Luminescence and photoconductivity in mononuclear ortho-platinatedmetallomesogens

Cornelia Damm, Gunter Israel, Torsten Hegmann{* and Carsten Tschierske

Received 17th January 2006, Accepted 3rd March 2006

First published as an Advance Article on the web 21st March 2006

DOI: 10.1039/b600680a

The electronic properties of series of mononuclear ortho-platinated and -palladated

metallomesogens were investigated by means of UV-vis absorption and fluorescence spectroscopy

in solution, fluorescence spectroscopy in the solid state as well as time-resolved photo-emf

measurements. The organoplatinum compounds show luminescence in solution and in the solid

state as well as photoconductivity caused by Pt…Pt closed shell interactions in comparison to the

non-luminescent and non-photoconductive organopalladium compounds. A significant influence

of the substitution pattern regarding luminescence and photoconductivity of the materials

(number, length and distribution of attached alkyl chains) was found for the organoplatinum

phenylpyrimidine and phenylpyridine series 1 and 3.

Introduction

Due to their applications as organic light emitting diodes

(OLEDs), and principally for use in display devices, emissive

materials are of great current interest.1 Many different

materials with various chemical structures are already in use

or have been intensively studied over the past two decades,2

e.g. 8-hydroxyquinoline-Al(III) (AlQ3),3 p-phenylene vinylene

monomers3 or polymers (PPV),4–6 1,3,4-oxadiazoles (PBD),3,4

poly(pyridine-2,5-diyls) (Ppy),6,7 poly(thiophene-2,5-diyls)

(PTh),6,8 poly(p-phenylethynyls) (PPE),6,9 tris(bipyridyl)10,11

and bis(terpyridyl) metal complexes,12 as well as porphyr-

ins13,14 and phthalocyanines and their metal complexes,15 to

point out some of the most important. Additionally, some

of these materials such as PPV,16 phthalocyanines and

their metal complexes17 act distinctly as conductors and/or

semiconductors.

Recently, we reported on series of mononuclear ortho-

palladated and ortho-platinated organometallic compounds

(series 1 and 2) showing thermotropic liquid crystalline (LC)

properties.18,19 Variation of the overall molecular shape in

series of these ortho-metalated compounds (see Table 1) was

achieved by connecting 2-phenylpyrimidine and 2-phenyl-

pyridine mesogenic units with 1,3-diphenylpropane-1,3-dio-

nato moieties having varying number and length of attached

peripheral alkyl tails via the central metal atom (Pd, Pt). In

these series, the type of liquid crystalline phase strongly

depends on both the number and the position of the

attached alkyl chains, allowing for easy control over the liquid

crystalline behavior (vide infra). Materials with only a few

chains (four and five alkyl chains) show smectic LC phases

(SmA, SmC), in which the molecules are organized in layers,

and nematic phases (N), whereas compounds with in total six

(depending on the distribution) or more chains can organize

in columns leading to hexagonal columnar phases (Colh). The

types and transition temperatures of the observed LC phases

are summarized in Table 1.19

However, in contrast to both series of ortho-metalated

2-phenylpyrimidines, none of the synthesized ortho-platinated

2-phenylpyridine derivatives (series 3) exhibits liquid crystal-

line behavior (Table 2).19 Nevertheless they are included in this

study to evaluate the influence of the ligand structure.

The increased stability of the LC phases of the platinum

derivatives (1a–1g) with respect to the related palladium

compounds (2a–2g) was explained by the higher polarizability

of the Pt(II) atoms and the existence of Pt(II)…Pt(II) d8–d8

closed shell interactions20 in comparison to Pd(II). Such

metal–metal interactions of d8-configured Pt(II) atoms,

initially investigated by Krogmann et al.,20a are a well known

phenomenon in square planar coordinated Pt(II) complexes.

A number of such complexes exhibit fluorescence in the

solid state (e.g., [Pt(CN)4]22)21 and can act as one-dimensional

conductors (molecular wires), e.g., partially oxidized tetra-

cyanoplatinates with the general formula M2[Pt(CN)4].22

With this in mind and in an attempt to combine liquid

crystalline behavior with emissive and photo(conducting) pro-

perties, we investigated the exchange of the palladium atom

with platinum in these series of ortho-metalated 2-phenyl-

pyrimidine(-pyridine)/1,3-diketonato compounds. Apart from

the quite similar mesophase behavior, the platinum(II)

compounds promise additional properties like luminescence

in the solid state and in solution by aggregation as well as

photoconductivity.

Luminescent properties arising from Pt…Pt d8–d8 closed

shell interactions have recently been described for series of

cyclometalated platinum compounds.23 For liquid crystalline

materials, emissive properties were also reported for a number

of metal-containing liquid crystals (metallomesogens) such

as liquid crystalline lanthanide complexes,24 2,29-bipyridine

metal complexes,25 dinuclear cyclometalated azobenzenes,26

Martin-Luther-University Halle-Wittenberg, Institute of OrganicChemistry, Kurt-Mothes-Str. 2, 06120 Halle, Germany{ Present address: Department of Chemistry, Universityof Manitoba, Winnipeg, Manitoba, Canada R3T 2N2.E-mail: hegmannt@cc.umanitoba.ca; Fax: +1 (204) 474-7608;Tel: +1 (204) 474-7535.

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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mononuclear cyclometalated 6-phenyl-[2,29]bipyridinyl deriva-

tives,27 and oxadiazole palladium complexes.28 Such photo-

luminescent liquid crystalline materials that act as active color

filters might well be used for liquid crystal display (LCD)

devices by enhancing the visual performance.29,30 Additionally,

cyclopalladated liquid crystals have also been discussed as a

new class of highly efficient photorefractive materials.31

In this contribution we discuss the results for these materials

upon electronic excitation. For this reason, luminescence

spectra were analyzed both in solution and in the solid state.

Further, photo-emf32,33 measurements were performed, as they

permit detailed insights into the electronic excited states of the

bulk solid material. These particular series of compounds

appeared to us as excellent candidates for comparative studies:

a) on the influence of substituents on the electronic properties

of the molecules, as the substitution pattern changes stepwise

via a successively increasing number of alkoxy chains, and b)

for comparative studies on the influence of the difference in

strength of metal–metal interactions in d8-configured, square

planar-coordinated Pt(II) and Pd(II) organometallic com-

pounds on the mesomorphic and electronic properties.

Results and discussion

Synthesis

The synthesis of compounds 1–3 with the exception of 1f and

2f was reported in a previous paper.19 The synthesis of

compounds 1f and 2f with a 3,4,5-trihexyloxybenzoyl group is

shown in Scheme 1. At this point it should be noted that all

compounds substituted with a different number (or length) of

O-alkyl chains on the diketonate aromatic rings (e.g., 1e, 1f,

3e, and 3f) were isolated as a mixture of cis : trans isomers in a

ratio of approximately 1 : 1.

Absorption and luminescence in solution

The Pt(II) organyls 1a–1g and 3a–3c are intense yellow solids

(the related Pd(II) organyls 2a–2g are pale yellow materials)

and luminescence in the solid state was expected for these

materials. The stepwise increase of the number of attached

alkyl chains as well as variations of length and distribution of

these chains in these materials opens the possibility to probe

the relationship between molecular structure and electronic

Table 1 Phase transition temperatures T [uC] of the mononuclear metallomesogens 1a–1g and 2a–2g19

R1 R2 R3 R4 R5 Compd. M T/uC

OC10H21 H H H H 1a Pt Cr 137 SmA 145 Iso2a Pd Cr 113 SmC 117 SmA 133 Iso

OC10H21 OC10H21 H H H 1b Pt Cr 126 (SmA 107 N 109) Iso2b Pd Cr 117 (SmA 99 N 101) Iso

OC10H21 OC10H21 H OC10H21 H 1c Pt Cr 117 Iso2c Pd Cr 115 Iso

OC10H21 OC10H21 OC10H21 H H 1d Pt Cr 62 Colh 75 Iso2d Pd Cr 59 Colh 76 Iso

OC10H21 OC10H21 OC10H21 OC10H21 H 1e Pt Cr 74 Colh 151 Iso2e Pd Cr 72 Colh 134 Iso

OC6H13 OC6H13 OC6H13 OC10H21 H 1f Pt Cr 78 Colh 134 Iso2f Pd Cr 75 Colh 116 Iso

OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 1g Pt Cr 76 Colh 170 Iso2g Pd Cr 79 Colh 163 Iso

a Abbreviations: Cr = crystalline solid; SmA = smectic-A phase; SmC = smectic-C phase; N = nematic phase; Colh = hexagonal columnarphase; Iso = isotropic liquid.

Table 2 Melting points, mp [uC], of the mononuclear organo-platinum compounds 3a–3c19

Compd. R1 R2 mp/uC

3a H H 1003b OC10H21 H 873c OC10H21 OC10H21 92

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properties via electronic spectra, both in solution and in the

solid state.

The solution spectra of 1a–1f (compound 1g decomposes

slowly in solution) in 1,2-dichloroethane (Table 3) show three

distinct absorption peaks and one shoulder at about 400 nm.

Two of these bands may be attributed to allowed p–p* (238–

272 nm) and the corresponding n–p* transition (353–375 nm)

of the platinum 1,3-diketonate moiety. The absorption band

centered at 297–316 nm may arise from the lowest ligand

field singlet–triplet transition of the Pt(II) complex.38,39 All

materials investigated show luminescence40 in solution.

However, no continuity was found for the increase of the

number of alkyl chains from 1a to 1f concerning wavelength

and intensity of the emission peaks. In a first approach, the

materials can be divided into two distinct classes of materials:

the first one includes materials with one emission band below

440 nm (1b, 1c and 1e) and the second one includes materials

with additional emission bands above 490 nm (1a, 1d and 1f).

The second class refers to all these compounds with either only

one alkoxy substituent at one of the diketonate aromatic rings

(1a and 1d, compound 1b however does not exhibit any

luminescence maximum above 426 nm), or with partially

shorter alkyl chains (compound 1f). Despite the fact that the

luminescence below 440 nm in all materials seems to arise from

the single square planar platinum organyls with large Pt–Pt

distances, the emission above 490 nm in 1a, 1d and 1f arises

from the Pt…Pt closed shell interactions of a dimer form

(excimer) of the platinum materials,23 as none of the related Pd

analogues 2a–2g shows luminescence in solution.41 Hence, in

the organoplatinum materials 1a, 1d and 1f, the substitution

pattern seems to enforce aggregation in solution in such way

that metal–metal interactions between the square planar

coordinated Pt(II) atoms become possible.

For the solution spectra of the 2-phenylpyridine platinum(II)

organyls 3a–3c, some differences were observed in comparison

to the 2-phenylpyrimidine derivatives 1. The absorption band

Scheme 1 Synthesis of compounds 1f and 2f. Reagents and conditions: (i) CH3Li, Et2O;34 (ii) 1. NaH, dimethoxyethane, 2. H+;35 (iii) TlOEt,

toluene;36 (iv) CH2Cl2, 25 uC.19,37

Table 3 Absorption and luminescence maxima of 1a–1f and 3a–3c in solutiona

Absorption Emission

Compd. c/mol l21 6 1026l1/nm(e/l mol21 cm21)

l2/nm(e/l mol21 cm21)

l3/nm(e/l mol21 cm21)

Shoulder at #400 nm,e400 nm/l mol21 cm21

l1max/nm

(I (rel)) l2max/nm (I (rel))

1a 8.77 272 (31 000) 298 (37 300) 353 (34 200) 13 200 420 (—)c 530b (140), 496 (158)1b 6.86 241 (32 900) 297 (26 200) 372 (42 700) 18 200 426 (86)1c 4.75 237.5 (43 800) 314 (32 800) 375 (43 600) 27 200 423 (114.5)1d 6.54 268.5 (28 800) 312.5 (25 200) 367 (24 800) 12 800 436 (13) 523 (42)1e 5.50 249 (64 300) 316 (60 900) 374 (91 600) 54 200 433 (200)1f 6.32 260 (26 000) 313 (30 700) 360.5 (28 500) 17 400 436 (38) 512 (191)3a 6.5 286 (36 900) — 342 (43 000) 541b (80), 515 (120)3b 3.5 285 (22 600) — 371 (33 900) 425 (25) 544b (10),c 514 (15)3c 4.2 280 (31 000) — 373 (41 700) 420 (70)a Solvent = 1,2-dichloroethane. b Shoulder. c Detectable maximum with low intensity.

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l2 centered at 300 nm is not present, and the absorption band

l1 is shifted to larger values (¢280 nm) for all pyridinepla-

tinum compounds 3. Nevertheless, similar trends can be found

for the luminescence behavior in solution for the pyrimidine-

and the pyridineplatinum organyls. Both compounds 3a and

3b (here also 3b in comparison to 1b in the pyrimidine series)

with only one alkoxy substituent at one of the aromatic rings

of the diketonate moiety show luminescence above 500 nm.

Compound 3a exhibits two distinct emission bands of low

intensity at 541 nm and 515 nm. For compound 3b with one

emission band at 425 nm, a second band was observed at

514 nm with a shoulder at 544 nm (see Table 3).

Luminescence in the solid state

The solid state spectra show emissions after electronic

excitation for all investigated Pt(II) organyls 1a–1f and

3a–3c. Two distinct bands of quite equal intensity, one

between 505–525 nm, and another one between 525–560 nm,

characterize emissions found in all materials (see for example

1f in Fig. 1).

As can be seen from the data collected in Table 4, differences

in the substitution patterns have a strong influence on the

positions of these two bands as well as on their corresponding

intensities. Compounds with only one alkoxy chain on the

aromatic rings of the 1,3-diketonate unit (compds. 1a, b, d and

3a, b) as well as compound 1f with shorter alkyl chains exhibit

strong intensities of their luminescence. Here, the pyrimidine

derivative 1a shows the strongest emission intensity in the

presented series. Generally, the intensity of the emitted

luminescence light depends on the substitution pattern and

increases with decreasing number of alkoxy substituents.

Remarkably, in the same manner the wavelength of both

emission maxima shifts to higher values. An explanation for

that behavior might be that either a reduced number of alkyl

chains at least at one of the aromatic rings of the diketonate

unit or shorter alkyl chains allow for a closer packing of the

molecules in the crystal. In this way, their rigid organometallic

centers come closer to each other resulting in enhanced p–p

interactions between the aromatic cores as well as in stronger

interactions of the square planar coordinated Pt(II) atoms

among themselves41 (d8–d8 closed shell interactions likely by

formation of a dimer).20,23

For comparison, we also investigated the analoguous Pd

compounds 2a–f, which did not exhibit any luminescence,

neither in the solid state nor in solution. Hence, the exchange

of the palladium atom with the platinum atom in these

mononuclear metallomesogens generates emissive materials

caused by d8–d8 closed shell interactions of square planar

coordinated Pt(II)-atoms.

Again, these observations prove that metal–metal interac-

tions between d8-configured platinum atoms are in fact

stronger in comparison to palladium.20,41 Such interactions

should be favorable for thermal or (photo)conductivity along

the stacking axis of the solid as is known for [Pt(CN)4]22

complexes.22c Therefore, we have carried out photo-emf

measurements for both the platinum compounds 1, 3 as well

as for the related organopalladium derivatives 2.

Photoconductivity

Pulse exposure generates in solid photoconductors a concen-

tration gradient of electron–hole pairs, which causes a

diffusion of charges h+ and e2 into the inner part of the solid.

Different mobilities of electrons and holes lead to a spatial

separation of the crucial points of charge and an inner

electric field builds up between exposed and non-exposed side

of the crystal. This integral temporary potential difference

appears as photo-emf or Dember voltage32 that can be

measured as a function of time. The voltage U(t) is normally

directly proportional to the number of separated charges. In

the case that no voltage can be measured after an actinic

exposure, as a rule, the sample does not have any semi-

conductor properties. The principle for the origin of a photo-

emf in combination with the measurement arrangements is

shown in Fig. 2.33

Photo-emf measurements are carried out without outer

electric fields and therefore, one can obtain evidence of the

natural behavior of the charges in the sample. For this reason

the method is very sensitive to changes of the chemical and/or

solid state structure. Additionally, the photo-emf parameters

are essentially determined by the surface of the photo-

conductor, as found in previous studies on silver halo-

genides,33,42 metal oxides43 as well as on organic pigments.44

Fig. 1 Solid state emission spectrum of the Pt(II) organyl 1f.

Excitation wavelength lexc. = 380 nm; slits (graphs): (a) 2.5/5, (b)

2.5/7.5 (excitation/emission monochromator). To prevent overloading

of the photomultiplier tube a 2% transmission filter is used in the

excitation light beam of the LS 50 fluorescence spectrometer.

Table 4 Solid-state luminescence data of 1a–1f and 3a–3ca

Compd. I (rel) l1/nm l2/nm

1a 11250 525 5601b 408 500 5251c 160 509 5301d 8250 507 5341e ,10 505 5271f 2250 506 5363a 300 517 5493b 1250 515 5403c 25 514 540a Excitation wavelength lexc. = 380 nm; slits 2.5/5 (excitation/emission monochromator).

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After pulse exposure, the photo-emf increases until a

maximum UMAX and fades out further to zero caused by

recombination or reaction of the charges with traps. In most

cases, this fade out process can be described by a biexponential

rate law (eqn (1)):33

U(t) = U10?exp(2k1t) + U2

0?exp(2k2t) (1)

UMAX = U10 + U2

0 (2)

The total amount of both partial photo-emfs U10 and U2

0

complies with the maximum value UMAX of the photo-emf.

Both U10 and U2

0 can have the same or a different sign in

eqn (1) and (2) and therefore eqn (1) describes either fading out

photo-emf signals as well as such, which intersect the U(t) axis

at zero with the same set of four parameters. By definition,

process 1 is the process that fades out faster (k1 . k2). More

detailed investigations of the influence of the trap distribution

close to the surface and in the bulk of organic photo-

conductors concerning the photo-emf parameters show that

this faster fade out process (parameters U10, k1) can be

associated with the photo-emf in near-surface areas.44

The platinum compounds 1a–1f and 3a–3c can be regarded

as potential photoconductors caused by their well-organized

structure and possible Pt…Pt interactions. To prove this

assumption we have measured the photo-emf of three platinum

compounds, one with an intense luminescence 1d and two with

a modest luminescence, 1c and 3c.

Compound 1d does not show an appreciable maximum

voltage, whereas for compounds 1c (Fig. 3 and Table 5) and

3c distinct transient voltages were found (Figs. 4, 5 and

Tables 6, 7). The maximum voltages and the appropriate

lifetimes are in the ranges typically found for organic solids.

From the quite similar rate constants k1 and k2 for the

deactivation of the photo-emf we assume that there exists only

a slight differentiation between near-surface and bulk areas of

these solids. From those observations one can conclude that

both compounds 1c and 3c behave like typical organic

photoconductors. In contrast, in the case of compound 1d,

no appreciable amount of free charges was detectable within

the timescale of the apparatus (tK . 50 ns). Most likely,

the photo-excited compound 1d, which emits in the range

.500 nm in the solid state, has fast emitting deactivation

Fig. 2 Principle for the origin of a photo-emf in combination with

the measurement arrangement (using an n-type photoconductor).

Labels— 1: transparent glass electrode, 2: insulating foils, 3: sample

(photoconductor), 4: metal rear electrode (ground).

Fig. 3 Photo-emf signals of powder samples of 1c (curve A), 1d

(curve B) and the Pd analogue 2b (curve C); lexc. = 337 nm,

approximately 2.7 6 1013 quanta/flash, virgin samples.

Table 5 Parameters of the photo-emf signals of 1c (acquisition),virgin sample

UMAX/mV U10/mV U2

0/mV k1/s21 k2/s21

+0.70 +51.2 250.5 41.1 40.5

Fig. 4 Photo-emf signals of powder samples of 3c with respect to

the side of the sample. Upon welding the material in between the

polyethylene foils one side of the sample partially melts. These sample

sides will be named melted side, otherwise non-melted.

Table 6 Parameters of the photo-emf signals of 3ca

Conditions (curve) UMAX/mV U10/mV U2

0/mV k1/s21 k2/s21

1. flash (black) 22.1 2447 445 45.4 45.2acq. (red) 21.8 2388 386 45.4 45.210 flashes (green) 22.0 2292 290 44.3 44.01. flash (blue)b ca. 20.2 — — — —acq. (purple)b ca. 20.2 — — — —a See Fig. 4. b Melted side.

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channels, whereas compounds 1c and 3c, which exhibit

almost no luminescence in the solid state above 500 nm (low

intensity), do not have such effectively competing deactivation

channels so that exciton dissociation yielding free charge

carriers becomes important (see Table 4). For compound 3c

the comparatively highest maximum voltage was measured in

these series. Furthermore, a dependence of this maximum

voltage on previous thermal treatments of the sample

was found. Sides of the samples without previous thermal

treatment (non-melted side) show higher transient voltages

than the thermally treated sides (melted side) as shown in

Fig. 4.

However, heating the complete sample of 3c above the

melting point into the liquid phase, followed by slow cooling to

room temperature, results in an even higher value for the

maximum transient voltage, UMAX. Once again, this maximum

voltage is influenced by the difference in the preceding thermal

treatment of both sides of the sample, i.e. the ‘top-side’ without

contact to the external heating stage shows a higher maximum

voltage (Fig. 5).

For comparison, we also have investigated the related

palladium compounds 2. As expected, none of the organo-

palladium derivatives gave any photo-emf signal (see for

example 2b in Fig. 3), similar to the observation of no

luminescence at longer wavelength after electronic excitation.

One can conclude that d8–d8 closed shell interactions caused

by relativistic effects of d8-configured square planar coordi-

nated Pt(II) atoms23,40 are essential for the occurrence of a

photo-emf as well as for the luminescence at longer wavelength

in the solid state in both series of the investigated mononuclear

ortho-platinated liquid crystals.

Summary

All presented platinum compounds exhibit luminescence in

the solid state and in solution. In these materials, both the

wavelengths and the intensities strongly depend on the

structure of the ligands, i.e. number, position and length of

the attached alkyl substituents. With decreasing the number or

with partially shorter alkyl chains, a closer packing of the rigid

organoplatinum central units enhances the Pt…Pt interactions

via the formation of dimers, which, in turn, results in a

stronger emission of light (higher intensity) in the solid state.

The same substitution patterns by formation of aggregates

lead to luminescence at longer wavelength also in solution. In

addition, some of the synthesized organoplatinum derivatives

(e.g.: 1c and 3c) exhibit transient voltages after pulsed

exposure with actinic light (photo-emf), i.e. they behave as

photoconductors or organic semiconductors. Such materials

are desirable for a variety of applications such as optical

storage materials or in organic light emitting diodes

(OLEDs).2 Within the series of organoplatinum compounds

a competition between emissive deactivation (luminescence)

and (photo)conductivity was observed. Compounds emitting

intense luminescence above 500 nm in the solid state as

well as in solution are non-conductive, whereas comparatively

less or non-luminescent materials in the solid state exhibit

transient voltages after pulse exposure, i.e. they act as

photoconductors.

Materials combining liquid crystalline behavior with

emissive and photoconductive properties, such as the here

presented ortho-platinated metallomesogens, are promising

candidates for potential technological applications, as liquid

crystalline phases can be used for the fabrication of large,

highly ordered, thin films for the use in solar cells and other

optical devices.45

Experimental

Methods

Electronic absorption spectra were obtained on a Shimadzu

3101 PC UV-vis-NIR spectrophotometer. Emission measure-

ments were taken on a Perkin-Elmer LS 50B fluorescence

spectrometer (lexc. = 380 nm). For the investigations in

solution the materials were dissolved in 1,2-dichloroethane

(concentrations ca. 4.8–8.8 6 1026 mol l21). Photo-emf

measurements were performed using a home-built apparatus

consisting of a nitrogen pumped dye laser UDL 210

(Lasertechnik Berlin, .30 mJ per pulse, lexc. = 337 nm, 2.7 61013 quanta/flash, tK = 0.5 ns, registration via 200 MHz

digital storage oscilloscope (Philips PM3394). All investigated

compounds (1a–g, 2a–g, and 3a–c) were purified by column

chromatography and repeated crystallization/filtration proce-

dures using pure solvents to remove any ionic and insoluble

inorganic and organic impurities. Purity was checked by

NMR, MS, and elemental analysis. DSC as well as polarized

optical microscopy showed sharp, constant phase transitions

for all compounds (liquid crystalline and crystalline solid), and

did not reveal traces of impurities. Prior to photo-emf

measurements, powdered samples of the compounds investi-

gated were placed between clean polyethylene foils and welded

Fig. 5 Photo-emf signals of compound 3c after heating above the

melting point (ca. 100 uC) and further cooling to room temperature.

The side of the sample which was placed directly on the heating stage is

named as ‘bottom-side’, otherwise ‘top-side’.

Table 7 Parameters of the photo-emf signals of 3c after melting andcooling to room temperaturea

Conditions (curve) UMAX/mV U10/mV U2

0/mV k1/s21 k2/s21

1. flash (black)b 20.7 21.1 0.4 71.6 71.5acq. (red)b 20.7 279.3 78.6 45.7 45.3acq. (green) 22.4 2420.0 418.0 47.5 47.2a See Fig. 5. b ‘Bottom-side’.

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inside without previous thermal treatment according to a

published procedure.42–44

Syntheses

1-(3,4-Didecyloxyphenyl)-3-(3,4,5-trihexyloxyphenyl)pro-

pane-1,3-dione. NaH (23.8 mmol, 0.71 g of an 80% suspension

in paraffin oil) was suspended in dimethoxyethane (25 ml).

3,4,5-Trihexyloxyacetophenone (11.9 mmol, 5.0 g) dissolved in

dimethoxyethane (25 ml) was added. After stirring for 10 min

at room temperature a solution of ethyl 3,4-didecyloxybenzo-

ate (11.9 mmol, 5.5 g) in dimethoxyethane (25 ml) was added

dropwise with stirring at 25 uC. The resulting mixture was

heated to reflux with stirring until the reaction was complete

(TLC, ca. 6 h). After cooling to 20 uC, water (2 ml) was added

dropwise with stirring, followed by HCl (10% aqueous) to

adjust the solution to pH = 2. The solvent was removed

in vacuo, and the residue was dissolved in diethyl ether (100 ml).

The solution was washed twice with water (50 ml) and dried

over Na2SO4. After removal of the solvent under reduced

pressure, the residue was purified by repeated recrystallization

from ethanol to yield 1.1 g (11%) 1-(3,4-didecyloxyphenyl)-3-

(3,4,5-trihexyloxyphenyl)propane-1,3-dione: mp 45 uC; 1H

NMR (400 MHz, CDCl3) d 17.10 (s, 1H, enol-OH), 7.58–

7.55 (m, 2H, H-ph), 7.17 (s, 2H, H-ph), 6.91 (d, 3J(H,H) =

8.4 Hz, 1H, H-ph), 6.67 (s, 1H, enol-CH), 4.51 (s, traces, keto-

CH2), 4.09–4.01 (m, 10H, CH2O), 1.88–1.71 (m, 10H,

OCH2CH2), 1.54–1.46 (m, 10H, OCH2CH2CH2), 1.38–1.22

(m, 36H, CH2), 0.93–0.82 (m, 15H, CH3); 13C NMR (100 MHz,

CDCl3) d 185.64, 184.48, 153.37, 153.28, 149.21, 142.48,

130.65, 128.40, 121.37, 112.54, 112.40, 106.18, 92.20, 73.60,

69.56, 69.50, 69.13, 31.81, 31.63, 31.48, 30.19, 29.52, 29.50,

29.48, 29.46, 29.32, 29.27, 29.24, 29.17, 29.02, 25.92, 25.88,

25.65, 25.60, 22.55, 22.50, 13.94, 13.92, 13.87; MS (EI), m/z

(%): 836 (100) [M]+, 752 (22) [M 2 C6H12]+, 417 (22), 321 (25),

277 (28), 137 (30).

1-(3,4-Didecyloxyphenyl)-3-(3,4,5-trihexyloxyphenyl)pro-

pane-1,3-dionatothallium(I). Prepared in analogy to a published

procedure36 from 1-(3,4-didecyloxyphenyl)-3-(3,4,5-trihexyl-

oxyphenyl)propane-1,3-dione (0.6 mmol, 0.5 g) and thallium

ethoxide (0.6 mmol, 0.15 g) in dry toluene (2 ml). Purified by

recrystallization from n-hexane, yield: 0.35 g (57%). Transition

temperatures/uC: K 75 (Colh 45) Iso; 1H NMR (400 MHz,

CDCl3) d 7.46–7.43 (m, 2H, H-ph), 7.06 (s, 2H, H-ph.), 6.83 (d,3J(H,H) = 8.9 Hz, 1H, H-ph), 6.31 (br s, 1H, CH), 4.03–3.95

(m, 10H, CH2O), 1.83–1.69 (m, 10H, OCH2CH2), 1.49–1.41

(m, 10H, OCH2CH2CH2), 1.32–1.25 (m, 36H, CH2), 0.89–0.84

(m, 15H, CH3); Elemental analysis: Calcd. for C53H87O7Tl: C,

61.29; H, 8.25. Found: C, 61.26; H, 8.22%.

[2-(4-Decyloxyphenyl-kC2)-5-heptylpyrimidine-kN]-1-

(3,4-didecyloxyphenyl)-3-(3,4,5-trihexyloxyphenyl)propane-1,3-

dionatoplatinum (1f). A suspension of 1-(3,4-didecyloxyphe-

nyl)-3-(3,4,5-trihexyloxyphenyl)propane-1,3-dionatothallium(I)

(71.2 mmol, 73.9 mg) in dry CH2Cl2 (50 ml) was added at

once to a suspension of di-m-chlorobis[2-(4-decyloxyphenyl-

kC2)-5-heptylpyrimidine-kN]diplatinum19 (35.6 mmol, 45.6 mg)

in dry CH2Cl2 (30 ml), and the resulting mixture was stirred

at room temperature. After consumption of the starting

materials (TLC), the mixture was filtered and the solvent

was removed under reduced pressure. The crude material

was purified by column chromatography (silica gel; CH2Cl2–

ethanol, 10 : 0.2) and repeated crystallization from

ethanol–ethyl acetate (5 : 1) to yield 75 mg (73%) 1f as an

orange–yellow solid. Transition temperatures/uC: K 78 Colh134 Iso; (cis : trans y 1 : 1) 1H NMR (400 MHz, CDCl3) d

8.99, 8.96 (2d, 4J(H,H) = 2.7 Hz, 2.5 Hz, 1H, H-6 py), 8.56

(d, 4J(H,H) = 2.5 Hz, 1H, H-4 py), 7.72 (d, 3J(H,H) =

8.4 Hz, 1H, H-69 pyph), 7.68–7.57 (m, 2H, ar-H), 7.26

overlapping, 7. 22 (2d, 4J(H,H) = 2.3 Hz, 1H, H-39 pyph),

7.25, 7.20 (2s, 2H, ar-H), 6.91, 6.88 (2d, 3J(H,H) = 8.2 Hz,

8.6 Hz, 1H, ar-H), 6.71, 6.69 (2dd, 3J(H,H) = 8.6 Hz,4J(H,H) = 2.1 Hz each, 1H, H-59 pyph), 6.60, 6.59 (2s, 1H,

CH), 4.12–4.01 (m, 12H, OCH2), 2.63–2.57 (m, 2H,

pyCH2), 1.86–1.73 (m, 12H, OCH2CH2), 1.68–1.63 (m, 2H,

pyCH2CH2), 1.50–1.42 (m, 12H, OCH2CH2CH2), 1.34–1.25

(m, 56H, CH2), 0.91–0.84 (m, 21H, CH3); 13C NMR

(100 MHz, CDCl3) d 179.23, 178.52, 174.28, 161.48 (C),

158.01 (CH), 153.29, 153.20 (C), 153.05 (CH), 152.11,

149.18, 141.22, 140.07, 135.47, 134.63, 133.94, 132.81,

132.01, 130.09 (C), 128.14, 120.69, 115.04, 113.33, 112.77,

111.10, 110.85, 106.42, 106.10, 96.78 (CH), 73.53, 69.50,

69.42, 69.16, 67.76, 31.83, 31.67, 31.59, 31.55, 30.68, 30.50,

30.26, 29.54, 29.42, 29.25, 29.13, 28.95, 26.11, 25.94, 25.79,

25.66, 22.57 (CH2), 13.96, 13.89 (CH3); Elemental analysis:

Calcd. for C80H128N2O8Pt: C, 66.68; H, 8.95; N, 1.94.

Found: C, 66.85; H, 8.79; N, 1.73%.

[2-(4-Decyloxyphenyl-kC2)-5-heptylpyrimidine-kN]-1-

(3,4-didecyloxyphenyl)-3-(3,4,5-trihexyloxyphenyl)propane-1,3-

dionatopalladium (2f). Synthesized as described above

from di-m-chlorobis[2-(4-decyloxyphenyl-kC2)-5-heptylpyrimi-

dine-kN]dipalladium19 (45 mmol, 50 mg) and 1-(3,4-didecyl-

oxyphenyl)-3-(3,4,5-trihexyloxyphenyl)propane-1,3-dionato-

thallium(I) (90 mmol, 93.5 mg). Purified by column chromato-

graphy (silica gel; n-pentane–ethyl acetate, 6 : 0.75) and

repeated crystallization from ethanol–ethyl acetate (1 : 1) to

yield 100 mg (82%) 2f as a yellow solid. Transition

temperatures/uC: K 75 Colh 116 Iso; (cis : trans y 1 : 1) 1H

NMR (400 MHz, CDCl3) d = 8.75, 8.72 (2d, 4J(H,H) = 2.5 Hz

each, 1H, H-6 py), 8.52 (d, 4J(H,H) = 2.7 Hz, 1H, H-4 py), 7.67

(d, 3J(H,H) = 8.4 Hz, 1H, H-69 pyph), 7.64–7.51 (m, 2H, ar-

H), 7.25, 7.22 (2d, 4J(H,H) = 2.5 Hz, 2.3 Hz, 1H, H-39 pyph),

7.22, 7.15 (2s, 2H, ar-H), 6.88, 6.87 (2d, 3J(H,H) = 8.8 Hz,

8.6 Hz, 1H, ar-H), 6.69, 6.67 (2dd, 3J(H,H) = 8.6 Hz each,4J(H,H) = 2.0 Hz each, 1H, H-59 pyph), 6.54, 6.53 (2s,

1H, CH), 4.13–4.00 (m, 12H, OCH2), 2.60–2.54 (m, 2H,

pyCH2), 1.87–1.72 (m, 12H, OCH2CH2), 1.66–1.61 (m, 2H,

pyCH2CH2), 1.56–1.42 (m, 12H, OCH2CH2CH2), 1.36–1.22

(m, 58H, CH2), 0.92–0.84 (m, 21H, CH3); 13C NMR (100 MHz,

CDCl3) d 182.16, 182.05, 181.79, 181.70, 171.08, 160.73 (C),

158.39 (CH), 154.67, 154.56, 153.00, 152.97, 148.97, 135.66,

135.47, 134.77, 134.71, 133.07, 130.95, 130.91 (C), 127.44,

120.87, 116.03, 115.66, 113.66, 113.11, 112.55, 112.11, 111.79,

106.72, 106.34, 94.91, 94.80 (CH), 73.57, 73.54, 69.57, 69.45,

69.33, 69.18, 67.91, 67.85, 60.30, 31.82, 31.68, 31.59, 31.55,

30.54, 30.37, 30.25, 29.61, 29.53, 29.42, 29.34, 29.24, 29.18,

1814 | J. Mater. Chem., 2006, 16, 1808–1816 This journal is � The Royal Society of Chemistry 2006

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29.00, 28.96, 26.09, 25.94, 25.81, 25.79, 25.67, 22.56, 22.54,

22.52, 22.46 (CH2), 14.05, 13.93, 13.87 (CH3); Elemental

analysis: Calcd. for C80H128N2O8Pd: C, 71.05; H, 9.54. Found:

C, 71.28; H, 9.29%.

Acknowledgements

This work was supported by the Deutsche Forschungs-

gemeinschaft and the Fonds der Chemischen Industrie.

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