% - Conjugated Polymers for Electrochromic and Photovoltaic Applications

Tanmoy Dey, Ph.D.

University of Connecticut, 2010

Abstract

Electrochromism is a process by which a material can change its electronic-

optical properties upon charge injection/removal. Conjugated polymers are an

interesting class of electrochromic materials because of their color tunability, high

optical contrasts, fast switching speeds, and processability. Poly(3,4-

propylenedioxy)thiophenes (PProDOT) are a substantial subclass of materials in

conjugated polymer electrochromics due to their high optical contract between

the bleached and colored states. Common derivatives of this molecule are

typically made at the beta position with respect to oxygen on the seven

membered ring. PProDOTs with methyl and benzyl substituents (beta position

with respect to oxygen) are two of the more successful due to their high contrast.

We have found that there is a much more substantial effect when PProDOT is

derivatized in the positions alpha to the oxygen. For example, two t-butyl groups

with each placed alpha to the oxygen in PProDOT incurs a 200 nm shift in the

lambda max (365 nm) compared to having two methyl groups with each placed

alpha to the oxygen. The dimethyl derivative is blue in color whereas the di-t-

butyl, dihexyl, diisopropyl is showing yellow, orange and red color respectively .

The polymer of this new derivative, P13ProDOT-TB2 and P13ProDOT-Hex2 is

organic-soluble and can be processed by a variety of solution methods, including

spray coating. Furthermore we have also studied some selenium based polymer

for electrochromic application. Poly(3,4-propylenedioxy)selenophenes

(PProDOS) is showing better optical contrast, stability and faster switching speed

as compared to their sulfur analogs.

Low band gap conducting polymers (CPs) have relatively low absorption in the

visible region, in their conducting states, making them promising candidates for

optically transparent electrode, hole- injection layer for light-emitting diodes and

suitable donor material for Photovoltaics. The monomer, Seleno[3,2-c]thiophene

and Seleno[3,4-£>]thiophene, were electrochemically and polymerized to produce

new low band gap conducting polymer, poly(Seleno[3,2-c]thiophene) (PS32cT)

and Poly(Seleno[3,4-ib]thiophene) (PS34bT), having a low band gap of 1.03 eV

and 1.50 eV respectively. Besides from the suitable energy gap, they also offer a

good match of the absolute energy levels with the other materials in the

photovoltaic device. The HOMO of the low band gap polymers agree with the

work function of ITO and LUMO matches with the acceptor level of PCBM. This

overlap is very important to the function of photovoltaic devices.

In a different approach we describe a new alternative route for the synthesis of

thieno[3,4-b]thiophene, alkyl derivatives thereof, seleno[3,4-b]thiophene, and

thieno[3,4-b]furan made from inexpensive starting materials, such as thiophene-

2-carboxylic acid and furan-2-carboxylic acid. Such fused heterocycles are of

great interest for low band gap organic semiconductors and applications

including OLEDs, organic photovoltaic cells, and electrochromic applications.

n - Conjugated Polymers for Electrochromic and Photovoltaic Applications

Tanmoy Dey, Ph.D.

B.Sc. Hons., University of Calcutta, 1999

B.Tech., University of Calcutta, 2002

A Dissertation

Submitted in Partial Fulfillment of the

Requirements for the Degree of

Doctor of Philosophy

At the

University of Connecticut

2010

UMI Number: 3451416

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2010

APPROVAL PAGE

Doctor of Philosophy Dissertation

7t - Conjugated Polymers for Electrochromic and Photovoltaic Applications

Presented By

Tanmoy Dey, B.Sc. (Hons.), B.Tech.

Major Advisor>^"^ \

^—RcQf^Gre^Qry A. Sotzing

Associate Advisor.

Prof. Amy Howell

Associate Advisor

Prof. Rajeswari Kasi

University of Connecticut

2010

<^WY^ O^W^e^L

/ ^ ^ A ^ ^ 7 K / / C

To my Mother

in

Acknowledgments

Leaving the home country for pursuing my Ph.D. studies was the most difficult

decision I ever had to make, and I would never have been able to go through this

without my mother's support. Of course this learning experience would not have

been so rich without the guidance of my research advisor Prof. G.A. Sotzing. He

handled the research group as a businessman, educating us very well for our

future industrial careers. I wish to express my sincere gratitude for his valuable

guidance, motivation, and suggestions. He allowed me to work on research

projects that I found particularly interesting and prepared me for life after

graduate school.I would like to thank my associate advisors, Professor Amy

Howell and Professor Rajeswari Kasi for their advice and direction during my

research. I also thank Professor Adamson and Professor Selampinar for being

my examiners and investing their valuable time. I would also like to thank

Professor Zeki Buyukmumcu's collaboration and contribution in the seleno[3,4-

b]thiophene and 1,3-ProDOT work.

A special thank you goes to my labmates for their help and support. I appreciate

Dr. Arvind Kumar, Dr. Selman Yavuz, Dr. Jayesh Bokria, Dr. Mathew Ombaba,

Dr. Yogesh Ner, Dr. Chris Asemota, Dr. Jia Choi and Dr. Mike Invemale for their

guidance, especially through the tough times. It was a great pleasure to work

with Ki-Ryong Lee, Daminda Navaratne, Yujie Ding, Donna Mamangun, John

Hyun Park and Yani Feng on several projects.

IV

I extent thanks to all the IMS and Chemistry staff for providing me with technical

and non-technical assistance in UCONN. I also thank all my friends for their

encouragement and just being there.

Last but not least, I want to thank my wife Madhumita. She has always been very

supportive of me, pushing me to reach higher and to achieve more than I thought

I could.

v

LIST OF FIGURES

Figure # Caption Page#

1.1 Common conjugated polymers. 3

1.2 Chemical Structures of Polyethylene dioxythiophene 6

(PEDOT) and Polypropylenedioxythiophene (PProDOT).

1.3 Three most common viologen redox states. 11

1.4 Poly(thiophene) oxidative doping 16

1.5 Electrochromic states of various conducting polymers 18

1.6 Energy band formation during polymerization of conjugated 20

Monomer.

1.7 Effect on the polymer optical band gap upon modification of 22

backbone rotational freedom

1.8 Aromatic and quinonoidal forms of conjugated polymers 23

1.9 Color tuning of thiophenes though intrachain twisting 25

1.10 Effect of electron donating or electron withdrawing 26

substituents on Egof Conjugated Polymers

1.11 Factors affecting the Eg of conducting polymer 27

1.12 Davydov splitting in interchain aggregates 29

1.13 The schematic structure and operation of an organic bulk 35

heterojunction solar cell

1.14 J-V measurement on a bulk heterojunction solar cell 36

VI

1.15 Solar Spectrum Comparison 38

2.1 Electrochemical set up for a three electrode cell 59

2.2 Conjugated polymer electrodeposition techniques : Cyclic 61

Voltammetry

2.3 Scan rate dependence of P13ProDOT-Me2 64

2.4 EQCM setup where gold coated piezoelectric quartz crystal 68

is used as working electrod

2.5 Experimental set up for in situ spectroelectrochemistry. 69

2.6 CIE u ^ coordinates of Polyl3ProDOT-R2 72

3.1 Chemical structures of 1,3 di substituted ProDOT 80

3.2 Overlays of UV-Vis absorption of neutral P13ProDOT-R2 82

3.3 Electrochemical polymerization of 13ProDOT-TB2 98

3.4 Electrochemical polymerization of 13ProDOT-IP2 99

3.5 Electrochemical polymerization of 13ProDOT-Me2 100

3.6 Electrochemical polymerization of 13ProDOT-Bz2 101

3.7 CV scans of polymer, P13ProDOT-TB2 deposited onto Pt 103

button electrode at different scan rates

3.8 CV scans of polymer, P13ProDOT-IP2, deposited onto Pt 104

button electrode at different scan rates

3.9 CV scans of polymer, P13ProDOT-Me2, deposited onto Pt 105

button electrode at different scan rates

3.10 CV scans of polymer, P13ProDOT-Hex2, drop casted onto 106

Pt button electrode at different scan rates

vii

3.11 CV scans of polymer, P13ProDOT-Bz2, deposited onto Pt 107

button electrode at different scan rates

3.12 Spectroelectrochemical data (350-2200 nm) for 110

P13ProDOT-TB2 film (100 nm thick) on ITO-coated glass

at different applied potentials

3.13 Spectroelectrochemical data for P13ProDOT-IP2 film (ca. 111

250nm thick) on ITO-coated glass at different applied

potentials

3.14 Spectroelectrochemical data for P13ProDOT-Me2 film (ca. 112

250nm thick) on ITO-coated glass at different applied

potentials

3.15 Spectroelectrochemical data for P13ProDOT-Bz2 film (ca. 113

250nm thick) on ITO-coated glass at different applied

potentials

3.16 Spectroelectrochemical data for P13ProDOT-Hex2 film (ca. 114

250nm thick) on ITO-coated glass at different applied

potentials

3.17 Spectra of P13ProDOT-TB2, electrochemically 117

(chronocoulometry at +1.1 V) deposited on ITO

3.18 Solution oxidation of chemically polymerized (FeCI3) 118

P13ProDOT-TB2 by UV-vis-NIR spectroscopy in THF as a

function of oxidant concentration using SbCI5.

3.19 Solution Spectra of P13ProDOT-Hex2, electrochemically 119

viii

(chronocoulometry at +1.1 V) deposited on ITO.

3.20 Spray coated P13ProDOT-Hex2 on ITO 119

3.21 CIE iT v' coordinate plot of the neutral states of 121

P13ProDOT-R2

4.1 Various combinations of fused five-membered heterocycles 132

4.2 Structure of thieno[3,4-b]thiophene (T34bT), its 135

derivatives, seleno[3,4-b]thiophene (S34bT), and

thieno[3,4-b]furan (T34bF).

5.1 Chemical structures of ethylenedioxythiophene (1), 168

thieno[3,4-b]furan (2), and thieno[3,4-b]thiophene

(3),ethylenedioxyselenophene (4), seleno[3,2-c]thiophene

(5), seleno[3,4-b]thiophene (6)

5.2 Cyclovoltammetric polymerization (electrochemical), 20 172

scans, of Selono[3,2-c]thiophene (S32cT) (A), Selono[3,4-

b]thiophene (S34bT) (B), (0.01 M) in 0.1 M

TBAP/acetonitrile electrolyte solution at a scan rate of 100

mV/s using a Pt button working electrode.

5.3 Scan rate dependency of poly(Seleno[3, 2- 176

c]thiophene)(A), poly(Seleno[3, 4- b]thiophene)(B)

5.4 Chronocoulometry obtained for redox switching (A) 177

PS32cT and (B) PS34bT

5.5 Spectroelectrochemistry spectrum of a 0.2 |iim thick 180

ix

PS32cT and PS34bT films on ITO glass.

5.6 Vis-NIR spectrum of a 0.05 |im thick PS32cT and PS34bT 183

film on ITO glass. The film was chemically reduced using

0.2 % v/v hydrazine solution in acetonitrile, and oxidized

using 0.2% v/v antimony (V) chloride solution in

acetonitrile.

5.7 1s t and 2nd scan of p- and n-doping cyclic voltammetry for 185

PS32cT and PS34bT

5.8 Optimized calculated structure of S32cT dimmer and 187

S34bT tetramer

5.9 Solar Spectrum Material Comparison 189

5.10 Band Levels (in eV vs. Vacuum) of a device based on 191

PS32cT (A) and PS34bT (B) as donar material and PCBM,

and movement of the electron (e) and hole (o) created as

a result of NIR absorption.

6.1 Chemical structures of ProDOS-Me2 and ProDOS-Hex2 202

6.2 Electrochemical polymerization of 10 mM ProDOS-Me2 and 210

ProDOS-Hex2

6.3 Swithing speed of PProDOS-Me2 212

6.4 Scan rate dependency of polymer, PProDOS-Me2 and 213

PProDOS-Hex2 deposited onto Pt button electrode at

different scan rates

6.5 In-situ spectroelectrochemistry of PProDOS-Me2 and 215

x

PProDOS-Hex2 deposited onto ITO-coated-glass

xi

LIST OF SCHEMES

Scheme # Caption Page#

3.1 Synthesis of meso-2,2,6,6-Tetramethyl-3,5-heptanediol 84

(TMHDiol)

3.2 Synthesis of 13ProDOT-TB2. 86

3.3 Synthesis of 5-hydroxy-2,6-dimethylheptan-3-one 87

(HDMH-One)

3.4 Synthesis of 13ProDOT-IP2 89

3.5 Synthesis of 13ProDOT-Me2. 90

3.6 Synthesis of pentadecane-7,9-diol 91

3.7 Synthesis of 13ProDOT-Hexyl2 92

3.8 Synthesis of 1,5-diphenylpentane-2,4-diol 94

3.9 Synthesis of 13ProDOt-Bz2 95

3.10 Chemical Polymerization of 13ProDOT-TB2 and 116

13ProDOT-Hex2

4.1 137 Synthetic procedure for thieno[3,4-£>]thiophene (T34bT)

and seleno[3,4-b]thiophene (S34bT).

4.2 Synthetic procedure for 2-substituted thieno[3,4- 138

£>]thiophenes

4.3 Synthetic procedure for thieno[3,4-/?]furan (T34bF). 139

5.1 Synthesis of seleno[3,2-c]thiophene 169

5.2 Synthesis of seleno[3,4-b]thiophene 171

xii

6.1

6.2

6.3

6.4

6.5

Synthetic of 3,5-DimethoxySelenophene (DMOS)

Synthetic of ProDOS-Me2(ProDOS-Me2)

Synthesis of 2,2-Dihexylmalonic Acid Diethyl Ester

Synthesis of 2, 2-Dihexyl Propane-1, 3-diol

Synthesis of ProDOS-Hex2

204

205

206

207

208

Xlll

LIST OF TABLES

Caption Page#

Color Cordinates (CIE u v) for ProDOT Polymers 73

Color Analysis of Polyl 3ProDOT-R2 121

HOMO and LUMO energy levels of monomers and dimers 125

3D Views of Monomers and Dimers : Optimised Structures 125

Color Cordinates (CIE u v ) for PProDOS-Me2 and 217

PProDOS-Hex2

XIV

TABLE OF CONTENTS

LIST OF FIGURES vii LIST OF SCHEMES xiii LIST OF TABLES xv TABLE OF CONTENTS xvi

CHAPTER 1: INTORDUCTION

1.1 p- Conjugated Polymers 1 1.2 Electrochromic Material 7

1.2.1 Inorganic Electrochromic Systems 8 1.2.2 Organic EC Systems 10 1.2.3 Conjugated Polymers 15

1.3 Energy Gap in Conjugated Polymers 18 1.3.1 Bond Length Alternation 23 1.3.2 Intrachain Interaction or Coplanarity Deviation 24 1.3.3 Resonance Energy Contribution 25 1.3.4 Substitution Effect 25 1.3.5 Interchain interactions 28

1.4 Photovoltaics: 30 1.4.1 Concept of a heterojunction solar cell 30 1.4.2 Fabrication aspects Polymer Solar Cells 33 1.4.3 Electrical Considerations 35 1.4.4 Optical absorption 37

1.5 Structure of this Thesis 39 1.6 References 42

CHAPTER 2: EXPERIMENTAL

2.1 Materials 55 2.2 Instrumentation 56 2.3 Electrochemistry for Organic Polymer Chemists 57 2.4 Techniques 58

2.4.1 Cyclicvoltammetry 58 2.4.2 Scan Rate Dependence 62 2.4.3 Square-wave Voltammetry 65 2.4.4 Electrochemical Quartz Crystal Microbalance 66 2.4.5 Spectroelectrochemistry 68 2.4.6 Colorimetry 71

2.5 References 74

1

CHAPTER 3: ELECTROCHROMIC CONJUGATED POLYMERS FROM 1, 3 -DISUBSTITUTED PROPYLENEDIOXYTHIOPHENE (P13ProDOT-R2)

3.1 Introduction 75 3.2 Experimental 82 3.3 Monomer Synthesis and Characterization 84

3.3.1 Electrochemical Synthesis and Characterization 96 3.3.2 Scan Rate Dependency and Redox Switching 101 3.3.3 Optical Properties 108 3.3.4 Chemical Polymerization 115 3.3.5 Color Analysis 120

3.4 DFT Analysis of Monomers and Dimers 121 3.5 Conclusions 126 3.6 References 128

CHAPTER 4: VERSATILE SYNTHESIS OF 3, 4-b HETEROPANTALENES

4.1 Introductio 132 4.2 Synthesis and Characterization of Thieno[3,4-6]thiophene and Seleno[3,4-

Z>]thiophe 142 4.3 Synthesis and Characterization of 2-alkylthieno[3,4-b]thiophene 147 4.4 Synthesis and Characterization of Thieno[3,4-b]furan 155 4.5 Conclusions 159 4.6 References 161

l i

CHAPTER 5: LOW BAND GAP CONDUCTING POLYMER, POLY(SELONO[3,2-c]THIOPHENE) and POLYSELENO[3,4-6]THIOPHENE

5.1 Introduction 165 5.2 Monomer Synthesis and characterization 168 5.3 Poly(Seleno[3,2-c]thiophene)(PS32cT) and Poly(Seleno[3,4-6]thiophene),

(PS346T) 172 5.3.1 Electrochemical Synthesis and Characterization 172 5.3.2 Scan Rate Dependency and Redox Switching 175 5.3.3 Optical Properties 179

5.4 Energy Gap Calculations 182 5.4.1 Spectroelectrochemistry 182 5.4.2 Cyclic Voltametry (p and n doping) 184 5.4.3 Theoritical Calcultions 186

5.5 Conductivity 193 5.6 Conclusions 193 5.7 References 195

CHAPTER 6: SELENIUM BASED ELECTROCHROMIC CONJUGATED POLYMER

6.1 Introduction 199 6.2 Monomer Synthesis and Characterization 202 6.3 Electrochemical Synthesis and Characterization 208

6.3.1 Cyclic Voltametry 208 6.3.2 Scan Rate Dependency and Redox Switching 212 6.3.3 Spectroelectrochemistry 214

6.4 Color Coordinates 217 6.5 Conclusions 217 6.6 References 218

in

CHAPTER 1

7i- Conjugated Polymers for Electrochromics and

Photovaltaics Applications

1.1 n- Conjugated Polymers

Interest in the field of conjugated polymers (CPs) has increased tremendously

following the discovery of iodine-doped polyacetylene as conducting polymeric

material.1 Heeger, Macdiarmid and Shirakawa were awarded with the Nobel

Prize in Chemistry in 2000 in recognition of this discovery as well as their

contribution in the field of conducting polymers.2 CPs have existed since 1962,

when the electrochemical synthesis of polyaniline was reported by H. Letheby

(PAni).3 Polyaniline is also known as "aniline black"; this material was formed by

oxidation of aniline under mild conditions and was used in the printing industry.4

The first polymerization of acetylene to form polyacetylene (PAc, a) was reported

in 1958 by Natta and coworkers.5 Because PAc obtained was insoluble and

infusible in nature, the material gained little interest at that time. The inspiration

that conjugated polymers could be good electrical conductors roots back to the

1960s when MacDiarmid and others revealed that poly(sulfurnitride) (SN)X, a

polymeric inorganic explosive,6 has a high conductivity.7 The remarkable high

1

electrical properties of (SN)X represented a important step in the field of

conjugated polymers.

The modern age of conjugated polymers started during late 1970s when films

of PAc were found to exhibit a 12 order of magnitude increase in electrical

conductivity when doped with iodine vapors.1"2 The synthetic procedure of

making PAc was based upon a route discovered in 1974 by Shirakawa by

accidental addition of 1,000 times more catalyst during the chemical

polymerization of acetylene. Although initially it was thought that PAc would

replace metals in air and space applications, its environmental instability as well

as its insolubitily created major obstacle for any practical use. However, PAc,

being the simplest model from this class, remains the archetype of conducting

polymers and is still subject to extensive research work.

2

Figure 1.1 Common conjugated polymers, (a) poly(acetylene), (b) poly(pyrrole),

(c)poly(thiophene), (d) poly(3,4-ethylenedioxythiophene), (e) poly(p-phenylene),

(f) poly(p-phenylene vinylene), (g) poly(aniline), (h) poly(fluorene), and (i)

poly(carbazole).

The prospect of synthesizing new conjugated polymers with improved

rheological and electrical properties began to draw the interest of synthetic

chemists in the early 1980s. At the same time, the discovery of polypyrrole (PPy,

b) with highly conducting and homogeneous, free-standing films via oxidative

electropolymerization 8 focused the research efforts towards the development of

conjugated poly(heterocycles). Followed by the invention of PPy,

electrochemical polymerization focusing film forming and conducting properties

3

was rapidly extended to other aromatic hetero cyclic compounds such as aniline,

thiophene, furan, indole, carbazole, indole, azulene, pyrene, and fluorene.9"10

Despite their poor conductivity compare to PAc, the interest in conjugated

poly(heterocycles) shown in Figure 1.1 (compounds b-i) was due to their

superior environmental stabilities in their p-doped state. Moreover

poly(heterocycles) showed better opto-electronic properties than that of PAc.11

Among these so-called "first generation" CPs, polythiophene (PT, c) has rapidly

gained considerable interest, mainly due to its structural versatility and enhanced

environmental stability both in neutral and p-doped states. However, PTs are not

stable at the electrical/chemical potentials required for their polymerization. This

effect is known as "PT paradox" which means that the polymer degradation

competes with its polymerization, leading to the formation of polymers with a high

content of overoxidized, non-electroactive material. The problem of

polythiophene polymerization can be circumvented by modifying chemical

structure of the monomer in order to stabilize the polymer to the degradation.

Another advantage of this approtch is other properties such as processabillity,

optical properties can also be tuned. This gives the motivation to the synthesize a

vast family of PT derivatives with varied properties.12"13

Among the 'second generation conjugated polymers1 the 3,4-ethylenedioxy

derivative of PT (PEDOT, d) proved itself as an excellent candidate for variety of

electro-optical applications such as electrochromic devices, LEDs, capacitors

and sensors. Primarily PEDOT was developed to give a processable polymer (as

an aqueous dispersion) with a high degree of structrural order due to the lack of

4

a-p and (3-(3 couplings. PEDOT has gained tremendous attention, both

industrially and in academia due to its excellent opto-electrical properties. In

addition to its high electrical conductivity, PEDOT also exhibits high

electrochromic contrast with the major advantage of being almost transparent in

its doped conducting state.14"15 Other advantages include low monomer oxidation

potential, high stability in the doped form and an ease of derivatization at the

ethylenedioxy ring, thus allowing for state-of-the-art tuning of the materials'

electronic and optical properties. The industrial use of the polymer as an

antistatic layer in photographic films makes it the most extensively used

conjugated polymer to the date.16

Derivatization of the monomer with long alkyl or alkoxy pendant groups affords

polymeric materials that can be processed by common solution processing

techniques including spraying, inkjet printing, spin coating or solution

casting.17'18 .One drawback of neutral PEDOT based polymers is their instability

towards atmospheric conditions (air, light and water). The polymers easily oxidize

in air and degrade over time. Similar to ethylenedioxythiophene (EDOT)-based

monomers, the oxygen atoms of the propylenedioxy bridge increase the electron

density of the thiophene ring and lower its oxidation potential. Indeed, the

ProDOT oxidation peak was reported16 around +0.98 V vs Fc/Fc+ while

thiophene oxidation was reported17 around +1.22 V vs Fc/Fc+ and the EDOT

oxidation peak is found18 at +0.88 V vs Fc/Fc+ for comparison. The effect of the

electron-donating oxygens on the oxidation potential is a bit less for ProDOTthan

5

for EDOT due to its twisting conformation, which diminishes the overlap between

the oxygen lone pairs and the aromatic thiophene ring.

Although the structural difference lies in the presence of the larger 7-

membered ring in place of the six-membered ring, PProDOTs (shown in Figure

1.2 ) are much more stable to atmospheric conditions and can be stored in air for

extended periods of time without oxidizing.

PEDOT PProDOT-R2

R = H, Alkyl

Figure 1.2 Chemical Structures of poly(3,4-ethylenedioxythiophene)5 (PEDOT)

and Poly(3,4-propylenedioxythiophene (PProDOT). There are two n's here

change one to for prodot n=4

In order to develop the range of substituted 3,4-alkylenedioxy thiophenes,

Reynolds group at Univ. of Fluorida studied monomers having different ring sizes,

(such as 7-membered (ProDOT) and 8-membered (BuDOT) rings).17"18 ProDOT

has gained special attention, as the monomer can be symmetrically derivatized

at the central carbon of the propylene bridge, resulting the polymer region-

symmetric in nature.

6

The band gap of ProDOT-containing polymers can be tuned by varying the

degree of TT-overlap along the polymer backbone via induced steric interactions

and by controlling the electronic character of the n-system with electron donating

or withdrawing units.19"22

The band gap and optical properties are controlled by using varied substituents

and co-repeat units that can adjust the energy of the highest occupied molecular

orbital (HOMO) or valence band and the lowest unoccupied molecular orbital

(LUMO) or conduction band, thus obtaining polymers with a broad range of

colors. Based on this concept, materials with higher energy gap than the PEDOT

parent have been made, some of which are proved to be anodically coloring

polymers in electrochromic devices.23"25 Further work using a donor-acceptor

methodology has led to low band gap polymers able to p- and n- dope along with

exhibiting multi-color electrochromism.26

1.2 Electrochromic Materials

Electrochromism can be defined as the ability of a material to

reversibly switch its optical properties (absorptance / transmittance) upon

application of electrical charge.27"29 There are many different classes of

compounds which exhibit electrochromism and ranges from mixed-valence metal

complexes, to inorganic metal oxides, to organic small molecules, and finally to

organic conjugated polymers. Generally, electrochromic (EC) materials switch

between a transparent or bleached state and a colored state. However, material

7

which can switch between two colored states or exhibiting "polyelectrochromism"

i.e. having more than two colored states are also possible 30

EC materials are of great interest to interdisciplinary group of scientists and

entrepreneurs for both their excellent spectroelectrochemical properties and their

potential commercial application including windows, mirrors, and displays.

Electrochromic materials that color upon ion insertion (intercalation/reduction) are

referred to as cathodically coloring electrochromes, while those that color upon

ion abstraction (deintercalation/oxidation) are known as anodically coloring

electrochromes. Electrochromic materials of the future look to incorporate several

attributes missing from today's electrochromic materials. Specifically, they must

possess a high degree of optical modulation (high coloration efficiency), high

optical contrat, faster switching speed and higher stability with decent

electrochromic memory.29b

In the following section some examples of EC materials are compared and

contrasted, leading to a discussion on organic small molecule and polymeric

electrochromes and their impact on development of this important field of applied

research.3031

1.2.1 Inorganic Electrochromic Systems

Metal oxides

Among all the EC materials, transition metal oxides, specifically high band gap

tungsten oxide (W03) has gain tremendous attention over the last 30 years.32

8

In 1815, Berzelius first discovered EC properties W03 by passing hydrogen over

gently heated W03..33 Similarly, Wohler was able to color WO3 upon cationic

intercalation with sodium metal.34 Electrochromism of W03 was discovered by

Deb in 1969, which motivated other researchers to study and incorporate W03

into electrochromic devices.35 Other metal oxide electrochromes include cobalt,

indium tin, iridium, molybdenum, nickel, and vanadium oxides. This group of

electrochromic materials owes their intense optical absorbance bands to

intervalence charge transfer reactions, similar to those found in Prussian Blue.

Based on the metal oxide, either cathodic or anodic coloration is observed. Metal

oxides of Ti, Nb, Mo, Ta, W, and V+5 exhibit electrochromism through cathodically

coloring mechanism while metal oxides of Cr, Mn, Fe, Co, Ni, Rh, Ir, and V+4 are

anodically coloring. Thin film of WO3 is highly transmissive, but upon

electrochemical ion intercalation (reduction) WVI is reduced to Wv resulting in an

intense blue color. The reduction is accompanied by uptake of a counter cation

(either H+ or Li+, represented by M+ in the equation) as shown in Equation 1-1.

W03 + x(M+ + e") -> MxWvl

(1.x)Wvx03 1-1

(pale yellow/transmessive) (blue)

The coloration mechanism was widely debated throughout the years, resulting

in two existing theories. Based on the first theory, at low x values, the films

display the intense blue color from a photoeffected intervalence charge transfer

(CT) between Wv and WVI sites. Recently the second theory was proposed by

9

Deb and co-workers proposed that the color change is affected by intervalence

transitions between Wv and WIV sites.36

A thin film of W03 can be deposited through a variety of techniques including

thermal evaporation (in vacuum), RF-sputtering, electrochemical oxidation of

tungsten metal, chemical vapor deposition (CVD), sol-gel methods, and spray

pyrolysis. Each method results in the deposition of amorphous tungsten oxide (A-

WOx) as a thin transparent film.37 W03's popularity as an electrochrome is driven

by the requirements of device applications, 38 specifically the development of

'smart window' technology for control of thermal conditions within a building.39

Other applications for tungsten, and other metal oxides include elements for

information display, light shutters, variable-reflectance mirrors, and variable-

emittance thermal radiators.32

1.2.2 Organic EC Systems

1.2.2.1 Viologens

Viologens40 is a class of organic electrochromic molecules that are obtained by

the diquatemization of 4,4'-bipyridyl to yield 1,1'-disubstituted-4,4'-bipyridilium

salts.41 Viologens are used as redox indicators in biological studies, as well as

herbicides.41 Several excellent reviews41"42 articles about the development of

viologen chemistry are available , so only a brief summary of their

electrochromism is given below. The most common viologen is 1,1'-dimethyl-4,4'-

bipyridilium, otherwise known as methyl viologen (MV) or paraquat (PQ) is shown

10

in Figure 1-3. Methyl groups can be substituted for a variety of alkyl and aromatic

groups.

Radical calion dimer ^„wwvv.......vv

(red)

2X~ 4 /r\ f=\ +

-e"

4 >r\ ..vvvv....vv H U f i — N > -

X"

«~e~

+@~

HQN-CH3

i

HgC~*N >—< N ~ G H 3

colorless

bfue-vfotet

very little cotor&to I

Figure 1.3 The three most common viologen redox states are shown above.

Stability as well as electrochromic properties of vilogene can be varied by

substituting methyl groups of MV with other alkyl group or by changing anion (x-).

Various anions available includes dihydrogen phosphate (H2P04~), sulphate,

fluoride, formate, acetate, tetrafluroborate, and perchlorate.43 Upon one electron

reduction, the highly stable radical cation is formed which shows an intense blue-

violet color in the visible region as a result of intramolecular optical charge

transfer between the positively charged and zero valent nitrogens. Stability of

viologenes is attributed to derealization of the radical electron over the

11

conjugated 7c-framework with the 1,1' substituents bearing some of the charge.

The color exhibited by the radical cation can be tuned to a certain extent by the

substitution of nitrogens with various alkyl and aryl groups. Specifically, 1,1'-

bis(4-cyanophenyl)-4,4'- bipyridilium exhibits a green-hued solution. The dinners

of the radical cations can also form; leading to the formation of a solution that is

more red in color.44 This phenomenon is most common in aqueous solution and

in non-aqueous solutions at low temperatures. The di-reduced viologens,

however, display low color absorption due to the lack of optical charge transfer or

internal transition corresponding to visible wavelengths region. Since both the

dication and radical cation of MV are soluble in aqueous electrolytes, its

coloration life span is finite. Upon one electro-reduction, the highly colored radical

cations are formed. However, once the electric potential is removed, the soluble

radical cation starts diffusing away from the electrode and diffuses into the bulk,

undergoing electron-transfer reactions with the equally soluble counter anion and

thus returning to the colorless dicationic form. This problem can be solved either

by using a semi-solid electrode such as poly(2-acrylamido-2-methylpropane-

sulfonic acid45 or by preparing viologens with longer alkyl chains. Semi-solid

electrodes slow the migration process of the radical cation into the bulk solution

while long alkyl chains result in insoluble radical cations that form well-adhered

films on the electrode surface. The most extensively studied viologen is 1,1'-di-

heptyl-4,4'-bipyridilium (heptyl viologen, HV) as the bromide salt. 46 The insoluble

crimson radical cation salt has been subsequently incorporated into devices that

12

have response times between 10 and 50 ms and lifetimes of >105 cycles

between the redox states.

A practical and cost-effective application of viologen's solution based

electrochromism is Gentex's automatic-dimming interior 'Night Vision Safety'

(NVS) mirrors. The device consists of a dual-coloring system where a

cathodically coloring viologen is in solution with an anodically coloring molecule.

The Optically transparent ITO-coated glass (with the conductive side facing

inward) is spaced micrometers apart from the reflective metallic surface with the

two electrochromic materials dissolved in a solvent.47 When the device is activate

(through a rear-facing photosensitive detector that senses the incident head light

from another vehicle) an applied potential reduces the viologen to the highly

colored radical cation that adheres to the ITO electrode provides night vision

functionality to device.

Moreover, this modern technology is also used in motorcycle visors, using a

material called electrochromic foil, which consists of thin oxide layers laminated

between two flexible polymer sheets. The foils are first coated with a transparent

electrically conducting layers followed by active electrochromic layers. The

lamination process includes a special ion-conducting electrolyte which makes it

possible to charge and discharge the electrochromic layers, thus getting them to

absorb or transfer visible light. This application is said to be able to greatly

reduce the amount of accidents caused by changing light conditions.

Furthermore, this technology is also used to manufacture windows that darken by

themselves upon changing light conditions. The windows have light sensors

13

installed in them and upon an increase in brightness from the outside, they will

transfer the light energy into electrical energy which will then be used to change

the structure of the electrochromic polymers in the windows, thus darkening the

windows in the process and preventing excess light from entering the structure.

Although, electrochromic materials based on transition metal oxides and

viologenes have received extensive research efforts and attempts toward

commercialization (some quite successfully), there remain drawbacks. These

include electrochromes that are solution-based, limiting application in devices,

such as some viologens, and transition-metal complexes, whereas other

materials have relatively slowswitching speeds, as with the metal oxides that

require anywhere from 30 s to several minutes to reach a full contrast,38 limiting

their application in devices that require a color change at shorter time frames.

Other drawbacks include lack of ease of processability to high surface area or

patterned films (as needed for windows and displays) and limit in range of colors

available. Through intensive synthetic efforts over the past 5-10 years,

conjugated electroactive polymers have come to address and remedy many of

these drawbacks, with efforts still underway, to allow for fully solution

processable, fast, and stable switching thin films that are available in colors that

span the entire visible region, switching to highly transmissive states. In that

sense, CP based electrochromics provides more potential.

14

1.2.3 Conjugated polymers

Electrochromism in conjugated polymers is particularly attractive from the

perspective that the visible absorption bands can be fine-tuned through organic

structure modifications. Controlling the electronic and steric environment by

substitution of aromatic monomers followed by polymerization results in

conjugated systems having large range of electrochromic responses. As an

outcome, electrochromic CPs have become the largest classe of electrochromic

materials studied. Most conjugated polymers are electrochromic in thin-film form,

with redox switching giving rise to new optical absorption bands in conjunction

with the transfer of electrons and counter anions.48 Among all the conjugated

polymers discussed previously, PTh, PPy, and PANI have attracted the most

attention as evidenced by the total volume of primary research articles and

reviews based on their study.49 PANI is the oldest of the conjugated polymers

with scientists referring to a material known as "aniline black". Its ability to form

processable, low cost conducting films exhibiting three distinctive color states,

depending on the extent of oxidation makes PANI as the most attractiove

electrochromic polymer.50"51 These polymer films exhibit polyelectrochromism

switching from a transparent, insulating leucoemeralidine state, to a

yellow/green, conducting emeralidine state, and finally to a blue/black

pernigraniline state.52Both electrochemical and chemical process can be used to

synthesize PANI, although the best films of PANI being prepared under constant

current conditions from aqueous acidic solutions.51

15

However, recently PANI has lost the attention of industrial and academic

groups due to the harsh acidic conditions required to make processable samples

as well as the possible formation of carcinogenic byproducts upon degradation.53

Fortunately, the formation of color states upon oxidation or reduction is a

property shared by most electractive conjugated polymers and as a result, other

heteroaromatic systems have essentially studied to replace PANI.

Poly(thiophene) (PTh) and its derivatives are the most extensively studied of all

of the polyheterocycles and several review articles have been written detailing

their properties.54 PTh was first reported in the early 1980's55 and has since been

synthesized by oxidative polymerization at an electrode surface proceeding

through the generation of a radical cation or through chemical polymerizations.56

Electrochromic films of PTh can be prepared that switch between a blue oxidized

state (max = 730 nm) and a red neutral state (Xmax = 470 nm).

R R* 3 H 4

i

B

\j \j \J s \J s

"xyV rvy\ s. A^^iX

Neutral State (fed)

Oxidized State (blue)

16

Figure 1.4 Poly(thiophene) oxidative doping. (A) Numbering system in thiophene

based conjugated polymers. R and R' can be alkyl, alkoxy, or aromatic groups.

(B) Structural changes and associated electrochromism of PTh.

Although PTh displays very attractive electronic and electrochromic properties,

it suffers from poor processability, which has hindered further exploration.

Substitution of thiophene with various alkyl, alkoxy, and aromatic groups in the 3-

and or 4-positions, (Figure 1.4) or through polymerization of dimers and trimers

of thiophene results in materials with significantly enhanced solubility and a

diverse set of optical and electrical properties. Color changes in PThs can be

dictated through structural modifications which play a important role in either

increasing or decreasing the effective conjugation length of the polymer as well

as the band gap of the polymers. Examples of the diversity in the electrochromic

polymer color states are illustrated in Figure 1.5. It has been observed that the

structure and doping level have an enormous effect on the color, as well as on

the band gap of the polymer. The entire rainbow colors can be realized through

structural and oxidative manipulation. With relevance to thesis on present study,

properties of CPs are detailed in next sections.

17

Figure 1-5 Electrochromic states of various conducting polymers based on PTs

in their 0, neutral; I, partially oxidized; +, oxidized; and - and --, reduced states.

Figure adapted from Argun et a/.19

1.3 Energy Gap in Conjugated Polymers

The electro-optical properties of conjugated polymers depend on the energy

difference between their Highest Occupied Molecular Orbital (HOMO) or valence

band and Lowest Unoccupied Molecular Orbital (LUMO) or conduction band.

In the field of organic chemistry the highest energy % orbital and lowest energy n*

orbital are designated as the HOMO and LUMO, respectively. The energy gap

18

(Eg), is defined as the energy difference between the HOMO and LUMO and it

decreases with an increase of effective conjugation length. Incorporation of

additional repeat units into a conjugated polymer backbone leads to the

formation of new energy bands of n and n* orbitals. These bands are also known

as valence and conduction bands, respectively. The width of the band energy

increases with an increase in the number of repeating units resulting in a

lowering of Eg. The band width also depends on the extent of % orbital overlap or

derealization of n electrons over the polymer back bone, and the breadth of the

band generally increases with an increase in n orbital overlap. The energy gap of

conjugated polymers corresponding to the n to n* transition, and the formation of

bands of molecular orbitals are shown in Figure 1.6. The oxidation of conjugated

polymer and formation of hole charge carriers involves the removal of electrons

from the HOMO energy leve, resulting in the p-doped form, while the reduction

process involves the addition of electrons to the LUMO energy level, resulting in

the n-doped form. The onset of the oxidation and reduction process is

representative of the energies of the HOMO and the LUMO.

19

i -o c

LU

n= 1 2 3 4 5 6 a Figure 1.6 Energy band formation during polymerization of conjugated Monomer.

Generally there are two common methods for the determination of the energies

of the energy gap. One of the methods is electrochemistry, by which the

potentials for electron removal from the HOMO and electron injection into the

LUMO are determined. The energy difference between these two electron

transfer events is shown in Equation 1-2. The electrochemical method assumes

negligible intermolecular interactions.57"58

E~(Eox-Ered) + (S++S-){l- [ l - ( l / * 2 ) ] i 1-2

Where, Eox and Ered are the polymer oxidation and reduction potential onsets,

respectively; S+ and S" are the solvation energy of the positive or negative ions

20

minus the solvation energy of the neutral molecule, respectively; 81 and e2 are the

dielectric constants of the solution and the solid, respectively.

The second common method for determining energy gap of conjugated

polymer is through obtaining an electromagnetic spectrum of the neutral polymer,

in which the to 71* transition generally lies, within the lower energy near infrared

or higher energy visible region. Through this technique, the energy gap is defined

as the onset for the n to n* transition. Where spectra are not reported, it is useful

to report both the onset and the maximum for the absorption. In case of narrow

or sharp onset of absorbance peak, the wavelength at which it deviates from the

absorbance baseline (at the longer wavelength side) is generally taken as energy

gap. While in case of broad onset absorption peak, the energy gap is determined

by extrapolating the onset of the n to n* absorbance to the background

absorbance.

The Eg for conjugated polymers depends on several parameters involving both

intra and intermolecular interactions. Intramolecular parameters such as the

nature of the substituents and stereochemistry, and intermolecular parameters

such as interchain interactions may alter Eg and can be expressed as Equation

1-3.

Eg = E5r + Ee + ERes + ESub + Ein 1-3

where E5r, E0, ERes, ESub and Eint represent the energy related to bond length

alternation, coplanarity deviation, resonance, substituent and interchain

interactions, respectively.

21

Figure 1.7 illustrates the concept of conformational freedom playing a key role

in the band gap of a conducting polymer. It can be seen that PA, with a band gap

of 1.4 eV due to Peierls distortion, as described above, when locked into a trans-

cisoid geometry via a sulfur atom (thus producing polythiophene), experiences a

band gap elevation to 2.0 eV.60 When the 3- and 4-positions are substituted via

an ethylenedioxy bridge, thus producing PEDOT, the band gap is seen to

decrease to 1.6 eV.

Lock into cisoid geometry

Increase Band Gap

G?oU9s

PoJyacetylene a^9

oO> ( A

E g - 1 . 6 e V

Polythiophene

Restrict Bond Rotation •

Increase Band Gap

PEDOT

Poly13ProDOT-TB2

Figure 1.7 Effect on the polymer optical band gap upon modification of backbone

rotational freedom.

The reason for this is two-fold. First, the electron donating groups effectively

raise the HOMO level with respect to thiophene. Second, the electron-rich

oxygen atoms coordinate to the d-orbitals of the sulfur atom of an adjacent

repeat unit, thus locking the confirmation of PEDOT into planarity.61 If adjacent

22

repeat units were to be conformationally restricted, a disruption of the TT-overlap

would occur, thus elevating the band gap, and will be the topic of discussion in

Chapter 3.

1.3.1 Bond Length Alternation:

E5r is the energy associated with bond length alternation, (BLA) which is related

to the difference between single and double bond length. BLA is also defined

quantitatively as 8r which is a measure of relative degree of aromatic and

quinonoid character in conjugated polymers. 5r > 0 or 8r < 0 indicates a

quinonoid or aromatic structure, respectively. The quinonoid form generally has a

lower Eg than the aromatic form.62 BLA is absent in poly(acetylene) due to two

degenerate ground states while the polyaromatic polymers such as

poly(thiophene), poly(pyrrole) and poly(p-phenylene) have nondegenerate forms,

aromatic and quinonoid, resulting in BLA as shown in Figure 1.8.63

Where X = S, NH, -CH=CH Figure 1.8 Aromatic and quinonoidal forms of conjugated polymers

1.3.2 Intrachain Interaction or Coplanarity Deviation

Ee, represents the energy related to the structure deviation from coplanarity,

and may arise from the interannular rotations of aromatic rings through the single

bond connecting them. Besides following the 4n + 2 n electron rule, coplanarity is

23

a prerequisite for a molecule to be aromatic (which means that there is a

derealization of n electrons). Rotational distortion limits the effective conjugation

length in oligothiophene, thereby increasing the Eg by Ee.64'66 When the

backbone of a conducting polymer is twisted out of planarity, the TT orbital overlap

decreases, resulting in a decrease of the effective conjugation length.'67 The

bandgap will be much higher in twisted polymers than in planar polymers. This

twisting leads to blue-shifted absorption Internal twisting has a dramatic effect on

conductivity and charge transport in the solid state. With increased twisting,

intrachain charge transport along polymer chains will decrease strongly. This

results in changes in interchain ordering which are highly dependent on the

individual chain conformations. Normally, polymer chains with a strongly twisted

backbone cannot order easily in the solid state. One exception is possible where

the twisting is regular leading to a formation of a helical structure.63'68 Figure 1.9

shows a series of different substituted polythiophenes with their emission

colors.64'69 Disubstitution of thiophene leads to a high degree of twisting arising

from steric repulsions between side chains on adjacent thiophenes, and also

steric interactions between side chains and the large sulfur atom. The polymers

then have shorter conjugation lengths, and their emission shifts to higher energy

(blue-shift) as the twisting increases.

24

Blue Green Red Green Blue

Figure 1.9. Color tuning of polythiophenes though intrachain twisting. The driving

force is steric repulsion between bulky groups.

1.3.3 Resonance Energy Contribution:

ERes, represents the energy associated with the resonance energy of the

monomer in the case of poly(aromatic)s. The derealization of n electrons along

the polymer backbone is in competition with their confinement within the aromatic

ring, owing to the resonance 70 with the former being responsible for higher

conjugation. Thus, higher resonance stabilization energy of aromatic monomers

(ERes) decreases the derealization of n electrons along the chain, resulting in a

higher Eg.

1.3.4 Substitution Effect

ESub, is the energy related to the inductive and mesomeric effect of the

substituents attached directly to the monomer ring. Electron withdrawing

substituents reduce the Eg by decreasing the LUMO level energy while electron

25

donating substituents increase the energy of the HOMO level resulting in Eg

reduction (Figure 1.10). As substitution generally involves simple chemistry, this

is very popular method among chemists to modify or tune the Eg.

LUMP

e- withdrawing t J — — • * • E

substituent | a

y I

HOMO

Figure 1.10. Effect of Substituents (electron donating or electron withdrawing

substituents) on the Eg of Conjugated Polymers.

By introducing substituents on the conjugated polymer backbone, HOMO and

LUMO levels can be controlled precisely. By attaching electron donating or

electron withdrawing chains in direct conjugation with the conjugated backbone,

the HOMO and LUMO will be increased and decreased, respectively. Poly(3,4-

alkylenedioxythiophenes) (PXDOTs) are examples of a donor substituent effect.

By introducing the oxygen at the 3- and 4- positions of the thiophene, Tr-donation

of the lone pairs into the thiophene ring occurs. As a result, the HOMO level is

raised (-4.1 eV for neutral PEDOT, -5.3eV for P3HT) and the bandgap is

decreased (1.6 eV for PEDOT, 2.35eV for P3HT). PXDOTs are therefore much

easier to oxidize than polythiophenes and a bathochromic shift is observed in

their absorption spectrum (polymers are more blue). If electron-withdrawing

LUMO LUMO

E e* releasing 9 *

substituent

HOIVIO

HOMO

26

species are introduced in conjugation with the conjugated backbone, the result is

a lower LUMO level and a smaller bandgap. The oxygens have also an inductive

electron withdrawing effect but it is suppressed by the resonance donating effect.

But if a methylene spacer is introduced between the backbone and the oxygen,

then the resonance effect no longer occurs. The inductive effect is dominant and

the oxidation potential is raised. Compared to poly(thiophenes), it was shown that

poly(3- alkyloxymethylthiophenes) have 100mV higher oxidation potential,

corresponding to 0.1 eV lower HOMO levels.71 Another method for disruption of

the conjugated backbone is by altering its rotational freedom: either through

rotational restriction or liberation.

All four terms above described the factors affecting the Eg of a single

conducting polymer chain. However, in reality it is also important to consider the

interchain interactions to define the Eg. The last term in Equation 1-3, Eint,

accounts for the energy related to the interchain interaction in the solid state.

27

Figure 1.11. Factors affecting the Eg of conducting polymer. Adapted from

reference 57.

1.3.5 Interchain interactions

Interchain interactions also have a strong effect on the bandgap and the

absorption/emission properties of conjugated polymers. Strong TT-stacking

interactions lead to significant shifts of the bandgap, The shift depends heavily on

the relative orientation of neighboring chains, as shown in Figure 1.12.7273 For

H-aggregates, formed when two chromophores are in cofacial arrangement, the

transition from the ground state to the lower level of the excited state is forbidden

but the transition from the ground state to the upper level is allowed, leading to a

blue-shifted absorption. For the same reason, the emission for such aggregates

is expected to be strongly quenched.74 For J-aggregates, formed when two

chromophores are in a staggered ("brick-work") arrangement, there is a

28

bathochromic shift, as the transition from the ground state to the upper level of

the dimmer excited state is forbidden and the transition from the ground state to

the lower level is allowed. In this type of aggregate, strong emission is seen.75 A

third type of aggregate, where each chromophores is tilted relative to its

neighbors, leads to little or no shift of the absorption since both transitions to the

excited state are allowed. Only if the Davydov splitting is large enough, it is

possible to observe splitting in the absorbance spectrum.

29

IN-LINE J-aggregat«

EE3EE3

OBLIQUE

MONO mumuomont nm-mm

(8«tlM«lmHntc)

PARALLEL H-aggregafes

MOMO dtCH&OMOPHORg HOMO 8f€H8OMOPK0f?£

mm mummm on mjrm& . 8Uf£*$Hlf¥ .

8ICHftOMQPHGi$g tW*V!$tSL€ 8!CMR0MQPH0ftjk U^VISISil SSCHROMOPH^RS iuV-VI$i8tE

Figure 1.12 Davydov splitting in interchain aggregates. The resulting energy

levels and absorption spectrum are also shown. Solid Lines: allowed transition;

dashedlines: forbidden transitions. Adapted from reference 77.

1.4 Photovoltaics:

1.4.1 Concept of a heterojunction solar cell

Photovoltaic cells (PVs) are one of the few viable renewable energy solutions

that could solve the impending energy issues facing our planet. Although

inorganic, primarily silicon-based, PVs have been moderately successful in this

regard, organic photovoltaics (OPVs) have gained much attention due to

potential large-area, flexible and low-cost solar cells.78 When light is absorbed by

the material, a photoexciton (electron-hole pair) is generated. This occurs as a

result of the excitation of an electron from the HOMO crossing the band gap to

30

the LUMO, thereby creating a hole, and an electron in the HOMO and LUMO

respectively. Since the exciton diffusion length is limited to a few nanometers

(10-20 nm), "splitting" of this Coulomb bound species has to be achieved and can

be done by carefully selecting an acceptor material whose LUMO (acceptor) level

lies below that of the LUMO level of the donor. The electron thus crosses the

barrier and moves into the acceptor region, continuing towards the cathode,

while the hole travels towards the anode. In order for the holes and electrons to

now cross the Semiconductor-Metal (Schottky) barrier, it is crucial that the work

functions of the selected metal match with the respective levels of the

semiconductor. The HOMO of the low band gap polymers should match with the

work function of ITO and LUMO with the acceptor level of PCBM along with

spectral energy match with solar spectrum.

Bulk heterojunction (BHJ) type conjugated polymer solar cells now play a

leading role in the field of OPV because of their high power conversion efficiency

(PCE). Presently, P3HT-PCBM-based solar cells (P3HT=poly(3-hexylthiophene),

PCBM= (6,6)-phenyl-C61-butyric acid methyl ester) have exhibited high

efficiencies of 4-5%. These are the highest efficiency one can obtain using

polymeric PVs. For any commercial viability of these systems efficiency more

than 10% are needed. The narrow absorption spectrum of P3HT in 300-650 nm

is one of the main challenges to further improve efficiencies of P3HT-based

devices. Therefore, organic materials harvesting solar photons in a broader

spectrum, mostly in the NIR region are needed. Yang Yangs group at UCLA

reported poly[4,8-bis-substituted-benzo [1,2-b:4,5-b0]dithiophene-2,6-diyl-alt-4-

31

substituted-thieno[3, 4-b]thiophene-2,6-diyl] (PBDTTT), a low band gap CP with a

highest efficiecy (7.4%) reported till date79. Low band gap CPs exhibit spectrum

extending into NIR region. Moreover, the low band gap polymeric donor material

should have the following essential properties for the achievement of high

efficiency in device performance: adequate driving force for exciton dissociation,

high charge hole mobility, sufficiently high output and good miscibility with the

electron acceptor to form an interpenetrating network.79,80 Short circuit current

(Jsc), open circuit voltage (Voc), and fill factor (FF) are three main parameters to

characterize a OPV device. In order to get better Jsc, some low band gap

polymers (LGB) were synthesized and applied to the OPV devices. Among

several band gap tuning strategies, conjugated polymers with fused heterocyclic

rings has been known to yield polymers with very low band gaps.81

Polythieno[3,4-b]thiophene (PTT) is one kind of LBG polymer in which the fused

thiophene moieties can stabilize the quinoid structure of the backbone, thereby

reducing the band gap of the conjugated system which is used for OPV. 82 The

conversion of solar light into electrical power requires the generation of both

negative (electron) and positive (hole) charges and a driving force (a potential)

that extracts these charges to an external circuit. In organic semiconductors,

absorption of photons results in the formation of excitons. An exciton can be

considered as of a strongly bound electron-hole pair with a binding energy of

about 0.4 eV caused by Coulomb interaction. These excitons, carrying energy

but no net charge, have to diffuse to the dissociation sites where their charges

can be separated.83"85

32

The interface between suited donor (D) and acceptor (A) materials provides

dissociation sites. At the D/A interface, the energy difference in the electron

affinities and the ionization potentials of those two materials is large enough to

overcome the exciton binding energy. The holes reside in the material with the

lower ionization potential or HOMO, and electrons are captured in the material

with the higher electron affinity or lowest LUMO. After this charge transfer

process the electrons and holes are still bound by Coulomb interaction across

the D/A interface. After breaking the binding energy of this bound electron-hole

(e-h) pair, meaning the electron in the acceptor and the hole in the donor, the

then free electrons and holes travel through the acceptor and donor phase,

respectively, to the electrodes of the device.86"87To obtain efficient photon to

charge conversion, many different device architectures have been developed

such as single layer cells, double layer cells and bulk heterojunction blend cells.

The bulk heterojunction (BHJ) solar cells based on the intimate mixing of

conjugated polymers and fullerene derivatives are mostly used today. Because of

the larger D/A interface, which provides the exciton dissociation sites, a BHJ cell

is more efficient than the other above-mentioned structures.88"97 Therefore, state-

of-the-art organic solar cells are based on so-called donor/acceptor (D/A) bulk

heterojunction.

1.4.2 Fabrication aspects Polymer Solar Cells

33

In Figure 1.13 the device structure is depicted, based on a blend of MDMO-PPV

and PCBM. The devices consist of active layere embeeded between two

electrodes i.e. anode and cathode.

In general, these electrodes of an organic solar cell must be conductive with

suited work function. This means that for extraction of the holes, the anode

should have the same work function (or deeper in energy) as the HOMO level of

the polymer used in the active layer. In ordes to match the work fuctions a thin

layer of poly(3,4-ethylene dioxythiophene) : polystryrenesulfonic acid

(PEDOT:PSS, H. C. Starck) is spin coated (about 50 nm) onto the anode that is

generally based on indium tin oxide (ITO) or gold (Au). The layer of PEDOT:PSS

stabilizes the work function of anode to about 5.1 eV. Furthermore, the PEDOT:

PSS improves the wetting between the electrode and the solution (blend of donor

and acceptor that are dissolved into chloroform or chlorobenzene) during

processing of the device. Additionally, PEDOT: PSS flattens the rough surface of

the ITO anode, which decreases the chance of shorting (anode in direct electrical

contact with the cathode) of the device. One of the two electrodes of the solar

cell has to be optically as transparent as possible, since the active layer is

processed between them. Therefore, indium tin oxide (ITO) is mostly used

because it is highly transparent and has high sheet conductivity for transporting

the holes extracted from the device under illumination. For the cathode of the

solar cell, a closed (-100 nm thick) layer of metals such as aluminum (Al) and

silver (Ag) is usually used. Electrically, the metallic cathode serves for transport

of the electrons extracted from the device under illumination. Optically, the

34

metallic cathode serves as a perfect mirror, which leads to more light trapping

(therefore more light absorption) inside the device. A very thin interlayer of

materials such as lithium fluoride (LiF) is normally used for two reasons; First, the

combination of LiF with above-mentioned metals has a low work function of

about 3.7 eV, which is nearly equal to the LUMO of the acceptor (PCBM) used.

This means that the cathode makes an Ohmic contact with the active layer in

order to extract the electrons from the device. The LiF also protects the soft

polymer surface (the surface of the blend) against the hot vapor-deposited

metallic atoms that can penetrate into the bulk film. This is important, because

metallic particles (Al, Ag,) that penetrate inside the active layer may serve as

exciton quenching sites or may react with the organic materials. Consequently,

the efficiency and lifetime of the device is lowered.

Aluminum

&&*

PEDOT" PSS

glass

35

Figure 1.13: The schematic structure and operation of an organic bulk

heterojunction solar cell. Adapted from reference 92.

1.4.3 Electrical Considerations

In order to measure the electrical performance of an organic solar cell , the

current density J vs. voltage V (J-V) characteristic has to be defined in dark and

under illumination by standard test condition (STC). The STC is 1000 W/m2

intensity, AM1.5 simulated solar spectrum and the substrate temperature equals

25 °C.

From such a measurement, the following parameters can be extracted: Open-

circuit voltage (V0c): the maximum voltage that a cell can produce under

illumination. At V0c the current-density (J) is zero. Short-circuit current (Jsc): the

current-density that a cell generates at zero applied voltage under illumination.

Photo-current (Jph): the difference between currents of the device in dark

and under illumination

Maximum Power (Pmax): The maximum power that a cell can produce is

when the product of the current-density (J) and voltage (V) is maximum.

Pmax = Vmax * Jmax

The Fill Factor (FF): a quantity that is defined as:

F F = ( V m a x * Jmax) / ( V o c * Jsc)

The power conversion efficiency (n): defined as the total power extracted

from the device under illumination divided by the total power of light:

H = Pout / Pin = Pmax / Plight = F F x ( V o c * Jsc) / Plight

36

A typical result of a J-V measurement on a bulk heterojunction solar cell is

shown in Figure 1.14

Photovoltaic Performance (Standard Testing Condition: Air Mass 1..5G at 1.00 m W / c n r )

-0.1 0J3 0.1 0,2 0,3 0.4 o!s 0,6 0 J Voltage (V)

Figure 1.14 J-V measurement on a bulk heterojunction solar cell. Adapted from

reference 92.

In an organic bulk heterojunction solar cell, the quantities fill factor (FF), open

circuit voltage (Voc), and short-circuit current-density (Jsc), and therefore the

efficiency (q) of the device are all dependent on morphology, light intensity,

thickness of the active layer, electrodes and temperature. A complete physical

description of single organic bulk heterojunction solar cells is given in

references.98"106

37

1.4.4 Optical absorption

All the bulk heterojunction organic solar cells are based on blends of

conjugated polymers.98106 (MDMO-PPV, PFDTBT, RR-P3HT and PTBEHT) with

the fullerene derivative PCBM. Current polymeric materials being used have a

high band gap (>2.0 eV) and therefore absorb light in the mid to high energy

visible region of the solar flux, limiting the photon harvesting to 30%. Latest

developments inlcude increases in hole mobilities for P3HTs upon annealing,

and this has been attributed to the alignment and packing of the thiophene

chains. Morphological considerations have also been elaborately studied, and

investigations have shown a dramatic influence of microstructure and phase

separation on all aspects of OPV device performance. Optimization of the

morphology and charge mobilities have lead to the highest reported value for a

OPV device: 6% efficiency.107 This device was a tandem solar cell, in which a a

high band gap CP (P3HT) was used in conjuction with poly[2,6-(4,4-bis-(2-

ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b,]dithiophene)-alt-4,7-(2,1,3

benzothiadiazole)] (PCPDTBT) separated by a thin Ti02 layer. Both the donors

(light harvesting dye), although complementing, absorb only in a narrow band of

the visble region and is thus unable to fully utilize the solar spectrum. Recently

benzo[1,2-b:4,5-b']dithiophene derivative PBDTTBT-PCBM system has been

reported, which exhibits efficiency of 7.73%. PBDTTBT has a band gap of 1.75

eV which is considerably higher than that of crystalline silicon cell.108

38

0 2

? LLii c

CM"

E

£ [oS

Loo^

Q | solar spectrum (AM 1.5-G, 1000 W/ms)

(5• y converted by crystalline silicon ceil

1100 nm - 1,1 eV » band gap of silicon

2400

Wavelength (nm)

Figure 1.15: Curves were normalized for relative absorbance but the

peaksremain at their correct wavelengths. (Blue = Seleno[3,2-c]thiophene,

Yellow = Thieno[3,4-b]thiophene, Red = MEH-PPV in acetone,Green = Poly(3-

hexylthiophene).

The use of novel, LBG polymers such as poly(thieno[354-b]furan) (PT34bF)109

poly(thieno[3,4-b]thiophene) (PT34bT)110 and poly(seleno[352-c]thiophene)111

help to improve the efficiency of future organic solar cells is shown in Figure

1.15. These materials exhibit a low energy gap of 0.85 eV (1459 nm), 1.04 eV

(1190 nm) and 1.05 eV(1200 nm) and 1.50 eV for PT34bT, PT34bF5 PS32CT

and PS34bT respectively. As a result of their low band gap, they absorb light

corresponding to this difference (NIR absorbing), showing a Xmax of 846 nm, 715

and 736 nm for PT34bT, PT34bF and PS32cT respectively. Aside from the band

39

gap, they also offer a good match of the absolute energy levels with the other

materials in the device. The Highest Occupied Molecular Orbital (HOMO) of the

low band gap polymers agree with the work function of ITO. Their Lowest

Unoccupied Molecular Orbital (LUMO) matches with the acceptor level of PCBM.

This overlap is crucial to the function of a device.

1.5 Structure of this Thesis

The main objective of this work is the combination of fundamental studies on

the electrical and optical properties of conducting polymers from the 13ProDOT,

Thieno[3,4-b]furan, Thieno[3,4-jb]thiophene, 2-Alkyl-Thieno[3,4-ib]thiophene,

Seleno[3,4-b]thiophene and Seleno[3,2-c]thiophene class with more application-

driven studies of electrochomic devices as well as organic solar cells.

This research work was extremely exciting as it allowed probing of theoretical

deductions through practical application in devices. Multiple and content-varied

projects as well as several useful research collaborations made this work broad

in the sense that it covers a wide range of concepts defining the conducting

polymer.

The first part of the thesis work is mainly focused towards modifying the

electronic band-gap of conjugated polymers consisting of 1,3- disubstituted

ProDOT as one of the repeat units.Common derivatives of this molecule are

typically made at the beta position with respect to oxygen on the seven

membered ring. PProDOTs with methyl and benzyl substituents (beta position

with respect to oxygen) are two of the more successful due to their high contrast.

40

We have found that there is a much more substantial effect when PProDOT is

derivatized in the positions alpha to the oxygen. For example, two t-butyl groups

with each placed alpha to the oxygen in PProDOT incurs a 200 nm shift in the

lambda max (365 nm) compared to having two methyl groups with each placed

alpha to the oxygen. The dimethyl derivative is blue in color whereas the di-t-

butyl, dihexyl, diisopropyl is showing yellow, orange and red color respectively .

The polymer of this new derivative, P13ProDOT-TB2 and P13ProDOT-Hex2 is

organic-soluble and can be processed by a variety of solution methods, including

spray coating. Furthermore we have also studied some selenium based polymer

for electrochromic application. Poly(3,4-propylenedioxy)selenophenes

(PProDOS) is showing better optical contrast, stability and faster switching speed

as compared to their sulfur analogs.

The second part of the thesis contains the synthesis and characterization of

new low band gap polymers towards the organic photovoltaic applications.Low

band gap conducting polymers (CPs) have relatively low absorption in the visible

region, in their conducting states, making them promising candidates for optically

transparent electrode, hole- injection layer for light-emitting diodes and suitable

donor material for Photovoltaics. The monomer, Seleno[3,2-c]thiophene and

Seleno[3,4-jb]thiophene, were electrochemically and polymerized to produce

new low band gap conducting polymer, poly(Seleno[3,2-c]thiophene) (PS32cT)

and Poly(Seleno[3,4-fe]thiophene) (PS34bT), having a low band gap of 1.03 eV

and 1.50 eV respectively. Besides from the suitable energy gap, they also offer a

good match of the absolute energy levels with the other materials in the

41

photovoltaic device. The HOMO of the low band gap polymers agree with the

work function of ITO and LUMO matches with the acceptor level of PCBM. This

overlap is very important to the function of photovoltaic devices.

In a different approach we describe a new alternative route for the synthesis of

thieno[3,4-t)]thiophene, alkyl derivatives thereof, seleno[3,4-b]thiophene, and

thieno[3,4-b]furan made from inexpensive starting materials, such as thiophene-

2-carboxylic acid and furan-2-carboxylic acid. Such fused heterocycles are of

great interest for low band gap organic semiconductors and applications

including OLEDs, organic photovoltaic cells, and electrochromic applications.

42

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96. Mihailetchi, V. D., Koster, L J. A.; Hummelen, J. C; Blom, P. W. M. Phys.

Rev.Lett. 2004, 93, 216601. 123

97. Mihailetchi, V. D., Koster, L. J. A.; Hummelen, J. C; Blom, P. W. M. Phys.

Rev.Lett. 2004, 85, 970.

98. Melzer.C; Koop, E. J.; Mihailetchi, V. D.; Blom, P. W. M. Adv. Fund. Mater.

2004, 14, 865.

99. Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Phys. Rev. Lett. 2005, 94,

126602.

100. Koster, L. J. A.; Mihailetchi, V. D.; Xie, H. X.; Blom, P. W. M. Appl. Phys.

Lett. 2005, 87, 203502.

101 Koster, L. J. A.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Appl. Phys.

Lett. 2005, 86, 123509.

102. (a) Thomas, C.A. Donor-Acceptor Methods For Band Gap Reduction in

Conjugated Polymers: The Role of Electron Rich Donor Heterocyles, University

of Florida, Ph. D. Dissertation, 2001; (b) Dubois, C. J., Donor-Acceptor Methods

for Band Gap Control in Conjugated Polymers, University of Florida, Ph. D.

Dissertation, 2003.; (c) Thompson, B. C. Variable Band-Gap Poly(3,4-

Alkylenedioxythiophene)-Based Polymers for Photovoltaic and Electrochromic

Applications, University of Florida, Ph. D. Dissertation, 2005.

103. N, Hoppe, N. Arnold, N. S. Sariciftci, D. Meissner, Solar Energy Mater. &

SolarCells 2003, 80, 105.

52

104. C. J. Brabec, A. Gravino, D. Meissner, N. S. Sariciftci, M. T. Rispens, L.

Sanchez, J. C. Hummelen, T. Fromherz, Thin Solid Films 2002, 403, 368.

105. I. Riedel, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande, J. C.

Hummelen,/Wi/. Fund Mater. 2004, 14, 38.

106. P. W. M. Blom, V. D. Mihailetchi, L J. A. Koster, and D. E. Markov, Adv.

Mater. 2007, 19, 1551.

107. Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.;

Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649

108. Huo, L; Hou, J.; Zhang, S.; Chen, H. Y.; Yang. Y. Angew. Chem. Int. Ed.

2010,45,1500.

109. (a) Kumar, A.; Buyukmumcu, Z.; Sotzing, G.A. Macromolecules 2006, 39,

2723. (b) Kumar, A.; Bokria, J.; Dey, T.; Buyukmumcu, Z.; Sotzing, G.A.

Macromolecules 2008, 341, 7098.

110. (a) Lee, K.; Sotzing, G. A. Macromolecules 2001, 34, 5746-5747. (b)

Sotzing, G. A.; Lee, K.Macromolecules 2002, 35, 7281-7286.

111. (a) Dey, T.; Navarathne, D.; Invemale, M. A.; Berghorn I. D.; Sotzing, G.A.

Tetrahedron Letter, 2010 (in press), (b) Dey, T.; Invemale, M. A.; Buyukmumcu,

Z.; Sotzing, G.A. Macromolecules ( in submission).

53

CHAPTER 2

EXPERIMENTAL

The work reported in this thesis involves the syntheses of new electroactive

monomers using multi-step organic synthesis and converting them into

conducting polymers by conventional electrochemical polymerizations or

chemical polymerizations.

All chemicals were purchased from Aldrich Chemicals or ACROS and used as

received unless mentioned otherwise. The monomer/polymer structure and purity

were determined by 1H-NMR, 13C-NMR, 77 Se-NMR5 elemental

analysis,GCMS, FTIR and GPC thus instrumentation details are only reported

for those techniques. Melting point measurements were also performed on solids

for complete characterization. For the deeper understanding of the newly

synthesized materials, specialized analytical techniques such as cyclic

voltammetry, square-wave voltammetry, electrochemical quartz crystal

microbalance, and in-situ spectroelectrochemistry are required.

54

2.1 Materials

All reactions were carried out in flame- dried flasks under inert atmosphere

unless stated otherwise. Furan-2-carboxylic Acid, Thiophene-2-carboxylic acid,

lithium aluminium hydride (LiAIH4), phosphorous tribromide, sodium sulfide,

selenium powder, 2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ), Iron (III)

chloride, Antimony (V) chloride, tetrabutylammonium bromide, hydrazine hydrate

and n-pentane were purchased from Fisher/Acros and used as such. Butyl

Lithium (BuLi) (2.5 M in hexanes), Ethyl magnesium bromide (1M solution in

THF), hexyl magnesium, pentane-2,4-diol, 3,3-dimethylbutan-2-one,

pivalaldehyde, 3-methylbutan-2-one, isobutyraldehyde, Diisobutylaluminium

hydride (DIBAL-H), 2,3-dimethoxybuta-1,3-diene, hydroquinone were used as

received from Aldrich. Acetonitrile and methylene chloride were purchased from

FISHER, and dried over calcium hydride (anhydrous) before use. Toluene was

purchased from FISHER and dried over sodium under nitrogen atmosphere

before use. THF (FISHER) was dried over potassium/ benzophenone ketyl under

nitrogen atmosphere, and distilled before use. Tetrahydrofuran (THF) was filtered

through 0.45 |iim PTFE whatmann filter paper prior to use for GPC

characterization. ITO-glass (unpolished float glass) (7 X 50 X 0.7 mm) with Rs =

5-15 CI was purchased from Delta Technologies Inc. for in-situ

spectroelectrochemistry.

55

2.2 Instrumentation

A HP 5890 series II GC-MS and Nicolet Magna-IR 560 FT-IR spectrometer

were used for the characterizon of monomers and polymers. A Bruker 400 FT-

NMR spectrometer was used to record 1H, 13C and 77Se nuclear magnetic

resonance (NMR) spectra using tetramethylsilane (TMS) as a reference.

Chemical shifts are reported as ppm downfield from TMS, and peak multiplicities

are reported as: br = broad signal, s = singlet, d = doublet, t = triplet, q = quartet,

dd = doublet of doublets, and m = multiplet. A CH instrument 400 potentiostat

was used for all electrochemical studies, Film thicknesses were obtained using a

Veeco DekTak mechanical profilometer. Spectroelectrochemistry was carried out

with a Varian Cary 5000 UV-Vis-NIR spectrophotometer with a 150 mm DRA

Integrating Sphere and Color software. Color data were calculated using a 10

degree standard observer angle, a measurement range of 360 nm to 860 nm in 1

nm intervals, and a standard D65 illuminant. Number-average molecular weight

and polydispersity index (PDI) of homopolymers and copolymers were

determined by using Waters 150-C plus gel permeation chromatograph (GPC)

and monodisperse polystyrene as a standard. GPC is equipped with UV/Vis,

refractive index, and evaporative light scattering detectors. Thermal properties of

polymers were measured using TA Instruments DSC Q-100, and TA Instruments

TGA Q-500.

56

2.3 Electrochemistry for Organic Polymer Chemists

General Experimental Preparation and Setup

There are several different methods which can be utilized to carry out the

electropolymerization of thiophene and pyrrole-based monomers including cyclic

voltammetry, constantcurrent, and constant potential methods.1

For all the electrochemical experiments the monomer is dissolved in a high

dielectric constant solvent (such as acetonitrile or propylene carbonate). It is

essential that the solvent be electrochemically inert (meaning that it will not

undergo electrochemical reactions itself within the potential window being used

for the experiment) and non-nucleophilic. In addition to the solvent, a supporting

electrolyte (such as the tetrabutylammonium (TBA+) and lithium (Li+) salts of

perchlorate, tosylate, tetrafluoroborate, and hexafluorophosphate) is also vital to

help current pass through the solution and to compensate charges that form on

the polymer during the redox process. Specifically, tetrabutylammonium

hexafluorophosphate (TBAPF6) was used for many of the electrochemical

experiments. For all of the electrochemical methods discussed within this

dissertation, an electrolyte concentration of 10 mM in solvent is used. It is

important to note that all solutions should be prepared from dry solvents as well

as pure electrolyte and monomer. Electrolyte and monomer purity is often

ignored, but can be a significant contributor to observed polymer

electrochemical properties. In addition, to minimize experimental error, fresh

solutions should be prepared at the start of the day's experiments because of the

high reactive nature of electron-rich heterocyclic monomers. Each polymerization

57

method discussed within this chapter utilizes a standard three-electrode setup

consists of a working electrode (i.e. Pt or ITO-coated glass), a counter electrode

(typically a Pt wire/ Pt Flag), and a reference electrode (in this dissertation a

Ag/Ag+ reference was utilized), connected to a potentiostat.2 The monomer

solution is placed inside the electrochemical cell in such a way that each

electrode is submerged properly into the solution. Prior to any experiment, the

solution is carefully sparged by bubbling with argon/nitrogen to remove incipient

oxygen. After through sparging, the needle attached to the argon line is removed

from solution and placed just above the surface of the liquid to maintain an inert

atmosphere blanket. By following this protocol, reproducible electrochemical data

can be obtained, or at the very least, an experimental inconsistency can be

minimized.

2.4 Techniques

2.4.1 Cyclicvoltammetry

Cyclic voltammetry (CV) is the most convenient and simple method for initial

studies on electrochemically polymerizeable compounds. In a typical CV

experiment, the potential is swept at a constant rate, while the current is

monitored. Commonly, a dilute monomer solution (c.a. 10 mmol in 0.1 M

electrolyte solution in acetonitrile, propylene carbonate, etc.) is used. Typical

electrochemical cell consists of a counter, working, and reference electrode.

Three electrodes system is used for the better control of the potential. However,

two electrodes cell can be used without a reference if exact potential values are

58

not important. The typical electrochemical set up for three electrodes cells is

shown in Figure 2.1.

Figure 2.1 Electrochemical set up for a three electrode cell

Electrochemical polymerization involves the oxidation of monomer to radical

cation and coupling of radical cations to produce polymers in a step-wise

manner. The onset of this process is known as the oxidation potential of a

monomer. For electrochemical polymerization, potential scanning starts at a

lower potential than the oxidation potential of the monomer, and thus no current

is observed at that potential. Upon scanning further in the cathodic direction, the

current response observed corresponds to the oxidative coupling of monomers to

form conjugated polymers. During this process, the concentration of monomer at

59

electrode interface begins to decrease over time. This is compensated by the

diffusion of monomer from solution towards the electrode, and a diffusion limited

peak is observed on going even further in the cathodic direction. By reversing the

scan in the anodic direction, a broad current response is observed corresponding

to the reduction of conjugated polymer deposited onto the working electrode. The

starting point is a reducing value with the potential being swept to positive

potentials until monomer oxidation and polymerization occurs. Monomer

oxidation is observed with a steep increase in current, followed by a peak, which

is referred to as the peak monomer potential (Ep,m). Following the Ep,m, a sharp

decrease in current is observed and shortly afterwards (typically abou 100 - 200

mV) the scan is reversed back to the starting potential. On the return scan, two

features are clearly visible and are indicative of the deposition of electroactive

species. The first feature is commonly known as the nucleation loop, where the

return scan after monomer oxidation crosses the anodic wave, and is exclusive

to conjugated polymers and metal deposition. Nucleation loops arise due to the

formation of electroactive polymer onto the working electrode, increasing its

electrically conducting surface area. The second feature has a maximum current

response at -0.2 V in Figure 2.2A and is attributed to reduction of the polymer

that was deposited onto the working electrode.

(A)

60

100

c

o

0.6 0.4 0.2 0.0

Potential (V)

-0.6

(B)

<

c

400 J

200-J

0

-200-

-400-

-600-

-800-J

-1000 — I — i — | — i — | — i — | — i — \ — i — | — i — | — i — | — i — | — i -

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

Potential (V)

61

Figure 2.2 Conjugated polymer electrodeposition techniques. (A) Cyclic

voltammetry (CV) deposition of P13ProDOT-Me2 (first scan only) at a scan rate

of 100 mV/s and (B) Cyclic voltammetry (CV) deposition of P13ProDOT-Me2 ( 20

scans ) at a scan rate of 100 mV/s.

Upon repeated scanning process, a thin insoluble polymer film forms on the

electrode surface. Figure 2.2 B shows the 1st to 20th scans of a 13ProDOT-Me2

polymerization. The peak current at 1.2 V remains while the current response

increases in value. Similarly, the polymer oxidation (+0.3 V) and polymer

reduction (-0.4 V) current responses (referred to as the polymer doping current)

increase in intensity indicating increasing electrode surface area. These features

point to a porous, conductive polymer film being deposited onto the electrode

surface.Following repeated scan electropolymerization, the electrode is removed

from the monomer solution and carefully rinsed with monomer-free electrolyte

solution. The rinsed electrode is then placed in another three-electrode cell filled

with monomer-free electrolyte solution. The region including the polymer doping

current is then isolated and scanned by cyclic voltammetry to ascertain other

electrochemical data such as the scan rate dependence and the half-wave

potential (E1/2) potential which is measured by taking the average of the peak

anodic and cathodic current potentials.

2.4.2 Scan Rate Dependence

As mentioned earlier, the polymer redox process (polymer doping current or

polymer electrochemistry) can be isolated and examined through cyclic

voltammetry experiments. In scan rate dependence experiments, the polymer

62

(adhered to the working electrode) is cycled between its oxidized and reduced

states at various scan rates while the peak anodic (ip,a) and cathodic (ip,c)

current responses are monitored as shown in Figure 2-3. For freely diffusing

species ( i.e. the ferrocene couple, Fc/Fc+), easily oxidized species near the

electrode surface react as the potential is swept anodically, generating a current

response. Reversal of the sweep to cathodic values reduces the same species

and another current response is measured. For diffusion-controlled solution-

based electrochemical reactions, the Randles-Sevick 40 equation2,4, 3,4

(Equation 2.1) dictates that the peak current is proportional to the square root of

the scan rate.

ip = (2.69 X105)n3/2AD1/2Cbv1/2 at 25 °C 2-1

where n is the number of electrons, A is the surface area of the working

electrode (cm2), D is the diffusion constant (cm2/s), Cb is the bulk concentration

of the electroactive species (mol/cm3), and v is the scan rate (V/s).

63

600

•600-1 1 * % * 1 1 1 * 1 s 1 * 1 0,6 0.4 0,2 0,0 -0.2 -0,4 -0.6

Potential (VI

Figure 2.3 Scan rate dependence of P13ProDOT-Me2 in 0.1 M TBAPF6/ACN,

a) 100 mV/s5 b) 200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s,

g) 700 mV/s, h) 800 mV/s, i) 900 mV/s, j) 1000 mV/s.

The case presented above does not hold for electrode adsorbed species, such

as electroactive polymers, and therefore a different approach must be taken. The

redox processes of electrode-bound conjugated polymers are not diffusion

controlled, so they cannot be described by the Randles-Sevick equation. Instead,

the following relationship for a surface-bound species is given in Equation 2-2

and dictates that if the electroactive species is electrode-confined, both the

anodic and cathodic current responses will scale linearly with scan rate.

64

ip=n2F2rv/4RT 2-2

where F is Faraday's constant (96,485 C/mol) and r is the concentration of

surface bound electroactive centers (mol/cm3). When examination of the

relationship between the peak current responses and increasing scan rate results

in a linear correlation the redox process is said to be non-diffusion controlled, and

the electroactive centers of the polymer are well adhered to the working

electrode surface. In general, the oxidation and reduction of conjugated polymers

is a fast process.However, there are several other factors influence the switching

speeds of a conjugated polymer (e.g film thickness, electrolyte, and the structure

of the polymer itself). Therefore, scan rate dependence studies are essential for

determining whether a polymer can be switched between redox states rapidly

without loss of current response or switching stability.

2.4.3 Square-wave Voltammetry

Square-wave Voltametry involves the switching of potential between two values

instead of a linear potential sweep used for cyclic voltammetry. This technique is

utilized in order to switch the conjugated polymer between the corresponding

redox states, and gives useful informations regarding switching speed, stability of

conjugated polymer as a function of the number of switches, charge transported

during redox processes, and other properties that are useful for practical device

applications. Square-wave Voltametry technique combined with gravimetric

techniques can be used to study the rate of polymerization, doping level and ion-

transport behavior of a conjugated polymer. Furthermore, this can be combined

65

with spectroscopic techniques to get more information about electro-optical

properties of conjugated polymers especially for electrochromic devices.

Generally, chronocoulometry or chronoamperometry are used for this purpose,

and the same electrochemical set up is used as shown in Figure 2.1. Sufficient

time should be given at each potential in order to reach the plateau before

switching to another potential. This technique is also used to perform

electrochemical polymerization at constant potential, generally at peak potential

observed during CV.

2.4.4 Electrochemical Quartz Crystal Microbalance

Electrochemical quartz crystal microbalance (EQCM) is the combination of

quartz crystal microbalance and electroanalytical techniques which gives more

meaning to electrochemical studies (e.g electrochemical polymerization and ion-

transport behavior) by simultaneously measuring the change in mass. Some of

the examples of EQCM studies of CPs are reported in the literature such as

polypyrrole,1 polyaniline,2 and polythiophene.3 The working principle of Quartz

crystal microbalance is based on piezoelectric effect, i.e. variation of resonance

frequency is associated to the applied electric field. EQCM is built with a quartz

crystal coated on both sides with gold and is coupled to the electric source which

then resonates at a given frequency. The deposition of any substance onto

quartz crystal results in the increase in mass, and hence, a dampening of any

oscillation. The relation between change in mass and change in frequency can

be explained by Sauerbrey's equation:

66

Af = -2f02Am/A(pn)1/2 = -Cfm 2-3

where, f0 is the resonant frequency of the fundamental mode of the crystal, p is

the density of the crystal (2.648 g/cm3) and M the shear modulus of quartz (2.947

X 1011 gcm'V2), A is the area of the gold disk coated onto the crystal. Af is the

frequency change caused by addition of mass per unit area of the crystal

surface, Am. It has been observed that 1 Hz change in frequency corresponds to

1.34 ng of material adsorbed/desorbed from the crystal surface of area 0.196

cm2 for a 7.995 MHz crystal. The simplified EQCM experimental set up for

studying electrochemical processes is shown below in Figure 2.4. All the

experiments were carried out using a 1 cm X 1 cm platinum flag as the counter

electrode, and a non-aqueous Ag-Ag+ reference electrode.

Polymers were deposited onto polished gold-coated quartz crystals from 0.01

M monomer solution in 0.1 M electrolyte/ ACN by applying the positive potential

corresponding to the oxidation of the monomer. It should be noted that all

solutions must be filtered before experiments. Polymer deposited onto the gold

coated quartz crystal was washed thoroughly with acetonitrile, and dried before

acquring the electrochemical measurement in a monomer free electrolyte

solution. It should be noted that the electrochemical characterization of polymer

was performed using the same electrolyte solution used for electrochemical

polymerization. In this work, EQCM has been used to determine rate of

polymerization, doping level and ion-transport behavior.

67

Reference electrode *^

Counter /'electrode

Working electrode (Au coated AT cut quartz)

/

Figure 2.4 EQCM setup where gold coated piezoelectric quartz crystal is used

as working electrode

2.4.5 Spectroelectrochemistry

The changes in electronic transitions that accompany the redox switching of

conjugated polymers can best be examined through spectroelectrochemistry

(SECHEM). These experiments reveal key properties of conjugated polymers

such as the electronic band gap (Eg) and the intergap states that appear upon

doping as well as give insights into fine structure. These properties have been

the focus of several publications and can be explained adequately through band

theory as introduced in Chapter 1

(A)

68

Reference electrode

Working electrode x

Detector

Counter electrode

hv

IF Light Source

* m a x = 5 4 6 n m

i — » — r * " 1 — i — ' — i — « — i — ' — i — * — i — • — i — • — i — '

400 600 800 1000 1200 1400 1600 1800 2000

Wavelength {nm}

Figure 2.5 (A) Experimental set up for in situ spectroelectrochemistry

69

(B) Spectroelectrochemical data for P13ProDOT-Me2 film (ca. 250nm thick) on

ITO-coated glass at applied potentials of a) -0.6, b) 0.0 c) 0.1, d) 0.2, e) 0.5 V

versus Ag/Ag+ reference (0.445 V vs. NHE).

In a typical SECHEM experiment, 0.1 M electrolyte (TBAP/ACN) solution and

blank ITO slides are placed in both sample and blank cuvettes. Both cuvettes are

sparged with argon for five minutes and are then placed in the

spectrophotometer for background collection. The background is collected over a

large range (1600 to 250 nm) so that both the visible and the near infrared (NIR)

transitions can be captured. The upper limit of 1600 nm is chosen since above

this value, significant absorptions attributed to water are present and are difficult

to remove. Data is collected every 1 nm but is typically displayed in terms of

electron volts (eV), where X (nm) = 1240/eV, resulting in the lower wavelength

region (above 1600 nm) being highly compressed (many points cover a small

range in eV), so very little data is lost during the experiment. Once the

background has been collected, the polymer film is placed in the sample cuvette

where leads are attached to a voltage source. The potential range for complete

doping/neutralization of the polymer is determined from CV experiments and then

applied to the SECHEM experiment. The potential is then held at a reducing

value so that the polymer is in its fully neutral state as the wavelength range is

scanned. This process is repeated, incrementally increasing the potential

anodically in 50 to 100 mV steps until the polymer is completely doped as shown

for a film of P13ProDOT-Me2 in Figure 2.5. The electronic band gap of the

70

polymer can be estimated from the onset of the % to n* transition and the

background absorbance as shown in Figure 2.5. In order for this value to be

valid, it is crucial that the polymer be completely neutralized, either

electrochemically or with the aid of a reducing agent additive such as hydrazine.

Upon electrochemical doping, the % to %* peak diminishes, while lower energy

transitions grow in which are attributed to the formation of radical cations that are

free to move along the polymer backbone. At intermediate doping levels, peaks

consistent with a and b (as predicted by Fesser, Bishop and Campbell or FBC

theory) are present (b lies beyond the range of the spectrophotometer, out into

the IR region). Since c and d are not allowed, only two transitions are observed.

Further oxidation can either result in the removal of additional electrons from the

valence band or can remove the unpaired electron from the polaron to form a

dication or bipolaron. Because the bipolaron is an unpaired state, only transitions

from the valence band can occur. Spectrally this is shown as a decrease in the

absorbance transition while a strong absorbance (Eb1) increases in intensity at

even lower energies (into the IR). FBC theory also suggests that the Eb1

transition is much stronger than Eb2 so the bipolaron transition shows as one

peak in the IR region of the spectrum.

2.4.6 Colorimetry

Colorimetry is a quantitative analytical tool that has proven an effective

objectivetechnique to compare and evaluate the optical responses of

electrochromic polymers and devices.4,5 Color can be described in terms of three

attributes; hue, saturation, and luminance, but because of the subjective nature

71

of the art, color matching can prove extremely difficult. Therefore, a standard

color system was developed and is now commonly used and referred to as the

CIE (The Commission Internationale de I'Eclairage) system of colorimetry.6 This

system utilizes matching functions to calculate the tristimulus values (L*, u*, v*)

which are used to define all of the recommended color spaces and take into

account how the human eye perceives color.7 Several researchers have utilized

colorimetry to describe the properties of electrochromic and light-emitting

polymers.

XrttrWtfrtt

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

U'

Figure 2.6 CIE if vx coordinate plot of the neutral states of P13ProDOT-TB2(a),

P13ProDOT-Hex2(b), P13ProDOT-IP2 (c), PProDOT-Me2 (d) and P22ProDOT-

Bz2 (e, square) reported by Reynolds eta.l

Table 2.1 Color Cordinates (CIE u' v ) for ProDOT Polymers. Situation : Black

Body @ 6000K.

Polymer

d) Poly13ProDOT-Me2

a) Poly13ProDOT-TB2

c) Poly13ProDOT-IP2

e) PolyProDOT-Bz2

b) Poly13ProDOT-Hex2

u'

0.2365

0.1975

0.4608

0.1641

0.2431

v '

0.1628

0.4772

0.4092

0.4104

0.5343

73

2.5 Reference

1. Baker, C. K.; Reynolds, J. R. J. Electroanal. Chem. 1988, 251, 307-322.

2. Orata, D.; Buttry, D. A. J. Am. Chem. Soc. 1987, 109, 3574-3581.

3. (a) Servagent, S.; Vieil, E. J. Electroanal. Chem. 1990, 280, 227-232. (b)

Hillman, A. R.; Swann, M. J.; Bruckenatein, S. J. Electroanal. Chem. 1990, 291,

147-162. (c) Sawyer, D. T.; Sobkowiak, A.; Roberts J., J. L. Electrochemistry

for Chemists, 2nd ed.; John Wiley & Sons: New York, 1995, Chapter 3

4. Kueni, R. G. Color: an Introduction to Practice and Principles; Wiley: New

York, 1996.

5. Berns, R. S. Billmeyer and Saltzman's Principles of Color Technology; 3rd

ed., JohnWiley & Sons: New York, 2000.

6. CIE. Colorimetry (Official Recommendations of the International Commission

on Illumination); CIE Publication No. 15, CIE: Paris 1971.

7. Wyszecki, G.; Stiles, W. S. Color for Science, Art, and Technology, Nassau,

K., Ed.; Elsevier: Amsterdam, 1998; p 31.

74

CHAPTER 3

ELECTROCHROMIC CONJUGATED POLYMERS FROM 1,3-

DISUBSTITUTED PROPYLENEDIOXYTHIOPHENE (13ProDOT-R2)

3.1 Introduction

Conjugated polymers offer a unique combination of properties which makes

them a suitable alternative to current materials used in many apllications such as

electrochromics,1 "4 organic gas sensors,5"7 non-linear optics,8 light-emitting

diodes (LEDs)9 energy storage batteries,10 charge dissipaters,11 corrosion

protectors, 12 and solar cells 13~14. For display applications, color and color tuning

are of prime importance. Monomer derivatization, often by incorporating electron

donating or electron withdrawing groups, is frequently employed to achieve color

tuning directly or to impart solubility which can result in an inadvertent color shift.

Herein, we describe a facile derivatization of a popular heterocycle, 3,4-

propylenedioxythiophene (ProDOT), resulting in a 200 nm shift in the Amax of the

absorbance spectrum for the neutral polymer compared to conventional

poly(ProDOT). Common derivatives of this molecule are typically made at the 2,2

position (the central carbon on the propylenedioxy bridge), however we have

accomplished 1,3-disubstitution with a t-butyl group, hexyl group, isopropyl

group, methyl group and benzyl group resulting in 1,3-di-t-butylProDOT

(13ProDOT-TB2), 1,3-dihexylProDOT (13ProDOT-Hex2) 1,3-diisopropylProDOT

(13ProDOT-IP2), 1,3-dimethylProDOT (13ProDOT-Me2) and 1,3-dibenzylProDOT

(13ProDOT-Bz2). The polymer from 2,2-dimethylProDOT, commonly abbreviated

75

PProDOT-Me2, was used for comparison with these new derivatives. Chemical

structures of the polymers used herein are shown in Figure 3.1.

Conjugated polymers based on 3,4-ethylenedioxythiophene (EDOT) and its

derivatives have gained much attention because of their low oxidation potentials,

high contrast and high conductivity.15'20 The electron-donating effect of the

oxygens at the 3 and 4 positions, coupled with favorable ring geometry, stabilize

the positive charge along the polymer backbone, rendering high stability of

poly(EDOT) (PEDOT) in its p-doped state. The electrochemical polymerization of

EDOT takes place at a low oxidation potential, and the polymer exhibits a band

gap (Eg) of 1.6eV.21"23 Polymers based on alkylenedioxythiophenes are popular

materials in electrochromics for their ability to change color from deep blue to

almost colorless. PEDOT has the ability to transit from deep blue to sky blue

upon oxidation with a Photopic contrast of 54%.24'25 Photopic contrast is a value

which takes into account the entire visible spectrum, weighted to the sensitivity of

the human eye. Higher Photopic contrasts and more colorless bleached states

can be obtained by the incorporation of an additional methylene unit into the

bridge between the oxygens, as with 3,4-propylenedioxythiophene (ProDOT). 26

Derivatization of ProDOT at the central carbon (so-called 2,2-disubstitution)

introduces a higher degree of disorder between polymer chains, reducing the

residual color of poly(ProDOT) (PProDOT) in the oxidized state while maintaining

an intense blue-purple in neutral states. 27 One polymer that exhibits enhanced

Photopic contrast (up to 75%) is PProDOT-Me2) whose monomer was

synthesized by the etherification of 3,4-dimethoxythiophene (DMOT) and

76

neopentyl glycol. The highest contrast (89%) reported to date was in the case

of the benzyl derivative, PProDOT-Bz2.29

Generally, there are three strategies for changing color transitions in

conjugated electrochromic systems. The first relies upon extent of

polymerization. The Eg of a conjugated polymer can be tuned by controlling the

number of repeat units. Eg decreases with increasing conjugation length, limited

by Peierls distortion. 30~31 Second, changing the electronic environment via

incorporation of electron donating substituents (red shift) or electron withdrawing

groups (blue shift). Third, introduce steric interactions between the polymer

backbone, causing an increase in the dihedral angle between aromatic rings,

thereby decreasing TT orbital overlap. This results in a blue shifted max and an

increased Eg observed both for unsubstituted ProDOT as well as 2,2-

disubstituted ProDOT. It is this approach that yielded the 200 nm blue shift for

P13ProDOT-TB2. The bulky t-butyl group disrupted the Tr-conjugated system in

the polymer; these interactions caused a severe shift in its coloration properties.

The ability to tune color transitions via simple derivatization would be an

attractive approach towards achieving a larger swath of the color gamut. Density

Functional Theory (DFT) calculations for these materials were carried out in

order to model these effects (see Supporting Information). The large difference in

band gap, 0.73 eV (experimentally measured), between PProDOT-Me2 and

P13ProDOT-TB2 can be explained by distortion of backbone planarity. The

dihedral angles of dimers were studied to give qualitative information about

dihedral angles in the polymer. The optimized calculated structure of the

77

13ProDOT-TB2 dimer shows a dihedral angle of 76.1°, resulting in a clear

distortion of planarity along the polymer backbone (Figure 1). Both ProDOT-Me2

and the hypothetical 22ProDOT-TB2 dimers show a 180° dihedral angle, offering

no such distortion. The 1,3-disubstitution approach, therefore, causes greater

changes in color properties than 2,2-disubstitution with the same moiety.

To date, studies of ProDOT have been carried out using 2,2-disubstition but

none have reported 1,3-disubstitution. These derivatives will provide a disruption

of conjugation in the polymer backbone, ultimately increasing the energy of the n-

71* transition. The steric interactions decrease interchain interactions of the

polymer thereby increasing the optical transparency in the bleached state. 32-34

Theoretically, 1,3 substituents would project over the conjugated polymer

backbone, thereby interacting to distort thiophenes out of planarity as compared

to unsubstituted ProDOT polymers. The distortion will be proportional to the size

of the substituents. Attaching larger groups may cause an even greater shift into

the UV, and current work is being undertaken to push the limits of these steric

effects. Potentially, a conjugated electrochromic material could become

transparent in the neutral state and colored in the oxidized state. Further, it may

also be possible to shift the peak transitions entirely out of the visible region for

both states, resulting in a polymer which is UV-absorbing in the neutral state and

NIR-absorbing in its oxidized state. One can envision an interesting series of

applications for such a material, particularly for coatings on windows.

Longer alkyl substituents would provide solubility in organic solvents. 35-37

Traditional solution processing may then be used (spray or spin coating).

78

Literature indicates a vast amount of conducting polymer based electrochromic

materials reflecting blue and red colors in their neutral state. To date, there are

few reports of the synthesis of a truly yellow polymer in the neutral state that

becomes green in the oxidized state. The class of green polymers 38-39 reported

are based on donor-acceptor theory; none have been from a single molecule,

such as 13ProDOT-TB2. In this chapter, we have accomplished a facile

transetherification procedure for 1,3-disubstitution using t-butyl groups, isopropyl

groups, methyl groups, hexyl groups and benzyl groups resulting in 1,3-di-t-

butylProDOT(13ProDOT-TB2), 1,3-di-iso-propylProDOT (13ProDOT-IP2) , 1,3-di-

methylProDOT (13ProDOT-Me2), 1,3-di-hexylProDOT(13ProDOT-Hex2) and 1,3-

dibenzylProDOT (13ProDOT-Bz2) respectively. The polymer obtained from 1, 3

disubstituted ProDOT (P13ProDOT) resulted upto 200 nm shift in the Amax as

compared to conventionally derivatized 2, 2 disubstituted PProDOT. The polymer

of this new derivative, P13ProDOT-TB2 and P13ProDOT-Hex2 are organic-

soluble and can be processed by a variety of solution methods, including spray

coating. The remarkable shift in the band gap resulted in a yellow (Y) to green

color transition from the neutral to oxidized states, respectively, whereas

P13ProDOT-IP2 is insoluble and showing intermediate band gap with a red color

in the neutral and transmissive light blue in the oxidised state. P13ProDOT-Me2

and P13ProDOT-Bz2 both undergo from a deep blue-purple color to light blue

during their redox cycles. Neutral P13ProDOT-Hex2 is orange in color but upon

oxidation it gives light green color. The chemical structures of these polymers

are shown in Figure 3.1

79

QO * *

QO

+CH -fOf.

CRH 6n13 CRH 6n13

S •» P13ProDOT-IP2

C6H5

S '" P13ProDOT-Me2

C6H5

*s' /n N s P13ProDOT-Hex2 P13ProDOT-Bz2

Figure 3.1 Chemical structures of 1,3 di substituted ProDOT

The monomer oxidation potential of 13ProDOT-TB2 was similar to other

alkylenedioxythiophenes (ca. +1.03 - 1.10 V), which indicates that the bulky

substituents do not have any significant effect on monomer oxidation. This was

expected because the t-butyl groups are neither directly attached to the

thiophene ring nor participatory in any inductive effects, so the electronic

properties of the monomer should remain similar to other derivatized ProDOTs.

The polymer obtained by the electrochemical polymerization of 13ProDOT-TB2

was soluble in common organic solvents, like chloroform and tetrahydrofuran.

The redox behavior of P13ProDOT-TB2 was studied through the recording of

80

cyclic voltammograms. The Ep values for each polymer increased upon an

increase in the scan rate, which indicated a quasireversible redox process. As

shown in Figure 3.2, spectroelectrochemical measurements were taken for a film

of P13ProDOT-TB2, P13ProDOT-Hex2, P13ProDOT-IP2, P13ProDOT-Me2 and

P13ProDOT-Bz2 at various oxidation potentials. The P13ProDOT-TB2 film

switches between an absorbing yellow neutral state to a green oxidized state. At

an applied potential of -1.0 V, the neutral form of the P13ProDOT-TB2 shows a

distinctive n-n* interband transition with onset occurs at 495 nm (2.50 eV) along

with a sharp peak at 365 nm (3.39 eV) (A,max). As the applied potential increases,

the absorption at 365 nm decreases while the polaron (800 nm) and bipolaron

peaks in the NIR region (2000 nm) increase. Upon stepwise increases in the

oxidation potential, the polaron peak reaches a maximum intensity and then

begins to decrease, while the bipolaron peak continues to increase. The band

gap of P13ProDOT-TB2 (assigned as the onset of the TT-TT* interband transition,

measured spectroelectrochemically) was found to be 2.50 eV (495 nm) and its

Amax was 365 nm, which is about 0.73 eV higher than that of corresponding

PProDOT-Me2 (Eg = 1.77, 700 nm). Color coordinates available in the

Supporting Information. Photopic transmittance of P13ProDOT-TB2 (100 nm thick

film) in the oxidized state (clear) and neutral (dark) state is 83.88% and 64.97%

respectively. The electrochromic contrast calculated at the Amax (365 nm) is 57%

with 100 nm thick film. The chemically polymerized 13ProDOT-TB2 gave a

number-average molecular weight 6,240 g mol-1 with polydispersity of 1.7,

whereas the electrochemical gave 2,700 g mol-1 with polydispersity of 1.5.

81

Wavelength (nm)

Figure 3.2 Overlays of P13ProDOT-R2

3.2 Experimental

Materials. Acetonitrile (ACN) and chloroform (CHCI3) were purchased from

Thermo-Fisher and distilled over calcium hydride before use. Ethyl acetate, n-

hexane and toluene were purchased from Thermo-Fisher; toluene was distilled

over sodium before use. Pinacolone, pivaladdehyde, tetra-n-butylammonium

hexafluorophosphate (TBAPF6), tetra-n-butylammonium perchlorate (TBAP), and

lithium diisopropylamide (LDA) were purchased from Aldrich and used as

received. 3,4-Dimethoxythiophene (DMOT) was purchased from TCI America

82

and used as received. Dodecylbenzene sulfonic acid (DBSA) was purchased

from Acros Organics and used as received. Indium-doped Tin Oxide (ITO)-

coated glass slides (Rs = 8-12 CI) were purchased from Delta Technologies and

cleaned by sonication in acetone prior to use. The reaction scheme for

synthesizing 13ProDOT-TB2 involves trans-etherification reactions between

DMOT and a (3S,5R)-2,2,6,6-tetramethylheptane-3,5-diol under acid catalyzed

(DBSA) conditions. Synthetic details of all the monomers and polymers have

been dicussed in the later part of this chapter.

Electrochemistry. Electrochemical polymerizations, cyclic voltammetry, scan

rate studies, and spectroelectrochemistry were carried out with CHI 400 and

660A potentiostats. Solution studies were carried out using 0.1 M TBAPF6 / ACN

solutions, with respect to a non-aqueous Ag/Ag+ reference electrode, calibrated

to 0.445 V vs. NHE. Detailed electrochemistry is discussed later.

Optical Characterization. Film thicknesses were obtained using a Veeco

DekTak mechanical profilometer. Spectroelectrochemistry was carried out with a

Varian Cary 5000 UV-Vis-NIR spectrophotometer with a 150 mm DRA

Integrating Sphere and Color software. Color data were calculated using a 10

degree standard observer angle, a measurement range of 360 nm to 860 nm in 1

nm intervals, and a standard D65 illuminant.

83

3.3 Monomer Synthesis and Characterization

Synthesis of meso-2,2,6,6-Tetramethyl-3,5-heptanediol (TMHDiol):

Q Q HO ^ x 1.2equ.LDA V L / V y A . | y ^ 2.2 equ. DIBAL-H

THF,-78°C,12hrs | | THF, -78° C, 8 hrs Pivalaldehyde I I O H O H

Yield = 80% Yield = 70%

HTMH-One TMHDiol

Scheme 3.1 Synthesis of meso-2,2,6,6-Tetramethyl-3,5-heptanediol (TMHDiol)

5-Hydroxy-2,2,6,6,-tetramethyheptan-3-one (HTMH-One): meso-2,2,6,6-

Tetramethyl-3,5-heptanediol (TMHDiol) was synthesized by modified procedure

with respect to that reported earlier. 1 To a solution of pinacolone ( 10 g, 100

mmol), in anhydrous THF (500 mL), at -78QC was added a 2.0 M solution of LDA

in hexane (60 mL, 120 mmol) over a period of 30 min. The reaction was stirred

at -78QC for another 30 min. To the resulting white suspension pivalaldehyde

(10.9 mL, 100 mmol) was added drop-wise via syringe and the reaction

continued for another 12 hrs at room temperature. The reaction was quenched

by adding 10 mL of water. Approximately 80% THF was removed and the

mixture was then poured into saturated aqueous solution of NH4CI. The aqueous

layer was extracted twice with diethyl ether (200 mL) and the organic layer was

washed with plenty of water. The organic layer was then dried over MgS04 and

concentrated to give a crude yellow solid of p-hydroxy ketone (17.2 g, 95%). The

crude product was recrystallized from hexane to give pure white solid with a yield

of 81%.

84

1H NMR (CDCI3, 400 MHz, 5): 3.64 (IH,ddd), 3.16 (IH, d), 2.74 (IH, dd), 2.46 (IH,

dd), 1.14 (9H, s) 0.91 (9H, s); 13C-NMR: 209.23, 75.26, 38.11, 34.45, 26.50,

25.93; FTIR (KBr): 3475, 2800, 1700, 1460, 1380, 1360, 1320, 1280, 1060, 1000

cm-l.

Meso-2,2,6,6-Tetramethyl-3,5-heptanediol (TMHDiol): To a solution of p-

hydroxy ketone (10 g, 53 mmol) in THF (250 ml_) was added 1 M Dibal in

hexane, (118 ml_, 118 mmol) at -789C, and the solution was stirred for 2 hrs at

this temperature. The reaction mixture was allowed to warm to room temperature

and continued for another 8 hrs at room temperature. After 8 hrs the reaction

mixture was quenched with 2 N aqueous HCL solution. The mixture was then

extracted twice with ether (200 mL), and the combined organic layer was washed

with saturated aqueous NaHC03 solution and with brine. Drying with anhydrous

MgS04 and concentration gave the TMHDiol (9 g, 90% yield) as a white solid.

1H NMR (CDCI3, 400 MHz, 5): 3.54 (2H, bs), 3.45 (2H5 dd), 1.74 (1H, ddd), 1.31

(1H, ddd), 0.91 (18H, s); 13C-NMR: 81.82, 35.23, 31.29, 25.99, 25.80; FTIR

(KBr): 3400 (b), 2910, 1480, 1130, 880 cm-1. Anal. Calcd for C„H2402: C, 70.16;

H, 12.85. Found: C, 70.08; H, 12.57. ; HRMS (ESI): m/z [M + Na]+ calcd for

CnH2402Na: 211.1259; found: 211.1263.

85

Synthesis of 13ProDOT-TB2:

-o o-YX OH OH ^ °W° \ Q / Anh. Toluene, 110°C, 48 hrs

DBSA Q Yield - 45 %

Scheme 3.2 Synthesis of 13ProDOT-TB2.

Synthesis of the 13ProDOT-TB2: The synthesis of 13ProDOT-TB2 monomers

was carried out by transetherification of 3,4-dimethoxythiophene with (3S, 5R)-2,

2, 6, 6-tetramethylheptane-3, 5-diol, as shown in Scheme 3.2. The desired diol

was synthesized by previously reported procedure.[40] A three-neck round

bottom flask, with a magnetic stir bar inside, was first vacuum dried to make it

moisture free, fitted with a drying tube and the whole set up was maintained

under an inert atmosphere of Argon. The reaction solvent, dry tolune (ca. 500

mL) was then transferred to the reaction flask with a cannula. 3, 4-

Dimethoxythiophene (DMOT) (2 g, 14 mmol), (3S, 5R)-2, 2, 6, 6-

tetramethylheptane-3, 5-diol (28 mmol) and dodecylbenzene sulfonic acid (0.67

g, 2.08 mmol) (DBSA), were combined and fitted with a Soxhlet extractor with

type 4A molecular sieves in the thimble. The molar proportion between the

reactants is thus maintained to DMOT : (3S,5R)-2,2,6,6-tetramethylheptane-3,5-

diol : DBSA = 1.00 : 2.00 : 0.15. The solution was heated to reflux and allowed to

reflux for 24 hrs. The reaction mixture was cooled and the toluene was removed

under vacuum. A viscous, oily (greenish/brownish) substance was obtained,

86

which was then dissolved into small amount of chloroform and extracted with

water three times to remove any water soluble component from the crude

reaction mixture. Due to the presence of DBSA, while doing the extraction with

water, an emulsifier results in. Solid NaCI was used as an emulsion breaker. The

organic layer (CHCI3) was collected and the solvent is subsequently evaporated

to obtain light brown oily liquid. The crude product was purified by column

chromatography on a silica gel with Ethyl acetate and n-Hexane (Ethyl acetate:

n-Hexane = 05:95) as eluent to obtain a white solid (45%, 1.36 g).

1H NMR (CDCI3, 400 MHz, 5): 6.50 (2H, s), 3.28 (2H, dd), 2.15 (1H, dd), 1.81

(1H, d), 1.03 (18 H, s); 13C NMR: 150.93, 106.40, 90.65, 35.49, 33.30, 29.91,

26.38; GC-MS (m/z): 268; EA: Anal, calculated for C15H24O2S: C 67.16, H 13.04;

found: C 66.01, H 13.24.

Synthesis of 13ProDOT-IP2

O \ ^ V . 1.2 equ. LDA ,

| ^ THF,-78°C, 12hrs I Isobutyraldehyde

Yield = 80% Yield = 70%

Scheme 3.3 Synthesis of 5-hydroxy-2,6-dimethylheptan-3-one (HDMH-One)

yj yjn 2.2 equ. DIBAL-H

THF, -78° C, 8 hrs

r\LJ A U

87

5-hydroxy-2,6-dimethylheptan-3-one (HDMH-One): To a solution of 3-

methylbutan-2-one ( 10 g, 116 mmol), in anhydrous THF (500 mL), at -78QC was

added a 2.0 M solution of LDA in hexane (70 mL, 140 mmol) over a period of 30

min. The reaction was stirred at -78QC for another 30 min. To the resulting white

suspension Isobutyraldehyde (8.4 mL, 116 mmol) was added drop-wise via

syringe and the reaction continued for another 12 hrs at room temperature. The

reaction was quenched by adding 10 mL of water. Approximately 80% THF was

removed and the mixture was then poured into saturated aqueous solution of

NH4CI. The aqueous layer was extracted twice with diethyl ether (200 mL) and

the organic layer was washed with plenty of water. The organic layer was then

dried over MgS04 and concentrated to give a crude white oil of p-hydroxy ketone

(16.5 g, 90%). The crude product was recrystallized from petroleum ether to give

pure white solid with a yield of 76%.

1H NMR (CDCI3, 400 MHz, 5): 3.61 (IH,ddd), 3.12 (IH, d), 2.68 (IH, dd)5 2.40 (IH,

dd), 1.12 (6H, s) 0.89 (6H, s); 13C-NMR: 209.45, 75.11, 38.03, 34.41, 26.35;

FTIR (KBr): 3475, 2800, 1700, 1460, 1380, 1360, 1320, 1280, 1060, 1000 cm-l.

(3R,5S)-2,6-dimethylheptane-3,5-diol (DMH-Diol): To a solution of (3-hydroxy

ketone (10 g, 63 mmol) in THF (250 mL) was added 1 M Dibal in hexane, (126

mL, 126 mmol) at -78QC, and the solution was stirred for 2 hrs at this

temperature. The reaction mixture was allowed to warm to room temperature and

continued for another 8 hrs at room temperature. After 8 hrs the reaction mixture

was quenched with 2 N aqueous HC1 solution. The mixture was then extracted

88

twice with ether (200 ml_), and the combined organic layer was washed with

saturated aqueous NaHCC>3 solution and with brine. Drying with anhydrous

MgS04 and concentration gave the crude DMHDiol (8.5 g, 84 % yield) as a white

solid. The crude diol was further purified by column chromatography using

petroleum ether and ethyl acetate mixture (80:20) to give purified product with a

yield of 72%.

1H NMR (CDCI3, 400 MHz, 5): 3.51 (2H, bs), 3.43 (2H, dd), 1.72 (IH, ddd), 1.29

(IH, ddd), 0.89 (12H, s); 13C-NMR: 80.23, 35.45, 31.42, 26.09, 25.89; FTIR

(KBr): 3400 (b), 2910, 1480, 1130, 880 cm-1. Anal. Calcd for C9H2002: C, 67.5;

H, 12.50. Found: C, 67.38; H, 12.41. ; HRMS (ESI): m/z [M + Na]+ calcd for

C9Hi902Na: 182.1259; found: 182.1265.

o o OH OH ^ \—/

Anh. Toluene, 110° C, 24 hrs DBSA

Yield - 47 %

Scheme 3.4 Synthesis of 13ProDOT-IP2

Synthesis of the 13ProDOT-IP2: Similar procedure was followed for the

synthesis of 13ProDOT-IP2 monomer as that of 13ProDOT-Me2, shown in

Supporting Scheme 3. The molar proportion between the reactants is maintained

to DMOT : (3R,5S)-2,6-dimethylheptane-3,5-diol: DBSA = 1.00 : 2.00 : 0.15. 3, 4-

Dimethoxythiophene (DMOT) (2 g, 14 mmol), (3R,5S)-2,6-dimethylheptane-3,5-

89

diol (28 mmol) and dodecylbenzene sulfonic acid (0.67 g, 2.08 mmol) (DBSA),

were reacted to yield 2.3 g of crude 13ProDOT-IP2. The crude product was

purified by column chromatography on a silica gel with toluene and hexane

mixture (25:75) as eluent to obtain a light yellow solid (47%, 1.56 g).

1H NMR (CDCI3, 400 MHz, 5): 6.50 (2H, s), 3.96 (2H, m), 2.10 (1H, m), 1.90 (1H,

m), 1.55 (3H, s), 1.25 (3H,s); 13C NMR: 150.63, 106.26, 90.45, 35.32, 23.63;

GC-MS (m/z): 240; EA: Anal, calculated for C9H12O2S: C 58.69, H 6.52; found: C

57.89, H 6.73.

Synthesis of 13ProDOT-Me2

OH OH Anh. Toluene, 110°C, 24 hrs

DBSA // w

S" Yield - 55 %

Scheme 3.5 Synthesis of 13ProDOT-Me2.

Synthesis of the 13ProDOT-Me2: The synthesis of 13ProDOT-Me2 monomers

was carried out by transetherification of 3,4-dimethoxythiophene with (2S,4S)-

2,4-Pantane-diol, as shown in Scheme 3.5. The desired diol was purchased from

Sigma Aldrich as used as received. The reaction procedure was similar as

described for 13ProDOT-TB2. The molar proportion between the reactants is

90

maintained to DMOT : (2S,4S)-2,4-Pantane-diol : DBSA = 1.00 : 2.00 : 0.15. 3, 4-

Dimethoxythiophene (DMOT) (2 g, 14 mmol), (2S,4S)-2,4-Pantane-diol (28

mmol) and dodecylbenzene sulfonic acid (0.67 g, 2.08 mmol) (DBSA), were

reacted to yield 1.6 g of crude 13ProDOT-Me2. The crude product was purified

by column chromatography on a silica gel with Toluene as eluent to obtain a

white solid (55%, 1.40 g).

1H NMR (CDCI3, 400 MHz, 5): 6.50 (2H, s), 3.96 (2H, m), 2.10 (1H, m), 1.90 (1H,

m), 1.55 (3H, s), 1.25 (3H,s); 13C NMR: 150.63, 106.26, 90.45, 35.32, 23.63;

GC-MS (m/z): 184; EA: Anal, calculated for C9H1202S: C 58.69, H 6.52; found: C

57.89, H 6.73.

Synthesis of pentadecane-7, 9-diol:

II || 1)DIBAL-H,Anh. Ether,-78 DegC C 6 H i 3 \ ^ ^ \ ^ C 6 H 1 3

CzHsO^^^^OCzHs 2) C6H13MgBr, RT, 2 Hrs ^ ^

Yield - 42 %

Scheme 3.6 Synthesis of pentadecane-7, 9-diol

A solution of 1M DIBAL-H in toluene (125.0 mL, 125 mmol) was slowly added

over a period of 10 mins, to a solution of diethylmalonate (10.0 mL, 62.5 mmol) in

Et20 (13.6 mL) at -78 °C under N2. The internal temperature was maintained -78

°C. The mixture was stirred for 1 h at -78 °C. A solution of hexylmagnesium

bromide in Et20 (65.0 mL, 130 mmol) was added at-78 °C. The reaction mixture

was then warmed to r.t. and stirred for 6 h. The mixture was quenched with

91

saturated NH4CI solution at 0 °C. A saturated solution of Rochelle's salt was

added at room temperature and the two-phase mixture was stirred for

approximately 8 h. The aqueous layer was extracted with ethyl acetate. The

combined organic layers were dried over Na2S04. The solvent was removed in

vacuum, and the resulting material was purified via column chromatography

(30% Ethyacetate - 70% hexanes) to provide the desired product (6.4 g, 42%).

1H-NMR (500 MHz, CDCI3,mixture of diastereomers): 3.96 (2H, m), 2.17 (2H, br),

1.64 (2H, m), 1.55-1.22 (16H, m), 0.96 (6 H, t); 13C-NMR (mixture of

diastereomers): 69.45, 42.69, 37.63, 34.04, 31.74, 29.23, 25.65, 22.49, 22.20,

13.86; HRMS (ESI): m/z [M + Na]+ calculated for Ci3H2602Na2: 260.17; found:

260.18.

Synthesis of 13ProDOT-Hexyl2

-O O - ^ 6 H I 3 N A X ^ S J ^ C 6 H 1 3 * r T * ^6Hi3\^^-v./CeH-13

\ { I I O O ^ OH OH . O ^ Anh. Toluene, 110°C, 24 hrs // \

DBSA ^ Q '

Yield - 44 %

Scheme 3.7 Synthesis of 13ProDOT-Hexyl2

92

Synthesis of 13ProDOT-Hexyl2: The 3-neck RBF, with a magnetic stir bar

inside, was first vacuum dried to make it moisture free, fitted with thermometer

and the drying tube and the whole set up was maintained under an inert

atmosphere of N2. The reaction solvent, dry tolune (~ 500 mL) was then

transferred to the RBF with a cannula. 3, 4-Dimethoxythiophene (DMOT) (2 g, 14

mmol), 1, 5-dihexyl-pentane-2, 4-diol (28 mmol) and dodecylbenzene sulfonic

acid (0.67 g, 2.08 mmol) (DBSA), were combined and fitted with a Soxhlet

extractor with type 4 A molecular sieves in the thimble. The molar proportion

between the reactants is thus maintained to DMOT: 1, 5-dihexyl-pentane-2, 4-

diol: DBSA = 1.00: 2.00: 0.15. The reaction is shown in scheme 3.7. The solution

was heated to reflux and allowed to reflux for 24 h. The reaction mixture was

cooled and the toluene was removed under vacuum. A viscous oily dark greenish

substance was obtained, which was then dissolved into small amount of

chloroform and extracted with water three times to remove any water soluble

component from the crude reaction mixture. Due to the presence of DBSA, while

doing the extraction with water, an emulsifier results in. Solid NaCI was used as

an emulsion breaker. The organic layer (CHCI3) was collected and the solvent is

subsequently evaporated to obtain a green/brown oily liquid. The crude product

was purified by column chromatography on a silica gel using Ethyl acetate and n-

Hexane (Ethyl acetate: n-Hexane = 05:95) solvent mixtures with 1.98 gm of

purified product.

1H-NMR (500 MHz, CDCI3): 6.37 (2H, s), 4.28 (2H, m), 2.02 (2H, t), 1.75 (4H, q),

1.53 (4H, m), 1.33 (12H, m) 0.94 (6H, t); 13C-NMR: 149.03, 103.55, 78.46,

93

43.30, 35.52, 31.69, 29.60, 29.05, 25.69, 22.46, 13.82; GC-MS (m/z): 324; FTIR

(liquid film): 3100, 1554, 1493, 1323, 1294, 1152, 1083, 809 and 768 cm-1.

Synthesis of 1, 5-diphenylpentane-2, 4-diol:

O O 1) DIBAL-H, Anh. Ether, -78 Deg C C e H s h ^ C x ^ ^ ^ v ^ ^ C I ^ C e H s II M 1) DIBAL-H, Anh. Ether, -78 Deg C u 6 M 5 n 2 ^ \ ^ / s \ ^

C2H5O^^T>C2H5 2)C6H13MgBr,RT,2Hrs J ^ ^

Yield - 46 %

Scheme 3.8 Synthesis of 1, 5-diphenylpentane-2, 4-diol

Synthesis of 1, 5-diphenylpentane-2, 4-diol: A solution of 1M DIBAL-H in

toluene (125.0 mL, 125 mmol) was slowly added over a period of 10 mins, to a

solution of diethylmalonate (10.0 mL, 62.5 mmol) in Et20 (13.6 mL) at -78 °C

under N2. The internal temperature was maintained -78 °C. The mixture was

stirred for 1 h at -78 °C. A solution of freshly prepared benzylmagnesium

chloride in Et20 (65.0 mL, 130 mmol) was added at -78 °C. The reaction is

shown in scheme 3.8. The reaction mixture was then warmed to room

temperature and stirred for 6 h. The mixture was quenched with saturated NH4CI

solution at 0 °C. A saturated solution of Rochelle's salt was added at r.t., and the

two-phase mixture was stirred for approximately 8 h. The aqueous layer was

extracted with ethyl acetate. The combined organic layers were dried over

Na2S04. The solvent was removed in vacuum, and the resulting material was

purified via column chromatography (30% Ethylacetate-70% hexanes) to provide

the desired product (7.2 g, 45%).

94

1H-NMR (500 MHz, CDCI3, mixture of diastereomers): 7.33-7.21 (12H, m), 4.24

(1H, m), 4.09 (1H, m), 3.12 (1 H, br), 2.81-2.77 (4H, m), 2.41 (1H, br) ; 13C-NMR

(mixture of diastereomers): 138.36, 138.09, 129.39, 129.32, 128.52, 128.50,

73.22, 70.07, 44.52, 44.14, 42.21, 41.81; HRMS (ESI): m/z [M + Na]+ calculated

for Ci7Hi802Na2: 300.11; found: 300.11.

Synthesis of 13ProDOt-Bz2:

-O CK C6H5H2CS^^CH2C6H5 Y^C \—/ L. L. Q o o OH OH w W

O ^ Anh. Toluene, 110°C, 24 hrs ^ DBSA ^Q'

Yield - 47 %

Scheme 3.9 Synthesis of 13ProDOt-Bz2

Synthesis of 13ProDOt-Bz2: The 3-neck RBF, with a magnetic stir bar inside,

was first vacuum dried to make it moisture free, fitted with thermometer and the

drying tube and the whole set up was maintained under an inert atmosphere of

N2. The reaction solvent, dry tolune (~ 500 mL) was then transferred to the RBF

with a cannula. 3, 4-Dimethoxythiophene (DMOT) (2 g, 14 mmol), 1, 5-dibenzyl-

pentane-2, 4-diol (28 mmol) and dodecylbenzene sulfonic acid (0.67 g, 2.08

mmol) (DBSA), were combined and fitted with a Soxhlet extractor with type 4 A

molecular sieves in the thimble. The reaction is shown in scheme 3.9. The molar

proportion between the reactants is thus maintained to DMOT: 1, 5-dibenzyl-

95

pentane-2, 4-diol: DBSA = 1.00: 2.00: 0.15. The solution was heated to reflux

and allowed to reflux for 24 h. The reaction mixture was cooled and the toluene

was removed under vacuum. A viscous oily (greenish/brownish) substance was

obtained, which was then dissolved into small amount of chloroform and

extracted with water three times to remove any water soluble component from

the crude reaction mixture. Due to the presence of DBSA, while doing the

extraction with water, an emulsifier results in. Solid NaCI was used as an

emulsion breaker. The organic layer (CHCI3) was collected and the solvent is

subsequently evaporated to obtain a green/brown oily liquid. The crude product

was purified by column chromatography using Ethyl acetate and n-Hexane (Ethyl

acetate: n-Hexane = 10:90) solvent mixtures.

1H-NMR (500 MHz, CDCI3): 6.28 (2H, s), 4.52 (2H, m), 3.04 (2H, m), 2.78 (2H,

m), 2.06 (2 H, m); 13C-NMR: 148.43, 137.84, 129.33, 129.21, 128.30, 128.26,

126.41, 104.20, 78.79, 41.82, 41.60; GC-MS (m/z): 336; FTIR (liquid film): 3100,

1554, 1493, 1323, 1294, 1152, 1083, 809 and 768 cm1-

3.3Poly13ProDOT-R2

3.3.1 Electrochemical synthesis and characterization :

All electrochemical experiments were performed using a three-cell

configuration in 0.1 M TBAPF6/ACN with a CHI 400a potentiostat. The reference

electrode was a nonaqueous Ag/Ag+ electrode consisting of a silver wire

immersed in a glass capillary body fitted with a Vycor tip and filled with 0.1 M

96

silver nitrate (AgN03)/ACN and 0.1 M TBAPFe/ACN solutions. The Ag/Ag+

reference electrode was calibrated to be 0.44V vs. the normal hydrogen

electrode (NHE) using a 10 mM ferrocene/ACN solution. A 1 cm2 platinum flag

was used as a counter electrode in all electrochemical measurements. We define

"conventional electrochemical polymerization" as the method by which the

monomer is dissolved in electrolyte solution and a potential is applied to the

working electrode by which oxidation of the monomer takes place to form

corresponding polymer. After polymerization, the polymer-coated Pt (working

electrode was washed with ACN to remove residual monomers and oligomers.

The cyclovoltammogram of the polymer was obtained in monomer-free solution

in order to isolate the electrochemical processes of the polymer. All the

electrochemical polymerizations were done by using a solution containing 0.1 M

monomer and 0.1 M TBAP as supporting electrolytes in ACN by the scanning

potential ranging from -1 to 1.3 V. In all cases the irreversible oxidation of the

monomer was observed at 1.1 V except for 13ProDOT-TB2 where the irreversible

monomer oxidation occurred at 1.3 V.

The monomer oxidation potentials are similar to other alkylenedioxythiophenes

indicates that the long and bulky alkyl chain substituents do not have significant

effect on the monomer oxidation potential. 1,3-ProDOT-Hex2 was unable to be

electrochemically polymerized/deposited onto a ITO coated glass slide working

electrode because of poor film quality which peel from the electrode after a few

scans. Cyclic voltametry of all the 13ProDOT-R2 (except 13ProDOT-Hex2) are

shown in Figure 3.3 - 3.6.

97

50

0

-50

-100

-150-

-200

-250

-300 -j

-350 J

1.5 1.0 0.5 0.0

Potential (V)

-0.5 -1.0

Figure 3.3 Electrochemical polymerization of 10 mM 13ProDOT-TB2 in 0.1 M

TBABF6/ACN at a scan rate of 100 mV/s using platinum button working

electrode. Potentials are reported vs Ag/Ag+ non-aqueous reference electrode

(0.44 V vs NHE).

98

-350 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

Potential (V)

Figure 3.4 Electrochemical polymerization of 10 mM 13ProDOT-IP2 in 0.1 M

TBABF6/ACN at a scan rate of 100 mV/s using platinum button working

electrode. Potentials are reported vs Ag/Ag+ non-aqueous reference electrode

(0.44 V vs NHE).

99

i c

o

-1000 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

Potential (V)

Figure 3.5 Electrochemical polymerization of 10 mM 13ProDOT-Me2 in 0.1 M

TBABF6/ACN at a scan rate of 100 mV/s using platinum button working

electrode. Potentials are reported vs Ag/Ag+ non-aqueous reference electrode

(0.44 V vs NHE).

100

50

0

-50

^ -100

1 -150

O -200

-250

-300-

-350-

1.5 1.0 0.5 0.0 -0.5 -1.0

Potential(V)

Figure 3.6 Electrochemical polymerization of 10 mM 13ProDOT-Bz2 in 0.1 M

TBABF6/ACN at a scan rate of 100 mV/s using platinum button working

electrode. Potentials are reported vs Ag/Ag+ non-aqueous reference electrode

(0.44 V vs NHE).

3.3.2 Scan Rate Dependency and Redox Switching

For this study, polymer was prepared by electrochemical polymerization using

the similar experimental parameters as described above except that only 5 scans

of cyclovoltammetry were performed instead of 20 scans. After electrochemical

polymerization, polymer redox behavior was observed by using the monomer

101

free electrolyte solution, and it should be noted that the electrochemical

characterization was performed using the same electrolyte as that used for

electrochemical polymerization.

The redox behavior of polymers was studied through the recording of cyclic

voltammograms at different scanning rates between -0.6 and 0.6 V in 0.1 M

TBAP in ACN. The linear relationship was observed between the peak current

and the scanning rate indicated the formation of a redox-active and well-adhered

polymer on the electrode. The Ep values of all the polymers increased upon an

increase in the scanning rate which indicated the quasireversible redox process.

EQCM was used in order to maintain similar film masses for the comparison of

scan rates between polymer systems. Scan rate dependency of all the

13ProDOT-R2are shown in Figure 3.7-3.11.

102

5 w :Z3-

o -200

-400-J

*S0O H

0*G -0+1 -Q.2

Potential (V)

Figure 3.7: CV scans of polymer, P13ProDOT-TB2, deposited onto Pt button

electrode at different scan rates varying from 100 mV/s to 10000 mV/s with an

interval of 100 mV/s in 0.1 M TBABF4/ACN electrolyte solution, a) 100 mV/s3 b)

200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s, g) 700 mV/s, h)

800 mV/s, i) 900 mV/s, j) 1000 mV/s.Potential reported vs Ag/Ag+ non-aqueous

reference electrode (0.44 V vs NHE)

103

Cur

rent

/ \iA

60- |

5 0 -

4 0 -

3 0 -

2 0 -

1 0 H

oJ -10 J

-20 J

-30 J -40 J

-50 J -60 J

-70 J -80 J

-I I I I I I I I I I I I I I I I

0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5

Potential / V

Figure 3.8 CV scans of polymer, P13ProDOT-IP2> deposited onto Pt button

electrode at different scan rates varying from 100 mV/s to 10000 mV/s with an

interval of 100 mV/s in 0.1 M TBABF4/ACN electrolyte solution, a) 100 mV/s, b)

200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s, g) 700 mV/s, h)

800 mV/s, i) 900 mV/s, j) 1000 mV/s.Potential reported vs Ag/Ag+ non-aqueous

reference electrode (0.44 V vs NHE)

104

€00

§J 0.4 0,2 0.0 -0,2 -0,4 -0,6

Potential m

Figure 3.9: CV scans of polymer, P13ProDOT-Me2) deposited onto Pt button

electrode at different scan rates varying from 100 mV/s to 10000 mV/s with an

interval of 100 mV/s in 0.1 M TBABF4/ACN electrolyte solution, a) 100 mV/s, b)

200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s, g) 700 mV/s, h)

800 mV/s, i) 900 mV/s, j) 1000 mV/s.Potential reported vs Ag/Ag+ non-aqueous

reference electrode (0.44 V vs NHE)

105

Potential (V)

Figure 3.10 CV scans of polymer, P13ProDOT-Hex2, drop casted onto Pt button

electrode at different scan rates varying from 100 mV/s to 10000 mV/s with an

interval of 100 mV/s in 0.1 M TBABF4/ACN electrolyte solution, a) 100 mV/s, b)

200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s, g) 700 mV/s, h)

800 mV/s, i) 900 mV/s, j) 1000 mV/s.Potential reported vs Ag/Ag+ non-aqueous

reference electrode (0.44 V vs NHE)

106

60

T—'—r— r*""~T—*"—i—'i •—i « i ' i •-- > » i ' i 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8

Potential |V)

Figure 3.11 CV scans of polymer, P13ProDOT-Bz2, deposited onto Pt button

electrode at different scan rates varying from 100 mV/s to 10000 mV/s with an

interval of 100 mV/s in 0.1 M TBABF4/ACN electrolyte solution, a) 100 mV/s, b)

200 mV/s, c) 300 mV/s, d) 400 mV/s, e) 500 mV/s, f) 600 mV/s, g) 700 mV/s, h)

800 mV/s, i) 900 mV/s, j) 1000 mV/s.Potential reported vs Ag/Ag+ non-aqueous

reference electrode (0.44 V vs NHE).

107

3.3.3 Optical Properties

3.3.3.1 In-situ Spectroelectrochemistry

Spectroelectrochemistry: The spectroelectrochemistry of all the polymers was

performed with ITO-coated glass as a working electrode.The polymers were

formed via cyclovoltammetric growth from 0.01 M monomer solution in 0.1 M

TBAP/acetonitrile by scanning the potential between -1.0 and 1.3 V for 20 cycles

at a scan rate of 100mV/s. The polymer films were then repeatedly washed with

ACN to remove the monomers and oligomers and then placed into a glass

cuvette with monomer free electrolyte. The UV-Vis spectra of the polymers were

recorded by using Ag/Ag+ nonaqueous reference electrode described in the

electrochemistry section, and the counter electrode was another ITO-coated

glass which allows for the passage of light. All optoelectrochemical experiments

were carried out under a nitrogen atmosphere.

At -1.0 V P13ProDOT-TB2 film was yellow in color and a peak was observed at

max value of 365 nm. Upon the stepwise increase of oxidation potential of

P13ProDOT-TB2 the absorbance of the K-K* transition decreased, and the peak

due to the polaron increased at a higher wavelength IR region. At 1.0 V the

polymer became green in color.

For P13ProDOT-Bz2, two peaks were observed at 573 and 623 nm at -1.0 V

which is due to the vibronic coupling. 15,19.At 1.0 V, the absorbance of n-n*

decreased, and the polymer became transparent.

108

The photopic contrast was calculated from the difference in percentage

transmittance (%T) between the completely reduced and oxidized states and the

contrast was 65% for P13ProDOT-Bz2.

The spectroelectrochemistry of all other polymers was performed by following

the similar procedure and P13ProDOT-Me2 also showed two peaks at Xvnax

values of 550 and 576 nm. P13ProDOT-Me2 also exhibit blue to transparent

upon stepwise oxidation with an optical contrast of 62 %. Since the chemically

polymerized DHP was soluble in organic solvents, we studied the

spectroelectrochemical properties of the thin films of these polymer. The

spectroelectrochemistry of the chemically synthesized soluble P13ProDOT-Hex2

was performed by spray casting polymer solution (5 mg/mL in toluene) onto the

glass coated ITO using an air brush followed by the vacuum drying of the

polymer film. The spray casted thin film of chemically synthesized P13ProDOT-

Hex2 showed a Xmax value of 435 nm. The solution spectra of P13ProDOT-Hex2

showed at X max value of 430 nm, 5 nm less than that of thin film because of the

better packing of the polymer chains in the solid state.

The thin film of P13ProDOT-Hex2 was electroactive when switched between

the two redox states with electrochemical switching methods indicated that the

chemical nature of the polymers remained unaffected, in spite of having different

polymerization route. The optical contrast of chemically synthesized P13ProDOT-

Hex2 was much lower that that of other polymer, which could be due to the poor

quality of the film obtained by spray casting from solution.

109

I • I ' I • I ' I • I • I ' I ' I • I

400 600 800 1000 1200 1400 1600 1800 2000 2200

Wavelength (nm)

Figure 3.12 Spectroelectrochemical data (350-2200 nm) for P13ProDOT-TB2 film

(100 nm thick) on ITO-coated glass at applied potentials of a) -1.1, b) -0.9, c) -

0.5, d) -0.1, e) 0.0, f) 0.1, g) 0.3 Ag/Ag+ reference (0.445V vs. NHE). The gap

between 1650 and 1750 nm is due to noise from the solvent which has been

deleted to show clean spectra.

110

3.2

— I ' 1 -' 1 ", 1 " T """ 1 •'* 1

400 600 800 1000 1200 1400 1800

Wavelength (ran)

Figure 3.13 Spectroelectrochemical data for P13ProDOT-IP2 film (ca. 250nm

thick) on ITO-coated glass at applied potentials of a) -0.2, b) -0.1 c) 0.0, d) 0.1, e)

0.2 V, f) 0.3 V versus Ag/Ag+ reference (0.445 V vs. NHE).

I l l

T_,»^^ ^ ^ y ^ ^ ^ 400 600 800 1000 1200 1400 1600 1800 2000

Wavelength (nm)

Figure 3.14 Spectroelectrochemical data for P13ProDOT-Me2 film (ca. 250nm

thick) on ITO-coated glass at applied potentials of a) -0.6, b) 0.0 c) 0.1, d) 0.2, e)

0.5 V versus Ag/Ag+ reference (0.445 V vs. NHE).

112

500 1000 1500

Wavelength (nm)

2000

Figure 3.15: Spectroelectrochemical data for P13ProDOT-Bz2 film (ca. 250nm

thick) on ITO-coated glass at applied potentials of a) -0.6, b) 0.0 c) 0.1, d) 0.2, e)

0.5 V versus Ag/Ag+ reference (0.445 V vs. NHE).

113

400 600 - i 1 1 1 1 1 1 1 1 1 1 , 1-

800 1000 1200 1400 1600 1800 2000

Wavelength (nm)

Figure 3.16 Spectroelectrochemical data for P13ProDOT-Hex2 film (ca. 250nm

thick) on ITO-coated glass at applied potentials of a) -0.6, b) 0.0 c) 0.1, d) 0.2, e)

0.5 V versus Ag/Ag+ reference (0.445 V vs. NHE).

114

3.3.4 Chemical Polymerization

Chemical polymerization of 13ProDOT-TB2 and 13ProDOT-Hex2:

Chemical polymerization of monomer (13ProDOT-TB2) was carried out by

using 3 equivalents of FeCl3 in dry chloroform. The reaction was continued for 48

hours followed by the addition of hydrazine hydrate to obtain the reduced form of

polymer. The reduced polymer was purified by Soxhlet extraction using methanol

as solvent for 24 hours. The final polymer was obtained by washing the Soxhlet

extractor by chloroform. The molecular weight of the purified polymer was

determined by GPC with polystyrene as the standard and THF as the mobile

phase. The GPC results gave number-average molecular weight 6240 and

weight average molecular weights 10628 g mol-1 with polydispersity 1.7. The 1H

NMR spectra of the soluble polymer showed broader peaks without having the

thiophene end groups indicated the formation of congugated polymer. The

solution oxidation of chemically polymerized P13ProDOT-TB2 was also studied

by UV-vis-NIR spectroscopy in THF as a function of oxidant concentration using

SbCI5. The polymerization reactions are shown is scheme 3.10.

115

o o

9 FeCI3, Dry CHCI3

3 Days

CfiH 6 n 1 3 \ ^ - \ / ^ 6 r , 1 3

o o

9 FeCI3, Dry CHCI3

3 Days " ^ 0 \ _ / 0

Yield - 45 %

Scheme 3.10 Chemical Polymerization of 13ProDOT-TB2and 13ProDOT-Hex2

116

0 o W w o m <

0,2 H

"i * r 400 600

"1 * I * f r—T 800 1000 1200 1400 1800 1800 2000

Wavelength (nirt)

Figure 3.17 Spectra of P13ProDOT-TB2, electrochemically (chronocoulometry at

+1.1 V) deposited on ITO. Pictures are for polymer dissolved in dichloromethane

(oxidized, green) and then reduced by 2% solution of hydrazine in

dichloromethane (reduced, yellow).

117

Abs

orba

nce

3.2-

2.8-

2.4-

2.0-

1.6-

1.2-

0.8-

0.4-

0.0- " ^ —""^Ifes^aBS^J ^ ^ - « ^ » * M ™ * ^ . . ^ . r J _ _ J J ^ . . . . . . ^ ^ . . . i ^ ^ ^ ^ ^ . „._,.... t.^ |

| 1 | 1 | 1 | 1 | 1 | . • • ! • • • • 400 600 800 1000 1200 1400 1600 1800 2000 2200

Wavelength (nm)

Figure 3.18 Solution oxidation of chemically polymerized(FeCI3) P13ProDOT-

TB2 by UV-vis-NIR spectroscopy in THF as a function of oxidant concentration

using SbCI5.

118

PolyDiHexylProDOT

0.0-

400

Oxidized Neutral

— i • 1— 600 800

Wavelength (nm)

1000

Figure 3.19 Spectra of P13ProDOT-Hex2, electrochemically (chronocoulometry

at +1.1 V) deposited on ITO. Pictures are for polymer dissolved in

dichloromethane (oxidized, green) and then reduced by 2% solution of hydrazine

in dichloromethane (reduced, yellow

"vrrlr \ ^ t ' i ;

119

Figure 3.20 Spray coated P13ProDOT-Hex2 on ITO.

3.3.5 Color Analysis

Table 3.1 Color Cordinates (CIE u' v ' ) for ProDOT Polymers. Situation: Black

Body @ 6000K.

Polymer

d) Poly13ProDOT-Me2

a) Poly13ProDOT-TB2

c)Poly13ProDOT-IP2

e) PolyProDOT-Bz2

b)Poly13ProDOT-Hex2

u'

0.2365

0.1975

0.4608

0.1641

0.2431

V'

0.1628

0.4772

0.4092

0.4104

0.5343

120

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

U'

Figure 3.21 CIE u' v' coordinate plot of the neutral states of P13ProDOT-TB2

(a), P13ProDOT-Hex2 (b), P13ProDOT~IP2 (c), PProDOT~Me2 (d) and

P22ProDOT-Bz2 (e, square) reported by Reynolds et a.l

3.4 DFT Analysis of Monomers and Dinners

In this part of the study, monomers subject to investigation and their dimers

have been analyzed employing hybrid density functional B3LYP[41] with basis

set 6-31G(d,p). All the calculations were performed using the Gaussian 03 suit of

programs[42]. Input files are prepared using GaussView 3.0[43] and visualization

121

of calculations results were done using the same program. All the structures of

the monomers were fully optimized without any geometrical restrictions. Full

optimization was checked by obtaining a vibrational spectrum without imaginary

frequencies. Optimized structures for the monomers 13ProDOT-TB2 and

ProDOT-Me2 are shown in Table 4. Both 13ProDOT-TB2 and ProDOT-Me2 have

symmetry Cs similar to unsubstituted ProDOT (its structure was optimized at the

same theory level but its geometry is not given in the figure). All four C atoms on

the thiophene part which will form conjugated backbone of polymer are in the

same plane for all the structures in this study. The oxygen atoms are nearly on

the same plane with the C atoms of thiophene ring for all the structures, since the

dihedral angle for C-C-C-0 is nearly same. Here, three C atoms belong to the

thiophene ring. The dihedral angle, defined with atoms of thiophene ring, C-C-C-

S, was the same for all the structures: -1.14, -1.07, -1.08 for ProDOT, ProDOT-

Me2 and 13ProDOT-TB2, respectively. The sulfur atom is slightly directed to the

same side as the C atoms to which the substituents are connected.

Since the valence and conduction bands are formed from the HOMOs and

LUMOs of the monomers on the chain, some predictions may be done on the

electronic properties of the polymers based on its monomers. HOMO and LUMO

energy levels of three monomers are given in Supporting Table 3. Having two

methyl substitution on the 2,2 positions causes nearly no differences in the

HOMO and LUMO energy levels. t-Butyl substitution into the 1,3 positions

causes a slightly larger effect, but approximately the same again. A 0.06 eV shift

in HOMO and LUMO energy levels occured, which resulted in the same energy

122

gap. So it may be concluded that experimental finding on the band gap

differences of polymers, given in this study, cannot be explained based simply on

the electronic properties of monomers, since all of them have approximately the

same electronic structure with nearly the same geometry. There must be another

factor causing the higher band gap value of P13ProDOT-TB2 with respect to

ProDOT-Me2.

Mean deviation from planarity affects also band gap value for the polymers[44].

The dihedral angle of dimers gives us enough qualitative information about

dihedral angles between consecutive units on the polymer chain. Thus, dimers of

the monomers studied in this work have been optimized with same method. Also

included was the dimer of 22ProDOT-TB2, which is a theoretical monomer (as no

one has synthesized it, to date) used as a comparison with calculations for

13ProDOT-TB2. The dimer of unsubstituted poly(ProDOT) was also optimized to

compare with these substituted ones. Optimized structures of the dimers are

given in Table 3.2. The dimer of unsubstituted poly(ProDOT) was not shown

since its structure is very similar to ProDOT-Me2. Both ProDOT and ProDOT-Me2

dimers have thiophene rings approximately in the same plane. But the dimer of

P13ProDOT-TB2 has a high deviation from planarity. The C-C-C-C dihedral

angle, including C atoms which are subject to inter-ring bonding is -76.1 degrees.

As seen from Table 3.3, HOMO and LUMO energy levels of ProDOT and

ProDOT-Me2 are nearly same. So substitution of methyl groups into the 2,2

position does not affect HOMO and LUMO energy levels. These values for

P13ProDOT-TB2 are rather different from previous ones. HOMO energy level is

123

lowered to -5.70 eV, since LUMO energy level is shifted to a higher value, -0.44

eV. This effect comes from the fact that t-butyl substituents cause twisting along

the backbone. The bond length between the thiophene rings is 1.446 and 1.445

A for ProDOT and ProDOT-Me2, respectively. This value is also altered as a

result of twisting and shifted to 1.458 A. The first C-C bond length after the inter­

ring bond in the ring was 1.382 and 1.381 A for ProDOT and ProDOT-Me2,

respectively. This value is 1.377 A for 13ProDOT-TB2j shorter than that of other

dimers. The stretching of the inter-ring bond and shortening of the next bond in

the ring is a measure of decreasing conjugation and opening of the gap between

HOMO and LUMO energy levels. 13ProDOT-TB2 has 1.12 eV higher gap value

with respect to other dimers. This extends as the reason for the higher band gap

value of P13ProDOT-TB2 with respect to other polymers. When we compare this

value with that of the polymers measured, there are two factors which should be

considered. These factors cause reduction in the energy gap value as going from

dimer to polymer into the solid state phase. First, Hthe HOMO level is shifted to

higher value and LUMO level is shifted to lower value with increasing chain

length. Due to this reason, the energy gap is lowered for a longer polymer chain.

Second, the inter-chain interaction resulting in the reduction of band gap value of

polymer in a solid state form. Considering these two factors, it may be concluded

that the higher band gap value of P13ProDOT-TB2 with respect to PProDOT-Me2

is due to a deviation from planarity.

124

Table 3.2 HOMO and LUMO energy levels of monomers and dimers.

8LUMO-

(5 E o c o

J—

CD

E b

ProDOT

22ProDOT-Me2

13ProDOT-TB2

22ProDOT- TB2

ProDOT

22ProDOT-Me2

13ProDOT-TB2

22ProDOT-TB2

eHOMO

(eV)

-6.02

-6.03

-5.97

-5.95

-5.05

-5.06

-5.70

-5.04

E L U M O

(eV)

-0.12

-0.13

-0.07

-0.11

-0.90

-0.91

-0.44

-0.89

E H O M O

(eV)

5.90

5.90

5.90

5.84

4.14

4.14

5.26

4.15

Dihedral Angle

(C-C-C-C)

180.0

-180.0

-76.1

-179.9

Table 3.3 3D Views of Monomers and Dimers in this study.

Monomer: ProDOT-Me2 Dimer: ProDOT-Me2

•r ***. ; ^ ™ * *

jj&ffZ.j

Monomer: 13ProDOT-TB2 Dimer: 13ProDOT-TB2

125

3.5 Conclusion

In conclusion; we have shown that 1,3-disubstituted ProDOT derivatives

exhibit excellent characteristics for EC applications, most notable being the 200

nm shift in Amax from PProDOT-Me2. Fine tuning of color is possible by simply

changing the size of the substituents at the 1,3 positions, which is one of the

most important aspects of this system. A distinct structure-property relationship

was observed for P13ProDOT-TB2 with respect to PProDOT-Me2. The polymer

with bulky t-butyl substituents had higher band-gap and color change occurred

within similar voltage range in comparison to other ProDOT polymers with linear

126

substituents. The bulky substituents used were designed to increase the

solubility of the polymer without compromising the electrochromic properties. The

higher band-gap and increased solubility can be explained by a limited effective

conjugation length in the solid state due to the presence of bulky side groups

which distorted chain planarity. Currently, we are working on other derivatives in

order to fully explore the color gamut that can be achieved by this method. This

simple synthetic approach to color tuning will make it easier for display

applications, among other color-transition based applications, to be realized in

the future.

127

3.6 References

1. Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. in Electrochromism:

Fundamentals and Applications, Wiley-VCH, Weinheim, Germany 1995.

2. Dyer, A. L; Reynolds, J. R. in Handbook of Conducting Polymers. Conjugated

Polymers: Theory, Synthesis, Properties, and Characterization, 3nd ed. (Eds: T.

A. Skotheim, J. R. Reynolds), CRC Press, Boca Raton, FL, USA 2007, Ch. 20.

3. Mortimer, R. J. Chem. Soc. Rev. 1997, 26, 147.

4. Rosseinsky, D. R.; Mortimer, R. J. Adv. Mater. 2001, 13, 783.

5. McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537.

6. Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.

7. Sotzing, G. A.; Briglin, S.; Grubbs, R. H.; Lewis, N. S. Anal. Chem. 2000, 72,

3181.

8. Ma, H.; Chen, .; Sassa, T.; Dalton, L. R.; Jen, A. K. Y. J. Am. Chem. Soc.

2001, 723,986.

9. Yu, G.; Heeger, A.J. Synth. Met. 1997, 85, 1183.

10. Ferraris, J. P.; Eissa, M. M.; Brotherston, I. D.; Loveday, D. C; Chem. Mater.

1998, 11, 3528.

11. Jonas, F.; Heywang, G.; Electrochim. Acta 1994, 39, 1345.

12. Perucki, M.; Chandrasekhar, P. Synth. Met. 2001, 119, 385.

13. Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Func. Mater. 2001, 11,

15.

14. Henckens, A.; Knipper, M.; Polec, I.; Manca, J.; Lutsen, L; Vanderzande, D.

Thin Solid Films 2004, 451 & 572.

128

15. Groenendaal, L; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R.;

AdV.Mater. 2000, 12,481.

16. Sotzing, G. A.; Reynolds, J. R.; Steel, P.J. Adv. Mater. 1997, 9, 795.

17. Schwendeman, I.; Gaupp, C. L; Hancock, J. M.; Groenendaal, L; Reynolds,

J. R. Adv. Funct. Mater.2003, 13, 541.

18. Reeves, B. D.; Thompson, B. C; Abboud, K. A.; Smart, B. E.; Reynolds, J.

R. Adv. Mater. 2002, 74,717.

19. Sonmez, G.; Meng, H.; Wudl, F. Chem. Mater. 2004, 16, 574.

20. Groenendaal, L; Zotti, G.; Aubert, P. H.; Waybright, S. H.; Reynolds, J. R.

Adv. Mater. 2003, 15, 855.

21. Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994,

369, 87

22. Bokria, J. G.; Kumar, A.; Seshadri, V.; Tran, A.; Sotzing, G. A. Advanced

Materials, 2008, 20, 1175.

23. Invemale, M. A.; Ding, Y.; Mamangun, D. M. D.; Yavuz, M. S.; Sotzing, G. A.

Advanced Materials, 2010, 22, 1-4.

24. Welsh, D. M.; Kumar, A.; Morvant, M. C; Reynolds, J. R. Synth. Met. 1999,

102, 967;

25. Welsh, D. M.; Kloeppner, L. J.; Madrigal, L; Pinto, M. R.; Thompson, B. C;

Schanze, K. C; Abboud, K. A.; Powell, D.; Reynolds, J. R. Macromolecules,

2002,35,6517.

26. Kumar, A.; Welsh, D. M.; Morvant, M. C; Piroux, F.; Abboud, K. A.; Reynolds,

J. R. Chem. Mater. 1998, 10, 896.

129

27. Welsh, D. M.; Kumar, A.; Meijer, E. W.; Reynolds, J. R.; Adv. Mater. 1999,

11,1379

28. Kobayashi, H.; Cui, H. Chem. Rev. 2004, 104, 5265.

29. Krishnamoorthy, K.; Ambade, A. V.; Kanungo, M.; Contractor, A. C; Kumar,

A. J. Mater. Chem. 2001, 11, 2909

30. Yamabe, T.; Tanaka, K.; Koike, T.; Ueda, M. Molecular Crystals and Liquid

Crystals, 1985, 117, 185-92.

31. Wu, C. Q.; Zhang, Y. Z.; Lin, H. Q.; Synthetic Metals, 2001, 119, 219.

32. Dyer, A. L; Grenier, C. R. G.; Reynolds, J. R. Adv. Fund. Mater. 2007, 17,

1480.

33. Cho, S. I.; Kwon, W. J.; Choi, S.; Kim, P.; Park, S.; Kim, J.; Son, S. J.; Xiao,

R.; Kim, S.; Lee, S.B. Adv. Mater. 2005, 17, 171.

34. Meng, H.; Tucker, D.; Chaffins, S.; Chen, Y.; Helgeson, R.; Dunn, B.; Wudl,

F. Adv. Mater. 2003, 75,146.

35. Sapp, S. A.; Sotzing, G. A.; Reynolds, J. R. Chem. Mater. 1998, 10, 2101.

36. Sonmez, G.; Sonmez, H. B.; Shen, K. F.; Jost, R. W.; Rubin, Y.; Wudl, F.

Macromolecules, 2005, 38, 669.

37. Reeves, B. D.; Grenier, C. R. G.; Argun, A. A.; Cirpan, A.; McCarley, T. D.;

Reynolds, J. R. Macromolecules, 2004, 37, 7559.

38. Sonmez, G.; Sonmez, H. B.; Shen, K. F.; Jost, R. W.; Rubin, Y.; Wudl, F.

Angew.Chem. Int. Ed. 2004, 43, 1498.

39. Sonmez, G.; Sonmez, H. B.; Shen, K. F.; Jost, R. W.; Rubin, Y.; Wudl, F. Adv.

Mater. 2004, 16, 1905.

130

40. Suzuki, I.; Kin, H.; Yamamoto, Y. J. Am. Chem. Soc. 1993, 115,10139.

41. Becke, A. D. J. Chem. Phys.1993, 98, 5648

42. Lee, C; Yang, W.; Parr, R. G. Phys. RevB, 1988, 37, 785

43. Gaussian 03, Revision D.01

44. Roncali, J. Chem.Rev. 1997, 97, 173-205.

131

CHAPTER 4

VERSATILE SYNTHESIS OF 3, 4-b HETEROPANTALENES

4.1 Introduction

During the last twenty-five years, many research groups have been

involved in the preparation of a variety of heterocycles and fused

heterocycles, including thienothiophenes 1 and thienofurans,2 for different

applications. Many of these methodologies have involved expensive starting

materials and difficult synthetic procedures, sometimes with undesirable

yields. Herein, we focus on the synthesis and characterization of the fused

heterocycles thieno[3,4-b]thiophene (T34bT), thieno[3,4-b]furan (T34bF),

seleno[3,4-b]thiophene (S34bT) and related compounds using inexpensive

chemical reagents, lowering synthetic difficulty, as well as providing a

versatile route to a myriad of fused heterocycles and derivatives thereof.

Selenium, oxygen, and sulfur were used as the heteroatoms, and different-

length alkyl chain derivatives were prepared; these routes are amenable to

the inclusion of different heteroatoms, various combinations thereof, and

other substituents, as well.

I II III IV

132

Figure 4.1 Various combinations of fused five-membered heterocycles.

Five member fused heterocyclic structures (Figure 4.1) are commonly

known as the A, B diheteropentalenes. They are created by replacing a CH

group from each of the pentalenerings with a heteroatom, typically oxygen,

nitrogen, sulfur, selenium or tellurium.6 A pair of electrons is donated by the

heteroatom, resulting in 10 7t-electron systems, which are isoelectronic with

the aromatic pentalene dianion.3,4,5 Structures I - III are represented by

uncharged, covalently bound species and are known as the "classical"

heteropentalenes. "Non-classical" heteropentalenes, IV, are represented by

diradical structures, except in the case of a sulfur or selenium heteroatom in

the B-position. In this case, multiple bonding to the heteroatom itself can be

used, as shown. The stability of the ^-electrons within the A, B-

diheteropentalene rings depends on the relative orientation of the two

heteroatoms.6'7,8 The resonance energy value and heats of formation

obtained from MNDO calculation suggested that the heterocycles with [2,3-

b] and [3,2-b] ring fusion, I and II respectively, would be more stable than

the [3,4-b] isomer III, while the [3,4-c]-fused isomer (IV) was predicted to be

extremely unstable. Research efforts in the thienothiophene series I - IV (A

= B = S throughout) confirmed these predictions; both thieno[2,3-

b]thiophene (T23bT, I) and thieno[3,2-b]thiophene (T32bT, II) were stable

and underwent electrophilic substitution9 while thieno[3,4-b]thiophene

(T34bT, III) was more reactive and more easily underwent oxidation in air,

though none of these heterocycles were completely immune to such

133

oxidation.10 The parent thieno[3,4-c]thiophene (T34cT, IV) has not been

isolated to date.

Any thienothiophene possesses two fused thiophene rings, the

orientations of which vary depending on the locations of the sulfur atom in

the peripheral thiophene. Whereas, in any thienofuran (TF, A = S, B = O), a

furan ring is attached to another thiophene. Thus far, two furan-containing

diheteropentalenes have been reported, namely thieno[3,4-b]furan11

(T34bF) and thieno[2,3-c]furan (T23cF).12 Approaches to the furo[2,3-

c]furan (F23cF) ring system have not been investigated, likely due to the

limits imposed on viable ring-closure reactions due to the relative instability

of furans as compared to thiophenes. Figure 4.2 displays the structures of

the various molecules synthesized herein.

T34bT S34bT T34bF

R= H, Hexyl, Octyl, Decyl, Dodecyl

Figure 4.2 Structure of thieno[3,4-b]thiophene (T34bT), its derivatives,

seleno[3,4-b]thiophene (S34bT), and thieno[3,4-b]furan (T34bF).

134

In the growing field of organic semiconductors, thiophene-based fused

heterocyclic materials play an important role. These materials display

promising optical and electrical properties for use in electrochromics,13

organic light-emitting diodes (OLEDs) 14 and organic photovoltaics

(OPVs).15 T34bT and T34bF are notable monomers as they can be used in

the preparation of intrinsically conducting, low band gap polymers. Indeed,

polymeric forms of T34bT have been used as conductors and as ion storage

layers for electrochromic devices.16'17 Derivatization of these heterocycles is

an effective method of property-tuning for the resultant polymers.

Limitations, with respect to derivatization, in the established syntheses of

these monomers have led to the development of the alternate routes

described herein.

In 1991, Brandsma18 reported the synthesis of T34bT via conversion of 3,4-

dibromothiophene to the monotrimethylsilyl acetylenic derivative, which was

then ring closed to form T34bT after forming the thiolate on the 3-position.

We reported a modified synthetic procedure where we obtained as high as

55-60% yield in the final ring closing step, however we were able to obtain a

maximum yield of only 8% using the conditions reported by Brandsma.19 In

1986, Moursounidi and coworkers first reported the synthesis of T34bF. For

many applications, derivatization at the 2-position is important, since the

substituent can affect the band gap and solubility of the resultant polymer.

This concept was shown by Pomerantz et al. with the synthesis of poly(2-

decylthieno[3,4-b]thiophene).20 Ferraris extended this concept by

introducing a phenyl group in the 2-position.21 Lu Ping Yu et al.22, has

135

synthesized T34bT with electron withdrawing ester groups in the 2-position;

however, thus far no one has reported substituents with an electron

donating character.

We were able to extend and expand the versatility of fused heterocycles by

our new routes. The alkyl substituents reported in this work are weak

electron donating groups, which will result in lower oxidation potentials for

polymerization. In addition, the lengthy alkyl groups will also contribute

towards making the resultant polymer organic soluble. Our use of the

selenium heteroatom was aimed at lowering the band gap.

The reported methods of making T34bT, 2-alkyl derivatized T34bT, and

T34bF23 require expensive chemicals such as 3,4-dibromothiophene and

3,6-di(pyridine-2'-yl)-s-tetrazine (DPT).24 This makes the polymers thereof

more costly, which is where the bulk of their applications lie. Moreover,

current synthetic procedures are limited because of the difficulty in

derivatizing the 2-position of T34bT. It is therefore necessary to develop

novel synthetic approaches for five-member fused heterocycles. We were

interested in developing a common synthetic methodology that combined

the strategies of T34bT with 2-alkyl T34bT to ultimately yield a series of 2-

substituted molecules. Further, we sought to incorporate another

heteroatom, selenium, into these fused to systems to explore the versatility

of our approach.

Synthetic schemes, including yields, for the new preparation of T34bT, its

derivatives, S34bT, and T34bF appear in Schemes 4.1-4.3. Scheme 4.1

shows the new synthesis for T34bT and S34bT, Scheme 4.2 shows the

136

modified procedure for our alkyl-derivatized T34bT (though it may be

adapted to the attachment of other substituents), and Scheme 4.3 shows

the new synthetic route to T34bF.

COOH j-— OH

/ F \ ^ 1)n.BuLit-78°C f/V^ 1)LiAIHd,THF ^ //Xi J» \ / ^ C O O H 2)C02 ^ \ / ^ C O O H 7 0 ° C ^ \ / ^ ^

S 1 2 h r s S 24hrs S

F.W.-128 (1) (2)

Yield = 56 % Yield = 81 %

r-C* r-<^s r-^s ? r ^ / / \ \ ,Br 1 ) N a 2 S , D M F ^ / / \ \ / 1) DDQ, PCM w / / \ — /

\ y / 6h^ \ ' 30mins m X g /

(3) (4) (5)

Yield = 80 % Yield = 51 % Yield = 62 %

1)NaHSe, DMF 12 hrs

\ s / " " " ^ 30mins ^ \ ^

(6) (7)

Yield = 49 % Yield = 53 % Note: DDQ: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

SCHEME 4.1. Synthetic procedure for thieno[3,4-fo]thiophene (T34bT) and

seleno[3,4-i)]thiophene (S34bT).

137

COOH COOH

/ / \ Y 1)n-BuLi,-78°C ^ / / \ V 1) n-BuLi, -78° C ^ Jj »\ N g X ^ C O O H 2)C0 2 • N g / ^ C O O H 2)Alkylbromide ^ R ^ s ^ 0 0 0 "

12hrs 12hrs 8a =C8Hi3

(2) (8) 8b = C8H17

8c = C10H21 Yield = 79% 8d = C12H25

1)LiAIH4,THF

••—OH r—Br

^ _ / / % V ^ 0 H 1)PBr3,Anh. Ether^^ / / \ V .B

- • R^SSX^/ ilh^ • R ^ % X ^ / 70° C ^ K ^S 1 2 h r s * ^S 24hrs

9a=C6H13 10a=C6H13 (9) 9b = C8H17 (10) 10b = C8H17

9c = C10H2i 10c = C10H21

Yield = 76% 9d = C12H25 Yield = 84% 10d = C12H25

1)Na2S, DMF w _ JJ M / 1) DDQ, PCM w / / \ — / ^ R ^ \ C X ^ 30"^ ^ R- XCX 6 hrs ^ R ^s 3 0 m , n s S*

11a=C6H13 12a=C6H13

(11) 11b = C8H17 (12) 12b = C8H17

11c = C10H21 12c = C10H21

Yield = 5 1 % 11d = C12H25 Yield = 58 % 12d = C12H25

SCHEME 4.2 Synthetic procedure for 2-substituted thieno[3,4-b]thiophenes.

138

COOH

/ M A . 1)LDA,-78°C W \ J^\^ 1)n-BuLi,-78°C ^ \ J ^ \ ^ N Q X ^ C O O H 2)TMS-CI ^ ^ S i - ^ \ 0 / ^ C O O H 2) C0 2 • ^ S r ^ ^ C O O H

12hrs \ 12hrs \ F.W.-112

(13) (14)

Yield = 85 % Yield = 81 %

/ — O H y — B r

1)LiAIH4,THF ^ C ^ ^ ° H 1 ) P B r 3 ' A n h E t h e t ^ ( F \ ^ y B r 1) Na2S, DMF ^ / M A / 70°C,24hrs ^ N / ^ * " ^ 127te ^ N r * * ' ^ 6hr1 ^ \ r ^

TBAF

(15) (16) (17)

Yield = 69 % Yield = 81 % Yield = 47 %

1)DDQ, PCM H x ' #S

30 mins C# (18)

Yield = 56 %

SCHEME 4.3 Synthetic procedure for thieno[3,4-b]furan (T34bF).

The first step, a carboxylation, is common for the syntheses of T34bT, 2-

alkyl-T34bT and S34bT. Isopropanol was used for the recrystallization of

thiophene-2, 3-dicarboxylic acid.

An initial direct attempt (nucleophilic aromatic substitution) to synthesize

the 2-alkyl-T34bT from 4, 6-dihydrothieno[3,4-b]thiophene intermediate (4)

using corresponding alkyl bromide was not successful . Coupling reaction

of 2-Br-4, 6-dihydrothieno[3,4-b]thiophene and appropriate Grignard

reagent, catalyzed with 1,3(diphenylphosphino)propanedichloronickel(ll)

was also failed. It was observed that this coupling reaction was not suitable

for fused 4,6-dihydrothieno[3,4-b]thiophene and 4,6-dihydrothieno[3,4-

b]furan compounds. We are not sure of the reasons at this moment, but the

rigidity change and electron distribution change may have contribution to

this result. Thus, an alternative approach where the alkyl groups were

139

added after carboxylation step. Alkylation of thiophene-2, 3-dicarboxylic acid

was carried out by using different alkyl bromide in excess.Since the 5

position of 2-furoic acid is more reactive than the 3 position, protection of

the 5 position by TMS was necessary to obtain the 5-TMS-2,3-

furandicarboxylic acid. The reductions of corresponding dicarboxylic acids

were carried out by using LiAIH4. In the case of T34bF, deprotection of the

TMS group was carried out by using TBAF at the 2,3-furandiol step to

optimize the yield of the ring-closing reaction. The bromination of alcohols

was accomplished by using phosphorus tribromide (PBr3). 2,3-

Bis(bromomethyl)thiophene is not stable at room temperature; the white

solid became dark brown upon exposure to ambient conditions. A number of

different conditions and reagents were attempted for the ring closing of 2,3-

bis(bromomethyl)thiophene/furan with sulfur. The best results were obtained

by using sodium sulfide in a DMF solution. In the case of 4,6-

dihydroseleno[3,4-b]thiophene, the ring closing was done by using NaHSe

in a DMF solution. The intermediates 4,6-dihydrothieno[3,4-b]thiophene,

4,6-dihydroseleno[3,4-b]thiophene and 4,6-dihydrothieno[3,4-b]furan

decomposed upon standing at room temperature, which was minimized by

keeping them cold, at -50QC, at all times. The alkyl-substituted 4,6-

dihydrothieno[3,4-b]thiophene is stable at room temperature, however. The

last step, in each case, is a DDQ oxidation of the dihydro compounds; this

was carried out at 0°C for 30 minutes and progress was monitored by GC-

MS and TLC. Higher temperatures and longer reaction times lead to the

formation of dark, greenish-blue material that was insoluble in organic

140

solvents, possibly due to the oxidative polymerization, initiated by DDQ, of

the product formed. A modified work-up procedure was carried out by using

dry flash column to improve the previous reported yield.

Thus, we have demonstrated an efficient, inexpensive, and versatile

synthetic approach for preparing T34bT, 2-alkylated T34bT, S34bT, and

T34bF. These new routes open the door to a wide variety of potential

derivatives. We intend to more fully explore these substituents at the 2-

position using this chemistry, particularly with other electron-donating

groups. By using the same synthetic methodology, we also intend to make

different annelated heterocyclic systems with N and Te. The range of

practical applications of isomeric TT or TF derivatives and their 0-, N-, S-,

Se-, Te- containing and fused analogues is vast, ranging from prospects in

the design of new medicines25 to the design of the previously unknown

liquid and clathrate crystals,26 charge-transfer complexes,27 conducting

polymers,28 nonlinear optical materials,29 and dyes,30 among other.

141

4.2 Synthesis and Characterization of thieno[3,4-b]thiophene and

Seleno[3, 4-b]thiophene:

COOH r^-ow

[ T \ 1)n-BuLi,-78°C fi\i 1)LiAIH„THF ^ /Tx^ JDH

1 2 h r s 24hrs

F.W.-128 (1) (2)

Yield = 56 % Yield = 81 %

1 )PBr 3 ,Anh .E ther^ / / \ \ Br 1) Na2S, DMF ^ / / \ \ / 1) DDQ, PCM ^ / / \ — /

12hrs ^ s ^ ^ ^ 6 ^ ^ ^ V " ^ " " ^ 30mins ^ \ ^

(3) (4) (5)

Yield = 80 % Yield = 51 % Yield = 62 %

1)NaHSe,DMF 12 hrs

y 7 \ \ / DDDQ.DCM^ / / \ — / X g ^ " " - 30mins ^ \ ^

(6) (7)

Yield = 49 % Yield = 53 %

Note : DDQ: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

Thiophene-2, 3-dicarboxylic acid (1): To a solution of thiophene-2-

carboxylic acid ( 20 g, 156 mmol), in anhydrous THF (500 mL), at -78QC was

added a 2.5 M solution of n-BuLi in hexane (137.2 mL, 342 mmol) over a

period of 90 mins. The reaction was allowed to stir at -78QC for another 30

mins at which time it was quenched with C02. Upon addition of C02, the

reaction became very thick and required some shaking for efficient mixing.

The reaction was allowed to warm at room temperature and stirring

continued for another 8 hrs. The reaction was quenched by 200 mL of water

and 80% of THF was removed by using a rotovap. The aqueous layer was

142

acidified with dilute HCI (until cloudy) and kept for 4 hrs. The crude acid was

precipitated out from the aqueous layer, filtered and dried. The crude acid

solid was recrystallized from isopropanol to obtain pure 1 (14.2 g, 56%

yield), which was confirmed by 1H-NMR, 13C-NMR and Melting Point.

Compound 1: 1H-NMR (500 MHz, DMSO-d6): 7.40(1 H, d, J = 5.1 Hz),

7.84(1 H, d, J = 5.1 Hz), 12.38 (2 H, br); 13C-NMR (500 MHz, DMSO-d6):

164.34, 164.14, 141.39, 140.03, 137.48, 129.59; Mp 270-272QC.

2, 3-Bis(hydroxymethyl)thiophene (2): To a slurry of LiAIH4 (16.5 g, 434

mmol), in THF (500 mL) at 0QC, under an atmosphere of nitrogen, was

added thiophene-2, 3-dicarboxylic acid (1, 15 g, 87.2 mmol). The reaction

was allowed to warm to 70QC and was stirred for another 24 hrs. After 24

hrs the reaction flask was kept in an ice bath and then 50 mL of diethyl ether

was added followed by the addition of 15 mL of water and 15 mL of 10%

NaOH solution. Addition of water was drop-wise. The organic layer was

dried over MgSCU, filtered and the filtrate concentrated to a crude solid 2

(10.1 g, 75 % yield) confirmed by 1H-NMR, 13C-NMR, GC-MS.

Compound 2 : 1H-NMR (500 MHz,CDCI3): 7.20 (1H, d, J = 5.1 Hz), 6.99

(1H, d ,J = 5.1 Hz), 4.65 (2 H, s), 4.52 (2 H, s), 4.2 (2 H, br); 13C-NMR (500

MHz,CDCI3): 140.88, 139.53, 129.32, 124.4, 58.94, 57.84; GC-MS (m/z):

128.

2, 3-Bis(bromomethyl)thiophene (3): To a solution of 2, 3-

143

bis(hydroxymethyl)thiophene (2, 10 g, 69.4 mmol) in anhydrous ether (400

mL) at 0QC, under a nitrogen atmosphere, was added PBr3 (9.8 mL, 104

mmol) drop-wise over a period of 15 mins. The reaction was continued for

another 6 hrs at room temperature and quenched by adding 50 mL of water.

The organic layer was extracted 4-5 times with water until the pH became

neutral. The organic layer was dried over MgS04, filtered and the filtrate

concentrated to a white solid 3 (15 g, 80% yield) confirmed by 1H-NMR, 13C-

NMR, GC-MS.

Compound 3: 1H-NMR (500 MHz,CDCI3): 7.27 (1H, d, J = 5.2 Hz), 7.01 (1H,

d , J = 5.2 Hz), 4.75 (2 H, s), 4.54 (2 H, s), 4.2 (2 H, br); 13C-NMR (500

MHz,CDCI3): 137.68, 137.24, 129.82, 126.27, 24.31, 23.76; GC-MS (m/z):

270.

Dihydrothieno[3,4-fo]thiophene (4): To a solution of sodium sulfide

nonahydrate (8.9 g, 37 mmol) in DMF (100 mL) at 30QC under a nitrogen

atmosphere, was added 2,3 bis(bromomethyl)thiophene (3, 10 g, 37 mmol)

solution in DMF (300 mL). 2, 3Bis(bromomethyl)thiophene addition was very

slow and the temperature monitored to gradually increase from 30QC to

70QC. The reaction was continued for another 12 hrs at 70QC during which

time the contents became dark in color. The product was extracted in diethyl

ether (200 mL) and washed with sufficient water to remove DMF. The

organic layer was dried over MgS04, filtered and the filtrate concentrated to

a crude dark brown liquid 4 (2.6 g, 51% yield). The product was confirmed

144

by 1H-NMR, 13C-NMR and GC-MS.

Compound 4: 1H-NMR (500 MHz, CDCI3): 7.27 (1H, d), 6.75 (1H, d), 4.16

(2H, s), 4.04 (2 H, s); 13C-NMR (500 MHz, CDCI3): 144.31, 139.93, 130.01,

123.39, 33.63, 33.52; GC-MS (m/z): 142;

Thieno[3,4-t»]thiophene (5): Dihydrothieno [3,4-b]thiophene (4, 2.0 g, 14

mmol) and anhydrous methylene chloride (100 mL) were taken in a dry 3-

neck flask, and cooled the solution to 0°C before adding 2,3-dichloro5,6-

dicyano-1,4-benzoquinone (DDQ, 3.17 g, 14 mmol). The reaction mixture

was stirred at 0°C for 30 minutes, and then passed through the silica. After

evaporating the solvent, the product was purified by liquid chromatography

using petroleum ether as an eluent. The purified thieno[3,4-£)]thiophene 5

was obtained (1.2 g, 62 % yield). The final product was confirmed by 1H-

NMR, 13C-NMR, GC-MS and FTIR.

Compound 5: 1H-NMR (500 MHz, CDCI3): 7.37 (1H, d), 6.95 (1H, dd), 7.27

(1H, dd), 7.36 (1H, d); 13C-NMR (500 MHz, CDCI3): 148.21, 140.00, 132.79,

117.19, 112.01, 110.98; GC-MS (m/z): 140; FTIR (liquid film): 3110 (w),

3070 (w), 2923 (w), 2850 (w), 2340 (w), 1732 (s), 1554 (s), 1490 (s), 1383

(w), 1020 (w), 770 (m), 665 (m) cm"1.

Dihydroseleno[3,4-b]thiophene (6): To a freshly prepared solution of

sodium hydrogen selenide7 in DMF (100 mL) at 30SC under a nitrogen

145

atmosphere, was added 2, 3 bis(bromomethyl)thiophene (3, 4 g, 14.8 mmol)

solution in DMF (200 ml_). 2, 3Bis(bromomethyl)thiophene addition was very

slow and the temperature monitored to gradually increase from 30QC to

50QC. The reaction was continued for another 12 hrs at 70QC during which

time the contents became dark in color. The product was extracted in diethyl

ether (200 mL) and washed with sufficient water to remove DMF. The

organic layer was dried over MgS04, filtered and the filtrate concentrated to

a crude dark brown liquid 4 (1.37 g, 49 % yield). The product was

confirmed by 1H-NMR, GC-MS. The compound decomposed during the 13C

NMR.

Compound 7: 1H-NMR (400 MHz, CDCI3): 7.04 (1H, d), 6.89 (1H, dd), 3.68

(2H, s), 3.50 (2 H, s); GC-MS (m/z): 190;.

Selono[3,4-b]thiophene (7): Dihydroselono[3, 4-b]thiophene (6, 1.0 g, 5.2

mmol) and anhydrous methylene chloride (40 mL) were taken in a dry 3-

neck flask, and cooled the solution to 0°C before adding 2, 3-dichloro5,6-

dicyano-1,4-benzoquinone (DDQ, 1.2 g, 5.2 mmol). The reaction mixture

was stirred at 0°C for 30 minutes, and then passed through the silica. After

evaporating the solvent, the product was purified by liquid chromatography

using petroleum ether as an eluent. The purified seleno[3,4-b]thiophene 7

was obtained (0.52 g, 53 % yield). The final product was confirmed by 1H-

NMR, 13C-NMR, 77Se- NMR and GC-MS.

146

Compound 7: 1H-NMR (400 MHz, CDCI3): 8.01 (1H, d), 7.88 (1H, dd), 7.43

(1H, dd), 7.79 (1H, d); 13C-NMR: 151.06, 141.36, 132.75, 117.52,

117.40,115.01; 77Se NMR: 432.72; GC-MS (m/z): 188; FTIR (liquid film):

3100, 1554, 1493, 1323, 1294, 1152, 1083, 809 and 768 cm1.

4.3 Synthesis and Characterization of 2-alkylthieno[3,4-b]thiophene

COOH COOH

U ^ 1 )n -BuLi , -78°C^ / M V 1) n-BuLi, -78° C w / / \ \ N s / ^ C O O H 2)CO; • \ s / ^ C O O H 2)Alkylbromide ^ R ^ N j ^ C O O H

12hrs 12hrs 8a =C6H13,Yield = 73 %

(2) (8) 8b = C8H17, Yield = 71 % 8c = C10H21, Yield = 78 % 8d = C12H25, Yield = 79 %

1) LiAIH4,THF

yT—OH y — B r

^ / / \ \ ^OH 1)PBr3,Anh. E t h e r ^ / / \ Y „ B r

70° C ^ K ^s 12hrs

9a =C6H13( Yield = 69 % 10a =C6H13, Yield = 80 % (9) 9b = C8H17lYield = 7 1 % (10) 10b = C8H17, Yield = 84 %

9c = C10H2i, Yield = 74 % 10c = C10H21, Yield = 84 % 9d = C12H25f Yield = 79 % 10d = C12H25, Yield = 86 %

1)Na2S, DMF ^ _ f M \ / 1) DDQ, PCM w Jj\zzJ ^ R ^ ^ y ^ 3 0 ^ ^ R ^ N ^ 6 hrs R S 3 0 m i n s

11a =C6H13, Yield = 50 % 12a =C6H13, Yield = 54 % (11) 11b = C8H17> Yield = 47% (12) 12b = C8H17) Yield = 57 %

11c = C10H21, Yield = 49 % 12c = C10H21l Yield = 57 % 11d = C12H25, Yield = 51 % 12d = C12H25, Yield = 58 %

Thiophene-2, 3-dicarboxylic acid (2) was prepared according to the

procedure outlined in Thieno[3,4-b]thiophene.

5-Alkyl-Thiophene~2, 3-dicarboxylic acid (8): To a solution of thiophene-

2,3-dicarboxylic acid (2, 10 g, 58.1 mmol), in anhydrous THF (500 ml_), at -

78QC was added a 2.5 M solution of n-BuLi in hexane (77 ml_, 192 mmol)

over a period of 30 mins. The reaction was allowed to stir at -78QC for

another 90 mins. After 90 mins, alkyl iodide (87.15 mmol) was added drop-

147

wise and the reaction continued for another 8 hrs at room temperature. The

reaction was quenched by adding 10 mL of water. Approximately 80% THF

was removed and made the aqueous layer basic by dilute NaOH solution

(color: Red-Orange). The aqueous layer was extracted twice with diethyl

ether to remove excess alkyl iodide which is soluble in ether. The cloudy

yellow aqueous layer was acidified with dilute HCI and extracted with diethyl

ether until the aqueous layer became clear. The organic layer was washed

with plenty of water to remove excess HCI. The combined organic layers

were dried over MgS04 and concentrated to a crude orange solid 8. No

purification was required in this step.

Compound 8a: 1H-NMR (500 MHz, DMSO-d6): 12.03 (br s, 2H), 7.13 (s, 1H),

2.67 (t, 2 H), 1.58 (m, 2 H), 1.25-1.15 (m, 6 H), 0.85 (t, 3 H). 13C-NMR (500

MHz, DMSO-d6): 164.55, 164.09, 146.53, 142.52, 137.70, 129.80, 31.91,

31.58, 30.28, 29.65, 22.56, 14.06.

Compound 8b: 1H NMR (500 MHz, DMSO-d6) 5 12.03 (br s, 2H), 7.14 (s,

1H), 2.76 (t, 2 H),1.59 (m, 2 H), 1.34-1.14 (m, 10H), 0.84 (t, 3 H). 13C-NMR

(500 MHz, DMSO-de) 164.54, 164.06, 146.46, 142.03, 137.58, 129.69,

31.75, 31.17, 29.73, 29.70, 29.65, 29.48, 22.58, 14.30.

Compound 8c: 1H NMR (500 MHz, DMSO-d6) 5 12.03 (br s, 2H), 7.15 (s,

1H), 2.77 (t, 2 H),1.59 (m, 2 H), 1.34-1.14 (m, 12 H), 0.84 (t, 3 H). 13C-NMR

(500 MHz, DMSO-d6) 164.55, 164.06, 146.46, 142.03, 137.58, 129.69,

148

31.75, 31.17, 29.73, 29.70, 29.67, 29.46, 29.39, 29.14, 22.56, 14.32.

Compound 8d: 1H-NMR (500 MHz, DMSO-d6): 11.99 (brs, 2H), 7.13 (s, 1H),

2.68 (t, 2 H), 1.56 (m, 2 H), 1.26-1.17 (m, 12 H), 0.85 (t, 3 H); 13C-NMR (500

MHz, DMSO-de) 164.54, 164.04, 146.46, 142.03, 137.58, 129.69, 31.75,

31.17, 29.73, 29.70, 29.67, 29.65, 29.48, 29.46, 29.39, 29.14, 22.53, 14.34.

FTIR: 3650 (w), 3600-3000 (b), 2952, 2925, 2852, 1571, 1524, 1465, 1435,

1377, 1347, 1171, 1086, 1023, 837, 814, 744, 691, 503 cm-1.

5-Alkyl-2, 3-Bis(hydroxymethyl)thiophene (9): To a slurry of LiAIH4 (7.5

g, 195 mmol), in THF (400 mL) at 09C, under an atmosphere of nitrogen,

was added 5alkyl-thiophene-2-3-dicarboxylic acid (8, 39 mmol). The

reaction was allowed to warm to 70QC and was stirred for another 24 hrs.

After 24 hrs the reaction flask was kept in ice bath and then 50 mL of diethyl

ether was added followed by the addition of 15 mL of water and 15 mL of 10

% NaOH solution. Addition of water was drop-wise. The organic layer was

dried over MgS04, filtered and the filtrate concentrated to a crude solid 9,

confirmed by 1H-NMR, 13C-NMR and GC-MS.

Compound 9a: 1H-NMR (500 MHz, CDCI3): 6.70 (s, 1H), 4.68 (s, 2H), 4.58

(s, 2 H), 2.72 (t, 2H), 1.64 (m, 2 H), 1.37-1.23 (m, 18 H), 0.89 (t, 3 H). 13C-

NMR (500 MHz, CDCI3): 145.27, 139.35, 137.51, 126.43, 59.40, 58.16,

31.Q1, 31.58, 30.28, 29.65, 22.56, 14.06; GC-MS (m/z): 228.

149

Compound 9b: 1H-NMR(500 MHz, CDCI3): 6.66(s, 1H), 4.66 (s, 2H), 4.55 (s,

2 H), 2.73 (t, 2H), 1.62 (m, 2 H), 1.39-1.16 (m, 10 H), 0.88 (t, 3 H) 13C-NMR

(500 MHz, CDCI3): 145.31, 139.40, 137.48, 126.36, 59.39, 58.12, 32.40,

32.10, 30.47, 30.14, 30.04, 29.84, 23.17, 14.48; GC-MS (m/z): 256.

Compound 9c: 1H-NMR(500 MHz, CDCI3): 6.66(s, 1H), 4.66 (s, 2H), 4.55 (s,

2 H), 2.73 (t, 2H), 1.62 (m, 2 H), 1.40-1.14 (m, 12 H), 0.88 (t, 3 H). 13C-NMR

(500 MHz, CDCI3): 145.29, 139.41, 137.46, 126.31, 59.35, 58.11, 32.40,

32.10, 30.48, 30.14, 30.04, 30.01, 29.85, 29.60, 23.17, 14.50; GC-MS (m/z):

284.

Compound 9d: 1H-NMR(500 MHz, CDCI3): 6.68 (s, 1H), 4.70 (s, 2H), 4.60 (s,

2 H), 2.73 (t, 2H), 1.63 (m, 2 H), 1.34-1.28 (m, 18 H), 0.86 (t, 3 H). 13C-NMR

(500 MHz, CDCI3): 145.31, 139.41, 137.46, 126.31, 59.35, 58.11, 32.40,

32.10, 30.47, 30.14, 30.04, 30.02, 29.84, 29.83, 29.60, 23.17, 14.50; GC-

MS (m/z): 312.

5-Alkyl-2, 3-Bis(bromomethyl)thiophene (10): To a solution of 5-alkyl-2,3

bis(hydroxymethyl) thiophene (9, 43 mmol) in anhydrous ether (300 mL) at

0SC, under a nitrogen atmosphere, was added PBr3 (6.15 mL, 65.7 mmol)

drop-wise over a period of 15 mins. The reaction was continued for another

6 hrs at room temperature and quenched by adding 50 mL of water. The

organic layer was extracted 4-5 times with water until the pH became

150

neutral. The organic layer was dried over MgSCU, filtered and the filtrate

concentrated to a crude solid 10, confirmed by 1H-NMR 13C-NMR and GC-

MS.

Compound 10a: 1H-NMR (500 MHz, CDCI3) 5 6.67(s, 1H), 4.69 (s, 2H), 4.45

(s, 2 H), 2.70 (t, 2H), 1.62 (m, 2 H), 1.40-1.25 (m, 6 H), 0.86 (t, 3 H). 13C-

NMR: (500 MHz, CDCI3): 147.21, 136.78, 134.80, 129.68, 126.47, 126.11,

31.80, 31.47, 30.17, 29.54, 22.45, 13.95; GC-MS (m/z): 354.

Compound 10b: 1H-NMR (500 MHz, CDCI3) 5 6.68(s, 1H), 4.70 (s, 2H), 4.46

(s, 2 H),2.73 (t, 2H), 1.64 (m, 2 H), 1.14-1.40 (m, 12 H), 0.88 (t, 3 H). 13C-

NMR: 147.32, 136.89, 134.85, 129.75, 126.63, 126.22, 32.33, 32.03, 30.40,

30.07, 29.77, 29.53, 23.10, 14.20; GC-MS (m/z): 382.

Compound 10c: 1H-NMR(500 MHz, CDCI3) 5 6.68(s, 1H), 4.70 (s, 2H), 4.46

(s, 2 H),2.73 (t, 2H), 1.64 (m, 2 H), 1.14-1.40 (m, 12 H), 0.88 (t, 3 H). 13C-

NMR: 147.30, 136.89, 134.91, 129.79, 126.58, 126.22, 32.33, 32.03, 30.40,

30.07, 29.97, 29.95, 29.76, 29.53, 23.10, 14.40; GC-MS (m/z): 410.

Compound 10d: 1H-NMR(500 MHz, CDCI3) 5 6.67(s, 1H), 4.69 (s, 2H), 4.45

(s, 2 H), 2.70 (t, 2H), 1.62 (m, 2 H), 1.40-1.25 (m, 18 H), 0.86 (t, 3 H). 13C-

NMR: 147.32, 136.89, 134.91, 129.79, 126.58, 126.22, 32.33, 32.03, 30.40,

30.07, 29.97, 29.95, 29.77, 29.76, 29.53, 23.10, 14.40; GC-MS (m/z): 438.

151

2-Alkyl-dihydrothieno[3,4-i)]thiophene (11): To a solution of sodium

sulfide nonahydrate (6.7 g, 28 mmol) in DMF (300 mL) at 30eC under a

nitrogen atmosphere, was added 5-alkyl-2,3 bis(bromomethyl)thiophene

(10, 28.2 mmol) solution in DMF (100 mL). 5-Alkyl-2,3

bis(bromomethyl)thiophene addition was very slow and the temperature

monitored to gradually increase from 30 eC to 70 SC. The reaction was

continued for another 12 hrs at 70 SC during which time the contents

became dark in color. The product was extracted in diethyl ether (200 mL)

and washed with sufficient water to remove DMF. The organic layer was

dried over MgSCU, filtered and the filtrate concentrated to a crude dark

brown liquid 11, confirmed by 1H-NMR,13C-NMR and GC-MS.

Compound 11a: 1H NMR(500 MHz, CDCI3) 5 6.45(s, 1H), 4.11 (t, 2H), 3.99

(t, 2 H), 2.72 (t, 2H), 1.63 (m, 2 H), 1.35-1.25 (m, 6 H), 0.86 (t, 3 H). 13C

NMR (500 MHz, CDCI3): 151.30, 143.10, 136.43, 119.11, 34.22, 33.99,

31.70, 31.37, 30.07, 29.44, 22.35, 14.01; GC-MS (m/z): 226.

Compound 11b: 1H NMR (500 MHz, CDCI3) 8 6.45(s, 1H), 4.11 (t, 2H), 3.99

(t, 2 H), 2.72 (t, 2H), 1.63 (m, 2 H), 1.35-1.26 (m, 10 H), 0.88 (t, 3 H). 13C

NMR (500 MHz, CDCI3): 151.35, 143.11, 136.52, 119.21, 34.22, 34.07,

32.42, 32.12, 30.49, 30.04, 29.85, 29.62, 23.19, 14.47 GC-MS (m/z): 254.

Compound 11c: 1H NMR(500 MHz, CDCI3) 5 6.45(s, 1H), 4.12 (t, 2H), 4.00

(t, 2 H), 2.72 (t, 2H), 1.64 (m, 2 H), 1.14-1.40 (m, 12 H), 0.88 (t, 3 H). 13C

152

NMR (500 MHz, CDCI3): 151.38, 143.12, 136.53, 119.22, 34.22, 34.09,

32.42, 32.12, 30.49, 30.16, 30.05, 29.86, 29.85, 29.62, 23.19, 14.45; GC-

MS (m/z): 282.

Compound 11d: 1H NMR (500 MHz, CDCI3) 5 6.45(s, 1H), 4.11 (t, 2H), 3.99

(t, 2 H), 2.72 (t, 2H), 1.63 (m, 2 H), 1.35-1.25 (m, 18 H), 0.86 (t, 3 H). 13C

NMR (500 MHz, CDCI3): 151.40, 143.13, 136.53, 119.21, 34.22, 34.09,

32.42, 32.12, 30.49, 30.16, 30.06, 30.04, 29.86, 29.85, 29.62, 23.19, 14.49;

GC-MS(m/z):310.

2-Alkyl-thieno[3,4-b]thiophene (12): 2-Alkyl-dihydrothieno[3,4-b]thiophene

(11, 22.7 mmol) and anhydrous methylene chloride (100 mL) were taken in

a dry 3-neck flask, and cooled the solution to 0°C before adding DDQ (5.2

g, 22.7 mmol). The reaction mixture was stirred at 0°C for 30 minutes, and

then passed through the silica. After evaporating the solvent, the product

was purified by liquid chromatography using petroleum ether as an eluent.

The purified product was obtained in -60% yield and confirmed by 1H-NMR,

13C-NMR, FTIR and GC-MS.

Compound 12a: 1H-NMR (500 MHz, CDCI3): 7.13 (s, 2 H), 6.61 (t, 1H),

2.76 (t, 2 H), 1.70 (t, 2 H), 1.2-1.4 (m, 6 H), 0.90 (t, 3 H). 13C-NMR (500

MHz, CDCI3): 153.01, 147.59, 138.77, 113.18, 110.19, 109.99, 31.91, 31.56,

30.29, 28.78, 22.57, 14.12; GC-MS (m/z): 224; FTIR (liquid film): 3108,

3057, 29521, 2927, 2849, 1569, 1521, 1467, 1434, 1376, 1347, 1172, 1090,

153

1022, 837, 814, 745, 693, 506 cm"1.

Compound 12b: 1H-NMR (500 MHz, CDCI3): 8 7.12 (s, 2 H), 6.59 (t, 1 H),

2.74 (t, 2 H),1.69 (m, 2 H), 1.42-1.25 (m, 10 H), 0.89 (t, 3 H). 13C-NMR (500

MHz, CDCI3): 152.44, 147.07, 138.27, 113.10, 110.92, 110.69, 32.08, 31.94,

29.73, 29.65, 29.36, 29.06, 22.71, 14.12 GC-MS (m/z): 252, FTIR (liquid

film): 3101, 3052, 2948, 2921, 2849, 1567, 1520, 1463, 1432, 1373 1347,

1171, 1083, 1023,834,811,742, 688, 501 cm"1.

Compound 12c: 1H NMR (500 MHz, CDCI3) 5 7.13(s, 2H), 6.61 (s, 1H), 2.75

(t, 2H), 1.68 (m, 2 H), 1.14-1.40 (m, 12 H), 0.88 (t, 3 H). 13C-NMR (500 MHz,

CDCI3): 152.39, 147.08, 138.27, 113.10, 110.92, 110.69, 32.08, 31.94,

30.12, 29.73, 29.67, 29.51, 29.36, 29.05, 22.69, 14.10; GC-MS (m/z): 280,

FTIR (liquid film): 3110, 3062, 2957, 2929, 2854, 1574, 1527, 1469, 1437,

1381, 1351, 1175, 1090, 1027, 840, 816, 747, 698, 507 cm"1.

Compound 12d: 1H-NMR: 7.10 (s, 2 H), 6.58 (t, 1 H), 2.73 (t, 2 H),1.69 (m, 2

H), 1.42-1.25 (m, 14 H), 0.89 (t, 3 H). 13C-NMR (500 MHz, CDCI3): 152.47,

147.07, 138.27, 113.10, 110.92, 110.69, 32.08, 31.94, 30.12, 29.73, 29.67,

29.65, 29.51, 29.36, 29.33, 29.06, 22.71, 14.12; GC-MS (m/z): 308; FTIR

(liquid film): 3106, 3056, 2952, 2925, 2852, 1571, 1524, 1465, 1435, 1377,

1347,1171, 1086, 1023, 837, 814, 744, 691, 503 cm"1.

154

4.4 Synthesis and Characterization of thieno[3,4-b]furan

COOH

A \V 1)LDA,-78°C w \ / / \V 1)n-BuLi,-78°C ^ \ If \Y X 0 / ^ C O O H 2)TMS-CI ^ ^ S i ^ \ 0 / ^ C O O H 2) C02 • ^Si-^N 0^^COOH

12hrs \ 12hrs \ F.W.-112

(13) (14)

Yield = 85 % Yield = 81 %

r-C°" r-C* r-T^s 1)LiAIH4,THF ^ lfy-^S0H 1 ) P B r 3 , A n h E t h e ^ (fy^SBr D Na2S, DMF ^ AMA I 70°C,24hrs ^ N T * * * ^ i T i ^ ^ N r " * " ^ Sh^ ^ N C T ^

TBAF (15) (16) (17)

Yield = 69 % Yield = 81 % Yield = 47 %

1)DDQ,DCIVI ———^ 30 mins

(18)

Yield = 56 %

5-TMS-Furan-2-Carboxylic acid (13): To a solution of furan-2-carboxylic

acid (12, 20 g, 178 mmol), in anhydrous THF (600 mL), at -78QC was added

a 2.0 M solution of LDA (Lithium Diisopropylamide) in hexane (178 mL, 357

mmol) over a period of 90 mins. The reaction was allowed to stir at -78QC for

another 60 mins. After 60 mins, TMS-CI (chloro trimethyl silane) (25 mL,

196 mmol) was added to the reaction mixture. The reaction was allowed to

warm at room temperature and continued the stirring for another 8 hrs. The

reaction was quenched by 200 mL of water and 80 % THF was removed by

using rotovap. The aqueous layer was acidified with dilute HCI (until cloudy)

and extracted with ethyl acetate until the aqueous layer became clear. The

organic layer was washed with sufficient water (at least 4-5 times) to

remove excess HCI and unreacted starting material. The organic layer was

155

dried over MgS04 and concentrated to a crude dark brown solid 13 (28 g,

85% yield), confirmed by 1H-NMR, 13C-NMR and GC-MS.

Compound 13: 1H-NMR (500 MHz, DMSO-d6): 7.16 (1H, dd, J = 5.0 Hz),

6.87 (1H, dd , J = 5.0 Hz), 12.25 (2 H, br), 0.00 (9 H, s); 13C-NMR: 166.79,

161.13, 150.76, 123.28, 119.02, 107.64, 0.10; GC-MS (m/z): 184.

5-TMS- Furan-2-3-dicarboxylic acid (14): To a solution of 5-TMS-furan-2-

carboxylic acid (13, 20 g, 108 mmol), in anhydrous THF (600 ml_), at -78QC

was added a 2.5 M solution of n-BuLi in hexane (100 mL, 250 mmol) over a

period of 30 mins. The reaction was allowed to stir at -78QC for another 2

hrs at which time it was quenched with C02. It should be noted that upon

addition of CO2, the reaction became very thick and required some shaking

for efficient mixing. The reaction was allowed to warm at room temperature

and continued the stirring for 8 hrs. The reaction was quenched by 200 mL

of water and 80 % THF was removed by using rotovap. The aqueous, layer

was acidified with dilute HCI (until cloudy) and extracted with ethyl acetate

until the aqueous layer became clear. The organic layer was washed with

sufficient water (at least 4-5 times) to remove excess HCI. The organic layer

was dried over MgS04 and concentrated to a crude light brown solid 14 (20

g, 81% yield), confirmed by 1H-NMR, 13C-NMR and GC-MS.

Compound 14: 1H-NMR (500 MHz, DMSO-d6): 7.11 (1H, s, J = 5.1 Hz),

12.48 (2 H, br), 0.00 (9 H, s); 13C-NMR: 166.61, 165.60, 161.30, 151.01,

156

125.90, 124.23; GC-MS (m/z): 246.

2, 3-Bis(hydroxymethyl)furan (15): To a slurry of LiAIH4 (12.5 g, 329

mmol), in THF (400 mL) at 0QC, under an atmosphere of nitrogen, was

added 5-TMS-furan-2-3-dicarboxylic acid (14, 15 g, 65.8 mmol). The

reaction was allowed to warm to 70QC and was stirred for another 24 hrs.

After 24 hrs the reaction flask was kept in an ice bath and then 50 mL of

diethyl ether was added followed by the addition of 15 mL of water and 15

mL of 10 % NaOH solution. Addition of water was drop-wise. The organic

layer was dried over MgS04, filtered and the filtrate concentrated to a crude

solid, which was further dissolved in THF. The solution was then stirred for

10 mins and 1.0 M TBAF (5.4 mL, 18.1 mmol) in THF was added drop-wise

over a period of 15 mins. The mixture was stirred for another 6 hrs at 70QC.

Then 80% THF was removed by rotovap and acidified with dilute HCI (until

cloudy). The aqueous layer was extracted with ethyl acetate until the clear

aqueous layer is obtained. The organic layer was washed with plenty of

water (at least 4-5 times) to remove excess HCI and dried over MgS04 and

concentrated to a crude brown liquid 15 (5.8 g, 69 % yield), confirmed by

1H-NMR, 13C-NMR and GC-MS.

Compound 15 : 1H-NMR (500 MHz,CDCI3): 7.29 (1H, d, J = 5.0 Hz), 6.56

(1H, d , J = 5.0 Hz), 4.55 (2 H, s), 4.43 (2 H, s); 13C-NMR: 150.26, 141.45,

120.10, 110.97, 55.17, 54.73; GC-MS (m/z): 128.

157

2, 3-Bis(bromomethyl)furan (16): To a solution of 2, 3-

bis(hydroxymethyl)furan (15,10 g, 32 mmol) in anhydrous ether (300 mL) at

0QC, under a nitrogen atmosphere, was added PBr3 (4.5 mL, 48 mmol)

drop-wise over a period of 15 mins. The reaction was continued for another

6 hrs at room temperature and quenched by adding 50 mL of water. The

organic layer was extracted 4-5 times with water until the pH became

neutral. The organic layer was dried over MgS04, filtered and the filtrate

concentrated to a crude solid 16 (11.3 g, 81 % yield), confirmed by 1H-NMR,

13C-NMR and GC-MS.

Compound 16 : 1H-NMR (500 MHz,CDCI3): 7.38 (1H, d, J = 5.0 Hz), 6.42

(1H, d , J = 5.0 Hz), 4.55 (2 H, s), 4.36 (2 H, s); 13C-NMR: 148.10, 141.12,

115.14, 110.98, 23.17, 22.89; GC-MS (m/z): 254.

Dihydrothieno[3,4-b]furan (17): To a solution of sodium sulfide

nonahydrate (7.3 g, 30.4 mmol) in DMF (150 mL) at 30QC under a nitrogen

atmosphere, was added 2, 3bis (bromomethyl)thiophene (16, 10 g, 30.67

mmol) solution in DMF( 100 mL). 5-TMS-2,3Bis (bromomethyl)furan addition

was very slow and the temperature monitored to gradually increase from

30QC to 50QC. The reaction was continued for another 12 hrs at 50QC during

which time the contents became dark in color. The product was extracted in

diethyl ether (200 mL) and washed with sufficient water to remove DMF.

The organic layer was dried over MgSCU, filtered and the filtrate

concentrated to a crude dark brown liquid 17 (2.8 g, 40% yield). No

158

purification required.

Compound 17 :1H-NMR (500 MHz, CDCI3): 7.23 (1H, d), 6.12 (1H, d), 3.75

(4H, m); 13C-NMR (500 MHz, CDCI3): 155.0, 147.5, 124.6, 108.4, 28.5, 28.3.

GC-MS(m/z):126.

Thieno[3,4-6]furan (18): Dihydrothieno[3,4-t>]furan (17, 1.3 g , 10.3 tnmol)

and anhydrous methylene chloride (100 ml.) were taken in a dry 3-neck

flask, and cooled the solution to 0°C before adding DDQ (2.35 g, 10.3

mmol). The reaction mixture was stirred at 0°C for 30 minutes, and then

passed through the silica. After evaporating the solvent, the product was

purified by liquid chromatography using petroleum ether as an eluent. The

purified thieno[3,4-6]furan 18 was obtained (0.67 g, 50 % yield) confirmed

by 1H-NMR, 13C-NMR, GC-MS and FTIR.

Compound 18: 1H-NMR (500 MHz, CDCI3): 7.58 (1H, d), 6.92 (1H, d), 6.74

(1H, dd), 6.41 (1H, dd); 13C-NMR (500 MHz, CDCI3): 156.8, 153.0, 134.8,

107.1, 103.0, 94.9. GC-MS (m/ z): 124; FTIR (liquid film): 3131, 3063, 2949,

2925, 1715, 1541, 1434, 1421, 1323, 1273, 1203, 1114, 1103, 1002, 809,

767 cm"1.

4.5 Conclusion

159

In conclusion, we have demonstrated an efficient, inexpensive, and

versatile synthetic approach for preparing T34bT, 2-alkylated T34bT,

S34bT, and T34bF. These new routes open the door to a wide variety of

potential derivatives. We intend to more fully explore these substituents at

the 2-position using this chemistry, particularly with other electron-donating

groups. By using the same synthetic methodology, we also intend to make

different annelated heterocyclic systems with N and Te. The range of

practical applications of isomeric thienothiophene or thienofuran derivatives

and their 0-, N-, S-, Se-, Te- containing and fused analogues is vast,

ranging from prospects in the design of new medicines to the design of the

previously unknown liquid and clathrate crystals, charge-transfer

complexes, conducting polymers, nonlinear optical materials, and dyes,

among others.

160

4.6 References

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Sudhir, V.; Phani Kumar, N. Y.; Nasir Baig, R. B.; Chandrasekaran, S. J.

Org. Chem., 2009, 74(19), 7588-7591. (c) Sen, S.; Kulkami, P.; Borate, K.;

Pai, N. R. Tet. Lett, 2009, 50(28), 4128-4131. (d) Scalzullo, S. M.; Islam, R.

U.; Morgans, G. L; Michael, J. P.; van Otterlo, W. A. L. Tet. Lett., 2008,

49(52), 7403-7405. (e) Majumdar, K. C; Chakravorty, S.; De, N. Tet. Lett.,

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C; Chen, L. X.; Yu, L. Macromolecules, 2009, 42(4), 1091-1098. (d) Liang,

Y.; Xiao, S.; Feng, D.; Yu, L. J. Phys. Chem. C, 2008, 112(21), 7866-7871.

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9. Buemi, G.; J. Chim. Phys.-Chim.-biol., 1987, 84, 1147.

10. Litvinov, V. P.; Gol'dfarb, Y. L. Adv. Heterocyclic Chem., 1976, 19, 123.

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17. Moursounidis, J.; Wege, D. Tet. Lett., 1986, 27, 3045-8.

13. (a) Friedrichsen, W.; Schoning, A. Heterocycles, 1986, 24, 307. 14.

Schwendeman, I.; Hwang, J.; Welsh, D. M.; Tanner, D. B.; Reynolds, J. R.

Adv. Mater., 2001, 13, 634. (b) Schottland, P.; Zong, K.; Gaupp, C. L;

Thompson, B. C; Thomas, C. A.; Giurgiu, I.; Hickman, R.; Abboud, K. A.;

Reynolds, J. R. Macromolecules, 2000, 33, 7051. (c) Thompson, B. C;

Schottland, P.; Zong, K.; Reynolds, J. R. Chem. Mater., 2000, 12, 1563.

15. Yu, G.; Heeger, A. J. Synth. Met, 1997, 85, 1183.

16. (a) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Func. Mater.,

2001, 11, 15. (b) Henckens, A.; Knipper, M.; Polec, I.; Manca, J.; Lutsen, L;

Vanderzande, D. Thin Solid Films, 2004, 451 & 572.

17. (a) Lee, B.; Seshadri, V.; Sotzing, G. A. Synth. Met, 2005, 152(1-3),

177-180. (b) Lee, B.; Seshadri, V.; Sotzing, G. A. Langmuir, 2005, 21(23),

10797-10802.

18. Invernale, M.A.; Seshadri, V.; Mamangun, D.M.D.; Ding, Y.; Filloramo,

F.; Sotzing, G.A. Chem. Mater., 2009, 21(14), 3332-3336.

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20. (a) Lee, K.; Sotzing, G. A. Macromolecules, 2001, 34, 5746-5747. (b)

Sotzing, G. A.; Lee, K. Macromolecules, 2002, 35, 7281-7286.

21. (a) Pomerantz, M.; Gu, X. Synth. Met, 1997, 84, 243-244. (b)

Pomerantz, M.; Gu, X.; Zhang, S.X. Macromolecules, 2001, 34, 1817-1822.

22. Neef, C. J.; Brotherston, I. D.; Ferraris, J. P. Chem. Mater., 1999, 11,

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Mater., 2007, 19, 3979-3983.

24. Buttery, J. H.; Moursounidis, J.; Wege, D. Aust. J. Chem., 1995, 48,

593-607.

25. Butte, W. A.; Case, F. H. J. Org. Chem., 1961, 26(11), 4690-4692.

26. Abdelmoty, S. G.; Heta, H. F. Bulletin of Pharm. Sci., Assiut Univ.,

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27. Ha, S.-T.; Koh, T.-M.; Lin, H.-C; Yeap, G.-Y.; Win, Y.-F.; Ong, S.-T.;

Sivasothy, Y.; Ong, L.-K. Liquid Crystals, 2009, 36(9), 917-925.

28. Hasegawa, M.; Fujioka, A.; Kubo, T.; Honda, T.; Miyamoto, H.; Misaki,

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29. Matsuo, Y.; Maruyama, M.; Gayathri, S. S.; Uchida, T.; Guldi, D. M.;

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163

164

CHAPTER 5

LOW BAND GAP CONDUCTING POLYMER, POLY(SELONO[3,2-

c]THIOPHENE) and POLYSELENO[3,4-*>]THIOPHENE

5.1 Introduction

The field of conjugated polymers (CPs) has been growing at a remarkable pace

since the discovery of iodine doped polyacetylene as the conducting polymeric

material1 owing to their light weight, flexibility, low cost, and better processability

as compared to inorganic materials. CPs also offer large chemical structure

variation having wide range of electrical and optical properties resulting in their

application in the field of electrochromics,2 volatile organic gas sensors,3 non­

linear optics,4 light-emitting diodes (LEDs),5 energy storage batteries,6 charge

dissipation film,7 protective coatings for corrosion prevention,8 to name a few.

The future perspective of CPs is in making cheap throw-away electronic devices

to advanced molecular electronic devices. The electrical and optical properties of

CPs are generally controlled by the energy gap between HOMO and LUMO of

the conjugated polymers.

There has been a great interest in low energy gap conjugated polymers for

their applications as an optically transparent electrode and hole-injection layer for

light emitting diodes.9 Low energy gap polymers are conjugated polymers having

energy gap equal or less than 1.5 eV.10They absorb > 600 nm in their neutral

state, and absorb in NIR region in their oxidized state making them optically

transparent in the visible region. The applications of low energy gap polymers

165

have also been recently demonstrated in the field of photovoltaic devices,11

electrochromic devices,12 near-infrared applications,13 and transistors.14

Poly(ethylenedioxythiophene) (PEDOT) is commercially available as

BAYTRON-P ® [PEDOT- poly(styrenesulfonate)] having low energy gap ranging

from 1.6 - 1.7 eV 15 which may be attributed to the electron rich oxygen atom in

conjugation with the polythiophene backbone that raises the highest occupied

molecular orbital. Optically transparency and stability in the conducting state

make PEDOT a material of choice for optically transparent electrode and also

used as hole-injection layer beside other numerous applications in the field of

electronic and optoelectronic devices. Polymerization of fused heterocyclic

monomers is one of the most promising ways to synthesize low energy gap

polymers. Wudl and coworkers reported the first low energy gap polymer,

poly(isothianaphtene) (PITN), with Eg = 1.0 - 1.2.16'17 In PITN, the stability of

the quinonoid form through aromatic benzene ring attributed to the low energy

gap. But environmental instability of PITN limits its application, and various

derivatives have been synthesized to improve the stability and relative

processability, and to lower the energy gap.18' 19 Although electrochemical

polymerization of 2-substituted thieno[3,4-jb]thiophene have been reported,20'21

Sotzing et al. first prepared conjugated polymers from thieno[3,4-b]thiophene

(T34bT) and observed a band gap of 0.85 eV for electrochemically generated

poly(thieno[3,4-jfc>]thiophene) (PT34bT).22 Sotzing et al. have reported aqueous

dispersion polymerizations of T34bT to produce dispersable PT34bTs with band

gaps ranging from 1.0 to 1.1 eV that are stable in water for over 12 months.23

166

Furthermore, we have demonstrated that insoluble PT34bT prepared via

oxidative chemical polymerization can be sulfonated to afford low band gap water

processable polymer, and they can be organized into thin films via layer-by-layer

deposition. 24 Sotzing et al. have reported that simultaneous

electropolymerization25 of T34bT and EDOT produce stable low band gap

conjugated copolymers. Sotzing et al. have reported also reported low band gap

polymer based on thieno[3,4-b]furan (T34bF) 26'27 for future photovoltaic

applications.

This chapter deals with the electrochemical synthesis and characterization of a

low energy gap polymer comprising seleno[3,2-c]thiophene (S32cT) seleno[3,4-

£>]thiophene (S34bT),26 a repeat unit bearing resemblance to both EDOS and

T34bT as shown in Figure 5.1. The polymer, S32cT, was found to have a very

low energy gap of 1.03 eV as calculated from the onset of absorption, and this

was in agreement with the theoretical value of 1.01 eV, wheseas the polymer

from S34bT has a band gap of 1.33 eV.

167

4 5 6

Figure 5.1 Chemical structures of ethylenedioxythiophene (1), thieno[334-£>]furan

(2), and thieno[3,4-fo]thiophene (3),ethylenedioxyselenophene (4), seleno[3,2-

c]thiophene (5), seleno[3,4-b]thiophene (6)

5.2 Seleno[3,2-c7thiophene and Seleno[3,4-b/thiophene Synthesis and

Characterization

Monomer Synthesis and Characterization. S32cT was synthesized according to

the reported procedure27 with modifications in the reaction conditions which are

necessary for the synthesis as shown in Scheme 5.1. The detailed systhesis of

S34bT (shown in Scheme 5.2) was discussed in the previous chapter 4.

Both S32cT and S34bT were characterized by using 1H-NMR, 13C-NMR, 77Se-

NMR and GC-MS, and results are in accordance with the reported values. 1H and

13C- NMR peaks positions were further confirmed by coupling between C atoms,

or C and H atoms via Heteronuclear Multiple Bond Correlation (HMBC) and

Heteronuclear Multiple- Quantum Coherence (HMQC) techniques.

168

5.2.1 Seleno[3,2-c]thiophene (T32cT) Synthesis :The synthesis of Seleno[3,2-

c]thiophene (T32cT) from 3,4-dibromothiophene was carried out in a manner

similar to our previously reported procedure.15 The first step involved palladium-

catalyzed coupling reaction of 3,4-dibromothiopene with trimethylsilylacetylene

according to the procedure reported by Brandsma15 in order to produce 3-bromo-

4-(trimethylsilyl)ethynylthiophene. First, lithium halogen exchange was carried

out via addition of n-butyllithium at -78° C and was followed by the warming of

the reaction mixture and addition of Selenium powder in order to produce the

selenium ion. The reaction was then cooled at -40° C and further reacted,

allowing for completion of selinium formation. After warming to approximately -10

°C, an extraction with brine cooled to -5 °C was carried out. Of paramount

importance is the separation of the aqueous layer from the organic layer. After

separation, the brine layer is heated to 70 °C, which after extraction and

purification via vacuum distillation results in a 65% yield of S32cT. Finally, the

overall purified yield obtained for S32cT starting from commercially available 3,4-

dibromothiophene is 54%.

IMS

1.n-BuLi,-78°C 2. Se, -40° C 3. Brine, 70° C

Scheme 5.1 Synthesis of seleno[3,2-c]thiophene,

169

Procedure: All equipment was vacuum-dried and argon-purged before the

reaction. A 500 mL round-bottom, three-necked flask equipped with a

thermometer was charged with 300 mL of dry diethyl ether and 10 g (38.6 mmol)

of 3-bromo-4 (trimethylsilyl)ethynylthiophene while under argon. This solution

was maintained at a temperature of -78 °C and stirred for an another 15 min,

after which 16.8 mL of n-BuLi (2.5 M in hexanes, 42.4 mmol) was slowly added

dropwise via a syringe over a period of 60 min while keeping the temperature of

the solution around -78 °C. After addition, the solution was stirred for 2 h at - 78

°C, and then the solution was allowed to warm to ca. -45 °C, which took

approximately 1 h to achieve. The reaction mixture was allowed to stir for an

additional hour while maintaining a temperature of -45 °C. Selenium powder (3.2

g, 40.6 mmol) was then added slowly over a 10 min period to the flask via a

solids addition funnel, and the reaction mixture was allowed to warm to ca. -35

°C in order to fully dissolve the selenium. After approximately 10 min the initially

cloudy solution that resulted immediately after selenium addition had changed to

clear light yellow. The reaction mixture was then cooled to -45 °C, and stirring

was continued for an additional 2.5 h. Then the reaction mixture was slowly

warmed to -10 °C over a period of 30 min, and 250 mL of the reaction mixture

was then added to a 500mL separatory funnel equipped with a cooling jacket

containing a brine/ice mixture held at -5 °C. It should be noted that the separatory

funnel was first charged with 200 mL of brine, and then this solution was cooled

to -5 °C before the extraction. During the extraction the aqueous layer turned

cloudy light yellow after 30 s. It is pertinent at this stage to separate the layers.

170

After the first brine wash, the same process was then repeated for the second

portion (250 mL) of the reaction mixture with a fresh brine solution (200 ml_). The

brine layers were then combined and placed into a single-neck 1000 mL round-

bottom flask under argon, and then this solution was heated to 70 °C and stirred

for 1 h at this temperature. After cooling to room temperature, the product was

extracted with ether (4 X 300 mL), and then the ether layers were combined,

dried over MgSCU and evaporated under vacuum. The crude product was

purified using vacuum distillation with an 4.6 g (65%) seleno[3,2-c]thiopene

fraction being collected from 45 to 55 °C at 0.05 Torr as a colorless liquid.

S32cT: 1H-NMR (400 MHz, CDCI3): 8.01 (1H, d), 7.88 (1H, dd), 7.79 (1H, dd),

7.43 (1H, d); 13C-NMR: 151.06, 141.36, 132.75, 117.52, 117.40,115.01; 77Se

NMR: 432.72; GC-MS (m/z): 188; FTIR (liquid film): 3100, 1554, 1493, 1323,

1294, 1152, 1083, 809 and 768 cm-1

Mwt-128

1)nBuli,-78 Peg C

COOH 2) c ° 2 12Hrs

COOH

COOH

Mwt-172 Yields - 50 %

y-*-Br

1)PBr3> Ether f T \ \ Br J !

12Hrs \ 0 / ^ ^

1)l_iAIH4>THF

70 Deg C 24Hrs

y^ -OH

Mwt-144 Yields - 70 %

Na2S, DMF^

6Hrs

// \\ / ^1P D Q D C M -Se

30 Mins

3 Mwt-270

Yields - 80 % Mwt-190

Yields ~ 50 %

cP Mwt-188

Yields ~ 50 %

Scheme 5.2 Synthesis of seleno[3,4-b]thiophene,

171

5-3 Poly(Seleno[3,2-c]thiophene)(PS32cT) and Poly(Seleno[3,4-6]thiophene), (PS346T)

5.3.1 Electrochemical Synthesis and Characterization

PS32cT and PS34bT were prepared via cyclovoltammetric polymerization of

0.01 M T34bF in 0.1 M electrolyte solution using acetonitrile (ACN) as the

solvent. Figure 5.2 depicts the cyclovoltammogram for the electrochemical

polymerization of T34bF in the presence of tetrabutylammonium tetrafluoroborate

(TBABF4) / ACN electrolyte solution using Pt button as working electrode in a

three electrode electrochemical cell containing Pt flag as counter electrode and

non-aqueous Ag-Ag+ as the reference electrode (0.445 eV vs NHE).

(A)

40 n ,

• 8 0 H 1 1 • 1 1 1 1 1 • 1 i 1 1 1 1 1 1 1 1 1 r

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

Potential / V

172

(B)

i • •*«

*-» c 0)

3

o

10-J

°1 -10 J

-20-]

-30 J

-40 J J

-50 J J

-60 -J

-70 J

-80 J

-90 J

-loo H — , — , — , — | — , — | — , — | — , — | — • — , — , — , — , — , — , — l 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8

Potential/V

Figure 5.2 Cyclovoltammetric polymerization (electrochemical), 20 scans, of

Selono[3,2-c]thiophene (S32cT) (A), Selono[3,4-ib]thiophene (S34bT) (B), (0.01

M) in 0.1 M TBAP/acetonitrile electrolyte solution at a scan rate of 100 mV/s

using a Pt button working electrode.

The electrochemical oxidative polymerization of S32cT (Figure 5.2A) shows

the onset for monomer oxidation at 0.85 V with a peak at 1.1 V. The oxidative

polymerization of S32cT is almost similar with respect to T34bT. The low

monomer oxidation potential may help to avoid over oxidation during the

electrochemical process. The current response at -0.2 V is attributed to the

neutral conjugated polymer to conductive polymer redox process, and its

increase in intensity during subsequent scans indicates the deposition of

173

conducting PS32cT onto the working electrode. The electrochemical

polymerization was accompanied with the formation of an insoluble dark blue

solid on the working electrode. Upon scanning towards the cathodic direction, a

broad reduction process was observed corresponding to the reduction of the

oxidized form of PS32cT deposited onto the working electrode during the

previous anodic scan. After the first scan, a new oxidation process was observed

at a lower oxidation potential, indicates the oxidation of a more conjugated

species formed during the first CV scan. This was further confirmed by the

increase in the current response corresponding to the reduction of oxidized form

of PS32cT to the neutral form. The increase in reduction and oxidation current

during successive CV scans indicated the deposition of additional conducting

polymer during each scan, and this behavior has been observed for 20 CV scans

as shown in the Figure 5.2. This indicates that PS32cT obtained is very stable

during the electrochemical CV scans between -1.0 V and +1.25 V. It should be

noted that the peak position for the oxidation of PS32cT shifted after 20 CV

scans, and this may be attributed to the increase in the film thickness of

conductive polymer deposited onto the electrode that slows the movement of

ions through the polymer matrix. Whereas the electrochemical oxidative

polymerization of S34bT shows the onset for monomer oxidation at 0.70 V with a

peak at 0.9 V as shown on Figure 5.2B. The oxidative polymerization of S32cT

is 0.15 V lower than that of T34bT, forms a dark brown polymer on the working

electrode.

174

5.3.2 Scan Rate Dependency and Redox Switching

For this study, polymer was prepared by electrochemical polymerization using

the similar experimental parameters as described above except that only 6 scans

of cyclovoltammetry were performed instead of 20. After electrochemical

polymerization, polymer redox behavior was observed by using the monomer

free electrolyte solution, and it should be noted that the electrochemical

characterization was performed using the same electrolyte as that used for

electrochemical polymerization. Polymer showed a broad oxidation process with

an onset at -0.40 V, similar to that observed during electrochemical

polymerization. An oxidation peak was observed at 0.3 V with a shoulder at

0.10 V corresponding to the oxidation of neutral form to the oxidized form of

PS32cT (Figure 5.3A). On scanning in the cathodic direction, a broad reduction

process was observed corresponding to the reduction of oxidized form of

PT34bF to the neutral form. The redox current response increased with the

increase in the scan rate for PS34bT is shown in Figure 5.3B and fits the

modified Randles-Sevcik equation.

175

(A)

(B)

8>

o

1000 mV/s

100mV/s

1000 mV/s

0.6 0.4 0.2 0.0 -0.2

Potential / V

-0.4 -0.6 -0.8

c <D

o

80

60 -I

40 J

20 J

0

-20

-40 H

-60

-80-|

-100

1000 mV/s

100mV/d

1000 mV/s

0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

Potential / V

176

Figure 5.3 Cyclic voltammogram of poly(Seleno[3, 2- c]thiophene)(A),

poly(Seleno[3, 4- b]thiophene)(B) films deposited by CV on Pt button electrode at

different scan rates varying from 50 mV/s to 1000mV/s in 0.1 M TBAP/ACN

electrolyte solution. Potential reported vs. Ag/Ag+ nonaqueous reference

electrode (0.44 V vs. NHE).

PS32cT and PS34bT were electrochemically switched between neutral (-0.8 V)

and oxidized state (+0.6 V) using monomer free 0.1 M electrolyte solution in

acetonitrile. A 9 - 10 % decrease in charge was observed after 200 double-

potential steps using all four electrolyte solutions, without further optimization.

(A)

CD <D i

O T -

<D O) i . (S -C O

0 20 40 60 80

Time / sec

(B)

177

-10 A

-15J

-20 A

Time / sec

Figure 5.4 Chronocoulometry obtained for redox switching (A) PS32cT and (B)

PS34bT between -0.8 and 0.6 V in 0.1 M TBAP pulse width of 8 s. Only the first

four double potential steps of a total 50 that were taken are shown.

178

5.3.3 Optical Properties

In-situ Spectroelectrochemistry

Polymer deposited onto ITO-coated glass was used to perform insitu

optoelectrochemistry by applying constant potential via chronocoulometry in

conjunction with the Vis-NIR spectrophotometer. A UV-Vis-NIR spectrum of the

PS32cT in the neutral state is shown in Figure 5.5. The onset for the 7u-to- 71*

interband transition occurs at 1.05 eV (1200 nm) with a peak at 1.72 eV (736 nm)

and therefore this polymer fulfils the defined criterion for being a low band gap

conjugated polymer.9 PS32cT has a band gap 0.55 eV lower than that of PEDOT

(Eg = 1.6 eV) and 0.2 eV higher than that of PT34bT (Eg = 0.85 eV). The fused

selenophene ring helps stabilize the quinoidal form of PS32cT results the

polymer having a band gap lower than that of PEDOT. The slightly higher band

gap of PS32cT as compared to PT34bT can be attributed to the difference in the

stability of the quinoidal state by selenophene and thiophene ring respectively.

The polymer has dark blue color in the neutral form and is transmissive light blue

in the oxidized state (p-doped). Whereas PS34bT has a very broad and shallow

peak in the visible region, which is in contrast to the sharp peak with a well-

defined A,max of PS32cT. This result can be explained by the overlapping of

several absorption peaks resulting from planar and twisted fragments of the

polymer chain. P34bT has brown color in the neutral form and is transmissive

light brown in the oxidized state (p-doped). Polymer color was further confirmed

by calculating [L a b] color coordinates of PS32cT and PS34bT using the spectral

179

data from Figure 5.5. Calculations were performed by assuming

blackbody@6000K. [u\ v'] coordinates for PS32cT and PS34bT in reduced states

were found to be [0.1678, 0.4489] and [0.2050, 0.4825] respectively.

(A)

0)

o m •O o m n <

0.6 A

600 800 1000 1200 1400

Wavelength (nm)

1601

180

(B)

' 1 « 1 « 1 ' 1 ' 1 «•

400 600 800 1000 1200 1400 1600

Wavelength (nm)

Figure 5.5. UV-vis-NIR spectrum of a 0.2 |xm thick PS32cT (A) and PS34bT(B)

films on ITO glass. The film were electrochemically reduced at -1.0 V in 0.1 M

TBAP/ACN, dipped in a solution of 0.1 M TBAP/ACN containing 0.2% hydrazine

by volume, and air-dried before obtaining the spectrum. The film was then

oxidized by dipping in a solution of 0.1 M TBAP/CAN containing 0.2% antimony

pentachloride (SbCI5) and air-dried before obtaining the spectrum.

181

5.4 Energy Gap Calculations

5.4.1 Spectroelectrochemistry

A 10 mM monomer/ 0.1 M TBAP/ACN solution was used to prepare a ca. 0.05

|jm thick polymer film, as measured by profilometry, on ITO coated glass at 1.15

V. The polymer film was reduced chemically by dipping it into a 0.2 vol %

hydrazine solution. A Vis-NIR spectrum of the polymer in the neutral state is

shown in Figure 5.6. The onset for the n- n* occurs at 1.03 eV (1200 nm) with a

peak at 1.72 eV (720 nm) and therefore this polymer meets the defined criterion

for being a low band gap polymer. PT32cT has a band gap 0.57 eV lower than

that of PEDOT (Eg = 1.6 eV) and 0.18 eV higher than that of PT34bT (Eg= 0.85

eV). The fused furan ring helps stabilize the quinoidal form of PS32cT as does

the selenophene benzene of isothianaphthene11 thereby leading to a polymer

having a band gap lower than that of PEDOT. The slightly higher band gap of

PS32cT as compared to PT34bT can be attributed to the difference in the

stability of the quinoidal state by selenophene and thiophene ring respectively.

The polymer has pale blue color in the neutral form and is transmissive light blue

in the oxidized state (p-doped). Polymer color was further confirmed by

calculating [L a b] color coordinates of PS32cT using the spectral data from

Figure 5.9.

(A)

182

CD O c CO L .

o W n <

1.8-

1.6-

1.4-

1.2-J

1.0-1

0.8 J J

0.6 J

J^SL / Oxidized ^

^ ^ Reduced

Li 1 1 1 1 1 1 1 1 1 1 1

600 800 1000 1200

Wavelength (nm)

1400 1600

(B)

183

0.5 -'

o 0.4-<o n o to

0.2 J

400 600 800 1000 1200 1400 1600

Wavelength (nm)

Figure 5.6 Vis-NIR spectrum of a 0.05 jim thick poly(thieno[3,4-b]furan) film on

ITO glass. The film was chemically reduced using 0.2 % v/v hydrazine solution in

acetonitrile, and oxidized using 0.2% v/v antimony (V) chloride solution in

acetonitrile.

5.4.2 Cyclic Voltametry (p and n doping):

Cyclic voltammetry (CV) was used to measure oxidation and reduction

potentials. PS32cT presents one reversible p-doping [Eox= - 0.22 V vs. Ag/Ag+

and -0.26 V ferrocene / ferrocenium (Fc/Fc+)] and one irreversible n-doping

(Ered= - 1 -25V vs Ag/Ag+ and - 1.29 V vs Fc/Fc+) process (Figure 5.7A.The

HOMO ( - 4.54 eV) and LUMO (-3.51 eV) of PS32cT could be calculated from

these electrochemical results. Bang Gap = 1.03 eV. Whereas PS34bT presents

184

one reversible p-doping [Eox= 0.55 V vs Ag/Ag+ and 0.51 V vs ferrocene /

ferrocenium (Fc/Fc+)] and one irreversible n-doping (Ereci= - 0.95 V vs Ag/Ag+

and -0.99 V vs Fc/Fc+) process (Figure 5.7B). The HOMO (- 5.31 eV) and

LUMO (-3.81 eV) were calculated base on CV, shown in Figure 5.7B

(A)

(B)

0.0 -0.5 -1.0

Potential (V) 0.0 -0.5 -1.0

Potentia/V -1.5 -2.0

Figure 5.7 1st and 2nd scan of p- and n-doping cyclic voltammetry for (A)

poly(S32cT) and (B) poly(S34bT) on a Pt button working electrode at a scan rate

of 100 mV/s in 0.1 M TBAP/acetonitrile showing the first scan (I) and the second

scan (II).

185

5.4.3Theoritical Calcultions

Theoritical calculations on PS32cT and P34bT were performed to calculate the

band gap value on the assumption that repeat units are connected thru positions

4 and 6. The ground state geometry was optimized using the density functional

theory (DFT) method treated with periodic boundary conditions at the B3PW91

level of theory with 6-31D(d,p) basis set.28 The Gaussian 03 program package29

was employed for this optimization. HOMO and LUMO energies for PS32cT were

calculated to be -4.08 eV and -3.02 eV with a band gap of 1.06 eV which is in

good agreement with the observed value of 1.05eV. Dihedral angles of the

optimized structure show that the structure can be assumed planar since

extremely small deviations from planar structure take place. The optimized

strucutures of S32ct (dimer) and S34bt (tetramer) is shown in Figure 5.8.

(A)

186

Figure 5.8 Optimized calculated structure of S32cT dimmer (A). The band gap

for this dimer was calculated to be 1.05 eV with a HOMO level of 4.08 and LUMO

of 3.02 eV. Optimized calculated structure of S34bT tetramer (B). The band gap

for this tetramer was calculated to be 1.33 eV with a HOMO level of 4.40 and

LUMO of 3.07 eV.

Future of PS32cT in Organic Photovoltaics:

All the bulk heterojunction organic solar cells are based on blends of

conjugated polymers (MDMO-PPV, PFDTBT, RR-P3HT and PTBEHT) with the

fullerene derivative PCBM. Current polymeric materials being used have a high

band gap (>2.0 eV) and therefore absorb light in the mid to high energy visible

region of the solar flux, limiting the photon harvesting to 30 %. However, 77 % of

these photons lie in the low energy end of the spectrum. Inorganic crystalline

silicon is capable of absorbing these photons, and in the OPV industry there is a

need for organic materials that can do the same.30 The absorption spectrum of

187

PS32cT in its neutral state is shown in Figure 5-9, and for comparison is overlaid

with spectra of P3HT, MEH-PPV, PT34bT along with the solar flux under air

mass 1.5 conditions and absorption of crystalline silicon.31 Figure 5.9 shows the

solar spectrum under Air Mass 1.5 conditions (AM1.5-G; i.e. sun light that passes

through the atmosphere at 42Q from horizon) along with overlaid spectra for the

light trapped (absorbed) by currently employed light harvesting materials:

crystalline silicon, P3HT, MEH-PPV, and that of PT34bT, and PT34bF. The Y-

axis of Figure 5.9 is in energy units (W/m2 nm) corresponding to solar flux, and

the absorption spectra of the organic materials were normalized to an arbitrary

height but the A,max of the materials was not altered. Thus, the Y-axis of the

materials should be in normalized dimensionless absorbance scale. However,

the figure is accurate in terms of the wavelength, i.e. abscissa, which means that

the new materials show a remarkable matching to that of the silicon spectrum. It

is clear that crystalline silicon has a broad absorption, and the currently used

organic polymers suffer both in terms of absorption range and the location of the

maximum: they have relatively sharp absorptions at the high energy end.

However, PS32cT exhibits broader absorptions and at lower energies - where

maximum number of photons are. The absorption spectra of PS32cT match well

with that of crystalline silicon and therefore have the ability to trap photons across

the spectrum, including the low energy end. This is a direct result of the low

band gap of these unique materials, and thus are a good match, in terms of

absorption characteristics, with the projected need for low band gap materials.

The potential to use both PS32cT and P3HT (or other higher band gap

188

materials), in tandem, will lead to improved light harvesting both in the low and

high energy end of the solar spectrum.

c CM"

E

(A)

fool L i

Q solar spectrum (AM 1.5-G, 1000 W/m2)

Cj converted by crystalline silicon ceil

1100 nm - 1.1 eV = band gap of silicon

2400

Wavelength (nm)

(/^ solar spectrum (AM 1.5-G, 1000 W/m2

£ j ) converted by crystalline silicon cell

1100 nm - 1.1 eV ~ band gap of silicon

24001

(B) Wavelength (nm)

189

Figure 5-9 Solar Spectrum Material Comparison . (A) Curves were normalized

for relative absorbance but the peak remains at their correct wavelengths. (Blue

= PolySeleno[3,2-c]thiophene, Yellow = PolyThieno[3,4-b]thiophene, Red =

MEH-PPV in acetone, Green = Poly(3-hexylthiophene). (B) Normalized

absorbance peak for PS34bT.

Aside from the energy gap, they also offer a good match of the absolute energy

levels with the other materials in the device. The HOMO of the low band gap

polymers agree with the work function of ITO and LUMO matches with the

acceptor level of PCBM. This overlap is crucial to the function of a device. When

light is absorbed by the material, a photoexciton (electron-hole pair) is generated.

This occurs as a result of the excitation of an electron from the HOMO crossing

the band gap to the LUMO, thereby creating a hole, and an electron in the

HOMO and LUMO respectively. Since the exciton diffusion length is limited to a

few nanometers (10-20 nm), "splitting" of this Coulomb bound species has to be

achieved and can be done by carefully selecting an acceptor material whose

LUMO (acceptor) level lies below that of the LUMO level of the donor. The

electron thus crosses the barrier and moves into the acceptor region, continuing

towards the cathode, while the hole travels towards the anode. In order for the

holes and electrons to now cross the Semiconductor-Metal (Schottky) barrier, it is

crucial that the work functions of the selected metal match with the respective

levels of the semiconductor.

190

Band Gap Energies (eV)

0

2

4

6

ITO:-4.2

(A)

0

2

4

6

Band Gap Energies (eV)

PS34bT PCBM

Ca/AI: - 2.9

ITO:-4.2

L.„.^.

(B)

Figure 5.10 Band Levels (in eV vs. Vacuum) of a device based on PS32cT (A)

and PS34bT (B) as donar material and PCBM, and movement of the electron (e)

and hole (o) created as a result of NIR absorption.

One method for reducing this barrier involves the use of additional layers like a

hole injection layer (e.g. PEDOT) and electron injection layer (e.g. LiF) placed

191

between the metal electrode and the semiconductor. In order to avoid this extra

level of construction, the design of an OPV device must include a careful

selection of materials with appropriate band levels. Figure 5-10 shows the band

levels of PS32cT and PS34bT as a donar material and PCBM along with the

work functions of ITO32"33 and Calcium/Aluminum. The HOMO level of PS32cT

was calculated from cyclic voltammogram (CV) measurements vs. ferrocene,33'36

and since the band gap is known, the LUMO level found. As explained earlier,

exciton formation will occur in the PT34bF layer after NIR absorption, and the

electron will move from the LUMO of PS32cT (-3.65 eV) into the acceptor level of

PCBM (-3.75 eV) and since the work function of cathode is -4.3 eV, will flow into

the Aluminum electrode. The hole generated in the HOMO of PS32cT (-4.58

eV), on the other hand, will move into the ITO anode having a work function of -

4.2 eV.

5.5 Conductivity

A 1.2 cm diameter pellet obtained from electrochemically polymerized PS32cT

and PS34bT were used for conductivity measurements. Five measurements of

each polymer were taken for each experiment by varying the applied current

between 1 x 10"5 and 1 x 10~7 A, and the resistance values are same at these

different current within 1- 2% error indicating the ohmic behavior. In neutral form,

both PS32cT and PS34bT showed a conductivity of ~10"5 S/ cm. After exposing

the pellet to iodine vapors for 30 min., the conductivity increased to ~10"2 S/cm.

192

5.6 Conclusions

This chapter includes a modified procedure for the synthesis of S32cT and

S34bT monomer with an emphasis to avoid purification procedure until the last

synthesis step. The lower oxidation potential of S32cT and S34bT helps in

avoiding overoxidation of polymer during electrochemical polymerization. S32cT

and S34bT showed stable redox behavior with broad oxidation and reduction

processes observed during cyclic voltammetric scans. PS32cT showed a A,max at

720 nm and an Eg of 1.04 eV which qualifies it as a low energy gap polymer. This

value is similar to that of 1.03 eV for chemically neutralized PS32cT. There is a

increase in the transmittance in the visible region on going from -0.6 to +0.6 V

indicating completely oxidized and reduced states at +0.6 V and -0.6 V,

respectively.

193

5.7 References

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2. (a) Schwendeman, I.; Hwang, J.; Welsh, D. M.; Tanner, D. B.; Reynolds, J. R.

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Macromolecules 2000, 33, 7051. (c) Thompson, B. C; Schottland, P.; Zong, K.;

Reynolds, J. R. Chem. Mater. 2000, 12, 1563.

3. (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537.

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7. Jonas, F.; Heywang, G. Electrochim. Acta 1994, 39, 1345.

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Winder, C; Sariciftci, N. S.; Hummelen, J. C; Dhanabalan, A.; Van H., Paul A.;

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16. Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382.

17. (a) Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J. J. Chem.

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Synth. Met. 2000, 110, 25. (b) Kisselev, R.; Thelakkat, M. Macromolecules 2004,

37, 8951. (c) Goris, L; Loi, M. A.; Cravino, A.; Neugebauer, H.; Sariciftci, N. S.;

Polec, I.; Lutsen, L; Andries, E.; Manca, J.; De Schepper, L; Vanderzande, D.

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197

CHAPTER 6

SELENIUM BASED ELECTROCHROMIC CONJUGATED POLYMER

6.1 Introduction

Interests in the field of conjugated polymers have increased tremendously after

the discovery of iodine doped polyacetylene as conducting polymeric material.1

Conducting Polymers offer a unique combination of properties that make them

suitable alternative to current materials in specific electrochromics,2 cheap light

weight volatile organic gas sensors,3 non-linear optics,4 light-emitting diodes

(LEDs),5 energy storage batteries,6 charge dissipaters,7 corrosion protectors,8

and solar cells 9 among other uses.

Conjugated polymers based on 3, 4-ethylenedioxythiophene (EDOT) and its

derivatives have gained lot of attention because of their lower oxidation potential,

better stability towards arial oxidation and at elevated temperature, highly

conducting and especially stable doped states. Moreover EDOT has a range of

optical properties with electronic band varying across the visible spectrum,

enhanced redox properties making the useful candidate for electrochromic

devices.10"11 The electron-donating effect of the oxygen atoms in the 3, 4

positions and favorable ring geometry stabilize the positive charge in the polymer

backbone, which causes the higher stability of poly-EDOT (PEDOT) in its p-

doped state. The electrochemical polymerization of EDOT takes place at a

relatively low oxidation potential, and the corresponding PEDOT exhibits an

optical bandgap of 1.6 eV.12 which can be explained by raising of the HOMO

198

(with little expense to raising of LUMO) due to electron rich oxygen atoms from

the cyclic ethylenedioxy ring and reduction of steric intereactions in the polymeric

backbone. Polymers based upon alkylenedioxythiophene are popular materials

in electrochromics for their ability to change color from blue to colorless. PEDOT

has the ability to transition from deep blue to sky blue upon oxidation with a

photopic contrast of approximately 54%.13 Higher photopic contrast (66%)13 and

a more colorless bleached state is obtained by incorporation of an additional

methylene unit into the EDOT repeat unit with 3,4-propyldioxythiophene

(ProDOT).14 Further derivatization of ProDOT at the central methylene unit

introduces a higher degree of disorder between polymer chains further reducing

the color of poly-ProDOT (PProDOT) in the oxidized state while maintaining an

intense blue in the reduced state.14"15 One of the first polymer that exhibit

enhanced photopic contrast (up to 75 %) along with more rapid switching was

poly-(ProDOT-Me2), whose monomer was synthesized by the traditional double-

Willison etherification of 3,4-dimethoxythiophene (DMOT) and neopentyl glycol

in presence of p-toluenesulfonic acid as a catalyst.16

Despite, being a member of five membered fused heterocylces, unlike

polythiophenes it is surprise that very little is known about polyselenophenes. It

have been shown that the performance of selenophene(as

tetramethyltetraselenafulvalene (TMTSF) and

bis(ethylenedithio)tetraselenafulvalene (ETS) based organic superconductors are

better than that of thiophene (tetrathiafulvalene (TTF) based superconductors.17

Theoretical studies and calculations18 indicate that selenophene based polymers

199

should have a lower band gap {Eg) than corresponding polythiophenes. Due to

the larger size of Seleniun, polyselenophenes are also expected to have some

advantages over polythiophenes, such as having lower oxidation and reduction

potentials, easier to polarize and more suitable for interchain charge transfer

(which is facilitated by the intermolecular contacts between Se atoms).

Recently, Cava et.al. and Patra et.al.19"20 synthesized poly(3,4-

ethylenedioxyselenophene) (PEDOS) and reported the band gap of 1.4 eV. It

also has lower band gap than PEDOT (1.6-1.7 eV). PEDOS is highly stable in its

oxidized state and has excellent electrochromic properties.21'22

Considering all the advantages of PProDOT-Me2 and selenium, we decide to

concentrate our study on poly(3, 4-propylenedioxyselenophene-Me2) (PProDOS-

Me2). The goal is to reduce the oxidation potential of the monomers for faster

switching speed and enhanced the electrchromic contrast by introducing steric in

the 3, 4-dioxy ring. The replacement of the sulfur atom in ProDOT by the more

polarizable selenium atoms is an alternate approach for lowering the oxidation

potential of monomer and consequently for optimizing the electro-optical

properties of corresponding conjugated polymer. Chemical structures of

ProDOS-Me2and ProDOS-Hex2 are shown in Fig 6.1.

200

ProDOS-Me2 ProDOS-Hex2

Fig 6.1 Chemical structures of ProDOS-Me2 and ProDOS-Hex2

6.2 Monomer Synthesis and Characterization

ProDOS-Me2 and ProDOS-Hex2 were prepared by Transetherification of

dimethoxyselenophene (DMOS) with neopentyl glycol and 2,2-Dihexyl Propane-

153-diol in the presence of a catalytic amount of p-toluenesulfonic acid (p-TSA) to

produce ProDOS-Me2 and ProDOs-Hex2 respectively. DMOS was synthesized

according to the reported procedure10 (20) with modifications in the reaction

conditions which are necessary for obtaining better yield. The selenophene ring

was constructed, by cycloaddition of 2,3-dimethoxy-1,3-butadiene with freshly

prepared SeCI2 in the presence of NaOAc. Chemical polymerization of

DibromoProDOS-Hex2 was done by using FeCI3.PolyProDOS-Hex2 is soluble in

common organic solvents with a number average molecular weight of 5800

g/mole. ProDOS-Me2j ProDOD-Hex2 and the soluble polymer obtained from

ProDOS-Hex2 were characterized by using 1H-NMR, 13C-NMR, 77Se-NMR ,

GC-MS and GPC.

*

201

Synthesis of 3,4-dimethoxyselenophene (DMOS):

SeCI2 was prepared by adding S02CI2 (2.0 g, 14.8 mmol) to selenium powder

(1.18 g, 14.8 mmol) over a period of 5 min at 10-20°C. After 30 min, 10 mL

hexane was added to it and the resulting reaction mixture was stirred for 4 h at

room temperature. A clear brown solution of SeCI2 was formed.23

To a well stirred solution of 2,3-dimethoxy-1,3-butadiene 24 (1.47 g, 12.9 mmol)

and CH3COONa (2.64 g, 32.25 mmol) in dry hexane (80 mL) at - 78°C (dry

ice/acetone bath), under an inert atmosphere, was added a solution of freshly

prepared SeCI2 in hexane over a period of 15 min. The resulting yellowish

solution was further stirred for 1 h at -78°C and then removed from the cooling

bath and the reaction mixture was brought to room temperature over a period of

2 h and further stirred for 5 h. The reaction mixture was filtered through neutral

alumina and washed with hexane. The residue was concentrated to give brown

yellow oil. The crude product was purified by flash column chromatography on

TLC grade silica gel (Hexane: Ethyl acetate - 95:5) to provide DMOS (0.85 g,

35%) as a white crystalline solid, mp. 43-45 "C. The synthesis of DMOS is shown

in Scheme 6.1 .

202

MeOv OMe _ _. M n f t MeO OMe \ / SeCU, NaOAc 7—( #\ 0 ** FA '' v Hexane,-78° C to RT N _ ^

' Se F.W-114 F.W-192

35%

Scheme 6.1 Synthetic of 3,5-DimethoxySelenophene (DMOS)

1H NMR (500 MHz, CDCI3): 5 6.55 (s, 2H), 3.85 (s, 6H); 13C-NMR : 5 148.9,

96.0, 57.0; MS El (70 eV), m/z: 192 (M+, 100%), 149, 134, 93; Anal calcd. for

C6H802Se: C, 37.71; H, 4.22; Found. C, 38.05; H, 4.23. 1H NMR spectrum of 4

was in good agreement with the reported data.3 77Se NMR (250 Hz, CDCI3) 5 =

377.9 ppm.

Synthesis of 3,4-propylenedioxyselenophene (ProDOS-Me2)

3,4-Dimethoxyselonophene (DMOS) (0.5 g, 2.60 mmol), neopentyl glycol (0.54

g, 5.20 mmol), dodecylbenzene sulfonic acid (DBSA) ( 0.17 g, 0.52 mmol) and

100 mL of dry toluene were combined in a 3-neck round bottom flask equipped

with a Soxhlet extractor with type 4 A molecular sieves in the thimble the

synthetic scheme is shown in Scheme 6.2 . The solution was heated to reflux

and allowed to reflux for 12 hrs. The reaction mixture was cooled, washed with

dilute NaHC03 solution and finally with water. Due to the presence of DBSA,

while washing with water, an emulsifier results in. Solid NaCI is used as an

emulsion breaker. The toluene was removed under vacuum, and the crude

product was purified by column chromatography on silica gel with 4:1

203

hexanes/ethylacetate as the eluent to yield Pro-DOS-Me2 as a white solid (0.39

g, 52%).

V MeO OMe O O

\ / Neopentyl Glycol^ \ / < ^ DBSA, Toluene ^ >

S e 70°C, 12Hrs Se

F.W-192 F.W-232

65%

Scheme 6.2 Synthetic of ProDOS-Me2

1H-NMR (500 MHz, CDCI3): 5 6.97 (s, 2H), 3.72 (s5 4H)5 1.02 (s, 6H); 13C NMR: 5

151.44, 108.37, 80.05, 39.18, 21.98. 77Se NMR (400 MHz, CDCI3) 6 = 394.9

ppm.

Synthesis of 2,2-Dihexylmalonic Acid Diethyl Ester

In a 500 mL flame dried three-neck round bottom flask equipped with an argon

inlet and condenser were combined 200 mL of dry THF, Hexyl bromide (0.00 g,

1.5 mole), and 3.5 mol of NaH. The flask was cooled to 0 °C, and 1.15 mol of

freshly distilled diethylmalonate was added dropwise via syringe. After the

addition of malonate, the mixture was refluxed for 12 h. The flask was then

cooled at 0 °C and the remaining sodium hydride was quenched by adding water

dropwise. The mixture was then poured into brine solution (2 L) and extracted

two times with ether. The ether layer was finally washed with brine and then with

water. The organic phase was dried over MgS04, and evaporated to give a light

204

yellow liquid. The crude product was further purified by vacuum distillation to

provide 2,2-Dihexylmalonic Acid Diethyl Ester (0.85 g, 70%) as a colorless oil.

p C 2 H 5

C 6 H 1 3 C 6 H 1 3

NaOEt, C6H13-Br C 2 H 5 0 N ^ > C J D C 2 H 5 C6H13-Br C 2 H 5 t X ^ X . ^ O C 2 H ,

1,24 hrs i f I f _ , Ethanol

bc2H5

F.W-160 F.W-328

72%

Scheme 6.3 Synthesis of 2,2-Dihexylmalonic Acid Diethyl Ester

1H NMR (500 MHz, CDCI3): 0.87 (t, 6H), 1.11-1.30 (m, 20H), 1.85 (t, 6H), 4.17

(q, 4H). 13CNMR (500 MHz, CDCI3): 14.08, 14.17, 22.63, 23.94, 29.59, 31.60,

32.21, 57.63, 60.94, 172.09. GC-MS: 311, 272, 244, 173. IR (KBr, cnrV1): 1733,

2859, 2928, 2957.

Synthesis of 2, 2-Dihexyl Propane-1,3-diol

A suspension of lithium aluminum hydride (LAH) (2 g, 52.6 mmol) in dry THF

(20 mL) was stirred at room temperature, and a THF solution of substituted

malonic acid diethyl ester (26.3 mmol) was added dropwise. The reaction was

allowed to reflux for 3 h and was quenched by the addition of cold water. The

compound was extracted in ethyl acetate. The organic layer was washed with

water, dried over Na2S04, and evaporated to produce either a sticky liquid or a

low-melting solid.

205

C6H1 3 A H 1 3 CeH^CeH^ C 2 H 5 0 ^ JC ^OC 2 H 5 LiAiH4 ^ * ^

i f T THF,24hrs I 1 0 0 OH OH

F.W - 328 F.W - 244

65%

Scheme 6.4 Synthesis of 2, 2-Dihexyl Propane-1, 3-diol

1H NMR (500MHz, CDCI3): 0.86 (t, 6H)5 1.28-1.39 (m, 20H), 3.84 (s, 4H), 6.41

(s, 2H).13C NMR (500 MHz, CDCI3): 14.24, 22.82, 22.94, 30.31, 31.91, 32.02,

43.89, 76.88, 104.80, 149, 89. GC-MS: 324, 169, 138, 116, 97, 69, 55.

ELEM.ANAL. Calcd. for C19H32O2S: C, 70.32; H, 9.94%; O, 9.86%; S, 9.88%.

Found: C, 70.24; H, 9.76%; 0,9.9%; S, 9.28%.

Synthesis of 2,2-Dihexyl(3,4-propylenedioxyseIenophene)(ProDOS-Hex2)

3,4-Dimethoxyselonophene (DMOS) (0.5 g, 2.60 mmol), 2,2-Dihexyl Propane-

1,3-diol (0.54 g, 5.20 mmol), dodecylbenzene sulfonic acid (DBSA) ( 0.17 g, 0.52

mmol) and 100 mL of dry toluene were combined in a 3-neck round bottom flask

equipped with a Soxhlet extractor with type 4 _ molecular sieves in the thimble.

The solution was heated to reflux and allowed to reflux for 6 h. The reaction

mixture was cooled, washed with dilute NaHC03 solution and finally with water.

Due to the presence of DBSA, while washing with water, an emulsifier results in.

Solid NaCI is used as an emulsion breaker. The toluene was removed under

vacuum, and the crude product was purified by column chromatography on silica

206

gel with 4:1 hexanes/ethylacetate as the eluent to yield ProDOS-Hex2 (5) as a

white solid (0.39 g, 65%).

c e H i 3 3 p 6 H i 3

MeO OMe O O \ / 2, 2-dihexylpropane-1, 3-diol ^ \ / <l ? DBSA, Toluene ^ i>

Se 70°C, 12Hrs S e

F.W-192 F.W-272

49%

Scheme 6.5 Synthesis of ProDOS-Hex2

1H-NMR (500 MHz, CDCI3) 5 6.97 (s, 2H), 3.72 (s, 4H), 1.02 (s, 6H); 13C NMR: 5

151.44, 108.37, 80.05, 39.18, 21.98.Spectral data of 5 were in agreement with

the reported data for 3,4-propylenedioxyselenophene(5).4 77Se NMR (400 MHz,

CDCI3) 6 = 394.9 ppm.

6.3 Electrochemical Synthesis and Characterization

6.3.1 Cyclic Voltametry

Electrochemical polymerizations were carried out by preparing a 0.01 M

solution of monomer in a 0.01 M tetrabutylammonium hexafluorophosphate

(TBAPF6) in ACN, as supporting electrolyte solution. A three electrode

electrochemical cell was used for all the electrochemistry experiments using

Ag/Ag+ as a reference, platinum flag (0.5" X 1") as a counter and Pt (2 mm

diameter) button as a working electrode. The reference electrode consisted of Ag

wire dipped in 0.01 M AgN03 in 0.01 M TBAPFe/ acetonitrile inside a glass tube

207

fitted with a Vycor tip, which was calibrated to be 0.458 V vs normal hydrogen

electrode (NHE) using ferrocene- ferrocenium redox.

Monomers (ProDOS-Me2 and ProDOS-Hex2) were electrochemically polymerized

from a solution containing 10 mM monomer and 0.1MTBAP as supporting

electrolytes in ACN by the scanning of the potential between - 0.6 and +1.3 V.

This resulted in the formation of polymer films on the electrode. For ProDOS-

Me2j the irreversible oxidation of the monomer was observed at 1.3 V, whereas

for ProDOS-Hex2, irreversible monomer oxidation occurred at 1.2 V. Sharp redox

peaks of the polymer were observed in both cases, being characteristic of the

growth of polymers of alkylenedioxythiophene.The growth of ProDOS-Me2 and

ProDOS-Me2 is shown in Figure 6.2. The polymer oxidation potentials for

PolyProDOS-Me2 and PolyProDOS-Hex2 in the second cycle were observed at -

40 and 0.0 mV respectively. Interestingly, both the polymer showed two sharp

oxidation. This kind of behavior has been observed for other

poly(3,4-alkylenedioxythiophene)s and polyalkylthiophenes.

The first CV scan showed the onset of monomer oxidation at a potential of 0.85

V (vs. Ag/Ag+ reference) with a diffusion limited peak at 1.30V corresponding to

the oxidation of ProDOS-Me2 to its radical cation, and thereafter coupling to form

PProDOS-Me2. These values of ProDOS-Me2 are 0.35 V lowers than that of

corresponding ProDOT-Me2. The low oxidation potential of ProDOS-Me2

prevents over-oxidation of the polymer formed during electrochemical

polymerization, thereby eliminating the possibilities of undesirable side reactions.

Upon scanning in the cathodic direction, a broad reduction process is observed

208

corresponding to the reduction of the oxidized form of PProDOS-Me2 deposited

onto the working electrode during the previous anodic scan. During the second

CV scan, a new oxidation process is observed at a lower oxidation potential with

an onset at -0.3 V, peaking at 1.30 V indicating the oxidation of a more

conjugated species formed during the first CV scan.

(A)

100

c 0) v. 3

o

-100 J

-150 J

-200 H

-250

(B)

0.5 0.0

Potential (V)

0.5

209

- i — i — i — i — i — i — i — i — i — i — , — i — i — i — i — i — r

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6

Potential (V)

Figure 6.2 Electrochemical polymerization of 10 mM (A) ProDOS-Me2 and (B)

ProDOS-Hex2 in 0.1 M TBAPF6/ACN at a scan rate of 100 mV/s using platinum

working electrode. Potentials are reported vs Ag/ Ag+ non-aqueous reference

electrode (0.445 V vs NHE).

On going further in the anodic direction, the current response corresponding to

the oxidative polymerization of ProDOS-Me2 increases indicating the deposition

of more electro active materials onto the working electrode. This was further

confirmed by the increase in the current response corresponding to the reduction

of oxidized form of PProDOS-Me2 to its neutral form and vice-versa. This

increase in reduction and oxidation current during successive CV scans indicates

the increase in the amount of electroactive material, viz. conducting polymer

210

PProDOS-Me2 on the electrode during each scan. Further, PProDOS-Me2 was

found to be stable in the potential window of 0.6 V and - 0.8 V as evident from 50

scans shown in Figure 6-3. (explan if peak shifted).To investigate the redox

stability of , the polymer was electrochemically switched 100 times in double-

potential steps between its neutral ( -1.0V) and oxidized state (+0.6V) in

monomer free 0.1 M electrolyte solution in ACN

F

o i

-i J

y -2 J

6 si 6 J

-7 4-

Figure 6.3 Swithing speed of PProDOS-Me2.

6.3.2 Scan Rate Dependency and Redox Switching. Both the polymer

PolyProDOS-Me2 and PolyProDOS-Hex2 were deposited on a Pt working

electrode using cyclicvoltammetry in a three electrodes cell. After

electrodeposition, the Pt button was removed and washed thoroughly with ACN.

The polymer redox behavior was investigated in a monomer free electrolyte

211

Cycle 1

- Cycle 49

solution using the same electrolyte as that used for electrochemical

polymerization while varying scan rates and is shown in Figure 6-4. The scan

rate was varied from 25 mV/s to 100 mV/s with 25 mV/s increments. The linear

increase of the redox current response with increase in scan rate fits the modified

Randles-Sevcik equation, and indicates that the electroactive species,

conductive polymer, is electrode surface-confined. CV of the polymers (in a

monomer free electrolyte bath) showed similar broad oxidation process with an

onset that observed during electrochemical polymerization on Pt button.

c 0)

60-I

40-1

20-1

0A

-20 H

-40 ^

-60 H

0.8 0.6

100mV/s

25 mV/s

100mV/s

0.4 0.2 0.0 — I • 1 • I •--0.2 -0.4 -0.6 -0.8

Potential (V)

212

B

c

3

o

1.0 0.8 0.6 0.4 0.2

Potential (V)

Figure 6.4 CV scans of polymer, (A) PProDOS-Me2 and (B) PProDOS-Hex2

deposited onto Pt button electrode at different scan rates varying from 25 mV/s

to 100 mV/s with an interval of 25 mV/s in 0.1 M TBABFVACN electrolyte solution

(A). Potential reported vs Ag/Ag+ non-aqueous reference electrode (0.44 V vs

NHE).

6.3.3 Spectroelectrochemistry

A ~ 0.1 mm thick PProDOS-Me2 and PProDOS-Hex2 film were deposited onto

the working ITO-coated glass electrode and used to perform insitu

optoelectrochemistry in monomer-free 0.1 M TBAPFe/ACN electrolyte solution. A

three electrode cell set up was used, with ITO-glass as counter and non-aqueous

Ag/Ag+ reference. A constant potential was maintained and simultaneously Vis-

NIR data acquired. Figure 6.5 shows absorbance of the PProDOS-Me2 and

213

PProDOS-Hex2 films on ITO at different potentials. In case of PProDOS-Me2j at -

1.0 V, the spectrum showed n-n* transition of the neutral polymer at 625 nm and

with a band energy gap Eg =1.62 eV. The decrease in absorbance in the visible

region was observed as potential increased from -1.0 V to +0.6 V indicating the

more transparent oxidized from of PProDOS-Me2. At higher potentials, with a

increase in the absorbance in the NIR region occurs due to the polaronic and

bipolaronic transitions indicating the formation of oxidized PProDOS-Me2.

o.o i » i > ( » i » " i » i i I 400 600 800 1000 1200 1400 1600

Wavelength (nm)

214

Wavelength (nm)

Figure 6.5 In-situ spectroelectrochemistry of (A) PProDOS-Me2 and (B)

PProDOS-Hex2 deposited onto ITO-coated-glass in 0.1 M TBAPF6/ ACN.

Potential reported vs non-aqueous Ag-Ag+ reference electrode (0.445 V vs

NHE).

On the other hand PolyProDOS-Hex2j at -1.0 V, two peaks were observed at

630 and 710 nm due to the vibronic coupling. Upon the stepwise oxidation of the

polymer, the absorbance of the n-%* transition decreased, and the peak due to

the polaron increased at a higher wavelength region. At 0.6 V, the absorbance of

K-K* decreased, and the polymer was transparent and light blue.

215

6.4 Color Cordinates (CIE u v' ) for PProDOS-Me2 and PProDOS-Hex2.

Table 6.1 Situation: Black Body @ 6000K.

Polymer

a) Poly Pro DOS-Me2

b) Poly13ProDOS-

Hex2

u'

0.1859

0.1552

v'

0.4048

0.4358

6.5 Conclusion

In conclusion, we have synthesized the first poly(3,4-

propylene)dioxyselenophene (ProDOS) by taking advantage of a novel method

for efficiently contracting the selenophene ring. Both PEDOS-Me2 and ProDOS-

Hex2 showed low band gap, very high stability in the oxidized state, and a well-

defined spectroelectrochemistry. We have demonstrated that chemical

polymerization of ProDOS-Hex2 could be a viable route for the synthesis of

processable polyselenophenes. We have also shown that polyselenophenes are

excellent candidates for future electrochromic applications. Applications of

PolyProDOS in electrochromic devices and studies of other polyselenophenes

are currently underway in our laboratory

216

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