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Masters Theses & Specialist Projects Graduate School

5-2015

Fused Arene-Based Molecular Systems as Additivesfor Organic PhotovoltaicsRachana NeesuWestern Kentucky University, rachana.neesu372@topper.wku.edu

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Recommended CitationNeesu, Rachana, "Fused Arene-Based Molecular Systems as Additives for Organic Photovoltaics" (2015). Masters Theses & SpecialistProjects. Paper 1471.http://digitalcommons.wku.edu/theses/1471

i

FUSED ARENE-BASED MOLECULAR SYSTEMS AS ADDITIVES FOR

ORGANIC PHOTOVOLTAICS

A Thesis

Presented to

The Faculty of the Department of Chemistry

Western Kentucky University

Bowling Green, Kentucky

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science

By

Rachana Neesu

May 2015

iii

ACKNOWLEDGEMENTS

I would like to express the sincere gratitude to my research advisor, Dr. Hemali

Rathnayake, for providing with all the necessary facilities for the research and for the

valuable guidance and encouragement extended to me. Without her supervision and

constant help, nothing would have been possible.

I would like to thank Dr. Edwin Stevens and Dr. Yan for serving on my committee and for

their valuable time.

I also place on record, my sense of gratitude to my friends, one and all who directly or

indirectly, have lent their hand in this venture. I thank my parents Parvathi and

Nagabhushanam for their support and love.

iv

CONTENTS

CHAPTER 1

INTRODUCTION...................................................................................................1

1.1 Overview…………………………………………………………………..2

1.2 General Goals of the Research…………………………………………….3

CHAPTER 2

BACKGROUND.....................................................................................................4

2.1 Recent Advances of Small Molecules-Based Organic Solar

Cells.........................................................................................................................4

2.2 Basic Principles of Organic Photovoltaics……………………………………7

2.3 Organic Solar Cell Mechanism……………………………………………….7

2.4 Bulk Heterojunction Approach………………………………………………..8

2.5 Challenges of Small Organic Molecules……………………………………..11

CHAPTER 3

RESULTS AND DISCUSSIONS………………………………………………..12

3.1 Overview…………………………………………………………………......12

3.1. 1 Synthesis and Characterization of pyren-1-ylmethyl

5-bromothiophene-2-carboxylate, (1)…………………………………………….12

3.1.2 Photophysical Studies of Compound (1)………………………………...…14

3.2. Synthesizing and Characterization of anthracen-9-ylmethyl

5-bromothiophene-2-carboxylate, (2)…………………………………………….15

3.2.1 Photophysical Studies of Compound (2)……………………………………16

3.3 Synthesis and characterization of compound (3) pyren-1-ylmethyl 5-(5 [

v

(anthracen-9-ylmethoxy) carbonyl]thiophen-2-yl)thiophene-2-carboxylate, (3).....17

3.3.1 Photophysical properties of Compound (3)………………………………….19

3.4. Synthesis and characterization of pyren-1-ylmethyl

anthracene-9-carboxylate (4)……………………………………………………....20

3.4.1 Photophysical Properties of Compound (4)……………………………….....21

4.1 Synthesis of (3,6-bis(anthracen-9-yl)-9-phenyl-9H-carbazole by

Kumada coupling…………………………………………………………………..22

CHAPTER 4

4.1 Procedures……………………………………………………………………...24

Materials………………………………………………………………………….24

Characterization…………………………………………………………………..24

4.2 Preparation of compound 1 by DCC coupling………………………………....25

4.3 Preparation of compound 2 by DCC coupling…………………………………26

4.4 Preparation of compound 3 by Kumada coupling……………………………...27

4.5 Preparation of compound 4 by DCC coupling…………………………………28

4.6 Preparation of compound 5 by Kumada coupling……………………………...30

Conclusion……………………………………………………………………….....32

APPENDIX………………………………………………………………………....34

References…………………………………………………………………………..40

vi

LIST OF FIGURES

Figure 2.1: Representation of organic photovoltaics mechanism……………………….8

Figure 2.2: Bulk heterojunction approach……………………………………………….9

Figure 3.2: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excite at 270

nm) of compound (1) in chloroform solution…………………………………………..14

Figure 3.4: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excite at 345

nm) of compound (2) in chloroform solution…………………………………………..16

Figure 3.6: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excite at 345

nm) of compound (3) in chloroform solution…………………………………………..19

Figure 3.8: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excite at 276

nm) of compound (4) in chloroform solution…………………………………………..21

vii

LIST OF SCHEMES

Scheme 3.1: Synthetic route for the preparation of Compound (1) ………….…….12

Scheme 3.5: Synthetic route for the preparation of compound (2)…………………15

Scheme 3.5: Synthetic route for the preparation of compound (3)…………………17

Scheme 3.7: Synthetic route for the preparation of compound (4)…………………20

Scheme 3.8: Synthetic route for the preparation of compound (5)…………………22

viii

LIST OF TABLES

Table 1: Molecular systems with their power conversion efficiencies………………..6

ix

FUSED ARENE-BASED MOLECULAR SYSTEMS AS ADDITIVES FOR

ORGANIC PHOTOVOLTAICS

Rachana Neesu May 2015 43 Pages

Directed By: Dr. Hemali Rathnayake, Edwin Stevens, Bangbo Yan

Department of Chemistry Western Kentucky University

Organic photovoltaics (OPVs) are mainly based on organic semiconducting small

molecules, macromolecules, and polymers, which form an active layer in photovoltaics.

They act as an active material in absorbing light and causing charge mobility to generate

electricity from sunlight. This thesis describes the molecular systems derived from fused

arenes such as anthracene, pyrene, carbazole and thiophene for use as either a donor or an

acceptor component of the active layer of OPVs. Two novel molecular systems (9-

anthracenecarboxy-1-methylpyrene, (1) and Py-bi-TH-ANT, (2) were prepared using

Steglich esterification and Grignard metathesis followed by Kumada coupling. The

molecular structure of each was confirmed by 1H-NMR and IR analysis respectively. The

photophysical properties of the products were also evaluated in solution. The potential

applicability of these two novel systems for OPVs will be studied in the future.

1

CHAPTER 1

INTRODUCTION

1.1 Overview

There is an increasing demand for photovoltaic technology with its properties of

being efficient and economical. This leads to increased interest in the development of

new solar cells. The types of solar cells include dye sensitized, organic and quantum dot

solar cells.1-4 Of all the types, organic solar cells have an upper hand in being

economical, as it utilizes materials which are abundant, ease of manufacturing and

potential for combining different technologies.5-8 Organic solar cells, using the

phenomenon of the photovoltaic effect, absorbs light from sunlight, resulting in charge

transportation, thus generating electricity.9-10

The use of organic materials in photovoltaics dates back to 1980.11 Organic

molecules and polymers, besides their advantages of being lightweight,12 flexibility,13

and easily fabricated,14 also have the advantage of being simple, in terms of chemical

modifications, to change their properties, especially the optical band gap.15, 16 For

several decades, organic solar cells have been constructed using polymers either as light

harvesting electron donors or acceptors. Polymers possess outstanding properties like

high optical density, optimized film morphology and other unique properties.17 Polymers

with their positive unique features, also hold negative traits like polydispersity, lack of

reproducibility in synthesis, batch-to-batch variation, and difficulty in purification.18 All

these factors lead to consideration of small molecules as alternatives.19 Small molecules

have advantages like ease of synthesis, molecular structures that are well ordered, high

charge carrier mobility, flexibility and ease of purification.20 Conjugated small

2

molecules have resilient intermolecular forces that enable electron delocalization, which

is an important factor for efficient charge transport. Initially, small molecules were not

competitive with polymers because of their low photovoltaic performance. Recently,

Heliatek, a German company, reported a tandem solar cell featuring small molecules

with a remarkable cell efficiency of 12% for a standard size of 1.1cm2.21 Based on this

development, there is hope that small-molecule organic solar cells will overtake their

polymer counterparts.

Anthracene, thiophene, pyrene, and carbazole are of particular interest as the

active organic layer in solar cells. Anthracene has been widely used in polymer based

solar cells, 22 dye sensitized solar cells, and organic thin film transistors. Anthracene has

a planar structure and is highly crystalline in nature. These factors could be the reason

for the higher carrier mobility and higher energy conversion efficiency, and therefore the

high power conversion efficiency (PCE) in small molecule organic solar cells

(SMOSCs). Thiophene based conjugated-π oligomers and polymers have been widely

used for their chemical and environmental stability and electronic tenability.23

Thiophenes are promising for designing and synthesizing new conjugated material

because they possess outstanding optic, electronic, and redox properties as well as

supramolecular behavior on solid surfaces or in bulk. Thiophene can be modified at the

β-positions, or even at the sulfur atoms to create flexible structures.

3

1.2 General Goals of the Research

The general goal of my research is to synthesize conjugated molecular materials

that are highly functionalized through structural modifications in order to improve their

photovoltaic properties. The main goal is to synthesize fused-arene based novel organic

compounds, which have efficient charge carrying mobility’s and can improve the power

conversion efficiencies in small molecule based organic solar cells (SMOSCs). This

thesis describes studies on four different fused-arene systems. These molecular systems

are mainly derived from thiophene, anthracene, pyrene, and carbazole derivatives.

4

CHAPTER 2

BACKGROUND

2.1 Recent advancements of small molecules-based organic solar cells

Jiaoyan Zhou, et al., (2012), reported the synthesis of two small molecules,

namely DCAO3TBDT and DR3TBDT, with two ethoxy substituted benzo[1,2-b:4,5-

b′]dithiophene (BDT) as the central building block and octyl cyanoacetate and 3-

ethylrhodanine as different terminal units with the same linkage of dioctyltert-thiophene.

DR3TBDT showed a PCE as high as 7.38%.24

Ye Rim Cheon, et al., (2014), synthesized the novel small molecule 2,7-bis(1,3-

di(thiophenyl)-5-octyl-4H-thieno [3,4-c]pyrrole-4,6[5H0-dione)-5,10-bis (4,5-

didecylthiophen-2-yl)benzol[1,2-b:4,5-b’]-dithieno [3,2-b] thiophene (DTBDT-TTPD).

This compound has an acceptor-donor-accepter (A-D-A) structure with DTBDT as the

core donor unit and thiophene-bridged thienopyrroledione (TPD) as the acceptor units.

This framework showed a small optical band gap with a low HOMO level with a high

hole mobility. The PCE was 4.98%.25

Pierre-Antoine Bouit, et al., (2013) reported the synthesis, optical and electrical

properties of new push pull molecules based on a 10-(1,3-dithiol-2-ylidene)-anthracene

core. Density functional theory and X-ray diffraction were used for the characterization

of the optical and electrochemical properties and molecular structures. They described

their application in dye solar cells. In this dye, the 10-(1,3-dithiol-2-ylidene)-anthracene

group acts as an electron donor and a conjugated π-bridge, 3,4-ethylenedioxythiophene

(EDOT) or phenyl and two more accepting units of an ester group and a

dicyanovinylene group for other molecules.26

5

Vellaiappillai Tamilvan, et al.,22 reported a novel anthracene derivative bearing

strong electron donating triphenylamine and thiophene units at the 9, 10 and 2,6

positions of anthracene. They were interested in incorporating cyano functional group on

the backbone. Among the reported cyano functional groups, the tricyanovinyl (TCV: -

C(CN)=C(CN)2) group was found to be efficient at lowering the band gap of organic

molecules, compared to monocyano (-CH=CHCN), dicyano (-CH=C(CN)2 AND DCBP)

and tricyanofuran (TCF) groups. Their studies showed that the band gaps of the

relatively less π-conjugated molecules are lowered with the use of the tricyanovinyl

(TCV) group. They investigated the linkage position by attaching the TCV group at the

phenyl group of the triphenylamine unit or at the thiophene unit of 9, 10 and 2,6-linked

anthracene-based small molecules bearing TCV at two different postions.22

Molecular structures of above systems are summarized with their power conversion

efficiencies in Table 1.

6

Table 1: Molecular systems with their power conversion efficiencies.22, 24-26

Compound

DTBDT-TTPD

A/D

A-D-A

PCE

4.98%

TCV-Tpa2,6T

A-D

1.67%

TCV-Tpa9,10T

A-D

1.04%

TCV-Tpa2,6TCV

A-D

1.95%

R1= 2-ethylhexyl

R2=n-octyl

D

7.3%

S

SS

S

NO

O

S

C8H17

S

C10H21

C10H21

S

SS

S

NO O

C8H17HS

C10H21C10H21

S

N

S

NC

CNNC

N

NC

CNNC

S

NS

NC

CNNC

OC2H7

C2H7O

CN

NCCN

NS

S

O S

SR2

S

R2

S

O

OR1

R1

SS

R2

SS

R2

SN

O

S

7

2.2 Basic Principles of Organic Photovoltaics

There was a general perception about organic solar cells being indolent for

electrical conductivity due to the presence of strong covalent bonds. But this assumption

was changed with the discovery of conducting polymers by H. Shirakawa et al. in

1977.27 They exposed trans-polyacetylene to chlorine, bromine or iodine vapor, which

resulted in a dramatic increase in electrical conductivity. This discovery led to wide-

ranging applications including organic semiconductors, optical displays, organic LEDs,

microelectronics, and organic photovoltaics.

Conjugated π- electron systems play a major role in organic photovoltaics since π-

electrons are greatly mobile compared with σ-electrons. Conjugated systems usually

consist of an alteration between double bonds and single bonds. Therefore, when energy

is absorbed by the solar cells in the form of photons, excitons are generated. The

Molecular π-π* gap corresponds to the Highest Occupied Molecular Orbital (HOMO)

and Lowest Unoccupied Molecular Orbital (LUMO) energy level difference.

2.3 Organic Solar Cells: Mechanism

A successful organic solar cell has five important steps to attain effective power

conversion efficiency.

1. Light absorption and generation of excitons

2. Exciton diffusion to an active surface

3. Separation of charges

4. Transport of the charges

5. Charge collection

8

The photoactive material (electron and donor) is sandwiched between the electrodes, in

which one electrode is transparent in order to collect the photo-generated charges. These

charges are separated and then they are transported to the respective electrodes. These

charges are directed towards the external circuit at the electrodes.

Fig 2.1. Representation of mechanism of organic photovoltaics

2.4 Bulk heterojunction approach

The bulk heterojunction was introduced by Tang in 1986. Tang defined a two-

layered device that had copper phthalocyanine (CuPc) as the donor and a perylene

tetracarboxylic derivative (PV) as the acceptor. The device had a power conversion

efficiency of 1%.28

To convert the solar energy into electrical energy, the generation of charges

(positive and negative), that is holes and electrons, is essential. A potential force is also

required for carrying these charges towards an external circuit. When light is absorbed in

the form of photons by organic semiconductors the result is the generation of a strongly

9

bound electron-hole pair called an exciton. These excitons diffuse to the dissociation

sites where the charges are separated.29 A dissociation site is found at the interface

between the donor and acceptor material. In the charge transfer process, the holes exist

in the material at the HOMO level and electrons exist in the material at the LUMO level.

The electrons and holes are still bound by the coulomb interaction at the D/A interface.

After breaking the binding energy of the bound electron-hole pair, the free electrons and

holes travel through the acceptor and donor phase, respectively, to the opposite

electrodes of the device.29 To obtain efficient photon to charge conversion efficiency,

many device architectures have been developed such as single layer cells, double layers,

and bulk heterojunction (BHJ) cells. The bulk heterojunction is a blend of conjugated

polymers of donors and acceptors. As the BHJ provides a larger D/A interface, it is

considered as the most efficient approach.

Fig 2.2. Bulk heterojunction approach

In the bulk heterojunction method, the electron donors (p-type) and electron

acceptors (n-type) are combined together in the active layer. It is an important factor that

the produced excitons must diffuse to the interface and undergo charge separation. The

10

diffusion length of excitons in organic semiconductors is very short ~ 10 nm to 100 nm.

This applies an important condition for the efficient charge generation.

The first materials to be used in bulk heterojunction cells were substituted

fullerenes (with the acronym PCBM) and DMOV-PPV (poly [2-methoxy-5-(3’-7’-

dimethyloctyloxy)-p-phenylene vinylene]). One of the challenges faced by the bulk

hetero junction approach is that the charges must be able to travel to the respective

electrodes through the combined material. As holes are transported by the p-type

material and electrons by the n-type material, these materials should be first mixed into a

bicontinuous interpenetrating network. In the Figure 2, the bulk heterjunction is

represented. The most commonly used n-type components are fullerene derivatives such

as P61BM ([6,6]-phenyl-C61 butyric acid methyl ester) and P71CBM due to their ability to

be soluble in organic solvents, their ability to undergo reversible electrochemical

reductions, and their property of low energy levels.30 Donor materials have undergone

substantial modification overt time resulting in a variety of polymers and molecules.31

The Bulk heterojunction is presently the most extensively used photoactive layer

for construction of organic solar cells. The name bulk-heterounction has been chosen

because the interface between the two dissimilar components (heterojunction) is

distributed throughout the bulk, which is opposite to the conventional bilayer junction. It

is an important factor that bulk heterojunction based solar cells are stable during the

process. Polymer-based solar cells have to be protected from the ambient air to prevent

degradation of the active layer and electrode material from effects caused by water and

oxygen. The materials must also be photochemically stable and the nanoscale uniformity

of the D/A blend has to be preserved.

11

2.5 Challenges of Small Organic Molecules

Small molecules, despite of their unique factors, have challenges including limited

solubility and tendency of aggregation in organic solvents. Solution casted small

molecules have the tendency to crystallize on a substrate leading to poor film formation

with polycrystalline domains, which are characterized by unfavorable grain connectivity

and suboptimal crystalline ordering.32

To overcome the challenges associated with small molecules, many researchers are

developing D-A molecular systems that lead to a well-defined long range D-A

heterojunctions through molecular D-A dyad interactions. In this active layer

construction, the two segments of donor and acceptor are constructed to be inter-spaced

in order to provide “highways” for electron and hole charge carriers leading to the

electrodes. The small molecular D-A dyads will be designed to improve the molecular-

polymer interactions (- interactions) to form two types of stacking, which consist of

either D-D and A-A or D-A and A-D arrangements.

12

CHAPTER 3

RESULTS AND DISCUSSION

3.1 Overview

Anthracene, thiophene, and pyrene have been promising as either donors or

acceptors in photovoltaics. The goal of this thesis work is to synthesize D-A dyads and

D-A-D triads. It is worthwhile to design molecular fragments by functionalizing the 5-

position of 5-bromo-2-thiophene carboxylic acid with pyrenemethanol and 9-antharcene

methanol. The products synthesized from these precursors were allowed to react by

Kumuda coupling to produce a C-C bonded conjugated structure. The 9-position of

anthracene was functionalized with a 1- pyrene unit. The 3,6- positions of carbazole

were functionalized with 2 units of anthracene. In this manner, three novel systems were

synthesized and characterized, System 1 (9-Anthracenecarboxy-1-methylpyrene),

System 2 (Pyrene-bi-Thiophene-Anthracene), and System 3 (Anthracene-carbazole-

anthracene). The photoluminescence behavior of these compounds was also studied.

3.1. 1 Synthesis and Characterization of pyren-1-ylmethyl 5-bromothiophene-2-

carboxylate – Compound (1)

OHS

BrCOOH

DCC/DMAP

DCM/RT/20 Hrs O C

O

S Br

Scheme 3.1: The Synthetic route for the preparation of Compound (1)

(1)

13

As depicted in Scheme 3.1, starting from 5-bromo-2-thiophenecarboxylic acid

and 1-pyrenemethanol, the Compound (1) was prepared using the well-known

esterification method called “Steglich esterification” in the presence of

dicyclohexylcarbodiimide (DCC).33 The reaction was performed in room temperature

under argon atmosphere to yield the crude product as a yellowish white solid. Further

purification was performed using column chromatography with a solvent mixture of

dichloromethane and hexane in a 3:2 ratios to give a white solid in 60% yield. The

structure and composition were confirmed by proton NMR and FTIR. The 1H- NMR

spectrum reveals the presence of the thiophene ring from the aromatic hydrogen peaks at

δ 7.02 (d) and δ 7.551 (d). The aromatic hydrogen peaks from the pyrene ring range

from δ 8.03 to δ 8.88 with two sets of multiplets and a set of doublet to doublet. The

presence of a strong intense singlet peak at δ 6.03 further confirms the successful

coupling of 1-pyrenemethanol with 5-bromo-2-thiophenecarboxylic acid. The FTIR

spectrum also supports the product formation from the presence of aromatic C-H

stretching frequencies at 3039 cm-1 and alkyl C-H stretching at 2920 cm-1. The presence

of an ester bond is confirmed from an ester carbonyl stretching found at 1695 cm-1.

Aromatic C=C stretching frequencies at 1527 cm-1 and 1460 cm-1 are also observed. The

peaks found at 1288 cm-1 and 843 cm-1 corresponds to the thiophene S-C stretching.

14

3.1.2 Photophysical Studies of Compound (1)

A photophysical study of compound (1) was carried out in solution phase. The

UV-Vis absorption spectrum was obtained in chloroform solution. As shown in Figure

3.2(a), two sets of distinct absorption peaks were observed. The lower absorption set

that ranges from 260 to 300 nm with absorption maxima of 265 nm and 276 nm, and the

higher absorption set ranging from 306 nm to 356 nm, reflects the presence of the

pyrene moiety in compound (1). In Overall, the absorbance spectrum of the compound

shows well-resolved absorption maxima of the pyrene unit with its three distinct higher

vibrational transitions at wavelengths of 313, 327, and 345 nm. The three vibrational

transitions are characteristic of pyrene absorbance itself.

Figure 3.2: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excitation at

270 nm) of compound (1) in chloroform solution.

The fluorescence emission spectrum of compound (1) in chloroform solution was

obtained and is shown in Figure 3.2 (b). The emission spectrum shows the expected

maximum emission at 396 nm with two well-resolved vibronic bands at 377 nm and 417

nm. These spectral patterns of the product agree well with the corresponding electronic

15

transitions of planar pyrene.34, 35 However, there is no evidence of a blue shift in the

spectrum due to the presence of thiophene unit.

3.2. Synthesis and Characterization of anthracen-9-ylmethyl 5-bromothiophene-2-

carboxylate – Compound (2)

(2)

Scheme 3.3 The synthetic route for preparation of compound (2)

As depicted in Scheme 3.3, starting from 5-bromo-2-thiophenecarboxylic

acid and 9-anthracene methanol, compound (2) was prepared in a similar manner to

compound (1) using the esterification reaction. The reaction was performed at room

temperature under an argon atmosphere to yield the crude product as a bright yellow

solid. Further purification was performed by washing the crude product with hexane to

give a bright yellow solid in 85% yield. The structure and its composition was

confirmed from proton NMR and FTIR. The 1H- NMR spectrum reveals the presence of

the anthracene unit from the aromatic hydrogen peaks consisting of two sets of

multiplets (δ 7.50 to 7.60), two sets of doublets (δ 8.02-8.05 and 8.39-8.41) and a singlet

at 8.54. Doublet from δ6.98 to 6.99 and a doublet at δ7.48-7.50, which overlaps with a

multiplet of anthracene peaks at δ7.50 -7.60, confirms the successful coupling of 5-

bromo-2-thiophenecarboxylic acid with 9-anthracenemethanol.The FTIR spectra also

support the product formation by the presence the ester group carbonyl stretching

SBr COOH

OH

DCC/DMAP

DCM , RT, 24hrs SBr C

O

O

16

frequency at 1704 cm-1. The peak at 1531 cm-1 is assigned to the aromatic C=C

stretching frequency. The peak at 2928 cm-1 corresponds to the alkyl C-H stretching.

The peaks at 1082 cm-1 corresponds to the ester C-O stretching frequencies. The S-C

bending was found at 822 cm-1 and 1247 cm-1.

3.2.1 Photophysical Studies of Compound (2)

Figure 3.4 (a) UV-Visible spectrum and (b) Fluorescence emission spectrum (excitation

at 345 nm) of compound (2) in chloroform solution.

The photophysical properties of the compound (2) were studied in chloroform

solution. As shown in the Figure 3.4 (a), the absorption spectrum exhibits typical spectral

characteristic features of anthracene with four characteristic bands at 330, 360, 380 and

400 nm, which correspond to vibronic transitions of S0-S1.36 The photoluminescence

emission spectrum of compound (2) excited at 345 nm shows peaks at 400 nm, 420 nm

and 440 nm with a shoulder peak at 460 confirming the presence of anthracene. There is

no indication of a spectral shift in either absorbance or photoluminescence due to the effect

of the thiophene ring.

17

The fluorescence emission spectrum of compound (2) in chloroform solution

was obtained and is shown in Figure 3.4 (b). The emission spectrum shows the expected

maximum emission at 420 nm with three well-resolved vibronic bands at 395 nm and

445 nm and a shoulder peak at 471 nm. These spectral patterns of the product agree well

with the electronic transitions of the anthracene unit. However, there is no evidence of a

blue shift in the spectrum due to the presence of thiophene unit.

3.3 Synthesis and characterization of pyren-1-ylmethyl 5-(5 [(anthracen-9-

ylmethoxy) carbonyl] thiophen-2-yl) thiophene-2-carboxylate - Compound (3)

O C

O

SBr S

Br C

OO Anhyd. THF/ 800C

t-BuMgCl

Reflux for 2 hours

Ni(dppp)Cl2in two portions at 30 min interval

1 hour Stirring

Quench with methanolO C

O

S SC

OO

(3)

Scheme 3.5 Synthetic route for the preparation of compound (3).

As shown in Scheme 3.5, compound (3) was prepared by Grignard metathesis

followed by a Kumada coupling reaction37 using compound (1) and compound (2) as

starting materials. Grignard reagent and Nickel catalyst, Ni(dppp)Cl2 transition metal

complex were used in the reaction. The Kumada coupling reaction is a type of cross

coupling reaction method used to generate carbon-carbon bonds. The Grignard reagent

18

used was t-butylmagnesium chloride and the nickel catalyst used was [1, 3-Bis

(diphenylphosphino)propane]–dichloronickel(II). Anhydrous tetrahydrofuran was used

as the solvent and the reaction was done under an argon atmosphere to yield an orange-

yellow colored solid. The product was purified by vacuum filtration using methanol to

yield the final product with 86% of yield. The 1H-NMR spectrum shows the presence of

pyrene with two sets of multilplets from δ 8.12-8.33, a doublet at δ 8.35. Two sets of

doublets at δ 6.99 and δ 7.03 are from the thiophene moiety (thiophene attached to

pyrene). A set of doublet of thiophene at δ 7.50 overlaps with the two sets of multiplets

of anthracene at δ 7.48 and δ 7.60. A singlet at δ 8.54 and doublet at δ 8.41 shows the

presence of anthracene. The peak at 1697 cm-1 corresponds to the ester carbonyl group

which confirms the esterification reaction. The ester C-O stretching frequencies is found

at 1082cm-1. The peak at 1412 cm-1 corresponds to the aromatic C=C stretching. The

peak at 1528 cm-1 corresponds to the aromatic C=C stretching. The alkyl C-H

stretchings are found at 2852 cm-1and 2924 cm-1. The peak at 843 cm-1 and 1247 cm-

1confirms the S-C stretching.

19

3.3.1 Photophysical properties of compound (3)

Figure 3.6: (a) UV-Vis spectrum and (b) Fluorescence emission spectrum (excitation at

345 nm) of compound (3) in chloroform solution.

The photophysical properties of compound (3) were studied in chloroform

solution. The absorption spectrum shows a broad absorbance ranging from 270 nm to

390 nm, which corresponds to both chromophores, pyrene and anthracene. As shown in

the Figure 3.6 (a), two sets of absorption with peaks maxima at 277 nm, 320nm, 330

nm, 345 nm corresponds to absorption from the pyrene ring, whereas the less intense

two peaks at 367 nm and 392 nm corresponds to anthracene absorption. Although

vibrational transitions of pyrene are well-resolved, the vibronic structures of anthracene

is poorly resolved.

The photoluminescence emission spectrum of compound (3) as shown in the

Figure 3.6 (b) when excited at 345 nm, compound (3) shows peaks at 376, 397, 418,

445 and 472 nm which corresponds to the anthracene ring. There should be a possibility

of pyrene emission overlapping with the emission of anthracene. So the peaks of pyrene

are not well resolved.34, 35

20

3.4. Synthesis and characterization of pyren-1-ylmethyl anthracene-9-carboxylate-

Compound (3)

(4)

Scheme 3.7 The synthetic route for the preparation of compound (4)

As depicted in Scheme 3.7, starting from 9-anthracenecarboxylic acid and 1-

pyrene methanol, the compound 4 was prepared using “Steglich esterification” 33 in the

presence of dicyclohexylcarbodiimide (DCC). The reaction was performed at room

temperature under an argon atmosphere to yield the crude product as a yellow solid.

Further purification was performed using column chromatography with a solvent

mixture of dichloromethane and hexane in a 3:2 ratios to give a pale yellowish solid in

66% yield. The structure and its composition was confirmed by proton NMR and FTIR.

The 1H NMR shows the presence of anthracene peaks as two sets of multiplets (doublet

of doublet) from δ 7.35-δ 7.41 and a doublet peak at δ 8.50 and δ 8.28. One set of

doublets from anthracene at δ 8.05 was overlapping with the pyrene peaks that occur

from δ 7.96-δ 8.30 as three sets of multiplets (doublet of doublets). Compound (4) was

also characterized by FTIR spectroscopy. A peak at 3048 cm-1 corresponds to aromatic

C-H stretching. The ester carbonyl peak was observed at 1715 cm-1. Peaks at 1196 cm-1

and 1150 cm-1 corresponds to ester C-O stretching frequencies. The peaks at 1417 cm-1

and 1525 cm-1 corresponds to the aromatic C=C stretching.

COOH

OH DCC / DMAP

DCM, RT , 20 hrsC

O

O

21

3.4.1 Photophysical properties of compound (4)

Figure 3.8: (a) UV- Vis spectrum and (b) Fluorescence emission spectrum (excitation

at 276 nm) of compound (4) in chloroform solution.

The absorption and photoluminescence properties of compound 4 was measured

in chloroform solution. As shown in the Figure 3.8 (a), there is a broad absorption

spectrum ranging from 270 to 390 nm representing the chromophores of pyrene and

anthracene. The two sets of peaks at 270, 316, 330, and 347 nm represents the

absorption from the pyrene and the two peaks at 365 nm and 387 nm represents the

absorption from anthracene. Although the vibrational transitions of pyrene are well

resolved, the anthracene transitions are not well resolved.

The fluorescence emission spectrum was obtained by exciting at 276 nm to give

a broad peak with an emission maxima at 467 nm. The emissions of anthracene and

pyrene are not well resolved and they occur as a single combined peak.

22

4.1 Synthesis of (3,6-bis(anthracen-9-yl)-9-phenyl-9H-carbazole by Kumuda

coupling-Compound (5)

Br

2

N

Br

Br

Anhy. THF/ 800C

t-BuMgCl

Reflux for 2 hours

N

(5)

Scheme 3.8: Synthetic route for the preparation of coompund (5)

As shown in the Scheme 3.8 compound (5) was prepared by Grignard metathesis

followed by a Kumada coupling reaction37 using 3,6-dibromo-9-phenyl carbazole in 1

equivalent and 9-bromoanthracene in two equivalents. The Grignard reagent used was t-

butylmagnesium chloride and the nickel catalyst used was [1, 3-Bis (diphenylphosphino)

propane]-dichloronickel(II), which is similar to the preparation method of compound

(3). Anhydrous tetrahydrofuran was used as the solvent and the reaction was done under

an argon atmosphere to yield an orange brownish colored solid. Based on the NMR data

it was confirmed that the product was not coupled successfully. Although the reaction

was repeated multiple times by changing the reaction parameters, all attempts to make

the product with both side coupled with anthracene units were unsuccessful. This may

be due to the steric hindrance of the molecule since both anthracene and carbazole are

bulky in their structure.

Ni (dppp)Cl2 added

in 2 portions at 30

minutes interval

1 hour stirring

Quench with

methanol

23

In overall, four compounds (1-4) were successfully synthesized and the procedures

developed to make these four compounds in gram scale. The future work on this project

will target studying the solid state molecular packing, as well as the electronic

properties, of these compounds, with the goal of using them as novel components of

organic solar cells.

24

CHAPTER 4

EXPERIMENTAL PROCEDURES

4.1 Procedure

Materials:

5-bromo-2-thiophenecarboxylic acid, 1-pyrenemethanol, dicyclohexylcarbodiimide

(DCC), 4-dimethylaminopyridine (DMAP), dichloromethane (DCM), 9-anthracene

carboxylic acid, 9-anthracene methanol, anhydrous tetrahydrofuran, methanol,

chloroform-D, bis(diphenylphosphino)propane]nickel(II) chloride, tert-butylmagnesium

chloride, 3,6-dibromo-9-phenylcarbazole, 9-bromonthracene were obtained from

Aldrich Chemicals.

Characterization:

A 500 MHz Jeol NMR spectrometer was used for recording the 1H NMR spectra using

deuterated chloroform. Perkin-Elmer Spectrum One FTIR was used for recording the IR

spectra. Perkin Elmer LS 55 spectrometer was used for recording the Fluorescence

spectra and Perkin Elmer, Lambda 35 spectrometer was used for recording absorbance

spectra.

25

4.2 Preparation of compound (1) by DCC coupling

O C

O

SBr

To a 100 mL three necked round bottom flask, 5-bromo-2-thiophene carboxylic

acid (1 eq, 0.52 g, 2.50 mmol), 1-pyrene methanol (1 eq, 0.58 g, 2.50 mmol),

dicyclohexylcarbodiimide (1.2 eq, 0.61 g, 3.0 mmol), and 4-dimethylaminopyridine (20

mg) were added. The flask was flushed with argon and degassed for 5 minutes. Under

argon atmosphere dichloromethane (~30 mL) was added with the help of a syringe

through one of the septa and the solution was stirred for 16 hours at room temperature.

A cloudy yellow colored solution was formed after 16 hours of stirring. The solution

was filtered and the filtrate was concentrated under vacuum evaporation to yield a

yellow solid (0.47 g). The precipitate on the filter paper was a white solid (0.62 g),

which was confirmed as excess DCC and a by-product of DCC. The proton NMR was

performed for both samples in deuterated chloroform. The NMR of the filtrated solid

showed an excess of pyrene methanol was present, and thus purification was needed.

TLC was performed for the samples 5-bromo-2-thiophene carboxylic acid, pyrene

methanol and the vacuum evaporated product, which were labelled as 1, 2 and 3

respectively on the TLC plates. These samples were dissolved in small amounts in DCM

and were spotted on TLC plates. Two TLC jars were prepared. In the first jar, the

solvent used was DCM and in the second jar the solvent mixture used was DCM:

Hexane in 3:2 ratio. Separation of spots was clearly noticed under UV-lamp on the TLC

26

plate, which used only DCM as solvent. The product was purified using column

chromatography using DCM as a solvent eluent to yield 0.42 g (60%) as a white solid.

1H NMR (500 MHz, CDCL3): δ 8.33 (d, 1H), 8.17-8.22 (m, 4H), 8.03-8.12 (m, 4H),

7.55 (d, 1H), 7.03 (d, 1H), 6.03 (s, 2H). FTIR (cm-1): 3039 (aromatic C-H stretching),

2920 (alkyl C-H stretching), 1695 (ester carbonyl stretching), 1527 (aromatic C=C

stretching), 1460 (aromatic C=C stretching), 1288 (thiophene S-C stretching), 843

(thiophene S-C stretching).

4.3 Preparation of compound (2) by DCC coupling

To a 100 ml three necked round bottom flask, 9-anthracene methanol ( 1 eq, 0.5 g,

2.4 mmol), 5-bromo-2-thiophene carboxylic acid (1 eq, 0.49 g, 2.3 mmol),

dicyclohexylcarbodiimide (1.2 eq, 0.59 g, 2.8 mmol), and 4-dimethylaminopyridine (20

mg) were added. The flask was flushed with argon and degassed for 5 minutes. Under

argon atmosphere, dichloromethane (~30 mL) was added to the flask through one of the

septa with the help of a syringe and the solution was stirred for 16 hours at room

temperature. A white suspension in a yellow colored solution was formed after 16 hours

of continuous stirring. The solution was filtered and the filtrate was concentrated under

vacuum evaporation to yield a yellow solid (0.59 g). The precipitate on the filter paper

was a yellowish white solid (0.54 g). Hexane washing was done for both the solids.

NMR was performed on both the precipitate and the filtrate. Since both portions

SBrC

O

O

27

contained the product, they were combined and further purification was performed. The

product was then washed with DI water followed by suction filtration to yield a yellow

solid of 0.81 g (85%).

1H NMR (500 MHz, CDCl3): δ 8.54 (s, 1H), 8.43 (d, 2H), 8.05 (d, 2H), 7.59-7.60 (m,

2H), 7.48-7.50 (m, 3H), 6.99 (d, 1H), 6.34 (s, 2H). FTIR (cm -1) 2928 (alkyl C-H

stretching), 1704 (ester carbonyl stretching), 1531 (aromatic C=C stretching), 1247

(thiophene S-C stretching), 1082 (ester C-O stretching), 822 (thiophene S-C stretching).

4.4 Preparation of compound (3) by Kumada coupling

O C

O

S SC

OO

One equivalent of compound (1) (0.39 g, 8.7 mmol) and one equivalent of

compound (2) (0.35 g, 8.7 mmol) were added to a 100 ml three-necked flask. The flask

was flushed with argon for 10 minutes and 30 mL of anhydrous THF was then added. A

clear solution was observed. This flask was immersed in an oil bath maintained at 800 C.

Tertiarybutylmagnesiumchloride (0.95 ml) was added to the round bottom flask through

a syringe slowly, drop-wise for 15 minutes, which turned the solution slight red and

cloudy. The flask was refluxed for 2 hours and then was cooled to room temperature.

The solution turned to dark brownish yellow. Bis(diphenylphosphino)propane]nickel(II)

chloride was added in two portions. The first portion of [1,3-

Bis(diphenylphosphino)propane]nickel(II) chloride (0.015 g, 0.28 mmol) was dissolved

in a few mL of dry THF and was added to the flask by syringe. The second portion

28

(0.007 g, 0.12 mmol) dissolved in a few mL of dry THF was added after half an hour

and stirring was continued for an hour. The reaction was quenched with few drops of

methanol. After 15 minutes, 30 ml of methanol was added to the solution and the

precipitate was vacuum filtered and dried to yield a brownish yellow solid of (0.62 g)

(86%).

Characterization was done using FTIR, and proton NMR.

1H NMR, (CDCL3, 500MHz): δ 8.54 (s,1H), 8.41 (d,1H), 8.35 (d, 1H), 8.24 (m, 5H),

8.12 (m,7H), 7.60 (m, 1H), 7.50 (d, 1H), 7.48 (m, 3H), 7.03 (d, 1H), 6.09(d, 1H), 6.34 (s,

2H), 6.03(s, 2H). FTIR (cm-1): 2924 (alkyl C-H stretching), 2852 (alkyl C-H stretching),

1528 (aromatic C=C stretching), 1412 (aromatic C=C stretching), 1247(thiophene S-C

stretching), 1082 (ester C-O stretching), 843 (thiophene S-C stretching).

4.5 Preparation of compound (4) by DCC coupling

To a 100 ml three necked round bottom flask, 9-anthracenecarboxylic acid (1

eq, 0.5 g, 2.2 mmol) and 30 mL dichloromethane (~30 mL) were added to the flask

through one of the septa with the help of a syringe. Approximately 5 mL of anhydrous

THF was also added .The mixture was allowed to stir for 10 minutes, yeilding a clear

solution. 1-Pyrene methanol (1 eq, 0.51 g, 2.1 mmol) was dissolved in a small amount of

DCM and added to the flask. Dicyclohexylcarbodiimide (1.2 eq, 0.54 g, 2.6 mmol), and

O

29

4-Dimethylaminopyridine (20 mg) were added in a similar way by dissolving them in

small amounts of DCM under an argon atmosphere. The flask was flushed with argon

and degassed for 5 minutes. The solution was stirred for 24 hours at room temperature

and the solution turned from a clear yellow solution to a cloudy yellow solution. The

precipitate on filter paper was a yellow solid (0.36 g), which was confirmed as excess

DCC based on the proton NMR. The proton NMR of the filtered solid showed an excess

of pyrene methanol and thus purification was needed. TLC was performed on samples of

9-anthracene carboxylic acid, pyrene methanol and the vacuum evaporated product,

which were labeled as 1, 2 and 3 respectively on TLC plates. These samples were

dissolved in small amounts in DCM and were spotted on TLC plates. The solvent

mixture used was DCM: Hexane in a 3:2 ratio. Separation of spots was clearly observed

under UV-lamp illumination of the TLC plate. The product was purified using column

chromatography using DCM:Hexane as a solvent eluent to yield 0.34 g (66%) as a

yellow solid.

1H NMR (500 MHz, CDCl3): δ 8.30 (d, H), 8.10 (d, H), 8.18-8.23 (m, 4H), 8.10 (d,

2H), 8.02-8.12 (m. H), 7.96-7.99 (m, 4H), 7.46-7.52 (m, H), 7.37-7.41 (m, 2H), 7.34-

7.36 (m, 2H), 6.38(s, 2H). FTIR (cm-1): 3048 (aromatic C-H stretching), 1715 (ester

carbonyl stretching), 1525 (aromatic C=C stretching), 1417 (aromatic C=C stretching),

1196 (ester C-O stretching), 1150 (ester C-O stretching).

30

4.6 Attempted preparation of compound (5) by Kumada coupling

One equivalent of 3,6-dibromo-9-phenyl carbazole (0.45 g, 1.1 mmol) was added to a

100 ml three-necked flask. The flask was flushed with argon and 30 mL of anhydrous

THF was then added and a clear solution was observed. Tertiary butyl magnesium

chloride (2.31 mL) was added to the round bottom flask through syringe slowly, drop-

wise for 15 minutes and was refluxed with a condenser for 2 hours with the flask

immersed in an oil bath maintained at 800 C. After 2 hours of stirring, a clear yellow

solution was observed. Through one of the septa, 2 equivalents of 9-Bromoanthracene

(0.58 g, 2.2 mmol) dissolved in THF were added with a syringe. A clear intense yellow

color was observed. The solution was allowed to stir for an hour. [1,3-

Bis(diphenylphosphino) propane]nickel(II) chloride was added in two portions. After the

flask was cooled, the first portion of [1,3-Bis(diphenylphosphino) propane]nickel(II)

chloride (0.015 g) dissolved in few ml of dry THF was added to the flask via syringe.

The second portion (0.007 g in few ml of dry THF) was added after one hour. The

reaction was allowed to run for an hour and then a few drops of methanol were added to

quench the reaction. After 15 minutes, 30 mL of methanol was added to the solution,

which was then vacuum filtered and dried to yield a brownish yellow solid of (1.14 g).

N

31

Characterization was done by 1H NMR, which indicated the reaction had failed to

couple completely.

32

CONCLUSION

The aim of this project was to synthesize small molecule based additives for

organic solar cells. The structural properties of these conjugated small molecules have

been studied. A major concentration was the optimization of the materials, so that they

could be successfully used as additives for the active layer in solar cells. Synthesizing

the pure compounds was a major challenge of this project. The recent advances of small

molecule additives and their power conversion efficiencies have also been discussed.

In conclusion, two systems consisting of anthracene-bithiophene-pyrene (D-D-A)

triad assembly and anthracene-pyrene (D-A) assembly were proposed and synthesized

using the small molecule components of anthracene, thiophene and pyrene. The

synthesized compounds were purified and their structural and photophysical properties

were studied by NMR, UV-Visible spectroscopy, fluorescence spectroscopy and IR

spectroscopy. Compound (1) was short in conjugation length. It has been designed and

synthesized, and the absorbance of the compound was well resolved, but there was no

evidence of a blue shift in the spectrum due to the presence of thiophene. The

photophysical properties of compound (2) were studied, and the absorbance of

anthracene was observed corresponding to the vibronic transitions of S0-S1. Again, there

was no indication of a spectral shift in either absorbance or emission due to effect from

thiophene ring. Compound (3) has the longest conjugation length when compared to the

other compounds with two thiophene units, anthracene and pyrene units. The vibrational

transitions of pyrene were well resolved, but not those of anthracene. In the emission

spectra the peaks of pyrene were not well resolved because of the possibility of pyrene

overlap with anthracene.

33

In compound (4) the vibrational transitions of pyrene in the absorption spectra were well

resolved but the anthracene transitions were poorly resolved. The emission spectra of

pyrene and anthracene were poorly resolved. Synthesis of compound (5), despite

multiple attempts made by changing the experimental parameters, failed in the coupling

of the desired components.

Future work will focus on molecular packing studies and evaluating the electrical

properties of these compounds.

34

APPENDIX

1H NMR

1H NMR of compound (1).

35

1H NMR of compound (2)

36

1H NMR of compound (3)

37

1H NMR of compound (4)

38

FTIR

IR of compound (1)

IR of compound (2).

843

39

IR of compound (3)

IR of compound (4)

15

25

14

17

843

40

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