DIFFERENT CHAIN LENGTH CONJUGATED POLYMERS: PHOTOPHYSICAL STUDIES AND DYE SENSITIZED SOLAR CELL APPLICATIONS By ZHENXING PAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILL MENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2015
Â© 2015 Zhenxing Pan
To my parents
4 ACKNOWLEDGMENTS I want to give my sincere appreciation to all those who he lped and supported me in the past five years at the University of Florida. First of all, I want to express my deepest thanks to my principal investigator , Dr. Kirk S. Schanze, for his guidance, encouragement and support through my PhD study. It has been a privilege for me to work with him whose wisdom is everlasting and passion for science fuels our research. Without his help, this work would not be possible. I would like to give my heartfelt thanks to my committee members, Dr. Ronald K. Castellano, Dr. Ste phen A. Miller, Dr. Nicolas C. Polfer and Dr. Jiangeng Xue , for their help, support and valuable time. I also want to thank Dr. John R. Reynolds, Dr. John M. Papanikolas, Dr. Franky So and Dr. Omar F. Mohammed Abdelsaboor for their advice and suggestions. Their expertise and wisdom inspire the work in this dissertation. I have been lucky to work with many great colleagues who are also my friends . Dr. Zhen Fang passed valuable DSSC fabrication and characterization technics to me. Dr. Fude Feng taught the tr icks of running reactions and doing work ups and shared with me numerous research ideas. Dr. Gyu Leem and I spent days and months on improving DSSC performance together. Dr. Galyna Dubinina and I worked on research proposals and initiative works on many ch allenging projects. Dr. Jan Moritz Koenen and I worked on polyelectrolyte project and hanged out a lot. The discussion topics between Dr. Dustin Jenkins and I went beyond just chemistry. Dr. Coralie Richard and Dr. Dan Patel, from the Reynolds group, are a mazing collaborators and friends to me as well. Mr. Robert J. Dillon and Ms. Amani Alsam also contributed a lot to my research projects.
5 There are a lot of great fellow graduate students in the Schanze groups I want to thank . Dr. Dongping Xie, Dr. Zhuo Ch en, Dr. Danlu Wu, Dr. Xuzhi Zhu and Dr. Jie Yang helped me a lot settle down in Gainesville and are always willing to give me assistance without hesitation whenever I need it. Dr. H sien Yi Hus , Russ ell W. Winkle, Subhadip Goswami and I joined the Schanze g roup at th e same year and had lots of fun working together. I also want to thank Dr. Randi S. Price, Junlin Jiang, Shanshan Wang, Yun Huang, Jiliang Wang, Yajing Yang, Ethan D. Holt and Bethy Kim for their valuable advice and friendship. I want to give spe cial thanks to my Turkish friends Dr. and Seda Cekli for the happy time we had together and their suggestions on my research. Last, but not least, I want to thank my parents for their love, support and encouragement through my entire life. This dissertation is dedicated to t hem.
6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 9 LIST OF FIGURES ................................ ................................ ................................ ........ 10 LIST OF ABBREVIATIONS ................................ ................................ ........................... 14 ABSTRACT ................................ ................................ ................................ ................... 17 C H A P T E R 1 INT RODUCTION ................................ ................................ ................................ .... 19 Conjugated Polymers ................................ ................................ .............................. 19 Linear Conjugated Polymers ................................ ................................ ............ 20 Conjugated Dendrimers and H yperbranched Conjugated Polymers ................ 21 Polymers with Interrupted Conjugation ................................ ............................. 22 Synthesis of Conjugated Polymers and Control of Molecular Weights ................... 23 Electropolymerization ................................ ................................ ....................... 23 Alkene and Alkyne Metathesis ................................ ................................ ......... 25 Pd Catalyzed Cross coupling Reactions ................................ .......................... 27 Photophysical Process of Conjugated Polymers ................................ .................... 30 Excitation, Fluorescence and Phosphorescence ................................ .............. 30 Energy an d Electron Transfer ................................ ................................ ........... 33 Energy transfer ................................ ................................ .......................... 33 Electron transfer ................................ ................................ ......................... 36 Energy and Electron Transfer in Conjugated Polymers ................................ .... 40 Conjugated Polyelectrolytes and Dye sensitized Solar Cells ................................ .. 47 Selective Applications of CPEs ................................ ................................ ........ 48 Dye Sensitized Solar Cells (DSSCs) ................................ ................................ 52 Operational principles of DSSCs ................................ ................................ 52 Solar cell characterization ................................ ................................ .......... 56 Materials used in DSSCs ................................ ................................ ........... 57 Scope of Present Study ................................ ................................ .......................... 63 2 CONJUGATED POLYELECTROLYTE SENSITIZED TIO 2 SOLAR CELLS: CHAIN LENGTH AND AGGREGATION EFFECTS ON EFFICIENCY ................... 65 Background ................................ ................................ ................................ ............. 65 Results and Discussion ................................ ................................ ........................... 67 Synthesis ................................ ................................ ................................ .......... 67 DLS Cha racterization ................................ ................................ ....................... 71 Optical Properties in Solution ................................ ................................ ........... 73
7 TiO 2 Film Characterization, Polymer Adsorption and Charge Injection ............ 75 Polymer Sensitized Solar Cells ................................ ................................ ........ 81 Summary ................................ ................................ ................................ ................ 83 Experiments and Materials ................................ ................................ ..................... 84 Materials ................................ ................................ ................................ ........... 84 Instrumentation ................................ ................................ ................................ . 84 TiO 2 Sol Preparation ................................ ................................ ......................... 86 Dev ice Fabrication ................................ ................................ ............................ 87 Synthetic Procedures ................................ ................................ ....................... 88 3 CHARGE SEPARATION IN DIFFERENT CH AIN LENGTH CONJUGATED POLYMERS ................................ ................................ ................................ ............ 94 Background ................................ ................................ ................................ ............. 94 Results and Discussion ................................ ................................ ........................... 96 Synthesis and Characterization ................................ ................................ ........ 96 Energetics and Optical Properties ................................ ................................ .... 99 Charge Recombination Study ................................ ................................ ......... 104 Summary and Future Work ................................ ................................ ................... 109 Experiments and Materials ................................ ................................ ................... 109 Materials ................................ ................................ ................................ ......... 109 Instrumentation ................................ ................................ ............................... 110 Synthetic Procedures ................................ ................................ ..................... 111 4 ULT RAFAST ENERGY TRANSFER IN VARIABLE CHAIN LENGTH CONJUGATED POLYMERS WITH ENERGY ACCEPTOR END CAPS .............. 117 Background ................................ ................................ ................................ ........... 117 Results and Discussion ................................ ................................ ......................... 119 Structures, Synthesis and Characterization ................................ .................... 119 Optical Properties in Solution ................................ ................................ ......... 123 Fluorescence Polarization Investigation ................................ ......................... 126 Energy Transfer Kinetics ................................ ................................ ................ 131 S ummary ................................ ................................ ................................ .............. 137 Experiments and Materials ................................ ................................ ................... 137 Materials ................................ ................................ ................................ ......... 137 Instrumentation ................................ ................................ ............................... 138 Synthetic Procedures ................................ ................................ ..................... 139 5 ..... 145 Background ................................ ................................ ................................ ........... 145 Results and Discussion ................................ ................................ ......................... 146 Synthesis and Characterization ................................ ................................ ...... 146 Optical Properties in Solution ................................ ................................ ......... 148 Summary and Future Work ................................ ................................ ................... 151 Experiments and Materials ................................ ................................ ................... 152
8 Materials ................................ ................................ ................................ ......... 152 Instrumentation ................................ ................................ ............................... 152 Synthetic Procedures ................................ ................................ ..................... 153 6 CONCLUSION ................................ ................................ ................................ ...... 157 LIST OF REFERENCES ................................ ................................ ............................. 160 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 176
9 LIST OF TABLES Table page 1 1 Name reactions using palladium catalysts ................................ .......................... 28 2 1 Molecular weight table of P1 O, P2 C and Model compound ............................. 70 2 2 G PC results and photophysical data ................................ ................................ .. 73 2 3 Calculated transient absorption lifetime ................................ .............................. 80 2 4 Summary of cell performance ................................ ................................ ............. 83 3 1 Molecular weight characterization of PPE NDI n and PPE ................................ . 99 3 2 Table of energetics ................................ ................................ ........................... 101 3 3 Summary of the photophysical properties ................................ ........................ 103 3 4 Charge recombination kinetics ................................ ................................ ......... 107 4 1 Photophysical Properties of Polymer Samples ................................ ................. 126 4 2 Average Anisotropy and Angle Displacement ................................ .................. 131 4 3 S ummary of lifetime measurements ................................ ................................ . 136 5 1 Photophysical data summary ................................ ................................ ............ 151
10 LIST OF FIGURES Figure page 1 1 Example s of linear conjugated polym ers ................................ ............................ 20 1 2 Ex amples of conjugated dendrimers ................................ ................................ .. 21 1 3 Structures and photophysical properties of conjugated small molecules and conj ugated interrupted polymers ................................ ................................ ........ 23 1 4 Electropolymerization ................................ ................................ ......................... 24 1 5 Alkene and alkyne metathesis ................................ ................................ ............ 26 1 6 Examples of ploy(arylene ethynylene)s s ynthesized by alkene metathesis ....... 27 1 7 Schematic representation of catalytic cycle of Pd cat alyzed cross coupling reactio ns ................................ ................................ ................................ ............. 28 1 8 Jablonski diagram illustrating th e basis photophysical processes ...................... 31 1 9 Schematic representation of electron spin in gro und, singlet ex cited and triplet excited state ................................ ................................ .............................. 33 1 10 Schematic presentation of th e mechanism of energy transfer ............................ 34 1 11 Schemati c repr esentation of electron transfer ................................ .................... 36 1 12 Potential energy surface diagra m for electron transfer process ......................... 37 1 13 Energy diagra m to illustrate splitting ................................ ................................ ... 38 1 14 Potential energy surface diagram for illust ration of reorganization energy ......... 39 1 15 Schematic rep resenta tion of molecular wire effect ................................ ............. 41 1 16 Comparison of polymer and smal l molecule fluorescence sensors .................... 41 1 17 Chemical struct ure of PPEAn and emission spectr a of PPEAn in solution and film ................................ ................................ ................................ ...................... 43 1 18 Pht otophysical properties of PPElp ................................ ................................ .... 44 1 19 Structure of PI F and el ectronic coupling distance plot ................................ ........ 46 1 20 Energy transfer in MEH PPV ................................ ................................ .............. 47 1 21 Molecular structure of CPEs ................................ ................................ ............... 48
11 1 22 Schematic representation of simplified Jablonski diagram and quenching ......... 49 1 23 Detection of avidins with MPS PPV vi a amplified quenching mech anism .......... 49 1 24 Mechanism of biocidal action ................................ ................................ .............. 50 1 25 Schemat ic illustration of drug release ................................ ................................ . 51 1 26 Schematic represen tation of DSSC and film images ................................ .......... 53 1 27 Reactions in DSSCs ................................ ................................ ........................... 54 1 28 Energy le vel ch ange with and without TBP ................................ ........................ 54 1 29 Regeneration of dyes and redox couples ................................ ........................... 55 1 30 A typical DSSC J V curve ................................ ................................ ................... 57 1 31 Examples of TiO 2 nanostructures ................................ ................................ ....... 59 1 32 Examples of Ruthenium dyes ................................ ................................ ............. 60 2 1 Structure of P1 O n, P2 C n and the Model compound ................................ ...... 67 2 2 Synthesis of polymers ................................ ................................ ........................ 68 2 3 GPC analysis of polymers ................................ ................................ .................. 69 2 4 1 H N MR characterization of polymers ................................ ................................ 70 2 5 Syn thesis scheme of model compound ................................ .............................. 71 2 6 DL S characterization of samples in DMF ................................ ........................... 72 2 7 The ground state absorption and emission spectra ................................ ............ 74 2 8 SEM characterization of TiO 2 f ilms ................................ ................................ ..... 75 2 9 Absorptan ce of polymer sensitized films ................................ ............................ 76 2 10 Calculated surface cover age of polymer sensitized films ................................ ... 77 2 11 AFM images of dye sensitized TiO 2 films ................................ ........................... 78 2 12 Film transient absorption and kinetics of dye sensitized TiO 2 films .................... 79 2 13 IPCE and current voltage (J V) characters of polymer cel ls ............................... 81 3 1 Synthesis scheme of PPE NDI n ................................ ................................ ........ 96
12 3 2 GPC and NM R characterization of PPE NDI n ................................ ................... 98 3 3 A) Energetics and B) CV of PPE NDI 8 ................................ ............................ 100 3 4 Uv vis absor ption and emission spectra in THF ................................ ............... 102 3 5 Transient spectra of the PPE an d PPE NDI 8 in DCM after 100 ps ................. 104 3 6 Time resolved tra nsient absorptio n spectra of PPE NDI n polymers ................ 105 3 7 Transient absorption spectra of PPE NDI n after 5 ns ................................ ...... 106 3 8 T ransient kin ................................ ................................ ..... 107 3 9 Nasosecond TA spectra of PPE NDI n, PPE and NDI in THF .......................... 108 4 1 Molecular structures ................................ ................................ ......................... 119 4 2 Synthesis scheme of polymers ................................ ................................ ......... 120 4 3 Synt hesis of OPE TBT model compound ................................ ......................... 121 4 4 GPC traces of PPE TBT n ................................ ................................ ................ 122 4 5 1 H N MR spectra of PPE TBT n and PEE ................................ ......................... 123 4 6 Steady state photophysi cal properties in THF solut ions ................................ ... 124 4 7 Anisotropy characterization and simplified representation of polymer chain conformation ................................ ................................ ................................ ..... 127 4 8 Structure simulation a nd calculated dipole of OPE TBT ................................ ... 130 4 9 Fluorescence lifetime characteriza tion in THF with streak camera ................... 132 4 10 Ultrafast trans ient absorption sp ectra of the PPE and PPE TBT 30 ................. 132 4 11 Time resolved ultrafas t transient absorption spectra ................................ ........ 133 4 12 TA decay kinetics ................................ ................................ ............................. 134 4 13 Schematic representation of the energy transfer pr ocesses in conjugated polymers ................................ ................................ ................................ ........... 135 4 14 Comparison of ultrafas t TA decay kinetics (circles) and time res olved fluorescence (lines) data ................................ ................................ .................. 136 5 1 Molecular stru ctures and synthesis procedures ................................ ................ 147
13 5 2 GPC trace of polymers ................................ ................................ ..................... 148 5 3 Steady state optical proper ties of P0, PPE and OPE in THF ............................ 149 5 4 Steady state absorpti on and emission of P1 n and P0 ................................ ..... 150
14 LIST OF ABBREVIATIONS A A cceptor ADMET Acyclic Diene M etathesis AFM Atomic Force M icroscopy BHJ Bulk H eterojunction Solar C ell CM Cross M etathesis CP Conjugated P olymer CPE Conjugated P ol yelectrolyte D D onor DCM D ichloromethane DLS D ynamic L ight S cattering DMF D imethylformamide DP Degree of P olymerization DSSC Dye S ensitized Solar C ells FF Fill F actor FMO Frontier Molecule Orbitals GPC Gel permeation C hromotography HCP Hyperbranc hed C onjugated P olymers HOMO Highest Occupied Molecular O rbital IPCE Incident Photon to Current E fficiency Jsc Short Circuit Current D ensity LED Light Emitting D iodes LUMO Lowest Unoccupied M olecular O rbital MEH PPV P oly[2 methyoxy 5 (2' ethyl hexylo xy) 1,4 phenylenevinylene]
15 Mn N umber E verage M olecular W eight MV Methyl V iologen NDI Naphthalene D iimide NIR N ear infrared OFET Organic Thin Film T ransistor OPE O ligo(phenylene ethynylene) PA P olyacetylene PDI Polydispersity I ndex PFO P oly(9,9 dio ctylfluorenyl 2,7 diyl) PIF P olyindenofluorene PMII 1 Methyl 3 (n propyl)imidazolium I odide PPE P oly(p phenylene ethynylene) PPV P oly(phenylene vinylene) PT P olythiophene RCM Ring closing M etathesis RMS R oot Mean S quare ROM Ring opening M etathesis SDS Sodium Dodecyl S ulfate TA Transient A bsorption TBAF Tributylammonium F luoride TBAT Tetrabutylammonium D ifluorotriphenylsilicate TBP 4 T ert butyl pyridine TBT 4,7 D i(thiophen 2 yl)benzo[c][1,2,5]thiadiazole TCO Transparent C onduct ing Optical G las ses THF T etrahydrofuran
16 TiO2 Titanium D ioxide TMSA Trimethylsilyl A cetylene Voc Open Circuit P otential
17 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the R equirements for the Degree of Doctor of Philosophy DIFFERENT CHAIN LENGTH CONJUGATED POLYMERS: PHOTOPHYSICAL STUDIES AND DYE SENSITIZED SOLAR CELL APPLICATIONS By Z henxing Pan May 201 5 Chair: Kirk S. Schanze Major: Chemistry C onjugated polymers have d rawn significant attention from the science community due to their interesting photophysical and optoelectronic pr operties. The polymer structure property relationship s have been well studied; howev er, the molecular weight effects on polymer properties hav e been less explored. In this dissertation, we focus on the synthesis of different molecular weight conjugated polymers using the end capping strategy , and the investigation of the en ergy/electron transfer behavior and application s in dye sensitized solar cells (DSSCs). First, two families of conjugated polyelectrolytes (CPEs) featuring the same backbone but with different side chain linkages were synthesized and applied as active materials for DSSCs. CPEs bearing oxygen linkage s ( O ) are more likely to a ggregate in solution , a nd the aggregation status depends s trongly on the molecular weight . In contrast, there is no obvious evidence showing that CPEs with methylene ( CH 2 ) linkage s aggregate in solution. The oxygen linkage family shows strong chain lengt h dependence on cell performance , while little difference can be observed in the methylene linkage family.
18 Second, a series of poly(p phenylene ethynylene)s (PPEs) having different chain length and naphthalene diimide derivative as end caps were synthesiz ed and their photophysical properties were investigated . The overall quenching efficiency increases with decreasing polymer chain length as evidenced by fluorescence quantum yield s . In addition , the charge recombination rate was investigated by ultra fast transient absorption. The charge recombination rate also depends strongly on the chain length: as the chain length increases, the charge recombination rate decreases. Third, a series of PPEs with different chain length and 4,7 di(thiophen 2 yl)benzo[c][1, 2,5]thiadiazole (TBT) end caps were synthesized. Under light irradiation, energy transfer from the PPE to TBT occurs via the Forster Resonance Energy Transfer (FRET) mechanism. The overall energy transfer efficiency increases with decreasing molecular weig ht. Ultra fast transient absorptio n studies show ed that the energy transfer happens in the pico second time scale fo r all the polymers , and the lowest molecular weight polymer ha s fastest transfer rate . Last, a seri es of s with different chain length and [2.2]paracy c lophane moiety end cap s with TBT were synthe sized. Despite the fact that conjugation is interrupted, very efficient energy transfer can still be observed and the overall energy transfer efficiency also depends on the molecular weight.
19 CHAPTER 1 INTRODUCTION Conjugated Polymers Conjugated polymers (CPs) feature polymer backbones with connected orbitals , in which electrons are delocalize d within many repeat units . The delocalization of electrons narrow s the b and gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and, therefore, lowers the energy required to promote one electron to the conducting band and improves the conductivity of CPs. Iodine doped p olyacetylene with high conductivity was discovered by Nobel laureates Alan J. Heeger, Alan MacDiamid and Hideki Shirakawa in 1977 . 1 Since then, this research area has received significant attention from the science and engineering communities. Research has mainly focused on the synthesis, functionalization and application of new materials. Man y synthetic strategies have been applied to tune the properties of CPs, such as changing the polymer backbone architectures 2 5 and incorporating donor acceptor units into the backbones 6 , 7 . Th e se strategies help researchers alter the band gap , molar extinction eff icient , electron/ hole mobility, thermo/ photo stability, etc. The r esulting polymers find applications in the fields of dye sensitized solar cells (DSSCs), 8 , 9 bulk heterojunction s olar cells (BHJs), 10 12 light emitting diodes (LEDs), 13 , 14 organic thin film transistors (OFETs), 15 fixed p n junctions, 16 , 17 chemo and biosensors, 18 20 cell imaging, 21 , 22 antimicrobials, 23 , 24 diagnosis, therapy, 21 , 25 etc .
20 Linear Conjugated Polymers C ommon types of conjugated polymers ha ve linear backbones with fully c onjugated double bonds, triple bonds or aromatic rings . Some classic examples of linear conjugated polymers are shown in Figure 1 1. 26 T he relative synthetic ease makes them more readily accessible , and their properties can feasibly be tuned by control ling the molecular weight, solubility, etc . Figure 1 1. Examples of linear conjugated polymers. Figure was reprinted from Heeger with per mission . 26 Copyright 2010 The Royal Society of Chemistry. In linear of conjugated polymers, such as polyacetylene (PA), polythiophene ( PT) and poly( phenylene acetylene ) (PPE), the linear polymer backbone provides the light harvesting properties. Since the entire backbone is connected by conjugated bonds, one may assume that the conjugation length will be equal to the chain length. Howev er, conjugation is actually broken into smaller segments with different conjugation
21 length s due to conformational disorder which limits the exciton migration length to ~ 10 nm. 27 30 But, very efficient intra /inter chain e nergy/ electron transfer can still h appen via a hopping mechanism. Conjugated Dendrimers and Hyperbranched Conjugated Polymers Dendrimers are highly branched organic macromolecules with well defined molecular structures featuring a central core, different degree of interior branches and fun ctional groups on the surface. Dendrimers can be categorized by generation (i.e. the number of branching ) (Figure 1 2) . 31 Dendrimers are monodis persed and can only be synthesized stepwise . T herefore, synthetic difficulty becomes one of the major drawbacks of dendritic systems. The photophysical properties of conjugated dendrimers largely depend on the conjugated core, however, other properties, li ke solubility and self assembly, are dominated by the surface functional groups. Figure 1 2. Examples of conjugated dendrimers. Figure was reprinted from Schanze et al with permission . 31 Copyright 2012 American Chemical Society. Hyperbranched conjugated polymers (HCPs) have three dimensional dendritic conjugated. Compared to dendrimers, the structures of HCPs are less well defined, but are easier to synthesize , which increases their accessibility . While t he degree of branching of HCPs can not be well control led , the accuracy of
22 branching is difficult to be predicted. Like dendrimers, the optoelectronic properties are largely controlled by the conjugated backbones, but other properties are provided by terminal fun ctional groups. Polymer s with Interrupted C onjugation Although, much attention has been paid to the investigation of fully conjugated systems, polymers with interrupted conjugation can be very interesting as well. There are two common methods to construct such a system: one is to break the conjugation of back bones into smaller segments with non conjugated groups /linkages 32 34 and another way is to conjugated chromophores onto non conjugated polymer backbones. 35 37 The photophysical properties of fully conjugated polymers depend strongly on the conjugation length. 38 , 39 I n inte rrupted conjugation systems, conjugation is limited to each aromatic segment; therefore, the polymers inherit most of the optoelectroni c properties of the chromophores. However, due to the fact that all the chromophores are in the vicinity of each other, they can still have very strong photophysical interaction among polymer chains. For example (Figure 1 3) , when oligo(phenylene ethynylen e) (OPE , energy donor ) and thiophene benzothia diazole (TBT , energy acceptor ) are grafted onto the polymer chain at the same time, ultra fast energy transfer can occur from donors to acceptors. And the overall energy transfer efficiency depends on the dono r/acceptor ratio. 37 At the same time, conjugation interrupted polymers also show some advantages over small molecules because of the ir polymer properties, like stronger te ndency to form films, superior mechanical properties, et al. These polymers have already found applications in the fields of transistors, 40 OLEDs, 41 DSSCs, 36 and so forth .
23 Figure 1 3. Structures of conjugated small molecules and conjugated interrupted polymers . F igure was reprinted from Chen et al wit h permission . 36 Copyright 2012 American Chemical Society. Synthesis of Conjugated Polymers and Control of Molecular Weights Many synthetic methodologies h ave been applied to b oos t the facile synthesis of conjugated polymers, such as electropolymerization , 42 alkyne/alkene m e tathesis 43 , 44 and transition metal catalyzed cross coupling reactions. 45 Electropolymerization In the 1970s, when electropolymerization w as first discovered, the application of this technique was very limited, because the media used for this reaction was water , which is a poor solvent for most aromatic molecules. It was Diaz and co workers who used acetonitrile (1% aqueous) which contained 0.1 M Et 4 NBF 4 electrolyte as the solvent for the electropolymer ization of pyrrole in 1979 that opened the gate for mass application of this polymerization method. 46 Since then, many conducting polymers have been synthesized in this way, such as polythiophene, poly(p p henylene), polypyrene, polyindole, polyazulene, polyfluorene , etc . 47 In a typical electropolymerization, three steps are involved (Figure 1 4 (A) ). 42 1) At the applied voltage, a molecule (R H 2 ) is oxidized to its radical cation [ RH 2 ] efficiently
24 when it approaches the electrode surface. Because the electro chemical reaction is very fast , a large number of radi cal cations accumulate around the electrode. 2) Two monomer radical cations [RH 2 ] can form a dimer dication [H 2 R RH 2 2+ ] through a coupling reaction and then lose two protons to yield a dimer [HR RH] . 3) The newly formed dimer [ HR RH ] can undergo electroo xidation to generate a dimer radical (HR RH ) , which reacts with a monomer radical cation [RH 2 ] to create a new dication [HR RH RH 2 2+ ], followed by losing two protons to produce a neutral trimer. The overall reaction is repeated to generate polymers. Figure 1 4. Electropolymerization A) General procedures for electropo lymerization; B) Electropolymerization of polythiophene. Depending on the stability of the radical cation s , many other reactions can occur. When the radi cal has a long lifetime (i.e . the radical is fairly stable ) , it can diffuse far away from the electrode into the solution and react with other molecules to form byproducts. However, when radicals are short lived, they oft en react with solvent molecules or other nearby molecules immediately after being generated. In order to
25 have the desired dimerization or polymerization reactions , radicals need to have suitable stability , which depends strongly on the nature of the molecules. In addition, there are many ot her factors that can affect the overall electrochemical reactions and, therefore, the polymer molecular weight as well. These factors include, but are not limited to, electrode materials, solvent, electrolytes and temperature . 48 Alkene and A lkyne M etathesis A lkene and alkyne metathesis has captured the atte ntion of the polymer synthesis community for a long time. Since its discovery, the application of th is method has gone way beyond the industrial manufacturing of polyol e fins , and several distinguishable processes have been identified: cross metathesis (CM) , 49 ring openin g metathesis (ROM), 50 ring closing metathesis (RCM) 51 and acyc lic diene metathesis (ADMET) (Figure 1 5) . 52 A large variety of catalysts have been developed , most of which are based on tungsten (W), molybdenum (Mo) or ruthenium (Ru) complexes. Compared to the more water and air sensitive W and Mo catalysts (Shrock types), R u based catalysts (Grubbs type) have more tolerance towards air, water and reaction substrates , making them more useful in the construction of functional materials . Because of the ir pioneer ing work in developing the synthetic methodology and various cataly sts , Grubbs, Schrock and Chauvin shared the Nobel Prize in Chemistry in the year of 2005. Besides being employed in the synthesis of non conjugated polymers, alkene metathesis has also been extensively used to synthesize ploy(arylene vinylene)s (PPVs) (Fi gure 1 6). 43 , 45 , 53 The polymerization reaction follows a typical st ep growth mechanism: At the beginning of the reaction, the monomer conversion rate is high, and large amount of oligomer formation is observed. Subsequently, polymer molecular
26 weights grow with time with the decrease in low molecular weight molecules. The expected polydispersity index (PDI) is around 2 (Flory Schulz distribution). However, there are examples that polymers synthesized by the metathesis show narrow PDI, although, the overall molecular weight is relatively low. 54 , 55 By performing the polymerization at optimized conditions (e.g., the right combination of catalyst, solvent, temperature and monomer concentration), high molecular weight polymers can be obtained. 56 , 57 Figure 1 5. Alkene and alkyne metathesis A) Schematic representation of a gen er al alkene metathesis reaction; B) e xamples of W, Mo and Ru based catalysts. Figure was reprinted from Bunz et al with permission. 43 Copyright 2012 John Wiley & Sons. In contrast to the application of alkene metathesis to build PPVs , there are fewer repor ts concerning the synthesis of poly(phenylene ethynylene)s ( PPEs ) via alkyne
27 metathesis. However, compared to other synthetic methodologies, alkyne metathesis can give very high molecular weight polymers. 58 Figure 1 6. Examples of ploy(a rylene ethynylene)s synthesized by alkene metathesis. Figure was reprinted from Bunz et al with permission. 43 Copyright 2012 John Wiley & Sons. Pd Catalyzed Cross coupling Reactions The development of cross coupling reactions, especially palladium catalyzed carbon carbon bond formation reactions, facilitates the synthesis of CPs. Compared to catalysts used in metathesis polymerization, most Pd catalysts have relatively higher tolerance to different functional groups and reaction conditions, and they are less sensitive to water , making them more popular choice s for the synthesis of CPs. Due to their significant contribution to the development of Pd catalyzed reactions, Heck, Negishi and Suzuki were awarded the Nobel Prize in Chemistry in 2010. There are severa l milestone name reactions using Pd catalysts and they are listed in Table 1 1. Although, these reactions involve different catalytic systems and substrates and have different application scopes, they share very similar catalytic cycles (Figure 1 7). The c atalytic cycle usually starts with oxidative addition of organic halides (Reactant B, Ar X, X= Cl, Br or I) onto the Pd catalyst to form a new complex, followed
28 by a transmetallation reaction of activated Reactant A (Nu ) with the newly generated complex t o put Nu on the same catalytic center. Then, trans cis isomerization will take place to put Ar and Nu at cis position. The final step is the production of the desired coupling compound along with the regeneration of the catalyst by reductive elimination. T able 1 1. Name reactions using p alladium catalyst s Name Reaction Reactant A Reactant B Catalyst Substrate Hybridization Sbustrate Hybridization Heck Alkene sp 2 R X sp 2 Pd Negishi R Zn X sp, sp 2 , sp 3 R X sp 2 , sp 3 Pd or Ni Suzuki R B(OR) 2 sp 2 R X sp 2 , sp 3 Pd Stille R SnR 3 sp, sp 2 , sp 3 R X sp 2 , sp 3 Pd Sonogashira Alkyne sp R X sp 2 , sp 3 Pd and Cu(I) Figure 1 7. Schematic representation of catalytic cycle of Pd catalyzed cross coupling reactions. Figure was reprinted from Amatore and Jutand with perm ission. 59 Copyright 2000 American Chemical Society. Because the catalyst can dissociate from the reactant, the reaction has no living polymerization characteristics. In fact, cross coupling polymerizations are typical step -
29 growth reactions with PDI around 2 . s possible to obtain high molecular weight polymers by using 1:1 ratio of Reactant A and Reactant B. The molecular weight is determined by degree of polymerization (DP) , is given by: DP=1/(1 p), where p is the extent of reaction P olymer molecular weight ca n be tuned via the control of DPs by introducing a stoichio metric imbalance of functional groups of Reactant A to that of Reactant B . The resulted DP is given by : DP=(1+r)/(1+r 2rp), where r is stoichiometric imbalance ratio . 60 If the stoichiometric imbalance is caused by the addition of a mono functioned end cap , the polymer molecular weight can be controlled , and the end group function can be introduced at the same time . This strategy has been successfully applied by many research groups to synthesize CPs with different molecular weight with end group functionality . 61 64 More recently, several groups developed a chain growth process for controlled synthesis of polythiophenes, 65 polyfluoren es, 66 , 67 polyphenylenes, 68 and poly(phenylene ethylene)s. 69 The chain growth reactions still undergo the same catalytic cycle as the step growth mecha nism reacti trans metalation, trans cis isomerization and reductive elimination. However, the oxidation addition happens in an intra chain manner due to the nature of the new catalytic systems, giving the polymerization living char acteristics . Due to the living polymerization nature, this method can be used to control molecular weight, build di block polymers and synthesize graft ing .
30 Photophysical Process of Conjugated Polymers Conjugated molecules can interact with a large region of the solar spectrum , from near U V to near conjugation differs in different conjugated systems. Despite the difference in absorption wavelength, the absorption process of all the molecules follow s the same principle, the Stark Einstein law, which says that each ab sorbed photon will cause only one primary chemical or physical change. The Stark Einstein law is also called the photochemical equivalence law, for it can be rephrased as: for every mole of quanta of light absorbed, one mole of substance will react. The fo rmula is given by : mol =N A h , where N A is 70 Molecules that can absorb light are called chromophores and they are responsible for the color s of molecules. W hen light irradiate s the molecule, a certain wavelength of light is absorbed and the molecule shows t he complementary color which is not absorbed. If all the visible light is absorbed by a substance, it will appear to be black. After light is absorbed, many interesting photophysical process can happen. Excitation, Fluorescence and Phosphorescence During t he photoexcitat ion process, an electron is excited from the ground state to an excited state and will remain in the excited states until the relaxation or transfer process occurs. A schematic diagram , named in honor of Alexsander Jablonski, has often been used to illustrate the photophysical process , as shown in Figure 1 8. A Jablonski diagram portrays the relative electronic and vibrational energy levels of the ground and excited states without any attempt to depict the nuclear and electron ic geometries.
31 Figure 1 8. Jablonski diagram illustrating the basis photophysical processes. Figure was reprinted from Lakowicz with permission. 71 Copyright 2007 Springer. When a molecule absorbs a photon with appropriate energy, an electron is promoted to a vibronically exci ted level of electronic singlet state, S 1 , from the lowest vibronic level of the electronic ground state, S 0 . This process is called ground state excitation and occurs on the time scale of 10 16 10 14 s . There are many vibrational levels within the S 1 s tate , and the chance of an electron being promoted to a specific energy level depends on the energy overlapping . However, the molecule will relax to the lowest vibronic level of electronic state S 1 , because the extra energy is transferred to solvent or los t due to molecule reorganization. The relaxation usually takes about 10 14 10 11 s. When a proper light source is applied (e.g. a high energy laser), that the molecule s will be promoted to higher energy electronic singlet states, S 2 S n . Without external interactions, two possible follow up conversions can occur for the singlet excited state : decay to S 0 or transfer to first excited triplet state, T 1 . There are two possible pathways to decay to S 0 : radiative decay and non radiative decay. In the
32 radiative decay, a photon is emitted to give fluorescence and the energy of the emitted photon depends on the energy difference between S 1 and S 0 . T here is usually a shift , called Stokes shift, to longer wavelength, between the absorption wavelengt h maximum and emission wavelength maximum due to the energy difference between photon absorbed and photon emitted. Radiative decay mostly happens in the time range of 10 9 10 7 s. On the other hand, the singlet excited state can relax to ground state wit hout emitting photons . Instead, the energy will be lost in the form of heat. Alternatively, the excited state can undergo intersystem crossing (ISC) from singlet excited state to triplet excited state. The intersystem crossing rate depends strongly on the system and can vary in a large range. After intersystem crossing, the system relax es to the lowest triplet excited state, T 1 . Similar to the process in the singlet excited state, the molecule will eventually decay to the ground state via either radiative decay which produce s phosphorescence or non radiative day which generates heat. However, in the singlet state, the electron spins are still paired the same as the electrons pairs in the ground state. In contrast, in the triplet state, th e electrons spins a re parallel (Figure 1 9) . Because of the difference in electron spin orientation in the ground state and triplet state , direct excitation of electrons from ground state to the triplet state is quantum mechanically for bidden. An electron must be excited to its singlet excited and undergo ISC to convert to the triplet excited state. For the same reason, the radiative decay rate (from T 1 to S 0 (phosphorescence , usually 10 3 10 2 s 1 ) is considerab ly slower compared to the rate of singlet emission (fluorescence ,10 9 10 7 s 1 ).
33 Figure 1 9. Schematic representation of electron spin in ground, singlet excited and triplet excited state. Energy and Electron Transfer When a photo excited molecule interact s with another substance , the excited state molecule can be q uenched. There are two distinct path ways for the quenching : energy transfer when excited electron transfers its energy to interacting molecules and returns back the ground state; electron transfer if the electron hops to the interacting molecules. For each pathway, t he molecule that gives energy or electron is called donor (D) while the molecule that accepts energy or electron is called acceptor (A) . The donor has higher excited state energy than the acceptor. The i nteraction mechanism s for energy and electron transfer differ significantly and are discussed below . Energy transfer Energy transfer can occur via either a radiative or non radiative process. In a radiative energy transfer process, the excited donor molecules (D*) emit photons which are directly absorbed by acceptors (A). Apparently, this re quires that the emission spectrum of D* must have some ove rlap with the absorption spectrum of A. The overall energy transfer efficiency depends on the quantum yield of the donor molecules, the overlap integral (J) of the emission and absorption spectra, the concentration of acceptor and the molar extinction coefficiency. In general, the radiative energy transfer efficiency is not very high.
34 On the other hand, non radiative energy transfer , which does not involve the emission and absorption of photons , but only energy can be very efficient . The energy transfer is a two step process : photoexcitation of the donor molecule Equation 1 1 and energy transfer from donor to acceptor Equati on 1 2 . D + h * 1 1 D * + A + A * 1 2 There are two diffe rent mechanisms for the energy transfer step : FÃ¶rster energy transfer which occurs via a dipole dipole interaction and Dexter energy transfer which involves an electron exchange interaction (Figure 1 10) . Figure 1 10. Schematic presentation of the mechanism of energy transfer. A) Dexter energy transfer; B) FÃ¶rster energy transfer. 72 Figure s are adapted with modification. In FÃ¶rster energy transfer, the driving force is the electronic coupling interaction between the dipole moment s of the excited donor and the acceptor (dipole dipole interaction) . During FÃ¶rster energy transfer, the energy of the excited electron in the lowest unoccupied molecular orbital (LUMO) of the donor (D*) is transferred to the acceptor (A) via a C olumbic inte raction , and the electron relax es to the highest occupied
35 molecular orbital (HOMO). At the same time, the acceptor acquires the energy and one of its electrons is promoted from the HOMO to LUMO. This mechanism does not involve the exchange of electrons and can occur over a long distance (30 100 Ã…) . The e lectrostatic interaction energy (E) between the two interacting dipole s is directly proportional to the strength of both transition dipoles ( Âµ D and Âµ A ) and inversely proportional to the cube of the distance between the donor and acceptor. (1 3) T he energy transfer rate (k ET ) can be derived from equation 1 3 and is proportional to the square of the electrosta tic interaction energy (E). (1 4) Energy transfer efficiency increase s with the magnitude of dipole moments and decre ase s significantly with increasing donor acceptor separation, since the rate is proportional to the inverse sixth power of the distance between donor and acceptor. 72 In Dexter ene rgy transfer, there is direct electron exchange between the donor and acceptor via the overlapping orbitals of D* and A. The excited electron in the LUMO of D* hops to the LUMO of A and one electron in the HOMO of A is transfer ed to the HOMO of D*. Another key difference between Dexter and FÃ¶rster energy transfer is that both singlet singlet and triplet triplet energ y transfer can take place in via Dexter energy transfer while FÃ¶rster theory can be applied in triplet triplet energy transfer. The energy tra nsfer rate in the Dexter mechanism is given by: (1 6)
36 Where K is a parameter related to specific orbital interactions, J is the spectral overlap integral, is the separation of D* and A when they are in van der Waals contact and is the distance between D and A. Because the transfer rate is exponential dependent of the distance of D and A ( ) , energy transfer is efficient o nly wh en D and A are very close to each other ( 5 10 Ã… ) . Electron t ransfer Electron transfer is a very important reaction in many biological processes 73 , 74 and photo electronic devices. 75 , 76 Unlike energy transfer, electron transf er involves actual electron transfers from a donor to an acceptor , which produces a charge separated state (Figure 1 11) . In photoinduced electron transfer , one electr on in the donor is excited to its LUMO and then transferred to the LUMO of the acceptor. As a result, the donor is oxidized to D + , while the acceptor is simultaneously reduced to A . The electron trans fer produces a charge separated state which may be deactivated to the ground state by charge recombination or may undergo follow up reactions if the charge separated state is long lived . Figure 1 11. Schematic representation of electron transfer. Rudolph A. Marcus developed a theory to explain electron transfer which treated the electron transfer process as a transition state. In the Marcus the ory the excited donor and acceptor pair (DA*) and the charge separation state (D + A ) are treated as the reactant and product , respectively (Figure 1 12) . 77
37 Figure 1 12. Potential energy surface diagram for electron transfer process. In the figure, DA, DA*, TS a nd D + A stand for the grand state, excited state, 0 are Gibbs free energy change and reorganization energy, respectively. Figure is adapted with modification. 72 The electron transfer rate can be derived from the theory: (1 7) w here, V if is the reorganization energy induced by the electron transfer and G 0 is the Gibbs free energy variation during the reaction . The electron transfer rate is controlled by two parameters: 1) the electronic matrix element and 2) the reorganization energy. In order to have high electron transfer rate, the electronic coupling needs to be maximized while the reorganization energy should be minimized. The electronic coupling can be understood as the tendency of the donor to transfer an electron to the acceptor, and therefore, transfer integral can be assigned to quantify the electron coupling matrix. The overall transfer integral depends on both the interacting Frontier Molecule Orbitals (FMOs) of the donor and acceptor and the relative positions of the interacting molecules. One method to estimate the tr ansfer integral is to
38 apply Koo , which use half the splitting of the HOMO to calculate th e hole transfer integral and half the splitting of the LUMO to calculate the electron transfer integral (Figure 1 13) . 78 In general, the HOMO splitting i s systematically larger than LUMO splitting, b ecause, the FMOs of the HOMO feature bonding characters while the FMOs of the LUMO are mostly anti bonding. When two molecules approach each other, the FMOs start to interact with each other , splitting the HOMO and LUMO into tw o energy levels. The splitting create s anti bonding characters in HOMO and some bonding in the LUMO and. The anti bonding character greatly increase s the energy level of HOMO and , therefore, enlarge s the magnitude of HOMO splitting. Because t he original LU MO has anti bonding characters , the significance of the splitting is smaller. In fact, most organic materials have higher hole mobility than electron mobility due to larger HOMO splitting . Figure 1 13. Energy diagram to illustrate splitting. The relativ e position of the interacting molecules also affects the electronic coupling. For example, co facial displacement of the two molecules usually provides the largest electronic interaction. However, lateral displacement can cause the splitting of the LUMO ev en larger than that of the HOMO. 79
39 The reorganization energy is the sum of the inner and outer contribution. Upon electron gain/loss, the geometry of the donor/acceptor changes, and the energy var iation associated with the change is the inner reorganization energy. The polarization, relaxation and stabilization effects of the surrounding environment on the donor/acceptor also cause an energy change , which is assigned as the out er reorganization ene rgy. I n many cases, the magnitude s of the inner and ou ter reorganization energy are of the same order. Figure 1 14. Potential energy surface diagram for illustration of reorganization energy. A p otential energy surface diagram can be used to illustrate the electron transfer process (Figure 1 14). During an electron transfer reaction, the donor (D) is oxidized to D+ and the acceptor (A) is reduced to A simultaneously. This step does not involve any geometry change. The following step is the relaxation of the product nuclear geometries. The energy required to overcome the barrier for the transition is pr ovided by photon irradiation in the case of photo induced electron transfer (See Figure 1 12). The reorganization energy greatly affects the overall electr on transfer efficiency since the electron transfer rate is exponentially proportional to the sum of reorganization energy and Gibbs free energy change.
40 Energy and Electron Transfer in Conjugated Polymers Compared to small molecules and oligomers, energy and electron migration is very efficient in conjugated polymers due to the molecular wir e effect which is proposed by Swager and co workers (Figure 1 15) . 80 The conjugate d polymer chains resemble molecular wires along which the exciton can delocalize and migrate along the wire efficiently. This effect contributes to the fact that the quenching efficiency of conjugated polymers is s everal order of magnitude higher compared to small molecules. For example, Zhou and Swager prepared a water soluble conjugated polymer featuring PPE backbone with a crown ether side chain on each repeat unit (Figure 1 16 ) . 80 In solution, the polymer shows strong emission which can be quenched by methyl viologen (MV 2+ ) about 50 100 fold greater efficiency compared to an oligomer with only three benzene ring s bearing the same crow n ether group. In the case of small molecules, each quencher can quench the fluorescen ce of only one small molecule . In contrast, in polymer emission quenching, all excitons generated by different chromophore un its on a polymer chain can be quenched , as long as they can migrate to the quenching site and encounter a quencher. The molecule wi re effect acts as an amplifier and greatly enhance s the quenching efficiency. Based on this strategy, many conjugated polymer sensors have been developed. 18 , 81 However, the photophysical process in conjugated polymers is very complicated. In polymer assemblies, there can be both interchain and intrachain energy and/or electron transfer competing with each other. In trachain transfer refers to energy/electron migration along a single polymer chain while interchain process means the exciton hops among multiple polymer chains. The conformation and assembly status of polymers depends on the solvent and temperature, which affect the energy and charge transfer
41 processes. Numerous researchers have continued to resolve the complexity and understand the mechanisms and kinetics of energy/electron transfer in CPs. Figure 1 15. Schematic representation of molecular wire effect. Figure was reprinted from Zhou and Swager with permission. 80 Copyright 1995 American Chemical Society. Figure 1 16. Comparison of polymer and small molecule fluorescence sensors. Green color indicates the molecule is emissive and grey color means the fluorescence is quenched. Figure was reprinted from Zhou and Swager with permission. 80 Copyright 1995 American Chemical Society. S wager and co workers contributed significantly in this a rea. 82 They demonstrated that, in a pure rigid rod system, like PPEs, the excit on hop along the
42 polymer backbone follows a 1D random walk mode l. The exciton migration does not have a preferred direction and wi ll pass a certain portion of the polymer chain multiple times when it travels back and forth. The exciton can be effectively quenched as long as it reaches the quenching site before decay ing to the ground state. The effective migration length depends on th e product of lifetime of excitation ( ) and transfer rate (v). In order to determine the effective excitation migration length, Zhou and Swager synthesized a series of PPEs with different chain length s . 80 In the quenchi ng experiments, they noticed that the quenching efficiency stopped increasing after the number of repeating unit s reached ~ 140. Therefore, they concluded that in s olution the effective exciton migration length of PPEs is about 14 0 (Ph CC ) unit s. Based on the conclusion, they calculated the total exciton travel distance is about 20,000 phenylethynyl unit s. However, one thing that needs to be pointed out here is that PPEs are not perfectly rigid rods and the persistence length is about 15 nm. 82 Wrighton and Swager reported energy transfer study in a system containing PPEs with anthracene substitution (PPEAn) at the polymer chain end s (Figure 1 17) . 83 The polymer backbone harvests energy and creates excitons which tr ansfer to anthracene, due to the fact that anthracene has a lower LUMO energy and acts as an acceptor. Compared to PPEs without anthracene substitution, the emission of PPEAn shows two distinct bands in solution (Figure 1 17). The higher energy band at 478 nm comes from the polymer backbone while the lower energy band at 510 nm is assigned to anthracene emission. The intensity ratio of the higher energy band to the lower band is about 2:1, indicating fairly efficient energy transfer. However, in a PPEAn fi lm, there is only one emission peak, and the emission from the PPE backbone is completed
43 quenched indicating the energy transfer efficiency is enhanced (Figure 1 17). The authors stated that in solution, energy migration is only one dimensional (e.g. the e xciton can only travel along the polymer chain). In contrast, polymer chains are closer to each other in films making interchain energy hopping possible, making energy transfer three dimensions. Figure 1 17. Chemical structure of PPEAn and emission spect ra of PPEAn in solution and film. Figure was reprinted from Swager. 82 Copyright 2011 John Wiley & Sons. The effective conjugation length is also believed to affect the energy transfer rate. Swager investigated the energy migration rate of PPEs containing iptycene units (PPEIp) in liquid crystalline (LC) solvents. 64 The effective conjugation length of PPE type polymers in solution (like tetrahydrofuran (THF) or dich loromethane (DCM)) is limited to around 9 10 repeating units. But, when the polymers are dissolved in a liquid crystalline solvent (e.g. 1 (trans 4 hexylcyclohexyl) 4 isothiocyanatobenzene , 6CHBT),
44 both absorption and emission spectra show significant bath ochromic shifts indicating increasing conjugation length in polymer (Figure 1 18 A & B). In addition, the fluoresce anisotropy increases from ~0.3 in DCM to ~0.72 in 6CHBT (Figuire 1 18 D). The possible reason is that the polymer chains are better aligned along the liquid crystalline direction. As a result, the authors observed improved energy transfer efficiency and they attributed it to increased conjugation length and better alignment. Figure 1 18. Ph o tophysical properties of PPElp. A) U V vis absorptio n and B) emission of PPEIp in DCM, film and LC solution; C) s chematic representation of extended polymer chain model; D) p olarized emission spectra of PPEIp. Figure was reprinted from Swager. 64 Copyright 2005 American Chemical Society. Mullen and Breda s work help ed explain the details of energy/electron transfer in conjugated polymers . 84 , 85 They studied the energy transfer in a donor acceptor system
45 featuring a polyindenof luorene (PIF) backbone (donor) and perylene derivatives as end group s (acceptor) (Figure 1 19A) . They stated that energy transfer in solution, which does not involve much interchain interaction, can be viewed as a two step process: exciton migration along the backbone and energy transfer to the acceptor. T he effective conjugation length is shorter than the total polymer chain length , and the pol ymer chain the exciton hops among these segments , which is a slow process due to the weak dipole coupling between chain segments . H opping means the exciton jumps to a near by segment which is a homomolecular self exchange process and does not involve energy change . Once the exciton reaches the segment which has close contact with the acceptor, ultrafast energy transfer take s place, and the e nergy transfer rate is determined by the reorganizatio n energy and electronic coupling matrix (see previous section). Energy migration is more efficient in rigid rods compared to flexible chains, due to smaller reorganization energy loss , and the electronic coupling matrix is calculated to decrease with incre asing D A distance (Figure 1 19B). Thus, the hopping process becomes the key step in determining the overall transfer rate. In film s , the polymer chains stack together , and this favors interchain energy transfer from the conjugated segments of one chain to the perylene unit in an o ther chain. As a result, the energy transfer efficiency is about 10 times higher in film s compared to in solution.
46 Figure 1 19. Structure of PIF and electronic coupling distance plot. A) Chemical structure of PIF ; B) D A dista nce and electronic coupling matrix plot. Figure was reprinted from Bredas. 86 Copyright 2002 United States National Academy of Sciences. Energy and electron transfer in more flexible conjugated polymers are even more complicated. Scholes, Tobert and Schwartz studied the energy transfer in poly[2 meth y oxy 5 ethyl hexyloxy) 1,4 phenylenevinylene] (MEH PPV) in solution and in a restricted matrix. 87 , 88 MEH PPV chain is more flexible compared to PPEs , and has both tightly coiled and open chain conformation s in solution (Figure 1 20 A ) . In the tightly coiled conformation zone, conformational subunit s (chain segments within which the repeating unit s are conjugated ) are close to each other , facilitating interchain energy transfer. In the open chain conf ormation region, conformational subunits are in the extended chain mode , which favors intrachain energy transfer. In a confined silica composite matrix , where the polymer chain conformation is confined, most polymer chains adapt the extended conformation (Figure 1 20 B). Interchain energy transfer is inhibited and the energy transfer prefers to occur through the intrachain hopping mechanism. Thus, the overall energy trans fer efficiency is lower in films compared to in solution.
47 Figure 1 20. Energy transfer in MEH PPV. A) Schematic representation of inter and intra chain energy transfer in MEH PPV; B) MEH PPV in silica co mposite matrix. Figures were reprinted from Scholes, Tobert and Schwartz. 87 , 88 Copyright Copyright 2001 John Wiley & Sons. Conjugated Polyelectrolyte s and Dye sensitized Solar Cells Conjugated polyelectrolytes (CPEs) are conjugated polymers with ionic pendant chains . Thy inherit the interesting electronic and optical p roperties of organic conjugated back bone s and are soluble in polar solvent s, such as water, ethanol, etc . Many ionic side chains have been proven to endow this solubility to CPs, including sulfonate ( SO 3 ), carboxylate ( CO 2 ), phosphonate ( PO 3 2 ) and quaternary ammonium ( NR 3 + ) groups (Figure 1 21) . The charged side groups can interact with many species, such as metal ions, metal oxides, polyelectrolytes, proteins, oligo an d polynucleic acids, making CPEs outstanding platform s for a variety of applica tions . 89 , 90 C harges also allow dir ect deposition of CPEs on top of neutral semiconductor surface , making CPEs applicable in device fabrications. 91 , 92 In addition, the use of some polar solvents, like methanol and wate r, to make CPE based devices is more environmentally friendly, as these solvents are regarded as green solvents.
48 Figure 1 21. Molecular structure of CPEs. Selective A pplications of CPEs Chemo and bio sensing are very impor tant applications of CPEs. The working principles of sensors are based on fluorescence quenching and/or recovery. T here are two quenching pathways, dynamic quenching and static quenching , which are shown in the simplified Jablonski diagram (Figure 1 22). 71 In a q uenching process, a ground state f luorophore (F) first absorb s light and is excited to the singlet excited state (F*). When F* returns to the ground state via photon emission, it produ ces fluorescence. H owever, quenching occurs when F* interacts with a spe cies which causes the fluorescence intensity or lifetime be reduced. In fact, quenching is a subcategory of energy/electron transfer (refer to section 1.3.2) and the molecular wire effect makes CPEs superior candidates for sensing applications compared to small dyes (see section 1.3.3). One of the early CPE based sensor s exploiting the amplified quenching mechanism wa s reported by Whitten and co workers (Figure 1 23). 93 Biotin functionalized viologens quench the emission of MPS PPV very efficiently in aqueous solution. But, the fluorescence is recovered by addition of avidin s to the solution. Avidins can bind ve ry strongly with biotins and prevent the viologen quencher s to approach the MPS PPV chain . Therefore, the quenching is stopped which induces the recovery of CPE emission. This system exhibits high sensitivity for the detection of avidin s .
49 Figure 1 22. Schematic representation of simplified Jablonski diagram and quenching. Figure was reprinted from Lakowicz with permission. 71 Copyright 2007 Springer. Figure 1 23. Detection of avidins with MPS PPV via amplified quenching mechanism. Figure was reprinted from Wh itten. 93 Copyright 1999 United States National Academy of Sciences. Polymer conformation change provides ano ther useful mechanism for sensing application s . Schanze and co workers studied the Ca 2+ induced aggregation of PPE CO 2 and the quenching behavior by methyl viologen (M V 2+ ) . 94 Divalent cation Ca 2+ serves as a bridge between negatively charged polymers and causes the polymer chains to aggregate. In the aggregated s tate, the overall quantum yield s of CPEs decreas es. Q uenching is more efficient, probably because the diffusion of the exciton is three dimensional within the aggregates , which increases the probability of quenching . Another important application of CPEs i s in antimicrobials. For several decades , quinolones, gylcopeptides, and streptogramins, which were introduced in the mid 20 th
50 century, were the only options for antimicrobial applications. One serious crisis can arise due to the bacterial resistance whic h limits the usefulness of these drugs. Therefore, new antibacterial materials are needed to resolve the crisis. Photo dynamic inactivation of bacteria , which was first introduced more than a century ago , is an alternative (Figure 1 24) . 95 Singlet oxygen ( 1 O 2 ) is believed to be the active species that cause s cell damage or death, because it degrades cell walls, lipid membranes, enzymes and nucleic acids in photodynamic inactivation. 96 Figure 1 24. Mechanism of biocidal action. ( i ) Reversible bacteria adhesion to the particles. ( ii ) Photoexcitation of CPE. ( iii ) Singlet oxygen generation. ( iv ) Killing bacteria by oxygen. ( v ) Aggregation of particles. Figure was reprinted from Whitten. 97 Copyright 2008 American Chemical Society. When irradiated , CPEs can generate singlet excited state s which can undergo intersystem crossing to produce triplet exciton s . The energy of the triplet exciton is can be used to convert groun d state dioxygen ( 3 O 2 ) to excited state singlet oxygen 1 O 2 . Since CPEs can bind closely to the bacteria, the 1 O 2 only has to travel a short diffusion pathway to reach the bacteria which will reduce its toxic effect. 98 Based on that, the Whitten and Schanze groups developed s everal CPE based antimicrobials (e.g. CPEs grafted silica particles ) . 97 , 99 Silane functionalized iodobenzene was first grafted onto silica particles which then reacted with 1,4 diiodobenzene and 1,4 diethynylbenzene in
51 s olution to give polymer coatings. When exposed to light , bacteria which were accompanying or in in the vicinity of the particles were killed effectively. Figure 1 25. Schematic illustration of drug release. A) PFO/PG complex and B) drug release process. Figures was reprinted from Wang. 100 Copyright 2010 American Chemical Society. CPEs are also outstanding candidates for drug and gene deli very, because the fluorescence change can be used to monitor the release process in vivo . 21 , 25 Wang and co workers de signed several drug delivery systems and one example is shown in Figure 1 25 . 100 Positively charged poly(9,9 dioctylfluorenyl 2,7 diyl ) ( PFO) forms a complex with anionic poly(L glutamic acid) which is conjugated with an anticancer drug through electrost atic interaction. The PFO alone shows very strong fluorescence, but upon interaction with drugs, the fluorescence is quenched by the drug v ia an electron transfer mechanism. However, upon hydrolysis of the poly(L glutamic acid), the drug is released, thereby stopping the electron transfer process and recovering the fluorescence of the PFO. The quenching and recovery of fluorescence make it ve ry convenient for monitoring drug release.
52 Dye S ensitized Solar Cells (DSSCs) Bec ause of the increasing demand for energy, the depletion of fossil fuels and the environmental impact associated with the use of non renewable energy resources, technology has drawn significant attention recently. D ye sensitized solar cells (DSSCs) are among the most important emerging phot ovoltaic devices which can be alternative s to traditional inorganic solid state solar cells. A typical DSSC structure is shown schematically in Figure 1 26 . It is made of a transparent nanoporous semiconductor electrode on a transp arent conducting optical glass ( TC O), a thin layer of light harvesting materials on the surface of the semiconductor electrode (usually ti tanium d ioxid e, TiO 2 ), a counter electrode, and redox electrolyte solution (I /I 3 , for example) filling the pores. Under working conditions, the light harvesting materials (dyes) absorb sunlight and promote ground state electrons to the excited state . The excited stat e electrons are injected into the conduction band of TiO 2 , l eaving the oxidized dyes behind . The electrons go through the circuit to deliver work. After losing energy, electrons can reduce I 3 to I . T he oxidized dyes are reduced by I and the same process is repeated. 101 Ope ration al p rinciples of DSSCs Under working condition s , many photo/electro chemical reactions occur as shown in a simplified energy diagram (Figure 1 27 (a)) with different time constant (Figure 1 27 (b)). There are both desired reactions, such as reactions 0, 2, 3, 4 and 7, and undesired reactions, listed as 1, 5 and 6, which need to be hindered.
53 Figure 1 26 . Schematic representation of DSSC and film images. A) Typical DSSC structure; B ) TiO 2 films deposited with sensitizers. Process es 1 and 2: C harge I njection vs D ecay . In order to have the cell working properly, the charge injection time constant (reaction 2) should be short er than that of exciton decay (reaction 1). Depending on the nature of dyes, the lifetime of some dyes can be as long as 10 6 s w hile some other dyes are very short lived with lifetime as short as 10 9 s. 71 The short excited state life time increases the chance of that electron will not be injected into the conducting band of TiO 2 , and decay to the ground state will occur instead . The mech anism of charge injection is still not very clear, but it is well accepted that the charge injection is an ultrafast process with a femtosecond component. 102 , 103 However, very slow injection was also observed in a DSSC device whose time scale is around 150 ps. 104 Such a slow injection process greatly reduce s the charge collection efficiency and reduce s the cell performance. The overa ll injection process has been an important research topic and is greatly influenced by additives in electroly te solutions, the Fermi energy level of TiO 2 , the dye binding model and the distance between the dye molecules and TiO 2 particles, etc . For example, the addition of 4 tert b utyl p yridine (TBP) into the electrolyte solution
54 increases the energy level of con duction band edge and increase s the energy overlap of the excited state dyes and the TiO 2 conducting band (Figure 1 28) . 75 Figure 1 27 . Reactions in DSSCs. A) E nergy diagram; B ) time constant. Figures were reprinted from Hagfeld. 101 Copyright 2010 American Chemical Society. Figure 1 28 . Energy level change wi th and without TBP. Figures were reprinted from Hagfeld. 101 Copyright 2010 American Chemical Society. Reaction s 3 & 7 : Regeneration of Dyes and Redox Couples . In order to have the cell working in a continuous manner , the oxidized dye has to be reduced to the ground state ( reg eneration of dyes, reaction 3 ) . The time constant in this reaction is limited by the diffusion of the redox couple . For example, in a non viscous solvent, like dimethylformamide (DMF), the diffusion rate constant is around 10 9 10 10 M 1 s 1 and the time cons tant of regeneration is about 1 10 ns with normal redox couple concentration s (~0.1 M). I odide/triiodide (I /I 3 ) is a common redox couple with iodide as the reducta nt
55 and triiodide as the oxidant and the reaction mechanism is proposed as follow s (Figure 1 29 ). The first step is the one electron transfer reaction between oxidized dye and iodide , followed by the addition of a second iodide. 105 After reduction of oxidized dyes, iodide is converted to diiodide (I 2 ) , which subsequently undergoes a disproportionation reaction and converts to triiodide and iodide. The last step is the reduction of triiodide back to iodide at the cathode interface (reaction 7) . O ther redox mediators have been applied in DSSCs as well , such as Br /Br 3 106 and Cobalt complex . 107 Figure 1 29 . Regeneration of d yes and r edox c ouples Reaction 4, 5 and 6: Charge Transport and Recombination . A fter injection from excited dye molecules to the conduction band of TiO 2 , charges will travel through the network of TiO 2 particles to the outside load (reaction 4). The charge density is higher at the surface of TiO 2 films compared to the bulk conducting substrate and c harge transport occurs by diffusion, driven by the charge density. Once charges reach the conducting substrate, the current can be detected. But, there are traps located inside the bulk TiO 2 parti cles, at the grain boundaries and at the interface of TiO 2 and electrolytes , which reduce the charge collection efficiency.
56 Moreover, charge recombination with either oxidized dye or a redox couple is another process which reduces charge collection efficiency. There are still debates on the driving force of recomb ination. Some results show that, like charge transport, charge recombination is also controlled by diffusion; i.e., the collision of electrons with oxidized dyes or redox molecules. 108 However, there are also studies showing that recombi nation kinetic s lies in the inverted Marcus region , smaller than the the recombination rate (Equation 1 7). 109 Redox couples have been found to have a huge impact on charge recombination in DSSCs. One way to characterize recombi nation is to determine the life time of electrons which can be obtained using transient absorption technique . I t has be en found that the electron life time in a system with iodide/triiodide is longer compared to most other systems which makes iodide/triiodide a very successful redox couple in DSSCs. 110 Solar c ell c haracterization There are several common parameters to characterize cell performance. The i ncident photon to curr ent efficiency (IPCE) indicates the efficiency of a device converting photons to electri city at a certain wavelength, which is given by: Where I is the photocurrent measured in A/m 2 , P is the in cid ent light power with unit of W/m 2 it is import ant to note that the actual number of photons absorbed is almost impossible to measure due to light scattering and transmission in the device . So the number calculated in the above equation represents the lower limit.
57 The p hotocurrent density voltage beha vior (J V) is another important criterion to characteriz e solar cells (Figure 1 30 ). Short circuit current density (J sc ) is the current for unit area under short circuit condition. The o pen circuit potential (V oc ) is the maximum potential that can be obtai n ed when a cell is under open circuit condition s . It is also the energy difference between the Fermi level of the semiconductor electrode under working condition s and the Nernst potential of the redox couple. The f ill factor (FF) is the ratio of the maximu m output , J opt V opt , to J sc V oc (FF=(J opt V opt )/(J sc V oc )) and is affected by many factors, such as is the ratio of the maximum output to the J opt V opt )/P in ). Figure 1 30 . A typical DSSC J V curve . Figure was reprinted from Huang et al. 111 Copyright 2007 Bentham Science Publishers Ltd. Materials u sed in DSSCs Metal Oxide Anode . The us e of mesoporous TiO 2 as anode is one of the most important reasons for the high efficiency of DSSCs. In nature, there are three types of TiO 2 crystals: rutile, anatase and brookite. Although rutile is the most thermodynamically st able form, anatase is the most used structure in DSSCs , due to larger bandgap and higher conduction band edge, E c , which increases the Fermi energy level and V oc . In
58 DSSC application s , mesoporous TiO 2 is used instead of single crystals , because mesoporous materials have larger surf ace area , which increases the amount of dyes absorbed. For better cell performance, many improvements have been made on the architectures of the TiO 2 layer . A typical high performance DSSC device usually employs multiple TiO 2 layers , and each layer has a d ifferent thickness and the size of TiO 2 in each layer may vary . First, a blocking layer (~50 nm thick) is coated on the TCO substrate to prevent direct contact of the redox couple with the substrate and reduce charge recombination. Second, an active dye ab sorbing layer (~10 20 Âµm thick) is deposited onto of the blocking layer. This layer utilizes mesoporous TiO 2 with diameter of ~20 nm , and offers a large surface area for efficient dye uptake . Third, a light scattering layer (~3 Âµm thick) consisting of ~400 nm TiO 2 particles is deposited to give effective scattering and to increase the chance of incident light being ha rvested by dyes. Last, the entire structure is treated with aqueous TiCl 4 to obtain an ultrapure TiO 2 shell coating , which increase electron l ifetime and lower the energy barrier for charge injection. In addtion , TiO 2 nanostructures with well defined morphology , such as nanorods, nanotubes and nanowires, have been developed to improve the charge transport efficiency . 112 The highly oriented nanostructures are expected to better allow electrons to reach the TCO e lectrode surface. Some other materials are also used as DSSC electrode materials, such as zinc oxide (ZnO), tin ( II ) dioxide (SnO 2 ) and niobium pentoxide (Nb 2 O 5 ). Zinc oxide was initially used in DSSC and has gained significant attention re cently. Its bandgap and conduction band edge are similar to those of TiO 2
59 and electron mobility is higher . However , the chemical stability hinders its application, since ZnO decomposes under both acidic and basic condition. Figure 1 3 1 . Examples of TiO 2 nanostructures. A) nano tubes, B) nano particles, C) nano rods, D) nano wires. Figure was reprinted from Chen et al. 112 Copyright 2007 American Chemical Society. Dyes . Development of novel dyes ha s been crucial in achieving high performance DSSC and the number of publications concerning this topic increases each year. Th e characteristics of ideal dyes should meet several requirements: (1) D yes should have high molar extinction coefficiency and broad absorption in the visible and near infrared (NIR) region to ensure efficient photon harvesting. (2) T hey have to be able to bind strongly with the metal oxide electrode and usually th is is achieved by functionalizing the molecule s with anchoring groups, such as CO 2 H, H 2 PO 3 or CN . (3) D yes must have suitable energy level s : the excited state energy level should be higher
60 than the conduction band edge of electrodes for electron injection purpose s , and the reduction potential of the oxidized dye has to be more positive than that of the redox couple for dye regeneration. (4) T he dyes need to be both photochemically and thermally s table for long term application s . (5) T he binding and aggregation behavior s also need to be optimized to boost cell performance. O rganometallic compounds , especially ruthenium (Ru( II )) complexes (Figure 1 32) , have found great success in DSSC application s due to their superb properties: broad absorption spect ra, high extinction coefficient , suitable energy level s , long lived excited state, fast electron injection and good stability. The light harvesting properties are largely attributed to the absorption in the visible region due to a metal to ligand charge transfer (MLCT). Therefore, the electronic interaction between d orbitals of the central metal and * ligands is the key to tune the absorption spectra. Ruthenium compounds are 6 coordinated , which means the MLCT absorption can be bonding donation. 113 Anchoring groups are necessary to ensure strong binding of dyes onto the TiO 2 surface and efficient charge injection. Figure 1 3 2 . E xamples of Ruthenium dyes. Compared to organometallic compounds, pure organic dyes have many advantages and are emerging as a class of competing materials for DSSC
61 applications. 111 In general, organic dyes have higher molar e xtinction coefficient than metal complexes , leading to increased light h arvesting efficie ncy and reducing the amount of dyes needed in the cell. In addition, the donor acceptor structure can be easily modified and the absorption spectra are easily tuned. With the development of computational chemistry, the photophysical properties and energy l evels of new dyes can be more accurately predicted. Furthermore , the cost of organic dyes is less than that o f organometallics and the supply is abundant. One approach for dye design is to construct donor acceptor (D A) structures and the charge transfer f rom donor to acceptor accounts for the absorption in the long wavelength region. The absorption spectra can be tuned by changing the relative strength of the donor/acceptor (i.e . stronger D A interaction will lead to more red shifted absorption). Tradition ally, organic dyes have the disadvantage of having sharp absorption peaks instead of a broad ab sorption across a large spectral region , decreasing light absorbing ability. However, upon careful design of the D A structure, black dyes which have strong abso rption across the entire visible region can be synthesized . 114 , 115 C o absorbing is another approach to overcome the shortcoming s of organic dyes. M ixtures of two or more dyes with complementary absorption spectr a are co a d sorbed onto TiO 2 , thereby broadening the overall absorption spectrum . 116 Electrolyte Solution . A typical electrolyte solution is made of a solvent, redox couples and additives. Iodide/triiodide was the first redox couple used in DSSC s and is still the most common redox couple. Iodide/triiodid e couple has a suitable redox potential, high diffusion co efficiency and can undergo fast redox reaction. Research on this combination is mainly focused on the effects of different cation on cell performance.
62 For example, it has been found that the V oc in creases with cation size in the alkali metal family: Li +
63 example, 4 tert butylpyridine (TBT) has been widely used in DSSC s which usually increases V oc significantly. Some investigation showed that TBP suppressed the dark current on the TiO 2 surface and improved the photovoltage , 119 while others claimed that TBP with the presence of lithium cations can increase the band edge of the TiO 2 electrode. 120 Scope of Present Study The purposes of present study are to design and synthesize variable chain length poly(phenylene e thynylene)s with end caps, investigate their aggregation properties in solution and ultrafast energy/electron transfer and apply these materials in dye sensitized solar cells. Chapter 2 describes the synthesis of two families of conjugated polyelectrolytes (CPEs) featuring the same backbone but different side chain linkages and demonstrates their application as active materials for dye sensitized solar cells (DSSCs ) . It is found that CPEs bearing an oxygen linkage ( O ) are more likely to form aggregates in solution , and that the aggregation status depends strongly on the molecular weights. In contrast, there is no obvious evidence showing that CPEs with methylene ( CH 2 ) linkage s aggregate in solution. In addition, the two families of polymer s show different trend s in ad sorbing onto mesoporous TiO 2 films and different overall cell efficiencies . The oxygen linkage family shows strong ch ain length dependence on film a d sorption and, therefore, the cell performance. In contrast, little difference can be observed in the methylene linkage family. In Chapter 3, a series of different chain length poly(p phenylene ethynylene)s (PPEs) with naphthalene diimid e derivative end ca ps were synthesized. When polymers are photo excited, the electron will transfer from the PPE backbones to naphthalene
64 diimid e , thereby quenching the polymer fluorescence. The overall quenching efficiency increases with decreasing polymer chain length , as e videnced by fluorescence quantum yield measurement. And the charge recombination rate was investigated by ultra fast transient absorption spectroscopy . The charge recombination rate also depends strongly on the chain length: as the chain length increases, the charge recombination rate decreases. In Chapter 4, a series of different chain length poly(p phenylene ethynylene)s (PPEs) with 4,7 di(thiophen 2 yl)benzo[c][1,2,5]thiadiazole (TBT) end caps were synthesized. Under light irrad iation, energy transfer fr om the PPE backbone to the TBT end groups occurs via the Forster Resonance Energy Transfer (FRET) mechanism. The overall energy transfer efficiency increases with decreasing molecular weight while the fluorescence lifetime remains almost the same. Ultra fa st transient absorption study shows that the energy transfer happens in pico second time scale for all the polymers and the lower molecular weight samples show faster decay in the initial stage. In Chapter 5, a series spa polymers with [2.2]paracylophane moiety end caped with TBT were synthesized. Despite the fact that the conjugation is interrupted, very efficient energy transfer can still be observed and the overall energy transfer efficiency also depends o n the molecular weight. Fluorescence lifetime study reveals that there is a rising time in the energy transfer process for the sample with longest chain length , which indicates there is distance limitation on exciton hopping.
65 CHAPTER 2 CONJUGATED POLYELE CTROLYTE SENSITIZED TIO 2 SOLAR CELL S : CHAIN LENGTH AND AGGREGATION EFFECTS ON EFFICIENCY Background Dye sensitized solar cells (DSSCs) are one of the most important low cost alternatives to conventional silicon based inorganic solar cells. 101 , 121 Since the first report of the technology, it has drawn significant attention from the scientific community. 122 A great d eal of effort has been made to understand the fundamental problems, improve the overall cell efficiency , and explore the possibi lity for commercialization. And there are several advantages of the DSSC format compared to the traditional solar cells , which i nclude the use of low cost materials, mechanical flexibility and possibility of large scale manufacture. Originally, metal organics , in particular ruthenium and metal porphyrin complexes, have been utilized as the sensitizers and demonstrated power convers ion efficiency (PCE) as high as 13% and incident photon to current efficiency (IPCE) higher than 85%. 123 Meanwhile, donor bridge acceptor (D A) type organic dyes are emerging as a new class of sensitizers and upon careful structure design, they can also attain efficiencies matching that of inorganic dyes. 124 , 125 More recently, highly effective hybrid perovskite cells have been made, achieving efficiency as high as 19.3 %. 126 Conjugated polyelectrolytes (CPEs) are conjugated polymers with ionic pendant chains. Some of the key advantages of CPEs a re strong absorption in the visible region, tunable bandgap, and processability from green solvents . 90 , 92 Despite the advantages , the application of CPEs as sensitizers in DSSCs has not been well studied. DSSCs. 127 130 In most cases, the work has been focused on the design of novel D A
66 structure CPEs, which are believed to facilitate intramolecular charge transfer (ICT) from d onor to acceptor resulting in lower bandgap. Such polymers show broad absorption and large extinction coefficients, both of which are crucial to efficient light harvesting. However, recently both Schanze and Ramakrishna groups noticed that polymer chain le ngth can also have a significant impact on the overall cell performance when CPEs were used as the sensitizers for DSSCs . 131 , 132 The authors attributed the decreasing of cell efficiency of large molecular weight polymers to the fact they have larger size which reduces the penetration ability into the TiO 2 layers and total amount of sensitizers absorbed. Very simil ar phenomenon has also been found in case of dendrimers where overall cell efficiency decreases with increasing dendrimer size. 133 , 134 In this work, we study the relationship between overall cell efficiency and molecular weight of two series of CPEs which feature the same conjugated backbone with alternating (1,4 phenylene) and (2,5 thienylene ethynylene) repea ting units, but different linkages between the backbone and carboxylic side chains, namely oxy methylene ( O CH 2 ) (P1 O n) and methylene ( CH 2 ) (P2 C n) , respectively. In addition, a model compound was synthesized to compare its properties with the polym ers. The polymer backbone structure was chosen mainly due to the ease of synthesizing building blocks and convenience of controlling molecular weight via the end capping strategy. The carboxylic side chains help the polymer s a d sorb onto the surface of TiO 2 films and ensure that polymers are in close proximity to the TiO 2 film , which facilitate s charge injection. Steady state absorption and emission spectra of polymers were taken to compare photophysical properties and dynamic light scattering was applied to investigate polymer aggregation in solution. When used in D SSCs, the efficiencies of
67 P1 O series showed a strong dependence on molecular weight , while that of P2 C series did not change regardless of chain length. The result s showed here clearly demonstra ted that slight change in side chain could have a huge impact on the aggregation behaviors of polymers and solar cell performance. Results and Discussion Synthesis In this study, two families of polymers, P1 O and P2 C, were synthesized which feature the same conjugated backbone with alternating (1,4 phenylene) and (2,5 thienylene ethynylene) repeating units, but have different linkages between the backbone and side chains, namely oxy methylene ( O CH 2 ) and methylene ( CH 2 ) , respectively. For comparison, a model oligomer which has similar structure to the polymers was also prepared. The structures of the polymers and oligomer are shown in Figure 2 1 . All the samples feature carboxylic acid side groups which help improve polymer solubility in solution and serve as anchoring groups for film adsorption. Figure 2 1 . Structure of P1 O n , P2 C n and the Model compound
68 Figure 2 2 . Synthesis of polymers. were protected as esters. The ester protected polymers were hydr olyzed and then acidified to obtain the carboxylic acid substituted polymers. Due the fact that 2,5 diethynyl thiophene is not stable, 2,5 bis((trimethyls ilyl)ethynyl)thiophene was used in the polymerization reaction and trimethylsilyl groups were deprotected in situ with t etrabutylammonium difluorotriphenylsilicate (TBAT). Polymerization between 3 and 1 or 2 afforded polymer P1 O DP n and P2 C DP n , respec tively. The p olymer chain length adding mono functioned end cap to the reaction. 60 , 62 Once end caps react with polymer chains ,
69 polymerization process is terminated . Polymer molecular weight decreases with increasing m olar ratio of end cap s in the reaction. The molecular weights and degree of polymerization (DP) of polymers were chara cterized by GPC using dodecyl ester protected polymer precursors. Figure 2 3 . GPC analysis of polymers . A) P1 O n ester n: P1 O ester 7 (black square, M n =5000, PDI=1.55), P1 O ester 9 (red circle, M n =6600, PDI=1.53), P1 O ester 14 (blue triangle, M n =990 0, PD I =1.74); B) P2 C n ester : P2 C 7 ester (black square, M n =5100, PDI=1.87), P2 C 12 ester (red circle, M n =8400, PDI=2.00), P2 C 18 ester (blue triangle, M n =13000, PDI=1.98). Proton NMR is also used to calcula te the number of repeating unit s and characte rize the molecular weight. C hemical shift of the aromatic protons of 1 iodo 4 (trifluoromethyl) benzene appear at around 7.6 4 ppm while the thiophene aromatic protons are at around 6.96 ppm in P1 O n esters polymers. Chemical shifts of tert butyl protons a re at around 1.45 ppm while the methylene protons are at around 1.25 ppm in P2 C n ester polymers. Proton signal integrations are used to calculate the molar ratio of functional groups and the number of repeating unit s is derived from the calculation . The results obtained by both methods are quite comparable and listed in Table 2 1.
70 Figure 2 4 . 1 H NMR characterization of polymers . A ) 1 H NMR of P1 O n ester at the aromatic region; B ) 1 H NMR of P2 C n ester at the alkane region. Table 2 1 . Molecular weight table of P1 O, P2 C and Model compound Mn( g/mol ) a Mw( g/mol ) a PDI DP(GPC) b DP(NMR) c P1 O 7 ester 5000 7700 1.55 7 6 P1 O 9 ester 6600 10200 1.53 9 8 P1 O 14 ester 9900 17200 1.74 14 13 Model ester 866 866 \ \ \ P2 C 7 ester 5100 9500 1.87 7 7 P2 C 1 2 ester 8400 17000 2.00 12 11 P2 C 18 ester 13000 26000 1.98 18 17 a Narrow dispersed polystyrenes were used as the standard. b Degree of polymerization was calculated using Mn obtained by GPC . c Degree of polymerization was calculated using signal inten sity integration ratio from 1 H NMR. The model compound was synthesized in a stepwise route (Figure 2 5 ) . Reaction of compound 4 with trimethylsilyl acetylene (TMSA), followed by deprotection, yielded compound 5 . And, compound 5 was reacted with 2,4 diiod othiophene to give compound 6 , which was used to couple with compound 7 to get compound 8 (ester protected model compound, Model ester). Finally, compound 8 was hydrolyzed under basic condition and acidified to afford the model compound.
71 Figure 2 5 . Synthesis scheme of model compound. DLS Characterization Dynamic Light Scattering (DLS) has been widely used for the characterization of polyelectrolyte samples. 135 , 136 In this work, DLS measurements were performed to study the size of the polymer chains in DMF and the concentration was set at 0.1 mg/ m L which was the same used for adsorption on TiO 2 films . As shown in Figure 2 6 , the model compound has smaller size (about 2 nm) compared to polymer samples. And, the sizes of the P2 C series remain fairly the same (about 7 nm). It is well known that the radius of gyration of polyelectrolytes stays constant in a wide range of different molecular weight which explains the trend for the P2 C series. 137 , 138 In contrast, the particle size of P1 O increases (from ~6 nm for P1 O 7, ~12 nm for P1 O 9, to ~13 nm for P1 O 14) with increasing molecular weight. The increasing particle size for P1 O
72 series might be caused by aggregation. Higher molecular weight po lymers are more likely to form bigger aggregate s , therefore, the average polymer size increases significantly as well. 62 Figure 2 6 . DLS characterization of samples in DMF ( solution concentration= 0.1 mg/ml ). Previous work from our lab show that despite the similarities in the ba ckbone structures, the side chain linkages can make a difference in the aggregation state of polymers in solution. 21 , 139 , 140 While polymers with O CH 2 linkages aggregate in solution, polymers which have CH 2 linkage show surprisingly reduced tendency of aggregati on in solution. The difference might be caused by the electronic effect of the oxygen atoms or the oxygen substituents being able to stabilize the stacked polymer chains. 139 It has been reported that P2 C series adopt ellipsoidal conforma tion instead of rigid worm like chains in solution. 21 O ur previous AFM results show ed that the diameter of the ellipsoidal polymer chain is about 47 Â±3 Ã… . The DLS particle size of the P2 C series is in good agreement with previous atomic force microscopy ( AFM ) result (7 nm vs 5 nm). Therefore, P2 C series are more like to be monomeric in solution while the P1 O series are aggregated. Although the end caps are different in the two series of polymers, they do not affect the aggregation status. 62 , 139 , 141
73 Optical Properties in Solution Steady state UV visibl e absorption and emission spectra of the samples were measured in dimethylformamide ( DMF ) (Figure 2 7 ), and the concentration of the samples was adjusted to 50 Âµg/ml (based on repeat units) . Molar extinction coefficiency and fluorescence quantum yield are listed in T able 2 2. As shown in Figure 2 7 a and c, the absorption spectra are similar for both series and the absorption maximum is red shifted as the molecular weight increases, because of the increasing in the conjugation length (424 nm, 425 nm and 432 nm for P1 O 7, P1 O 9, P1 O 14, respectively; 388 nm, 398 nm and 407 nm for P2 C 7, P2 C 12 and P2 C 18, respectively). The absorption spectra of the P1 O series red shift about 25 nm compared to the P2 C series, due to the fact that oxy methylene is a st ronger donor than the methylene group , which increases the HOMO energy level . Finally, the absorption maximum of the model compound is blue shifted about 20 nm compared to P1 O series, due the significant decrease in conjugation length. Table 2 2. GPC resu lts and photophysical data Mn a PDI DP abs (nm) 4 cm 1 M 1 ) em (nm) fl b P1 O 7 5000 1.6 7 424 3.1 528 0.14 P1 O 9 6600 1.5 9 425 3.5 531 0.091 P1 O 14 9900 1.7 14 432 3.8 542 0.071 Model 866 \ \ 408 9.9 470 0.39 P 2 C 7 5100 1.9 7 388 5.1 456 0.12 P2 C 12 8400 2.0 12 398 5.5 456 0.12 P2 C 18 13000 2.0 18 407 5.9 457 0.13 a narrow dispersed polystyrene was used as the standard. b measurement was done in pH=8 water with the P1 O n salt and P2 C n salt samples, and Quinine Sulfate in 0.1 M H 2 SO 4 fl =0.545.
74 Figure 2 7 . The ground state absorption and emission spectra. A ) UV vis absorption and B ) emission spectra of P1 O 7 (black square), P1 O 9 (red circle), P1 O 14 (blue up triangle) and model compound ( dark cyan down triangle) ; C ) UV vis absorption and D ) emission spectra of P2 C 7 (black square), P2 C 12 (red circle) and P2 C 18 (blue triangle) . Figure 2 7 b and d show the fluorescence emission of P1 O and P2 C series respectively. Despite the increasing chain length, the emission peak shows only a slight change for the P 2 C series with emission maximum at around 456 nm. In comparison, the emission maximum of P1 O 14 is red shifted 10 nm compared to P1 O 9 which indicates the formation of aggregates of P1 O 14. In addition, P2 C series show more structured emission compare d to the P1 O series; this should also be due to the fact that P2 C is more likely to be monomeric in solution. 142 Moreover, the emission intensity of the vibrational band of the P2 Cs increases with molecular weight. Finally, the model compound has more blue shifted and narrower emission band compared to P1 O n
75 polymers . The aggregation status has a significant effect on the overall fluorescence quantum yields as well. The fluorescence quantum yields of P1 O n polymers decrease with increasing molecular weight. The quantum yield of P1 O 7 is as high as 14% while that of P1 O 14 decreases to 7.1%. In contrast, P2 C n polymers show almost the same quantum yield (~12 13%) regardless the molecular weights and the model has a quantum yield of 39%. TiO 2 Film Characterization , Polymer Adsorption and Charge In jection The detailed information for making nanocrystalline TiO 2 colloids and DSSC cell fabrication are provided in the experimental section. It is worth noting that in most literature reports , the size of TiO 2 particle used varies from 20 nm to 30 nm to a chieve high efficiency and most researchers have applied a three layer TiO 2 film: a blocking layer, an active layer and a scattering layer. However, it was not our goal to produce high performance cells, only a single layer TiO 2 was used to facilitate phot ophysical characterization of film s . Because, t he single layer film is more transparent than multi layer films and makes UV vis absorption and transient absorption measurements easier . The TiO 2 particle ha s an average size of 20 nm and the film thickness i s around 13 Âµm, according to the scanning electron microscope (SEM) images (Figure 2 8) . Figure 2 8. SEM characterization of TiO 2 films. A) Top view of the film and B) side view of the film intersection.
76 UV visible absorption spectroscopy was used to mon itor the adsorption of the P1 O n and P2 C n polymers. All the bare TiO 2 films have little absorption beyond 380 nm due to light scattering and all the films have very similar thickness according to SEM images. For film adsorption, a solution of 0.1 mg/mL of polymer in DMF was stirred for 24 h before the TiO 2 electrodes were immersed into it and soaked for 36 h. The UV vis absorption of the result ing polymer coated film was measured and the film absorptance, ( which can be calculated as absorptance = 1 10 A , where A is the film absorption ) , was plotted in Figure 2 9. he peak absorptance of P1 O n sensitized films increases with decreasing molecular weight while the P2 C n sensitized films show almost identical peak absorptance. Fig ure 2 9. Absorptance of polymer sensitized films. A) Absorptance of P1 O n films and the time dependent absorp tion (inserted plot): P1 O 7 (black square), P1 O 9 (red circle) and P1 O 14 (blue triangle); B) Absorptance of P2 C n films: P2 C 7 (black square ), P2 C 12 (red circle) and P2 C 18 (blue triangle ). The absorptance is direct proportional to the amount of polymers adsorbed, because each polymer within a polymer series has very similar molar extinction coefficient and the amount of polymers adsorbed i s only fact that changes the absorptance. For the P1 O n polymers, lower molecular weight samples form smaller aggregates and are more
77 likely to penetrate deep into the TiO 2 films, which increase the amount of materials absorbed onto the TiO 2 films . 131 In contrast, the P2 C n polymers do not aggregate and have essential ly the same size in solution, therefore, show very similar adsorption behaviors. In addition, the absorptance curves resemble the absorption spectra, with higher molecular weight polymers show more red shifted curves. In order to further characterize the film absorption properties of P1 O n series, time dependent film absorption measurements were performed. TiO 2 films with same film thickness were contained in polymer solution wi th same concentration for various amounts of time and the film absorption was measured. The absorption values at 440 nm were recorded and the calculated absorptance was plotted in Figure 2 9, inset figure . Absorpt ance of all films increases with time with the first 15 h showing the most significant enhancement. Then, it reaches a plateau after 30 h , indicating that the a d sorption is almost saturated and the final absorp tance is consistent with film absor p tance data. In addition, at any given time, absorptio n intensity increases with decreasing molecular weight (P1 O 7 > P1 O 9 > P1 O 14). Figure 2 10. Calculated surface coverage of polymer sensitized films .
78 The amount of polymers adsorbed can also be characterized by surface coverage ( , based on polymer repeat units), which can be calculated as: where A is the film absorption and is the molar extinction coefficient. In the calculation, there are two assumptions upon polymer adsorption onto TiO 2 films: 1) the polymer molar extinction coefficient does not change and 2) the changing of UV vis absorption spectrum of polymer is negligible. The unit of 2 /mol and has the unit of mol/cm 2 . The P1 O 7 has the largest surface coverage in the P1 O n polymers whil e the P2 C n polymers are almost the same, which is consistent with the film absorptance results. The amount of polymers adsorbed is about 1.5 x 10 1 6 cm 2 , which is lower compared to small metal organic or organic dyes (5~20 x 10 16 cm 2 ). 133 , 134 , 143 Figure 2 1 1 . AFM images of dye sensitized TiO 2 films. AFM ima ges of A) P2 C 18 sensitized TiO 2 film; B) P1 0 14 sensitized TiO 2 film; C) bare TiO 2 film . Root mean square values ( RMS ) of P2 C 18, P1 0 14, and bare TiO 2 are 18.6 nm, 20.5 nm and 17.4 nm, respectively . The surface morphology of TiO 2 films was characteri zed by AFM (Figure 2 1 1 ). Upon dye deposition on top of TiO 2 films, morphology will change which can be characterized by the root mean square (RMS) roughness of the surface. Compared to
79 the RMS value of bare TiO 2 (17.4 nm), that of P2 C 18 sensitized film increased slightly (from 17.4 nm to 18.6 nm), indicating that P2 C 18 polymers successfully absorbed onto the film and there is no obvious aggregation on the film surface. However, the RMS value of film sensitized with P1 O 14 (20.5 nm) is significantly hi gher than that of bare TiO 2 which was caused by adsorbing of aggregated polymer particles. It has been reported that dye aggregation reduced electron lifetime and lowered overall cell performance. 144 , 145 Considering the fact that P2 O n series show lower absorptance which means fewer materials absorbed (Figure 2 s proposed that the aggregated P1 O n polymers will cover the film surface and prevent more materials from film morphology and affect the total amount of materials can be absorbed which ultimately affect the cell performance significantly. Figure 2 1 2 . Film transient absorption and kinetics of dye sensitized TiO 2 films. A ) Transient Absorption spectra after initial 15 ns camera delay of P1 O 7 (black square), P1 O 9 ( red circle), P1 O 14 (blue up triangle) and model compound (dark cyan down triangle) sensitized films ; B ) kinetic decay of P1 O DP and Model compound sensitized TiO 2 films : P1 O 7 (black), P1 O 9 (red), P1 O 14 (blue) and model compound (dark cyan) .
80 Film t ransient a bsorption (TA) measurements (Figure 2 1 2 ) were performed on P1 O n sensitized films to have insight information on charge injection and regeneration . Polymer sensitized films were contained in sealed cuvettes containing 0.1 M LiClO 4 in acetonitri le and degassed for 30 min before measurements. Immediately after subjection to 355 nm laser excitation pulses, strong transient absorption of polymer radical cation was observed. Polymers became radical cations after losing electrons due to charge injecti on to TiO 2 conducting bands . The spectra of all investigated films are very similar with the radical cation signal rises from approximately 500 nm and the peak is around 675 nm. The signal intensity of P1 O polymers decreases with molecular weight, which i s consistent with the trend of the film absorptance. Charge recombination occurs between the polymer cations and the TiO 2 conducting band electrons and the signal decay kinetics were measured at 650 nm and plotted in Figure 2 1 2 B. The lifetime of polymers is about several hundred microseconds , which is generally shorter than organic metal complexes. 146 , 147 Howeve r, a strong electronic coupling between the polymer sensitizers and the TiO 2 conducting band can facilitate the ultrafast electron injection. 148 All the sensitizers have very similar averag O 14 which has the largest aggregation size (Table 2 3) . The decreasing lifetime of the excited electrons can decrease the overall cell performance . 149 Table 2 3. Calculated transient absorption lifetime a A 1 1 A 2 2 A 3 3 ave P1 O 7 0.49 13 0.32 139 0.17 977 217 P1 O 9 0.50 32 0.26 817 \ \ 228 P1 O 14 0.58 21 0.23 570 \ \ 143 Model 0.55 6 0.30 80 0.19 927 203 a ave was calculated usi ng .
81 Polymer Sensitized Solar Cells The scope of this work is to understand the effect s of polymer molecular weight and aggregation on DSSCs performance rather than achieving high cell efficiency, therefore, all the cells were fabricated and characterized under the same condition without seeking to optimize overall performance (varying electrolyte, etc.) . The active area s of cells are 0.2 cm 2 . Three different batches of devices were prepared for each sample and the results were consistent for all. Figure 2 1 3 . IPCE and current voltage (J V) characters of polymer cells. A) IPCE and B) J V curves of P1 O n and model compound: P1 O 7 (black square), P1 O 9 (red circle), P1 O 14 (blue up triangle) and model compound (magenta down triangle); C) IPCE and D) J V curves of P2 C n: P2 C 7 (black square), P2 C 12 (red circle) and P2 C 18 (blue triangle).
82 I ncident photon to current efficiency (IPCE) of both P1 O n and P2 C n series were plotted in Figure 2 1 3 . A significant difference in the phot ovoltaic performance is observed for the P1 O series while the P2 C n polymers show little change despite the difference in molecular weight (Figure 2 11 A) . In the P1 O n polymers , P1 O 7, which has the lowest molecular weight polymer , shows the highest p eak IPCE value (~50%) and the IPCE value decreases with increasing molecular weight . The IPCE of P1 O 14 is only about half of P1 O 7 (~25%) , indicating that the charge injection efficiency will also be lower . In contrast, all P2 C n polymers show very hig h peak IPCE (~48%), indicating that photoinduced charge injection will be quite efficient at short circuit conditions. The trend in peak IPCE value is consistent with the film absorptance ( F igure 2 9 ). The difference in current density of devices is also d ue to change in total amount of polymers adsorbed. Although, the model compound shows hi gh est peak IPCE value, the IPCE response region is narrower. The same trend can be observed in the J V cu rve of the P1 O series (Figure 2 1 3 B & D ), where P1 O 7 show s the highest open circuit voltage value ( V oc , 0.49 V ) and short circuit current (J sc , 2.70 mA/cm 2 ), and P1 O 14 has the lowest V oc (0.45 V ) and J sc (1.56 mA/cm 2 ) . This may also be caused by polymer aggregation which affect s surface coverage: less aggregat ed samples can better cover the TiO 2 surface which will reduce the charge recombination rate between electrons in the conducting band of TiO 2 and I /I 3 electrolyte s or dye cations at the interface of TiO 2 particle, and increase the open circuit voltage an d short circuit current . 150 , 151 The P2 C n polymers all have the same V oc and J sc , but have decrease d V oc and J sc compared to P1 O 7 , which are probably due to lower LUMO energy level and narrower photo response region , respectively. 123
83 A lthough the model compound does not aggregate, the surface coverage is less efficient than P1 O n polymers and a s lightly lower V oc is observed. The absorbed light to photon to current efficiency was calculated and the trend is also consistent with film absorption. Table 2 4 . Summary of cell performance a V oc (V) J sc (mA/cm 2 ) FF(%) IPCE max (%) APCE max (%) b cell (%) P1 O 7 0.49 2.70 55.8 4 6 .0 99.2 0.74 P1 O 9 0.48 2.05 58.3 40.6 8 8.8 0.57 P1 O 14 0.45 1.56 57.7 24.4 45.2 0.40 Model 0.44 2.26 57.4 62.6 N.A. 0.57 PC C 7 0.41 2.32 49.9 47.4 73.0 0.47 P2 C 12 0.41 2.26 52.1 47.6 71.8 0.48 P2 C 18 0.40 2.39 50.1 47.4 65.2 0.49 a Three cells for each polymer were made and the all t he number reported were the average values ; b was calculated using . Summary Two series of conjugated polyelectrolytes featuring the same alternating (1,4 phenylene) and (2,5 thienylene ethynylene) repeating units, but with different molecular weight, were synthesized and u tilized as light harvesting materials in DSSCs. DLS and steady state photophysical studies showed that P1 O series which had oxy methylene ( O CH 2 ) linkage between the backbone and carboxylic acid group were aggregated in solution while P2 C series which had methylene ( CH 2 ) instead were in the monomeric state in solution. The aggregation state determines the film morphology and affect s the total amount of materials can be absorbed . As a result, the cell efficiency of P1 O series decreased with increasing molecular weight while cell performance of P2 C series was independent of molecular weight. Although, the fact causing the difference between alkyl and oxygen substituents are still not very clear, the results still provide us with
84 guidance of designing m olecular structure and controlling molecular weight in achieving high efficiency DSSCs. Experiments and Materials Materials Unless specified, all compounds and solvents were purchased from commercial sources (Aldrich, Acros, Strem Chemicals, et al) and use d without further purification. For all palladium catalyzed reactions, the solvents were carefully degassed with argon for at least 30 min. 1 H and 13 C NMR spectra were recorded on either Inova2 (500 MHz) or Varian Gemini 300 spectrometer (300 MHz). The che parts per million (ppm) using the residual solvent signals as internal standards. Instrumentation 1 H and 13 C NMR spectra were measured on Varian Mercury 300, Gemini 300, or Inova 500 spectrometers . Chemical shifts were re ferenced to the residual solvent peaks. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclotron Resonance mass spectrometer (Bruker Daltonics, MA). Gel Permeation Chromatography (GPC) data was collected on a system composed of a Shimadzu LC 6D pump, an Agilent mixed D column, and a Shimadzu SPD 20A photodiode array (PDA) detector, with was calibrated against linear narrow dispersed polystyrene standards in THF. 1800 dual beam absorption spectrophotometer. Photoluminescence spectra were recorded on a spectrofluorimeter from Photon Technology International (PTI). P hoto luminescence lifetimes were obtained by time correlated single photon counting
85 (TCSPC) using a Fluo Time 100 (Picoquant), and excitation was provided using a P DL 800 B Picosecond Pulsed Diode Laser (375 nm). Dynamic light scattering (DLS) characterization was performed on a Zetasizer Nano (Malvern Instruments, Worcestershire, United Kingdom) at 25Â°C. The concentration of the samples was adjusted to 0.1 mg/ml fo r this measurement. Three measurement cycles were run for each sample. The data were averaged from 10 light scattering periods of 10 s for each cycle. Average diameter values were calculated using the Malvern Instruments DTS software. Film transient absorp tion measurements were conducted on a home built apparatus. The excitation wavelength was generated by a Continuum Surelite OPO Plus pumped with the third harmonic (355 nm) of a Continuum Surelite II 10 Nd:YAG laser. Xenon arc lamp was used as a probe sour ce. Triax 180 Monochromator and Si amplified photodetector from Thorlabs (PDA8A) were used for detection at single wavelength. Films were merged in sealed cuvettes containing 0.1 M LiClO 4 in acetonitrile and degas s ed with Ar for 30 min before measurements. Kinetic trace was measured at a wavelength of 650 nm. Exponential decay 2 or 3 was used for fitting ( or ) and the average lifetime was calculated as . AFM images of polymer coated TiO 2 films and bare TiO 2 films were obtained using Digital Instruments (Vecco) in ambient atmosphere equipped with Nanoscope controller (Bruker ) . Tapping mode by an integrated tip/cantilever ( 125 Âµ m in length with 325 kHz resonant frequency, Mikromasch USA) was used at a sca n rate of 1 .2 Âµ m/s
86 and 0.1 Âµ m/s with a range of 3 Âµ m Ã— 3 Âµ m and 0.5 Âµ m Ã— 0.5 Âµ m, respectively, at room temperature in air. TiO 2 Sol Preparation The nanocrystalline titanium dioxide (TiO 2 ) electrode and platinum counter ele c trode were prepared according to li terature with modifications . 152 Detailed information is pr ovided as below. Titanium isopropoxide ( 35.52 g) was added to a separatory funnel containing 10 mL isopropanol. The Ti(i OPr) 4 /isopropanol solution was then added dropwise to acetic acid/H 2 O (80 mL acetic acid, 250 mL deionized H 2 O) that had been pre chilled to 0 o C and was stirred rapidly . Ti(i OPr) 4 / isopropanol was added over a period of ~20 30 minutes . The reaction solution was then heated to 80 o C using and r apidly stirred the entire time. Upon heating to this temperature, the reaction solution becomes a thick gel and then later becomes a loos e, white colloidal solution. The reaction solution was heated at 80 o C for 8 hours and stirred the entire time. At the end of the 8 hours, the reaction solution was cooled to room temperature and ultrasonicated (70% power, pulse on for 1 sec, off for 0 .5 sec) for five minutes to break up TiO 2 aggregates. Upon ultrasonication, the colloidal solution becomes cl earer with hints of blue color. The total volume of the colloidal solution was measured at this time to determine the amount of TiO 2 per unit volum e . A portion of the colloidal solution was then placed in a Teflon cup which in turn was placed in an acid digestion bomb and autoclaved for 13 h at 420 o C (ramp rate = 5 o C/minute). After cooling the bomb to room temperature, the TiO 2 colloid (white soli d + colloidal solution) was transferred to a clean 100 mL beaker for ultrasonicating (5 minutes, see above conditions). The resulting white solution was then concentrate the TiO 2 to a final concentration of 12 wt%. For example, if 80 mL out of a total 370 mL TiO 2 colloid is autoclaved, it gives 2.16 g/80 mL
87 TiO 2 post autoclave. It is pr eferable to concentrate beyond then add deionized water to give a final weight . It is important to minimize the amount of TiO 2 way. It is also impor tant to avoid rotovapping to dryness or partial dryness (i.e. allowing a significant amount of TiO 2 to precipitate out of solution); this seems to significantly affect DSSC performance. After acquiring a 12 wt% colloidal solution, hydroxypropylcellulose (HPC) was added slowly over ~1 minute to a rapidly stirred solution. The HPC concentration is 6 wt% (with respect to total weight of solution) and 50 wt% (with respect to TiO 2 weight). The solution is stirred well for 12 24 hours and then stirred less rap idly for 3 5 days (stir 2 paste is stored in a brown amber vial until use and stirred continuously. The paste appears as a semi viscous white fluid. Device Fabri cation FTO substrates are cleaned by sonication in pH=13 water containg 2 wt% sodium dodecyl sulfate (SDS) , followed by isop rop anol and acetone (20 minutes each). For depositing thin films, tape thickness o f the fil m depends on the number of tape stripes used on either side of the exposed substrate surface . For example, two layers of tape stripes will give a TiO 2 film TiO 2 paste is applied to the substrate by pipette and spread usin g a razor blade. The thin film is allowed to air dry at room temperature for 10 20 minutes; the films should be crack free and fairly transparent. The films are then sintered by slowly heating the films at a rate of 5 o C/min to a final temperature of 450 o C and maintaining this temperature for 30 minutes. The fil ms are then cooled at a rate of 10 o C/min.
88 And, two holes were drilled at the Pt cathode. A solution of 0.1 mg/ml of polymer in DMF was stirred for 24 h before the TiO 2 electrodes were immersed into it. After 36 h of absorption, the electrodes were rinsed with dry DMF and acetone to remove the unabsorbed dye s , and then placed under vacuum for 2 h for furt her drying. The platinum cathode and TiO 2 anode were sealed together with surlyn (Solaronix Melto nix 1170 25). Electrolyte solution containing 0.05 M I 2 , 0.1 M LiI, 0.6 M 1 methyl 3 (n propyl)imidazolium iodide (PMII) , and 0.5 M 4 tert butylpyridine (TBP) in butyronitrile was injected into the sealed device from the holes on the platinum cathode . For incident photon to current efficiency (IPCE) characterization, a n Oriel Cornerstone monochromator was used as light source , and the device current response was recorded under short circuit conditions at 10 nm intervals using a Keithley 2400 source meter . T he light intensity at each wavelength was calibrated with an energy meter (S350, UDT Instruments). The current voltage characteristics of the cells were measured with a Keithley 2400 source meter under AM1.5 (100 mW/cm 2 ) solar simulator. The active area of the TiO 2 film is 0.5 cm * 0.5 cm for IPCE and current voltage measurements. Synthetic Procedures T ert butyl 2 (4 iodophenyl)acetate 140 and Compound 1, 141 2, 141 3, 139 4 153 and 7 141 were synthesized accroding to reported method in literature. Compound 5. Compound 4 (2.00 g, 6.90 mmol) was dissolved in a mixed solvent of THF (20 ml) and (i Pr) 2 NH (10 ml) in a round bottom flask and degased for 30 min. Then, Pd(PPh 3 ) 2 Cl 2 (28 mg, 0.4 mmol) and CuI (7.5 mg, 0.4 mmol) were added the flask and degassed for another 30 min. Afterwards , trimethylsilyacetylene (0.75 g, 7.65 mmol) was injected into the solution and rea cted at room temperature for
89 overnight. The solvent was evaporated and the residue was purified with a flash column. The eluent was evaporated under vac u um and the solid was dissolved in THF (10 ml) and TBAF (7 ml, 1 M in THF) was added to the solution. Af ter two hours, 10 ml water was added and the solution wa s extracted with dichloromethane. The organic layer was washed with saturated ammonium chloride, water and brine, and then dried over anhydrous sodium sulfate. The solvent was removed under vacuum and the crude product was purified by silica chromatography to yield compound 5 (1.17g, 90%). 1 H NMR (300 MHz, CDCl 3 2H), 7.46 (d, 2H). 13 C NMR (75 MHz, CDCl 3 3.55, 121.04, 129.42, 132.43, 135.07, 171.23. MS (ESI) m/z ([M + H] + ) , calculated 189.092; found 189.0910. Compound 6 . Compound 5 (1.00 g, 5.32 mmol) and 2, 5 diiodothiophene (1.79 g, 5.32 mmol) were dissolved in a mixed solvent of THF (15 ml) and (i Pr) 2 N H (10 ml) and degased for 30 min. Then, Pd(PPh 3 ) 2 Cl 2 (28 mg, 0.4 mmol)and CuI (7.5 mg, 0.4 mmol) were added and the reaction was run for 12 h under argon protection. After the reaction, the solvent was removed under vacuum and solid was purified with silic a chromatography to give compound 6 (0.93 g, 44%). 1 H NMR (300 MHz, CD 2 Cl 2 (t, 3H), 3.62 (s, 2H), 4.14 (q, 2H), 6.95 (d, 1H), 7.18 (d, 1H). 7.27 (d, 2H), 7.49 (d, 2H). 13 C NMR (75 MHz, CD 2 Cl 2 131.42, 133.30, 135.21, 137.21, 139.23, 172.82. MS (ESI) m/z ([M + NH 4 ] + ) , calculated 414.001; found 414.00 2 . Compound 8 . Compound 6 (0.50 g, 1.26 mmol) and compound 7 (0.39 g, 0.63 mmol) were added into flask containing THF (15 ml) and (i Pr) 2 NH (10 ml) and the
90 solution was degased for 30 min. Then, Pd(PPh 3 ) 2 Cl 2 (14 mg, 0.2 mmol)and CuI (3.75 mg, 0.2 mmol) were added. After reaction overnight, the solvent was removed under vacuum and the solid was purified by silica chromatography to give compound 8 (0.60 g, 92%). 1 H NMR (300 MHz, CD 2 Cl 2 (s, 4H), 4.14 (q, 4H). 4.22 (t, 4H), 4.72 (s, 4H). 6.97 (s, 2H). 7.20 (d, 2H), 7.23 (d, 2H), 7.29 (d, 4H), 7.50 (d, 4H). 13 C NMR (75 MHz, CD 2 Cl 2 29.34, 29.50, 29.58, 29.63, 29.64, 31.91, 41.13, 60.91, 65.57, 66.70, 82.07, 88.48, 89.82, 94.15, 113.97, 117.27, 121.12, 124.11, 125.26, 129.50, 131.45, 132.03, 132.46, 135.35, 153.05, 168.35, 170.84. MS (MALDI) m/z ([M + H] + ) , calculated 1146.53; found 1146.5 1 . Model compound . Comp ound 8 (0.20 g, 0.17 mmol) was dissovled in THF (5 ml) which was added NaO H solution (2 ml, 1M) dropwise and heated at 45 Â°C. After 6 h, THF was removed under vacuum and 5 ml water was added to the flask and the solution was stirred for anther 6 h. Then, t he solution was concentrated and added to acetone. The precipitate was collected and re dissolved in 2 ml water and HCl (2M, 5 ml) was added. Then, the precipitate was collected and washed several times with water. Last, the solid wad dried under vacuum to give the model compound (90 mg, 70%). 1 H NMR (500 MHz, DMSO d 6 4H), 7.45 (dd, 4H), 7.56 (d, 4H). 13 C NMR (125 MHz, DMSO d 6 88.16, 91.31, 94.93, 112.86, 120.05, 123.93, 124.45, 130.31, 130.59, 131.55, 131.82, 133.77, 137.03, 152.83, 170.35, 172.71. MS (APCI) m/z ([M 2H + Na] + ) , calculated 7 76 . 09 2; found 775.07 1 .
91 General method for polymerization. Monomer 2 (45.0mg, 0.163mmol) was dissolved in 15ml dry THF in a round bottom flas k at room temperature and the solution was degased for 30min. Then, Tetrabutylammonium difluorotriphenylsilicate (264mg, 0.489mmol) was added into the flask under argon protection. After 6 hours, monomer 1 or 3 (0.163mmol), a different amount of 1 iodo 4 ( trifluoromethyl)benzene or tert butyl 2 (4 iodophenyl)acetate (10%, 20% or 30%, molar ratio) and 15ml dry (i Pr) 2 NH were added. The solution was degased for another 30min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulted reaction mixture was heated to 40 Â°C for 24 hr. The yellow solution was flashed through a silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100ml of methanol and the polymer precipitated out immediately. This process was repeated twice. Last, the precipitate was collected and dried under vacuum. Typical yield of this reaction is 50% 70%. P1 O ester 7. GPC (THF, polystyrene standard): M w =7700, M n =5000, PDI=1.55. 1 H NMR (300 MHz, CDCl 3 (br, 4H) , 7.56 ( br, 4H ), 7.21 (br, 12 H), 6.96 (br, 1 2H), 4.71 (br, 24 H), 4.23 (br, 2 4H), 1.67 ( br , 2 4H), 1.31 1.25 ( br , 216 H), 0.87 ( br , 36 H). P1 O ester 9. GPC (THF, polystyrene standard): M w =10200, M n =6600, PDI=1.53. 1 H NMR (300 MHz, CDCl 3 7. 64 (br, 4H) , 7.56 ( br, 4H ), 7.21 (br, 16 H), 6.96 (br, 16 H), 4.71 (br, 32 H), 4.23 (br, 32 H), 1.67 ( br , 32 H), 1.31 1.25 ( br , 288 H), 0.87 ( br , 48 H). P1 O ester 14. GPC (THF, polystyrene standard): M w =17200, M n =9900, PDI=1.74. 1 H NMR (300 MHz, CDCl 3 7.64 (b r, 4H) , 7.56 ( br, 4H ), 7.21 (br, 26 H), 6.96
92 (br, 26 H), 4.71 (br, 52 H), 4.23 (br, 52 H), 1.67 ( br , 52 H), 1.31 1.25 ( br , 468 H), 0.87 ( br , 78 H). P2 C ester 7. GPC (THF, polystyrene standard): M w =9500, M n =5100, PDI=1.87. 1 H NMR (300 MHz, CDCl 3 14 H) , 7.18 (br, 14 H), 4.12 (br, 28 H), 3.83 (br, 28 H), 1.61 ( br , 28 H), 1.40 (s, 18H), 1.31 1.25 ( br , 252 H), 0.87 ( br , 42 H). P2 C ester 12. GPC (THF, polystyrene standard): M w =17000, M n =8400, PDI=2.00. 1 H NMR (300 MHz, CDCl 3 22 H), 7.18 (br, 2 2 H), 4. 12 (br, 44 H), 3.83 (br, 44 H), 1.61 ( br , 44 H), 1.40 (s, 18H), 1.31 1.25 ( br , 396 H), 0.87 ( br , 66 H). P2 C ester 18. GPC (THF, polystyrene standard): M w =26000, M n =13000, PDI=1.98. 1 H NMR (300 MHz, CDCl 3 34H), 7.18 (br, 34 H), 4.12 (br, 68 H), 3.83 (br, 68 H), 1.61 ( br , 68 H), 1.40 (s, 18H), 1.31 1.25 ( br , 612 H), 0.87 ( br , 102 H). General method for hydrolysis. To 100 mg of P1 O ester polymer was in 30 mL of THF and 5 eq. of NaOH, pre dissolved in 5 ml di water, was added drop wisely. The reaction mi xtu re was stirred at 45 Â°C overn ight. Then THF was removed under vacuum and another 5ml of di water was added. The reaction mixture was heated at 45 Â°C for another 6 hr. The solution was concentrated and the product was precipitated from a mixture of acetone/ methanol (95:5 volume ratio). The precipitate was collected and dissolved in basic water (pH=8), followed by filtration with a dialysis filter (D=0.45Âµm). Finally, the solution was dialyzed for two days and freeze dried. To 100 mg of P2 C ester was dissol ved in 20 mL chloroform in a flask and 5 mL trifluoreacetic acid was added dropwise over 10 min. The mixture was stirred for 2 h under R.T. and all the solvent was removed under vacuum. The residual solid was dissolved in 30 mL of THF and 5 eq. of NaOH, p re dissolved i n 5 ml di water, was
93 added dropwise . The reaction mi xture was stirred at 45 Â°C overn ight. Then THF was removed under vacuum and another 5ml of di water was added. The reaction mixture was heated at 45 Â°C for another 6 hr. The solution was con centrated and the product was precipitated from a mixture of acetone/methanol (95:5 volume ratio). The precipitate was collected and dissolved in basic water (pH=8), followed by filtration with a dialysis filter (D=0.45 Âµm). Finally, the solution was dialy zed for two days and freeze dried. P1 O 7 1 H NMR (500 MHz, D 2 H ), 4.59 (br, 4H). P1 O 9 1 H NMR (500 MHz, D 2 H ), 4.59 (br, 4H). P1 O 14. 1 H NMR (500 MHz, D 2 H ), 4.59 ( br, 4H). P2 C 7 1 H NMR (500 MHz, D 2 6.80 (br, 2 H ), 3.64 (br, 4H) P2 C 12 1 H NMR (500 MHz, D 2 (br, 2 H ), 3.64 (br, 4H) P2 C 18 1 H NMR (500 MHz, D 2 (br, 2 H ), 3.64 (br, 4H) General method for a cidification. A 20mg sample of hydrolyzed polymer was dissolved in 5ml basic water (pH=8) and 5 ml of pH=1 hydrochloric acid was added to the solution dropwise. The mixture was stirred for 2 hr and centrifuged. The solid at the bottom of centrifuge tube wa s collected and dried under vacuum. No other purification was performed before use as a sensitizer for DSSCs.
94 CHAPTER 3 CHARGE SEPARATION IN DIFFERENT CHAIN LENGTH CONJUGATED POLYMERS Background Since the ir discovery, c onjugated chromophores have been o utstanding candidates for many artificial electronic device applications, such as solar cells, light emitting diodes, field effect transistors , etc . 8 , 10 , 14 , 15 Compared to small organic molecules, conjugated polymers have many advantages, such as being more stable, ea sier to synthesize and more feasible for device processing. Charge transport plays an important role in these devices. For example, in bulk heterojunction solar cells , it is crucial to have fast charge separation and long lived charge separated state s in o rder to build high performance d evices. Further , it has been proved that molecular weight can significantly affect device performance and quenching efficiency . 62 , 154 156 Many work s ha ve already furthered the understanding of the effect of the molecular weight on charge transport behavior while charge recombination receives less attention. To the knowledge of authors, most of the charge recombination studies have been focused on systems containing small molecular weight molecules, due to the fact that small molecules have well defined structures and molecular weight is mono dispersed. Otsubo and co workers reported cha rge separation and recombination studies in system s of oligothiophene fullerene dy ads. 157 , 158 In a system consisting of tetrathiophene fullerene, the c harge separation rate was faster in highly polar solvents (on the order of 10 10 s 1 ) , such as THF and benzonitrile , and decreased in less polar solvents (on the order of 10 9 s 1 ), such as anisole and toluene. And, the charge separation state was not observed in non polar benzene. T he chain length effect of oligothiophene on charge separation and recombination rate was also investigated .
95 With increasing cha in length , from octathiophene fullerene to dodecathiophene fullerene , the charge separation rate increased while the charge recombination rated decrea s ed. And, in charge recombination, there were both fast process es which originated from direct charge reco mbination and slow process es which were due to indirect charge recombination after charge migration. Albinsson and collaborators investigated charge separation and recombination in donor bridge acceptor systems where donors and acceptors were porphyrin com pounds . 159 , 160 It was interesting that, in a system with a broken conjugation bridge, the singlet excited sta te of the donor was quenched via a singlet energy transfer mechanism and the quenching rate was conjugated, the quenching occurred via an electron transfer mechanism . Both charge separation and rec conjugated bridge systems decreased with increasing bridge length and the charge separation rate decreased more rapidly compared to that of charge recombination. In this work, we reported the synthesis of a series of polymers (PPE NDI n) featuring a poly (phenylene ethynylene) (PPE) (donor) backbone and naphthalene diimide derivative (NDI, acceptor) end caps , and charge recombination studies on the polymers . The polymer structure was chosen due the fact that there is no overlap of the e mission spectra of PPE and absorption of NDI . Further, the NDI is almost non fluorescent which minimize of energy transfer. The polymer chain length was varied to study the effect polymer molecular weight on charge transfer and recombination behaviors. Fem tosecond (fs) time resolved transient absorption (TA) spectroscopy was applied to investigate charge recombination kinetics and the long lived charge
96 separation state was also studied by nanosecond TA. The results of this work provide insight about the cha rge recombinatio n kinetics and charge separated state. Results and Discussion Synthesis and C haracterization In this study, a series of polymers, PPE NDI n, were synthesized which feature the same conjugated poly(phenylene ethynylene) backbone and napht halene diimide derivative end group substitution , but different molecular weight . The synthetic procedure was outlined in Figure 3 1. Figure 3 1. Synthesis scheme of PPE NDI n. The synthesis of monomer started with the re action of 1,4 dic h lorobenze ne with n bromohexane under K umada coupling reaction condition s to give compound 1 . Then,
97 typical iodination reaction was performed on compound 1 to get compound 2 , which was further reacted with ethynyltrimethylsilane under Sono gashira reaction condition s to obtain compound 3 . Deprotection of the trimethylsily l group by strong base generated compound 4 . The synthesis of end groups started with reacting naphthalene diimide with one equivalent of octan 1 amine to afford compound 5 . Due the possibility of having both mono and di substitution products and tough work up, the yield was very low. Compound 5 was further reacted with 4 iodoaniline to generate compound 6 with decent yield. Reaction of compound 6 with ethynyltrimethylsilane under Sonogashira reaction condition s produced compound 7 which was deprotected by tributylammonium fluoride (TBAF) to get compound 8 . In this step, strong base was not used to avoid hydrolyzing the imide functional groups. The p olymers were synthesized u nder Sonogashira polycondensation reaction condition s polymerization reactions. In a typical reaction, compound 2 and 4 ( 1:1 molar ratio ) and various amounts of compound 6 were add ed into flask. Without the presence of mono functionalized compound 6 , the polymer chain will keep growing. However, the addition of compound 6 changed the stoichiometric balance of the functional groups and lower ed the overall molecular weight. Compound 8 was added at the end of polymerization to ensure efficient end capping at both chain ends. In addition, one PPE polymer without electron acceptor was synthesized as a model compound. The synthesis of OPE 8 was reported by Mr. Junling Jiang from the Schanz e group (unpublished results). Polymer molecular weights were characterized by both GPC and 1 H NMR (Figure 3 2). The GPC trace shows a clear decrease in the retention time with
98 increasing polymer molecular weight (Mn (PPE NDI 8) < Mn (PPE NDI 14) < Mn (PPE NDI 22) < Mn (PPE NDI 39)). In the 1 H NMR spectrum , both proton signals from the naphthalene diimide and polymer backbone appear . Chemical shift at 8.82 ppm was assigned to the aromatic protons on the naphthalene rings and shifts at 7.73 ppm and 6.98 ppm came from the phenyl ring next to the nap hthalene unit . Aromatic protons of the PPE backbone occur at 7.38 ppm an d the methylene unit which was directly connected to the phenyl ring had a chemical shift at around 2.84 ppm. Signal integration of naphthalene protons and m ethylene proton s were used to calculate the molar ratio of naphthalene unit s to phenyl rings. Assuming there are two naphthalene groups on each polymer chain, the numb er of phenyl rings and repeat unit s can be determined by calculating the ra tio of the signal integration at 7.38 ppm and 2.84 ppm . Figure 3 2. GPC and NMR characterization of PPE NDI n. A) GPC and B) 1 H NMR of PPE NDI 8 (black), PPE NDI 14 (red), PPE NDI 22 (blue) and PPE NDI 39 ( dark cy an) The number of repeat unit s calculat ed from NMR di ffers significantly from the that derived from GPC when molecular weight is high (Table 3 1). And, the reason might be
99 stacking and aggregate in solution , shielding the NMR signal; or b) high molecular weight polymers were obtained using a reduced amount of end caps in the reaction syst ems, which resulted in a decreased 1 H NMR signal of the end cap protons and cause more error in signal integration . Table 3 1. Molecular weight characterization of PPE NDI n and PPE GPC a NMR Mn (g/mol) Mw (g/mol) PDI DP b Mn DP c PPE NDI 8 3100 4800 1.52 8 3100 8 PPE NDI 14 4800 9000 1.87 14 6000 19 PPE NDI 22 7300 1610 2.21 22 8700 29 PPE NDI 39 11700 23800 2.03 39 17500 62 PPE 5400 8200 1.40 17 3700 12 d a N arrow dispersed polystyrene was used as the standard. b Degree of polymerization was calculat ed using Mn obtained by GPC. c Degree of polymerization was calculated using signal intensity integration ratio from 1 H NMR. d Calculated using the 1 H NMR integration of the t butyl and PPE backbone signals. Energetics and Optical Properties The objectiv e of this project is to investigate the charge recombination in conjugated polymers. It is essential to have the redox state information, in order to und erstand the excited and charge separated state energies. The singlet excited state energy of PPE NDI n polymers is available from the fluorescence emission spectra . T he reduction and oxidation potentials of the NDI end caps and PPE backbone are obtained from cyclic voltammetry (CV) , and the charge separated state energy is calculated from the difference in the PPE backbone oxidation and NDI reduction potentials. Cyclic voltammetry was performed on all polymers, but reasonable results were obtained only on the shortest chain length polymer, namely, PPE NDI 8 (Figure 3 3), due to the intrinsic difficulty of pe rforming CV on high molecular weight polymers . However, the energetics of other PPE NDI n polymers should be v ery similar to that of PPE NDI 8 ,
100 because they have the same polymer structure and almost identical absorption and emission (Figure 3 4). Figure 3 3. A) Energetics and B) CV of PPE NDI 8. Cyclic voltammagram of PPE NDI 8 in methylene chloride with 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as electrolyte, vs . SCE. The cyclic voltammagram of PPE NDI 8 shows two quasi reversible reduction p eaks with E red = 0.61 V and E red = 1.01 V , along with a single irreversible oxidation peak with E ox = 1. 4 9 V and onset at 1.22 V (potentials vs . SCE). One thing needs to be pointed out here is that each reduction wave on the CV very likely corresponds to one electron transfer to both NDI end caps which is supported by previous studies. 161 , 162 The charge se parated state energy is calculated using E cs = E ox E red DA , where E ox is the oxidation potential of the PPE backbone, E red is the first reduction potential of the NDI end caps dielectric constant and R DA is the distance between the don or and acceptor units. The 14.4 DA term is the Coulombic stabilization energy in the charge separated state and is estimated to be 0.05 eV for PPE NDI 8 in DCM, assuming the average distance between the positive charge and the NDI is about the length of 4 repeat units . In the case of PPE NDI 8, the charge separated state energy ( E cs ) is calculated to be ~ 1.90 eV and the singlet energy is
101 ~2.94 eV. In PPE type conjugated polymers, the triplet state energy is found to be 0.7 Â± 0.1 eV below the singlet state . 163 Thus, the triplet state energy level in PPE NDI n polymers is about 2. 1 Â± 0.1 eV, which is within the range of reported value for PPE type polyme rs ( 1.95 2.26 eV ) . 164 , 165 The triplet energy level of the NDI compound has been estimated to be 2.03 eV, using the phosphorus nm) . 166 Table 3 2. Table of energetics E singlet /eV c E red /V d E ox /V d E cs /eV OPE8 a 2.99 N.A. 1.31 N.A. NDI H b 3.21 N.A. 0.53 N.A. PPE NDI 8 2.94 0.65 1. 27 e 2.14 a . b Taken from Ref 162 . c Estimated from fluorescence emission. d Value reported vs . SCE . e Calculated using the average number of E ox (OPE8) and the onset of PPE NDI 8 oxidation potential. UV visible absorption and emission spectra of the samples were measured in THF , and the concentration of the samples was adjusted to 50 Âµ M (based on repeating unit for polymers) (Figure 3 4 ) . The N DI has two major absorptio n peaks at 358 nm and 378 nm. T he PPE backbone has a broad absorption from 300 nm to 425 nm, with the absorption maximum at 377 nm. Although, the PPE and NDI absorption spectra have a large overlap (300 395 nm), the absorption of the PPE NDI n polymers is stil l dominated by the PPE backbone because the absorption coefficient of the PPE backbone (8.8*10 6 for PPE NDI 39 ) is significantly higher than that of the NDI end caps (~5*10 5 for NDI) . The NDI does not have any absorption beyond 395 nm while the emissio n of the PPE backbone starts near 390 nm. The mismatch of the PPE emission and NDI absorption ensures that the energy transfer in the PPE NDI system is minimized. In addition, the NDI is almost non emissive in solution compared to the PPE polymer , with a q uantum
102 , while the quantum yield of the PPE is about 76%. Therefore, the emission of PPE NDI n polymers mostly comes from the PPE backbone. Figure 3 4. U V vis absorption and emission spectra in THF. A) U V vis absorption of the PPE (black ) and NDI (red), normalized according to the absorption coefficient. B) Fluorescence emission of the PPE (black) and NDI (red), normalized according to fluorescence quantum yields. C) U V vis absorption and D) emission of PPE NDI 8 (black square), PPE BDI 1 4 (red circle), PPE NDI 22 (blue up triangle) and PPE NDI 39 (dark cyan down triangle), and the signals are normalized according to the quantum yields. The excitation wavelength for all samples is 375 nm. Absorption spectra in C are offset for clarity. In the absorption spectr a of PPE NDI n polymers , the NDI absorption band at 362 nm can be easily identified in PPE NDI 8 and this feature becomes less pronounced as the polymer molecular weight increases. However, despite overlapping with the PPE backbone abs orbance , the NDI absorption band at 379 nm can clearly be distinguished even in longest chain length polymer. Meanwhile, the PPE backbone absorption becomes stronger as molecular weight increases, due to the fact that the
103 molar extinction coefficient incre ases with molecular weight. 140 The overall absorption spectra increasingly resemble the PPE model polymer as the molecular weight increases due to the increase d contribution of the PPE backb one . Despite the changing molecular weight, the absorption maximum remains essentially the same (at 379 nm), indicating the conjugation length is saturated for all polymer samples. In many donor a charge transfer absorption band can also be observed. However, it was not the case for PPE NDI n polymers , due to weak electronic coupling . Table 3 3. Summary of the photophysical properties. a With anthrathene as quantum yield standard, =0.27 in ethanol at room temperature. b Energy (PPE380 525nm)/ (PPE), in which (PPE) was the only polymer PPE. All polyme rs have almost identical emission spectra with emission maxima at 422 nm which is the same as the PPE model polymer. However, the emission intensity is significantly weaker compared PPE model d ue to charge transfer. Although there is only 5% NDI (molar rat io) in PPE NDI 39, more than 80% of the emission of polymer backbone is quenched. Further , the quenching is more efficient in PPE NDI 22 and this trend continues as NDI content increases ( T able 3 3 ). The overall energy transfer efficiency was calculated us ing the ratio of PPE NDI n polymer fluorescence quantum yield s to the PPE model polymer quantum yield, and the trend is clear that charge transfer efficiency decreases with increasing M n . Polymers NDI Content (molar %) abs (nm) em (nm) 4 cm 1 M 1 ) Lifetime at 420 nm (ns) Quantum Yield a e transfer Efficiency % b PPE NDI 39 4.9 379 422 2.25 0.18 0. 15 80.5 PPE NDI 22 8.3 379 422 2.14 0.15 0.0 93 87.9 PPE NDI 14 12.5 379 422 2.16 0.15 0.0 44 94.3 PPE NDI 8 20 379 422 2.24 0.13 0. 013 98.3 PPE 0 3 80 422 2.12 0.38 0.77 N.A.
104 Charge R ecombination S tudy Femtosecond (fs) transient absorption (TA ) spectroscopy was used to characterize the intrachain charge recombination kinetics in PPE NDI n polymers . Figure 3 5 compares the transient spectrum of the PPE and PPE NDI 8 . The transient spectrum of PPE has negative signals (bleach) from 420 nm to 510 nm which are the combination of ground state bleaching and stimulated emission . An intense excited state absorption also 510 nm. The transient spectrum of PPE NDI n is very different compared to that of PPE. The spectrum is dominated by three strong absorption bands with one at around 480 nm , which is attributed to the absorption of naphthalene diimide radical anion (NDI ) , and a broad band from about 57 0 nm to 650 nm, which is attributed to a combinat ion of NDI radical anion and PPE r adical cation (PPE + ) absorption s . 161 , 166 The appearance of the 480 nm band clearly indicates the format ion of NDI radical anion which is generated by one electron transfer from the PPE backbone to the NDI end group. In the spectrum of PPE NDI 8, the bleaching from 420 nm to 510 nm, observed in the spectrum of the PPE, is completely covered by the NDI abso rption. Figur e 3 5. Transient spectra of the PPE and PPE NDI 8 in DCM after 100 ps.
105 The time dependent transient absorption spectrum of PPE NDI n polymers is plotted in Figure 3 6. As the molecular weight increases, the feature of NDI peak at ~480 nm b ecomes less structured due to the fact that the NDI content decreases with increasing molecular weight and the NDI signal intensity , which is proportional to the NDI content, decreases as well. In addition, charge transfer is less efficient in high mo lec ular weight polymer and the bleaching signal from the PPE backbone becomes more significant. T he broad peak from ~550 to 65 0 nm also starts to blend with the PPE excited state absorption as molecular weight increases. Another interesting finding is that ab sorption signals for PPE NDI 8 and PPE NDI 14 decay to zero after ~5 ns while there are still signals at ~480 nm in the spectra of PPE NDI 22 and PPE NDI 39 which means the NDI lives longer in high molecular weight polymers. Figure 3 6 . Time resolved t ransient absorption spectra of PPE NDI n polymers .
106 Figure 3 7. Transient absorption spectra of PPE NDI n after 5 ns. Detailed information of charge transfer kinetics can be obtained by monitoring signals at specific wavelength s . Because the signal at 480 nm is dominated by NDI absorption, this wavelength was exclusively used t o probe the charge separated state. In the TA spectrum , a rise in absorbance was not observed due to polymer relaxation and reorganization and therefore, th e charge transfer dynami cs can not be resolved . 167 , 168 In general, the decay k inetics has two components: fast component ( with 100 ps) which is attributed to polymer structure relaxation and reorganization and a relatively slow component (with on the order of several hundred ps) reflecting the process of charge recombination. More careful review of the charge recombination process shows that there is an increasing contribution from the slow recombination process as the molecular weight increases (Table 3 4). One explanation is that the holes generat ed by electron transfer from PPE t o NDI end groups can delocalize along the conjugated backbone and become further removed from the end NDI as the polymer chain length increases. The charge recombination is driven by the C oulomb
107 interaction between the cation and anion radicals , and the strength of which follows proportional to the inverse square of the distance between the interacting charges. The distance between the cation and anion radicals increases with molecular weight as the hole delocalized further away fro m the NDI end caps. Therefore, Columbic interaction s decrease dramatically. The residue signals in the kinetic trace in crease with molecular weight which is also consistent with the transient absorption spectrum. Table 3 4. Charge recombination kinetics t 1 (ps) A 1 t 2 (ps) A 2 y 0 PPE NDI 8 100.00 0.65 436.00 0.27 0.05 PPE NDI 14 64.00 0.41 414.00 0.36 0.10 PPE NDI 22 155.00 0.57 764.00 0.24 0.13 PPE NDI 39 121.00 0.37 676.00 0.30 0.20 Equation was used for fitting. Figure 3 8 Nano second (ns) TA spectroscopy was used to further investigate the long lived charge separated state of PPE NDI n polymers in THF (Figure 3 9) . However, no PPE
108 radical cation or NDI radical anion signal is detected after an initial 15 ns camera delay, because the lifetimes of the radicals are shorter than 15 ns. But interestingly, intense PPE triplet absorption is detected. Figure 3 9. Nasosecond TA spectra of PPE NDI n, PPE and ND I in THF. The optical density of all samples at 355 nm is adjusted to 0.6. The triplet excited states can be generated by either intersystem crossing from the singlet excited state or induced by charge recombination. 169 , 170 Considering the fact that more than 80% of the fluorescence is quenched in PPE NDI n polymers, charge recombination should be the major cont ribution to induced tri plet formation. However, more careful investigation is needed for better understanding of the triplet formation. In addition, the absorption intensity increases with molecular weight across the polymer
109 series. The PPE NDI 8 does not show any PPE triplet absorption while that of the PPE NDI 39 is almost the same as the PPE model polymer. Summary and Future Work A series of polymers (PPE NDI n) with different molecular weight were synthesized which featured the same poly (phenylene eth ynylene) (PPE) conjugated backbone and naphthalene diimide derivative (NDI) substitution at the polymer chain ends. The energetics of polymers were investigated by cyclic voltammetry and steady state absorption and emission spectroscopy . Despite the increa sing of molecular weight, all PPE NDI n polymers show similar energy levels. But, the fluorescence emission quan tum yield measurements indicate very efficient electron transfer from the PPE backbone to the NDI end groups, and t he transfer efficiency increa ses with decreasing molecular weight. Femtosecond transient absorption (fsTA) analysis showed that charge recombination rate also increased with decreasing chain length. Nanosecond TA (nsTA) results shows the formation of the PPE triplet excited state and more careful investigation is needed to understand this phenomenon . Experiments and Materials Materials Unless specified, all compounds and solvents were purchased from commercial sources (Aldrich, Acros, Strem Chemicals, et al) and used without further p urification. For all palladium catalyzed reactions, the solvents were carefully degassed with argon for at least 30 min. 1 H and 13 C NMR spectra were recorded on either Inova2 (500 MHz) or Varian Gemini parts per million (ppm) using the residual solvent signals as internal standards.
110 Instrumentation 1 H and 13 C NMR spectra were measured on a Mercury 300, a Gem ini 300, or an Inova 500. Chemical shifts were referenced to the residual solvent peaks. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclotron Resonance mass spectrometer (Bruker Daltonics, MA). Gel Perme ation Chromatography (GPC) data was collected on a system composed of a Shimadzu LC 6D pump, an Agilent mixed D column, and a Shimadzu SPD 2 0A was calibrated against linear narrow dispersed polystyrene standards in THF. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performe d in a dry methylene chloride (CH 2 Cl 2 ) solution containing 0.1 M tetra n butylammonium hexafluorophosphate (TBAH). The three electrode setup consisted of a platinum microdisk (2 mm 2 ) working electrode, a platinum wire auxiliary electrode, and a silver wire reference electrode. Solutions were degassed with argon flow prior to measurements, and positive argon pressure was maintained during the measurements. The concentration of PPE NDI 8 in the solutions was 0.5 mM. A 100 mV/s scan rate was used. All potentia ls were calibrated by using a ferrocene internal standard (E(Fc/Fc + ) = 0.43 V vs SCE in CH 2 Cl 2 ), and potentials are reported vs SCE. 1800 dual beam absorption spectrophotometer. Photolumi nescence spectra were recorded on a spectrofluorimeter from Photon Technology International (PTI). P hoto luminescence lifetimes were obtained by time correlated single photon counting (TCSPC) using a Fluo Time 100 (Picoquant), and excitation was provided u sing a PDL 800 B Picosecond Pulsed Diode Laser (375 nm).
111 Nanosecond triplet triplet 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 LS 1130 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. 6 at the ex citation wavelength in a continuously circulating 1 cm pathlength flow cell (volume = 10 mL). transient absorption spectroscopy with broadband capabilities. Detailed information of the experimental setup can be found elsewhere. 171 Briefly, Ultrafast Systems Helios femtosecond transient absorption spectrometer equipped with UV Vis and near IR detectors was used to measure the samples in this study. White light continuum probe pulse was generated in a 2 mm thick sapphire plate in an Ultrafast System LLC regenerative amplifier operating at 800 nm with 35 fs pulses and a repetition rate of 1 kHz. The pum p pulses at 355 nm were created from fs pulses generated in an optical parametric amplifier (Newport Spectra Physics). The sample solution was constantly stirred to avoid photodegradation in scanned volume. The pump and probe beams were overlapped both spa tially and temporally on the sample solution, and the transmitted probe light from the samples was collected on the broad band UV visible near IR detectors to record the time resolved excitation induced difference spectra. Synthetic Procedures T ert butyl 2 (4 ethynylphenyl)acetate and tert butyl 2 (4 iodophenyl)acetate were synthesized according to literature. 140
112 1,4 D ihexylbenzene (Compound 1) . To a solution of hexylmagnesiumbromide [prepar ed from magnesium (1.375 g, 75 mmol) and n bromohexane (10.5 mL, 75 mmol)] and NiCl 2 in 50 mL anhydrous Et 2 O was added dropwise 1,4 dichlorobenzene (5 g, 34 mmol) in 50 mL anhydrous Et 2 O. The solution was refluxed for 12 h. After coo ling to R.T., the mixture was hyd rolyzed with HCl under ice bath and extracted with Et 2 O (50 mL, 3 times). The organic layers were combined, dried with Na 2 SO 4 and evaporated under vacuum. The residue oil was distilled to afford compound 1 as colorless oil (13.3 g, 80%). 1 H NMR (300 MHz, CDCl 3 ) 7.08 (s, 4 H), 2.57 (t, 4 H), 1. 60 (m, 4 H ) , 1.42 1.24 (m, 12 H), 0.88 (t, 6 H). 1,4 Dihexyl 2,5 diiodobenzene (compound 2). Compound 1 (5 g, 20 mmol), I 2 (5.6 g, 22 mmol) and HIO 3 (1.76 g, 10 mmol) were dissolved in a mixture solvent of HOAC (100 mL), H 2 SO 4 (5 mL) and water (2 mL) and refluxed for 8 h at 120 Â°C . After the mixture was cooled to R.T., it was poured into ice water (200 mL). The mixture was then extracted with chloroform (100 mL, 3 times) and the orga nic layers were collected, washed with water and tried with Na 2 SO 4 . All the solvent was removed under vacuum and the crude product was purified by column with hexane as the eluent to give compound 2 (8 g, 86%). 1 H NMR (300 MHz, CDCl 3 ) 7.59 (s, 2 H), 2.59 (t, 4 H), 1.52 (m, 4 H), 1.38 1.28 (m, 12 H) , 0.89 (t, 6 H). ((2,5 dihexyl 1,4 phenylene)bis(ethyne 2,1 diyl))bis(trimethylsilane) ( compound 3 ) . Compound 2 (4 g, 8 mmol) was dissolved in a mixed solvent of THF and diisopropylamine and degased was argon for 30 min. PdCl 2 (PPh 3 ) 2 (100 mg, 0.14 mmol) and CuI (50 mg, 0.26 mmol) were added under argon protection. The mixture was degased for another 30 min before TMSA (2 g, 20 mmol) was added. The reaction
113 was stirred at R.T. for overnight. T he mixture was filtered and the solution was evaporated under vacuum. The solid was purified by column with hexane as eluent to yield compound 3 ( 3.3 g, 95% ) . 1 H NMR (300 MHz, CDCl 3 ) 7.24 (s, 2 H), 2.68 (t, 4 H), 1.58 (m, 4 H), 1.40 1.25 (m, 12 H), 0.89 (t, 6 H), 0.25 (s, 18 H). 1,4 diethynyl 2,5 dihexylbenzene ( compound 4 ) . Compound 3 (2 g, 4.56 mmol) was dissolved in a mixed solvent of THF/MeOH (2:1 ratio, 120 mL) and degases with argon for 30 min. Anhydrous K 2 CO 3 (6.9 g, 50 mmol) was added to the mixture and stirred for 4 h. The mixture was filter and extracted with DCM and the organic layer was washed with water and brine. Then, the solvent was evaporated under vacuum and the resi due solid was passed through a short column with hexane as the eluent to yield compound 3 ( 1.2 g, 94% ) . 1 H NMR (300 MHz, CDCl 3 ) 7.30 (s, 2 H), 3.28 (s, 2 H), 2.71 (t, 4 H), 1.60 (m, 4 H), 1.40 1.26 (m, 12 H), 0.89 (t, 6H). 7 O ctyl 1H isochromeno[6,5,4 def]isoquinoline 1,3,6,8(7H) tetraone ( compound 5 ) . 1,4,5,8 Naphthalenetetracarboxylic acid dianhydride (10 g, 37.3 mmol) was dis solved in anhydrous DMF at 120 Â°C . 1 Aminooctane (4.82 g, 37.3 mmol) in 20 mL of anhydrous DMF was added to the mixture dropwise over 5 min. The reaction was fluxed for 15 h. After reaction, solvent was removed under vacuum. The crude product was purified by column with DCM to yield compound 5 (3.1 g, 22% ) . 1 H NMR (300 MHz, CDCl 3 ) 8.82 (s, 4 H), 4.20 (t, 4 H), 1.44 1.28 (m, 12 H), 0.88 (t, 3 H). N octyl iodophenyl)naphthalene 1,4,5,8 tetracarboxylic acid bisimide ( compound 6 ) . Compound 5 (1 g, 2.68 mmol) dissolved in 50 mL anhydrous DMF at 120 Â°C . 4 iodoaniline (0.88 g, 4 m mol) in 5 mL of anhydrous DMF was added to the mixture dropwise over 5 min. The reaction was fluxed for 15 h. After reaction, solvent
114 was removed under vacuum. The crude product was purified by column with DCM to yield compound 6 (1 g, 65%). 1 H NMR (300 MH z, CDCl 3 ) 8.79 (s, 4 H), 7.91 (d, 2H) 7.09 (d, 2 H), 4.20 (t, 2 H), 1.75 (m, 2 H), 1.4 4 (m, 2 H), 1.37 (m, 2 H), 1.35 1.25 (m, 6 H), 0.88 (t, 3 H). N octyl trimethylsilylethynylphenyl)naphtalene 1,4,5,8 tetracarboxylic acid bisimide ( compound 7 ) . Compound 6 (1 g, 1.74 mmol) was dissolved in a mixed solvent of THF and diisopropylamine and degased was argon for 30 min. PdCl 2 (PPh 3 ) 2 (100 mg, 0.14 mmol) and CuI (50 mg, 0.26 mmol) were added under argon protection. The mixture was degased for another 30 min befor e TMSA (0.26 g, 2.6 mmol) was added. The reaction was stirred at R.T. for overnight. The mixture was filtered and the solution was evaporated under vacuum. The solid was purified by column with DCM as eluent to yield compound 7 (766 mg, 80%). 1 H NMR (300 M Hz, CDCl 3 ) 8.79 (s, 4 H), 7.65 (d, 2 H), 7.26 (d, 2 H), 4.21 (m, 2 H), 1.76 (m, 2 H), 1.42 (m, 2 H), 1.40 1.23 (m, 6 H), 0.88 (t, 3 H), 0.28 (s, 9 H). N octyl ethynyl phenyl)naphthalene 1,4,5,8 tetracarboxylic acid bisimide ( compound 8 ) . Compound 7 (600 mg, 1.1 mmol) was dissolved in 15 mL chloroform and degased under argon for 30 min. TBAF (1.65 mL, 1 M solution in THF) was added into the solution and the mixture was stirred for 2 h at room temperature. The reaction mixture was washed with water a nd dried with Na 2 SO 4 and the solvent was removed under vacuum. The residue solid was purified by column with DCM as the eluent to yield compound 8 (473 mg, 90%). 1 H NMR (300 MHz, CDCl 3 ) 8.78 (s, 4 H), 7.68 (d, 2 H), 7.30 (d, 2 H), 4.14 (m, 2 H), 3.17 (s, 1 H), 1.75 (m, 2 H), 1.41 (m, 2 H), 1.39 1.23 (m, 8H), 0.87 (t, 3 H).
115 General method for polymerization of PPE NDI n. Compound 2 (84.7 mg, 0.17 mmol), compound 4 (50 mg, 0.17 mmol) and various amount of compound 6 (0.1 0.6 equivalent) were added to a mixed solvent of 20 mL THF and 10 mL piperidine and the mixture was degased for 30 min. The solution was degased for another 30min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mm ol) of CuI were added. The resulted reaction mixture was heated to 40 Â°C for 24 hr. Then, various amount of compound 8 (0.1 0.6 equivalent) was added to the mixture and the solution was stirred at 40 Â°C for another 6 h. The pale yellow solution was flashed through a silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100ml of methanol and the polymer precipitated out immediately. This process was repeated twice. Last, the precipitate was collected and d ried under vacuum. Typical yield of this reaction is 50% 70%. PPE NDI 8. GPC (THF, polystyrene standard): Mn= 3.1 kDa, Mw=4.8 kDa, PDI=1.51. 1 H NMR ( 5 00 MHz, CDCl 3 ) 8.82 (s, 8 H), 7.73(d, 4 H), 7.38 (b, 20 H), 4.22 (b, 2 H), 2.84 (b, 32 H), 1.8 1.2 (b, 128 H), 0.89 (b, 48 H). PPE NDI 14 . GPC (THF, polystyrene standard): Mn= 4.8 kDa, Mw=9.0 kDa, PDI=1.87. 1 H NMR ( 5 00 MHz, CDCl 3 ) 8.82 (s, 8 H), 7.73(d, 4 H), 7.38 (b , 40 H), 4.22 (b, 2 H), 2.84 (b, 76 H), 1.8 1.2 (b, 304 H), 0.89 (b, 114 H). PPE NDI 22 . GPC (THF, polystyrene standard): Mn= 7.3 kDa, Mw=16.1 kDa, PDI= 2.21. 1 H NMR ( 5 00 MHz, CDCl 3 ) 8.82 (s, 8 H), 7.73(d, 4 H), 7.38 (b, 59 H), 4.22 (b, 2 H), 2.84 (b, 116 H), 1.8 1.2 (b, 442 H), 0.89 (b, 128 H).
116 PPE NDI 39 . GPC (THF, polystyrene standard): Mn= 11.7 kDa, Mw=23.8 kDa, PDI=2.03. 1 H NMR ( 5 00 MHz, CDCl 3 ) 8.82 (s, 8 H), 7.73(d, 4 H), 7.38 (b, 120 H), 4.22 (b, 2 H), 2.84 (b, 240 H), 1.8 1.2 (b, 930 H), 0.89 (b, 341 H). PPE. Compound 2 (84.7 mg, 0.17 mmol), compound 4 (50 mg, 0.17 mmol) and tert butyl 2 (4 iodophenyl)acetate ( 16 mg, 0.05 mmol ) were added to a mixed solvent of 20 mL THF and 10 mL piperidine and the mixture was degased for 30 min. The solution was degased for another 30min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulted reaction mixture was heated to 40 Â°C for 24 hr. Then, t ert butyl 2 (4 ethynylphenyl)acetate ( 10.8 mg, 0.05 mmol ) was added to the mixtur e and the solution was stirred at 40 Â°C for another 6 h. The pale yellow solution was flashed through a silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100ml of methanol and the polymer precipitate d out immediately. This process was repeated twice. Last, the precipitate was collected and dried under vacuum. Typical yield of this reaction is 50% 70%. PPE. GPC (THF, polystyrene standard): Mn= 5.4 kDa, Mw=9.2 kDa, PDI=1.40. 1 H NMR (500 MHz, CDCl 3 7.49 (d, 4H), 7.38 (br, 22H), 7.28 (d, 4H), 3.55 (s, 4H), 2.84 (br, 44 H) , 1.45 (br, 64H), 1.34 (br , 90 H), 0.89 (br, 66 H).
117 CHAPTER 4 ULTRAFAST ENERGY TRANSFER IN VARIABLE CHAIN LENGTH CONJUGATED POLYMERS WITH ENERGY ACCEPTOR END CAPS Background Since the discovery, conjugated polymers (CPs) have found numerous applications, such as organic light emitting diodes (OLEDs), 2 , 172 bulk heterojunction solar cells (BHJs), 173 175 chemo / biosensors, 18 , 81 , 89 cell imaging 22 , 100 and etc . Compared to sma ll organic dyes, CPs have higher extinction coefficient , better stability and superior processing properties. The synthetic ease of alternating the building blocks for the construction of conjugated polymers makes it convenient to change polymer architectu res, modify functional groups and tune optical properties, such as the HOMO and LUMO energy levels. More interestingly, ultrafast energy transfer in conjugated polymer systems has been discovered, which is essential for the development of highly efficient sensors, electroluminescent devices and photovoltaics. 86 H owever, energy transfer is very complicated in conjugated systems, in which both intrachain and interchain energy migration processes can take place and the dominant pr ocess depends strongly on the nature of the systems. In order to resolve the complexity, many systems have been designed to investigate the energy transfer process. One approach is to synthesize energy acceptor capped CPs which incorporate chromophores wit h low LUMO energy level (acceptors) onto the end of polymer backbone with high LUMO (donor). In such a system, after selective excitation of the donor chromophores, energy will transfer from the donor s to the acceptor s . Swager and Wrighton reported studies investigating energy transfer in polymers with poly (phenylene ethynylene) (PPE) (donor) backbone and anthracene (acceptor) substitution (PPEAn) at the polymer chain ends. 83 In solution, the energy transfer from
118 the PPE backbone to the anthracene moieties is o nly modestly efficient , because anthracene is not a strong acceptor and there is a mismatch of the PPE emission and anthracene absorption spectra . M Ã¼ llen and Br Ã© das compared interchain and intrachain energy transfer in acceptor capped conjugated polymers. 84 , 85 They found that electronic matrix elements, which affects the energy transfer rate, decreased with i ncreasing donor acceptor distance in both interchain and intrachain energy transfer. In addition, the energy transfer was more efficient in film than in solution. Recently, we reported very efficient energy transfer along flexible polymer chain with conjug ated oligo(phenylene ethynlene) (OPE) donor and thioph e ne benzothiadiazole (TBT) acceptor side pendants. 37 The energy transfer rate varies from 0.02 0.5 ps 1 depending on t he distance between the acceptors and the initially formed donor excitons . In this chapter , we present energy transfer study in a series of conjugated polymers featuring PPE backbones end capped with TBT. This system is chosen because of the perfe ct match of the emission spectrum of the PPE with the absorption spectrum of the TBT . The quantum yields of both the PPE and TBT moieties are reasonably high . The intrachain e nergy transfer is very efficient, with dynamic and overall efficiency comparable to simila r systems. 84 , 85 Femtosecond (fs) time resolved transient absorption (TA) spectroscopy is applied to inves tigate the rate of energy transfer which provides insight information about the energy transfer kinetics. The results of this work help us better understand the intramolecular energy transfer processes in conjugated polymers and that the transfer rate depe nds on the D A distance in conjugated polymers.
119 Results and Discussion Structures, Synthesis and C haracterization The structures of polymers investigated in this study all feature the PPE backbone and are shown in Figure 4 1 . The PPE TBT n polym ers are all end capped with TBT units while the PPE polymer has phenyl end groups (the same polymer in Chapter 3) . The PPE backbon e adopts a rod like conformation in solution and there is minimal structure distortion along the backbone . In diluted solution , the inter chain energy transfer is eliminated , which makes the intrachain energy transfer process easier to investigate. 82 , 86 Figure 4 1. Molecular structures. In comparison, poly(p phenylene vinylene) (PPV), whose chain is more flexible, has both tightly co iled conformation and open chain conformation in solution. And, interchain energy transfer dominates in the tightly coiled conformation zone while intrachain energy transfer is favored in open chain conformation regain. 87 , 88 In addition, the PPE backbone and TBT moieties have distinct singlet excited state energy level: the singlet energy level of the PPE backbone is relatively higher (~3.01 eV) which makes it a good donor and the TBT serves as an acceptor which has lower singlet energy level (~2.07 eV). The emission spectrum of the PPE match es well with the absorption of the TBT
120 which facilitates the energy transfer . And the overlap of the PPE and TBT absorption is minimal which allows selective photoexcitation of the PPE backbone . Moreover, both PPE and TBT have high fluorescence quantum yields, which allows quantificati on of energy transfer efficiency. In summary, polymers studied in this work are perfect candidates for intramolecular energy transfer investigation and the molecular weights of polymers are varied to study the chain length dependent behavior s . Figure 4 2. Synthesis scheme of polymers. The general synthesis route for the preparation of polymers is outlined in Figure 4 2 . Palladium (Pd) catalyzed Sonogashira cross coupling polymerization was applied for the synthesis of polym ers. Cross coupling polymerization is a typical poly condensation reaction which proceeds via the step growth mechanism . T he molecular weights of resulting polymer s are determined by the degree of polymerization (DP) , which is given by DP=1/(1 p) ( where p is the extent of reaction and assuming stoichiometric balance of the reacting functional groups). 60 Polymer molecular weight can be tuned via the control of DPs by introducing stoichiometric imbalance ratio of
121 functional groups and DP is given by DP=(1+r )/(1+r 2rp) ( where r is stoichiome tric imbalance ratio ). 60 Adding variable amount of m ono functionalized end groups to the reaction mixture at the beginning has been widely applied to introduce stoichiometric imbalance and end group functionality. 62 At the beginning of reaction, reactants 1 and 2 (1:1 molar ratio) and different amount of A1 (0.1 0.3 molar ratio) or B1 (0.3 molar ratio) were added to the flask. After 24 h, ce rtain amount of extra end groups A2 (0.1 0.3 molar ratio) or B2 (0.3 molar ratio) were added and the reaction continued for another 12 h in order to make sure polymers were end capped at both chain ends. 64 Figure 4 3. Synthesis of OPE TBT model compound. The OPE TBT model compound was synthesized stepwise. Reaction of 1,4 diiodobenzen e with 1 equivalent trimethylsilyl acetylene (TMSA) and 1 equivalent triisopropylsilyl acetylen e (TIPSA) afford ed compound 3 . Deprotection reaction was performed on compound 3 with potassium carbonate to yield compound 4 , which was further reacted with compound 1 to make compound 5 . Under Sonagashira reaction conditions, the coupling of compound 5 w ith compound A1 yields compound 6 . Deprotection of the triisopropylsilyl group on compound 6 produces compound 7 with high yield. The last step is to couple compound 7 with compound 5 . The reaction yields
122 of compound 3 and 5 are relatively low due to the p ossibility of creating both di and mono substituted products. The polymer molecular weights and structure were characterized by gel permeation chromatography ( GPC ) and 1 H NMR. I t is known that GPC usually overestimates the molecular weight of conjugate d p olymers, but it is still a well accepted method for relative molecular weight characterization in this field . 80 The number average molecular weights (M n ) of polymers vary from 5.7 to 14 kDa which correspond to 19 to 50 r epeating units. Polymers are name d after the number of repeating units, PPE TBT 19, PPE TBT 30, PPE TBT 37 and PPE TBT 50, respectively. The PPE polymer has a molecular weight of 5.4 kDa, which is comparable to the shortest chain length TBT end capped poly mer, namely PPE TBT 1 9 . Figure 4 4 . GPC traces of PPE TBT n. PPE TBT 19 (black squares, Mn=5.7 kDa, PDI=2.24), PPE TBT 30 (red circle, Mn=8.7 kDa, PDI=2.57), PPE TBT 37 (blue up triangle, Mn=10.6 kDa, PDI=2.55), PPE TBT 50 (dark cyan down triangle, Mn=1 4.0 kDa, PDI=2.87) and PPE (left triangle magenta, Mn=5.4 kDa, PDI=1.70).
123 1 H NMR was used to confirm the chemical structure and molecular weight. Resonances at = 8.15 ppm (label ed as T) belong to benzothiadiazole protons and the resonance at = 2.85 ppm (labeled as M) is assigned to the methylene protons ( CH 2 ) next to phenyl groups on the conjugated backbone (Figure 4 5) . And, since there are two TBT units o n each polymer chain, the number of methylene groups can be determined using the ratio of the peak integrations of T to M . Each repeat unit has methylene groups and, therefore, the number of repeat is determined. However, as molecular increases, the NMR si gnal integration method becomes less accurate. 62 Figure 4 5 . 1 H NMR spectra of PPE TBT n and PEE . A ) Zoomed in area and B ) whole spectra . Optical P roperties in S olution The absorption and fluorescence spectra of the TBT TIPS , OPE TBT and PPE polymer were measured in THF solutions and they are plotted in Figure 4 6 . The PPE polymer features a broad single absorption band at around 387 nm while the OPE TBT and TBT TIPS have both charge transfer absorption bands (Figure
124 4 6 A ). Compared to TBT TIPS, both absorption bands of OPE TBT are red shifted due to increasing HOMO energy level. Figure 4 6 . Steady state photophysical properties in THF solutions. A ) Norm alized Uv vis ab sorption and B ) fluorescence emission spectra of PPE(black), TBT TIPS (red) and OPE TBT (blue); C ) n ormalized Uv vis absorption of and D ) e mission spectra of PPE TBT 19 (black squares), PPE TBT 30 (red circle), PPE TBT 37 (blue up triangle ), PPE TBT 50 (dark cyan down triangle) and PPE (left triangle magenta); emission spectra w ere normalized according to the quantum yield. Inset plot in Figure C is the zoom in area of 425 600 nm. Inse t plot in figure D shows the energy transfer efficiency. The excitation wavelength is 370 nm for all samples. The PPE backbone and TBT moiety have a great mismatch in absorption spectra , which allow s selectively excitation of the PPE chromophore and study of the energy transfer efficiency. The PPE backbone and TBT moiety also show distinct emissions with the PPE emitting at 420 nm and the TBT featuring a broad emission peak at 590
125 nm (Figure 4 6 B ). But, both chromophores have fairly high quantum yield ( PPE = 0.92 and TBT TIPS = 0.73). The OPE TBT shows little donor emission due to very efficient energy transfer (> 99.5%) and the acceptor emission also red shifts compared to that of TBT TIPS. The steady state absorption of PPE TBT n polymers was measur ed in THF and the concentr ation ( based on repeat units) . And, the spectra were plotted in Figure 4 6 ( C ) and normalized at absorption maximum (~390 nm). The PPE , which caps , has no absorption beyond 430 nm. But, an additional absorption band, from 470 nm to 550 nm, shows up in the spectra of TBT PPE polymers, which is attributed to the TBT absorption. The intensity of the TBT absorption increases with decreasing molecular weight , indicating increasing TBT content. But, compared to the dominating absorption band 390 nm , which is attributed to the PPE backbone absorption, the TBT absorption is still less intense. Since the effective conjugation length of PPE type polymers in isotropic solvent (like THF or DCM ) is lim ited to be aro und 9 10 repeat unit s, the absorption maximum of PPE backbone remains essentially the same. 64 Steady state emission spectra of polymers (PPE TBT n and PPE) at the same g unit s) were plotted in Figure 4 6 ( D ) and normalized according to their quantum yield. The PPE shows a single peak at around 420 nm and the PPE TBT n polymers have an additional band at about 600 nm , which is attributed to the TBT emission. The emission i ntensity of the 420 nm peak decreases with increasing TBT content while the 600 nm peak increases, which indicates more energy transfer from the PPE backbone to the end TBT acceptors. Compared to the
126 PPE, more than 60% of the donor emission (420 nm) in PPE TBT 50 is quenched and the quenching is more significant in PPE TBT 37 with efficiency reaching 70%. This trend continues as the molecular weight of polymers decreases as evidenced in the PPE emission fluorescence quantum yield (Table 4 1). The overall en ergy transfer efficiency was calculated using the quantum yield of emission from 380 nm to 525 nm (the donor emission) in PPE TBT n polymers over the quantum yield of PPE . T he trend is clear that energy transfer efficiency decreases with TBT content. The t rend is supported by the changing of fluorescence lifetime at 420 nm which decreases with molecular weight: the average lifetime at 420 nm is about 357 ps, 299 ps, 290 ps and 222 ps for PPE TBT 50, PPE TBT 37, PPE TBT 30 and PPE TBT 19, respectively. The d ecreasing in donor lifetime indicates more efficient quenching , which is caused by energy transfer to the acceptors. Table 4 1 . Photophysical Properties of Polymer Samples Polymer abs (nm) em (nm) lifeti m e at 420 nm (ps) a Lifetime at 600 nm (ns) a quantum yield b Energy Transfer Efficiency % c PPE (380 nm 525 nm) TBT (525 nm 750 nm) Overall (380 nm 750 nm) TBT PPE 19 389 423 222 4.45 0.082 0.52 0.6 91.1 TBT PPE 30 393 423 29 0 4.45 0.16 0.55 0.71 82.6 TBT PPE 37 393 423 299 4.45 0.27 0.4 0.67 70.7 TBT PPE 50 396 423 357 4.45 0.34 0.39 0.72 63.0 PPE 397 423 462 N.A. 0.92 N.A. 0.92 N.A. a Data was collected on a streak camera. b With anthrathene as quantum yield standard, =0.27 in ethanol at room temperature. c Energy transfer efficiency (PPE ( 380 525nm) ) / (PPE), in which only polymer PPE. Fluorescence Polarization Investigation Fluorescence polarization studies of polymers ( PPE TBT n and PPE) were performed at room temperature using THF as the solvent while the anisotropy of OPE TBT was measured in glycerin (Figure 4 7 ) . A 3 8 0 nm excitation wavelength was
127 chosen to selective ly excite the PPE chromophore. The fluorescence emission polarization, which can be characterized by anisotropy (r), is calculated from the emission intensity through a polarizer using equation : (4 1) where I VV and I VH are the intensity observed when the emission polarizer is parallel and perpendicular to the direct ion of the polarized excita tion respectively. Figure 4 7. Anisotropy characterization and simplified representation of polymer chain conformation. A) Anisotropy of PPE TBT 19 (black), PPE TBT 30 (red), PPE TBT 37 (blue), PPE TBT 50 (dark cyan) and PPE (magenta); B) anisotropy of PPE TBT 19 in DCB (filled black square) and THF (hollow black square) and PPE TBT 50 in DCE (filled blue circle) and THF (hollow blue circle); C) anisotropy of PPE (black), PPE TBT 19 (red) and OPE TBT (blue); D) simplified representation of polymer chain conformation and dipoles. Polymers are dissolved in THF while the OPE TBT is dissolved in glycerin.
128 Two interesting trends can be observed in the emission anisotropy of PPE TBT n polymers: 1) the emission polarization of PPE backbone (400 500 nm) increases with decreasing molecular weight ; 2) the donor emission (400 500 nm) polarization of the OPE TBT is higher than that of polymers and the acceptor emission (550 700 nm) polarization is almost zero and 3) the acceptor emission polarization of the OPE TBT is lower compared to the polymers . The emission anisotropy reveals the angular displacement of the polymers that occur between absorption and emission of a photon, which depends strongly upon the relative time scales of fluorescence emission lifetime and ro tation diffusion of polymers. When the time scales of the two are on the same order, an increase of emission lifetime or decreasing of rotation diffusion rate will result a decreasing anisotropy. 71 The l ifetime of PPE emission increases wi th molecular weight (Tab le 4 1), but , it should not be the cause here. Because the model PPE polymer and PPE TBT 19 have similar molecular weights and they show very similar anisotropy value as well, despite the fact the lifetime of the PPE is two time longer compared to that of PPE TBT 19. An increasing of molecular weight will lower the rotation diffusion rate and increase the ani sotropy, which is the opposite of the observation . But , i n order to rule out the possibility that the change in anisotropy is caused by the changing of either fluorescence emission li fetime or rotation diffusion , anisotropy of PPE TBT 19 and PPE TBT 50 were measured in ortho dic h lorobenzene (DCB) , which is a more viscous solvent and can slow down the polymer rotation diffusion rate . It is obvious that th e emission anisotropy does not change with solvent, which demonstrates that the emission lifetime is way shorter compared to the rotation diffusion rate. Therefore, the change of emission anisotropy
129 should not be caused by the changing of fluorescence life time or rotation diffusion , which leads to the speculation that the change of PPE backbone emission anisotropy is caused by the polymer chain conformation variation . The anisotropy can be calculated according equation: (4 2) In which ( ) is the angle between the absorption and emission dipole moments. The anisotropy is 0.4 w hen the dipoles are aligned with each other ( = 0) and a mis alignment reduces the overall anisotropy . And, the emission anisotropy of the polymer backbone in PPE T n polymers is significantly lower than 0.4 and decreases with increasing molecular weight. It has been demonstrated that in isotropic solution s, PPEs ad segments with different conjugation lengths, which have different segment orientation (Figure 4 7 D) . 64 results in angular displacement variation along the polymer backbones . After photoexcitation, the exciton migrates along the polymer backbone There are more possible dipole orientation s i n lo nger chain length polymers and a greater possibility that the emission dipole is misaligned with the original excitation polymerization direction. As a result, the anisotropy of the PPE backbone emission is lower in high molecular polymers. The anisotropy of the OPE TBT donor emission is significantly higher, because the structure of the OPE TBT is more rigid and there is less conformation variation. After energy transfer, t he anisotropy of the TBT emission is almost zero, which is due to the angular displa cement between the OPE and the TBT dipoles. The molecular
130 structure of the OPE TBT was simulated using DFT calculation s (detailed information can be found in the experimental part) and the result is shown in Figure 4 8 . The dipole angle is calculated by us ing the unit vectors, given in the output of a Gaussian 09 geometry optimization, and setting them in a right triangle. This triangle has the overall dipole vector (0.4147 Debye) as the hypotenuse, and the unit vectors as the other sides of the triangle. T he x vector (0.2521 Debye) runs along the oligo phenylacetylide axis, while the y vector (0.3278 Debye) lies perpendicular, but still in the aromatic plane. The acute dipole angle relative to the oligo phenylacetylide axis is found by calculating the angle using sin 1(y vector/overall dipole) = sin 1(0.3278/0.4147) = sin 1(0.7905) = 52.2 degrees. T he anisotropy is calculated to be 0.024 ( E quation 4 2 ) . And, t he measured anisotropy of the OPE TBT acceptor emission is about 0.038 which agrees well with the ca lculated result. Figure 4 8 . Structure simulation and calculated dipole of OPE TBT. The anisotropy of TBT emission in the polymer is significantly higher compared to OPE TBT compound , which is also very likely due to the polymer chain conformation varia tion . In polymers, the conjugated segments have different orientation , which increases the chance that the absorption and emission dipoles of T BT are aligned with each other and increases the overall anisotropy . A nd the average angle, , can also be determ ined using the average anisot ropy values according to equation 4 2 and are listed in Table 4 2 .
131 The energy transfer efficiency in conjugated polymers is affected by polymer chain conformation. The le ss planarized polymer chains lead to less efficient exc iton migration due to reduced alignment of transition dipole mo men ts. 176 Therefore, the change of energy transfer efficient in different chain length polymers can be the results of two factors: the increased distance between the effective donor and acceptors and reduced e xciton migration efficiency , which is caused by conformation disorder. Table 4 2. Average Anisotropy and Angle Displacement r ave (400 500 nm) r ave (550 700 nm) o ) a PPE TBT 19 0.265 0.099 45.1 PPE TBT 30 0.251 0.100 45.0 PPE TBT 37 0.236 0.106 44.5 PPE TBT 50 0.212 0.113 43.8 PPE 0.249 N.A. N.A. OPE TBT 0.346 0.038 50.8 a Calculated using Energy Tra nsfer Kinetics Time resolved fluorescence lifetime of the PPE TBT n polymers and PPE were investigated to understand the relative long time energy transfer behaviors , using the streak camera . Figure 4 9 A compares the lifetime of the donor emission (PPE, 4 20 nm) of the PPE TBT n polymers and the PPE. The fluorescence of the PPE TBT n polymers decays faster than that the PPE, indicating that the energy transfer is still occurring even at long time scale. T he decay rate depends on the chain length, with the s hortest chain decaying fastest (Table 4 1) . The PPE decays slower and has a rising time. In contrast, the fluorescence lifetime of the acceptor emission (TBT, 600 nm) remains the same for the PPE TBT n polymers regardless of the chain length. But the life time is shorter compared to the TBT TIPS compound and the reason should be that the conjugation system of the TBT is disturbed in the PPE TBT n polymers. Close analysis
132 of early time fluorescence decays reveals that the majority of the energy transfer must happen within the instrument response of the camera (~15 ps), since there is no 9 B, inset plot). Figure 4 9. Fluorescence lifetime characterization in THF with streak camera. A) Donor emission lifetime (420 nm) of PPE TBT 19 (black ), PPE TBT 30 (red), PPE TBT 37 (blue ), PPE TB T 50 (dark cyan ) and PPE (magenta); B) acceptor emission lifetime (600 nm) of PPE TBT 19 (black), PPE TBT 50 (dark cyan) and TBT TIPS (magenta). Inset figure shows the early time of PPE TBT 19 and PPE TBT 50 decay. Figure 4 10. Ultrafast transient absorption spectra of the PPE and PPE TBT 30. In order to further investigate the fast component of the energy transfer dynamics, ultrafast transient absorption measurements were performed on pol ymers.
133 Figure 4 10 compares the transient absorption spectra of the PPE and PPE TBT 30 at t = 100 ps. The transient spectrum of the PPE (black line) shows negative band (bleach) from 350 nm to 520 nm, which is the result of a combination of ground state bl eaching and stimulated emission. There are also indication of polymer relaxation in the spectrum, evidenced by the subtle red shift of stimulated emission (420 nm and 450 nm) compared to the steady state fluore scence emission (Figure 4 6). Both the transie nt absorption spectra of the PPE and the PPE TBT 30 also feature a strong broad induced absorption band starting from 520 nm. Figure 4 11. Time resolved ultrafast transient absorption spectra. A) PPE and B) PPE TBT 30. The time resolved ultrafast transi ent absorption spectra provide more evidence on polymer relaxation, since the induced absorption is shifting to lower energy and broadening as well (Figure 4 11 A). The ultrafast transient absorption spectrum of the PPE TBT 30 mostly resembles that of the PPE, indicating that laser pulse predominantly excite the PPE backbone. But, there are additional peaks in the spectrum, which indicates the energy transfer feature. The negative signal at 480 nm corresponds to the bleaching of the TBT end caps while the s timulated TBT emission
134 (negative signal, ~600 nm) is overlapping with the induced absorption of the PPE and the TBT. The stimulated emission of TBT can be observed after 1 ns (Figure 4 11 B). These features arise due to energy transfer from the PPE backbon e to the TBT end caps, since only the PPE backbone is excited. The energy transfer dynamics was examined at the wavelength of 428 nm and plotted in Figure 4 12 A. The wavelength was chosen because it has the least contribution from the TBT related spectral features as well as minimal effects from the PPE relaxation. This wavelength is also between the two PPE stimulated emission peaks (~420 nm and ~440 nm), so the spectrum shifting does not have a significant effect. The signal from the PPE polymer evolves slower and there is a slight growth at the beginning of the decay, which is also observed in the time resolved fluorescence lifetime. The PPE TBT 19 polymer decays much faster compared to the PPE and the decay kinetics of the PPE TBT n polymers begin to re semble the PPE polymer as the polymer chain gets longer. Figure 4 12. TA decay kinetics. A) Kinetic trace and B) early time decay of PPE TBT 19 (black ), PPE TBT 30 (red), PPE TBT 37 (blue ), PPE TB T 50 (dark cyan ) and PPE (magenta).
135 A careful examination of the decay kinetics reveals that there are ultrafast component s (< 2 ps) in the e nergy transfer process, which are attributed to the rapid energy transfer from the excited PPE segments that are close to the TBT end caps (Figure 4 12 B). Therefore, the P PE TBT 19 has greater amplitude to the fast time component, since the chance of having an excited PPE segment close to the TBT end caps is higher. There are also relatively slow component in the energy transfer process, which is attributed to the exciton h opping along the polymer backbone. For longer chain length polymer, the possibility of creating an exciton which is far away from the acceptor is higher, thus, the exciton hopping is more likely to happen before the energy transfer. Figure 4 13. Schema tic representation of the energy transfer processes in conjugated polymers. A) Direct energy transfer and B) exciton hopping and energy transfer. A comparison of the time resolved fluorescence data and ultrafast TA data shows that these two data sets match up very well and they are plotted on top of each other in Figure 4 1 4 . The fluorescence experiment completely misses the ultrafast component for the decay kinetics due to the instrument limitation. There are other
136 noticeable difference s in the two, probab ly due to the fluorescence decay being the integrated decay over the whole emission spectrum , whereas the ultrafast TA decay is solely the signal of one wavelength. The calculated lifetime is listed in Table 4 3. The 2 a nd t 3 were taken from fits to the time resolved fluorescence. The ultrafast components for the PPE TBT 19, PPE TBT 30 and PPE TBT 37 polymers are within 1 ps and the amplitude decreases with increasing polymer chain length. The PPE TBT 50 has negative comp onent (signal rising time) due to the contribution of the PPE backbone fluorescence. The calculated average lifetime clear ly shows that the energy transfer rate decreases with increasing polymer chain length. Figure 4 1 4 . Comparison of ultrafast TA decay kinetics ( circles ) and time resolved fluorescence ( lines ) data. PPE TBT 19 (black ), PPE TBT 30 (red), PPE TBT 37 (blue ), PPE TB T 50 (dark cyan ) and PPE (magenta). Table 4 3. Summary of lifetime measurements. a 1 t 1 (ps) a 2 t 2 (ps) a a 3 t 3 (ps) a t ave (ps) b PPE TBT 19 0.4 0.47 0.31 55 0.29 348 117 PPE TBT 30 0.25 0.46 0.29 73 0.46 374 193.4 PPE TBT 37 0.15 0.64 0.31 78 0.53 378 225.7 PPE TBT 50 0.19 42.5 0.52 154 0.68 420 355.4 a Data t aken from the fluorescence lifetime obtained via the steak camera . b n* t n .
137 Summary Energy transfer in conjugated donor acceptor polymers was studied in a system consisting of poly (phenylene ethynylene) (PPE) end capped with thiophne benzothiadiazole (TBT). A series of polymers with different chain leng th were Sonogashira polycondensation reaction. Highly efficient energy transfer from PPE donor to TBT acceptor was observed by fluorescence emission measurements and the overall ene rgy transfer efficiency decreases with increasing polymer molecular weights, with the lowest molecular weight polymer achieving an overall of more than 90% energy transfer efficiency . The decreased energy transfer efficiency is most likely due to longer di stance between the donor and the acceptor, which has been observed in many FRET energy transfer systems. Fluorescence emission anisotropy demonstrates that polymers , which lower the energy migration rate and the energy transfer efficiency . Ultrafast transient absorption and time resolved fluorescence lifetime investigation s confirm that m ost of the energy transfer happens with the first 10 20 ps and the energy transfer rate decreases with increasing polym er chain length . In addition, there is evidence showing energy transfer is also occurring on long time scale (~ns). Experiments and Materials Materials Unless specified, all compounds and solvents were purchased from commercial sources ( Aldrich, Acros o r Strem Chemicals ) and used without further purification. For all palladium catalyzed reactions, the solvents were carefully degassed with argon for at least 30 min.
138 Instrumentation 1 H and 13 C NMR spectra were recorded on either Varian Inova2 (500 MHz) o r Gemini million (ppm) using the residual solvent signals as internal standards. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion C yclotron Resonance mass spectrometer (Bruker Daltonics, MA). Gel Permeation Chromatography (GPC) data was collected on a system composed of a Shimadzu LC 6D pump, an Agilent mixed D column, and a Shimadzu SPD 20A photodiode array (PDA) detector, with THF a against linear narrow dispersed polystyrene standards in THF. 1800 dual beam absorption spectrophotometer. Photoluminescence spec tra were recorded on a spectrofluorimeter from Photon Technology International (PTI). Computational calculation was carried out using DFT as implemented in Gaussian 091 Rev. C.0 2 . 177 The geometry was optimized using the B3LYP functiona l along with the 6 31G(d) basis set for all atoms. Silicon isopropyl groups and solubilizing hexyl moieties were replaced with methyl groups for computational efficiency. This or OPE TBT is termed as OPE without symmetry constraints. The optimized structure was characterized by a vibrational frequency calculation and was shown to be at a minimum by the absence of ima ginary frequencies. Spectroscopy carried out at UNC Chapel Hill by Robert J. Dillon. In the photo luminescence lifetimes experiments, light emanating from the sample is re collected by
139 the objective, transmitted through a dichroic beam splitter (Semrock: F F670), and focused onto the entrance slit of a streak camera (Hamamatsu: Streakscope). The instrument response of the streak camera is approximately 17 ps. 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 z 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 Âµ J/cm 2 ) were necessary to achieve linear behavior of the transients. Kinetics w ere monitored by a weak continuum probe pulse generated by focusing a small portion of the 775 nm fundamental into a translating CaF 2 window. Spectra were collected at a rate of 1 kHz 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 pump off spectra. Synthetic Procedures Tetrakis(triphenyl phosphine) palladium (Pd(PPh 3 ) 4 ) was from Strem Chemical and triisopropylsilyl acetylene (TIPSA) was from TCI. Copper (I) iodide (CuI) , diisopropylamine ((i Pr) 2 NH), tetrahydrofuran (THF) and all other chemicals were purchase d from either Sigma Aldrich or Fisher Chemicals. All reagents were used without further purification unless specified. 1,4 dihexyl 2,5 diiodobenzene ( 1 ) , 1,4 diethynyl 2,5 dihexylbenzene ( 2 ) and PPE were synt hesized following the procedures detailed in the previous chapter . 4 (5 ethynylthiophen 2 yl) 7 (thiophen 2 yl)benzo[c][1,2,5]thiadiazole ( A1 ), 4 (5 iodothiophen 2 yl) 7 (thiophen 2 yl)benzo[c][1,2,5]thiadiazole ( A2 ), 178 tert butyl 2 (4 -
140 ethynylphenyl)acetate ( B1 ) 140 and tert butyl 2 (4 iodophenyl)acetate ( B2 ) 140 were synthes ized according to published proc edures. General Procedure for Polymerization . Monomer 1 (84.6 mg, 0.17mmol), 2 (50 .0 mg, 0.17 mmol) and A1 (0.1 0.3 molar ratio) were dissolved in a solution of 20 m L dry THF and 10 m L (i Pr) 2 NH in a round bottom flask at room temperature . The solution was degas s ed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was heated to 4 5 Â°C for 24 hr. Then, A2 (0.1 0.3 molar ratio) was added to the solution and the reaction was allowed for another 12 h. The solution was flashed through a silica gel column and the eluent wa s collected and concentrated. The concentrated solution was poured into 100 m L of methanol and the polymer precipitated out immediately. This process was repeated twice. Last, the precipitate was collected and dried under vacuum. Typic al yield of this reac tion is 50 70%. PPE TBT 19. GPC (THF, polystyrene standard): M n = 5.7 kDa, M w = 12.9 kDa, PDI = 2.24. 1 H NMR (500 MHz, CDCl 3 8.15 (d, 2H), 8.05 (d, 2H), 7.90 (s, 4H), 7.69 (dd, 2 H), 7.38 (br. 28H), 7.36 (d, 2H), 7.24 (dd, 2H), 2.85 (t, 56 H) , 1.73 (m, 56 H), 1. 43 (m, 56H), 1.34 ( br , 112H), 0.89 (t, 84 H). PPE TBT 30. GPC (THF, polystyrene standard): M n = 8.7 kDa, M w = 22.5 kDa, PDI = 2.57. 1 H NMR (500 MHz, CDCl 3 8.15 (d, 2H), 8.05 (d, 2H), 7.90 (s, 4H), 7.69 (dd, 2 H), 7.38 (br. 40H), 7.36 (d, 2H), 7.24 ( dd, 2H), 2.85 (br, 80 H) , 1.73 (br, 80 H), 1. 43 (br, 80H), 1.34 (br , 160 H), 0.89 (br, 120 H). PPE TBT 37. GPC (THF, polystyrene standard): M n = 10.6 kDa, M w = 27.2 kDa, PDI = 2.55. 1 H NMR (500 MHz, CDCl 3 8.15 (d, 2H), 8.05 (d, 2H), 7.90 (s, 4H), 7.69
141 (dd, 2 H), 7.38 (br. 52H), 7.36 (d, 2H), 7.24 (dd, 2H), 2.85 (br, 104 H) , 1.73 (br, 104 H), 1. 43 (br, 104H), 1.34 ( br , 208 H), 0.89 (br, 156 H). PPE TBT 50. GPC (THF, polystyrene standard): M n = 14.0 kDa, M w = 40.3 kDa, PDI = 2.87. 1 H NMR (500 MHz, CDCl 3 8.15 (d, 2H), 8.05 (d, 2H), 7.90 (s, 4H), 7.69 (dd, 2 H), 7.38 (br. 60H), 7.36 (d, 2H), 7.24 (dd, 2H), 2.85 (br, 120 H) , 1.73 (br, 120 H), 1. 43 (br, 120H), 1.34 ( br , 240 H), 0.89 (br, 180 H). Compound 3 . 1,4 D iiodobenzen e ( 5 g, 15.2 mmol ) was dissolved in a mixed solve nt of THF (40 mL) and (i Pr) 2 NH (40 mL) and the solution was degassed for 30 min before 150 mg (0.13 mmol) Pd(PPh 3 ) 4 and 50 mg (0. 26 mmol) of CuI were added . Then, trimethylsilyl acetylene (1.49 g, 15.2 mmol) and triisopropylsilyl acetylene (2.76 g, 15.2 m mol) were added. The resulting mixture was stirred under Ar protection at R.T. for 4 h. After the reaction, the solvent was removed under vacuum and the residue solid was purified by column with hexane as the eluent to yield compound 3 (2.26 g, 42%) . 1 H NM R (500 MHz, CDCl 3 Compound 4 . Compound 3 (2 g, 5.6 mmol) was dissolved in a mixed solvent of THF (40 mL) and MeOH (10 mL). Potassium carbonate (1.5 g, 11.3 mmol), which was dissolved in 5 mL of water, was added to the mixture and the resulting mixture was stirred at R.T. for 2 h. After the reaction, the mixture was extracted with DC M (3 x 20 mL ) and the organic layers were combined. After removing the solvent under vacuum, the residual solid was purified with a flash column wit h hexane as the eluent to yield compound 4 ( 1.50 g, 95% ). 1 H NMR (500 MHz, CDCl 3 7.42 (s, 4H), 3.16 (s, 1H), 1.13 (s, 21H).
142 Compound 5 . Compound 4 (1 g, 3.5 mmol) and compound 1 (1.77 g, 3.5 mmol) were dissolved in a solution of 20 ml dry THF and 10 ml (i Pr) 2 NH in a round bottom flask at room temperature . The solution was degas s ed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was stirred at R.T. for 24 hr. After the reaction, the solvent was removed under vacuum and the residue solid was purified by silica column with 1:1 ratio of hexane and DCM as the eluent to yield compound 5 (0.93 g, 40%). 1 H NMR (500 MHz, CDCl 3 7.67 (s, 1H), 7.44 (dd, 4H), 7.30 (s, 1H), 2.73 (t, 2H), 2.65 (t, 2H), 1.69 1.55 (m, 4H), 1.43 1.27 (m, 12H), 1.14 (s, 21H), 0.89 (m, 6H). Compound 6 . Compound 5 (0.8 g, 1.2 mmol) and compound A1 (0.4 g, 1.2 mmol) were dissolved in a solution of 20 ml dry THF and 10 ml (i Pr) 2 NH in a round bottom flask at room temperature . The solution was dega s sed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was stirred at R.T. for 24 hr. Aft er the reaction, the solvent was removed under vacuum and the residue solid was purified by column with 1:1 ratio of hexane and DCM as the eluent to yield compound 6 (0.92 g, 88%). 1 H NMR (500 MHz, CDCl 3 7.46 (dd, 4H), 7.37 (s, 2H), 7.34 (d, 1H), 7.22 (d, 1H), 2.81 (t, 4H), 1.72 (m, 4H), 1.49 1.21 (m, 4H), 1.15 (s, 21H), 0.90 (m, 6H). 1 3 C NMR (125 MHz, CDCl 3 1 32.69, 132.51, 132.21, 132.15, 131.36, 128.21, 127.87, 127.47, 127.19, 126.57, 125.83, 125.79, 125.36, 124.91, 123.50, 123.45, 122.72, 122.51, 106.83, 94.43, 94.02, 93.02, 90.46, 87.72, 34.33, 34.31, 31.93, 31.90, 30.84, 30.78, 29.42, 29.40, 22.84, 22.78, 18.83, 14.33, 14.27, 11.48.
143 Compound 7 . Compound 6 (0.8 g, 0.94 mmol) was dissolved in chloroform (15 mL) and degassed for 30 min. Then, 1 mL TBAF in THF (1 M) was added and the solution was stirred for 4 h at R.T. After the reaction, the solution was wash ed with water and extracted with DCM (3 x 15 mL ) and the organic layer was collected and dried with anhydrous sodium sulfate (Na 2 SO 4 ) . The solvent was removed under vacuum and the solid was purified by column with 1:1 ratio of hexane and DCM as the eluent to yield compound 7 (0.59 g, 91%). 1 H NMR (500 MHz, CDCl 3 1H), 7.86 (dd, 2H), 7.46 (dd, 4H), 7.37 (s, 2H), 7.34 (d, 1H), 7.22 (d, 1H), 3.21 (s, 1H), 2.81 (t, 4H), 1.72 (m, 4H), 1.49 1.21 (m, 4H), 0.90 (m, 6H). 1 3 C NMR (125 MHz, CD Cl 3 ) 131.46, 128.19, 127.87, 127.45, 127.16, 126.55, 125.79, 125.74, 125.32, 124.88, 124.09, 122.63, 122.60, 122.04, 94.40, 93.73, 90.66, 87.79, 83.45, 79.09, 34.33, 34.29, 3 1.92, 31.91, 30.82, 30.76, 29.42, 29.39, 22.84, 22.77, 14.32, 14.25. OPE TBT . Compound 7 (0.4 g, 0.57 mmol) and compound 5 (0.38 g, 0.57mmol) were dissolved in a solution of 20 ml dry THF and 10 ml (i Pr) 2 NH in a round bottom flask at room temperature . The solution was degased for 30min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was stirred at R.T. for 24 hr. After the reaction, the solvent was removed under vacuum and the residue solid wa s purified by column with 1:1 ratio of hexane and DCM as the eluent to yield compound OPE TBT (0.36 g, 72%) . 1 H NMR (500 MHz, CDCl 3 8.13 (d, 1H), 8.01 (d, 1H), 7.85 ( dd , 2H), 7.52 (s, 4H), 7.46 (m, 5H), 7.3 8 (d, 4H), 7.34 (d, 1H), 7.20 (m , 3H), 2.82 (t, 8H), 1.72 (m, 8H), 1.49 1.21 (m, 8H), 1.15 (s, 21H), 0.90 (m, 12H). 1 3 C NMR (125 MHz, CDCl 3
144 140.85, 139.38, 132.69, 132.50, 132.20, 132.15, 131.57, 131.35, 128.20, 127.86, 127.45, 127.17, 126.53, 125.80, 125.76, 125.32, 124.88, 123.48, 123.42, 122.70, 122.67, 122.64, 122.54, 106.82, 94.43, 94.07, 93.93, 93.01, 90.60, 90.43, 87.77, 34.34, 34.31, 31.93, 31.92, 31.90, 30.84, 30.80 , 29.44, 29.40, 22.85, 22.79, 22.78, 18.82, 14.34, 14.27, 11.47. MS (MALDI) m/z ([M + H] + ), calculated 1216.623, found 1216.621.
145 CHAPTER 5 ENERGY TRANSFER IN CONJUGATED POLYMERS Background Light harvesting polymers , which can convert solar energy to either electrical energy or chemical energy , have drawn a lot of attention fr om the science community . 179 181 They also show very intere sting light/electro response activities which help them find applications in areas such as light emitting diodes , 2 , 182 optical data storage 183 , 184 and optical limitin g 185 , 186 . Both fundamental study and application of light harvesting polymers have been focused on materials with conjugated backbones. However , polymers with interrupted conjugated backbone or conjugated side chains can be very interesting as well . 187 , 188 They inherit many typical polymer properties , such as high mechanical strength and better stability, but their photo optical properties resemble individual chromophores. The [2,2]p aracyclophane moiety brings two phenyl ring s in close proximity to each other and enforces a cofacial overlap , which causes partial overlap of the two electrons . And, when incorpora ted into conjugated structures, interesting photophysical properties have been discovered. The main contribution of absorption comes from the monomer state (the structure mimics stilbenes). The emission can come from two different excited states: the ph ane state whose conjugation has contribution from the entire [2,2]cyclophane unit and the monomer state which only involves the stilbene structure . In [2,2]cyclophane containing oligomers and polymers, the conjugation length increase s with number of rep eating unit s, but, the conjugation is less efficient compared to fully conjugated polymers with similar structure. 33 , 189 191 This special conjugation behavior has been termed . The
146 [2,2]cyclophane containing polymers can be v iewed as a series of stacked electron systems and is valuable for intermolecular interaction study . Traditionally, intermolecular interaction study was performed on bulk materials, such as polymer films. However, spectroscopic measurements on bulk materi als do not really reveal the average status of molecules, rather the lowest energy state. In dilute solution, intermolecular interaction is minimized which is very difficult to probe. ning [2,2]cyclophane moieties backbone and 4,7 di(thiophen 2 yl)benzo[c][1,2,5]thiadiazole (TBT) end caps were synthesized. This system is chosen because of the perfe ct match of the emission spectrum of polymer back bone with the absorption spectrum of the TBT and high quantum yield of both the backbone and the TBT end caps . Energy transfer is very efficient, with dynamic and overall efficiency comparable to fully conjugated systems which proves that this type of polymers can serve as molecular wires. Resul ts and Discussion Synthesis and C haracterization The molecular structures and synthesis route are outlined in F igure 5 1. 4,16 Diethynyl[2,2]paracyclophane ( Monomer 1 ) was synthesized in three steps, starting with reaction of [2,2]paracyclophane with c once ntrated bromine. The resulting compound 1 was further converted to 4,16 bis[(trimethylsilyl)ethynyl][2,2] paracyclophane ( compound 2 ) under typical Sonogashira reaction condition s . Deprotection of compound 2 with TBAF in chloroform gave monomer 1 with high yield. Reaction of hydroquinone with n bromooctane under basic condition s yield s compound 3 which was easily converted to monomer 2 under typical iodination reaction condition s .
147 Figure 5 1. Molecular structures and synt hesis procedures.
148 was applied for the synthesis of polymers and controlling the molecular weight. In order to determine the absorption and emission states of [2,2]paracyclophane c ontaining polymers (P1 n) , a fully conjugated polymer (PPE) and a model compound (OPE) have been synthesized and the detail synthetic information can be found in experiment al section. The molecular weights of P1 n polymers vary from 5.8 kDa to 10.6 kDa and 23.5 kDa which corresponds to 9, 19 and 39 repeat unit s, respectively. The molecular weights of the P0 and the PPE are 12 kDa and 61.2 kDa, respectively, which co rrespond to 20 and 136 repeat unit s. All polymers should reach saturated conjugation, for the maximum conjugation length of [2,2]paracyclophane containi ng polymers is 9 repeating unit s while that of PPE is about 9 10. 191 Figure 5 2. GPC trace of polymers. P1 9 ( black square ), P1 19 ( red circle ), P1 39 ( blue triangle ) and P0 ( dark cyan down triangle ) . Optical Properties in Solution The absorption and emission spectra of the P0 , PPE and OPE were plotted in F igure 5 3 . The main absorption band of P0 is at 385 nm which is about 30 nm blue shifted compared to that of PPE (417 nm) , indicating the effective conjugation length is
149 shorter in P0 nt than the completely delocali electrons in fully conjugated systems. However, the partially overlapped elect ron in [2,2]paracyclophane unit does contribute to the conjugation, for the absorption maximum of P0 is about 20 nm red shifted compared to the OPE model compound , whose absorption maximum is at 364 nm. T he emission maximum of P0 resemble s that of OPE, probably due to the fact that the state containing cyclophane core has higher energy . Thus, the emission does not involve the which is consistent with literatu re report. 191 Figure 5 3 . Steady state optical properties of P0, PPE and OPE in THF . A ) Absorptio n and B) emission spectra of P0, PPE and OPE. P0 ( black square ), PPE ( red circle ) and OPE ( blue triangle ) . Excitation wavelength has been set at the absorption maximum for each sample. The steady state absorption spectra of P1 n and P0 at the same concentr ation s) were plotted in Figure 5 3 ( A ) and normalized at absorption maximum (~3 87 nm). P0 , which does not contain the TBT moiety , has no absorption beyond 4 25 nm. An absorption band centered at 470 nm shows up in the spectra of P1 n series and increases with decreasing molecular weight , which is consistent with TBT content (Table 5 1) . But, compared to the dominating absorption
150 band 3 87 nm which is attributed to backbone absorption, the 470 nm band is still less intense . Since t he effective conjugation length of [2,2]paracyclophane containing polymers in solution is limited to be around 9 repeating unit s, the absorption maximum of the backbone remains essentially the same. 191 Figure 5 4 . Steady state absorption and emission of P1 n and P0 . A) Absorption and B) emission spectra of P1 9 ( black square ), P1 19 ( red circle ), P1 39 ( blue triangle ) an d P0 ( dark cyan down triangle ) . Steady state emission spectra of P1 n polymers s) are plotted in Figure 5 4 ( B ) and normalized according to their quantum yield, respectively. In addition to the polymer backbone emission (~ 420 nm ) , the P1 n polymers have an additional peak at about 600 nm which is attributed to the TBT emission. The intensity of the backbone emission (~ 420 nm ) decreases with increasing polymer chain length while the 600 nm peak increases, which indicates more effi cient BBBBBBB energy transfer from the polymer backbone to the end TBT acceptors. Compared to the P0 polymer , more than 8 0% of the donor emission (420 nm) in the P1 3 9 is quenched and the quenching efficiency of P1 19 is slightly higher than P1 39 . However, the quenching efficiency reaches ~95% in the P1 9 polymer . The overall energy transfer efficiency was calculated using the quantum yield of emission
151 from 380 nm to 525 nm (the donor emission) over that of the P 0 and the trend is clear that energy transfer efficiency decreases with increasing polymer chain length . The trend of changing energy transfer efficiency is compared with the changing of fluorescence lifetime at 420 nm which decreases with molecular weight: average lifetime at 420 nm is about 910 ps , 491 ps, 308 ps and 163 ps for P0, P 1 39, P1 19 and P1 9 , respectively. The decreasing in donor lifetime indicates more efficient quenching which is caused by energy transfer to the acceptors. Compared to the fully conjugated systems discussed in the Chap ter 4, the presence of acceptors has a more significant impact on the lifetime of donors. At the same time, the lifetime of acceptor remains un touched. Table 5 1. Photophysical data summary TBT content mol% Lifetime at 420 nm (ps) Lifetime at 600 nm (ns) Fluoresc en ce QY % a E nergy tansfer efficiency b Donor (400 525 nm) Acceptor (525 700 nm) Overall (400 750 nm) P1 9 18.2 163 3.08 4.41 25.3 29.7 93.9 P1 19 9.5 308 3.2 0 11.9 22.7 34.6 83.6 P1 39 4.9 491 3.22 14.3 22.1 36.4 80.3 P0 0 910 N.A. 72.6 N.A. 72.6 N.A. a With anthracene as quantum yield standard, = 0.27 in ethanol at room temperature. b Energy transfer e ciencies ( were calculated as: ( P1 n 400 525nm)/ ( P0), in which ( P0) was the only polymer. Summary and F uture W ork ugated polymers were synthesized and the energy transfer properties were investigated . Polymers featu re backbones containing the [2, 2]paracyclophane moieties, which serve the energy donor, and the TBT end caps, which serve as the energy acceptors. Polymers have different chain length which in Palladium catalyzed Sonogashira polycondensation reaction s . E nergy transfer from donor to acceptor was
152 invest igated by fluorescence emission and time correlated s ingle photon counting (TCSPC) . Although the system does not have system along the polymer backbone, very efficient energy transfer was still observed. In order to further investigate the energy transfer kinetics, ultrafast transi ent absorption study will be conducted on the polymers . Experiments and Materials Materials Unless specified, all compounds and solvents were purchased from commercial sources (Aldrich, Acros, Strem Chemicals, et al) and used without further purification. For all palladium catalyzed reactions, the solvents were carefully degassed with argon for at least 30 min. 1 H and 13 C NMR spectra were recorded on either Inova2 (500 MHz) or Varian Gemini parts per million (ppm) using the residual solvent signals as internal standards. Instrumentation 1 H and 13 C NMR spectra were measured on a Mercury 300, a Gem ini 300, or an Inova 500. Chemical shifts were referenced to the residual solvent peaks. High resolution mass spectrometry was performed on a Bruker APEX II 4.7 T Fourier Transform Ion Cyclotron Resonance mass spectrometer (Bruker Daltonics, MA). Gel Perme ation Chromatography (GPC) data was collected on a system composed of a Shimadzu LC 6D pump, an Agilent mixed D column, and a Shimadzu SPD 20A was calibrated against linea r narrow dispersed polystyrene standards in THF. 1800 dual beam absorption spectrophotometer. Photoluminescence spectra were recorded
153 on a spectrofluorimeter from Photon Technology Inter national (PTI). P hoto luminescence lifetimes were obtained by time correlated single photon counting (TCSPC) using a Fluo Time 100 (Picoquant), and excitation was provided using a PDL 800 B Picosecond Pulsed Diode Laser (375 nm). Synthetic Procedures Tetra kis(triphenyl phosphine) palladium (Pd(PPh 3 ) 4 ) was from St rem Chemical and triisopropylsilyl acetylene (TIPSA) was from TCI. Copper (I) iodide (CuI) , diisopropylamine ((i Pr) 2 NH), tetrahydrofuran (THF) and all other chemicals were purchased from either Sig ma Aldrich or Fisher Chemicals. All reagents were used without further purification unless specified. 4 (5 ethynylthiophen 2 yl) 7 (thiophen 2 yl) B enzo[c][1,2,5]thiadiazole ( A1 ), 192 4 (5 iodothiophen 2 yl) 7 (thiophen 2 yl)benzo[c][1,2,5]thiadiazole ( A2 ) , 178 4,16 diethynyl[2,2]paracyclophane 193 ( Monomer 1 ) and 1,4 diiodo 2,5 dioctylbenzene 194 ( Monomer 2 ) were synthesized according to literature. Iodobenzene and ethynylbenzene were purchase d from Sigma. General Procedure for the synthesis of P1 n . Monomer 1 (84.6 mg, 0.17 mmol), Monomer 2 (50.0 mg, 0.17 mmol) and A1 (0.1 0.3 molar ratio) were dissolved in a solution of 20 m L dry THF and 10 m L piperidine in a round bottom flask at room temper ature. The solution was degas s ed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were a dded. The resulting reaction mixture was heated to 45 Â°C for 24 hr. Then, A2 (0.1 0.3 molar ratio) was added to the solution and the reac tion was allowed for another 12 h. The solution was passed through a short silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100 mL of methanol and the polymer precipitated out
154 immediately. This proc ess was repeated twice. Last, the precipitate was collected and dried under vacuum. Typical yield of this reaction is 50 70%. P1 9 . GPC (THF, polystyrene standard): M n = 5. 8 kDa, M w = 9.2 kDa, PDI = 1.59 . 1 H NMR (500 MHz, CDCl 3 8.15 (d), 8.05 (d ), 7.90 (s), 7. 4 9 (d ), 7.38 ( d ), 7. 23 ( m ), 7. 16 7.06 ( m , 4 H), 6.65 (m , 4 H) , 6.54 (m, 4 H), 4.14 ( br , 4 H), 3.82 (br , 2H), 3.35 (br , 2 H), 3.05 2.93 (br, 4 H), 1.96 (br, 4H), 1.61 1.25 (br, 40H), 0.88 (br, 6H). P1 19. GPC (THF, polystyrene standard): M n = 10.6 kDa, M w = 21.0 kDa, PDI = 1.97 . 1 H NMR (500 MHz, CDCl 3 ) 8.15 (d), 8.05 (d), 7.90 (s), 7.49 (d ), 7.38 (d), 7.23 (m), 7.16 7.06 (m, 4H), 6.65 (m, 4 H) , 6.54 (m, 4 H), 4.14 (br, 4H), 3.82 (br , 2H), 3.35 (br, 2H), 3.05 2.93 (br, 4H), 1.96 (br, 4H), 1.61 1.25 (br, 40H), 0.88 (br, 6H). P1 3 9. GPC (THF, polystyrene standard): M n = 23.5 kDa, M w = 53.0 kDa, PDI = 2.2 5 . 1 H NMR (500 MHz, CDCl 3 ) 8.15 (d), 8.05 (d), 7.90 (s), 7.49 (d ), 7.38 (d), 7.23 (m), 7.16 7.06 (m, 4H), 6.65 (m, 4 H) , 6.54 (m, 4 H), 4.14 (br, 4H), 3.82 (br , 2H), 3.35 (br, 2H), 3.05 2.93 (br, 4H), 1.96 (br, 4H), 1.61 1.25 (br, 40H), 0.88 (br, 6H) . P0. Monomer 1 (84.6 mg, 0.17 mmol), Monomer 2 (50.0 mg, 0.17 mmol) and B1 ( 1.7 mg, 0.017 mmol ) were dissolved in a solution of 20 m L dr y THF and 10 m L piperidine in a round bottom flask at room temperature. The solution was degased for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was heated to 45 Â°C for 24 hr. Then, B 2 ( 3.4 mg, 0.017 mmol ) were added to the solution and the reaction was allowed for another 12 h. The solution was flashed through a silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100 m L of metha nol and the
155 polymer precipitated out immediately. This process was repeated twice. Last, the precipitate was collected and dried under vacuum. Typical yield of this reaction is 65 % P0 . GPC (THF, polystyrene standard): M n = 12.2 kDa, M w = 23.4 kDa, PDI = 1. 83. 1 H NMR (500 MHz, CDCl 3 7.16 7.06 (m, 4H), 6.65 (m, 4 H) , 6.54 (m, 4 H), 4.14 (br, 4H), 3.82 (br , 2H), 3.35 (br, 2H), 3.05 2.93 (br, 4H), 1.96 (br, 4H), 1.61 1.25 (br, 40H), 0.88 (br, 6H). PPE. 1,4 Diethynylbenzene (21.4 mg, 0.17 mmol) and Monomer 2 (5 0.0 mg, 0.17 mmol) were dissolved in a solution of 20 m L dry THF and 10 m L piperidine in a round bottom flask at room temperature. The solution was degas s ed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The res ulted reaction mixture was heated to 45 Â°C for 24 hr. The solution was flashed through a silica gel column and the eluent was collected and concentrated. The concentrated solution was poured into 100 ml of methanol , and the polymer precipitated out immedia tely. This process was repeated twice. Last, the precipitate was collected and dried under vacuum. Typical yield of this reaction is 80 % . PPE . GPC (THF, polystyrene standard): Mn = 61.2 kDa, Mw = 126 kDa, PDI = 2 . 06 1 H NMR (500 MHz, CDCl 3 7.51 (br, 4H), 7.02 (br, 2H), 4.04 (br, 4H), 1.86 (br, 4H), 1.55 (br, 4H), 1.20 1.42 (m, 16H), 0.88 (br, 6H). OPE . Ethynylbenzene (34.7 mg, 0.34 mmol) and Monomer 2 (50.0 mg, 0.17 mmol) were dissolved in a solution of 20 m L dry THF and 10 m L piperidine in a round bottom flask at room temperature. The solution was degas s ed for 30 min before 16.2 mg (0.014 mmol) Pd(PPh 3 ) 4 and 5.0 mg (0.026 mmol) of CuI were added. The resulting reaction mixture was heated to 45 Â°C for 24 hr. after the reaction, the solvent was
156 removed unde r vacuum and the residual solid was purified by column chromatography to yield OPE (79.8 mg, 88%). 1 H NMR (500 MHz, CDCl 3 7.02 (s, 2H), 4.04 (t, 4H), 1.85 (m, 4H), 1.53 (m, 4H), 1.22 1.45 (m, 16H), 0.87 (t, 6H). 1 3 C NMR (125 MHz, CDCl 3 94.83, 85.98, 69.69, 31.84, 29.42, 29.40, 29.33, 26.11, 22.69, 14.10.
157 CHAPTER 6 CONCLUSION Conjugated polymers are among the most promising materials for applications in fields of or ganic photoelectronic devices. The investigation of this category of materials attracts a lot of attention from the science community. The studies detailed in this dissertation focus on the investigation of molecular weight effect s on energy/electron trans fer efficiency in conjugated polymers and the application in dye sensitized solar cells. In Chapter 2 , two families of conjugated polyelectrolytes (CPEs) featuring the same backbone but different side chain linkages were synthe sized and their application as light harvesting materials for dye sensitized solar cells (DSSCs) was investigated. CPEs bearing an oxygen linkage ( O ) (P1 O n) are more likely to aggregate in solution and , the aggregate status depends strongly on the molecular weights. In contrast, there is no obvious evidence showing that CPEs with methylene ( CH 2 ) (P2 C n) linkage aggregate in solution. In addition, the two families of polymer also show different behaviors in a d sorbing onto mesoporous TiO 2 films . The resulting films show different overall cell efficiency when applied in DSSCs . The P1 O n TiO 2 films s how strong chain length dependence in the absorption and, therefore, the cell performance. In contrast, little difference can be observed in the P2 C n TiO 2 films . Once the aggregated P 1 O n polymers a d sorbed onto the surface of TiO 2 films, they block the pores in the film and prevent more materials from penetrating into the inside layer. As a result, it slows down the kinetics of dye a d sorbing and total amount of dyes that are absorbed which ultimately lower s the overall cell efficiency.
158 In Chapter 3, a series of different chain length poly(p phenylene ethynylene)s (PPEs) end capped by naphthalene diimid e derivatives were synthesized. When polymers are excited, electron s transfer from th e PPE backbones to the naphthalene diimid e end caps, and the polymer fluorescence is quenched. The overall quenching efficiency increases with decreasing polymer chain length as evidenced by fluorescence quantum yield measurement s . And, the charge recombin ation rate is investigated by femtosecond transient absorption. The charge recombination rate also depends strongly on the chain length: as the chain length increases, the charge recombination rate decreases. In addition, the formation of the PPE triplet a bsorption was detected in the polymers . The overall triplet absorption intensity increases with molecular weights. More careful investigation is needed to understand the origin of the triplet formation. In Chapter 4 , a series of different chain length pol y(p phenylene ethynylene)s (PPEs) end capped by 4,7 di(thiophen 2 yl)benzo[c][1,2,5]thiadiazole (TBT) were synthesized. Under light irradiation, energy transfer from the PPE to TBT occurs mainly via the Forster Resonance Energy Transfer (FRET) mechanism. T he overall energy transfer efficiency increases with decreasing molecular weight. The overall lifetime of PPE backbones, which is the donor, decreases with molecular weight while the lifetime of acceptors, which are TBT, remains constant. Ultra fast transi ent absorption study shows that the energy transfer happens in pico second time scale for all the polymers and the lower molecular weight samples show faster decay in the initial stage. An interesting trend in steady state fluorescence anisotropy was also discovered that fluorescence anisotropy in the donor emission is significantly higher than that of acceptors. It has been proposed that polymers are not perfect rigid -
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176 BIOGRAPHICAL SKETCH Zhenxing Pan was born in 1987 in Sihong, Jiangsu, China, where he grew up and finished junior high school. At the age of 15, he went to Nanjing and stayed there for 3 years for senior high school. Then, at the age of 18, he attended Soochow University and received a bachelor of engineering degree in material science and enginee ring in the year of 2010. Immediately after graduation, he continued his graduate school work at the University of Florida, pursuing a D octor of P hilosophy degree in chemistry. Under the supervision of Dr. Kirk S. Schanze, he focused his research on conjug ate d polymers and optoelectronic devices. After his graduation, he will go back to China and pursue a career in chemical industry.