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SYNTHESIS AND FINE-TUNING THE EMISSION

PROPERTIES OF NEW AMPHIPHILIC CONJUGATED

POLYMERS

CHINNAPPAN BASKAR

NATIONAL UNIVERSITY OF SINGAPORE

2004

SYNTHESIS AND FINE-TUNING THE EMISSION

PROPERTIES OF NEW AMPHIPHILIC CONJUGATED

POLYMERS

CHINNAPPAN BASKAR

(M.Sc., IIT MADRAS)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY

NATIONAL UNIVERSITY OF SINGAPORE

2004

i

Dedicated to my beloved parents

ii

Dedicated to my beloved teachers and inspirational minds

“If I have been able to see further, it was only because I stood on the shoulders of giants.”

- Sir Isaac Newton (1642-1727)

iii

Acknowledgements

Life on Earth is a journey, starts as well as ends with Almighty, like cyclic reactions.

During this journey, we are blessed with invaluable teachers and well wishers. It is very

difficult to forget important events, ups and downs, achievements, excellent

collaborators, contributors, great inspirational minds, and the land of harvest. At the end

of my journey to PhD, it is a great pleasure to acknowledge people, who have supported

my growth.

First and above all I would like to thank Dr. Suresh Valiyaveettil for his invaluable

guidance throughout my PhD research work.

I thank Prof Lai Yee Hing and Prof Leslie Harrison for their interest in serving on my

advisory committee. I would like to thank Prof Jagadese J. Vittal, Prof Chuah Gaik

Khuan, Dr. John Yip and Dr. Yang Daiwen for their support as my thesis committee.

My heartfelt thanks to Prof Hardy Chan (Vice Dean, Faculty of Science), Prof Andrew

Wee (Vice Dean), Prof Xu Guo Qin (Vice Dean), Prof Tan Eng Chye (Dean), Prof Lai

Choy Heng and Prof Andy Hor for their support and encouragement during my

contributions in Science Graduate Committee (SGC), Graduate Students Society (GSS),

and Chemistry Graduate Club (CGC). My special thanks to Prof Hian Kee Lee (Head,

Chemistry), Prof Ng Siu Choon (Deputy Head) and Prof Leung Pak Hing (Deputy Head).

iv

My sincere gratitude to Prof Seeram Ramakrishna (Dean, Faculty of Engineering), Prof

Senthil Kumar (Assitant Dean, FoE), Prof Goh Suat Hong (Chemistry), Prof Ji Wei

(Physics), Prof Perera Conrad (Chemistry), Prof B. V. R. Chowdari (Physics), Prof G. V.

Subba Rao (Physics), Prof K. Swaminathan (DBS) and Dr. Ignacio Segarra (S*Bio).

During this period of my doctoral research program, I was certainly blessed to meet many

great minds including Prof Roald Hoffmann (1981 Nobel Laureate in Chemistry), Prof

Carl Djerassi (Stanford University, USA), Prof C. N. R. Rao (President, JNCAR,

Bangalore), Prof Alan Heeger (2000 Nobel Laureate in Chemistry), Prof Hideki

Shirakawa (2000 Nobel Laureate in Chemistry), Prof John C. Warner (University of

Massachusetts Boston, USA), Dr. Paul Anastas (Director, Green Chemistry Institute,

American Chemical Society, USA), Dr. Dennis Hjeresen (Former Director, Green

Chemistry Institute, American Chemical Society, USA) and Dr. Mary Kirchhoff

(Assistant Director, Green Chemistry Institute, American Chemical Society, USA). My

sincerest thanks to all of them for their suggestion, motivation and inspiration.

My thanks are also to Prof K. V. Ramanujachary (Rowan University, USA), Prof R. K.

Sharma (University of Delhi, India), Prof B. Viswanathan (IIT Madras), and Prof G.

Sundararajan (IIT Madras) for their informal discussion and encouragements during their

journey in Singapore.

I would like to thank Prof Bengt Nordén (Member, The Royal Swedish Academy of

Sciences, Chairman, The Nobel Committee for Chemistry in 2000, Nobel Foundation),

v

Ms. Birgitta Sandell (Assistant, The Royal Swedish Academy of Sciences) and Ms. Elin

Stenbom (Assistant, The Royal Swedish Academy of Sciences) for their support to

include the year 2000 Nobel Prize Presentation in Chemistry in my thesis and regular

Nobel Posters.

My sincerest thanks also go to Prof M. S. Subramanian (My graduate mentor, IIT

Madras) and Prof Xavier Machado (My undergraduate teacher, St. Joseph’s College,

Trichy, India) for their invaluable suggestion, motivation and encouragement.

I want to thank many people without whom I would not have been able to complete the

work presented in this thesis. I want to warmly thank all the support staff of the chemistry

department in the main office, NMR, MS, Elemental Analysis, X-ray crystallography

facilities, chemical stores, Honors lab, analytical lab, organic lab, and in the glassblowing

shops. I would like to acknowledge the Department of Chemistry for their hospitality and

encouragement on my graduate study.

I wish to thank all of my past and present colleagues of the Dr. Suresh Group.

I extend my special thanks to my friends especially Felix Lawrence, Lakshmanan,

Skanth, Karen, Nacha, Hendry Elim, Kangueane, Arockiam and Peter, classmates and

housemates.

vi

I would like to specially thank my parents, brothers (Doss and Julian), and my uncle

Sebastian for all the moral and financial support selflessly provided throughout my

career. I would like to thank my sister, Ammu Margaret, who stayed up with me over the

phone when I was stressed out, encouraged me when I was down, prayed for me when I

didn’t think to pray for myself and believed in me when I didn’t believe in myself.

Last but not least, I would like to thank God. “So, whatever you eat or drink, or whatever

you do, do everything for the glory of God.” – I Corinthians 10:31 (Holy Bible)

CHInNaPPaN BaSKAr

May 22, 2004 Saturday

vii

Table of Contents

Dedication

Acknowledgements

Table of Contents

Summary

List of Monomers and Polymers Synthesized in this Thesis

List of Figures

List of Schemes

List of Tables

Glossary of Abbreviations and Symbols

Opening Quotations

i

iii

vii

xii

xvi

xxi

xxiii

xxiv

xxv

xxxii

Chapter 1 Introduction: The Art and Science of Conjugated Polymers 1

1.1

1.2

1.3

1.4

1.5

Prologue

Genesis of Conjugated Polymers

A Case History of Poly(p-phenylene)s PPPs

Pyridine incorporated conjugated polymers

Bipyridine incorporated conjugated polymers

2

6

13

23

28

viii

1.6

1.7

1.8

Poly(m-phenylene)s (PMPs)

Aim of the project

References

33

37

38

Chapter 2 Amphiphilic Poly(p-phenylene)s 75

2.1

2.2

2.3

2.4

2.5

2.6

Introduction

Synthesis of polymers

Characterization of polymers

Optical and ionochromic properties of polymers

Conclusions

References

76

77

79

81

89

90

Chapter 3 Pyridine Incorporated Amphiphilic Conjugated Polymers

94

3.1

3.2

3.3

3.4

3.4.1

Introduction



Optical Properties

Influence of hydroxyl groups

95

98

101

103

103

ix

3.5

3.6

3.4.2

3.4.3

3.4.4

3.4.5

3.4.6

Comparison of properties of polymers

Solvatochromic behavior of polymers

Effect of protonation and deprotonation of polymers

Influence of base

Metal complexation of polymers

Conclusions

References

107

107

110

113

115

117

118

Chapter 4 Bipyridine Incorporated Conjugated Polymers

125

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

Introduction



Optical properties of polymers

Solvatochromic behavior of polymers

Ionochromic effects of polymers

Conclusions

References

126

129

132

133

134

134

137

138

x

Chapter 5 Experimental Section 142

5.1

5.2

5.3

5.4

5.5

5.3.1

5.3.2

5.3.3

5.3.4

5.3.5

5.3.6

5.3.7

5.4.1

5.4.2

5.4.3

5.4.4

5.4.5

5.4.6

Materials

Measurements

Synthesis of polymers 201a-c

2,5-Dibromohydroquinone (203)

2,5-Dibromo-4-dodecyloxy phenol (204a)

2,5-Dibromo-1-benzyloxy-4-dodecyloxy benzene (205a)

1-Benzyloxy-4-dodecyloxyphenyl-2,5-bisboronic acid

(206a)

1-Benzyloxy-4-dodecyloxy phenyl-2,5-bis(trimethylene

boronate) (207a)

Poly(1-benzyloxy-4-dodecyloxy-p-phenylene) (208a)

Poly(1-hydroxy-4-dodecyloxy-p-phenylene) (201a)

Synthesis of polymers 301-306

2,5-Dibromo-1, 4-dibenzyloxy benzene (312)

1,4-Dibenzyloxy-2,5-bisboronic acid (313)

Synthesis of Polymer 304




References

143

143

145

145

145

147

148

150

151

152

153

153

153

154

155

156

156

157

xi

Chapter 6 Conclusions and Suggestions for the future work

158

6.1

6.2

6.2.1

6.2.2

Conclusions

Suggestions for the future work

Applications of new amphiphilic conjugated polymers

Design of new polymer structures: Evolution of

hydroxylated polyphenylenes (HPPs)

159

160

160

160

List of Publications 162

Recent Publications

Unpublished Papers

International Conference Papers

International Conference Presentations

National Publications

National Presentations

163

163

164

166

168

169

Appendix 171

Absorption maxima of non-hydroxyl-containing

conjugated polymers

TG curves of 301-403

172

178

xii

Summary

SYNTHESIS AND FINE-TUNING THE EMISSION PROPERTIES OF NEW

AMPHIPHILIC CONJUGATED POLYMERS

By

Chinnappan Baskar

May 2004

Since the discovery of conducting polymers in the late 1970’s, research efforts

were focused on synthesis and characterization of novel polymers with π-conjugated

backbone due to their interesting optical, electrochemical and conducting properties and

possible applications in electroluminescent devices, nonlinear optical materials, lasing

materials, solar cells, fuel cells, batteries, photoconductors, field effect transistors,

chemical and biosensors, nanoscience and nanotechnology, and biomedical applications.

A variety of conjugated polymers have been investigated and reported in literature.

Among these polymers, poly(p-phenylene) (PPP) and its derivatives have found

considerable interest in blue light-emitting diodes over the last ten years.

The present work reports on syntheses and fine-tuning the emission properties of

a series of new amphiphilic poly(p-phenylene)s PPPs containing free hydroxyl groups

and hydrogen bond acceptor groups such as nitrogen atoms on polymer back bone

capable of forming an inter/intra molecular hydrogen bonding. This allows us to

planarize the neighboring aromatic rings on the polymer backbone and thereby extending

the π-conjugation of the polymer backbone. These hydroxyl and nitrogen sites also act as

potential binding sites for complexation with metal ions.

xiii

Three types of new amphiphilic conjugated polymers were prepared using Suzuki

coupling reaction in good yields. These polymers are: amphiphilic PPPs (201a-c),

pyridine incorporated PPP (2,5-linkage) and poly(m-phenylene) PMP (2,6-linkage) (301-

306), and bipyridine incorporated polyphenylene (both 2,5 and 2,6-linkage) (401-403).

Their structures were confirmed by Nuclear Magnetic Resonance (NMR), infrared (IR),

and elemental analysis. All polymers showed good solubility in common organic solvents

such as chloroform, tetrahydrofuran (THF), dimethyl formamide (DMF), toluene, formic

acid (HCOOH) and trifluoroacetic acid (TFA). Thermogravimetric analysis (TGA)

results showed that they had good thermal stability in both nitrogen and air atmosphere.

The optical properties of these novel polymers were closely related to the

architectures of the backbone and studied using different solvents. Polymers with

pyridine and bipyridine were showed positive solvatochromic effect. The target polymers

exhibited different absorption/emission properties based on the nature and type of solvent

used. The ionochromic effect of polymers was investigated using various metal salts

added to the polymer solutions. The color of the polymers solution was changed from

light yellow to blue, green, or reddish brown depending on the type of metal ions added.

Polymers with pyridine and bipyridine were found to exhibit reversible and tunable

optical properties depending on metal complexation and protonation-deprotonation

process.

In conclusion, a novel series of optically tunable amphiphilic conjugated

polymers have been successfully synthesized and studied in detail. All the derived

polymers showed good solubility in common organic solvents. The emission color could

be tuned by introducing different linked polymer backbones and by using different

xiv

solvents and metal ions. The characterization of these polymers suggested that they were

promising candidates for application in polymeric light emitting diode (PLED), nonlinear

optical properties (NLO), sensors for metal ions, catalytic studies and other properties.

Style of thesis:

Chapter 1 focuses on the introduction and historical perspectives of conjugated

polymers, illustrated with numerous examples (up-to-date). This chapter is divided into

seven major parts: Prologue (with the year 2000 Nobel Prize Presentation in Chemistry),

classification of conjugated polymers, a case history of PPPs with the examples of PPP

and PPP related structures, pyridine incorporated conjugated polymers, bipyridine

incorporated conjugated polymers, PMPs, and aim of the project.

Chapter 2 is focused on a series of optically tunable amphiphilic conjugated

polymers, poly(2-hydroxy-5-alkoxy-p-phenylene) (201a-c) containing long alkyl chains

prepared by Suzuki polycondensation using 2,5-dibromo-1-benzyloxy-4-alkoxybenzene

and bis(boronic ester) monomers. Optical properties of all polymers were investigated in

THF at room temperature under neutral condition and emission maxima were observed in

the violet region (λemi = 401- 403 nm). By the addition of stoichiometric amount of a base

(e.g. aqueous NaOH solution), absorption maxima shifted to the blue region (λemi = 474 –

468 nm). Ionochromic effect of target polymers with transition metal ions such as Fe3+,

Cu2+, and Co2+ was also reported. In the presence of metal ions, the optical properties of

polymers showed interesting tunability of emission maxima, ∆λmax (140 nm to 26 nm).

Chapter 3 is focused on three fluorescent amphiphilic π-conjugated polymers with

donor and acceptor groups prepared by Suzuki polycondensation method. The resulting

xv

polymers containing long alkyl chains showed good solubility in common organic

solvents such as chloroform, toluene, THF, DMF and formic acid. The absorption and

emission wavelength of the synthesized copolymers gave positive solvatochromism in

solvents of varying polarity. The polymers 301-303 dissolved in chloroform showed a

large stokes shift, presumably due to excited-state intramolecular proton transfer (ESIPT)

mechanism. The precursor polymers 304-306 exhibiting large stokes shift due to

intramolecular charge transfer (ICT). We also explored the ion responsive properties of

the target polymers with different metal ions such as Cu2+, Co2+, Ni2+, and Fe3+. Polymers

complexed with metal ions indicated large metal-to-ligand charge transfer (MLCT).

Chapter 4, three types of conjugated copolymers containing bipyridine and 1,4-

phenylene units in an alternative sequence (401-403) were prepared by Suzuki

polycondensation. The resulting polymers showed good solubility in common organic

solvents such as chloroform, toluene, THF and DMF. Optical properties of synthesized

copolymers were investigated using chloroform, THF and HCOOH. All the polymers

showed interesting optical properties and possessed sensitivity to various metal ions such

as Cu2+, Mn2+, and Fe3+. It was found that the absorption and emission maxima of the

polymers could easily be fine-tuned by varying solvents and metal ions.

Chapter 5 focuses on the experimental section of all polymers and compounds

synthesized in this work.

Chapter 6 focuses on conclusion and suggestions for the future work.

xvi

List of Monomers and Polymers Synthesized in this Thesis

Table 1. The polymers and main compounds prepared in this thesis

Chapter

No.

Main monomers (compounds)

Polymers

Chapter 2

R = CH3(CH2)11R = CH3(CH2)15R = CH3(CH2)17

Bn = C6H5CH2

205a-c

BrBr

OBn

RO


Bn = C6H5CH2

207a-c

OBn

RO

B

O

O

B

O

O

OBn

RO

n


Bn = C6H5CH2

208a-c

OH

RO

n


201a-c

xvii

Table 1. The polymers and main compounds prepared in this thesis (Continued)

Chapter

No.


Polymers

Chapter

3

R = CH3(CH2)11Bn = C6H5CH2

311

B(OH)2(HO)2B

OBn

RO

313

B(OH)2(HO)2B

OBn

BnO

R=CH3(CH2)11Bn=C6H5CH2

OBn

BnO

N

305

n

OBn

RO

N

304

n

O

HO

N

H

302

n

O

RO

N

H

301

n

xviii


Chapter

No.


Polymers

Chapter 3


303

N

O

RO

OH

OR

H

n

306

N

OBn

RO

OBn

OR

n

xix


Chapter

No.


Polymers

Chapter 4


406

B(OH)2(HO)2B

OBn

RO

409

B(OH)2(HO)2B

OR

RO

N N

OBn OBn

RO OR

n401

N N

O ORO OR

H H

n402


xx


Chapter

No.


Polymers

Chapter 4

N N

Br Br405

N NRO

OR RO

OR

n403

R = CH3(CH2)11

xxi

List of Figures

Figure 1-1

Figure 1-2

Figure 1-3

Figure 1-4

Figure 1-5

Figure 1-6

Figure 2-1

Figure 2-2

Figure 2-3

Figure 2-4

The art and science of conjugated polymers

Examples of conjugated polymers, note the bond-

alternated structures

Examples of poly(p-phenylene)s (PPP)s

Examples of pyridine incorporated conjugated polymers

Examples of bipyridine incorporated conjugated polymers

Examples of poly(m-phenylene)s (PMPs)

Structures of amphiphilic poly(p-phenylenes) 201a-c

Absorbance and emission spectra of Polymer 201a

X-ray powder diffraction pattern of polymer 201a

Illustration of the polymer lattice indicating alkyl chain

packing and interchain hydrogen bonding or metal

complexation

1

8

14

24

28

34

77

82

84

86

xxii

List of Figures

Figure 3-1

Figure 3-2

Figure 3-3

Figure 3-4

Figure 3-5

Figure 3-6

Figure 3-7

Figure 4-1

Figure 4-2

Figure 6-1

Figure A 1-9

Molecular structures of target polymers 301-306

Absorbance and emission spectra of polymers 304 and 301

in chloroform

Excited-state intramolecular proton transfer (ESIPT) for

polymer 301

UV/Vis spectra of Protonation and Deprotonation of

polymers 301-303 with aqueous HCl and aqueous NaOH

in THF

Proton Transfer from the excited cation of polymer 301 to

a base B

UV/Vis spectra of polymers 301 and 303 without and with

aqueous NaOH in DMF

Emission spectra of polymers 301 and 303 without and

with aqueous NaOH in DMF

Molecular structure of the polymers 401-403

Absorbance and emission spectra of polymers 401 and 402

in THF

Evolution of hydroxylated polyphenylenes (HPP)s

TG curves of 301-403

97

104

106

111

112

113

114

128

133

161

178

xxiii

List of Schemes

Scheme 2-1

Scheme 3-1

Scheme 3-2

Scheme 4-1

Scheme 4-2

Synthesis of polymers 201a-c

Synthesis of polymers 301 and 302

Synthesis of polymer 303

Synthesis of polymers 401 and 402

Synthesis of polymer 403

78

99

100

130

131

xxiv

List of Tables

Table 2-1

Table 2-2

Table 3-1

Table 3-2

Table 3-3

Table 4-1

Table 4-2

Table 4-3

Table A-1

Molecular weights of target polymers 201a-c observed

from GPC analysis

Absorption and emission responses of polymers 201a-c

with and without metal ions

Molecular weights of polymers 301-306 observed from

GPC analyses

Solvatochromic behavior of polymers 301-306

Absorption and emission responses of polymers 301-303

with metal ions

Molecular weights of polymers 401-403 observed from

GPC analyses

Solvatochromic behavior of polymers 401-403

Absorption responses of polymers 401-403 with metal ions

Absorption maxima of non-hydroxyl-containing

conjugated polymers

80

88

102

108

116

132

135

136

172

xxv

Glossary of Abbreviations and Symbols

(Arranged in alphabetical order of abbreviations and symbols)

Abbreviation Description

Å angstrom

abs. absolute

AcOH acetic acid

anhyd. anhydrous

aq. aqueous

BBL poly(benzimidazobenzophenanthroline)

bpy bipyridine

br broad

°C degree Celsius (centigrade)

calcd. calculated

CB conduction band

conc. concentrate

CP conjugated polymer

CT charge transfer

δ chemical shift (ppm)

d doublet

distd. distilled

DMF dimethyl formamide

EL electroluminescence

xxvi

ESIPT excited-state intramolecular proton transfer

Et ethyl

EtOH ethanol

eV electron volt

FTIR fourier transform infrared

GPC gel permeation chromatography

g gram

gl. glacial

h hour

HCOOH formic acid

HH head-to-head

HOMO highest occupied molecular orbitol

HPP hydroxylated polyphenylene

HPPP hydroxylated poly(p-phenylene)

HT head-to-tail

Hz Hertz

ICT intramolecular charge-transfer

IR Infrared

λmax absorption wavelength at band maximum (nm)

λemi emission wavelength at band maximum (nm)

L liter

LED light emitting diode

LPPP ladder-type poly(p-phenylene)

xxvii

LUMO lowest unoccupied molecular orbital

m multiplet

m- meta-

M molar

Mn number average molecular weight

Mw weight average molecular weight

Me methyl

MeOH methanol

mg milligram

MHz Megahertz

MEH-PPV poly(2,5-dialkoxy)paraphenylene

mL milliliter

MLCT metal-to-ligand charge transfer

mmol millimole

mol mole

MS mass spectrum

ν frequency (cm-1)

N normality

n- normal

near-IR near-infrared

NLO nonlinear optical properties

nm nanometer

NMR nuclear magnetic resonance

xxviii

o- ortho-

OBn benzyloxy

OLED organic light emitting diode

OR alkoxy

p- para-

P3AT poly(3-alkyl thiophene)

P3HT poly(3-hexyl thiophene)

PA polyacetylene

PABTz poly(alkylbithiazole)

PANI polyanaline

Pant poly(anthrylene)

PAz polyazulene

PAzb poly(azobenzene-4,4’-diyl)

PBnap polybinaphthalene

PBI poly(benzimidazole)

PBIm poly(N,N’-dialkyl-2,2’-biimidazole-5,5’-diyl)

PBO poly(p-phenylene benzobisoxazole)

PBT poly(p-phenylene benzobisthiazole)

PBtd poly(benzo-[d] [2.1.3] thiadiazole-4,7-diyl)

PBpy poly(2,2’-bipyridine-5,5’-diyl)

PBPym poly(2,2’-bipyrimidine-5,5’-diyl)

PCyh poly(1,3-cyclohexadiene-1,4-diyl)

PCz polycarbazole

xxix

PDA polydiacetylene

PDF poly(dithiafulvene)

PDI polydispersity index

PDPA poly(diphenylamine-4,4’-diyl)

PDT polydithiathianthrene

PECz poly(N-ethylcarbazole)

PEDOT polyethylenedioxythiophene

PEPPB poly(2-ethynyl-N-propargylpyridinium bromide)

PF polyfluorene

PFu polyfuran

Ph phenyl

PHT polyheptadiene

PI polyindole

PIm poly(imidazole-2,5-diyl)

PITN polyisothianaphthene

PLED polymeric light emitting diode

PMP poly(m-phenylene)

PMPS poly(m-phenylene sulphide)

PNap polynaphthalene

PNBO poly(nonylbisoxazole)

POD poly(1,3,4-oxadiazole)

PPE poly(p-phenyleneethylene)

PPhen poly(1,10-phenanthroline-3,8-diyl)

xxx

ppm part per million

PPP poly(p-phenylene)

PPyrr polypyrrole

PPS poly(p-phenylene sulphide)

PPSA poly(p-phenylene sulfide-phenyleneamine)

PPSAA poly(p-phenylene sulfide-phenyleneamine-phenyleneamine)

PPV poly(p-phenylenevinylene)

PPy poly(pyridine-2,5-diyl)

PPyrim poly(pyrimidine-2,5-diyl)

PPyrz poly(pyrazine-2,5-diyl)

PQ polyquinoline

PQx polyquinoxaline

PRPyrr poly(N-alkyl)pyrrole

PSPE poly(salphenyleneethylene)

PT polythiophene

PTHP poly(4,9-dialkyl-4,5,9,10-tetrahydropyrene-2,7-diyl)

PTP poly(triphenylene)

py pyridine

q quartet

r.t. room temperature

R alkyl

s singlet

S Siemens (conductance)

xxxi

soln. solution

t triplet

t- tertiary-

TFA trifluoroacetic acid

TGA thermogravimetric analysis

THF tetrahydrofuran

TMS tetramethylsilane

TLC thin layer chromatography

UV-vis Ultraviolet-visible

XRD X-ray diffraction

xxxii

Opening Quotations

"Ask, and it will be given you; search, and you will find; knock, and the door will be opened for you."

-Matthew 7:7 (Holy Bible)

“Fortunately science, like that nature to which it belongs, is neither limited by time nor by space. It

belongs to the world, and is of no country and of no age. The more we know, the more we feel our

ignorance; the more we feel how much remains unknown; and in philosophy, the sentiment of the

Macedonian hero can never apply,- there are always new worlds to conquer.”

– Sir Humphry Davy (1778-1829)

"I am young and avid for glory." – Antoine Lavoisier (1743-1794)

Chapter 1: The Art and Science of Conjugated Polymers

1

Chapter 1

Introduction: The Art and Science of

Conjugated Polymers

Figure 1-1. The art and science of conjugated polymers.

Inside the square: Classical structure of conjugated polymer (CP) backbone and types of

CP; Outside the square: Applications of CP


2

1.1 Prologue1

“Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

Chemistry! We all associate chemistry with test tubes, stinking laboratories and

explosions - Alfred Nobel's dynamite was born in such an environment. Perhaps the

development of new knowledge in chemistry, more than any other science, has been

characterized as a sparkling interplay between theory on one hand, the safe and

predictable, and, on the other hand, the explosive and surprising reality. When we by

chance discover something that may become valuable, we talk about "serendipity" - after

the tale about the three princes of Serendip, who traveled widely and had the gift of

drawing far-reaching conclusions from whatever they encountered. This year's Nobel

Prize in Chemistry is being awarded to three scientists, whose unexpected discovery gave

birth to a research area of great importance.

But let us go back to the beginning. In Japan, in 1967, a group of scientists were

studying the polymerization of acetylene into plastics - acetylene was the gas that the

Swedish engineer Gustaf Dalén once tamed to bring light in the dark for sailors in the

form of blinking buoys (1912 Nobel Prize in Physics). Polymerization is the process by

which many small molecules react to form a long chain - a polymer. Professors Ziegler

and Natta were awarded the 1963 Nobel Prize in Chemistry for a technique for

polymerizing ethylene or propylene into plastics; the Japanese scientists used the same

catalyst for polymerizing acetylene. One day a visiting researcher in the laboratory, the

story goes, added more catalyst than written in the recipe: actually one thousand times

too much! Imagine the surprise among your invited dinner guests if, rather than using a

few drops of Tabasco in the soup, you had added the whole bottle! The result was a


3

surprise also to the scientists. Instead of the expected black polyacetylene powder that

normally was obtained, and that was of no use, a beautifully lustrous silver colored film

resulted.

It was, however, only its appearance that was metallic. The material did not

conduct electricity. The breakthrough was not made until ten years later in collaboration

between physicist Alan Heeger and chemists Alan MacDiarmid and Hideki Shirakawa,

continuing the experiments with the silver colored film. They tried to oxidize the film

using iodine vapor, and - Bingo! The conductivity of the plastic increased by as much as

ten million-fold; it had become conductive like a metal, comparable to copper. This was a

surprising discovery, to the researchers as well as to others - we are all used to plastics, in

contrast to metals, being insulators, which is why we cover electrical cords in plastic.

The discoverers started pondering what had happened. In order to conduct

electricity the plastic would somehow have had to mimic metals, making their electrons

easily mobile. Polyacetylene can be seen as beads on a string made up of carbon atoms

linked by chemical bonds, alternatingly single and double bonds. It is the electrons of the

double bonds that give rise to the electrical conductivity. But this only happens after

oxidizing the polymer chain a little here and there, for example using iodine. And why is

that? The iodine removes one electron from a carbon atom, thus creating a hole in the

electronic structure into which an electron from a neighboring atom can jump, whereupon

a new hole is formed and so on. A hole, i.e. lack of electron, corresponds to a positive

charge, and the movement of the hole along the chain gives rise to a current.

The exciting idea of being able to combine the flexibility and low weight of

plastics with the electric properties of metals has stimulated scientists all over the world,


4

resulting in a novel research field bordering physics and chemistry. Various theoretical

models and new conductive, but also semi-conductive, polymers followed during the

1980s in the wake of the first discoveries. Today we can see several possible applications.

How about electrically luminous plastic that may be used for manufacturing mobile

phone displays or the flat television screens of the future? Or the opposite - instead using

light to generate electric current: solar-cell plastics that can be unfolded over large areas

to produce environmentally friendly electricity. Finally, lightweight rechargeable

batteries may be necessary if we are to replace the combustion engines in today's cars

with environmentally friendly electric motors - another application where electrical

polymers might find use.

In parallel with the development of conducting polymers, there is an ongoing

development of what we might call "molecular electronics," where the very molecules

perform the same tasks as the integrated circuits we just heard about in the Nobel Prize in

Physics, with the difference that these could be made incomparably smaller. In

laboratories around the world, scientists are working hard to develop molecules for future

electronics. And among test tubes and flasks, and in the interplay between theory and

experiment, we may some day again be astonished by something unexpected and

fantastic. But this is a different story, and perhaps a different Nobel Prize...

Professors Heeger, MacDiarmid and Shirakawa. You are being rewarded for your

pioneering scientific work on electrically conductive polymers.”2,3

With these elegant words Professor Bengt Nordén, Member, The Royal Swedish

Academy of Sciences, Chairman, The Nobel Committee for Chemistry, proceeded to

introduce Alan Heeger, Alan MacDiarmid and Hideki Shirakawa at the Nobel


5

Ceremonies in 2000, the year in which Heeger, MacDiarmid and Shirakawa received the

coveted prize for the discovery and development of conducting polymers.

This description and praise for conducting polymers resonates today with equal

validity and appeal; most likely, it will be valid for some time to come. Indeed, unlike

many one-time discoveries or inventions, the endeavor of new conjugated polymers is in

a constant state throughout the second part of the twentieth century and continues to

provide fertile ground for new discoveries and inventions. The practice of conjugated

polymers demands the following virtues from, and cultivates the best in, those who

practice it: ingenuity, artistic taste, experimental skill, persistence, and character. In turn,

the practitioner is often rewarded with discoveries and inventions that impact, in major

ways, not only other areas of chemistry, but most significantly material science, biology,

and medicine. The harvest of chemical syntheses for polymers enables scientists, today to

design materials, which touch upon our everyday lives in myriad ways: the controlled

delivery of drugs, high-tech materials for electronics and tools for biological processes.

A number of excellent reviews have been published on conducting (conjugated)

polymers, covering synthesis, processing and applications. 4-32 The goal of this chapter is

to provide a survey of up-to-date examples of reported conjugated polymers especially

poly(p-phenylene)s (PPPs) and poly(m-phenylene)s (PMPs) (with reference to our on

going project). For the detailed discussions of all these polymers is referred to specialized

reviews or papers.

For the purpose of this chapter, the art and science of conjugated polymers,

illustrated with numerous examples, is divided into six main headings: the genesis of

conjugated polymers, a case history of poly(p-phenylene)s, pyridine incorporated


6

conjugated polymers, bipyridine incorporated conjugated polymers, poly(m-phenylene)s

and aim of the project.

1.2 Genesis of Conjugated Polymers

The genesis of conjugated polymers can be traced back to the mid 1970s when the

first polymer namely polyacetylene capable of conducting electricity was reportedly

prepared by accident by Shirakawa.33,34 The subsequent discovery by Heeger and

MacDiarmid35 that the polymer would undergo an increase in conductivity of 12 orders

of magnitude by oxidative doping quickly reverberated around the polymer and created a

new field of research in the scientific community and brighten up a number of

opportunities in both academia and industry due to many potential applications such as

light emitting diodes,36-52 field effect transistors,53-61 inkjet-printing,62-65 solar cells,66-72

fuel cells,73-75 rechargeable batteries,76 lasers,52,77-80 molecular electronics,81-88

spintronics,89 nonlinear optical properties,90-98 optical power limiting,99 chemical and

biosensors,100-110 actuators,111-115 radical scavengers,116 membrane based separations,117

biomedical applications,118-120 nanoscience and nanotechnology,118,121-125 and catalysts.126-

129 Different types of conjugated polymers such as polyacetylene (PA),130-139 poly(p-

phenylene) (PPP),140-148 poly(p-phenylenevinylene) (PPV),140,149-159 poly(p-

phenyleneethylene) (PPE),81,151,160-169 poly(salphenyleneethylene) (PSPE),126

polythiophene (PT),170-177 poly(3,4-ethylenedioxythiophene) (PEDOT),178-180 polypyrrole

(PPyrr),181-185 polyaniline (PANI),186-193 polyfluorene (PF),194-200 ladder-type PPP

(LPPP),201-207 poly(pyridine-2,5-diyl) (PPy),208-214 poly(2,2’-bipyridine-5,5’-diyl)

(PBpy),208-214 poly(pyrimidine-2,5-diyl) (PPyrim),215 poly(2,2’-bipyrimidine-5,5’-diyl)


7

(PBPym),216-219 poly(pyrazine-2,5-diyl) (PPyrz),220 poly(1,10-phenanthroline-3,8-diyl)

(PPhen),221-223 polyquinoline (PQ),224-229 polyquinoxaline (PQx),230-234 polyindole

(PI),235,236 polycarbazole (PCz),237-244 poly(fluoren-9-one-2,7-diyl),245,246 poly(p-

phenylene benzobisoxazole) (PBO),247-252 poly(p-phenylene benzobisthiazole)

(PBT),247,248, poly(p-phenylene sulfide) (PPS),253-255 poly(m-phenylene sulfide)

(PMPS),254 poly(p-phenylene sulfide-phenyleneamine) (PPSA),256,257 poly(p-phenylene

sulfide-phenyleneamine-phenyleneamine) (PPSAA),258 polydithiathianthrene (PDT),259

polyheptadiene (PHT),260,261 poly(2-ethynyl-N-propargylpyridinium bromide)

[PEPPB],262 poly(1,3-cyclohexadiene-1,4-diyl) (PCyh),263 polynaphthalene (PNap),264-266

polybinaphthalene (PBnap),267-270 poly(anthrylene) (Pant),140 poly(phenanthrene),140

polypyrene,140 poly(4,9-dialkyl-4,5,9,10-tetrahydropyrene-2,7-diyl) (PTHP),271

poly(benzimidazobenzophenanthroline) (BBL),272,273 polyazulene (PAz),274-276

poly(diphenylamine-4,4’-diyl) (PDPA),277,278 poly(azobenzene-4,4’-diyl) (PAzb),277,279-

281 polyazomethine,282 poly(dithiafulvene) (PDF),283-287 poly(phthalocyanine),288,289

conjugated metallophorphyrin,290-293 polyanthraquinone,294 polyquinone,295 polyfuran

(PFu),296-299 polytellurophene,300 polyphosphole,296,301 poly(naphthodithiophene),302

poly(1,3,4-oxadiazole) (POD),303-308 poly(benzimidazole) (PBI),309-311 poly(benzo-[d]

[2.1.3] thiadiazole-4,7-diyl) (PBtd)312-315 poly(alkylbithiazole) (PABTz),316-319

poly(nonylbisoxazole) (PNBO),320 poly(imidazole-2,5-diyl) (PIm),321 poly(N,N’-dialkyl-

2,2’-biimidazole-5,5’-diyl) (PBIm),321 poly(isothianaphthene) (PITN),322-324

polydiacetylene (PDA)325-328 have been developed and intensively investigated. The

examples of few conjugated polymers are listed in Figure 1-2.


8

nPolyacetylene (PA)

OR

R'On

Poly(2,5-dialkoxy)paraphenylene (e.g. MEH-PPV)

S nPolythiophene (PT)

S

R

nPoly (3-alkyl) thiophene (P3AT)

S

OR

nPoly(3-alkoxy)thiophene (P3AT)

S nPolyisothianaphthene (PITN)

S

O O

n

Polyethylenedioxythiophene (PEDOT)

N

H

N N N

H

n

Polyaniline (PANI)

n

Poly(p-phenylene) (PPP)

n

Poly(p-phenylene) (PPV)

Sn

Poly(p-phenylene sulphide) (PPS)

N

H

n

Polypyrrole (PPyrr)

N

R

n

Poly(N-alkyl)pyrrole (PRPyrr)

Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures.


9

N n

Poly(pyridine-2,5-diyl) (PPy)

Poly(alkyl pyridine-2,5-diyl) (PRPy)

R

N n N n

Poly(isoquinoline-1,4-diyl) P(1,4-iQ)

N N n

Poly(2,2'-bipyridine-5,5'-diyl) (PBpy)

Poly(dialkyl-2,2'-bipyridine-5,5'-diyl) (PRBpy)

N N n

R R

N

nPoly(quinoline-5,8-diyl) P(5,8-Q)

N N

n

Poly(quinoxaline-5,8-diyl) P(5,8-Qx)

N N

ArAr

n

Poly(2,3-diarylquinoxaline -5,8-diyl) P(5,8-diArQx)

N n

Poly(quinoline-1,4-diyl) P(2,6-Q)

N

N nPoly(quinoxaline-2,6-diyl) P(2,6-Qx)

N

N n

Poly(1,5-naphthyridine -2,6-diyl) P(2,6-Nap)

N N n

Poly(1,10-phenanthroline -3,8-diyl) (PPhen)

Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures

(Continued).


10

N

N

n

Poly(pyrimidine-2,5-diyl) (PPyrim)

N

N

N

N

n

Poly(2,2'-bipyrimidine-5,5'-diyl) (PBPym)

N NH

n

Poly(benzimidazole -4,7-diyl) P(4,7-Bim)

N NH

R

n

Poly(benzimidazole-4,7-diyl) and its derivatives P[4,7-Bim(R)]

n

Poly(1,3-cyclohexadiene -1,4-diyl) (PCyh)

NS

N

n

Poly(benzo-[d][2,1.3] thiadiazole-4,7-diyl) P(4,7-Btd)

OO

n

Poly(2-methyl-anthraquinone -1,4-diyl) P(1,4-AQ)

O

O NO2

NO2

Poly(4,8-dinitro anthraquinone-1,5-diyl) P(4,8-NO2-1,5-AQ)

n

Polyheptadiyne (PHT)

nPolymetaphenylene (PMP)

S

n

Poly(thiophene-2,4-diyl) P(2,4-Th)

N

Nn

Poly(pyrazine-2,5-diyl) (PPyrz)

n

Poly(9,10-dihydro-pheanthrene-2,7-diyl) (PH2Ph)


(Continued).


11

n

Polynaphthalene (PNap)

S

S

CHn

Poly(dithiafulvene) (PDF)

N

S n

N

S

R

n

N

S

R

Poly(nonylbithiazole) PNBTz (R = Nonyl)

N

O

R

n

N

O

R

Poly(nonylbisoxazole) PNBO (R = Nonyl)

N

O

N

N

O

N n

Polybenzimidazolebenzophenanthroline (BBL)

O

NN

O n

Poly(p-phenylene benzobisoxazole) (PBO)

S

NN

S n

Poly(p-phenylene benzobisthiazole (PBT)

R

R

n

PTHP

n

Poly(anthrylene) (Pant)

Se nPolyselenophene

O n

Polyfuran (PFu)

Te n

Polytellurophene

N

N

n

Poly(azobenzene-4,4'-diyl) (PAzb)

N

N

R

n

N

N

R

PBIm (R)


(Continued).


12

R

R

R

R

R

R

n

Poly(2,5-dialkyl p-phenyleneethylene) (dialkyl-PPE)

C C C C C C

Polydiacetylene (PDA)

[]n

Polyazulene (PAz)

O

N N

O

RO

OR

M

n

Poly(salphenyleneethylene)s (PSPE)s

N

R

n

Polyindole (PI)

+NBr-

nPEPPB

nN

R

Polycarbazole (PCz)

N N

O

OR

n

Polyoxadiazole (POD)


(Continued).


13

1.3 A case history of Poly(p-phenylene)s PPPs

Poly(p-phenylene) is one of the simplest polymers being exclusively composed of

benzene rings. The first PPP namely tridecaphenyl was prepared by Gold – Schmidt in

1886 from 1,4 – dibromo benzene with sodium at around 300 °C for 130 h using Wurtz –

Fitting reaction. In 1936, Busch and co-workers increased the number of phenyl units up

to 16 using the same kind of monomer. Till the end of 1989, PPP has been synthesized by

various methods and studied as thermally resistant polymer but not extended their

applications into Light emitting diodes.15 In the late 1990s, the absorption wavelength of

PPP has been found that 336 nm and emitted in the blue region.140 This result gave the

high excitement and opened up new era for the PPP as blue light emitting diodes. Due to

poor solubility, the synthesized PPP was not able to process for further applications. To

increase the solubility, verities of alkyl and alkoxy groups have been introduced in the

polymer back bone. The drawback of having a soluble side group on the PPP is that the

additional substituents twist the substituted phenylene rings considerably out of the plane.

This drastically decreases the interaction of the aromatic π-electron system, and

unwanted additional blue-shift in the emission spectrum compared with that for PPP is

usually accompanied by a drop in fluorescence quantum yield.330,346,347 Thus, for the PPP

contains alkyl side group, the wavelength reduced to 300 nm. For alkoxy group, the

absorption wavelength was increased to 335 nm. In the early 1998s, the Yamamoto group

synthesized the poly(dihydroxyphenylene).350 The π-π* absorption peak was shifted to

longer wavelength due to the extension of π-conjugation. But it’s soluble only in DMF.

They have also observed the hypsochromic shift by the addition of base. A series of PPP

derivatives have been synthesized and reported in literature. The examples of a few PPP


14

are shown in Figure 1-3.329-401 Examples given in Figure 1-3 are arranged in the form of:

PPP, PPP with side groups (mono alkyl, aryl, alkoxy, dialkyl, dialkoxy, ethyloxy, water

soluble groups, esters, copolymer, ladder-type PPP, and dendrimers).

n

1

R

n

2

R= C12H25 (1995)

OR

n5

R= C10H21 (1996)R= C12H25 (1996)R= 2-ethyl-hexyl (1996, 2003)

OCN

n

6(1996)

O (CH2)6 O CN

n

7

(2003)

n

4

(1999)

(1990)

R

Rny

10

R= C6H13, C12H25

R

R

n

9

R= H, CH3, C4H9, C6H13R= (CH2)2C(CH3)3R= C7H15, C8H17R= C12H25, C16H33

(1990) (1993)

RO

OR

n

11

R= C6H13, C6H4CN

H13C6

C6H13

O

O

n

12 (1996, 1997)

H13C6

C6H13

O

O

n

14 (1997)

H13C6

C6H13

O

n

13 (1997, 2000)

3 (2000)

C6H13

n

C6H13

H13C6

n

8 (1989)

Figure 1-3. Examples of poly(p-phenylene)s (PPP)s.329-343,419,483


15

X

XH13C6

C6H13

n

15 (1997)

X= N(CH3)2, NO2

X

XH25C12

C12H25

n

18 (1997)

X= N(CH3)2, F

X= NO2, X'= H

X = X'= NO2

R= C6H13

X'

R

R X

n

16 (1997)

R= C12H25X= CN, CF3

X

R

R

n

17 (1997)

19 (1999)

(CH2)6

(CH2)6

O

O

n

20 (1999)

x= 1,6

R= H, CH3

(CH2)x

(CH2)xH13C6

C6H13

O

O

COOR

COOR

n

OR

ROH9C4O

OC4H9

n

22 (1994)

R= C8H17, C12H2524 (1998)

O

O

n

*

*

OCH3

H3COH15C7

C7H15

n

23 (1994)

R= C4H9, C8H17, C12H25R= 3-methyl butyl (isopentyl)

21 (1994, 1996, 1999)

OR

RO

n

25 (1999)

OH

HO

n

26 (1999)

O

O

n

Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).344-350


16

O

O

O(CH2CH2O)xCH3

CH3(OCH2CH2)xO

n

27 (1997, 1998)

x = 1-5 x = 1-5

O

O

O(CH2CH2O)xCH3

CH3(OCH2CH2)xO

O

O

CH3(OCH2CH2)yO

O(CH2CH2O)yCH3

n m

y = 1-5

28 (1997, 1998) x = y/

OR

RO

n

29 (1998)

R= CH

CH2O(CH2CH2O)3CH3

CH2O(CH2CH2O)3CH3

30 (1998)

R= CH

CH2O(CH2CH2O)3CH3

CH2O(CH2CH2O)3CH3

OR

RO

O

O

CH3(OCH2CH2)5O

O(CH2CH2O)5CH3

n m

O

O

OC12H25

CH3(OCH2CH2)5O

n

32 (2001)

O

O(CH2CH2O)4CH3

OC12H25

n

31 (2001)



17

33 (2001, 2003)

n

OC16H13

O

O(CH2CH2O)4CH3

34 (2001, 2003)

n

OC16H13

O

O(CH2CH2O)xR

R= SiPh2tBuR= Hx = 2, 4

35 (2001, 2003)

n

OC16H13

O

O(CH2CH2O)xR

R= THPR= Hx = 2, 4

36 (2001, 2003)

n

O

O

(CH2)6

(CH2)6

O(CH2CH2O)4R

O(CH2CH2O)4R

OR

RO

CF3

F3C

n

37 (1998)

R = C16H33

O

3R =

O

O

(CH2)6

(CH2)6O

O CN

NC

n

38 (2003)

39 (1994)

n

OR

RO

R = C6H13, C8H17, C11H23, C16H33

Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).353-356,334,346


18

COOH

HOOC

n

40 (1991)

41 (1994)(H2C)6

(CH2)6 O COOH

OHOOC

n

R = H, CH3, C6H13R' = H, C6H13, C12H25M = N(CH3)4, Na

R'

R

SO3-M+

n

43 (1999, 2000, 2001)

R'

R

SO3-M+

+M-O3S

n

44 (1999, 2000, 2001)

46 (1999)

O

O

R2N

NR2

n

R = Me, Et

47 (1999, 2000)

O

O

R3N

NR3

n

+ Br-

Br-+

R = Me, Et

t-Bu

t-Bu

R =

CH3

H25C12

SO3R

RO3S

n

42 (2001)

O

O

SO3Na

NaO3S

nx

45 (1994, 1998)

x = 1, 2



19

49 (1996)

(H2C)6

(CH2)6

N+(C2H5)3

N+(C2H5)3

n

I-

I- 48 (1996)

(H2C)6

(CH2)6

I

I

n

50 (1996)

(H2C)6

(CH2)6

n

I-

I-

N

N

+

+

51 (2000)(H2C)6

(CH2)6

N+(C2H5)3

N+(C2H5)3

n

-O3S (CF2)7 CF3

F3C (CF2)7 SO3-

53 (1998)

(H2C)6

(CH2)6

N

N

CH3

CH2H3C

CH3

H3C CH2

CH2 N C2H5

CH3

CH3

CH2 N C2H5

CH3

CH3

n

+

+ +

+

I- I-

I-

I-(H2C)6

(CH2)6

N

N

CH3

CH2H3C

CH3

H3C CH2

CH2 N

CH3

CH3

CH2 N

CH3

CH3

n

+

+I-

I-

52 (1998)

54 (1996)

(H2C)6

(CH2)6

I

I

C6H13

H13C6

n

55 (1996)

I-

I-

(H2C)6

(CH2)6

N+(C2H5)3

N+(C2H5)3

C6H13

H13C6

n

56 (1996)

I-

I-

N

N

+

+

(H2C)6

(CH2)6 C6H13

H13C6

n



20

O

X

n

58 (1995)

X = H, F, Cl, C(CH3)3

O

OR

n

57 (1995)

R = CH3R = CH(CH3)2R = 2-ethyl hexyl

62 (1996)

R'

R'

R'

n m

61 (1996)

m

R'

R'

R'

n

60 (1996)

R' = COC6H5R' = CO(p-t-BuC6H4)R' = CO(o-FC6H4)R' = CO(m-FC6H4)R' = CO(p-FC6H4)R' = CH3

O

N

n

64 (2000)

59 (2003)

O

O R

n R = H, BrR = PO(OEt)2, PO(OH)2

C

O

O

(CH2)n

O

CN

m

66 (1999)

n = 2-12

O

O

n

67 (1999)

MeOOC COOMe

n

R

63 (2003)

R = CH2CF2CF3R = (CH2)2CH3R = (CH2)2(CF2)5CF3R = (CH2)7CH3

COO(CH2)2O

COO(CH2)6O

COO COOCH

CH3

*(S)

CN

x 1-x

68 (2000)

O

O

SO3H

n

65 (1998)



21

n

69 (2003)71 (2003)

n

H9C4O

OCH3

m

O

O

O

O

R

R

H13C6 C6H13

n

R = O(CH2)5CH3R = O(CH2)7CH3R = O(CH2)9CH3R = CNR = NO2

73 (2003)

R R'

n

R = R' = HR = H, R' = OCH3R = R' = OCH3R = R' = S-PhR = R' = Ph72 (1991)

OC6H13

OC6H13

OH

RO

[

]n

R = HR = C6H13

76 (2003)

R = Boc, C11H23 (2000)

R = O-tBu (1996)

R = C (CH2)7CH3

CH3

CH3

(1996)

N

N

N

N

R

O

R

O

H

H

n

74

N

N

N

N

H3C

CH3

OtBu

O

tBuO

O

H

H

n

75 (1996)

n

70 (1994)



22

R

R

S

n77

OR

RO

OR

RO

O

O

OR

RO

(CH2)6

(CH2)6

OH

OH

32 2

n

79

R1

R1

R1

R1

R1

R1

R1

R1H R2

R2 H R2 H

H R2

x' y'n

Ladder-type PPP

80

n

RO

OR

H13C6

C6H13

R-dendrimer

81

NO2

n

78 (2000)



23

1.4 Pyridine incorporated conjugated polymers

Pyridine-based conjugated polymers are considered to be promising candidates

for light emitting devices. As compared to phenylene-based analogues, one of the most

important features of the pyridine based polymers is the higher electron affinity. As a

consequence, the polymer is more resistant to oxidation and shows better hole transport

properties.429 Due to poor solubility, pyridine-containing polymers received minimum

attention. The examples of some pyridine incorporated conjugated polymers are

illustrated in Figure 1-4.406-452 Examples given in Figure 1-4 are arranged in the form of:

PPy, PPy with side groups (mono alkyl), copolymer, and copolymers with alkyl groups

and alkoxy groups.


24

N

CH3

n

83(1992, 1994)

N

H3C

n

84(1992, 1994) (1992, 1994)

N

H3C

n

85

(2000, 2002)

91

R

Nn

R= n-C6H13

(2000, 2002)

R= n-C6H13

92

R

N

OCH3

H3CO

n

OC8H17

H17C8ON

n

94

(2000, 2001)

OC8H17

H17C8O

N

n

95

(2000, 2001)

N

H25C12

C12H25

n

90 (1997)

Nn

87 (1996) 88 (1996)

N nm N N n

89 (1996)

NH17C8 C8H17

m n

96 (2003)

N

RN

C7H15

n

97 (2002)R= C8H17 R= 2-ethyl-hexyl

100 (2002)

NN N

N[

]n

98 (2002)

N

N

m

n

(1993, 1994, 2001)

N

H13C6

n

86

N

N

N

N1-n

n

99 (2002)

N

NO2

n

101 (2000)

(1996, 1998, 1999, 2000, 2003)

N n

82

N

OR

RO

n

93

R= C4H9, C8H17, C12H25 = C16H33 = 2-ethyl-hexyl

(2000, 2001, 2002, 2003)

Figure 1-4. Examples of pyridine incorporated conjugated polymers.406-430,344


25

N n113

N

N N

n114

NN

NN

N

115

(2003)

N

n102

(1996)

N R

R

n106

R= C12H25 = OC16H33 = COOC12H25(1996, 1997, 2002)

N

R Rn

R = CH3(CH2)5 (1999)

108

(2003)

110N

O

O

HOC11H22

n

N

CH3+

n103 (1995)

N

H+

n104 (1995)

N

Bu

n105 (1995)

N

OC12H25

H25C12O

n111 (1996)

N n

109 (2003)

(1997, 1999)

107

N

R

R N

O

O

R= OC16H33 = C12H25

(1994)

112n

NS

C6H13

Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).431-445


26

(1996, 1999)

117

nN S N S Nm n

118

(1996, 1999)

S

R

N S

R

n

120

(2003)

R= C6H13 = C8H17 = C12H25

(2003)

121

S

R

N S

R

S

R

N S

R

n

R= C12H25

(2003)

122

S

R

N S

R

S

R N

S

R

n

R= C12H25

N

N

N

H13C6

C6H13

H13C6

C6H13

n

125 (2001)

(1996, 1999)

116

(1996)

Se Nn

123

NN

O

H

N

H

Om n

126

(1995)

S Nn

S

O O

NS

OO

S

N

O O

S

OO

(

)n119

(1999, 2002)

Se Nm n

124

(1996)

Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).446-

451,418


27

N

S

n

132(2002)

N

S

N

N

R

R

n

131(2002)

127

(2002)

n

NSS

OR

R = H, Me, n-hexyl, benzyl R = Me, n-pentyl, phenyl, NEt2

128(2002)

n

NSS

O

O

R

129(2002)

n

NSS

O-

R

+

R = H, OMe130

(2002)

n

NSS

R

O-

+

N

S S

n

134

(2002)

Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).452


28

1.5 Bipyridine incorporated conjugated polymers

(1992)

M=Ru, Ni

l m

M

NNNN

Lx

136

N N

R R

n

139R= C6H13 (2003)R= C8H17 (2001)

H13C6 C6H13RO

N N

n

R= HR= Bn141(2003)

(1990, 1992, 1996, 1997)

N N n

135

R= CH3R= C6H13

(1995)

N N

R R

n

137

(2001)M=Ru138

MLx

nNN

H13C6

C6H13

140

n

Re(CO)3Cl

N N

H13C6C6H13H13C6

C6H13

NN m

(2003)

Figure 1-5. Examples of bipyridine incorporated conjugated polymers.453-464


29

143

N N

OR

H3COn

R= 2-ethyl-hexyl (2000)

R= C10H21 (1997)R= 2-ethyl-hexyl (1999)

144

N N

OR

RO

[

]n

R= C10H21 (1997)R= 2-ethyl-hexyl (1999)

[

]

N N

OR

RO

OR

ROn

(

)2

145

R= 2-ethyl-hexyl (2000)

N N

OR

RO

[

]nM

146R= 2-ethyl-hexyl (2000)

[

]

N N

OR

RO

OR

ROn

(

)2

M 147

Si

C6H13

CH3

N N

[

n](2000)

149

148H17C8 C8H17 N N

n

(2001)

(2000)

142N N

n

Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).465-

469,462


30

R= OC18H37 (2001)

N N

OR

RO

n

150

(2001)

n

H17C8 C8H17

N N

151

N N

S

S

CHn

(2001)155

(2001)

156

N N

S

S

CHx N N

S

S

CHyRu

S

OO

N N S

O O

S

O O

N N

S

OO

n

154 (1999)

(2000)152

4S

C8H17

N N n

(2000)153

4S

C8H17

N N n

Ru

Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).470-

473,462


31

(2002)

157

N NS

EH

(

)nEH= 2-ethyl-hexyl (2002)158

N NS

C10H21

(

)n

(2002)159

(

)n

S

H21C10

N NS

C10H21

(2002)160

(

)n

S

H21C10

N NS

C10H21

N N

N

C12H23

(

)n

161(2002)

Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).474


32

N

N

n

163 (1998, 2001)

164 (1998)

N

Nn

[][ ]m

N

N[ ]n

165 (1998)

166 (2001)

N

Nn3

N

N

n

167 (2001)

R= H = OC12H25 = OC18H37

169 (2000)

n

N

R

R

N

N

N

C6H13

H13C6

n

162 (2001)

R= H = OC8H17 = OC18H37

N

N

R

R

n

168 (2000)

Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).475-478


33

1.6 Poly(m-phenylene)s (PMPs)

Poly(m-phenylene)s and their oligomers have found considerable interest in non

linear optical applications due to the formation of helical structures and sufficient

delocalization.482 Due to poor solubility, PMP has received less attention compared to

PPPs. Examples of poly(m-phenylene)s are illustrated in Figure 1-6.479-504


34

174 (1996)

n

N O 175 (1996)

n

N

O

176 (1996)

n

N

O

CH3

OC12H21

CH3

S x n

177 (1996)

CH3

OCH3

OCH3

CH3

178 (2000)

CH3

OCH3

CH3

OCH3

179 (2000)

X =

X =

X

X

n

180 (2001)

181 (2001)

NY

n

+ BF4-

Y = (CH2)3CH3Y = (CH2)15CH3

Y =

Y =

n170

OC12H25

n171

(1997)

n

172 (2000)

n

173 (1996)

Figure 1-6. Examples of poly(m-phenylene)s PMPs.479-488


35

R = t-ButylR = C6H13R = C12H25

R

R

n 182(1999, 2000)

R1

R1

R1R1

R n

183(1999, 2000)

R = t-ButylR = C6H13R = C12H25

R1 = C11H23

C12H23R1 =

OC8H17

OC8H17

n

184(1997, 2000, 2001, 2003)

186(2001, 2002, 2003, 2004)

CH3

ORn

R = n-C4H9, n-C6H13, n-C8H17

185(2001, 2002, 2003)

R = n-C4H9, n-C6H13

OR

RO

n

187(2001, 2002, 2003, 2004)

R = n-C4H9, n-C6H13, n-C8H17

CH3

ORn

Figure 1-6. Examples of poly(m-phenylene)s PMPs (Continued).489-500


36

R = HR = OC10H21

OC4H9

R

R

H9C4O

[

]n 189(2003)

188 (2000)

OC4H9

H9C4O

n

CH CH CH CH

n 190 (2003)

n 191 (2003)

R = COOCH3R = H

C6H13H13C6COOR

x y

194(2003)

n

192(2004)

OC6H13

H13C6O

n 193(2004)

Figure 1-6. Examples of poly(m-phenylene)s PMPs (Continued).492,501-504


37

1.7 Aim of the project

The aim of this project was to test the hypothesis that the presence of hydroxyl

groups on conjugated polymer backbone will improve the planarity of polymer and fine-

tune the optical properties. The main goals of this project are:

(i) Designing and synthesizing novel amphiphilic conjugated polymers

containing free hydroxyl groups and hydrogen bond acceptor groups

such as nitrogen atoms on polymer back bone capable of forming an

inter/intra molecular hydrogen bonding.

(ii) Investigating the optical properties of polymers

(iii) Investigating the ionochromic effect, solvatochromic effect and

protonation-deprotonation process of polymers

(iv) Designing and synthesizing novel precursor polymers (Salen type)

(v) Preliminary probing the potential applications of the derived polymers

A few series of novel, soluble amphiphilic conjugated polymers were designed

and synthesized. They are: amphiphilic PPPs (201a-c), pyridine incorporated conjugated

polymers (301-306), and bipyridine incorporated conjugated polymers (401-403). All the

polymers were prepared by Suzuki polycondensation. All polymers were examined by

spectroscopy to confirm their structures. The optical properties of all polymers were

studied.


38

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2001, 202, 980-984.

476. Eichen, Y.; Nakhmanovich, G.; Gorelik, V.; Epshtein, O.; Poplawski, J. M.;

Ehrenfreund, E. J. Am. Chem. Soc. 1998, 120, 10463-10470.

477. Narayan, K. S.; Geetha, K. V.; Nakmanovich, G.; Ehrenfreund, E.; Eichen, Y.

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478. Grummt, U. W.; Birckner, E.; Klemm, E.; Egbe, D. A. M.; Heise, B. J. Phys.

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479. Yamamoto, T.; Wu, B.; Choi, B. –K.; Kubota, K. Chem. Lett. 2000, 720-721.

480. Yamamoto, T.; Abe, M.; Takahashi, Y.; Kawata, K.; Kubota, K. Polym. J.

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481. Williams, D. J.; Colquhoun, H. M.; O’Mahoney, C. A. J. Chem. Soc., Chem.

Commun. 1994, 1643-1644.

482. Panda, M.; Chandrasekar, J. J. Am. Chem. Soc. 1998, 120, 13517-13518.

483. Reddinger, J. L.; Reynolds, J. R. Macromolecules 1997, 30, 479-481.

484. Tsuie, B.; Reddinger, J. L.; Sotzing, G. A.; Soloducho, J.; Katritzky, A. R.;

Reynolds, J. R. J. Mater. Chem. 1999, 9, 2189-2200.

485. Mathias, L. J.; Tullos, G. Polymer 1996, 37, 3771-3374.


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486. Kang, B. S.; Seo, M. –L.; Jun, Y. S.; Lee, C. K.; Shin, S. C. Chem. Commun.

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487. Dridi, C.; Said, A. H.; Ouerghi, O.; Chikhi, M.; Legrand, A. P.; Gamoudi, M.;

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489. Bumgarten, M.; Yuksel, T. Phys. Chem. Chem. Phys. 1999, 1, 1699-1706.

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491. O’Brien, D.; Bleyer, A.; Lidzey, D. G.; Bradley, D. D. C.; Tsutsui, T. J. Appl.

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492. Cacialli, F.; Chuah, B. S.; Friend, R. H., Moratti, S. C.; Holmes, A. B. Synth.

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495. Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2001, 34, 7300-

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497. Liao, L.; Ding, L.; Karasz, F. E.; Pang, Y. J. Polym. Sci., Part A: Polym.

Chem. 2004, 42, 303-316.

498. Drury, A.; Maier, S.; Davey, A. P.; Dalton, A. B.; Coleman, J. N.; Byrne, H.

J.; Blau, W. J. Synth. Met. 2001, 119, 151-152.


74

499. Lipson, S. M.; Cadby, A. J.; Lane, P. A.; O’Brien, D. F.; Drury, A.; Bradley,

D. D. C.; Blau, W. J. Monatsh. Chem. 2001, 132, 151-158.

500. Liao, L.; Pang, Y.; Ding, L.; Karasz, F. E. Macromolecules 2002, 35, 6055-

6059.

501. Zhang, H.; He, L.; Liu, X.; Li, Y.; Ma, Y.; Shen, J. Synth. Met. 2003, 135-136,

207-208.

502. Sarker, A. M.; Gürel, E. E.; Ding, L.; Styche, E.; Lahti, P. M.; Karasz, F. E.

Synth. Met. 2003, 132, 227-234.

503. Li, J.; Pang, Y. Synth. Met. 2004, 140, 43-48.

504. Ling, Q. D.; Kang, E. T.; Neoh, K. G.; Huang, W. Macromolecules 2003, 36,

6995-7003.

Chapter 2: Amphiphilic Poly(p-phenylene)s

75

Chapter 2

Amphiphilic Poly(p-phenylene)s1

“An investigator starts research in a new field with faith, a foggy idea, and a few wild

experiments. Eventually the interplay of negative and positive results guides the work. By the

time the research is completed, he or she knows how it should have been started and

conducted.” - Donald J. Cram (1987 Nobel Laureate in Chemistry)

“I am making everything new! .......these words are trustworthy and true." – Revelation 21:5 NIV (Holy Bible)


76

2.1 Introduction

During the last two decades, design and synthesis of new conjugated polymers

attracted attention due to their interesting optical, electrochemical, and electrical

properties, which led to the fabrication of optoelectronic and electronic devices,2-5

photovoltaic cells,6 biosensors,7 to name a few. Synthesis and characterization of a large

number of multifunctional conjugated polymers such as poly(p-phenylene)s (PPPs),8-10

poly(p-phenylenevinylene)s (PPVs),11,12 poly(pyridine-2,5-diyl) (PPy),13,14

polythiophenes,15,16 polyfluorenes,17,18 polyacetylenes,19 have been reported in the

literature. Among these polymers, PPP and its derivatives have found considerable

interest over the last twenty years due to their high photoluminescence efficiency in blue

light-emitting diodes.20 In order to enhance the solubility and processability of PPPs,

various substituents were introduced along the conjugated polymer backbone, in

particular the pioneering synthetic efforts by Wegner et al.21-24 Owing to the steric

interaction between ortho-H atoms of consecutive aryl rings, the extent of π-conjugation

was relatively low for these polymers.25,26 There have been numerous research efforts in

planarising the polymer backbone through covalent bond modification27-29 or

incorporation of weak interactions such as hydrogen bonds.30,31

We have designed and synthesized multifunctional poly(p-phenylene)s with many

free hydroxyl groups on the polymer backbone and explored the possibility of fine-tuning

the optical properties by changing environmental conditions such as pH or presence of

metal ions. A general structure of amphiphilic poly(p-phenylene)s 201a-c is shown in

Figure 2-1. In our design strategy, we explored the use of the hydroxyl groups

incorporated on the polymer backbone as a hydrogen bonding functionality to planarize


77

the backbone as well as potential ligand sites for complexation with metal ions. In this

chapter, we report on the synthesis and characterization of three novel polymers and

discuss the optical properties in detail.

OH

RO

n

201a: R = CH3(CH2)11201b: R = CH3(CH2)15201c: R = CH3(CH2)17

Figure 2-1. Structures of amphiphilic poly(p-phenylene)s 201a-c

2.2 Synthesis of polymers

Synthesis of monomers, 2,5-dibromo-1-benzyloxy-4-alkoxybenzene 205,

bis(boronic ester) 207 and amphiphilic conjugated polymers, poly(2-hydroxy-5-alkoxy-p-

phenylene) 201a-c, are described in Scheme 2-1.


78

or

(i) (ii)

(iii)

(iv)(v)

a: R = CH3(CH2)11b: R = CH3(CH2)15c: R = CH3(CH2)17 Bn = C6H5CH2

208

n

OBn

RO

OBn

RO

208

n

OBn

RO

OR

BnO

(vi)

(vii)

n

OH

RO

OH

RO

201

n

OH

RO

OR

HO

or

OH

HO 202

OH

HO

BrBr

203

OH

RO

BrBr

204

OBn

RO

BrBr

205

OBn

RO

B(OH)2(HO)2B

206

OBn

RO

B

O

O

B

O

O

207OBn

RO

BrBr

205

Scheme 2-1. Synthesis of polymers 201a-c: (i) Br2 in gl. AcOH, 85%; (ii) NaOH in abs.

EtOH, RBr, 60 °C for 10 h, 60%; (iii) anhyd. K2CO3 in abs. EtOH, BnBr, 40-50 °C for

10 h, 95%; (iv) BuLi in hexanes (1.6 M soln), THF/Et2O at –78 °C, B(OiPr)3, water

stirred at RT for 10 h, 80%; (v) 1,3-propanediol, toluene, reflux, 3 h, 80%; (vi) 2N

Na2CO3, Toluene, 3.0 mol % Pd(PPh3)4, reflux for 3 d, (vii) H2, 10% Pd/C, EtOH/THF.


79

Bromination of hydroquinone 202 was achieved using a standard procedure.32 The

monoalkylation of dibromocompounds 203 was carried out at 60 °C for 10 h using 1.0

equivalent of dibromohydroquinone and 0.9 equivalent of alkyl bromide in presence of a

base, sodium hydroxide (1.5 equivalent) with ethanol as solvent.33 The crude product was

purified by column chromatography using a 3 : 2 mixture of hexane and dichloromethane

solvents and the benzylation of 204 was performed in the presence of anhydrous K2CO3

and benzyl bromide to yield compound 205 in 95% yield. Bis(boronic ester) 207 was

prepared from momomer 205 by reaction with butyllithium and triisopropyl borate,34

followed by esterification with 1,3-propanediol.35

The benzylated precursor polymers were synthesized by Suzuki polycondenzation

under standard conditions.36-38 The polymerization was carried out using equimolar

quantities of the monomers 205 and 207 in the biphasic medium of toluene and aqueous

2M sodium carbonate solution with tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] as

the catalyst under vigorous stirring for 48 h. The standard work-up afforded O-benzylated

polymers (208a-c) as a yellowish precipitate. Hydrogenolysis of precursor polymers

(208a-c) using palladium adsorbed on carbon as catalyst gave the target polymers (201a-

c) in quantitative yield. Polymers (201a-c) were further purified using fractional

precipitation from methanol.

2.3 Characterization of polymers

All polymers showed good solubility in common organic solvents such as

tetrahydrofuran (THF), chloroform, toluene and dimethylformamide (DMF). Molecular-

weight of fractionated polymers was determined by gel permeation chromatography


80

(GPC) with reference to polystyrene standards using THF as solvent (Table 1). Most of

these fractionated polymers have molecular weights in the range of 2 kg/mol to 4 kg/mol

and polydispersivity index (PDI) around 1.11. However, Müllen et al.39 reported that

GPC results for rigid rod polymers using polystyrene standards were not completely

reliable. Similar results were observed for PPPs with polar functional groups on the

polymer backbone.40 For understanding the structure and complexation properties, PPPs

with low molecular weights and narrower distribution (PDI close to 1) are preferred.

Table 2-1. Molecular weights of target polymers 201a-c observed from GPC analysisa

Polymer 201 Mn (g/mol) Mw (g/mol) Mw/ Mn

201a 3600 4200 1.16

201b 3600 4000 1.11

201c 2700 2900 1.07

aThe GPC analysis was done at RT using polymers dissolved in THF and filtered with

reference to polystyrene standards.


81

All neutral polymers were highly stable at room temperature. The thermal

properties of all polymers were investigated by thermogravimetric analyses using a

heating rate of 10 °C/min under nitrogen. The initial temperature of decomposition of

precursor polymers 208a-c ranged from 260 to 294 °C owing to the decomposition of

benzyl protecting group. For the target polymers 201a-c, the initial decomposition starts

from 325 to 350 °C. This may be due to the presence of large alkyl chains along the

polymer backbone.

2.4 Optical and ionochromic properties of polymers

Optical properties of all polymers were studied using polymer dissolved in doubly

distilled THF. The absorption wavelength of benzylated polymers 208a-c showed no

significant variation with respect to the length of the side chain from dodecyl (C12) to

hexadecyl (C16) groups. [λmax = 326 nm for 208a and λmax = 326 nm for 208b]. The

absorption maxima of polymers 201a and 201b appeared in the longer wavelength region

(λmax = 338 nm for 201a, λmax = 336 nm for 201b, presumably due to interchain hydrogen

bonding of hydroxyl groups (Figure 2-2). For polymer 201c, apparently no changes were

observed (λmax = 322 nm for 208c λmax = 322 nm for 201c). It is anticipated that longer

alkyl chains, such as octadecyl (C18) group were difficult to reorganize as compared to

shorter ones and this could be the reason for this lack of change in absorption maxima.

On addition of a stoichiometric amount of a base such as aqueous solution of sodium

hydroxide (NaOHaq), the polymers exhibited a stronger hypsochromic shift to blue region

(λmax = 364 nm for 201a, λmax = 364 nm for 201b and λmax = 354 nm for 201c, all in

NaOHaq/THF) as shown in Figure 2-2. This may be due to the formation of phenolate


82

anions on the polymer backbone.41 The absorbance and emission spectra of neutral

polymer (201a) with and without an added stoichiometric amount of NaOHaq are shown

in Figure 2-2.

Figure 2-2. Absorbance and emission spectra of Polymer 201a: a – absorption spectrum;

b – absorption spectrum in the presence of base NaOH; c – emission spectrum; d –

emission spectrum in the presence of base NaOH.


83

The emission spectra of the polymers were recorded using polymers dissolved in

freshly distilled THF. The emission maxima of polymers in the presence of a

stoichiometric amount of a base such as NaOHaq, showed significant differences

compared to the neutral polymer as shown in Figure 2-2. For example, polymer 201a in

THF showed an emission maximum (λemi) at 403 nm but in the presence of NaOHaq the

emission maximum of 201a was shifted to 474 nm. A similar trend was also observed for

polymers 201b and 201c. All absorption and emission maxima of the target polymers are

given in Table 2-2.

The observed shift in the absorption and emission maxima of polymers (201a-c)

could be explained using the planarized structure of the polymer backbone (Figure 2-2).

Both hydrogen bonding and alkyl chain crystallization promote the formation of a layer-

type morphology for the polymer lattice. This was evident from the X-ray powder

diffraction pattern of the polymers. A strong peak at the low angle region corresponding

to a d-spacing of 34.4 Å (for 201a with dodecyl group as side chain) indicates a layer-

type morphology as shown in Figure 2-3.


84

0100

200

300400

1.5 10 18.5 272θ

Inte

nsity

(cps

)

d = 34.42

Figure 2-3. X-ray powder diffraction pattern of polymer 201a. The powder pattern was

taken using powdered polymer samples placed on the sample pan without preannealing.


85

Since each phenyl ring along the polymer backbone carries one alkoxygroup, the

observed d-spacing value of 34.4 Å implies a noninterdigitated packing of alkyl chains.

Similar results were observed for polymers 201b and 201c. During this investigation, no

attempts were made to isolate possible isomers such as head-to-head or head-to-tail

coupling of AA/BB type monomers. In either case, it is expected that the hydroxyl groups

and alkyl chains do not mix with each other, but get separated to the same side of the

polymer backbone. After considering the X-ray diffraction data, an illustration of the

expected polymer lattice indicating the possible alkyl chain packing and hydrogen

bonding or metal complexation is given in Figure 2-4.


86

d

n

n

O

O

O

O

O

O

HH H HHH

O

O

O

O

O

O

Region of H-bonds or metal complexation

Region of alkylchain crystallization

Figure 2-4. Illustration of the polymer lattice indicating alkyl chain packing and

interchain hydrogen bonding or metal complexation


87

The ionochromic effect of polymers 201a-c was characterized by the addition of

metal salts to the polymer solutions. Color of the polymer solution in THF changed from

originally light yellow (metal free polymers) to green, blue or reddish brown, dependent

on the type of metal ions added (Table 2-2). According to the spectroscopic results on

metal complexes of the polymers, both 201.Cu2+ and 201.Co2+ complexes emitted in the

blue region (λemi = 464 nm for 201a.Cu2+, 463 nm for 201a.Co2+) and complex 201a.Fe3+

showed a strong emission in the green region (λemi = 509 nm). Similar results were

observed for polymers 201b and 201c (Table 2-2). The absorption wavelength of

previously reported PPP derivatives containing alkyl and alkoxyfunctional groups were

varied from 335 nm to 280 nm.6,14,19 It is interesting to note that by changing the metal

ions and thereby the nature of the complex, significant changes in the optical properties

of the polymers can be obtained.

To the best of our knowledge, such ionochromic effect was observed only in

conjugated polymers containing bipyridyl units42,43 and polythiophenes functionalized

with oligoethyleneoxide side chains.44 So far, none of the PPP derivatives reported in

literature showed ionochromic effect and our results indicated that target polymers (201a-

c) have strong interaction with metal ions. The absorption and emission maxima of

polymers 201a-c with various metal ions are shown in Table 2-2, indicating that emission

of polymers can be fine-tuned in the blue to green region using stoichiometric amount of

base or metal ions. These results may be of interest to people working in the area of

fabricating sensors for metal ions or polymeric light emitting diodes (PLED) with tunable

emission properties.


88

Table 2-2. Absorption and emission responses of polymers 201a-c with and

without metal ionsa

Polymer 201a Polymer 201b Polymer 201c

λmax (nm)/

E in eV

λemi (nm)/

E in eV

λmax (nm)/

E in eV

λemi

(nm)/E in

eV

λmax

(nm)/E

in eV

λemi (nm)/

E in eV

ion – free 338/3.67 403/307 336/3.69 402/3.08 328/3.78 402/3.08

Na+ 364/3.40 474/2.61 364/3.40 479/2.59 354/3.50 468/2.65

Cu2+ 384/3.23 436/2.84 444/2.79 518/2.39 433/2.86 499/2.44

Co2+ 416/2.98 436/2.84 416/2.98 471/2.63 427/2.90 489/2.53

Fe3+ 446/2.78 509/2.43 476/2.60 551/2.25 448/2.77 519/2.39

aConcentration: polymer 201a : 0.011 g in 100 mL THF, polymer 201b: 0.011 g in 100

mL THF , polymer 201c: 0.016 g in 100 mL THF, Base: stoichiometric amount of base

from 1M aqueous solution of NaOH, 0.1 mL of 1M metal ion (Cu2+, Co2+& Fe3+)

solution in methanol added to the polymer. [Under neutral conditions, benzylated

polymers showed absorption maxima (λmax) at 208a = 326 nm, 208b = 326 nm; 208c =

322 nm and emission maxima (λemi) at 208a = 400 nm, 208b = 399 nm, 208c = 397.5

nm.]


89

2.5 Conclusions

In conclusion, a novel series of optically tunable amphiphilic conjugated

polymers is synthesized using Suzuki polycondenzation. All polymers showed good

solubility in common organic solvents and emission properties in the blue to green region

in presence of base and metal ions. This optical tunability would allow such polymers as

good candidates for fabricating PLED devices. Metal chelating effect of these polymers

induced significant changes in emission properties and could be used for sensing metal

ions. Presence of free hydroxyl groups (phenolic) on the polymer backbone is expected to

show interesting electrochemical properties and self-assembly at the liquid-metal

interface.


90

2.6 References

1. Part of this work was presented at The International Chemical Congress of Pacific

Basin Societies, Pacifichem 2000, Honolulu, Hawaii, December 2000:

Valiyaveettil, S.; Chinnappan, B. Macromolecular Chemistry session, Abs. No.

0072.

2. Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804.


Taliani, C.; Bradley, D. D.C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.;


4. McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan,

G. C.; Srdanov, V. I.; Heeger, A. J. Adv. Mater. 1999, 11, 1349-1354.


1997, 30, 430-436.

6. Halls, J. J. M.; Walls, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;

Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498-500.

7. Heeger, P. S.; Heeger, A. J. Proc. Natl. Acad. Sci. USA 1999, 96, 12219-12221.

8. Berresheim, M.; Muller, M.; Müllen, K. Chem. Rev. 1999, 99, 1747-1785.


10. Goodson, F. E.; Wallow, T. I.; Novak, B. M. Macromolecules 1998, 12, 2047-

2056.


12. Feast, W. J.; Tsibouklis, J.; Pouwer, K. L.; Groenendaal, L.; Meijer, E. W.

Polymer 1996, 37, 5017-5047.


91

13. Blatchford, J. W.; Jessen, S. W.; Lin, L. B.; Gustafson, T. L.; Fu, D. K.; Wang, H.

L.; Swager, T. M.; MacDiarmid, A. G.; Epstein, A. J. Phys. Rev. B 1996, 54,

9180-9189.

14. Yamamoto, T.; Maruyama, T.; Zhou, Z. H.; Ito, T.; Fukuda, T.; Yoneda, Y.;


Chem. Soc. 1994, 116, 4832-4845.

15. Leclerc, M. Adv. Mater. 1999, 11, 1491-1498.

16. Goldenberg, L. M.; Bryce, M. R.; Petty, M. C. J. Mater. Chem. 1999, 9, 1957-

1974.

17. Setayesh, S.; Marsitzky, D.; Müllen, K. Macromolecules 2000, 33, 2016-2020.

18. Uckert, F.; Setayesh, S.; Müllen, K. Macromolecules 1999, 32, 4519-4524.

19. Goodson, F. E.; Novak, B. M. Macromolecules 1997, 30, 6047-6055 and further

references are sited therein.

20. Cimrova, V.; Schmidt, W.; Rulkiens, R.; Schulze, M.; Meyer, W.; Neher, D. Adv.

Mater. 1996, 8, 585.

21. Rehan, M.; Schluter, A. –D.; Wegner, G. Polymer 1989, 30, 1060.

22. Schülter, A. –D.; Wegner, G. Acta Polym. 1993, 44, 59.

23. Vanhee, S.; Rulkens, R.; Lehmann, U.; Rosenauer, C.; Schulze, M.; Kohler, W.;

Wegner, G.; Macromolecules 1996, 29, 5136.

24. Lauter, U.; Meyer, W. H.; Wegner, G. Macromolecules, 1997, 30, 2029.

25. Martin, R. E.; Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1350-1377.

26. Lamba, J. J. S.; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11723-11736.

27. Yao, Y.; Tour, J. M. Macromolecules, 1999, 32, 2455-2461.


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28. Satayesh, S.; Marsitzky, D.; Müllen, K. Macromolecules, 2000, 33, 2016-2020

and the references sited therein.

29. Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Müllen, K.; Meghadadi,

F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc., 2001, 123, 946-953.

30. Delnoye, D. A. P.; Sijbesma, R. P.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am.

Chem. Soc. 1994, 118, 8717-8718.


Chem. Eur. J. 2000, 6, 4597-4603.

32. Tietze, L. F. Reactions and Syntheses in the Organic Chemistry Laboratory.

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33. Rehahn, M.; Schluter, A. D.; Wegner, G. Macromol. Chem. 1990, 191, 1991-

2003.

34. Hensel, V.; Schluter, A. D. Chem. Eur. J. 1999, 5, 421-429.

35. Tanigaki, N.; Masuda, H.; Kaeriyama, K. Polymer 1997, 38, 1221-1226.

36. Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513-515.

37. Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schluter, A. D. J. Am.

Chem. Soc. 1997, 119, 3296-3301.

38. Yamamoto, T.; Kimura, T.; Schiraishi, K. Macromolecules 1999, 32, 8886-8896.

39. Kreyenschmidt, M.; Uckert, F.; Müllen, K. Macromolecules 1995, 28, 4577.

40. Wright, M. E.; Toplikar, E. G.; Lackritz, H.; Subrahmanyan, S. Macromol. Chem.

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41. Remmers, M.; Schulze, M.; Wegner, G. Macromol. Chem. 1996, 17, 239-252.


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42. Bouachrine, M.; Lere-Porte, J. P.; Moreau, J. J. E.; Serein-Spirau, F.; Torreilles. J.

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Chapter 3: Pyridine incorporated amphiphilic conjugated polymers

94

Chapter 3

Pyridine Incorporated Amphiphilic Conjugated

Polymers1

“Discovery consists of seeing what everybody has seen and thinking what nobody has

thought.” - Albert von Szent-Györgyi (1937 Nobel Laureate in Physiology or Medicine)

“Everything belongs to God, and all things were created by his power.” – Hebrews 2:10 CEV (Holy Bible)


95

3.1 Introduction

In recent years, design and synthesis of conjugated polymers with donor-

acceptor groups on the main chain are of great interest due to potential applications as

charge transport materials for light-emitting diodes,2-10 photovoltaic devices,11-13

lasers,14-17 xerographic imaging photoreceptors,18 field effect transistors,19 non linear

optical properties,20-23 chemical and biosensors.24-27 A large number of conjugated

polymers with and without donor-acceptor architectures have been reported in the

literature.29-41

The planarization of poly(p-phenylene) (PPP) polymer backbone attracted

considerable interest due to its influence on increasing the optical and conducting

properties. A few groups have successfully demonstrated that covalent ladder type

polymers, synthesized via covalent linkages, have better stability and properties.42

Meijer et al. have shown that ladder type structures formed via intramolecular

hydrogen bonds can planarize the polymer backbone.35 Previously we reported the

synthesis and characterization of hydroxylated poly(p-phenylenes) (HPPPs) [Chapter

2] and demonstrated the role of various weak interactions such as hydrogen bonding

and alkyl chain crystallizations on the property of such polymers.43

Due to high electron affinity of the pyridine rings as compared to the benzene

rings, pyridine-based conjugated polymers are considered to be promising candidates

for the fabrication of LEDs.44 Moreover, the presence of basic nitrogen atoms on the

polymer backbone allows fine-tuning of optical properties through protonation-

deprotonation processes.45,46 So far only a few pyridine-based polymers such as

poly(pyridine-2,5-diyl) (PPy) and poly(pyridine-2,5-diyl vinylene) (PPyV) have been

synthesized and characterized47-53 (examples of pyridine incorporated conjugated

polymer are given in Chapter 1). Most of the above polymers are soluble only in


96

formic acid. Due to poor solubility, PPy received minimal attention compared to

PPPs. Similarly, few literatures have been found for poly(m-phenylene)s (PMPs),

poly(m-phenylenevinylene)s and substituted PPPs with alternating meta and para

linkages.54-62 PMPs and their oligomers have found considerable interest in non linear

optical applications due to the formation of helical structures and sufficient

delocalization.54-62

Excited-state intramolecular proton transfer (ESIPT) is a photochemically

induced proton transfer process of molecules having a cyclic hydrogen bond.63 The

chemical structure of these compounds usually contains a phenolic group which is

intramolecularly hydrogen bonded to a hetero atom such as nitrogen or oxygen in the

same chromophore. Photochemical excitation of the ground state of such molecules is

followed by an extremely rapid tautomerization process to give energetically more

stable excited-state tautomers which results a large Stokes shift and shows efficient

fluorescent properties.64-68

By using our previously reported synthetic strategy,1,43 we have synthesized

pyridine incorporated soluble PPPs. The introduction of hydroxyl groups and the

pyridine rings on the polymer backbone may result in the formation intramolecular

hydrogen bonds between two adjacent phenol and pyridyl rings and planarize the

backbone. It also helps to facilitate ESIPT and act as a potential ligand sites for

complexation with metal ions. Moreover, a comparison of the properties of linear

(1,4- for benzene rings and 2,5- for pyridine rings) and twisted (1,4- for benzene and

2,6- for pyridine) polymer backbone with alternating donor-acceptor groups is given.

The introduction of alkyl chains on the alternating benzene ring on the polymer

backbone, enhances the solubility as well as the organization of polymer chains


97

through van der Waal’s interaction. Molecular structures of target polymers 301-306

are shown in Figure 3-1.69


OBn

BnO

N

305

n

OBn

RO

N

304

n

O

HO

N

H

302

n

O

RO

N

H

301

n

303

N

O

RO

OH

OR

H

n

306

N

OBn

RO

OBn

OR

n

Figure 3-1. Molecular structures of target polymers 301-306.69


98

In this chapter, we report the synthesis and optical properties of a few

pyridine-incorporated hydroxylated polyphenylenes (Py-HPPs).


The syntheses of monomers, bis(boronic acid) 311 and 313 and new pyridine-

incorporated hydroxylated polyphenylenes (Py-HPPs) 301-303, are described in

Scheme 3-1 and 3-2.69 The bisboronic acids were synthesized from hydroquinone

using previously reported procedure with good yields.43 The precursor polymers 304-

306 were synthesized using a Suzuki polycondenzation.43,55 The polymerizations were

carried out with a stoichiometric amount of the corresponding bisboronic acid 311 and

2,5-dibromo pyridine (314) or 2,6-dibromopyridine. These reactions were conducted

in the heterogeneous system of THF and aqueous 2M sodium carbonate solution with

tetrakis(triphenylphosphine)palladium as the catalyst. The O-benzylated polymers

(304, 305 and 306) were isolated as light yellowish solids. The target polymers (301,

302 and 303) were prepared from the respective precursors (304, 305 and 306) via

hydrogenation on 10% Pd/C and purified by fractional precipitation from methanol in

quantitative yield.


99


OH

HO 307

OH

HO

BrBr

308

OBn

BnO

BrBr

312

OH

RO

BrBr

309

OBn

RO

BrBr

310

OBn

BnO

B(OH)2(HO)2B

313

OBn

RO

B(OH)2(HO)2B

311

N

BrBr

314

OBn

BnO

N

305

n

N

BrBr

314

OBn

RO

N

304

n

(i) (ii)

(iii)(iii)

(iv) (iv)

(v)

O

HO

N

H

302

n

O

RO

N

H

301

n

(vi)

(vii)

(viii)

Scheme 3-1. Synthesis of polymers 301 and 302: (i) Br2 in gl. AcOH, 85%; (ii)

NaOH in abs. EtOH, RBr, 60 °C for 10 h, 60%; (iii) anhyd. K2CO3 in abs. EtOH,

BnBr, 40-50 °C for 10 h, 95%; (iv) BuLi in hexanes (1.6 M soln), THF/Et2O at –78 °C

, B(OiPr)3, stirred at RT for 10 h, 80%; (v) 2M Na2CO3 solution, THF, 3.0 mol %


100

Pd(PPh3)4, reflux for 3 d, (vi) H2, 10% Pd/C, EtOH/THF; (vii) 2M K2CO3 solution,

Toluene, 3.0 mol % Pd(PPh3)4, reflux for 3 d, (vi) H2, 10% Pd/C, MeOH/THF, 40 °C.

OBn

RO

B(OH)2(HO)2B

311

NBr Br

315

+

N

OBn

RO

OBn

OR

n

306

N

O

RO

OH

OR

H

n

303

(i)

(ii)


Scheme 3-2. Synthesis of polymer 303: (i) 2M Na2CO3, THF, 3.0 mol % Pd(PPh3)4,

reflux for 3 d, (ii) H2, 10% Pd/C, EtOH/THF.


101


The polymers incorporated with long alkyl chains (301, 303, 304 and 306)

showed good solubility in common organic solvents such as chloroform, THF,

toluene, DMF, and formic acid. Polymers 305 and 302 were partially soluble in THF

and methanol. The molecular weights of all polymers were determined by GPC with

reference to polystyrene standards using chloroform for polymers 301, 303, 304 and

306 and THF for polymers 302 and 305 as the eluents (Table 3-1). Most of these

fractioned polymers have molecular weights in the range of 2-5 kg/mol and a

polydispersity around 1.22. Similar results were observed for PPPs with polar

functional groups.43,70 For a better understanding of their structural and complexation

properties, PPPs and their copolymers with low molecular weights and narrower

distributions (PDI close to 1) are preferred.


102

Table 3-1. Molecular weights of polymers 301-306 observed from GPC analyses

Polymer Mn Mw Mw/ Mn

301 3500 4300 1.22

302 2100 2600 1.23

303 1800 1900 1.05

304 3800 5400 1.42

305 2300 2800 1.21

306 3000 4000 1.33

The GPC analyses were done by using the eluents chloroform for polymers 301, 303,

304 & 306 and THF for polymer 302 & 305 at room temperature with reference to

polystyrene standards.


103

The resonance peaks in the NMR spectra of all parent polymers 304-306

showed overlapping signals due to the presence of both the head-to-head (HH) and

head-to-tail (HT) units. Similar properties were observed in other reported polymers

containing pyridine rings.29-32 Due to high planarity of polymers 302, we have

observed pyridinyl proton peaks at δ (ppm) 7.71 (b), 7.55 (b), 6.97 (s), 6.70 (s).

Similar observations were reported for the rest of the polymers.

All polymers were stable at room temperature. The thermal properties of all

polymers were determined by thermogravimetric analyses (TGA) with a heating rate

of 10 °C/min under nitrogen atmosphere. The initial decomposition temperature of

precursor polymers 304-306 was at 250 - 300 °C due to the decomposition of the

benzyl protecting group. For the polymers 301-303, two decomposition peaks were

observed at 79 °C to 233 °C, respectively. The initial peak may be due to the

evaporation of traces of solvent in the polymer sample.

3.4 Optical Properties

3.4.1 Influence of hydroxyl groups

The optical properties of all polymers 301-306 were studied in chloroform and

summarized in Table 3-2. All absorption spectra of polymers showed two maxima

located in the range of 275-320 nm and the other above 350 nm. The signal at shorter

wavelength is attributed to π → π* transitions whereas that a longer wavelength is

attributed to charge-transfer (CT) process.71,72

The absorption wavelength of the π → π* absorption bands of polymers 301-303

were practically the same as these of the precursor polymers 304-306 with shifts of

less than 15 nm. However, there were significant changes in CT bands with shifts in


104

the range of 25-50 nm. The absorption and emission spectra of target polymer 301

and its precursor 304 are displayed in Figure 3-2.

Figure 3-2. Absorbance and emission spectra of polymers 304 and 301 in chloroform:

a & b: absorption spectra of polymer 304 (λmax = 364 nm) and 301 (λmax = 390 nm); c

& d: emission spectra of polymer 304 (λemi = 429 nm) and 301 (λemi = 493 nm).

Concentrations: Polymer 301: 0.0147 g in 100 mL of CHCl3; Polymer 304: 0.005 g in

100 mL of CHCl3.


105

The absorption maximum of polymer 301 occurred at a longer wavelength

(λmax = 390 nm) as compared to that of polymer 304 (λmax = 364 nm). This is the

result of planarization of the polymer backbone through intramolecular hydrogen

bonding between the adjacent hydroxyl group and N-atom of the pyridine in 301. The

polymer 301 emits in the blue-green region (λemi = 493 nm) with a large Stokes shift

(103 nm), which indicates the possibility of ESIPT as observed in intramolecular H-

bonded small molecules, oligomers, and some polymers.63,73 On the basis of our

observed results and the published reports, an intramoelcular proton-transfer

mechanism for the target polymers in the electronically excited state is proposed in

Figure 3-3.


106

NO

RO

H

n

E1K1

NO H

RO

n

K0

NO H

RO

n

emission

NO

RO

H

n

E0

absorption

Intramolecular

Proton Transfer

Proton Transfer

back

Figure 3-3. Excited-state intramolecular proton transfer (ESIPT) for polymer 301: E-

enol; K-keto; E0 – ground state (enol); E1 – excited state (enol).


107

The values of Stokes shift of the polymers are given in Table 3-2. The

observed Stokes shift of polymer 302 is exhibiting a large as compare to 301 and 303.

This may be due to the presence of two hydroxyl groups on the backbone for the

formation of intramolecular hydrogen bonds with neighboring pyridine units.74

3.4.2 Comparison of properties of polymers (301, 302 and 303)

The optical properties of soluble alkoxy substituted polymers showed similar

effects in all solvents. For example, both polymers 301 and 303 in chloroform showed

emission properties in the blue-green region (301.λmax = 390 nm; λemi = 493 nm and

303.λmax = 380 nm; λemi = 500 nm). Polymer 302 (λmax = 396 nm; λemi = 543 nm)

showed larger shift as compare to polymer 301 and 303. This may be due to induced

planarization of the polymer backbone through intramolecular hydrogen bonds

between the hydroxyl groups and pyridyl rings on the polymer chain.

3.4.3 Solvatochromic behavior of polymers

Solvatochromic measurements were performed for all polymers by using three

solvents as shown in Table 3-2. On varying the solvent polarity, a positive

solvatochromism (i.e bathochromism) of the absorption band is observed, which is

consistent with an intramolecular charge-transfer (ICT) transition.65,71,72 For example,

polymer 301 emits blue-green region (λemi = 493 nm) in chloroform, violet region

(λemi = 429 nm) in THF and blue region (λemi = 447 nm) in DMF. The

solvatochromism of all polymers are illustrated in Table 3-2.


108

Table 3-2. Solvatochromic behavior of polymers 301-306a

Chloroform (CHCl3) Tetrahydrofuran(THF) Dimethylformamide(DMF) Polymerb

λmax(nm)/

E in eV

λemi(nm)/

E in eV

Stokes

Shift (nm)

λmax(nm)/

E in eV

λemi(nm)/

E in eV

λ.H+max(nm)c/

E in eV

λmax(nm)/

E in eV

λemi(nm)/

E in eV

301 390/3.18 493/2.51 103 380/3.26 429/2.89 434/2.85 382/3.24 447/2.77

302 396/3.13 543/2.28 147 398/3.11 450/2.75 412/3.01 392/3.16 446/2.78

303 380/3.26 500/2.48 120 364/3.40 425/2.91 374/3.31 368/3.37 434/2.85

538/2.30

304 364/3.40 429/2.89 65 362/3.42 428/2.89 406/3.05 362/3.42 438/2.83

305 348/3.56 428/2.89 80 354/3.50 426/2.91 408/3.04 350/3.54 432/2.87

306 358/3.46 437/2.83 79 362/3.42 408/3.04 420/2.95 358/3.46 425/2.91

aNo absorption of <320 nm are listed here

bConcentrations:

Polymer 301: 0.0147 g in 100 mL of CHCl3; 0.001 g in 100 mL of THF; 0.0147 g in 100 mL of DMF. Polymer 302: 0.003 g in 100 mL of

CHCl3; 0.002 g in 100 mL of THF; 0.005 g in 100 mL of DMF. Polymer 303: 0.0147 g in 100 mL of CHCl3; 0.0134 g in 100 mL of THF; 0.005


109

g in 100 mL of DMF. Polymer 304: 0.005 g in 100 mL of CHCl3; 0.005 g in 100 mL of THF; 0.007 g in 100 mL of DMF. Polymer 305: 0.0134

g in 100 mL of CHCl3; 0.003 g in 100 mL of THF; 0.002 g in 100 mL of DMF. Polymer 306: 0.0162 g in 100 mL of CHCl3; 0.0173 g in 100 mL

of THF; 0.003 g in 100 mL of DMF.

cAbsorption maxima of polymers with aqueous HCl


110

3.4.4 Effect of protonation and deprotonation of polymers

The pyridine units of the copolymers (301, 302 and 303) can be protonated

using aqueous HCl solution. This transformation is accompanied by changes in the

color of the solution depends on the donor-acceptor structure of the polymer

backbone.75,76 This may result in possible charge transfer from the electron rich

phenyl ring to the electron poor pyridine, which is enhanced by protonation of the

nitrogen in the pyridine ring.45 Selected data of protonated polymers 301-306 are

summarized in Table 3-2. The absorbance spectra of polymers 301-303 in low pH are

given in Figure 3-3. Protonation of polymer 301 results in a λmax shift to 434 nm as

compared to the observed band at 382 nm before protonation. Upon neutralization of

the solution with base (e.g. NaOH solution), the λmax was shifted back to 382 nm

(Figures 3-4 and 3-5). Similar effects were also observed for the polymers 302 and

303.


111

Figure 3-4. UV/Vis spectra of protonation and deprotonation of polymers 301-303

with aqueous HCl and aqueous NaOH in THF: a: polymer 301 in THF (λmax = 380

nm); b: polymer 301 with 20 ppm 1M aqueous HCl/THF (λmax = 434 nm); c: polymer

302 with 20 ppm 1M aqueous HCl/THF (λmax = 412 nm); d: polymer 303 with 20

ppm 1M aqueous HCl/THF (λmax = 374 nm); e: polymer 301 with 20 ppm 1M

aqueous HCl and 30 ppm 1M aqueous NaOH/THF (λmax = 380 nm). Concentrations:

Polymer 301: 0.001 g in 100 mL of THF; Polymer 302: 0.002 g in 100 mL of THF;

Polymer 303: 0.0134 g in 100 mL of THF.


112

N+O

RO

HH

n

C1 K1

NO H

RO

n

Proton Transfer+ B BH++

N+O

RO

HH

n

C0

H+ NO

RO

H

n

E0

absorption

Figure 3-5. Proton transfer from the excited cation of polymer 301 to a base B: E-

enol; C-cation; K-keto.


113

3.4.5 Influence of base

When the polymers 301 and 303 were treated with excess aqueous NaOH, a

strong hypsochromic shift due to the formation of electron rich phenolate anions

along the polymer backbone was observed. For example, polymer 301 in the presence

of a base showed a λmax at 442 nm (∆λmax = 60 nm) and emitted in the yellow-green

region (λemi = 560 nm) as shown in Figures 3-6 & 3-7.

Figure 3-6. UV/Vis spectra of polymers 301 and 303 without and with aqueous

NaOH in DMF. a: polymer 301 in DMF (λmax = 382 nm); b: polymer 303 in DMF

(λmax = 368 nm); e: polymer 301 with 20 ppm 1M aqueous NaOH/DMF (λmax = 442

nm); d: polymer 303 with 20 ppm 1M aqueous NaOH/DMF (λmax = 398 nm).

Concentrations: Polymer 301: 0.0147 g in 100 mL of DMF; Polymer 303: 0.005 g in

100 mL of DMF.


114

Figure 3-7. Emission spectra of polymers 301 and 303 without and with aqueous

NaOH in DMF. a: polymer 301 in DMF (λemi = 447 nm); b: polymer 303 in DMF

(λemi = 434 nm); c: polymer 301 with 20 ppm 1M aqueous NaOH /DMF (λemi = 560

nm); d: polymer 303 with 20 ppm 1M aqueous NaOH/DMF (λemi = 534 nm).

Concentrations: Polymer 301: 0.0147 g in 100 mL of DMF; Polymer 303: 0.005 g in

100 mL of DMF.


115

Similar effects were also observed for polymer 303 (λmax = 398 nm, λemi = 534 nm,

stokes shift = 136 nm). Due to oxidation or quenching, the optical properties of

polymer 302 in the presence of a base could not be optimized. The absorption and

emission properties of polymers 301 and 303 can be tuned over a wide range (382 <

λmax < 442 nm; 434 < λemi < 560 nm) simply by varying the quantity of base added.

3.4.6 Metal complexation of polymers

There has been considerable interest in polynuclear transition-metal

complexes containing multichromophoric units capable of performing light-induced

processes. In particular, metal complexes exhibiting long-lived metal-to-ligand charge

transfer (MLCT) in the excited states have received much attention recently.77-80 The

ionochromic effect of polymers 301-303 was investigated using various metal salts

added to the polymer solutions. The color of the polymers solution was changed from

light yellow to blue, green, or reddish brown depending on the type of metal ions

added. The optical properties (λmax and λemi) of polymers 301-303 in the presence of

different metal ions are summarized in Table 3-3.


116

Table 3-3. Absorption and emission responses of polymers 301-303 with metal ionsa

Polymer 301 in THF Polymer 302 in MeOH Polymer 303 in THF

λmax(nm)/

E in eV

λemi(nm)/

E in eV

λmax(nm)/

E in eV

λemi(nm)/

E in eV

λmax(nm)/

E in eV

λemi(nm)/

E in eV

Fe3+ 434/2.80 499/2.48 480/2.58 563/2.20 426/2.91 493/2.51

Cu2+ 398/3.11 460/2.69 450/2.75 521/2.38 388/3.19 423/2.93

Ni2+ 424/2.92 457/2.71,

487/2.54

392/3.16 492/2.52 366/3.39 417/2.97

Co2+ 382/3.24 432/2.87,

451/2.75

392/3.16 446/2.78,

469/2.64

366/3.39 399/3.10,

417/2.97

aConcentrations: Polymer 301: 0.001 g in 100 mL of THF; Polymer 302: 0.0128 g in

100 mL of MeOH; Polymer 303: 0.0134 g in 100 mL of THF. metal ions: 100 ppm

of metal ions (Cu2+, Co2+, Ni2+ & Fe3+) in methanol.


117

The metal ion complexation induces a significant bathochromic shift of the absorption

band, which is sensitive to the nature of the added metal ion. Signals in the range

(λmax) of 380 nm to 480 nm (Table 3-3) due to metal to MLCT81 could be observed.

Polymer complexes with Fe3+ and Ni2+ ions showed strong emission in the blue-green

to yellow-green region. For example, polymer 301.Fe3+ showed a λmax at 434 nm and

emitted in the blue-green region (λemi = 499 nm). Polymers complexed with Co2+ ions

showed strong emission in the blue region (Table 3-3).

3.5 Conclusions

In conclusion, we have synthesized three amphiphilic π-conjugated pyridine-

incorporated polyphenylenes containing free hydroxyl group and long alkyl chain

using Suzuki polycondensation method. All polymers showed good solubility in

common organic solvents. The optical properties of all polymers were studied using

different solvents and showed positive solvatochromic effect. The target polymers

exhibited different absorption/emission properties based on the nature and type of

solvent used. All polymers (301, 302 and 303) were found to exhibit reversible and

tunable optical properties depending on metal complexation and protonation-

deprotonation process.


118

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Davenas, J.; Maaref, H. Synth. Met. 2000, 115, 97-101.

57. Cacialli, F.; Chuah, B. S.; Friend, R. H., Moratti, S. C.; Holmes, A. B. Synth.

Met. 2000, 111-112, 155-158.

58. Panda, M.; Chandrasekar, J. J. Am. Chem. Soc. 1998, 120, 13517-13518.

59. Li, C. J.; Slaven IV, W. T.; Chen, Y. P.; John, V. T.; Rachakonda, S. H. Chem.

Commun. 1998, 1351-1352.


61. Kang, B. S.; Seo, M. L.; Lee, C. K.; Shin, S. C. Chem. Commun. 1996, 1167-

1168.

62. Williams, D. J.; Colquhoun, H. M.; O’Mahoney, C. A. Chem. Commun. 1994,

1643-1644.

63. Rios, M. A.; Rios, M. C. J. Phys. Chem. A 1998, 102, 1560-1567.

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67. Cohen, B.; Huppert, D. J. Phys. Chem. A 2001, 105, 7157-7164.


123

68. Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19-27.

69. The structures of polymers in Figure 3-1 were used for simplicity. The

polymers probably contain both head-to-tail and head-to-head structures with

respect to C-C bond formation of phenyl and pyridyl moiety. No attempts to

introduce strict control on the regioselectivity in synthesis or purification of

the various isomers are carried out during this study.

70. Wright, M. E.; Toplikar, E. G.; Lackritz, H.; Subrahmanyan, S. Macromol.

Chem. Phys. 1995, 196, 3563.

71. Bosch, P.; Fernandez-Arizape, A.; Mateo, J. L.; Lozano, A. E.; Noheda, P. J.

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72. Bruni, S.; Cariati, E.; Cariati, F.; Porta, F. A.; Quici, S.; Roberto, D.

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73. Tarkka, R. M.; Zhang, X.; Jenekhe, S. A. J. Am. Chem. Soc. 1996, 118, 9438-

9439.

74. Yang, C-J.; Jenekhe, S.; Meth, J. S.; Vanherzeele, H. Ind. Eng. Chem. Res.

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75. Eichen, Y.; Nakhmanovich, G.; Gorelik, V.; Epshtein, O.; Poplawski, J. M.;

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76. Scheiner, S.; Kar, T. J. Phys. Chem. B. 2002, 106, 534-539.

77. Sun, S. S.; Lees, A. Organometallics 2002, 21, 39-49.

78. Manners, I. Science 2001, 294, 1664-1666.

79. Wang, Y.; Liu, S.; Pinto, M. R.; Dattelbaum, D. M.; Schoonover, J. R.;

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124

81. Ley, K. D.; Schanze, K. S. Coord. Chem. Rev. 1998, 171, 287-307.

Chapter 4: Bipyridine incorporated conjugated polymers

125

Chapter 4

Bipyridine Incorporated Conjugated Polymers

“There is no greater joy than that of feeling oneself a creator. The triumph of life is expressed

by creation.” - Henri Bergson (1927 Nobel Laureate in Literature)

“Wisdom is the principal thing; therefore get wisdom: and with all thy getting get understanding.”

- Proverbs 4:7 KJV (Holy Bible)


126

4.1 Introduction

Chemosensors based on conjugated polymers recently have attracted considerable

interests due to their merits over the sensor system based on small molecules in the

enhanced sensitivity, many transduction methods and facile processibility for condensed

phase applications.7,8 The conjugated polymers functionalized with electron-donor groups

such as crown ethers, aza crown ethers and calixarenes as side chains for sensory metal

ions have been the most dominantly studied sensory systems. However, these conjugated

polymers are adequate to recognize small size alkali metal ions such as Li+, Na+, and K+.

To be sensitive to various metal ions including transition metal ions, a 2,2’-bipyridyl

group, one of the well-known bidentate ligands, has been employed in the main chain of

conjugated polymers.16-19

In this chapter, we report the synthesis and characterization of new copolymers

containing bipyridine and 1,4-phenylene units in an alternative sequence. Bipyridyl

incorporated conjugated polymers with metal ions are cable of photoinducing electron

and energy transfer and are used as molecular arrays displaying nonlinear optical

properties. 20A general structure of target polymers 401-403 are shown in Figure 4-1.14

Owing to their ability to complex with a wide variety of transition metal ions,

salens have become one of the most widely studied groups of ligands. Various

applications of the metal-salen complexes have been demonstrated such as catalysts21-39

for oxidation, aziridination formation, epoxide opening and cycloaddition, both in the

asymmetric and non-stereo selective versions, sensing,40,41 DNA cleavage,42,43 and

optoelectronics.44 Incorporation of metal-salen complexes into a polymeric system

offered some advantages in certain applications.45 Salen-type complexes have been


127

known since 1933 and they constitute a standard system in coordination chemistry. In

salen, the ligand backbone and the coordinated metal ion can be easily varied which

make these catalysts especially useful in catalytic studies. The investigation of salen

complexes has been very active during the last decades, especially following the

discovery of salen-catalyzed enatioselctive epoxidation of olefins by the groups of

Jacobsen and Katsuki.46 Numerous salen-type complexes have been synthesized and

investigated in relation to a wide variety of reactions.47 However, the drawback of most

of the complexes has been in their limited solubility in aqueous solutions as well as more

sensitive in the strong acid conditions. In order to overcome the existing problems, we

prepared a new precursor copolymer 402 as shown in Figure 4-1. In this chapter, we

report on synthesis and characterization of new copolymers (bipyridine incorporated

conjugated polymers) 401-403 and discuss the optical properties in detail.


128

N N

OBn

OBnRO OR

N N NNR=CH3(CH2)11Bn=C6H5CH2

401

N N

OORO OR

N N NN

H H

R=CH3(CH2)11

402

N N

N N NN

RO

OR RO

OR

R=CH3(CH2)11

403

Figure 4-1. Molecular structure of the polymers 401-403.14


129


Synthesis of monomers, bis(boronic acid) 406 & 409 and new conjugated

polymers, bipyridyl incorporated conjugated polymers 401-403, are described in Scheme

4-1 and 4-2. The bisboronic acids were synthesized from commercially available starting

material hydroquinone using previously reported procedure with good yield.48 6,6’-

dibromo-2,2’-dipyridyl 405 was synthesized according to the literature.49 The polymers

401 & 403 were synthesized by Suzuki polycondenzation under standard conditions.48

The polymerizations were carried out with equivalent amount of corresponding

bisboronic acids (406 & 409) and 6,6’-dibromo-2,2’-dipyridyl 405 in the heterogeneous

system toluene and aqueous 2M sodium carbonate solution with

tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] as a catalyst precursor under vigorous

stirring for 72 hours. The standard work-up afforded polymers (401 & 403) as a light

yellowish precipitate. The polymer with free hydroxyl groups (402) was prepared from

precursor polymer (401) by hydrogenation using palladium adsorbed on carbon. Polymer

(402) was purified by fractional precipitation from methanol.


130

NBr Br

404

N N

Br Br405

B(OH)2(HO)2B

OBn

RO406

N N

NN N N

OBnRO OBn

OR

401

N N

NN N N

ORO O OR

H H

402


(i)

(ii)

(iii)

Scheme 4-1. Synthesis of polymers 401 and 402: (i) BuLi in hexanes (1.6 M soln), Et2O

at –60 0C; SOCl2, -40 0C (ii) 2N K2CO3, Toluene, 1.5 mol % Pd(PPh3)4, reflux for 3 d,

(iii) H2, 10% Pd/C, EtOH/THF.


131

N N

Br Br405

R=CH3(CH2)11

OH

HO

Br Br

407

OR

RO

Br Br

408

N N

N NNN

RO

OR RO

OR

403

(i) (ii)

OR

RO

(HO)2B B(OH)2

409

(iii)

Scheme 4-2. Synthesis of polymer 403: (i) NaOH in abs. EtOH, RBr, reflux for 10 h; (ii)

BuLi in hexanes (1.6 M soln), THF/Et2O at –780C , B(OiPr)3, water stirred at RT for 10

h; (iii) 2N K2CO3, Toluene, 1.5 mol % Pd(PPh3)4, reflux for 3 d.


132


All the polymers are showed good solubility in common organic solvents such as

chloroform, toluene, THF, and DMF. Molecular weight of fractionated polymers (401-

403) was determined by GPC with reference to polystyrene standards using THF as the

eluent (Table 4-1).

Table 4-1. Molecular weights of polymers 401-403 observed from GPC analyses

Polymer Mn Mw Mw/ Mn

401 9900 13000 1.31

402 4800 6700 1.39

403 3800 5400 1.42

The GPC analyses were done by using the eluent THF at room temperature with

reference to polystyrene standards.

All the polymers were highly stable at room temperature. The thermal properties

of all polymers were determined by thermogravimetric analyses with a heating rate of 10

0C/min under nitrogen. The initial temperature of decomposition of polymer 401 ranged

from 293 0C due to the decomposition of benzyl protecting group. For the polymer 402,

there are two kinds of initial decomposition and starts from 100 0C to 303 0C.


133

4.4 Optical properties of polymers

The optical properties of the polymers were studied using different solvents such

as THF, CHCl3, and HCOOH. The absorption maximum (λmax) of the polymers 402 was

observed at 388 nm (in HCOOH) and emission maximum (λemi) at 514 nm in HCOOH as

shown in Table 4-2. The larger stokes shift (~ 126 nm) indicates the presence of

intramolecular hydrogen bonding between two neighboring aromatic rings. We have

observed the same kind of optical responses of other polymers as shown in Table 4-2.

The UV-Vis absorbance and emission spectra of polymers 401 and 402 are displayed in

Figure 4-2.

Figure 4-2. Absorbance and emission spectra of polymers 401 and 402 in THF: a & b:

absorption spectra of polymer 401 (λmax = 348 nm) and 402 (λmax = 350 nm); c & d:

emission spectra of polymer 401 (λemi = 403 nm) and 402 (λemi = 413 nm).


134

Concentrations: Polymer 401: 0.016 g in 100 mL of THF; Polymer 402: 0.005 g in 100

mL of THF.

4.5 Solvatochromic behavior of polymers

Solvatochromic measurements were performed for all polymers by using three

solvents as shown in Table 4-2. On varying the solvent polarity, a positive

solvatochromism (i.e bathochromism) of the absorption band is observed, which is

consistent with an intramolecular charge-transfer (ICT) transition.50-52 For example,

polymer 402 emits violet region (λemi = 418 nm) in chloroform, violet region (λemi = 413

nm) in THF and green region (λemi = 514 nm) in HCOOH. The solvatochromism of all

polymers are illustrated in Table 4-2.

4.6 Ionochromic effects of polymers

The ionochromic effect of polymers 401-302 were investigated using various

metal salts added to the polymer solutions. There has been considerable interest in

polynuclear transition-metal complexes containing multichromophoric units capable of

performing light-induced processes. In particular, metal complexes exhibiting long-lived

metal-to-ligand charge transfer (MLCT) in the excited states have received much

attention recently.53-55 The ionochromic effect of all polymers 401-402 with different

metal ions were studied using THF as solvent as shown in Table 4-3.


135

Table 4-2. Solvatochromic behavior of polymers 401-403a

Polymer 401 Polymer 402 Polymer 403 Solvents

λmax(nm) λemi(nm) Stokes

Shift (nm)


Shift

(nm)


Shift

(nm)

Chloroform (CHCl3) 348 405 57 344 418 74 344 410 66

THF 348 403 55 350 413 63 346 404 58

HCOOH 392 514 122 388 514 126 400 506 106

aNo absorption of <320 nm are listed here

bConcentrations: Polymer 401: 0.007 g in 100 mL CHCl3; 0.016 g in 100 mL of THF; 0.005 g in 100 mL HCOOH. Polymer 402:

0.005 g in 100 mL CHCl3; 0.005 g in 100 mL of THF; 0.005 g in 100 mL HCOOH. Polymer 403: 0.005 g in 100 mL CHCl3; 0.015 g

in 100 mL of THF; 0.005 g in 100 mL HCOOH.


136

Table 4-3. Absorption responses of polymers 401-403 with metal ionsa

Metal Ions 401 in THF

λmax(nm)

402 in THF

λmax(nm)

403 in THF

λmax(nm)

Metal ion-free 348 350 346

Fe3+ 460 468 464

Cu2+ 406 398 400

Mn2+ 350 350 348

aConcentrations: Polymer 401: 0.016 g in 100 mL of THF; Polymer 402: 0.005 g in 100

mL of THF; Polymer 403: 0.015 g in 100 mL of THF.


137

4.7 Conclusions

In conclusion, three types of new bipyridine incorporated conjugated polymers

were synthesized using Suzuki-coupling reaction. All polymers were soluble in common

organic solvents. The synthesized copolymers showed interesting optical properties. The

metal ion recognition property with new copolymers was evaluated. Due to increasing

solubility compared to conventional bipyridine-based conjugated polymers, the

synthesized target polymers will be promising candidates for LEDs, nonlinear optical

properties, chemical sensors for metal ions, and catalytic studies.


138

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probably contain both head-to-tail and head-to-head structures with respect to C-C

bond formation of phenyl and bipyridyl moiety. No attempts to introduce strict


139

control on the regioselectivity in synthesis or purification of the various isomers

are carried out during this study.

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3227.

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19. Adam, W.; Roschmann, K. J.; Saha-Moller, C. R. Eur. J. Org. Chem. 2000, 3519-

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21. Jacobsen, H.; Cavallo, L. Chem. Eur. J. 2001, 7, 800-807.

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23. Evans, D. A.; Janey, J. M.; Magomedov, N.; Tedrow, J. S. Angew. Chem. Int. Ed.

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24. Bandini, M.; Cozzi, P. G.; Melchiorre, P.; Morganti, S.; Umani-Ronchi, A. Org.

Lett. 2001, 3, 1153-1155.

25. Belokon, Y. N.; Green, B.; Ikonnokov, N. S.; Larichev, V. S.; Lokshin, B. V.;

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26. Breinbauer, R.; Jacobsen, E. N. Angew. Chem. Int. Ed. 2000, 39, 3604-3607.

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Soc. 1991, 113, 7063.

32. Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T. Tetrahedron Lett. 1991, 32,

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33. Canali, L.; Sherington, D. C. Chem. Soc. Rev. 1999, 28, 85-93.

34. Ganjali, M. R.; Poursaberi, T.; Hosseini, M.; Salavati-Niasary, M.; Yousefi, M.;

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35. Mao, L.; Yamamoto, K.; Zhou, W.; Jin, L. Electroanal. 2000, 12, 72-77.

36. Bhattacharya, S.; Mandal, S. S. Chem. Commun. 1995, 2489-2490.

37. Steullet, V.; Dixon, D. W. Bioorg., Med. Chem. Lett. 1999, 9, 2935-2940.

38. Bella, S. D.; Fragala, I.; Ledoux, I.; Garcia, M. D.; Marks, T. J. J. Am. Chem. Soc.

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41. Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37-52.



141

43. Ucida, Y.; Echikawa, N.; Oae, S. Heteroat. Chem. 1994, 5, 409-413.


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Chapter 5: Experimental Sections

142

Chapter 5

Experimental Sections

"The test of all knowledge is experiment.” – Richard Feynman (1965 Nobel Laureate in Physics)

“There is gold, and a multitude of rubies: but the lips of knowledge are a precious jewel.” - Proverbs 20:15 (Holy Bible)


143

5.1 Materials

All chemicals, reagents, and solvents were used as received from Aldrich Chemical Co.,

Fluka or Merck. All reactions were carried out with dry, freshly distilled solvents under

anhydrous conditions. THF was refluxed over sodium and distilled under nitrogen

atmosphere. HPLC grade toluene, chloroform, DMF, hexanes and MeOH were purchased

from J. T. Baker Company. Hydroquinone, 2,5-dibromopyridine and 2,6-

dibromopyridine were purchased from Fluka and used without further purification.

5.2 Measurements

1H and 13C NMR spectra were recorded using a Bruker AC 300 instrument at 300 MHz

for 1H and 75.47 MHz for 13C respectively. Thermogravimetric analyses (TGA) were

done using TA Instruments SDT 2960 with a heating rate of 10 K/min under nitrogen

atmosphere. Gel permeation chromatography (GPC) was used to obtain the molecular

weight of polymers with reference to polystyrene standards using THF as eluent.

Absorption and emission spectra of polymers were obtained using Hewlett Packard

Diode Array spectrometer and Perkin Elmer LS 50B Luminescence spectrometer,

respectively. IR spectra were recorded using a BIO-RAD FT-IR spectrophotometer. MS

spectra were obtained using Finnigan TSQ 7000 spectrometer with ESI ionization

capabilities. Elemental analyses were performed at the elemental analysis laboratory,

Department of Chemistry, National University of Singapore. X-ray powder patterns were


144

obtained using a D5005 Siemens X-ray diffractometer with Cu-Kα (1.54 Å) radiation (40

kV, 40 mA). Samples were mounted on a sample holder and scanned with a step size of

2θ = 0.01° between 2θ = 1.5° and 35°.


145

5.3 Synthesis of polymers 201a-c

The synthetic scheme for the monomers and the polymers are outlined in Scheme 2-1.

The experimental procedure for compounds containing hexadecyl and octadecyl alkyl

groups is same as compounds with dodecyl alkyl chains. 2,5-dibromohydroquinone was

synthesized according to the literature.1

5.3.1 2,5-Dibromohydroquinone (203)

Bromine (102.50 mL, 2 mol) in 100 mL of glacial acetic acid was added dropwise to a

stirred suspension of hydroquinone (110.00 g, 1 mol) in 1 L of glacial acetic acid at RT.

The temperature is raised to 30-40 0C and a clear solution formed. Stirring was continued

for 3 h. After this, the reaction mixture was keep it outside for 3 h , filtered and the solid

was washed with cold water. The mother liquor was reduced to half volume and allowed

to stand for 12 h to crystallize more products. Repeated volume of reduction and

crystallization gives a third drop. Recrystallization can be done from glacial acetic acid.

The total yield was 80%. (203) 1H NMR (DMSO-d6, ppm): 9.83 (s, 2H), 7.02 (s, 2H). 13C

NMR (DMSO-d6, ppm): 147.34, 119.53, 108.32.

5.3.2 2,5-Dibromo-4-dodecyloxy phenol (204a)

2,5-Dibromohydroquinone 203 (40.2 g, 0.15 mol) was dissolved in a solution of sodium

hydroxide (9.2 g, 0.23 mol) in 1.5 L of abs. ethanol at room temperature under nitrogen

atmosphere. The reaction mixture was carried out at 50 – 60 °C with constant stirring.

The dodecylbromide (36 mL, 0.15 mol) was added dropwise to the above reaction

mixture at 60 °C. After 10 h of stirring under nitrogen atmosphere, the reaction mixture


146

was cooled and the precipitate formed was filtered off and washed with methanol. This

precipitate was identified as dialkylated-2,5-dibromohydroquinone as a side product. The

filtrate was evaporated to remove the solvents. 2 L of distilled water was added to the

residue and the reaction mixture was acidified with 36% HCl, boiled gently for 1 h and

cooled. The resulting precipitate was collected by filtration, washed with water and

dried. The crude product was purified by column chromatography using a mixture of

solvents (CH2Cl2 : hexanes, 4 : 6) to get the pure product in 60% yield. (204a) 1H NMR,

(CDCl3, ppm): 7.25 (s, 1H,), 6.97 (s, 1H), 5.16 (s, 1H), 3.92 (t, J = 6 Hz, 2H), 1.62 (q,

2H), 1.4 (m, 18H); 0.88 (t, J = 6 Hz, 3H). 1H NMR (CDCl3 & D2O, ppm): 7.25 (s, 1H,),

6.97 (s, 1H), 3.92 (t, J = 6 Hz, 2H), 1.80 (q, 2H), 1.4 (m, 18H); 0.87 (t, J = 6 Hz, 3H). 13C

NMR (CDCl3, ppm): 149.95, 146.64, 120.16, 116.49, 112.34, 108.26, 70.25, 31.81,

29.55, 29.47, 29.26, 29.20, 28.97, 25.82, 22.60, 14.04. Elemental analysis calcd. for C18

H28 Br2 O2: C, 49.56; H, 6.47; Br, 36.63. Found: C, 49.17; H, 6.59; Br, 37.31. FT-IR

(KBr, cm-1): 3241, 2911, 2853, 2384, 2337, 1498, 1434, 1386, 1211, 1062, 855, 792, 718.

MS (ESI): m/z: 438, 437, 435, 433.

(204b) 1H NMR (CDCl3, ppm): 7.24 (s, 1H,), 6.97 (s, 1H), 5.14 (s, 1H), 3.92 (t, J = 6 Hz,

2H), 1.80 (q, 2H), 1.4 (m, 26H), 0.86 (t, J = 6 Hz, 3H). 1H NMR (CDCl3 & D2O, ppm):

7.24 (s, 1H,), 6.97 (s, 1H), 3.92 (t, J = 6 Hz, 2H), 1.80 (q, 2H), 1.4 (m, 26H); 0.86 (t, J =

6 Hz, 3H). 13C NMR (CDCl3, ppm): 149.5, 146.66, 120.16, 116.53, 112.39, 108.26,

70.28, 31.84, 29.61, 25.84, 22.60, 14.04. Elemental analysis calcd. for C22 H36 Br2 O2:

C, 53.67; H, 7.37; Br, 32.46. Found: C, 51.93; H, 8.10; Br, 33.89. FT-IR (KBr, cm-1):

3427, 2911, 2841, 2609, 1641, 1503, 1472, 1430, 1385, 1213, 1060, 860, 794, 717. MS

(ESI): m/z: 494, 493, 491, 489.


147

(204c) 1H NMR (CDCl3, ppm): 7.24 (s, 1H,), 6.97 (s, 1H), 5.19 (s, 1H), 3.92 (t, J = 6 Hz,

2H), 1.82 (q, 2H), 1.4 (m, 30H); 0.87 (t, J = 6 Hz, 3H). 1H NMR (CDCl3 & D2O, ppm):

7.24 (s, 1H,), 6.97 (s, 1H), 3.92 (t, J = 6 Hz, 2H), 1.82 (q, 2H), 1.47 (m, 26H); 0.87 (t, J =

6 Hz, 3H). 13C NMR (CDCl3, ppm): 149.96, 146.66, 120.17, 116.53, 112.37, 108.26,



(KBr, cm-1): 3225, 2917, 2848, 2359, 1498, 1466, 1434, 1386, 1211, 1062, 855, 722. MS

(ESI): m/z: 522, 521, 519, 517.

5.3.3 2,5-Dibromo-1-benzyloxy-4-dodecyloxy benzene (205a)

Benzyl bromide (3.8 mL, 0.031 mol) was added dropwise to a stirred solution of 2,5-

dibromo-4-dodecyloxy phenol (204a) (6.95 g, 0.015 mol) and anhyd. K2CO3 (3.28 g,

0.023 mol) in 700 mL of abs. ethanol at 40- 50 °C. After 10 h, the mixture was cooled

and evaporated to remove the solvent. An equal volume of distilled water was added to

the residue and the mixture was stirred for one hour at 0 °C. The resulting precipitate was

collected by filtration, washed with water, and dried under vacuum. Recrystallization

was done from methanol. Yield is typically 95%.

(205a) 1H NMR (CDCl3, ppm): 7.46 (m, 5H), 7.21 (s, 1H), 7.15 (s, 1H), 5.11 (s, 2H),

3.99 (t, J = 6 Hz, 2H), 1.85 (q, 2H), 1.32 (m, 18H), 0.95 (t, J = 6 Hz, 3H). 13C NMR

(CDCl3, ppm): 150.51, 149.49, 136.16, 128.50, 128.10, 127.17, 119.32, 118.31, 111.53,




148

(KBr, cm-1): 2922, 2848, 2359, 1493, 1466, 1355, 1200, 1073, 1004, 855, 802, 754. MS

(ESI): m/z: 528, 526, 453, 451, 425.

(205b) 1H NMR (CDCl3, ppm): 7.45(m, 5H), 7.16 (s, 1H), 7.10 (s, 1H), 5.06 (s, 2H), 3.95

(t, J = 6 Hz, 2H), 1.80 (q, 2H), 1.26 (m, 26H), 0.88 (t, J = 6 Hz, 3H). 13C NMR (CDCl3,

ppm): 150.43, 149.41, 136.13, 128.49, 127.15, 119.32, 118.31, 111.52, 110.00, 71.98,



(KBr, cm-1): 2917, 2848, 1500, 1365, 1216, 1062, 1014, 850, 738. MS (ESI): m/z: 584,

582, 573, 571, 563, 473, 441, 417, 405.

(205c) 1H NMR (CDCl3, ppm): 7.37 (m, 5H), 7.14 (s, 1H), 7.09 (s, 1H), 5.05 (s, 2H),

3.93 (t, J = 6 Hz, 2H), 1.79 (q, 2H), 1.46 (m, 30H), 0.88 (t, J = 6 Hz, 3H). 13C NMR

(CDCl3, ppm): 150.48, 149.46, 136.13, 128.47, 127.15, 119.30, 118.29, 111.52, 110.00,

71.98, 70.17, 31.81, 28.98, 25.82, 22.59, 14.00. Elemental analysis calcd. for C31 H46 Br2

O2: C, 60.99; H, 7.59; Br, 26.18. Found: C, 60.63; H, 7.20; Br, 26.72. FT-IR (KBr, cm-1):

2911, 2848, 2359, 1503, 1466, 1365, 1264, 1222, 1057, 1025, 844, 733. MS (ESI): m/z:

612, 610, 599, 571, 283.

5.3.4 1-Benzyloxy-4-dodecyloxyphenyl-2,5-bisboronic acid (206a)

Dibromide 205a (11.57 g, 0.022 mol) was dissolved in a mixture of diethylether (150

mL) and THF (150 mL). A 1.6 M solution of butyllithium in hexanes (55 mL, 0.088

mol) was added at –78 °C. After warming to RT and cooling again to –78 °C,

triisopropylborate (51 mL) was added within 2 h. After complete addition, the mixture

was warmed to RT and stirred overnight. Water was added and the mixture stirred for 24


149

h. The crystalline mass was recovered by filtration. The product was recrystallized from

acetone in 80% yield.

(206a) 1H NMR (DMSO-d6, ppm): 7.80 (s, 2H), 7.75 (s, 2H), 7.46 (m, 5H), 7.29 (s, 1H),

7.17 (s, 1H), 5.11 (s, 2H), 3.99 (t, J = 6 Hz, 2H), 1.73 (q, 2H), 1.24 (m, 18H), 0.85 (t, J =

6 Hz, 3H). 13C NMR (DMSO-d6, ppm): 157.00, 156.22, 137.16, 128.38, 127.77, 127.52,

118.28, 117.70, 70.05, 68.30, 31.2, 28.89, 25.38, 22.00, 13.87. Elemental analysis calcd.

for C25 H38 B2 O6: C, 65.84; H, 8.34; B, 4.74. Found: C, 66.09; H, 8.37; B, 4.68. FT-IR

(KBr, cm-1): 3496, 3352, 2917, 2848, 2359, 1493, 1413, 1392, 1296, 1200, 1052, 796,

727. MS (ESI): m/z: 456, 455, 454, 453, 437.

(206b) 1H NMR (DMSO-d6, ppm): 7.81 (s, 2H), 7.76 (s, 2H), 7.46 (m, 5H), 7.29 (s, 1H ),

7.16 (s, 1H ), 5.10 (s, 2H), 3.99 (t, J = 6 Hz, 2H), 1.69 (q, 2H), 1.23 (m, 26H), 0.84 (t, J =



H46 B2 O6: C, 68.01; H, 8.99; B, 4.22. Found: C, 67.59; H, 8.98; B, 3.50. FT-IR (KBr,

cm-1): 3492, 3352, 2917, 2848, 2364, 1498, 1429, 1386, 1296, 1195, 1083, 1052, 795,

727. MS (ESI): m/z: 512, 511, 510, 421.

(206c) 1H NMR (DMSO-d6, ppm): 7.82 (s, 2H), 7.76 (s, 2H), 7.47 (m, 5H), 7.29 (s, 1H),

7.17 (s, 1H), 5.11 (s, 2H), 3.99 (t, J = 6 Hz, 2H), 1.73 (q, 2H), 1.23 (m, 30H), 0.83 (t, J =



H50 B2 O6: C, 68.93%; H, 9.26; B, 4.00. Found: C, 68.14; H, 8.75; B, 3.35. FT-IR (KBr,

cm-1): 3448, 3363, 2917, 2853, 2359, 1498, 1429, 1392, 1296, 1195, 1057, 781, 722. MS

(ESI): m/z: 540, 539, 538, 449.


150

5.3.5 1-Benzyloxy-4-dodecyloxy phenyl-2,5-bis(trimethylene boronate) (207a)

Diboronic acid 206a (8.2 g, 0.018 mol) and trimethylene glycol (5.2 mL , 0.072 mol)

were added to toluene (150 mL) at RT. Then the reaction mixture was refluxed for 3h.

After evaporation of the solvent, the residue was dissolved in CHCl3, dried over sodium

sulfate and filtered. The solution was evaporated and the residue was recrystallized from

hexanes. The recrystallized product was used without further purification for

polymerization.

(207a) 1H NMR (CDCl3, ppm): 7.35 (m, 5H), 5.05 (s, 2H), 4.16 (d, 8H), 3.85 (t, J = 6

Hz, 3H), 2.02 (m, 4H), 1.57 (m, 2H), 1.27 (m, 18H), 0.88 (t, J = 6 Hz, 3H). 13C NMR

(CDCl3, ppm): 157.73, 156.92, 138.28, 128.06, 127.00, 120.42, 119.79, 71.70, 69.70,

61.91, 31.81, 29.55, 27.22, 25.98, 22.57, 14.01. FT-IR (KBr, cm-1): 3363, 2917, 2848,

2359, 2337, 1498, 1392, 1296, 1190, 1052, 780, 718. MS (ESI): m/z: 536, 531, 530, 449.

(207b) 1H NMR (CDCl3, ppm): 7.35 (m, 5H), 7.21 (s, 1H), 7.14 (s, 1H), 5.04 (s, 2H),

4.17 (d, 8H), 3.84 (t, J = 6 Hz, 3H), 1.97 (m, 4H), 1.79 (m, 2H), 1.26 (m, 26H), 0.87 (t, J

= 6 Hz, 3H). 13C NMR (CDCl3, ppm): 157.72, 156.91, 138.06, 128.06, 127.02, 120.43,

119.79, 71.73, 69.70, 61.91, 33.95, 31.81, 29.60, 27.22, 25.97, 22.59, 14.01. FT-IR (KBr,

cm-1): 3358, 2911, 2853, 2359, 1493, 1418, 1397, 1296, 1190, 1052, 781, 717. MS (ESI):

m/z: 592, 552, 551, 531, 530.

(207c) 1H NMR (CDCl3, ppm): 7.34 (m, 5H), 7.21 (s, 1H), 7.14 (s, 1H), 5.04 (s, 2H),

4.14 (d, 8H), 3.85 (t, J = 6 Hz, 3H), 2.01 (m, 4H), 1.81 (m, 2H), 1.25 (m, 30H), 0.87 (t, J

= 6 Hz, 3H). 13C NMR (CDCl3, ppm): 157.72, 156.91, 138.27, 128.06, 127.02, 120.42,

119.79, 71.70, 69.62, 62.08, 33.97, 31.81, 29.60, 27.22, 25.98, 22.59, 14.01. FT-IR (KBr,


151

cm-1): 3358, 2911, 2848, 2364, 1503, 1413, 1397, 1291, 1195, 1052, 782, 727. MS (ESI):

m/z: 620, 595, 565, 530.

5.3.6 Poly(1-benzyloxy-4-dodecyloxy-p-phenylene) (208a)

Diboronic ester 207a (4.29 g, 8.02 mmol) and dibromocompound 205a (4.21 g, 8.02

mmol) were added to dry toluene (22 mL). The solution was degassed and flushed with

nitrogen repeatedly. 2M Na2CO3 (70 mL) was added to this followed by palladium

catalyst tetrakis(triphenylphosphino)palladium (1.5 mol % with respect to monomer

205a). The mixture was then heated to 80 °C for 48 h with vigorous stirring. The reaction

mixture was precipitated twice from methanol to yield a yellowish polymer, which was

recovered by filtration and dried in an oven. The yield was 5 g.

(208a) 1H NMR (CDCl3, ppm): 7.28(b), 4.97 (b), 3.91 (b), 1.57 (b), 1.27 (b), 0.91 (b). 13C

NMR (CDCl3, ppm): 150.57, 149.73, 137.79, 128.07, 127.03, 118.06, 116.89, 71.62,

69.41, 31.83, 29.59, 22.59, 14.01. FT-IR (KBr, cm-1): 2916, 2858, 2367, 1413, 1117,

1114, 727, 715.

(208b) 1H NMR (CDCl3, ppm): 7.26(b), 4.94 (b), 3.87 (b), 1.5 (b), 1.27(b), 0.89 (b). 13C

NMR (CDCl3, ppm): 150.72, 149.70, 137.79, 128.07, 127.03, 118.05, 116.86, 71.62,

69.32, 31.83, 29.63, 26.01, 22.59, 14.01. FT-IR (KBr, cm-1): 2922, 2852, 2362, 1453,

1200, 726, 694.

(208c) 1H NMR (CDCl3, ppm): 7.19 (b), 7.05 (b), 6.9 (b), 4.94 (b), 3.85 (b), 1.5 (b),

1.25(b), 0.87 (b). 13C NMR (CDCl3, ppm): 149.70, 148.77, 134.77, 128.17, 127.56,

126.99, 119.35, 71.80, 69.96, 31.83, 29.62, 25.99, 22.59, 14.01. FT-IR (KBr, cm-1): 2911,

2846, 2362, 1469, 1200, 1017, 726, 688.


152

5.3.7 Poly(1-hydroxy-4-dodecyloxy-p-phenylene) (201a)

Precursor polymer 208a (1.32g) was dissolved in a mixture of dry THF (50 mL) and abs.

ethanol (50 mL) at RT. 10 % Pd/C (3 g) was added to the above solution. The mixture

was flushed with nitrogen gas three times. Three drops of conc. HCl were added to

enhance the debenzylation. The reaction was carried out at RT under positive pressure of

hydrogen for 24 h with constant stirring. The reaction mixture was filtered through celite

powder and the precipitate was washed with abs. ethanol. The filtrate was evaporated and

dried to yield the desired polymer (0.8 g).

(201a) 1H NMR (CDCl3, ppm): 7.04 (b), 6.88 (b), 3.90 (b), 1.77 (b), 1.22 (b), 0.85 (b).

FT-IR (KBr, cm-1): 3416, 2922, 2848, 2359, 1647, 1466, 1200, 1025, 802. (201b) 1H

NMR (CDCl3, ppm): 7.04 (b), 6.87 (b), 3.90 (b), 1.77 (b), 1.23 (b), 0.87 (b). FT-IR (KBr,

cm-1): 3379, 2914, 2848, 2359, 1615, 1466, 1206, 1052, 807, 722. (201c) 1H NMR

(CDCl3, ppm): 7.03 (b), 6.89 (b), 3.91 (b), 1.75 (b), 1.22 (b), 0.86 (b). FT-IR (KBr, cm-1):

3397, 2916, 2848, 1625, 1469, 1406, 1200, 1054, 796, 720.


153

5.4 Synthesis of polymers 301-306

The synthetic scheme for the monomers and the polymers are illustrated in Scheme 3-1 &

3-2. Monomer 311 was synthesized by methods described previously.2 The experimental

procedure for 303 and 306 was analogous to the one described for 301 and 304.

5.4.1 2,5-Dibromo-1, 4-dibenzyloxy benzene (312)

Benzyl bromide (28.03 mL, 0.236 mol) was added dropwise to a stirred solution of 2,5-

dibromohydroquinone (309) (31.62 g, 0.118 mol) and anhydrous K2CO3 (48.92 g, 0.472

mol) in absolute ethanol (800 mL) at 40 °C. After 10 h, the mixture was cooled and

evaporated to remove the solvent. An adequate amount of distilled water was added to

the residue and the mixture was stirred for one hour at RT. The resulting precipitate was

collected by filtration, washed with water and dried. Recrystallization was done from

methanol. Yield is 95%. 1H NMR (CDCl3, ppm): 7.44 (m, 10H), 7.17 (s, 2H), 5.07 (s,

4H). 13C NMR (CDCl3, ppm): 149.96, 136.04, 128.5, 127.13, 119.19, 111.45, 71.89.

Elemental analysis calcd. for C20 H16 Br2 O2: C, 53.60; H, 3.60; Br, 35.66. Found: C,

53.39; H, 3.61; Br, 35.44. FT-IR (KBr, cm-1): 3063, 3036, 1491, 1450, 1387, 1363, 1224,

1208, 1066, 1010, 912, 856, 844. MS (ESI): m/z: 489, 488, 487, 469, 437, 432.

5.4.2 1,4-Dibenzyloxy-2,5-bisboronic acid (313)

Dibromide 312 (14.78 g, 0.033 mol) was dissolved in THF (300 mL) and a 1.6 M

solution of butyllithium in hexanes (82.50 mL, 0.132 mol) was added at –78 °C. After

warming to RT and cooling again to –78 °C, triisopropylborate (75.87 mL, 0.33 mol) was

added within 2 h. After complete addition, the mixture was warmed to RT and stirred


154

overnight. Water was added and the mixture stirred for 24 h. The crystalline mass was

filtered and recrystallized from acetone in 60% yield. 1H NMR (DMSO-d6, ppm): 7.31 (s,

8H), 7.21 (m, 10H), 5.03 (s, 4H). 13C NMR (DMSO-d6, ppm): 156.44, 137.17, 128.26,

126.80, 118.17, 70.06. Elemental Analysis Calcd. for C20 H20 B2 O6: C, 63.53; H, 5.33; B,

5.72. Found: C, 65.53; H, 5.96; B, 5.02. FT-IR (KBr, cm-1): 3365, 3031, 2929, 2864,

1496, 1195, 1050, 860, 749. MS (ESI): m/z: 377, 376, 347, 332.

5.4.3 Synthesis of Polymer 304

Diboronic acid 311 (5.19 g, 11.40 mmol) and 2,5-dibromopyridine (3.58 g, 11.40 mmol)

were dissolved in freshly distilled tetrahydrofuran (100 mL). The solution was degassed

and flushed with nitrogen repeatedly. After the addition of a solution of Na2CO3 (2M, 100

mL) and catalyst tetrakish(triphenylphosphino)palladium (3.0 mol % with respect to

monomer), the reaction mixture was stirred for 72 h at reflux temperature, poured into

methanol and the yellowish polymer precipitate was recovered by filtration. The obtained

yield was 5.0 g. (304) 1H NMR (CDCl3, ppm): 8.99 (1H, s), 8.14-7.64 (3H, m), 7.36 (5H,

b), 7.17 (1H, s), 5.26 (2H, m), 4.10 (2H, b), 1.79 (2H, b), 1.24 (18H, b), 0.86 (3H, b). 13C

NMR (CDCl3, ppm): 155.0, 150.31, 138.06, 136.59, 131.81, 128.47, 127.29, 116.18,

115.56, 71.63, 69.38, 31.80, 29.53, 26.08, 22.57, 13.99. Elemental Analysis Calcd. for

(C30 H37 N O2)n: C, 81.27; H, 8.34; N, 3.16; Br, 0; Calcd. for Br-(C30 H37 N O2)8-Br: C,

77.76; H, 7.98; N, 3.02; Br, 4.31. Found: C, 76.47; H, 7.64; N, 3.39. FT-IR (KBr, cm-1):

2922, 2852, 1590, 1506, 1456, 1231, 1196, 1026, 842. (306) 1H NMR (CDCl3, ppm):

7.99 (2H, b), 7.76 (1H, b), 7.26 (7H, b), 5.19 (2H, b), 4.11 (2H, b), 1.75 (2H, b), 1.22

(18H, b), 0.85 (3H, b). 13C NMR (CDCl3, ppm): 156.20, 153.78, 151.45, 150.64, 137.90,


155

136.88, 128.33, 127.46, 126.03, 124.01, 115.94, 71.69, 69.44, 31.81, 29.55, 26.21, 22.59,

14.01. Elemental Analysis Calcd. for (C30 H37 N O2)n: C, 81.27; H, 8.34; N, 3.16, Br, 0;

Calcd. for Br-(C30 H37 N O2)6-Br: C, 76.66; H, 7.87; N, 2.97; Br, 5.67. Found: C, 75.67;

H, 7.56; N, 3.30. FT-IR (KBr, cm-1): 2927, 2852, 1572, 1426, 1383, 1130.


Precursor polymer 304 (3.00g) was dissolved in a mixture of dry THF (200 mL) and

absolute ethanol (50 mL) at RT. 10 % Pd/C (9.00 g) was added to the above solution.

Three drops of conc. HCl was added to enhance the debenzylation. The mixture was

flushed with nitrogen gas. The flask was fitted with a hydrogen gas balloon and the

mixture was stirred at RT for 24 h. The reaction mixture was filtered through celite and

the excess solvent was removed under reduced pressure. The obtained precipitate was

washed with abs. ethanol. The filtrate was evaporated and dried under vacuum. The yield

was 1.00 g. (301) 1H NMR (CDCl3, ppm): 7.80-7.46 (m, all aromatic H), 3.98 and 3.63

(b, OCH2), 1.75 (b, CH2), 1.26 (b, CH2), 0.87 (b, CH3). Elemental Analysis Calcd. for

(C23 H31 N O2)n: C, 78.20; H, 8.77; N, 3.96; Br, 0; Calcd. for Br-(C23 H31 N O2)8-Br: C,

74.01; H, 8.30; N, 3.75; Br, 5.35. Found: C, 74.82; H, 8.04; N, 3.02. FT-IR (KBr, cm-1):

3429, 2924, 2853, 1465, 1200, 722. (303) 1H NMR (CDCl3, ppm): 7.84-7.31 (m, all

aromatic H), 4.06-3.63 (b, OCH2), 1.78 (b, CH2), 1.25 (b, CH2), 0.85 (b, CH3). FT-IR

(KBr, cm-1): 3426, 2924, 2846, 1610, 1452, 1204, 1066, 871, 731.


156


Diboronic acid 313 (4.91 g, 13.00 mmol) and 2,5-dibromopyridine (3.07 g, 13.00 mmol)

were dissolved in toluene (90 mL). The solution was degassed and flushed with nitrogen

repeatedly. After the addition of a solution of K2CO3 (2M, 45 mL) and the catalyst

tetrakish(triphenylphosphino)palladium (3 mol % with respect to monomer), the reaction

mixture was stirred for 72 h under reflux temperature, poured into methanol and filtered.

The yield was 3.27 g. 1H NMR (CDCl3, ppm): 8.93 (1H, s), 8.12-7.78 (2H, m), 7.32-7.10

(12H, b), 5.12 (4H, m). Elemental Analysis Calcd. for (C25 H19 N O2)n: C, 82.20; H, 5.20;

N, 3.83; Calcd. for C25 H19 N O2. 1.2CHCl3: C, 61.88; H, 3.97; N, 2.97. Found: C, 62.26;

H, 3.99; N, 2.91. FT-IR (KBr, cm-1): 2868, 1453, 1222, 1195, 1001, 839, 726, 694.


Precursor polymer 305 (3.00 g) was dissolved in a mixture of dry THF (300 mL) and

methanol (50 mL) at RT. 10 % Pd/C (10.00 g) was added to the above solution. The

mixture was flushed with nitrogen gas. 1 mL of conc. HCl was added to enhance the

debenzylation. The reaction was carried out at 50 °C for 48 h under hydrogen

atmosphere. The reaction mixture was cooled to RT and filtered through celite powder.

The precipitate was washed with abs. ethanol. The filtrate was evaporated and dried. The

yield was 1.00 g. 1H NMR (DMSO-D6, ppm): 8.75 (b, 1H), 8.25 (b, 2H), 7.65-7.57 (b,

2H). FT-IR (KBr, cm-1): 3087, 2688, 1619, 1605, 1432, 1275, 1228, 885, 779.


157

5.5 References

1. Tietze, L. F. Reactions and Syntheses in the Organic Chemistry Laboratory.

University Science: Mill Valley, California, 1989, pp 253.


Chapter 6: Conclusions and suggestions for the future work

158

Chapter 6

Conclusions and Suggestions for the future work

“The future belongs to those who prepare for it.” - Ralph Waldo Emerson (1803-1882)

“There is a time for everything, and a season for every activity……” – Ecclesiastes 3:1 RSV (Holy Bible)


159

6.1 Conclusions

A series of new amphiphilic conjugated polymers containing free hydroxyl groups

and hydrogen bond acceptor groups such as nitrogen atoms on polymer back bone

capable of forming an inter/intra molecular hydrogen bonding have been successfully

synthesized with good yields by using Suzuki coupling reaction. All the derived polymers

showed good solubility in common organic solvents such as chloroform, toluene, THF,

DMF and formic acid. The emission color could be tuned by introducing different linked

polymer backbones and by using different solvents and metal ions. All the derived

polymers showed that they had good thermal stability in both air and nitrogen. Based on

their characterization results obtained in previous chapters, some main conclusions are

summarized here:

The introduction of long alkoxy chain improved the solubility of all polymers.

The emission properties could easily be fine tuned by using different solvents,

base and metal ions

Incorporation of heterocyclic compounds, namely pyridine and bipyridine on the

polymer backbone has tremendous changes in the optical properties.

Pyridine- and bipyridine- incorporated conjugated polymers gave positive

solvatochromism in solvents of varying polarity

Most of the reported pyridine-incorporated conjugated polymers are soluble only

in formic acid but our synthesized polymers are soluble in all solvents and easy to

process for further applications.


160

Metal chelating effect of all derived polymers induced significant changes in

emission properties and could be used for sensing metal ions

The optical tunability would allow such derived polymers as good candidates for

fabricating polymeric light emitting diode (PLED) devices

Presence of free hydroxyl groups (phenolic) on the polymer backbone is expected

to show interesting electrochemical properties and self-assembly at the liquid-

metal interface

The observed ICT, ESIPT and MLCT effects on polymers suggest that they are

very promising materials for many potential applications such as LEDs, NLO,

chemical sensors and catalytic studies.

6.2 Suggestions for the future work

6.2.1 Applications of new amphiphilic conjugated polymers

Due to good solubility, thermal stability, and the characterization of these

polymers suggested that they were promising candidates for potential application in

materials science. Their applications in PLED, NLO, langmuir-blodgett films, chemical

and biosensors, and catalytic studies could be investigated.

6.2.2 Design of new polymer structures: Evolution of hydroxylated polyphenylenes

(HPPs)

The new conjugated polymers with free hydroxyl groups, namely hydroxylated

polyphyenylnes (HPPs) could be extended by incorporating different heterocyclic

compounds on the polymers backbone. The evolution of HPP is illustrated in Figure 6-1.


161

By manipulating the functional groups and/or side groups on the polymer backbone with

the hydroxyl groups, these polymers can be used for all the applications in Light emitting

diodes, electroluminescent devices, photoconductors, field effect transistors, solar cells,

fuel cells, hydrogen storage, rechargeable batteries, lasers, inkjet-printing, xerographic

imaging photo receptors, piezoelectric and pyroelectric materials, optical data storage,

optical switching and signal processing, molecular electronics, spintronics, nonlinear

optical properties, optical power limiting, MEMS and BioMEMS, actuators, membrane

based separations, transparent antistatic coating, scintillators, catalysis, chemical and

biosensors, molecular wires, nanoscience and nanotechnology, and biomedical

applications.

Figure 6-1. Evolution of hydroxylated polyphenylenes (HPPs)

Inside the square: Classical structure of HPP backbone and types of CP; Outside the

square: Possible applications of HPPs

List of Publications

162


“Let us learn to dream, gentlemen: then perhaps we will find the truth. But let us beware of

publishing our dreams until they have been tested by the waking understanding.”

– August Kekule von Stradonitz (1829-1896)

“In our science endeavor, the thrill of discovery is the real fuel for taking off but the flight becomes satisfactory and

enjoyable when recognition by peers, perhaps the most significant reward, becomes evident.”

– Ahmed Zewail (1999 Nobel Laureate in Chemistry)


163

Recent Publications

1. Ji, W.; Elim, H. I.; He, J. F.; Fitrilawati, F.; Baskar, C.; Valiyaveettil, S.; Knoll,

W. Photo-physical and nonlinear-optical properties of new polymer: hydroxylated

pyridyl para-phenylene. J. Phys. Chem. B 2003, 107(40), 11043-11047.

2. Ravindranath, R.; Valiyaveettil, S.; Baskar, C.; Putra, A.; Fitrilawati, F.; Knoll,

W. Design and Characterization of Nanoarchitectures from Multifunctional

Polyparaphenylenes. Mat. Res. Soc. Symp. Proc. 2003, 776, Q11.5.1-Q11.5.5.

3. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Synthesis of a Novel Optically Tunable

Amphiphilic Poly(p-phenylenes): Influence of Hydrogen Bonding and Metal

Complexation on Optical Properties. Macromolecules 2001, 34(18), 6255-6260.

4. Valiyaveettil, S.; Baskar, C. A Novel class of polyphenylenes: Synthesis and

Characterization. Polym. Mater. Sci. Eng. 2001, 84, 1079-1080.

5. Valiyaveettil, S.; Baskar, C.; Wenmiao, S. A Novel blue light emitting

polyhydroxy polyparaphenylenes. Polym. Prepr. 2001, 42(1), 432-433.

Unpublished Papers

1. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Synthesis and Optical Tuning of Pyridine

Incorporated Amphiphilic Conjugated Polymers with Donor-Acceptor

Architectures.

2. Baskar, C.; Valiyaveettil, S. Evolution of Amphiphilic Hydroxylated

Polyphenylenes.


164

International Conference Papers

1. Fitrilawati, F.; Baskar, C.; Elim, H. I.; Ji, W.; Valiyaveettil, S.; Knoll, W. Optical

properties of hydroxylated pyridyl PPP. International Symposium on Modern

Optics and Its Applications (IS-MOA 2002) Indonesia, July 3-5, 2002.

2. Valiyaveettil, S.; Arockiam, J.; Baskar, C.; Lee, H. K. Multifunctional Polymers

for Nano- and Microsensor Applications. NUS-JSPS Joint Symposium on

Analytical Sciences: Challenges of the New Century, NUS, Singapore February

28 – March 1, 2002.

3. Baskar, C.; Valiyaveettil, S. Synthesis and fine-tuning the emission properties of

novel conducting polymers. Singapore International Conference-2: Frontiers in

Chemical Design and Synthesis, December 18 - 20, 2001. Singapore, Abstract

No. 245.

4. Min, T. W.; Baskar, C.; Valiyaveettil, S. Synthesis and characterization of a

novel nitrogen containing heteroaromatic rings incorporated poly(p-phenylenes).

Singapore International Conference-2: Frontiers in Chemical Design and

Synthesis, December 18 - 20, 2001. Singapore, Abstract No. 331.

5. Mien, T. H.; Baskar, C.; Valiyaveettil, S. Design and synthesis of novel

conjugated polymers as possible molecular wires. Singapore International

Conference-2: Frontiers in Chemical Design and Synthesis, December 18 - 20,

2001. Singapore, Abstract No. 335.

6. Valiyaveettil, S.; Baskar, C. Novel amphiphilic conducting polymers: Use of

backbone functionalization and self-assembly to finetune the structure-property

relationship. 2001 MRS Fall Meeting symposium, November 26-30, 2001,


165

Boston, Massachusetts. Division of Organic Optoelctronic Materials, Processing,

and Devices, Abs. No. BB10.14.

7. Valiyaveettil, S.; Baskar, C.; Wenmiao, S. Synthesis and characterization of

multifunctional oligo- and polypararphenylenes as building blocks for novel

materials. Abstracts of Papers of the American Chemical Society 2001, 222, 108-

IEC.

8. Valiyaveettil, S.; Baskar, C. Novel class of polyphenylenes: Synthesis and

characterization. Abstracts of Papers of the American Chemical Society 2001,

221, 595-PMSE.

9. Valiyaveettil, S.; Baskar, C.; Wenmiao, S. Novel blue light emitting polyhydroxy

polyparaphenylenes. Abstracts of Papers of the American Chemical Society 2001,

221, 6-POLY.

10. Valiyaveettil, S.; Chinnappan, B. Amphiphilic polyparaphenylenes: Novel

building blocks for multifunctional materials. The International Chemical

Congress of Pacific Basin Societies, Pacifichem 2000. Macromolecular Chemistry

Session, Abs. No. 0072.


166

International Conference Presentations

1. Ji, W.; Elim, H. I.; Fitrilawati, F.; Baskar, C.; Valiyaveettil, S. High third-order

nonlinear-optical susceptibilities in a new amphiphilic conjugated polymer

measured with Z-scan technique. International Conference on Materials for

Advanced Technologies (ICMAT 2003), Singapore, December 7-12, 2003. Oral

presentation (Invited).

2. Fitrilawati, F.; Baskar, C.; Elim, H. I.; Ji, W.; Valiyaveettil, S.; Knoll, W. Optical

properties of hydroxylated pyridyl PPP. International Symposium on Modern

Optics and Its Applications (IS-MOA 2002) Indonesia, July 3-5, 2002.

3. Baskar, C.; Valiyaveettil, S. Synthesis and fine-tuning the emission properties of

novel conducting polymers. Singapore International Conference-2: Frontiers in

Chemical Design and Synthesis, December 18 - 20, 2001. Singapore, Poster

Presentation.

4. Min, T. W.; Baskar, C.; Valiyaveettil, S. Synthesis and characterization of a

novel nitrogen containing heteroaromatic rings incorporated poly(p-phenylenes).

Singapore International Conference-2: Frontiers in Chemical Design and

Synthesis, December 18 - 20, 2001. Singapore, Poster Presentation.

5. Mien, T. H.; Baskar, C.; Valiyaveettil, S. Design and synthesis of novel

conjugated polymers as possible molecular wires. Singapore International

Conference-2: Frontiers in Chemical Design and Synthesis, December 18 - 20,

2001. Singapore, Poster Presentation.

6. Valiyaveettil, S.; Baskar, C. Novel amphiphilic conducting polymers: Use of

backbone functionalization and self-assembly to finetune the structure-property


167

relationship. 2001 MRS Fall Meeting symposium, November 26-30, 2001.

Boston, Massachusetts. Division of Organic Optoelctronic Materials, Processing,

and Devices, Poster Presentation.

7. Baskar, C.; Valiyaveettil, S. 221st ACS National Meeting April 1-5, 2001,

SanDiego, CA, USA. Oral Presentation, Polymeric Materials: Science &

Engineering Division, April 05, 2001.


SanDiego, CA, USA. Oral Presentation, Polymer Division, April 01, 2001.


SanDiego, CA, USA. Poster Presentation (Invited), Polymeric Materials: Science

& Engineering Division, April 02, 2001.

10. Valiyaveettil, S.; Chinnappan, B. Amphiphilic polyparaphenylenes: Novel

building blocks for multifunctional materials. The International Chemical

Congress of Pacific Basin Societies, Pacifichem 2000. Macromolecular Chemistry

Session.

International Workshop

1. Baskar, C. Indo-US Science and Technology Forum Workshop on Green

Chemistry, University of Delhi, New Delhi, India November 17-20, 2003.

(Advisory Committee)

2. Baskar, C. CHEMRAWN XIV Conference and the Green Chemistry Pre-

Conference Workshop, University of Colorado, Boulder, Colorado, USA, June 6-

14, 2001. (Invited)


168

National Publications

1. Mien, T. H.; Baskar, C.; Valiyaveettil, S. Synthesis and Manipulation of a novel

conjugated polymers towards a new direction. Applied Chemistry Honors Year

Project Report, NUS. 2001/2002.

2. Min, T. W.; Baskar, C.; Valiyaveettil, S. Synthesis and Characterization of a

Novel Pyridine-Based Conjugated Polymer. Applied Chemistry Honors Year

Project Report, NUS. 2001/2002.

3. Baskar, C. Evolution of Polyhydroxy polyparaphenylenes in Chemical and

Biosensors. Advanced Polymeric Materials Symposium (Course Work

Assignment), NUS. September 26, 2001.

4. Fransiska, C. K.; Baskar, C.; Valiyaveettil, S. A Novel Pyridyl-based

Oiligomers: Synthesis and Characterization. Advanced Undergraduate Research

Opportunity Programme in Science, NUS. September 2001.

5. Fung, N. W.; Baskar, C.; Valiyaveettil, S. A Novel Class of Polyphenylenes:

Synthesis and characterization. Advanced Undergraduate Research Opportunity

Programme in Science, Department of Chemistry, NUS. June 2001.

6. Kiat, C. C.; Baskar, C.; Valiyaveettil, S. Synthesis and characterization of a

novel blue emitting polyphenylenes and its oligomer. Advanced Undergraduate

Research Opportunity Programme in Science, Department of Chemistry, NUS.

June 2001.

7. Mei, Y. C.; Baskar, C.; Valiyaveettil, S. Synthesis and characterization of a novel

Polymer. Advanced Undergraduate Research Opportunity Programme in Science,

Department of Chemistry, National University of Singapore. June 2001.


169

8. Baskar, C.; Valiyaveettil, S.; Eng, C.H.; Vincent, H.H.B.; Pin, L.W. Syntheis of

Multifunctional Monomers for Conducting Polymers. Hwa Chong Junior

College-National University of Singapore 3rd Chemistry Mentornet Symposium

August 14, 1999 pp 9-20.

Magazine

1. Baskar, C. The Graduate Reminisces 2002, A Publication of Science Graduate

Committee, Faculty of Science, National University of Singapore. (Chief Editor)

National Presentations

1. Baskar, C. State of Science Graduate Committee 2001-2002. Presented for

Welcome Tea for All New Graduate Students, Faculty of Science, National

University of Singapore, Singapore. October 1, 2002.

2. Fitrilawati, F.; Baskar, C.; Renu, R.; Valiyaveettil, S.; Tamada, K.; Knoll, W.

Organized films from asymmetrically substituted poly(paraphenylene)s. Temasek

Professorship Programme, Department of Materials science and Department of

Chemistry, National University of Singapore. (Poster Presentation) July 2002.

3. Baskar, C. Evolution of Polyhydroxy polyparaphenylenes in Chemical and

Biosensors. Advanced Polymeric Materials Symposium (Course Work Seminar),

National University of Singapore, Singapore. October 23, 2001.


170

4. Baskar, C. As I Remember: Science, Religion, and Philosophy. Presented for

Pursuit of Higher Degrees in the Faculty of Science, National University of

Singapore, Singapore. September 12, 2001.

5. Baskar, C. Welcome Address. Presented for Welcome Tea for All New Graduate

Students, National University of Singapore, Singapore. August 3, 2001.

6. Baskar, C.; Arockiam, J. Aspartate Proteases and HIV. Course Work Seminar,

Department of Chemistry, National University of Singapore, Singapore. April 11,

2001.

7. Baskar, C. Biosensors: Enzyme Sensors for Environmental Analysis. Course

Work Seminar, Department of Chemistry, National University of Singapore,

Singapore. March 21, 2001.

8. Baskar, C. Synthesis and Characterization studies of Novel Conjugated

Polymers. PhD Project Proposal Seminar, Department of Chemistry, National

University of Singapore, Singapore. October 13, 2000.

Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers

171

Appendix

“The progress of science today is not so much determined by brilliant achievements of

individual workers, but rather by the planned collaboration of many observers.”

– Emil Fischer (1902 Nobel Laureate in Chemistry)

“Success is knowing that you have done your best and have exploited your God-given or gene-given abilities to the next

maximum extent. More than this, no one can do."

– Alan G. MacDiarmid (2000 Nobel Laureate in Chemistry)


172

Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymersa

Conjugated Polymers

Absorption maxima (nm) References

n

λmax = 336 nm 1,2

OR

n

R = C8H17 λmax = 336 nm

R = C12H25 λmax = 334 nm

R = C16H33 λmax = 334 nm

3-5

R

R

n

R = C6H13 λmax = 247 nm (in cyclohexane)

R = C6H13 λmax = 300 nm

2

6

OR

RO

n

R = H λmax = 345 nm (in DMF)

R = C4H9 λmax = 336 nm (in CH2Cl2)



R = λmax = 335 nm (in CH2Cl2)

R =

λmax = 331 nm (in CHCl3)

7

6

6

6

8

9

aExamples given here based on the derived polymers mentioned on the thesis


173

Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymers

(Continued)

Conjugated Polymers

Absorption maximum (nm) References

N n

λmax = 373 nm (in HCOOH)

λmax = 360 nm [in (CF3)2CHOH]

10-12

N

CH3

n


12

N

H3C

n


12

N

H3C

n



λmax = 323 nm (in THF)

λmax = 321 nm (in benzene)

12, 13


174


(Continued)

Conjugated Polymers


N n

λmax = 382 nm (in HCOOH) 14

N N n

λmax = 366 nm (in HCOOH) 14

N n

C6H13


15

n

C6H13

N

OMe

MeO


15

nN

OC8H17

H17C8O


16-19


175


(Continued)

Conjugated Polymers


N N n


λmax = 380 nm

20,21

N N

H3C CH3

n


22

N N

C6H13 C6H13

n


λmax = 320 nm (in CH2Cl2)

22

N

N

C6H13

C6H13

n


23

OC12H25

n

λmax = 313 nm (in THF)

24,25


176

References

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Applications; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M., Gresser,

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178

Figure A-1. TG curve of 301; heating rate: 10 K/min under nitrogen


179



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181



182



183



184



185



186


187

Concluding Quotations

“Science serves humanity only when it is joined to conscience” - Pope John Paul II

“This is not the end. It is not even the beginning of the end. But it is, perhaps, the

end of the beginning.” – Sir Winston Churchill

"With the spirit of love, dedication, will power, creativity, and hard work; everything is possible in the

world.” – BaSKAr, C.

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