obtained as a yellowish oil. 1H NMR (CDCl3): δ 7.80 (m, 2H), 7.67 (m, 2H), 7.35 (d, J = 9.1 Hz, 2H), 6.74 (d, J = 9.1 Hz, 2H), 4.57 (m,

1H), 3.95 (t, J = 9.9 Hz, 1H), 3.80 (t, J = 6.6 Hz, 2H), 3.60-3.45 (m, 2H), 3.40-3.25 (m, 1H), 2.15-2.00 (m, 1H),

1.65-1.10 (m, 10H), 0.92 (dd, J = 8.5, 6.3 Hz, 6H). 13C NMR (CDCl3): δ 168.73, 158.17, 133.80, 132.16,

131.96, 123.06, 116.26, 112.53, 70.74, 70.49, 68.02, 49.38, 37.46, 29.41, 29.00, 25.79, 25.64, 25.10, 23.21,

21.73.

(S)-1-[6-(4-Bromophenoxy)hexyloxy]methyl-3-methylbutylamine (5). A solution of 4 (1.34 g, 2.7

mmol) and hydrazine monohydrate (0.19 mL, 3.9 mmol) in methanol (25 mL) was refluxed for 42 h under

argon. After cooling to room temperature the solvent was evaporated in vacuo. The residue was added to ethyl

acetate and subsequently washed with 2N HCl, 1N NaOH, water (dist.), and brine and dried over Na2SO4. After

evaporation of ethyl acetate in vacuo 0.94 g (95 %) of 5 was obtained as a slightly yellow oil. The product was

used without any further purification. 1H NMR (CDCl3): δ 7.35 (d, J = 9.2 Hz, 2H), 6.76 (d, J= 9.2 Hz, 2H),

3.91 (t, J = 6.4 Hz, 2H), 3.44 (m, 3H), 3.20 (t, J = 8.6 Hz, 1H), 3.11 (m, 1H), 1.76 (m, 3H), 1.61 (m, 2H), 1.43

(m, 4H), 1.25 (m, 2H), 0.91 (dd, J = 8.8, 6.6 Hz, 6H). 13C NMR (CDCl3): δ 158.20, 132.18, 116.28, 112.58,

75.02, 71.25, 68.10, 49.15, 42.06, 29.55, 29.12, 25.92, 25.87, 24.57, 23.14, 22.20.

N,N´-Bis{(S)-1-[6-(4-bromophenoxy)hexyloxy]methyl-3-methylbutyl}perylene-3,4:9,10-

tetracarboxylic-bisimide (6). A mixture of 5 (0.050 g, 0.13 mmol), perylene-3,4:9,10-tetracarboxylic

dianhydride (0.026 g, 0.007 mmol), imidazole (0.35 g), and a few grains of zinc acetate were heated to 180 °C

and stirred for 1h. After cooling the reaction mixture was poured in a mixture of 2N HCl and ethyl acetate. The

organic layer was separated and was washed with 2N HCl, water (dist.), and brine and dried over Na2SO4. After

filtration the solvent was evaporated in vacuo. The product was purified by column chromatography (silica gel,

CH2Cl2/methanol 9.5:0.5) and by washing the solid with n-hexane. Compound 6 was obtained as a dark red

solid in the amount of 0.040 g (55 % yield). 1H NMR (CDCl3): δ 8.65 (br s, 4H), 8.52 (d, 4H, J = 8.4 Hz), 7.14

(br s, 4H), 6.46 (br s, 4H), 5.57 (m, 2H), 4.23 (t, J = 9.9 Hz, 2H), 3.73 (dd, J = 10.1, 5.3 Hz, 2H), 3.55 (m, 6H),

3.40 (m, 2H), 2.20 (m, 2H) 1.62 (m, 4H), 1.48 (br s, 8H), 1.22 (br s, 8H), 0.98 (dd, J = 17.6, 6.2 Hz, 6H). 13C

NMR (CDCl3): δ 164.20, 163.65, 134.20, 131.87, 131.66, 130.88, 129.30, 126.05, 123.66, 123.04, 122.77,

115.83, 112.31, 70.87, 70.68, 67.82, 51.48, 38.32, 29.47, 28.96, 25.95, 25.64, 25.52, 23.19, 22.44. MALDI-TOF

MS (MW = 1100.30) m/z = 1100.69 [M•]-. Anal. Calcd. for C60H64Br2N2O8: C 65.46, H 5.86, N 2.54. Found: C

65.55, H 5.91, N 2.58.

((S)-1-Hydroxymethyl-3-metylbutyl)carbamic acid tert-butyl ester (7). Di-tert-butyl dicarbonate (4.61

g, 21 mmol) was dissolved in THF (30 mL). (S)-(+)-leucinol (3.0 mL, 23 mmol) was added to this solution. The

reaction mixture was stirred for 5 h at room temperature under argon. After removal of the solvent in vacuo the

residue was dissolved in diethyl ether and washed with 1N HCl, water (dist.), and brine. After drying over

Na2SO4 the solvent was removed in vacuo to give 4.42 g (97 %) of a clear oil. 1H NMR (CDCl3): δ 4.55 (br s,

Chapter 4

96

1H), 3.69 (m, 2H), 3.49 (m, 1H), 2.42 (br s, 1H), 1.64 (m, 1H), 1.45 (s, 9H), 1.31 (t, J = 6.2 Hz, 2H), 0.93 (dd, J

= 6.5, 1.2 Hz, 6H). 13C NMR (CDCl3): δ 156.66, 79.75, 66.77, 51.17, 40.64, 28.43, 24.88, 23.08, 22.26.

(S)-[1-(4-Bromophenoxymethyl)-3-methylbutyl]carbamic acid tert-butyl ester (8). Alcohol 7 (0.69

g, 3.2 mmol), 4-bromophenol (0.55 g, 3.2 mmol) and triphenylphosphine (1.26 g, 4.8 mmol) were dissolved and

stirred in toluene (10 mL). A solution of diethyl azodicarboxylate (DEAD) (0.75 mL, 4.8 mmol) in toluene (7

mL) was slowly added to the reaction mixture such that the temperature did not exceed 35 °C. The reaction

mixture was stirred under argon for 22 h. The reaction mixture was washed with 1N HCl (3 times), water, and

brine and was dried over Na2SO4. After filtration and removal of the solvent in vacuo column chromatography

(silica gel, n-heptane/ethyl acetate 4:1, Rf = 0.5) was performed. The product was crystallized in n-hexane. 0.27

g (23 %) of 8 was obtained as white crystals. 1H NMR (CDCl3): δ 7.36 (d, J = 9.2 Hz, 2H), 6.77 (d, J = 8.8 Hz,

2H), 4.66 (br s, 1H), 4.05-3.85 (m, 3H), 1.68 (m, 1H), 1.45 (m, 1H), 1.44 (s, 9H), 0.94 (dd, J = 6.4, 4.2 Hz, 6H). 13C NMR (CDCl3): δ 157.95, 155.40, 132.25, 116.30, 113.05, 79.40, 70.39, 48.11, 40.94, 28.37, 24.82, 22.99,

22.24. Anal. Calcd for C17H26BrNO3: C 54.84, H 7.04, N 3.76. Found: C 54.83, H 7.05, N 3.71.

(S)-1-(4-Bromophenoxymethyl)-3-methylbutylamine (9). Compound 8 (0.88 g, 2.4 mmol) was stirred

with TFA (5 mL, 65 mmol) in CH2Cl2 (5 mL) for 4.5 h under argon at room temperature. Subsequently

NaHCO3 and water were added and the phases were separated. The organic layer was washed with water and

brine and was dried over Na2SO4. After filtration and evaporation in vacuo, 0.49 g (76 %) of a slightly yellow

oil was obtained. The product was used without any further purification. 1H NMR (CDCl3): δ 7.37 (d, J= 8.9 Hz,

2H), 6.79 (d, J = 9.2 Hz, 2H), 3.87 (dd, J = 8.8 ,3.7 Hz, 1H), 3.65 (dd, J = 9.0, 7.5 Hz, 1H), 3.23 (m, 1H), 1.78

(m, 1H), 1.31 (t, J = 7.0 Hz, 2H), 0.95 (dd, J = 11.0, 6.6 Hz, 6H). 13C NMR (CDCl3): δ 158.07, 132.28, 116.32,

112.93, 73.97, 48.49, 43.14, 24.66, 23.44, 22.08.

N,N´-Bis[(S)-1-(4-bromophenoxymethyl)-3-metylbutyl]perylene-3,4:9,10-tetracarboxylic-

bisimide (10). Amine 9 (0.161 g, 0.59 mmol), perylene-3,4:9,10-tetracarboxylic dianhydride (0.11 g, 0.28

mmol), zinc acetate (0.007 g, 0.04 mmol), and imidazole (1.34 g) were heated to 180 °C for 2 h. After cooling to

room temperature, column chromatography (silica gel, CH2Cl2/MeOH 95:5, Rf = 0.9) was used to purify the

product. 0.165 g (65 %) of 10 was obtained as a red solid. 1H NMR (CDCl3): δ 8.80-8.55 (m, 8H), 7.29 (d, J =

9.1 Hz, 4H), 6.76 (d, J = 9.1 Hz, 4H), 5.73 (m, 2H), 4.72 (t, J = 8.9 Hz, 2H), 4.32 (dd, J = 9.5, 5.6 Hz, 2H), 2.26

(ddd, J = 14.1, 9.1, 5.4 Hz, 2H), 1.62 (ddd, J = 14.0, 8.5, 5.6 Hz, 2H), 1.64 (m, 2H) 1.00 (dd, J = 12.8, 6.5 Hz,

12H). 13C NMR (CDCl3): δ 163.73, 157,76, 134.15, 132.15, 131.71, 130.86, 129.26, 125.98, 123.40, 122.77,

116.63, 113.05, 68.96, 51.46, 38.54, 25.56, 23.11, 22.46. MALDI-TOF MS (MALDI) (MW = 900.12) m/z =

900.45 [M•]-. Anal. Calcd. for C48H40Br2N2O6: C 64.01, H 4.48, N 3.11. Found: C 63.79, H 4.77, N 2.98.

(E,E)-1,4-bis{4-boronic acid-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]ben-

zene (12). A solution of dibromide 11 (2.0 g, 2.1 mmol) in THF (50 mL) was cooled to –78 °C. To this

solution n-BuLi (3.6 mL, 1.6 M in hexane, 5.76 mmol n-BuLi) was added. The mixture was stirred at this

temperature and after 3 h trimethyl borate (2.4 mL, 21.4 mmol) was added, later followed by another portion of

THF (20 mL). The mixture was stirred for 3 h at –78 °C and then heated to room temperature and poured into

water (700 mL). The product was extracted with chloroform and dried over Na2SO4. After filtration and removal

of the solvent in vacuo, the product was crystallized twice in acetone. Compound 12 0.945 g (51 %) was


97

obtained as a yellow/green solid. 1H NMR (DMF-d7): δ 7.86 (s, 4H), 7.69 (s, 4H), 7.45 (s, 2H), 7.36 (m, 4H),

4.15-3.80 (m, 12H), 2.05-1.85 (m, 6H), 1.75-1.50 (m, 6H), 1.45-1.25 (m, 6H), 1.15-0.95 (m, 36H). 13C NMR

(DMF-d7): δ 158.71, 151.73, 151.33, 130.55, 127.99, 127.65, 125.13, 124.33, 120.74, 111.11, 109.34, 74.39,

73.95, 35.44, 35.39, 35.14, 26.62, 26.57, 26.46, 16.84, 16.79, 16.61, 11.47, 11.26. MALDI-TOF MS (MW =

886.59). m/z = 886.68 [M•]+.Anal. Calcd. for C52H80B2O10: C 70.43, H 9.09. Found: C 71.03, H 9.15.

(E,E)-1,4-bis{4-{4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl-}-2,5-bis[(S)-2-methylbutoxy]styryl}-

2,5-bis[(S)-2-methylbutoxy]benzene (13). Diboronic acid 12 (0.878 g, 0.99 mmol) and pinacol (0.38 g,

3.2 mmol) were refluxed in CH2Cl2 using a Dean-Stark setup. After 16 h extra pinacol (0.27 g, 2.27 mmol) was

added. After a total 24 h of reflux, the reaction mixture was cooled to room temperature and the solvent was

evaporated in vacuo. The residue was crystallized three times in acetone. Product 13 was obtained as a yellow

solid in a yield of 0.599 g (58 %). 1H NMR (CDCl3): δ 7.55 (d, J = 16 Hz, 2H), 7.50 (d, J = 17 Hz, 2H), 7.18 (s,

2H), 7.17 (s, 2H), 7.13 (s, 2H), 3.90-3.75 (m, 12H), 2.00-1.85 (m, 6H), 1.75-1.60 (m, 6H), 1.40-1.25 (m, 6H),

1.35 (s, 24H), 1.15-1.05 (m, 18H), 1.00-0.90 (m, 18H). 13C NMR (CDCl3): δ 158.48, 151.15, 150.45, 130.98,

127.43, 123.71, 122.99, 120.72, 118.26, 110.10, 109.16, 83.31, 74.44, 74.27, 73.96, 35.20, 35.07, 26.36, 26.05,

24.90, 16.83, 16.48, 11.44. MALDI-TOF MS (MW = 1050.75) m/z = 1050.84 [M•]+. Anal. Calcd. for

C64H100B2O10: C 73.13, H 9.59. Found: C 73.34, H 9.55.

(E,E)-1,4-bis{4-(4-methoxyphenyl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)2-methylbut-

oxy]benzene (M2). Diboronic ester 13 (25.0 mg, 0.024 mmol), 4-bromoanisol (30 µl, 0.24 mmol) and

Pd(PPh3)4 (1.2 mg, 0.0010 mmol) were purged with argon in THF (6 mL) for 15 min. An aqueous solution of

K2CO3 (0.14 M) was purged with argon for 20 minutes. With a syringe 0.7 mL of the aqueous K2CO3 solution

was added to the mixture in THF and the whole mixture was again purged with argon for 15 min. at room

temperature. Subsequently, the temperature was raised to 90 °C and the reaction mixture was refluxed

overnight. The solvents were evaporated in vacuo and the residue was dissolved in CH2Cl2. The solution was

washed with water (dist.) and brine and dried over Na2SO4. After filtration and evaporation of the solvent, the

product was purified by column chromatography (silica gel, n-hexane/CH2Cl2 ranging from 3:1 to 1:1) to yield

0.018 g (75 %) of M2 as a yellow solid. 1H NMR (CD2Cl2): δ 7.56 (s, 4H), 7.53 (d, J = 8.8 Hz, 4H), 7.24 (m,

4H), 6.94 (d, J = 8.8 Hz, 4H), 6.90 (s, 2H), 4.00-3.70 (m, 18H), 2.10-1.90 (m, 4H), 1.90-1.75 (m, 2H), 1.75-1.60

(m, 4H), 1.60-1.45 (m, 2H), 1.45-1.30 (m, 4H), 1.30-1.20 (m, 2H), 1.20-0.85 (m, 36H). 13C NMR (CDCl3): δ

158.63, 151.08, 150.96, 150.47, 130.94, 130.70, 130.58, 127.40, 126.64, 122.70, 122.62, 115.67, 113.28,

110.32, 110.05, 74.56, 74.26, 74.11, 55.28, 35.09, 35.07, 34.86, 26.38, 26.35, 26.14, 16.81, 16.73, 11.47, 11.26.

MALDI-TOF MS (MW = 1010.66) m/z = 1010.59 [M•]+. IR (ATR): ν 3058, 2959, 2916, 2874, 1610, 1519,

1497, 1464, 1421, 1389, 1344, 1298, 1249, 1202, 1177, 1109, 1038, 965, 859, 850, 829, 730, 670 cm-1.

P1. Diboronic ester monomer 13 (0.0948 g, 0.090 mmol), dibromide monomer 6 (0.0993 g, 0.090 mmol) and

Pd(PPh3)4 (0.004 g, 0.003 mmol) were dissolved in THF (25 mL). The solution was purged with argon for 20

min. An aqueous K2CO3 solution (0.14 M) was purged with argon for 15 min. Subsequently, this K2CO3

solution (2.5 mL, 0.35 mmol K2CO3) was added to the reaction mixture, which was purged again with argon for

15 min. The reaction mixture was stirred for 19 h at 90 °C in the dark. After cooling to room temperature the

solvents were evaporated in vacuo. The soluble fractions were dissolved in chloroform (5mL) and were

precipitated in methanol (200 mL). The solid was washed with methanol. The precipitation was repeated one

Chapter 4

98

time. Acetone was used to extract the low molecular weight fractions with a Soxhlet-setup. The polymer was

obtained as a dark red/black solid in 0.052 g (33 %) yield. 1H NMR (CDCl3): δ 8.70-8.50 (m, 8H), 7.53 (s, 4H),

7.47 (d, J = 8.1 Hz, 4H), 7.25 (m, 4H), 6.88 (s, 2H), 6.81 (d, J = 8.8 Hz, 4H), 5.56 (m, 2H), 4.19 (t, J = 9.5 Hz,

2H), 4.00-3.65 (m, 18H), 3.60-3.45 (m, 2H), 3.45-3.30 (m, 2H), 2.25-2.10 (m, 2H), 2.05-0.80 (m, 94H). 13C

NMR (CDCl3): δ 164.34, 158.07, 151.08, 150.98, 150.44, 134.56, 131.92, 131.11, 130.70, 130.50, 129.58,

127.40, 126.61, 126.45, 123.85, 123.10, 122.67, 115.65, 113.74, 110.26, 110.04, 74.57, 74.25, 74.06, 71.01,

70.81, 67.77, 51.54, 38.41, 35.11, 35.07, 34.85, 29.54, 29.18, 26.38, 26.36, 26.14, 25.94, 25.76, 23.22, 22.44,

16.83, 16.75, 11.49, 11.26. SEC (CHCl3, versus polystyrene): Mw= 26.0 kg/mol, Mn = 12.6 kg/mol.

M1. The compound was separated from the crude fraction of polymer P1 by extraction with acetone.

Subsequently, column chromatography (SiO2, acetone/CH2Cl2 7:3) and size exclusion chromatography

(Biobeads SX1, CH2Cl2) afforded the product in pure form. 1H NMR (CDCl3): δ 8.51 (br s, 2H), 8.44 (d, 2H, J

= 8.0 Hz), 8.37 (d, J = 6.0 Hz, 2H), 8.30 (br s, 2H), 7.70-7.50 (br s + d, J = 5.6 Hz, 6H), 7.40-7.15 (m, 6H), 6.99

(d, J = 8.1 Hz, 4H), 6.92 (s, 2H) 5.21 (br s, 2H), 4.25 (t, J = 6.0 Hz, 4H), 3.90-3.60 (m, 14 H), 3.60-3.30 (m,

2H), 3.30-3.00 (m, 4H), 2.04 (m, 2H), 1.95-1.10 (m, 38 H), 1.10-0.75 (m, 48H). 13C NMR (CDCl3): δ 163.46,

157.72, 150.91, 150.56, 134.45, 131.92, 131.33, 130.67, 129.44, 126.33, 126.14, 123.52, 122.96, 122.22,

115.84, 115.51, 109.65, 74.28, 74.09, 73.87, 72.97, 70.91, 68.16, 51.72, 39.10, 35.11, 35.06, 34.95, 29.74,

29.47, 28.47, 26.35, 26.22, 25.40, 25.00, 23.31, 22.23, 16.84, 16.80, 16.74, 11.49, 11.45, 11.32. MALDI-TOF

HRMS (MW = 1738.034) m/z = 1738.021 [M•]+.

P2. Dibromide 10 (0.079 g, 0.0088 mmol), diboronic ester 13 (0.092 g, 0.0088 mmol), and Pd(PPh3)4 (0.004 g,

0.003 mmol) in THF (15 mL) were purged with argon for 15 min. A solution of K2CO3 in water (0.14 M) was

purged with argon for 15 minutes. This solution (2.5 mL, 0.35 mmol K2CO3) was added to the reaction mixture,

which was purged again with argon for 15 min. The reaction mixture was stirred at 90 °C for 18 h in the dark.

Precipitates were clearly observed in the reaction mixture. The reaction mixture was cooled to room temperature

and the liquid phase was removed using a pipette. The solid phase was dissolved in chloroform (4 mL) by

stirring and sonication. The chloroform solution was precipitated in methanol (250 mL) and the liquid phase was

separated by centrifugation and filtration. The solid was washed with methanol and was precipitated out of 3 mL

chloroform into 270 mL of acetone. The solid was filtered and was washed with acetone using the Soxhlet-setup

to remove low-molecular weight substances. From the polymer 0.042 g (31 %) was obtained as a dark red to

black solid. 1H NMR (CDCl3): δ 8.80-8.50 (m, 8H), 7.60-7.40 (m, 8H), 7.25-7.15 (m, 4H), 6.91 (br s, 4H), 6.81

(br s, 2H), 5.78 (br s, 2H), 4.77 (br s, 2H), 4.39 (br s, 2H), 3.90-3.60 (m, 12 H), 2.28 (br s, 2H), 2.00-0.80 (m,

70H). 13C NMR (CDCl3): δ 163.97, 157.77, 151.06, 150.92, 150.43, 134.58, 132.17, 131.13, 130.50, 129.53,

127.37, 126.62, 126.41, 123.71, 123.09, 122.67, 122.59, 116.67, 115.66, 114.16, 110.24, 110.02, 74.55, 74.23,

74.04, 68.70, 51.68, 38.65, 35.08, 35.05, 34.82, 26.36, 26.33, 26.12, 25.64, 23.19, 22.51, 16.81, 16.74, 11.47,

11.24. SEC (CHCl3, versus polystyrene): Mw = 26.0 kg/mol, Mn = 10.8 kg/mol.


99

4.10 References and notes

(1) G. J. Kavarnos, Fundamentals of Photoinduced Electron Transfer; Wiley-VCH, Weinheim, 1993.

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(3) Photoinduced Electron Transfer; Volumes 1-4, M. A. Fox, M. Chanon, (Ed.), Elsevier Amsterdam, 1988.

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(6) D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22.

(7) J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Holmes, Nature 1995, 376, 498.

(8) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789.

(9) C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001, 11, 15.

(10) A. Marcos Ramos, M. T. Rispens, J. K. J. van Duren, J. C. Hummelen, R. A. J. Janssen, J. Am. Chem. Soc. 2001, 123, 6714.

(11) G. Zerza, A. Cravino, H. Neugebauer, N. S. Sariciftci, R. Gomez, J. L. Segura, N. Martin, M. Svensson, M. R. Andersson, J. Phys. Chem. A 2001, 105, 4172.

(12) A. Cravino, G. Zerza, H. Neugebauer, N. S. Sariciftci, M. Maggini, S. Bucella, M. Svensson, M. R. Andersson, Chem. Commun. 2000, 2487.

(13) F. Zhang, M. Svensson, M. R. Andersson, M. Maggini, S. Bucella, E. Menna, O. Inganäs, Adv. Mater. 2001, 13, 1871.

(14) S. Wang, J. Yang, Y. Li, H. Lin, H. Guo, S. Xiao, Z. Shi, D. Zhu, H.-S. Woo, D. L. Caroll, I.-S. Kee, J.-H. Lee, Appl. Phys. Lett. 2002, 80, 3847.

(15) D. M. Russel, A. C. Arias, R. H. Friend, C. Silva, C. Ego, A. C. Grimsdale, K. Müllen, Appl. Phys. Lett. 2002, 80, 2204.

(16) B. de Boer, U. Stalmach, P. F. van Hutten, C. Melzer, V. V. Krasnikov, G. Hadziioannou, Polymer 2001, 42, 9097.

(17) U. Stalmach, B. de Boer, C. Videlot, P. F. van Hutten, G. Hadziioannou, J. Am. Chem. Soc. 2000, 122, 5464.

(18) M. A. Hempenius, B. M. W. Langeveld-Voss, J. A. E. H. van Haare, R. A. J. Janssen, S. S. Sheiko, J. P. Spatz, M. Möller, E. W. Meijer, J. Am. Chem. Soc. 1998, 120, 2798.

(19) J. Liu, E. Sheina, T. Kowalewski, R. D. McCullough, Angew. Chem. Int. Ed. 2002, 41, 329.

(20) H. Wang, H. H. Wang, V. S. Urban, K. C. Littrell, P. Thiyagarajan, L. Yu, J. Am. Chem. Soc. 2000, 122, 6855.

(21) P. R. L. Malenfant, L. Groenendaal, J. M. J. Fréchet, J. Am. Chem. Soc. 1998, 120, 10990.

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(22) H. Wang, M.-K. Ng, L. Wang, L. Yu, B. Lin, M. Meron, Y. Xiao, Chem. Eur. J. 2002, 8, 3246.

(23) J.-F. Nierengarten, J.-F. Eckert, J.-F. Nicoud, L. Ouali, V. Krasnikov, G. Hadziioannou, Chem. Commun. 1999, 617.

(24) J.-F. Eckert, J.-F. Nicoud, J.-F. Nierengarten, S.-G. Liu, L. Echegoyen, F. Barigelletti, N. Armaroli, L. Ouali, V. Krasnikov, G. Hadziioannou, J. Am. Chem. Soc. 2000, 122, 7467

(25) E. Peeters, P. A. van Hal, J. Knol, C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, R. A. J. Janssen, J. Phys. Chem. B 2000, 104, 10174.

(26) A. El-ghayoury, A. P. J. H. Schenning, P. A. van Hal, J. K. J. van Duren, R. A. J. Janssen, E. W. Meijer, Angew. Chem. Int. Ed. 2001, 40, 3660.

(27) J. J. M. Halls, R. H. Friend, Synth. Met. 1997, 85, 1307.

(28) L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, 1119.

(29) J. J. Dittmer, E. A. Marseglia, R. H. Friend, Adv. Mater. 2000, 12, 1270.

(30) J. J. Dittmer, K. Petritsch, E. A. Marseglia, R. H. Friend, H. Rost, A. B. Holmes, Synth. Met. 1999, 102, 879.

(31) M. A. Angadi, D. Gosztola, M. R. Wasielewski, J. Appl. Phys.1998, 83, 6187.

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(34) L. M. Herz, C. Silva, R. H. Friend, R. T. Phillips, S. Setayesh, S. Becker, D. Marsitsky, K. Müllen, Phys. Rev. B. 2001, 64, 195203.

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(36) N. Harada, K. Nakanishi, Circular Dichroic Spectroscopy. Exciton Coupling in Organic Stereochemistry; University Science Books, Mill Valley, California, 1983.

(37) B. Köhler, V. Enkelmann, M. Oda, S. Pieraccini, G. P. Spada, U. Scherf, Chem. Eur. J. 2001, 7, 3000.

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(41) It is important to note that small concentrations of fluorescent impurities, such as monomers, can mask the true excitation spectra of these weakly fluorescent systems, and the data should be interpreted with caution.

(42) J. Salbeck, J. Electroanal. Chem, 1992, 340, 169.


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101

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3893.

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102

103

5

Conjugated oligo(p-phenylene vinylene) polymers with

dangling perylene bisimides

Abstract

Two new donor-acceptor copolymers that consist of an oligo(p-phenylene

vinylene) main chain with dangling perylene bisimides have been synthesized by

Suzuki polymerization. Circular dichroism spectroscopy in toluene solution has

demonstrated that these copolymers show donor-acceptor exciton coupling, even

at elevated temperatures. Fluorescence quenching studies show that in toluene an

efficient charge transfer occurs upon excitation of either the donor or the

acceptor. Detection of the charged species was accomplished with femtosecond

and near steady state photoinduced absorption spectroscopy. Charges are formed

within 1 ps after excitation and the recombination decay time is 45-60 ps, both in

solution and solid state. The short distance between the two chromophores

rationalizes this behavior. The photovoltaic effect of these donor-acceptor

polymers in devices is limited, which is attributed to the fast recombination

kinetics.

Chapter 5

104

5.1 Introduction Ordering of donor and acceptor moieties in organic solar cells is crucial for achieving a

photoactive layer that is able to convert absorbed photons efficiently into electrons and holes

and subsequently for transporting these charges to the electrodes. An increasing number of

donor-acceptor molecules and polymers is being reported that are designed to create such

beneficial ordering in the solid state.

One strategy is the preparation of ‘double-cable’ polymers.1 These polymers consist of a

conjugated donor main chain (donor-cable) bearing a number of closely spaced acceptor

moieties (acceptor-cable). Double-cable polymers are designed to achieve an intimate contact

between donor and acceptor segments and to avoid clustering and phase separation of the

donor and acceptor phase. By virtue of the covalently linked structure, a bicontinuous

network on a molecular scale is formed. In working devices based on double-cable polymers,

holes are expected to move away from the electron acceptors by intrachain and interchain

migration, while the electrons can hop between the acceptor units. The majority of double

cables reported in literature have fullerene C60 as the dangling acceptor unit. These materials

have been synthesized by either electrochemical polymerization,2-5 chemical

polymerization,6,7 or by grafting a functionalized conjugated polymer with fullerene C60.3,8,9

In addition to fullerene-based double cables, polythiophenes with pendant tetracyano-

anthraquinodimethane moieties have been reported.10,11 Although some of these double-cable

polymers were incorporated in working photovoltaic devices,6,7 their photophysical properties

have not been studied on short time scales. Hence, the detailed kinetics of charge separation

and recombination in double-cable polymers is unknown.

In Chapter 4, alternating copolymers of oligo(p-phenylene vinylene) (OPV) and perylene

bisimide were reported that gave an efficient charge separation in solution and in the solid

state. The photophysical properties in the solid state indicated that these polymers probably

form stacks of alternating OPV and perylene bisimide units in the solid state. Another attempt

to mesoscopically organize OPV and perylene bisimide chromophores has been reported for a

triad consisting of two tridodecyloxy-OPV units attached to a central perylene bisimide core

that possesses a liquid-crystalline mesophase and, hence, provides a handle to achieve

ordered morphologies upon annealing.12 Indeed, charge recombination was reduced in more

ordered phases in consequence of improved charge transport. Altering the polymer

architecture from a linear array of alternating donors and acceptors as used in Chapter 4, to a

long donor with pendant acceptors, could induce differences in the photophysical properties.

With this in mind, new polymers involving OPV and perylene bisimide were designed.

Conjugated oligo(p-phenylene vinylene) polymers with dangling perylene bisimides

105

In this chapter the synthesis, photophysical, and photovoltaic properties are presented of

two polymers that consist of donor OPV segments in the main chain with pendant perylene

bisimide units (P1 and P2, Figure 5.1). The structural difference between these polymers lie

in the substitution pattern of the phenyl ring that connects two OPV units. For P1, the OPV

units are placed para with respect to each other while in P2 they are positioned in a meta

fashion.

O

N

N

O

O

O

O

O

O

O

O

O

O

O

n

N

N

O

O

O

O

O

O

O

O

O

O

O

n

P1 P2

Figure 5.1. Structures of donor-acceptor copolymers P1 and P2.

5.2 Synthesis In order to synthesize polymers P1 and P2, two different perylene bisimide monomers

were prepared that were subsequently reacted with an OPV monomer in a Suzuki

polymerization.

The first step in the synthesis of the perylene bisimide monomer 8 (Scheme 5.1) was the

Williamson etherification of 2,5-dibromobenzene-1,4-diol 113 with racemic 2-ethylhexyl

bromide, yielding alcohol 2. Subsequently, alcohol 2 was coupled under Mitsunobu

conditions with ((S)-1-hydroxymethyl-3-methylbutyl)carbamic acid tert-butyl ester 314

(described in Chapter 4) to the BOC-protected diether 4, which was deprotected with

trifluoroacetic acid (TFA) and NaHCO3 to the free amine 5. N,N´-bis(1-ethylpropyl)perylene-

3,4:9,10-tertacarboxylic-bisimide 615 was partially hydrolyzed with potassium hydroxide in

tert-butanol to the anhydride imide 7.16,17 The final reaction of amine 5 with anhydride 7

resulted in perylene bisimide 8.

Chapter 5

106

O NOH

O

H

O NO

O

H

Br

O

BrNH2

O O

Br

Br

N NO O

Br

O

O

O

O

Br

OH OH

Br

Br

OH

Br

O

Br

N O

O

O

O

O

N N

O

O

O

O

+

1 2 3

4 5

7

a

b c

6

ed

8

Scheme 5.1. Synthesis of the perylene bisimide monomer 8; (a) K2CO3, 2-ethylhexyl bromide,

ethanol, reflux, 28 %; (b) PPh3, DEAD, toluene, 36 %; (c) 1. TFA, CH2Cl2, 2. NaHCO3, 91

%; (d) KOH, tert-butanol, 20 %; (e) DMF, 155 °C, 39 %.

The second perylene bisimide monomer 12 was synthesized starting with a Mitsunobu

reaction of 2,4-dibromophenol 9 with alcohol 3, that lead to the formation of ether 10

(Scheme 5.2). After deprotection of 10 with TFA and NaHCO3, amine 11 was reacted with

the anhydride 7 to afford the perylene bisimide 12. Monomer 13, which is used as a reference

compound for monomer 12, was obtained after the Williamson etherification of phenol 9 with

racemic 2-ethylhexyl bromide.


107

O NO

OBr

H

Br

NH2O Br

Br

N NO Br

Br

O

O

O

O

OH Br

Br

O Br

Br

Br

f g

h

9 10 11

12

9 +i

13

3 +

7

Scheme 5.2. Synthesis of the perylene bisimide monomer 12 and the reference monomer 13;

(f) PPh3, DEAD, toluene, 28 %; (g) 1. TFA, CH2Cl2, 2. NaHCO3, 83 %; (h) DMF, 160 °C, 69

%; (i) K2CO3, ethanol, reflux, 24 %.

For all polymerization reactions bisboralane (E,E)-1,4-bis{4-{4,4,5,5-

tetramethyl[1,3,2]dioxaborolan-2-yl-}-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-

methylbutoxy]benzene 1414 (Chapter 4) was coupled, using the palladium-catalyzed Suzuki

reaction, to the different dibromo aryl monomers (Scheme 5.3). Reaction of monomer 14

with perylene bisimide monomers 8 and 12 resulted in the formation of perylene bisimide

substituted polymers P1 and P2 respectively. Likewise, polymerization of 14 with 1,4-

dibromo-2,5-di(2-ethylhexyloxy)benzene 1518,19 and 13 provided the corresponding reference

polymers RP1 and RP2 that have the same main chain, but lack the dangling perylene

bisimide chromophores. The polymers were characterized using 1H NMR, 13C NMR and size

exclusion chromatography (SEC). The molecular weights and polydispersities (PDI)

determined by SEC in chloroform versus polystyrene standards are Mn = 8.2 kg/mol for P1

and Mn = 8.0 kg/mol for P2 with PDI of 3.3 and 2.8, respectively. The reference polymers

have molecular weights of Mn = 24.6 kg/mol (RP1, PDI = 4.5) and Mn = 13.8 kg/mol (RP2,

PDI = 3.5). The reason for the difference in Mn of the donor-acceptor polymers and the

reference polymers is not known, but solubility of the polymers might play a role since the

products precipitate in the reaction mixture. The integrals of the 1H NMR spectra show that

the perylene bisimide monomers, or their respective reference monomers, and the OPV

monomer are incorporated in an equimolar fashion in all four polymers.

Chapter 5

108

BO

O

BO

O

O

O

O

O

O

O

O

OO

O

O

O

O

O

O

O

OO

O

O

O

n

O

OO

O

O

O

O

O

n

O

Br Br

O

14

8 12

13

15

P1

RP1

P2

RP2

j k

l m

16

15

Scheme 5.3. Overview of the synthesis of the perylene bisimide substituted polymers P1 and

P2, their reference polymers RP1 and RP2 and the structure of model compound 16; (j)

Pd(PPh3)4, K2CO3, water, THF, 90 °C, 72 %; (k) Pd(PPh3)4, K2CO3, water, THF, 90 °C, 75

%; (l) Pd(PPh3)4, K2CO3, water, THF, 90 °C, 51 %; (m) Pd(PPh3)4, K2CO3, water, THF, 90

°C, 55 %.

5.3 Absorption spectra in solution The polymers and model compound 6 were also characterized by UV/Visible absorption

spectroscopy in toluene solution (Figure 5.2). The perylene bisimide segment (PERY)

absorbs at 458 nm, 489 nm and 526 nm, as demonstrated by 6. The maximum of the

absorption of RP1 is at 431 nm, whereas for RP2 it is at 426 nm. The dissimilar effective


109

conjugation length in these polymers can explain this difference. The para connectivity of

adjacent OPV segments at the benzene rings in the main chain of RP1 results in a more

effective conjugation than in RP2, where the OPV segments are linked via a meta substituted

benzene ring. The absorption spectra of P1 and P2 clearly show that both the OPV backbone

and the perylene bisimide are incorporated. It seems, however, that the absorption intensity in

the 400-470 nm region is too high in both cases, compared to the perylene absorption at 528

nm. It is unlikely that this is due to a significant deviation from the equimolar incorporation

of the OPV and perylene bisimide monomers in the polymer chain, because 1H NMR

spectroscopy confirmed that their ratio is 1:1 in polymers P1 and P2. A possible explanation

is an electronic interaction between the two different chromophores, which was also

suggested for a macrocycle containing OPV and perylene bisimide (see Chapter 4).14 Because

of the relatively short distance between the backbone and the perylene bisimides, such

electronic coupling might also be possible in polymers P1 and P2. Absorption spectra in

toluene were recorded as function of temperature in the range from 20 to 90 °C, with heating

steps of 10 °C (Figure 5.3a and 5.3c). The UV/Visible spectra show a small blue shift with

increasing temperature, which is commonly observed for dialkoxy-substituted PPVs.20 Also,

the intensity of the band around 430 nm decreases slightly relative to the band around 530

nm, which might point to a decreasing electronic interaction. Because the original spectra at

20 °C were retrieved after cooling it can be concluded that the spectral changes with

temperature are reversible.

300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 b

P2 6 RP2 6 + RP2

Nor

mal

ized

Abs

orba

nce

(-)

Wavelength (nm)

300 350 400 450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 a

Wavelength (nm)

P1 6 RP1 6 + RP1

Nor

mal

ized

Abs

orba

nce

(-)

Figure 5.2. Absorption spectra in toluene of P1, 6, RP1 (a) and P2, 6, RP2 (b) together with

the 1:1 summation of the spectra of the reference polymers and 6. The spectra are normalized

at ~527 nm. The spectra of RP1 and RP2 are normalized with respect to the normalized

spectra of the summation spectra.

Chapter 5

110

300 350 400 450 500 550 600-10

-5

0

5

bT increase

CD

(m

deg)

Wavelength (nm)

0.0

0.5

1.0

1.5

a

P1T increase

Abs

orba

nce

(O.D

.)

20 ºC 40 ºC 60 ºC 90 ºC

300 350 400 450 500 550 600-10

-5

0

5

10

d T increase

CD

(m

deg)

Wavelength (nm)

0.0

0.5

1.0

1.5

c

P2T increase

Abs

orba

nce

(O.D

.)

20 ºC 40 ºC 60 ºC 90 ºC

Figure 5.3. Temperature dependence of the UV/Visible absorption (a and c) and circular

dichroism spectra (b and d) of solutions of P1 (left) and P2 (right) in toluene, recorded upon

heating from 20 °C to 90 °C in steps of 10 °C.

5.4 Circular dichroism spectra in solution Electronic interaction in the ground state between the perylene bisimide and the OPV

segments was investigated by circular dichroism (CD) spectroscopy. In these polymers CD

signals may originate from the exciton coupling between main chain and pedant

chromophores when they are in a preferential (helical) orientation with respect to each other.

When chirality in the molecules favors one type of helicity, the interaction is expressed in a

CD signal. In this case the preferential helicity can originate from the interaction between the

perylene bisimide, having an enantiomerically pure (S)-leucinol linker to the backbone, and

the OPV with its enantiomerically pure (S)-2-methylbutoxy side chains. CD spectra of the

polymers P1 and P2 were recorded in toluene as a function of temperature in heating steps of

10 °C, starting at 20 °C and ending at 90 °C (Figure 5.3b and 5.3d). Both polymers exhibit a

negative signal for the OPV-backbone and a positive signal for the perylene bisimide. This is

an indication for exciton coupling between the donor and the acceptor with a positive

helicity, i.e. a right-handed rotation will superimpose the two transition dipole moments. This

effect has also been observed for the OPV-perylene bisimide macrocyle, which showed a


111

similar spectrum (Chapter 4).14 For P1 the bisignated signal around 530 nm points to an

additional perylene-perylene coupling. With increasing temperatures the shape of the CD

spectra remains, but becomes gradually less intense. As for the absorption experiments, also

the temperature dependent CD measurements were reversible.

5.5 Charge separation The Gibbs free energy of the intramolecular charge-separated state (GCS) can be

calculated via the Weller equation.21

−

+−−−= −+

sref0

2

ccs0

2

redoxCS1111

84))A()D((

εεπεεπε rr

e

R

eEEeG (5.1)

The first reduction potential Ered(A) of compound 6 is determined at –0.65 V in

dichloromethane (εref = 8.93) and the first oxidation potential Eox(D) of an OPV model

compound 16 is at +0.80 V (both vs SCE).14 Although the backbones of P1 and P2 are fully

conjugated, the comparison with 16 is justified because the absorption spectra are similar

(λmax = 425 nm for the model compound). This indicates that the conjugation length in the

polymers P1 and P2 is similar to that of the model compound 16. The radius of the perylene

bisimide anion r– was estimated at 4.7 Å and the radius of the OPV cation r+ at 5.1 Å. The

distance between the centers of the donor and acceptor segments Rcc was estimated with

molecular modeling to be within the range of 4 Å to 16 Å for both P1 and P2. This wide

range of distances is a consequence of the flexible spacer between the OPV and the perylene

bisimide. Substituting these parameters in Equation 5.1 shows that the intramolecular charge-

separated state in the polymers in toluene (εs = 2.38) lies lower in energy than the OPV and

PERY singlet excited state (S1 OPV = 2.56 eV, S1 PERY = 2.32 eV, Table 5.1). In fact, ∆GCS

is negative up to a donor-acceptor distance of 150 Å, which is much larger than the actual

distance that will occur in these two polymers. The calculations show that electron transfer in

P1 and P2 is an exergonic reaction, irrespective of whether it originates from the OPV or

perylene bisimide S1 states.

Chapter 5

112

Table 5.1. Free energy of intramolecular and intermolecular charge-separated states

calculated from Equation 5.1 in toluene relative to the ground state and the singlet-excited

state of OPV and PERY.

∆GCS = GCS – E00

Rcc (Å) GCS (eV) S1 OPV S1 PERY

4 0.85 –1.71 –1.47

16 1.98 –0.57 –0.34

150 2.32 –0.24 0

∞ 2.36 –0.20 0.04

5.5.1 Fluorescence spectra and fluorescence quenching in solution

Figure 5.4 shows the photophysical processes, such as energy transfer and the charge

separation that can take place upon exciting the OPV-donor. Two possible pathways exist to

go to the charge-separated state (CSS): direct charge transfer (kCSd) from the OPV S1 state or

an initial energy transfer to the singlet excited state of the perylene bisimide with subsequent

(indirect) charge transfer (kET + kCSi). Selective excitation of the perylene bisimide can also

lead to the CSS with kCSi as the rate.

kRD

kET

kCSikCS

d

kRA

S0 S0

S1

S1

CSS

OPV PERY

kNRD

kNRA

kRD

kET

kCSikCS

d

kRA

S0 S0

S1

S1

CSS

OPV PERY

kNRD

kNRA

Figure 5.4. Jablonski diagram representing the different photophysical events that can take

place in the donor-acceptor (D-A) polymers P1 and P2 upon excitation of the OPV-backbone

(open arrow). The singlet energy transfer (kET), the direct (kCSd) and indirect (kCS

i) electron

transfer, the radiative emission (kR) and non-radiative emission (kNR) are indicated.


113

For P1 and P2, Equation 5.1 has indicated that both pathways are possible of but

experimentally it is not always possible to distinguish between the direct and indirect

pathway, especially for cases when direct and indirect charge separation occur with similar

rate constants and the energy transfer with a large rate constant.

The occurrence of intramolecular photoinduced energy an electron transfer in the

polymers was investigated by photoluminescence experiments in both toluene (Figure 5.5)

and ortho-dichlorobenzene (ODCB). The polymers P1 and P2 were excited at either 433 nm

or 427 nm and 529 nm, where mainly the polymer backbones and perylene bismide absorb,

respectively. The photoluminescence intensity was compared to the fluorescence signals of

the reference compounds RP1, RP2, and 6 (λmax = 482-485 nm, 485-486 nm and 534 nm

respectively), which were excited at the same wavelengths as P1 and P2. The quenching

factors Q were determined by comparing the photoluminescence intensities at the wavelength

of maximum emission. In toluene, the fluorescence intensity of P1 was quenched with a

factor QOPV of at least 390 with respect to the backbone fluorescence of RP1 around 490 nm

and QPERY = 540 with respect to the perylene bisimide fluorescence of 6 around 535 nm. P2

showed quenching factors of QOPV = 200 and QPERY = 440. In ODCB the quenching of P1

was similar as in toluene, now with QOPV = 280 around 494 nm and QPERY = 600 at 538 nm.

For P2 these factors are QOPV = 200 with respect to RP2 at 494 nm and QPERY = 400 around

539 nm with respect to compound 6.

450 500 550 600 650 700

0

500

1000

1500

2000

PL

Inte

nsity

(-)

a

Wavelength (nm)

P1 RP1 6

450 500 550 600 650 700

0

500

1000

1500

2000

P2 RP2 6

b

PL

Inte

nsity

(-)

Wavelength (nm)

Figure 5.5. Fluorescence emission spectra in toluene of polymers P1 and RP1 upon

excitation at 433 nm and compound 6 upon excitation at 529 nm (a) and of polymers P2 and

RP2 upon excitation at 427 nm and 6 upon excitation at 529 nm (b). The spectra are

corrected for the optical density at the excitation wavelength.

Chapter 5

114

The observed fluorescence quenching shows that very efficient charge separation takes

place in P1 and P2 upon either exciting the backbone or the perylene bisimides in toluene and

ODCB, because almost all fluorescence of both chromophores is quenched. One should bear

in mind that the small residual fluorescence of P1 and P2 may originate from minute amounts

of highly fluorescent impurities such as monomers and therefore the quenching factors should

be taken not as absolute values for polymers P1 and P2, but as a lower limit of the actual

numbers.

From the quenching factors Q in toluene it is possible to estimate the rates of charge

separation via the direct and indirect pathway (kCSd and kCS

i) and the rate of energy transfer

kET (Figure 5.4, Table 5.2). The sum of kET and kCSd can be estimated with Equation 5.2 from

the OPV fluorescence quenching QOPV and the fluorescence lifetime of the polymer

backbones (τOPV), which are 0.9 ns for RP1 and 1.1 ns for RP2 in toluene (determined by

single-photon counting).

OPV

OPVdCSET

1

τ−=+ Q

kk (5.2)

Using this equation values of kET + kCSd = 4.3 × 1011 s-1 for P1 and kET + kCS

d = 1.8 × 1011

s-1 for P2 were obtained.

From the fluorescence quenching after excitation of the perylene bisimide QPERY and its

fluorescence lifetime τPERY, which is 4.0 ns for compound 6,14 kCSi can be calculated with

Equation 5.3, giving kCSi = 1.3 × 1011 s-1 for P1 and kCS

i = 1.1 × 1011 s-1 for P2.

PERY

PERYiCS

1

τ−= Q

k (5.3)

Because the quenching factors are a lower limit, these rates must also be considered as

lower estimates.


115

Table 5.2. Rate constants for energy transfer, direct and indirect charge separation, and

charge recombination as obtained from photoluminescence quenching and pump-probe PIA

spectroscopy for P1 and P2 in toluene solution and solid state.

PL quenchinga PIA OPV PIA PERY

QOPV QPERY kET + kCSd

(ns-1)

kCSi

(ns-1)

kCSi or

kET + kCSd

(ns-1)

kCR

(ns-1)

kCSi

(ns-1)

kCR

(ns-1)

P1 Toluene 390 540 430 130 ≥ 2000 22 ≥ 2000 18

Film ≥ 2000 18 ≥ 2000 16

P2 Toluene 200 440 180 110 ≥ 2000 22 ≥ 2000 16

Film ≥ 2000 16 ≥ 2000 18 a Rate constants determined from PL quenching are lower limits to the actual values as

explained in the text.

5.5.2 Photoinduced absorption in solution

A technique to detect photogenerated charged species is photoinduced absorption (PIA)

spectroscopy. In order to investigate the charge separation and recombination processes,

toluene solutions of P1 and P2 were studied by femtosecond pump-probe spectroscopy. The

polymers were excited with a ~150 fs pulse at 450 nm and 525 nm.

0 200 400 600 800 1000

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Time delay (ps)

450 nm P1 450 nm P2 525 nm P1 525 nm P2

∆T/T

Figure 5.6. Normalized differential transmission dynamics of toluene solutions of P1 and P2

at room temperature, recorded at 1450 nm (low-energy absorption of OPV radical cations)

after excitation at 450 nm and 525 nm.

Chapter 5

116

Figure 5.6 shows the normalized transient change in transmission ∆T/T at the probe

wavelength of 1450 nm (0.86 eV), which is a wavelength at which the OPV+• radical cations

absorb. The fact that a negative signal is observed is consistent with the absorption of a

transient species formed after photoexcitation. The OPV+• radical cations are formed within

0.5 ps (kCS ≥ 2 × 1012 s-1) after photoexcitation. In the time range between 0 and 250 ps after

the formation of the charges upon excitation with 450 nm, the recombination has a time

constant of ~45 ps for both polymers. When the perylene bisimide chromophores of P1 and

P2 are selectively excited at 525 nm, the recombination of the charges seems slightly slower,

resulting in a lifetime of ~60 ps.

As a reference experiment, transient photoinduced absorption experiments were

performed on toluene solutions of RP1, RP2, and compound 6. The solutions were excited at

450 nm (RP1 and RP2) and 525 nm (6), and were probed at 1450 nm. The signals obtained

for all three solutions have lower intensities (~10 to 20 %) and longer decay constants. This

indicates that the photoinduced absorptions detected for polymers P1 and P2 have a different

origin than the photoinduced absorptions of the model compounds. This supports the

proposal that charges are created intramolecularly in solutions of donor-acceptor polymers P1

and P2 upon photoexcitation.

Pump-probe measurements can give reliable rate constants, although limited by the

resolution of the excitation pulse. The fluorescence quenching experiments are extremely

sensitive for impurities, mainly because the fluorescence quantum yields of an impurity and

of the bulk of the molecules may differ by several orders of magnitude. Comparing the rates

of charge separation measured with the femtosecond pump-probe spectroscopy kCS ≥ 2 × 1012

s-1, with the calculated kET + kCSd and kCS

i (1 - 4 × 1011 s-1) from the fluorescence quenching

studies, shows that they are in satisfactory agreement with each other. Because the rate of

charge separation detected by pump-probe spectroscopy is very fast and is limited by the

resolution of the pump-probe set-up, it is not possible to distinguish whether the charge

separation from the OPV excited state follows the direct or indirect pathway.

5.5.3 Photoinduced absorption in solid state

In solid state it was possible to study the process of photoinduced charge transfer on long

and short timescales using different PIA set-ups. Near steady state PIA spectra of thin films

of P1 and P2 on quartz were recorded at 80 K upon excitation with a wavelength of 458 nm

(Figure 5.7). The distinct absorptions of the PERY–• radical anions at 1.28, 1.54, and 1.72 eV

are clearly visible in the spectra.22 The absorptions at 0.70 eV and in the region of 1.5-2.1 eV


117

are attributed to OPV+• radical cations, based on previous results for OPV3+• and OPV4+•.23

The 1.5-2.1 eV absorption of OPV+• overlaps with the absorptions of the PERY–• radical

anions.

The PIA signals at 0.70 and 1.54 eV of both P1 and P2 show a sublinear dependence

with increasing intensity of the excitation. When the data are fitted to a power-law expression

for ∆T as function of the pump intensity, exponents between 0.38 and 0.47 are obtained. This

indicates that the charged species associated with these signals probably decay via a

bimolecular recombination mechanism, for which an exponent of 0.5 would be expected.

Exponents of less than 0.5 can be explained when the steady-state concentration of charges is

trap limited.24 The signal intensities at 0.70 eV and 1.54 eV for P1 and P2 were found to

decrease with increasing modulation frequency in an identical fashion, closely following a

power-law decay with the modulation frequency. This indicates that the signals in the

spectrum have the same origin and that a distribution of lifetimes exists, extending in the

millisecond time domain. The distribution of lifetimes is consistent with a trap-filling model

for the long-lived charges observed in this experiment.

0.5 1.0 1.5 2.0

-0.0002

-0.0001

0.0000

0.0001

0.0002

OPV+•

PERY-•

PERY-•OPV+•

∆T/T

Energy (eV)

Figure 5.7. Near steady state photoinduced absorption spectra of P1 (solid line) and P2

(dashed line) as thin films on quartz recorded at 80 K. The excitation wavelength is 458 nm,

modulation frequency 275 Hz.

The actual charge formation and recombination processes in films of P1 and P2 were

studied at room temperature with femtosecond pump-probe spectroscopy (Figure 5.8). The

Chapter 5

118

pump wavelengths were 450 nm and 525 nm and the signals were probed at 1450 nm (0.86

eV). The kinetics of the two polymers are similar and show an identical behavior for the two

excitation wavelengths. Like in solution (Figure 5.6), the grow-in of the photoinduced

absorption signals occurs extremely fast and is completed within 1 ps. The recombination of

the positive and negative charges between 0 and 250 ps after charge formation occurs with a

time constant of ~60 ps. This lifetime corresponds to the value obtained for the solutions

excited with 525 nm wavelength.

0 200 400 600 800 1000

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

450 nm P1 450 nm P2 525 nm P1 525 nm P2

∆T

/T

Time delay (ps)

Figure 5.8. Normalized differential transmission dynamics of films of P1 and P2 at room

temperature, recorded at 1450 nm (low-energy absorption of OPV radical cations) after

excitation at 450 nm and 525 nm.

5.6 Photovoltaic devices Photovoltaic devices were prepared that contain P1 and P2 as active layer. Toluene

solutions of the polymers were spin cast on indium tin oxide covered with a layer of

polyethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS) as the transparent front

electrode. As back electrode LiF (1 nm) and Al (100 nm) were deposited in vacuum. The J-V

characteristics of the ITO/PEDOT:PSS/polymer/LiF/Al devices were measured in dark and

under illumination. A rather poor photovoltaic behavior was obtained. The open circuit

voltage VOC is around 0.66 V, the short circuit current density JSC is ~7 µA cm-2, and the fill

factors FF is around 0.25 (Figure 5.9). Also devices that contain P2 with an additional

equivalent of donor (MDMO-PPV) or acceptor (6) were prepared The addition of MDMO-

PPV gave a slightly better device performance (VOC ~0.9 V, JSC ~11 µA cm-2, FF ~0.24), but

the extra fraction of 6 made hardly a difference. Although the origin of the poor performance


119

is not understood, it could at least partly be ascribed to the fast charge recombination that has

been observed with photoinduced absorption spectroscopy.

0.0 0.5 1.0 1.5 2.0-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

J (m

A c

m-2)

Bias (V)

Figure 5.9. Current density – voltage characteristics of photovoltaic devices

(ITO/PEDOT:PSS/polymer/LiF/Al) of polymer P2 in the dark (●) and under illumination (○).

5.7 Conclusion Two new polymers consisting of a main chain with OPV segments and pendant perylene

bisimides have been synthesized and studied. Photoluminescence quenching studies show

that efficient charge transfer takes place between the donor backbone and its pendant

acceptors in toluene and ODCB solutions. Consistent with estimated values for the Gibbs free

energy or charge separation, the charge separation occurs irrespective from which of the two

chromophores is initially excited. Femtosecond pump-probe spectroscopy on these polymer

solutions allowed monitoring the rate of formation of the OPV+• radical cations. The charge

separation occurs within 1 ps and the charge-separated state decays with a time constant of 45

- 60 ps. The time constants for charge separation and recombination in solid state are similar

to the corresponding time constants in toluene. This result suggests that the relative

orientation of the OPV and perylene bisimide chromophores in solution is similar to that in

solid state. The CD spectra give a strong indication of exciton coupling between the OPV and

perylene bisimide. This implies that they are in close proximity and have a preferential

helical orientation. The combination of these results suggests that both in solution and in film

the OPV and perylene bisimide are in close proximity, which leads to efficient and fast

charge separation and eventually to fast recombination. Fast recombination of charges is

detrimental for the charge collection in solar cells.

Chapter 5

120

5.8 Experimental section For general information about experimental procedures and instumentation see sections 3.7 and 4.9. The

synthesis of 3 was described in section 4.9. 2,5-Dibromo-4-(2-ethylhexyloxy)phenol (2). 2,5-dibromo-benzene-1,4-diol 1 (1.34 g, 5.0 mmol) was

dissolved in ethanol (50 mL) in argon atmosphere. After stirring with K2CO3 (0.69 g, 5.0 mmol), 2-ethylhexyl

bromide (0.9 mL, 5.0 mmol) was added. The reaction mixture was heated to reflux and stirred for 22 h. After

this, the reaction mixture was cooled to room temperature and evaporated in vacuo. The residue was dissolved

in CH2Cl2 and was washed two times with water (dist.) and once with brine. After drying on MgSO4 the solution

was filtered and the solvent was evaporated in vacuo. After purification by column chromatography (silica gel,

CH2Cl2/n-pentane 3:1) 0.54 g (28 %) of 2 was obtained as a brown oil. 1H NMR (CDCl3, 400 MHz): δ 7.26 (s,

1H), 6.97 (s, 1H), 5.13 (s, 1H), 3.82 (d, J = 5.5 Hz, 2H), 1.75 (m, 1H), 1.60-1.40 (m, 4H), 1.40-1.25 (m, 4H),

0.94 (t, J = 7.5 Hz, 3H), 0.98 (t, J = 7.0, 3H). 13C NMR (CDCl3, 75 MHz): δ 150.27, 146.62, 120.25, 116.24,

112.42, 108.30, 72.54, 39.41, 30.45, 29.03, 23.87, 23.00, 14.06, 11.15. Anal. Calcd. for C14H20Br2O2: C 44.24,

H 5.30. Found: C 44.27, H 5.21.

{(1S)-[2,5-Dibromo-4-(2-ethylhexyloxy)phenoxymethyl]-3-methylbutyl}carbamic acid tert-butyl

ester (4). To a solution of ((S)-1-hydroxymethyl-3-methylbutyl)carbamic acid tert-butyl ester 3 (0.39 g, 1.79

mmol), compound 2 (0.69 g, 1.82 mmol), triphenlyphosphine (0.71 g, 2.71 mmol) in toluene (15 mL) under

argon atmosphere, a solution of diethyl azodicarboxylate (DEAD) (0.43 mL, 2.73 mmol) in toluene (5 mL) was

added slowly such that the reaction mixture did not exceed 35 °C. After stirring overnight at room temperature,

the reaction mixture was filtered and washed with 1 M KHSO4, water (dist.) and brine and was subsequently

dried over Na2SO4. After filtration and evaporation of the solvent in vacuo, column chromatography purification

was performed (silica gel, ethyl acetate/n-heptane 1:4, Rf = 0.5). The product 4 was obtained as a clear sticky oil

in a yield of 0.39 g (36 %). 1H NMR (CDCl3, 400 MHz): δ 7.07 (s, 1H), 7.00 (s, 1H), 4.79 (br d, 1H), 4.05-3.85

(m, 3H), 3.83 (d, J = 5.5 Hz, 2H), 1.80-1.60 (m, 2H), 1.60-1.20 (m, 10 H), 1.45 (s, 9H), 1.00-0.85 (m, 12H). 13C

NMR (CDCl3, 75 MHz): δ 155.38, 150.74, 149.56, 118.89, 117.95, 111.28, 111.17, 79.38, 72.49, 48.33, 40.96,

39.40, 30.49, 29.02, 28.40, 24.86, 23.85, 23.00, 22.90, 22.39, 14.06, 11.14. MALDI-TOF MS (MW = 579.14)

m/z = 602.10 [M + Na]+.

(1S)-[2,5-Dibromo-4-(2-ethylhexyloxy)phenoxymethyl]-3-methylbutylamine (5). To a solution of

compound 4 (0.253 g, 0.44 mmol) in CH2Cl2 (4 mL), TFA (2.5 mL, 32 mmol) was added. The mixture was

stirred for 16 h under argon atmosphere at room temperature. Subsequently NaHCO3 was added to the reaction

mixture until the mixture became basic. After addition of CH2Cl2 (20 mL) the mixture was washed three times

with water (dist.) and once with brine. After drying over Na2SO4 the solution was filtered and the solvent was

removed in vacuo, 0.19 g (91 %) of a brown oil was obtained. The product was used without further

purification. 1H NMR (CDCl3, 300 MHz): δ 7.10 (s, 1H), 7.08 (s, 1H), 3.94 (dd, J = 8.5, 3.6 Hz, 1H), 3.83 (d, J

= 5.5 Hz, 2H), 3.68 (t, J = 8.2 Hz, 1H), 3.30 (m, 1H), 1.85-1.60 (m, 4H), 1.60-1.40 (m, 4H), 1.40-1.25 (m, 6H),

1.00-0.85 (m, 12 H). 13C NMR (CDCl3, 75 MHz): δ 150.65, 149.61, 118.84, 117.98, 111.23, 111.14, 75.77,

72.49, 48.70, 42.81, 39.41, 30.44, 29.03, 24.70, 23.86, 23.27, 23.00, 22.18, 14.06, 11.15. MALDI-TOF MS

(MW = 479.09) m/z = 480.04 [M + H]+.

N-(1-ethylpropyl)perylene-3,4:9,10-tetracarboxylic-3,4-anhydride-9,10-imide (7). A mixture of

N,N´-di(1-ethylpropyl)perylene-3,4:9,10-tertacarboxylic-bisimide 6 (2.87 g, 5.4 mmol), potassium hydroxide

(0.91 g, 0.016 mol) in tert-butanol (100 mL) was heated at 100 °C for 30 min. The reaction mixture was then


121

poured into 400 ml of 10 % HCl and precipitate was filtered. The residue was stirred in 200 mL of a warm

aqueous solution containing potassium hydroxide (20 g, 0.36 mol) and potassium chloride (16 g, 0.21 mol). The

solid was filtered and subsequently washed with the aqueous solution until the solution no longer colored

yellow/green. The solid was then stirred in water and subsequently filtered. The dark red colored filtrate was

precipitated by addition of hydrochloric acid to a final total percentage of 10 % HCl concentration. The

precipitate was filtered, washed with water and dried at 90 °C under vacuum to yield 0.50 g (20 %) of a black

solid. 1H NMR (CDCl3) δ 8.80-8.60 (m, 8H), 5.07 (m, 1H), 2.35-2.20 (m, 2H), 2.00-1.90 (m, 2H), 0.93 (t, J =

7.5 Hz). 13C NMR (CDCl3) δ 160.15, 136.61, 133.86, 133.76, 131.81, 124.10, 123.35, 123.24, 119.23, 58.03,

57.86, 25.16, 11.50, Electron impact MS (MW = 461) m/z = 461 [M]+.

N-(1-Ethylpropyl)-N´-[(1S)-(2,5-dibromo-4-(2-ethylhexyloxy)phenoxymethyl)-3-methylbutyl]

perylene-3,4:9,10-tetracarboxylic bisimide (8). Compound 5 (0.16 g, 0.33 mmol) and N-(1-

ethylpropyl)perylene-3,4:9,10-tetracarboxylic-3,4-anhydride-9,10-imide 7 (0.15 g, 0.33 mmol) were stirred at

155 °C in DMF (20 mL) under argon atmosphere. After 24 h the solvent was removed in vacuo. The product

was purified by column chromatography (silica gel, CH2Cl2) and was obtained in a yield of 0.12 g (39 %) as a

red solid. 1H NMR (CDCl3, 300 MHz): δ 8.64 (d, J = 8.0 Hz, 4H), 8.56 (d, J = 8.0 Hz, 4H), 7.14 (s, 1H), 6.90 (s,

1H), 5.80 (m, 1H), 5.07 (m, 1H), 4.71 (t, J = 8.9 Hz, 1H), 4.34 (dd, J = 8.9, 5.6 Hz, 1 H), 3.80-3.60 (m, 2H),

2.20-2.05 (m, 3H), 1.96 (m, 2H), 1.85-1.55 (m, 3H), 1.55-1.15 (m, 8H), 1.15-0.80 (m, 18H). 13C NMR (CDCl3,

75 MHz): δ 164.14, 150.57, 149.34, 134.57, 134.40, 131.98, 131.48, 129.55, 126.37, 124.83, 123.67, 123.05,

118.88, 117.78, 111.09, 72.39, 70.32, 57.73, 51.27, 39.33, 38.25, 30.36, 28.97, 25.61, 25.02, 23.79, 23.15,

22.96, 22.46, 14.02, 11.37, 11.10. MALDI-TOF MS (MW = 922.20) m/z = 921.90 [M•]-. Anal. Calcd. for

C49H50Br2N2O6: C 63.78, H 5.46. Found: C 63.14, H 5.41.

[(1S)-(2,4-Dibromophenoxymethyl)3-methylbutyl]carbamic acid tert-butyl ester (10). In toluene

(15 mL), ((S)-1-hydroxymethyl-3-methylbutyl)carbamic acid tert-butyl ester 3 (1.017 g, 4.73 mmol), 2,4-

dibromophenol 9 (1.19 g, 4.72 mmol) and trihenylphosphine (1.86 g, 7.09 mmol) were dissolved and stirred

under argon flux. Slowly, a solution of DEAD (1.1 mL, 6.99 mmol) in toluene (10 mL) was added drop wise to

the reaction mixture such that the temperature did not exceed 35 °C. After stirring overnight at room

temperature, white precipitate was filtered off the reaction mixture. The remaining solution was washed two

times with 1 M KHSO4, three times with water (dist.) and one time with brine. After the organic phase was dried

on Na2SO4, it was filtered and the solvent was evaporated in vacuo. Cyclohexane and diethyl ether were added

to the residue to yield precipitation. The solid was filtered off and the solvents in the remaining solution were

evaporated in vacuo. After purification by column chromatography (silica gel, ethyl acetate/n-heptane 1:4, Rf =

0.4) the product was crystallized in n-hexane. The product 10 was obtained as white crystals in a yield of 0.84 g

(39 %). 1H NMR (CDCl3, 300 MHz): δ 7.66 (d, J = 2.2 Hz, 1H), 7.36 (dd, J = 8.9, 2.1 Hz, 1H), 6.76 (d, J = 8.8

Hz, 1H), 4.75 (br d, 1H), 4.05-3.90 (m, 3H), 1.70-1.60 (m, 1H), 1.60-1.40 (m, 2H), 1.44 (s, 9H), 0.96 (dd, J =

6.4, 2.1 Hz, 6H). 13C NMR (CDCl3, 100 MHz): δ 155.40, 154.47, 135.42, 131.27, 114.45, 113.23, 79.49, 71.50,

48.13, 40.93, 28.38, 24.85, 22.93, 22.31. MALDI-TOF MS (MW = 451.02) m/z = 474.08 [M + Na]+.

(1S)-(2,4-Dibromophenoxymethyl)-3-methylbutylamine (11). Compound 10 (0.76 g, 1.68 mmol) was

dissolved in CH2Cl2 (5 mL) and was stirred with TFA (5 mL, 65 mmol) under argon. After 15 h, NaHCO3 was

added to the reaction mixture until it was basic and it was subsequently washed four times with water (dist.) and

once with brine. The solution was dried over Na2SO4 and after filtration and evaporation in vacuo 0.49 g (83 %)

of 11 was obtained as a pink oil. The product was used without any further purification. 1H NMR (CDCl3, 300

MHz): δ 7.66 (d, J = 2.5 Hz, 1H), 7.35 (dd, J = 8.8, 2.5 Hz, 1H), 6.75 (d, J = 8.8 Hz, 1H), 3.96 (dd, J = 8.8, 3.6

Chapter 5

122

Hz, 1H), 3.70 (dd, J = 8.7, 7.6 Hz, 1H), 3.30 (m, 1H), 1.79 (m, 1H), 1.34 (t, J = 7.0 Hz, 2H), 0.96 (t, J = 6.7 Hz,

6H). 13C NMR (CDCl3, 100 MHz): δ 154.56, 135.42, 131.21, 114.40, 113.18, 113.06, 75.10, 48.48, 43.06,

24.70, 23.36, 22.11. MALDI-TOF MS (MW = 350.97) m/z = 351.85 [M + H]+.

N-(1-Ethylpropyl)-N´-[(1S)-(2,4-dibromophenoxymethyl)-3-methylbutyl] perylene-3,4:9,10-tetra-

carboxylic bisimide (12). Compound 11 (0.34 g, 0.97 mmol) and N-(1-ethylpropyl)perylene-3,4:9,10-

tetracarboxylic-3,4-anhydride-9,10-imide 7 (0.45 g, 0.98 mmol) were stirred in DMF (25 mL) at 140 °C in

argon atmosphere. After 2.5 h, the temperature was elevated to 160 ºC. After subsequent 17 h of stirring, the

reaction mixture was cooled to room temperature and the solvent was evaporated in vacuo. After purification by

column chromatography (silica gel, CH2Cl2) 0.53 g (69 %) of 12 was obtained as a red solid. 1H NMR (CDCl3,

300 MHz): δ 8.80-8.60 (m, 8H), 7.51 (d, J = 2.5 Hz, 1H), 7.34 (dd, J =8.7, 2.3 Hz, 1H), 6.80 (d, J = 8.8 Hz, 1H),

5.83 (m, 1H), 5.07 (m, 1H), 4.73 (t, J = 8.9 Hz, 1H), 4.39 (dd, J = 9.1, 5.8 Hz, 1H), 2.28 (m, 3H), 1.95 (m, 2H),

1.78 (m, 1H), 1.67 (m, 1H), 1.02 (dd, J = 13.6, 6.5 Hz, 6H), 0.93 (t, J = 7.4 Hz, 6H). 13C NMR (CDCl3, 75

MHz): δ 164.00, 154.24, 135.29, 134.73, 134.44, 131.65, 131.16, 129.61, 126.45, 123.50, 123.14, 114.59,

113.17, 69.44, 57.72, 50.97, 38.17, 25.58, 25.01, 23.15, 22.41, 11.34. MALDI-TOF MS (MW = 794.08) m/z =

793.88 [M•]-. Anal. Calcd. for C41H34Br2N2O5: C 61.98, H 4.31. Found: C 61.76, H 4.23.

2,4-Dibromo-1-(2-ethylhexyloxy)benzene (13). To a stirring solution of 2,4-dibromophenol 9 (1.03 g,

4.09 mmol) in ethanol (25 mL), K2CO3 (0.57 g, 4.12 mmol) and subsequently 2-ethylhexyl bromide (0.73 mL,

4.10 mmol) were added under argon atmosphere. After refluxing for 2 h, the reaction mixture was cooled to

room temperature and the solvent was evaporated in vacuo. The residue was dissolved in CH2Cl2 and washed

with 1 M NaOH, water (dist.) and brine and dried over Na2SO4. Column chromatography purification was

performed (silica gel, CH2Cl2) and the residue was heated at 80 °C under vacuum. The resulting product was

obtained as a clear liquid in a yield of 0.36 g (24 %). 1H NMR (CDCl3, 300 MHz): δ 7.65 (d, J = 2.5 Hz, 1H),

7.34 (dd, J = 8,7, 2.3 Hz, 1H), 6.75 (d, J = 8.8 Hz, 1H), 3.87 (d, J = 5.8 Hz, 2H), 1.77 (m, 1H), 1.60-1.40 (m,

4H), 1.40-1.20 (m, 4H), 1.00-0.85 (m, 6H). 13C NMR (CDCl3, 75 MHz): δ 155.02, 135.37, 131.06, 114.08,

113.17, 112.47, 71.75, 39.31, 30.45, 29.03, 23.87, 22.99, 14.05, 11.14. Anal. Calcd. for C14H20Br2O: C 46.18, H

5.54. Found: C 45.64, H 5.36.

P1. In distilled THF (10 mL) a mixture of monomer 8 (0.040 g, 0.043 mmol), monomer (E,E)-1,4-bis{4-

{4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl-}-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-

methylbutoxy]benzene 14 (0.0456 g, 0.043 mmol) and Pd(PPh3)4 (0.0025 g, 0.002 mmol) was purged with

argon for 15 min. To this mixture a solution, which was also purged with argon for 15 min., of K2CO3 (0.024 g,

0.17 mmol) in water (1.2 mL) was added with a syringe. The whole mixture was purged with argon for 15 min.

before stirring in the dark at 90 °C under argon flow. After 22 h, the reaction mixture was cooled to room

temperature. The reaction mixture was dried in vacuo and was dissolved in toluene (3 mL). After precipitation

in methanol (300 mL) the solid was filtrated. The yield was 0.048 g (72 %) of a red solid. 1H NMR (CDCl3, 300

MHz): δ 8.80-8.10 (m, 8H), 7.70-6.40 (m, 12H), 5.90-5.60 (br signal, 1H), 5.10-4.90 (m, 1H), 4.80-4.50 (br

signal, 1H), 4.50-4.10 (br signal, 1H), 4.00-3.20 (m, 14H), 2.40-1.20 (m, 34H), 1.20-1.10 (m, 6H), 1.10-0.70 (m,

48H). 13C NMR (CDCl3, 75 MHz): δ 163.98, 163.53, 150.96, 150.61, 150.39, 149.61, 149.43, 134.56, 134.22,

131.42, 130.42, 129.62, 129.42, 129.26, 128.43, 127.62, 127.14, 126.78, 126.32, 125.28, 123.62, 123.02,

122.40, 117.10, 116.69, 115.80, 110.90, 110.39, 110.06, 108.67, 74.43, 73.81, 71.81, 57.54, 51.07, 39.53, 38.14,

35.12, 34.89, 30.47, 29.02, 26.40, 26.19, 26.02, 25.57, 25.01, 23.83, 23.15, 23.00, 22.57, 16.83, 16.70, 14.03,

11.48, 11.39, 11.32, 10.97. SEC (CHCl3, versus polystyrene): Mw = 27.2 kg/mol, Mn = 8.2 kg/mol.


123

P2. Monomer 12 (0.151 g, 0.19 mmol), monomer 14 (0.200 g, 0.19 mmol) and Pd(PPh3)4 (0.009 g, 0.008

mmol) were purged with argon in distilled THF (30 mL) for 20 minutes. After purging a solution of K2CO3

(0.10 g, 0.72 mmol) in water (5 mL) for 30 minutes, the aqueous solution was added to the THF solution with a

syringe. The whole solution was again purged with argon for 30 minutes. Then, the reaction mixture was heated

to 90 °C and was stirred under argon atmosphere in the dark for 18 h. After cooling to room temperature, the

solvents were evaporated in vacuo. The residue was dissolved in CHCl3 (7 mL) and was precipitated in

methanol (500 mL). The precipitate was filtered off and the filtrate was dried in vacuo after which the

precipitation procedure was repeated from toluene (4 mL) in methanol (250 mL). The polymer was obtained as

red solid in an amount of 0.202 g (75 %). 1H NMR (CDCl3, 400 MHz): δ 8.80-8.05 (m, 8H), 7.70-6.70 (m,

13H), 5.95-5.65 (br signal, 1H), 5.20-4.90 (br signal, 1H), 4.80-4.60 (m, 1H), 4.60-4.30 (br signal, 1H), 4.10-

3.00 (m, 12H), 2.30-0.60 (m, 73H). 13C NMR (CDCl3, 100 MHz): δ 163.96, 150.94, 134.60, 132.00, 130.39,

129.40, 127.78, 126.32, 123.66, 123.12, 122.38, 115.57, 110.31, 109.99, 108.72, 74.51, 74.22, 57.50, 50.73,

38.08, 35.10, 34.95, 34.80, 30.92, 26.34, 26.17, 25.94, 25.54, 25.02, 23.10, 22.53, 16.78, 16.32, 11.43. SEC

(CHCl3, versus polystyrene): Mw = 22.2 kg/mol, Mn = 8.0 kg/mol.

RP1. 1,4-Dibromo-2,5-di-(2-ethylhexyloxy)benzene 15 (0.018 g, 0.037 mmol) was dissolved in distilled THF

(7 mL) together with 14 (0.038 g, 0.036 mmol) and Pd(PPh3)4 (0.002 g, 0.001 mmol). The solution was purged

with argon for 15 minutes, after which a 15-minutes-purged solution of K2CO3 (0.020 g, 0.14 mmol) in water (1

mL) was added with a syringe. This whole solution was purged with argon for 15 minutes and then the

temperature was elevated to 90 °C and the reaction mixture was stirred for 23 h in the dark. After cooling to

room temperature the solvents were evaporated in vacuo and the residue was dissolved in toluene (4 mL) and

poured into methanol (300 mL). The precipitate was filtered off and was washed with water and methanol.

Finally, 0.021 g (51 %) of a yellow solid was obtained. 1H NMR (CDCl3, 400 MHz): δ 7.57 (br s, 4H), 7.40-

7.20 (br s, 4H), 7.01 (s, 2H), 6.98 (s, 2H), 4.00-3.60 (m, 16H), 2.05-1.85 (br s, 4H), 1.85-1.05 (m, 36H), 1.05-

0.75 (m, 48H). 13C NMR (CDCl3, 100 MHz): δ 151.08, 150.95, 150.36, 150.13, 127.55, 123.11, 122.50, 117.10,

110.47, 110.07, 74.38, 74.30, 71.88, 39.57, 35.09, 34.90, 30.54, 29.70, 29.04, 26.39, 26.12, 23.85, 23.02, 16.82,

16.72, 14.07, 11.47, 11.28, 11.00. SEC (CHCl3, versus polystyrene): Mw = 110.8 kg/mol, Mn = 24.6 kg/mol.

RP2. Monomer 13 (0.0279 g, 0.077 mmol), monomer 14 (0.0805 g, 0.077 mmol) and Pd(PPh3)4 (0.0035 mg,

0.003 mmol) were dissolved in distilled THF (13 mL) and the solution was purged with argon for 20 minutes. A

solution of K2CO3 (0.04 g, 0.029 mmol) in water (2 mL) was purged with argon for 15 minutes and was

subsequently added to the organic solution. The reaction mixture was purged with argon for 30 minutes before

heating to 90 °C. After stirring in the dark for 18 h, the reaction mixture was dried by evaporation in vacuo. The

solid residue was washed two times with water and was subsequently dissolved in toluene (4 mL). The solution

was poured in methanol (100 mL) and the precipitate was filtered off. A yellow solid was obtained in the yield

of 0.042 g (55 %). 1H NMR (CD2Cl2, 300 MHz): δ 7.60-7.40 (m, 6H), 7.30-7.05 (m, 4H), 7.00-6.80 (m, 3H),

4.00-3.80 (m, 14H), 1.95 (m, 4H), 1.80 (m, 2H), 1.75-1.20 (m, 21H), 1.20-0.70 (m, 42H). 13C NMR (CD2Cl2,

100 MHz): δ 156.29, 151.45, 151.36, 150.79, 134.40, 132.71, 131.03, 130.19, 129.19, 127.82, 126.77, 126.51,

123.25, 122.69, 117.02, 115.76, 112.60, 111.61, 110.27, 110.11, 74.81, 74.71, 74.27, 71.23, 39.77, 35.40, 35.25,

35.17, 30.83, 29.35, 26.67, 26.47, 26.38, 24.16, 23.33, 16.87, 16.60, 14.14, 11.55, 11.30, 11.14. SEC (CHCl3,

versus polystyrene): Mw = 48.8 kg/mol, Mn = 13.8 kg/mol.

Chapter 5

124

5.9 References

(1) A. Cravino, N. S. Sariciftci, J. Mater. Chem. 2002, 12, 1931.

(2) T. Benincori, E. Brenna, F. Sannicolò, L. Trimarco, G. Zotti, P. Sozzani, Angew. Chem. Int. Ed. Engl. 1996, 35, 648.

(3) J. P. Ferraris, A. Yassar, D. C. Loveday, M. Hmyene, Opt. Mater. 1998, 9, 34.

(4) A. Cravino, G. Zerza, M. Maggini, S. Bucella, M. Svensson, M. R. Andersson, H. Neugebauer, N. S. Sariciftci, Chem. Commun. 2000, 2487.

(5) A. Cravino, G. Zerza, H. Neugebauer, M. Maggini, S. Bucella, E. Menna, M. Svensson, M. R. Andersson, C. J. Brabec, N. S. Sariciftci, J. Phys. Chem. B 2002, 106, 70.

(6) F. Zhang, M. Svensson, M. R. Andersson, M. Maggini, S. Bucella, E. Menna, O. Inganäs, Adv. Mater. 2001, 13, 1871.

(7) A. Marcos Ramos, M. T. Rispens, J. K. J. van Duren, J. C. Hummelen, R. A. J. Janssen, J. Am. Chem. Soc. 2001, 123, 6714.

(8) S. Wang, S. Xiao, Y. Li, Z. Shi, C. Du, H. Fang, D. Zhu, Polymer 2002, 43, 2049.

(9) S. Xiao, S. Wang, H. Fang, Y. Li, Z. Shi, C. Du, D. Zhu, Macromol. Rapid. Commun. 2001, 22, 1313.

(10) G. Zerza, A. Cravino, H. Neugebauer, N. S. Sariciftci, R. Gómez, J. L. Segura, N. Martín, M. Svensson, M. R. Andersson, J. Phys. Chem. A 2001, 105, 4172.

(11) F. Giacalone, J. L. Segura, N. Martín, M. Catellani, S. Luzzati, N. Lupsac, Org. Lett. 2003, 5, 1669.


(13) T. Vahlenkamp, G. Wegner, Macromol. Chem. Phys. 1994, 195, 1933.





(18) A. P. Monkman, L.-O. Pålsson, R. W. T. Higgins, C. Wang, M. R. Bryce, A. S. Batsanov, J. A. K. Howard, J. Am. Chem. Soc. 2002, 124, 6049.

(19) A. L. Ding, J. Pei, Z. K. Chen, Y. H. Lai, W. Huang, Thin Solid Films 2000, 363, 114.

(20) E. Peeters, A. Delmotte, R. A. J. Janssen, E. W. Meijer, Adv. Mater. 1997, 9, 493.


(22) J. Salbeck, J. Electroanal. Chem. 1992, 340, 169.


(24) P. A. van Hal, M. P. T. Christiaans, M. M. Wienk, J. M. Kroon, R. A. J. Janssen, J. Phys. Chem B. 1999, 103, 4352.

125

6

Singlet-energy transfer in quadruple hydrogen-bonded

oligo(p-phenylenevinylene)-perylene bisimide dyads*

Abstract

The photophysical properties of a supramolecular donor-acceptor dyad

consisting of an oligo(p-phenylene vinylene) unit and a perylene bisimide unit are

described. The dyad is created by functionalizing the two chromophores with

quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidinone units, which provide

a high association constant (K ≈ 108 M-1 in toluene). This feature made it possible

to study the time-resolved photoinduced singlet-energy transfer reaction between

the two chromophores in dilute solution with transient pump-probe spectroscopy.

This energy transfer occurs with a time constant of 5.1 ps. Also, an oligo(p-

phenylene vinylene) unit was functionalized with two urea groups. This molecule

was studied with STM.

*Part of this work has been published: E. E. Neuteboom, E. H. A. Beckers, S. C.

J. Meskers, E. W. Meijer, R. A. J. Janssen, Org. Biomol. Chem. 2003, 1, 198.

Chapter 6

126

6.1 Introduction Non-covalent interactions between organic molecules are ubiquitous in nature and serve

to assemble, position and organize extended architectures that fulfill complex functions. The

photosynthetic reaction center is an intriguing example of such a structural design. Here,

photoinduced energy and electron transfer reactions between carefully aligned molecular

arrays of photo- and redox-active components work in concert to convert and store solar

energy.1 Because hydrogen bonding is often a key element in structuring the natural systems,

the study of photoinduced energy and electron transfer reactions between donors and

acceptors assembled via hydrogen bonding interactions has attracted considerable interest in

recent years.2,3 Hydrogen bonds, however, represent a fairly weak interaction and generally

result in low association constants. Consequently, only a small fraction of the donors and

acceptors remain associated, while the remaining molecules are free to diffuse in solution,

especially at low concentrations. This problem can be alleviated by using multiple hydrogen-

bonded arrays designed to gain strength and directionality.4 The 2-ureido-4[1H]-

pyrimidinone (UP) quadruple hydrogen-bonding unit, for example, dimerizes in a self-

complementary array of four cooperative hydrogen bonds and provides association constants

in excess of 107 M-1 in organic solvents.5 Utilizing these high association constants it is

possible to construct supramolecular polymers.6 Recently the UP unit has been utilized to

create hydrogen-bonded donor-acceptor dyads7 based on photo- and redox-active oligo(p-

phenylene vinylene) (OPV)8 and [60]fullerene (C60)9 derivatives. Photovoltaic cells based on

supramolecular OPV polymers have demonstrated that the hydrogen bonds of the UP unit can

be incorporated into working opto-electronic devices. 10

Perylene bisimides have been extensively studied as organic semiconductors in electronic

and optical applications such as field-effect transistors,11 fluorescent solar collectors,12

electrophotographic devices,13 photovoltaic devices,14 dye lasers,15 and molecular switches.16

These perylene bisimides have outstanding chemical, thermal and photochemical stability.17

Recently photoinduced electron transfer in liquid crystalline oligo(p-phenylene vinylene)-

perylene bisimide-oligo(p-phenylene vinylene)s (OPV-PERY-OPV) arrays and in π-stacks of

triple hydrogen-bonded OPV-PERY-OPV trimers was reported.18,19 In this chapter these

studies are extended and describe the synthesis of a perylene bisimide with the donor-donor-

acceptor-acceptor (DDAA) four-point hydrogen bonding motif UP-unit (PERY-UP) and

study its photophysical properties in hetero-assemblies with OPV-UP (Figure 6.1) using

fluorescence spectroscopy and femtosecond pump-probe spectroscopy. Because of the high

Singlet-energy transfer in quadruple hydrogen-bonded OPV-perylene bisimide dyads

127

association constant of the heterodimer, the singlet-energy transfer reaction in this

supramolecular system could be time resolved by femtosecond pump-probe spectroscopy.

N

N

O

O

O

O

OC12H25

OC12H25

OC12H25

NNO

HO

HH

C13H27

N N

N

N

O

HH

NN

O

O

O

O

H

O

OPV-UP

PERY-UP

Figure 6.1. Heterodimer of PERY-UP and OPV-UP bound by quadruple hydrogen bonds.

6.2 Synthesis The preparation of OPV-UP has been described previously.8 The synthetic route to

PERY-UP is shown in Scheme 6.1. Perylene anhydride monoimide 1 (which was described

in Chapter 5) was reacted with 2,5-di-tert-butyl-1,4-benzenediamine20 in presence of zinc

acetate in molten imidazole to afford the asymmetric perylene bisimide 2. PERY-UP was

obtained by reaction of 2 with carboxamide 321 in chloroform.

N O

O

O

O

O

N NH2N

O

O

O

O

NN

N

N

ON

O

H

H

1 2

PERY-UPa b

3

Scheme 6.1. Synthesis of PERY-UP; (a) 2,5-di-tert-butyl-1,4-benzenediamine, imidazole,

Zn(OAc)2, 160 °C, 3.5 h, 67 %; (b) CHCl3, reflux, 18 h, 54 %.

6.3 Keto-enol equilibrium The UP-unit of OPV-UP and PERY-UP can exist in two different tautomeric forms

(Figure 6.2).22 Apart from the pyrimidinone tautomer (keto) with a DDAA motif, the

Chapter 6

128

pyrimidinol tautomer (enol) exists. Both keto and enol forms are self-complementary with

DDAA and DADA motifs, respectively, but cannot associate with each other. The keto-enol

equilibrium depends on the polarity of the solvent and the nature of the substituent on the

isocytosine moiety. The keto tautomer is known to have a higher dimerization constant (K = 6

× 107 M-1 in water-saturated chloroform; 6 × 108 M-1 in toluene) than the enol tautomer (K =

~105 M-1 in chloroform),22,23 as a result of more favorable secondary interactions.24,25

NN

O

H N

NO

R2

R1

H

HN

N

O

HN

NO

R2

R1

H

HN

N

O

R1

N HN

O

H

R2

H

NN

O

R1

NHN

O

H

R2

H

4[1H]-pyrimidinone pyrimidin-4-ol

Figure 6.2. Tautomerization of the dimer of the UP unit (left, keto form) into the enol

tautomer (right)

1H NMR spectroscopy has been used to determine the relative amounts of keto and enol

of PERY-UP in different of solvents by comparing the integrated signals of the N-H protons.

For PERY-UP the amount of keto is 80 % in CDCl3, while it decreases to 50 % in toluene-d8.

These ratios are >99 % (CDCl3) and 90 % (toluene-d8) for OPV-UP.7 It is assumed that in a

1:1 mixture of PERY-UP and OPV-UP, both homodimers and the heterodimer are

simultaneously present.

6.4 Photophysical measurements

6.4.1 Optical properties

The absorption and photoluminescence spectra of PERY-UP, OPV-UP and a 1:1

mixture of the two compounds in toluene are shown in Figure 6.3. The characteristic

absorption peaks of the perylene bisimide are at 459 nm, 490 nm, and 527 nm for PERY-UP.

The absorption spectrum of OPV-UP is not structured and maximizes at 432 nm. Both

compounds fluoresce in toluene solution with very high quantum yields.7,26 The fluorescence

spectra reveal that the singlet-excited state (S1) of OPV-UP (E(S1) = 2.52 eV) is higher in

energy than the singlet-excited state of PERY-UP (E(S1) = 2.32 eV). Fluorescence lifetimes


129

(τ) for OPV-UP and PERY-UP are 1.23 and 3.61 ns, respectively, as determined from time-

correlated single photon counting (TCSPC) with excitation at 400 nm. The absorption

spectrum of the mixture closely resembles the superposition of the spectra of the two

compounds and gives no evidence of a significant electronic interaction. In dichloromethane,

the oxidation potential of OPV-UP is Eox = 0.71 V vs SCE and the first reduction potential of

PERY-UP is Ered = –0.61 V vs SCE.

350 400 450 500 550 600

ba

Abs

orba

nce

(a.u

.)

Wavelength (nm)

450 500 550 600 650 700

Wavelength (nm)

PL

Inte

nsity

(a.

u.)

Figure 6.3. UV/Visible absorption (a) and emission (b) spectra of PERY-UP (solid line),

OPV-UP (dashed line) and a 1:1 mixture of PERY-UP and OPV-UP (dotted line) in toluene

solution. The excitation wavelength is 527 nm for PERY-UP and 410 nm for OPV-UP and

the mixture.

6.4.2 Fluorescence quenching

Fluorescence quenching has been used to study the steady-state photophysics of OPV-

UP/PERY-UP heterodimers. As a result of the ordering of the singlet states and the

difference between oxidation and reduction potentials, the singlet-excited state of OPV-UP

can act as a donor towards PERY-UP in both photoinduced energy and photoinduced

electron transfer reactions. Both processes are expected to quench the OPV-UP fluorescence,

when their rates are competitive with the intrinsic decay of OPV-UP.

The OPV-UP fluorescence quenching was studied by adding a mixture of 2 × 10-5 M

PERY-UP and 10-6 M OPV-UP in small aliquots to a solution of 10-6 M OPV-UP in

toluene. Almost selective excitation of OPV-UP in the mixtures was achieved by irradiation

at 410 nm and the fluorescence signal at 493 nm (OPV-UP fluorescence only) was followed

as a function of the PERY-UP concentration in the mixture. A significant quenching of the

OPV-UP fluorescence was observed. The quenching factor Q, i.e. the ratio of initial OPV-

UP fluorescence (I0) and that of the mixture (I) is plotted in Figure 6.4 after correction for

Chapter 6

130

absorption by PERY-UP.27 The Stern-Volmer constant, defined as KSV = (Q-1)/[PERY-UP],

amounts to KSV = 5.9 × 105 M-1 for concentrations of PERY-UP up to 1.13 × 10-6 M. Two

control experiments were performed, in each experiment one of the two chromophores lacks

the hydrogen bonding UP-group. The first control used N,N´-bis(1-ethylpropyl)perylene-

3,4:9,10-tertacarboxylic-bisimide (PERY-REF)26 instead of PERY-UP together with OPV-

UP and in the second experiment PERY-UP was mixed with a methyl end-capped oligo(p-

phenylene vinylene) with four phenyl rings (OPV4)28 instead of OPV-UP. In both cases no

fluorescence quenching was observed (Figure 6.4). This indicates that dynamic or collisional

quenching is not important at these low concentrations and that the quenching can

unambiguously be ascribed to the formation of OPV-UP/PERY-UP heterodimers.

0 5 10 15 20

1

2

3

4

5

6

7

I 0/I

[PERY-UP] x 106 / M

Figure 6.4. Stern-Volmer plot of OPV fluorescence quenching by PERY derivatives in

toluene solution. Curves are shown for OPV-UP + PERY-UP (squares); OPV-UP + PERY-

REF (circles); and OPV4 + PERY-UP (triangles). The OPV chromophore concentration is

10-6 M in each experiment and the excitation wavelength 410 nm. The detection wavelength is

493 nm for OPV-UP and 500 nm for OPV4.

The origin of the OPV fluorescence quenching in the OPV-UP/PERY-UP heterodimers

can be inferred from the photoluminescence spectrum (Figure 6.3b) of the mixture. When the

1:1 mixture is excited at 410 nm, the fluorescence of the PERY-UP unit dominates the

photoluminescence spectrum even though the absorption at 410 nm of this chromophore is

negligible. Hence the OPV-UP unit acts as a sensitizer for the PERY-UP fluorescence. This

indicates that a singlet-energy transfer, rather than electron transfer, occurs in the heterodimer

in toluene in which the photoexcited OPV-UP S1 state evolves to PERY-UP S1.


131

6.4.3 Transient photoinduced absorption

Sub-picosecond transient pump-probe spectroscopy (150 fs pulses) was performed at

room temperature on OPV-UP/PERY-UP solutions to assess the temporal evolution of the

singlet-energy transfer on short timescales. Upon preferential photoexcitation of the OPV-UP

moiety at 450 nm in a mixture of 4 × 10-5 M OPV-UP and 2 × 10-4 M PERY-UP in toluene,

a negative differential transmission was observed by probing at 900 nm (Figure 6.5). At 900

nm the Sn←S1 photoinduced absorption of OPV chromophore is present.29 The dynamics in

the low picosecond time range (Figure 6.5, open circles) show a rapid decay which can be

fitted to a mono-exponential decay with a time constant of 5.1 ps, i.e. a rate of k = 1.9 × 1011

s-1. On longer time scales (Figure 6.5, solid circles) a much slower decay is visible with a

mono-exponential time constant of 1.1 ns. The two different decay times account for the fact

that under these conditions both OPV-UP/PERY-UP heterodimers and OPV-UP/OPV-UP

homodimers are present. It is important to note that a control experiment confirmed that the

Sn←S1 photoinduced absorption of the PERY chromophore has only a small (~10 %)

contribution to the photoinduced absorption at 900 nm. The fast (5.1 ps) decay is attributed to

the singlet-energy transfer reaction from OPV-UP(S1) to PERY-UP in the heterodimer,

while the slow (1.1 ns) decay represents the intrinsic relaxation of OPV-UP(S1) in the

homodimer. The lifetime of the latter process is close to the photoluminescence lifetime of

1.23 ns of OPV-UP homodimers, determined by TCSPC.7

0.1 1 10 100 1000

-6

-4

-2

0

∆T/T

x 1

03

Time delay (ps)

Figure 6.5. Differential transmission dynamics of the OPV (Sn ← S1) absorption at 900 nm

as function of the pump-probe time delay after photoexcitation at 450 nm. The concentration

PERY-UP is 2 × 10-4 M and the concentration OPV-UP is 4 × 10-5 M in toluene. Solid lines

are fits to decays with time constant of 5 ps and 1100 ps.

Chapter 6

132

6.5 Discussion

6.5.1 Förster energy transfer

The fluorescence quenching studies provide a lower limit for the rate for singlet-energy

transfer via the Stern-Volmer constant via:

τ

1maxET

−= Qk (6.1)

Using the highest quenching observed in the titration experiment, Qmax = 6.6, and τ =

1.23 ns, the lower limit for kET is 4.5 × 109 s-1. Of course, Qmax is limited by the presence of

OPV-UP homodimers and the highest ratio of PERY-UP:OPV-UP (21.6 : 1) used in the

experiment. The actual value, kET = 1.9 × 1011 s-1, determined from the transient

spectroscopy, is over 40 times faster.

The Förster equation30 can be used to estimate the distance d between the centers of the

donor and acceptor chromophores in the energy transfer process.

( ) 6

c64

A5

PL2

ET1

128

10ln90001

=⋅=

d

RJ

dnNπk F τ

φκτ

(6.2)

In Equation 6.2, the parameters NA and n represent Avogadro’s number and the refractive

index of the medium, φPL is the fluorescence quantum yield of the donor in the absence of

transfer, Rc the Förster radius and d is the actual center-to-center distance of the two

chromophores involved in the energy transfer. JF represents the overlap between the

absorption ( )(νε ) of the acceptor (PERY-UP) and the fluorescence ( )(νF ) of the donor

(OPV-UP) on an energy scale (cm-1) defined as:

∫

∫=νν

νννεν

dF

dFJ

)(

])()([ 4

F (6.3)

The factor κ is the so-called orientation factor, which incorporates the dependence of the

energy transfer rate on the mutual orientation of the transition dipole moments of donor and

acceptor chromophores. It can be expressed as:


133

)cos()cos()cos( ARDRDA φφκ −= Θ (6.4)

where ΘDA is the angle between the transition dipole moments of donor and acceptor and φDR

(φAR) is the angle between the transition dipole moment of the donor (acceptor) chromophore

and the line joining the centers of the two chromophoric units. In calculating values for κ2,

the assumption was that rotation around the bond joining the UP unit and the methylene

group connecting the UP and the OPV moieties is possible. For various conformations values

for κ2 were found ranging from 2.5 to 3.5. Using the following numbers for the OPV-

UP/PERY-UP combination, JF = 1.55 × 10-13 cm6 mol-1, kET = 1.9 × 1011 s-1, τ = 1.23 ns, φPL=

0.84,7 κ2 = 3, and Equations 6.2 and 6.3, values of Rc = 64 Å and d = 26 Å were obtained. The

latter value is slightly less than the center-to-center distance estimated from molecular

modeling (~33 Å). The difference can be rationalized by considering that the singlet-excited

states delocalize and effectively reduce the center-to-center distance. A similar result has

been found for energy transfer reactions in covalent OPV4-C60 donor-acceptor systems.29

6.5.2 Electron transfer

Why does singlet-energy transfer occur from the OPV-UP singlet state rather than

electron transfer and is electron transfer possible subsequent to energy transfer from the

singlet-excited state of PERY-UP? To address these questions it is instructive to consider the

change in free energy for charge separation using the Weller equation:31,32

( )

−

+−−−−=∆ −+

sref0

2

ccs0

2

00redoxCS1111

84)A()D(

εεπεεπε rr

e

R

eEEEeG (6.5)

Table 6.1, shows that according to Equation 6.5 photoinduced electron transfer is

energetically possible for the OPV-UP/PERY-UP heterodimer in solvents of varying

polarity from either the OPV-UP or the PERY-UP singlet-excited states.

To investigate the possibility for electron transfer in the heterodimer in more detail, the

fluorescence signal of PERY-UP at 578 nm was monitored after selective photoexcitation of

PERY-UP at 527 nm as function of an increasing amount of OPV-UP. For this purpose, a

solution of 2 × 10-5 M OPV-UP and 10-6 M PERY-UP was added in steps to a 10-6 M

solution of PERY-UP. Irrespective of the polarity of the solvent (toluene, ε = 2.38; ortho-

dichlorobenzene (ODCB), ε = 9.93; benzonitrile, ε = 25.18) the PERY-UP fluorescence

Chapter 6

134

signal remained constant. From this experiment it can be concluded that photoinduced

electron transfer does not occur in the heterodimer from the PERY-UP S1 state.

If electron transfer is absent but thermodynamically feasible, it is likely hampered

kinetically. The expression for the rate constant for non-adiabatic charge separation:

∆−

=

Tk

GV

Tkhk

B

‡2

21

B2

3

CS exp4

λπ

(6.6)

shows that kCS is determined by the coupling (V) between donor and acceptor in the excited

state, the reorganization energy λ and the barrier for charge separation

( λλ 4/)( 2CS

‡ +∆=∆ GG ). The reorganization energy is the sum of internal (λi) and solvent

(λs) contributions. The latter can be estimated from:33

−

−

+= −+

s2

cc0

2

s11111

2

1

4 επελ

nRrr

e (6.7)

and the results for OPV-UP/PERY-UP are collected in Table 6.1. The internal

reorganization energy λi is probably not very high for these extended conjugated systems and

estimated to be in the range of 0.2 to 0.5 eV. Within these limits the barrier ‡G∆ for charge

separation from PERY-UP(S1) remains less than 0.01 eV in ODCB and benzonitrile, and is

only slightly higher in toluene (< 0.04 eV).

Table 6.1. Change in free energy for charge separation ∆GCSa (eV) from OPV(S1) and

PERY(S1), and solvent reorganization energy λs (eV) for OPV-UP/PERY-UP heterodimers in

different solvents from Equations 6.5 and 6.7.

∆GCS OPV ∆GCS

PERY λs

toluene –0.47 –0.27 0.07

ODCB –1.28 –1.08 0.79

benzonitrile –1.43 –1.23 0.98 a ∆GCS

OPV uses E00 = 2.52 eV; ∆GCSPERY uses E00 = 2.32 eV.


135

The absence of electron transfer in each of these solvents is therefore ascribed to a very

weak electronic coupling V of donor and acceptor in the excited state, rather than a high

barrier. V is known to be exponentially dependent on the distance between donor and

acceptor. Molecular modeling and the value derived from the Förster equation, indicate an

appreciable distance between OPV-UP and PERY-UP units in the heterodimer, causing a

low V. In agreement with this proposition, photoinduced electron transfer reaction does occur

between OPV and PERY chromophores in systems were the two redox-active chromophores

are at a much shorter distance.18,19

6.6 Hydrogen bonding by the bisurea motif In order to get more control over the separation of different chromophores in for instance

copolymers of donors and acceptors, an attempt was made to introduce urea functionalities in

a chain. From the literature it is known that units that are of equal length and that are

substituted with urea groups on both ends can form hydrogen-bonded ribbons.34 This

principle has been used in both non-conjugated and conjugated units, e.g. n-dodecyl chains34-

36 and oligothiophenes.37-39 In this section the synthesis of a bis-urea-OPV (OPV-BU) and

STM imaging of these molecules are presented.

6.6.1 Synthesis and infrared characterization

The OPV-BU was synthesized in three steps, starting with the etherification reaction40 of

4-iodophenol 4 with 3-bromopropylamine, affording amine 5 (Scheme 6.2). The urea

functionality was introduced by reaction of the amine of compound 5 with n-butylisocyanate

into compound 6. In the last step OPV-BU was formed via a double Suzuki coupling between

the aromatic iodine of 6 with the bisborolane 7 (which was described in Chapter 4).

Infrared spectroscopy was used to study the hydrogen bonding of the urea groups in solid

state. The vibrations at 3331 (N-H stretching), 1634 (C=O stretching) cm-1 suggest that the

urea groups form ordered hydrogen bonds, since the corresponding vibrations of a non-

hydrogen bonded urea group are around 3450 and 1690 cm-1, respectively.41

Chapter 6

136

I OH I O NH2 I O N N

O

HH

ONN

O

H HO N N

O

HH

OO

O

OO

O

OO

O

OO

B

B

OO

O

O

O

4 5 6

OPV-BU

c d

e

7

Scheme 6.2. Synthesis of OPV-BU; (c) NaOH, DMF, 88 %; (d) n-butylisocyanate, CH2Cl2,

72 %; (e) Pd(PPh3)4, K2CO3, water, THF, 7 %.

6.6.2 STM experiments

Scanning Tunneling Microscopy (STM) was performed by Dr. A. Gesquière (TU/e) at

the University of Leuven, on OPV-BU that was adsorbed on highly oriented pyrolytic

graphite (HOPG) from a solution in 1-heptanol. The images in Figure 6.6 show that stable

adsorption layers are formed and that long range ordering can be observed. Preliminary

analysis of the data gives values for the intermolecular distance of ~1.5 nm, a lamella width

of ~3.8 nm and an angle of –13° between lamella direction and symmetry axis of HOPG. The

intermolecular distance indicates that the OPV units are not π-stacked and that the (S)-2-

methylbutoxy chains of the OPV can lie extended on the surface without interdigitating with

the chains of a neighboring OPV. The 1-butyl-3-propylurea side chains however, are most

likely extended on the surface and are interdigitating in a manner that is depicted in either

Figure 6.7b or 6.7c. This would explain the lamella width of ~3.8 nm, since the OPV core has

a length of ~ 2.6 nm and each 1-butyl-3-propylurea side chain has a length of ~1.2 nm. Given

that the hydrogen bonding distance for urea groups is typically 0.46 nm,34,42 hydrogen

bonding might not be taking place in the case of organization like in Figure 6.7b, or it is far

from optimal. Alternatively, when the 1-butyl-3-propylurea chains make an angle with


137

respect to the lamella direction while interdigitating (Figure 6.7c), hydrogen-bonding pairs of

the 1-butyl-3-propylurea chains could possibly be formed.

3.8 nm 1.5 nm

3.8 nm

1.5 nm3.8 nm 1.5 nm

3.8 nm

1.5 nm

Figure 6.6. STM images of OPV-BU monolayers physisorbed at the 1-heptanol/HOPG

interface. Left: white bar in the left corner represents 20 Å, total picture size is 134 × 134 Å2,

range black to white is –0.13 - –1.56 nA. Right: white bar represents 20 nm, total picture size

is 154 × 154 nm2, range black to white is –0.37 - –0.86 nA.

O N N

O

HH

OO

O

OO

O

ONN

O

H H

O N N

O

HH

OO

O

OO

O

ONN

O

H H

O N N

O

HH

OO

O

OO

O

ONN

O

H H

=

a b c

O N N

O

HH

OO

O

OO

O

ONN

O

H H

O N N

O

HH

OO

O

OO

O

ONN

O

H H

O N N

O

HH

OO

O

OO

O

ONN

O

H H

=

a b c Figure 6.7. Schematic representation of possible organization of OPB-UB. Top and (a)

ribbon; (b) interdigitating 1-butyl-3-propylurea chains that are parallel to the lamella

direction; (c) interdigitating 1-butyl-3-propylurea chains that make an angle with the lamella

direction.

Chapter 6

138

6.7 Conclusion Photoluminescence studies reveal that a singlet-energy transfer reaction occurs in

quadruple hydrogen-bonded OPV-UP/PERY-UP heterodimers (Figure 6.1) after excitation

of the OPV chromophore. As a result of the high association constant of the quadruple

hydrogen bond, significant amounts of heterodimers and homodimers are present, even in

dilute solutions. This feature made it possible to follow the temporal evolution of the singlet-

energy transfer reaction using sub-picosecond transient absorption spectroscopy in the

heterodimers on short timescales. The time constant of 5.1 ps that was obtained for this

reaction is in fair agreement with Förster theory. The singlet-excited in OPV-UP

homodimers, is much longer lived and decays with a time constant of 1.1 ns. Although

exergonic, electron transfer does not occur after photoexcitation as a result of a too weak

electronic coupling between OPV and PERY chromophores in the excited state. To achieve

photoinduced electron transfer in quadruple hydrogen-bonded systems based on the UP unit a

judicious design of the relative positioning of donor and acceptor will be required.

The STM results on OPV-BU show that it is a promising approach to obtain more order

between electron donor-units. The infrared studies indicate that hydrogen bonding between

the urea groups takes place. The STM images show that ordered assemblies can be formed,

however ribbons of OPV were not observed. This suggests that incorporation of OPV-BU in

an alternating copolymer, in order to obtain ribbon-like separation of the OPV units demands

an improved design of the OPV-BU.

6.8 Experimental section For general information about experimental procedures and instrumentation see sections 3.7 and 4.9. The

synthesis of compounds 1 and 7 are described in sections 5.8 and 4.9, respectively.

Scanning Tunneling Microscopy (STM)

Prior to imaging, OPV-BU was dissolved in 1- heptanol and a drop of this solution was applied on a freshly

cleaved surface of highly oriented pyrolytic graphite. The STM images were acquired in the variable current

mode (constant height) under ambient conditions with the tip immersed in the liquid. STM was performed using

a Discoverer scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA) along with an external

pulse/function generator (Model HP 8111 A), with negative sample bias. Tips were electrochemically etched

from Pt/Ir wire (80 %/20 %, diameter 0.2 mm) in a 2 N KOH/6 N NaCN solution in water.

N-(1-Ethylpropyl)-N´-(4-amino-2,5-di-tert-butylphenyl)perylene-3,4:9,10-tetracarboxylic-bisimide

(2). A mixture of N-(1-ethylpropyl)perylene-3,4:9,10-tetracarboxylic-3,4-anhydride-9,10-imide 1 (0.048 g, 0.10

mmol), 2,5-di-tert-butyl-1,4-benzenediamine (0.14 g, 0.64 mmol), imidazole (2.5 g) and a few grains of zinc

acetate were stirred at 160 °C for 3.5 h under argon. After cooling to room temperature the reaction mixture was


139

purified extensively by column chromatography (SiO2, CH2Cl2/ethanol 95:5, CH2Cl2/methanol 99:1 – 97.5:2.5,

ethyl acetate/n-hexane 4:1 and ethyl acetate/n-hexane 3:2). After washing the solid with n-hexane 0.046 g (67

%) of 2 was obtained pure as a red solid. 1H NMR (CDCl3) δ 8.80-8.65 (m, 8H), 6.87 (s, 1H), 6.82 (s, 1H), 5.08

(m, 1H), 3.93 (s, 2H), 2.40-2.20 (m, 2H), 2.10-1.90 (m, 2H), 1.54 (s, 18H), 0.94 (t, 6H, J = 7.3 Hz). 13C NMR

(CDCl3) δ 164.88, 145.24, 145.07, 135.08, 134.71, 133.07, 131.99, 130.05, 129.79, 129.40, 126.96, 126.66,

124.02, 123.73, 123.31, 123.24, 118.18, 100.34, 57.92, 34.04, 31.76, 29.73, 25.21, 11.51. MALDI-TOF MS

(MW = 663.31) m/z = 663.02 [M•]-.

N-(1-Ethylpropyl)-N´-{4-[2-ureido-6-(1-ethylpentyl)-4[1H]-pyrimidinone]-2,5-di-tert-

butylphenyl}perylene-3,4:9,10-tetracarboxylic-bisimide (PERY-UP). Compound 2 (0.0262 g, 0.04

mmol) and N-[6-(1-ethylpentyl)-4-oxo-1,4-dihydro-pyrimidin-2-yl]imidazole-1-carboxamide 3 (about 0.02 g,

0.07 mmol) were refluxed in dry chloroform (dried over molsieves) for 18 h under argon. After cooling to room

temperature the reaction mixture was washed with 1 N HCl (5 mL), saturated aqueous NaHCO3, water, brine

and was dried over Na2SO4. The product was triturated with acetone to yield 0.020 g (54 %) of a red solid. 1H

NMR (CDCl3) δ two tautomers: 13.27 (s, 1H), 12.59 (s, 1H), 11.67 (s, 1H), minor 11.49 (s), 8.77 (d, 2H, J =

8.06 Hz), 8.69 (d, 2H, J = 8.06 Hz), 8.67 (d, 2H, J = 8.06 Hz), 8.66 (d, 2H, J = 8.06 Hz), minor 7.63 (s), 7.47 (s,

1H), 7.10 (s, 1H), minor 6.27 (s), 5.89 (s, 1H), 5.08 (m, 1H), minor 2.53 (br s), 2.45-2.10 (m, 3H), 1.98 (m, 2H),

1.80-1.50 (m, 4H), 1.44 (s, 9H), 1.30 (s, 13H), 1.00-0.80 (m, 12H). 13C NMR (CDCl3) δ two tautomers: 164.50,

156.06, 155.13, 145.53, 135.22, 134.61, 132.12, 130.07, 129.73, 126.94, 126.60, 123.69, 123.43, 123.27, 57.88,

45.43, 32.88, 31.68, 31.02, 29.41, 26.73, 25.15, 22.65, 14.03, 11.83, 11.57. MALDI-TOF MS (MW = 898.44)

m/z = 898.43 [M•]-.

3-(4-Iodo-phenoxy)-propylamine (5). After addition of NaOH powder (4.51 g, 0.11 mol) of to a solution of

4-iodophenol 4 (1.24 g, 5.6 mmol) in DMF (dried on molsieves, 150 mL), the mixture was stirred for 30

minutes in argon atmosphere at room temperature. Subsequently 3-bromopropylamine hydrobromide (1.36 g,

6.2 mmol) was added and the mixture was stirred for another two hours. After evaporation of the solvent the

residue was dissolved in CH2Cl2 and was extracted with water and brine and was dried over Na2SO4. After

filtration and evaporation of the solvent, 1.37 g (88 %) of the product was obtained as a clear wax. 1H NMR

(CDCl3, 300 MHz) δ 7.54 (d, J = 8.8 Hz, 2H), 6.68 (d, J = 8.8 Hz, 2H), 4.01 (t, J = 6.2 Hz, 2H), 2.90 (t, J = 6.8

Hz, 2H), 1.91 (quintet, J = 12.8 Hz, 6.3 Hz, 2H). 13C NMR (CDCl3, 100 MHz) δ 158.82, 138.15, 116.87, 82.58,

65.92, 39.10, 32.88.

1-Butyl-3-[3-(4-iodo-phenoxy)-propyl]-urea (6). To a solution of compound 5 (0.427 g, 1.54 mmol) in

CH2Cl2 (10 mL) under argon at room temperature, was added drop wise a solution of n-butylisocyanate (0.16

ml, 1.62 mmol) in CH2Cl2 (2 mL) over a period of 2 minutes. The reaction mixture was stirred and after 5

minutes again n-butylisocyanate (0.02 mL, 0.18 mmol) was added. After 2 h the isocyanate was disappeared

(observed with FT-ATR) and the reaction mixture is filtered. The white powder was washed with cold CH2Cl2

and dried in vacuo at room temperature. The product was obtained as a white solid with a yield of 0.42 g (72 %). 1H NMR (DMSO-d7, 300 MHz) δ 7.57 (d, J = 8.5 Hz, 2H), 6.77 (d, J = 8.2 Hz, 2H), 5.83 (br t, 1H), 5.76 (br t,

1H), 3.93 (t, J = 6.0 Hz, 2H), 3.11 (d, J = 6.0 Hz, 2H), 2.95 (d, J = 6.3 Hz, 2H), 1.78 (t, J = 6.3 Hz, 2H), 1.40-

1.10 (m, 4H), 0.85 (t, J = 6.9, 3H). 13C NMR (CDCl3, 75 MHz) δ 158.58, 158.24, 138.26, 116.85, 82.91, 65.96,

40.41, 37.85, 32.24, 29.69, 20.01, 13.77.

Chapter 6

140

(E,E)-1,4-Bis{4-(4-(3-(1-butylurea)-propoxy)phenyl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-

bis[(S)2-methylbutoxy]benzene (OPV-BU). A mixture of compound 6 (0.0284 g, 0.0074 mmol), (E,E)-

1,4-bis{4-{4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl-}-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-

methylbutoxy]benzene 7 (0.0369 g, 0.0037 mmol), Pd(PPh3)4 (0.003 g, 0.003 mmol) in distilled THF (10 mL)

was purged with argon for 15 minutes. An aqueous solution of K2CO3 (0.14 M) was purged for 20 minutes.

With a syringe 1 mL of the aqueous solution was added to the THF solution and the whole was purged with

argon for 15 minutes. After this the temperature was raised to 90 °C and was stirred in the dark for 17 h. The

reaction mixture was cooled to room temperature and was dried in vacuo. The residue was dissolved in CH2Cl2

and was washed with water and brine. After drying over Na2SO4 and filtering, the solvent was removed by

evaporation in vacuo. Purification was performed by using column chromatography (silica gel, first ethyl acetate

and later CH2Cl2/methanol 91:9) and crystallization in acetone. The product was obtained as a yellow solid

(0.007 g, 7 %). 1H NMR (CDCl3, 300 MHz) δ 7.56 (m, 8H), 7.25 (m, 4H), 6.95 (d, J = 8.5 Hz, 4H), 6.90 (s,

2H), 4.62 (br t, 2H), 4.34 (br t, 2H), 4.12 (t, J = 5.8 Hz, 4H), 4.00-3.70 (m, 12 H), 3.44 (dd, J = 12.4 Hz, 6.3 Hz,

4H), 3.17 (dd, J = 12.8 Hz, 6.7 Hz, 4H), 2.10-1.20 (m, 30 H), 1.12 (t, J = 6.7 Hz, 12H), 1.05-0.80 (m, 30 H). 13C

NMR (CDCl3, 100 MHz) δ 158.07, 157.48, 150.90, 150.76, 150.26, 131.12, 130.52, 130.37, 127.27, 126.61,

122.64, 122.48, 115.57, 113.73, 110.14, 109.95, 89.88, 74.58, 74.26, 74.04, 66.03, 40.56, 38.28, 35.20, 34.96,

32.35, 29.85, 26.50, 26.48, 26.27, 20.16, 16.98, 16.96, 16.91, 13.93, 11.64, 11.43. MALDI-TOF MS (MW =

1294.85) m/z = 1294.88 [M•]+. IR (ATR): ν 3331, 2959, 2928, 2874, 1634, 1609, 1579, 1519, 1497, 1466, 1422,

1389, 1345, 1244, 1203, 1177, 1113, 1047, 966, 860, 850, 831, 682 cm-1.

6.9 References and notes

(1) J. Deisenhofer, J. R. Norris, The Photosynthetic Reaction Center, Academic Press, New York, 1993.

(2) For reviews see: (a) V. Balzani, F. Scandola, Supramolecular Photochemistry, Ellis Horwood, Chichester, 1991; (b) J. L. Sessler, B. Wang, S. L. Springs, C. T. Brown, in Comprehensive Supramolecular Chemistry, ed. J. L. Atwood, J. E. D. Davies, D. D. MacNicol, F. Vögtle, Y. Murakami, Pergamon, Oxford, 1996, Vol. 4; (c) M. D. Ward, Chem. Soc. Rev. 1997, 26, 365; (d) P. Piotrowiak, Chem. Soc. Rev. 1999, 28, 143; (e) C. J. Chang, J. D. Brown, M. C. Y. Chang, E. A. Baker, D. G. Nocera, in Electron Transfer in Chemistry, ed. V. Balzani, Wiley-VCH, Weinheim, 2001, Vol. 3; (f) M. D. Ward, C. M. White, F. Barigelletti, N. Amaroli, G. Calogero, L. Flamigni, Coord. Chem. Rev. 1998, 171, 481.

(3) (a) P. Tecilla, R. P. Dixon, G. Slobodkin, D. S. Alavi, D. H. Waldeck, A. D. Hamilton, J. Am. Chem. Soc. 1990, 112, 9408; (b) A. Harriman, D. J. Magda, J. L. Sessler, J. Phys. Chem. 1991, 95, 1530; (c) A. Harriman, Y. Kubo, J. L. Sessler, J. Am. Chem. Soc. 1992, 114, 388; (d) C. Turro, C. K. Chang, G. E. Leroi, R. I. Cukier, D. G. Nocera, J. Am. Chem. Soc. 1992, 114, 4013; (e) J. L. Sessler, B. Wang, A. Harriman, J. Am. Chem. Soc. 1993, 115, 10418; (f) J. L. Sessler, B. Wang, A. Harriman, J. Am. Chem. Soc. 1995, 117, 704; (g) P. J. F. de Rege, S. A. Williams, M. J. Therien, Science 1995, 269, 1409; (h) J. A. Roberts, J. P. Kirby, D. G. Nocera, J. Am. Chem. Soc. 1995, 117, 8051; (i) J. P. Kirby, J. A. Roberts, D. G. Nocera, J. Am. Chem. Soc. 1997, 119, 9230; (j) E. Prasad, K. R. Gopidas, J. Am. Chem. Soc. 2000, 122, 3191; (k) A. J. Myles, N. R. Branda, J. Am. Chem. Soc. 2001, 123, 177; (l) M. A. Smitha, E. Prasad, K. R. Gopidas, J. Am. Chem. Soc. 2001, 123, 1159; (m) J. L. Sessler, M. Sathiosatham, C. T. Brown, T. A. Rhodes, G. Wiederrecht, J. Am. Chem.


141

Soc. 2001, 123, 3655; (n) M. A. Smitha, K. R. Gopidas, Chem. Phys. Lett. 2001, 350, 86.

(4) For a recent review see: D. C. Sherrington, K. A. Taskinen, Chem. Soc. Rev. 2001, 30, 83.

(5) R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer, J. H. K. K. Hirschberg, R. F. M. Lange, J. K. L. Lowe, E. W. Meijer, Science 1997, 278, 1601.

(6) L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev. 2001, 101, 4071.

(7) E. H. A. Beckers, P. A. van Hal, A. P. H. J. Schenning, A. El-ghayoury, E. Peeters, M. T. Rispens, J. C. Hummelen, E. W. Meijer, R. A. J. Janssen, J. Mater. Chem. 2000, 12, 2054.

(8) A. El-ghayoury, E. Peeters, A. P. H. J. Schenning, E. W. Meijer, Chem. Commun. 2000, 1969.

(9) (a) M. T. Rispens, L. Sánchez, J. Knol, J. C. Hummelen, Chem. Commun. 2001, 161; (b) J. J. González, S. González, E. María Priego, C. Luo, D. M. Guldi, J. de Mendoza, N. Martín, Chem. Commun. 2001, 163; (c) L. Sánchez, M. T. Rispens, J. C. Hummelen, Angew. Chem. Int. Ed. 2002, 41, 838.

(10) A. El-ghayoury, A. P. H. J. Schenning, P. A. van Hal, J. K. J. van Duren, R. A. J. Janssen, E.W. Meijer, Angew. Chem. Int. Ed. 2001, 40, 3660.

(11) P. R. L. Malenfant, C. D. Dimitrakopoulos, J. D. Gelorme, L. L. Kosbar, T. O. Graham, A. Curioni, W. Andreoni, Appl. Phys. Lett. 2002, 80, 2517.

(12) G. Seybold, G. Wagenblast, Dyes Pigm. 1989, 11, 303.

(13) K. Y. Law, Chem. Rev. 1993, 93, 449.

(14) (a) D. Wöhrle, D. Meissner, Adv. Mater. 1991, 3, 129; (b) D. Schlettwein, D. Wöhrle, E. Karmann, U. Melville, Chem. Mater. 1994, 6, 3; (c) S. Ferrere, A. Zaban, B. A. Gregg, J. Phys. Chem. B 1997, 101, 4490; (d) L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, 1119.

(15) M. Sadari, L. Hadel, R. R. Sauers, S. Husain, K. Krogh-Jespersen, J. D. Westbrook, G. R. Bird, J. Phys. Chem. 1992, 96, 7988.

(16) M. P. O’Neil, M. P. Niemczyk, W. A. Svec, D. Gosztola, G. L. Gaines, M. R. Wasielewski, Science 1992, 257, 63.

(17) W. A. Fisher, in Pigment Handbook Volume I: Properties and Economics; ed. T. C. Patton, John-Wiley & Sons, New York, 1973.


(19) A. P. H. J. Schenning, J. van Herrikhuyzen, P. Jonkheijm, Z. Chen, F. Würthner, E. W. Meijer, J. Am. Chem. Soc. 2002, 124, 10252.

(20) F. Bell, J. Chem. Soc. 1958, 120.

(21) H. M. Keizer, R. P. Sijbesma, E. W. Meijer, Eur. J. Org. Chem., submitted.

(22) F. H. Beijer, R. P. Sijbesma, H. Kooijman, A. L. Spek, E. W. Meijer, J. Am. Chem. Soc. 1998, 120, 6761.

(23) S. H. M. Söntjens, R. P. Sijbesma, M. H. P. van Genderen, E. W. Meijer, J. Am. Chem. Soc. 2000, 122, 7487.

Chapter 6

142

(24) W. L. Jorgensen, J. Pranata, J. Am. Chem. Soc. 1990, 112, 2008.

(25) J. Pranata, S. G. Wierschke, W. L. Jorgensen, J. Am. Chem. Soc. 1991, 113, 2810.


(27) The data points are corrected for the simultaneous excitation of the PERY-UP at 410 nm and for the re-absorption of the OPV-UP emission by PERY-UP at 493 nm (especially important at high excess of PERY-UP).

(28) E. Peeters, A. Marcos, S. C. J. Meskers, R. A. J. Janssen, J. Chem. Phys. 2000, 112, 9445.

(29) P. A. van Hal, R. A. J. Janssen, G. Lanzani, G. Cerullo, M. Zavelani-Rossi, S. De Silvestri, Phys. Rev. B 2001, 64, 075206.

(30) T. Förster, Discuss. Faraday Soc. 1959, 27, 7.


(32) In Equation 6.4, –e is the electron charge, ε0 the vacuum permittivity, εs the polarity of the solvent, εref the polarity of the solvent used to determine the redox potentials Eox(D) and Ered(A), Rcc the center-to-center distance of positive and negative charges and r+ and r– the radii of the positive and negative ions. E00 is the energy of the excited state from which electron transfer occurs. For OPV-UP, r+ = 5.1 Å (Ref. 29). For the perylene bisimide segment of PERY-UP, r– = 4.7 Å was taken from the density (ρ = 1.59 g cm-1) of N,N´-dimethylperylene-3,4:9,10-tertacarboxylic-bisimide derived from the X-ray crystallographic data (E. Hädicke and F. Graser, Acta Cryst. C 1986, 42, 189) via r– = [3M/(4πρNA)]1/3. Rcc was set to 33 Å.

(33) H. Oevering, M. N. Paddon-Row, M. Heppener, A. M. Oliver, E. Cotsaris, J. W. Verhoeven, N. S. Hush, J. Am. Chem. Soc. 1987, 109, 3258.

(34) J. van Esch, S. De Feyter, R. M. Kellogg, F. De Schryver, B. L. Feringa, Chem. Eur. J. 1997, 3, 1238.

(35) J. van Esch, R. M. Kellogg, B. L. Feringa, Tetrahedron Lett. 1997, 38, 281.

(36) S. De Feyter, P. C. M. Grim, J. van Esch, R. M. Kellogg, B. L. Feringa, F. C. De Schryver, J. Phys. Chem. B 1998, 102, 8981.

(37) F. S. Schoonbeek, J. H. van Esch, B. Wegewijs, D. B. A. Rep, M. P. de Haas, T. M. Klapwijk, R. M. Kellogg, B. L. Feringa, Angew. Chem. Int. Ed. 1999, 38, 1393.

(38) D. B. A. Rep, R. Roelfsema, J. H. van Esch, F. S. Schoonbeek, R. M. Kellogg, B. L. Feringa, T. T. M. Palstra, T. M. Klapwijk, Adv. Mater. 2000, 12, 563.

(39) A. Gesquière, M. M. S. Abdel-Mottaleb, S. De Feyter, F. C. De Schryver, F. Schoonbeek, J. van Esch, R. M. Kellogg, B. L. Feringa, A. Calderone, R. Lazzaroni, J. L. Brédas, Langmuir 2000, 16, 10385.

(40) G. Etemad-Moghadam, L. Ding, F. Tadj, B. Meunier, Tetrahedron 1989, 45, 2641.

(41) M. M. Coleman, M. Sobkowiak, G. J. Pehlert, P. C. Painter, Macromol. Chem. Phys. 1997, 198, 117.

(42) L. Born, H. Hespe, Colloid Polym. Sci. 1985, 263, 335.

143

7

Multiple-perylene bisimide compounds

Abstract

The synthesis and characterization of three compounds bearing multiple perylene

bisimides is presented. The compounds have two, three, or four chromophores in

quasi-linear, trigonal, and tetrahedral orientations, respectively. The

photophysical properties of the latter two are similar to that of an isolated

perylene bisimide unit. In contrast, the bis(perylene bisimide), in which the two

chromophores are linked to a central fluorene, shows a considerable quenching

of the perylene bisimide fluorescence that is ascribed to an intramolecular charge

transfer reaction from the fluorene to the perylene bisimide in the excited state.

The fluorescence quenching is strongly dependent on the nature of the solvent but

does not increase monotonously with the relative the dielectric constant. The

electron transfer has also been inferred from fluorescence lifetime measurements

and femtosecond photoinduced absorption spectroscopy. The origin of unusual

solvent dependence of the rate for charge separation is not fully understood. 1H

NMR experiments indicate that specific solvent interactions or conformations

may play a role.

Chapter 7

144

7.1 Introduction Perylene bisimides easily crystallize to form ordered structures that possess appealing

optical and electronic properties such as a large exciton diffusion length (2.5 ± 0.5 µm)1,2 and

a high charge-carrier mobility (0.6 cm2 V-1 s-1)3 in field-effect transistors. These properties

are advantageous for device applications that rely on large domains of n-type materials. On

the other hand, such large domains are less beneficial in bulk heterojunction solar cells. Here,

phase separation of n-type and p-type materials into nanoscopic domains will generally

improve the device performance with respect to active layers that contain large domains,

because the increased interfacial area in a nanoscopic blend enhances the efficiency of

dissociation of excitons into free charge carriers. Hence, the degree of crystallization is an

important parameter that allows controlling the device performance when crystalline

materials are used because it enables balancing charge generation and charge mobility.4 A

drawback of crystalline materials, however, is that the as-prepared morphology is usually not

stable and prone to further changes with time or changing temperature.

Functional molecular glasses are amorphous materials that may alleviate some of the

problems associated with crystallization of small organic molecules. Generally, these

molecules consist of a manifold of linked chromophores and have a non-planar structure that

prevents crystallization.5 Their topology can for instance be X-shaped or branched. In fact

molecular glasses already found use in organic light-emitting diodes (OLEDs), where they

provide morphologically and thermally stable amorphous organic materials that permit the

formation of uniform films without pinholes.6 Interesting materials for OLEDs are

amorphous low-molecular-weight organic compounds that have a high glass-transition

temperature. Several starburst,6 tetrahedral,7 and spiro8 compounds have been investigated

that contain conjugated chromophores. Also in solid-state dye-sensitized mesoporous TiO2

solar cells, amorphous organic materials have been successfully applied,9 e.g. 2,2’,7,7’-

tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene was used as an amorphous

hole conductor to replace the liquid electrolyte.

In this chapter three branched compounds containing multiple perylene bisimides

PERY2, PERY3, and PERY4 (Figure 7.1) are described that can be considered as a first step

towards amorphous molecular materials with n-type properties. One potential problem with

multichromophoric perylene bisimides is that intramolecular electron transfer reaction may

occur. In section 2.8.1 several examples were given of non-planar compounds containing two

perylene bisimides that show electron transfer from one perylene bisimide to the other. In

some cases the electron transfer was facilitated by the introduction of N-pyrrolidinyl


145

substitutents on the bay-position, which decreases the oxidation potential of the perylene

bisimide.10,11 However, intramolecular electron transfer between adjacent perylene bisimides

was also reported for molecules in which two perylene bisimides without such substituents

are linked via oligo(p-phenylene) units of different length.12 To investigate this issue in more

detail, the photophysical properties of PERY2, PERY3, and PERY413 have been

investigated with UV/Visible, (time-resolved) fluorescence, and ultrafast pump-probe

spectroscopy.

NN

N NO

O

O

OO

O

O

O

C6H13

C6H13

H13C6

H13C6

N

N N

N

N N

OO

O

O O

O

O

O O

O

O O

N

N

N

N

N

N

N

N

O O

OO

H13C6

O

O

O

OH13C6

H13C6

O

O

O

OH13C6C6H13

O

OO

O C6H13

C6H13

C6H13

PERY2

PERY3

PERY4

Figure 7.1. Compounds containing multiple perylene bisimides.

7.2 Synthesis Three compounds were synthesized that contain two, three, or four perylene bisimides.

The perylene bisimides in these polychromophores were linked by 2,7-fluorene (PERY2,

Scheme 7.2), 1,3,5-benzene (PERY3, Scheme 7.3), and para-substituted tetraphenylmethane

(PERY4, Scheme 7.4), respectively.

In order to ensure the solubility of PERY2 in organic solvents, 1-hexylheptyl groups

were introduced on the imide position of the perylene and a didodecyl-substituted fluorene

was selected as linker. First, 1-hexylheptylamine 2 was synthesized in a two-step procedure

Chapter 7

146

as reported by Semenov and Skorovarov.14 In the first step a Leuckart reaction was

performed with the dihexylketone 1 and formamide in nitrobenzene to produce N-(1-

hexylheptyl)formamide. After removal of the starting compounds by distillation, N-(1-hexyl-

heptyl)formamide was hydrolyzed with HCl to form amine 2, which was purified by

distillation. The preparation of the perylene bisimide 315 and monoimide 416 has been

described by Langhals et al. and involves a condensation reaction of amine 2 with perylene

dianhydride in imidazole to 3 and conversion to 4 by reaction with KOH and subsequently

HCl.

N N

O

O

O

O

O N

O

O

O

O

O NH2

1 2

3 4

a

bc

d

Scheme 7.1. Synthesis of perylene monoanhydride 4; (a)14 formamide, nitrobenzene, 185 °C;

(b)14 HCl; (c)15 perylene-3,4:9,10-tetracarboxylic dianhydride, imidazole, 180 °C; (d) 1.

KOH, tert-butanol, 2. HCl, 53 %.

The synthesis of PERY2 started with the formation of bisphthalimide fluorene 6 by reaction

of 9,9-didodecyl-2,7-dibromofluorene 5 with potassium phthalimide in dimethylacetamide in

presence of copper(I) iodide.17 Diaminofluorene 7 was formed in reaction of 5 with hydrazine

monohydrate. PERY2 was obtained by reaction of monoanhydride 4 with the

diaminofluorene 7 and was purified by column chromatography and preparative High

Performance Liquid Chromatography (HPLC).


147

Br Br

H25C12 C12H25

N N

H25C12 C12H25

O

O

O

O

NH2 NH2

H25C12 C12H25

5 6

7

e f

gPERY2

4

Scheme 7.2. Synthesis of PERY2 triad; (e) potassium phthalimide, Cu(I), DMA, 180 °C, 28

%; (f) N2H4·H2O, ethanol, reflux, 83 %; (g) imidazole, Zn(OAc)2, 180 °C, 24 %.

Variations of trichromophoric compound PERY3, containing longer alkyl side chains

than 1-ethylpropyl, have been reported by Langhals et al.18 The synthesis of PERY3 was

performed by J. Caeiro Rodríguez (Universiteit van Santiago de Compstela, TU/e) and started

with the reduction of 3,5-dinitroaniline 8 with hydrazine monohydrate and palladium on

activated carbon, producing 1,3,5-triaminobenzene 9.19 This compound was immediately

reacted with the perylene monoanhydride 10 (Chapter 5)20 to obtain the tris(perylene

bisimide) substituted compound PERY3.

NH2

NO2

NO2 NH2

NH2

NH2

N O

O

OO

O

h i

8 9 10

PERY3+

Scheme 7.3. Synthesis of PERY3; (h)19 N2H4·H2O, 5 % Pd/C, ethanol; (j) imidazole,

Zn(OAc)2, 180 °C, 11 %.

Langhals et al. have also reported the synthesis of the tetrachromophoric PERY4.13 The

actual synthesis by Dr. N. Stuhr-Hansen (TU/e) was performed with a small modification,

saving an intermediate step to a tetrakis(4-formylaminophenyl)methane. The first step is the

nitration of tetraphenylmethane 11 with nitric acid in acetic anhydride and acetic acid,

affording tetrakis(4-nitrophenyl)methane 12.21 After reduction of the nitro groups with Raney

nickel to 13, direct reaction of perylene monoanhydride 4 with the amines of 13 resulted in

the formation of tetrakis(perylene bisimide) PERY4. Reaction of 13 with 1-ethylpropyl

substituted perylene bisimide 10 produced a less soluble compound (not shown).

Chapter 7

148

NO2

NO2

O2N

O2N NH2

NH2

NH2

NH2

j k

12 13

l

11

PERY44

Scheme 7.4. Synthesis of PERY4; (j)21 HNO3, acetic anhydride, acetic acid; (k) Raney Ni,

N2H4·H2O, THF; (l) imidazole, Zn(OAc)2, 180 °C.

7.3 Photophysical properties of PERY3 and PERY4 UV/Visible absorption and fluorescence spectra of chloroform solutions of PERY4 and

PERY3 (with longer alkyl chains) have been reported previously.13,18 To study the effect of

polarity, spectra of PERY3 and PERY4 were recorded in toluene (ε = 2.38) and

chlorobenzene (ε = 5.69). The UV/Visible spectra of PERY3 are identical in both solvents

and similar to that of the perylene bisimide 3. In contrast, the absorption spectrum of PERY4

in toluene shows more tailing and a higher relative intensity of the bands at 493 nm and 463

nm than in chlorobenzene (Figure 7.2). This observation points to some extent of aggregation

of the perylene bisimides in toluene.

Fluorescence studies were performed by excitation of the solutions at 490 nm and 531

nm, and the fluorescence intensities were compared to that of 3. These fluorescence studies

revealed that within the error of the experiment, no fluorescence quenching occurs for

PERY3 and PERY4 in both solvents. This has also been reported for a chloroform solution

of PERY4.13 These results indicate that no charge transfer occurs in the case of PERY3 and

PERY4 in these solvents.

The fluorescence lifetimes of solutions of PERY3 in toluene and chlorobenzene were

equal with a value of 3.2 ns. For PERY4, the lifetime at 534 ns was 3.4 ns in both solvents.

These results confirm the results from steady state fluorescence that the decay processes of

the singlet-excited states of PERY3 and PERY3 in both solvents are equal, and not

significantly different from 3.


149

450 500 550 6000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

(-)

Wavelength (nm)

toluene chlorobenzene

Figure 7.2. UV/Visible absorption spectra of PERY4 in toluene and chlorobenzene.

7.4 Photophysical processes in PERY2

7.4.1 UV/Visible absorption spectroscopy

UV/Visible absorption and fluorescence experiments on PERY2 were performed in eight

different solvents with varying dielectric constants: toluene, chloroform (ε = 4.81),

chlorobenzene, 2-methyltetrahydrofuran (2-MeTHF, ε = 6.97), dichloromethane (ε = 8.93),

ortho-dichlorobenzene (ODCB, ε = 9.93), pyridine (ε = 12.3), and benzonitrile (ε = 25.2).

The UV/Visible absorption spectrum of PERY2 in toluene solution has characteristic

bands at 308, 459, 491 and 529 nm (Figure 7.3a). The first band is ascribed to the absorption

of the fluorene moiety and the others to the perylene bisimide segments. The molar

absorption coefficient of PERY2 (15.7 × 104 L mol-1 cm-1) is about the double of the

extinction coefficient of 3 (7.3 × 104 L mol-1 cm-1). The wavelength of maximum absorption

of PERY2 depends on the solvent and ranges from 523 nm in 2-MeTHF to 533 nm in

ODCB. This solvatochromic behavior is similar to that of 3. After correction for the slightly

different λmax, the profiles of vibrational progressions of the absorption spectra of PERY2 are

almost identical in all eight solvents. No significant differences in relative height or width of

absorption bands are present (Figure 7.3b). This result excludes that the aggregation of

perylene bisimide segments of PERY2 differs strongly among these solvents.

Chapter 7

150

300 400 500 6000.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105

1.4x105

1.6x105

1.8x105

a

mol

ar a

bsor

ptio

n co

effic

ient

(L

mol

-1 c

m-1)

Wavelength (nm)

PERY2 3 2 x ε3

-150 -100 -50 0 500.0

0.5

1.0

b

Nor

mal

ized

abs

orba

nce

(-)

Relative wavelength (nm)

toluene chloroform chlorobenzene 2-MeTHF dichloromethane ODCB pyridine benzonitrile

Figure 7.3. UV/Visible absorption spectra of PERY2. (a) Molar absorption coefficients in

toluene solution of PERY2, 3, and for clarity the doubled molar absorption coefficient of 3.

(b) Absorption spectra of PERY2 in all solvents relative to λmax.

7.4.2 Fluorescence experiments

The fluorescence spectra of PERY2 in selected solvents are shown in Figure 7.4. In

contrast to the almost identical absorption spectra, the fluorescence intensity strongly differs

among the various solvents. The fluorescence quenching was calculated from the

fluorescence intensity of PERY2 with respect to the fluorescence intensity of 3 in the same

solvent with excitation at the same wavelength. The solutions were all excited at 490 nm and

at a wavelength around the maximum of 529 nm, depending on the position of this maximum

in the different solvents. The fluorescence quenching factors Q [= IPL(3)/IPL(PERY2)] were

similar for both excitation wavelengths in the identical solvent. However, as indicated above,

the quenching strongly depends on the solvent. For instance, the fluorescence of PERY2 is

quenched with a factor of Q = 1.5 in toluene, while in 2-MeTHF the quenching is much

stronger with Q = 66.6 (Figure 7.4a). In Figure 7.4b the quenching factors Q in all eight

solvents are plotted versus the dielectric constant. This graph clearly shows that quenching

depends on the solvent, but does not increase with increasing dielectric constant.

The origin of the quenching of the perylene bisimide fluorescence cannot be ascribed to a

singlet-energy transfer from the perylene bisimide to fluorene, because this is strongly

endergonic. Likewise, quenching via aggregation of the perylene bisimide is excluded

because the UV/Visible spectra are similar in all solvents and do not show any sign of

aggregation (Figure 7.3b). Hence, photoinduced charge transfer seems the only process that is

causing the fluorescence quenching in this system.


151

500 550 600 650 700

0

500

1000

1500

2000 a

PERY2 2-MeTHF 3 2-MeTHF PERY2 toluene 3 toluene

PL

Inte

nsity

(-)

Wavelength (nm)

5 10 15 20 250

10

20

30

40

50

60

70b

pyrid

ine

dich

loro

met

hane

chlo

robe

nzen

e

tolu

ene

chlo

rofo

rm

2-M

eTH

F

OD

CB

benz

onitr

ile

PL

Que

nchi

ng fa

ctor

(-)

dielectric constant solvent

Figure 7.4. (a) Fluorescence spectra of PERY2 and 3 in 2-MeTHF and toluene, λex = 490

nm. (b) Fluorescence quenching factors for PERY2 (with respect to 3) in different solvents

versus the dielectric constant of the solvents.

The temperature dependence of the quenching factor Q was studied for solutions of

PERY2 in chloroform and chlorobenzene. These solvents were selected because they have a

similar relative dielectric constant but give strongly different quenching factors (Figure 7.5).

Only a small temperature effect is observed in both solvents. For chloroform Q decreases

slightly with increasing temperature, whereas for chlorobenzene Q increases with rising

temperatures.

10 20 30 40 50 60 70 80 90 100 110

6

7

22

23

24

PL

Que

nchi

ng fa

ctor

(-)

Temperature (oC)

chloroform chlorobenzene

Figure 7.5. Temperature dependence of the fluorescence quenching factors for PERY2 (with

respect to 3) in chloroform (●) and chlorobenzene (○) solution.

Chapter 7

152

7.4.3 Fluorescence lifetime experiments

In order to study the fluorescence more in detail, the fluorescence lifetimes were recorded

with time-correlated single photon counting (TCSPC) at 534 nm, after excitation at 400 nm.

The fluorescence lifetimes of PERY2 (τPERY2) depend on the solvent and can be correlated

together with the fluorescence lifetime of 3 (τ3) via Equation 7.1 with the rate of charge

transfer kCS (Table 7.1).

3PERY2

CS11

ττ−=k (7.1)

The rate of charge transfer kCS can also be derived from the fluorescence-quenching

factor Q of PERY2 via Equation 7.2 (Table 7.1). For most solvents, the values of kCS,

calculated either with the fluorescence lifetime of PERY2 and with the quenching factor, are

similar. Table 7.1 shows that the rate of charge transfer is the largest in 2-MeTHF and the

smallest in toluene.

3

CS1

τ−= Q

k (7.2)

Table 7.1. Rate constants for charge transfer as obtained from photoluminescence

quenching, photoluminescence lifetimes, and pump-probe PIA spectroscopy for PERY2 in

various solvents.

Solvent Quenching PL lifetime PIA

τ3

(ns)

Q kCS (Q)

(ns-1)

τPERY2

(ps)

kCS (τ)

(ns-1)

τ

(ps)

kCS (PIA)

(ns-1)

Toluene 3.99 1.5 0.13 2500 0.15

Chloroform 3.88 21.9 5.4 46 21.5

Chlorobenzene 3.90 5.5 1.2 850 0.9 1000 0.7

2-MeTHF 3.69 66.6 17.8 43 23.0 130 7.4

Dichloromethane 4.39 37.8 8.4 45 22.0

ODCB 3.95 7.4 1.6 694 1.2

Pyridine 3.91 31.8 7.9 96 10.2

Benzonitrile 3.84 21.1 5.2 172 5.6


153

7.4.4 Transient photoinduced absorption spectroscopy

Sub-picosecond transient pump-probe spectroscopy was performed on solutions of

PERY2 in chlorobenzene and 2-MeTHF and compared to that of N,N´-bis(1-ethylpropyl)-

3,4:9,10-perylenebiscarboximide 14 (Chapter 3, structure not shown here)22 in chlorobenzene

as a reference. These experiments were carried out to gain more insights in the kinetics of the

processes after photoexcitation at 520 nm. The detection wavelength was set at 700 nm, in

order to detect the possible formation of perylene bisimide radical anions.23

The grow-in of the transient absorption at 700 nm occurs for all samples within 1 ps, i.e.

close to the width of the excitation pulse and the time resolution of the pump-probe

spectrometer (Figure 7.6). The absorption of 14 in chlorobenzene is long-lived and is

attributed to the excited state photoinduced absorption (Sn←S1) of the perylene bisimide

chromophore. The decay of the signal of PERY2 has a lifetime τ of 1.00 ns (k = 1.00 × 109

s-1) in chlorobenzene and 0.13 ns (k = 8.09 × 109 s-1) in 2-MeTHF. The signals of 14 and

PERY2 have similar intensities. The decay rate constants of the photoinduced absorption of

PERY2 at 700 nm are of the same order of magnitude as the rates determined from

fluorescence quenching and fluorescence lifetime measurements, although the value in 2-Me-

THF is somewhat different. These rates are significantly lower than the rate of the grow-in of

the signal. These observations suggest that the photoinduced absorption signals of PERY2 in

both solvents predominantly originate from the Sn←S1 absorption of the perylene bisimide,

and not from the radical anion. This may occur when the magnitude of the extinction

coefficient of the absorption of the perylene bisimide radical anion is much smaller than that

of the Sn←S1 absorption.

The decay rate of the signal at 700 nm observed with the photoinduced absorption

spectroscopy can be correlated with the rate of fluorescence decay determined with

fluorescence lifetime measurements. By definition, the rate of disappearance of the Sn←S1

absorption is equal to the decay constant determined by fluorescence S1→S0, since both

experiments monitor the concentration of the same S1 state. However, the signal at 700 nm is

the sum of the transient absorptions of the neutral perylene bisimide Sn←S1 band and the

absorption of the perylene bisimide radical anion. If the perylene bisimide anion is formed

from the singlet-excited state, the lifetime of the 700 nm transient absorption is likely to be

increased compared to the fluorescence lifetime. This effect, might explain some of the

differences in Table 7.1.

Chapter 7

154

-2 0 2 4 6 8

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 14 chlorobenzene PERY2 chlorobenzene PERY2 2-MeTHF

Nor

mal

ized

-∆T

/T (

-)

Time (ps)

0 200 400 600 800 1000

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

Nor

mal

ized

-∆T

/T

Time (ps)

Figure 7.6. Normalized differential transmission dynamics of solutions of 14 in

chlorobenzene and PERY2 in 2-MeTHF and chlorobenzene; recorded at 700 nm, λex = 520

nm.

7.5 Discussion The results strongly suggest that photoinduced charge transfer occurs within PERY2.

Calculations of the Gibbs free energy of intramolecular charge-separated state (GCS), using

the Weller equation24 (Equation 7.3), have been performed to check if this is a plausible

process for PERY2 in the solvents with different dielectric constants (εs). Cyclic

voltammetry measurements were performed in dichloromethane (εref = 8.93) to determine the

oxidation potential Eox and the reduction potential Ered of PERY2. Values of Eox = 1.63 V

and Ered = –0.68 V were measured (vs Ag/AgCl, Fc/Fc+ = 0.35 V). From literature it is known

that fluorene has a lower oxidation potential than perylene bisimide 3 (1.51 V and 1.61 V

respectively in acetonitrile vs SCE).25,26 Hence, it is assumed that also in PERY2 this is the

case.

The center-to-center distance between the charges Rcc was set at 11Å, which is the

distance between the center of the perylene bisimide and the center of the fluorene. The

radius of the fluorene cation (r+ = 3.8 Å) was estimated from the molecular volume (V= 229

Å3) of fluorene from the X-ray crystallographic data via r+ = [3V/(4π)]1/3.27 The radius of the

perylene bisimide anion was determined at r– = 4.7 Å (see Chapter 4).

−

+−−−= −+

sref0

2

ccs0

2

redoxCS1111

84))A()D((

εεπεεπε rr

e

R

eEEeG (7.3)


155

Table 7.2. Free energy of intramolecular charge separate state calculated using Equation

7.3 in the eight solvents and relative to the ground state and the singlet-excited states of

fluorene and perylene bisimide.a

Solvent εs GCS (eV) S1 perylene bisimide

Toluene 2.38 2.83 0.51

Chloroform 4.81 2.37 0.05

Chlorobenzene 5.69 2.30 –0.02

2-MeTHF 6.97 2.24 –0.09

Dichloromethane 8.93 2.17 –0.15

ODCB 9.93 2.14 –0.18

Pyridine 12.3 2.10 –0.22

Benzonitrile 25.5 2.01 –0.31 a The following parameters were used Eox(D) = 1.63 V, Ered(A) = –0.68 V, Rcc = 11 Å, r+ =

3.8 Å, r– = 4.7 Å, εref = 8.93. For perylene bisimide (S1): E00 = 2.32 eV.

Using these parameters charge separation from the singlet-excited state of perylene

bisimide is not favorable in apolar solvents, and should not occur in toluene and chloroform

(Table 7.2). Assuming that the negative charge on the perylene bisimide chromophore is not

situated in the center of the unit, but on the imide functionality close to the fluorene instead

(with a maximum distance Rcc of 5.80 Å), charge separation is favorable in all solvents.

The assumption that photoinduced electron transfer occurs from the fluorene unit to the

perylene bisimide seems plausible. However, the extent of fluorescence quenching does not

monotonously increase with increasing dielectric constant of the solvent. This behavior is not

in accordance with Equation 7.3. The results presented in this chapter cannot elucidate the

reason for this deviation. An explanation might be that the conformation of the alkyl chains

of the molecule depends on the solvent and therefore influence the local dielectric constants,

which may deviate from that of the solvent.

Whether this happens or not, the solvent has a large influence on the photoinduced

behavior of PERY2. Therefore the interaction of PERY2 and 3 with different solvents was

studied by 1H NMR spectroscopy. The chemical shifts in the aromatic region of the fluorene

moiety was relatively constant in THF-d8, toluene-d8, CD2Cl2, CDCl3 and ODCB-d4, but the

chemical shifts of the perylene bisimide in the aromatic region varied significantly. For

instance, in deuterated toluene the aromatic protons of the perylene bisimide have signals

around 8.50 ppm (4H) and around 7.65 (3) or 7.35 ppm (PERY2) (4H). In CDCl3 however,

Chapter 7

156

all signals of the perylene bisimide are in the region of 8.80-8.60 ppm. These examples show

that the environment of the perylene bisimide can be influenced by the nature of the solvent.

7.6 Conclusion The compounds PERY2, PERY3, and PERY4 show mutually diverse photophysical

behavior. In the two compounds where the perylene bisimides are linked to a benzene ring,

PERY3 and PERY4, the fluorescence experiments indicate that in these multichromophores,

the decay of the excited state is similar to that of a single perylene bisimide unit. For PERY2,

where the perylene bisimides are attached to a central fluorene moiety, the fluorescence

quenching shows that PERY2 behaves different from a single perylene bisimide upon

photoexcitation. This behavior is ascribed to intramolecular electron transfer from the

fluorene to the perylene bisimide. The extent of the electron transfer in the eight different

solvents is not in agreement with theory, and is at this moment not fully understood. An

explanation may be sought in the interaction with the solvent, as 1H NMR experiments have

indicated that the different solvents have an effect on the environment of the perylene

bisimide moiety.

7.7 Experimental section For general information about experimental procedures and instrumentation see sections 3.7 and 4.9.

Compounds 2,14 3,15 9,19 12,21 and 1415 were synthesized according to literature procedures. The synthesis of

compound 10 is described in Chapter 5. Preparative HPLC was performed with a Shimadzu system using a LC-

8A preparative liquid chromatograph pump (5 ml/min), a SIL-10AD-VP auto injector, a SPD-10AV-VP UV-Vis

detector (522 nm) and a SCL-10A-VP system controller, using an eluent gradient of THF/water 8:2 to pure

THF. An Alltima C18 5µm column with a length of 150 mm and an internal diameter of 10 mm was used.

Cyclic voltammetry was measured in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a

supporting electrolyte in dichloromethane using an Ecochemie Autolab PGSTAT30 potentiostat. The working

electrode was a Pt disk, the counter electrode was an Ag bar, and an Ag/AgCl electrode was used as reference

electrode.

N-(1-Hexylheptyl)perylene-3,4:9,10-tetracaboxylic-3,4-anhydride-9,10-imide (4) The synthesis and

work-up have been performed with alterations from the literature procedure16, following partially the procedure

of Nagao et al.28 Perylene bisimide 3 (16.32 g, 21.6 mmol) and KOH powder (3.64 g, 64.9 mmol) were refluxed

under argon in tert-butanol (500 mL) for 50 minutes. After cooling of the reaction mixture, it was poured into 1

L of 10 % aqueous HCL. The precipitate was isolated by filtration of the mixture through a P3 glass filter. The

solid was filtered extensively with a warm 9 wt-% KOH and 7.5 wt-% KCl aqueous solution and subsequently

with water. The remaining solid was washed with 2 N HCl and subsequently dissolved in CH2Cl2 and extracted

with a 15 % aqueous HCl solution and brine. After drying on Na2SO4 and filtering the solvent was removed by

evaporation in vacuo. The solid was triturated with acetone and dried in a vacuum oven at 80 ºC. Of the product

6.61 g (53 %) was obtained as a black/red solid. Characterization is according to literature data.16


157

9,9’-Didodecylfluorene-2,7-bisphthalimide (6). To dimethylacetamide (dried on molsieves, 50 mL), 9,9-

didodecyl-2,7-dibromofluorene 5 (2.96 g, 4.48 mmol) was added. After addition of potassium phthalimide (1.66

g, 8.96 mmol) and copper(I) iodide (dried in oven, 1.71 g, 8.96 mmol) the mixture was stirred under argon for

five minutes at room temperature and subsequently stirred at 180 °C. After 18 h the reaction mixture was cooled

to room temperature and was poured into 4 N HCl (300 mL). To this solution CH2Cl2 was added and the organic

layer was isolated and subsequently washed with water and brine and was dried over Na2SO4. After filtration

and evaporation of the solvent the product was purified by column chromatography (silica gel, CH2Cl2/n-hexane

3:1, Rf = 0.3). A yellowish/brownish sticky glass was obtained with a yield of 1.00 g (28 %). 1H NMR (CDCl3,

300 Hz): 7.98 (dd, J = 5.5 Hz, 3.0 Hz, 4H), 7.84 (d, J = 8.8 Hz, 2H), 7.81 (dd, J = 5.5 Hz, 3.3 Hz, 4H), 7.46 (m,

4H), 2.00-1.95 (m, 4H), 1.30-1.00 (m, 36 H), 0.85-0.70 (m, 10 H). 13C NMR (CDCl3): 167.54, 152.11, 139.97,

134.55, 132.01, 130.89, 125.23, 123.84, 121.46, 120.45, 40.17, 32.05, 30.25, 29.77, 29.72, 29.44, 24.11, 22.82.

MALDI-TOF MS (MW = 792.49) m/z = 793.28 [M+H]+. Anal. Calcd. for C53H62N2O4: C 80.26, H 8.13, N 3.53.

Found: C 79.91, H 7.95, N 3.50.

9,9’-Didodecylfluorene-2,7-diamine (7). Compound 6 (0.75 g, 0.95 mmol) and hydrazine monohydrate

(0.46 ml, 9.5 mmol) were stirred under argon in refluxing ethanol for 18 h. After this period again hydrazine

monohydrate (0.50 ml, 10 mmol) was added and it was refluxed for another 4 h. After cooling to room

temperature, the mixture was filtered and the solvent was removed in vacuo. The residue was dissolved in

diethyl ether and again filtrated. The organic phase was washed with water and brine and dried over Na2SO4.

After filtration and evaporation of the solvent 0.42 g (83 %) of a brown oil was obtained as the product. 1H

NMR (CDCl3, 300 MHz) δ 7.33 (dd, J = 6.3 Hz, 2.5 Hz, 2H), 6.62 (d, J = 2.2 Hz, 2H), 6.61 (d, J = 6.3 Hz, 2H),

3.65 (s, 4H), 1.80-1.75 (m, 4H), 1.30-0.95 (m, 36 H), 0.87 (t, J = 6.9 Hz, 6H), 0.75-0.60 (br signal, 4H). 13C

NMR (CDCl3 75MHz): δ 151.63, 144.47, 133.12, 119.01, 113.81, 110.04, 54.59, 40.90, 31.90, 30.23, 29.70,

29.62, 29.38, 29.33, 23.76, 22.68, 14.11. MALDI-TOF MS (MW = 532.48) m/z = 532.43 [M•]+. Anal. Calcd. for

C37H60N2: C 83.39, H 11.35, N 5.26. Found: C 82.96, H 11.23, N 5.26.

2,7-Bis(N-(1-hexylheptyl)-3,4:9,10-perylene-bisimide-N´-yl))-9,9’-didodecylfluorene (PERY2).

Diamine 7 (0.34 g, 0.64 mmol), anhydride 4 (0.73 g, 1.27 mmol), imidazole (8.85 g) and zinc acetate (0.062 g,

0.34 mmol) were heated to 180 °C and were stirred under argon atmosphere. After 16 h no anhydride was

present anymore and the reaction mixture was cooled to room temperature. The reaction mixture was dissolved

in CH2Cl2 and was washed with 2 N HCl and brine. The solution was dried on Na2SO4 and was subsequently

filtered and dried in vacuo. Column chromatography was performed twice (silica gel, 1: CH2Cl2 with 0-3 %

methanol and 2: a] ethyl acetate, b] CH2Cl2/methanol 95:5, Rf = 0.95). The obtained solid was subsequently

purified by Soxlet extraction with ethyl acetate and precipitation from CH2Cl2 into ethyl acetate. 0.25 g (24 %)

of red solid was obtained, which was around 10 % impure according to HPLC characterization. A small amount

of was purified by preparative HPLC (THF/water) to obtain 12 mg of pure product. 1H NMR (CDCl3, 400

MHz): δ 8.80-8.60 (m, 16H), 7.95 (d, J = 8.4 Hz, 2H), 7.40-7.35 (m, 4H), 5.20 (m, 2H), 2.30-2.20 (m, 4H),

2.05-2.00 (m, 4H), 1.95-1.80 (m, 4H), 1.40-1.10 (m, 72H), 1.00-0.95 (m, 4H), 0.84 (m, 18H). 13C NMR (CDCl3,

100 MHz): δ 164.63, 163.59, 152.32, 140.71, 135.03, 134.42, 134.12, 131.80, 131.25, 129.86, 129.58, 127.36,

126.71, 126.47, 124.07, 123.73, 123.54, 123.25, 123.09, 120.70, 55.60, 54.83, 39.98, 32.38, 31.95, 31.76, 30.12,

29.70, 29.67, 29.39, 29.28, 29.21, 26.94, 23.95, 22.69, 22.57, 14.11, 14.03. M.p. = 285 °C. MALDI-TOF MS

(MW = 1643.96) m/z = 1644.04 [M•]-. Anal. Calcd. for C111H126N4O8: C 81.08, H 7.72, N 3.41. Found: C 80.57,

H 7.58, N 3.45.

Chapter 7

158

1,3,5-tri-(N-(1-ethylpropyl)perylene-3,4:9,10-tetracarboxylic-bisimide-N´-yl)benzene (PERY3).

1,3,5-triaminobenzene 9 (0.042 g, 0.29 mmol), N-(1-ethylpropyl)perylene-3,4:9,10-tetracarboxylic-3,4-

anhydride-9,10-imide 10 (0.473 g, 0.88 mmol) and a few grains of zinc acetate were heated under argon at 180

°C in imidazole (3.0 g) for 18 h. After cooling to room temperature, the reaction mixture was dissolved in

dichloromethane and was washed with 1 N HCL, water and brine and was dried on Na2SO4. Extensive

purification was performed by repetitive column chromatography (Al2O3, CH2Cl2/methanol 99:1 to 8:2) and

repetitive size exclusion chromatography (Bio Beads, SX-3 and SX-1, CH2Cl2) to yield 55 mg (11 %) of red

product. Attempted recording of 1H NMR and 13C NMR was not successful. Only extremely broad signals were

observed with 1H NMR in CDCl3 and CDCl3/methanol-d4. M.p. > 400 °C. MALDI-TOF MS (MW = 1453.43)

m/z = 1452.84 [M•]-. PERY3 gives one peak in GPC.

Tetra-4-[N-(1-hexylheptyl)perylene-3,4:9,10-tetracarboxylic-bisimide-N´-yl]phenylmethane

(PERY4).13 To a slurry of Raney Nickel (1 g) and tetrakis(4-nitrophenyl)methane 12 (0.15 g, 0.3 mmol) in THF

(20 mL) was dropwise added hydrazine hydrate (0.2 g, 4 mmol) in such a rate that foaming could be controlled.

After complete addition the reaction mixture was allowed to reflux for 3 h. The reaction was then filtered hot

and the filtrate was further washed with THF. The solvent was evaporated under argon to give white crystalline

mass. N-(1-hexylheptyl)perylene-3,4:9,10-carboximide 4 (0.70 g, 1.1 mmol), anhydrous zinc acetate (30 mg, 0.2

mmol), and imidazole (20 g) were added. The reaction mixture was then heated at 180 °C for 12 h under

vigorous stirring and poured into ice (200 g). The product was allowed to settle overnight, filtered off, and dried

in a vacuum oven (120 °C, 10 mmHg) to give a raw yield of 0.5 g (approximately 64 %) as a purple crystalline

mass. Pure samples of PERY4 were obtained by purification of 0.1 g material by column chromatography

(silica gel, CH2Cl2/methanol 99.5:0.5). Purity >98 % determined by GPC. M.p. > 400 °C. Pure according to

MALDI-TOF MS (MW = 2603.17) m/z = 2603.00 [M•]-. Other characterization data are described in

literature.13

7.8 References

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(5) (a) Y. Shirota, J. Mater. Chem. 2000, 10, 1; (b) P. Strohriegl, J. V. Grazulevicius, Adv. Mater. 2002, 14, 1439.

(6) Y. Kuwabara, H. Ogawa, H. Inada, N. Noma, Y. Shirota, Adv. Mater. 1994, 6, 677.

(7) (a) W. J. Oldham, R. J. Lachicotte, G. C. Bazan, J. Am. Chem. Soc. 1998, 120, 2987; (b) M. R. Robinson, S. Wang, G. C. Bazan, Y. Cao, Adv. Mater. 2000, 12, 1701; (c) M. R. Robinson, S. Wang, A. J. Heeger, G. C. Bazan, Adv. Funct. Mater. 2001, 11, 413.


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160

161

Summary

Photoinduced electron transfer at the interface of a donor and acceptor material is the

primary step in organic solar cells in which photons are used to create free charge carriers.

Because the lifetime and diffusion length of photoexcitations in organic materials is limited,

efficient charge separation can only be obtained when the electron donor and acceptor

materials are in close, nanometer, proximity. A second requirement for efficient solar cells is

that the generated charges can be transported to the two electrodes. Hence, it is important that

both materials form a continuous phase, extending from the interface to the electrode. Both

conditions can be fulfilled in composites of electron donor and acceptor materials. However,

the morphology of these composite organic semiconductors is difficult to control. Often,

large domains of the components are formed, which have a small interfacial area precluding

efficient charge generation. In contrast, too well mixing provides a large interface but is

prone to give discontinuities in the transport pathways, resulting in charge recombination.

Creating and maintaining nanoscale bicontinuous order of the two chromophores are

therefore important to obtain efficient organic solar cells.

A possibly elegant approach to control the morphology of donor and acceptor is by

incorporating the two components into block copolymers that are able to provide a predefined

bicontinuous nanostructure via self-organization, since the covalent bond between donor and

acceptor defines the dimension of the two phases. This thesis describes the synthesis and

photophysics of such novel donor-acceptor polymers and related architectures based on

electron deficient perylene bisimides and –primarily– electron rich oligo(p-phenylene

vinylene)s.

Chapter 1 gives a short introduction into the field of organic solar cells, briefly

summarizing the present state of the art and providing the aim of the research. In addition, the

contents of the remaining chapters are briefly outlined.

Chapter 2 provides an extensive literature overview on photoinduced energy and electron

transfer reactions involving functionalized perylene monoimides and bisimides. The synthesis

and intrinsic properties of perylene bisimides are addressed and basic concepts of energy and

electron transfer processes are described. The main part discusses photophysical properties of

perylene imides connected to various photoactive groups. The attention is focused on the

parameters such as redox potentials, excited state energies, and relative orientation and

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distance of the chromophores in controlling and discriminating between, photoinduced

energy and charge transfer processes.

Chapter 3 describes the properties of perylene bisimide units incorporated in polymer

main chains. Three alternating perylene bisimide – polytetrahydrofuran copolymers, differing

in the length of the polytetrahydrofuran segment, have been studied in ortho-dichlorobenzene

solution. Using temperature dependent absorption and fluorescence spectroscopy it has been

established that the polymers form H-like aggregates in solution. The extent of aggregation

depends on the length of the nonconjugated segments and temperature. Additional time-

resolved photoluminescence experiments gave evidence for excimer emission in these

polymers.

Chapter 4 deals with two polymers in which perylene bisimides and oligo(p-phenylene

vinylene)s are incorporated in an alternating fashion in the main chain via two different

spacers. This is a first step towards donor-acceptor block copolymers. The photophysical

properties of these materials and a cyclic model compound have been studied in detail in

solution and in the solid state with femtosecond transient pump-probe spectroscopy. All

compounds exhibit a very efficient photoinduced electron transfer from the oligo(p-

phenylene vinylene) to the perylene bisimide. The rates for the photoinduced charge

separation and recombination are related to the orientation of the donor and acceptor units in

the polymer chains. From the results it is concluded that an alternating ordering of donor and

acceptors occurs in the solid state. The copolymers have been incorporated in photovoltaic

devices.

Chapter 5 describes the synthesis and properties of two conjugated polymers in which an

oligo(p-phenylene vinylene) main chain polymer carries pendant perylene bisimides.

Absorption, fluorescence, and photoinduced absorption spectroscopy indicate efficient charge

transfer. As a result of the reduced distance between the chromophores, the rates of charge

separation and recombination in these polymers are higher than the rates obtained for the

alternating copolymers in Chapter 4.

Chapter 6 explores connecting chromophores by hydrogen bonding. In a first example an

ureidopyrimidinone functionalized perylene bisimide is linked by four hydrogen bonds to an

ureidopyrimidinone-functionalized oligo(p-phenylene vinylene). In contrast to the systems in

Chapter 4 and 5, this supramolecular donor-acceptor dyad exhibits only photoinduced energy

transfer and no charge transfer. The high binding constant of the quadruple hydrogen-

bonding motif allowed, for the first time, to time-resolve the photoinduced energy transfer

process in a supramolecular dyad in dilute solution. A second example employs urea units to

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interconnect oligo(p-phenylene vinylene)s. Scanning Tunneling Microscopy experiments

reveal a very regular pattern of these molecules on a graphite surface. In future systems, urea

functionalities might be used to direct the three-dimensional ordering of donor and acceptors

in multichromophoric systems, preventing the formation of alternating stacks.

Chapter 7 describes the synthesis and fluorescence properties of three multichromophoric

perylene bisimides, linked in pseudo-linear, trigonal, and tetrahedral orientations. Charge

transfer reactions occur in a variety of solvents only in case of linear system, in which two

perylene bisimides are connected via a fluorene group.

The results described in this thesis show that the type and orientation of the connectivity

of perylene bisimides with electron donors have a large influence on the photoinduced

processes. Absorption, (time-resolved) fluorescence, and (femtosecond) photoinduced

spectroscopy are powerful techniques to investigate photoinduced charge separation and

recombination between perylene bisimide and oligo(p-phenylene vinylene), but also provide

relevant information about the conformation of these polymers. Studying isolated chains in

solution is effective for explaining and predicting the behavior of the polymers in solid state.

Aggregation of the perylene bisimides, attraction between donors and acceptors in the ground

state, and the influence of the distance between them have shown to play important roles

concerning their properties. These results are of particular interest for the design of future

donor-acceptor block copolymers that are more effective in polymer photovoltaic cells. These

second-generation donor-acceptor polymers may have for instance segments that differ

strongly in size, or hydrogen bonding anchoring units at the polymer chain. This design

anticipates on the tendency of donor and acceptor segments to give alternating stacks, and

therefore may result in less charge recombination.

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Samenvatting

De lichtgeïnduceerde elektronoverdracht aan het grensvlak van een donormateriaal en

een acceptormateriaal is de primaire stap in organische zonnecellen waarin fotonen worden

gebruikt om vrije ladingen te creëren. Omdat fotoexcitaties in organische materialen een

beperkte levensduur en diffusieweglengte hebben, wordt efficiënte ladingsscheiding alleen

verkregen alleen als de elektrondonor en elektronacceptor zich op korte, nanometer, afstand

van elkaar bevinden. Een tweede voorwaarde voor een efficiënte zonnecel is dat de

gegenereerde positieve en negatieve ladingen getransporteerd worden naar de twee

elektroden. Een belangrijke vereiste hiervoor is dat beide materialen een continue fase

vormen vanaf het grensvlak tot aan de elektrode. Het blijkt dat aan beide voorwaarden

voldaan kan worden in composieten van elektrondonor en elektronacceptor materialen. De

morfologie van deze organische halfgeleidercomposieten laat zich echter moeilijk sturen.

Veelal vormen de individuele componenten grote domeinen, met een beperkt grensvlak,

waardoor ladingsscheiding inefficiënt is. Zeer goed gemengde componenten, daarentegen,

leiden tot een groot grensvlak, maar zijn vatbaar voor discontinuïteiten in de transportfase,

resulterend in ladingsrecombinatie. Voor het maken van efficiënte organische zonnecellen is

het creëren en behouden van twee continue fasen van de twee chromoforen op

nanometerschaal essentieel.

Een mogelijk elegante methode om de morfologie van de donor en acceptor te sturen, is

het integreren van de twee componenten in blokcopolymeren die via zelforganisatie de

gewenste, voorgeprogrammeerde, bicontinue nanostructuur geven omdat de covalente

binding tussen donor en acceptor de dimensie van de twee fasen definieert. Dit proefschrift

beschrijft de synthese en fotofysica van dergelijke nieuwe donor-acceptor polymeren en

verwante architecturen, gebaseerd op elektrondeficiënte peryleenbisimides en –

voornamelijk– elektronrijke oligo(p-fenyleenvinylenen).

Hoofdstuk 1 geeft een korte inleiding over polymere zonnecellen en beschrijft de huidige

stand van zaken van dit onderzoeksgebied. Voorts worden het doel van het onderzoek en de

inhoud van de overige hoofdstukken kort uiteengezet.

Hoofdstuk 2 geeft een uitgebreid literatuuroverzicht over lichtgeïnduceerde energie- en

elektronoverdrachtsreacties waarbij gefunctionaliseerde peryleenbisimides betrokken zijn. De

synthese en intrinsieke eigenschappen van peryleenbisimides worden beschreven en

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elementaire concepten van energie- en elektronoverdrachtsprocessen worden uitgelegd. De

meeste aandacht gaat uit naar de fotofysische eigenschappen van peryleenimides die

gebonden zijn aan diverse foto-actieve groepen. De nadruk ligt hierbij op parameters als

redoxpotentialen, aangeslagen-toestandenergieën, en de relatieve oriëntatie en afstand tussen

de chromoforen voor de beheersing van, en het onderscheid tussen, lichtgeïnduceerde

energie- en elektronoverdracht.

Hoofdstuk 3 behandelt de eigenschappen van peryleenbisimides die ingebouwd zijn in

polymere hoofdketens. Drie alternerende peryleenbisimide – polytetrahydrofuraan

copolymeren die verschillen in de lengte van het polytetrahydrofuraan segment, zijn

bestudeerd in ortho-dichloorbenzeen oplossing. Met temperatuurafhankelijke absorptie en

fluorescentie spectroscopie is vastgesteld dat de polymeren een soort H-aggregaten vormen in

oplossing. De mate van aggregatie hangt af van de lengte van de niet-geconjugeerde

segmenten en van de temperatuur. Aanvullende tijdsopgeloste fluorescentie-experimenten

geven bewijs voor excimeeremissie in deze polymeren.

Hoofdstuk 4 beschrijft twee polymeren waarin peryleenbisimides en oligo(p-

fenyleenvinylenen) op een alternerende wijze zijn geïntegreerd in polymere hoofdketens met

verschillende verbindingsstukken. Dit is een eerste stap om donor-acceptor blokcopolymeren

te verkrijgen. De fotofysische eigenschappen van deze materialen en een cyclische

modelverbinding zijn in oplossing en in de vaste fase bestudeerd met behulp van

femtoseconde tijdsopgeloste lichtgeïnduceerde absorptie. Alle verbindingen geven een zeer

efficiënte lichtgeïnduceerde elektronoverdracht van oligo(p-fenyleenvinyleen) naar

peryleenbisimide. De snelheden van de lichtgeïnduceerde ladingsscheiding en

ladingsrecombinatie zijn gerelateerd aan de onderlinge oriëntatie van de donoren

acceptoreenheden in de polymeerketens. Op basis van de resultaten is geconcludeerd dat een

alternerende stapeling van donoren en acceptoren optreedt in de vaste fase. De copolymeren

zijn toegepast in een zonnecellen.

Hoofdstuk 5 behandelt de synthese en eigenschappen van twee polymeren waarbij de

geconjugeerde oligo(p-fenyleenvinyleen) hoofdketens peryleenbisimides als zijgroepen

hebben. Absorptie, fluorescentie en lichtgeïnduceerde absorptiespectroscopie laten zien dat

de ladingsoverdracht efficiënt is. Door een kleinere afstand tussen de chromoforen zijn de

snelheden van de ladingsscheiding en -recombinatie in deze polymeren hoger dan de

snelheden die bepaald zijn voor de polymeren in Hoofdstuk 4.

Hoofdstuk 6 beschrijft mogelijkheden om chromoforen door waterstofbruggen aan elkaar

te koppelen. Een eerste voorbeeld hiervan is een peryleenbisimide dat gekoppeld is aan een

166

oligo(p-fenyleenvinyleen) via viervoudige waterstofbruggen. In tegenstelling tot de systemen

in Hoofdstuk 4 en 5, geeft deze supramoleculaire donor-acceptor diade alleen

lichtgeïnduceerde energieoverdracht. Door de grote bindingsconstante van de viervoudige

waterstofbrug bleek het voor het eerst mogelijk om een lichtgeïnduceerde energieoverdracht

in een supramoleculaire diade in verdunde oplossing te volgen in de tijd. In een tweede

voorbeeld zijn ureumeenheden gebruikt om oligo(p-fenyleenvinylenen) te verbinden.

Scanning Tunneling Microscopy experimenten laten een zeer regelmatig patroon zien van

deze moleculen op een grafietoppervlak. In toekomstige systemen kunnen ureumeenheden

wellicht gebruikt worden om een driedimensionale ordening van donoren en acceptoren in

multichromofore systemen te creëren waarbij de vorming van een alternerende stapeling

voorkomen kan worden.

Hoofdstuk 7 beschrijft de synthese en fluorescente eigenschappen van drie

multichromofore peryleenbisimides in een pseudo-lineaire, trigonale of tetraëdrische

oriëntatie. Alleen het lineaire molecuul, waarin twee peryleenbisimides zijn verbonden via

een fluoreengroep, geeft lichtgeïnduceerde ladingsoverdrachtsreacties in verschillende

oplosmiddelen.

De resultaten die in dit proefschrift beschreven zijn, tonen aan dat de aard en oriëntatie

van de binding tussen peryleenbisimides en elektrondonoren een grote invloed hebben op de

efficiëntie van lichtgeïnduceerde processen. Absorptie, (tijdsopgeloste) fluorescentie en

(femtoseconde) lichtgeïnduceerde absorptie blijken krachtige technieken om

lichtgeïnduceerde ladingsscheiding en ladingsrecombinatie tussen peryleenbisimide en

oligo(p-fenyleenvinyleen) te onderzoeken, en tevens om informatie te verkrijgen over de

conformatie van de polymeren. Het bestuderen van geïsoleerde ketens in oplossing is van nut

voor het begrijpen en voorspellen van het gedrag van de polymeren in de vaste fase.

Aggregatie van peryleenbisimides, aantrekking tussen donoren en acceptoren in de

grondtoestand en de invloed van de afstand tussen beiden blijken een grote invloed te hebben

op hun eigenschappen. Deze resultaten zijn van belang voor het ontwerpen van toekomstige

donor-acceptor blokcopolymeren, die effectiever zijn in polymere zonnecellen. Deze tweede-

generatie donor-acceptor copolymeren kunnen bijvoorbeeld bestaan uit segmenten die sterk

in afmeting verschillen of waterstofbrugvormende ankerpunten hebben op de polymeerketen.

Met een dergelijk ontwerp zou de neiging van donoren en acceptoren om een alternerende

stapeling te geven onderdrukt kunnen worden, waardoor ladingstransport verbetert en

ladingsrecombinatie afneemt.

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Curriculum Vitae

Edda Neuteboom werd op 23 maart 1975 geboren in Odijk. In 1993 behaalde ze het VWO diploma aan de Openbare Scholengemeenschap Schoonoord te Zeist. In hetzelfde jaar begon ze aan de studie Chemische Technologie aan de Universiteit Twente. Tijdens haar studie liep ze vier maanden stage bij de groep ‘Polymeren en Organische Chemie’ van Philips Research te Eindhoven. Het afstudeeronderzoek werd verricht in de vakgroep ‘Supramoleculaire Chemie en Technologie’ van prof. dr. ir. D. N. Reinhoudt. Daarna volgde een stage van drie maanden in de groep

van prof. dr. A. Pochini aan de Universiteit van Parma (Italië) in het kader van het Socrates uitwisselingsprogramma. In augustus 1999 werd het doctoraaldiploma behaald. In oktober 1999 werd ze assistent in opleiding binnen de capaciteitsgroep ‘Macromoleculaire en Organische Chemie’ aan de faculteit Scheikundige Technologie van de Technische Universiteit Eindhoven. Onder leiding van prof. dr. ir. R. A. J. Janssen en prof. dr. E. W. Meijer werkte ze daar tot november 2003 aan het onderzoek waarvan de resultaten beschreven zijn in dit proefschrift. Vanaf december 2003 is ze werkzaam als instituutssecretaris van het FOM-Instituut voor Atoom- en Molecuulfysica (AMOLF) te Amsterdam. Edda Neuteboom was born in Odijk, the Netherlands, on 23 March, 1975. She graduated in 1993 from the Openbare Scholengemeenschap Schoonoord in Zeist. In the same year she started studying Chemical Technology at the University of Twente. During her studies she did a four-month traineeship in the group ‘Polymers and Organic Chemistry’ at Philips Research in Eindhoven. Her undergraduate project was in ‘Supramolecular Chemistry and Technology’ under the supervision of prof. dr. ir. D. N. Reinhoudt. Subsequently, in the Socrates student exchange program, she did a three-month internship in the laboratory of prof. dr. A. Pochini at the University of Parma (Italy). In August 1999, she graduated from the University of Twente. She started her Ph. D. in October 1999 in the group ‘Macromolecular and Organic Chemistry’ in the department of Chemical Engineering at the Eindhoven University of Technology. Under the guidance of prof. dr. ir. R. A. J. Janssen and prof. dr. E. W. Meijer, she studied photoinduced processes of functionalized perylene bisimides until November 2003. The results of her research are described in this thesis. As of December 2003 she works as the secretary of the FOM-Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam.

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List of publications Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant. P. van de Witte, E. E. Neuteboom, M. Brehmer, J. Lub, J. Appl. Phys. 1999, 85, 7517.

Aggregation of perylenebisimid-polytetrahydrofuran copolymers. E. E. Neuteboom, R. A. J. Janssen, E. W. Meijer, Synth. Met. 2001, 121, 1283.

Kinetic stabilities of double, tetra-, and hexarosette hydrogen-bonded assemblies. L. J. Prins, E. E. Neuteboom, V. Paraschiv, M. Crego-Calama, P. Timmerman, D. N. Reinhoudt, J. Org. Chem. 2002, 67, 4808. Singlet-energy transfer in quadruple hydrogen-bonded oligo(p-phenylenevinylene)perylene-diimide dyads. E. E. Neuteboom, E. H. A. Beckers, S. C. J. Meskers, E. W. Meijer, R. A. J. Janssen, Org. Biomol. Chem. 2003, 1, 198. Alternating oligo(p-phenylene vinylene) – perylene bisimide copolymers: Synthesis, photophysics, and photovoltaic properties of a new class of donor-acceptor materials. E. E. Neuteboom, S. C. J. Meskers, P. A. van Hal, J. K. J. van Duren, E. W. Meijer, R. A. J. Janssen, H. Dupin, G. Pourtois, J. Cornil, R. Lazzaroni, J.-L. Brédas, D. Beljonne, J. Am. Chem. Soc. 2003, 125, 8625. Photoluminescence of self-organized perylene bisimide polymers. E. E. Neuteboom, S. C. J. Meskers, E. W. Meijer, R. A. J. Janssen, Macromol. Chem. Phys., in press.

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Dankwoord

De afgelopen vier jaar heb ik ontzettend veel geleerd en genoten. Dit is mogelijk geweest door het fascinerende onderzoek dat binnen SMO wordt verricht, de goede faciliteiten en de geweldige sfeer.

Allereerst zou ik graag mijn promotoren René Janssen en Bert Meijer willen bedanken voor het vertrouwen dat ze de afgelopen vier jaar in mij gehad hebben en voor de mogelijkheid die ze mij hebben gegeven om mijzelf te ontwikkelen. René, ik had me geen betere en leukere begeleider kunnen wensen. Je hebt me altijd gestimuleerd in mijn werk en in het maken van de juiste beslissingen. Ik heb veel van je geleerd en ik hoop dat in de toekomst te blijven doen. Bert, ik heb veel bewondering voor de manier waarop je leiding geeft aan SMO. Het is uitgegroeid tot een zeer grote groep, maar de sfeer is nog steeds als vanouds. Ook al begeleidde je me op de achtergrond, ik heb het altijd erg plezierig en nuttig gevonden om ook deel uit te maken van jouw eigen groep.

Stefan Meskers zou ik graag willen bedanken voor de tijd die hij steeds vrijmaakte om lastige en voor mij nieuwe concepten uit te leggen. Stefan, je hebt een zeer originele manier om mensen te stimuleren en te interesseren voor het zoeken naar fotofysische processen. Jouw interpretatie van mijn experimenten heeft een grote bijdrage geleverd aan mijn uiteindelijke resultaten. Ik wil je hiervoor bedanken en ik wens je veel succes met je carrière.

I would like to thank the group in Mons, especially dr. Hélène Dupin and dr. David Beljonne, for their contribution to the JACS paper and Chapter 4. That project has become more complete because of your calculations and efforts.

Dr. André Gesquière wil ik graag bedanken voor de STM experimenten die beschreven staan in Hoofdstuk 6. Ook al was dat project ‘preliminary’, ik ben erg blij met de behaalde resultaten.

I would like to thank professor Würthner and professor Hofstraat for reading the draft of my thesis and for participating in my Ph. D. committee.

Voor de synthese van verschillende produkten heb ik hulp gekregen van veel mensen. Ik wil Michel Fransen, Jolanda Spiering, Ron Versteegen en Abdelkrim El-ghayoury bedanken voor de synthese en donatie van OPV3, polyTHF en OPV-UP. Vele malen heb ik hulp van Xianwen Lou, Joost van Dongen en Ralf Bovee gekregen voor het karakteriseren en scheiden van mijn produkten. Hartelijk bedankt daarvoor. Research stagiair Martijn van Gerwen wil ik bedanken voor de moeilijke synthese van een peryleen trimeer en de reactanten daarvan. I would like to thank Jorge Caeiro Rodríguez, Corinne Martineau and Nicolai Stuhr-Hansen for their help with the synthesis of some of the compounds that are described in Chapter 7. I know that the purification of perylene bisimides can be very difficult, but you all were very brave and successful. Antonino Rizzuti, thank you for sharing your knowledge and enthusiasm in fluorene and UP chemistry. Thanks to you, I got some nice results. Jef Vekemans en Rint Sijbesma wil ik graag bedanken voor hun advies over syntheseroutes.

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Paul van Hal, Jeroen van Duren, Edwin Beckers, Stefan Meskers, Martijn Wienk en Alicia Marcos Ramos wil ik graag bedanken voor hun vele metingen en advies over spectroscopie, zonnecellen en synthese. Dankzij jullie hulp hebben we belangrijke resultaten verkregen. I also would like to thank Jayakannan for the advice on his beloved Suzuki reaction.

Dankzij de inspanningen van Hans Damen, Hanneke Veldhoen, Ingrid Dirkx, Emma Eltink, Joke Rediker, Carine van der Vaart, Henk Eding en Hannie van der Lee was alles altijd prima geregeld binnen de vakgroep.

Joke, Paul, Stefan, Patricia en Martijn hebben mij gezelschap gehouden in STO 4.29. Dat was niet alleen erg gezellig, maar het gaf me ook de gelegenheid om vaak even een vraagje tussendoor te kunnen stellen. I would like to thank all (former) members of SMO, and in particular of Lab 2, for the pleasant atmosphere.

De (zeer!) aardige ontwerper Erik Hoogendorp zou ik graag willen bedanken voor het ontwerpen van de hippe omslag van deze 178 pagina’s.

In these years I have made some great friends in the lab. Thank you Alicia, Fiorella, Daniela, Mitsutoshi, and Corinne for your company, support and most of all many amusing moments.

Met mijn vriendinnen Annemieke, Astrid en Naomi heb ik al vanaf de middelbare school menig hoogte- en dieptepunt meegemaakt. Ik hoop dat we in de toekomst nog veel hoogtepunten met elkaar kunnen delen.

Ellen, je ben een fantastische vriendin. Sinds onze eerste (ladies)dag op de UT klikte het en ik ben erg blij dat jij mijn paranimf wilt zijn.

Mijn ouders en mijn broer Olaf wil ik bedanken voor hun onvoorwaardelijke steun. Ik kon altijd op jullie rekenen en ik hoop van harte dat ik dat nog lang kan blijven doen. Olaf, je neemt je taken van paranimf zeer serieus. Ik reken erop dat je mijn redder in nood bent op de 15e!

En Esko, wat moet ik zonder jou? Bedankt voor alle fantastische jaren. Ik weet zeker dat na onze beide promoties er nog vele zullen volgen.

Edda

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functionalized perylene bisimides