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
Dedicated to my beloved parents
i
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)
ii 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).
iii 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),
iv 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.
v 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
vi Table of Contents
Dedication i
Acknowledgements iii
Table of Contents vii
Summary xii
List of Monomers and Polymers Synthesized in this Thesis xvi
List of Figures xxi
List of Schemes xxiii
List of Tables xxiv
Glossary of Abbreviations and Symbols xxv
Opening Quotations xxxii
Chapter 1 Introduction: The Art and Science of Conjugated Polymers 1
1.1 Prologue 2
1.2 Genesis of Conjugated Polymers 6
1.3 A Case History of Poly(p-phenylene)s PPPs 13
1.4 Pyridine incorporated conjugated polymers 23
1.5 Bipyridine incorporated conjugated polymers 28
vii 1.6 Poly(m-phenylene)s (PMPs) 33
1.7 Aim of the project 37
1.8 References 38
Chapter 2 Amphiphilic Poly(p-phenylene)s 75
2.1 Introduction 76
2.2 Synthesis of polymers 77
2.3 Characterization of polymers 79
2.4 Optical and ionochromic properties of polymers 81
2.5 Conclusions 89
2.6 References 90
Chapter 3 Pyridine Incorporated Amphiphilic 94 Conjugated Polymers
3.1 Introduction 95
3.2 Synthesis of polymers 98
3.3 Characterization of polymers 101
3.4 Optical Properties 103
3.4.1 Influence of hydroxyl groups 103
viii 3.4.2 Comparison of properties of polymers 107
3.4.3 Solvatochromic behavior of polymers 107
3.4.4 Effect of protonation and deprotonation of polymers 110
3.4.5 Influence of base 113
3.4.6 Metal complexation of polymers 115
3.5 Conclusions 117
3.6 References 118
Chapter 4 Bipyridine Incorporated Conjugated 125 Polymers
4.1 Introduction 126
4.2 Synthesis of polymers 129
4.3 Characterization of polymers 132
4.4 Optical properties of polymers 133
4.5 Solvatochromic behavior of polymers 134
4.6 Ionochromic effects of polymers 134
4.7 Conclusions 137
4.8 References 138
ix Chapter 5 Experimental Section 142
5.1 Materials 143
5.2 Measurements 143
5.3 Synthesis of polymers 201a-c 145
5.3.1 2,5-Dibromohydroquinone (203) 145
5.3.2 2,5-Dibromo-4-dodecyloxy phenol (204a) 145
5.3.3 2,5-Dibromo-1-benzyloxy-4-dodecyloxy benzene (205a) 147
5.3.4 1-Benzyloxy-4-dodecyloxyphenyl-2,5-bisboronic acid 148
(206a)
5.3.5 1-Benzyloxy-4-dodecyloxy phenyl-2,5-bis(trimethylene 150
boronate) (207a)
5.3.6 Poly(1-benzyloxy-4-dodecyloxy-p-phenylene) (208a) 151
5.3.7 Poly(1-hydroxy-4-dodecyloxy-p-phenylene) (201a) 152
5.4 Synthesis of polymers 301-306 153
5.4.1 2,5-Dibromo-1, 4-dibenzyloxy benzene (312) 153
5.4.2 1,4-Dibenzyloxy-2,5-bisboronic acid (313) 153
5.4.3 Synthesis of Polymer 304 154
5.4.4 Synthesis of Polymer 301 155
5.4.5 Synthesis of Polymer 305 156
5.4.6 Synthesis of Polymer 302 156
5.5 References 157
x Chapter 6 Conclusions and Suggestions for the 158 future work
6.1 Conclusions 159
6.2 Suggestions for the future work 160
6.2.1 Applications of new amphiphilic conjugated polymers 160
6.2.2 Design of new polymer structures: Evolution of 160
hydroxylated polyphenylenes (HPPs)
List of Publications 162
Recent Publications 163
Unpublished Papers 163
International Conference Papers 164
International Conference Presentations 166
National Publications 168
National Presentations 169
Appendix 171
Absorption maxima of non-hydroxyl-containing 172
conjugated polymers
TG curves of 301-403 178
xi 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.
xii 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
xiii 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
xiv 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.
xv List of Monomers and Polymers Synthesized in this Thesis
Table 1. The polymers and main compounds prepared in this thesis
Chapter Main monomers (compounds) Polymers
No.
OBn OBn OBn OH O O Br Br B B n Chapter 2 O O n RO RO RO RO R = CH3( CH2)11 R = CH3( CH2)11 R = CH ( CH ) 3 2 11 R = CH3( CH2)11 R = CH3( CH2)15 R = CH3( CH2)15 R = CH ( CH ) 3 2 15 R = CH3( CH2)15 R = CH3( CH2)17 R = CH3( CH2)17 R = CH ( CH ) 3 2 17 R = CH3( CH2)17 Bn = C6H5CH2 Bn = C6H5CH2 Bn = C6H5CH2 201a-c 205a-c 207a-c 208a-c
xvi Table 1. The polymers and main compounds prepared in this thesis (Continued)
Chapter Main monomers (compounds) Polymers
No.
O B n O H O H N N
( HO ) 2B B(OH)2 n n Chapter 302 R O 311 R = CH3( CH2)11 RO 301 HO 3 Bn = C6H5CH2 OBn OBn O B n N N
( HO ) B B( O H) 2 2 n n RO 304 BnO 305 B n O 313 R=CH3( CH2)11 Bn=C6H5CH2
xvii Table 1. The polymers and main compounds prepared in this thesis (Continued)
Chapter Main monomers (compounds) Polymers
No.
RO OR N Chapter 3 H O O H n 303 R=CH3( CH2)11 Bn=C6H5CH2
RO OR N
OBn O Bn n 306
xviii Table 1. The polymers and main compounds prepared in this thesis (Continued)
Chapter Main monomers (compounds) Polymers
No.
OBn N N
( HO) B B(OH) 2 2 RO OBn O OR Chapter 4 Bn RO R = CH3( CH2)11 n 406 401 Bn = C6H5CH2 OR R = CH3( CH2)11 Bn = C6H5CH2 N N (HO)2B B(OH)2 H H RO O O OR RO 409 402 n
xix Table 1. The polymers and main compounds prepared in this thesis (Continued)
Chapter Main monomers (compounds) Polymers
No.
RO N N OR Chapter 4 N N Br 405 Br n OR 403 RO R = CH3( CH2)11
xx
List of Figures
Figure 1-1 The art and science of conjugated polymers 1
Figure 1-2 Examples of conjugated polymers, note the bond- 8
alternated structures
Figure 1-3 Examples of poly(p-phenylene)s (PPP)s 14
Figure 1-4 Examples of pyridine incorporated conjugated polymers 24
Figure 1-5 Examples of bipyridine incorporated conjugated polymers 28
Figure 1-6 Examples of poly(m-phenylene)s (PMPs) 34
Figure 2-1 Structures of amphiphilic poly(p-phenylenes) 201a-c 77
Figure 2-2 Absorbance and emission spectra of Polymer 201a 82
Figure 2-3 X-ray powder diffraction pattern of polymer 201a 84
Figure 2-4 Illustration of the polymer lattice indicating alkyl chain 86
packing and interchain hydrogen bonding or metal
complexation
xxi
List of Figures
Figure 3-1 Molecular structures of target polymers 301-306 97
Figure 3-2 Absorbance and emission spectra of polymers 304 and 301 104
in chloroform
Figure 3-3 Excited-state intramolecular proton transfer (ESIPT) for 106
polymer 301
Figure 3-4 UV/Vis spectra of Protonation and Deprotonation of 111
polymers 301-303 with aqueous HCl and aqueous NaOH
in THF
Figure 3-5 Proton Transfer from the excited cation of polymer 301 to 112
a base B
Figure 3-6 UV/Vis spectra of polymers 301 and 303 without and with 113
aqueous NaOH in DMF
Figure 3-7 Emission spectra of polymers 301 and 303 without and 114
with aqueous NaOH in DMF
Figure 4-1 Molecular structure of the polymers 401-403 128
Figure 4-2 Absorbance and emission spectra of polymers 401 and 402 133
in THF
Figure 6-1 Evolution of hydroxylated polyphenylenes (HPP)s 161
Figure A 1-9 TG curves of 301-403 178
xxii
List of Schemes
Scheme 2-1 Synthesis of polymers 201a-c 78
Scheme 3-1 Synthesis of polymers 301 and 302 99
Scheme 3-2 Synthesis of polymer 303 100
Scheme 4-1 Synthesis of polymers 401 and 402 130
Scheme 4-2 Synthesis of polymer 403 131
xxiii
List of Tables
Table 2-1 Molecular weights of target polymers 201a-c observed 80
from GPC analysis
Table 2-2 Absorption and emission responses of polymers 201a-c 88
with and without metal ions
Table 3-1 Molecular weights of polymers 301-306 observed from 102
GPC analyses
Table 3-2 Solvatochromic behavior of polymers 301-306 108
Table 3-3 Absorption and emission responses of polymers 301-303 116
with metal ions
Table 4-1 Molecular weights of polymers 401-403 observed from 132
GPC analyses
Table 4-2 Solvatochromic behavior of polymers 401-403 135
Table 4-3 Absorption responses of polymers 401-403 with metal ions 136
Table A-1 Absorption maxima of non-hydroxyl-containing 172
conjugated polymers
xxiv 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
xxv 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)
xxvi 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
xxvii o- ortho-
OBn benzyloxy
OLED organic light emitting diode
OR alkoxy p- para-
P3AT poly(3-alkyl thiophene)
P3HT poly(3-hexyl thiophene)
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
xxviii 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)
xxix 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)
xxx 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
xxxi 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)
xxxii Chapter 1: The Art and Science of Conjugated Polymers
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
1 Chapter 1: The Art and Science of Conjugated Polymers
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
2 Chapter 1: The Art and Science of Conjugated Polymers
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,
3 Chapter 1: The Art and Science of Conjugated Polymers
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
4 Chapter 1: The Art and Science of Conjugated Polymers
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
5 Chapter 1: The Art and Science of Conjugated Polymers
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)
6 Chapter 1: The Art and Science of Conjugated Polymers
(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.
7 Chapter 1: The Art and Science of Conjugated Polymers
OR
n Pol yacetyl ene n ( PA) n Pol y( p-phenyl ene) R' O ( PPV) Pol y(2, 5-dial koxy)paraphenyl ene (e. g. MEH-PPV) R
n S S n n Pol y( p-phenyl ene) Pol ythi ophene Pol y (3-al kyl) thi ophene ( PPP) ( PT) ( P3AT)
OR O O
S n S S n n Pol y(3-al koxy)thi ophene ( P3AT) Pol yisothianaphthene Pol yethyl enedi oxythi ophene ( PI TN) ( PEDOT)
S N n N n n H R Pol y( p-phenyl ene sul phi de) Pol ypyrrol e Pol y( N-al kyl)pyrrol e ( PPS) ( PPyrr) ( PRPyrr)
H H N N N N n
Pol yanili ne ( PANI)
Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures.
8 Chapter 1: The Art and Science of Conjugated Polymers
R
N n N n N n Pol y(pyri di ne-2, 5-di yl) Pol y(al kyl pyri di ne-2, 5-di yl) Pol y(isoqui noli ne-1, 4-di yl) ( PPy) ( PRPy) P(1, 4-i Q)
R R
n N N N N n N N n Pol y(2, 2' -bi pyri di ne-5, 5' -di yl) Pol y(dial kyl -2, 2' - Pol y(1, 10-phenanthroli ne ( PBpy) bi pyri di ne-5, 5' -di yl) -3, 8-di yl) ( PRBpy) ( PPhen)
Ar Ar
N N N N N
n n n Pol y(qui noli ne-5, 8-di yl) Pol y(qui noxali ne-5, 8-di yl) Pol y(2, 3-diaryl qui noxali ne P(5, 8-Q) P(5, 8-Qx) -5, 8-di yl) P(5, 8-di Ar Qx)
N N
N N n n N n Pol y(qui noli ne-1, 4-di yl) Pol y(qui noxali ne-2, 6-di yl) Pol y(1, 5-naphthyri di ne P(2, 6-Q) P(2, 6-Qx) -2, 6-di yl) P(2, 6-Nap)
Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures
(Continued).
9 Chapter 1: The Art and Science of Conjugated Polymers
N N N N
n N n N N n N Pol y(pyri mi di ne-2, 5-di yl) Pol y(2, 2' -bi pyri mi di ne-5, 5' -di yl) Pol y(pyrazi ne-2, 5-di yl) ( PPyri m) ( PBPym) ( PPyrz)
R S N NH N NH N N
n n n Pol y(benzi mi dazol e Pol y(benzi mi dazol e-4, 7-di yl) Pol y(benzo-[d][2, 1. 3] -4, 7-di yl) and its deri vati ves thiadiazol e-4, 7-di yl) P(4, 7-Bi m) P[4, 7-Bi m( R)] P(4, 7-Btd)
NO2 O O O
n n O NO Pol y(2-methyl -anthraqui none 2 Pol y(9, 10-di hydro- Pol y(4, 8-di nitro -1, 4-di yl) pheanthrene-2, 7-di yl) anthraqui none-1, 5-di yl) P(1, 4-AQ) P(4, 8-NO -1, 5-AQ) ( PH2Ph) 2
n
n S n n Pol ymetaphenyl ene Pol y(thi ophene-2, 4-di yl) Pol y(1, 3-cycl ohexadi ene Pol yheptadi yne ( PMP) P(2, 4-Th) -1, 4-di yl) ( PHT) ( PCyh)
Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures
(Continued).
10 Chapter 1: The Art and Science of Conjugated Polymers
R
n n n
PTHP R Pol ynaphthal ene ( PNap) Pol y(anthryl ene) ( Pant)
N N S n CH Pol y(azobenzene-4, 4' -di yl) ( PAzb) S n Pol y(dithiaful vene) ( PDF) N N
O O O O n Pol y( p-phenyl ene benzobisoxazol e) N N ( PBO)
N N n N N Pol ybenzi mi dazol ebenzophenanthroli ne ( BBL) S S n Pol y( p-phenyl ene benzobisthiazol e ( PBT)
R R R R N N N N N N
S S n O O n N N n Pol y(nonyl bithiazol e) Pol y(nonyl bisoxazol e) R R PNBTz ( R = Nonyl) PNBO ( R = Nonyl) PBI m ( R)
N
S n O n Se n Te n Pol yfuran ( PFu) Pol ysel enophene Pol ytell urophene
Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures
(Continued).
11 Chapter 1: The Art and Science of Conjugated Polymers
R R R
C C C C C C Pol ydiacetyl ene ( PDA) n R R R Pol y(2, 5-dial kyl p-phenyl eneethyl ene) (dial kyl -PPE)
N N RO M O O n OR Pol y(sal phenyl eneethyl ene)s ( PSPE)s
[ n ] N n N n R R Pol ycarbazol e ( PCz) Pol yi ndol e ( PI)
Pol yazul ene ( PAz)
OR O Br- +N N N n Pol yoxadiazol e ( POD) n PEPPB
Figure 1-2. Examples of conjugated polymers, note the bond-alternated structures
(Continued).
12 Chapter 1: The Art and Science of Conjugated Polymers
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
13 Chapter 1: The Art and Science of Conjugated Polymers 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).
R C6H13
n n n n 4 1 2 3 (2000) (1999) R= C12H25 (1995)
OR CN O
n 5 n R= C10H21 (1996) 6 O (CH) O CN R= C12H25 (1996) (1996) 2 6 R= 2-ethyl -hexyl (1996, 2003) n 7 (2003)
RO
C6H13 R R
n n n y n H13C6 8 (1989) R 9 (1990) R 10 (1990) 11 (1993) OR R= H, CH3, C4H9, C6H13 R= C6H13, C12H25 R= ( CH2)2C( CH3)3 R= C6H13, C6H4CN R= C7H15, C8H17 R= C12H25, C16H33
O O O O O
C H C H C6H13 6 13 6 13
n n n H C H C H13C6 13 6 13 6 12 (1996, 1997) 13 (1997, 2000) 14 (1997)
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s.329-343,419,483
14 Chapter 1: The Art and Science of Conjugated Polymers
C H X R X X' R X 6 13
n n n R 16 (1997) R 17 (1997) H13C6 X 15 (1997) R= C6H13 R= C12H25 X= NO2, X' = H X= CN, CF3 X= N( CH3)2, NO2 X = X' = NO2 COOR
O O C H X (CH) (CH)x 12 25 2 6 C6H13 2
n n n
H25C12 X (CH) H C 2 6 13 6 (CH2)x 18 (1997) O 19 (1999) O X= N( CH3)2, F x= 1, 6 20 (1999) R= H, CH3
COOR
* OR OC H OR 4 9 O
n n n RO H9C4O RO O 21 (1994, 1996, 1999) 22 (1994) 24 (1998) * R= C4H9, C8H17, C12H25 R= C8H17, C12H25 R= 3-methyl butyl (isopentyl)
OH C7H15 OCH3 O
n n n HO H15C7 H3CO 25 (1999) O 23 (1994) 26 (1999)
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).344-350
15 Chapter 1: The Art and Science of Conjugated Polymers
CH(OCH CH)xO CH(OCH CH)xO CH(OCH CH)yO 3 2 2 3 2 2 3 2 2
O O O
n n m O 27 (1997, 1998) O O x = 1-5 x = 1-5 y = 1-5
O(CH CH O)xCH 2 2 3 O(CH2CH2O)xCH3 O(CH2CH2O)yCH3 28 (1997, 1998) x/ = y
CH(OCH CH) O 3 2 2 5
OR OR O
n n m RO 29 (1998) RO O 30 (1998) CH O(CH CH O) CH 2 2 2 3 3 CH O(CH CH O) CH 2 2 2 3 3 R= CH CH R= O(CH2CH2O)5CH3 CH2O(CH2CH2O)3CH3 CH O(CH CH O) CH 2 2 2 3 3
OC H OC H 12 25 12 25
O O
n n
31 (2001) 32 (2001) O O(CH2CH2O)4CH3
CH3(OCH2CH2)5O
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).351-353
16 Chapter 1: The Art and Science of Conjugated Polymers
OC H OC H 16 13 16 13 OC16H13
n n n O O O 33 (2001, 2003) 34 (2001, 2003) 35 (2001, 2003)
O(CH CH O) CH O(CH CH O)xR 2 2 4 3 2 2 O(CH2CH2O)xR R= Si Ph2t Bu R= THP R= H R= H x = 2, 4 x = 2, 4
O(CH2CH2O)4R
(CH2)6 OR CF O 3
n n
O 36 (2001, 2003) RO 37 (1998) F3C
(CH2)6 R = C16H33 O(CH CH O) R 2 2 4 O R = 3
O (CH2)6 O CN
n 38 (2003) NC O (CH2)6 O
OR
n RO 39 (1994)
R = C6H13, C8H17, C11H23, C16H33
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).353-356,334,346
17 Chapter 1: The Art and Science of Conjugated Polymers
COOH
(CH) O COOH n 2 6
40 (1991) HOOC n
HOOC O (H2C)6 41 (1994)
CH3 SO3R
n H C RO S 25 12 3 t-Bu
R = 42 (2001) t-Bu - + - + R' SO3 M R' SO3 M
n n + - R R M O3S 43 (1999, 2000, 2001) 44 (1999, 2000, 2001)
R = H, CH3, C6H13 R' = H, C6H13, C12H25 M = N( CH3)4, Na SO3Na
R N+ Br- R2N 3 O
O O x n O n n x = 1, 2 O O R = Me, Et R = Me, Et
NaO S 46 (1999) + 3 Br- NR NR2 3 45 (1994, 1998) 47 (1999, 2000)
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).357-368
18 Chapter 1: The Art and Science of Conjugated Polymers
I - + I I - N+ N (C2H5)3 (CH) (CH) (CH2)6 2 6 2 6
n n n (H C) (H C) (H2C)6 2 6 2 6 I - + I 48 (1996) + - N (C2H5)3 N I 49 (1996) 50 (1996)
F C (CF ) SO - + 3 2 7 3 N (C2H5)3
(CH2)6
n 51 (2000) ( H2C)6 + -O S (CF ) CF N (C2H5)3 3 2 7 3
CH I - CH I - - 3 3 CH3 I CH3 + + + H3C N CH2 CH2 N C2H5 H3C N CH2 CH2 N (CH2)6 CH3 (CH2)6 CH3
n n - (H2C)6 CH3 I (H C) CH + 2 6 3 H C N+CH CH N C H H C N+CH CH N 3 2 2 2 5 3 2 2 CH I - CH I - 3 3 CH3 CH3 52 (1998) 53 (1998)
- N I - + I I + N (C2H5)3 (CH) C H (CH) C H (CH2)6 C6H13 2 6 6 13 2 6 6 13
n n n ( H C) H C (H C) H C (H2C)6 H13C6 2 6 13 6 2 6 13 6 I - + I 54 (1996) N + I - N (C2H5)3 56 (1996) 55 (1996)
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).369-371
19 Chapter 1: The Art and Science of Conjugated Polymers
O O O OR X O R
n n n R = H, Br 57 (1995) R = PO( OEt)2, PO( OH)2 58 (1995) 59 (2003) R = CH 3 X = H, F, Cl, C( CH3)3 R = CH( CH3)2 R = 2-ethyl hexyl
R' R' R' R' R' = COC6H5 R' = CO( p- t-BuC6H4) R' = CO( o-FC6H4) n R' = CO( m-FC H ) 6 4 m n m R' = CO( p-FC6H4) 60 (1996) 61 (1996)R' R' R' = CH3 62 (1996)
R R = CH2CF2CF3 R = ( CH2)2CH3 O R = ( CH2)2( CF2)5CF3 n R = ( CH2)7CH3 MeOOC COO Me 63 (2003) n
SO3H 67 (1999) O
O N m
C O O 66 (1999) O (CH)n O 2 O n = 2-12 n n 64 (2000) 65 (1998)
(S) * COO(CH) O COO COOCH 2 2
CH3 CN
x 1-x COO(CH) O CN 68 (2000) 2 6
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).372-381
20 Chapter 1: The Art and Science of Conjugated Polymers
H9C4O
n n n m OCH 69 (2003) 3 71 (2003) 70 (1994)
R
n
R = R' = H O R = H, R' = OCH3 R = R' = OCH H C C H 3 13 6 6 13 O R = R' = S-Ph R R' 72 (1991) R = R' = Ph
n O R = O( CH2)5CH3 R = O( CH2)7CH3 O R = O( CH2)9CH3 R = CN [ R = NO2 73 (2003) R OH RO OC H R t BuO 6 13 O O
N H H3C H N R = H N N R = C6H13 ] n 76 (2003) OC H n 6 13 N n N
H N CH3 N H 74 O 75 (1996) O R Ot Bu R = O- tBu (1996)
CH3 (1996) R = C (CH2)7CH3
CH3 R = Boc, C11H23 (2000)
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).382-389
21 Chapter 1: The Art and Science of Conjugated Polymers
R NO2 S
n n R 77 78 (2000)
OH
OR (CH2)6 OR O OR
3 RO 2 2 RO O RO n (CH) 2 6 79 OH
R2 H R H R R R 2 R 1 1 1 1
x' n y' R R R R 1 1 1 R 1 H R2 H 2 Ladder-type PPP 80 OR C H 6 13
n H C 13 6 RO R-dendri mer
81
Figure 1-3. Examples of poly(p-phenylene)s (PPP)s (Continued).390-405
22 Chapter 1: The Art and Science of Conjugated Polymers
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.
23 Chapter 1: The Art and Science of Conjugated Polymers
CH 3 H3C
N n N n N n N n N n 82 83 84 H3C 85 H13C6 86 (1996, 1998, 1999, (1992, 1994) (1992, 1994) 2000, 2003) (1992, 1994) (1993, 1994, 2001)
n N m N n N N n 87 (1996) 88 (1996) 89 (1996)
OCH C12H25 R 3 R
n n N n N N 91 H C H CO 92 25 12 90 (1997) R= n-C6H13 3 R= n-C6H13 (2000, 2002) (2000, 2002)
OR OC H 8 17 OC8H17 N n N n n N RO 93 H C O 94 17 8 H17C8O 95 R= C4H9, C8H17, C12H25 (2000, 2001) (2000, 2001) = C16H33 = 2-ethyl -hexyl C H (2000, 2001, 2002, 2003) 7 15 N N N n R m 97 (2002) N R= C8H17 98 (2002) n m n R= 2-ethyl -hexyl N NO H C 2 17 8 C8H17 96 (2003)
N n N 101 (2000) N ] N n N N n 1-n N N N 99 (2002) [ 100 (2002)
Figure 1-4. Examples of pyridine incorporated conjugated polymers.406-430,344
24 Chapter 1: The Art and Science of Conjugated Polymers
CH H + 3 + Bu N N N
n N 102 n n n 103 (1995) 104 (1995) 105 (1995) (1996)
N R
N n 106 n 108 R R R R= C H R = CH ( CH ) 12 25 3 2 5 (1999) = OC16H33 = COOC12H25 (1996, 1997, 2002)
R
N n N O 109 (2003) R 107 N R= OC16H33 OC H = C12H25 (1997, 1999) O 12 25
N n 111 (1996) H C O 25 12
HOC H O 11 22 O C6H13
n S n N 112 N 110 (2003) (1994)
N N N N N N N n N N 113 114 115 n (2003)
Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).431-445
25 Chapter 1: The Art and Science of Conjugated Polymers
S n S n S m n 116 N N N N 117 118 (1996, 1999) (1996, 1999) (1996, 1999)
O O R R ( O O S S S N S n N S S R= C H ) 6 13 120 N n = C8H17 O O 119 = C12H25 (2003) (1999, 2002) O O
R R R R
n S S S N S Se N N n 123 121 R= C H (1996) 12 25 (2003)
R R R N R
n Se m N S N S S S n 124 122 R= C H (1996) 12 25 (2003)
O H H C 13 6 N H C 13 6 N N N n N m n H O N C6H13 C H 126 125 (2001) 6 13 (1995)
Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).446-
451,418
26 Chapter 1: The Art and Science of Conjugated Polymers
O
OR O R S S S S N N 127 n 128 n (2002) (2002) R = H, Me, n-hexyl, benzyl R = Me, n-pentyl, phenyl, NEt2
O- R S + S N S + S N R n O- n 129 130 (2002) R = H, OMe (2002)
R N R N 132 S N (2002) n N S S N 134 (2002) 131 S n (2002) n
Figure 1-4. Examples of pyridine incorporated conjugated polymers (Continued).452
27 Chapter 1: The Art and Science of Conjugated Polymers
1.5 Bipyridine incorporated conjugated polymers
N N n N N l N N m 135 136 M=Ru, Ni M (1990, 1992, 1996, 1997) Lx (1992)
H13C6
n N N N N n 137 138 R R C H M=Ru M 6 13 R= CH3 (1995) Lx (2001) R= C6H13
N N n 139 R= C6H13 (2003) R R R= C8H17 (2001)
N N m N N n
C6H13 Re( CO)3Cl C6H13 H13C6 H13C6 140 (2003)
n
H C RO 13 6 C6H13 R= H N N (2003) 141 R= Bn
Figure 1-5. Examples of bipyridine incorporated conjugated polymers.453-464
28 Chapter 1: The Art and Science of Conjugated Polymers
OR
N N N N 142 n 143 (2000) n H3CO R= 2-ethyl -hexyl (2000)
OR OR [ [ ( OR N N ) N N 2 ] 145 144 n RO ] RO n R= C10H21 (1997) RO R= 2-ethyl -hexyl (1999) R= C10H21 (1997) R= 2-ethyl -hexyl (1999)
OR [ ( OR OR N N ) [ 2 M 147 N N RO ] M ] n 146 n RO RO R= 2-ethyl -hexyl (2000) R= 2-ethyl -hexyl (2000)
H C C H N N 17 8 8 17 148 n (2001)
C6H13 [ Si CH 3 N N ] 149 n (2000)
Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).465-
469,462
29 Chapter 1: The Art and Science of Conjugated Polymers
OR
N N n 150 RO R= OC18H37 (2001)
N N n H C C H 151 17 8 8 17 (2001)
C8H17 C8H17
4 4 S N N n S N N n 153 152 (2000) (2000) Ru
O O O O
S S
N N S N N S n O O O O 154 (1999)
S N N CH 155 S n (2001)
S N N CH S S x 156 N N CH (2001) Ru S y
Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).470-
473,462
30 Chapter 1: The Art and Science of Conjugated Polymers
EH ( C H ( 10 21 N N S ) N N 157 S ) EH= 2-ethyl -hexyl n 158 (2002) (2002) n
C H ( 10 21 S N N (2002) S ) 159 n H21C10
C H ( 10 21 S N N S ) 160 (2002) n H21C10
( N N 161 ) (2002) n N
C12H23
Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).474
31 Chapter 1: The Art and Science of Conjugated Polymers
C6H13 N N n N n N 162 (2001) H13C6 163 (1998, 2001)
[ N ][ ] n m N 164 (1998)
[ N ] n
165 (1998) N
N N n 3 n N N 166 (2001) 167 (2001)
R N R= H N = OC12H25 N n = OC18H37 R 168 (2000) R= H R = OC8H17 = OC18H37 N n 169 (2000) R
Figure 1-5. Examples of bipyridine incorporated conjugated polymers (Continued).475-478
32 Chapter 1: The Art and Science of Conjugated Polymers
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
33 Chapter 1: The Art and Science of Conjugated Polymers
OC H 12 25 n
n n 170 171 (1997) 172 (2000) n 173 (1996)
n n n O O 175 N N O (1996) 176 N (1996)
CH3 174 CH (1996) OCH3 3
OC H CH 12 21 CH3 3
S x n OCH3 CH3 177 179 (1996) 178 CH OCH (2000) 3 OCH3 (2000) 3 X
Y = ( CH2)3CH3 Y = ( CH2)15CH3 X Y =
n Y = 180 + Y BF - (2001) N 4 X =
n X = 181 (2001)
Figure 1-6. Examples of poly(m-phenylene)s PMPs.479-488
34 Chapter 1: The Art and Science of Conjugated Polymers
R
182 n (1999, 2000) R = t-Butyl R = C6H13 R = C H 12 25 R
R n R = t-Butyl R = C6H13 R = C12H25 R1 = C11H23
R 1 R = C H 183 1 12 23 R R 1 1 (1999, 2000) R1
OR
OC8H17 n
RO n OC H R = n-C4H9, n-C6H13 8 1 7 184 (1997, 2000, 2001, 2003) 185 (2001, 2002, 2003)
CH3 CH3
n OR OR n R = n-C4H9, n-C6H13, n-C8H17 187 (2001, 2002, 2003, 2004) 186 (2001, 2002, 2003, 2004) R = n-C4H9, n-C6H13, n-C8H17
Figure 1-6. Examples of poly(m-phenylene)s PMPs (Continued).489-500
35 Chapter 1: The Art and Science of Conjugated Polymers
[ OC H R = H 4 9 R = OC10H21 OC H 4 9 R H C O 9 4 H9C4O n 188 189 ] (2000) n (2003) R
CH CH CH CH n n 190 191 (2003) (2003)
OC6H13
n n 193 192 H13C6O (2004) (2004)
x y
H13C6 C6H13 COOR 194 R = COOCH3 (2003) R = H
Figure 1-6. Examples of poly(m-phenylene)s PMPs (Continued).492,501-504
36 Chapter 1: The Art and Science of Conjugated Polymers
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.
37 Chapter 1: The Art and Science of Conjugated Polymers
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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.
74 Chapter 2: Amphiphilic Poly(p-phenylene)s
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)
75 Chapter 2: Amphiphilic Poly(p-phenylene)s
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
76 Chapter 2: Amphiphilic Poly(p-phenylene)s
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
n RO 201a: R = CH3( CH2)11 201b: R = CH3( CH2)15 201c: 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.
77 Chapter 2: Amphiphilic Poly(p-phenylene)s
OH OH OH
(i) (ii) Br Br Br Br
HO 202 HO 203 RO 204
(iii)
OBn OBn OBn O O (v) (i v) B B ( HO) B B(OH) Br Br 2 2 O O RO 207 RO 206 RO 205 OBn
a: R = CH ( CH ) Br Br 3 2 11 (vi) b: R = CH3( CH2)15 c: R = CH3( CH2)17 RO 205 Bn = C6H5CH2
OBn OBn OBn OR
or n n RO RO RO BnO 208 208
(vii)
OH OH OH OR
or n n RO RO RO HO
201
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.
78 Chapter 2: Amphiphilic Poly(p-phenylene)s
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
79 Chapter 2: Amphiphilic Poly(p-phenylene)s
(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.
80 Chapter 2: Amphiphilic Poly(p-phenylene)s
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
81 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
82 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
83 Chapter 2: Amphiphilic Poly(p-phenylene)s
400 d = 34.42 300 200 100 Intensity (cps) 0 1.51018.527 2θ
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.
84 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
85 Chapter 2: Amphiphilic Poly(p-phenylene)s
Region of H-bonds or metal complexation O O O
n O O O H H H H HH O O O
n O O O d
Region of alkyl chain crystallization
Figure 2-4. Illustration of the polymer lattice indicating alkyl chain packing and interchain hydrogen bonding or metal complexation
86 Chapter 2: Amphiphilic Poly(p-phenylene)s
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
2+ 2+ 3+ blue region (λemi = 464 nm for 201a.Cu , 463 nm for 201a.Co ) and complex 201a.Fe 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.
87 Chapter 2: Amphiphilic Poly(p-phenylene)s
Table 2-2. Absorption and emission responses of polymers 201a-c with and without metal ionsa
Polymer 201a Polymer 201b Polymer 201c
λmax (nm)/ λemi (nm)/ λmax (nm)/ λemi λmax λemi (nm)/
E in eV E in eV E in eV (nm)/E in (nm)/E E in eV
eV 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.]
88 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
89 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
3. Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.;
Taliani, C.; Bradley, D. D.C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.;
Salaneck, W. R. Nature 1999, 397, 121-128.
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.
5. Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc. Chem. Res.
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.
9. Yang, Y.; Pei, Q.; Heeger, A. J. J. Appl. Phys. 1996, 79, 934-939.
10. Goodson, F. E.; Wallow, T. I.; Novak, B. M. Macromolecules 1998, 12, 2047-
2056.
11. Scherf, U. Top. Curr. Chem. 1999, 201, 163-222.
12. Feast, W. J.; Tsibouklis, J.; Pouwer, K. L.; Groenendaal, L.; Meijer, E. W.
Polymer 1996, 37, 5017-5047.
90 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.;
Begum, F.; Ikeda, T.; Sasaki, S.; Takeoe, H.; Fukuda, A.; Kubota, K. J. Am.
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.
91 Chapter 2: Amphiphilic Poly(p-phenylene)s
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.
31. Pieterse, K.; Vekemans, J. A. J. M.; Kooijman, H.; Spek, A. L.; Meijer, E. W.
Chem. Eur. J. 2000, 6, 4597-4603.
32. Tietze, L. F. Reactions and Syntheses in the Organic Chemistry Laboratory.
University Science: Mill Valley, California, 1989, pp 253.
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.
Phys. 1995, 196, 3563.
41. Remmers, M.; Schulze, M.; Wegner, G. Macromol. Chem. 1996, 17, 239-252.
92 Chapter 2: Amphiphilic Poly(p-phenylene)s
42. Bouachrine, M.; Lere-Porte, J. P.; Moreau, J. J. E.; Serein-Spirau, F.; Torreilles. J.
Mater. Chem. 2000, 10, 263-268.
43. Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 12-21.
44. Swager, T. M. Acc. Chem. Res. 1998, 31, 201 and the references sited therein
93 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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)
94 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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
95 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers 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
96 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers through van der Waal’s interaction. Molecular structures of target polymers 301-306 are shown in Figure 3-1.69
O H O H N N RO OR N n n H O O n RO 301 HO 302 H 303
OBn OBn N N RO OR N n n 304 OBn O RO BnO 305 Bn n R=CH3( CH2)11 306 Bn=C6H5CH2
Figure 3-1. Molecular structures of target polymers 301-306.69
97 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
In this chapter, we report the synthesis and optical properties of a few pyridine-incorporated hydroxylated polyphenylenes (Py-HPPs).
3.2 Synthesis of polymers
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.
98 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
OH OH OH
(i) (ii) Br Br Br Br
HO 307 HO 308 RO 309
(iii) (iii) OBn OBn
Br Br Br Br R=CH3( CH2)11 Bn=C6H5CH2 BnO 312 RO 310
(i v) (i v) OBn OBn
( HO) B B(OH) ( HO)2B B(OH)2 2 2
BnO 313 RO 311
N N (v) Br Br (vii) Br Br
314 314 OBn OBn N N
n n BnO 305 RO 304
(viii) (vi)
O H O H N N
n n 301 HO 302 RO
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 %
99 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
RO OR Br N Br N 315 (i) OBn O + n OBn Bn 306
(HO)2B B(OH)2 (ii) RO 311
R=CH3( CH2)11 Bn=C6H5CH2 RO OR N H O O H n 303
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.
100 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
3.3 Characterization of polymers
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.
101 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
102 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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
103 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers 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.
104 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
105 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
O H N Intramolecular OH N n Proton Transfer RO E1 n RO K1
emission absorption OH N
back n
O H RO K0 Proton Transfer N
n RO E0
Figure 3-3. Excited-state intramolecular proton transfer (ESIPT) for polymer 301: E- enol; K-keto; E0 – ground state (enol); E1 – excited state (enol).
106 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
107 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
Table 3-2. Solvatochromic behavior of polymers 301-306a
b Polymer Chloroform (CHCl3) Tetrahydrofuran(THF) Dimethylformamide(DMF)
+ c λmax(nm)/ λemi(nm)/ Stokes λmax(nm)/ λemi(nm)/ λ.H max(nm) / λmax(nm)/ λemi(nm)/
E in eV E in eV Shift (nm) E in eV E in eV E in eV E in eV 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
108 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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
109 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
110 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
111 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
H O H OH + N Proton Transfer N + B + BH+ n n RO RO K C1 1
absorption
H H O H O + N N H+
n n RO RO C0 E0
Figure 3-5. Proton transfer from the excited cation of polymer 301 to a base B: E- enol; C-cation; K-keto.
112 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
113 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
114 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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.
115 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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)/ λemi(nm)/ λmax(nm)/ λemi(nm)/ λmax(nm)/ λemi(nm)/
E in eV E in eV E in eV E in eV E in eV 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, 392/3.16 492/2.52 366/3.39 417/2.97
487/2.54
Co2+ 382/3.24 432/2.87, 392/3.16 446/2.78, 366/3.39 399/3.10,
451/2.75 469/2.64 417/2.97
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.
116 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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
81 (λmax) of 380 nm to 480 nm (Table 3-3) due to metal to MLCT could be observed.
Polymer complexes with Fe3+ and Ni2+ ions showed strong emission in the blue-green
3+ to yellow-green region. For example, polymer 301.Fe showed a λmax at 434 nm and
2+ emitted in the blue-green region (λemi = 499 nm). Polymers complexed with Co 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.
117 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
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75. Eichen, Y.; Nakhmanovich, G.; Gorelik, V.; Epshtein, O.; Poplawski, J. M.;
Ehrenfreund, E. J. Am. Chem. Soc. 1998, 120, 10463-10470.
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.;
Schanze, K. S. J. Phys. Chem. A. 2001, 105, 11118-11127.
80. Chan, S. C.; Chan, M. C. W.; Wang, Y.; Che, C. M.; Cheung, K. K.;
Nianyong, Z. Chem. Eur. J. 2001, 7, 4180-4190.
123 Chapter 3: Pyridine incorporated amphiphilic conjugated polymers
81. Ley, K. D.; Schanze, K. S. Coord. Chem. Rev. 1998, 171, 287-307.
124 Chapter 4: Bipyridine incorporated conjugated polymers
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)
125 Chapter 4: Bipyridine incorporated conjugated polymers
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
126 Chapter 4: Bipyridine incorporated conjugated polymers
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.
127 Chapter 4: Bipyridine incorporated conjugated polymers
N N
RO OBn O OR Bn N N N N R=CH3( CH2)11 Bn=C6H5CH2 401
N N H H RO O O OR
N N N N R=CH3( CH2)11
402
RO N N OR
N N OR RO N N
R=CH3( CH2)11 403
Figure 4-1. Molecular structure of the polymers 401-403.14
128 Chapter 4: Bipyridine incorporated conjugated polymers
4.2 Synthesis of polymers
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.
129 Chapter 4: Bipyridine incorporated conjugated polymers
(i)
Br N Br N N 404 Br Br 405 OBn
(ii) (HO)2B B(OH)2
RO 406
N N
RO OBn O OR Bn N N N N 401
R=CH3( CH2)11 (iii) Bn=C6H5CH2
N N H H RO O O OR
N N N N 402
Scheme 4-1. Synthesis of polymers 401 and 402: (i) BuLi in hexanes (1.6 M soln), Et2O
0 0 at –60 C; SOCl2, -40 C (ii) 2N K2CO3, Toluene, 1.5 mol % Pd(PPh3)4, reflux for 3 d,
(iii) H2, 10% Pd/C, EtOH/THF.
130 Chapter 4: Bipyridine incorporated conjugated polymers
OH OR OR (ii) (i) ( HO) B B(OH) Br Br Br Br 2 2
HO RO RO 407 408 409
(iii) N N Br Br 405
RO N N OR
403
N N OR RO N N
R=CH3( CH2)11
Scheme 4-2. Synthesis of polymer 403: (i) NaOH in abs. EtOH, RBr, reflux for 10 h; (ii)
0 BuLi in hexanes (1.6 M soln), THF/Et2O at –78 C , B(OiPr)3, water stirred at RT for 10
h; (iii) 2N K2CO3, Toluene, 1.5 mol % Pd(PPh3)4, reflux for 3 d.
131 Chapter 4: Bipyridine incorporated conjugated polymers
4.3 Characterization of polymers
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.
132 Chapter 4: Bipyridine incorporated conjugated polymers
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).
133 Chapter 4: Bipyridine incorporated conjugated polymers
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.
134 Chapter 4: Bipyridine incorporated conjugated polymers
Table 4-2. Solvatochromic behavior of polymers 401-403a
Solvents Polymer 401 Polymer 402 Polymer 403
λmax(nm) λemi(nm) Stokes λmax(nm) λemi(nm) Stokes λmax(nm) λemi(nm) Stokes
Shift (nm) Shift Shift
(nm) (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
b Concentrations: 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.
135 Chapter 4: Bipyridine incorporated conjugated polymers
Table 4-3. Absorption responses of polymers 401-403 with metal ionsa
Metal Ions 401 in THF 402 in THF 403 in THF
λmax(nm) λmax(nm) 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.
136 Chapter 4: Bipyridine incorporated conjugated polymers
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.
137 Chapter 4: Bipyridine incorporated conjugated polymers
4.8 References
1. McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574.
2. Leclerc, M. Adv. Mater. 1999, 11, 1491.
3. Gerard, M.; Chaubey, A.; Malhotra, B. D. Biosens. Bioelectron. 2002, 17, 345-
359.
4. Dai, L.; Mau, W. H. J. Phys. Chem. B 2000, 104, 1891-1915.
5. Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Mathieu, H. J.; Xanthopoulos, N.;
Viswanathan, B. Chem. Commun. 2003, 2022-2023.
6. Steuerman, D. W.; Star, A.; Narizzano, R.; Choi, H.; Ries, R. S.; Nicolini, C.;
Stoddart, J. F.; Heath, J. R. J. Phys. Chem. B 2002, 106, 3124-3130.
7. Dai, L.; He, P.; Li, S. Nanotechnology 2003, 14, 1081-1097.
8. Smela, E. Adv. Mater. 2003, 15, 481-494.
9. More references are cited in Chapter 1
10. Wang, B.; Waiselewski, M. R. J. Am. Chem. Soc. 1997, 119, 12.
11. Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568.
12. Zotti, G.; Zecchin, S.; Schiavon, G.; Berlin, A.; Penso, M. Chem. Mater. 1999,
11, 3342.
13. Liu, B.; Yu, W. –L.; Pei, J.; Liu, S. –Y.; Lai, Y. –H.; Huang, W. Macromolecules
2001, 34, 7932.
14. The structures of polymers in Figure 4-1 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 bipyridyl moiety. No attempts to introduce strict
138 Chapter 4: Bipyridine incorporated conjugated polymers
control on the regioselectivity in synthesis or purification of the various isomers
are carried out during this study.
15. Morris, G. A.; Zhou, H.; Stern, C. L.; Nguyen, S. T. Inorg. Chem. 2001, 40, 3222-
3227.
16. Daly, A. M.; Renehan, M. F.; Gilheany, D. G. Org. Lett. 2001, 3, 663-666.
17. Ready, J. M.; Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123, 2687-2688.
18. Sabater, M. J.; Corma, A.; Domenech, A.; Fornes, V.; Garcia, H. Chem. Commun.
1997, 1285-1286.
19. Adam, W.; Roschmann, K. J.; Saha-Moller, C. R. Eur. J. Org. Chem. 2000, 3519-
3521.
20. Pozzi, G.; Cinato, F.; Montanari, F.; Quici, S. Chem. Commun. 1998, 877-878.
21. Jacobsen, H.; Cavallo, L. Chem. Eur. J. 2001, 7, 800-807.
22. Woltinger, J.; Backvall, J. E.; Zsigmond, A. Chem. Eur. J. 1999, 5, 1460-1467.
23. Evans, D. A.; Janey, J. M.; Magomedov, N.; Tedrow, J. S. Angew. Chem. Int. Ed.
2001, 40, 1884-1888.
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.;
Moscalenko, M. A.; North, M.; Orizu, C.; Peregudov, A. S.; Timofeeva, G. I. Eur.
J. Org. Chem. 2000, 2655-2661.
26. Breinbauer, R.; Jacobsen, E. N. Angew. Chem. Int. Ed. 2000, 39, 3604-3607.
27. Chapman, J. J.; Day, C. S.; Welker, M. E. Eur. J. Org. Chem. 2001, 2273-2282.
139 Chapter 4: Bipyridine incorporated conjugated polymers
28. Shimakoshi, H.; Ninomiya, W.; Hisaeda, Y. J. Chem. Soc., Dalton Trans. 2001,
1971-1974.
29. Haikarainen, A.; Sipila, J.; Pietikainen, P.; Pajunen, A.; Mutikainen, I. J. Chem.
Soc., Dalton Trans. 2001, 991-995.
30. Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37-52.
31. Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am. Chem.
Soc. 1991, 113, 7063.
32. Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T. Tetrahedron Lett. 1991, 32,
1055.
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.;
Shamsipur, M. Anal. Sci. 2002, 18, 289-292.
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.
1997, 119, 9550-9957.
39. Yao, X.; Chen, H.; Lu, W.; Pan, G.; Hu, X.; Zheng, Z. Tetrahedron Lett. 2000,
41, 10267-10271.
40. Haikarainen, A.; Sipila, J.; Pietikainen, P.; Pajunen, A.; Mutikainen, I. J. Chem.
Soc., Dalton Trans. 2001, 991-995.
41. Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37-52.
42. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255-6260.
140 Chapter 4: Bipyridine incorporated conjugated polymers
43. Ucida, Y.; Echikawa, N.; Oae, S. Heteroat. Chem. 1994, 5, 409-413.
44. Pieterse, K.; Vekemans, J. A. J. M.; Kooijman, H.; Spek, A. L.; Meijer, E. W.
Chem. Eur. J. 2000, 6, 4597-4603.
45. Delnoye, D. A. P.; Sijbesma, R. P.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am.
Chem. Soc. 1996, 118, 8717-8718.
46. Zhang, Q. T.; Tour, J. M. J. Am. Chem. Soc. 1997, 119, 9624-9631.
47. Roncali, J. Chem. Rev. 1997, 97, 173-205.
48. Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mullen, K.; Meghdai,
F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123, 946-953.
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141 Chapter 5: Experimental Sections
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)
142 Chapter 5: Experimental Sections
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
143 Chapter 5: Experimental Sections
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°.
144 Chapter 5: Experimental Sections
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.
1 13 The total yield was 80%. (203) H NMR (DMSO-d6, ppm): 9.83 (s, 2H), 7.02 (s, 2H). C
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
145 Chapter 5: Experimental Sections
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
1 solvents (CH2Cl2 : hexanes, 4 : 6) to get the pure product in 60% yield. (204a) H 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,
1 2H), 1.4 (m, 18H); 0.88 (t, J = 6 Hz, 3H). H 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.
1 (204b) H NMR (CDCl3, ppm): 7.24 (s, 1H,), 6.97 (s, 1H), 5.14 (s, 1H), 3.92 (t, J = 6 Hz,
1 2H), 1.80 (q, 2H), 1.4 (m, 26H), 0.86 (t, J = 6 Hz, 3H). H 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 =
13 6 Hz, 3H). C 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.
146 Chapter 5: Experimental Sections
1 (204c) H NMR (CDCl3, ppm): 7.24 (s, 1H,), 6.97 (s, 1H), 5.19 (s, 1H), 3.92 (t, J = 6 Hz,
1 2H), 1.82 (q, 2H), 1.4 (m, 30H); 0.87 (t, J = 6 Hz, 3H). H 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 =
13 6 Hz, 3H). C NMR (CDCl3, ppm): 149.96, 146.66, 120.17, 116.53, 112.37, 108.26,
70.27, 31.84, 29.60, 29.60, 28.98, 25.84, 22.60, 14.04. Elemental analysis calcd. for C24
H40 Br2 O2: C, 55.39; H, 7.75; Br, 30.71. Found: C, 55.26; H, 7.74; Br, 32.14. FT-IR
(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%.
1 (205a) H 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,
111.01, 71.99, 70.19, 31.83, 29.56, 25.84, 22.60, 14.02. Elemental analysis calcd. for C25
H34 Br2 O2: C, 57.05; H, 6.51; Br, 30.36. Found: C, 57.17; H, 7.31; Br, 28.41. FT-IR
147 Chapter 5: Experimental Sections
(KBr, cm-1): 2922, 2848, 2359, 1493, 1466, 1355, 1200, 1073, 1004, 855, 802, 754. MS
(ESI): m/z: 528, 526, 453, 451, 425.
1 (205b) H NMR (CDCl3, ppm): 7.45(m, 5H), 7.16 (s, 1H), 7.10 (s, 1H), 5.06 (s, 2H), 3.95
13 (t, J = 6 Hz, 2H), 1.80 (q, 2H), 1.26 (m, 26H), 0.88 (t, J = 6 Hz, 3H). C NMR (CDCl3, ppm): 150.43, 149.41, 136.13, 128.49, 127.15, 119.32, 118.31, 111.52, 110.00, 71.98,
70.19, 36.39, 31.81, 29.58, 29.19, 25.82, 22.59, 14.01. Elemental analysis calcd. for C29
H42 Br2 O2: C, 59.80; H, 7.27; Br, 27.44. Found: C, 55.61; H, 7.29; Br, 26.72. FT-IR
(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.
1 (205c) H 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
-1 O2: C, 60.99; H, 7.59; Br, 26.18. Found: C, 60.63; H, 7.20; Br, 26.72. FT-IR (KBr, cm ):
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
148 Chapter 5: Experimental Sections
h. The crystalline mass was recovered by filtration. The product was recrystallized from acetone in 80% yield.
1 (206a) H 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 =
13 6 Hz, 3H). C 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.
1 (206b) H 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 =
13 6 Hz, 3H). C NMR (DMSO-d6, ppm): 157.02, 156.24, 137.15, 128.36, 127.51, 118.31,
117.74, 70.04, 68.31, 31.20, 28.93, 25.36, 21.98, 13.84. Elemental analysis calcd. for C29
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.
1 (206c) H 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 =
13 6 Hz, 3H). C NMR (DMSO-d6, ppm): 157.04, 156.28, 137.15, 128.38, 127.52, 118.34,
117.76, 70.06, 68.33, 31.21, 28.94, 25.38, 22.00, 13.84. Elemental analysis calcd. for C31
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.
149 Chapter 5: Experimental Sections
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.
1 (207a) H 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.
1 (207b) H 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
13 = 6 Hz, 3H). C 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.
1 (207c) H 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
13 = 6 Hz, 3H). C 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,
150 Chapter 5: Experimental Sections
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.
1 13 (208a) H NMR (CDCl3, ppm): 7.28(b), 4.97 (b), 3.91 (b), 1.57 (b), 1.27 (b), 0.91 (b). C
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.
1 13 (208b) H NMR (CDCl3, ppm): 7.26(b), 4.94 (b), 3.87 (b), 1.5 (b), 1.27(b), 0.89 (b). C
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.
1 (208c) H NMR (CDCl3, ppm): 7.19 (b), 7.05 (b), 6.9 (b), 4.94 (b), 3.85 (b), 1.5 (b),
13 1.25(b), 0.87 (b). C 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.
151 Chapter 5: Experimental Sections
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).
1 (201a) H 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
-1 (CDCl3, ppm): 7.03 (b), 6.89 (b), 3.91 (b), 1.75 (b), 1.22 (b), 0.86 (b). FT-IR (KBr, cm ):
3397, 2916, 2848, 1625, 1469, 1406, 1200, 1054, 796, 720.
152 Chapter 5: Experimental Sections
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
1 methanol. Yield is 95%. H NMR (CDCl3, ppm): 7.44 (m, 10H), 7.17 (s, 2H), 5.07 (s,
13 4H). C 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
153 Chapter 5: Experimental Sections
overnight. Water was added and the mixture stirred for 24 h. The crystalline mass was
1 filtered and recrystallized from acetone in 60% yield. H NMR (DMSO-d6, ppm): 7.31 (s,
13 8H), 7.21 (m, 10H), 5.03 (s, 4H). C 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
1 yield was 5.0 g. (304) H 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):
1 2922, 2852, 1590, 1506, 1456, 1231, 1196, 1026, 842. (306) H 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
13 (18H, b), 0.85 (3H, b). C NMR (CDCl3, ppm): 156.20, 153.78, 151.45, 150.64, 137.90,
154 Chapter 5: Experimental Sections
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.
5.4.4 Synthesis of Polymer 301
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
1 was 1.00 g. (301) H 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):
1 3429, 2924, 2853, 1465, 1200, 722. (303) H 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.
155 Chapter 5: Experimental Sections
5.4.5 Synthesis of Polymer 305
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.
1 The yield was 3.27 g. H 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.
5.4.6 Synthesis of Polymer 302
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
1 yield was 1.00 g. H 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.
156 Chapter 5: Experimental Sections
5.5 References
1. Tietze, L. F. Reactions and Syntheses in the Organic Chemistry Laboratory.
University Science: Mill Valley, California, 1989, pp 253.
2. Baskar, C.; Lai, Y. H.; Valiyaveettil, S. Macromolecules 2001, 34, 6255-6260.
157 Chapter 6: Conclusions and suggestions for the future work
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)
158 Chapter 6: Conclusions and suggestions for the future work
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:
¾ The introduction of long alkoxy chain improved the solubility of all polymers.
¾ The emission properties could easily be fine tuned by using different solvents,
base and metal ions
¾ Incorporation of heterocyclic compounds, namely pyridine and bipyridine on the
polymer backbone has tremendous changes in the optical properties.
¾ Pyridine- and bipyridine- incorporated conjugated polymers gave positive
solvatochromism in solvents of varying polarity
¾ 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.
159 Chapter 6: Conclusions and suggestions for the future work
¾ Metal chelating effect of all derived polymers induced significant changes in
emission properties and could be used for sensing metal ions
¾ The optical tunability would allow such derived polymers as good candidates for
fabricating polymeric light emitting diode (PLED) devices
¾ 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
¾ 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.
160 Chapter 6: Conclusions and suggestions for the future work
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
161 List of Publications
List of Publications
“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)
162 List of Publications
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.
163 List of Publications
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,
164 List of Publications
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.
165 List of Publications
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
166 List of Publications
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.
8. Baskar, C.; Valiyaveettil, S. 221st ACS National Meeting April 1-5, 2001,
SanDiego, CA, USA. Oral Presentation, Polymer Division, April 01, 2001.
9. Baskar, C.; Valiyaveettil, S. 221st ACS National Meeting April 1-5, 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)
167 List of Publications
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.
168 List of Publications
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.
169 List of Publications
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.
170 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
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)
171 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymersa
Conjugated Polymers Absorption maxima (nm) References
λmax = 336 nm 1,2 n
OR R = C8H17 λmax = 336 nm 3-5
n R = C12H25 λmax = 334 nm
R = C16H33 λmax = 334 nm
R R = C6H13 λmax = 247 nm (in cyclohexane) 2
6 n R = C6H13 λmax = 300 nm R
OR R = H λmax = 345 nm (in DMF) 7
6 n R = C4H9 λmax = 336 nm (in CH2Cl2) RO 6 R = C8H17 λmax = 336 nm (in CH2Cl2)
6 R = C12H25 λmax = 336 nm (in CH2Cl2)
R = 8 λmax = 335 nm (in CH2Cl2)
R = 9 λmax = 331 nm (in CHCl3)
aExamples given here based on the derived polymers mentioned on the thesis
172 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymers
(Continued)
Conjugated Polymers Absorption maximum (nm) References
λmax = 373 nm (in HCOOH) 10-12 N n λ = 360 nm [in (CF ) CHOH] max 3 2
CH 3
λmax = 320 nm (in HCOOH) 12 N n
H C 3
λmax = 310 nm (in HCOOH) 12 N n
λmax = 340 nm (in HCOOH) 12, 13 N n λmax = 319 nm (in CHCl3) H3C
λmax = 323 nm (in THF)
λmax = 321 nm (in benzene)
173 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymers
(Continued)
Conjugated Polymers Absorption maximum (nm) References
λmax = 382 nm (in HCOOH) 14 N n
λmax = 366 nm (in HCOOH) 14 N N n
C H 6 13
λ = 327 nm (in HCOOH) 15 N n max
O Me C H 6 13
λmax = 396 nm (in HCOOH) 15 N n MeO
OC H 8 17
λmax = 373 nm (in CHCl3) 16-19 N n
H17C8O
174 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Table A-1. Absorption maxima of non-hydroxyl-containing conjugated polymers
(Continued)
Conjugated Polymers Absorption maximum (nm) References
λmax = 373 nm (in HCOOH) 20,21 N N n λ = 380 nm max
N N n λmax = 349 nm (in HCOOH) 22 H3C CH3
λmax = 350 nm (in HCOOH) 22 N N n λmax = 320 nm (in CH2Cl2) C6H13 C6H13
C H 6 13 N
λmax = 322 nm (in CHCl3) 23 N n
C6H13
OC H 12 25
λmax = 313 nm (in THF) 24,25
n
175 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
References
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176 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
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177 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-1. TG curve of 301; heating rate: 10 K/min under nitrogen
178 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-2. TG curve of 302; heating rate: 10 K/min under nitrogen
179 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-3. TG curve of 303; heating rate: 10 K/min under nitrogen
180 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-4. TG curve of 304; heating rate: 10 K/min under nitrogen
181 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-5. TG curve of 305; heating rate: 10 K/min under nitrogen
182 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-6. TG curve of 306; heating rate: 10 K/min under nitrogen
183 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-7. TG curve of 401; heating rate: 10 K/min under nitrogen
184 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-8. TG curve of 402; heating rate: 10 K/min under nitrogen
185 Appendix: Synthesis and Fine-tuning the Emission Properties of New Amphiphilic Conjugated Polymers
Figure A-9. TG curve of 403; heating rate: 10 K/min under nitrogen
186 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.
187