1 LIGHT HARVESTING POLYMERS: ENERGY TRANSFER AND MATERIALS APPLICATIONS By ZHUO CHEN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF D OCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013
2 2013 Zhuo Chen
3 To my parents and grandparents
4 ACKNOWLEDGEMENTS I would like to express my deepest appreciation to many people who have helped and suppor ted me during my time in Florida for seven years. First I want thank my supervisor, Dr. Kirk S. Schanze for his invaluable guidance, patience, and generous support though my PhD study He is a great mentor to me, and always encourages and cha l lenge s me thr oughout my research. It has been a great experience and honor to work with him. I am sincerely grateful to my other committee members, Prof. Ken Wagner Prof. Stephen Miller Prof. Adam Veige and Prof. Anthony Brennan for their help, support and for thei r valuable time providing suggestions and revisions on the writing I want also give my thanks to Prof. John Reynolds Prof. John Papanikolas, Prof. Bruce Parkinson and Prof. Thomas Meyer for their advice and collaboration Their wisdoms and expertise on r esearch added many inspirations to the work Special thanks should be given to Prof. Charles Beatty, my master advisor in Department of Materials Science and Engineering. Prof. Beatty helped me a lot when I arrived in US, and he encouraged me to do the res earches I am really interested. I have been working with a lot of wonderful postdocs, who shared their opinion help, support and friendship with me. Dr. Hui Jiang helped me set up in Schanze s group and acted as my big brother. Dr. Kastu Ogawa taught me t o run the first column in my life. Dr. Chen Liao, Dr. Yali Sun, Dr. Zhen Fang and Dr. Fude Feng offered a lot of experience and useful tips not only in synthesis but also in life. Dr. Alec Nepomnyashchii provided the beautiful AFM images in this dissertati on. Dr. Erik Grumstrup Dr. Gyu Leem and Dr. Dustin Jenkins are my teammates in UNC EFRC project; it is a great experience working with them.
5 Many thanks to the former and current members in Dr. Schanze group for their friendship helpful discussion and precious contributions to build such a wonderful research environment. I want to express my special thanks to Dr. Dongping Xie, Dr. Jie Yang, Xuzhi Zhu and Zhengxing Pan, who was always ready to help without any hesitation Hsien Yi Hsu and Randi Price sp ent hundreds hours to help me with laser experiments. I also want to warmly thank Dr. Anand Parthasarathy, Dr. J an Moritz Koenen Dr. Galyna Dubinina Dr. Seoung Ho Lee, Dr. Enkyung Ji, Dr. Jonathan Somme r, Dr. Julia Keller Dr. Abigail Shelton Ali Gundogan, Russ Winkel, Aaron Eshbaugh, Subahdip Goswami, Junlin Jiang and Shanshan Wang for their valuable advice and friendship. Last but not the least, I am feeling lucky to be born and raised in a hap py and warm family I deeply appreciate to my parents and my grandparents for their unconditional love, support and encouragement. They make me who I am. This dissertation is dedicated to them.
6 TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ................................ ................................ ............................... 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBREVIATIONS ................................ ................................ ........................... 16 ABSTRACT ................................ ................................ ................................ .............. 20 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 23 1.1 Light Harvesting Systems ................................ ................................ ................. 23 1.1.1 Conjugated Polymers ................................ ................................ ............ 24 1.1.2 Light Harvesting Dendri mers ................................ ................................ 25 1.1.3 Side Chain Conjugated Polymers ................................ .......................... 27 1.2 Photophysical Processes in Light Harvesting Polymers ................................ ... 29 1.2.1 Mechanism of Energy Transfer ................................ .............................. 30 1.2.2 Energy Transf er in Light Harvesting Polymers ................................ ...... 32 22.214.171.124 Intermolecular energy transfer ................................ .................. 33 126.96.36.199 Intramolecular energy transfer ................................ .................. 34 188.8.131.52 Energy migration in light harvesting polymers .......................... 35 184.108.40.206 The antenna effect in light harvesting polymers ....................... 40 1.3 Preparation of Side Chain Conjugated Polymer: Direct Polymerizat ion of Functional Monomer ................................ ................................ ......................... 44 1.3.1 Conventional Free Radical Polymerization ................................ ............ 44 1.3.2 Anionic Polymerization ................................ ................................ .......... 47 1.3.3 Controlled Radical Polymerization ................................ ......................... 50 220.127.116.11 Atom transfer radical polymerization (ATRP) ............................ 51 18.104.22.168 Nitroxide mediated polymerization (NMP) ................................ 53 22.214.171.124 Reversible addition fragmentation chain transfer (RAFT) polymerization ................................ ................................ ........... 60 1.3.4 Metathesis Polymerization ................................ ................................ ..... 67 126.96.36.199 Ring opening metathesis polymerization (ROMP) .................... 68 188.8.131.52 Acyclic diene metathesis (ADMET) ................................ ........... 72 1.4 Preparation of Side Chain Conjugated Polymer: Post Polymerization Modification ................................ ................................ ................................ ...... 73 1.4.1 S N 2 Reaction ................................ ................................ ......................... 75 1.4.2 Amide Coupling ................................ ................................ ..................... 76 1.4.3 Reaction with Active Ester ................................ ................................ ..... 77 1.4.4 Metal Coordination Reaction ................................ ................................ 78
7 1.4.5 Palladium Catalyzed Coupling and Cross Coupling Reactions ............. 80 1.4.6 ................................ ................................ ................... 81 1.5 Scope of Present Study ................................ ................................ .................... 84 2 NONLINEAR ABSORPTION POLYMERIC ARRAY FROM CONTROLLED RY ................................ ... 86 2.1 Background ................................ ................................ ................................ ....... 86 2.2 Polymer Design and Preparation ................................ ................................ ...... 90 2.2.1 Synthesis of FBPt ................................ ................................ .................. 90 2.2.2 Preparation of polymer backbone and poly FBPt ................................ .. 92 2.3 Photophysical Characterization in Solution ................................ ....................... 97 2.3.1 Steady state Absorption and Emission ................................ .................. 97 2.3.2 Triplet triplet T ransient A bsorption ................................ ....................... 101 2.3.3 Nonlinear A bsorption R esponse ................................ .......................... 102 2.4 Photophysica l Characterization of Thin Film ................................ ................... 104 2.5 Summary ................................ ................................ ................................ ........ 107 2.6 Experimental ................................ ................................ ................................ ... 107 2.6.1 Instrumentation and Methods ................................ .............................. 107 2.6.2 Materials ................................ ................................ .............................. 109 2.6.3 Synthesis ................................ ................................ ............................. 110 3 ULTRAFAST ENERGY TRANSFER IN POLYSTYRENE BASED ARRAYS OF CONJUGATED CHROMOPHORES ................................ ................................ .. 114 3.1 Background ................................ ................................ ................................ ..... 114 3.2 Polymer Design and Preparation ................................ ................................ .... 115 3.2.1 Preparation of clickable polymer backbones ................................ ....... 115 3.2.2 Synthesis of C hromophores ................................ ................................ 117 3.2.3 Preparation of p oly c hromophores and model compounds ................. 118 3.2.4 Structural Characterization ................................ ................................ .. 119 3.3 Steady State Absorption and Emission ................................ ........................... 122 3.4 Ultrafast Transient Absorption ................................ ................................ ........ 126 3.5 M olecular D ynamics S imulations ................................ ................................ .... 132 3.6 Summary ................................ ................................ ................................ ........ 133 3.7 Experimental ................................ ................................ ................................ ... 134 3.7.1 Instrumentation and Methods ................................ .............................. 134 3.7.2 Materials ................................ ................................ .............................. 135 3.7.3 Synthesis ................................ ................................ ............................. 136 4 TRIPLET TRIPLET ENERGY TRANSFER IN POLYSTYRENE BASED PLATINUM ACETYLIDE ARRA YS ................................ ................................ ....... 147 4.1 Background ................................ ................................ ................................ ..... 147 4.2 Polymer Design and Preparation ................................ ................................ .... 148 4.2.1 Synthesis of P latinum Acetylides ................................ ......................... 149 4.2.2 Preparation of P oly Platinums ................................ ............................. 151
8 4.3 Steady State Absorption and Emission ................................ ........................... 156 4.4 Transient Absorption Characterization ................................ ............................ 164 4.5 Time Re solved Emission ................................ ................................ ................ 166 4.6 Energy Transfer Pathway ................................ ................................ ............... 169 4.7 Summary ................................ ................................ ................................ ........ 171 4.8 Experimental ................................ ................................ ................................ ... 171 4.8.1 Instrumentation and Methods ................................ .............................. 171 4.8.2 Materials ................................ ................................ .............................. 173 4.8.3 Synthesis ................................ ................................ ............................. 173 5 POLYSTYRENE BASED ARRAYS OF POLYPYRIDINE RUTHENIUM CHROMOPHORES WITH ACID END GROUP ................................ .................... 181 5.1 Background ................................ ................................ ................................ ..... 181 5.2 Polymer Design and Synthesis ................................ ................................ ....... 185 5.2.1 Synthesis of NMP Initiator ................................ ................................ ... 186 5.2.2 Preparation of Polypyridine Ruthenium Functionalized Polymers and and Model Compound ................................ ................................ ......... 187 5.3 Absorption and Photoluminescence ................................ ................................ 192 5.4 Amplified Quenching ................................ ................................ ....................... 196 5.5 Surface Absorption on Titanium Dioxide Surface ................................ ........... 198 5.6 Solar Cell Performance Characterization ................................ ........................ 204 5.7 Summa ry ................................ ................................ ................................ ........ 205 5.8 Experimental ................................ ................................ ................................ ... 206 5.8.1 Instrumentation and Methods ................................ .............................. 206 5.8.2 Materials ................................ ................................ .............................. 209 5.8.3 Synthesis ................................ ................................ ............................. 210 6 CONCLUSION ................................ ................................ ................................ ...... 218 LIST OF REFERENCES ................................ ................................ ............................. 222 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 241
9 LIST OF TABLES Table Page 2 1 Photophysical characteristics of small molecular chromophore, FBPt, and NLA pol ymer, Poly FBPt. ................................ ................................ .......................... 100 3 1 Photophysical Characteristics of Polymers ( P 0 to P 20 ). ................................ ..... 131 4 1 Photophysical characteristics of model c ompounds and poly platinums. .............. 163 4 2 Liftimes of p oly platinums ................................ ................................ ..................... 169 5 1 The photophysical and electrochemical properties of 2, P1 and P2. ..................... 195
10 LIST OF FIGURES Figure Page 1 conjugated polymers. ................................ ................................ ...... 25 1 2 Examples of light harvesting dendrimers ................................ ................................ 27 1 3 Different morphologies of a polymer f ilm ................................ ................................ 29 1 4 Comparison of the Frster and Dexter mechanisms of electronic energy transfer ................................ ................................ ................................ .............. 30 1 5 Types of energy transfer in light harvesting polymers ................................ ............. 33 1 6 Superquenching of a dye polymer by energy acceptor ................................ ........... 34 1 7 Utilization of FRET in biomacromolecules. ................................ .............................. 35 1 8 A conceptual comparison between dilute solutions of polymer and small molecules ................................ ................................ ................................ ........... 36 1 9 Intramolecular energy exchange in poly mers ................................ .......................... 37 1 10 Possible energy transfer steps in intramolecular migration ................................ ... 38 1 11 Stern Volmer plots for emission quenching of Ru polymer and monomeric Ru complex ................................ ................................ ................................ .............. 40 1 12 Emission spectra of the mixture of polystyrene and poly(1 vinylphthalene) and the corresponding copolymer ................................ ................................ ............. 41 1 13 Mechanism of the singlet antenna effect. The polymer is a naphthalene substituted polymer, containing an anthracene trap ................................ ........... 42 1 14 Structure and energy transfer model of the copolymer [co PS 4 CH 2 CH 2 NHC(O) (Ru II ) 17 )(Os II ) 3 ](PF 6 ) 40 ................................ .............................. 44 1 15 Example of free radical polymerization. ................................ ................................ 45 1 16 Side chain conjugated polymers made by conventional free radical polymerization. ................................ ................................ ................................ ... 46 1 17 Mechanism of living anionic polymerization. ................................ ......................... 48 1 18 Living anionic polymerization of styrene derivatives para substituted with conjugated oligo(fluorene) moieties. ................................ ................................ ... 49 1 19 General concept of controlled radical polymerization (CRP) ................................ 50
11 1 20 Mechanism of tra nsition metal catalyzed ATRP. ................................ ................... 51 1 21 Common nitrogen based ATRP ligands. ................................ ............................... 52 1 22 Examples of polymers prepared via ATRP. ................................ .......................... 53 1 23 Mechanism of the NMP process ................................ ................................ ........... 54 1 24 Structures of commonly used nitroxides and alkoxyamines. ................................ 55 1 25 Selected monomers polymerized by NMP for electronic applications .................. 56 1 26 Various amorphous crystalline donor acceptor block copolymers synthesized by NMP ................................ ................................ ................................ ............... 57 1 27 Polymerization of donor acceptor block copolymers via NMP .............................. 57 1 28 Synthesis of dendronized initiator and subsequent polymer p repared via NMP ... 59 1 29 Block copolymer prepared via NMP for LED application ................................ ....... 6 0 1 30 Mechanism of RAFT polymerization ................................ ................................ ..... 62 1 31 Structural features of RAFT agents ................................ ................................ ....... 63 1 32 Examples of RAFT agents ................................ ................................ .................... 63 1 33 Preparation of light harvesting polymers via RAFT polymerization ....................... 64 1 34 Monomers with pendant functionality polymerized by RAFT and used in optoelectronic applications. ................................ ................................ ................ 66 1 35 Chauvin mechanism of olefin metathesis. ................................ ............................. 67 1 36 Types of olefin metathesis reactions ................................ ................................ ..... 67 1 37 General mechanism for ROMP. ................................ ................................ ............ 69 1 38 Commonly used olefin metathesis catalysts. ................................ ........................ 69 1 39 ROMP of a blue emitting poly mer with Mo catalyst ................................ ............... 70 1 40 Preparation of polymers with pendant Ru complexes by ROMP ........................... 71 1 41 Examples of polymers with pendant m etal complexes prepared via ROMP ......... 72 1 42 Preparation of electroactive polymers via ADMET and post functionalization ...... 73 1 43 Synthe sis of polymers by post polymerization modifications ............................... 75
12 1 44 Post polymerization modification via S N 2 reactions ................................ .............. 76 1 45 Post poly merization modification via amide coupling ................................ ............ 77 1 46 Structures of active esters ................................ ................................ ..................... 77 1 47 Post polymerization modification via the re action between amine and active ester ................................ ................................ ................................ ................... 78 1 48 Examples of metal coordination reactions in post polymer modification ............... 79 1 49 P allad ium catalyzed coupling reactions in post polymerization modification. ....... 81 1 50 Copper(I) catalyzed azide alkyne cycloaddition ................................ .................... 82 1 51 Application of CuAAC click reaction in post polymerization modification .............. 83 1 52 Preparation of copolymers with different functional groups with orthogonal click reactions ................................ ................................ ................................ ............. 84 1 53 Synthetic strategy of side chain conjugated polymers in this dissertation. ............ 85 2 1 Mechanisms of nonlinear absorption ................................ ................................ ....... 87 2 2 Examples of platinum acetylides with TPA/ESA mechanisms. 148 149 ....................... 89 2 3 Platinum acetylides used in NLA materials. ................................ ............................ 89 2 4 Structures of FBPt and Poly FBPt ................................ ................................ ........ 90 2 5 Synthe tic route of FBPt ................................ ................................ .......................... 91 2 6 Synthesis of Poly FB Pt ................................ ................................ ......................... 93 2 7 GPC of polymers ................................ ................................ ................................ ..... 94 2 8 1H NMR spectra of PGMA and PHAZPMA. ................................ ............................ 95 2 9 Comparison of absorption of PGMA and PHAZPMA ................................ ............. 95 2 10 1 H NMR spectra of PHAZPMA Poly FBPt and FBPt ................................ ......... 96 2 11 FTIR spectr a of polymers, PGMA PHAZPMA and Poly FBPt ............................. 97 2 12 Ground state absorption and steady state emission of FBPt and Poly FBPt ....... 98 2 13 Transient absorption spectra of FBPt and P oly FBPt in deoxygenated THF .... 101 2 14 Schematic diagram of a simple open aperture z scan apparatus ....................... 102
13 2 15 NLA response of 1 mM solutions of blank, FBPt Poly FBPt and T2 ................ 104 2 16 Structure of z scan benchmark, T2. ................................ ................................ .... 104 2 17 Photos of Poly FBPt solution and film under visible and UV light ....................... 105 2 18 Ground state absorption and photoluminance of Poly FBPt thin film. ................ 105 2 19 Transient absorption of Poly FBPt thin film ................................ ........................ 106 3 1 Structures of Polymers (P 0 to P 20) and model compounds (1a and 1b). ........... 115 3 2 Synthesis of PVBC and PVBA ................................ ................................ ............. 116 3 3 Synthesis of conjugated chromophores with terminal alkyne ............................... 117 3 4 Preparation of p oly c hromophores ( P 0 to P 20 ). ................................ ................... 119 3 5 Synthesis of OPE and TBT model compounds 1a and 1b ................................ ... 119 3 6 1 H NMR spect ra for polymers, PVBC PVBA and P 0 to P 20 ............................. 120 3 7 GPC traces of polymers ................................ ................................ ....................... 121 3 8 Steady state absorption and emission of model compounds and polymers. ......... 122 3 9 Comparison of measured ( brown hollow diamond s) and calculated ( cyan hollow star s) absorption spectra of P 20 ................................ ................................ ...... 125 3 10 Excitation spectra of copolymers with both donor and acceptor ......................... 126 3 11 Transient absorption spectra showing early time ( t = 175 fs) comparison betw e en P 5 and the pure don or polymer P 0 Also shown are transient spectra at t = 1.15 ns comparing P 5 with the TBT acceptor moiety ................ 127 3 ................................ .... 128 3 13 Molecular dynamics simulation of the OPE TBT copolymer ............................... 133 4 1 Structures of poly chromophores ( P 0 to P 100 ) and model compounds. ............. 149 4 2 Preparation of PE2 Pt and Py Pt chromophores with terminal alkynes ( 5 and 9 ). 151 4 3 Preparation of p oly platinum s ( P 0 to P 100 ). ................................ ....................... 152 4 4 GPC traces of Poly Platinums and PVBA ................................ ............................ 152 4 5 1 H NMR Spectra of Poly Platinums ( P 0 to P 100 ). ................................ .............. 155
14 4 6 Absorption of platinum acetylides and Poly Platinums in THF. ............................. 156 4 7 Comparison of measured (brown) and simulated (violet circle) absorption spectra of P 20 ................................ ................................ ................................ 158 4 8 Emission of model compounds and poly platinums in THF ................................ ... 159 4 9 Quantum yields and energy transfer efficiency for poly platinum copolymers. ...... 160 4 10 Excitation spectra of poly platinums ................................ ................................ .... 162 4 11 Transient absorption spectra of model compounds ................................ ............. 164 4 12 Transient absorption spectra of copolymers ( P 3 to P 20 ) at different time and comparison with homopolym ers ( P 0 and P 100 ) ................................ ............. 165 4 13 Phosphorescence decay of poly platinums at 520 nm ................................ ........ 167 4 14 Transient kinetics from = 6 00 nm for fiv e polymers P 0 to P 20 on the timescale of 0 to 0.92s. ................................ ................................ ................... 168 4 15 Jablonski Diagram for energy transfer in copolymers ................................ ......... 170 5 1 Illus tration of functional metallopolymer assemblies with controlled pattern adsorbed on photonic electrode ................................ ................................ ....... 182 5 2 Polystyrene based Ru(II) arrays prepared by RAFT polymerization and NMP. .... 183 5 3 Structure of NMP initiator ( 1 ), the model ruthenium complex ( 2 ) and ruthenium functionalized polymers ( P1 and P2 ). ................................ ............................... 185 5 4 Synthesi s of NMP initiator ( 1 ) with triester group. ................................ ................. 186 5 5 Preparation of polypyridine ruthenium functionalized polymers. ........................... 188 5 6 GPC tra ces of polymers 7 8 and 9 ................................ ................................ ....... 189 5 7 Synthesis of model Ru(II) complex, 2 ................................ ................................ ... 189 5 8 1 H NMR spectra of polymes. ................................ ................................ ................. 191 5 9 Ground state absorption of model complex and polymers ................................ .... 192 5 10 Steady state emission of model complex and polymers ................................ ..... 193 5 11 Emission quenching of polymers ( P1 Cl and P2 Cl ) and model complex ( 2 Cl ). 197 5 12 Stern Volmer plots for emission quenching of P1 Cl P2 Cl and 2 Cl ................. 198
15 5 13 Non contact tapping mode AFM images of TiO 2 (110) with different resolutions and cross section analysis of C ................................ ................................ ........ 199 5 14 N on contact tapping mode AFM images of P2 Cl deposited on TiO 2 (110) surface from solutions of different concentrations. ................................ ........... 202 5 15 Cross section analysis for AFM images for Figure 5 17A, B and H ................... 202 5 16 IPCE spectra for a TiO 2 (110) electrode dipped into MeOH with various concentrations of P2 ................................ ................................ ......................... 202 5 17 AFM image of the P2 polymer molecules at the surface of TiO 2 (110) from ................................ ................. 203 5 18 Photocurrent action spectrum (IPCE) and J V curve of DSSC made from P2 Cl and nan ocrystalline TiO 2 ................................ ................................ ............. 205
16 LIST OF ABBREVIATIONS acac Acetoacetate A DMET Acyclic diene metathesis AFM Atomic force microscopy AIBN A zobisisobutyronitrile AIBN Azobisisobutyro nitrile AO Acryloyl oxime AQS 9,10 A n thraquinone 2,6 disulfonate ATRP Atom transfer radical polymerization BHJ Bulk heterojunctions BOP Benzotriazol 1 yloxy tris(dimethylamino)phosphonium hexafluorophosphate BPO Benzoyl peroxide bpy Bypyridine CB Chlorobenzene CM Cross metathesis CPDN 2 C yanoprop 2 yl 1 dithionaphthalate CPE Conjugated polyelectrolyte CRP Controlled radical polymerization CTA Chain transfer agent CuAAC Copper( I ) catalyzed azide alkyne cycloaddition CuBr Copper ( I ) boromide CuI Copper( I ) iodide DMAP 4 ( D im ethylamino)pyridine DMF Dimethyl fomamide
17 DNA Deoxyribonucleic acid DP Degree of polymerization DSSC Dye sensitized solar cell EFRC Energy frontier research center ESA Excited state absorption FB Benzothiazolylfluorene FF F ll factor FRET Frste r resonance energy transfer FTIR Fourier transform infrared spectroscopy FTO Fluorine doped tin( IV ) oxide GMA Glycol methacrylate GPC Gel permeation chromatography HOMO Highest occupied molecular orbital IPCE Internal photon to current efficiency i Pr2NH Diisopropylamine ITO Indium tin oxide LED Light emitting devices LUMO Lowest unoccupied molecular orbital MA Methacrylate MD Molecular dynamics MLCT Metal to ligand charge transfer MMA Methyl methacrylate Mn(salen) ( s ,s) (+) n,n bis(3,5 di tert butylsalicylidene) 1,2 C yclohexanediaminomanganese(iii) chloride MW Molecular weight
18 MWD Molecular weight distribution NDI Naphthalene diimides NHS N hydroxysuccinimide NHSVB N succinimide p vinyl benzoate NLA Nonlinear absoprtion NMM 4 M ethylmorpholine NMP Nitroxide mediated polymerization NMR Resonance NP Nitrophenyl OLED Organic light emitting devices OPE Oligo(phenylene ethynylene) OXA Oxadiazole P3HT Poly(3 hexylthiophene) PDA Photodioide array PDI Polydispersity index PF Polyfluorene PFP Petafluorephenyl PGMA Poly(glycol methacrylate) PHAZPMA Poly(hydroxyazidopropyl methacrylate) PMA Poly(methacrylate) PMDETA N, N N N N P entamethyldiethylenetriamine PMMA Poly(methyl methacrylate) PPE Poly(phenylacetylene) PRE Persistent radical effect PVBA Poly(4 vinylbenzyl azide)
19 PVBC Poly(4 vinylbenzyl chloride) PvTPA Poly(vinyltriphenylamine) Py Pyrene RAFT Reverse addition fragmentation transfer RCM Ring closing metathesis ROM Ring opening metathesis ROMP Ri ng opening metathesis polymerization RSA Reverse saturable absorption TA Transient absorption TBAF Tetrabutyl ammonium fluoride TBT Thiophene benzothiadiazole TCSPC Time correlated single photon counting TEMPO 2,2,6,6 tetramethylpiperidinyl 1 oxy terpy Terpyridine TFA Trifluoroacetic acid TFP Tetrafluorephenyl THF Tetrahydro furan TIPNO 2,2,5 trimethyl 4 phenyl 3 azahexane 3 nitroxide TIPSA Trisisopropylsilyl acetylene TPA Two photon absorption TT Thiazolidine 2 thione VB 4 V inyl benzo ate VBA 4 V inylbenzyl azide VBC 4 V inylbenzyl chloride
20 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy LIGHT HAR VESTING POLYMERS: ENERGY TRANSFER AND MATERIALS APPLICATIONS By Zhuo Chen August 2013 Chair: Kirk S. Schanze Major: Chemistry Side chain conjugated polymers combine the intrinsic film forming and mechanical propert ies of polymer s and well defined electr onic, photonic, and morphological properties of monodisperse oligomer moieties. In this dissertation a post polymerization modification synthetic strategy involving controlled radical polymerization, S N 2 substitution and copper(I) catalyzed azide alkyne click cycloaddition (CRP S N 2 click ) is used to prepare side chain conjugated polymers Several families of well defined light harvesting polymers have been prepared, featuring a non conjugated and flexible poly mer backbone having desired molecular wei ght and narrow polydispersity, and pendant organic or organometallic chromophores A p oly acrylate with pendant nonlinear absorption (NLA) chromophores was prepared via the RAFT S N 2 click synthetic strategy. Platinum acetylides that undergo NLA via both t wo photon absorption (TPA) and exited state absorption (ESA) mechanisms were utilized as chromophores attach ed to clickable poly acrylate backbone. The resulting polymer exhibits similar photophysical properties as platinum acetylide precursor including st eady state absorption and emission, triplet triplet
21 transient absorption, and nonlinear absorption properties In addition, t he resulting polymers can be easily drop or spin coated to afford optically transparent film. Singlet energy transfer along a non conjugated polymer chain was studied with graft copolymers having different conjugated side groups as energy donor (OPE) and acceptor (TBT). The graft copolymers were also prepared from the reversible adition fragmentation transfer polymerization ( RAFT ) S N 2 click route. The singlet energy transfer from donor to accept was char acterized employing both time resolved and steady state fluorescence spectroscopy as well as ultrafast time resolved transient absorption spectroscopy The ultrafast energy transfer from OPE to TBT occurs within 50 picosecond with remarkably high efficien cy. There were two energy migration processes existing : ultrafast neighboring OPE TBT quenching within 2 4 ps and OPE OPE hopping within 2 5 5 0 ps. In a similar approach, the triplet energy transport has also been studied in Pt acetylide chromophore arr ays that were assembled by polystyrene backbone When the Pt) was doped into the assembly along with high energy PE2 Pt on the polymer backbone. The triplet triplet energy transfer from PE2 Pt to P y Pt was found to be very efficient occurring within 50 ns. Finally, a ruthenium(II) functional polymer with carboxylic acid bearing end group was prepard via nitroxide mediated polymerization ( NMP ) S N 2 click strategy T he polymer shows typical absorptio n and emissiton as Ru(bpy) 3 2+ complex and also exhibits amplifed que n ching effect. Absorption of the acid end group polymer onto single crystal TiO 2 has been characterized by AFM and photocurrent action spectra.
22 The results indicate Ru functional polymer i s able to anchor to TiO 2 surface with the acid end groups and inject electrons to TiO 2 to produce current. DSSCs made from the acid end group Ru functional polymer and nanocrystalline TiO 2 shows photon to current conversion with a low overall efficiency. F urther efforts, including tuning polymer chain length, adjusting structures of pendant ruthenium complex and optimizing DCCS fabrication technique, etc. should be made to improve the solar cell performances.
23 CHAPTER 1 INTRODUCTION 1.1 Light Harvesting Systems Light harvesting systems bear repetitive chromophores which can absorb photons to mimic the natural photosynthe tic approaches 1 2 M ultichromopho r ic light harvesting systems have attracted a lot of interest in the research communities of chemistry, materials science and chemical engineering There are several reasons for the attention paid to light harvesting systems. First, they are capable of converting sun light energy into electrical energy for example, they can be used in solar cells. 3 Second, these systems can also act as catalyst to convert solar energy into easily storable chemical energy for example, molecular hydrogen by water splitting hydrocarbons methanol and formic acid by water reduction of CO 2 3 8 Third, they make valuable contributions to light/electro responsive materials such as organic light emitting devices (OLEDs), 9 12 optical data storage 13 14 and optical limiting 15 Among the structures of light harvesting systems, light harvesting polym ers are particularly attractive because numerous synthetic methods make it possible to construct all kinds of polymeric architectures Also, from a structural point of view, light harvesting polymers can be grouped into three categories polymers with co njugated backbones ( normally known as conjugated polymers), 16 dendrimers 17 and polymers with a non conjugated backbone (e.g., polyst yrene and polyacrylate) but pendant conjugated chromophores as side chains (also termed as side chain conjugated polymers). 2 18 The major topic i n this dissertation will be side chain conjugated polymers, including their preparation, characterization and application.
24 1.1.1 Conjugated Polymers In general, the electronic structure of conjugated polymers originates from the sp 2 p z hybridized wave functions of the carbon atoms in the repeat unit s T he cloud of one monomer is in conjugation with all other repeat units around it. This extended conjugation lowers the energy required to promote an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the conjugated units. 16 In conjugated polymers, such as poly(3 hexylthiophene) (P3HT) and polyfluorene (PF) the backbone provides the light harvesting function. Such materials exhibit good processability extraordinarily high extinction c oefficients, and the possibility to tune optical gaps and HOMO and LUMO energies. Some representative e xamples of conjugated polymers are shown in Figure 1 1. 16 However, the fully conjugated polymer chain was broken into a series of segment chromophores with different conjugation lengths because of conformational disorder The conjugation difference leads to exciton self trapping which limits exciton diffusion lengths to ~10 nm, and results in low fluorescence quantum yields. 19 27 From the synthetic point of view, it is difficult to control the chain length of conjugated polymers, which have broad molecular weight distribution. Thus it makes the reproducibility from batch to batch to be very poor.
25 Figure 1 1 Examples of conjugated polymers. Adapted with permission from r ef. 16. Copyright 2010 The Royal Society of Chemistry. 1.1.2 Light Harvesting Dendrimers Dendrimers are perfectly highly branched synthetic macromolecules having three architectural components: a central cor e, interior branches, and surface functional groups D endrimer s have attracted great scientific interest because of their unique molecular architecture. 17 Dendrimer s are symmetric and monodisperse molecules and are synthesized through a stepwise repetitive reaction sequence, which gives rise to
26 different generations of the dendrimers. Such distinguishe d frameworks induce relatively rigid conformation in terms of size and shape compared to linear polymers Dendrimer s featuring decreasing number of chromophores from periphery to core make an attractive candidate for light harvesting applications. T he dend ron acts as a well defined scaffold to link chromophores and restricts the position and distance between different chromophores so that the energy transfer from periphery to core can be oriented Numerous dendritic designs with different kinds of light c ollecting chromophores at periphery and an energy sink at the core have demonstrated high energy transfer efficiency and an example is presented in Figure 1 2A 28 The scaffold of dendrimers can also be conjugated; in this case the conjugated dendr ons act both light absorbing chromophores and delocalized energy/electron transfer pathways ( Figure 1 2B). 29 30 Because of the precise structure and high energy transfer efficiency, light harvesting d endrimers are now being developed for applications such as light emitting diodes, frequency converters and other photonic devices. 28 30 The disadvantage of dend rimers lies in the difficulty of synthesis. T he step by step synthesis leads to large amount of work. With the increasing size in the higher generation, the reactivity of reacting sites become lower, leading to low yields of high generation dendrimer produ cts; meanwhile, it also brings difficulty of purification.
27 Figure 1 2 Examples of light harvesting dendrimers A) Dendrimers with non conjugated scaffold Reproduced with permission from r ef. 28. Copyright 2000 The Royal So ciety of Chemistry. B) Dendrimers with conjugated scaffold Reproduced with permission from r ef. 30. Copyright 2006 Springer. 1.1.3 Side Chain Conjugated Polymers From the perspective of structure, s ide chain conjugated polymers are considered to be polymers wi th a non conjugated backbone (e.g., polystyrene and polyacrylate) and pendant conjugated organic or organometallic chromophores. 2 The research of such polymeric systems was pioneered by Fox, Gu i llet, Webber and coworkers to mimic the process of natural photosynthesis, 1 2 31 from which the term light harvesting polymer originally came. For main chain conjugated polymers the optoelectronic properties depend strongly on the length of the conjugated system, 24 32 while in side chain conjugated polymers the electronic properties of the individual chromophores are ma inly unaffected Therefore, the side chain conjugated polymers combine the typical polymer properties (e.g., film forming ability, mechanical stability, and processing advantages) with the well defined electronic, photonic, and morphological properties of the monodisperse
28 oligomer moieties. In addition, the generally low solubility of the oligomers can be improved significantly by th eir incorporation to a polymer structure. 33 35 With the develop ment of controlled radical polymerization (CRP) and living polymerization techniques, side chain c onjugated polymers can be obtained with precisely controlled polymer length, which means excellent reproducibility of side chain conjugated polymers from batch to batch. 31 32 36 B ecause of the controllability of CRP and living polymerization, it is practical to prepare random copolymers and block co polymers bearing different chromophores. T here are a lot of variables for the copolymer structure allowing one to tune the properties of the copolymers and devices based upon them. First of all, the chromophores installed into the polymer side chains can b e adjusted and optimized for different properties. F or example, energy donor and acceptor can be attached to the polymer backbone at the same time to induce energy transfer. 34 37 Secondly, by tuning relative concentrations of different chromophores the properties of copolymers and devices can also be adjusted. Fr chet et al. reported that by tuning the concentra tion of blue emissive platinum complex in a random terpolymer a high external quantum efficiency of 4.6% can be achieved in the polymer based white OLED. 10 Finally, the morphologies of polymer films can be controlled by adjusting relative length of different blocks in block copolymers to achieve different goals for different types of devices. T he possible morphologies of polymer layer, polymer blends and copolymers are illustrated in Figure 1 3. For copolymers, the domain size can be much smaller than polymer blends. H owever, the domain sizes in copolymer films can still be
29 adjusted. It can be adj usted to be in the range of around 15 nm to meet the requirements for polymer solar cells (equal to exciton diffusion length); 38 40 or it can fall into the range of 26 to 49 nm to suppress the energy transfer between different emitting chromophores to achieve the site isolation effect, which meets the requirements for white LEDs. 9 Figure 1 3 Different morphologies of a polymer film. Adapted with permission from r ef. 40. Copyright 2006 John Wiley & Sons In this dissertation, the main focus will be on the side chain conjugated polymers and light harvesting polymers will be particularly referred to the side chain conjugated polymers This chapter will continue to discuss the energy transfer in side chain conjugated polymers and the preparation strategies of such polymers. 1.2 Photophysical Processes in Light Harvesting Polymers Many kinds of light harvesting polymer ic assemblies have been made, with one or more chromophores covalently attached to a single polymer strand. T he chromophor es attached to the backbone can be the same or different. In polymers with different chromophores, often one act s as light absorbing sensitizer, or energy donor (as it often has higher excited state energy), and another chromophore acts as quencher (or ene rgy acceptor as it often has lower excited state energy ). Energy
30 transfer plays an important role in the properties and applications in light harvesting polymers, and this is the photophysical processes people of most interested. 1.2.1 Mechanism of Energy Tran sfer An electronic energy transfer process can be simply described by Equation 1 1 : (1 1) There are two different mechanisms for energy transfer T he first case is the el ectron exchange interaction (also known as orbital overlap mechanism or electron exchange mechanism ) which is often referred as Dexter energy transfer ( Figure 1 4 A ); and the second case is the dipole dipole interaction (also termed as the Columbic or mechanism for electronic energy transfer in the literature) which is known as F rster energy transfer ( Figure 1 4 B ). Figure 1 4 Comparison of the F rster and Dexter mechanisms of electronic energy transfer A) De xter (exchange) mechanism B ) F rster (dipole dipole,) mechanism The spin of the electrons exchanged must obey the spin conservation rules. Adapted with permission from ref. 71. Copyright 2009 University Science Books
31 In Figure 1 4 the interacting elect rons were labeled as 1 and 2. A key difference between the two mechanisms is that, for the F rster mechanism, the interaction between *D and A is through space by the interaction of dipolar electric fields of *D with A, while for the Dexter m echanism, the interaction between *D and A is through the overlap of the orbitals of *D and A. The dipole dipole interaction operates through an oscillating electric field produced by *D and does not require a van der Waals contact of *D and A or an overlap of the orbit als for *D and A. From Figure 1 4A it is seen that electrons 1 and 2 exchange positions between A and D for electron transfer; whereas from Figure 1 4B it is seen that electron 1 stays on D and electron 2 stays on A. 41 The rate of energy transfer ( k E n T ) of the Dexter mec hanism can be expressed as: ( 1 2) W here K is a parameter related to the specific orbital interactions, J is the normalized spectral overlap integral, and is the separation of *D and A when they are in van der Waals contact. From the equation, the exponential dependence of the efficiency on the R DA (distance between D and A) ensures that energy transfer is only efficient over fairly short distanc es on the order of R DA = 5 10 So the Dexter mechanism requires donor and acceptor to be very close to each other, if not in direct contact 41 On the other hand, t he rate of energy transfer ( k ET ) of the F rster mechanism is proportional to the inverse sixth power of the separation between *D and A: (1 3)
32 According to the equation in favorable cases the range of separation for *D and A energy transfer by the dipole dipole mechanism can be very large, i n the order of R DA > 30 41 1.2.2 Energy Transfer in Light Harvesting Polymers Energy transfer between groups attached to a polymer backbone occur s by the two mechanisms described in the previous section ; however, it is more complicated than the bimolecular energy transfer occ urring between small molecules. Three major types of energy transfer in a polymer system can be distinguished : intermolecular energy transfer between polymer and small molecul e intramolecular energy transfer and energy migration, as presented in Figure 1 5 31 Intermolecular energy transfer involves the transfer of excitation energy from or to a small molecule from a large molecule. T he exciton can be originally localize d on a small molecule (donor), and transfer to a polymer chain, causing a sensitized photochemical reaction. Alternatively the exciton can initially be localized on a chromophore on a polymer chain and transfer to a small molecule (acceptor) thus quenching a photochemical or photophysical process (as shown in Figure 1 6 A ). The second ty pe of energy transfer in light harvesting polymers, which is of special interest to polymer chemists is energy transfer between chromophores in the same polymer chain. In this case, an exciton localized in donor group can be quenched by an acceptor in the same polymer chain (i.e., intramolecular energy transfer, as presented in Figure 1 6 B ). Additionally, exciton localized on a sequence of chromophores may be transferred from one chromophore to the next by a hopping mechanism in a process that is termed en ergy migration ( Figure 1 5 C ). Although these energy transfer
33 processes are described separately in convenience, two or three energy transfer mechanisms normally co exist in a polymeric system. 31 Figure 1 5 Types of energy transfer in light harvesting polymers. A) Energy transfer between polymer and small molecule. B) Intramolecular energy transfer. C) Energy migration. Adapted with permission from r ef. 31. Copyright 1985 John Wiley & Sons 184.108.40.206 Intermolecular energy transfer A n e xample of energy transfer from polymer to small molecule is that exciton from polymer is quench ed by energy acceptor quenchers. 42 43 I n light harvesting polymer s with multiple chromophores the quenching process also involves energy migration along the polymer chain, resulting in a much more pronounced quenching effect compared to that of monomeric model compound. This phenomenon is termed amplified o r super Whitten et al. 42 illustrated quenching of a polymer with pendant cationic cyanine dye s on a L lysine scaffold ( P in Figure 1 7 A ) by a negative charged cyanine ( A in Figure 1 7A ). The anionic cyanine dye ( A ) quenches the fluor escence of P at very low concentrations and gives a very high Stern Volmer constant ( K SV ) of 4 x 10 7 M 1 In addition to decreasing intensity of fluorescence
34 peak of P a new fluorescence peak at 630 nm appeared, which was assigned to o a A that is selectively activated by an energy transfer process from photoexcited P Figure 1 6 Superquenching of a dye polymer by energy acceptor. A) Structures of the dye polymer P and acceptor A (b) Absorption and emissio n spectra of dye polymer P upon sequential addition of cyanine acceptor A : solid line, polymer P absorption; dotted line, polymer P emission with no dye A added; dot dash and dashed lines, emission spectra recorded on sequential addition of A Adapted with permission from r ef. 42. Copyright 2001 American Chemical Society 220.127.116.11 Int ra molecular energy transfer Intramolecular energy transfer means energy transfer from one chromophore to another in the same polymer chain. An important example is F rster resonance en ergy transfer (FRET). A donor and an acceptor can be installed at both and ends of a polymer, and then FRET between the donor and acceptor can act as the probe to study end to end distance, chain dynamics and conformation. 44 47 Zentel et al. utilized reverse addition fragmentation transfer (RAFT) polymerization to prepare dye
35 functionalized polymers with narrow molecular weight distribution, in which Orego n Green Cadaverin served as donor and Texas Red acted as acceptor. The calculated end to end distance of the polymer with FRET was in reasonable agreement with data obtained from light scattering and gel permeation chromatography. 45 FRET is very useful for the characterizations of biomacromolecules such as peptides and nucleic acids ( Figure 1 8 ) 46 As a probe, it is used to study peptide/DNA length and conformational distributions. By FRET, the length of rig id biomacromolecules, such as rigid peptides and double strand DNAs, can be accurately determined; and the dynamical properties of flexible peptides can also be determined. Figure 1 7 Utilization of FRET in biomacromolecules. Adapted with permission from r ef. 46. Copyright 2001 American Chemical Society. 18.104.22.168 Energy migration in light harvesting polymers In the cases of a polymer bearing multiple chromophores along the polymer chain the energy transfer behavior will be much more complicated compar ed to the polymers described in the previous section. Polymers enhance absorptivity by increasing the number of sensitizers (usually act as donor) bound to a polymer backbone.
36 A polymer with pendant chromophores is illustrated in Figure 1 8. Even though the polymer solution is diluted, the concentration of groups of chromophores along the polymer backbone remains relatively constant. Additionally, the distance between neighboring chromophores will be determined by the geometry and flexi bility of the polymer chain. This is an important factor that differentiates between inter and i ntramolecular energy transfer, since intermolecular transfer depends on the polymer concentration while intramolecular processes are relatively independent. 31 Figure 1 8 A conceptual comparison between dilute solutions of polymer and small molecules. A) Dilute polymer solution. B) Dilute smaller molecular solution. Adapted with permission from r ef. 31. Copyright 1985 John Wil ey & Sons In a molecule where the chromophores are sufficiently close to each other the transfer of excitons from one chromophore to another can be viewed as a random walk on the chromophores. The re are mainly three possible types of intramolecular ener gy exchange, as shown in Figure 1 9 : (1) the hopping of exciton from one chromophore group to another, often adjacent group along the polymer backbone ( Figure 1 9A) ; (2) movement of exciton along the conjugated backbone by an exciton band mechanism
37 ( Figure 1 9 B ) ; and (3) movement of exciton across loops in a single polymer chain, which could be formed by folding of polymer chain to cause a temporary collision ( Figure 1 9 C ) Figure 1 9 Intramolecular energy exchange in polymer s. Adapted with permission from r ef. 31. Copyright 1985 John Wiley & Sons In principle, the exciton can be localized on a particular chromophore group, at least for some finite period of time, before it moves to another group in the chain. This type of e nergy delocalization is refer red as energy migration And normally a single step between chromophore unites is termed energy transfer while more than one such step in sequence constitute energy migration Three distinct types of energy transfer step s can contribute to energy migration in a polymer chain, which are illustrated in Figure 1 1 0 T h e first one is the nearest neighbor transfer, which is defined as transfer between chromophores where n, the number of monomer units between those bearing th e transfer sites, is equal to zero. T his is important in polymers containing small chromophores with a flexible backbone. 31
38 The second type is non nearest neighbor transfer, where n = 1, 2, 3. T his type of energy transfer occurs with chromophores wh ich are prohibited by steric or structural effects from approaching an adjacent chromophore by faci le bond rotation. T his type of polymer often has large substituent chromophore groups attaching to backbone. In order to achieve the most stable conformation large chromophores will separate as far apart as possible, thus may preclude nearest neighbor interactions, especially when the lifetime of the excited states is shorter than the rotational relaxation time of the polymer chain and side groups. I n this ca se, the interposition of a single repeat unit (n=1) may give the closest approach of two chromophores. 31 Figure 1 10 Possible energy transfer steps in intramolecular migration. A) Nearest neighbor transfer (n=0). B) N on nearest neighbor transfer (n=1, 2, 3). C) L oop transfer (n>3). n is the number of monomer unites between those bearing the transfer sites. Adapted with permission from r ef. 31. Copyright 1985 John Wiley & Sons T he third type of process is termed loop (n>3). It happens wh en there is strong solvent effect on the polymer conformation. And it may also happen when the polymer chain is long enough to allow the polymer to fold, thus two chromophores with n>3 may approach close enough to exchange ex cited state energy.
39 E xperimental detection of energy migration includes quenching the polymer with small molecules. If there is energy migration the Stern Volmer constant will be much larger than that of quenching of small molecular chromophores. This ph enomenon is termed amplified que n ching or superque n ching T his concept was first studied by Swager and coworkers in main chain conjugated poly( phenylene ethynylene )s (PPEs). 48 49 Whitten Schanze and coworkers studied the amplified quenching effect extensively in conjugated polyelectrolytes (CPEs). CPEs can be quenched by small amount of oppositely charged quencher ions This process has been attributed to two ma in factors: ( 1) ion pairing between the (oppositely) charged quencher ion and repeat units in the polyelectrolyte chain effectively increases the local concentration of the quencher ion, and possibly more important, (2) the fact that excitons in the poly c hromophore are able to undergo rapid diffusive transport along the polymer chain, increasing the effective sphere of action of the quencher ion. 50 T hese two factors can also be used in the light harvesting side chain conjugated polymers, which have a non conjugated backbone. 51 52 S chanze and coworkers studied the amplified quenching effect in a polymer with pendant ruthenium compl exes and a polystyrene backbone ( Figure 1 1 2 ). T he polymer was quenched by 9,10 anthraquinone 2,6 disulfonate ( AQS ) an electron acceptor. For polymer with 20 repeat units, the Stern Volmer constant is 15 times larger than the monomeric model Ru complex (8 .7 x 10 5 M 1 vs. 6.3 x 10 4 M 1 ). As the polymer length increased, the Stern Volmer constant also increased 52 T hese experimental results clearly indicate that even with a no n conjugated polymer backbone, excited energy can
40 migrate efficiently. Calculation of similar structure reveals than the energy migration is site to site through space hopping. 8 53 54 Figure 1 11 Stern Volmer plots for emission quenching of Ru polymer and monomeric Ru complex. Adapted with permission from r ef. 46. Copyright 2012 American Chemical Society. 22.214.171.124 T h e antenna effect in light harvesting polymers I n 1969, Fox et al. found that efficient phosphore scence emission occurred from small amounts of copolymerized chemically bond energy traps in polymer chains (styrene vinylnaphthalene copolymer). Emission of naphthalene phosphorescence from the copolymers was much higher than that of the mixture of equiva lent amounts of the two homopolymers in solution ( Figure 1 1 3 ). 55 56 Later, Schneider and Springer found the similar phenomenon in styrene acenaphthalene copolymer. 57 The enhanced acceptor emission was explained to be due to energy migration along the polymer chain. Because it mimics that observed in the ordered chlorophyll regions of green plant chloroplasts, i.e., the antenna chlorophyll pigments, this effect is termed to be the antenna effect 31
41 Figure 1 12 Emission spectra of the mixture of polystyrene and poly(1 v inylphthalene) and the corresponding copolymer. Adapted with permission from r ef. 55. Copyright 2012 American Chemical Society. Guillet et al. studied singlet energy migration and transfer in a variety of copolymers containing naphthalene and phenanthrene donors with anthracene energy traps, and proposed that the antenna effect for singlet energy transfer wa s not exclusively due to energy migration among chromophores making up to the antenna but to a combination of energy migration and direct F rster trans fer to the acceptor (trap), as illustrated in Figure 1 1 4 Assuming only one trap occurs in a long antenna chain, on a short time scale, i.e., a typical singlet lifetime ( < 100 ns), collisional energy transfer is a relatively minor factor, as the conformational relaxation time is much l onger than the singlet lifetime. Thus energy migration and transfer should be dominated by the long range F rster dipole dipole mechanism 31
42 Figure 1 13 Mechanism of the singlet antenna effect. The polymer is a naphthalene substituted polymer, containing an anthracene trap. Adapted with permission from r ef. 31. Copyright 1985 John Wiley & Sons In Guillet s theory, the radius R F defines a sphere around the acceptor inside of which the direct F rster transfer from the absorbing chromophore to the trap is favored. Outside of R F at least one energy migration step will occur before the energy is trapped by th e acceptor. T he radius R 0 is the standard F rster radius. Although a substantial of donor donor transfer may occur in this region the energy would be transferred to trap in any case as long as the radius was less than the F rster radius. So the donor don or energy migration in this region would not be expected to contribute to the efficiency of energy collection. T he only increase in efficiency of energy collection due to energy migration process lie s in the region between R 0 to R N In this region, the ene rgy migration occur s between the donors at a rate corresponding to that in the absence of the acceptor (energy trap). In addition, the lifetime of singlet exciton allow it has sufficient time to hop into the R 0 radius and finally transfer the energy to the acceptor.
43 Outside of R N the hopping distance, or hopping steps, is too large for the singlet exciton to transfer to the acceptor before it relaxes back to the ground state of donor. 31 As the optical transitions from ground state (S 0 ) to the lowest tr iplet state (T 1 ) are spin forbidden, the F rster dipole dipole mechanism is excluded in the triplet energy migration and energy transfer. As a matter of fact, the short range Dexter electron exchange mechanism works in the triplet energy migration and ener gy transfer. 41 Th erefore the mechanism of triplet antenna effect is much simpler than that of the singlet energy transfer, which is mainly due to the triplet energy migration along the polymer chain. Meyer, Papanikolas and co workers studied the energy transfer in the copo lymer based on polystyrene backbone and with fully loaded Ru(II) and Os(II) metal complexes, [co PS 4 CH 2 NHC(O) (Ru II ) 17 )(Os II ) 3 ](PF 6 ) 40 T he Monte Carlo simulation illustrated the structural influence of the large excluded volumes of the complexes resulti ng in rod like, spatially extended structures, in which the transition metal complex cations are presented with large spheres with 14 diameter. Each complex has 4 to 5 nearest neighbors, and the average distance between peripheries is 2 3 T he rate c onstant of < k E n T > ~ 2.5 x 10 9 s 1 for the nearest neighbor Ru II to Os II energy transfer and < k mig > ~ 2.5 x 10 8 s 1 to 1 x 10 9 s 1 for Ru II to Ru II energy migration (lifetime of 1 4 ns, 50 times faster than Ru II excited state decay) were observed Exp erimental results and Monte Carlo simulation s conclude that the intrapolymer energy transfer quenching involves a combination of random walk, energy migration ( k mig ), and energy transfer ( k E n T ) events. O ver 80% of the energy transfer quenching events utili ze one or more Ru II to Ru II energy migration steps with contributions to energy transfer from pathways
44 in which there a re at least 100 migration steps. I t is demonstrated that the 2 3 average periphery to periphery distance between nearest neighbors in the polymer is sufficient to promote facile through space energy migration and energy transfer. 8 54 58 Figure 1 14 Structure and energy transfer model of the copolymer [co PS 4 CH 2 CH 2 NHC(O) (Ru II ) 17 )(Os II ) 3 ](PF 6 ) 40 (a) Chemical structures of the copolymer and ligands. (b) Molecular structure of the copolymer from a Monte Carlo simulation. (c) Model for the energy transfer quenching. Adapted with permission from r ef. 54 and 55. Copyright 2001 and 2002 American Chemical Society. 1.3 Preparation of Side Chain Conjugated Polyme r: Direct Polymerization of Functional Monomer 1.3.1 Conventional Free Radical Polymerization The use of macromolecular structures for the assembly of arrays of chromophores attached to a single polymer backbone was pioneered by Fox et al. 55 and Schneider et al. 57 They prepared polymers with naphthalene chromophores by
45 copolymerization of styrene and vinylnaphthalene or acenaphthalene using free radical polymerization. T his kind of light harvesting polymer was further studied by Guillet, 59 Webber and many other researchers, 2 with chromophores such as biphenyl, ph enanthrene, carbazole, pyrene, anthracene, etc. Synthetically these polymers were prepared via conventional free radical polymerization, either by homopolymerization of chromophore co ntaining vinyl monomers or copolymerization of chromophore containing monomers and other monomers, such as styrene and methacrylate s T he advantage of free radical polymerization is that this technique has been well developed and easy to handle. Figure 1 15 Example of free radical polymerization. While the earlier work s of preparing side chain conjugated polymers were focused on the photophysical studies, efforts were also made to the device a pplication. Due to the easy solution processing of side chain conjugated polymers, they are used to prepare single and multi layer LED devices. 60 64 Vinyl grou ps can be installed onto functional chromophores in several ways. The functional chromophores can be converted to urethane/methacrylate or urea/methacrylate monomer by reaction between amine groups on chromophores and isocyanate group of 2 isocyanatoethyl methacrylate ( Figure 1 1 7 1 2 7 ) 61 62 Hydroxyl group bearing chromophores can react with methacryloyl chloride to make functional methacrylates ( Figure 1 1 7 3 4 5, 6 ). 60 In
4 6 addition, Wittig reaction can make carbon carbon double bonds directly attached to chromophores ( Figure 1 1 7 8 9 10 ). 64 Figure 1 16 Side chain conjugated polymers made by conventional free radical polymerization. Unfortunately, due to the inevitable, near diffusion controlled bimolecular radical coupling (chain termination) and disproportionation (chain transf er) reactions, the polymers made by conventional radical polymeri z ation usually have a broad molecular weight distribution without molecular weight and architecture control. Chain polymerization without chain breaking is referred as living polymerization. Living polymerization provide s well defined polymers with desired molecular weight, low polydispersity indexes (PDIs) and controlled architecture s. 65
47 Anionic polymerization is the first living polymerization studied by polymer chemist s but it is difficult to handle and extremely sensitive to oxygen and moisture and not functional group tolerant 65 With the development o f controlled/living radical polymerization (CRP) in the 1990s, it is possible to prepare well defined polymers with desired molecular weight and narrow molecular weight distribution effectively. Several CRP approaches, including atom transfer radical polym erization (ATRP) 66 68 nitroxide mediated polymerization (NMP) and reversible addition fragmentation chain transfer polymerization (RAFT) 69 70 have been reported. In addition, metathesis polymerization techniques, such as ring opening metathesis polymerization (ROMP), are also considered as controlled/living polymerization when proper catalysts are used 1.3.2 Anionic Polymerization propagating species are anion s which can be initiated by nucleophilic initialization and electron transfer. While two pro pagating polymer chains have the same negative charge, the inter chain bimolecular termination is impossible. Varieties of base can act as nucleophilic initiators, including covalent or ionic metal amides such as NaNH 2 and LiN(C 2 H 5 ) 2 alkoxides, hydroxides cyanides, phosphine s amines O rganometallic compounds such as n C 4 H 9 Li and Ph MgBr and alkyllithium compounds are most useful of these initiators ( Figure 1 1 7 a). Electron transfer initiation is initiated by radical anions such as sodium naphthalene or l ithium in liquid ammonia ( Figure 1 1 7 a). As the propagating species are anions in the living anionic polymerization, the monomers are restricted to those with electron withdrawing functional groups to delocalize the negative charge and stabilize the anions These monomers includes styrene dienes methacryl ate vinyl pyridine aldehydes epoxide episulfide,
48 cyclic siloxane lactones acrylonitrile cyanoacrylate propylene oxide vinyl ketone acrolein vinyl sulfone, vinyl sulfoxide, vinyl silane and isocyanate If there is no impurity in the polymerization system, or no transfer agents added, propagation o ccurs with complete consumption of monomer to form living polymers. W hen more monomer is added, the polymerization will continue. T he non termination is a key feature for the living polymerization. However, the polymerization can be quenched by water or al cohol when the polymerization is finished ( Figure 1 1 7 e). Carbon dioxide, epoxy, isocyanate and other reagents can be added after complete polymerization to form functional end groups Figure 1 17 Mechanism of living anionic polymerization.
49 As a living polymerization, anionic polymerization was utilized to prepare side chain conjugated polymers with controlled molecular weight and narrow molecular weight distribution. Hirao, Chen and coworkers employed anionic polymerization to prepared a family of homo and copolymers of polystyrene with pendant conjugated oligo( fluorine ) chromophore moieties with PDI less than 1.08, and the molecular weight of the polymers were ranging from 3500 to 72400 g/mol by varying the monomer/initiator ratio. 71 Figure 1 18 Living anionic polymerization of s tyrene d erivatives para s ubstituted with c onjugated o ligo(fluorene) m oietie s. Adapted with permission from r ef. 71 Copyright 2009 American Chemical Society
50 The photopolymer with different oligof l uorene length were used to fabricate non volatile memory devices. 72 Hirao, Chen and coworkers reported different oligof l uorene length affected the turn on threshold voltages. Meanwhile, different surface morpholog ies of polymers were achieved by using good solvent (chlorobenzene, CB) and mixture of good/poor solvents (CB/DMF). T he results showed the polymer thin film from CB/DMF mixture gave larger polymer aggregation domains which promoted the diffusion of the Al atoms into the polymer film and formed the conduction channel and sign ificantly reduced the turn on threshold voltage on the studied polymer memory devices. T he polymer memory characteristics could be efficiently tuned through the pendent conjugated chain length and surface structures. 72 1.3.3 Controlled Radical Polymerization Controlled radical polymerization, or living radical polymerization ha s been achieved by minimizing normal bimole cular termination and prolonging the lifetime of living polymers into hours or longer through the introduction of dormant states for the propagating species ( Figure 1 20 ). This is achieved through alternate modes of reaction for the propagating radicals, s pecially, by either reversible termination (ATRP and NMP) or reversible transfer (RAFT). 65 Figure 1 19 General concept of controlled radical polymerization (CRP). Reproduced with permission from r ef. 73. Copyright 2013 Elsevier.
51 126.96.36.199 A to m transfer radical polymerization (ATRP) Since the initial discovery in 1995, 66 68 a tom transfer radical polymerization (ATRP) has become one of the most powerful an d robust CRP techniques, which result in unprecedented control over the preparation of many new well defined (co)polymers with predictable molecular weight (MW) and narrow molecular weight distribution (MWD). 73 74 Figure 1 20 Mechanism of transition metal catalyzed ATRP Adapted with permission from ref. 7 6 Copyright 2009 American Chemical Society T he ATRP mechanism 75 77 ( Figure 1 2 1 ) involves h o molytic cleavage of the alkyl (pseudo)halogen bond (R X) by a transition metal/ligand complex in its lower oxidation state (M n Y/L m ) to generate the corresponding higher transition metal complex (XM n +1 Y/L m ) and an alkyl radical (R ), with a rate constant of k act The resulting alkyl radicals (R ) initiate the polymerization by adding across the double bond of a vinyl monomer. Once the polymerization is initialized the radicals propagate ( k p ), and terminate by coupling or disproportionat ion ( k t ), or by reversibly deactivated in this equilibrium by XM n +1 Y/L m ( k deact ). I n a well controlled ATRP system, radical term ination
52 is diminished as a result of the persistent radical effect (PRE) 78 79 that strongly shifts the equilibrium towards the dormant species R X (i.e., k act << k deact ). Efficient ATRP employs the transition metal/ligand complex as the catalyst to obtain good control over the molecular weight and MWD. The majority of publications on ATRP deal with Cu mediated process L igands for Cu to form active and inexpensive catalytic complexes have been developed most of which are nitrogen based. 75 80 The commonly used ligands are summarized in Figure 1 2 2 81 One advantage of ATRP is that all ATRP reagents, including initiators, copper salts and ligands are commercially available and relatively inexpensive Figure 1 21 Common nitrogen based ATRP ligands. Zhao et al. 82 reported the application of ATRP in the preparation of a series of liquid crystalline diblock c opolymers composed of a polystyrene block and a polymethacrylate block with an azobenzene moiety in the side chain and the PDI was controlled to be less than 1.3. The homopolymer of azobenzene containing polymer PAzo had also been prepared with a PDI o f 1 .23 ; however, the molecular weight of PAzo is much higher than the calculated value based on the feed ratio of
53 monomer/initiator. One possible explanation proposed by the authors wa s that part of the initiator was not active in inducing polymerization of t he azobenzene monomer Lin et al. 83 synthesized side chain conjugated polymer with 4, 4 bis(biphenyl) fluorene pendants with commercialized ethyl 2 bromo 2 methylpropanoate initiator and polystyrene macroinitiator. All polymers were obtained with PDIs less than 1.30. The structures of the polymers are shown in Figure 1 22. Figure 1 22 Examples of polymers prepared via ATRP. However, ATRP has its own drawbacks. First, the monomer/initiator/copper ratio needs to be optimi zed for each monomer. Second, the tolerance for monomer is not good enough S ome nitrogen bearing monomer s halogen bearing monomer and some other monomers cannot be polymerized by ATRP. Third, as copper is involved in the polymerization system, it may aff ect device performance of the resulting polymer. 65 188.8.131.52 Nitroxide mediated polymerization (NMP) Nitroxide mediated polymerization (NMP) 84 89 is another CRP technique based on the reversible termination mechanism. The growing p ropagating (macro)radical is terminated by the nitroxide, acting as a control agent, to yield a ( macro)alkoxyamine as the predominant species. This dormant ( macro)alkoxyamine functionality generates
54 back the propagating radical and the nitroxide by a simpl e homolytic cleavage upon temperature increase. The activation deactivation equilibrium between dormant and active species is established Unlike ATRP or RAFT, this equilibrium takes the advantage of being a purely thermal process with neither catalyst nor bimolecular exchange being required. T he polymerization kinetics is controlled by both the activation deactivation equilibrium (with an activation deactivation constant K= k a / k d ) and the persistent radical effect (PRE). 89 Figure 1 23 Mechanism of the NMP process. Reproduced with permission from ref. 73. Copyright 2013 Elsevier. T he initiation system of NMP can be divided into two categories The first contains a conventional thermal initiator, s uch as 2,2 azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO), and a stable free nitroxide radical such as 2,2,6,6 tetramethylpiperidinyl 1 oxy (TEMPO) 85 In this bimolecular system, conventional radical polymerization process conditions a re employed with the only additions of free nitroxides, which is both economic and practical desirable. However, fine tuning of the
55 nitroxide/initiator ratio is required to control the polymerization kinetics for each polymerization system. Rizzardo 84 and Hawk er 88 90 92 developed the concept of unimolecular initiator, alkoxyamine, the second initiation system. It dec omposes into both the initiating radical and the nitroxide. Upon thermal dissociation 1: 1 initiating radical and the nitroxide releases and shows better control over molar masses and MWDs than bimolecular systems. A nother advantage of alkoxyamine is that the structure can be tuned to allow advanced macromolecular synthesis or post modification. The most used alkoxyamines are SG1 based and TIPNO based initiators. T he SG1 based alkoxyamine BlocBuilder (also referred as MAMA SG1) was commercialized by Arkem a in kilogram scale in 2005 and TIPNO based initiators have been commercialized by Sigma Aldrich recently. 88 The structures of common used nitroxides and alkoxyamines are presented in Figure 1 25. Figure 1 24 Structures of commonly used nitroxides and alkoxyamines. The alkoxyamines need high temperature, normally around 120 o C, to dissociate to initiating radicals and nitroxides, it may cause some side reaction s during the polymerization. However, the monomer tolera nce of NMP is much better than that of ATRP. Vinylbenzyl hal ide and 1,3 dienes can also be polymerized by NMP NMP controles the polymerization of styrene and styrene derivatives best among the publications ; however, it shows low controllability over m e thacrylates monomers, due to
56 slow combination of nitroxides with sterically hindered propagating poly(m e thacrylate) radicals and degradation of poly( methacrylate ) radicals via hydrogen abstraction by nitroxides. 88 As there is no metal catalyst existing in NMP systems, NMP has been used in preference to ATRP for polymerization of monomers which are intended for electronic application, suc h as arylamine functionalized monomers, oxadiazole, and carbozoles ( Figure 1 2 6 ). 88 Figure 1 25 Selected monomers polymerized by NMP for electronic applications. Reproduced with permission from r ef. 89 Copyright 2013 Taylor & Francis Thelakkat et al. 38 40 93 prepared a series of donor acceptor block copolymers using NMP. T hese polymers contain a poly(vinyltriphenylamine) s egment (PvTPA) as hole transport block and a polya c rylate segment bearing perylene bisimide side groups as electron transport and light harvesting block (the structures of the block copolymers are shown in Figure 1 26). The polymers exhibit nanostructured bulk heterojunctions (BHJ) required for photovoltaic applications when casting into thin films. T he nanodomains are 15 nm in diameter, which is in the same range as the exciton diffusion.
57 Figure 1 26 Various amorphous cryst alline donor acceptor block c opolymers synthesized by NMP. Adapted with permission from r ef. 38 and 39. Copyright 2010 The Royal Society of Chemistry and 2007 John Wiley & Sons. Figure 1 27 Polymerization of donor acceptor block copolymers via NMP. Adapted with permission from r ef. 39. Copyright 2007 John Wiley & Sons
58 The synthesis of the block copolymers are shown in Figure 1 2 8 The donor block, PvTPA, was prepared using the common unimolecular alkoxyamine, N tert Butyl N (2 methyl 1 phenyl propyl) O (1 phenyl ethyl)hydroxylamine as the initiator and 10 mol% to 50 mol% of TIPNO free nitroxide were added to obtain low polydi spersity T he resulting macroinitiator ( 26 27 28 ) had PDIs from 1.14 to 1.26. T he second block, PPerAcr, was initiated from the macroinitiators. In order to achieve long block of PPerAcr, the polymerizations were undertaken in concentrated PerAcr in the absence of free nitroxide. However, 5 mol% of styrene was added in the system to maintain sufficient control. T he resulting block copolymers ha d ~80 wt% of PPerAcr block and the PDIs were controlled to be from 1.14 to 1.26. These results show that NMP is c apable of preparing block copolymers even with monomers having rigid side groups. 39 As for the G2 b PPerAcr copolymer, it was synthesized from the dendronized alkoxyamine initiator, G2 TIPNO. 93 Starting from the functionalization of the commercial available alkoxyamine, TIPNO BzCl, G2 TIPNO w as prepared via nucleophilic displacement of chloride under basic conditions to form ether linkage with G2 dendron containing TPA units. T he resulting dendronized initiator controlled the polymerization of PerAcr monomer, obtaining polymers with different molecular weight s but all with PDIs around 1.1 ( Figure 1 2 9 ). This example demonstrated the possibility of alkoxyamine initiat or functionalization and preparation of polymers with functionalized end group via NMP.
59 Figure 1 28 Synthesis of dendronized initiator and subsequent polymer prepared via NMP. Adapted with perm ission from r ef. 93. Copyright 2009 John Wiley & Sons Fr chet et al. 9 also employed NMP to prepare b lock copolymers and applied them in white emitting LEDs. The block polymer, (TPA r Blue) b (OXA r Red), contains two blocks of random copolymers. T h e first copolymer block includes blue emitting Ir(dfppy) 2 (tpzs) and hole transporting triphenylamine (TPA) m oieties, while the second block consists of red emitting Ir(pq) 2 (tpzs) and hole transporting oxadiazole (OXA) moieties. By controlling the molecular weight of the polymer and relative content of two blocks, phase separation of the blue and red blocks can b e tuned to make the nanostructured domain size large enough to achieve the isolation of the two
60 phosphorescent emitters and suppress energy transfer between the two emitters. During the polymerization, the blue block was prepared first, resulting the mac roinitiator 36 T he red block was initialized by the macroinitiator subsequently. T he final block copolymers had PDIs varying from 1.2 to 1.5. The phase separation domain sizes were controlled to be from 26 nm to 49 nm by the structures of block copolyme r, which was larger than the exciton diffusion length and surpressed the energy transfer between two iridium complexes. Figure 1 29 Block copolymer prepared via NMP for LED application. Ada pted with permission from r ef. 9. Copyright 2006 John Wiley & Sons 184.108.40.206 R eversible addition fragmentation chain transfer (RAFT) polymerization While ATRP and NMP are controlled radical polymerizations based on r eversible termination mechanisms RAFT is a CRP technique based on the reversible chain transfer mechanism. 69 70 The key feature of the mechanism of RAFT po lymerization is a sequence of addition fragmentation equilibriums as shown in Figure 1 3 0 T he i nitiation occur s as in conventional free radical polymerization and
61 conventional initiator such as AIBN and BOP are utilized In the early stages of the polymer ization, addition of a propagating radical (P n compound (RSC(Z)=S, 38 referred as RAFT agent or chain transfer agent ( CTA ) produces the intermediate radical ( 39 ), which is followed by fragmentation and provides a polymeric thioca rbonylthio compound (PnS(Z)C=S, 40 T reacts with monomer and forms a new propagating radical (P m between the active propagating radicals (P n and P m thiocarbonylthio compounds ( 40 ) provides equal probability for all chains to grow and allows for the production of polymers with narrow polydispersity. When the polymerization is complete (or stopped), most chains retain the thiocarbonylthio end group and can be isolated a s stable materials. T he polymer with thiocarbonylthio end group can act as macro chain transfer agent and is able to control the polymerization of the second monomer to prepare another block 94
62 Figure 1 30 Mechanism of RAFT polymerization. Reproduced from ref. 94 Copyright 20 06 CSIRO. RAFT polymerization has the same condition as conventional free radical polymerization with only the addition of thio carbonylthio RAFT agents to induce rapid addition fragmentation transfer. A wide range of thiocarbonylthio compounds can be used as RAFT agents, including certain trithiocabonates, di ithio ester s xanthates, dithiocarbanmates and other compounds. T he struct ure features of RAFT agents are shown in Figure 1 3 1 and examples of commonly used RAFT agents are presented in Figure 1 3 2 The effectiveness of the RAFT agents depends strongly on monomers and the properties of the free radical leaving group R and the gr oup Z which can be chosen to activate or deactivate the thiocabonyl double bond and modify the stability of the intermediate radicals. Tuning structures of R and Z groups allow s introducing functional
63 groups into the RAFT reagents, thus mak ing end group po st modification of resulting polymer possible. 94 Figure 1 31 Structural features of RAFT agents. Reproduced from ref. 94 Copyright 20 06 CSIRO. Fig ure 1 32 Examples of RAFT agents. Adapted from ref. 95 Copyright 2011 The Royal Society of Chemistry. RAFT has the most tolerance over monomers among the three CRP processes ; in principle, all monomers that can be polymerized by conventional free radical polymerization can also be polymerized by RAFT polymerization in the presence of efficient chain transfer reagents 36 Other advantages of RAFT over ATRP and NMP include no metal catalyst involving and normal free radical polymerization conditions employing (high reaction temperature is not required). 95 O ne disadvantage of RAFT is that polymers obtained by RAFT polymerization have dithioester groups, which have
64 some associated odors and colors. 65 The sulfur containing end group s may have side effects on the performance when the polymers via RAFT polymerization are utilized in optoelectron ic applications. D ithioester groups are known to be efficient fluorescence quencher s 96 Even though dithioester groups can be easily hydrolyzed to thiol group, it can also quench excited states of pendant chromophores. 97 However, RAFT polymerization has still been employed in the preparation of functional polymers for optoelectronic applications 95 Figure 1 33 Preparation of light harvesting polymers via RAFT polymerization Adapted with perm ission from r ef. 36. Copyright 2010 The Royal Society of Chemistry. Ghiggino and Thang et al. 36 prepared a light harvesting polymer by RAFT polym erization containing acenaphthyl energy donors and a terminal anthryl energy acceptor (P(AcN) AN, 42 ). T he homopolymer has a low PDI of 1.08. Because of the energy transfer from acenaphthyl to dithioester, the energy transfer efficiency from acenaphthyl to anthryl is only 15% T his va lue can be increased up to 70% by us ing P(AcN) AN as a macroCTA to install a poly(methyl acrylate) block to separate dithioester group from the poly acenaphthyl block. T he synthesis of the polymers is illustrated in Figure 1 3 3
65 Many vinyl monomers with pendant functionalit ies were polymerized by RAFT polymerization, either by homopolymerization or copolymerization with other monomers such as styrene and methyl acrylate, and were employed in the optoelectronic application. Thes e monomers include organometallic compounds, carbozole and triphenylamine derivatives, arylene diimides and boron cantaining monomers. T he structures of examples of these functional monomers are presented in Figure 1 34.
66 Figure 1 34 Monomers with pendant functionality polymerized by RAFT and used in optoelectronic applications. Adapted from ref. 95 Copyright 2011 The Royal Society of Chemistry.
67 1.3.4 Metathesis Polymerization Metathesis polymerization is based on olefin metathesis reaction, in which two carbon carbon double bonds are reacted to form two new olefins following Chauvin s exchange mechanism ( as shown in Figure 1 3 5 ). 98 99 Initially reported in 1950s, olefin metathesis has been widely used from small molecular synthesis to polymer preparation O lefin metathesis can be used to form complex cyclic systems and medium and large rings difficult to be achieved previously which is particular ly useful in pharmaceutical chemistry. 98 99 When cyclic olefins or acyclic dienes are involved polymer s are formed in the metathesis reactions, ref erred as ring opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) respectively Figure 1 35 Chauvin mechanism of olefin metathesis. Reproduced with permission from r ef. 99. Copyright 2005 Springer. Figure 1 36 Types of olefin metathesis reactions. Reproduced with permission from r ef. 99. Copyright 2005 Springer.
68 220.127.116.11 R ing opening metathesis polymerization (ROMP) From the structural nature of cyclic olefin, r ing opening meta thesis polymerization (ROMP) is a chain polymerization process in which the cyclic olefin monomers are converted to a polymeric chain. D uring the polymerization, any unsaturation associated with the monomer is conserved. Figure 1 3 8 illustrate s t he mecha nism of ROMP. T he polymerization Initiat es with coordination of a transition metal alkylidene complex (i.e. a catalyst for olefin metathesis) Then a four membered metallacyclobutane intermediate is formed by the [2+2] cycloaddition. This intermediate undergoes a cycloreversion reaction to afford a new metal alkylidene the beginning of a growing polymer chain. T he reactivity of the believe d to be similar to the initiator although it has increased in size due to the incorporated monomer Hence, analogous steps are repeated during the propagation process until the polymerization is stop ped (e.g., all the monomer is consumed completely or the reaction is quenched) The polymerization can be stopped or quenched by a chain transfer agent (CTA) T he CTAs are usually acyclic olefins which undergo a cross metathesis reaction with the end grou p of the growing polymer chain to cleave the metal alkylidene Thus there is a good chance for chain end modification during the polymer termination. 98 100 101
69 Figure 1 37 General mechanism for ROMP. Reproduced with permission from r ef. 98 Copyright 20 07 Elsevier. When proper transition metal catalyst is chosen to initiate the polymerization rapidly and convert to growing polymer chains quantitatively, the ROMP can be considered as a living polymerization. 98 The normally used living ROMP catalysts include Schrock type catalysts and Grubbs catalysts, which have molybdenum and ruthenium metal center, respectively. The examples of the catalysts are presented in Fig ure 1 39. Figure 1 38 Commonly used olefin metathesis catalysts. A s the driving force for ROMP is the releasing of ring strain from cyclic olefin monomer to linear polymer, a huge variety of monomers have been successfully polymeri z ed using ROMP. Typical strained cyclic olefins monomer s for ROMP include
70 norbornenes, norbornadienes, 7 oxonorbornenes, azanorbornenes, cyclobutenes cyclooctenes cyclooctadienes cyclooctatetraenes and many othe rs. For advanced functional polymer preparation, norbornene derivatives are doubtless the preferred monomers 98 100 101 ROMP has been widely used in the preparation of electro active polymers with pendant chromophores. Schrock and Rubner et al. 102 synthesized electroluminescen t polymer by ROMP in 1997. T he blue emitting norbornene derivative was polymerized via ROMP by Schrock type [Mo] catalyst; the polymerization had a 95% con version yields and produced a polymer with a low PDI of 1.04 (as illustrated in Figure 1 40 ). T he same strategy was also applied to the polymerization of electron transporting and hole transporting norbornene derivatives, in which polymers with low PDI val ues of 1.02 1.08 were obtained. Figure 1 39 ROMP of a blue emitting polymer with Mo catalyst. Adapted with permission from ref. 102 Copyright 2009 American Chemical Society An advantage of ROMP is its tolerance of metal containing monomers when using Grubbs ruthenium catalysts. Sleiman et al. 103 104 prepared polymers with pendant ruthenium tris(b ypridine) (Ru(bpy) 3 ) complexes ( 44 Figure 1 4 1 ). T he homopolymerzation of 44 finished in 20 min utes with the third generation Grubbs catalyst ( 45 ). T he monomer conversion was observed to be linear depended on reaction time. 103 B lock copolymer containing an alkyl substituted group ( 47 ) were also prepared.
71 B y varying the ratio of alkyl block and Ru block, the block copolymer self assembled into different shapes such as vesicles, tubes, bowls and stars. 104 Other cycli c olefin monomers containing r uthenium 105 iridium, 106 107 platinum 108 109 and aluminum 110 complexes have also been polymerized via RO MP. Figure 1 40 Preparation of polymers with pendant Ru complexes by ROMP. Adapted with permission from ref. 103 and 104 Copyright 200 4 and 2007 American Chemical Society
72 Figure 1 41 Examples of polymers with pendant metal complexes prepared via ROMP. 105 110 18.104.22.168 A cyclic diene metathesis (ADMET) ADMET polymerization is performed on dienes to produce strictly linear polymers with unsaturated polyethylene backbone. While ROMP is chain growth polymerization, ADMET is step growth polymerization, which is a thermally neutral process and driven by the release of ethylene. Based on the nature of step growth polymerization, ADMET is not living polymerization H owever, ADMET creates perfect region regular polyethylene s with precisely placed braches, because of near quantitative monomer to polymer conversion and few side reaction s 99 Due to the perfectly region regularity, ADMET is noteworthy in the side chain polymer prepara tion An example of electroactive polymers prepared via ADMET was reported by Reynolds and Wagener et al 11 12 T he parent polymer with boron esters was prepared b y ADMET
73 polymerization first, and then blue, green and red emitting chromophores were attached to the polymer via Suzuki coupling reactions. Figure 1 42 Preparation of electroactive polyme rs via ADMET and post functionalization. Adapted with permission from ref. 12 Copyright 20 10 American Chemical Society 1.4 Preparation of Side Chain Conjugated Polymer: Post Polymerization Modification With the development of living polymerization, it allow s for the synthesis of functional polymers with precisely defined molecular weight, composition and architecture Direct polymerization of functional monomers is apparently a more attractive strategy ; however, there are still disadvantages of this strategy The first one is monomer tolerance A lthough there is significant improvement over tolerance of functional groups in monomers in CRP and ROMP, there is still a broad range of monomers with side chain functionalities that cannot be direct ly polymeriz ed us ing any currently available controlled polymerization techniques. Such functional groups may either completely prevent controlled polymerization or participate in side reactions that lead to loss of control over the polymerization. A n d in some cases, the l arge monomer
74 will introduce steric hindrance so that the molecular weight of resulting polymer is too low. 111 The second is economic consideration of monomers with functional groups synthesis and purification. Gene ral ly living polymerizations such as CRP will lose control if the conversion of monomer is high. S ome monomers, which t ake a lot of time and effort to prepare, will waste during the polymerization. T h is is economically undesirable especially for functiona l groups with rare earth metals. On the other hand, because of the high purity requirement for monomers in the living polymerization, the cost of purification of synthetic monomers may be very high. 112 Post polymerization modification, also known as polym er analogous modification, 112 113 is an alternate strategy that can overcome the technical and economical limitation of direct polymerization of functional mon omers. As illustrated in Figure 1 43, post polymerization modification is based polymerization of monomers with functional groups that are inert towards the polymerization conditions but can be quantitatively converted in a subsequent reaction step into a broad range of other functional groups. 112 As the function al monomers to be polymerized are inert to the corresponding polymerization, the monomer tolerance limitation has been overcome In addition, the post polymerization modification strategy is also highly attractive for combinatorial materials discovery. Whe n a single reactive polymer precursor is prepared, a diverse library of functional polymers with identical chain lengths and chain length distributions can be generated based upon the parent precursor With orthogonal modification reactions, copolymers wit h different functional groups can also be prepared. 112 In this section, different reactive polymer precursors and post
75 polymerization modification reactions used in the preparation of side chain conjugated polyme rs will be briefly reviewed. Figure 1 43 Synthesis of polymers by post polymerization modifications. Reprinted with permission from r ef. 112. Copyright 2006 John Wiley & Sons 1.4.1 S N 2 Reaction S N 2 nucleophil ic displacement of a leaving group in the polymer precursor might be the easiest post polymerization modification reaction. Meyer et al. 8 114 118 prepared a variety of functional polymers stating from styrene p (chloromethyl)styrene copolymers, in which chloride is a good leaving group. Via S N 2 reaction with bypridine, amine, alcohol and acid, conjugated functional groups were att ached to the parent copolymers with C N, ether and ester linkage s as illustrated in Figure 1 44. However, the S N 2 reactions with these nucleophiles are not quantitative.
76 Figure 1 44 Post polym erization modification via S N 2 reactions. 8 114 118 1.4.2 Amide Coupling Papanikolas and Meyer et al. 53 54 58 119 also reported the preparation of amide linked polypy ridylruthenium deviate d polystyrenes from the amide coupling reaction between amine containing polystyrenes and carboxylic acid bearing polypyridylruthenium complexes. T he parent polymer precursors were prepared by conventional free radical polymerization 58 119 and living anionic polymerization. 53 54 T he reaction condition was borrowed from peptide chemistry, in which the amide coupling reaction between primary amine and carboxylic acid was catalyzed by the combination of 4 (dimethylamino)pyridine (DMAP), 4 methylmor pholine (NMM) b enzotriazol 1 yloxy tris(dimethylamino)phosphonium hexafluorophosphate (BOP) and 1 hydroxybenzo triazole hydrate (HOBT) as presented in Figure 1 45. T he coupling reaction was quantitative according to 1 H NMR characterization. 53 119
77 Figure 1 45 Post polymerization modification via ami de coupling. 53 54 58 119 1.4.3 Reaction with Active Ester S ince the pioneered work by Ferrti and Ringsdorf et al. 120 121 a broad variety of active ester s ha ve been developed introduced into vinyl monomers and polymerized by different living polymerization techniques. T he commonly used active ester includes N hydroxysuccinimide (NHS), petafluorephenyl (PFP), tetrafluorephenyl (TFP), thiazolidine 2 thione (TT), acryloyl oxime (AO) and nitrophenyl (NP) groups, in which NHS a nd PFP are most frequently employed. 113 The structures of these active esters are shown in Figure 1 46. On the other hand, the vinyl monomers of active esters are normally based on methacrylates ( MAs) and 4 vinyl benzoate (VB). Figure 1 46 Structures of active esters. 113 The reaction between active ester polymers with amines is the most frequently used in the post polymerization modification to react with active esters and make amide linkage Because of their good nucleophilicity, amines are able to react selectiv ely even
78 in the presence of other weaker nucleophiles such as alcohols. 112 113 Because of the high re activity of activ e esters, the conversion from active ester to amide linkage is normally quantitative. An example of employing activ e ester polymer as the precursor to prepare side chain conjugated polymer was reported by Tew et al. ; 122 N succinimide p vinyl benzoate (NHSVB) was polymerized by RAFT, and then the resulting polymer was rea cted with an amine functionalized terpyridine ( terpy ) creating a new homopolymer containing terpy ligands on every monomer as demonstrated in Figure 1 47. T he pendant terpy group can be further utilized for metal coordination to install metal complexes o nto the polymer. Figure 1 47 Post polymerization modification via the reaction between amine and active ester. Adapted with permission from r ef. 122. Copyright 2006 John Wiley & Sons 1.4.4 Metal Co ordination Reaction As discussed in the previous section, one way to prepare metallopolymers with metal containing pendants is to direct ly polymerize monomers with pendant metal complexes. 95 However, the common way to prepare this type of metallopolymers is to prepare polymer precursor s with metal coordination (also referred as metal ligation) functionalities and introduce metal species by post functionaliz ation T he most
79 frequently employed metal coordination funct ional groups are terpyridine (t er py, coordinates with ruthenium) 123 a nd acetoacetate (acac, coordinates with platinum 10 and iridium 124 ). Examples of the reaction are presented in Figure 1 48. The yield of metal coordination in the polymeric system can hardly reach 100%. It is reported that the coordination between terpyrid ine and ruthenium was 75%, according to elemental analysis; 123 and the functionalization of acac with iridium was around 88% from 1 H NMR characterization. 124 Figure 1 48 Examples of metal coordination reactions in pos t polymer modification. 10 123 124
80 1.4.5 Palladium Catalyzed Coupling and Cross Coupling Reactions T he Palladium catalyzed Heck, Sonogoshira, Suzuki, and Stille reaction produce C C bonds in high yield under relatively mild reaction conditions, Thus these reactions ha ve also been employed in post polymerization modifications. Chan et al. 125 reported Heck reaction on the polystyrene b polyisoprene block copolymers, in which the functionalization yield varied from 22% to 44%. Reichmanis et al. 126 127 reported the stannylation of polystyrene and subsequent Stille reaction. T he yield of stannylation can reach more than 96% and the final Stille coupling was reported to be quantitative. Grubbs et al. 128 have explored the optimization of reaction conditions for the modification of poly( p bromostyrene) with phenylacetylene and 1 hexyne by Sonogashira coupling. With (PhCN) 2 PdCl 2 as catalyst and Tri tert butylphosphine as additional ligand, the conversion of bromide can reach up to 100% and 89% for phenylacetylene and 1 hexyne. As for Suzuki coupling, the preparation of boron containing polymers has been achieved by ATRP 129 131 and RAFT; 132 however, there is no report of S u zuki coupling based on these polymer s yet.
81 Figure 1 49 P alladium catalyzed coupling reactions in post polymerization modification. 1.4.6 Click Chemistry Considered as click reaction t h e copper (I) catalyzed Huisegen 1, 3 dipolar cycloaddition (also referred as Cu(I) catalyzed azide alkyne cycloaddition, CuAAC) has been exten sively employed in post polymerization modification, because it gives high yields under mild and simple reaction conditions in both aqueous and in organic media, generating only inoffensive byproducts. 133 136 The brief description of the mechanism of CuAAC is illustrated in Figure 1 50. 137
82 Figure 1 50 Copper(I) catalyzed azide alkyne cycloaddition. Reproduced with permission from ref. 137. Copyright 2013 The American Association for the Ad vancement of Science T he polymer precursor s for CuAAC, bearing azide, alk yne, and protected alkyne groups have been polymerized by a variety of living polymerization techniques. 112 113 136 Certain functional groups, such as epoxy 133 138 and benzyl halides, 97 can be converted to azide groups quantitatively. Figure 1 51 presents examples of post polymerization modification using CuAAC click rea ction. 97 139
83 Figure 1 51 Application of CuAAC click reaction in post polymerization modification. 97 139 Besides CuAAC click reaction, there are several other r esections belong ing to click chemistry such as Cu free azide alkyne cycloaddition, thiol ene, thiol yne, Diels Alder, thiol bromo, etc. It is possible to emerge different clickable functional groups into one polymer chain, and utilize different click reactions subsequently and orthogonally. 140
84 Figure 1 52 Preparation of copolymers with different functional groups with orthogonal click reactions. Reprinted with permission from ref. 139 Copyright 2009 American Chemical Society 1.5 Scope of Present Study In this dissertation, the author intends to utilize the post polymerization modification strategy to prepare several families of side chain conjugated polymers for fundamental photophysical studies and materials ap plications. T he goal for the polymer preparation is to synthesize polymers with desired molecular weight and low polydispersity, and quantitative chromophores loading during the post polymerization modification process. To this end, a synthetic strategy is designed, as demonstrated in Figure 1 53. T he polymerizations started with commercial available monomers, glycol methacrylate (GMA) and 4 vinylbenzyl chloride (VBC), because they are economical ly desirable and easy to purify. T he polymerization techniques employed were RAFT and NMP. The epoxy group in GMA and chloride in VBC can be converted to azide group quantitatively in mild conditions via S N 2 reactions. Finally, organic and organometallic
85 chromophores bearing a terminal alkyne group were graft ed on to the azide containing clickable polymer precursors. Figure 1 53 Synthetic strategy of side chain conjugated polymers in this dissertation. Several families of side chain conjugated polymers wer e prepared employing this synthetic strategy. First of all, a polymer bearing pendant non linear absorption chromophores (poly FBPt) was prepared. T he resulting polymer conserved all the photophysical properties of the small molecular chromophore, includin g the non linear absorption property. T he polymer can be cast into films easily and the polymer film still showed triplet triplet absorption and non linear absorption. The results of this work provide insight regarding the introduction of platinum acetylid es into polymers for optical applications. Secondly, different chromophores were installed onto polystyrene based backbone, acting as energy donor and acceptor, respectively. T he singlet and triplet energy transfer was characterized and studied in polymeri c systems with organic and organometallic chromophores. Finally, arrays of polypyridine ruthenium(II) complexes with polystyrene backbone were prepared and employed in dye sensitized solar cells (DSSCs)
86 CHAPTER 2 NON LINEAR ABSORPTION POLYMERIC ARRAY FROM CONTROL LED RADICAL RY 2.1 Background Nonlinear absorption (NLA) refers to the change in transmittance of a material as a function of light intensity or fluence. At sufficiently high intensities, the probability of a material absorbi ng more than one photon before relaxing to the ground state can be greatly enhanced. 141 So a n optical device based on NLA materials exhibits a linear transmittance below a specific input intensity or fluence level but, above this level, its output intensity varies non linearly with input NLA optical materials play a major role in the photonic te chnolog ie s and have been used in several applications, such as passive mode locking, pulse compression, and the most popular application: eye and sensor protection in optical system s (e.g., telescopes and night vision systems). 142 There are mainly two mechanisms for nonlinear absorption, including two photon absorption (TPA) and ex cited state absorption (ESA). Two photon absorption is the simultaneous absorption of two photons resulting in the excitation of a chromophore from the ground state to a higher lying state as demonstrated in Figure 2 1a. 141 143 This process involves different selection rules from those of single photon absorption. TPA strength of a chromophor e depends on the two photon absorption cross section ( 2 ). General guidelines of designing a TPA chromophore include (1) extended conjugation length with planar chromophores; (2) motifs such as D D, A A, and D A (where D = donor, A = acceptor and = conjugated spacer); (3) increasing the donor or a cceptor strength; (4) introduction of more polarizable unsaturated bonds; and (5) variation in the nature of the conjugated
87 bridge 143 145 TPA chromophores propert ies can be installed into organic molecules, liquid crystals, conjugated polymers, fullerenes, coordination and organometallic compounds, porphyrins and metalloporphyrins, nanoparticles, and biomolecules and derivatives. 143 146 Figure 2 1 Mechanisms of nonlinear absorption. A) Jablonski diagram for a typical chromophore that exhibits tw o photon absorption, intersystem crossing, and triplet triplet absorption. B) Schematic illustrating idealized ground state and excited state di ff erence absorption spectra for a chromophore that exhibits reverse saturable absorption. Adapted with permissio n from ref. 142 Copyright 20 11 American Chemical Society In addition to two photon absorption, reverse saturable absorption (RSA) is another pathway for nonlinear absorption. RSA chromophores feature a small but non zero ground state absorption cross se ction ( 1 ) and a large excited state absorption (ESA) cross section ( ESA ) at the same wavelength region of the ground state absorption ( Figure 2 1b). 143 145 The ESA originates mostly from the lowest energy singlet or triplet (S 1 or T 1 ), but can be originated from all possible transient states. Generally, as the light intensity increases, the population of the singlet and triplet excited states increases. In RSA, the excited state exhibits more absorption than the ground state, resulting in more absorption as the incident optical flux increases. The magnitude of RSA is determined by t he ratio ESA / 1 determines; the larger the ratio,
88 the stronger the RSA. RSA chromophores includes porphyrins, indanthrones, metal cluster compounds, fullerenes, cyanines, phthalocyanines, and naphthalocyanines 143 It is possible to combine TPA and ESA mechanisms for the nonlinear absorption and platinum acetylides are suitable candidate chromophores. Typical platinum acetylide complexes feature a four coordinated sq uare planar platinum(II) center with the general formula PtL 2 (C CR) 2 where L is typically a phosphine ligand ( e.g., P Bu 3 ) and C CR is an aryl acetylide unit (i.e., R = aryl). 147 When R is a TPA chr omo phore, such as benzothiazole 2, 7 fluorene (FB) and diphenylamino 2,7 fluorene (DPAF), both simultaneous TPA and one photon absorption can populate the singlet excited state (S 0 S 1 ) As a heavy metal, platinum can induce singlet to triplet (S 1 T 1 ) inters ystem crossing (ISC) via spin orbit coupling with a rapid dynamics and high efficiency ( ISC > 90%) The populated T 1 state will result in the strongly allowed T 1 T n transitions (ESA pathway) in the time of 10 to 400 ps time scale. T he combination of TPA a nd ESA will give strong overall nonlinear absorption in fs to s scale, because T PA occurs instantaneously (fs to ps), and then the triplet state is produced by ISC and persists into the ns to N onlinear absorption via T PA/ESA is most efficie nt in chromophores where the 2PA absorption maximum coincides spectrally with ESA. 143 Some examples of platinum acetylide complexes exhibiting TPA/ESA nonline ar absorption properties are presented in Figure 2 2. 148 149
89 Figure 2 2 Examples of platinum a cetylides with TPA/ESA mechanisms. 148 149 To utilize p latinum acetylide s for practical NLA materials it is necessary to i ncorporate into polymeric materials Pt acetylides can be doped into poly(methyl methacrylate) (PMMA) matrix to obtain a guest host solid Pt acetylides with multi acrylate functionality ( Figure 2 3 A ) can also be cross linked with MMA monomers. S ol gel processes are also employed to form gla ssy materials ( Figure 2 3 B ). Figure 2 3 Platinum acetylides used in NLA materials. Efforts have also be en made to prepare linear polymer s containing platinum acetylide units Our group has p repare d copolymers with platinu m acetylide containing acrylate monomer s and methyl methacrylate (MMA) monomers via free radical polymerization. However, it was found the maximum loading of platinum acetylide
90 chromophores c an only reach 12%. 111 In order to obtain polymers with high NLA chromophore loading, a post polymerization modification strategy was utilized A clickable polymer precursor was constructed first, and the nonlinear absorption platinum acetylide ch romophores were installed onto the polymer backbone via quantitative yield copper(I) catalyzed azide alkyne cycloaddition ( click reaction ) T he resulting polymer ha s chromophores in each repeat; it has typical polymer properties such as film forming abi lity and processing advantages and NLA properties conserved from small molecular chromophores. 2.2 Polymer Design and Preparation The well defined nonlinear absorption polymeric array prepared in this chaptor features a flexible, atactic and non conjugated po lyacrylate backbone and pendent small molecular NLA platinum acetylide c hromophore s The structure of the NLA chromophore ( FBPt ) and the polymer ( Poly FBPt ) are shown in Figure 2 4 Figure 2 4 Structures of FBPt and Poly FBPt 2.2.1 Synthesis of FBPt The structure of the NLA chromophore ( FBPt ) combines a fluorene benzothiazolyl (FB) unit and platinum(II) metal center. FB is a well known D D motif, yielding materials that have significant near IR TPA cross section Platinum is used to induce rapid intersystem crossing from singlet to triplet with high yields. As stated in the
91 previous section, FBPt has the nonlinear absorption property wit h a combination of TPA and ESA pathways. 150 Figure 2 5 Synthe tic route of FBPt The synthetic route of FBPt is shown in Figure 2 5 St arting from the trimethylsilyl protected benzothiazolylfluorene (FB), d eprotection of compound 1 result ed in compound 2 with a terminal alkyne. Efficient synthesis of monoalkynyl platinum(II) complex ( 3 ) was synthesized through reaction between compound 2 and cis Pt(PPh 3 ) 2 Cl 2 T he Hagihara coupling in the absence of copper(I) species allowed mono substitution of a chloride of cis Pt(PPh 3 ) 2 Cl 2 with alkyne Subsequent coupling between compound 3 and the mono protected diethynylbenzene ( 4 ) generate d the asymm etric dialkyny l Pt(II) complex ( 5 ) under mild conditions with a catalytic amount of copper(I) iodide. Deprotection of 5 with MnO 2 and KOH result ed in the FBPt c hromophore which had a terminal alkyne and was ready for the Cu(I) catalyzed azide alkyne cyclo addition ( click reaction )
92 2.2.2 Preparation of p olymer b ackbone and p oly FBPt The route used to synthesize Poly FBPt is outlined in Figure 2 6, which started from a commercially available monomer, GMA I t wa s with low cost and easy to purify The epoxy ring i n GMA can be opened with nucleophile azide salts (such as sodium azide) to make it clickable The narrow molecular weight distribution poly( glycal methacrylate ) PGMA ( 7 ) was polymerized via reversible addition fragmentation transfer (RAFT) polymerization of monomer, with 2 cyanoprop 2 yl 1 dithionaphthalate ( CPDN 6 ) 151 as chain transfer agent (CTA). T he structure of the chain transfer agent, CPDN invol ves a isobutylnitrile group (R), a good free radical leaving group both in absolute terms and relative to the propagating species derived from the monomer being polymerized and a naphthyl group as Z group, which enhance the reactivity of the C=S double bo nd. T he synthesis of CPDN was a one pot reaction which started from Grignard reaction 1 b T he resulting Grignard reagent was oxidized by DMSO, and then the disulfide bond was cleaved via free radical reaction with AIBN to give the chain transfer agent, CPDN as illustrated in Figure 2 6. 151 GMA was polymerized in the presence of both CPDN and AIBN (4:1 molar ratio). T he degree of polymerization (DP) of the resulting PGMA can be estimated from its molecular weight, which wa s 42. The pendent epoxy groups were ring opened with excess NaN 3 in the presence of NH 4 Cl, affording poly (hydroxyazidopropyl methacrylate) ( PHAZPMA 8 ) bearing one hydroxyl group and one azido group in each repeat unit. 152
93 Figure 2 6 Synthesis of Poly FBPt GPC analysis of 8 reveal ed a monomoda l and symmetric peak, indicating there wa s no cross linking or branching during the ring opening reaction with NaN 3 ( Figure 2 7 ) The ring opening reaction can also be evidenced by the 1 H NMR spectra ( Figure 2 8 ) After reaction, resonance signals characte ristic of epoxy moieties, 2.61 and 2.75 ppm ( C H 2 O in the epoxy group), 3.16 ppm ( C H O in the epoxy group) and 3.68 and 4.27 ppm ( OCH 2 C H next to the epoxy group), completely disappeared. On the other hand, the arising of new peaks, 3.35 ppm ( C H 2 N 3 ), 3.8 7 ppm ( OC H 2 CHOH) and 5.49 ppm ( CHO H ), further confirm ed the consumption of the epoxy group in the ring opening reaction with NaN 3 The FTIR spectra ( Figure 2 10 ) can also evidence the ring opening transformation. After reaction, the absorption peak at 90 9 cm 1 which wa s corresponding to the epoxy group, disappeared. And on the other hand, a strong band
94 at 2100 cm 1 was present in PHAZPMA which is the absorption band of the azido moieties. The dithioester group is believed to be removed affording a thio l end group 52 153 Although it is difficult to prove it from 1 H NMR spectra, it can be evidenced by UV vis absorp tion. The dithioester absorption peak in PGMA which is at 310 nm, disappeared after treating with sodium azide, as shown in Figure 2 9. T he color change of polymers, from pink ( PGMA ) to white ( PHAZPMA ), is caused by the dithioester end group removal. The 282 nm peak in the spectrum of PHAZPMA is due to absorption of azide groups. 154 155 Figure 2 7 GPC of polymers, PGMA (black, Mn = 8 30 kg/mol PDI = 1.21), PHAZPMA (red, Mn = 10 1 kg/mol PDI = 1.33) and Poly FBPt (blue, Mn = 4 69 kg/mol PDI = 1.31). With polystyrene standard.
95 Figure 2 8 1H NMR spectra of PGMA and PHAZ PMA. Figure 2 9 Comparison of absorption of PGMA and PHAZPMA
96 Figure 2 10 1 H NMR spectra of PHAZPMA Poly FBPt and FBPt The click reaction between PHAZPMA an d FBPt was catalyzed with CuBr under mild condition, g iving the polymer, Poly FBPt The proton NMR signal s for the polymer back bone w ere difficult to tell, as they were buried in the hydrogen signal of FBPt side groups; however, the broadening of FBPt peak s indicated the formation of polymer ( Figure 2 10 ) On the other hand, the reaction c ould be strongly evidenced by FTIR ( Figure 2 11 ) The azido absorption band (2100 cm 1 ) disappeared almost completely after the click reaction, and the band for the triazo le ring appeared in the spectrum of P oly FBPt which was located at 1650 cm 1 Noteworthy, the IR spectrum of Poly FBPt has been expanded along y axis for clarity purpose ; the OH stretching
97 bands around 3400 cm 1 for PHAZPMA and Poly FBPt should have the s ame intensity. In addition, the GPC trace of the resulting P oly FBPt shif ted to higher molecular weight (as shown in Figure 2 7), which fu r ther proved the successful grafting reaction between FBPt and PHAZPMA Figure 2 11 FTIR spectra of polymers, PGMA (black), PHAZPMA (red) and Poly FBPt (blue). 2.3 Photophysical Characterization in Solution 2.3.1 Steady state A bsorption and E missio n The photophysical properties of both the NLA c hromophore FBPT and the polymer Poly FBPt were first ly investigated by steady state absorption and photo lumin escence measurement in THF, a good solvent for both small molecular c hromophore and polymer. Figure 2 12 presents the ground state abs orption and steady state emission of the platinum acetylide chrom ophore ( FBPt ) and the NLA polymer ( Poly FBPt ).
98 Figure 2 12 Ground state absorption and steady state emission of FBPt (blue) and Poly FBPt (red). The ground state absorption spectrum of the polymer ( Poly FBPt red) in solut ion is nearly identical to spectrum of the small molecular chromophore ( FBPt blue). Both small molecular chromophore and polymer display strong absorption in the near UV region, which has a maximum around 378 nm. T he extinction coefficient of both chromo phore and polymer were measured (shown in Table 2 1) and the grafting efficiency of the post functional click reaction were calculated to be 98% based on the absor bance of polymer and the molar absorptivity of small molecular chromophore at 378 nm
99 The photoluminescence emission of the FBPt and Poly FBPt were examined via excitation at the ground state absorption maxima. The measurements were c onducted in deoxygenated THF solutions at room temperature and t he photoluminescence emission spectra are shown in Figure 2 12 FBPt and Poly FBPt exhibit both similar fluorescence (430 and 431 nm) and phosphorescence (567 and 568 nm) emissions. However, their quantum yields have significant differences. FBPt has a high phosphorescence quantum yield of 0.78, but thi s value of Poly FBPt decreased to 0.052; on the other hand, the fluorescence quantum of Poly FBPt is higher than FBPt (0.020 vs. 0.012). The quantum yield data are summarized in Table 2 1. These results indicate there are strong chromophore chromophore int eractions existing in the Poly FBPt most likely triplet triplet annihilation and caused delayed fluorescence
100 Table 2 1. Photophysical characteristics of small molecular chromophore, FBPt, and NLA polymer, Poly FBPt. Fl /n s d (A) Ph / s d ( ) FBPt 378 84500 430 0.0 12 1 = 10.20 ns (0.012) 2 = 0.85 ns (0.99) = 0.97 ns 567 0. 78 = 336 s 670 Poly FBPt (solution) 379 83800 431 0.0 20 1 = 9.98 ns (0.01 4) 2 = 0.63 ns (0.99) = 0.77 ns 568 0.0 52 1 = 11.19 s (0.42) 1 = 1.25 s (0.58) = 5.45 s 675 Poly FBPt (thin film) 375 ---452 ---1 = 4.47 ns ( 0.13 ) 1 = 0.76 ns ( 0.87 ) = 1.23 ns 567 ---1 = 17.38 s (0.4 1 ) 1 = 3.00 s ( 0.59 ) = 8.95 s 675 Ground state absorption maxima Fluorescence spectra, obtained by excitation at ground state absorption maxima With anthracene as quantum yield standard, Triplet excited state lifetimes, from t ime c orrelated s ingl e p hoton c ounting (TCSPC), and is median lifetime calculated as = A i i Phosphorescence spectra, obtained by excitation at ground state absorption maxima Triplet triplet transient absorption maxima
101 2.3.2 Triplet triplet T ransient A bsorption Figu re 2 13 Transient absorption spectra of FBPt ( blue ) and P oly FBPt ( red ) in deoxygenated THF Excited at 355 nm The triplet excited state is further studied via triplet triplet transient absorption (TA) as presented in Figure 2 1 3 Near UV excitation at 355 nm generates strongly absorbing transients for the small molecular platinum acetylide FBPt and the NLA polymer Poly FBPt in THF. T he shapes of TA spectra for FBPt and Poly FBPt are similar ; both show negative bands from 350 400 nm correspond ing to bleaching of the ground state absorption and positive triplet triplet (T 1 T n ) excited state absorption bands
102 across most of the visible region, with maxima at 670 and 675 nm T he polymer has a slightly red shift in the transient absorption On the other hand, the triplet excited state lifetime of Poly FBPt is much smaller than that of FBPt (5.45 s vs. 336 s ). This result agrees with the significant decrease of phosphorescence quantum yields, and further confirms the chromophore quenching interaction for Poly FBPt However, the existing of triplet triplet (T 1 T n ) excited state absorption of the polymer suggests it can still be a candidate NLA material 2.3.3 Nonlinear A bsorption R esponse All ground state absorption, steady state photo l uminescen ce and triplet triplet transient absorption have suggested that the resulting polymer, Poly FBPt exhibits similar photophysical characteristics as the small molecular NLA chromophore FBPt ; in addition, NLA response of the Poly FBPt polymer were e xamined by nanosecond open aperture z scan. 143 Figure 2 14 Schematic diagram of a simple open aperture z scan apparatus Adapt ed with permission from ref. 142 Copyright 2009 American Chemical Society O pen aperture z scan technique is a nonlinear transmission method and a typical setup of a simple open aperture z scan apparatus is illustrated in Figure 2 14. A beam splitter (B S) is utilized to divide the single laser beam into two equal parts; one will be detected by Detector 1 as the reference and the other will be focused by the
103 plano convex lens, pass through the sample and finally collected Detector 2. T he signal strength r atio by D e tectors 2 and 1 (I 2 /I 1 ) can be considered as the nonlinear absorption response. T he sample is placed in the light path between convex lens and Detector 2, and it is able to move one directional along the light path (z axis), in which the light f lux intensity is different at different positions. When the sample is placed at the focal point of the convex lens (z = 0), it receives highest light intensity; while at positions other than the focal point, the light intensities are lower; the farther awa y from z = 0 position, the lower the light intensity. In summary a V shape curve of I 2 /I 1 vs. z will be obtained for a NLA material, and I 2 /I 1 should has the lowest value when z = 0. 111 143 NLA measurements of FBPt and Poly FBPt were conducted using 1 mM solutions in benzene, and the known DPAF capped diplatinum acetylide, T2 ( Figure 2 1 5 ) was used as the be nchmark 143 The solution concentration of poly FBPt was based on the platinum acetylide containing repeat unit. An excitation wavelength of 600 nm was selecte d due to the lack of appreciable ground state absorption at this wavelength. T he Poly FBPt solution clearly display s ~ 7% attenuation of the transmittance at z = 0 position whi ch is nearly identical to FBPt. This observation strong evident that Poly FBPt conserved the non linear absorption property from FBPt.
104 Figure 2 15 NLA response of 1 mM solutions of blank (black square), FBPt (blue triangle), Poly FBPt (red inverted triangle) and T2 (pink circle) Figure 2 16 Structure of z scan benchmark, T2. 2.4 Photophysical Characterization of Thin Film A major goal of designing the side chain conjugated polymer is to utilize the film forming property of polymers. Poly FBPt w as spin coated to thin film and characterized via UV vis ible absorption, steady state photolumin escence and triplet triplet transient absorption and the photophysical parameters of the thin film are summarized in Table 2 1 The pictures of Poly FBPt solut ion and thin film under visible light and UV light (365 nm) are presented in Figure 2 1 7 THF solution of Poly FBPt is light green, and
105 exhibits week fluorescence under UV radiation. T he Poly FBPt thin film from spin coating is transparent and colorless. Figure 2 17 Photos of Poly FBPt solution and film under visible and UV light. A) Poly FBPt in THF solution under visible light. B) Poly FBPt in THF solution under UV light. C) Poly FBPt film under visible light. D) Poly FBPt film under UV light. Figure 2 18 Ground state absorption and photoluminance of Poly FBPt thin film.
106 Figure 2 1 8 displays ground state absorption and photo luminescen ce of Poly FBPt thin film T he absorption of the thin film is nearly identical to that of solution, with an absorption maximum at 375 nm. The film has a ~20 nm red shift for fluorescence comparing to the polymer solution. The red shift may due to the chromophore interactions in the polymer thin film. However the film has the same phosphorescence maximum as the solution. The lifetimes of fluorescence and phosphorescence of the polymer thin film exhibit higher value than those of polymer solution, but they are still in the same magnitude Figure 2 19 Transient absorption of Poly FBPt thin film T he transient absorption of Poly FBPt thin film is recorded in Figure 2 1 9 The positive bands which present the triplet triplet (T 1 T n ) excited state absorption, are similar to the solutions o f FBPt and Poly FBPt displaying strong and broad absorption across most of the visible region, with maxim um at 675 nm. Unfortunately the setup of nanosecond open aperture z scan was not optimized for thin film samples. However, UV vis ible absorption, st eady state photolumin escenc e a nd triplet triplet transient absorption have suggested the Poly FBPt thin film have
107 similar photophysical characteristics as its solution, which exhibits non linear absorption property as the small molecular chromophore FBPt 2.5 Summary In this chapter, the design and preparation of polymer with non linear absorption property were described. T he NLA polymer, which features a flexible, non conjugated polyacrylate backbone and pendant platinum acetylide NLA chromophores, was prepared via the RAFT S N 2 click strategy, starting from a commercial available monomer. T he resulting polymer, Poly FBPt can be achieved with desired molecular weight, low polydispersity index and high loading yields of NLA chromophores. The photophysical propert ies of Poly FBPt were characterized and the results showed that the polymer conserve d the properties of the small molecular chromophore FBPt displaying non linear absorption characteristics. 2.6 Experimental 2.6.1 Instrumentation and Methods NMR spectra were measure d on a Gemini 300 FT NMR, a Mercury 300 FT NMR, or an Inova 500 FT NMR. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer (Bruker Daltonics, Billerica, MA). FT IR was measure d in a Perkin Elmer FTIR spectroscopy. Gel permeation chromatography (GPC) analyses were carried out on a system comprised of a Shimadzu LC 6D pump, Agilent mixed D column and a Shimadzu SPD 20A ph oto diode array (PDA) detector, with THF as eluent at 1 m L /m in flow rate. The system was calibrated against linear polystyrene standards in THF. UV visible absorption was carried out on a Shimadzu UV 1800 dual beam absorption spectrophotometer using 1 cm quartz cells. Photoluminescence
108 measurements were obtained on a fluorometer from Photon Technology International (PTI) using 1 cm quartz cells. Luminescence lifetimes were obtained with a multichannel scaler/photon counter system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime S pectrophotometer. A high performance Coherent CUBE diode laser provided the excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford Research Systems DG535 digital decay and pulse generator with four independent delay channels. At least four narrow band pa ss filters were used for measurements followed by global fit processing (FluoroFit software). Decays were obtained using the multi exponential fitting p arameters (FluoroFit software). Nanosecond triplet triplet transient absorption measurements were acquir ed with excitation at 355 nm (10 mJ/pulse) using the third harmonic of a Continuum Surelite II 10 Nd:YAG laser. Perkin Elmer LS1130 3 pulsed xenon lamp was used as a probe source and the transient absorption signal was detected with a gated intensified CCD mounted on a 0.18 M spectrograph (Princetor PiMax/Acton Pro 180). Samples were prepared to an optical density of 0.7 at the excitation wavelength in a continuously circulating 1 cm pathlength flow cell (volume = 9 mL). Triplet lifetimes were calculated wi th a single exponential global fitting of the transient absorption decay data using SpecFit analysis software. Nonlinear transmission measurements were performed using an open aperture z scan apparatus. The excitation wavelength, 600 nm, was generated by a Continuum Surelite OPO Plus pumped with the third harmonic (355 nm) of a Continuum Surelite II 10 Nd:YAG laser. The laser beam was split with a 50:50 beam splitter to two
109 pyroelectric detectors which measured the transmitted pulse energy through the sampl e as a function of the input pulse energy using an Ophir Laserstar dual channel optical laser energy meter. The beam was focused with a 25.4 mm diameter, 50.8 mm focal length concave lens. A ThorLabs motorized translation stage (Z825B and TDC001) allowed m illimeter movement along the z axis. Solution samples were prepared with dry benzene to a concentration of 1 mM and put into a 1 mm pathlength quartz cuvette for analysis. 2.6.2 Materials Copper (I) iodide (CuI), copper (I) boromide (CuBr), manganese metal, 1 b r omonaphane diethylamine, diisopropylamine ( i Pr 2 NH), potassium hydroxide (KOH), tetrabutyl ammonium fluoride (TBAF, 1 M in THF), azobisisobutyro nitrile (AIBN), sodium azide, pentamethyldiethylenetriamine (PMDETA), glycol methacrylate ( GMA ) magnesium metal and ammonium chloride were from Sigma Aldrich. GMA was dried over GaH 2 and distilled under reduced pressure to remove inhibitors prior to use. CuBr was washed with acetic acid 1 b romonaphane and anhydr ous sodium sulfate before use All other reagents were uses as recived without further purification. 2 (9,9 diethyl 2 (2 (trimethylsilyl)ethynyl) 9 H fluoren 7 yl)benzo[ d ]thiazole ( 1 ) and 2 (9,9 diethyl 9 H fluoren 7 yl)benzo[ d ]thiazole ( 2 ) 156 cis Pt(PBu 3 ) 2 Cl 2 157 and 3 (4 ethynylphenyl)prop 2 yn 1 ol ( 4 ) 158 was synthesized according to the literature procedu r e.
110 2.6.3 Synthesis Compound 3 In a 25 mL round bottom flask was placed in 38 0 mg of 2 670 mg of cis Pt(PBu 3 ) 2 Cl 2 and 10 mL of Et 2 NH. After the solution was degassed with argon for 30 mins, it was refluxed at 60 o C for 2 h. The solvent was removed by rotary evaporation, and the crude product was purified by gradient column chromat ography from 25% to 33% CH 2 Cl 2 in hexane. Yield: 640 mg, 63%. 1 H NMR (300 MHz, CDCl3 ppm: 8.06 8.11 (m, 3H), 8.00 (d, 1H), 7.91 (d, 1H), 7.72 (d, 1H), 7.60 (d, 1H), 7.5 (t, 1H), 7.38 (t, 1H), 7.24 (s, 1H), 1.94 2.18 (m, 12H), 1.36 1.7 (m, 24H), 0.95 (t, 18H), 0.34 (t, 6H). 31 P NMR (121 MHz) ppm 7.97 ( J Pt P = 2369 Hz) Compound 5 In a 25 mL round bottom flask was placed in 300 mg of 3 46.2 mg of 4 and 10 mL of Et 2 NH. After the solution was degassed with argon for 30 mi ns, 2.8 mg of CuI was added and the solution was refluxed at 60 o C for overnight The solvent was removed by rotary evaporation, and the crude product was purified by gradient column chromatography from 2 5% of hexane in CH 2 Cl 2 to pure CH 2 Cl 2 Yield: 200 mg 6 0 %. 1 H NMR (300 MHz, CDCl3 ppm: 8.06 8.11 (m, 2 H), 8.00 (d, 1H), 7.91 (d, 1H), 7.72 (d, 1H), 7.60 (d, 1H), 7.5 (t, 1H), 7.38 (t, 1H), 7.3 (d, 2H), 7.20 (t, 2H), 7.17 (d, 2H) 4.49 (s,
111 2H), 2.0 2.28 (m, 12H), 1.38 1.73 (m, 24H), 0.94 (t, 24H), 0.34 (t, 6H). 31 P NMR (121 pp m: 4.16 ( J Pt P = 2356 Hz) FBPt In a 200 mL three neck round bottom flask was placed in 400 mg of compound 4 and 40 mL of anhydrous ether. The solution was degassed with argon for 30 min before addition of 307 mg of activa ted MnO 2 and 200 mg of powdered KOH. The solution was stirred at room temperature overnight. The solvent was removed by rotary evaporation, and the crude product was purified by gradient column chromatography Yield: 380 mg, 98%. 1 H NMR (300 MHz, CDCl3 ppm: 8.06 8.11 (m, 2 H), 8.00 (d, 1H), 7.91 (d, 1H), 7.72 (d, 1H), 7.60 (d, 1H), 7.5 (t, 1H), 7.3 5 ( d 1H), 7.31 (d, 2H), 7.24 (t, 2H), 7.17 (d, 2H), 3.07 (s, 1H), 2.0 2.28 (m, 12H), 1.38 1.73 (m, 24H), 0.94 (t, 24H), 0.34 (t, 6H). 2 cynoprop 2 yl 1 d ithionaphthalate ( CPDN 6 ) A solution of 1 bromonaphthalene (0.11 mol 22.5 g) in THF ( 90 m L) was added to a 250 ml bottle containing magnesium (2.88 g, 0.118 mol) within 1 h and refluxed for 6 h until the majority of ma gnesium disappeared Carbon disulfide (0.11 mol 8.36 g) was added to the solution at room temperature and then refluxed for 8 h. 100mL 10wt% hydrochloric acid was added to t he mixture Then the organic part was extracted with
112 chloroform (40 m L 3) and coll ected The solvent was evaporated under vacuum; the resident was added with 18 m L ethyl acetate and reacted with 4.7 g DMSO under argon protection at room temperature for 1 2 h. The mixture was added with 12.24 g AIBN and refluxed for another 1 2 h. After ev aporation of the solvent, crude CPDN was obtained. The pure CPDN was obtained as dark red oil by chromatography on silica gel column with ethyl acetate /hexane ( 1 5 : 85) as eluent The product was kept in the refrigerator at 20 o C 151 Yield: 14.32 g ( 4 8 %). 1 H NMR (300MHz, CDCl3 ): 1.9 6 (s, 6H) 7. 39 7.58 (m, 4 H) 7. 79 7.92 (m, 2H) 8.1 2 8.17 (m, 1H). Poly(glycol methacrylate) (PGMA, 7) To a 10 mL Schleck tube with a stirring bar was added GMA (45 mmol, 6.40 g), CPDN (0.45 mmol, 0.122 g), and AIB N (0.15 mmol, 0.025 g). The solution was deoxygenated by 3 freeze pump thaw cycles Then the flask was immersed in an oil bath with setting temperature of 60 o C for 3h. Then the flask was put in ice bath to stop the polymerization. T he product was dissolve d in THF and then precipitated from ethyl ether. The precipitation was re dissolved and precipitated for three times and then dried in vacuo for 24 hours to give pink powder (Mn = 6000, PDI = 1.33). Yield: 2.43 g (38%).
113 Poly (2 hydroxy 3 azidopropyl met hacrylate) (PHAZPMA, 8) PGMA (1.0 g, 7 mmol of GMA units) was dissolved in DMF (20 mL), then sodium azide (1.37 g, 21 mmol) and ammonium chloride (1.12 g, 20.9 mmol) were added to this solution, and the mixture as stirred at 50 o C for 24 hours. The resultant polymer was precipitate in water, washed with water on the filter paper and fried under vacuum overnight. Yield: 1.12 g (86%) Poly FBPt (9) PHAZPMA (18.5 mg, 0.10 mmol of azide unit), FBPt (132.3 mg, 0.12 mmol) and PMDETA (10 L, 8.66 mg, 0.05 mmol) were dissolved in THF (10 mL), and the solution was deoxygened by bubbling argon for 30 minutes CuBr (7.05 mg, 0.05 mmol) was added in argon flow. The mixture was stirred at room temperatur e for 24 hours at argon atmosphere. Then the mixture was diluted with THF and passed through a neutral alumina column to get rid of copper catalyst. T he solution was concentrated and the resultant polymer was precipitated in ethyl ether and dried in vacuum overnight. Yield: 135 mg (89.7%).
114 CHAPTER 3 ULTRAFAST ENERGY TRANSFER IN POLYSTYRENE BASED ARRAYS OF CONJUGATED CHROMOPHOR ES 3.1 Background Multichromopho r ic light harvesting macromolecules have been targeted for solar energy applications. 1 2 159 In conjugated polymers, such as poly(3 hexylthiophene) (P3HT) and polyfluorene (PF) the backbone provides the light harvesting fun ction. Such materials can exhibit good processability extraordinarily high extinction coefficients, and the ability to tune optical gaps and HOMO and LUMO energies. However, fully conjugated polymers are plagued by conformational disorder that breaks the chain into a series of segment chromophores with different conjugation lengths. This result in excited state dynamics characterized by exciton self trapping, exciton diffusion len gths that are limited to ~ 10 nm and low fluorescence quantum yields. 19 27 A lternative light harvesting macromolecular architecture s utiliz e polymer s 2 10 12 18 52 53 160 161 or dendrimer s 161 164 as scaffolds, bringing multiple pendant chr omophores into close proximity while leaving their molecular electronic states intact 165 166 With the development of controlled radical polymerization technique s, it is possible to prepare such polymers with well defined chain structure and low polydispersity Interchain and intrachain energy migration and energy transfer have been observed in polymers with poly(methacrylate) backbone and oligo(phenylene ethynyle ne) (OPE) side chains. 34 35 In the chapter we report the stud y of a series of light harvesting polymers that consist of a non conjugated polystyrene backbone fun ctionalized with oligomeric conjugated chromophores. We apply both time resolved and steady state fluorescence spectroscopy as well as ultrafast time resolved transient absorption spectroscopy to
115 study the efficiency and dynamic s of energy transfer in th ese macromolecular chromophore systems. Energy transfer in these polychromophores is highly efficient, with dynamics and overall efficiency approaching that seen in fully conjugated polyme rs 167 3.2 Polymer Design and Preparation 3.2.1 Preparation of c lickable p olymer b ackbones A well defined macromolecular structure was designed to study energy migration in the conjugated chromophore arrays, which featured an atactic polystyrene backbone and pendent conjugated chromophore side groups and the structures of the polymers are shown in Figure 3 1 Polymers with different donor / acceptor ratios ( poly chromophores ) are name d as P x where x represents the percentage of acceptor in the polymer Figure 3 1. S tructure s of Polymers (P 0 to P 20) and model compounds (1a and 1b). The clickable polystyrene backbone, poly (4 vinylbenzyl az ide ) ( PVB A ) was synthesized with reversible addition fragmentation transfer (RAFT) polymerization and followed by chloride to azide S N 2 reaction, as depicted in Figure 3 2. 4 V inylbenzyl chloride ( VBC ) was chos en as the starting monomer because it wa s comm ercially available with low cost and easy to purify; meanwhile, from safety concern, the chloride
116 to azide S N 2 transformation is quantitative in mild er condition (room temperature ) than epoxy ring in glycol methacrylate used in Chapter 2 (50 o C). RAFT was used here as the controlled radical polymerization technique due to its monomer tolerance easy handling and it gave desired molecular weight and narrow polydispersity. Figure 3 2. Synthesis of PVBC and PVBA T he resulting PVBC was a pink polymer with a molecular weight (Mn) of 9800 (DP ~ 64) and a low polydispersity index (PDI) of 1.22. The pink color was caused by the dithioester end group introduced by the chain transfer agent CPDN T he pink color disappeared as PVBC was treated with sodium azide, i ndicating the low energy, non fluorescent dithioester quencher was removed simultaneously during the post functionalization by the nucleophilic azide anion; meanwhile, chlorides at each repeat unit were substituted by azido groups via S N 2 reaction, which m ade the polymer to be clickable PVBA T he transformation from PVBC to PVBA was quantitative, which can be observed from 1 H NMR ( Figure 3 6 ) The complete loss of the peak at 4.52 ppm for chloromethyl ene and the presence of the peak at 4.27 ppm assigned t o azidomethyl ene indicate the Cl was completely substituted by N 3 The GPC traces indicating no branching or cross linking during the S N 2 post functionalization ( Figure 3 7 )
117 3.2.2 Synthesis of C hromophores The two chromophores along the polymer chain have dif ferent excited state energy, in which the one with higher energy, oli go(phenylene ethynylene) ( OPE ) servers as energy donor while the lower energy thio phene benzothiadiazole ( TBT ) is the acceptor The syntheses of the precursors for both chromophores 7 and 12 which have terminal alkyne, are shown in Figure 3 3 Figure 3 3. Synthesis of conjugated chromophores with terminal alkyne T h e unsymmetrical protected diethynylbenzene ( 2 ) was prepared f rom Sonagshria coupling be tween 1 iodo 4 ((trimethylsilyl)ethynyl) benzene ( 1 ) which was first prepared following the literature, and trisisopropylsilyl acetylene (TIPSA) Then the TMS group was selectively deprotected with KOH in DCM/MeOH (v/v, 1/1) mixed
118 solution to give 3 with a 99% yield, as a building block for 7 Compound 5 was prepared via the coupling reaction between 4 and phenylacetylene (1:1 ratio) in the presence of catalytic amount of Pd(Ph 3 ) 4 and CuI in the mixture solution of tetrahydrofuran and diisopropylamine. Pur ified by silica gel chromatography, compound 5 was obtained with a 44.6% yield. Sonagshria coupling between compound 3 and compound 5 gave compound 6 which can be further deprotected to obtain compound 7 the OPE precursor bearing a terminal alkyne. The p reparation of compound 12 started with the synthesis of 4,7 di(thiophen 2 yl)benzo[ c ][1,2,5]thiadiazole ( 9 ) Compound 9 was synthesized via Stille coupling of 4,7 dibromobenzothiadiazole ( 8 ) and 2 (tribytylstannyl)thiophene with a yield of 86.7%. Brominati on of 9 gave compound 10 which under went Sonagshria coupling to give compound 11 Deprotection of 11 with TBAF gave compound 12 the TBT precursor bearing a terminal alkyne. 3.2.3 Preparation of p oly c hromophores and m odel c ompounds P oly chromophores with di fferent OPE/TBT ratios were prepared, with the name of P x where x represents the percentage of TBT groups in the polymer ( Figure 3 1 ). T he two conjugated chromophores bearing terminal acetylene group on one side ( 7 and 12 ) w ere grafted onto PVBA via copper (I) catalyzed azide alkyne click reaction as shown in Figure 3 4 All five polymers were derived from the same PVBA precursor with Mn = 1 0 1 kg/mol a n d PDI = 1.31, so they all have the same chain length estimated of degree of polymerization (DP) around 60 The x value was controlled by of x were deter mined by UV Vis absorption ( Figure 3 8 ) In order to provide reference data for the isolated chromophores, OPE and TBT model compounds 1a and 1b were
119 also synthesized via click reaction between the two compounds with terminal alkyne group on one side ( 7 an d 12 ) and benzyl azide ( Figure 3 5 ). Figure 3 4. Preparation of p oly c hromophores ( P 0 to P 20 ). Figure 3 5. Synthesis of OPE and TBT model compounds 1a and 1b 3.2.4 Structural Characterizatio n The synthetic transformation from PVBC to the poly chromophores was monitored by 1 H NMR and GPC ( Figure 3 6 and 3 7 ) During the S N 2 reaction (from
120 PVBC to PVBA ), The peak at 4.52 ppm which is assigned to chloromethyl ene protons completely disappeared an d the peak at 4.27 ppm assigned to azidomethyl ene protons arises, indicat ing the Cl was completely substituted by N 3 During the click reaction, the peak at 4.27 ppm ( azidomethyl protons ) shifts to 5.30 ppm (triazolemethylene protons) and a peak at 3.98 ppm arise, which belongs to protons in oxy methylene groups in OPE side groups. Figure 3 6. 1 H NMR spectra for polymers, PVBC PVBA and P 0 to P 20
121 The GP C traces showed there is no branching or cross linking during the S N 2 post functionaliz ation A ll P x polymers had significant molecular weight increase comparing to PVBA precursor but still remaining low PDI values. Figure 3 7 GPC traces of polymers, P 0 (black hollow square, Mn = 29 2 kg/mol PDI=1.23), P 1 (red hollow circle, Mn=29 2 kg/mol PDI=1.17), P 5 (green hollow triangle, Mn=28 7 kg/mol PDI=1.23), P 10 (blue hollow inverted triangle, Mn=28 1 kg/mol PDI=1.23), P 20 (brown hollow diamond, Mn=26 8 kg/mol PDI=1.23), PVBC (black solid square, Mn= 9.80 kg/mol PDI=1.22) and PVBA ( red solid circle, Mn=10 1 kg/mol PDI=1.31). With polystyrene as standard.
122 3.3 Steady State Absorption and Emission Figure 3 8. Steady state absorption and emission of model compounds and polymers. A ) Steady state absorption of model compounds 1a (green s olid square s 375 nm = 58000 M 1 cm 1 ) and 1b (red solid circle s 470 nm = 24000 M 1 cm 1 ). B ) Fluorescence emission of OPE ( fl =1.0) and TBT ( fl = 0.75). C ) Steady state absorption of polymers, P 0 (black hollow square s ), P 1 (red hollow circle s ), P 5 (green hollow triangle s ), P 10 (blue hollow inverted triangle s ) and P 20 (brown hollow diamond s ). All polymers were dissolved in THF. The inset is the absorbance at 470 nm for five polymers. D ) Fluorescent spectra of polymers, P 0 to P 20 All polymers were dissolved in THF with an OD = 0.8 at 370 nm. T he inset shows quantum yields of OPE emission ( green solid square s 370 nm to 510 nm) and TBT emission ( red solid circle s 510 nm to 775 nm) for the five polymers E) Fluorescence photographs of five polymers.
123 T he absorpti on and fluorescence spectra of model compounds 1a and 1b are shown in Figure 3 8 A and 3 8 B OPE oligomer 1a features two near UV absorption bands (322 nm and 370 nm) and a single fluorescen ce band (407 nm) with a quantum yield of unity, fl ~1.0. The TBT o ligomer 1b exhibits a near UV absorption band at 330 nm and a visible band at 470 nm. T he fluorescen ce band from 1b has a maximum at ~ 600 nm, with fl ~ 0.75. The absorption spectra of the polychromophores are shown in Figure 3 8 C The donor only polym er, P 0 has essentially the same spectrum as that of OPE model compound 1a The spectra of P 1 to P 20 are dominated by the OPE based transitions (especially at 370 nm) ; however, they show increasing TBT character as the loading of the lo wer energy chromo phores in the click grafting is increased ( Figure 3 8C inset) This feature allows straightforward determination of the TBT loading in the co polymer from the ratio of the ab sorbance at 370 and 470 nm (see Table 3 1 ). The simulated absorption spectrum of P 20 ( Figure 3 9 ), calculated from the absorption spectra and extinction coefficients of 1a and 1b corresponds nicely with the measured P 20 absorption Taken together, the absorption data for the model compounds and polymers allow several conclusions. F irst, the fractional loading of OPE a nd TBT units in the polymers closely corresponds to the stoichiometry used in the feed for the click reaction s. Second, the absorption at 370 nm is due almost exclusively to the OPE (donor) making it possible to select ively excite this chromophore Finally, the polymer spectra are accurately simulated as a linear combination of the spectra of the OPE and TBT chromophores, indicating that there is not a strong ground state interaction among the individual units in the p olymers.
124 Figure 3 8 D shows the fluorescen ce spectra of all five polymers. In this experiment t he concentration of the polymer solutions was adjusted such that the absorption at 370 nm excitation wavelength was identical The fluorescence of the OPE (dono r) only polymer P 0 ( = 413 nm) appears as a single band which is slightly red shifted and broadened compared to that of the OPE model 1a ; this result suggests that there is some interchromophore interaction in the singlet excited state Interestingly, in P 1 the emission from the OPE chromophore at 413 nm is quenched ~ 4 0% relative to the intensity of P 0 and fluorescence from the TBT units at 600 nm is evident ; indicating that OPE to TBT energy transfer is efficient Calculations b ased on the fluoresce nce quantum yields of the donor and acceptor reveal that the energy transfer efficien cy in P 1 is ~ 55 %. I n P 5 the OPE emission is quenched to a greater extent, and the energy transfer efficiency from OPE to TBT approaches 85 %. This trend continues throug h the series as seen in Figure 1 d (inset) where the quantum yields of OPE and TBT fluorescence in the P x series is plotted Note that t h e OPE emission yield decrease s sharply from P 0 to P 5 followed by a more gradual decline from P 10 to P 20 By contra st, t he TBT emission yield increase s sharply from P 0 to P 5 ; however, it then decrease s slightly from P 5 to P 20 The latter trend is likely due to self quenching of the TBT chromophore as its concentration in the polymer s increases possibly due to inte rchromophore charge transfer interaction between TBT units. The self quenching mechanism is supported by the fact that the lifetime of the TBT fluorescence also decreases from P 5 to P 20 ( see Table 3 1 ). The five polymers display different colors upon UV radiation because of the different ratios of 413 nm and 600 nm emissions ( Figure 3 8 E ).
125 Figure 3 9 Comparison of measured ( brown hollow diamond s) and calculated ( cyan hollow star s ) absorption spectra of P 20 The calculated spectrum was derivate from th e absorption spectra and extinction coefficients of OPE model compound 1a and TBT model compound 1 b based on the equation (Calculated P 20 ) = 0.8 x ( 1a ) + 0.2 x ( 1b ). The results were then normalized to the spectrum ( cyan hollow star s ) above. Additiona l evidence for OPE to TBT energy transfer in from P 1 to P 20 is provided by e xcitation spectra collected while monitoring emission at 610 nm ( Figure 3 10 ) T he spectrum of the TBT model compound 1b shows that the emission of 610 nm comes from both 330 nm and 470 nm; however, emission at 6 1 0 nm of poly chromophores bearing TBT side groups ( P 1 to P 20 ) are mostly originated from OPE absorption ( 370 nm ). The excitation spectra of P 1 to P 20 match the absorption spectra of these polymers. These data clearly show that the TBT emission excitation of the OPE chromophores
126 Figure 3 10 Excitation spectra of copolymers with both donor and acceptor. P 1 (red hollow circle s ), P 5 (green hollow triangle s ), P 10 (blue hollow inverted triangle s ), P 20 (brown hollow diamond s ) and model compound 1b (red solid circle s ). The emission wavelength was set at 6 1 0 nm 3.4 Ultrafast Transient Absorption Ultrafast transient absorption experiments were carried out on the polychromophores to characterize the dynamics of the intrapoly mer energy transfer process. Figure 3 11 and 3 12 compare the ultrafast transient absorption o f the donor acceptor co polymers ( P 1 to P 20 ) with those of the donor only polymer ( P 0 ) and the TBT acceptor unit as modeled by 1a The transient spectrum of P 0 ( Figure 3 11 red filled) shows a negative feature (bleach) at 400 nm that results from a combination of ground state bleach and stimulated OPE emission, as well as an intense excited state absorption centered near 600 nm The single wavelength kinetic t race of P 0 ( Figure 3 12 black hollow squares) measured at = 665 nm shows biphasic decay. The minor component ( 0 .20) has a 50 ps lifetime and is attributed to relaxation dynamics within the singlet excited state, likely due to planarization of the OPE u nit. This decay is
127 accompanied by a slight blue shift of the excited state absorption. The major decay component ( 0 .80) has a 1.2 ns lifetime and is consistent with the excited state lifetime measured from the fluorescence decay kinetics. As the laser flu ence is increased above 40 J/cm 2 the excited state absorption begins to exhibit intensity dependent kinetics, presumably due to exciton exciton annihilation events that occur when more than one excited state is created on each chain. The transient data r eported here were collected at pulse energies below this threshold (25 J/cm 2 ), thus avoiding these exciton exciton processes. Figure 3 11. Transient absorption spectra showing early time ( t = 175 fs) comparison between P 5 (hollow triangles) and the pu re donor polymer P 0 ( red filled ). Also shown are transient spectra at t = 1.15 ns comparing P 5 ( hollow squares ) with the TBT accep tor moiety ( blue filled )
128 Figure 3 12. five polymers P 0 (black hollow square s ), P 1 (red hollow circle s ), P 5 (green hollow triangle s ), P 10 (blue hollow inverted triangle s ), P 20 (brown hollow diamond s ). Examination of the transient absorption of P 5 reveals the importance of energy transfer to the excited state dynamics when both OPE donor and TBT acceptor moieties are present on a single chain Figure 3 11 shows the transient absorption spectrum of the P 5 donor acceptor copolymer at 175 fs ( hollo w triangles ). The spectrum is nearly identical to that of the donor only polymer P 0 ( red filled ), indicat ing that photoexcitation at 388 nm predominantly creates OPE excited states (OPE*) However, by 1.1 ns following excitation the transient spectrum of P 5 ( hollow squares ) has evolved now containing absorption and bleach features that are very similar to those of the excited TBT chromophore (TBT*, compare with transient spectrum of TBT model 1b blue filled ) These transient absorption results are in accord with the fluorescence spectra, confirming the occurrence of efficient intrachain OPE to TBT energy transfer.
129 The dynamics of energy transfer can in principle be followed either through the disappearance of OPE (i.e. donor decay ) or appearance of TB T (acceptor rise). However, monitoring the appearance of TBT* via its ground state bleach at = 470 nm is problematic due to the stimulated emission from OPE in this spectral region We have instead focused our analysis on the decay of OPE absorption a t 665 nm, where the OPE* stimulated emission and TBT* contribution s are minimized. The single wavelength kinetic trace of P 5 ( Fig ure 3 12 green hollow triangles) exhibit s much faster decay dynamics than donor only P 0 The accelerated decay is primarily caused by an additional fast component ( 1 ps ), which is assigned to quenching of OPE* excitons that are formed on a chain in close spatial proximity to a TBT acceptor unit, and undergo direct energy transfer to the acceptor In addition to this fas test component, there is also an intermediate component ( 2 ps) likely reflecting two processes: relaxation of the initially formed excited state that is observed in the donor only polymer as well as multi step energy transfer in which the excited st ate first has to migrate through a random walk, site to site hopping process to a position along the chain that is in close proximity to an acceptor. The overall timescale for this latter process will depend on the OPE OPE hopping rate ( k hop ), the number o f hops need ed to reach the trap, as well as the OPE TBT energy transfer rate ( k EnT ). Across the polymer series, a trend of faster OPE* quenching is observed as loading of the TBT acceptor units increases. The additional fast ( 1 ) decay component as well as greater contribution from intermediate rate processes (relative to that seen in P 0 ) is found for each of the other co polymers in the series. Time constants and
130 normalized amplitudes recovered from the triexponential modelin g of the 5 polymers ( P 0 P 20 ) are summarized in Table 3 1 Increasing the loading of acceptor chromophores in the copolymer has two effects. First, the probability of photoexcitation producing excited states within the quenching radius of an acceptor in creases. Second, for those excitons created far from an acceptor, the number of steps necessary to migrate near enough for quenching to occur decreases. Both of these effects will result in qualitatively faster quenching of the excited state, a result whic h can be seen by comparing the kinetics at 665 nm for the polymer series in Table 3 1 As the loading increases from 1% to 20%, the amplitude of the fast component increases from 0 .07 to 0 .38 with little change in the time constant itself, consistent with the direct OPE* to TBT energy transfer assignment. The time constant associated with the intermediate processes (25 50 ps) trend s downward as loading increases reflecting the shorter amount of time necessary for an excited state to migrate to the TBT trap
131 Table 3 1. Photophysical Characteristics of Polymers ( P 0 to P 20 ). P 0 29200 1.23 0 2.63 (0.28) 1.00 (0.72) ( ) ( ) 44.9 4.7 (0.20 0.01) 1148 35 (0.80 0.01) P 1 29200 1.17 1 2.02 (0.31) 0.58 (0.69) 10.60 (0.9) 3.04 (0.1) 2.6 0.7 (0.07 0.01) 56.8 4.6 (0.31 .01) 960 34 (0.61 0.01) P 5 28700 1.23 5.5 2.04 (0.24) 0.35 (0.76) 10.10 (1.0) 3.4 0.5 (0.23 0.02) 44.8 3.7 (0.44 0.02) 1240 112 (0.33 0.01) P 10 28100 1.23 11 1.90 (0.24) 0.36 (0.76) 9.67 (1.0) 2.1 0.2 (0.26 0.01) 27.7 1.5 (0.43 0.01) 1160 66 (0.30 0.01 ) P 20 26800 1.23 22 1.78 (0.29) 0.36 (0.71) 8.94 (1.0) 2.0 0.1 (0.38 0.01) 26.1 2.1 (0.33 0.01) 1465 110 (0.29 0.01) a TBT content determined by UV Vis absorption The ratio of absorbance of P x polymers at 370 nm and 400 nm follows the equ ation: So In which = 58000 M 1 cm 1 = 2640 M 1 cm 1 and = 24000 M 1 cm 1 b Emission lifetime measured by TCSPC. c Measured from single wavelength transient absorption kinetics at = 665 nm
132 3.5 M olecular D ynamics S imulations To examine the relationship between energy transfer and polymer structure, we performed m olecular dynamics (MD) simulations of a 30 unit subsection of the P 0 P 5 P 10 and P 20 polymers with explicit THF solvent ( details in Supporting Information ). On average, the nearest neighbor chromophore distance (regardless of whether it was an OPE OP E pair or an OPE TBT pair) was found to be 0 .99 0 .23 nm. This distance used in the F rster expression predicts rates that are far greater than those observed experimentally: a hopping rate of k hop = 6.1 10 12 s 1 and an energy transfer rate k EnT = 7.5 10 1 3 s 1 However, given that F rster theory tends to overestimate the energy transfer rates at such short distances (and between chromophores where the point dipole approximation is no longer valid), quantitative comparison with experimentally determined rates is invalid unless more elaborate expressions are used for the Coulomb interaction. 168 169 Nevertheless, the average inter chromophore distance calculated from MD s imulations is well within the F rster radii calculated from spectral overlap and the OPE quantum yield for both energy transfer (R 0 = 6.63 nm) and hopping processes (R 0 = 4.37 nm). This suggests that both direct ( 1 ) and hopping ( 2 ) routes are efficient m echanisms of energy migration and transfer. As a result, the polymer motif serves to efficiently couple donor and acceptor chromophores, creating a network that quickly shuttles energy and may be a promising model for light harvesting applications.
133 Figur e 3 13. M olecular dynamics simulation of the OPE TBT copolymer. Green: polymer backbone; gray: OPE donor; yellow: TBT acceptor. 3.6 Summary In summary, energy transport in a light harvesting polymer consisting of ~ 60 OPE chromophores have been examined. Phot oexcitation of OPE sites gives rise to site to site energy transfer and ultimately sensitization of a trap site ( TBT ) doped into the polymer at low concentration, on the picosecond time scale and with remarkably high efficiency. The energy transfer process proceeds via ultrafast neighborhood OPE TBT quenching in 2 ~ 4 picosecond s energy hopping in 25 ~ 50 picosecond s
134 3.7 Experimental 3.7.1 Instrumentation and Methods NMR spectra were measured on a Gemini 300 FT NMR, a Mercury 300 FT NMR, or an Inova 500 FT NMR. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer (Bruker Daltonics, Billerica, MA). Gel permeation chromatography (GPC) analyses were carried out on a system compri sed of a Shimadzu LC 6D pump, Agilent mixed D column and a Shimadzu SPD 20A ph otodioide array (PDA) detector, with THF as eluent at 1 m L /min flow rate. The system was calibrated against linear polystyrene standards in THF. UV visible measurements were carr ied out on a Shimadzu UV 1800 dual beam absorption spectrophotometer using 1 cm quartz cells. Photoluminescence measurements were obtained on a fluorimeter from Photon Technology International (PTI) using 1 cm quartz cells. P hotoluminescence lifetimes were obtained by using a single photon counting Fluo Time 100 (Picoquant) Fluorescence Lifetime Spectrometer and excitation was provided using a PDL 800 B Picosecond Pulsed Diode Laser. For transient absorption spectroscopy, samples were dissolved in THF to an OD of between 0 .4 and 0 .5 in a 2 mm cuvette. Femtosecond pulses were derived from a Clark MXR CPA 2210 Ti: Sapphire laser which produces ~ 150 fs pulses centered at 775 nm with a 1 kH hz repetition rate. A portion of the output was frequency doubled (to 388 nm) in a BBO and used for photoexcitation of the donor. Low fluences (25 mJ/cm 2 ) were necessary to achieve linear behavior of the transients. Kinetics was monitored by a weak continuum probe pulse generated by focusing a small portion of the 775 nm fundam ental into a translating CaF 2 window. Spectra were collected at a rate of 1 kHz
135 with pump on and pump off spectra interleaved by mechanical chopping, are chirp corrected for delay times < 20 ps, and are each the average of 8000 individual pump on and pu mp off spectra. Molecular dynamics (MD) simulations were performed on 30 repeat unit polymers with 0, 2, 3, and 6 TBT chromophores placed randomly in the structure to represent the P 0 P 5 P 10 and P 20 copolymers, respectively. Monomer repeat units wit h TBT and OPE chromophores were geometric ally optimized with the B3LYP DFT functional and a 6 31G+ basis set as implemented in Gaussian09 version 09a02. 170 The structures and DFT determined a tomic charges were imported into the Materials Studio software package for MD. Unit cells consisting of the polymers along with explicit THF solvent were annealed with 10 temperature cycles ranging from 300 to 500 K using the Universal Force Field 171 as implemented in the Forcite module of Materials Studio The final minimum of the annealing cycle was taken as the starting point for subsequent MD runs. MD on each of the 4 polymers showed the minimum distance between chromophores was independent of chromophore identity (i.e. TBT OPE distance or OPE OPE distance), so to boost statistical relevance of the dynamics runs, all neighboring OPE OPE distances of the P 0 polymer were tracked to find the minimum distance reported in the manuscript. Molecular dynamics were performed on t wo separate P 0 polymers (last and penultimate energy minima of the annealing cycling). The first structure was run for a total of 20 ns. The second was run for 6 ns. 3.7.2 M aterials Copper (I) iodide (CuI), copper (I) boromide (CuBr), diisopropylamine ( i Pr 2 NH), potassium hydroxide (KOH), phenyl acetylene, tetrabutyl ammonium fluoride (TBAF, 1 M in THF), 2 tributylstannyl thiophene, N bromo succinimide (NBS),
136 azobisisobutyronitrile (AIBN), sodium azide, pentamethyldiethylene triamine (PMDETA), 4 vinylbenzyl chloride (VBC) and benzyl chloride were from Sigma Aldrich. Tetrakis(triphenyl phosphine) palladium (0) (Pd(PPh 3 ) 4 ) was from Stern Chemical and trisisopropylsilyl acetylene (TIPSA)was fr om TCI chemical. VBC was passed a short neutral alumina column to remove inhibitors prior to use. CuBr was purified according to literature. All reagents were uses as received without further purification. 1 iodo 4 ((trimethylsilyl)ethynyl) benzene ( 1 ) 172 1 iodo 2,5 bis(octyloxy) 4 (phenylethynyl)benzene ( 4 ), 173 and 4,7 dibromobenzothiadiazole ( 8 ) 174 were prepared according to literatures. 3.7.3 Synthesis 1 ((triisopropylsilyl)ethynyl) 4 ((trimethylsilyl)ethynyl) benz ene (2) A solution containing 1 (3.0 g, 0.01mol), tetrahydrofuran (THF, 80 mL) and i Pr 2 NH, (20 mL) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 3 ) 4 (115.5 mg, 0. 1 mmol) and CuI (38.2 mg, 0.2 mmol) were ad ded in a argon flow. TIPSA (2.19 g, 0.012 mol) was added dropwise with a syringe. The mixture was stirred at room temperature overnight. After reaction, the solvent was evaporated in reduced pressure, and the remaining was extracted by ethyl acetate and br ine. The organic phase was collected and washed by brine and water for three times and dried over anhydrous sodium sulfate. After the solvent was evaporated under reduced pressure, the residue was purified through column chromatography using hexane as the eluent (Rf = 0.50) to yield a light yellow liquid (Yield: 3.2 g, 91.4%). 1 H NMR ( 300 (ppm) 7.40
137 (s, 4H), 1.11 (m, 21H), 0.24 (s, 9H). 13 C NMR (75 (ppm) 131.76, 123.59, 122.99, 106.61, 104.67, 96.16, 92.81, 18.71, 11.33, 0.03. 1 ethynyl 4 ((triisopropylsilyl)ethynyl) benzene ( 3 ) To a deoxygenated solution of 2 (1.24 g, 3.5 mmol) in dichloromethane (DCM, 50 mL) and methanol (MeOH, 50 mL), potassium carbonate (KOH, 0.59 g, 10.5 mmol) was added u nder argon flow. The mixture was stirring at room temperature for 1 hour. Then deionized water (100 mL) was added to extract unreacted KOH. The organic layer was separated and washed 3 times with brine and deionized water, and was dried over sodium sulfate The solvent was evaporated to obtain colorless oil. The crude product was purified by column chromatography on silica gel using hexane as the eluent (Rf = 0.47) to yield colorless liquid. Yield: 0.98 g (99%). 1 H NMR (300 (ppm) 7.40 (s, 4 H), 3.14 (s, 1H) and 1.10 (m, 21H). 13 C NMR (75 (ppm) 132.09, 124.16, 122.08, 106.56, 93.19, 83.48, 79.06, 18.89, 11.52. 1 iodo 2,5 bis(octyloxy) 4 (phenylethynyl)benzene (5) A solution containing 4 (5.86 g, 0.01 mol), THF (100 mL) and i Pr 2 NH (50 mL) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 4 ) 3 (0.23 g, 0.02 mmol) and CuI (76 mg, 0.04mmol) were added in a argon flow. Phenylacetylene (1.02 g, 0.1
138 mmol) was added dropwise. The mixture wa s stirred at room temperature overnight. After reaction, the solvent was evaporated in reduced pressure, and the remaining was extracted by ethyl acetate and brine. The organic phase was collected and washed by water for three times and dried over anhydrou s sodium sulfate. After the solvent was evaporated under reduced pressure, the residue was purified through column chromatography using 85/15 hexane/dichloromethane (DCM) as the eluent (Rf = 0.31) to yield a white powder (Yield: 2.5 g, 44.6%). 1 H NMR (300 (ppm) 7.53 (m, 2H), 7.33 (m, 3H), 7.30 (s, 1H), 6.91 (s, 1H), 3.98 (t, 4H), 1.82 (m, 4H), 1.53 (m, 4H), 1.40 1.25(m, 16H), 0.88 (t, 6H) 13 C NMR (75 (ppm) 154.60, 152.09, 131.76, 128.51, 128.47, 124.18, 123.64, 116.23, 113.93, 94.41, 81.71, 85.79, 70.26, 32.03, 29.58, 29.50, 29.43, 26.28, 22.89, 14.32. 2,5 bis(octyloxy) 1 (phenylethynyl) 4 (triisopropylsilyl) ethynyl)phenyl ethynyl) benzene (6) A solution containing 5 (1.4 g, 2.5 mmol), 4 (0.78 g, 2.75 mmol), THF (20 mL) and i Pr 2 NH (20 m L ) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 4 ) 3 (57 mg, 0.05 mmol) and CuI (19.1 mg, 0.1 mmol) were added in a argon flow. The mixture was stirred at room temperature overnight. After reaction, the solvent was evaporated in reduced pressure, and the remaining was extracted by ethyl acetate and brine. The organic phase was collected and washed by brine and water for three times and dried over anhydrous sodium sulfate. After the solvent w as evaporated under
139 reduced pressure, the residue was purified through column chromatography using 80/20 hexane/DCM as the eluent (Rf = 0.33) to yield a light yellow solid (yield: 1.69 g, 94.5%). 1 H NMR (300 (ppm) 7.54 (d, 2H), 7.45 (d, 4H), 7.33 (m, 3H), 7.01 (d, 2H), 4.03 (t, 4H), 1.85 (p, 4H), 1.53 (m, 4H), 1.40 1.25(m, 16H), 1.13 (m, 21H), 0.87 (t, 6H) 13 C NMR (75 (ppm) 153.90, 131.80, 128.51, 123.57, 117.13, 114.50, 113.90, 106.95, 95.22, 94.77, 92.98, 88.09, 86.15, 69.87, 32.05, 29.63, 29.58, 29.32, 26.32, 22.90, 18.88, 14.32, 11.33. 2,5 bis(octyloxy) 1 (phenylethynyl) 4 ethynyl)phenylethynyl) benzene (7) A THF solution of 6 (0.7 g, 0.98 mmol) was degassed by bubbling argon for 20 minutes, and then tetrabutylamonium fluoride (TBAF, 2 mL, 2 mmol) was added dropwise via a syringe. The mixture was stirred at room temperature for 1 hour. After reaction, the solvent was evaporated in reduc ed pressure, and the residue was purified through column chromatography using 70/30 hexane/DCM as the eluent (Rf = 0.42) to yield a light yellow solid (Yield: 0.53 g, 97%). 1 H NMR (300 (ppm) 7.54 (d, 2H), 7.45 (d, 4H), 7.33 (m, 3H), 7.01 (d, 2H), 4.03 (t, 4H), 1.85 (p, 4H), 1.53 (m, 4H), 1.40 1.25(m, 16H), 0.87 (t, 6H). 13 C NMR (75 (ppm) 153.90, 132.26, 131.71, 128.51, 124.22, 123.65, 122.03, 117.14, 114.61, 113.76, 95.25, 94.43, 88.30, 86.12, 83.55, 79.10, 69.87, 32.04, 29.61, 29.58, 29.53, 26.31, 22.89, 14.31. MS: Calculated: 558.35; (ACPI TOF): [M+ CH3OH+ H] + : 591.3649
140 4,7 di(thiophen 2 yl)benzo[ c ][1,2,5]thiadiazole (9) A mixture containing 8 (1 g, 3.42 mmol), 2 (tribytylstannyl)thiophene (2. 82 g, 7.54 mmol), DMF (50 mL), was bubbled with argon for 20 minutes, and then Pd(PPh 3 ) 4 (79 mg, 0.068 mmol)was added in the argon flow. The mixture was stirred at 70 o C overnight. After reaction, the solvent was evaporated in reduced pressure, and the rem aining was extracted by ethyl acetate and brine. The organic phase was collected and washed by water for three times and dried over anhydrous sodium sulfate. After the solvent was evaporated under reduced pressure, the residue was purified through column c hromatography using 80/20 hexane/DCM as the eluent (Rf = 0.29) to yield a red orange solid (Yield: 0.89 g, 86.7%). 1 H NMR (300 (ppm) 8.12 (d, 2H), 7.88 (s, 2H), 7.46 (d, 2H), 7.22 (d, 2H) 13 C NMR (75 (ppm) 126.87, 139.56, 128.22, 127.72, 127.01, 126.23, 126.01. 4 (5 bromothiophen 2 yl) 7 (thiophen 2 yl)benzo[ c ][1,2,5]thiadiazole (10) A mixture containing 9 (0.60 g, 2 mmol), N bromosuccinmide (NBS, 0.36 g, 2 mmol), chloroform (CHCl 3 40 mL) and acetic acid (CH 3 COOH, 20 mL) was stirred for 4 hours at 0 o C. The mixture was poured into water, extracted by CHCl3 and washe d by water. After the solvent was distilled out under reduced pressure, the residue was purified by column using 20/80 hexane/DCM as the eluent (Rf = 0.35) to yield a dark red
141 solid (Yield: 0.68 g, 89.5%). 1 H NMR (300 (ppm) 8.12 (d, 1H), 7.85 (d, 2H), 7.78 (d, 2H),7.46 (d, 2H), 7.22 (t, 2H), 7.15 (d, 1H) 13 C NMR (75 (ppm) 152.69, 152.48, 140.88, 139.38, 130.86, 128.26, 127.89, 127.30, 127.21, 126.53, 125.82, 125.35, 125.17, 114.71. 4 (thi ophen 2 yl) 7 (5 ((triisopropylsilyl)ethynyl)thiophen 2 yl)benzo[c][1,2,5] thiadiazole (11) A solution containing 10 (0.38 g, 1 mmol), DMF (20 mL) and i Pr 2 NH (10 mL) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 4 ) 3 (23 mg, 0.02 mmol) and CuI (3.8 mg, 0.02 mmol) were added in a argon flow. Trimethylsilylacetylene (0.91 g, 5 mmol) was added dropwise. The mixture was stirred at 70 o C overnight. After reaction, ethyl acetate and brine was added to the mixture. The organic phase was collected and washed by water for three times and dried over anhydrous sodium sulfate. After the solvent was evaporated under reduced pressure, the residue was purified through column chromatography using 80/20 hexane/DCM as the eluen t (Rf = 0.42) to yield an orange solid (Yield: 0.43 g, 89.6%). 1 H NMR (300 (ppm) 8.12 (d, 1H), 7.93 (d, 1H), 7.85 (d, 2H), 7.46 (d, 1H), 7.30 (d, 1H), 7.21 (dd, 1H), 1.15 (m, 21H) 13 C NMR (75 (ppm) 152.76, 152.69, 140.40, 139.44, 133.39, 128.27, 127.91, 127.24, 127.06, 126.61, 125.93, 125.85, 125.44 125.15, 99.67, 97.90, 18.88, 11.54.
142 4 (5 ethynylthiophen 2 yl) 7 (thiophen 2 yl)benzo[ c ][1,2,5]thiadiazole (12) A THF solution of 11 (0.36 g, 0.75 mmol) was degassed by bubbling argon for 20 minutes, and then TBAF (1. 5 mL, 1.5 mmol) was added dropwise. The mixture was stirred at room temperature for 1 hour. After reaction, the solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using 80/20 hexane/DCM as the eluent (Rf = 0.26) to yield a red orange solid (Yield: 0.23 g, 96%). 1 H NMR (300 (ppm) 8.14 (d, 1H), 7.96 (d, 1H), 7.86 (d, 2H), 7.48 (d, 1H), 7.36 (d, 1H), 7.22 (dd, 1H), 3.49 (s, 1H) 13 C NMR (75 (ppm) 152.68, 141.09, 139.38, 134.10, 1 28.30, 128.01, 127.78, 127.35, 127.09, 126.87, 126.17, 125.81, 125.23, 123.34, 83.11, 77.31. MS: Calculated: 323.98; (ACPI TOF): [M+H] + : 324.9927 Poly(4 vinylbenzyl chloride) ( PVBC ) To a 10mL Schleck tube with a stirrin g bar was added 4 vinylbenzyl chloride (VBC, 26.86 mmol, 4.10 g ), CPDN (0.54 mmol, 0.146 g), and AIBN (0.13 mmol, 0.022 g). The solution was deoxygenated by freeze pump thaw for 3 cycles. Then the flask was immersed in an oil bath with setting temperature of 80 o C for 16 h. Then the flask
143 was put in liquid nitrogen to stop the polymerization. The product was dissolved in THF precipitated from methanol. The precipitation was re dissolved and precipitated for three times and then dried in vacuo for 24 hours t o give white powder. (Yield: 2.10 g, 50%). 1 H NMR (500 (ppm) 7.75 6.80 (sb, 2H), 6.75 6.25 (mb, 2H), 4.52(sb, 2H), 2.52 1.02 (mb, 3H). GPC: Mn = 9800, PDI = 1.22. Poly(4 vinylbenzyl azide) ( PVBA ) A solution of PVBC (1.0 g, 6.58 mmol of Cl), NaN 3 (19.4 mmol, 1 .26 g) in DMF (10 mL) was stirred at room temperature for 24 hours. Then the polymer was precipitated from methanol. The precipitation was re dissolved in THF and precipitated for three times and then dried in vacuo for 24 hours to yield white powder. (Yie ld: 0.85 g, 90%). 1 H NMR (500 (ppm) 7.75 6.80 (sb, 2H), 6.75 6.25 (mb, 2H), 4.27 (sb, 2H), 2.52 1.02 (mb, 3H). GPC: Mn = 10100, PDI = 1.31. General procedure for synthesis of P 0 ~ P 20: Take P 5 as an example A solution of PVBA (20 mg, 0.126 mmol N 3 ), 7 (73.5 mg, 0.131 mmol), 11 (2.3 mg, 0.007 mmol), PMDETA (6 mg, 0.034 mmol) and THF (10 mL) was deoxygenated by bubbling argon for 30 min, then CuBr (5 mg, 0.034 mmol) was added in argon flow. The mixture was stirred under under argon at room temperature for 24 hours. The resultant mixture was passed a short neutural alumina column to remove copper
144 catalyst. The solution was concentrated and the resulting polymer was precipitated in methanol. The precipitate was collected and re dissolvend in THF, and then re precipitated i n methanol for three times and then dried in vacuo for 24 hours to give orange powder. (Yield: 81 mg, 86.2%) Benzyl azide (15) A solution of benzyl chloride (1. 26 g, 10 mmol), NaN 3 (1. 95 g 30 mmol,) in DMF (10 mL) was st irred at room temperature for 24 hours. Then the mixture was extracted from water and ethyl acetate The organic phase was washed with brine and then distilled water The organic phase was then collected and dried over anhydrous Na 2 SO 3 The solvent was rem oved in reduced pressure and a colorless liquid was obtained (Yield: 1.22 g 9 2 %) and used in next step without further purification 1 H NMR ( 3 00 (ppm) 7. 44 7.33 ( m 5 H), 4.35 ( s 2H ) 13 C NMR ( 7 5MHz, CDCl3): (ppm) 135.43, 128.88, 128.34, 128.26, 54.84. ( Caution : this compound is explosive, should be stored at 20 o C and dealt with care.) 1a A solution of benzyl azide ( 15 17 mg, 0.128 mmol N 3 ), 7 (74.2 mg, 0.133 mmol), PMDETA (6 mg, 0.034 mmol) and THF (1 0 mL) was deoxygenated by bubbling argon
145 for 30 min, then CuBr (5 mg, 0.034 mmol) was added in argon flow. The mixture was stirred under under argon at room temperature for 24 hours. The resultant mixture was passed a short neutural alumina column to remov e copper catalyst. The solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using DCM as the eluent (Rf = 0.26) to yield a light yellow green solid. 1 H NMR (300 (ppm) 7.77 (d, 2H), 7.65 (s, 1 H), 7.55 (d, 2H), 7.52 (m, 2H), 7.36 7.40 (m, 3H), 7.29 7.35 (m, 5H), 7.00 (d, 2H), 5.57 (s, 2H), 4.01 (t, 4H), 1.83 (p, 4H), 1.48 1.56 (m, 4H), 1.21 1.40 (m, 16H), 0.83 (t, 6H). 13 C NMR (75 MHz, CDCl3): (ppm) 132.27,131.82, 129.45, 129.11, 128.53, 128. 55, 125.76, 119.93, 117.21, 77.50, 77.25, 76.99, 69.91, 54.56, 32.06, 29.64, 29.62, 26.34, 22.92, 14.34. MS: Calculated: 691.41; (ACPI TOF): [M+H] + : 692.4208. 1b A solution of benzyl azide ( 15 17 mg, 0.128 mmol N 3 ), 11 ( 74.2 mg, 0.133 mmol), PMDETA (6 mg, 0.034 mmol) and THF (10 mL) was deoxygenated by bubbling argon for 30 min, then CuBr (5 mg, 0.034 mmol) was added in argon flow. The mixture was stirred under under argon at room temperature for 24 hours. The resultant m ixture was passed a short neutural alumina column to remove copper catalyst. The solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using DCM as the eluent (Rf = 0.26) to yield a light yellow green solid. 1 H NMR (300 (ppm) 8.12 (d, 1H), 8.09 (d, 1H), 7.88 (s, 2H), 7.66 (s,
146 1H), 7.44 7.47 (m, 2H), 7.38 7.43 (m, 3H), 7.32 7.36 (m, 2H), 7.19 (dd, 2H), 5.59 (s, 2H). 13 C NMR (75 (ppm) 152.74, 139.57, 134.61, 134.28, 129.48, 129.18, 128.42, 128 .29, 128.27, 127.80, 127.13, 126.04, 125.73, 125.24, 129.43. MS: Calculated: 457.05; (ACPI TOF): [M+H] + : 458.0572.
147 CHAPTER 4 TRIPLET TRIPLET ENERGY TRANSFER IN POLYSTYRENE BASED PLATINUM ACETYLIDE ARRA YS 4.1 Background Triplet triplet energy transfer is an en ergy transfer process from an electronically excited triplet donor to produce an electronically excited acceptor in its triplet state 1 75 Triplet triplet energy transfer plays an important role in chemistry (such as photosensitizers 176 ), biology (such as photosynthesis, 176 singlet oxygen formation and quenching 177 178 ) and material science (such as organic light emitting diodes, OLEDs 9 10 ). The triplet triplet energy transfer process follows the Dexter exchange mechanism and can be considered as two simultaneous electron transfers with different spin, as shown in Equation 4 1: ( 4 1) s o triplet triplet energy transfer can only occur in a short distance on the order of R DA = 5 10 Many experimental and theoretical efforts have been made to investigate triplet triplet energy transfer. Schanze and coworkers have investigated triplet triplet energy transfer intensively in platinum acetylide polymers, in which platinum acetylides wi th lower triplet energy (Pt thien ylene acceptor) were doped in the polymer of platinum acetylides with higher triplet energy (Pt phenylene, donor). I t was found the energy transfer from donor to acceptor was efficient and rapid, even though in the copol ymer with only 5% of acceptor. 179 Triplet triplet energy transfer has also been investigated in platinum acetylide oligomers. Casterllano, Ziessel and coworkers reported the ultrafast
148 energy transfer between 3 T and 3 MLCT in picoseconds range. 180 181 Schanze, Miller and coworkers put a triplet energy trap (N aphthalene diimides NDI) at the ends of platinum acetylides and found the triplets transport ed diffu siv ely by a site to site hopp ing mechanism with hopping time of 27 ps each step. 182 183 Meyer, Papanikolas and co workers studied the energy transfer between 3 MLCT states induced by Ru and Os in the copolymer based on polystyrene backbone and with pendant Ru(II) and Os(II) metal complexes T he intrapolymer energy transfer que n ching involes a combination of random walk, energy migration ( k mig ), and energy transfer ( k EnT ) events T he rate consta nt of < k E n T > ~ 2.5 x 10 9 s 1 for the nearest neighbor Ru II to Os II energy transfer and < k mig > ~ 2.5 x 10 8 s 1 to 1 x 10 9 s 1 for Ru II to Ru II energy migration (lifetime of 1 4 ns, 50 times faster than Ru II excited state decay) were observed 8 54 58 In th is chapter the preparation of graft copolymers with a polystyrene backbone and pedant platinum acetylide chromophores (poly platinums) are reported. In poly platinums different Pt acetylides act as triplet energy donor and acceptor, respectively. Both time resolved and steady state fluorescence spectr oscopy as well as time resolved transient absorption spectroscopy was applied to study the efficiency and dynamic s of energy transfer in these macromolecular triplet donor acceptor systems. T he results indicate that triplet triplet e nergy transfer between different platinum acetylides in these polymers is highly efficient occurring on the nanosecond timescale 4.2 Polymer Design and Preparation A well defined polystyrene based macromolecular structure (poly platinums) was designed and prepared to study the tr iplet triplet energy transfer in platinum acetylide arrays. T he preparation of clickable polystyrene precursor with pendant azides ( PVBA )
14 9 was described in Chapter 3. T he two platinum acetylides designed for triplet energy transfer have a PE2 unit and pyren e unit attached to platinum, respectively (referred as PE2 Pt and Py Pt in this chapter). The triplet of PE2 Pt has a higher energy than that of Py Pt (2.35 eV vs. 1.88 eV, calculated from phosphorescence emission peak wavelengths) so in the polymer PE2 P t act as triplet energy donor and Py Pt act as the acceptor. The structure s of designed polymers are demonstrated in Figure 4 1. Polymers with different PE2 Pt ( donor ) / Py Pt ( acceptor ) ratios ( poly chromophores ) are identified as P x where x represents the percentage of Py Pt in the polymer P 0 is the homopolymer of PE2 Pt and P 100 is the homopolymer of Py Pt Figure 4 1 Structures of poly chromophores ( P 0 to P 100 ) and model compounds. 4.2.1 S ynthesis of Platinum Acetylides The preparation of two platinum acetylides ( 5 and 9 ) with terminal alkynes, which are ready to be clicked onto polymer with pendant azides (PVBA) is shown in Figure 4 2. T he synthesis started from compound 1 which had one TIPS protected acetylene group and an unprotected acetylene group. M onoalkynyl platinum(II) complex ( 2 ) wa s synthesized through the direct Hagihara coupling reaction between compound 1 and
150 cis Pt(PPh 3 ) 2 Cl 2 in the absence of copper(I) halide The mon oalkyn yl platinum(II) complex 2 was the precursor for both 5 and 9 Sonag a shria coupling between compound 1 and 1 iodobenzne afforded compound 3 which can be de protected by TBAF to give compound 4 (PE2). Hagihara coupling between 4 and 2 in the presence of CuI gave the protected PE2 Pt ( M1 ), in which the TIPS protecting group can be removed by TBAF to obtain clickable 6 T he preparation of terminal alkyne bearing Py Pt ( 9 ) began from 1 bromopyrene ( 6 ). S onagashira coupling installed a TIPS protected acetylene o nto pyrene to make compound 7 Deprotection of 7 by TBAF gave compound 8 (Py) Compound 8 is not very stable in air, and some of it will involve self coupling to make dark colored products. However, the self coupling side products did not affect the subseq uent Hagihara coupling between 8 and 2 during which the TIPS protected Py Pt ( M2 ) was prepared. T he clickable terminal alkyne bearing Py Pt ( 9 ) was obtained by deprotection of M2 by TBAF.
151 Figure 4 2 Preparation of PE2 Pt and Py Pt chromophores with terminal alkynes ( 5 and 9 ). 4.2.2 Preparation of P oly Platinums P oly Platinums with different PE2 Pt / Py Pt ratios were prepared, identified as P x where x represents the percentage of Py Pt groups i n the polymer ( Figure 4 1 ). T he two platinum ac e t y lides bearing terminal acetylene group on one side ( 5 and 9 ) w ere grafted onto PVBA via copper (I) catalyzed azide alkyne click cycloaddition as shown in Figure 4 3 All five polymers were derived from t he same PVBA precursor with Mn = 10100 a n d PDI = 1.31, so they all have the same chain length estimated of degree of
152 polymerization (DP) around 60 The x value was controlled by adjusting the platinum ac e t y lides nd the actual values of x were determined by UV visible absorption spectroscopy Figure 4 3 Preparation of p oly platinum s ( P 0 to P 10 0 ). Figure 4 4 GPC traces of Poly Platinums and PVBA P 0 (black, Mn= 31900 PDI=1. 31 ), P 3 ( dark yellow Mn= 320 00, PDI=1. 30 ), P 5 ( blue Mn= 343 00, PDI=1.2 4 ), P 10 ( red Mn= 338 00, PDI=1.2 6 ), P 20 (brown, Mn= 3060 0, PDI=1. 31 ), P 100 ( green solid square, Mn= 300 00, PDI=1.2 3 ) and PVBA ( black dash Mn=10100, PDI=1.31).
153 The click reaction can be monitored by GPC and 1 H NMR, as presented in Figure 4 4 and 4 5 After click grafting, the molecular weights increased significantly, with GPC traces shifting to lower retention time (i.e., highe r molecular weight) from PVBA to P x polymers. Peaks in the 1 H NMR spectrum of P 0 can be divided into several groups The first comes from the polystyrene backbone, including two peaks (6.35 and 6.82 ppm) assigned to phenyl protons in the backbone ( a and b in Figure 4 5) and a peak (5.42 ppm) corresponding to triazole methylene protons ( c in Figure 4 5). T he complete disappearance of 4.27 ppm for the azidomethylene protons in PVBA and arising of the 5.42 ppm peak for P x polymers is evidence of complete chr omophore grafting d uring the azide alkyne click cycloaddition (the 1 H NMR spectrum of PVBA is shown in Figure 3 7). The second group of peaks arises from the n butyl phosphine protons in the PE2 Pt side groups, at 2.11, 1.60, 1.42, and 0.90 ppm ( d e f a nd g in Figure 4 5). The third group includes resonances corresponding to aromatic protons from PE2 Pt (6.35 to 7.72 ppm, h n in Figure 4 5 ). The first group peaks belonging to the polymer backbone protons are broad as the grafted side chains limit the f ree rotation of backbone, while the second and third group peaks assigned to PE2 Pt side chain protons are relatively sharp. From P 0 to P 3 the first and second groups of peaks do not change. However, new and small peaks appear in the third group, which arise from pyrene protons in Py Pt side chains (7.80 8.80 ppm, o w in Figure 4 5). From P 3 to P 100 signals from o to w become stronger as the Py Pt content increases, while signals from k to n become weaker. In P 100 the Py Pt homopolymer, signals from k to n completely disappeared.
154 Noteworthy, a ruler for Py Pt content is the single peak at 8.71 ppm, which is assigned to the 10 position proton of pyrene ( w in Figure 4 5). This peak arises significantly along with the Py Pt content and the peak i ntegration is used to calculate the actual Py Pt content in the copolymers. However, the calculation has large error in P 3 P 5 and P 10 because the single at 8.71 ppm is too small to be integrated accurately. Accurate calculation relies on UV visible ab sorption. For P 20 and P 100 the Py Pt contents calculation from NMR agree well with the results from absorption.
155 Figure 4 5 1 H NMR Spectra of Poly Platinums ( P 0 to P 100 ).
156 4.3 Steady State Absorption and Emission Figure 4 6 Absorption of platinum acetylides and Poly Platinums in THF. The absorption of the two model platinum acetylides PE2 Pt model compound M1 and Py Pt model compound M2, and six poly platinums ( P 0 to P 100 ) are presented in Figure 4 6 The energy donor model M1 has two near UV peaks at 302 nm and 350 nm. The acceptor model compound M2 has a near UV absorption band at 292 nm and three visible absorption band s at 368 nm 387 nm and 398 nm, and Py Pt also has a modulate absorpti on from 300 nm to 350 nm. P 0 and P 100 is the homopolymer of PE2 Pt and Py Pt, respectively and the absorption of the two polymers is nearly identical to the two model compounds. T he extinction coefficients of P 0 and M1 are similar which are less than 5 % difference ( see Table 4 1). It is the same for P 100 and M2 The extinction coefficients are another evidence of complete grafting during the azide alkyne click reaction.
157 The spectra of P 3 to P 20 are dominated by the PE2 Pt based transitions (especiall y at 3 48 nm) ; however, they show increasing Py Pt character as the loading of the lower energy chromophores in the click grafting is increased which features the absorption at 398 nm. This feature allows straightforward determination of the Py Pt loading in the copolymer from the ratio of the absorbance at 3 5 0 and 398 nm C onsidering the absorption of P 3 to P 20 is a linear combination of P 0 and P 100 the absorption spectra are used to calculate the accurate x value in the polymers. The absorption at 35 0 nm arises from both PE2 Pt and Py Pt side chains; while the absorption at 398 nm is solely from Py Pt side chain. So the absorption ratio at 350 nm and 400 nm will follow Equation 4 2 : (4 2 ) And from Equation 4 2 the x value is calculated as: (4 3 ) The calculation results ( summarized in Table 4 1) indicate that the fractional loading of PE2 Pt and Py Pt corresponds closely to the stoichiometry used in the click reactio n On the other hand, t he simulated absorption spectrum of P 20 ( Figure 4 6 ), calculated from the absorption spectra and extinction coefficients of P 0 and P 100 corresponds nicely with the measured P 20 absorption. Taken together, the absorption data for the model compounds and polymers allow several conclusions. First, the fractional loading of PE2 Pt and Py Pt units in the polymers closely corresponds to the stoichiometry used in the feed for the click
158 reactions. Second, the absorption at 3 5 0 nm is due almost exclusively to the PE2 Pt (donor), making it possible to selectively excite this chromophore. Finally, the polymer spectra are accurately simulated as a linear combination of the spectra of the PE2 Pt and Py Pt chromophores, indicating that there is not a strong ground state interaction among the individual units in the polymers. Figure 4 7 Comparison of measured ( brown) and simulated (violet circle ) absorption spectra of P 20 The calculated spectrum was derivate fro m the absorption spectra and extinction coefficients of P 1 and P 100 based on the equation P 20 ) = 0.8 x ( P 1 ) + 0.2 x ( P 100 ). The results were then normalized to (0, 1) to obtain the spectrum (violet circle ) above. The spectra of P 0 and P 100 in this figure are normalized based on their relative molar absorptiv it ies and relative PE2 Pt/Py Pt content in P 20 copolymer The phosphorescence spectra of P 0 to P 100 are shown in Figure 4 8 T he phosphorescence measurement was carried out in THF solutions and the solutions were degassed via bubbling argon for 45 minu tes. T he phosphorescence of the PE2 Pt (donor) only polymer P 0 appears at 52 7 nm, with a shoulder at 562 nm, which is nearly
159 identical to that of M1 Similarly, the P y Pt (acceptor) only polymer P 10 0 and acceptor model M2 also have similar phosphorescenc e spectra, with peaks of 660 and 737 nm. Figure 4 8 Emission of model compounds and poly platinums in THF. The solutions had OD round 0.8 and were deoxygenated by bubbling argon for 45 minutes. The spectra were normalized acc ording to their quantum yields. The excitation wavelength was set at 350 nm for P 0 to P 20 and M1 and 385 nm for P 100 and M2 As for the copolymers ( P 3 to P 20 ), t he excitation wavelength was selected to be 350 nm, which can selectively excite PE2 Pt side chains. In P 3 which has only 3 percent of Py Pt loaded onto the co polymer, the phosphorescence from the PE2 Pt chromophore at 52 7 nm is quenched ~ 85% relative to the intensity of P 0 and emission from Py Pt at 660 nm and 737 nm is evident indicat ing the PE2 Pt* to Py Pt energy transfer is efficient The energy transfer efficiency can be calculated with the quantum yield s according to Equation 4 4 : 58
160 (4 4 ) In Equation 4 4 is the phosphorescence quantum yield of P 0 and is quantum yield of phosphorescence emission from PE2 Pt* (480 to 628 nm) in the c opolymers. The quantum yields are listed in Table 4 1. According to Equation 4 4 the energy transfer efficiency is ~ 86% in P 3 In P 5 the PE2 Pt emission is quenched to a greater extent, and the energy transfer efficiency from PE2 Pt* to Py Pt approach es 95 %. When Py Pt content increased to 20% ( P 20 ), the PE2 Pt emission is quenched almost completely, with ~ 100%. Figure 4 9 Quantum yields and energy transfer efficiency for poly platinum copolym ers. Figure 4 9 shows the quantum yields from PE2 Pt and Py Pt emission, respectively, and the energy transfer efficiencies in the polymers P 0 to P 20 T h e PE2 Pt emission yield decrease s sharply from P 0 to P 3 followed by a more gradual decline from P 5 to P 20 By contrast, t he Py Pt emission yield increase s sharply from P 0 to P
161 3 ; however, it then decrease s slightly from P 5 to P 20 The latter trend is likely due to self quenching of the Py Pt chromophore as its concentration in the polymer s increa ses possibly due to triplet triplet annihilation between Py Pt units. The energy transfer efficiency increased sharply from P 0 to P 3 (86.3%), and then increased gradually to approach 100% in P 20 T he quantum yield and energy transfer efficiency data ar e also listed in Table 4 1. T he phosphorescence emission s clearly demonstrate the triplet triplet energy transfer from PE2 Pt (donor) chromophore to Py Pt (acceptor) chromophore Additional evidence for PE2 Pt to Py Pt energy transfer in from P 3 to P 20 is provided by excitation spectra as presented in Figure 4 10 which was collected while monitoring emission at 660 nm the Py Pt emission maximum The spectrum of the Py Pt only polymer P 100 shows that the phosphorecence of 6 6 0 nm comes from the three v isible absorption bands of Py Pt, 3 68 nm 387nm and 398 nm H owever, emission at 6 6 0 nm of polymers bearing both PE2 Pt and Py Pt side groups ( P 3 to P 20 ) are mostly originated from PE2 Pt absorption ( ~ 347 nm). The excitation spectra of P 3 to P 20 match the absorption spectra of these polymers. These data clearly show that the Py Pt emission s are coming from excitation of the PE2 Pt chromophores.
162 Figure 4 10 Excitation spectra of poly platinum s. emission = 6 6 0 nm.
163 Tab le 4 1. Photophysical c haracteristics of model compounds and p oly platinums /nm b /% ------292 368 387 398 60,900 58,000 66,800 57,100 412 0.002 660 737 -----388 <0.0001 527 660 737 -388 <0.0001 527 660 737 -388 <0.0001 527 660 737 -388 <0.0001 527 660 737 292 368 387 66,800 5 6,100 63,500 54,100 412 0.0013 660 737 --a Determined by UV visible absorption b Abs max : Ground state absorption maxima; FL max : Fluorescence emission maxima; Ph max : Phosphorescence emission maxima c With anthracene as quantum yield standar d, = 0.27 in ethanol at room temperature
164 4.4 Transient Absorption Characterization T ransient absorption (TA) experiments were carried out on the poly platinums to characterize the dynamics of the intrapolymer energy transfer process. Figure 4 11 presents T A spectra for the two model compounds M1 and M2 The spectrum of M1 ( Figure 4 11A ) shows a negative feature (bleach) at 365 nm that results from ground state bleach of M1 as well as an intense excited state absorption centered near 580 nm Meanwhile, the TA spectrum of M2 ( Figure 4 1 1B ) shows strong negative bleach at ~ 387 nm, a modulate intense excited state absorption at ~ 443 nm due to T 1 T n absorption, and a broad absorption centered at 548 nm, which may come from the excimer as pyrene moiety exists i n the compound. T he TA spectra of M1 and M2 are similar to previously reported spectra of platinum acetylides with similar structures. 149 Figure 4 11 Transient absorption spectra of model compounds. A) M1 and B) M2 Figure 4 13 compare s the transient absorption o f the donor acceptor co polymers ( P 3 to P 20 ) with those of the donor only polymer ( P 0 ) and the acceptor only polymer ( P 10 0 ) The TA spectr a of P 0 ( Figure 4 12 black ) and P 100 ( Figure 4 12, pink) show similar features as those of M1 and M2
165 Figure 4 12 Transient absorption spectra of copolymers ( P 3 to P 20 ) at different time and comparison with homopolymers ( P 0 and P 100 ) Examination of the transient absorption of copolymers P 3 to P 20 clearly reveals the presence of energy transfer when both PE2 Pt donor and Py Pt acceptor moieties are present on a sing le chain Figure 4 12a shows the transient absorption spectrum of the P 3 donor acceptor copolymer at 60 n s ( red ) and 2000 ns (blue) The spectrum at 60 ns is nearly identical to that of the donor only polymer P 0 ( black ), indicat ing that photoexcitation at 3 55 nm predominantly creates PE2 Pt excited states ( PE2 Pt *) ; however, a small peak around 460 nm arises, which is coming from Py Pt excited states ( Py Pt *) suggesting certain amount of triplet energy have transferred from PE2 Pt to Py Pt within 60 ns In addition, by 2000 ns following excitation the transient spectrum of P 3 ( blue ) has evolved the spectrum is dominated by the positive absorption around
166 450 nm that are coming from the T 1 T n absorption of the excited Py Pt* The 543 nm absorption peak does not show as in the copolymer with low Py Pt content, excimer is difficult to form. Investigations of other copolymers, P 5 to P 20 reveal similar phenomenon as P 3 Note the delay time needed for the 460 nm small shoulder to appear is getting shorter from P 3 to P 20 which means the energy transfer from donor to acceptor becomes faster as Py Pt content increases. 4.5 Time Resolved Emission Time resolved emission can be characterized by both t ime correlated single photon counting (TCSPC) and transient abs orption (TA). The working mechanisms for these two instrumentations are different. TCSPC counts photons emitted from T 1 to S 0 transition directly. On the other hand, TA deals with excitons staying in T 1 state at different times, thus studying T 1 to S 0 tran sition indirectly. 184 Both methodologies w ere applied to study the kinetics of energy transfer from PE2 Pt* to Py Pt. Figure 4 13A shows emission decays at 520 nm (peak value of PE2 Pt phosphorescence emission) of PE2 Pt containing polymer P 0 to P 20 T he decay of PE2 Pt only polymer P 0 is bi exponential with a fast decay ( 1 = 1.89 s) and a slow decay ( 2 = 62 s); the two components have nearly equal amplitude. As 3% of Py Pt is loaded, the lifetime of fast decay decreases by 30% ( 1 = 1.22 s ) and the amplitude increases up to 0.90. As the acceptor content increases, the time constants for both decay compon ents are shortened; and the time constant of the fast decay ( 1 ) deceases to 360 ns for P 20 with a amplitude of 0.96. The trend is more clear in the expanded view from 0 to 100 s, as displaced in Figure 4 11B. The lifetimes and amplitudes obtained from the time resolved emission are listed in Table 4 2.
167 Figure 4 13 Phosphorescence decay of poly platinums at 520 nm. A) Complete decay from 0 to 500 s. B) Decay from 0 to 100 s. T he decrease of the fast decay time constant a nd increasing of its amplitude clearly indicate the energy transfer from PE2 Pt* to Py Pt, and this quenching process is a dynamic process. T he increasing of Py Pt (acceptor) in the copolymer speeds up the quenching process. F rom the lifetime values, we co uld conclude the energy transfer happens with in 1 s. More detailed study about the energy transfer within the 1 s timescale was further studied by transient absorption, by following the single wavelength at = 6 00 nm This wavelength is close to t he peak value of T 1 to T n absorption of PE2 Pt. The decays of polymers are presented in Figure 4 14. Interestingly, comparing to the long time scale decay, biphasic decays are also observed in the polymers, with much shorter lifetimes when examining the decay kinetics within 1 s. T he decay of P 0 exhibits a fast component ( 1 = 92.7 ns and A 1 = 0.43) and an intermediate component ( 2 = 817 ns and A 2 = 0.57). As the loading of Py Pt increases from 3 % to 20% the time constants of fast components ( 1 ) decreases from 53.0 ns to 15.9 ns, with the amplitude increased from 0.56 to 0.75. Meanwhile,
168 time constants of the second component ( 1 ) decrease from 395 ns to 222 ns, with a decreased amplitude from 0.44 to 0.25. The first component ( 1 ) can be assigned to the direct energy transfer from nearest neighbor PE2 Pt* to Py Pt, with a timescale from 10 to 50 ns. The second component ( 2 ) is attributed to energy migration through a random walk, site to site hopping process of PE2 Pt exited energy to a position along the chain that is in close proximity to an acceptor Figure 4 14 Transient kinetics from = 6 00 nm for five polymer s P 0 to P 20 on the timescale of 0 to 0.92 s. Increasing the loading of acceptor chromophores in the copolymer has two effects. First, the probability of photoexcitation produc ing excited states next to an acceptor increases. Second, for those excitons created far from an acceptor, the number of steps necessary to migrate near enough for quenching to occur decreases. Both of these effects will result in qualitatively faster quen ching of the excited state
169 which agrees with the amplitude change for the direct energy transfer and energy migration processes. Table 4 2 Liftimes of p oly platinums Long time Scale Short time Scale 1.89 (0.49) 61.6 (0.51) 92.7 (0.43) 817 (0.57) 1.29 (0.90) 43.6 (0.10) 53.0 (0.56) 395 (0.44) 1.11 (0.92) 36.5 (0.77) 31.5 (0.54) 295 (0.46) 0.48 (0.95) 31.1 (0.05) 22.6 (0.64) 252 (0.36) 0.36 (0.96) 24.0 (0.04) 15.9 (0.75) 222 (0.25) 4.6 Energy Transfer Pathway In the system with triplet state, the energy transfer may have two distinct mechanisms. After the donor in the copolymer is excited selectively to produce 1 (PE2 Pt), a s show in Figure 4 14 path 1, the first mechanism involves rapid singlet energy transfer from 1 (PE2 Pt) to 1 (Py Pt) ( Figure 4 15 path 2 ) as described in Chapter 3 Following singlet transfer, 1 (Py Pt) can relax either radiatively (fluorescence) or via int ersystem crossing to produce 3 (Py Pt) (paths 3 and 4 in Figure 4 15 ). The second mechanism, which is triplet energy transfer, involves intersystem crossing on PE2 Pt unit to produce 3 (PE2 Pt) ( Figure 4 14 path 5), followed by
170 intrachain triplet energy tra nsfer from 3 (PE2 Pt) to 3 (Py Pt). Radiative decay of 3 (Py Pt) emits phosphorescence photons. To determine which mechanism the energy transfer in the copolymer follows the relative rates of intersystem crossing, singlet energy transfer and triplet energy t ransfer should be considered. Previously studies reveal that the intersystem crossing of platinum acetylides is i n the timescale less than 10 ps. 149 180 Studies in Chapter 3 indicate the singlet energy transfer lies in 20 50 ps. Therefore, energy transfer following the first mechanism, involving singlet energy transfer should occur in picoseconds range. Consi dering the energy transfer in P 3 to P 20 has time constants in nanoseconds range, the first mechanism is ruled out as the energy transfer in the copolymers is much slower than intersystem crossing and singlet transfer. Thus we can conclude the energy tran sfer in the copolymers follows triplet triplet energy transfer mechanism. Figure 4 15 Jablonski Diagram for energy transfer in copolymers. The energy level is calculated from emission peak values.
171 4.7 Summary In summary, a serie s of polymeric arrays consisting of ~ 60 platinum acetylide chromophores have been synthesized with RAFT S N 2 click strategy, and energy transport in the polymer s ha s been examined. Photoexcitation of PE2 Pt sites gives rise to site to site triplet energy transfer and ultimately sensitization of a trap site ( Py Pt ) doped into the polymer at low concentration, on the nano second time scale and with remarkably high efficiency. The energy transfer process proceeds following triplet triplet transfer pathway, vi a a fast neighborhood (PE2 Pt) (Py Pt) quenching in 10 ~ 50 nano second s (PE2 Pt) (PE2 Pt) energy hopping in 200 ~ 400 nano second s 4.8 Experimental 4.8.1 Instrumentation and Methods NMR spectra were measured on a Gemini 300 FT NMR, a Mercury 300 FT NMR, or a n Inova 500 FT NMR. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer (Bruker Daltonics, Billerica, MA). Gel permeation chromatography (GPC) analyses were carried out on a sy stem comprised of a Shimadzu LC 6D pump, Agilent mixed D column and a Shimadzu SPD 20A photodioide array (PDA) detector, with THF as eluent at 1 m L /min flow rate. The system was calibrated against linear polystyrene standards in THF. For UV visible absorpt ion measurements samples were dissolved in THF and were carried out on a Shimadzu UV 1800 dual beam absorption spectrophotometer using 1 cm quartz cells. Photoluminescence measurements were obtained on a fluorimeter from Photon Technology International (P TI) using 1 cm quartz cells. For
172 phosphorescen ce measurement the sample solutions were degassed via bubbling argon for 45 minutes. Luminescence lifetimes were obtained with a multichannel scaler/photon counter system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime S pectrophotometer. A high performance Coherent CUBE diode laser provided the excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford Research Systems DG535 digital decay and pulse generator with four independ ent delay channels. At least four narrow band pass filters were used for measurements followed by global fit processing (FluoroFit software). Decays were obtained using the multi exponential fitting p arameters (FluoroFit software). Nanosecond triplet tripl et transient absorption measurements were acquired with excitation at 355 nm (10 mJ/pulse) using the third harmonic of a Continuum Surelite II 10 Nd:YAG laser. Perkin Elmer LS1130 3 pulsed xenon lamp was used as a probe source and the transient absorption signal was detected with a gated intensified CCD mounted on a 0.18 M spectrograph (Princetor PiMax/Acton Pro 180). Samples were prepared to an optical density of 0.7 at the excitation wavelength in a continuously circulating 1 cm pathlength flow cell (volu me = 9 mL). Triplet lifetimes were calculated with a single exponential global fitting of the transient absorption decay data using SpecFit analysis software. Single wavelength kinetics measurements were done in a home built system, with the same laser sou rce and xenon lamp described above. T he detected wavelength was controlled by a monochromator (Instruments SA, H 20) and the intensity of the monitoring light is detected by a photomultiplier tube (Humamatsu, R928). 185
173 4.8.2 Materials Copper (I) iodide (CuI), copper (I) boromide (CuBr), diisopropylamine ( i Pr 2 NH), phenyl acetylene, tetrabutyl ammonium fluoride (TBAF, 1 M in THF), pentamethyldiethylenetriamine (PMDETA), and 1 bromopyrene were from Sigma Aldrich. Tetrakis(triphenyl phosphine) palladium (0) (Pd(PPh 3 ) 4 ) was from Stern Chemical and trisisopropylsilyl acetylene (TIPSA) was from TCI chemical. CuBr was stirr ed in acetic acid overnight, washed with acetone and then dried over vacuum All reagents were uses as received without further purification. cis Pt(PBu 3 ) 2 Cl 2 157 was synthesized according to the literature procedu r e. T he synthesis of 1 ethynyl 4 ((triisopropylsilyl)ethynyl) benzene ( 1 ) and poly(vinylbenzyl azid e) were described in Chapter 3. 4.8.3 Synthesis Compound 2 In a 25 mL round bottom flask was placed in 282 mg of 1 ( 0.1 mol) 670 mg of cis Pt(PBu 3 ) 2 Cl 2 (0.1 mol) and 10 mL of Et 2 NH. After the solution was degassed with argon f or 30 mins, it was refluxed at 60 o C for 2 h. The solvent was removed by rotary evaporation, and the crude product was purified by column chromatography from 25% CH 2 Cl 2 in hexane. Yield: 750 mg (82.0%). 1 H NMR (300 MHz, CDCl3 ) ppm 7.30 (2H, d), 7.12 (2H, d), 1.98 (m, 12H), 1.36 1.57 (m, 24H), 1,10 (m, 21H), 0.90 (t, 18H). 1 3 C NMR (125 MHz, CDCl3 ) (ppm) 131.78, 130.59, 129.14, 119.97, 107.71, 101.45, 90.94,
174 86.90, 26.22, 24.45, 22.13, 18.82, 13.94, 11.48. 31 P NMR (121 MHz) ppm 6.93 ( J Pt P = 236 5 Hz) Compound 3 A solution containing 1 ( 2.82 g, 0.01mol), tetrahydrofuran (THF, 80 mL) and iPr 2 NH, (20 mL) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 3 ) 4 (115.5 mg, 0.1 mmol) and CuI (38.2 mg, 0.2 mmol) were added in a argon flow. I odobenzene ( 1.22 g, 0.012 mol) was added dropwise with a syringe. The mixture was stirred at room temperature overnight. After reaction, the solvent was evaporated in reduced pressure, and the re sidu re was extracted by ethyl acetate and brine. The organic phase was collected and washed by brine and water for three times and dried over anhydrous sodium sulfate. After the solvent was evaporated under reduced pressure, the residue was purified through co lumn chromatography using hexane as the eluent to yield a light yellow liquid (Yield: 3.3 g, 92.2%). 1 H NMR (500 MHz, CDCl3 ) (ppm) 7.53 (2H, m), 7.45 (4H, m), 7.35 (5H, m), 1.13 (21H, m). 1 3 C NMR (125 MHz, CDCl3 ) (ppm) 132.20, 131.78, 131.69, 123.92, 123.08, 122.21, 91.50, 88.96, 83.42, 79.03, 18.83, 15.50.
175 Compound 4 A THF solution of 3 ( 1.0 g, 2.8 mmol) was degassed by bubbling argon for 20 minutes, and then tetrabutylamonium fluoride (TBAF, 1 M solution in THF, 5.6 mL, 5.6 mmol) was added dropwise via a syringe. The mixture was stirred at room temperature for 1 hour. After reaction, t he solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using 70/30 hexane/DCM as the eluent to yield a light yellow solid (Yield: 0.5 5 g, 97 .2 %). 1 H NMR (500 MHz, CDCl3 ) (ppm) 7.54 (2H, m), 7.48 (4H, m), 7.35 (5H, m), 3.17 (1H, s). 1 3 C NMR (125 MHz, CDCl3 ) (ppm) 132.21, 131.78, 131.68, 123.93, 123.08, 122.22, 91.51, 88.97, 83.43, 79.02. M1 In a 25 mL round bottom flask was placed in 3 00 mg of 2 (0.33 mmol) 69.5 mg of 4 (0.34 mmol) and 10 mL of Et 2 NH. After the solution was degassed with argon for 30 mins, 2.8 mg of CuI (0.02 mmol) was added and the solution was stirred at room temperature overnight The solvent was removed by rotary e vaporation, and the crude product was purified by gradient column chromatography from 2 5% of hexane in CH 2 Cl 2 to pure CH 2 Cl 2 Yield: 310 mg, 87.6 %. 1 H NMR (500 MHz, CDCl3 ) (ppm) 7.52 (2H, dd), 7.37 (2H, d), 7.34 7.31 (5H, m), 7.22 (2H, d), 7.18 (2H, d), 2.12 (12H, dt), 1.60 (12H, m), 1.44 (12H, s), 1.12 (21H, m), 0.92 (18H, t). 1 3 C NMR (125 MHz, CDCl3 ) (ppm)
176 134.77, 131.61, 131.35, 130.80, 130.61, 129.25, 128.45, 128.14, 123.73, 119.70, 119.35, 109.48, 107.88, 90.71, 90.19, 89.91, 26.51, 24.55, 24.10, 18.84, 13.96, 11.51. 31 P NMR (121 MHz) ppm 3.17 ( J Pt P = 23 4 9 Hz) Compound 5 A THF solution of M1 ( 250 m g, 0.23 mmol) was degassed by bu bbling argon for 20 minutes, and then tetrabutylamonium fluoride (TBAF, 1 M solution in THF, 0.5 mL, 0.5 mmol) was added dropwise via a syringe. The mixture was stirred at room temperature for 1 hour. After reaction, the solvent was evaporated in reduced p ressure, and the residue was purified through column chromatography using 70/30 hexane/DCM as the eluent to yield a light yellow solid (Yield: 190 m g, 89.2 %). 1 H NMR (500 MHz, CDCl3 ) (ppm) 7.52 (2H, dd), 7.38 (2H, d), 7.34 7.31 (5H, m), 7.23 (2H, d), 7.20 (2H, d), 2.12 (12H, m), 1.60 (12H, m), 1.44 (12H, s), 0.92 (18H, t). 1 3 C NMR (125 MHz, CDCl3 ) (ppm) 131.74, 131.47, 131.22, 130.67, 130.57, 128.31, 123.58, 119.23, 117.98, 109.99, 90.04, 89.79, 84.25, 26.37, 24.40, 23.95, 13.82. 31 P NMR (121 MHz) ppm 3.19 ( J Pt P = 23 47 Hz)
177 1 (( T riisopropylsilyl)ethynyl) pyrene (7) A solution containing 1 bromopyrene ( 6 0. 28 g, 1 mmol), DMF (20 mL) and i Pr 2 N H (10 mL) was degassed by bubbling argon for 20 minutes, and then Pd(PPh 4 ) 3 (23 mg, 0.02 mmol) and CuI (3.8 mg, 0.02 mmol) were added under a n argon flow. Tri isopropylsilyl acetylene (0.91 g, 5 mmol) was added dropwise and t he mixture was stirred at 70 o C o vernight. After reaction, ethyl acetate and brine was added to the mixture. The organic phase was collected and washed by water for three times and dried over anhydrous sodium sulfate. After the solvent was evaporated under reduced pressure, the residue wa s purified through column chromatography using 80/20 hexane/DCM as the eluent (Rf = 0.42) to yield an orange solid (Yield: 0. 30 g, 78.9 %). 1 H NMR (300 MHz, CDCl3 ) (ppm) 8.59 (1H, d), 8.11 7.94 (8H, m), 1.12 (21H, m). 1 3 C NMR (75 MHz, CDCl3 ) (ppm) 132.5, 131.7, 131.5, 131.3, 130.2, 128.8, 128.6, 127.4, 126.6, 125.7, 124.8, 124.6, 124.4, 117.9, 104.5, 100.7, 18.8, 11.5. 1 E thynyl pyrene (8) A THF solution of 7 ( 250 m g, 0.65 mmol) was degassed by bubbling argon for 20 minu tes, and then tetrabutylamonium fluoride (TBAF, 1.5 mL, 1.5 mmol) was added dropwise via a syringe. The mixture was stirred at room temperature for 1 hour. After
178 reaction, the solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using 70/30 hexane/DCM as the eluent to yield a light yellow solid (Yield: 110 m g, 74.9 %). 1 H NMR (300 MHz, CDCl3 ) (ppm) 8.57 (1H, d), 8.11 7.94 (8H, m), 3.69 (1H, s). 1 3 C NMR (75 MHz, CDCl3 ) (ppm) 132.3, 131.4, 131.0, 130.8, 129. 9, 128.4, 128.2, 127.0, 126.1, 125.6, 125.5, 125.1, 124.2, 123.9, 116.3, 82.8, 82.6. M2 In a 25 mL round bottom flask was placed in 2 00 mg of 2 (0.22 mmol) 51.9 mg of 4 (0.23 mmol) and 10 mL of Et 2 NH. After the solution was degassed with argon for 30 mins, 1.8 mg of CuI (0.013 mmol) was added and the solution was stirred at room temperature overnight The solvent was removed by rotary evaporation, and the crude product was purified by gradient column chromatography from 2 5% of hexane in CH 2 Cl 2 to pure CH 2 Cl 2 Yield: 151 mg, 62.3 %. 1 H NMR (500 MHz, CDCl3 ) (ppm) 8.73 (1H, d), 8.11(2H, t), 8.01(2H, q), 7.97 7.94(4H, m), 7.34(2H, d), 7.22(2H, d), 2.19(12H, m), 1.68(12H, m), 1.46(12H, s), 1.12(21H, m), 0.92(18H, t). 1 3 C NMR ( 125 MHz, CDCl3 ) (ppm) 131.80, 131.76, 131.63, 131.36, 131.13, 130.68, 129.29, 129.12, 128.74, 121.57, 121.06 126.65, 126.37, 125.91, 124.98, 124.74, 124.64, 124.58, 124.49, 119.72, 109.67, 109.51, 107.90, 90.72, 26.66, 24.58, 24.27, 18.85, 14.02, 11.52. 31 P NMR (121 MHz) ppm 3.69 ( J Pt P = 23 53 Hz)
179 9 A THF solution of M2 ( 120 m g, 0.11 mmol) was degassed by bubbling argon for 20 minutes, and then tetrabutylamonium fluoride (TBAF, 0.3 mL, 0.3 mmol) was added dropwise via a syringe. The mixture was stirred at room temperature for 1 hour. After reaction, the solvent was evaporated in reduced pressure, and the residue was purified through column chromatography using 70/30 hexane/DCM as the eluent to yield a light yellow solid (Yield: 85 m g, 81.7 %). 1 H NMR (500 MHz, CDCl3 ) (ppm) 8.72 (1H, d), 8.11(2H, t), 8.01(2H, q), 7.98 7.94(4H, m), 7.36(2H, d), 7.24(2H, d), 2.19(12H, m), 1.68(12H, m), 1.46(12H, s), 0.92(18H, t). 1 3 C NMR (125 MHz, CDCl3 ) (ppm) 131.75, 131.62, 130.64, 128.61, 127.53, 126.90, 126.51, 126.24, 125.77, 124 .82, 124.50, 124.44, 124.36, 90.56, 89.86, 88.86, 87.99, 85.56, 84.28, 26.52, 24.43, 24.12, 13.87. 31 P NMR (121 MHz) ppm 3.74 ( J Pt P = 23 47 Hz) General procedure for synthesis of P 0 ~ P 20: Take P 20 as an example A solution of PVBA ( 8 mg, 0. 05 mmol N 3 ), 5 ( 40.8 mg, 0. 044 mmol), 9 ( 10.5 mg, 0.0 1 1 mmol), PMDETA ( 0.88 mg, 0.0 05 mmol) and THF (10 mL) was deoxygenated by bubbling argon for 30 min, then CuBr ( 0.74 mg, 0.0 05 mmol) was added in argon flow. The mixture was stirred under under argon at room temp erature for 24 hours. The resultant mixture was passed a short neutural alumina column to remove copper catalyst. The solution was concentrated and the resulting polymer was precipitated in
180 methanol. The precipitate was collected and re dissolvend in THF, and then re precipitated in methanol for three times and then dried in vacuo for 24 hours to give orange powder. (Yield: 4 1 mg, 75.9 %)
181 CHAPTER 5 POLYSTYRENE BASED ARRAYS OF POLYPYRIDINE RUTHENIUM CHROMOPHORES WITH ACID END GROUP 5.1 Background As world demand fo r energy rapidly expands, it is a global challenge to transform the way to generate suppl y transmit, store, and use energy Essentially this challenge is a scientific one. In August 2009, 46 Energy Frontier Research Centers (EFRCs) w ere established by Office of Science to conquer this energy crisis challenge 186 A s part of a coordinated effort within Solar Fuels Energy Frontier Research Center at University of North Carolina Chapel Hill (UNC EFRC) 187 we are interested in developing light harvesting metallopolymer assemblies with controlled structures as shown in Figure 5 1, which can be used in photovoltaic applications and artificial photosynthesis towards the development of so lar fuel cells. In this system, ruthenium plays an important role. Ruthenium (II) polypyridyl complexes are broadly studied 188 and applied in photovoltaic, biosensor and energy transfer processes due to their specific photo chemical, photophysical and electrochemical properties with their intrinsically broad and inte nse light harvesting properties 189 194
182 Figure 5 1 Illustration of functional metallopolymer assemblies with controlled pattern adsorbed on photonic electrode. Adapted with permission from r ef. 1 87 Copyright 201 1 Springer. T he synthesis, photophysical and electrochemical properties of polym ers that feature pendant metal complexes groups have been previous ly investigated by many research groups These studies have provided insight concerning the mechanism and dynamics of charge and exciton transpo rt in chromophore assemblies featuring long li ved triplet metal to ligand charge transfer (MLCT) excited states 54 58 118 124 195 Our group reported the photophysics and luminescence quenching of polyelectrolytes with poly(phenylene ethynylene) (P PE) backbone and pendant Ru complexes 196 T hese polymers exhibited an amplified quenching effect which was interpreted as evide nce for exciton transport within the polymers via the conjugated polymer backbone and/or by self exchange hopping between adjacent metal complex chromophores. 49 197 198 Recently, Reynolds, Papanikolas and coworkers reported the synthesis and photophysics of a Ru(II) assembly consisting of metal polypyridyl complexes linked sca ffo ld In this polymer energy transfer from PF backbone to
183 Ru(II) complexes happened within 500 fs; while electron transfer between PF and Ru(II) was also observed in picoseconds range. 26 By the incorporation of ruthenium complexes into the insulating polystyrene backbone, the resulting me tallopolymer structures can function as a light harvesting antenna with the combination of high absorptivity, robust photophysical stability and efficient electron/exciton transport processes between the excited states of transition metals over long distan ce in a one dimensional pattern with high efficiencies. Meyer Papanikolas and coworkers have reported studies of a series of copolymer of styrene and 4 substituted styrene with variation of ruthenium/osmium pendants through the ether or amide linkage s F or the amide linked polymers, both experimental photophysical properties and Monte Carlo simulations concluded that the energy hopping between the excited state (Ru*) to the ground state ( Ru) happen ed with in a time constant falling between 1 4 ns This fast energy migration process can guarantee that the polymer system will not lose energy in its overall efficiency even with a longer dimension. 21 87 89 Figure 5 2 Polystyrene based Ru(II) arrays prepared by RAFT polymerization (A) and NMP (B).
184 With the utilization of r eversib le addition fragmentation chain transfer (RAFT) polymerization and copper(I) catalyzed azide alkyne cycloaddition ( click reaction), our group was able to construct polystyrene based arrays of polypyridine ruthenium(II) chromophores with precisely controlled structu re ( Figure 5 2A). This polymer also exhibited amplified quenching as well as the polymers with conjugated backbones, which was interrupted to be contributed by the combination of MLCT exciton diffusion along the chain and combined with the close proximity and rapid diffusion of the quencher ion along the polyelectrolyte chain However, this polymer suffered from self que n ching (reductive quenching ) caused by the thiol ( SH) end group, which was introduced from RAFT polymerization. Compared to small molecula r model Ru(II) complex, the emission quantum yield and lifetime of the polymers were significantly shortened, which would limit its application in dye sensitized solar cell (DSSC) applications. 52 In order to overcome the self quenching by the thiol group, an alternative controlled radical polymerization (CRP) technique, nitroxide m ediated polymerization (NMP) was selected to prepare the polymer backbone. With NMP, there was no thiol group introduced; and the nitroxide end group from NMP can be easily removed by oxidant mCPBA. 92 199 T here was no self quenching observed i n the Ru(II) functional polymers ( Figure 5 2B) based on quantum yields and lifetime data. 200 It is necessary for an or ganic/organometallic dye to have an acid anchoring group to attach to TiO 2 nanoparticles in dye sensitized solar cells (DSSCs). 201 204 Therefore an end group bea ring three carboxylic acids was introduced in the ruthenium functional polymer in this chapter T he photophysical properties of the polymer were
185 characterized. The absorption of the triacid functionalized polymer onto TiO 2 was observed by AFM and the polym er performance in DSSC was also characterized. 5.2 Polymer Design and S ynthesis The structure of proposed polymer ( P2 Figure 5 3) features a well defined polystyrene backbone, pendant polypyridine ruthenium (II) chromophores and a triacid end group. In this p olymer, polypyridine ruthenium (II) chromophores act as dyes to absorb sunlight, and the triacid end group is used to anchor to titanium dioxide (TiO 2 ) surface in dye sensitized solar cells (DSSCs). Different from previous chapters the controlled radical p olymerization (CRP) technique employed in this chapter is nitroxide mediated polymerization (NMP), other than reversible addition fragmentation transfer (RAFT) polymerization, to avoid the intrachain self quenching of the thiol end group from RAFT polymeri zation. 52 Figure 5 3 Structure of NMP initiator ( 1 ), the model ruthenium complex ( 2 ) and ruthenium functionalized polymers ( P1 and P2 ).
186 5.2.1 Synthesis of NMP Initiator The triacid end group in the polymer was introduced from the NMP initiator 1 which had a tri ester group that was able to be converted to triacid during post polymerization modification. The synthetic route started from the nucleophil ic addition between nitromethane and t butyl acrylate, which gave the nitro triester ( 2 ). Reduction of the nitro triester 3 wit h hydrogen gas and Raney nickel gave the amino triester (also called 4 ), 205 207 which was ready for the amide coupling with 4 benzoic acid. Figure 5 4 Synthesis of NMP initiator ( 1 ) with triester group.
187 The amide coupling first involved the transformation from acid to acetyl chloride ( 5 ), by the reaction between vinylbenzoic acid and o xalyl chloride T he reaction went on smoothly in dry DCM with several drops of DMF as catalyst. Without purification, the resulting vinylbenzyl acetyl chloride ( 5 ) can be used in the next step to react with the aminotriester ( 4 ) form styrene derivat ive w ith triester ( 5 ). T he NMP initiator, 1 was then prepared by coupling the styrene derivative 5 and 2,2,5 t rimethyl 4 phenyl 3 azahexane 3 nitroxide (TIPNO) via the Mn(salen) method similar as described by Hawker and coworkers with a yield of 29%. 92 208 5.2.2 Preparation of Polypyridine Ruthenium Functionalized Polymers and and M odel C ompound The p olypyridine r uthenium f unctionalized p olymers were synthesized by the sequence as shown in Figure 5 5. 4 Vinylbenzyl chloride (VBC) was polymerized at the presence of alkoxyamine 1 affording poly(vinylbenzyl chloride) ( 7 ). The molecule weight (Mn) of 7 was ~ 4500 (DP ~ 25 ) and the PDI was 1.17, which suggested the good control during NMP proce ss. The nitroxide end group was then removed by oxidation with mCPBA 92 199 to avoid any possible interference to the final polymer caused by the nitroxide end group Subsequent to end group removal, the resulting polymer 8 (Mn = 4500, PDI = 1.16 ) was converted to the corresponding azidomethylsubsituted polymer ( 9 Mn = 4600, PDI = 1.18 ) by treatment with sodium az ide. GPC analyses of polymer 7 to 9 reveals that all three polymers have narrow molecular weight distributions (see Figure 5 6). Then the polystyrene based Ru array ( P1 ) was assembled by the copper (I) catalyzed azide alkyne click reaction between azide functional 9 and alkyne functionalized Ru(II) complex ( 10 ). Finally, the t butyl protecting group can be removed by trifluoroacetic acid (TFA), which converting the
188 tri ester end group to triacid end group ( P2 ). T he counter ions of P1 and P2 can be intercha nged between PF 6 and Cl to adjust their solubility in organic solvents (such as acetonitrile acetone and methanol) and water. In order to provide reference data for the isolated Ru(II) complex, a model compound ( 2 ) was also synthesized via the click rea ction between benzyl azide and the alkyne functionalized Ru(II) complex ( 10 ), as demonstrated in Figure 5 7. Figure 5 5 Preparation of p olypyridine r uthenium f unctionalized p olymers
189 Figure 5 6 GPC traces of polymers 7 (black, Mn=4500, PDI=1.17), 8 (red, Mn=4500, PDI=1.16), and 9 (blue, Mn=4600, Mn=1.18). Figure 5 7 Synthesis of model Ru(II) complex, 2 The transformation of the five polymers ( 7 8 9 P1 and P2 ) was monitored by 1 H NMR spectroscopy, which was taken in deuter ated acetone (acetone d6). I n the spectrum of polymer 7 a sharp peak at 1.42 ppm is assigned to protons from t butyl groups at t he triester end group ( c ), and the two peaks at 2.15 and 2.28 ppm are assigned to protons in the methylene groups ( d and e ) at the triester end group. The peak at 4.65 ppm is corresponding to the methylene group next to chloride (ph C H 2 Cl, f ). T he spectru m also features two aromatic peaks at 7.07 ppm and 6.47 ppm ( g and h ),
190 and a set of small peaks with chemical shifts smaller than 1 ppm from nitroxide end group ( a ). After mCPBA treatment, the spectrum from 0.5 ppm to 1 ppm became smooth, indicating the di sappearance of nitroxide in the polymer 8 From polymer 8 to 9 the complete substitution of chloride to azide is evidenced by the peak from 4.65 ppm (ph C H 2 Cl) to 4.37 ppm (ph C H 2 N 3 ). After click reaction the peak assigned to the benzyl methylene ( f ) c ompletely shifts from 4.37 ppm to 5.38 ppm, indicating the complete loading of Ru chromophores during the click reaction. F or P1 polymer, it features the sharp peak at 1.33 ppm, which is assigned to t butyl group at the triester end group. T he signals from methylene in the end group are now buried into the signals from the methyl group from Ru(II) complexes ( j ,2.40 ppm). T he peak at 4.57 ppm is corresponding to the methylene group between triazole ring and amide ( i ). T he two aromatic peaks from polystyrene backbone ( g and h ), 6.47 ppm and 7.02 ppm, are broadened as the pendant Ru complexes restrict the free rotation of the backbone. T he peaks above 7.30 ppm are coming from hydrogen of bipyridine ligands. After deprotection of t butyl group to form the triaci d end group functionalized polymer P2 the signal of t butyl group completely disappears, indicating the complete transformation from trester to triacid. Other features in the 1 H NMR spectrum of P2 remain the same as P1
191 Figure 5 8 1 H NMR spectra of polymes.
192 5.3 Absorption and Photoluminescence The absorption of the ruthenium model complex and polymers were characterized by a UV vis absorption spectrometer in acetonitrile (with PF 6 counter ion) or methanol (with Cl counter ion) at room temperature, as presented in Figure 5 8. T he model complexes and all four polymers displayed similar spectra with characteristic s of ruthenium(II) polypyridine complexes. The absorption spectra featured strong absorption band around 288 nm, due to an intense intra ligand transition, along with a moderate ly intense band at ~ 455 nm in the visible range, which arises from the d (Ru) to *(ligand) metal to ligand charge transfer (MLCT) 188 Polystyrene backbone has little contribution to the absorption of the ruthenium arrays. T he extinction coefficients of model complexes and polymers are listed in Table 5 1. Comparing P1 (with triester end group) and P2 (with tr iacid end group), there was no significant difference observed. Figure 5 9 Ground state absorption of model complex and polymers. 2 PF6 ( black dash), 2 Cl ( red dash), P1 PF6 ( black solid), P2 PF6 ( red solid), P1 Cl ( red solid), and P2 Cl ( green solid),
193 Steady state emission spectra were characterized in deoxygenated acetonitrile or methanol solutions at room temperature upon 45 7 nm excitation. The model complex and polymer s featured a broad and moderately intense luminescence with a maximum at max around 655 to 660 nm arising from the ruthenium localized MLCT excited state. T he counter ion and solvent effects for the emission was observed. Compared with polymers with Cl counter ion ( P1 Cl and P2 Cl ) in methanol solution, polymers with PF 6 cou nter ion ( P1 PF6 and P2 PF6 ) in acetonitrile exhibit a 5 nm red shift. P1 and P2 show similar emission behavior Figure 5 10 Steady state emission of model complex and polymers. T he spectra were normalized based on the relati ve quantum yields. Quantum yields and decay lifetimes for the MLCT emission for the complex and the two polymers are listed in Table 5 1. Compared with the model complexes ( 2 PF6 and 2 Cl ), the polymers exhibit lower the quantum yields, which may be due t o self quenching from inter chromophore interactions, as the density of Ru chromophores is
194 high in a polymer chain. However, the significant quantum yield decrease was not observed as in the polymers prepared via RAFT polymerization. 52 Polymers with different end groups ( P1 and P2 ) have similar quantum yields around 0.05. The emissi on lifetimes of model complexes and polymers were recorded by t ime c orrelated s ingle p hoton c ounting (TCSPC). Both model complexes ( 2 PF6 and 2 Cl ) exhibit single exponential decay; however, 2 Cl has shorter lifetime. As for the polymers ( P1 and P2 ), they all show bi exponential decays, indicating some intra chain interactions exist. The medium lifetimes of polymers are slightly shorter than those of model complexes, but the decrease is not large. All polymers exhibit similar lifetime values. Combining qu antum yield and lifetime data, we can conclude that although some intrachain interactions existing there is no significant self quenching effect existing in these polymeric systems. In addition, end group s (triester vs. triacid) did not affect the absorpt ion and emission behavior of these ruthenium arrays.
195 Table 5 1. The photophysical and electrochemical properties of 2, P1 and P2. Compound/ Polymer Abs max /nm a ( / 10 4 M 1 1 ) Emi max /nm b c / s ( A) d K SV /10 4 M 1 d 2 PF6 287(8.80) 455( 1.82) 653 0.1 2 =1.37 s -2 Cl 287(5.73) 455(1.23) 654 0.08 4 =1.02 s 2.67 P1 PF6 28 8 ( 8.70 ) 45 7 (1. 98 ) 6 65 0.07 6 1 =0.94 s (0.69) 2 =0.27 s (0.31) < >=0.74 s -P1 Cl 28 8 ( 5.99 ) 45 7 (1. 37 ) 6 60 0.066 1 =0.94 s (0.69) 2 =0.27 s (0.31) < >=0.7 4 s 283 P2 PF6 288(8.72) 45 8 ( 2.00 ) 6 65 0.06 5 1 =1.26 s (0.58) 2 =0.41 s (0.42) < >=0.91 s -P2 Cl 28 8 ( 5.70 ) 45 7 (1. 32 ) 6 60 0.06 5 1 =0.96 s (0.70) 2 =0.39 s (0.30) < >=0.79 s 282 a Ground state absorption maxima. b Emission spectra, obtained by excitation at 455 nm. c With Ru(bpy) 3 Cl 2 as quantum yield standard, = 0.0379 in air saturated water. d Emission lifetimes, from time correlated single photon counting (TCSPC), and is median lifetime calculated as = A i i
196 5.4 Amplified Quench ing T he concept amplified was first studied by Swager and coworkers in main chain conjugated poly (phenylacetylene)s (PPEs). 48 49 When a quencher is bin ding to PPEs with host guest interaction, the Stern Volmer constant is much larger than that in the interaction between quencher and small molecular monomer. Whitten Schanze and coworkers expanded this concept to conjugated polyelectrolytes (CPEs). CPEs can be quenched by small amount of oppositely charged quencher ions This process has been attributed to two main factors: (1) the ion pairing between the charged quencher ion and the polyelectrolyte chain effectively to increase the local concentration of the quencher ; and more important (2) the exciton in the polyelectrolyte are able to undergo rapid diffusive transport along the polymer chain, in effect increasing the effective sphere of action of the quencher ion. 50 Amplified quenching can also be expanded to polyelectrolytes with non conjugated backbone and pendant chromophores and it is an effective method to examine the energy migration (by hopping) along the non conjugated chain. The amplified quenching experi ments of the two polymers with the Cl counter ion ( P1 Cl and P2 Cl ) were taken in water with sodium 9 10 anthraquinone 2,6 disulfonate (AQS) as the quencher. Figure 5 11 demonstrates the quenching studies of P1 Cl and P2 Cl T he polymer concentrations we re controlled at 10 M in water based on the repeat units and quenched by addition of AQS. The quenching of both polymer emissions was similar. Both quenching were very efficient, with 50% quenching (I 0 /I = 2) were observed around 0.4 M of AQS, meaning on e AQS molecule could quench around 12 Ru chromophores. T he Stern Volmer plots were also obtained, which were linear until around 75% of the
197 initial emission intensities were quenched (I 0 /I = 3.8), and Stern Volmer constants around 2.8 x 10 6 M 1 were obtain ed (Table 5 1). Above this value, the plots curved upwards, indicating higher concentration of opposite charged AQS may induce aggregation of Ru arrays. In comparison the quenching of model complex 2 Cl was also studied ( Figure 5 11, right) and a small St ern Volmer constants of 2.67 x 10 4 M 1 was obtained. T his comparison reveals that there is significant amplification of quenching in both polymers, with a ~100 fold magnification. T his result clearly indicates the MLCT exciton diffusions along the non con jugated polymer backbone. In addition, end groups do not have significant effect on the amplified quenching behavior Figure 5 11 Emission quenching of polymers ( P1 Cl left, and P2 Cl middle) and model complex ( 2 Cl right)
198 Figure 5 12 Stern Volmer plots for emission quenching of P1 Cl (black squares), P2 Cl (red circles) and 2 Cl (blue triangles). 5.5 Surface Absorption on Titanium Dioxide Surface T he main objective of installing carboxylic ac ids on Ru functional polymer end group is to utlize them to anchor to surfaces. Therfore, their ability of P2 to absorb onto TiO 2 surface and inject electrons into TiO 2 was investigated with n type rutile TiO 2 (110) single crystals. Single crystals of ruti le TiO 2 are good candidates for the study of the morphology structure interactions as dyes, conjugated polyelectrolytes and quantum dots show efficient electron injection into the single crystals 209 214 Morphology studies were carried out to correspond to the photocurrent results with the surface coverage. The unmodified single crystals of TiO 2 (110) are flat with terraces of different size present. Morphology p hotocurrent interactions were also successfully investigated by Ginger et al. for the bulk heterojunction materials 215 216 Nanocrystalline TiO 2 will also be use d to correlate the efficiency of the electron injection with the data obtained for single crystals. Atomic
199 force microscopy ( AFM ) results for the flat rutile single crystals show the presence of clean terraced surfaces without impurities present. Different magnification images are shown to clearly demonstrate the homogeneous nature of the surface. The height of the terraces is around 100 200 pm with the width of around 60 nm, as illustrated in Figure 5 16. Figure 5 13 Non co ntact tapping mode AFM images of TiO 2 (110) with different resolutions ( A C ) and cross section analysis of C (D) Addition of the dyes to the surface causes appearance of the certain features with range of sizes at the TiO 2 (110) surface Hereby two serie s of studies have been carried out. The first is to soap single crystal TiO 2 into P2 Cl in methanol solutions with
200 different concentrations with a constant time (12 hours). T he second study is to immerse single crystal TiO 2 into P2 Cl in methanol solution with a constant concentration ( L) for different soaking times After removing single crystal TiO 2 out of solution, the substrates were rinsed with methanol, dried and characterized with AFM and photo current measurement (characterized with i nternal photon to current efficiency IP CE). Morphology change of P2 Cl on TiO 2 (110) surface along with polymer concentration is demonstrated in Figure 5 1 4 A t lower concentration (10 L) single particles are formed along the TiO 2 (110) terraces ( Figure 5 1 4 A and B). Most particles have s ized from 1 to 2 nm, and small amount of particles have larger sizes around 8 nm, as demonstrated in cross section analysis ( Figure 5 1 5 A and B). The small particle sizes are consistent with the radius for the monomer units, suggesting there may be single polymer chains lying down on TiO 2 (110) surface. As polymer concentration increases (30 80 L ), polymer layer begins to form and covers TiO 2 surface ( Figure 5 1 4 C E). When polymer concentration is larger than 150 L, larger aggregates of P2 Cl start to form ( Figure 5 1 4 F H), and the sizes of 5 10 nm are observed ( Figure 5 1 5 H). Th ese polymer films on TiO 2 were also characterized via photoelectrochemical study. The IPCE results show the presence of a substantial signal which is close to the ground state absor ption ( Figure 5 1 6 A ). Increase of the concentration of the solution causi ng increase of the photocurrent response (internal photon to current efficiency, IPCE) as a consequence of higher surface coverage ; however, th ere is saturation
201 noticed at certain concentration ( Figure 5 1 6 B ) which may be caused by the formation of the mo nolayer.
202 Figure 5 14 Non contact tapping mode AFM images of P2 Cl deposited on TiO 2 (110) surface from solutions of different concentrations ( A ) and ( B ) 10, ( C ) 30, ( D ) 50, ( E ) 80, ( F ) an ( G ) 150 ( H L MeOH was th e solvent used for deposition with a dipping time of 12 hours; ( B ) and ( G ) are zoom in version of ( A ) and ( F ). Figure 5 15 Cross section analysis for AFM images for Figure 5 17A, B and H Figure 5 16 (a) IPCE spectra for a TiO 2 (110) electrode dipped into MeOH with various concentrations of P2; numbers by the arrow indicates the concentrations used L ; (b) IPCE values as a function of the dipping solution concentration for curves shown in A When immersing single crystal TiO 2 in dilute P2 Cl solution (1 L) the amount of polymer seems to be insufficient to cover the whole TiO 2 surface to form the monolayer, so small particles are formed. Cross section analysis of the AFM image in initial stage ( Figure 5 17 A) reveals that the height polymer particle is around 1 nm, suggesting single polymer chains lying down on TiO 2 (110) surface. A s the dipping time increases, the polymer aggregates grow around the small particle nucleus to form
203 larger particles. The m aximum IPCE value (at 456 nm) also increases with dipping time as more polymers are absorbed onto TiO 2 surface ( Figure 5 17 F). Figure 5 17 AFM image of the P2 polymer molecules at the surface of TiO 2 (110) from solutions of 1 36 hours, (d) 48 hours; (e) cross section of (a) ; (f) IPCE values as a function of the dipping time. The AFM images of P2 Cl onto single crystal TiO 2 along with the photocurrent action c haracterizations clearly demonstrated that the Ru functional polymer is able to
204 anchor to TiO 2 surface and inject electrons to TiO 2 to produce current. The a d sorption of the polymer onto TiO 2 initials with single particles, and then the polymer will cover the whole surface to form a monolayer. After that more polymer may aggregate onto the monolayer; however, the photocurrent reaches a limit after the monolayer is formed, because the electrons produced from aggregates above the monolayer may not be able to inject into TiO 2 5.6 Solar Cell Performance Characterization As there is a saturation effect for single crystal TiO 2 after polymer monolayer is formed ( Figure 5 1 6 ), n anocrystalline TiO 2 film (with ~20 nm particle size) on fluorine doped tin(IV) oxide (FTO) sustrate was employed to fabricate the dye sensitized solar cells (DSSCs). Figure 5 18 a illustrates the photocurrent action spectr um (IPCE) of the polymer sensitized solar cells obtained using monochromatic illumination under short circuit conditions T h e trend of peak efficiency is largely consistent with IPCE spectra of a TiO 2 (110) electrode. T he solar cell has a peak IPCE value of 1.76% at 460 nm. This value is 25 times larger than that from single crystal TiO 2 indicating much better polymer absorpti on on n anocrystalline TiO 2 film. The J V characteristics under AM1.5 illumination (100 mW / cm 2 ) of the polymer sensitized DSSCs are shown in Figure 5 18 b. The performance of DSSCs are characterized with of short circuit current density (J sc ), open circuit voltage (V oc factor (FF), and overall power conversion e ff iciency cell ) For the cell made from P2 Cl it has relative high open circuit voltage ( V oc = 464 mV) and ll factor (FF = 0.62); however, the short circuit current density is low ( J sc = 0. 34 mA/cm 2 ), resulting a low overall power conversion e ff iciency cell =0.10% )
205 Figure 5 18 P hotocurrent action spectr um (IPCE A ) and J V curve (B) of DSSC made from P2 Cl and n anocrystalline TiO 2 Although the IPCE and o verall power conversion e ff iciency of the cell from the polymer P2 Cl is relatively low, the characterizations have clearly shown the Ru functional polymer can act as a polymeric dye in DSSCs. Further efforts, including tuning polymer chain length, adjusti ng structures of pendant ruthenium complex and optimizing DCCS fabrication technique, etc, should be made to improve the solar cell performances. 5.7 Summary In this chapter, we have reported the preparation and characterization of ruthenium(II) functional pol ymer with acid end group. The polymer was prepared via NMP S N 2 click strategy, and featured a well defined polystyrene backbone, pedant ruthenium(II) chromophores and a triacid end group. T he polymer shows typical absorption and emission as the Ru(bpy) 3 2+ complex, and it also exhibits the amplifed que n ching effect. Absorption of the acid end group polymer, P2 Cl onto single crystal TiO 2 has been characterized by AFM and photocurrent action spectra. The results indicate Ru functional polymer is able to anch or to TiO 2 surface with the acid end
206 groups and inject electrons to TiO 2 to produce current. DSSCs made from P2 Cl and nanocrystalline TiO 2 shows photon to current conversion with a low overall efficiency. Further efforts, including tuning polymer chain le ngth, adjusting structures of pendant ruthenium complex and optimizing DCCS fabrication technique, etc, should be made to improve the solar cell performances. 5.8 Experimental 5.8.1 Instrumentation and Method s NMR spectra were measured on an Inova 500 FT NMR. High r esolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclone Resonance mass spectrometer (Bruker Daltonics, Billerica, MA) For UV visible absorption measurements samples were dissolved in acetonitrile or methanol and were carried out on a Shimadzu UV 1800 dual beam absorption spectrophotometer using 1 cm quartz cells. Photoluminescence measurements were obtained on a fluorimeter from Photon Technology International (PTI) using 1 cm quartz cells For phosphorescen ce me asurement the sample solutions were degassed via bubbling argon for 45 minutes. Luminescence lifetimes were obtained with a multichannel scaler/photon counter system with a PicoQuant FluoTime 100 Compact Luminescence Lifetime S pectrophotometer. A high per formance Coherent CUBE diode laser provided the excitation at 375 nm (P < 10 mW). The laser was pulsed using an external Stanford Research Systems DG535 digital decay and pulse generator with four independent delay channels. At least four narrow band pass filters were used for measurements followed by global fit processing (FluoroFit software). Decays were obtained using the multi exponential fitting p arameters (FluoroFit software).
207 Cyclic voltammograms were collected using IVIUM Compact potentiostats (Eind hoven, The Netherlands). The three electrodes setup with platinum electrode disc (0.0314 cm 2 ) or ITO working electrodes, platinum wire as a counter electrode and silver wire as a reference electrode for experiments in acetonitrile and Ag/AgCl as reference electrode for aqueous solution studies. The potential for experiment in acetonitrile was calibrated by applying ferrocene/ferrocenium redox couple and assuming the reference potential +0.342 V vs SCE or 0.297 V vs Ag/AgCl. 217 Solutions for investigations in acetonitrile were prepared in anhydrous oxygen free Varian glovebox (Agilent, Santa Clara, CA). AFM measurements have been done in non contact AC mode using As ylum Research (Santa Barbara, CA) microscope. Olympus silicon rectangular probes with a 42 N/m force constant, tip radius of 9+/ 2 nm and resonant frequency of approximately 300 kHz were applied as tips for topographical investigations in air. The TiO 2 (11 0) crystals were mounted on magnetic pucks using double sided scotch tape. Different parts of single crystals were analyzed to obtain reproducible results. Static method and dipping electrode into the solution with certain concentration of the polyelectrol yte for certain amount of time was used for deposition. The solution was rinsed with water and dried prior the AFM investigations. Filter with the size of 200 nm (Nalgene, Rochester, NY) was used to get rid of large particles prior the AFM and photoelectro chemical measurements. Photoelectrochemical investigations were carried out in aqueous solutions with 0.1 M KNO 3 and 20 mM KI. Three electrode configuration with TiO 2 (110) single crystal
208 as a working electrode, platinum mesh or wire as a counter electrode and Ag/AgCl as a reference electrode was used in all photocurrent experiments. The dye sensitized solar cells were fabricated as a sandwich with fluorine doped tin(IV) oxide (FTO) conducting glass, nanocrystalline TiO 2 as the wide bandgap semiconductor w ith the dyes adsorbed, I /I 3 electrolyte for charge regeneration, and a Pt counter electrode as a catalyst. The TiO 2 paste was doctor bladed onto a clean FTO glass slide followed by sintering at 500 o C for 30 min with 1 o C /min of the heating and cooling ra te. The TiO 2 2 layer thickness and scanning electron microscopy (SEM). Afterwards, the dried TiO 2 active cell was then dipped int o the polymer solution in methanol for 72 h. The TiO 2 active cell area was controlled as 0.35 cm 2 to allow for consistent measurements of IPC E and J V characteristics. A Pt counter electrode was prepared by spinning 0.01 M H 2 PtCl 6 in isopropyl alcohol on FTO substrates with two holes created using a drill and by sintering 450 o C for 30 min. A Surlyn (25 between TiO2 photoanode and a Pt counter electrode Finally, an electrolyte solution containing 0.05 M I 2 and 0.1 M LiI in anhydrous ethanol was injected into two holes on the Pt counter electrode side. The current voltage characteristics of the cells were measured with a Keithley 2400 source meter under AM1.5 (100 mW/cm 2 ) solar simulator. For IPCE measurements, the cells were illuminated by monochromatic light from an Oriel Cornerstone spectrometer, and the current response under short circuit conditions was
209 recorded at 10 nm intervals using a Keithley 2400 source meter. The light intensity at each wavelength was calibrated with an energy meter (S350, UDT Instruments). 5.8.2 Materials C opper (I) b romide (CuBr), ( S, S ) (+) Bis(3,5 di tert butylsalicylidene) 1,2 cyclohexanediaminomanganese(III) chloride (Mn(Salen) ), sodium boronhydride di tert butylperoxide, nitromethane, tert butyl acrylate, Triton B, Raney nickel 4 vinylbenzoic acid, o xalyl chloride pentamethyldiethylenetriamine (PMDETA), 2,2,5 t rimethyl 4 phenyl 3 azahexane 3 nitroxide (TIPNO) and sodium azide were from Sigma Aldrich. CuBr was stirred in acetic acid overnight, washed with acetone and then dried over vacuum All reagents were us e d as received without further purification. Flat surface from commercial TiO 2 (110) single crystals (MTI Corporation, Richmond, CA) was prepared in several steps. The initial as bought electrodes was first polished with 20 and 50 nm silica suspension in water (Buehler, Lake Bluff, IL) The crystals was also sonicated in acetone, water and cleaned from impurities in 10 % THF for 20 minutes and annealed for 1 hour at 700 C. Vacuum doping was accomplished by using RTI furnace (Pfeifer Vacuum, Asslar, Germa ny). The vacuum doping procedure consists of several steps which include: 1) temperature ramp to 700 C in five minutes, 2) holding this temperature for minutes, 3) ramp to 1000 C in one minute, 4) holding this temperature at 1000 C for 6 minutes, 5) ram p to 700 C in one minute, 6) holding this temperature at 700 C for 5 minutes, 7) ramp to room temperature in five minutes. The real ramp to room temperature was slower from temperature around 150 200 C. Vacuum was maintained throughout the whole procedu re of doping. The crystal structure was constantly checked throughout the whole experimental cycle
210 Compound 10 was synthesized according to literature, 26 and benzyl azide was synthesized according to Chapter 3. 5.8.3 Synthesis Di tert butyl 4 [2 (tert butoxycarbonyl)eth yl] 4 nit roheptanedicarboxylate ( 3 ) 205 207 A stirred solution of MeNO 2 ( 12.2 g, 2 00 mmol), Triton B (benzyltrimethylammonium hydroxide 40% in MeOH; 1.0 mL) in dimethoxyethane (DME 20 mL) was heated to 65 70 o C. tert Butyl acrylate ( 79.4 g, 620 mmol ) was added portionwise to maintain the temperature at 70 80 o C. Additional Triton B (2 X 1 mL) was added when the temperature started to dec rease; when the addition was completed, the mixture was maintained at 70 75 o C for 1 h. After concentration in vacuo, the residue was dissolved in DCM (200 mL), washed with 10% aqueous HCl (50 mL) and brine (3 X 50 mL), and dried over anhydrous sodium sulf ate Removal of solvent in vacuo gave a pale yellow solid, which was crystallized in EtOH to afford (72%) triester 3 as white microcrystals Yield: 49.0 g (55%). 1 H NMR ( 300 MHz, CDCl3 (ppm) 2.20 (m, 12 H), 1.44 (s, 27 H). 13 C NMR (75 MHz, CDCl3 (ppm ) 170.9 92.1, 80.9, 30.2, 39.7, 27.9.
211 Di tert butyl 4 [2 (tert butoxycarbonyl)ethyl] 4 aminoheptanedicarboxylate ( 4 ) 205 207 A solution of nitrotriester 3 (6.00 g, 13.5 m mol) in ethanol/dichloromethane (13:1, 140 mL) in a round bottom flask with ~ 4 g of Raney Nickel was stirred at 60 psi H 2 (a balloon with H 2 gas was attached to the flask) for 48 h ours at room temperature. The suspension was carefully filtered through Celite, and the removal of the solvent under reduced pressure gave the crude product. The residue was dissolved in d ichloromethane (120 mL), and the resulting organic solution was washed sequ entially with saturated NaHCO 3 (120 mL) and water (120 mL) and dried over anhydrous Na 2 SO 4 The solvent was evaporated under reduced pressure to yield aminotriester 4 (5. 37 g, 93%). 1 H NMR ( 3 00 MHz, CDCl3 ): (ppm) 2.43 (t, 6H), 1.95(t, 6H), 1.44 (s, 27H), 13 C NMR ( 75 MHz, CDCl3 ): (ppm) 172.30 80.96 56.99 31.47 29.46 27.98 4 V inylbenzoyl chloride ( 5 ) 500 mg (3.4 mmol) of 4 vinylbenzoic acid was dissolved in 20 mL of dry dichloromethane, then 470 mg (3.7 mmol) of oxalate chloride was added dropwise with a syringe. Several drop of dry DMF was then added in the mixture as catalyst. T he solution was stirred at room temperature for 4 hours, and gas releasled by the reacti on
212 was collected by a balloon. T he removal of the solvent under reduced pressure gave the crude product of 4 vinylbenzoyl chloride ( 5 ). The crude product was used in the next step without further purification. D i tert butyl 4 (3 (tert butoxy) 3 oxopropyl ) 4 (4 vinylbenzamido)heptanedioate (6) T he crude 4 vinylbenzoyl chloride ( 5 ) and 1.4 g (3.4 mmol) was dissolved in a dry DCM/Et 3 N (30 mL/5 mL) mixed solvent, and the mixture was stirred at room temperature overnight. The solvent was then removed under reduced pressure. The crude product was then extracted with DCM and brine. T he organic phase was collected and dried with rotary evaporation and the residue was purified through column chromatography using hexane as the elue nt to yield white microcrystals. (Yield: 1.39 g, 75 % over two steps ). 1 H NMR ( 5 00 MHz, CDCl3 ): (ppm) 7.75(d, 2H), 7.43(d, 2H), 6.84(s, 1H), 6.73(dd, 1h), 5.83(d, 1H), 5.34(d, 1H), 2.31(t, 6H), 2.11(t, 6H), 1.4 3 (s, 27H), 13 C NMR ( 125 MHz, CDCl3 ): (ppm) 221.46, 173.32, 166.50, 140.63, 136.22, 134.35, 127.45, 126.36, 115.93, 80.98, 58.01, 30.42, 30.13, 28.29.
213 D i tert butyl 4 (3 (tert butoxy) 3 oxopropyl) 4 (4 (1 (( tert butyl(2 methyl 1 phenylpropyl)amino)oxy)ethyl)benzamido)heptanedioate ( 1 ) Compound 6 (1.25 g, 2.28 mmol) and TIPNO (0.48 g, 2.17 mmol) was dissolved in a toluene /EtOH (40 mL/40 mL) mixture and the solution was stirred vigorously A suspension of Mn(Salen) (0.30 g, 0.43 mmol) in toluene /EtOH (10 mL/10 mL) was added. Then d i tert butyl peroxide (0.51 g, 3.44 mmol) and sodium boronhydride (0.26 g, 6.88 mmol) was added subsequently. The mixture was stirred at room temperature for 3 hours. Then the solvent was removed by rotary evaporation and the residue was purified through column chromatography using hexane /ethyl acetate (6/1, v/v) as the eluent to yield off white solid. (Yield: 490 mg, 29.4%). T he product was diastereomers. 1 H NMR ( 5 00 MHz, CDCl3 ): (ppm) 7.7 8 (d, 2H), 7.71 (d, 2H), 7.47(d, 2H), 7. 3 3(d, 2H), 7.27 7.15 (m, 10H), 6.8 3 (s, 1H), 6. 68 ( s 1 H ), 4.96 (s, 2H), 3.42 (d, 1H), 3.32 (d, 1H), 2.31( m 12 H), 2.1 3 ( m 12 H), 1.62 (m, 6H), 1.53 (d, 3H), 1.4 2 ( d 54 H), 1.30 1.24 (m, 6H), 1.04 (s, 9H), 0.96 (d, 3H), 0.77 (s, 9H), 0.54 (d, 3H), 0.25 (d, 3H). 13 C NMR ( 125 MHz, CDCl3 ): (ppm) 173.31, 173.25, 166.88, 166.76, 149.57, 131.10, 130.98, 127.61, 127.48, 127.09, 127.00, 126.60, 126.45, 126.29, 83.56, 82.51, 80.96, 80.94, 72.43, 72.38, 60.68, 58.00, 57. 95, 32.26, 31.96, 30.43, 30.39, 30.14, 30.12, 28.61, 28.44, 28.35, 28.29, 28.28, 24.95, 23.64, 22.19, 22.10, 21.35, 21.33 MS: Calculated: 766.51 ; ( ESI TOF): [M + H] + : 767.5205; [M + Na ] + : 789.5024.
214 Polymer 7 A solution of 1 (200 mg, 0.26 mmol) and 4 vinylbenzyl chloride (2.0 g, 13 mmol) in 8 mL of xylene was place in a 25 mL Shlenck flask, and the solution was deoxygenated by 3 cycles of freeze pump thaw. Then the flask was immersed in a 120 o C oil bath and stirred for 40 min utes. T he polymerization was quenching by immering the flask into liquid nitrogen. T he polymer was precipitate in large excess of methanol. T he precipitation was then dissolved in minimum amount of THF and re pricipitated in methanol. T he dissolve precipit ate process was repeated for 3 times. T he product was then dried in vacuo for 24 hours to yield white powder (440 mg, 20 %). GPC: Mn = 4500, PDI = 1.17. 1 H NMR ( 5 00 MHz, acetone d 6 ): (ppm) 7.19(sb, 2H), 6.64(sb, 2H), 4.67(s, 2H), 2.30(s, 0.40H), 2.16(s, 0.40H), 1.95 1.25 (mb, 3H), 1.42(s, 1.8H), 1.02 0.31(m, 0.60H). Polymer 8 420 mg of polymer 7 was dissolved in 10 mL of toluene, and 400 mg of mCPBA was added. The solution was stirred at room temperature for 24 hours. Then the mixture
215 was precipitated in large excess of methanol. T he precipitation was then dissolved in minimum amount of THF and re pricipitated in methanol. T he dissolve precipitate pr ocess was repeated for 3 times. T he product was then dried in vacuo for 24 hours to yield white powder (300 mg). GPC: Mn=4500, PDI=1.16. 1 H NMR ( 5 00 MHz, acetone d 6 ): (ppm) 7.19(sb, 2H), 6.64(sb, 2H), 4.67(s, 2H), 2.30(s, 0.40H), 2.16(s, 0.40H), 1.95 1.25 (mb, 3H), 1.42(s, 1.8H). Polymer 9 125 mg (0.82 mmol of chloride) of polymer 8 was dissolved in 5 mL of DMF, and then 0.5 g (76.9 mmol) of NaN 3 was added. T he mixture was stirred ar room temperature for 24 hours, and then was precipitated in water. T he precipitate was collected by filtration, washed with water several times, and dried in vacuo for 24 hours to yield white powder produ ct (112 mg). GPC: Mn = 4600, PDI = 1.18. 1 H NMR ( 5 00 MHz, acetone d 6 ): (ppm) 7.19(sb, 2H), 6.64(sb, 2H), 4.37(s, 2H), 2.30(s, 0.40H), 2.16(s, 0.40H), 1.95 1.25 (mb, 3H), 1.42(s, 1.8H).
216 P1 PF6 A solution of polymer 9 ( 16 mg, 0.1 mmol N 3 ), Ru complex 10 ( 100 mg, 0.1 05 mmol), PMDETA (6 m g, 0.034 mmol) and DMF ( 5 mL) was deoxygenated by bubbling argon for 30 min, then CuBr (5 mg, 0.034 mmol) was added in argon flow. The mixture was stirred under under argon at room temperature for 24 hours. Then the mixture was precipitated in large excess of methanol. T he precipitate was collected and re dissolved in munimum amount of acetonitrile, and re precipitated in methanol. T he T he dissolve precipitate process was repeated for 3 times to yield red precipitates which was dried in vacuo for 24 hours. Yield: 96 mg (84.2%). 1 H NMR ( 5 00 MHz, acetone d 6 ): (ppm) 9.01 6.29(mb, 31H), 5.41(sb, 2H), 4.54(s, 2H), 2.38(s, 3H), 1.42(s, 2H) P1 Cl 20 mg of P1 PF6 was dissolved in acetone, and a saturated tertraammonium chloride (50 mg) in acetone solution was ad ded dropwise. T he red precipitate was collected, washed with acetone for several times and dry in vacuo to yield red brown polymer (14 mg).
217 P 2 PF6 50 mg of P1 PF6 was dissolved in acetonitrile and then 2 mL of trifluoroac etic acid was added. 50 mg of Bu 4 NPF 6 was also added to adjust the counter ion. T he mixy=ture was stirred at room temperature overnight. Then the mixture was precipitated in large excess of methanol. T he precipitate was collected and re dissolved in munimu m amount of acetonitrile, and re precipitated in methanol. T he dissolve precipitate process was repeated for 3 times to yield red precipitate which was dried in vacuo for 24 hours. Yield: 34 mg. 1 H NMR ( 5 00 MHz, acetone d 6 ): (ppm) 9.01 6.29(mb, 31H), 5. 41(sb, 2H), 4.54(s, 2H), 2.38(s, 3H). P 2 Cl 15 mg of P1 PF6 was dissolved in acetone, and a saturated tertraammonium chloride (30 mg) in acetone solution was added dropwise. T he red precipitate was collected, washed with acetone for several times and dry in vacuo to yield red brown polymer (11 mg).
218 CHAPTER 6 CONCLUSION Conjugated polymers and oligomers have attracted a lot of interest; it is not only due to their optical and electronic properties, but also because conjugated organic materials provide the poss ibility for device performance with freedom to control over material s and thereby, device properties. These control are from synthesis of novel designed molecules, to appropriate morphologies and interfacial interaction in device architectures. Based on t he structures, the conjugated organic materials can be divided into two categories, conjugated polymers and small conjugated molecules (or oligomers). One of the most attractive aspects of conjugated polymers is their solution processbility. However the reproducibility from batch to batch is difficult to control, due to the broad molecular weight distribution nature of condensation polymerization. Conjugated oligomers, on the other hand, offer more precisely defined structures, simpler purification methods, easier modification and functionalization, no end group contamination, and more reproducible synthesis. However, the poor film forming ability is the disadvantage for the small molecular conjugated oligomers. The side chain conjugated polymers wh ich structurally feature a non conjugated and flexible polymer backbone with pendant conjugated chromophores, combine the intrinsic film forming and mechanical propert ies of polymer s and well defined electronic, photonic, and morphological properties of th e monodisperse oligomer moieties. In addition, the generally low solubility of the oligomers can be improved significantly With the development of controlled radical polymerization methodologies it is possible to precisely control the length and molecul ar weight distribution of the polymer
219 backbone and thereby, architecture s of side chain conjugated polymers. In this dissertation a post polymerization modification synthetic strategy was focused and employed to prepare side chain conjugated polymers with well defined structures. The strategy involves controlled radical polymerization, S N 2 substitution and copper(I) catalyzed azide alkyne cycloaddition which abbreviates as CRP S N 2 click In this dissertation it is demonstrated that the CRP S N 2 clic k strategy is a versatile route to prepare side chain conjugated light harvesting polymers. T he monomers utilized to construct the clickable precursor used in this dissertation ( GMA and VBC ) were co mmercially available, inexpensive and easy to purify. W hen a single reactive polymer precursor (such as PVBA in Chapter 3 and Chapter 4) is prepared, a diverse library of functional polymers with identical chain lengths and chain length distributions can be generated based upon the parent precursor T he functi onal chromophores can be organic (Chapter 3 and 4) and organometallic compounds (such as platinum acetylides and ruthenium complexes, Chapter 2 and Chapter 5). T he resulting side chain conjugated light harvesting polymers may have one or several different chromophores. In polymers with different chromophores, the chromophore ration in the polymers closely corresponds to the stoichiometry used in the feed for the click reactions (Chapter 3 and 4). Different CRP techniques (such as RAFT and NMP) can be employ ed to synthesize the well defined polymer backbone, and end group modification can be achieved through functional monomers (Chapter 5). Singlet and triplet energy transfer has been studied in copolymers with both donor and accepter in the same polystyrene backbone. The energy transfer from donor to accept was characterized employing both time resolved and steady state
220 fluorescence spectroscopy as well as time resolved transient absorption spectroscopy The dynamics of both singlet and triplet energy transf er were explored. The ultrafast energy transfer from donor ( OPE ) to acceptor ( TBT ) occurs within 50 picosecond s with remarkably high efficiency. There were two energy migration processes existing : ultrafast neighboring OPE TBT quenching within 2 4 ps and OPE OPE hopping within 2 5 5 0 ps. In a similar approach, the triplet triplet energy transfer from donor ( PE2 Pt ) to ( Py Pt ) was found to be also very efficient occurring within 50 ns. The singlet energy transfer follows both F ster and Dexter mechanisms while the triplet energy transfer only follows Dexter mecha nism. The applications of light harvesting polymer made from the CRP S N 2 click strategy were also investigated A p oly acrylate with pendant nonlinear absorption (NLA) chromophores was prepare d via the RAFT S N 2 click synthetic strategy. Platinum acetylides that undergo NLA via both two photon absorption (TPA) and exited state absorption (ESA) mechanisms were utilized as chromophores attach ed to clickable poly acrylate backbone. The resulting p olymer exhibits similar photophysical properties as platinum acetylide precursor including steady state absorption and emission, triplet triplet transient absorption, and nonlinear absorption properties In addition, t he resulting polymers can be easily d rop or spin coated to afford optically transparent film. A ruthenium(II) functional polymer with carboxylic acid bearing end group was prepard via NMP S N 2 click strategy T he polymer shows typical absorption and emissiton as Ru(bpy) 3 2+ complex and also exhibits amplifed que n ching effect. Absorption of the acid end group polymer onto single crystal TiO 2 has been characterized by AFM and photocurrent action spectra. The results indicate Ru
221 functional polymer is able to anchor to TiO 2 surface with the acid end groups and inject electrons to TiO 2 to produce current. DSSCs made from the acid end group Ru functional polymer and nanocrystalline TiO 2 shows photon to current conversion with a low overall efficiency. Further efforts, including tuning polymer chain length, adjusting structures of pendant ruthenium complex and optimizing DCCS fabrication technique, etc, should be made to improve the solar cell performances. In summary, a versatile methodology to prepare side conjugated light harvesting polymer has be en developed and employed in the preparation of several families of light harvesting polymers. T hese polymers can be utilized in both photophysical studies and materials applications.
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241 BIOGRAPHICAL SKETCH Zhuo Chen was born in Gaoan, Jiangxi, a small town in east China. At the age of 16, he went to Fudan Universit y in Shanghai, where he earned his Bachelor of Science degree in Macromolecular Materials and Engineering in the summer of 2006. He also studied in the University of Hong Kong as exchange student for half a year in 2005. Zhuo then attended the University o f Florida for graduate school in 2006. He first worked with Prof. Charles Beatty in Department of Materials Science and Engineering where he got his Master of Science degree in December 2007. During the 16 months in MSE department, Zhuo studied modificati on and strengthen of thermoplastic elastomers with the aid of super critical carbon dioxide. Zhuo transfer red to Department of Chemistry in 2008 in the area of organic chemistry under the supervision of Prof. Kirk Schan z e. His research focused on preparati on of well defined polymeric arrays with pendant organic and organometallic chromophores for fundamental energy transfer research and functional materials application. Zhuo was awarded Butler Polymer Research Award in 2012 He received his Ph.D. degree fro m the University of Florida in the summer of 201 3.