Synthesis and fine tuning the emission properties of new amphiphilic conjugated polymers

PROPERTIES OF NEW AMPHIPHILIC CONJUGATED POLYMERS CHINNAPPAN BASKAR NATIONAL UNIVERSITY OF SINGAPORE... PROPERTIES OF NEW AMPHIPHILIC CONJUGATED POLYMERS CHINNAPPAN BASKAR M.Sc., IIT

PROPERTIES OF NEW AMPHIPHILIC CONJUGATED

POLYMERS

CHINNAPPAN BASKAR

NATIONAL UNIVERSITY OF SINGAPORE

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

Dedicated to my beloved parents

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)

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 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)

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),

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

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

Chapter 1 Introduction: The Art and Science of

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

Pyridine incorporated conjugated polymers Bipyridine incorporated conjugated polymers

26132328

References

767779818990

Chapter 3 Pyridine Incorporated Amphiphilic

Influence of hydroxyl groups

9598101103103

Metal complexation of polymers Conclusions

References

107110113115117118

Chapter 4 Bipyridine Incorporated Conjugated

References

126129132133134134137138

Synthesis of polymers 201a-c 2,5-Dibromohydroquinone (203) 2,5-Dibromo-4-dodecyloxy phenol (204a) 2,5-Dibromo-1-benzyloxy-4-dodecyloxy benzene (205a)

Synthesis of polymers 301-306 2,5-Dibromo-1, 4-dibenzyloxy benzene (312) 1,4-Dibenzyloxy-2,5-bisboronic acid (313) Synthesis of Polymer 304

Synthesis of Polymer 301 Synthesis of Polymer 305 Synthesis of Polymer 302

References

143143145145145147148

150

151152153153153154155156156157

Unpublished Papers International Conference Papers International Conference Presentations National Publications

National Presentations

163163164166168169

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

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

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

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

Table 1 The polymers and main compounds prepared in this thesis

O B n

R O

4 0 9

B ( O H ) 2( H O ) 2 B

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

Examples of pyridine incorporated conjugated polymers Examples of bipyridine incorporated conjugated polymers

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

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

Absorbance and emission spectra of Polymer 201a X-ray powder diffraction pattern of polymer 201a

Illustration of the polymer lattice indicating alkyl chain packing and interchain hydrogen bonding or metal complexation

18

1424283477828486

polymer 301

UV/Vis spectra of Protonation and Deprotonation of

polymers 301-303 with aqueous HCl and aqueous NaOH

Emission spectra of polymers 301 and 303 without and

with aqueous NaOH in DMF

Molecular structure of the polymers 401-403 Absorbance and emission spectra of polymers 401 and 402

in THF Evolution of hydroxylated polyphenylenes (HPP)s

TG curves of 301-403

97104

161178

Synthesis of polymers 401 and 402 Synthesis of polymer 403

7899100130131

Absorption and emission responses of polymers 201a-c

with and without metal ions

Molecular weights of polymers 301-306 observed from

GPC analyses

Solvatochromic behavior of polymers 301-306 Absorption and emission responses of polymers 301-303

with metal ions

Molecular weights of polymers 401-403 observed from

132

135136172

(Arranged in alphabetical order of abbreviations and symbols)

Et ethyl

FTIR fourier transform infrared

GPC gel permeation chromatography

λmax absorption wavelength at band maximum (nm)

λemi emission wavelength at band maximum (nm)

LED light emitting diode

LPPP ladder-type poly(p-phenylene)

m multiplet

Mn number average molecular weight

Mw weight average molecular weight

PPP poly(p-phenylene)

PPS poly(p-phenylene sulphide)

PPSA poly(p-phenylene sulfide-phenyleneamine)

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

t triplet

TFA trifluoroacetic acid

TGA thermogravimetric analysis

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

-Matthew 7:7 (Holy Bible)

“Fortunately science, like that nature to which it belongs, is neither limited by time nor by space It belongs to the world, and is of no country and of no age The more we know, the more we feel our ignorance; the more we feel how much remains unknown; and in philosophy, the sentiment of the Macedonian hero can never apply,- there are always new worlds to conquer.”

– Sir Humphry Davy (1778-1829)

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

Chapter 1 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

1.1 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

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

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

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, 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,66-72fuel cells,73-75 rechargeable batteries,76 lasers,52,77-80 molecular electronics,81-88spintronics,89 nonlinear optical properties,90-98 optical power limiting,99 chemical and biosensors,100-110 actuators,111-115 radical scavengers,116 membrane based separations,117biomedical 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),126polythiophene (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)