1 CONJUGATED PHOTOACTIVE POLYELECTROLYTES AND HYBRID MATERIALS BASED ON CONJUGATED OLIGOMERS: SYNTHESIS, PHOTOPHYSICS AND APPLICATION I N SOLAR CELLS By DONGPING XIE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIV ERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 201 2
2 2012 Dongping Xie
3 To M y parents
4 ACKNOWLEDGMENTS I want to express my deepest appreciation to many people who have shared their support, advice, love and friendship during my time here in Florida. First I give my sincerest gratitude to my advisor Kirk Schanze. With his teaching which is full of wisdom and knowledge I have learnt a lot more than just chemistry With hi s unending patience and support, I have had chance to explore myself as a person, instead of worrying about turning into an obedient machine whose data matters larger than life. Having been working with him is indeed an invaluable asset to me. I am very mu ch obliged to those who have contributed to this dissertation. My collaborator s Dr. John Reynolds, Dr. Jiangeng Xue, and Dr. Paul Holloway have all been extremely supportive T heir wisdom s and expertise on research added many inspirations to the work. My c ommittee professors Dr. Ronald Castellano Dr. Valeria Kleiman and Dr. Sukwon Hong have spared their valuable time providing suggestions and revisions on the writing, to which I am sincerely grateful. I also owe my appreciat e to the student researchers who have worked side by side with me for almost the entire graduate study: Romain Stalder is a handsome intelligent man who also becomes a energy seems endless. Those late nigh t (or early morning) experiments, meetings before deadlines on the turf, and of course, the happy conversations on the highways with them are always be cherished I would like to thank my former and current colleague s who have shared their advice and friendship s Dr. Zhen Fang, Dr. Fude Feng and Dr. Chen Liao offered many useful tip s and trainings that paved my way to this career point. Dr. Anand Parthasarathy tirelessly assisted me editing my writings, and kindly discussed pro blems
5 that I encountered during research. this work by conducting many transient absorption experiments. Zhuo Chen, Xuzhi Zhu, Zhenxing Pan, Jie Yang and Danlu Wu, who are also thousands miles away fro m home s have provided many joyful times. I am grateful to my best friends who are always there for me Yiheng Huang is the smartest person I have ever met. His funny yet witty ideas really ha ve catalyzed my process of self recognition, and his unremitting ly pursue on his goal s has been encouraging all the time. Jinhua Tu is a self taught astronomist and engineer. He shared with me a great deal of happy time of childhood and teenage, and this friendship has never ceased growing up with us. I also want to gi ve Yiheng and Jinhua my innermost gratitude for taking care of my parents while I am oceans apart. Special thanks go to the great composers Frdric Chopin, Ludwig van Beethoven et al whose fantastic music have conquered the hard times along the way. Las t but not the least, I am feeling lucky to be endowed with a happy and warm family. I thank my parents and grandparents for their constant supports and love during the past 27 years. I give my heart whole acknowledgement to my mother Hui Peng. Beyond any w ord, this work is dedicated solely to her.
6 TABLE OF CONTENTS Page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 15 Conj ugated Polyelectrolytes ................................ ................................ ................... 15 Synthesis Approaches ................................ ................................ ...................... 16 Pd catalyzed cross coupling reactions ................................ ....................... 16 Other approaches used in CPE synthesis ................................ ................. 18 Synthetic methodologies ................................ ................................ ............ 20 Photophysical Properties: Aggregatio n and Amplified Quenching of CPEs ..... 21 Aggregation and self assembly ................................ ................................ .. 22 Amplified photoluminescence quenching ................................ ................... 25 Application of CPEs as Functional Materials ................................ .................... 28 Bio/chemo sensory materials ................................ ................................ ..... 28 Dye sens itized solar cells ................................ ................................ ........... 31 Summary for CPEs ................................ ................................ ........................... 36 Conjugated Polymer/Oligomer Semiconductor Nanocrystal Hybrid Materials ........ 36 Synthetic Strategies ................................ ................................ ......................... 37 ................................ ................................ ........................ 38 ................................ ................................ ................... 41 Characterization Interfaces of the Hybrids ................................ ....................... 43 Compositional analysis ................................ ................................ .............. 43 Photoph ysical and electrochemical analysis ................................ .............. 46 ................................ ........................... 48 Summary for Polymer/Oligomer Semiconductor Nano particle Hybrids ............ 50 2 AGGREGATION INDUCED AMPLIFIED QUENCHING IN CONJUGATED POLYELECTROLYTES WITH INTERRUPTED CONJUGATION .......................... 52 Conju gated Polyelectrolytes ................................ ................................ ................... 52 Results and Discussion ................................ ................................ ........................... 54 Synthesis and Structural Characterization ................................ ........................ 54 Photophysics ................................ ................................ ................................ .... 57 Summary and Conclusions ................................ ................................ ..................... 67 Experimental ................................ ................................ ................................ ........... 68 Materials and Methods ................................ ................................ ..................... 68
7 Synthetic Procedures ................................ ................................ ....................... 68 3 SYNTHESIS AND CHARACTERIZATION OF BODIPY BASED POLYELECTR OLYTES AND THEIR INTERACTIONS WITH QUENCHERS ........ 73 Incorporating BODIPY into Polymer Backbones ................................ ..................... 73 Results and Discussion ................................ ................................ ........................... 76 Synthesis and Structural Characterization ................................ ........................ 76 Photophysical Properties ................................ ................................ .................. 80 Int eraction with Quenchers Amplified Quenching and Ion Sensing .................. 84 Application of PB a in Dye sensitized Solar Cells ................................ ............ 90 Summary and C onclusions ................................ ................................ ..................... 94 Experimental ................................ ................................ ................................ ........... 95 Materials and Methods ................................ ................................ ..................... 95 Synthetic Pr ocedures ................................ ................................ ....................... 97 4 CONJUGATED PHOTOACTIVE OLIGOMERS: PHOTOPHYSICAL PROPERTIES, INTERACTIONS WITH CDSE NANOCRYSTALS, AND APPLICATION IN DYE SENSITIZED SOLAR CELLS ................................ ................................ ................................ .................. 104 Hybrid Materi als Based on Conjugated Oligomers ................................ ............... 104 Results and Discussions ................................ ................................ ....................... 107 Synthesis and Structural Characterizations ................................ .................... 107 Photophysics of the Oligomers and CdSe NCs ................................ .............. 110 E chem Characterizations ................................ ................................ .............. 112 Photolum inescence Quenching of the Oligomers and NCs ............................ 114 Oligomer CdSe Hybrids and IPCE Results ................................ .................... 118 Application of the Oligomers in Dye s ensitized Solar Cells ............................ 125 Summary and Conclusions ................................ ................................ ................... 127 Experimental ................................ ................................ ................................ ......... 128 Materials and Methods ................................ ................................ ................... 128 Synthetic Procedures ................................ ................................ ..................... 130 5 CONCLUSIONS AND FUTURE WORK ................................ ............................... 136 Aggregation Induced Amplified Quenching ................................ ........................... 136 BODIPY in the Backbone ................................ ................................ ...................... 137 Conjugated Oligomers with Mono Phosph onic Acid Functionality ........................ 138 LIST OF REFERENCES ................................ ................................ ............................. 140 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 152
8 LIST OF TABLES Table page 1 1 Palladium catalyzed cross coupling reactions ................................ .................... 17 1 2 Examples of CPEs that were synthesized by Pd catalyzed cros s coupling reactions ................................ ................................ ................................ ............. 18 2 1 Photophysical data of C PPE and O PPE ................................ .......................... 59 3 1 Photophysical data of PB e PB a and PB Na ................................ ................... 83 4 1 Contributions made by several groups to the project ................................ ........ 106 4 2 Photophysical data for the oligomers and CdSe NCs in CHCl 3 solution. .......... 112 4 3 Optical data of the oligomer/CdSe hybrids in CHCl 3 solution. .......................... 123
9 LIST OF FIGURES Figure page 1 1 Examples of typical CPEs ................................ ................................ .................. 16 1 2 A general catalytic cycle for Pd catalyzed cross coupling. ................................ 17 1 3 Electropolymerizatio n of thiophenes ................................ ................................ ... 19 1 4 Synthetic approach for PPV type of CPE ................................ ........................... 20 1 5 Photophysics of CPEs with variable bandgaps ................................ .................. 22 1 6 Schematic representation of the aggregative interactions between polymer 1 and different diamine analytes ................................ ................................ ............ 24 1 7 Equations of Photoluminesc ence Quenching. ................................ .................... 26 1 8 .......................... 27 1 9 Stern Volmer plots of polyfluorene qu enched by 5 nm gold nanoparticles. ....... 30 1 10 ................................ ......................... 31 1 11 Principle of operation of DSSCs. ................................ ................................ ........ 33 1 12 J V characteristics under AM1.5 conditions for solar cells sensitized with PPE, PT, and PPE/PT mixture. ................................ ................................ .......... 34 1 13 Structures o f the repeat units for variable gap PPE based CPEs used in TiO 2 sensitized photovoltaic devices. ................................ ................................ 35 1 14 Schematic presentation of molecular recognition between MHT functionalized CdSe NCs and DAP modified P3HT. ................................ ........... 39 1 15 Schematic presentations of different routes the oligomers/polymers are attached to the NC surfaces. ................................ ................................ .............. 40 1 16 ................................ ........ 42 1 17 In situ synthesis of CdS nanorods template by P3HT. ................................ ........ 43 1 18 1 H NM R broadening as free oligothiophenes bind to CdSe NCs. ...................... 45 1 19 TEM characterizations of the hybrid films ................................ ........................... 46 1 20 Energy level ali gnment in a conjugated polymer/CdSe NC hybrid film facilitating charge separations. ................................ ................................ ........... 47
10 1 21 Schematic presentation of the proposed energy level alignment i n CdSe oligothiophene complexes ................................ ................................ .................. 48 2 1 Structures of C PPE and O PPE ................................ ................................ ........ 54 2 2 Synthetic scheme of C PPE and O PPE ................................ ............................ 55 2 3 1 H NMR and peak assignment for C PPE and O PPE ................................ ...... 56 2 4 Absorption and fluorescence spectra of C PPE and O PPE in MeOH.. ............. 58 2 5 Fluorescence spectra of C PPE and O PPE in a mixture of methanol and water ................................ ................................ ................................ ................... 59 2 6 Fluorescence spectra of O PPE with added MV 2+ ................................ .............. 60 2 7 Fluorescence quenching of O PPE by adding DOC.. ................................ ......... 61 2 8 Fluorescence quenching of O PPE by DODC. ................................ ................... 62 2 9 Stern Volme r plots of C PPE and O PPE with MV 2+ in MeOH Water.. ............... 63 2 10 Distribution of particle sizes of C PPE and O PPE in MeOH and H 2 O ............... 65 2 11 Interaction between Ru(bpy) 2 (dppz) 2+ and O PPE .. ................................ ........... 66 2 12 Photophysics of O PPE at different concentrations ................................ ........... 67 2 13 Fluorescence spectra of O PPE at different temperatures in MeOH. ................. 67 3 1 Chemical structure of BODIPY ................................ ................................ ........... 74 3 2 Tw o examples of conjugated polymers incorporating BODIPY. ......................... 75 3 3 Synthesis of monomer 6 ................................ ................................ .................... 76 3 4 Synthesis of monomer 10 ................................ ................................ ................... 77 3 5 Polymerization and deprotection.. ................................ ................................ ...... 78 3 6 1 H NMR spectra and peak assignment for monomer 6 monomer 10 PB e and PB a ................................ ................................ ................................ ............ 79 3 7 Normalized a bsorption and f luorescence spectra of PB e PB a and PB Na .... 81 3 8 Absorption and Fluorescence spectra of PB Na in a mixture of m ethanol and water. ................................ ................................ ................................ .................. 82 3 9 Absorption and fluorescence response of PB Na in MeOH upon adding MV 2+ .. 85
11 3 10 CV and DPV of PB a ................................ ................................ .......................... 87 3 11 Fluorescence quenching of PB Na by Cu 2+ ................................ ........................ 88 3 12 Chemical structure of DOTC ................................ ................................ ............... 89 3 13 Fluorescence quenching of PB Na by DOTC ................................ ..................... 90 3 14 PB a on TiO 2 films. ................................ ................................ ............................. 91 3 15 PB a TiO 2 film transient absorption and decay profile at 700 nm. ..................... 93 3 16 Transient absorption setup for films. ................................ ................................ ... 97 4 1 Chemical structures of conjugated oligomers stu died in this work. ................... 107 4 2 Synthesis of OPE E and OPE A ................................ ................................ ....... 109 4 3 Photophysical characterization of the oligomers. ................................ .............. 111 4 4 Energy level alignment of the ol igomers and CdSe ................................ ......... 113 4 5 Fluorescence quenching of the oligomers by CdSe. ................................ ........ 115 4 6 Evolution of the fluorescence of CdSe NCs upon addition of OPE E and OPE A ................................ ................................ ................................ .............. 117 4 7 Absorbance comparison of the hybrids with parental CdSe NCs.. ................... 11 9 4 8 TGA thermograms of the pristine CdSe NCs and the hybrids. ......................... 121 4 9 Comparison of absorption of T6 A /CdSe hybrid and the free compone nts. ..... 122 4 10 IPCE of OPE /CdSe T6 /CdSe and T4BTD /CdSe hybrids films. ....................... 125 4 11 J V curve and IPCE of T6 and T4BTD in TiO 2 base DSS Cs. ........................... 127
12 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONJUGATED PHOTOACTIVE POLYE LECTROLYTES AND HYBRID MATERIALS BASED ON CONJUGATED OLIGOMERS: SYNTHESIS, PHOTOPHYSICS AND APPLICATION I N SOLAR CELLS By Dongping Xie May 201 2 Chair: Kirk S. Schanze Major: Chemistry This dissertation focus es on fun damental investigation s of the struct ure property relation ship s of conjugated polyelectr olytes (CPE) and conjugated oligomers as well as their application to the dye sensitized solar cells (DSSCs) First, a pair of conjugated polyelectrolytes with sulfonate side groups that contain three ri 2 tether ( C PPE and O PPE respectively) were studied. The linkers serve d along the polymer backbone. Fluorescence spectroscopy reveal ed that O PPE forms a fluores cent aggregate in methanol and water; however, the fluorescence of C PPE is much weaker in water, and C PPE exhibits only weak aggregate fluorescence. Fluorescence quenching of the polymers was examined using methylviologen (MV 2+ ) as a cationic quencher. C PPE show ed only a weak amplified quenching effect, with a Stern Volmer quenching constant of K SV ~ 6 10 5 M 1 in methanol. Interestingly, for O PPE in methanol, the aggregate emission is strongly quenched with K SV ~ 5 10 6 M 1 which is comparable to th conjugated polyelectrolytes. By contrast, the monomer emission is quenched much less efficiently,
13 with K SV ~ 2 10 5 M 1 The results are explained by a model in which O PPE is able to fold into a helical stacked aggregates for long distance exciton transport. Second, a novel type of CPE bearing BODIPY phenylene ethynylene copolymer backbone and branched sodium carboxylate side chains ( PB Na ) was synthesized by a precursor route via Sonogashira coupling reaction. PB Na was less likely to aggregate in both MeOH and H 2 O, result ing in moderate fluorescence quantum yield s The fluorescence quenching experiment showed that PB Na could be qu enched by Cu 2+ selectively among a series of metal ions in an amplified quenching manner (K SV ~ 10 6 M 1 ). Also, fluorescence quenching of PB Na was observed with cyanine dye DOTC (K SV ~ 10 6 M 1 ) by an energy transfer mechanism However, treatment with the co mmonly used electron acceptor MV 2+ caused little change to the photophysical propert ies of the CPE. Electrochemical analysis show ed that MV 2+ is not a sufficient ly strong oxidant to allow the electron transfer process to occur presumably due to low LUMO e nergy level of the novel CPE PB Na On the other hand, t he carboxylic acid form PB a wa s utilized as sensitizer in DSSC cells with TiO 2 Evidence for electron injection between PB a and TiO 2 wa s provided by fluorescence lifetime measurement s and transient absorption experiment s on PB a TiO 2 films. The l ow transient absorption signal and inadequate IPCE data suggest ed that there was insufficient electron injection yield. Possible solutions towards optimization of PB a TiO 2 DSSCs were pointed out. Third, in a collaborati ve project, we conducted a systematic study of functionalized conjugated oligomers with variable band gaps and their interactions with CdSe nanocrystals (NCs). The chromopho res oligo(phenylene ethynylene ) ( OPE ),
14 oligothiophene ( T6 ) and donor a cceptor donor oligothiophenes with a benzothiadiazole acceptor ( T4BTD ) (the latter two synthesiz ed by Romain Stalder from Reynolds group) were designed with decreasing HOMO LUMO energy gaps so that increasing amounts of light could be abs orbed toward long er wavelengths up to 600 nm. In this dissertation, the work performed by Schanze group is reviewed in detail S ynthetically, a newly designed route which is less tedious without compromising the overall yield was employed to replace the conventional OPE synthesis. Photoluminescence quenching studies on both the oligomers and CdSe NCs indicated that charge separation occurr ed at the interface of the oligomer/ NCs, and this photophysical process was assisted by the phosphonic acid group s on the oligomers. Hy brid materials based on the oligomer/ NCs complex were synthesized. The incident photon to current efficiency (IPCE) provided solid evidence for an electron transfer process in the hybrids. Fina l l y studies on the performance of the oligomers in two differ ent solar cell formats indicated that the oligomers have promising application potential in dye sensitized solar cells.
15 CHAPTER 1 INTRODUCTION Conjugated P olyelectrolytes Conjugated polyelectrolytes (CPEs) are a class of polymers that contain conjugate d backbones and ionic side chains. 1 Incorporati on of ionic side chains such as carbox ylate and sulfonate on the back bones usually enable s the polymers which are othe rwise hydrophobic, to dissolve in water and other polar solvents (eg. MeOH) The advantage of this modification is obvious First, fundamental studies on area s such as charge transport amplified quenching, and self assembl y bene fit from the aqueous solubi lity of the CPEs Second the drastic change in the photophysical properties upon binding to opposite ly charged ion s give s CPEs great potential for applications i n sensing, labeling and other biological studies Third solubility in aqueous media allows CP Es to be processed in a n environmental friendly manner Since Wudl and co poly(phenylene vinylene)s, 2 3 the field of CPEs has experienced phenomenal growth leading to development of new polymers with broad applications in optoelectronic devices and bio/chemo sensory materials. 4 12 Figure 1 1 exhibits some typical CPEs that have been subjects of intensive research E ngineering of CPEs has been active both on the polymer backbones by including different electrochemical properties and the divers e ionic side groups that add functionalities to the polymers. In this chapter we will first introduce the synthes e s of CPEs, then the self a ssembly and photophysical property relations of CPEs will be discussed, and last a few examples of their application s i n several areas such as bio/chemo sensors and dye sensitized solar cells (DSSCs) will be describ ed.
16 Figure 1 1 Examples of typical CPEs 10 11 13 16 Synthesis Approaches In this section we briefly review the synthetic methodologies for CPE s, including b s Pd catalyzed c ross coupling r eactions The fac ile syntheses of CPEs are largely attributed to the development of transition metal catalyzed cross coupling reactions Specifically, Palladium catalyzed carbon carbon bond formation is one of the most utilized transformations in organic chemistry T he bro ad substrate scope, tolerance of variable functional group s and mild reaction conditions associated with the P alladium method impart the m noticeable importance in organic synthesis As a matter of fact, t he 2010 Nobel Prize in C hemistry was awarded to H eck, Negishi and Suzuki for palladium catalyzed cross couplings in Table 1 1 present s a summary of some common name reactions that are related to Pd catalyzed cross couplings and t he general mechanism for this
17 type of reaction is illu strated in Figure 1 2 T he reactions usually begin with oxidative addition of organic halides (R 2 X) onto the Pd, followed by substitution of the partner substrate R 1 on the same catalytic center via transmetallation. The final step is the formation of the coupling product along with the regeneration of the catalyst by reductive elimination. Table 1 1. Palladium catalyzed cross coupling reactions Reaction Reagent A Reagent B C atalyst substrate hybridization substrate hybridization Heck alkene s p 2 R X s p 2 Pd Negishi R Zn X sp,s p 2 ,sp 3 R X s p 2 ,sp 3 Pd or Ni Suzuki R B(OR) 2 s p 2 R X s p 2 ,sp 3 Pd Stille R SnR 3 sp,s p 2 ,sp 3 R X s p 2 ,sp 3 Pd Sonogashira RC CH sp R X s p 2 ,sp 3 Pd and Cu(I) Figure 1 2 A general catalytic cycle for Pd catalyzed cross coupling Figure was reprinted from Suzuki et al. 17
18 Listed i n Tabl e 1 2 is a series of CPEs that catalyzed cross emphasizing the impor tance of the Pd chemistry in CPE syntheses Table 1 2. E xamples of CPEs that were synthesized by Pd catalyzed cross coupling reactions CPE structure s Name/abbreviation Reaction Reference P oly( para phenylene) / PPP Suzuki 18 Poly(phenylene ethynylene) /PPE Sonogashira 14 Polyfluorene / PF Suzuki 19 Poly(phenylenevinyle ne) /PPV Heck 20 Other a pproaches u sed in CPE s ynthes i s Admittedly that the Pd catalyzed cross coupling reactions are the most utilized means of preparing CPEs, it is necessar y to point out that alter native approaches have
19 been investigated as well In the synthesis of water soluble polythiophene, Wudl and co workers employed electro construct the backbone of the CPE 2 The mechanism of the thiophenes polymerization is illustrated in Figure 1 3. When a n electrical potential is applied across a solution containing thiophene and an electrolyte a t hiophene monomer is oxidized to produce a radical cation, which is either coupled with another radical cation to form a dication dimer, or coupled with a monomer to form a radical cation dimer which is further oxidized to yield the dication dimer. Bithiop hene is obtained upon losing two equivalents of protons. This process is repeated before polythiophenes are deposited in the form of polymer films on the anodes. Figure 1 3. Electropolymerization of thiophenes Another impo rtant class of CPEs is poly ( phenylenevinylene) (PPV). Table 1 2 shows that the PPV type of CPE c an be prep ared by the Heck reaction, where di viny l benzene s and di iodobenezes were coupled via Pd catalysis 20 Nevertheless the first reported CPE that bear PPV backbone was synthesized via a route 3 as shown in Figure 1 4 In this work, a precursor polymer with sulfonyl chloride side groups was first obtain ed by anionic or radical addition polymerization before it was hydrolyzed in DMF and water for conversion into a fully conjugated system.
20 Figure 1 4. Synthe tic approach for PPV type of CPE Figure was reprinted with permission from Wudl et al 3 In all, the development of versatile synthetic approaches has endowed the CPE field with a large variety of molecules with many interesting functionalities and promising applications which will be reviewed later in the chapter. Synthetic m ethodologies Regardless of the synthetic route employed the g eneral goal of engineering the CPEs is always focused on the extension of conjugated backbones and installation of the appropriate pendant ionic side groups to meet the desired specific p roperties As described above, the extension of the conjugated backbo ne can be achi eved via a number of approaches. F urthermore, the installation of ionic side groups is usually
21 realized via either of two pathways. One approach is dir ect polymeriz ation of monomers containing ionic side groups that are soluble in water or po lar organic solvent s Alternatively, in 1 the polymer precursors are first prepared in organic solvent s before the protecting groups are removed (usually via hydrolysis) to give rise to the functional ionic side groups. Comparing the two methods, the former is synthetically straightforward, but has drawbacks that it is difficult to obtain precise information on the molecular weight of the CPEs. 21 The more steps for protection of the ionic groups and removal of the protecting groups, but this method has the advantage of facile purification of the precursor polymers which can be characterized by gel permeation chromatography ( GPC ) before t hey are hydrolyzed to form CPEs. In addition, the completeness of hydrolysis can be monitored by NMR, IR and UV vis spectra. Photophysical Properties: Aggregation and Amplified Quenching of CPE s The applications of CPEs i n either sensory materials or optoelectronic devices are closely related to their unique photophysical properties Fundamentally, the photophysical properties of CPEs are determined by the chemical structures of the conjugated b ackbones CPE s bearing the same chromophores in the backbones should show similar absorption and photoluminescence spectra Figure 1 5 gives a typical example for PPE type of CPEs containing chromophores with variable band gaps. Both absorption and fluores cence spectra of the CPEs were drastically red shifted as lower band gap chromophore was incorporated. 22 A nother factor that can have a major effect on CPE photophysical behavior is the tendency to form secondary structure s in aqueous or polar organic solvents, due to the amphiphilic nature of CPEs (hydrophobic backbone and hydrophilic side groups) The formation of the aggregates in many cases
22 significant ly changes the photophysical properties of the CPEs. Examples will be shown in the following discussions. Figure 1 5. Photophysics of CPEs with variable bandgaps. A) Norma lized absorption and B) fluorescence of PPE (red), PPE Py (black), PPE Th (green), PPE EDOT (blue), and PPE BTD (yellow) in methanol solution. Figure was reprinted with permission from Schanze et al 22 Aggregation and s elf assembly It is not possible to discuss the photophysical properties of CPEs without mentioning the agg regation and self assembling of the CPEs under specific solution conditions. As described above, due to their amphiphilic nature CPEs have a tendency to aggregate in aqueous solution or polar organic solvents, and the aggregation process usually induces si gnificant changes in both the absorption and fluorescence spectra. 21 23 25 Taking the poly (arylene ethynylene) type of CPEs as an example, the CPEs with anionic side groups such as sulfonate ( SO 3 ) or carboxylate ( CO 2 ) exist as
23 molecularly dissolved chains in methanol but when the solvent is changed to water, the CPEs aggregate. 21 24 The formation of aggregates is reflected by photophysical change s which usually include a red shift in the absorption and fluorescence spectra, a decrease in the fluorescence quantum yield, and an increase in the fluorescence lifetime While the formation of CPE aggregates is largely dependent on the nature of the solvent other factors can induc e aggregation in CPE systems. For example when a divalent cation, such as Ca 2+ is added to the methanol solution of anionic PPE type of CPEs bearing carboxylate side groups PPE CO 2 aggregates form 23 a nd as a result, the spectral changes mimicking the effect of adding water to the CPE solutions are observed It is believed that Ca 2+ induces aggregation of the CPE by cross link ing PPE CO 2 chains wh ile complexing with the carboxyl ate side groups of the polymer. More direct evidence of the inducing aggregation was observed in a study of analyte directed polymer aggregation 26 In this clever design a series of diamines varying successively by only one methylene unit separating the amines were allowed to interact with polythiophenes with carboxylate side group s N ot onl y did the diamines induce the aggregation of the polyt hiophenes, but the extent of communication between polymer chains w as controlled by the degree of flexibility of the tether between the two amines. 26 Illustrated in Figure 1 6 is a s chematic representation of how the different diamines affected the aggregativ e interactions with polymer 1 giving rise to different absorption sp ectra Similarly, when CPEs with cationic side groups were treated with an anionic di carboxylate analyte 27 th e formation of aggregation was dependent on the length of the tethers be tween carboxylates. When short tethered oxalic acid was added,
24 Figure 1 6, path A) formed, and by contrast ( Figure 1 6, path C) resulte d from the addition of long tethered glutaric acid. Figure 1 6. Schematic representation of the aggregative interactions between polymer 1 (colored rods) and different diamine analytes (A C). Different colored aggregates are formed depending on the added diamines Figure was taken from John Lavigne et al 26 Many other factors that are related to aggregate formation of CPEs have been in vestigated For example, CPEs substituted with weakly ionized polyelectrolyt e groups, such as phosphonate ( PO 3 2 ), aggregate in water at neutral pH but the aggregates dispers e when the pH increases above 8. 25 In addition factors such as concent ration, temperature, solution ionic strength, and added surfactant, have also been carefully studied. 28 Furthermore, studies of CPE aggregation have not been limited to the formation of the aggregates and the resulting photophysical property changes. Additional efforts have been made to understand the conformation s of the secondary stru ctures of the CPEs and to explore their applications One typical example is the meta linked PPE ( m PPE) system. Initiated by Moore and co workers, 29 several studies were reporte d on
25 investigations o f m PPE polyelectrolytes. 30 32 Direct evidence for the helical conformation was afforded by the negative chirality in the bisignate CD spectrum e xhibited by the polymer in solution. 31 The helical structure was found to serve as a host supermolecule to bind with planar guests in a n intercalation manner. Based on this property, the helical CPE was used as templat e for the formation of supramolecular helical aggregates of cyanine dyes. I t is also necessary to point out that although there have been extensive inves tigations on aggregates of CPEs, there is no for predict ing the status of agg regation for a given CPE structure. Thus studies of aggregation of CPEs are still case dependent. For example, in many cases MeOH is a good solvent for CPEs where they exist as monomeric states and H 2 solvent. However, excep tions were also seen in literature where a mixture of MeOH and H 2 O solvates the CPE optimally 19 33 Amplified p hotoluminescence q uenching Fluorescence quenching of CPE is one of the most popular research topics in this field from both the fundamental and application viewpoints 1 Fluorescence quenching occurs by two limiting mechanisms, 34 namely dynamic quenching and static quenching. The quenching processes are depicted in Figure 1 7. Dynamic quenching also known as collisional quenching occurs when the excited fluorophore experiences contact with a quencher that can facilitate non radiative transitions to the ground state (Figure 1 7, equation 1). Static quenching, by contrast, involves formation of a stable non emissive complex between the fluorophore and quencher (Figure 1 7, equation 2). Treatment of fluorescence intensity q uenching data by the standard Stern Volmer method yields equation 3), where I 0 and I are the fluorescence intensities in the absence and presence
26 of Q, respectively, and K SV is the Stern Volmer quenching constant. According to equation 3), the Stern Volmer plot of I 0 / I versus quencher concentration is expected to be linear. However, in many situations with quencher CPE systems, the Stern Volmer plots are curved upward (i.e., superlinear). Many factors could be responsible for this observation such as var iation in the association constant with quencher concentration, mixed static and dynamic quenching, and CPE aggregation. Figure 1 7. Equations of P hotoluminescence Q uenching F* is an excited state chromophore, Q is a quencher, k q is the bimolecular quen ching rate constant, and Ka is the association constant for formation of the ground state complex [F,Q]. Equations were taken from Lakowicz. 34 Th workers in 1995. 35 In th at work the author s reported t he fluorescence quenching of PPE polymers bearing cyclo phane receptor s on the repeat units. Methylviologen (MV 2+ ) was chosen as the quencher for its ability to associat e with the cyclophanes. Interestin gly, the quenching effect was much more pronounced for the polymers compar ed to that of monomeric model compound. Furthermore, the Stern Volmer quenching constant K SV increased with polymer chain length. To rationalize the observation, the authors propose d a theory involving the delocalization and migration of exciton s along the polymer chain, called 8 indicates the mechanism of the molecular wire effect. When an exciton is generate d the polymer chain acts as a condui t that allows the exciton to migrate rapidly along the chain. Within its lifetime, t he
27 exciton is quenched once it reache s the repeat unit with a quencher attached acceptor. Given the efficient exciton migration along the polymer chain, one quencher molecu le binding to the acceptor can cause quenching of the excitons that are generated many o ther places, leading to the amplified quenching response of the polymer to the quencher. Figure 1 gure was taken from Swager et al 35 The study of amplified quenching effect in CPE systems was initiated b y Whitten and co workers with their investigation of fluor escence quenching o f PPV type CPE by MV 2+ 10 It was found in this study that in aqueous solution, the PPV is quenched by MV 2+ with an extremely large K SV value ( ~ 10 7 M 1 ) aggregation of CPE further enhance d amplified quenching by facilitating exciton hopping between polymers chains. 10 More direct stud ies o f aggregation amplified quenching relations w ere conducted by Schanze and co workers. As previously reviewed, Ca 2+ ions induce aggregation of PPE pol yelectrolytes. More importantly, the
28 fluorescence quenching response to MV 2+ was found to have a positive cor relation with the concentration of Ca 2+ 23 In other words, the amplified quenching effect is more pronounced in the systems with more aggregation underlying the contribution of aggregation to the exciton hopping between polymer chains. To conclude, amplified photoluminescence quenching of CPEs a rises from the rapid exciton migration either along a single polymer chain or between different chains This unique property of CPE paves its way to many ultra sensitive sensory applications. Application of CPEs as Functional Materials The applications of CPEs have widely expanded into many fields such as bio/ chemo sensory materials, 1 36 37 polymer light emitting diodes 38 42 photovoltaic devices 43 cell imaging 44 45 and biocidal materials 46 47 Space limit ations make it impossible to review all the works related to the CPE applications. I nstead, this dissertation wi ll use a few examples to elucidate the property application relation of the CPE s. Bio/ chemo sensory materials CP Es are ideal materials for bio/ chemo sensors for several reasons. 1) They are generally fluorescent. This basic photophysical nature of CPEs m akes it possible to detec t target molecules/ ions from the change of photoluminescence. 2) They usually have considerable solubilit y in aqueous solutions. This feature largely broadens the applications of CPEs as sensory materials since water is the most co mmonly applied medi um for many molecules/ions of biological importance. 3) They bear ionic functional groups on their side chains. Besides enhancin g solubilities, the ionic groups provide substantial affinity between CPEs and the targets through electrosta tic 6 or/ and coordinating 48 interactions. 4) The amplified quen ching properties. As described above, ex citon delocalization and transport within / between the polymer chains lead to an
29 amplified quenching effect, which allows the fluorescence of CPEs to be quenched at very low quencher concentration s When applied to sensory materials, this property afford s t he CPEs substantially high sensitivity and low detecti on limits for analyte detection Depending on how the sensor systems are designed, there are two separate modes based CPEs to respond to the target molecule s. In the s is quenched by the added analyte Generally the analytes quench via energy/ charge transfer 49 or/and by inducing a change in the physical state of the CPE (aggregation and conformation) 48 By contrast, in the recovers the fluorescence of CPE. Specifically, the CPEs first in teract with quenchers to form non fluorescent CPE Quencher (C Q) complex es When analytes are introduced to the system, the quencher molecules redistribute between the CPEs and analytes, and thus the fluorescence of CPEs is Illustrated in Figure s 1 9 and 1 10 are two CPE sensor examples based modes, respectively. In 2003, Heeger and co workers r eported that cationic poly fluorene (PF) was quenched with extraordinarily high efficiency by gold nanoparticles (K SV ~ 8.3 10 10 M 1 ) in water (Figure 1 9) 49 In this work, a series of gold nanoparticles w ere treated with tertiary amine functionalized PF The quenching response was significantly reduced in solutions of high ionic strength, indicati ng that the PF interacted with gold nanoparticles via electrostatic attractions. Nanoparticles with diameters of 5, 10, and 20 nm, which all absorbed strongly in the region of polymer emission, prod uced similar quenching response However, t he smaller gold
30 nanoparticles (2 nm in diameter) showed approximate ly 10 4 lower quenching efficiency due to poor overlap of the absorption spectra of 2 nm nanoparticles with the PF fluorescence. Based on th is observation, the authors concluded that resonance energy tran sfer was the dominant quenching mechanism. On the other hand the monomeric model compound oligofluorene was 10 times less sensitive to the addition of nanoparticles, clearly indicating that exciton migra tion along the polymer backbones contributed to the amplified quenching. This work demonstrated that strong electrostatic nanoparticle polymer complexation, long distance energy transfer quenching and conjugated polymer based excitation mobilization are all critical factors in the amplified quenching effect s, which are fundamental to the design of sensor materials. Figure 1 9. Stern Volmer plots of polyfluorene quenched by 5 nm gold nanoparticles. Figure was taken from Heeger et al 49 portrayed in Figure 1 quencher tether ligand for detection of avidin was introduced by Whitten and co workers in 1999. 10 The authors designed a biotin functionalized viologen quencher that quenched the fluorescence of the PPV type of CPE in aqueous solution. The biotin functionality on the quencher was able to bind with avidin, so when avidin was added to the system, the avidin bound quencher was prevented to be associated with CPEs due
31 to enha nced spatial hinderance, result ing in the recovery of the CPE fluorescence. A c ontrol experiment was conducted to investigate the rol e tether lig in CPE fluorescence recovery. Avidin alone did not modulate the intensity of the CPE fluorescence and the CPE quenched by methyl viologen quencher (which has no affinity to avidin) showed no fluorescence recovery upon avidin addition All the evidenc the specific biotin avidin interaction. This pioneering work triggered a tremendous amount of research interest i n CPE based biosensing materials. Figure 1 10. orescence sensor for avidin. Figure was reprinted with permission from Whitten 10 and Swager et al 36 D ye sensitized solar cells The d ye sensitized solar cell (DSSC ) is one of the major inventions towards util izing solar energy. Since they were i ntroduced by Gra t zel in 1991 50 DSSCs have evoked increasing research interest both from the fundamental view point s and exploration of the commercializ ation opportunities. 51 Compar ed to the traditional solid state solar cells, the advantage s of this phot ovoltaic device format include the use of
32 relatively inexpensive materials and the possibility that mechanically flexible modules can be deployed on a large scale for low overall cost. 50 52 A schematic presentation of the operating mechanism of the DSSCs 52 i s illustrated in Figure 1 11. On the top of a thin film of fluorine doped tin oxide glass plate, a mesoporous layer of TiO 2 is placed. A monolayer of dye (sensitizer) is adsorbed to the surface of the nanocrystalline film. Excitons are generated when the dye is irradiate d with photons, and electrons are injected to the co sensitizer is restored to its ground state by electron donation from the mediator, usually a redox coupl e such as iodide / triiodide. The reducing species is then regenerated by reduction o f the oxidizing species at the electrode. Overall, the cell converts light to electric power without chemical transformation. To date, great success has been achieved on DSSC s with transition metal complex based sensitizers. A few specific metal (e g Ru, Pt and Zn) complex sensitizers exhibit broad absorption throughout the UV vis and near IR regions with high (> 85%) incident photon to current efficiency (IPCE) With careful engineering, these devices achieve up to 13% p ower conversion efficiency (PCE). 53 54 In addition metal free small molecule dyes as alternatives also have attracted attention with the concern s that the source of transition metals is limited. 55 Moderat e PCE s (6 9%) have been reported for these devices 55 56 Conjugated polyelectrolytes are also important sensit izers utilized in DSSCs. 14 57 59 Motivation to apply CPEs as sensitizers in DSSC come s from their capacity to offe r effective light harvesting efficiency due to large absorption coefficients and tunable optical band gaps. 22 Most recently, donor acceptor approach was employed to engineer
33 the energy levels of the CPE so that high light harvesting efficiency and charge injection efficiency was realized. 60 61 Several research cases conducted in the Schanze group will be discussed here to illustrate the CPE based DSSCs. Figure 1 11. Principle of operation o f DSSC s Figure was taken from Gra tzel et al 52 In 2006, Schanze, Reynolds and co workers demonstr ated the concept of ra l CPE sensitizer in a TiO 2 DSSC format. 14 As shown in Figure 1 12 the authors combined poly (phenylene ethynylene) (PPE) and polythiophene (PT) types of CPEs in one cell, and the performance of the hybr id cell was compared to the individual cell s based on one CPE. Transient absorption studies indicated that the electron injection from each CPE to TiO 2 was favored, and the photocurrent and PCE obtained for the dual CPE cell w ere the sum of the correspondi ng response s from each individual CPE. As part of their ongoing interest in constructing low bandgap CPE sensitizers, Schanze e t al. reported a series of poly (arylene ethynylene) CPEs with carboxylic acid
34 side groups (Figure 1 13) 58 The HOMO LUMO gap of the CPEs varied with absorption maxima ranging fr om 400 nm to 500 nm. The CPEs were assembled with TiO 2 films and the cell performances were tested under AM1.5 illumination. The photocurrent and PCE increased in the order PPE< TH PPE< EDOT PPE, indicating that the lower band gap CPE is able to accomplish more efficient solar energy conversion. Interestingly, the IPCE and PCE for the BTD PPE (Figure 1 13) which bears the longest absorption wavelength, were substantially less than those for the other CPEs. The authors attributed this decrease in efficiency to exciton trapping in polymer aggregates. In other words, instead of injecting into TiO 2 the excitons were hoppin g in the CPE aggregates far from the CP E/ TiO 2 interfaces. This argument emphasized the importance of subtle changes in device morphology on p erformance of the solar cells. Figure 1 12. J V characteristics under AM1.5 conditions for solar cells sensitized with PPE, PT, and PPE/PT mixture Figure was taken from Schanze et al 14 Recently, the Schanze group used the donor accept or approach to achi eve low band gap CPE sensitizers for DSSCs. 61 In this work, electron poor 1 2 ,3 benzothiadiazole (BTD) was incorporated with electron donor terthiophene segment to
35 yield a polymer exhibiting an absorption onset at 625 nm correspondin g to a ~1.9 eV bandgap. The polymers were modified with carboxylic acid groups which were able to attach to the TiO 2 surface. T he CPE turned out to be a promising candidate for DSSC sensitizer by offering ~ 3% PCE The best result was ~ 6 5% peak IPCE with Jsc ~ 12.6 mA cm 2 under AM1.5 illumination, which is the record for CPE based DSSCs. Furthermore, the author s also investigated the relationship between the CPE molecular weight and the cell performance. It was observed in this study that CPE s wi th lower molecular weight (~ 4 kD) yield better cell performance than those with higher molecular weight (~1 0 kD) and the difference was believed to come from the different extent of adsorption of the dyes on the TiO 2 The shorter CPE chains are able to access a gr eater fraction of the TiO 2 surface for adsorption due to effective penetration into smaller pores, leading to the observed greater surface coverage. By contrast, the longer chains are excluded from a substantial fraction of the surface and thus are not abl e to give rise to as much surface coverage. 61 Figure 1 13. Structures of the repeat units for variable gap PPE based CPEs used in TiO 2 sensitized photovoltaic devices. Figure was taken from Schanze et al 58
36 Summary for CPEs In this section we reviewed the synthesis, photophysi cal properties and applications of conjugated polyelectrolytes. Synthetically, the CPEs are readily accessible thanks to the development of transition met al catalyzed organic reactions, albeit other synthetic methodologies also share the credit for the va st variety of CPEs bearing different properties. The photophysical properties of the CPEs are dependent on several factors. 1) Conjugated backbones allow excitons to delocalize and migrate, which is the inherent character of CPE. 2) The amphiphilic nature of CPE chains give s rise to aggregation and self assembly properties in water or polar organic solvents, thereby producing dramatic changes i n the photophysical properties. The application o f CPEs in highly sensitive bio/ chemo sensors direct ly results fr om their amplified quenching capability Based on how the sensory systems are designed, the presence of analytes. The application of CPEs in DSSC s was also reviewed. E xamples were discussed to show the importance of engineering the bandgaps of CPEs in the se photovoltaic devices. Additionally it is also critical to optimize the morphology and structure s on the nanometer scale for optimum device performance Conjugated Polymer/Oli gomer Semiconductor Nanocrystal Hybrid Materials T he study of conjugated polymer/ oligomer and inorganic se miconductor nanocrystals (NCs) had little overlap until early 1990s. 62 The investigation into the interfaces between the two types of functional materials with different chemistry was fueled by Alivisatos and co workers s on optoelectronic devices based on
37 p oly(phenylene vinylene) ( PPV )/ CdSe NCs hybrids 63 and poly thiophene (PT) /CdSe NCs hybrid s 64 It was found in these works that a combination of two conventional materials would give rise to novel properties that are unma tchable by their components. To date, several research areas such as light emitting diodes, photodiodes, and photovoltaic cells have witness the promising applicati ons of these hybrids materials. 65 76 In this review, the materials will be organized to elucidate three aspects regarding the hybrid materials. Frist, the synthetic strategies of the hybrids are categorize d into two classes. Second, the means of characteriz ing the physical process at the interface of the hybrids are to be introduced. Third, a few examples will be exhibited to illustrate the application of the hybrids in the optoelectronic devices. Synthetic Strategies The syntheses of hybrids are generally i semiconductor preparations of inorganic NCs have been reviewed by a number of recent articles in details. 77 79 Concerning the scope of this dissertation, we are not going into this topic any further. Depending on how the inorganic NCs are incorporated with organic semiconductor s, the synthetic strategies c an be categorized into two classes, namely organic oligomer/polymers are synthesized before they are mixed with NCs w the p recursors of one component are f irst modified on the other complete components, and then the precursors are allowed to grow with more building blocks into a new phase Details are exhibited to compare the two methodologies.
38 t ch component prior to mixing them together. The construction of conjugated polymer / oligomer backbones are similar to those of CPEs (as reviewed in the first section ) which largely benefit from the development of Pd catalyzed cross coupling reactions The simplest form of hybrid material using the directly result ing from blending inorganic NCs with unfunctionalized polymers/ oligomers. 64 However, s im ple casting from a common solvent leads to hybrid films with phase separation of the components on a micrometer scale 80 This phenom enon is originated from the tendency of the inherent ligands on the surface of NCs to pack upon removal of solvents 81 82 a major imped iment for interaction between organic SCs and inorganic NCs Modifying the conjugated polymer/oligomer with anchoring groups that are capable of binding to the NC surface is a solution towards monodispersity of the hybrid material s. 68 Upon mixing, the anchoring group functionalized macromolecules undergo a so nds such as trioctylphosphine oxide ( TOPO ) and oleic acid, at the NC surfaces The driving force for ligand exchange is that anchoring groups usually hav e a larger affinity to the NCs or that anchoring groups are in relatively higher concentrations To d ate, a series of anchoring groups ha s been reported which include phosphine oxide, 75 76 thiol groups, 83 84 carbodithioic acid, 85 86 carboxylic acid, 87 89 phosphonic acid, 67 90 amines, 68 and anilines. 73 These anchoring groups attach to the NC surfaces via covalent bonds. Nonetheless interaction between conjugated polymers/oligomers and inorganic NCs is not limited to the covalent bonding The molecular recognition approach via hydrogen
39 bonding, for example, has been utilized to connect the NCs with conjugated polymers. 91 In a work repo rted by Pron and co workers, regioregular poly(3 hexylthiophene 2,5 diyl) (P3HT) that bears diaminopyri midine (DAP) side groups are able to molecular recognize 1 (6 mercaptohexyl) thymine (MHT) capped CdSe NCs via three hydrogen bonds (depicted in Figure 1 14) and as a result hybrid film with uniform distribution of the NCs in the polymer matrix is achieved. Figure 1 14. Schematic presentation of molecular recognition between MHT functionalized CdSe NCs and DAP modified P3HT Figure was taken from Pron et al. 91 Regarding the pattern of how the polymers/oligomers are attached to the NCs, there are mainly five circumstances. Figure 1 15 shows a schem atic presentation of the five different routes through which the hybrids are synthesized. For oligomers or polymers with low molecular weight, the anchoring groups can reach the NC surface either as terminal or side functionalities (Figure 1 15, routes 1 and 2). 67 For higher molecular mass polymers, the terminal anchorin g groups are less available for binding to the NCs due to the fact that the ratio of the end groups to the polymer chain length decreases (Figure 1 15, route 4). In this case direct ligand
40 exchange process is harder to realize. Frechet and co workers addre ssed this problem b y applying a two step procedure. F irst, pyridine was used to replace the initial ligands on CdSe NCs, and then the temporary pyridine ligands w ere exchanged with the polymer functional groups. 68 By contrast, introducing the anchoring functionalities as side groups is more favorable for the NC connection (Figure 1 15, route 5). For example, Reiss and co workers synthesize d polythiophenes that are side functionalized with carbodithioic groups. These polymers are attached to the CdSe surfaces under relatively mild conditions. 86 Finally, dendritic oligomer/ poly mer has also been reported to incorporate with NCs (Figure 1 15, route 3). For example, Advincula and co workers demonstrated the connection between CdSe NCs and dendritic polythiophenes containi ng phosphonic acid groups. 90 Figure 1 15. Schematic presentations of different routes the oligomers/polymers are attached to the NC surfaces. Figure taken from Pron et al. 62
41 In all, functionalized oligomers/ polymers have been proven to attach to the NC surfaces in different patterns via either covalent or other specific interactions between anchoring groups on the oligomers/ polymers and inorganic NCs Such d esign helps t o diminish the phase separation and thu s facilitates the communication between different components at m olecular level Better device performances based on the hybrids are expected. f is another way t o obtaining intimate blends of NCs and conjugated polymers. This method is, so to speak, to grow one component on the other. Both means that ing polymer from NCs NCs from polymer s been used to synthesize hybrid nanocomposites 69 75 92 Grafting polymer from NCs. This strategy was first intro duced by Emrick and co workers 69 In th at work, the authors first synthesized a bromide functionalized surfactant DOPO Br (compound 1, Figure 1 16) which mimics the role of TOPO (which cap s with CdSe since its nucleation and growth process). After the surfactant was m odified on the CdSe NCs, a PPV shell was allowed to grow in situ on the surface of CdSe by copolymerizing 1,4 divinyl benzene and 1,4 dibromobenzene derivatives (Figure 1 16). The advantage of this method is that it reduces the preparation steps of hybrid m aterials inevitable for this approach to weaken the control of the hybrids dispersity due to the fact that t he chain length s of the polymers on the CdSe NCs are varyin g. Based on this argument, the strategy is not quite he lpful to the well defined oligomer/ NC systems.
42 Figure 1 CdSe hybrids. Figure was taken from Emrick et al. 69 Grafting NCs from polymer This method is to i n situ grow NCs in the polymer matrix. By coordinating with the NC precursors, the polymers can not only form uniform hybrids with the NCs, but also act as templates to control the shape of the NCs. This method was employed by Liu and co workers to synthes ize a P3HT/ CdS hybrid system. 92 Specifically, t he authors used the sulfur atoms in the polythiophenes as the coordinating site for Cd 2+ cations. As CdS NCs nucleated and grew, the backbone of the polythiophene oriented the NCs to form the nano r od shaped cry stals (Figure 1 17). The as prepared hybrids were applied to the solar cells, and promising power conversion efficiency (~3%) was reported. In a separate research, Zhou and co workers prepared a PT and zinc methacrylate based copolymer by means of atom tr ansfer radical polymerization (AT RP ). Subsequent hydrolysis of zinc methacrylate groups gave rise to a hybrid
43 nanocomposite based on the PT/ ZnO. Photoluminescence quenching of PT was observed, which provided evidence for efficient interaction between the t wo components. 93 Figure 1 17. In situ synthesis of CdS nanorods template by P3HT. Figure was take n from Liu et al. 92 Characterization Interfaces of the Hybrids Many techni que s have been employed to analyze the properties of hybrids for the reason that they are relatively complex systems and the numbers of parameters are large 62 The characterization can be generally sorted in to two categories: compositional analysis and photophysical / electrochemical analysi s. In this review, several most used characterizations of the hybrids will be d iscussed by exhibiting research cases. Compositional analysis When a hybrid system is prepared, it is necessary to decipher the stoich iometry, distribution and morpholog y of each component, and the nature of interaction at the components interface First FTIR spectroscopy usually provides qualitative information on establishment of communication be tween the conjugated oligomers/ polymers and the inorganic NCs.
44 unveils repla cement of the initial ligands by anchoring groups from the oligomer s / polymer s given that the initial and latter ligands have separable diagnostic signals. 89 Second, NMR spectroscop y also provides evidence for success of anchoring from o ligomers/ polymers to NCs. For exa mple, in Frechet and co work oligothiophene / CdSe NCs hybrids, 67 the 1 H NMR of the hybrid s colloidal solutions and the free oligothiophene ligands were registered (Figure 1 18). Compared to the free oligothiophene ligands, the lines of aromatic region 1 H NMR for the hybrids w ere broadened and shifted The au thors attributed this phenomenon to the limited molecular mobility of the molecules coordinated to the NC surfaces. O ther works also use NMR as a semi quantitative approach to estimate 94 95 Third, e lemental analysis is another technique to determine the composition of the hybrids. It is especially helpful in cases where a specif ic element is present in the anchoring groups and absent in the initial ligands, or vice versa Forth, thermogravimetric analysis (TGA) can provid e information of the mass ratios of organic and inorganic component. Generally, the organic specie s should ha ve much less toleran ce to high temperature than the inorganic ones, thus the relative mass percentage of the organic component can be readily accessible at the plateau where the ligands are fully decomposed while the inorganic NCs remain intact. Fifth, the most direct image of the hybrid materials can be exhibited by electron and atomic force microscopy (EM and AFM, respectively). Taking advantage of these
45 nanometer scale. F igure 1 19 shows an example of a morphology study of P3HT/CdSe NCs conducted by Frechet and co workers. 68 In this work, the authors synt hesized two types of P3HT polymers, one with amine anchoring groups and the other without. The polymers were blended with CdSe NCs to make hybrid films. Using transmission electron microscopy (TEM), it was shown unambiguously that the films with functional ized P3HT polymers had a much more uniform dispersity than those with unfunctionalized P3HT polymers. Accordingly, the hybrid film with better dispersity offered better device performance when solar cells were assembled. Figure 1 18. 1 H NMR broadening as free oligothiophenes bind to CdSe NCs A) 1 H NMR of free oligothiophene. B) 1 H NMR of oligothiophene/CdSe hybrids. Figure was taken from Frechet et al. 67
46 Finally, it is necessary to point out that the above methods are usually applied in a combining manner so that the most accurate compositional information of hybrid ma terials can be obtained. Figure 1 19. TEM characterizations of the hybrid films. A) image of unfunctionalized P3HT/CdSe (wt% 40%) film and B) functionalized P3HT/ CdSe (wt% 40%) film. Figure was taken from Frechet et al. 68 Photophysical and electrochemical analysis The applications of the conjugated oligomer/polymer and semiconductor NC hybrids in electronic devices are solely dependent on their photophysical and electrochemica l properties. These properties are determined by the electrochemical/photochemical nature of the corresponding parental components. For example, i n a photovoltaic device with conjugated polymers as donors and CdSe N Cs as acceptors, the energy levels of the species should be staggered ( as depicted in Figure 1 20 ) for the photo induced e lectrons/ holes to move to the electrodes for electric currents generat ion So before hybrid materials are synthesized and applied to d evices, it is necessary to conduct clear characterization on the individual components and carefully design the combination of the two. In this review, we selectively discuss the characterization s that are applied in our researches ( C hapter 4 ) and more co mplete
47 recent review article. 62 Figure 1 20. Energy level alignment in a conjugated polymer ( donor)/ CdSe NC (acceptor) hybrid film facilitating charge separations. UV vis absorption and cyclic voltammetry (CV) study. The energy levels of the conjugated oligomers/ polymers and inorganic NCs can be determined by UV vis absorption spectroscopy and electroche mical analysis. T he onset of the UV vis spectra deciphers the HOMO LUMO energy difference of the study system. However in this spectrum, the absolute position of the energy levels is not accessible. Detailed information is given by electrochemical studies In CV measurement, the HOMO and LUMO positions can be estimated by measure of the onset of the oxidation and reduction wave onsets, respectively. In the systems that require higher sensitivity and resolution, differential pulse voltammetry (DPV) can be a pplied. 96 Generally the HOMO LUMO energy difference from electrochemical studies agrees with th at is calculated from optically measured one, albeit excep tions do exist where the lowest energy optical tran sition does not reflect the HOMO LUMO tr ansition. A typical example is given by donor acceptor type of polymers where the HOMO and the LUMO orbitals are located on different moieties. 97
48 Photoluminescence quenching stud ies Most conjugated oligomers/ polymers and inorganic semiconductor NCs are photoluminescent. Depending on the energy level s of the components, charge or/and energy transfer process can happen when attached together. 67 98 One direct indication for these physical processes to happen is photoluminescence quenching. For example, an early work conducted by Frechet and co workers used photoluminescence quenching as an indicator to deduct the physical process occu rring at the interface of oligothiophenes and CdSe NCs. 67 In this w ork, terthiophene (T3) and pentathiophene (T5) that contained phosphonic acid groups as anchoring groups were allowed to interact with CdSe NCs in solution. Both fluorescence of T3 and T5 were quenched upon addition of CdSe, but when the fluorescence of th e CdSe was studied, the CdSe NC fluorescence increased after complexing with T3 while it decrease with T5. From these observations, a plausible proposal of the energy level alignment between the species was p ropose d ( Figure 1 21). Figure 1 21. Schematic presentation of the proposed energy level alignment in CdSe oligothiophene complexes. 67 Applications in Solar Cells The application of the conjugated oligomer/polymer inorganic NCs in solar cells is originated from Alivisatos and co 99 and PT 64 polymers with CdSe NCs Although the early record was fairly lo w, they did arouse a
49 great deal of research interest on this solar cell format Optimizing the device performance in volve engin eering both conjugated polymers and semiconductor NCs. As the aspect of polymer engineering Frechet and co workers demonstrated the role of functionality on the polythiophenes in optimiz ing the morphology of the resulting hybrid films. The mo re uniform hybrid films turned out to exhibit higher power conversion efficiency. 68 Besides increasing affinity to semiconductor NCs, fine tuning the energy levels of the polymers towards harvesting more solar energy has been pursued consistently. Rece ntly, Dayal and co workers applied a low bandgap thiophene benzothiadiazole base copolymer in the CdSe NC hybrid films, and a efficiency ~3% was achieved. 100 Regarding the optimization of NCs works have been focused on tailori ng the shape of the NCs. The morphology of the ino rganic component is important because the inorganic component is the main contributor to charge transport in the hybrid cells (the charge mobility for conjugated polymers is usually low 101 ). For example, rod shaped CdSe NCs w ere incorporated with P3HT and improved the power conversion efficiency compared to the devices based on spherical CdSe NCs 64 which was attr ibuted to the fact that rod shaped NCs facilitates charge transport better than spherical ones. However, as the nanorods become longer, improvement in device performance is limited since the increasing charge transport characteristics are trade off with d ecreasing solubility of NCs in the processing solution, which give rise to phase separation. In another work tetra pod shaped CdSe NCs and PPV hybrids offered an efficiency close to 3%. 102 Finally, it should be mentioned that the engineering on NC
50 shapes were initiated from the synthetic work by Peng et al., who first introduced the concept of shape anisotropy to inorganic NCs. 103 To sum up, the application of conjugated polymer/semiconductor NC hybrids in solar cells are widely investigated. Abundan t improvement has been achieved from both the organic molecular design and the inorganic NC engineering aspects. And of course, the state of art hybrid cells are yet to be developed. Summary for Polymer/Oligomer Semiconductor Nanoparticle Hybrids In this section, we provided an overview of synthesis, characterization, and application of a class of hybrid materials based on conjugated polymer/oligomer and inorganic semiconductor NCs. First, synthetically, the development of the hybrid materials involves bo th engineering conjugated polymers/oligomers and inorganic semiconductors. The construction of conjugated polymer/oligomer backbone is similar to that for conjugated polyelectrolytes (described in the previous section), which are fueled by the transition m etal catalyzed cross coupling reactions. The synthetic method of inorganic NCs was not reviewed in this dissertation. Regarding incorporating the two components, there are o one component on the prepared other species, followed by in situ growing of the precursors. Compared to s teps synthetically, but it keeps the advantage of the capability to offer well defined oligomer NC hybrids. Second, the hybrid materials are usually complicated systems. Characterization of the materials includes compositional analysis and electrochemical/ photophysical
51 analysis. In the compositional analysis, the stoichiometry, distribution, and morphology of each component and the nature of interaction at the component interface s are deciphered. While in the electrochemical/photophysical analysis, the ener gy level alignment of the organic and inorganic phase and the physical process in terms of the components interactions are determined, which are the basis of their practical application in electronic devices. Finally, the application of the hybrid material s in solar cells was discussed. A few examples were shown to elucidate the effects of engineering both conjugated polymers and inorganic NCs.
52 CHAPTER 2 AGGREGATION INDUCED AMPLIFIED QUENCHING IN CONJUGATED POLYELECTROLYTES WITH INTERRUPTED CONJUGATION Conjugated P olyelectrolytes Conjugated polyelectrolytes (CPEs) are a class of polymers that contain conjugated backbones and ionic side chains. 1 Since Wudl and co work on water soluble polythiophene and poly(phenylene vinylene)s, 2 3 the field of CPEs has experienced phenomenal growth leading to devel opment of new polymers that have broad applications in optoelectronic devices and bio/chemo sensory materials. 4 10 12 104 Specifically, the strong fluorescence of poly(phenylene ethynylene) (PPE) based CPEs coupled with their high quenching response to biomolecules and metal ions have attracted significant research inter est in their photophysical properties both from fundamental and application viewpoints. 1 5 7 105 107 Studies have shown that CPEs self assemble into a variety of supramolecular architectures in aqueous solution. 1 21 30 108 109 In addition, it h as been demonstrated that the photophysical properties of CPEs are very sensitive to the solvent environment. 1 21 For instance, CPEs form stacked aggregates in water 24 and divalent cations 23 in duce aggregation in polar organic solvents which results in a dramatic change in the photophysical properties of the CPEs. 110 Another property that is characteristi c of CPEs is amplified quenching. 10 21 24 35 The fluorescence of CPEs is efficiently quenched by low concentration of oppositely charged quencher ions. This amplified quenching is attributed to delocalization and migration of the excitons along the polymer backbone which is o 111 Evidence shows that aggregation of CPEs further enhances the amplified
53 quenching effect by enabling exciton t ransport between polymer chains. 24 In the case where a meta linked poly(phenylene ethylene) with sulfonate side chains (m PPESO 3 ) was studied, the fluorescence quenching beha vior indicated that the quenching efficiency for aggregates present in the helical conformation of the polymer was higher than for the random coil conformation, 30 once again demonstrating the role of inter chain exciton transfer. It is with such properties like efficient fluorescence and amplified quenching that the applications of the CPEs to ultra sensitive chemo / bio sensors were developed. Now that the amplified quenching effect has been w ell established in CPE systems, we consider the question: Is amplified quenching limited to fully conjugated polyelectrolytes, or can it also occur in systems t conjugated? We note that previous stud ies have explored this concept. In par ticular, Whitten and coworkers synthesized a series of poly(L lysines) with varying chain length that contained cationic cyanine dye units on every repeat. 112 These polymers exhibited amplified quenching to an extent that varied with the poly ( lysine) chain length, and they attributed it to J aggregate formation and exciton transport among the pendant cyanine dyes. In a separate study, Heeger, Bazan and co workers reported amplified quenching in supramolecular aggregates formed from a sulfonate substituted (phenylene vinylene) oligomer. 113 In the present chapter we describe an investigation of two anionic polyelectrolytes that contain discrete oligo(phenylene ethynylene) chromophore units that are tethered into linear chains by weak ly o r non conjugated linker groups. These
54 and we find that the difference is closely related to the ability of the chromophores to self assemble into fluorescent aggr egates. Results and Discussion Synthesis and Structural Characterization The chemical structures of the CPEs investigated are shown in Figure 2 1 These polymers feature chromophores consisting of three ring phenylene ethynylene segments functionalized wit h two p ropylene sulfonate side groups. The units are linked into a polymeric array via a (CH 2 ) or O link ( C PPE and O PPE respectively). Both polymers were synthesized by Sonogashira coupling and characterized by 1 H NMR spectroscopy. Figure 2 1 Structures of C PPE and O PPE T he s yntheses of the two CPEs are presented in Figure 2 2 First, the starting material diphenylmethane or diphenyl ether was iodinated at para position relative to the linkers. T he reaction condit ion for diphenyl ether was milder compar ed to that for diphenylmethane, p resumably due to stronger electron donating ability of O linker than that of CH 2 linker. Second, the iodine was transformed into trimethysilyl (TMS) protected alkynes via Sonogash ira coupling in high yield, and removal of TMS group by Na 2 CO 3 in methanol / dichloromethane solution provided monomer 5 or 6 Third, the
55 monomer 7 was synthesized following a reported procedure 21 Finally, the pol ymerization of the monomers was carried out in H 2 O/ DMF mix ed solvent at 70 o C via Sonogashira coupling. Precipitation and dialysis were conducted to p urif y the polymers. Figure 2 2 Synthe tic scheme of C PPE and O PPE Afte r purification, the 1 H NMR spectr a of C PPE and O PPE were recorded in DMSO d 6 which are shown in Figure 2 3 A ll peaks are clearly assigned, indicating purity of the samples.
56 The purified CPEs were stored in stock solutions at 0 o C under dark in DI wate r (Millipore Nanopure TM ) with p H 8.5, and the concentrations were ca. 1 mg/ml. The stock solutions were diluted for spectroscopy studies. Figure 2 3 1 H NMR and peak assignment for A ) C PPE and B ) O PPE
57 Photophysics The absorption and fluorescence spe ctra of C PPE and O PPE in methanol are shown in Figure 2 4 as aggregation is minimal in this solvent. 21 The absorption spectra of the polymers sho w a very similar pattern; their band maxima are in the near UV region, which is typical for 3 ring OPEs, 114 indicating the two chromophores are very similar. The shorter wavelength for the absorption maximum in C PPE and O PPE in comparison to fully conjugated PPE type polymer is attributed to the interruption of conjugation enforced due to the presence 2 similar, the fluorescence spectra of C PPE and O PPE exhibit an interesting difference. The fluorescence emission of C PPE and O PPE show a similar structured band at short wavelength ( 400 nm); however, in addition to showing a well resolved 0 0 band at short wavelength, O PPE exhibits a second broad band at longer wavelength ( 450 500 nm) suggesting the formation of emissive aggregates in pure methanol. 21 24 Such aggregate emission is not observed for C PPE in methanol. More insight into the influence of solvent environment on the aggregation of the polymer s comes from the solven t dependence of fluorescence. To understand the role of solvent medium on the aggregation behavior of C PPE and O PPE their fluorescence was investigated in a mixture of methanol and water by systematically varying the solvent ratio ( Figure 2 5 ). As the v olume percent of water increases in the medium, the fluorescence of C PPE decreases significantly in intensity; however, no clear evidence for a separate aggregate emission a t longer wavelength can be seen (Figure 2 5 a ) By contrast, O
58 PPE exhibits complex behavior as the solvent composition is changed. Importantly, it is obvious from the dual emission of O PPE (i.e., the appearance two emission bands, Figure 2 5 b black solid line) that it forms aggregates in pure MeOH. Interestingly, as the fraction of wa ter is increased to 20%, the aggregate band almost disappears, and the monomer emission intensity increases significantly (5 fold increase). With further increase in the volume percent of water in the medium, the monomer fluorescence gradually disappears and is replaced by the aggregate emission at longer wavelength The 80% methanol water mixture appears to be the best solvent as the least polymer aggregation is seen in this mixture. Similarly, methanol is an intermediate solvent and water could be consi dered as a poor solvent as O PPE is present in an entirely aggregated state in this medium. Similar solvent dependent aggregation behavior has been observed for other CPE systems. 1 19 Table 2 1 consolidates the photophysical data of C PPE and O PPE in H 2 O and MeOH. Figure 2 4 Absorption and fluorescence spectra of C PPE (dashed) and O PPE (solid) in MeOH. Fluores cence spectra are area normalized to reflect relative quantum yields.
59 Figure 2 5 Fluorescence spectra of A ) C PPE and B ) O PPE in a mixture of methanol and water (Intensities reflect relative quantum yields). Table 2 1. Photophysical data of C PPE and O PPE Polymer /Solvent (nm) max (M 1 cm 1 ) (nm) Fl a fl (ns) 400 nm 500nm C PPE / MeOH 365 24500 396 0.075 0.85 1.01 C PPE / H 2 O 365 22800 473 0.005 1.45 1.50 O PPE / MeOH 370 69000 396 0.10 0.73 1.36 O PPE / H 2 O 371 67400 480 0.02 0.24 1.30 a Measured using quinine sulfate in 0.1 M sulfuric acid ( F = 0.54) as standard The fluorescence quenching behavior of the two polyelectrolytes in MeOH and H 2 dimethyl bipyridinium (MV 2+ ) was chosen a s a quencher to examine whether the amplified quenching effect could be observed. In MeOH solution, C PPE is quenched with moderate efficiency, with a Stern Vomer constant (K SV ) of 10 5 M 1 which is slightly higher than reported for an anionic 3 ring phen ylene ethynylene oligomer ( 10 4 M 1 ), 21 but much lower than typical for fully conjugated polyelectrolytes ( 10 6 10 7 M 1 ). This result suggests that exciton migration
60 is not as effective in C PPE compared to typ ical CPEs, presumably due to the interrupted conjugation. However, the quenching behavior for O PPE is quite unusual. As shown in Figure 2 6 a in MeOH where the emission of O PPE consists of both aggregate and monomer fluorescence quenching of the aggreg ate is seen at very low quencher concentration (<< 1 M), with K SV 10 7 M 1 ; such a large quenching efficiency is similar to that of typical CPEs. Figure 2 6. Fluorescence spectra of O PPE with added MV 2+ [ O PPE ] = 10 M. Note that A ) at lower concentr ations ([MV 2+ ] = from 0.02 to 1.0 M) the aggregate emission is quenched first and B ) with increasing concentrations of MV 2+ ([MV 2+ ] = from 1.0 to 4.0 M) further quenching of monomer emission was observed. Interestingly, while the emission of the aggregat es is quenched by MV 2+ the intensity of the monomer emission increases slightly with increasing quencher concentration, suggesting that interaction of the divalent quencher cation with the anionic polymer affects the balance betwee n monomer and aggregate state. Notably, quenching of the monomer emission does not begin until the aggregate emission is
61 entirely quenched ( Figure 2 6 b ); monomer emission is quenched only at much higher concentration of the quencher, with a quenching efficiency ~200 times less th an quenching of the aggregate emission (K SV ~ 10 5 M 1 ) This selective fluorescence quenching of the aggregate state in CPEs has been reported previously, 30 but the highly selective aggregate quenching exhibited by O PPE is unprecedented, and clearly underscores the importance of exciton delocalization and diffusion in the aggregates of conjugated chromophores. 1 106 111 Importantly, the unusual quenching effects seen for O PPE are not limited to MV 2+ diethyloxacarbocyani ne iodide diethyloxadicarbocyanine iodide (DODC) quench the fluorescence of O PPE via energy transfer, and a quenching trend similar to MV 2+ is observed, i.e. quenching of aggregate emission is much more efficient compared to the monomer que nching ( Figure 2 7 and 2 8 ). Figure 2 7 Fluorescence quenching of O PPE (10 M ) by adding DOC. A ) A t a lower DOC concentration [ 0.1 M 1.0 M ] and B ) at a higher DOC concentration [ 1.0 M 5.0 M ]. Comparison of the Stern Volmer plots for C PPE and O PPE quenching by MV 2+ in MeOH and H 2 O is illustrated in Figure 2 9 This presentation makes it very clear the
62 considerably higher efficiency for aggregate vs. monomer quenching of O PPE in MeOH and the intermediate efficiency exhibited by C PPE In water, the quenching of C PPE is slightly more efficient than in MeOH; however, for O PPE the quenching efficiency is very similar to that for the aggregat e state of the polymer in MeOH. This is not surprising, given that the emission of O PPE in water is domina ted by the aggregate band ( Figure 2 5 b ). Figure 2 8 Fluorescence quenching of O PPE (10 M ) by DO D C. A ) A t a lower DO D C concentration [ 0.1 M 1.0 M ] and B ) at a higher DO D C concentration [ 1.0 M 5.0 M ]. The dramatically different quenching behavio r of C PPE and O PPE is clearly tied to the difference in ability of the chromophore units to assemble into aggregate structures that give rise to a distinct aggregate fluorescence coupled with exciton delocalization and migration in the aggregate. The dif ference in aggregate structures is reflected by dynamic light scattering data obtained on the polymers in methanol and water (Fig ure 2 10 ). Specifically the two polymers exhibit a similar hydrodynamic radius in methanol (~10 nm); however, in water the radi us of C PPE is considerably larger than that of O PPE (55 vs 18 nm), indicating a much more compact aggregate for the latter
63 polymer in the aqueous environment. The difference in aggregate structures must be related the structural or electronic difference s imparted by the CH 2 linker in C PPE and the O linker in O PPE Figure 2 9. Stern Volmer plots of C PPE and O PPE with MV 2+ in A ) MeOH and B ) Water. (Polymer concentration = 10 M in each case). The linker difference could manifest in two possible w ays: 1) the electron donating effects of the oxygen linker may favor formation of compact emissive aggregate structures; or 2) the difference in linker structure may induce differences in secondary structure as the polymers fold in solution. Molecular mode ling studies do not suggest that there is a significant difference in conformation induced by the two linkers. However, we believe that in solution conju gated chromophore segments to form a compact stacked aggr egate structure is stronger for the oxygen linked system ( O PPE ), either due to steric or electronic factors. Seeking evidence for a difference in the secondary structures of the two CPEs we explo red their interaction with the cationic luminescent metallointercalator Ru(bpy) 2 (dppz) 2+ (Ru bipyridine and dppz = dipyrido[3,2 a:2',3'
64 c]phenazine). 115 In previous work we have shown that this complex intercalates into the stacked aromatic residues in the helical conformation of anionic meta linked poly(phenylene ethynylene) CPEs 30 31 Intercalation of Ru dppz is signaled by the appearance of its strong metal to ligand charge transfer (MLCT) luminescence, which is quenched when the complex is in water in the absence of a host Interestingly, in stacked helical conformation for O PPE we find that addition of the polymer to a solution of the Ru dppz gives rise to significant enhancement in the MLCT emission of the metal complex ( Figure 2 11 ). A si milar effect is not seen concomitant with addition of C PPE to the metal complex. This result stacking (and possibly helix formation) within the aggregate state of O PPE which is consistent with the observati on of significant aggregate emission and enhanced amplified quenching. I t is believed that the fluorescence lifetime of Ru(bpy) 2 (dppz) 2+ MLCT emission is positive ly correlated to how well they are shielded from water 116 In this sense, it is interesting to investigate the fluorescence lifetime of Ru(bpy) 2 (dppz) 2+ / O PPE com plex and compare it with the established Ru(bpy) 2 (dppz) 2+ / DNA 116 sys tems. T he fluorescence decay of Ru(bpy) 2 (dppz) 2+ / O PPE complex was recorded on time correlated single photon counting (TCSPC) instrument, and the result is shown in Figure 2 11d. The multi exponential decay consist s of two shorter components at 5 ns and 25 ns and a longer component at 150 ns. Since free Ru(bpy) 2 (dppz) 2+ is non emissive in water, all decays come from the interaction of Ru(bpy) 2 (dppz) 2+ and O PPE The shorter decay may be due to external binding, while the longer one should be the result of intercalation of Ru(bpy) 2 (dppz) 2+ into O PPE secondary structure. It is
65 necessary to point out that the average fluorescence lifetime of Ru(bpy) 2 (dppz) 2+ / O PPE complex is shorter than that of Ru(bpy) 2 (dppz) 2+ / DNA 116 (121 ns) complex, indicating that the secondary structure of O PPE is loose packed comparing to DNAs and t hus not able to shield Ru(bpy) 2 (dppz) 2+ from water as well. Figure 2 10. D istribution of particle sizes of C PPE and O PPE in MeOH and H 2 O (concentration at 20 M) measured by dynamic light scattering. A ll samples were passed through a 0.2 m PTFE membrane, and measured under 25 o C Indeed, the aggregate state of O PPE is in equilibrium with its monomer state. We have shown above that the emission ratio of aggregat e band to monomer band varies in solvents that contain different amount s of MeOH and H 2 O. Furthermore the concentration and temperature dependent photophysics was studied. 1) Figure 2 12 presents the absorption and fluorescence spectra of O PPE at differe nt concentrations. While the concentration of the CPE increases the ratio of monomer to aggregate emission decreased presumably because higher concentration favors the formation of aggregates. 2) Figure 2 13 records the emission spectra of O PPE A s the temperature
66 increases, the absorption is almost identical While both monomer and aggregate emission s decrease, the ratio of monomer emission to aggregate emission changes. It turns out that the monomer emission is more sensitive to temperature change than that of the aggregates. O ne plausible reason for this phenomenon is that the monomers tend to aggregate at higher temperatures. Taken together, the equilibrium between aggregate and monomer of O PPE is sensitive to concentration, solvent, and temperature, which is consistent with our conclusion that the O PPE forms loose secondary structures. Figure 2 11. Interaction between Ru (bpy) 2 (dppz) 2+ and O PPE A ) T he enhancement of Ru (bpy) 2 (dppz) 2+ (concentration 5 M) emission in water upon adding O PPE B ) pl ot showing variation of the emission intensity of Ru (bpy) 2 (dppz) 2+ with the concentration of C PPE and O PPE C ) the structure of Ru (bpy) 2 (dppz) 2+ a nd D ) the decay curve of Ru(bpy) 2 (dppz) 2+ with 20 M O PPE in H 2 O.
67 Figure 2 12. Photophysics of O PPE at different concentrations. A ) Absorption and B) fluo rescence spectra of O PPE at various concentrations in MeOH. (a) 2.5 M (solid line), (b) 5.0 M (dashed line) and (c) 10 M (dotted line). Figure 2 13. Fluorescence spectra of O PPE at different temperatures in MeOH. Summary and Conclusion s In of conjugated polyelectrolytes. Polymer C PPE does not exhibit emission from an aggregate state in either MeOH or H 2 O, and while the quenching of this polymer is enhanced slightly compared that of a monomeric chromophore, the polymer exhibits only a mod est amplified quenching effect. By contrast, polymer O PPE exhibits both monomer and
68 aggregate emission, and the a ggregate state is quenched 200 fold more efficiently compared to the monomer state with a K SV value rivaling the maximum efficiency seen for full y conjugated polyelectrolytes. The results underscore the important role played by chromophore aggregation in p romoting efficient exciton transport which is key to the amplified quenching effect. Experimental Materials and Methods All chemicals were purchased from either Acros or Aldrich and used as received without further purification. NMR spectra were recorded o n a VarianVXR 300 or a Varian Gemini 300. UV visible absorption spectra were obtained either on a Varian Cary 100 or a PerkinElmer Lambda 25 dual beam absorption spectrometer using 1 cm quartz cells and corrected for background due to solvent (HPLC grade) Fluorescence spectra were recorded on a PTI fluorimeter. Fluorescence lifetime data was recorded on a PicoQuant Picoharp 300 TCSPC instrument. Dynamic light scattering results were obtained from Brookhaven ZetaPlus The reported concentrations refer to the concentration of polymer repeat units (PRU). Solutions for spectroscopic studies were prepared by dilution of stock solutions. The C PPE st ock concentration was 0.72 mg/m L which corresponds to 1.2 mM and O PPE stock concentration was 0.89 mg/m L which corresponds to 1.4 mM. Synthetic Procedure s bis(4 iodophenyl)methane (1) To a stirring mixture of 8.4 g (0 .05 mol) diphenylmethane, 100 m L acetic acid and 10 m L deionized water, 10.5 g iodine and 4.3 g KIO 3 were added. The resulting mixture was heated to above 90 o C before 5 m L concentrated H 2 SO 4 were charged dropwise. The reaction was allowed 12 h r under
69 reflux. Saturated Na 2 S 2 O 3 solution was added to quench the excess I 2 the color of the solution turned from dark brown to light yellow. After cooling, th e reaction mixture was poured into 1 L ice cold water and then further cooled at 0 o C for 1 h r The precipitates were isolated and the crude product was recrystalized by Hexane. 1 was obtained as white crystals. Yield 6g (30%) 1 H NMR ( 300 MHz, CDCl 3 ppm ) 3.96 (s, 2H), 7.07 (d, 4H), 7.39 (d, 4H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 41.81, 94.03, 129.08, 132.20, 140.91. 4,4' oxybis(iodobenzene) (2) To a 100 mL round bottom flask, 5.6 g of N iodosuccinimide was dissolved in 40 m L of acetonitrile. 2 g diphenyl ether (11.7 mmol) was charged before 5 drops of CF 3 COOH was added to the solution. The reaction was allowed to run for 5 h r under room temperature and the process was monitored by TLC plate After reaction was done, the solvent was evaporated and the solid was redissolved in DCM, the solution was washed with saturated Na 2 S 2 O 3 solution twice, and then brine. Organic layers were combined and evaporated. The crude product was recrystalized by acetone. 2 was obtained as white crystals. Yield 3 g (60%) 1 H NMR ( 300 M Hz, CDCl 3 ppm ) 6.91 (d, 4H), 7.43 (d, 4H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 84.32, 120.04, 137.24, 153.86. bis(4 ((trimethylsilyl)ethynyl)phenyl)methane (3) To a 200 m L round bottom flask, 3 g of compound 1 (7.2 mmol) was charged. A mi xture of 50 m L dry THF and 20 m L tri ethylamine (TEA) was used to dissolve the solid. The solution was degassed by argon flow for 20 min before 80 mg Pd(PPh 3 ) 4 and 60 mg CuI were added quickly to the stirring solution. After 5 min, 1.67 m L (16 mmol) trimethylsilyl acetylene was added to the reaction mixture by syringe. Precipitates formed immediately. The reaction was allowed to run for overnight under room temperature. After reaction was done, the
70 precipitates were filtered. The filtrate was evaporated and the solid was redissol ved in DCM. The solution wa s washed by diluted HCl (30 m L 2), brine (30 m L 2) and water (30 m L ). The organic layers were combined and dried over anhydrous Na 2 SO 4 After DCM was evaporated, the crude product was purified by flash chromatography (eluent He xanes). 3 was obtained as white solid. Yield 2.3 g (90%) 1 H NMR ( 300 MHz, CDCl 3 ppm ) 0.24 (s, 18H), 3.96 (s, 2H), 7.07 (d, 4H), 7.39 (d, 4H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 0.22, 41.87, 94.78, 124.05, 129.06, 132.36, 141.32. (( oxybis ( 4 ,1 phenylene))bis(et hyne 2,1 diyl))bis(trimethylsilane) (4) This compound was synthesized by the same procedure used to prepare 3 except that the amounts of reagents used were: 2 (1.5 g 3.55 mmol), trimethylsilyl acetylene (0.88 m L 8.0 mmol), Pd(PPh 3 ) 4 (40 mg) and CuI (30 mg). The product was purified by elution through a plug of silica gel with a mix solvent of Hexanes and DCM (10:1 v/v ). Yiel d 1 g (78%) 1 H NMR ( 300 MHz, CDCl 3 ppm ) 0.25 (s, 18H), 6.91 (d, 4H), 7.43 (d, 4H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 0.28, 114.95, 117 .54, 132.21, 156.03 bis(4 ethynylphenyl)methane (5) To a 200 m L round bottom flask, 1.0 g of 3 (2.77 mmol) was charged. A mixture of 15 m L DCM, 10 m L Hexane, and 15 m L CH 3 OH were added to dissolve the reactant. The solution was degassed by argon for 15 m in before 2.21 g of K 2 CO 3 (16 mmol) was added portion wise to the solut ion. The reaction was done in 2 h r (monitored by TLC plate ). After filtration, the filtrate was concentrated. The solid was purified by flash chromatography ( Hexane / DCM 1/1 v / v) and 5 w as obtained as a white solid. Yield 0.52 g (87%) 1 H NMR ( 300 MHz, CDCl 3 ppm ) 3.04 (s, 2H), 3.97 (s, 2H), 7.12 (d, 4H), 7.43 (d, 4H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 41.89, 83.77, 120.30, 129.15, 132.57, 141.57
71 4,4' oxybis(ethynylbenzene) (6) This compound was synthesized by the same procedure used to prepare 5 except that the reagent used w as 4 (1.02 g 2.8 mmol). Yield 0.45 g (75%) 1 H NMR ( 300MHz, CDCl 3 ppm ) 3.15 (s, 2H) 6.92 (d, 4H), 7.42 (d, 4H) 13 C NMR ( 75MHz, CDCl 3 ppm) 81.23, 115.78, 117.32, 132. 21, 152.56 S odium 3,3' ((2,5 diiodo 1,4 phenylene)bis(oxy))bis(propane 1 sulfonate) (7) 7.24 g (20.0 mmol) 2,5 diiodohydroquinone was dissolved i n a solution that contained 2.0 g (50.0 mmol) sodium hydroxide in 200 m L water in a Erlenmeyer flask under ar gon. A solution of 6.1 g (50.0 mmol) of 1 3 propanesultone in 40 m L of dioxane was added to the former solution at once. The resulting mixture was then stirred at room temperature for overnight, during which time a thick pink slurry formed. The reaction mi xt ure was then stirred at 80 100 C for another 30 min and then cooled in a water/ice bath. The suspension obtained was vacuum filtered, and the retained solid was washed with cold water followed by acetone, and crystalli zed twice from water. Yield 8.7 g (6 6%) 1 H NMR ( 300MHz, DMSO d6 ppm ) 2.00 (t, 4H); 2.64 (t, 4H); 4.05 (t, 4H); 7.30 (s, 2H) 13 C NMR ( 75MHz, DMSO d6, ppm) 25.37, 48.14, 68.99, 86.99, 122.44, 152.32 C PPE was synthesized by a modified literature procedure. 21 0.735 g (1.16 mmol) monomer 7 and 0.250 g (1.16 mmol) monomer 5 were dissolved in a mixture of 10 m l water and 10 m L DMF in a Schlenk flask with a gentle flow of argon with stirring. The resulting clear solution was deoxygenated by several cycl es of vacuum argon cycling. Another solution comprised of 60.0 mg of Pd(PPh 3 ) 4 and 50.0 mg CuI in a mixture of 10 m L triethylamine and 10 m L of DMF was likewise deoxygenated and was subsequently added to the former solution by means of a syringe. The final mixture was again deoxygenated by vacuum argon cycling and was then warmed to 70 o C and stirred
72 under a positive pressure of argon for 24 h. The solution was cooled and then slowly added to 1 L of a methanol/ether mixture (30:70 v / v). The polymer pr ecipita ted as greenish fibers. It was redissolved in 200 m L of water/DMSO ( 98:2 v/v) treated with 0.08 g of sodium sulfide (Na 2 S), and then the solution was filtered through quantitative filter paper, followed by a 10 membrane. The polymer was precipitated by addition to a large volume of methanol /ether (30:70). The polymer was dissolved in water/DMSO and reprecipitated from methanol/ether two more times. Finally, the polymer was dissolved in 200 m L of water with 1 m L DMSO and the resulting solution was dialyzed against water (Millipore Nanopure TM ) using a 10 12 kD MWCO cellulose membrane. After the dialysis, the p olymer concentration was 1.0 mg/ m L (or 1.67 mM for polymer repeat unit) The polymer wa s stored in this format and diluted as appropriate for spectroscopic studies. 30 m L polymer solution was dried by means of freeze drying and redissolved in DMSO d6 for NMR characterization. 1 H NMR ( 300 MHz, DMSO d6, ppm) 2.04(broad 4H), 2.72(broad 4H), 4.0 0 4.18 (broad 6H), 7.09 (broad 2H), 7.31(broad 4H), 7.50 (broad 4H). O PPE was synthesized under exactly the same condition with that of C PPE except that the monomer used were 6 and 7 After dialysis, the polymer concentration was 0.9 mg / m L (or 1.50 mM f or polymer repeat unit). 1 H NMR ( 300 MHz, DMSO d6 ppm ) 2.04 (broad 4H), 2.70 (broad 4H), 4.13(broad 4H), 7.06 7 .17 (broad 6H), 7.62 (broad 4H)
73 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION OF BODIPY BASED POLYELECTROLYTES AND THEIR INTERACTIONS WITH QUENCHE RS Incorporating BODIPY into Polymer Backbones 4,4 4 bora 3a,4a diaza s indacene (BODIPY) dyes (structure shown in Figure 3 1 ) and their derivatives have been receiving continuous research interest since first introduced by Treibs and Kreuzer 117 for many reasons. First, the ir molar extinction coefficient s are very high ( usually around 70 000 80 000 M 1 c m 1 ) 118 Second, the y feature relative ly sharp emission and high photoluminescence quantum yield s Thi rd, the ir excellent photochemical and t hermal stabilit y and good solubilit y in many solvents offer them great potential s as functional materials 119 Most important their photophysical properties are easily tuned as desired upon modif ication of chemical structures 120 Based on these properties, the applications of BODIPYs in many field s that include biological labe ling 121 ion sensing 122 light harvesting 123 and solar cells 118 124 have been explored. While there is a good amount of research that succeed s in fine tuning the optical properties of BODIPYs by chemical structure modifications 117 125 most work is engaged in small molecules, and only a few report s have been presented in which the dyes are incorporated into conjugated polymer backbones 120 124 126 In 2008, Nagai e t al reported a novel set of polymers that included BODIPY into poly(phenylene ethynylene) or poly(fluorene ethynylene) back bones (Figure 3 2 a ) 120 achieving better processability comparin g to the small molecules. These polymers however, bear almost identical absorption and emission spectra as those of typical BODIPY dyes. 120 This is because that t he p arylene ethynylenes were attached to the 4,4 positions (Figure 3 1) on t he BODIPYs, which does not offer e fficient conjugation between the arylene
74 ethynylene units and the BODIPY cores. Similar phenomena were also observed in other researches. 127 129 In 2009, Liu and co workers modified the co polymer patterns by connecting BODIPYs at 2,6 positions (Figure 3 1) with a series of aromatic units featuring different band gaps (Figure 3 2 b ) 126 As a result, polymeric BODIPY dyes that emits at deep red and near I R regions were develope d, due to significant extension of conjugation between BODIPYs and other aromatic units. In parallel their cooperating group ( Frechet et al .) explored the potential of these BODIPY containing polymers in bulk heterojunction solar cells and achieved about 2% power conversion efficiency 124 In a separate study, Thayumanavan and co works synthesized a series of conjugated donor acceptor polymers based on the BODIPY s core and acceptors with different HOMO and LUMO levels such as quinoxaline benzothiadiazole and rylene acceptors 130 T he electrochemistry of the polymers w as carefully investigated, and preliminary results revealed the strong potential for such BODIPY based polymers to function as either p type or n type semiconductors depending on the electrochemical nature of the comonomer s 130 Figure 3 1. Chemical structure of BODIPY Now that it has been established that conjugated polymers with B ODIPYs in the conjugated backbone combine the unique properti es of both BODIPY and conjugated polymers, 120 124 126 130 we propose that the scope of the application of such polymers would be further expand ed if they could be render ed with water solub i l ity In fact, several fields such as ion sensing, bio labeling, and the stud y of amplified quenching
75 behavior require good solubilit y in water or protic organic solvent s Although there are a few cases in which the water soluble BODIPY dyes were introduced 131 136 the studies were all limited to small molecules T o the best of our knowledge, the water soluble polymeric BODIPY electrolytes have not been reported P reviously, we reported a series of conjugated polyelectrolytes (CPEs) with di fferent band gaps that feature sterically congested, branched polyionic side groups 137 I t was found that highly charged ionic functional groups serve to enhance the solubility of the CPE s in aqueous solution by increasing electrostatic repulsion betwe en polymer chains and thus reduc ing the hydrophobic interchain interactions 137 Following the sam e method, we have synthesized the BODIPY and phenylene ethynylene based CPE s that bear branched polyionic side groups. In this chapter, we will first intr oduce the synthesis of the CPE that features BODIPY and phenylene ethynylene backbones and dendritic side groups ; then the photophysical character and t he amplified quenching behavior of the CPE will be discussed. Finally, the preliminary results of i ts ap plication in dye sensitized solar cells will be described. Figure 3 2. Two examples of conjugated polymers incorporating BODIPY A ) attaching BODIPY at 4,4 positions 120 and B ) attaching BODIPY at 2,6 positions 126
76 Results and Discussion Synthesis and Struct ural Characterization In this section we introduce the synthesis of BODIPY phenylene ethynylene type of CPE carrying polyionic side chains using the precursor strategy namely, the precursor polymers are synthesized and purified in organic solvents b efo re the protecting groups (t b utyl in this case ) are hydrolyzed to yield the corresponding CPEs. T he synthesis of compound 5 was carried out following literature procedures 138 As shown in Figure 3 3, the di iodobenzene with dendritic side chains ( 5 ) was prepared from 4 step reactions with an overall yield of 50%. C ompound 5 was then reacted with trimethylsilyl (TMS) acetylene under Sonogashira coupling conditio n and trans form ed into TMS protected di ethynylene benzene, which was subsequently deprotected by t etra n butylammonium fluoride ( TBAF ) to give monomer 6 Figure 3 3 Synthesis of monomer 6 (i) DME, Triton B, t b utyl acryl ate 80 o C ; (ii) Raney nickel, EtOH, H 2 48hr ; (iii) SOCl 2 DMF, 90 o C 2hr ; (iv) TEA, DCM, 0 o C to r.t. overnight ; ( v 1 ) TMSA, Pd(0), CuI, THF r.t. 13hr ; ( v 2 ) TBAF 30 min
77 On the other hand, the BODIPY monomer was synthesized following the scheme presented i n Figure 3 4. Triethylene glycol mono methyl ether was tosylated to give compound 7 which underwent an S N 2 reaction attacked by deproton ated 4 hydroxy b enzaldehyde under basic condition to yield compound 8 The BODIPY core ( 9 ) was obtained from 8 in a thre e step one pot synthesis: condensation of benzylaldehyde with 2,4 dimethylpyrrole to give dipyrro monophenyl methane followed by oxidation by 2,3 Dichloro 5,6 dicyano 1,4 benzoquinone ( DDQ ) and complexation with trifluoro b oron ( BF 3 ) The overall yield fo r three steps was around 20%, which is consistent with similar reactions reported. 117 The iodination at 2,6 positions of compound 9 by I 2 /HIO 3 system was done unde r fairly mild condition with considerable yield (64%) The purity of monomer 6 and 10 was prove n by NMR ( 1 H and 13 C) and MS spectroscopy before they were polymerized. Figure 3 4 Synthesis of monomer 10 ( i ) Na OH, THF, 0 o C to r.t. ; ( i i) 7 K 2 CO 3 DMF, 80 o C overnight ; ( iii 1 ) 2 4 Dimethylpyrrole DCM TFA ( cat.) r.t. overnight ; ( iii 2 ) DDQ 3hr ; ( iii 3 ) TEA BF 3 OEt 2 r.t., 30min ; (iv) I 2 HIO 3 EtOH r.t. overnight The monomers 6 and 10 were polymerized under Sonogashira cross coupling condition s (Figure 3 5 ). The reaction was allowed to run for 36 hr under 60 o C and the
78 mixture was passed through a short alumina oxide column to remove the catalysts before it was concentrate d and then precipitated by addition of MeOH. The precipitate (polymer product) was redissolved in minimum amount of CHCl 3 and reprecipitated with MeO H. Such dissolving precipitati on process was repeated four times to yield the polymer protected by t butyl esters (named as PB e ), which was analyzed by GP C and 1 H NMR. Figure 3 5 Polymerization and deprotection (i) Pd(0), CuI, THF, 60 o C 36hr; (ii) TFA, r.t. overnight; (iii) NaOH (10 4 M in MeOH) GPC result suggested that the PB e has a number averaged molecular weight of 28 kD (DP around 1 8 ) with PDI = 1. 6 The t butyl groups on PB e were hydrolyzed by t rifluoroacetic acid ( TFA ) using t etrahydrofuran ( THF ) as co solvent ( THF/TFA v/v=1:1) T he deprotected polymer ( where H replaced the t butyl ester groups, named as PB a ) was precipitated and washed with acetone, and characterized by 1 H NMR in CDCl 3 The 1 H NMRs of the monomer 6 and 10 the PB e and PB a are compared in Figure 3 6. It is clearly shown that the chemical shifts of protons in PB e appear in the same regions a s those of the monomers, except that the peaks are broadened due to polymerization.
7 9 Protons on t butyl groups appear as a strong singlet at = 1.43 ppm. After hydrolysis, the PB a shows almost no peak around 1.4 1.5 ppm, indicating that the t butyl groups were completely removed from the polymer. Minimum amount of MeOH that contained 0. 1 mM NaOH was used to dissolve PB a and the result ing sol ution was slowly added to H 2 O with p H=9. The water soluble polymer was dialyzed against a NaOH/ water (Millipore Nanopure TM ) solution ( p H = 8.5) in the dark for 3 days, resulting pure polyelectrolyte (named as PB Na ) water solution. The concentration of the CPE was measured by gravimetric analysis and the stock solution was degassed and stored away from light at 4 o C Figure 3 6. 1 H NMR spectra (in CDCl 3 ) and peak assignment for monomer 6 monomer 10 PB e and PB a
80 Photophysical Properties The absorption and fluorescence spectra of PB e and PB a in THF and PB Na in methanol are shown in Figure 3 7. Compared to BODIPY monomer 9 both absorption and emission spectra of PB e are red shifted and broadened, which comes from the extension of conjugation on BODIP Y units. The absorption of PB e exhibits dual peak s at 400 nm and 591 nm respectively with high extinction coefficient featuring a low band gap of 2.10 eV The fluorescence spectr um of PB e is a single emission at 631 nm. Compared to the literature report ed BODIPY phenylene ethynylene copolymers 126 PB e has a similar shaped spectrum However the wavelengths of absorption and fluorescence for PB e are around 20 nm hypsochromically shifted than the reported polymer A plausible rationalization of this observation is that the dendritic side groups that are applied to PB e have given rise to much larger spatial hinderance than the reported polymer (which has linear side chains), so that the coplanarity of PB e backbone decrease s and thus the effective conjugation length i s short er As PB e is hydrolyzed, the acid form of the polymer PB a exhibits dramatic hypsochromic shift on absorption spectrum, increasing Stokes shift and lowered fluorescence quantum yield compared to PB e All the evidence is pointing to the formation of H a ggregates of the polymer, and we believe that the aggregation is caused by intermolecular hydrogen bonding between the polymer chains. When PB a was reacted with NaOH and transformed into the sodium salt counterpart PB Na which is dissolved in MeOH, it tu rns out that both absorption and emission wavelength shift bathochromically, with an increase in fluorescence quantum yield. While presumably the addition of NaOH could break up H bonding and thus diminish H aggregates, the absorption and emission waveleng th of PB Na remains hypsochromically shifted
81 compared to PB e One possible explanation is that with dendritic ionic side groups, the electrostatic repulsion induces even more torsional strain than that exists in the ester precursors, where steric repulsio n has already impeded the polymer backbone from being coplanar. Similar observations have also been reported by other works. 137 139 Figure 3 7. Normalized a bsorption ( A ) and f luorescence ( B ) spectra of PB e (black solid lines ) in THF, PB a (red dash dot lines ) in THF and PB Na (blue dash lines ) in MeOH ( Fluorescence intensity reflect s relative Fl quantum yie ld ) To further characterize the aggregation photophysical property relation of PB Na experiments with variable solvents were conducted. It has been discussed in the where the CPEs exhibit single chain properties, while H 2 CPEs form aggregates due to their amphiphilic nature. In this study, we examined the photophysical behavior of PB Na in MeOH, H 2 O, and two co solvent systems where MeOH and H 2 O ar e mixed (MeOH/ H 2 O v/v 1/1 and 1/2, respectively). The absorption and fluorescence spectra of the PB Na in different solvents are recorded in Figure 3 8.
82 As the amount of H 2 O increases, both absorption and fluorescence of PB Na decreases. This indicates t hat the addition of H 2 O induces some aggregation. Figure 3 8. Absorption ( A ) and Fluorescence ( B ) spectra of PB Na in a mixture of methanol and water ( Fluorescence i ntensities reflect s relative quantum yield ) More insights come from the comparis on between PB Na and CPEs with linear side ionic groups. Since reports of CPEs that contain BODIPYs with linear side chain ionic groups are lacking in the literature (in fact, we have tried in vain to make such structures, but all attempts resulted in poly mers that were not soluble in H 2 O), we refer here to other CPEs that have linear side chains. In previous studies from Schanze and other groups 22 24 30 it was reported that CPEs with linear side chains tend to form aggregates upon addition of H 2 O, resulting in the following characteristic features: 1) r ed shift of absorption spectra with appearance of a pronounced shou lder; 2) l oss of well structured emission with broadened, red shifted structures; and 3) substantial decrease in fluorescence quantum yield coupled with an increase in the fluorescence lifetime. In this study, none of the above effect s were observed for PB Na In particular,
83 as the solvent is changed from MeOH to H 2 O, a limited change is seen in the absorption and emission spectr um This observation suggests that PB Na is less likely to form aggregates even in H 2 O. It has been discussed pre v iously that th e coplanarity of the CPE backbone is impeded by the branched, dendritic ionic side groups by both steric and electrostatic repulsions, 137 thus it could be deduced that such CPE structures do not favor aggregation via a stacking manner. Furthermore, the hydrophobic polymer backbones are well protected by high charge density of the dendritic ionic side groups, so that PB Na is less likely to self assemble as clusters in aqueous solution. Table 3 1 summarizes all the photophysical data for PB e PB a a nd PB Na noting that the photophysical behavior for PB Na in both MeOH and H 2 O is similar. Table 3 1. Photophysical data of PB e PB a and PB Na Polymer /Solvent nm 10 4 M 1 cm 1 nm (ns) PB e /THF 591 3.45 631 0.1 9 1.78(45.8%) 0.87(54.2%) PB a /THF 51 2 2.57 6 09 0.0 7 2.40(11.3%) 0.93(45.2%) 0.32(43.5%) PB Na /MeOH 521 2.49 61 4 0. 1 3 2.40(13.6%) 0.99(52.1%) 0.33(34.4%) PB Na /H 2 O 52 2 2.13 630 0.0 5 2.70(20.0%) 1.09(47.2%) 0.38(32.8%) a Ru(bpy) 3 Cl 2 in H 2 O ( F = 0. 0 56 degassed ) as standard Above all, the first water soluble BODIPY based CPE with modest fluorescence quantum yield has been introduced. The CPE is less likely to form aggregate in water and remains fluorescent, such property opens many opportunities for the CPE being applied to studies that are conducted in water or polar organic solvents. Some examples are (not limited to) amplified quenching, ion sensing, and layer by layer films.
84 Interaction with Quenchers Amplified Quenching and Ion Sensing With PB Na in hand, we investigated the fluorescence quenching behavior of the CPE in the presence of several quenchers to determine whether the amplified quenching eff ect is observable First, dimethyl bipyridinium (MV 2+ ) wa s applied to examine quenching by photo i nduced electron transfer MV 2+ is used in a number of works as electron acceptor to quench the photoluminescence of other anionic CPEs with a K SV ~ 10 6 M 1 1 21 23 25 Experimentally, a 5 M PB Na study soluti on was obtained by diluting the CPE stock solution. In parallel, MV 2+ in MeOH solution was prepared with relatively higher concentration so that negligible volume of the solutio n could cause the designed concentration change of the que ncher in the study solution. The evolution of absorption and fluorescence spectra of PB Na in MeOH were recorded as varying amount of MV 2+ was added, which is shown in Figure 3 9. S urprisingly, neither absorption nor emission of PB Na was affected by addit ion of MV 2+ T he absorption spectrum remain s identical except that the absorption from MV 2+ ( less than 300 nm) gradually increases with added MV 2+ while the fluorescence intensity decreased less that 10% when up to 10 M MV 2+ is present in the solution. This result shows that the quenching of PB Na at best occurs t o only a limited extent. This relatively inefficient quenching (K SV ~ 1.5 10 4 M 1 ) is contrary to the common CPE systems with PPE type of backbones, 12 21 24 where the fluorescence intensities are quenched a t substantially high efficiency ( K SV usually at ~ 10 6 M 1 ) Two possible explanations could be put forward to explain this phenomenon The first possibility is that the singlet excitons are localized within each repeat unit so that the amplified quenching process is prohibit ed. However this is less likely to happen due
85 to the fact that the transition dipole of the BODIPY unit 140 is aligned with that of the phenylene ethynylene units ( i.e. p arallel to the backbone of the CPE). Thus theoretically the exciton migration is allowed through the backbone of PB Na which should give rise to amplified quenching effect as observed in other CPE system s 1 21 23 25 To clarify this point more quenchers were applied to the PB Na system as discussed later in this chapter Figure 3 9. Absorption ( A ) and fluorescence ( B ) response of PB Na in MeOH (5 M) upon adding MV 2+ (concentration range from 0 10 M) A second factor that may preclude the quenching of the PB Na fl uorescence by MV 2+ is that the electron transfer reaction from the singlet excited state of PB Na to MV 2+ is thermodynamically un favor able To examine this possibility electrochemic al characteristics of the polymer w ere determine d ( this experiment was carried out by Romain Stalder from Dr. Reynolds laboratory at University of Florida ). PB a was used in the experiment because of its good solubility in the electrochemical solvent (CH 2 C l 2 ) Here, we assum e that the electrochemical properties of PB a and PB Na are similar Figure 3 10 shows the c yclic voltammetry ( CV ) and differential pulse voltammetry ( DPV ) data of PB a in DCM solution, from which an oxidati on potential of ~ 1. 0 V vs. F c/Fc + is
86 observed. Given that Fc + /Fc is 0.46 V vs. s aturated calomel electrode ( SCE ) 141 the oxidati on potential of PB a is 1.46 V vs. SCE. On the other hand, the reducti on potential of MV 2+ / MV + is 0.45 V vs. SCE, 142 and the energy that is calculated from the fluorescence wavelength of PB a (or PB Na ) is around 2.0 eV. PB* + MV 2+ PB + + MV + 1) G et = E ox ( donor) E red ( acceptor) E fl 2) Considering a photo induced electron transfer process that occurs between the singlet excited state of PB Na and MV 2+ as illustrated in equation 1), the free energy change of the process c an be estimated by equation 2) noting that with the polar solvent (MeOH) used in this case, the Coulombic term (e 2 / r) is considered negligible. Plugging in all the numbers discussed above, a free energy of ~ 0. 10 eV was calculated for the process described in equation 1 This thermodynamic calculation shows that the electron transfer from the singlet excited state of PB Na to MV 2+ is at best only weakly exothermic This result suggests that the weak quenching observed with MV 2+ is due to the fact that the electron tran sfer reaction from PB Na to MV 2+ does not have sufficient driving force. For electron transfer from PB Na to an electron acceptor to be efficient the quencher must have a reduction potential less negative compared to MV 2+ (E red > 0.4 eV). Further fluore scence quenching experiments that unveil the interactions between PB Na and other quenchers via either charge transfer or energy transfer mechanisms were also conducted. Cupric ions ( Cu 2+ ) has been found to be an efficient singlet exciton quencher in many anionic CPE systems 6 143 144 with quenching occur via charg e
87 and/or energy transfer pathways. In this study, Cu 2+ was chosen as a quencher ion due to its high affinity f o r CO 2 groups. Figure 3 11 show s the fluorescence response of 5 PB Na aqueous solution to increment al addition of Cu 2+ It is shown that the f luorescence of the CPE is quenched with a Ksv of ~ 1.0 10 6 M 1 ( For a curved Stern Volmer plot, the first few point s at low quencher concentration were used for K SV calculation, Figure 3 11b), which is a typical value for Cu 2+ quenching the CPEs that has b een reported previously 6 Without the appearance of an the emission spectra, we believe that the quenching involves energy and/or electron transfer from PB Na to Cu 2+ Fig ure 3 10 CV (solid line) and DPV (dash dot) of PB a recorded in a 0.1M TBAPF 6 in DCM solution calibrated agains t Fc/Fc + Interestingly, the fluorescence of PB Na has very limited quenching (like the case of MV 2+ K SV in the range of 10 3 M 1 ) when other metal ions such as Ca 2+ Co 2+ Ni 2+ Zn 2+ Cd 2+ and Pb 2+ were titrated into the system with exactly the same condit ion of Cu 2+ These divalent ions were believed to induce aggregation of PPE base d polyelectrolytes with linear side groups by bridging different polymer chains. 23 In
88 presumably because the dendritic side groups that are modified on the polymer backbones offer a reasonable chelation to the metal ions within one unit and thus the interaction between the polymer chains is diminished. Figure 3 1 1 Fluorescence quenchin g of PB Na by Cu 2+ A ) f luorescence spectra of PB Na (5 M) with added Cu 2+ (concentration range from 0 8.0 M), and B ) the S V plot of the quenching process. (Ksv~1.0 10 6 M 1 ) T he amplified quenching effect was also investigated with the energy transfer quencher 3,3' diethyloxatricarbocyanine iodide ( DOTC ). ( The structure of DOTC is illustrated in Figure 3 12 ) The dye has been chosen for several reasons which include: 1) Good absorption spectr al overlap with the fluorescence emission of PB Na which give s rise to efficient Foster energy transfer. 2) H igh fluorescence quantum yield so that energy transfer c an be monitored at relati vely low quencher concentration. 3) Positively charge d which facilitates the electrostatic binding between the polymer and the dye The fluorescence intensity of the PB Na study solution (H 2 O, 5 M) was examined as a function of DOTC concentration with excitation at 450 nm, where no absorption from DOTC was observed.
89 Figure 3 12. Chemical structure of DOTC Figure 3 13 exhibits the evolution of the fluorescence of the study solution as DOTC was added It is evident that t he emission from PB Na is quenched while the emission from DOTC increases For comparison, a control experiment was conducted where the same amount of DOTC wa s titrated into pure H 2 O in the absence of PB Na When excited at 450 nm, there is almost no fluorescence detected f rom DOTC observed. This experiment clearly show s that energy transfer occur s from excited PB Na to DOTC i.e. the emission from DOTC is sensit ized by the CPE Based on the S V plot of fluorescence quenching (Figure 3 13 B ), t he quenching efficiency of PB Na study solution is calculated to be 5. 0 10 5 M 1 which is comparable to that of Cu 2+ quenching (1.0 10 6 M 1 ) discussed before. In all the qu enching experiments, the fluorescence lifetime of PB Na study solution was monitored as quenchers were added. In all cases, no obvious evidence is found for lifetime quenching, which indicates that the fluorescence quenching occurs by a static quenching pr ocess. Above all, we have systematically investigated the fluorescence quenching behavior of this novel red emitting CPE system featuring BODPIY phenylene ethynylene copolymer backbone. Amplified quenching has been observed both with metal ion Cu 2+ and cy anine dye DOTC. The quenching efficiency is comparable to typical PPE type of polyelectrolyte systems. Nonetheless, the interaction between the
90 PB Na with commonly used electron acceptor MV 2+ is not as prono unced as the literature reports, and this observa tion is attributed to the relative ly high oxidation potential of the CPE. Figure 3 1 3 Fluorescence quenching of PB Na by DOTC. A ) f luorescence spectra of PB Na (5 M) with added DOTC (concentration range from 0 4.0 M), and B ) the S V plot of the quen ching process. (K SV ~ 5.9 10 5 M 1 ) Application of PB a in D ye sensitized Solar Cell s As part of our efforts aimed at developing organic semi conductive opto electronic materials, we applied the PB system to the TiO 2 DSSCs, based on the following ideas : 1) T h e polymer has an ex tend ed absorption with onset at 600 nm with considerable molar absorptivity. 2) The electrochemical propert ies of the polymer suggest that there is a reasonable driving force for both electron injection and oxidized polymer regeneration in a TiO 2 based DSSC format. Specifically, t he reductive potential E red of PB a ( 1.4 V vs. Fc + /Fc, Figure 3 10) is sufficiently more negative as compared to the TiO 2 conduction band ( 0.9 V vs. Fc + /Fc) 145 such that electron injection from PB a into TiO 2 is expected. On the other hand, the oxidati on potential E ox of PB a (1.0 V vs.
91 Fc + /Fc, from Figure 3 10) is substantially more positive than the reducti on potential of the iodine /iodide couple (0.24 V vs. Fc + /Fc) 146 which allows the oxidized polymers to be reduced by iodide. 3) The PB a is substituted with carboxylic acid groups which are widely applied as anchoring groups to bind organic dyes to the TiO 2 surfaces. 147 148 Figure 3 1 4 PB a on TiO 2 films A ) film absorption ; B ) f luorescence lifetime comparison of PB a i n THF solution, on a silica film, and on a TiO 2 film ; C ) IPCE ; D ) J V curve under solar AM 1.5 (PCE 0. 7 % FF 4 3 %) Experimentally, the PB a coated TiO 2 films were tested in a modified reported DSSC format 61 using a propylene carbonate solution of I 2 /LiI as the electrolyte and a Pt/FTO counter electrode. T he IPCE and J V curve of the as prepared DSSC are exhibited in Figure 3 14 (Figure b and d, respectively). Comparing the IPCE curve with the film absorption profile
92 (Figure 3 14a), it is concluded that the main contribution to photocurrent comes from the BODIPY unit. Despite of that, the cell suffers from low photo current conversion efficiency which leads to a rather poor cell performance (PCE~ 0.7%). To rationalize this observation and make suggestion for further improvement on the cell performances, several aspects are pointed out here. 1) It has been discussed in the photophysics of the polymer that the dendritic side group containing carboxylic acid is very likely to induce the dye aggregation. Although dye aggregation is preferable in terms of favoring light harvest ing by broadening the absorption spectra, it is believed to hinder the electron injection efficiency 149 150 which would result in low power conversion efficiency. A g eneral solution to this problem is either to co adsorb the dye with additives that break the aggregation, 151 or to structurally modify the dyes to prevent aggregation. 152 2) A recent study that was conducted in Schanze lab has revealed a correlation between polymer chain length and cell performance in DSSCs. 61 It was demonstrated that unlike the conjugated polymer based bulk heterojunction solar cells, where the cell performance increases with polymer molecular weight, 153 154 better preforming cells were realiz ed by low molecular weight polym ers ( PD ~ 5 ) which tend to achieve more surface coverag e than the high molecular weight ones. By comparison, the PB a used in this study has a r elatively high molecular weight (DP ~18). Based on this argument, it might be worth while engineering the chain length of PB a to optimize the dye coverage on the TiO 2 surface, and therefore increase the DSSC performance. 3) Another barrier for electron injection may come from the chemical structure of PB a (Figure 3 5) It is obvious that i nstead of directly attach ed to the TiO 2 the backbone of PB a is spaced
93 rboxylic acid groups. Such insulation of the dye from TiO 2 surface could be a source preventing electron injection. 155 Finally, we looked into the photophysics of TiO 2 film a dsorbed with PB a for evidence of charge injection. Time resolved fluorescence spectroscopy was applied, and the fluorescence lifetime data of PB a in THF solution, on a non quenching silica gel, and on a TiO 2 film were obtained, as shown in Figure 3 14c. The fluorescence decay kinetics of PB a in THF solution and adsorbed on silica are very similar, with median lifetimes of 1.51 and 1.39 ns, respectively. By contrast, when adsorbed on TiO 2 films, the PB a fluorescence exhibits a 0.35 ns lifetime, indicatin g that the singlet excited state of PB a is quenched by TiO 2 More insight is given by transient absorption (TA) measurement of PB a / TiO 2 film. Figure 3 15 records the TA profile attributed to the radical cation of PB a which is formed as a result of elect ron injection. 61 156 The lifetime of the radical cation, which reflects the rate of charge recombination, is calc ulated to be 118 Compared to the literature values (200 300 ) 61 156 th e lifetime of the radical cation is relatively short which is provides another reason for low cell performance. Figure 3 1 5 PB a TiO 2 film transient absorption A ) Transient absorption signal and B ) decay profile at 700 n m The lifetime of radical cation of PB a (reflect the rate of charge recombination) w a s calculated to be 118 s
94 The time resolved fluorescence measurement and transient absorption of the PB a / TiO 2 film have provided concrete evidence for electron injection, and depicted the dynamics of charge recombination. Nonetheless, the relatively low TA signal compared to the literature reports 61 indicates that the charge injection yield is low, agreeing with the conclusion from IPCE results. Summary and Conclusion s To conclude this chapter, we have synthesized a novel CPE featuring low band gap BODI PY phenylene ethynylene copolymer backbone and branched anionic (CO 2 ) side chains. The polymerization was effected via a precursor route utilizing a Sonogashira cross coupling reaction that yielded the precursor polymer PB e Hydrolysis of PB e was conduc ted under THF/TFA condition, which give rise to the carboxylic acid form of the polymer PB a The latter was treated with dilute NaOH solution and transformed to the salt form PB Na The photophysics of PB Na in MeOH and H 2 O is similar, indicating that the CPE is well dispersed, and thus applications of PB Na in aqueous surroundings could be anticipated. The fluorescence quenching experiment s show that PB Na is quenched by Cu 2+ selectively among a series of metal ions in an amplified quenching manner (Ksv ~ 10 6 M 1 ), and also, the energy transfer process was observed from PB Na to cyanine dye DOTC However, the electron acceptor MV 2+ does not quench the CPE very efficiently Electrochemical analysis shows that MV 2+ is not a sufficient ly strong oxidant to al low photo induced ele ctron transfer under scoring the low HO MO energy level of the novel CPE PB Na The carboxylic acid form PB a wa s utilized as a sensitizer in DSSC cells with TiO 2 E lectron injection from PB a to TiO 2 is proved by fluorescence lifetime measurement and transient absorption experiment on PB a TiO 2 film. However, low transient
95 absorption signal together with relatively low IPCE suggest that insufficient electron injection yield is resulting from the cell. Structure property discussions were initiated and future development on the PB a based DSSCs are pointed out Experimental Materials and Methods All chemicals were purchased from either Fisher or Aldrich and used as received without further purification. NMR spectra were recorded either on a Varian Gemini 300 or a Mercury 300 spectrometer. Chemical shifts were referenced to residual signals from C H Cl 3 ( 1 H 7.26 ppm and 13 C 77.23 ppm ) UV visible absorpt ion spectra were obtained on a Varian Cary 100 absorption spectrometer using 1 cm quartz ce lls and corrected for background due to solvent (HPLC grade) Fluorescence spectra were recorded on a Photon Technology International ( PTI ) fluorimeter. Fluorescence lifetime data was recorded on a PicoQuant Picoharp 300 TCSPC instrument. E chem measurem ent was conducted by Romain Stalder from Dr. Reynolds group. The electrochemistry was performed under inert atmosphere (Ar filled glovebox) using a 3 electrode cell containing a solution of PB a in 0.1 M t etrabutylammonium hexafluorophosphate ( TBAPF 6 ) in D CM. The working electrode was a Pt button, the counter electrode was a Pt flag and we used a Ag/Ag + reference electrode calibrated against Fc/Fc + Film transient absorption experiment was conducted by Randi Price from Dr. Schanze group. The instrument setu p is illustrated in Figure 3 16. A Surelite I 10 Nd: YAG laser with second and third harmonic generators delivered nanosecond pulses for excitation a t 532 or 355 nm, respectively. A 250 watt quartz tungsten halogen lamp and power supply (Newport catalog #6 334NS lamp and #69931 radiometric power supplies)
96 was used as the probe source. Light exposure of the sample is contr olled by a series of shutters. Wavelengths of detection are isolated using a Cornerstone 130/m Motorized Monochromator. Transient absorptio n is collected using a Hamamatsu R928 PMT with an in house custom wired base utilizing 5 of the 9 stages powered with 730 volts. Dye sensitized solar cells were fabricated as follows. First, a TiO 2 compact layer was spin coated onto a clean FTO glass slide 157 The TiO 2 nanocrystalline paste (20 nm in diameter), which was received as a gift from Dr. Zhen Fang at Duke University, was doctor bladed onto the slide with compact layer and sintered at 400 o C for 30 min. The sintered electrode was immersed into the PB a solution in DMF ( ~ 0.2 mM, based on repeat units) for 36 h to allow for polymer adsorption onto the TiO 2 film. The counter electrode was prepared by sputtering Pt on FTO glass with a thickness of 10 nm. Finally, an electrolyte solution containing 0.05 M I 2 and 0.1 M LiI in dry propylene carbonate was drawn into the sandwich between the TiO 2 working electrode and the Pt counter electrode by capillary action. The active area of the cell was 0.3 cm 2 The performance of the cells was measured immediately after they were assembled. The current voltage characteristics of the cells were measured with a Keithley 2400 source meter under AM1.5 (100 mW/cm 2 ) solar simulator. As for the IPCE measurements, the c ells were illuminated by monochromatic light from an Oriel Cornerstone spectrometer, and the current response under short circuit conditions was recorded at 10 nm intervals using a Keithley 2400 source meter. The light intensity at each wavelength was cali brated with an energy meter (S350, UDT Instruments).
97 Figure 3 16. Transient absorption setup for films. Components : A Surelite I 10 Nd: YAG laser; B Seco nd and Third Harmonic Generator; C 250W QTH Lamp; D Slow Shutter ; E Fast Shutter; F Cornerstone 130/m Monochromator; G PMT; H APD. Optics : 1,2 3,4,10 5,6,7 10cm f l. Plano convex ; 8 10 cm fl. Concave mirror; 9 5 ; 10 5 cm f l. concave mirror Synthetic Procedure s Compound 1 4 were sy nthesized by following the literature procedures 158 Compound 5 This compound was synthesized by a modified literature procedure. 159 To a 250 m L round bottom flask were charged compound 2 (5 g, 12.1 mmol), 5 m L of Et 3 N and 50 m L distilled DCM T he mixture was coo led with an ice/water bath before 45 mg 4 ( 5. 5 mmol) in 30 m L dry DCM was added via a syringe. After 2 hr, the reaction mixture was allowed to warm to room temperature and further stirred for 10 hr. The solvent was removed by vacuo, and the crude product was purified by flash chromatography (silica g el, EtOAc/hexane (1/3 v/v )) to give 5 as white powder (yield 8 5 %). 1 H NMR ( 300 MHz CDCl 3 ppm): 7.13 ( s 2H), 6.60 (s, 2H), 4.35 (s, 4H), 2.25 (m, 12H), 2.03 (m, 12H), 1.42 (s, 27H). 13 C NMR ( 75 MHz, CDCl 3 ppm): 172.1 8 165.7 2 151.57, 122.63, 86.29, 80.6 3, 68.83, 57.80, 30.17, 29.74, 28.09. Compound 6 This compound was synthesized in two steps. Step 1 is the Sonogashira cross coupling reaction between compound 5 and TMS acetylene. To a
98 100 m L round bottom flask was charged compound 5 (2.55 g 2 mmol) 20 m L THF and 10 m L di isopropyl amine ( DIPA ) With stirring, the solution was degassed by a n argon flow for 30min before Pd(PPh 3 ) 4 (10 mg) and CuI (8 mg) were charged into the reaction mixture. After the system was degassed for another 5 min, 0.84 mL ( 8 mmol ) of trimethylsilyl acetylene was added to the reaction mixture by syringe. The reaction was allowed to run for 5 h r at room temperature before the precipitates were filtered. The filtrate was evaporated by vacuo and the solid was redissolved in DCM. The s olution was washed by saturated NH 4 Cl solution (30 mL 2), brine (30 mL 2) and water (30 mL). The organic layers were combined and dried over anhydrous Na 2 SO 4 before they are passed through a short column (silica gel) to remove residue catalyst. Then DCM was evaporated, the crude product was dried under vacuum and directly used in the next step, without further purification. Step 2 is the deprotection of TMS group to yield compound 6 The crude product of step 1 was dissolved in 30 m L DCM, the solution wa s degassed for 20 min before excess amount of t etra n butylammonium fluoride ( TBAF ) (2 m L ) was charged into the round bottom flask. The solution turned brown immediately. After 10 min 5 m L MeOH was added by syringe and the reaction was allowed to run for another 20 min before all solvent was removed in vacuo. The crude product was applied to flash chromat ography (silica gel) using DCM/ h exane (2:1 v/v) as eluent Compound 6 was obtained as white powder after dried under vacuum. Yield 1.54 g (72% for two ste ps) 1 H NMR (300 MHz, CDCl 3 6.94 (s, 2H), 6.63 (s, 2H), 4.37 (s, 4H), 3.63 (s, 2H), 2.23 (t, 12H), 2.01 (t, 12H), 1.42 (s, 54H). 13 C NMR (75 MHz, CDCl 3 172.46, 166.47, 153.12, 117.87, 114.07, 85.05, 80.87, 79.00, 68.44,
99 57.78, 30.39, 29.85, 28.30 ESI TOF MS calcd for C 58 H 88 N 2 O 16 Na 1091.6026 found [M+Na] + at 1091.6070 2 (2 (2 M ethoxyethoxy)ethoxy)ethyl p tosylate (7) This compound was prepared by a modified reported procedure. 160 To a 500 mL round bottom flask was added sodium hydroxide ( 8 g, 0.2 mol) in water ( 30 mL) with vigorous stirring When NaOH was fully dissolved, triethylene glycol mo nomethyl ether ( 16 g, 0.1 mol) in THF ( 50 mL) was charged. The reaction mixture was cooled to 0 C by ice bath before a solution of p tosyl chloride ( 19 g, 0.1 mol) in THF ( 50 mL) was added drop wise over 30 min. The reaction mixture was then warm ed to room temperature and stirr ed for overnight Water ( 1 00 mL) was then added to the resulting solution which was acidified by sulfuric acid ( 3 M). The solution was extracted with DCM ( 3 5 0 mL), the combined organic layers were washed with brine (3 10 0 mL), dri ed with sodium sulfate anhydrous and concentrated in vacuo to give 27.67 g ( 0.087 mol, 87%) of pure 7 as a colorless oil. 1 H NMR ( 3 00 MHz, CDCl 3 7.79 (d, J = 8.2 Hz, 2H), 7.33 (d, J = 8.7 Hz, 2H), 4.16 4.14 (m, 2H), 3.69 3.67 (m, 2H), 3.59 3.56 (m 6H), 3.51 3.49 (m, 2H), 3.35 (s, 3H), 2.43 (s, 3H). 13 C NMR ( 75 MHz, CDCl 3 ppm) 144.62, 132.66, 129.54, 127.82, 71.60, 70.42, 69.02, 68.26, 58.59, 21.30 4 (2 (2 (2 M ethoxyethoxy)ethoxy)ethoxy)benzaldehyde ( 8 ) This compound wa s synthesized by a modifi ed literature procedure. 161 To a 500 mL round bottom flask was added 2.05 g (20.5 mmol) 4 hydroxybenzaldehyde 8.4 g (60 mmol) anhydrous K 2 CO 3 powder and 15 0 mL DMF The reaction mixture was heated to 85 o C before 6.85 g (21 mmol) compound 7 was charged to the reaction mixture. The reaction was allowed to run for overnight before 200 mL H 2 O was added. The mixture was extracted
100 with DCM (3 80 mL), the organic layer s w ere combined and washed by 0.1 M HCl solution (2 30 mL) and brine (2 30 mL) before dr ying with anhydrous Na 2 SO 4 The crude product was purified by flash ch romatography (silica gel) u sing DCM as eluent. Yield 4.23g ( 79% ) 1 H NMR (300 MHz, CDCl 3 9.81 (s, 1H), 7.77 (d, 2H), 6.96 (d, 2H), 4.16 (t, 2H), 3.83 (t, 2H), 3.70 3.67 (m, 2H), 3.64 3.58 (m, 4H), 3.50 3.47 (m, 2H), 3.31 (t, 3H) 13 C NMR (75 MHz, C DCl 3 ppm) 72.06, 71.04, 70.80, 70.72, 69.62, 67.93, 59.18 BODIPY compound ( 9 ) This compound wa s prepared by a modified literature procedure. 126 To a 500 mL round bottom flask was charged 2.30 g (8.57 mmol) of compound 8 and 200 mL of fresh ly distilled DCM The reaction mixture was degassed for 30 min before 2,4 dimethylpyrrole ( 1.64 g, 17.2 mmol ) was added quickly to the solution along with a few drops of trifluoroacetic acid (TFA) The reaction was covered by aluminum foil and allo wed to run for 12 h r Then 1.8 g of 2,3 d ichloro 5,6 dicyano 1,4 benzoquinone (DDQ) was quickly added to the reaction mixture with vigorous stirring. The reaction was continued for 1 h r before 18 mL of triethylamine ( TEA ) and 18 mL of BF 3 OEt 2 was charged in to the solution. The reaction was quenched by adding 200 mL H 2 O after another hour stirring. The organic layer was washed by saturated NH 4 Cl solution ( 3 80 mL ) and H 2 O ( 2 50 mL ) and dried with anhydrous Na 2 SO 4 After filtration, DCM was evaporated un der reduced pressure. The dark crude product was allowed to pass t hrough a 3 inch column (silica gel) quickly under pressure. The eluent was a mixture of DCM and h exane (1:8 v/v). The resulting brown oil (0.7 g, ~ 1.4 mmol) was directly used without furthe r purification in the preparation of compound 10
101 BODIPY monomer ( 10 ) This compound was prepared using a modified literature procedure. 126 All compound 9 (0.7 g, 1.4 mmol) from the last step (could contain some impurity) was charged in to a 100 mL round bottom flask with 45 m L of absolute EtOH With stirring, 0.5 g of I 2 (4 mmol I atom) and 0.5 g of (2.8 mmol) HIO 3 were charged in to the flask. 30 min later the color of the reaction mixture turned from green to red orange. The reaction was allowed to run under room temperature for 5 h r before 50 mL of DCM w as added to the solution. With stirring, saturated Na 2 S 2 O 3 aqueous solution was added to quench the excess I 2 the aqueous layer was extracted with DCM which is later combined with the organic layer. The organic layer was was hed with saturated Na 2 SO 3 solution (2 50 mL), saturated Na 2 CO 3 solution (2 50 mL) and H 2 O (2 50 mL) before drying with anhydrous Na 2 SO 4 The organic solvent was evaporated and the crude product was purified by flash chromatography (s ilica gel) using ethyl acetate/ h exane (1:4 v/v) as eluent. Yield 0.66 g ( 64% ) 1 H NMR (300 MHz, CDCl 3 7.12 (d, 2H), 7.05 (d, 2H), 4.20 (t, 2H), 3.92 (t, 2H), 3.78 3.66 (m, 6H), 3.57 (t, 2H), 3.39 (s, 3H), 2.63 (s, 6H), 1.43 (s, 6H). 13 C NMR (75 MHz, CDCl 3 160.01, 156.78, 145.60, 141.77, 131.92, 129.26, 127.07, 115.73, 85.73, 72.17, 71.13, 70.90, 70.82, 69.92, 67.81, 59.29, 17.41, 16.23 ESI TOF MS calcd for C 26 H 31 BF 2 I 2 N 2 O 4 Na 761.0 3 32, found [M+Na] + at 761.0434 PB e was synthesized by a modified literature procedure. 126 To a 25 mL Schlenk flask, 150 mg (0.14 mmol) of dendritic monomer 6 and 103 mg (0.14mmol) of BODIPY monomer 10 were dissolved in a mi xed solvent of 5 mL of THF and 5 mL of DIPA. The system was degassed by bubbling argon flow for 30 min. The system was subsequen tly frozen by liquid N 2 before 10 mg of Pd(PPh 3 ) 4 and 8 mg of CuI were charged to the flask.
102 The reaction mixture was allowed to go another two freezing argon cycle before they were heated to 60 o C The reaction was vigorously stirred avoiding light under the protection of argon for 36 h r A dark red fluorescent solution resulted. The reaction mixture was pass ed through a 4 inch aluminum oxide column ( with argon pressure and flushed by THF) to remove the catalysts and ionic side products (ammonium iodide sa lts) before it was concentrated under vacuum to around 3 mL. The concentrated solution was added to 30 mL of MeOH whe reupon a dark purple precipitate formed. The precipitate w as washed by MeOH and redissolved in 3 5 mL of CHCl 3 The CHCl 3 solution was prec ipitated by MeOH again. Such precipitati on redissolving process w as repeated for 3 times before GPC and NMR characterization suggested achieving of pure polymer. Yield 118 mg. GPC: Mn = 28 15 0 PDI = 1.6. 1 H NMR (300 MHz, CDCl 3 7.23 6.96 (broad, Ph H, 6 7 H) 6.35 (broad, NH H, 2H), 4.45 (broad, 4 5 H), 4.18 (broad, 2H) 3.90 (broad, 2 3 H), 3.75 3.65 (broad, 12H), 3.59 3.55 (broad, 3 4H), 3.38 (s, 3H), 2.70 2.63 (broad, 6H), 2 .14 2.05 (broad, 14 15H), 2.00 1.88 (broad, 1 4H), 1.59 (broad, 6H), 1.37 (broad, 72H) PB a In a 50 mL round bottom flask, 80 mg of PB e was dissolved in a mix ed solvent of 10 mL of THF and 10 mL of TFA. With vigorous stirring, the reaction was allowed to run for overnight. The dark red fluorescence was almost quenched. The reaction mixture was concentrated and PB a was precipitated and washed with acetone. The product was dried under vacuum before it was characterized by NMR. 1 H NMR (300 MHz, CDCl 3 7.00 (broad Ph H 6H) 6.56 6.26 (broad, N H 2H), 4.52 4.30 (broad, 12H), 4.23 4.11 (broad, 8H), 3.95 3.48 (broad, 36H), 3.38 3.33 (broad 18H), 2.86 2.57 (broad, 24H), 2.20 1.76 (broad, 50H)
103 PB Na MeOH (2 mL) that contained 0. 1 mM NaOH was use d to dissolve PB a and the result ing solution was slowly added to 50 mL H 2 O with p H 9. The water soluble polym er was dialyzed against a NaOH/ water (Millipore Nanopure TM ) solution ( p H 8.5) under dark for 3 day s. The resulting polyelectrolyte ( PB Na ) aqueous solution was concentrated by means of freeze drying to ~10 mL. The concentration of the CPE was measured by gravimetric analysis which was calculated as 0.8 mg/ mL, or 0.6 mM (repeat unit). T he stock solution was degassed and stored from light under 4 o C
104 CHAPTER 4 CO NJUGATED PHOTOACTIVE OLIGOMERS: PHOTOPHYSICAL PROPERTIES, INTERACTIONS WITH CDSE NANO CRYSTAL S, AND APPLICATION I N DYE SENSITIZED SOLAR CELLS Hybrid Materials Based on Conjugated Oligomers O rganic/ inorganic hy brid materials are broadly defined as mixtures of organic mole cules and inorganic materials at molecular level. 162 Fir st introduced in the late 1980s, th ese materials have show n promising functionalities and applications in photocatalysis 163 solar fuels 164 and photovoltaics 165 166 Specifically, organic semiconductor / inorganic nanocrystal (NC ) hybrids are material s that not only inherit character istic s from both the organic and inorganic compone nts, but also exhibit new features due to the intera ctions (such as energy transfer / charge transfer ) between different ph ases 62 Inspired by Alivisatos and co workers opto electronic devices based on conjugated polymer / CdSe NCs hybrid films 63 64 a great deal of research interest has focus ed on developing organic semiconductor/ inorganic NC hybrids and exploring their applications in photovoltaic energy harvesting and optoelectr onic device s 65 76 The major challenge that these devices faced w as that d evice performance was the dramatic effect of morphology of the mixtures on device performa nce 68 It was demonstrated that inorgani c NCs tend to aggregate to form NC cluster islands in the organic polymer matrix. 167 the interaction between components thus largely limit ing device performance. Coating surfactants on the surface of inorganic NCs facilitated their dispersion in organic phases but the insulating nature of the surfactants largely reduce d the efficiency of charge transfer between the organic SC and inorganic NCs. 99 Another strategy to improve
105 communication between the NCs and organic SCs was to engineer the shape s of the inorganic NCs on a three dimensional scale so that the interpenetration of th e organic and inorganic phases w as optimize d. 79 168 169 Unfortunately, state of art intimate nanocomposite s of organic semiconductor and inorganic NC hybrids w ere hardly achieved 68 A third approach has prove n to provide better control of bo th the morphology of the hybrid films and the surfactant composition on the NC s urfaces. I nstead of physically blending the components, the organic molecules were functionalized with through covalent bonds. For instance, in an early work that studied the morphology of regio regular poly(3 hexylthiophene) (P3HT) / CdSe NC films, Fre chet and co workers discovered that end functional ized P3HT enhanced the performance of P3HT/CdSe solar cells by increasing t he dispersion of CdSe nanocrystals without introducing insulating surfactants. 68 To date, a number of have been emplo yed to facilitate the communication between organic SCs and inorganic NCs. Among them, phosphine oxide 75 76 thi ol groups 83 84 carbodithioic acid s 73 85 86 carboxylic acid s 87 89 ph osphonic acid s 67 90 amines, 68 and anilines 73 are commonly described in the literatures. Regarding the synthetic methodology of these hybrid materials, both 67 68 71 73 75 76 ( by synthesis of organic oligomer/polymers prior to linking to NCs) 69 ( p recursor s modifi cation on NC s prior to formation of oligo mer/polymer phases) met hods have been developed. As introduced in Chapter1, the although relatively more tedious in terms of synthes i s, ha s the advantage of provid ing
106 mono dispersed hybrid materials resulting in better processability 67 energy 70 75 / charge transfer 67 at the molecular level and increased device performance 68 In this chapter, we will discuss a series of conjugated oligomers with different band gaps. These oligomers are all functionalized with a phosphonic acid group on terminal for attach ment to the surface s of CdSe NCs The interactions between the oligomers and CdSe NCs are studied both in solution and in the form of solid hybrid films. As part of our efforts to develop high efficiency photo voltaic s, the functionalized oligomers are also utilized in dye sensitized solar cells (DSSCs) with TiO 2 NC films. Preliminary results show that the se conjugated oligomers have promising applications in DSSCs. The p roject being described in this chapter involves a combi nation of efforts from several research groups at the University of Florida as list ed in Table 4 1. The focus of this chapter will be on the work conducted in the Schanze group, yet to maintain the integrity of the entire story and the flow of the paper, data and figures from the other two grou ps will also be briefly covered ( w ith consent of the collaborators ) Table 4 1. Contributions made by se veral groups to the project Research group (PI / Researcher) Kirk Schanze / Dongping Xie John Reynolds / Romain Stalder Jiangeng Xue / Renjia Zhou Contributions Synthesis of OPE Photophysic al and PL quenching measurement s Syntheses of oligomer CdSe hybrids DSSC devices Syntheses of T6 and T4BTD Electro chemi cal measurements TGA Syntheses of CdSe NCs Hybrid device fabrications and IPCE measurements TEM imaging
107 Results and Discussions Synthesis and S tructural C haracterizations The oligomer s were designed a s electroactive rods bearing a phosphonic a cid functional group at one end (Figure 4 1 ) Achieving such asymmetric structure s normally requires stepwise synthesis in which the aromatic units are installed one after another onto the conjug ated backbones a process 170 In this work, we employ ed an alterna tive method in which a series of sub units was synthesized before in a one pot reaction to provide the final product. T his method is much less tedious without compromising product yield, thanks to the large difference i n polarity between the products and side products. In Figure 4 2, we use the synthesis of OPE oligomer as an example to discuss the methodology. Figure 4 1. Chemical s tructures of conjugated oligomers studied in this work
108 S ynthes is of the OPE oligomer bearing one phosphonic acid group ( OPE A ) begins with the alkylation of hydroquinone into 1 followed by iodination of the benzene ring to afford 2 which is the precursor to the central benzene ring in OPE A The two other precursors to OPE A the extended phenylene ethynylene moieties 4 and 7 were both synthesized from 1,4 diiodobenzene U nder Sonogashira cross coupling conditions, phenylene acetylene and propargyl alcohol were added to yield 3 which was subsequently deprotected un der basic condition s to afford the di(phenylene ethynylene) 4 In a nickel mediated Arbuzov type reaction with on e equivalent of triethylphosphite, 1,4 diiodobenzene was converted to diethyl (4 iodophenyl)phosphonate 5 Similar condition s as described for 3 w ere applied to transform 1,4 diiodobenzene into intermediate 6 which wa s then coupled with 5 to generate the di(phenylene ethynylene) 7 bearing a phosphonate group on one end and a terminal alkyne on the other Precursors 2 4 and 7 were then reacted in a one pot reaction to yield the phosphonate mono functionalized oligo ( phenylene ethynylene ) OPE E The symmetrical side products (the non functionalized and the di functionalized OPEs) were readily removed by column chromatography, based on the substant ial difference in polarities of the compounds. The last steps towards the phosphonic acid OPE A involve d treatment of the phosphonate OPE E with trimethylsilyl bromide in DCM followed by hydrolysis with methanol. The detail ed synthetic procedures for the T6 and T4BTD oligomers are not discussed in this work (Table 4 1) In brief the oligomer backbones were built under Pd catalyzed Suzuki or Stille coupling conditions. The s ame methodology used in OPE synthesis was applied to prepare the T6 and T4BTD oligo mers. In each case, the
109 phosphonic acid group was first introduced as ethyl ester T6 E or T4BTD E which was subsequently hydrolyzed to yield the acid products T6 A or T4BTD A I n all, we collect ed six conjugated oligomers end capped with either phosph onat e or phosphonic acid groups (Figure 4 1). The purity of the oligomers w as demonstrated by NMR and MS spectr oscopy Figure 4 2 Synthesis of OPE E and OPE A (i) KOH, C 8 H 17 Br,DMF; 80 o C overnight ; (ii) I 2 KIO 3 AcOH H 2 SO 4 H 2 O; reflux 24 hr ; (iii) propargyl alcohol, phenylene acetylene Pd(PPh 3 ) 4 CuI, THF, r.t; overnight ; (iv) MnO 2 KOH, Et 2 O r.t ; 5h ; (v) P(OEt) 3 NiCl 2 neat; 1 45 o C 30min ; ( vi 1 ) trimethylsilyl acetylene, propargyl alcohol, Pd(PPh 3 ) 4 CuI, THF, r.t; over night; ( vi 2 ) K 2 CO 3 DCM, MeOH, r.t. 4h ; (vii) Pd(PPh 3 ) 4 CuI, THF, r.t; overnight ; (viii) trimethylsilyl bromide, MeOH
110 The CdSe NCs used in this study were at the University of Florida (Table 4 1), following a previously rep orted procedure with modifications. 171 172 The NCs were nano spher ically shaped with 6.5 nm diameter (by TEM) c arrying trioctylphosphine oxide (TOPO) as the surfactant. Photophysics of the O ligomers and CdSe NCs T he UV vis absorption and fluorescence spectra of OPE A T6 A T4BTD A and CdSe NCs were obtained as shown in Figure 4 3 All the absorption spectra feat ure a strongly allowed long wavelength absorption band, which shifts systematically from 39 3 nm for OPE to 425 nm for T6 to 505 nm for T4BTD High molar absorptivities of 30,000 50,000 M 1 cm 1 were calculated as summarized in Table 4 2 From the absorpt ion onset of OPE A and T6 A in solution, relatively high energy gap s of 2.8 eV and 2.4 eV respectively were calculated, as expected of oligomers with homogeneous electron systems. The T4BTD oligomer, on the other hand, features a longer wavelength absorption onset corresponding to a lower HOMO LUMO gap of 2.0 eV. This is due to the donor acceptor (DA) interaction attribut ed to the mixing of the BTD acceptor unit with the flanking bithiophene donors. 173 The extinction coefficient of T4BTD is lower compar ed to th at for OPE or T6 which is consistent with the DA transition. The photoluminescence of each oligomer, in both the ester and acid form s were recorded in dilute chloroform solution. All oligomers exhibit intense fluorescence with quantum yield near or above 50%. Th e peak emission wavelengths red shifted proceeding from OPE to BTD 434 nm for OPE A 565 nm for T6 A and 676 nm for T4BTD A T his trend agrees with the phenomena observed in UV vis spectra. Additionally, t he emission band width and the Stokes shift are greater for BTD, reflecting the charge transfer nature of the excited state s Fluorescence lifetimes were determined
111 for the olig omers. In each case, the fluorescence decay exhibits single exponential kinetics L ittle difference b etween the acid and ester forms for each oligomer was observed, indicating that the acid forms of the oligomers are well dispersed in dilute solutions. Whi le the OPE and T6 oligomers show short fluorescence lifetime of 0.9 1.0 ns, the T4BTD oligomers exhibit sign ificantly longer lifetimes of 5 ns. The longer lifetime for T4BTD results from a much lower radiative rate (ca 0.16 ns 1 ), and this is again cons istent with the D A nature of the fluorescent excited state. Figure 4 3. Photophysical characterization of the oligomers. A) Normalized UV vis absorption and B) photoluminescence spectra of OPE A (black), T6 A (blue), T4BTD A (red) and the CdSe NCs (das hed) in chloroform. The CdSe NCs have a long wavelength absorption peak at 624 nm characteristic of the quantum confinement effect and the absorption rise s steadily towards the UV region of the spectrum. A n optical energy gap of 1.9 eV is calculated from the onset of absorption which is in accordance with the size of the NCs. 174 The low P L quantum yield of CdSe NPs is presumably due to formation of defect states at the CdSe NC surfaces during the removal of excess TOPO and oleic acids (reagents tha t are used
112 during the syntheses of the NCs). 171 Compar ed to passivated CdSe NCs that are surrounded by TOPO groups, the CdSe NCs used in this study ha ve relatively both shorter PL lifetime and lower P L quantum yield. 175 Table 4 2 summarizes all the photophysical characterizations of the oligomers and the CdS e NCs in chloroform solution Table 4 2. Photophysical data for the oligomers and CdSe NCs in CHCl 3 solution. a from low energy onset of absorption in the solution UV vis spectra. b calculated by the method described in ref 174 c quinine sulfate in 0.1 M sulfuric acid ( F = 0.54) as standard d Ru(bpy) 3 Cl 2 in H 2 O ( F = 0. 0 5 6, degassed ) as standard e average lifetime from a multi exponential decay E chem C haracteriza tions The measurement of redox properties of the oligomers w as conducted in Dr. Reynolds group and d etailed experimental procedures c an be found in Romain of the oligom ers in solution were investigated by means of both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) OPE A has a HOMO energy level at 5.84 eV, T6 A at 5.32 eV and T4BTD A at 5.50 eV vs vacuum. In the case of T4BTD A the LUMO energy is m easured from the reductive DPV onset to be at 3.55 eV, giving an
113 electrochemical energy gap of 1.95 eV, which is quite close to the optical energy gap value of 2.0 eV calculated from its UV vis spectrum. Unfortunately, the reductive DPV for OPE A or T6 A w ere difficult to measure due to the electron rich nature of the two oligomers. Thus the electrochemical estimation of LUMO energies for the latter two oligomers is not available Given the fact that for T4BTD the energy gap measured electronically is reas onably close to that calculated from the optical spectra, we make the assumption that the spectral values are valid in the cases OPE and T6 oligomers. Therefore the corresponding optical energy gaps listed in Table 4 2 were employed to deduce the energies of the LUMOs for OPE A and T6 A which were calculated at 3.04 eV and 2.92 eV vs vacuum, respectively. Figure 4 4 depicts the position of the HOMO and LUMO energy levels for the three oligomers. Figure 4 4. Energy level alignment of the oligomers and CdSe. Band structure diagram comparing the HOMO and LUMO levels of OPE A T6 A and T4BTD A and their offsets relative to the CdSe NCs. According to the literature, the positions of the conduction band (CB) and valence band (VB) of the CdSe NCs used in thi s study are set between 4.3 and 4.5 eV, and between 6.22 and 6.3 eV respectively. 171 The energy diagram of CdSe NCs is drawn parallel with those of the oligomers in Figure 4 4. In each case, the LUMO level of the
114 oligomer is more than 1 eV higher than the C B of the NCs and th e HOMO level of the oligomer is more than 0.5 eV higher than the VB of the NCs. Based on such energy configuration, i t is expected that upon formation of hybrids between the oligomers and the CdSe NCs, photoexcitation of either the oligomer or the NC will give rise to electron transfer from the oligomer to the NC. Photoluminescence Q uenching of the O ligomers and NCs As described in Chapter 1, photoluminescence quenching has been used as a between chromophores. Applying the same method, we sought evidence of energy /charge transfer at t he oligomer/ CdSe NCs interface s by examining the evolution of the fluorescence spectra of the oligomers upon incremental addition of CdSe NCs in CHCl 3 solution. F urthermore the difference b etween evolution s for the phosphonate ester protected oligomers was compared with the oligomers modified by phosphonic acid groups Figure 4 5a shows the PL evolution for OPE E and OPE A in dilute chloroform (5 M) as CdSe NCs ( chloroform solution ) was tit rated in to the study solution The increments were chosen so that each addition doubled the concentration of CdSe NCs in the oligomer solution, until an oligomer: CdSe molar ratio of 50:1 was reached The relative concentrations of the starting solutions ( oligomer and CdSe NCs) were such that only microliters of CdSe solution were ne ed ed for a fixed volume of 2 mL of oligomer solution to achieve the designed ratio Therefore the effect of dilution on the PL intens ity could be neg lected. For OPE E the addi tion of the CdSe reduced the oligomer fluorescence by 25% at the 50:1 ratio Since the fluorescence lifetime of the result solution remained the same with that of the pure OPE E diffusional quenching could be ruled out. We believe that the reduction of OP E E fluorescence is due to the absorption of CdSe NCs at 436 nm
115 ( acting as a neutral density filter). By comparison, w hen t he ac id form OPE A was subjected to the same procedure significant luminescence quenching wa s observed Over 90% of fluorescence wa s quenched when the molar ratio of OPE A : CdSe NCs equal ed 50:1 Figure 4 5. Fluorescence quenching of the oligomers by CdSe. A ) Evolution of the fluorescence of OPE E and OPE A upon addition of CdSe N C s and B ) evolution of the peak flu orescence intensities for the ester (squares) and acid (circles) forms of OPE (black ), T6 (blue ) and T4BTD (red ) upon addition of CdSe NCs. The concentration for each oligomer solution wa s 5 M The same set of experiments w as carried out for T6 and T4BTD oligomers as well. Figure 4 5 b summarizes the evolution of the peak fluorescence intensities for all six oligomer s T he fluorescence intensity of T4BTD E does not decrease as much as those of T6 E and OPE E This observation can be explained by referring to Figure 4 3a T here is almost no absorption for CdSe NCs at the emission wavelength of T4BTD E while the absorption is much stronger at wavelengths where T6 E and OPE E emit. Furthermore, i t is commonly true that the fluorescence of the ester form s decreases by a limited amount upon addition of the CdSe NCs, while the fluorescence of the acid form is quenched by more than 9 0 % under the same conditions. Noting that the
116 substantial fluorescenc e quenching of the acid form of the oligomers occurs at very low quencher concentration (in the range of nanomolar) and d iffusional quenching could be ruled out, it can thus be deduc ed that any evolution in the PL intensity would be due to oligomer/NC com plex formation, i.e. direct interaction between the two. Th e PL quenching results suggest that the phosphonate ester forms have at best, a weak interaction and thus there is little bind ing of the oligomers to the NCs, while the phosphonic acid forms bind strongly to the NC surface. In the experiments described above no luminescence increase was observed w h ile carefully monitoring the 640 to 6 6 0 nm r egion for any enhancement of the emission from the C dSe in the oligomer/CdSe mixture C onsidering both the l ow fluorescence quantum yield and the negligible concentration of CdSe NCs used in the study this observation alone was not sufficient to rule out the possibility of energy transfer from oligomers to the CdSe NCs To further decipher the mechanism of inte ractions between the oligomers and CdSe NCs, the r everse experiment was conducted, which involved i nvestigati on of the change of photoluminescence intensity of CdSe NCs emission upon addi tion the oligomers. For OPE the procedure began with mixing 0.5 mL o f a 1 M CdSe solution with 1.5 mL of oligomer solution of the following concentrations: 16. 7 M (for 50:1 oligomer: CdSe ); 3 3. 4 M (for 100:1 oligomer: CdSe ) and 6 6. 8 M (for 200:1 oligomer: CdSe ). The solutions were stirred and irradiated with light at the CdSe p eak absorption wavelength ( where no absorption for OPE was observed ). Figure 4 6 record s the change of CdSe NCs emission upon titration with OPE A as described above. As a control experiment, the same process was applied to OPE E T he photoluminescence of CdSe decreased upon addition of OPE A while the
117 same amount of OPE E had almost no influence on the CdSe emission. This result again supports the proposal that OPE A binds strong ly to the surface of the CdSe NCs so that the interaction between the two sp ecies is possible. Furthermore, the mutual quenching behavior of CdSe NCs and OPE A indicate s that when either component in the complex is photoexcited, a charge transfer process results. Either electron s are inject ed from OPE A to CdSe NC or holes migrat e in the other direction. Similar results were observed for the T6 derivatives. Figure 4 6.Evolution of the fluorescence of CdSe NCs upon addition of OPE E and OPE A E xcitation at 630 nm Unfortunately the same experiment was difficult t o perform on T4BTD since the absorption and emission of both organic and inorganic species overlap ped significantly making it impossible to look into the emission of CdSe without interference from the emission of T4BTD (The charge transfer between T4BTD and CdSe NCs will be demonstrated later in the incident photon to electron conversion e fficiency ( IPCE ) resu lts )
118 Referring back to the energy diagram of the oligomers and CdSe, it is true that the interaction between the oligomers and CdSe NCs occurs via electron transfer and this interaction is strongly facilitated by attaching the oligomers onto CdSe NCs with binding groups (in this case the phosphonic acid). Oligomer CdSe H ybrids and IPCE R esults Now that it has been demonstrated that the acid forms of oligomers tend to bind with CdSe NCs to form complex in solution, we further explored the possibility of synthesizing hybrid m aterials based on the oligomer/ CdSe complex. From the PL quenching experiments, a 200 :1 molar ratio of oligomer:NC was sufficie nt to quench more than half of the luminescence of the NCs. We thus stipulated that such a ratio or higher would be reasonable for the synthesis of hybrids consist ing of CdSe NC cores saturated by the oligomers at the surfaces Since phosphonic acid groups have higher affinity to the CdSe NCs than TOPO group s (the init ial ligands on the CdSe NCs), t he hybrid preparation utilizes the exchange of the superficial TOPO groups of the NCs with OPE A T6 A or T4BTD A oligomers As a general procedure, oligomers a nd CdSe NCs were mix ed in chloroform solutions followed by precipitation of the NC/oligomer hybrid in an appropriate solvent and centrifugation to remove the supernatant containing any unbound organic ligands (TOPO and excess oligomers) Experimentally, 1 0 mg of the oligomer was dissolved in 5 mL of degassed chloroform, to which was added a solution of the Cd Se NCs in chloroform at the appropriate concentration fo r an excess of 200:1ratio ( oligomer:NCs M:M) The mixture was stirred vigorously in the absen ce of light at room temperature for 30 minutes T he mixture was then precipitated in a poor solvent for the NCs/oligomer hybrid s which still dissolves the unbound surfactants and excess oligomer s For the
119 T6 /CdSe hybrid ethyl acetate was used to precipita te the hybrids, while methanol was employed to precipitate the OPE /CdSe and T4BTD /CdSe hybrid s After centrifugation of the suspension and removal of the supernatant, the precipitates were redissolved in chloroform and precipitated once again in the proper solvent. This was repeated several times, and meanwhile the UV vis absorption spectrum of the chloroform solutions was recorded in each step. As unbound oligomers were removed after each precipitation, the overall absorption profile of the redissolved pre cipitates featured less absorption contribution from the oligomers. Once the relative absorption intensities of the NCs versus that of the oligomer stabilized, the chloroform solution containing the redissolved oligomer/NC hybrid was consi dered free of unb ound oligomers (This usually required 3 4 precipitation redissol ve cycle s ) The UV vis absorption spectra of as prepared hybrids solutions are shown in Figure 4 7 Figure 4 7. Absorbance comparison of the hybrids with parental CdSe NCs. Absorption spectr a of the OPE CdSe (black ), the T6 CdSe (blue ) and the T4BTD A CdSe (r ed ) along with the spectr um of free CdSe NCs (dashed ) in solution
120 Compared to the absorption of pure NC solution ( dashed line ) the profile of the OPE /CdSe hybrid (black line) shows an absorption band emerging at 410 nm towards shorter wavelengths, and peaking at 325 nm with a shoulder at 370 nm, indicating the contribution of the bound OPE A The T6 /CdSe hybrid (blue line) has a broad absorption band centered at 426 nm from the contribu tion of the bound T6 PA oligomers. Likewise the absorption profile for the T4BTD /CdSe hybrid s hows the contribution of the NC bound T4BTD A peaking at 360 nm and 508 nm. In all three spectra, the contribution of the NCs to the overall absorption is observe d as a peak or shoulder around 625 nm. With the hybrids in hand, a quantitative estimation of the average number of oligomers at the NCs surface was attempted. Thermogravimetric analysis is carried out to determine the total weight loss difference between the pristine CdSe NCs and the ones functionalized with the electroactive oligomers. In principle, during the ligand exchange process, if a native surfactant such as TOPO (MW = 415 g/mol) is replaced by OPE A (MW = 815 g/mol), T6 A (MW = 827 g/mol) or T4BT D A (MW = 797 g/mol), then an oligomer/NC hybrid should have a higher organic content by weight than the original NCs. Displayed in Figure 4 8, the TGA thermograms for a CdSe sample before ligand exchange (dashed line) and after ligand exchange with OPE A (black line), T6 A (blue line) or T4BTD A (red line) were plotted. Weight loss differences in the 4% to 8% range at 500 o C were observed for the hybrids compared to the pristine CdSe sample. This implies that the ligand exchange process did increase the org anic content in the hybrid, supporting the presence of higher molecular weight species bound to the surface of the NCs. One obvious limitation to this method is that it is in fact very difficult
121 to determine the exact number of native surfactants before li gand exchange. The results from a TGA experiment on hybrids are thus, at best, qualitative. Figure 4 8. TGA thermograms of the pristine CdSe NCs (dashed line) and the hybrids with OPE A (black line), T6 A (blue line) and T4BTD A (red line) under nitrogen flow, 10 o C/min heating rate. A more quantitative way to estimate the number of surface bound oligomers involves ne NCs and the free oligomer s Figure 4 9 illustrate s the method of such estimation using the T6 /CdSe hybrid system as an example. The relative absorbance of the free oligomer T6 A (dash dot line) and the free CdSe NCs (dashed line) was adjusted so that : 1) the absor bance from the free CdSe NCs is identical to that of T6 /CdSe hybrid at 634nm ( n o absorption contribution from the T6 A ); 2) the sum of their absorption spectra (black solid line) resulted in a profile for which the intensities at the respective absorption olid blue line). This was achieved for an absorbance of 0.912 at 426 nm for T6 A and 0.087 at 624 nm for the CdSe and referring to the extinction coefficients listed in Table 4
122 2, concentrations of 18.7 m for T6 A and 136 nM for CdSe NCs were calculated respectively result ing in an oligomer to CdSe NC ratio of 137 :1. The same spectral analysis and calculations were applied to the OPE and T4BTD hybrids, suggesting ratio s of 200:1 for OPE /CdSe hybrid and 140:1 for T4BTD /CdSe hybrid, respectively While we succeeded in estimating the ratio between organic ligands and CdSe NCs, it is necessary to point out that t he major limitation of this estimati on is application of linear region of th e absorption concentration function. Based on the down curvature of the function, the actual oligomer ratio should be higher than the estimated values Overall, however, the estimation offers direct evidence that a significant coverage of the CdSe NCs was achieved using phosphonic acid functionalized oligomers Figure 4 9. Comparison of absorption of T6 A /CdSe hybrid and the free components. Absorption profiles of the T6 A /CdSe hybrid (blue line), the free T6 A (dash dot line), the free CdSe NCs (dashed l ine) and the sum of the latter two (black solid line). The optical properties of the hybrids are summarized in Table 4 3. Extremely weak fluorescence was detected for chloroform solutions of the hybrids, with quantum yields
123 below 0.1% at 433 nm for the OPE hybrid solution, 564 nm for the T6 hybrid solution and at 676 nm for the T4BTD hybrid solution. Seeking evidence for the dynamics of the hybrids fluorescence, we measured the fluorescence lifetime s of the hybrids solutions as recorded in Table 4 3. Two c omponents are contributing to the average lifetime of the hybrids fl uorescence. O ne from the ligand emission and the other from the CdSe emission. Compared with the fluorescence lifetime of the free ligands or CdSe NCs (Table 4 2) it is clear that the com ponents of hybrids fluorescence have almost identical lifetime with their corresponding parental oligomer or CdSe NCs. This observation suggest s that the fluorescence of the hybrids come s from the residu al unb ound free ligands and CdSe NCs, while the hybri ds themselves are non emissive, presumably due to rapid charge transfer between the components. Transient absorption was measured for the hybrid materials to investigate the rate of the charge transfer. Unfortunately we were not able to capture any inform ation on the nano second scale instrument al setup, suggesting that a rapid photophysical process occurred. Table 4 3. Optical data of the oligomer/CdSe hybrids in CHCl 3 solution. max abs ( nm ) max Fl ( nm ) Fl Fl ( ns ) OPE hybrid 325/370 /62 5 433 <0.1% 0. 88(450nm)/1.24(6 5 0nm) T6 hybrid 426/62 5 564 <0.1% 0.83(560nm)/1.23(6 5 0nm) T4BTD hybrid 360/ 508/62 5 676 <0.1% 5.2(680nm) / 1. 6 8(6 5 0nm) As the mechanism of photo induced charge transfer process is concerned, either (or both) of the following scenario are to be considered 1) D irect excitation of the organic ligands (CdSe NCs) followed by the electron injection (hole migration) from the ligands (CdSe NCs) to the CdSe NCs (the ligands), or 2) Energy transfer happens from the singlet excited state of the lig ands to the CdSe NCs generating excitons in the NCs
124 before the hole migration from NCs back to the ligands. In all events, charge separation between the components of the hybrid materials is involved, which is demonstrated by the IPCE of the hybrids (descr ibed later in this chapter) Further study (eg. u ltrafast transient absorption) is needed to discover the dynamics of the photophysical process at the interface of the hybrids. To further demonstrate photo induced electron transfer between the oligomers an d CdSe NCs, and to study the potential application of these hybrid material s for photovoltaics (PV), we fabricated hybrid PV devices with a structure of ITO/PEDOT: PSS/Oligomer: CdSe/ZnO/Al in which the oligomer:CdSe is the active layer, and ZnO N C s serv e as optical spacer and hole blocking layer 176 The IPCE was recorded for each hybrid as displayed in Figure 4 10. The IPCE for the OPE /CdSe hybrid peaks at 350 nm to 10%,with little response below 3% at wavelen gths higher than 500 nm except for a small shoulder at 640 nm that is likely due to the contribution of CdSe NCs. For the T6 based hybrid, a broader band reaching 12% appears in the IPCE from 400 nm to 550 nm corresponding to photons absorbed by the T6 lig ands bound on the CdSe NC surface. A shoulder at 640 nm wa s observed, implying the contribution from the CdSe NC absorption to the overall photocurrent. The IPCE for the T4BTD hybrid is broadened further to 650 nm, with two bands centered at 371 nm and 505 nm, and a value of 13%. The IPCE parallels the absorption of the T4BTD /CdSe hybrid, and a slight shoulder around 650 nm can be distinguished for CdSe contribution. Overall the IPCE spectra for the three hybrids indeed indicate that the photocurrent contr ibutions are from bo th oligomers and CdSe NCs, providing a dditional evidence of charge separation happening at the interface of the hybrid material.
125 Unfortunately, the device performances of the hybrids di scussed above are disappointing ( l ess than 0. 3 %) M any possible factors both from molecular design and engineering aspects are to be optimized for better devices. For exa mple, the oligomer based hybrid active layers do not have sufficient viscosity for the devices to be successfully processed. This problem could be potentially solved by blending the hybrids with a high mole cular weight conjugated polymer (eg. P3HT). Other wise the functionalized oligomers applied in DSSC format achieve d reasonabl e power conversion efficiencies (This topic will be cover ed la ter in this chapter). In all, the oligomers developed in this project are promising candidates for high efficiency photovoltaics. Figure 4 10. IPCE of OPE /CdSe (black line), T6 /CdSe (blue line) and T4BTD /CdSe (red line) hybrids films in a simple device a rchitecture under AM1.5 illumination. Application of the Oligomers in D ye sensitized Solar Cells It has been reviewed in C hapter 1 that d ye sensitized solar cells (DSSCs) have gained striking progress with transition metal based complex es as sensitizers. R ecently, metal free organic molecules are receivin g increasing research attention due to the
126 concern that the long term availability of the transition metals is limited. 55 To date, various organic dyes for DSSCs have been developed, such as coumarin dyes, 151 merocyanine derivatives, 177 and polyene dyes. 178 Power conversion efficienc y (PCE) that is comparable with transition metal dyes ha s been achieved so far (over 10%). 55 Previously we applied the BODIPY based conjugated polyelectrolyte as sensitizers in TiO 2 DSSC devices. As our con tinuing investigation, we constructed DSSCs using the phosphonic acid oligomers. T6 and T4BTD oligomers were chosen to build the devices for their capability of utilizing the majority of visible lights. Additionally, the phosphonic acid functional groups p rovide connection of the oligomers to the TiO 2 NCs. Experimentally, the oligomer coated TiO 2 films were tested in a modified reported DSSC format 61 using a propylene carbonate solution of I 2 /LiI as the electrolyte and a Pt/FTO count er electrode. Figure 4 11 shows the J V curve and IPCE plot of T6 and T4BTD dyes. It is clear that the IPCE for both oligomers mimics their absorption spectra, indicating sufficient utilization of photons absorbed. Noting that without any device optimizati on involved, the oligo mers already offer moderate PCE (1.4% and 2.2% for T6 and T4BTD respectively) I t is expect ed that the oligomers developed in this project are promising candidates for photovoltaic applications, which is endorsed by their capability of interacting with semiconductor NCs via electron transfer. Finally, the performance of the oligomers in the CdSe based hybrid solar cells and those in the TiO 2 based DSSCs are surprisingly different. We propose that this different is caused by the natur e of the devices. It was proven that in heterojunction cells, the device performance increase with polymer molecular weight, 153 154 due to increased
127 structural order in the films. Such structural order facilitates the charge mobility in the hybrid cells. On the contrary, in DSSCs, inverse correlation between polymer chain length and cell performance was obser ved, 61 presumably due to the fact that small molecules have better coverage on the TiO 2 surfaces. Indeed, the up to date efficient sensitizers used in DSSCs are all small molecules/oligomers. Based on the above arguments, it is reas onable that the oligomers developed in this project fit better for DSSC cells. More studies on morphologies need to be done to support the conclusion. Figure 4 11. J V curve ( A ) and IPCE ( B ) of T6 (blue squares) and T4BTD (green triangles) in TiO 2 base d D SSCs. Summary and Conclusion s T his chapter describ ed three mono functionalized conjugated oligomers with variable band gaps and their interactions with CdSe nanocrystals. First, the synthes e s of phosphonic acid functionalized oligo phenylene ethynylene OP E A w as introduced. Instead of applying the conventional synthetic procedure for the OPE backbones, we developed a subunit core one pot assembling methodology where the desired product and side product were easily separated by the polarity differences.
128 The novel methodology wa s less tedious in terms of synthesis, without compromising the overall yield of the oligomers. Second, the photophysics and electrochemi cal properties of the OPE T6 and T4BTD were carefully characterized. By means of donor acceptor d onor designing, the HOMO LUMO gaps of the oligomer were varied, from 2.8 eV for OPE to 2.0 eV for T4BTD Third, it has been demonstrated in the study from several aspects that electron transfer occur s from the oligomers to the CdSe NCs T he process is assi sted by the phosphonic acid group modified on the oligomers as evidenced by two observations 1) The fluorescence of the acid form of the oligomers are effectively quenched at lo w CdSe concentration ( n ano molar range) while the ester forms remain unquenched. Like wise addition of oligomers with acid groups to CdSe NCs quenche s the CdSe fluorescence. The mutual quenching of the oligomers and CdSe NCs is not surprising, since electrochemical studies have indicated that the charge transfer process between the species is favored. 2) IPCE measurement s on the oligomer/ CdSe hybrid materials offer solid evidence for photon induced current to occur, which is a direct result of charg e separation at the ol igomer/ CdSe interface Finally, the application of the oligomers in DSSC devices w as investigated in parallel. Preliminary results show that the oligomers are promising candidate s for high efficiency DSSCs. Experimental Materials and Methods All chemicals were purchased from either Fisher or Aldrich and used as received withou t further purification. NMR spectra were recorded on either a Varian Gemini 300
129 or a Mercury 300 spectrometer. Chemical shifts were referenced to residual signals from C H Cl 3 ( 1 H 7.26 ppm and 13 C 77.23 ppm ) UV visible absorpt ion spectra were obtained on a Varian Cary 100 absorption spectrometer using 1 cm quartz cells and corrected for background due to solvent (HPLC grade) Fluorescence spectra were recorded on a Photon Technology International ( PTI ) fluorimeter. Fluorescence lifetime data was recorded on a PicoQuant Picoharp 300 TCSPC instrument. Dye sensitized solar cells were fabricated as follows. First, a TiO 2 compact layer was spin coated onto a clean FTO glass slide. 157 The TiO 2 nanocrystalli ne paste (20 nm in diameter), which was received as a gift from Dr. Felix N. Castellano at Bowling Green State University was doctor bladed onto the slide with compact layer and sintered at 400 o C for 30 min. The sintered electrode was immersed into the ol igomer solution in DMF ( ~ 0. 5 mM) for 36 h to allow for oligomer adsorption onto the TiO 2 film. The counter electrode was prepared by sputtering Pt on FTO glass with a thickness of 10 nm. Finally, an electrolyte solution containing 0.05 M I 2 and 0.1 M LiI in dry propylene carbonate was drawn into the sandwich between the TiO 2 working electrode and the Pt counter electrode by capillary action. The active area of the cell was 0.3 cm 2 The performance of the cells was measured immediately after they were assem bled. The current voltage characteristics of the cells were measured with a Keithley 2400 source meter under AM1.5 (100 mW/cm 2 ) solar simulator. As for the IPCE measurements, the cells were illuminated by monochromatic light from an Oriel Cornerstone spect rometer, and the current response under short circuit conditions was recorded at 10 nm intervals using a Keithley 2400 source meter. The light intensity at each wavelength was calibrated with an energy meter (S350, UDT Instruments).
130 Synthetic Procedure s 1, 4 B is(octyloxy)benzene (1) This compound was prepared by a modified literature procedure. 179 8.6 g KOH powder and 50 m L DMSO were added to a 200 m L round bottom flask T he mixture was degassed for 15 min before 5.5 g (0.05 mol) hydroquinone was added. The system was heated to 80 o C After the hydroquinone dissolved, 19.3 g (0.1 mol) 1 bromooctane was added via syringe. The reaction was then allowed to run for 12 hours. After the cooled down, the mixture was poured into 600 mL ice cold water and the crude product was precipitated as tan solid. The crude product was recrystalized by ethanol and then dried as colorless crystals. Yield 89% 1 H NMR (300 MHz, CDCl 3 4H), 3.90 (t, 4H), 1.75 (pentet, 4H), 1.45 1.30 (m, 20H), 0.88(t, 6H) 13 C NMR ( 75 MHz, CDCl 3 153.15, 115.34, 68.61, 31.80, 31.49, 29.37, 29.24, 26.03, 22.64, 14.09 1,4 D iiodo 2,5 bis(octyloxy)benzene (2) 180 14.5 g (0.043 mol) of 1 and a mixture of 200 m L of a cetic acid and 40 m L of DI water was heated to 80 o C in a 500 mL 3 necked round bottom flask with a condenser installed After 30 min compound 1 was well mixed with acetic/H 2 O ( Since the melting point of 1 was around 60 o C it actually melted while heating) Then 13 g (0.05 mol) I 2 and 6.9 g KIO 3 was added while the system was kept stirring. After attaining reflux, a solution of su lfuric acid (4 mL in 10 mL water) was added slowly to the reaction. The reaction was then allowed to run 24 h r while the color of the iodine (purple) faded. After cooling, a saturated aqueous solution of Na 2 S 2 O 4 was added until the color of the reaction mixture turned pale yellow. The mixture was poured into ice cold water and the crude product precipitated as a light yellow solid. The crude product was dissolved in DCM and washed with DI water twice. DCM was then evaporated and the crude product was recrystal l ized in ethanol as a
131 white solid, yield 18.8g (75%) 1 H NMR (300 MHz, CDCl 3 4H), 1.80 (pentet 4H), 1.45 1.30 (m, 20H), 0.89 (t, 6H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 3 (4 ( P henylethynyl)phenyl)prop 2 yn 1 ol (3) 181 1,4 diiodobenzene (5 g, 15.2 mmol) was dissolved in a mixture of 50 mL of THF and 15 mL of d iisopropylamine (DIPA) in a 250 mL round bottom flask. The solution was degassed under argon f low for 40 min before 20 mg Pd(PPh 3 ) 4 and 15 mg CuI were charged. Under the protection of argon, propargyl alcohol (15 mmol) was injected slowly via syringe. The reaction mixture turned cloudy after 30 min, and then phenylene acetylene (15.2 mmol) was adde d to the mixture under argon. The reaction was allowed to run overnight before the mixture was filtered T he filtrate was concentrated to remove free amines. The crude product was dissolved in DCM and washed with satur a ted ammonium chloride (20 mL 2) and brine (20 mL 3). The organic layer s w ere combined and dried, and the crude product was p urified by flash chromatography (DCM: h exane 2:1 v/v ) Yield 2.2g ( 45% ) 1 H NMR (300 MHz, CDCl 3 13 C N MR ( 75 MHz, CDCl 3 131.6 2, 131.4 8 128.4 4 128.37, 123.43, 122.93, 122.28, 91.24, 88.87, 85.37, 51.64 1 E thynyl 4 (phenylethynyl)benzene (4) 182 500 mg (2.2 mmol) of co mpound 3 was dissolved by 50 mL of diethyl ether in a 200 mL round bottom flask. The system was degassed for 30 min. Then, 10 equivalents of MnO 2 (22 mmol) and KOH (22 mmol) powder were well mixed and divided into three portions. One portion was added ever y one hour with vigorous stirring. The reaction was allowed to run for another 2 h r after the addition of the last portion of MnO 2 /KOH. The reaction mixture was filtered and
132 passed through a short silica gel column by DCM and dried under vacuo. Compound 4 was obtained as a pale white solid. Yield 89% 1 H NMR (300 MHz, CDCl 3 (m, 2H), 7.49 (s, 4H), 7.37 (m, 3H), 3.19 (s, 1H). 13 C NMR ( 75 MHz, CDCl 3 132.15, 131.72, 131.56, 128.62, 128.46, 123.85, 122.99, 91.46, 88.91, 83.35, 78.96 D iethyl (4 iodophenyl)phosphonate (5) 183 16.5 g (50 mmol) of 1,4 d iiodobenzene and 0.33 g (2.5 mmol) of anhydrous NiCl 2 were charged into a 100 mL 3 necked round bottom flask with an argon inlet an d outlet. The mixture was degassed for 20 min and then melted (the temperature should not exceed 145 o C ) Whi le stirring, 10.4 m L (60 mmol) of P(OEt) 3 was slowly added to the flask via syringe with argon flow. 30 min later, the reaction mixture turned brow n. The product was purified by flash chromatography. H exane was employed first to remove free P(OEt) 3 and 1,4 diiodobenzene, and then the product was eluted with DCM. Evaporation of solvent s provided pure product as light yellow oil. Yield 70% 1 H NMR (300 MHz, CDCl 3 7.83 (m, 2H), 7.52 (m, 2H), 4.10 (m, 4H), 1.30 (m, 6H) 13 C NMR ( 75 MHz, CDCl 3 ppm) 31 P NMR (CDCl 3 against H 3 PO 4 ppm ): 19.1 3 (4 E thynylphenyl)prop 2 yn 1 ol (6) 184 This compound was synthesized in a two step reaction. Step 1 was a Sonogashira cross coupling reaction that was c arried out under similar condition s as compound 3 except that trimethylsilyl acetylene (instead of phenylene acetylene) was add ed. The intermediate was subsequently deprotected in step 2 where mild basic condition s w ere employed 500 mg of intermediate ( 3 (4 ((trimethylsilyl)ethynyl)phenyl)prop 2 yn 1 ol) was dissolve d in DCM (20 mL) and MeOH (20 mL) mix ed solvent and the system was degassed for 30 min. Excess amount of
133 K 2 CO 3 (3.5 g) solid was added and the reaction was vigorously stirred for 4 h r The rea ction mixture was filtered and the filtrate was concentrated under vacuo to yield the title compound as a white solid. Yield for 2 steps is 40% 1 H NMR (300 MHz, CDCl 3 7.35 (m, 4H), 4.51 (d, 2H), 3.18 (s, 1H), 1.69 (t, 1H) 13 C NMR ( 75 MHz, CD Cl 3 Diethyl (4 ((4 ethynylph enyl)ethynyl)phenyl)phosphonate (7) This compound was synthesized in a two step reaction. Step 1 : compound 5 (150 mg, 0.44 mmol) and 6 (68.8 mg 0.44 mmo l) were dissolved in a mix ed solvent of 15 mL of THF and 5 mL of DIPA in a 50 mL round bottom flask. The system was degassed with argon for 25 min before 8 mg Pd(PPh 3 ) 4 and 5 mg CuI were add ed. The reaction was gently heated to 50 o C and allowed to run for 12 h r The reaction mixture was filtered and the filtrate was concentrated and redissolved in DCM. After washing with saturate d ammonium chloride ( 2 0 mL 2 ) and brine ( 20 mL 3 ), the crude product was dried under vacuo and p urified by flash chromatograph y (DCM: MeOH v/v 20:1) Step 2 : the intermediate was deprotected at the propargyl alcohol site using the same condition s described for compound 4 The deprotected product was obtained as pale yellow powder. Yield for 2 steps 68% 1 H NMR (300 MHz, CDCl 3 pp 4.16 (m, 4H), 3.19 (s, 1H), 1.35 (t, 3H) 13 C NMR (75 MHz, CDCl 3 132.00, 131.89, 131.78, 131.58, 129.65, 127.40, 122.89, 91.73, 82.42, 62.57, 51.77, 16.60 31 P NMR (CDCl 3 against H 3 PO 4 ppm ) 19.0 ESI TOF MS calcd for C 20 H 19 O 3 P Na (M +Na ) 361.0970 found 361.0958 Anal. calcd for C 20 H 19 O 3 P : C, 71.00 ; H, 5.66 found: C, 69.85; H, 5.70
134 OPE E (8) In a 100 mL round bottom flask, compounds 2 (176 mg 0.3 mmol) and 7 (100 mg, 0.296 mmol) were dissolved by a m ix ed so lvent of 20 mL THF and 10 mL DIPA. The system was degassed with argon for 40 min before 10 mg Pd(PPh 3 ) 4 and 8 mg CuI were add ed. The mixture was stirred at room temperature for 1 h r and then 4 (60.7 mg 0.3 mmol) was quickly added to the system under argon. The reaction was then gently heated to 50 o C and allowed to run overnight. The reaction mixture was filtered and the filtrat e was concentrated and redissolved in DCM. After washing with satur ated ammonium chloride (20 mL 2) and bri ne (20 mL 3), the crude product was dried under vacuo and purified by flash chromatography using DCM as eluent. Yield 80 mg ( 31% ) 1 H NMR (300 MHz, CDCl 3 7.35 (m, 4H), 7.03 (s, 2H), 4.15 (m, 4H), 4.05 (t, 4H), 1.86 (pentet, 4H), 1.50 1.30 (m, broad, 20H), 1.31 (t, 6H), 0.88 (t, 6H) 13 C NMR ( 75 MHz, CDCl 3 23.1, 26.0, 29.3, 29.6, 33.2, 61 .8, 67.9, 96.0, 120.0, 128.4, 124.0, 133.1, 134.4 31 P NMR (300 MHz CDCl 3 HRMS ( APCI ) m/z calcd for C 58 H 63 O 5 P H (M H + ) 871.4491 fo und 871.4482. Anal. calcd for C 58 H 63 O 5 P : C, 79.97; H, 7.29 found C, 79.80 ; H, 7.36. O PE A (9) To a round bot tom flask equipped with stirring bar, OPE E was dissolved in DCM. Upon stirring, excess amount (10 mL) of trimethylsilyl bromide (TMSBr) was added via syringe. The reaction was allowed to run for 30 min before nitrogen inlet and outlet were set up. After a ll the DCM and free TMSBr were evaporated under nitrogen flow, MeOH was added and the mixture was kept stirring for another 30 min. The crude product was dried under vacuo and dissolved in a few drops of CHCl 3 10 mL MeOH was added to obtain the title comp ound as yellow precipitate,
135 which was collected by centrifuge. Yield (95%) 1 H NMR (300 MHz, CDC1 3 10.25 (broad, 2H) 7 8 (broad, 16H), 4.20 (broad, 4H), 1.0 1.8 (broad, 22H) 31 P NMR (300 MHz CDCl 3 Anal. c alcd for C 5 4 H 55 O 5 P : C, 79. 5 8 ; H, 6.80 found C, 79.57 ; H, 6.95.
136 CHAPTER 5 CONCLUSIONS AND FUTURE WORK The studies described in this dissertation involve conjugated polyelectrolyte s and conjugated oligomers based on phenylene/ arylene ethynylene backbones. In detail, we have discus sed t he synthesis, characterization, and photophysical properties of several CPEs and oligomers with newly designed chemical structures. Meanwhile, their applications on solar cells were explored. In C hapter 2, two CPEs with interrupted conjugation were sy nthesized and their structure property relations were investigated. In C hapter 3, the first CPE with BODIPY unit in the backbone was prepared by applying dendritic ionic side groups. Fundamentally the interaction of the CPE and a series of quenchers were systematically studied As the aspect of application, the CPE was used as sensitizer in DSSC cells. In C hapter 4, the interfaces of conjugated oligomers with variable band gaps and inorganic nano sized materials were explored. In the first part, hybrid ma t erials with oligomer/ CdSe NCs were studied. The charge transfer between the species was demonstrated from several aspects, which nominates them potential candidates in organic inorganic photovoltaic materials. In addition the oligomers were applied to DSS C devices with considerable power conversion efficiencies. Finally, possible reasons were discussed Aggregation Induced Amplified Quenching Two anionic conjugated polyelectrolytes that conta in three ring (phenylene 2 tether ( C PPE and O PPE respectively) were characterized. Polymer C PPE d id not exhibit emission from an aggregate state in either MeOH or H 2 O, and while the quenching of this poly mer was
137 enhanced slightly compared to that of a monomeric chromophore, the polymer exhibit ed only a mod est amplified quenching effect. By contrast, polymer O PPE exhibit ed both monomer and aggregate emission, and the aggregate state wa s quenched 200 fold m ore efficiently compared to the monomer state with a K SV value rivaling the maximum efficiency seen for full y conjugated polyelectrolytes. T his study provides insight am plified quenching of conjugated polyelectrolytes. Evidence has shown that O PPE like emission, while C PPE forms non emissive aggregates with much larger sizes. The se results underscore the important role playe d by chromophore aggregation in promoting efficient exciton transport which is key to the amplified quenching effect. More work is needed to discover how the subtle structur al difference between the two CPEs has induced the major differences in terms of photophysical properties. Molecular modeling and computation can be conducted to assist understanding the two are dissolved in organic solvent s like CHCl 3 ) of the CPEs, so that the aggregation effect can be minimized. Photophysical study of the polymer forms will reveal the role of chemical structure difference of the two polymers in their photophysical natures. BODIPY in the Backbone The BODIPY phenylene ethynylene copolymer was co nverted to CPE by installing branched anionic side groups ( PB Na ). The similar photophysics of PB Na in both MeOH and H 2 O indicates that the CPE is less likely to aggregate which result in moderate fluorescence quantum yield s in water. The fluorescence qu enching experiment show ed that PB Na could be quenched by Cu 2+ selectively among a series
138 of metal ions in an amplified quenching manner (Ksv ~ 10 6 M 1 ). Also the energy transfer process was observed from PB Na to cyanine dye DOTC (Ksv ~ 10 6 M 1 ) However treatment with commonly used electron acceptor MV 2+ caused little change i n the photophysical propert ies of the CPE. Electrochemical analysis show ed that MV 2+ is not a sufficient ly strong oxidant to allow the electron transfer process to occur underlyin g low LUMO energy level of the novel CPE PB Na The development of this novel CPE has opened many opportunities for applications in studies conducted in aqueous environment s. The carboxylic acid form PB a was utilized as sensitizer in DSSC cells with TiO 2 Evidence for e lectron injection between PB a and TiO 2 was pr ovide d by fluorescence lifetime measurement s and TA experiment s on PB a TiO 2 film s The l ow transient absorption signal together with relatively inadequate IPCE suggest that insufficient electro n injection yield is resulting from the cell. Structure property discussions were initiated and future development on the PB a based DSSCs were pointed out. Further work can be designed from two aspects. First, for DSSC applications, the side groups and p olymer chain length can be engineered to optimize the device performance. Second, for CPE application in aqueous media, ionic side groups can be introduced into the BODIPY unit. For example, the phenyl group at the meso position of the BODIPY unit can be r eplaced by hydrophilic groups. Although the proposed structure will be harder to achieve synthetically, it really can help to yield BODIPY based CPEs with high fluorescence quantum yields. Conjugated Oligomers with Mono Phosphonic Acid Functionality Throug h synthetic tailoring of the conjugated chromophore, the absorption of the conjugated oligomers was varied from 380 to 600 nm in solution The a symmetrically
139 modified phosphonic acid group on the oligomers offered strong anchoring of the electroactive liga nds at the CdSe NCs surface. Effective photoluminescence quenching was observed for both the oligomer and CdSe while the other species was titrated. H ybrid materials based on the oligomers and CdSe NCs were prepared and purified, leading to composites with significant coverage of the NCs by more than a hundred electroactive ligands. This effectively increased the absorption of the hybrids in the visible region of the spectrum especially as the donor acceptor donor molecule was used, and the photocurrent re sponse of hybrid thin films reflect ed the increased absorption at wavelengths longer than 400 nm. We anticipate that the phosphonic acid functionalized conjugated molecules described here are good candidates as active layer components for efficient hybrid organic inorganic solar cells, especially as the absorption is further increased by taking advantage of the donor acceptor approach. The oligomers were also used as sensitizer s in DSSC format. The cell performance was encouraging, especially for the low ba nd gap T4BTD oligomer. We believe that the difference on the architecture of oligomer/CdSe hybrid solar cells and TiO 2 based DSSC devices is responsible for explaining the disparity of the cell performance using the same oligomers.
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152 BIOGRAPHICAL SKETCH Dongping Xie was born in 1985 in Sichuan, China, where he grew up and finished high school. At the age of 18 he attended Fudan University in Shanghai majored in a pplied c hemistry. In 2007, he graduated with honor and continued immediately on to graduate work at the University of Florida, pursuing a doctorate in c hemistry. Under the supervision of Dr. Kirk Schanze, he conducted a series of research es on conjugated polyelect rolytes and conjugated oligomers After graduation in May, 2012, Dongping will be starting his professional career as an investment analyst for the industry of chemistry and chemical engineering in Beijing, China.