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Conjugated Polyelectrolytes Based on Poly(arylene ethynylene)

Permanent Link: http://ufdc.ufl.edu/UFE0021677/00001

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Title: Conjugated Polyelectrolytes Based on Poly(arylene ethynylene) Synthesis, Solution Photophysics, and Applications to Sensors and Solar Cells
Physical Description: 1 online resource (201 p.)
Language: english
Creator: Zhao, Xiaoyong
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this dissertation, we present the research that was focused on the design, synthesis and photophysical properties of conjugated polyelectrolytes (CPEs) based on poly(arylene ethynylene) (PAE). Applications of these materials for anion sensing and dye-sensitized solar cell (DSCs) were also explored. First, a meta-linked poly(phenylene ethynylene) featuring chiral anionic groups was designed and synthesized. Because of the presence of chiral side chains, the conjugated backbone of this polymer folds preferentially into a left-handed helix in water, as proven by absorption, fluorescence and circular dichroism spectroscopy. Similar to the helix formed by double-strand DNA, the helical conformation of the synthetic polymer interacts with a metallointercalator Ru(bpy)2(dppz)2+ and turns on the emission from the complex. Cationic cyanine dyes can also bind to the helical conformation of the polymer in a 'groove-binding' manner. A chiral and optically active aggregate of cyanine dyes is formed by transferring the chirality of the polymer template. Second, we have systematically investigated the photophysical properties of para-linked poly(phenylene ethynylene)s (PPEs). These CPEs are shown to undergo a solvent-driven aggregation in solution. By chemically tuning the polymer structure, the influence of charge density and polymer chain length to the aggregate formation was carefully examined. Further, the fluorescence quenching by metal ions and organic cations (methyl viologen derivatives) was also studied. It was found that aggregate formation and polymer chain length both have a strong effect on the quenching efficiency of these polymers by the quencher molecules. Based on these results, a highly selective and sensitive sensor for pyrophosphate (PPi) was developed. Third, we have successfully synthesized a series of poly(arylene ethynylene)s with variable absorption and emission properties. The photoluminescence of PAEs with linear ionic groups is strongly quenched in water due to aggregation. By replacing the linear ionic groups with dendritic ionic groups, the aggregation-induced self-quenching is suppressed. PAEs with dendritic ionic groups show high quantum efficiency in water and interesting optical properties when the solution conditions (pH, ionic strength) are varied. The application of PAEs with linear ionic groups in dye-sensitized solar cells was explored and the relationship between the polymer band gap and the cell performance provides valuable information regarding the further improvement of the cells.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Xiaoyong Zhao.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021677:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021677/00001

Material Information

Title: Conjugated Polyelectrolytes Based on Poly(arylene ethynylene) Synthesis, Solution Photophysics, and Applications to Sensors and Solar Cells
Physical Description: 1 online resource (201 p.)
Language: english
Creator: Zhao, Xiaoyong
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In this dissertation, we present the research that was focused on the design, synthesis and photophysical properties of conjugated polyelectrolytes (CPEs) based on poly(arylene ethynylene) (PAE). Applications of these materials for anion sensing and dye-sensitized solar cell (DSCs) were also explored. First, a meta-linked poly(phenylene ethynylene) featuring chiral anionic groups was designed and synthesized. Because of the presence of chiral side chains, the conjugated backbone of this polymer folds preferentially into a left-handed helix in water, as proven by absorption, fluorescence and circular dichroism spectroscopy. Similar to the helix formed by double-strand DNA, the helical conformation of the synthetic polymer interacts with a metallointercalator Ru(bpy)2(dppz)2+ and turns on the emission from the complex. Cationic cyanine dyes can also bind to the helical conformation of the polymer in a 'groove-binding' manner. A chiral and optically active aggregate of cyanine dyes is formed by transferring the chirality of the polymer template. Second, we have systematically investigated the photophysical properties of para-linked poly(phenylene ethynylene)s (PPEs). These CPEs are shown to undergo a solvent-driven aggregation in solution. By chemically tuning the polymer structure, the influence of charge density and polymer chain length to the aggregate formation was carefully examined. Further, the fluorescence quenching by metal ions and organic cations (methyl viologen derivatives) was also studied. It was found that aggregate formation and polymer chain length both have a strong effect on the quenching efficiency of these polymers by the quencher molecules. Based on these results, a highly selective and sensitive sensor for pyrophosphate (PPi) was developed. Third, we have successfully synthesized a series of poly(arylene ethynylene)s with variable absorption and emission properties. The photoluminescence of PAEs with linear ionic groups is strongly quenched in water due to aggregation. By replacing the linear ionic groups with dendritic ionic groups, the aggregation-induced self-quenching is suppressed. PAEs with dendritic ionic groups show high quantum efficiency in water and interesting optical properties when the solution conditions (pH, ionic strength) are varied. The application of PAEs with linear ionic groups in dye-sensitized solar cells was explored and the relationship between the polymer band gap and the cell performance provides valuable information regarding the further improvement of the cells.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Xiaoyong Zhao.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Schanze, Kirk S.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-06-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021677:00001


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1 CONJUGATED POLYELECTROLYTES BASE D ON POLY(ARYLENE ETHYNYLENE): SYNTHESIS, SOLUTION PHOTOPHYSICS A ND APPLICATIONS TO SENSORS AND SOLAR CELLS By XIAOYONG ZHAO A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

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2 2007 Xiaoyong Zhao

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3 To my parents To my wife To my daughter

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4 ACKNOWLEDGMENTS At this point of my academic career, there ar e many people I want to acknowledge. First, I would like to thank my advisor, Dr. Kirk Schanze. Without his support, advice and encouragement, my work at the University of Florida would not have been possible. I also want to acknowledge all the former and current members of Schanze group who have helped and contributed to my projects. Specifically, Dr. Mauric io Pinto taught me many tricks of polymer synthesis and characteriza tion, which helped me pass the most difficult induction period when I started my training here as an organi c chemist. Dr. Hui Jiang has been a wonderful co-worker and is always willing to teach and share his knowledge in photochemistry. Dr. Jeremiah Mwaura trained me how to fabricate solar cells and light emitting diodes. I also want to thank many other pe ople, Dr. Xiaoming Zhao, Dr. Fengqi Guo, Dr. Thomas Cardolaccia, Emine Demir, Dr. Kye-Young Kim, Dr. John Peak, Youngjun Li, Eunkyung Ji, Johnathan Sommer for their willingness to share their success and mistakes in their experiments. I give my thanks to, Kye-Young Kim and Julia Keller for managing the orders for the group, Emine Demir and Yan Liu fo r organizing the group meetings. I would like to thank Dr. John Reynolds and Dr. William Dolbier for their great help in my career and my life. And my committee members, Dr. Ronald Castellano, Dr. Elliot Douglas and Dr. Valeria Kleiman are appreciated for their su ggestions. I also thank Dr. Stephen Hagen for letting me use the CD spectrometer in his la b, giving me instructions and answering my questions. Finally, I am grateful to my fa mily for their love and support. I would not be able to come this far without my parent, they have provided me the best conditions they can to let me continue my education. I want to thank my wife for her love and understanding. I also thank her family for taking care of our daught er in the past year.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES................................................................................................................ .........9 ABSTRACT....................................................................................................................... ............15 CHAPTER 1 INTRODUCTION..................................................................................................................17 Conjugated Polymers............................................................................................................ ..17 Poly(Arylene Ethynylene)s.....................................................................................................18 The Pd-Methodology.......................................................................................................18 Alkyne Metathesis...........................................................................................................22 Poly(Phenylene Ethynylene)s.................................................................................................24 Poly( para -phenylene ethynylene): Truly a Rigid Rod?..................................................24 Meta / Ortho -Linked Phenylene Ethynyl enes: Helical Folding......................................27 Conjugated Polyelectrolytes...................................................................................................29 Amplified Quenching of C onjugated Polyelectrolytes...........................................................30 Stern-Volmer (SV) Quenching........................................................................................30 Molecular Wire Effect.....................................................................................................32 Fluorescence Quenching in Conjugated Polyelectrolytes...............................................35 Aggregation of CPEs.......................................................................................................36 Applications of Conjugated Polyelectrolytes.........................................................................38 Optical Biosensors...........................................................................................................38 Quenching-unquenching mechanism.......................................................................38 Chain conformation perturbation mechanism..........................................................39 Fluorescence resonance energy tr ansfer (FRET) mechanism..................................41 Dye-sensitized Solar Cells (DSCs)..................................................................................42 Scope of the Present Study.....................................................................................................44 2 SELF-ASSEMBLY OF META -LINKED POLY(PHENYLENE ETHYNYLENE).............47 Introduction................................................................................................................... ..........47 Results and Discussion......................................................................................................... ..48 Synthesis...................................................................................................................... ....48 Model Compound (9)......................................................................................................52 Solvatochromic Properties of Meta -Linked PPEs...........................................................53 UV-Vis absorption spectroscopy.............................................................................53 Steady state fluorescence spectroscopy....................................................................54 Circular dichroism spectroscopy..............................................................................56 Guest Binding with Meta -Linked PPEs..........................................................................58

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6 Intercalation of [Ru(bpy)2(dppz)]2+..........................................................................60 Groove binding of cyanine dyes...............................................................................62 UV-Vis absorption...................................................................................................62 Fluorescence spectroscopy.......................................................................................63 Circular dichroism spectroscopy..............................................................................64 Experimental................................................................................................................... ........66 Materials...................................................................................................................... ....66 Instrumentation................................................................................................................67 General Methods.............................................................................................................67 Synthetic Procedures.......................................................................................................68 3 POLY(PHENYLENE ETHYNYLENE) CARBOXYLATE.................................................76 Introduction................................................................................................................... ..........76 Results and Discussion......................................................................................................... ..77 Synthesis...................................................................................................................... ....77 Polymer Characterization................................................................................................82 1H NMR spectra.......................................................................................................82 Infrared spectra.........................................................................................................83 Photophysical Characterization.......................................................................................84 Fluorescence Quenching Properties................................................................................90 Quenching with metal ions in MeOH......................................................................91 Quenching with metal ions in the HEPES buffer.....................................................93 Quenching by methyl viologen (MV2+) in MeOH mediated by Ca2+: an explanation of superlinear quenching behavior for CPE-quencher system..........96 Application to Pyrophosphate (PPi) Sensing..................................................................98 Selectivity...............................................................................................................101 Sensitivity...............................................................................................................102 Sensing mechanism................................................................................................103 Experimental................................................................................................................... ......104 Materials...................................................................................................................... ..104 Instrumentation..............................................................................................................104 General Methods...........................................................................................................105 Synthetic Procedures.....................................................................................................105 4 VARIABLE CHAIN LENGTH POLY(PHENYLENE ETHYNYLENE) CARBOXYLATE.................................................................................................................112 Introduction................................................................................................................... ........112 Results and Discussion.........................................................................................................113 Synthesis...................................................................................................................... ..113 Polymer Characterization..............................................................................................115 Gel permeation chromatography............................................................................115 NMR spectra..........................................................................................................116 End group analysis by NMR..................................................................................117 Structural characterization of water-soluble polymers...........................................119 Photophysical Characterization.....................................................................................120

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7 In methanol.............................................................................................................120 In water...................................................................................................................123 Fluorescence Quenching with Electron Acceptors........................................................125 Quenching of PPE-CO2Na-7 by MV+ and MV2+...................................................126 Quenching of the series by m onovalent cationic quenchers..................................127 Quenching of the series by divalent cationic quenchers........................................129 Quenching of PPE-CO2Na-108 by MV+ and MV2+...............................................130 Chain length dependence of Ksv.............................................................................132 Experimental................................................................................................................... ......134 Materials...................................................................................................................... ..134 Instrumentation..............................................................................................................134 General Methods...........................................................................................................135 Synthetic Procedures.....................................................................................................135 5 VARIABLE BAND GAP POLY(ARYLENE ETHYNYLENE)S......................................139 Introduction................................................................................................................... ........139 Results and Discussion.........................................................................................................140 Synthesis of PAEs with Linear Side Chains..................................................................140 Synthesis of PAEs with Dendritic Side Chains.............................................................143 Monomer synthesis................................................................................................143 Synthesis of PAEs carrying anionic side chains....................................................145 Synthesis of PAEs carrying cationic side chains...................................................147 Optical Properties of PAEs with Linear Carboxylate Side Chains...............................150 Optical Properties of PAEs F eaturing Dendritic Side Chains.......................................152 Absorption and fluorescence properties.................................................................153 Solvent effects on absorption and fluorescence.....................................................154 Acidity effects on the absorption and fluorescence...............................................156 Salt Effects on the absorption and fluorescence.....................................................160 Applications of PAEs cont aining Linear Carboxylate Groups in Dye-sensitized Solar Cells..................................................................................................................161 Experimental................................................................................................................... ......165 Materials...................................................................................................................... ..165 Instrumentation..............................................................................................................166 General Methods...........................................................................................................166 Synthetic Procedures.....................................................................................................167 6 CONCLUSIONS..................................................................................................................178 APPENDIX A HELICAL FOLDING OF META -LINKED POLY(PHENYLENE ETHYNYLENES).....182 B NMR SPECTRA...................................................................................................................185 LIST OF REFERENCES.............................................................................................................190 BIOGRAPHICAL SKETCH.......................................................................................................201

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8 LIST OF TABLES Table page 4-1. GPC and 1H NMR end-group analysis of the precursor polymers.......................................115 4-2. Photophysical properties of the CPEs with different ch ain lengths in methanol and water.......................................................................................................................... .......125 5-1. UV-Vis absorption and photoluminescent properties of PAEs containing linear carboxlate side chains......................................................................................................152 5-2. UV-vis absorption and photoluminescent pr operties of PAEs containing dendritic carboxlate side groups......................................................................................................156 5-3. Summary of solar cell performance......................................................................................164 5-4. Redox potentials of studied c onjugated polyelectrolytes.....................................................165

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9 LIST OF FIGURES Figure page 1-1. Molecular structures of commonly seen CPs.........................................................................17 1-2. Historical aspect of CPs.................................................................................................. ........18 1-3. Molecular structures of a few PAEs.......................................................................................18 1-4. Mechanism of Pd (0) and Cu I catalyzed Sonogashira reaction..............................................20 1-5. Acyclic diyne metathesis (ADIMET) as an alternative approach to prepare PPEs................22 1-6. Structures of PPEs with diffe rent backbone configurations...................................................24 1-7. Radius of gyration Rg of a PPE sample with a broad mo lecular weight di stribution (PDI = 1.4) in THF solution, plotted as a function of molecular weight (Mw)..........................25 1-8. STM current images of oligo-PE (left) and PPE (right) at the so lid-liquid interface on the graphite substrate.........................................................................................................26 1-9. Meta -linked oligo phenylene ethynyl enes studied by Moore group......................................27 1-10. A space-filling model showing the folding process for an oligo-PE (n=18). The side chains were omitted for clarity..........................................................................................28 1-11. The transoid cisoid interconversion existing in meta-linked PPEs...................................29 1-12. Helical folding of ortholinked oligo-phenylene ethynylenes..............................................29 1-13. Molecular structures of some common CPEs.......................................................................30 1-14. Molecular structures of the CPs studied by Swagers group................................................32 1-15. Stern-Volmer plot of Swagers polymers with MV2+..........................................................33 1-16. Schematic illustration of the molecu lar wire effect expressed by conjugated polymers....................................................................................................................... ......34 1-17. Absorption and fluorescence of PPV-SO3 in water with and without 100 nM MV2+.........35 1-18. Absorption and fluorescence of PPE-SO3 in methanol, methanol:water (50:50) and water.......................................................................................................................... .........36 1-19. Biosenser applications of CPEs by the quencher-tethered-ligand (QTL) approach.........39 1-20. Molecular structures of so me poly(thiophene) derivatives..................................................39

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10 1-21. Schematic illustration of the form ation of polythiophene/ss-DNA duplex and polythiophene/ds-DNA triplex...........................................................................................40 1-22. Molecular structures of CPEs based on polyfluorene (PF) reported by Bazan and coworkers........................................................................................................................ .......41 1-23. Schematic illustration of the PN A/PF1 assay for ss-DNA detection...................................42 1-24. Schematic representation of the principle of dye-sensitize d solar cells (DSCs) based on nanocrystalline TiO2..........................................................................................................43 1-25. Molecular structures of some CPs with car boxylic acid functionalities that were used in DSCs........................................................................................................................ ......44 2-1. Structures of meta -linked PPE reported in the literature........................................................47 2-2. Synthesis of monomers..................................................................................................... ......49 2-3. Synthesis of the model compound 9 .......................................................................................50 2-4. Synthesis of the homopolymer.............................................................................................. .51 2-5. Synthesis of co-polymers.................................................................................................. ......51 2-6. Absorption and emission spectra of model compound 9 in methanol () and water (---).......................................................................................................................... ...........52 2-7. UV-Visible absorption of w -P2 in methanol, water and methanol/water mixtures...............54 2-8. Fluorescence spectra of w -P2 in MeOH, water and MeOH/water mixtures..........................55 2-9. Circular dichroism spectra of model compound 9 in water and w -P2 in methanol, water and in methanol/water mixtures.........................................................................................57 2-10. Chemical structures of the organic dyes...............................................................................60 2-11. Emission spectra of [Ru(bpy)2(dppz)]2+ in the absence and presence of wP2 ....................61 2-12. Schematic representation of the interaction between w -P2 and [Ru(bpy)2(dppz)]2+...........61 2-13. UV-Visible absorption of HM IDC in water titrated with w -P2 ...........................................63 2-14. Fluorescence spectra of HMIDC in water titrated with w -P2 excitation wavelength is 610 nm......................................................................................................................... ......64 2-15. Circular dichroism spectra of HMIDC alone and HMIDC with w -P2 in aqueous solution....................................................................................................................... ........65 2-16. Schematic representation of the interaction between w -P2 and HMIDC.............................65

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11 2-17. UV-Vis absorption and fluorescence emissi on spectra of HMIDC in methanol with titrations of w -P2 ................................................................................................................66 3-1. Synthesis of the monomer 8 and 10 ........................................................................................78 3-2. Synthesis of the neutral polymer precursors...........................................................................79 3-3. GPC chromatograms of P4 and P5 .........................................................................................80 3-4. Hydrolysis of the neutral polymers to prepare the wate r-soluble polymers, w -P4 and w -P5 ............................................................................................................................... ....81 3-5. 1H NMR spectra of monomer 8 organic polymer P5 and the water-soluble polymer wP5 ............................................................................................................................... ....82 3-6. FT-IR spectra of monomer 8 organic polymer P5 and the water-soluble polymer w -P5 .....84 3-7. Normalized absorption and fluorescence spectra of w -P4 in MeOH, (1:1) H2O-MeOH and H2O..............................................................................................................................85 3-8. Normalized absorption spectrum of P5 in THF solution, normalized absorption spectra of w -P5 in MeOH/H2O mixtures and variation of Amax/A415 in the absorption spectra of w -P5 with the volume % H2O.......................................................................................87 3-9. Fluorescence spectra of w -P5 in MeOH, water and MeOH/water mixtures..........................89 3-10. Schematic representation of the stacking interaction in CPE aggregates......................90 3-11. Absorption and fluorescence spectra of w -P5 in MeOH with added Ca2+ or Cu2+.............92 3-12. Absorption and emission spectra of w -P5 in 10 mM HEPES buffer solution.....................93 3-13. Absorption and fluorescence spectra of w -P5 in the HEPES buffer (pH = 7.5) with added Ca2+ or Cu2+.............................................................................................................94 3-14. Stern-Volmer plots of w -P5 emission quenching by different metal ions in 10 mM HEPES buffer and comparison of Ksv values for different metal ions. The inset shows a photograph of w -P5 /M2+ (5 M/10 M) solutions illuminated with a UV lamp........................................................................................................................... .........95 3-15. Stern-Volmer quenching of 10 M w -P5 emission by MV2+ in water ( ) and in MeOH with 0 M ( ), 2.5 M ( ), 5.0 M ( ), 7.5 M ( ) 10 M ( ) CaCl2.........................97 3-16. Fluorescence spectra of a solution of w -P5 /Cu2+ (5 M/10 M) titrated with PPi in 10 mM HEPES buffer at pH 7.5, 25 C Intensity enhancement (I/I0) at 530 nm titrated with PPi....................................................................................................................... .....100 3-17. Fluorescence response of w -P5 /Cu2+ (5 M/10 M) to various anions at 50 M concentration in 10 mM HEPE S buffer at pH 7.5, 25 C................................................101

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12 3-18. Intensity enhancement (I/I0) at 530 nm of w -P5 /Cu2+ (5 M/10 M) titrated with PPi....102 3-19. Schematic representation of the sensing of PPi by w -P5 /Cu2+ system...............................103 4-1. Synthetic route for conjugated polyele ctrolytes with different chain lengths......................114 4-2. GPC chromatograms of the precursor polymers..................................................................116 4-3. 1H NMR spectra of PPE-CO2R-7 in CDCl3.........................................................................117 4-4. The aromatic region in the 1H NMR spectra for the polymers with variable chain length..118 4-5. 1H NMR and FT-IR spectra of the water-sol uble polymers with different chain length.....120 4-6. Absorption and emission spectra of the se ries in methanol. Dependence of molar extinction coefficient ( ) and fluorescence quantum yield ( ) on the polymer chain length......................................................................................................................... .......121 4-7. Absorption and Emission spectra of the series in water.......................................................124 4-8. Molecular structure of the quenc hers used in the current study...........................................125 4-9. Absorption and emission spectra of PPE-CO2Na-7 with the addition of MV+ or MV2+.....126 4-10. SV Plots of the series in MeOH quenching by (a) MV+ and (b) HV+................................128 4-11. SV plots of the series quenching in MeOH by (a) MV2+ and (b) HV2+.............................129 4-12. UV-visible absorpti on spectra of PPE-CO2Na-108 with the addition of (a) MV+ and (b) MV2+...........................................................................................................................130 4-13. Emission spectra and SV-plot of PPE-CO2Na-108 with the addition of MV+...................131 4-14. Comparison of the Ksv values obtained from SV plots for the series with MV+ and HV+............................................................................................................................... ...133 5-1. Synthesis of variable band gap PAEs with linear carboxyl ate side chains..........................142 5-2. Synthesis of monomers ( 19 20 ) carrying dendritic side chains...........................................144 5-3. Syntheis of PAEs carrying denditic anion side groups.........................................................145 5-4. 1H NMR and FT-IR spectra of monomer 19 (a), PPE-R1 (b) and PPE-dCO2Na (c)..........146 5-5. Syntheis of PAEs carrying dendritic cationic side groups...................................................148 5-6. 1H NMR and FT-IR spectra of monomer 20 (a), PPE-R2 (b) and PPE-dNH3Cl (c)...........149

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13 5-7. Absorption and emission spectra of PAEs containing linear carboxyla te side chains in MeOH........................................................................................................................... ...151 5-8. Normalized absorption and fluorescence spectra of PPE-dCO2Na (solid line) and BTDPPE-dCO2Na (dotted line) in MeOH. Norm alized absorption and fluorescence spectra of PPE-dNH3Cl (solid line) and BTD-PPE-dNH3Cl (dotted line) in MeOH.....154 5-9. Absorption and fluorescence spectra of PPE-dCO2Na (solid line) and BTD-PPE-dCO2Na (dotted line). Absorption and fluorescence spectra of PPE-dNH3Cl (solid line) and BTD-PPE-dNH3Cl (dotted line)......................................155 5-10. Absorption and emission spectra of PPE-dCO2Na in aqueous solution as a function of pH. Absorption and emission spectra of PPE-dNH3Cl in aqueous solution as a function of pH................................................................................................................. .157 5-11. Absorption and emission spectra of BTD-PPE-dCO2Na in aqueous solution as a function of pH. Absorption and emission spectra of BTD-PPE-dNH3Cl in aqueous solution as a function of pH.............................................................................................159 5-12. Absorption and emission spectra of PPE-dCO2Na in aqueous solution as a function of NaCl concentration. Absorpti on and emission spectra of PPE-dNH3Cl in aqueous solution as a function of NaCl concentration...................................................................161 5-13. IPCE spectra and Current-potential (I-V) cu rves for CPE sensitized solar cells under AM 1.5 condition.............................................................................................................163 A-1. Absorption and fluorescence of w -P3 in methanol, water and methanol/water mixtures...182 A-2. Emission spectra of [Ru(bpy)2(dppz)]2+ in the absence and presence of w -P3 ...................182 A-3. UV-visible absorption of DiSC2(5) in water titrated with m PPE-Ala. Circular dichroism spectra of DiSC2(5) in water and DiSC2(5) in water mixed with w -P2 ..........183 A-4. Plot of absorbance at 675 nm (A675) vs. [PRU]/[DiSC2(5)] ratio........................................184 A-5. Fluorescence spectra of DiSC2(5) ([DiSC2(5)] = 5.0 M) in water titrated with w -P2 ......184 B-1. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 3 (chapter 2).....................................185 B-2. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 3 (chapter 2).......................................185 B-3. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 7 (chapter 2).....................................186 B-4. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 7 (chapter 2).......................................186 B-5. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 8 (chapter 3).....................................187 B-6. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 8 (chapter 3).......................................187

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14 B-7. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 19 (chapter 5)...................................188 B-8. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 19 (chapter 5).....................................188 B-9. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 20 (chapter 5)...................................189 B-10. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 20 (chapter 5)...................................189

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15 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONJUGATED POLYELECTROLYTES BASE D ON POLY(ARYLENE ETHYNYLENE): SYNTHESIS, SOLUTION PHOTOPHYSICS A ND APPLICATIONS TO SENSORS AND SOLAR CELLS By Xiaoyong Zhao December 2007 Chair: Kirk S. Schanze Major: Chemistry In this dissertation, we presen t the research that was focuse d on the design, synthesis and photophysical properties of conjugated polyel ectrolytes (CPEs) based on poly(arylene ethynylene) (PAE). Applications of these materials for anion sensing and dye-sensitized solar cell (DSCs) were also explored. First, a meta -linked poly(phenylene ethynylene) feat uring chiral anionic groups was designed and synthesized. Because of the pres ence of chiral side chains, the conjugated backbone of this polymer folds preferentially into a left handed helix in water, as proven by absorption, fluorescence and circul ar dichroism spectroscopy. Similar to the helix formed by double-strand DNA, the helical conformation of the synthetic polymer interacts with a metallointercalator [Ru(bpy)2(dppz)]2+ and turns on the emission from the complex. Cationic cyanine dyes can also bind to the helical conf ormation of the polymer in a groove-binding manner. A chiral and optically active aggregat e of cyanine dyes is formed by transferring the chirality of the polymer template. Second, we have systematically inves tigated the photophysic al properties of para -linked poly(phenylene ethynylene)s (PPEs). These CP Es are shown to undergo a solvent-driven

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16 aggregation in solution. By chemically tuning th e polymer structure, th e influence of charge density and polymer chain length to the aggregat e formation was carefully examined. Further, the fluorescence quenching by metal ions and orga nic cations (methyl viologen derivatives) was also studied. It was found that aggregate form ation and polymer chain length both have a strong effect on the quenching efficiency of these polyme rs by the quencher molecules. Based on these results, a highly selective and sensitive se nsor for pyrophosphate (PPi) was developed. Third, we have successfully synthesized a seri es of poly(arylene ethynylene)s with variable absorption and emission properties. The photolum inescence of PAEs with linear ionic groups is strongly quenched in water due to aggregation. By replacing the line ar ionic groups with dendritic ionic groups, the aggr egation-induced self-quenching is suppressed. PAEs with dendritic ionic groups show high quantum efficiency in water a nd interesting optical properties when the solution conditions (pH, ionic strength) are varied. The app lication of PAEs with linear ionic groups in dye-sensit ized solar cells was explored and the relationship between the polymer band gap and the cell performance provide s valuable information regarding the further improvement of the cells.

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17 CHAPTER 1 INTRODUCTION Conjugated Polymers In the past 30 years, a class of fascinati ng materials (i.e. conjugated polymers) has been developed.1 The ground-breaking work is the discovery that the conductiv ity of polyacetylene (PA) was increased 10 million times with chemical doping by I2 vapor in 1977, which was recognized by the Chemistr y Nobel Prize in 2000.2,3 Hideki Shirakawa of the University of Tsukuba in Japan, Alan MacDiarmid of the Univer sity of Pennsylvania at Philadelphia and Alan Heeger of the University of Ca lifornia at Santa Barbra shared the prize for the discovery of conducting polyacetylene. Ever since, the interest of scientists has expanded to a variety of other conjugated polymers (CPs),4 including polypyrrole (PPy),5 poly( para -phenylene) (PPP),6 polythiophene (PT),7 polyanaline (PANI),8 poly(phenylene vinylene) (PPV),9 poly(phenylene ethynylene) (PPE)10 and polyfluorene (PF),11 as shown in Figure 1-1. Figure 1-1. Molecular structures of some commonly seen CPs. One of the most important breakthroughs in the field is the first polymer light-emitting diode (PLED) fabricated w ith poly(phenylene vinylene).12 In the past thr ee decades, conjugated polymers have found numerous applications, including light-emitting diodes (LEDs),9 lightemitting electrochemical cells (LECs),13 plastic lasers,14 solar cells,15 field-effect transistors

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18 (FETs)16 and chemical and bio-sensors.17,18 Figure 1-2 highlights the most important developments related to this field. Figure 1-2. Historical aspect of CPs. Poly(Arylene Ethynylene)s The Pd-Methodology Compared with other types of conjugated pol ymers, poly(arylene ethynylene) (PAEs) did not receive much attention un til their applications in the area of explosive detection,19 molecular wire20 and polarizers for LC displays were discovered in mid-1990s.21,22 Figure 1-3. Molecular stru ctures of a few PAEs. In the past decade, a wide variety of PAEs with different chemical structures have been synthesized and studied,10 some examples of which are s hown in Figure 1-3. Despite their 1972 Metallatic films of polyacetylene were accidently discovered in Japan; 1977 Hideki Shirakawa, Alan G. Macdiarm id and Alan J. Heeger prepared the first conductive polymers, I2-doped polyacetylene; 1990 The first polymer light emitting diod e was prepared by Richard Friend and Andrew Holmes; 1995 The first polymer phot ovoltaic cell was fabricat ed in Heegers lab; 1995 Molecular wire effect in CPs was first demonstrated by Swager and coworkers; 1999 The first biological sensor was demonstrated by Whitten and co-workers; 2000 Nobel Prize in Chemistry was awarded to Hideki Shirakawa, Alan G. MacDiarmid and Alan J Heeger; 2003 Displays using CPs were first commercialized by Philips Inc.

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19 structural variety, they all share the same bac kbone feature, i.e. th ey are conjugated through ethynyl linked aromatic or hetero-aromatic rings. To date, the most common approach used to prepare poly(arylene ethynylene)s is the Palladium-catalyzed cross-coupli ng reactions between terminal al kyne and aromatic bromides or iodides in the presence of a catalytic amount of copper (I ) iodide, known as Sonogashira coupling or Sonogashir a-Hagihara coupling.23,24 Actually, earlier in the same year of 1975, Heck25 and Cassar26 independently reported similar coupli ng reactions. Heck used phosphatepalladium as the catalyst and triethylamine or pipe ridine as the base and also the solvent. In Cassars procedure, sodium methoxide was used as the base and the reaction was performed in DMF. In general, both reactions required high temperature up to 100 C. In the SonogashiraHagihara reaction, the addition of copper (I) iodide as the co-catalyst enables the reaction to be run at a lower temperature or even room temperat ure. Therefore, it provides a mild method and is more compatible with a wide vari ety of functional groups and solvents. The mechanism of Sonogashira coupling has been under considerable dispute for many years and the generally accepted mechanistic pathway involves two independent catalytic cycles, as shown in Figure 1-4. In the Pd-cyc le, the active catalyst, 14-electron Pd0L2 is generated from the initial pre-catalyst, commonly a palladium (II) complex by reduction with either solvent or substrate. Then, a fast oxidative addition of an aromatic bromide or iodide occurs at the electron deficient Pd center. At this point, the Pd-cyc le connects with the coppe r co-catalyst cycle (Cucycle) and a transmetalati on from copper acetylide to th e Pd-center generates the R1Pd(-C CR2)L2 species. The final coupled alkyne is produced by reductiv e elimination after trans/cis isomerization and the active catalyst is rege nerated. In the Cu-cycle the base (usually

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20 amine) promoted formation of copper acetylide is believed to occur via a -alkyne-Cu complex based on the fact that the amines are not basic enough to deprotonate the free alkyne. Figure 1-4. Mechanism of Pd (0) a nd CuI catalyzed Sonogashira reaction.24 In most examples of Sonogashira coup ling, the commercially available Pd(PPh3)2Cl2 is the source of Pd. Since Pd (II) is inactive, it is believed that the copper acetylide formed in the Cu-cycle is involved in the genera tion of the initial active Pd (0) sp ecies. In such a process, Pd(C CR2)2L2 is formed and subsequent reductive elimination affords the Pd0L2 and some amounts of diacetylene byproduct. This consumption of alkyne is not a problem in the synthesis of small organic molecules. However, since the polycondensation based on Sonogashira coupling follows a step-growth mechanism, the molecular we ight of the polymer is strongly related to the stoichiometry between the monomers.10 Such an activation step will cause a stoichimetric

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21 imbalance between the two monomers, leading to polymers with reduced molecular weight. To circumvent the problem, several methods have be en proposed including the addition of a small excess of the alkyne monomer,27 and/or using the co mbination of Pd(PPh3)2Cl2 and triphenylphosphine, etc.10 Most commonly, a Pd (0) complex, such as Pd(PPh3)4 is employed in a polymerization to reduce the problem and give rise to polymers with satisfactory molecular weights. In the synthesis of low molecular weight or ganic molecules, both aromatic bromides and iodides have been used. Since the oxidativ e addition step in the catalytic cycle is thermodynamically more favorable for aromatic iodides than aromatic bromides, the reactions involving aromatic iodides occur considerably fa ster and proceed at r oom temperature. By contrast, when aromatic bromides are used as the reagents, the r eactions require higher temperatures and a longer reaction time. Thus in a typical polymerization, when aromatic iodides are used, the reaction generally produces higher molecular weight polymer with less defects. Although Sonogashira coupling has been widely used to synthesize a variety of PAEs, there are several shortcomi ngs of this Pd-methodology.28 First, the addition of copper (I) iodide catalyzes the homocoupling of the terminal al kyne when the copper acetylides are formed in situ which generates the diacetylene byproduct and/or diyne defects in the polymer chain. Use of Pd (0) complexes and careful exclusion of air could circumvent the formation of structural defects based on the literature and the authors own experience.10 Second, the molecular weight of PAEs prepared from polycondensation based on Sonogashira coupling is strongly dependent on the monomer structure. Typically, when di alkoxyand dialkyldii odobenzenes were used, polymers with degrees of polymeriza tion (DPs) less than 100 were obtained.10 In fairly few

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22 examples, polymers with DP around 200 were reported.10 To date, PAEs with the highest molecular weight using this methodology were obtained by Swager et al. when acceptorsubstituted monomers were polymerized.27 Third, since dehalogena tion and substitution of iodides by triphenylphosphine can oc cur in all of the Pd-catalyzed reactions, thus polymerization based on Sonogashira coupling may lead to the form ation of polymer chains with poorly defined end groups.29 Alkyne Metathesis An alternative approach to prepare PAEs is the alkyne metathesis reaction, which refers to the exchange of substituents between two al kynes. Alkyne metathesis is commonly catalyzed by molybdenum or tungsten based complexes.30 The first homogenous catalyst for alkyne metathesis was reported by Mortreux et al. in 1974.31 They found that a mixture of Mo(CO)6 and simple phenol additives could catalyze the metath esis of alkynes. This system has been used widely and is referred as the M ortreux system. However, the nature of the catalytically reactive species formed in situ from the mixture remains unknown. Structurally defined catalysts were later developed by Schrock et al. which involves the usage of tungsten and molybdenum alkylidynes of the type (RO)3M C-CMe3, where M is W or Mo and R is Me3C or aryl groups.32 These catalysts have been found to be remarkably active, but they are extr emely air and moisture sensitive, which poses some difficulties for large-scale synthesis and polymerization.28,30 Figure 1-5. Acyclic diyne metathesis (ADIMET) as an alternative approach to prepare PPEs. The application of the alkyne metathesis to prepare PAEs was first demonstrated by Mllen and co-workers in 1997.33 Based on this work, the term acyclic diyne metathesis

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23 (ADIMET) was termed, as s hown in Figure 1-5. Bunz et al. successfully prepared poly(Phenylene ethynylene)s (PPEs) with DP of 100 and 1260 using Schrocks molybdenum alkylidyne and Mortreux system, respectively.28 In the latter case, when a combination of Mo(CO)6 and 4-chlorophenol was employed, the obtai ned polymer is the PPE with the highest molecular weight synthesized so far. In anot her example, a group of alkyne-bridged carbazole polymers was synthesized using the same reactio n condition, which demons trated the Mortreux system is not only effici ent to prepare hydrocarbon PPEs.34 Although Mortreux system shows the advantages that a comm ercially available metal complex is used directly and no strict purification of solvent is neede d, unfortunately the system requi res a high activation temperature and is only compatible with limited functiona l groups. Recently, Moore and his co-workers developed an in situ strategy to prepare trialkoxymo lybdenum alkylidyne (Schrocks catalyst)35,36 from readily available mol ybdenum complex Mo[N(t-Bu)Ar]3 (Ar = 3,5-C6H3Me2). They found that the resulting alkyl idyne is highly active and is compatible with a variety of functional groups. They utili zed it to catalyze the polymeri zation of thiophene-containing monomers, which are not reactive in the presence of any earlier alkyne metathesis catalysts, and successful prepared poly(2,5-thienylen e-ethynylene)s with alkyl chains.37 Although ADIMET shows its potential to produc e PAEs with high molecular weight and well-defined end groups, this methodology also has se veral major drawbacks. First, a catalytic system thats meet multiple requirements (stabi lity toward moisture and air; low activation temperature and wide compatibilities with mo st functional groups) is still not available.38 Second, since alkyne metathesis is an equilibrium process,30 to get polymers with a high molecular weight, the polymerization has to be conducted under open-driven conditions (high vacuum or continuous air flow) to remove the alkyne byproducts. Thus the polymerization is

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24 required to be carried out in a high boiling so lvent, such as 1,2-dichlorobenzene or 1,2,4trichlorobenzene, which is another factor that impedes the applica tion of alkyne metathesis in the synthesis of PAEs. Besides Sonogashira coupling and alkyne meta thesis, Mori and co-w orkers reported a copper-free method to prepare PAEs in 2001.39 In their reactions, trimethylsilyl-protected diacetylene compounds and diiodoarenes were poly merized in THF in the presence of Pd(0) and silver (I) oxide. Recently, Watson reported a tr ansition-metal-free synthesis of PAEs that contain alternating aren es and perfluoroarenes.40,41 Although these methods have led to the preparation of PAEs with high molecular we ights, they have drawbacks of using an stoichiometric amount of Ag2O or limited applicability to fluorinated substrates, which impedes their general use to prepare PAEs. Poly(Phenylene Ethynylene)s The most common type of PAEs is poly(pheny lene ethynylene)s, where all of the aromatic groups are benzene rings. Based on the main chain conformation, there are three isomers, namely, para, metaand orthopoly(phenylene ethynylene)s (Figur e 1-6). These three isomers display extraordinarily different el ectronic and physical properties. Figure 1-6. Molecular structures of PPEs with different backbone configurations. Poly( para -phenylene ethynylene): Truly a Rigid Rod? Poly( para -phenylene ethynylene)s ( para -PPEs) are ideally shape persistent structures because the bond angle between the phenyl carb on and sp carbon (triple bond) is 180 C. Initially because of their rod-like structur es, oligo-phenylene ethynyl enes (oligo-PEs) and para -

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25 PPEs have been synthesized and explored as promising materials for nonlinear optics,42 liquidcrystal displays22 and molecular wires to bridge nanoelectrodes.43 While low-molecular-weight PPEs and oligo-PEs are certainly rigid rods,44 substantial researches have led to the conclusion that PPEs above certain chain le ngth actually behave as worm like chains and should not be considered as rigid rods.45,46 Figure 1-7. Radius of gyration Rg of a PPE sample with a broad molecular weight distribution (PDI = 1.4) in THF solution, plotted as a function of molecular weight (Mw). Figure was taken from Cott et al .45 Cotts and Swager45 synthesized a series of PPEs with different molecular weights and investigated the solution propertie s of PPE chains using a variety of light scattering techniques, coupled with size exclusion chromatography. One of the most important conclusions of their work is that PPEs can be described as coil -like polymers at higher molecular weight (Mn > 50,000 kD). In a diluted polymer solution, th e root-mean-square radius of gyration (Rg) is often used to measure the size of a polymer molecule.47 This term is described being dependent on the molecular weight of the polymer in the form of Rg ~ M, in which the exponent indicates the rigidity of the polymer chain.47 For a random-coil polymer, the exponent is 0.5; while for rod-

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26 (a) (b) like polymer, is 1.0. Figure 1-7 shows the log-log plot of Rg vs Mw for a PPE sample with a broad molecular weight dist ribution. The results were obtained by size exclusion chromatography coupled with an on-line light sc attering detector, which measures the molecular weight simultaneously and instantaneously as th e narrow distribution fractions of the polymer is separated by the colum n. The dependence of Rg on the molecular weight shows an exponent near 0.5, indicative of a random coil conformation. Also the plot shown in Figure 1-7 can be fitted with Kratky-Porod worm-like chain model and the persistent length of PPEs is estimated to be around 15 nm, which equals 20 PE repeating units. Figure 1-8. STM current images of oligo-PE (a ) and PPE (b) at the so lid-liquid interface on the graphite substrate. L is the distance between the neighbouring backbones. Figure was taken from Samor et al .48 In another study carried out by Rabe and Mllen,48 the self-assembly of PPEs with different molecular weights at the solid/liqui d interface was investigated by scanning tunneling microscopy (STM). The rigid rod-like structures of PPE and oligo-PE ar e directly visualized with molecular resolution, as shown in Figure 1-8. At the interface between an oligo-PE solution and a highly oriented pyrolytic gr aphite (HOPG) substrate, an ep itaxial 2D crystal structure is observed (left). For the polydispersed PPE, a 2D nematic-like molecular order is revealed (right). The stiffness of the oligomer (trimer) and the polymer (DP = 9) is apparent in these images and is believed to play a key role for the molecular packi ng. In both cases, the

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27 conjugated skeleton and the side gr oups lie flat on the HOPG subs trates. The distance between the backbones is around 1.6 nm. This number is sm aller than the distance of 1.9 nm calculated for the case with the side chains (hexyl) extende d, indicating a slightly disordered arrangement of side groups between the backbones. Further studies of the self-a ssembly on the mica substrate shows that the morphology of the dry samples prepared by evaporation of PPE solutions on the mica surfaces is dependent on the molecular weight of the polymers. Fo r polymers with DPs of 9, 11, 20, 22, ribbons are observed, while for high molecular weight sa mples (DP = 28, 42), a grainy morphology is obtained. The cut-off molecular weight of the polymers for different morphologies corresponds also to about 20 phenylene ethynylene repeat un its, which in another way supports Cotts and Swagers result that PPEs with chain length bey ond 20 repeat units behave differently from PPEs with low molecular weight. Meta / Ortho -Linked Phenylene Ethynylenes: Helical Folding Compared with para -linked poly(phenylene ethynylene)s, meta -linked oligo-phenylene ethynylenes (oligo-PEs) and poly( phenylene ethynylene)s received much less attention before their folding properties were discover ed by Moore and co-workers in 1997.49 Figure 1-9. Meta -linked oligo-phenylene ethynyl enes studied by Moore group. In a series of well-designed work, meta -linked oligo-PEs with oli go(ethylene glycol) side chains were synthesized (shown in Figure 1-9) and their foldi ng process was carefully studied using 1H NMR, UV absorption, fluorescence spec troscopy and computational modeling.50

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28 Figure 1-10. A space-filling model showing the fo lding process for an oligo-PE (n=18). The side chains were omitted for clarity. Figure was taken from Prince et al .50 As shown in Figure 1-10, these oligo-phenylene ethynylenes are able to fold into a helical structure in a polar solvent, su ch as acetonitrile. In a typi cal non-polar solvent, such as chloroform or methylene chloride, the oligomer s are well-solvated and exist in an expanded form. When the solvent composition (CHCl3/CH3CN) is changed, the polar side chains are extended to the environment and the non-polar backbone folds into a helical conformation, which is stabilized by the favorable interaction between the phenyl rings. The ability of such PE systems to fold and unfold is believed to ar ise from the fact that the free rotation around the ethynylene linkers allows for the switching between the transoid and cisoid state of the conjugated chains,50 which is illustrated in Figure 111. In the helical conformation, the cisoid state is populated. The coil-helix transition is al so found to be strongly dependent on the oligoPEs chain length and the environment temperature.

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29 Figure 1-11. The transoid cisoid interconversion existing in meta -linked PEs. Recently, Tew and co-workes synthesized ortho -linked oligo-PEs and demonstrated that these ortho isomers with very short sequences can also undergo the folding process. By theoretical modeling51 and experimental studies using 1D and 2D NMR methods,52 the helical folding in ortho -linked oligo-PEs was confirmed. In a polar solvent such as acetonitrile, a helical conformation with 3 rings per turn is expected, as shown in Figure 1-12. Figure 1-12. Helical folding of ortholinked oligo-phenylene ethynylenes. Figure was taken from Blatchly et al .51 Conjugated Polyelectrolytes Conjugated polyelectrolytes (CPEs) are -conjugated polymers with ionic solubilizing side groups, such as sulfonate (-SO3 -), carboxylate (CO2 -), phosphate (PO3 2-) and alkyl ammonium (NR3 +).53 Some examples of these materials are show n in Figure 1-13. CPEs retain the intrinsic electronic and optical prope rties of their organic -conjugated backbones; in addition, their charged side chains integrate water solubility/ hydrophilicity to the polymer s. Because of this

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30 unique combination, CPEs feature a variety of usef ul material properties. First, CPEs can be processed into films from water or other polar solvents, such as methanol, which is much more environment-friendly compared with the proc essing of conjugated polymers from organic solvents. Second, CPEs interact strongly with other ionic species,18 such as metal ions, anions, polyelectrolytes, proteins and DNA, which provides a useful pl atform for developments of chemoand bio-sensors. Third, due to their in trinsic amphiphilicity, CP Es could self-assemble into supramolecular assemblies such as colloids and polyelectrolyte laye r-by-layer films, which could provide functional materials with internal molecular order. Figure 1-13. Molecular struct ures of some common CPEs. Amplified Quenching of Conjugated Polyelectrolytes Stern-Volmer (SV) Quenching Much of the excitement regarding the pr operties and applications of CPEs is associated with the observation of the amplified quenching effect, i.e. very efficient fluorescence quenching at low concentration of the quenching species. Be fore discussing this phenomenon, we provide a brief overview of fluorescence quenching and standa rd mechanisms used to explain the effect.

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31 Fluorescence quenching can occur by two limiting mechanisms, dynamic (eq. 1-1) and static (eq. 1-2).54 In eqs. 1a and 1b, F* is an exc ited-state fluorophore, Q is a quencher, kq is the bimolecular quenching rate constant and Ka is the association constant for formation of the ground state complex [F,Q]. Treatment of fluorescence intensit y quenching data according to th e Stern-Volmer (SV) equation yields, 0 SVI 1 K[Q] I (1-3) where I and I0 are the fluorescence intensity with and without Q, respectively, and KSV is the SV quenching constant. In the limit where quenching is dominated by the dynamic pathway (eq. 11), KSV = kqo, where o is the fluorescence lifetime of F *, whereas in the limit where static quenching dominates, KSV = Ka. There are several ways to distinguish betw een dynamic and static quenching. First, note that kq cannot exceed the diffusion rate constant (ca. 1010 M-1s-1) when quenching is fully dynamic. Since KSV = kqo, for a fluorophore that has a 1 ns life time this places an upper limit on KSV 10 M-1 for dynamic quenching (i.e., 1010 M-1s-1 x 10-9 s = 10 M-1). Thus, static quenching is likely to be important if the experimentally observed KSV is significantly greater than 10 M-1. Another method for distinguishing when static quen ching is important is to compare the ratio of emission lifetimes ( o/ ) and emission intensities (Io/I ) vs. quencher concentration. If the

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32 KSV obtained from intensity quenching is greater th an that from lifetime quenching, then static quenching is occurring. When the quenching is dominated by either a purely static or dynamic pathway, the quenching behavior follows eq. 1-2 and consequently SV plots of I0/I vs. [Q] are linear. However, in many situations (as shown belo w) the SV plots are curved upward (i.e., superlinear). Superlinear SV plots can arise from a variety of processes, including mixed static and dynamic quenching, variation in the association constant w ith quencher concentration, and chromophore (or polymer) aggregation. Molecular Wire Effect The concept of amplified fluorescence quenching in conjugated polymers was first described by Swager and co-workers in 1995.55,56 Although their work was focused on neutral, organic soluble poly(arylene ethynylene)s, the concept is also very important for the development and applications of conjugated polyelectrolytes. Figure 1-14. Molecular structures of the CPs studied by Swagers group.

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33 In the work, a series of organic soluble PPE s with a cylcophane receptor on each repeating unit were synthesized using Pd (0)/CuI cataly zed Sonogashira coupling as shown in Figure 114. The polymers have the same chemical stru cture but different molecular weights; the number-average molecular weights for 6, 7, 8 are 31 kD, 65 kD and 122 kD respectively with a similar polydispersity (PDI = 2.0). Figure 1-15. Stern-Volmer plots of Swagers polymers 6 7 8 with MV2+. Figure was taken from Zhou et al .55 Because N,N-dimethyl-4,4-bipyridinum (PQ++ or MV2+) could bind to the cyclophane unit, the fluorescence of the polymer is quenched by MV2+ via the electron transfer mechanism. As shown in Figure 1-15, linear Stern-Volm er (SV) relationships are observed and Ksv values in the order of 105 M-1 are obtained for all the polymers. By comparing intensity quenching and lifetime quenching, a static quenching mechanism was confirmed in these systems. Compared with the quenching by MV2+ of the model compound 2 polymers 6-8 show greatly enhanced Ksv values (66-fold increase relative to 2 at most), which suggests that there is an amplified response 0 50 100 150 200 250 [PQ++] 106 M-1 9 8 7 6 5 4 3 2 1 2 Ks = 1, 600 M-1 6 Ks = 75, 000 M-1 7 Ks = 101, 000 M-1 8 Ks = 105, 000 M-1 F0/F

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34 for the polymers. Even more, the amplif ication factor, defined as the ratio of Ksv for the polymer divided by Ksv for the model compound, is also shown to be dependent on the chain length of the polymers. When the molecular weight of the polymer is increased from 31 kD ( 6 ) to 65 kD ( 7 ), there is an additional 1.4-fold enhancement of Ksv value. Although further increase of the molecular weight ( 8 Mn = 122 kD) results in a Ksv value close to that of 7 it still demonstrates that the amplified response is an inhere nt property of the conjugated polymers. Figure 1-16. Schematic illustration of the molecular wire effect expressed by conjugated polymers. Figure was taken from Zhou et al .55 The amplification of quenchi ng efficiency by the polymer ( 6 7 and 8 ) compared with the model compound ( 2 ) is ascribed to the extended electr onic communication a nd transport by the conjugated polymer chain,56 as shown in Figure 1-16. Upon the absorption of light, an exciton (bound electron-hole) is generate d randomly and migrates along the polymer backbone. At the time when the exciton reaches a viologen occupied receptor, it is then quenched. Due to the extremely rapid exciton diffusion in the polymer ex cited state (molecular wi re effect), a single MV2+ bound to the receptor site can quench many repeat units in the polymer chain. Thus the response of the polymer to the target (here, th e viologen molecules) is amplified. Based on the + + eeET Quencher h receptor analyte Molecular Wire Receptor Assembly

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35 PPV-SO3 PPV-SO3 -/MV2+ Abs Flu concept, successful sensors for nitroaroma tics have been developed by Swagers group,19 which led to the commercialization of explosive de tectors, Fido by the ICx Technologies, Inc. Fluorescence Quenching in Conjugated Polyelectrolytes The amplified quenching effect in CPEs was first observed by Chen and Whitten in their study of quenching of th e fluorescence of PPV-SO3 (structure shown in Figure 1-13) by MV2+.57 This work has stimulated intensive investigatio ns concerning CPE fluorescence quenching with a variety of organic, inorgani c analytes and biomolecules.18,58,59 Figure 1-17. Absorption (left) and fluorescence (right) of PPV-SO3 in water with and without 100 nM MV2+. Figure was taken From Chen et al .57 As shown in Figure 1-17, in the initial study of PPV-SO3 -, the addition of 100 nM of MV2+ to a solution of the polymer (c 10-5 M in polymer repeat units) results in a very efficient quenching of the polymers fluorescence and a distin ct red-shift in the absorption spectrum. The SV plot for quenching of the PPV-SO3 fluorescence by MV2+ is linear at very low concentrations of MV2+ (0 1 M), with KSV 2 107 M-1. Compared with the quenching constants of stilbene in sodium lauryl sulfate micelles and in dilute solutions, it is four orders of

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36 300400500 Absorbance 0.0 0.2 0.4 0.6 0.8 1.0 W a v e l e n g t h / n m 450550650 Emission Intensity 0.0 0.4 0.8 1.2 1.6 2.0 H2O increase H 2 O increase magnitude and six orders of magnitude greater, respectively. Such a million-fold amplification of Ksv value is attributed to two effects: the i on-pair complex formation between the oppositely charged polymer and quencher and the ultrafast exciton migration along the conjugated polymer chain (molecular wire effect).57 The mechanism of fluorescence quenching in CPEs has been intensively studied by many other research gr oups in the following years. The quenching constant ( Ksv) is shown to be dependent on various factors, such as the polymer concentration,60 quencher properties (charge,61 hydrophobocity62 and size63), solution properties (pH,64,65 ionic strength65,66 and buffer66), and additives.67-70 Aggregation of CPEs As pointed out earlier, CPEs are inherently amphiphilic materials and their photophysical properties are strongly in fluenced by the tendency of the materi als to self-assemble In a series of work, Schanze et al. have carefully studied the aggregation of PPE-SO3 and its effect on fluorescence quenching by small molecules via electron or energy transfer mechanism.71,72 Figure 1-18. Absorption (left) and fluorescence (right) of PPE-SO3 in methanol, methanol:water (50:50) and water. Arrows show the di rection of change with increasing water content. Figure was taken from Tan et al .71

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37 As shown in Figure 1-18, as the amount of wa ter in the solvent increases, the absorption and fluorescence of PPE-SO3 undergo a red-shift. The most pronounced changes are seen in the fluorescence. In methanol the fluorescence appear s as a sharp, narrow band that has a very small Stokes shift relative to the absorption, whereas in water the fluorescence appears as a very broad and red-shifted band. The struct ured emission seen for PPE-SO3 in methanol is very similar to that which has been reported fo r structurally -related, neutral PPEs in d ilute solutions of good organic solvents such as CHCl3 or THF, where the polymer chains surely exist in an unaggregated monomeric state.73 Thus, it is concluded th at in methanol the PPE-SO3chains are well solvated and the material exists in a m onomeric state. Consequen tly in this solvent the fluorescence is dominated by excitons that are confin ed to single chains. By contrast, in aqueous solution it is believed that PPE-SO3 is strongly aggregated, an d the fluorescence emission is dominated by excitons that are trapped in aggreg ate states arising from interactions between two or more PPE chains in the aggreg ate. It is important to note that the aggregate emission observed for PPE-SO3 in water is very similar to that observed from excimers (excited -dimers) of small aromatic fluorophores such as naphthalene or pyrene.74 From this they concluded that a dominant structural feature in the aggregates must be stacking between two or more polymer chains. Most importantly, when the quenching of PPE-SO3 -with MV2+ was studied in methanol and water, a larger Ksv was observed in water, which indicated that the fluorescence quenching was further amplified in the aggregate. Bazan and co-workers75 also studied the solvent-depende nt aggregation of cationic watersoluble poly(fluorene) and found th at the aggregate structure of the polymer has a strong effect on the efficiency of fluorescence resonance energy transfer from the polymer to the dye-labeled DNA. In another publication,76 they found that for a copolymer consisting of fluorene and 2,1,3-

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38 benzothiadiazole (BTD) segments, efficient energy transfer from the fluorene units to the lower energy BT sites occurs when the polymer is a ggregated by changing the concentration or adding ssDNA. Applications of Conjugated Polyelectrolytes Optical Biosensors One of the most exciting applications of conjugated polyelectrolytes is their biological sensing in aqueous environments for many analytes,18 including small biomolecules, nucleic acids and various proteins. Because of th e signal amplification imparted by conjugated backbone of these polymers, biosensors with ex traordinary sensitivity ar e obtained. Typically the detection limits for CPE-based biosensors are in the nanomolar concentration range58 and in a few cases, sensors that can detect the target an alytes at zeptomole le vel were also reported.77 To fabricate these sensors, different formats of conjugated polyelectrolytes have been used, including homogenous aqueous solutions,78 glass slide supported CPEs79 and particle supported CPEs.58 Regardless of the sensing targets and th e formats of the fluorescent CPEs, there are three general sensing mechanisms that have been employed to build the sensors, i.e., quenchingunquenching mechanism, chain conformation perturbation mechanism and fluorescence resonance energy transfer mechanism. Quenching-unquenching mechanism Sensors that are built using the quenchingunquenching strategy take advantage of the superquenching property of CPEs by electron or energy accepting quenchers.58 By connecting such a quencher to biologically interesting ligan ds, a fluorescence response is produced when the ligands bind to their specific targets. Chen and Whitten reported the first example of a CPEbased biosensor using this strategy.57 They covalently linked MV2+ via a flexible tether chain to biotin, a small ligand that binds specifically to avidin. Initia lly such a quencher-tether-ligand

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39 (QTL) system is mixed w ith a solution of PPV-SO3 (structure shown in Figure 1-13). Because of the superquenching effect, the polymers fluor escence is substantially quenched at very low concentration of the QTL system. The fluores cence is recovered by adding small amounts of avidin, which disrupts the asso ciation between the polymer a nd the QTL system through binding to the biotin unit, as shown in Figure 1-19. Us ing this strategy, Whitte n and co-workers have developed a sensor platform based on CPE-co ated polystyrene microspheres for detecting enzymatic activity and DNA hybridization.58 Figure 1-19. Biosenser applicatio ns of CPEs by the quencher-tet hered-ligand (QTL) approach. Figure was taken from Chen et al .57 Chain conformation perturbation mechanism CPE-based sensors that rely on signal tran sduction by the conforma tional change of the polymer have mainly focused on wate r-soluble poly(thiophene) derivatives.59,80 Figure 1-20. Molecular structures of some poly(thiophene) derivatives.

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40 Poly(thiophene)s with pendant carboxylic ac id groups (as an example, PT1, Figure 1-20) show interesting ionchromatic sensing in wa ter, which is ascribed to the ion-induced conformation change of polymer backbone.81 Leclerc pioneered the application of a cationic regioregular poly(thiophene) (PT2, Fi gure 1-20) in the detection of DNA.59,82 Figure 1-21. Schematic illustration of the formation of polythiophene/ss-DNA duplex and polythiophene/ds-DNA triplex. Figure was taken from Ho et al .59 As shown in Figure 1-21, before the addition of oligonucleotide, PT 1 exists as a randomcoil polyelectrolyte; upon the addition of the single-strande d oligonucleotide (ssDNA), an electrostatic complex, called duplex, is formed between PT1 and ssDNA, in which PT1 takes a more planar conformation; a he lical conformation of PT1 results when a complementary ssDNA is added to the duplex solution. Correspondi ng to the conformational change, PT1 shows completely different absorption and fluorescence spectra. By monitoring either the absorption spectra or fluorescence spectra, oligonucleotide hybridization can be detected with a high sensitivity (10-14 M) and oligonucleotides with one mismatch can be discriminated from the perfect complementary oligonucleotide. More recently, Nilsson and co-workers synthesized a

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41 new poly(thiophene) with zwitterionic side groups (PT3) and studied its interaction with a wide variety of biomolecules.83,84 Fluorescence resonance energy transfer (FRET) mechanism Another method for DNA detection using CPEs was developed by Bazan, Heeger and coworkers.78,85 Their method is based on the amplified FRET from a CPE to a signaling chromophore attached to a probe oligonucleotide. Cationic CPEs based on polyfluorene (Figure 1-22) were synthesized and utilized in these sensors. These CPEs exhibit relatively high fluorescence quantum efficiencies in aqueous environment compared to other CPEs, which is very important for increased sensitivity. Figure 1-22. Molecular structures of CPEs based on polyfluorene (PF) reported by Bazan and coworkers.78 Figure 1-23 is a schematic representation of a sensing system based on the FRET from PF1 to a dye-labeled oligonucleotide probe.85 In the system, a peptide nucleic acid (PNA) which can hybridize with ssDNA is labeled at 5 -end with fluorescein and used as the probe. Since PNA is neutral, there is no electrostatic interaction between PNA and PF1. Thus the average distance between PNA and PF1 is too large for effectiv e FRET. Upon the addi tion of complementary ssDNA to the PNA probe, a strong PNA/ss-DNA complex is formed with a net negative charge, which enables the binding of the complex to PF1. The reduced distance between PNA and PF1 allows for the efficient FRET to take place betw een the polymer and the fluorescein. Thus the enhance emission from fluorescein signals the presence of target ssDNA. For

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42 noncomplementary ssDNA, because the hybridization with PNA does not occur, there is still no electrostatic interaction between PNA and PF1, and the distance between them remains too large for FRET. Following the strategy, a hom ogenous three-color DNA-sensing array76 that utilized PF2 was later developed and a solid-stat e assay was also fabricated using PF3.79 Figure 1-23. Schematic illustration of the P NA/PF1 assay for ss-DNA detection. Figure was taken from Gaylord et al .85 Dye-sensitized Solar Cells (DSCs) Nanocrystalline TiO2-based dye-sensitized solar cells invented by Grzel in 1991 have attracted much attention and are promisi ng for large-scale technological development.86 Figure 1-19 shows a schematic representation of the operating principle of such a device.87 On the top of a thin film of fl uorine-doped tin oxide supported by gl ass or polymer, a mesoporous layer of TiO2 (typically 5 10 m thick) is placed. A monolayer of sensitizer is attached to the surface of the nanocrystalline f ilm. After photoexcitation, elec trons are injected to the conduction band of the semiconductor from the sensit izers excited state. The sensitizer is

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43 restored to its ground state by electron donation fr om the mediator, usually a redox couple, such as iodide/triiodide. The reduc ing species is then regenerate d by reduction of the oxidizing species at the electrode. Overall, the cell conve rts light to electric power without chemical transformation. Figure 1-24. Schematic representa tion of the principle of dye-sens itized solar cells (DSCs) based on nanocrystalline TiO2. Figure was take n from Hagfeldt.87 The highest light conve rsion efficiency ( ) of DSCs till today is 11%, which was obtained using ruthenium (II) complex as the sensitizer.88 Because of the high co st of transition metal complexes and the long-time availability of nobl e metal materials, DSCs based on metal-free organic dyes have also been pursued.89 To date, values of 6% 9% using organic dyes, such as coumarin derivatives, indoline, etc. under so lar simulator (AM 1.5) have been attained.89 Conjugated polymers have been widely used in polymer-based solar cells. Bulk heterojuction solar cells have b een prepared from CPs and differ ent electron-accepting materials such as C60 derivatives, CdSe and TiO2 nanparticles.15,90 However, conjugated polymer-based

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44 DSCs have received much less attention in the past years. Given the tunable band gaps, high molar absorption coefficients and multiple binding sites to the oxide of conjugated polymers, it is possible that CPs could be valuab le alternatives for ruthenium complexes in DSCs. Figure 1-25 shows some CPs with carboxylic ac id functionalities that have been employed in DSCs without ruthenium complexes. In 2003, Kumar and co-workers91 reported DSCs that were based on carboxylated polythiophenes (PTAA or H-PURET) with a power conversion efficiency of ~ 1.5%. Using the same polymer, PTAA, Yangida et al92 showed that the power conversion efficiency could be doubled by simply adding an electron-donating ioni c liquid to the liquid electrolyte (0.5 M LiI/0.05 M I2). Recently, a collaborated work from Schanze and Reynolds labs demonstrated the concept of spectral br oadening to enhance the performance of polymer based DSCs using CPs.93 In the work, two CPs (PT-CO2H and PPE-CO2H), which absorb at the red and blue regions of the solar spectrum resp ectively, were mixed and co-adsorbed to the nanocrystalline TiO2 films. Unoptimized DSCs using these f ilms show a light conversion efficiency of 1.5%, which is among the hi ghest for CP based DSCs. Figure 1-25. Molecular structures of some CPs with carboxylic acid functionalities that were used in DSCs. Scope of the Present Study The aims of the present study are to design and synthesize poly(aryl ene ethynylene)s with different chemical structures, investigate thei r photophysical properties in solution and develop their applications in the areas of sensors and solar cells.

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45 In Chapter 2, to further confirm that meta -linked poly(phenylene ethynylene)s carrying ionic side chains self-assemble into a helical conformation in water, a polymer featuring chiral ionic side groups derived from L -alanine was designed and synt hesized. The self-assembly behavior of the polymer in solution was st udied using absorption, emission and circular dichroism spectroscopy. Due to the strong anal ogy between our polymer and DNA, interaction of the polymer helix with some metalloand organic dyes were also studied in the aqueous solution. In Chapter 3, two poly(phenylene ethynylen e)s with different carboxylate substitution patterns were synthesized via the precursor rout e. The photophysical pr operties of the new polymers were investigated and compared to each other, as well as to those of the perviously studied PPE-SO3 -. Moreover, the carboxylate groups on the polymer chain provide receptor sites for metal ions, which lead to the possibility of tuning the polymer fluores cence with metal ions. Interactions of the polymer with different me tal ions were studied by absorption and emission spectroscopy in methanol, as well as in (2-hyd roxyethyl)-1-piperaziethanesulfonic acid (HEPES) buffer solution. Quenching of the polymer fluorescence with MV2+ was studied in methanol in the presence of different amounts of Ca2+. And based on these studi es, a highly selective and sensitive sensor for pyrophosphate was developed. In Chapter 4, to further understand the am plified fluorescence quenching in conjugated polyelectrolyte system, poly(phe nylene ethynylene)s with 5 di fferent chain lengths were synthesized. By controlling the loading ratio of monofunctional monomer in the initial mixture of AA and BB type monomers, the molecular weight of the neutral polymer was systematically varied. The water soluble polymers were subs equently obtained by hydr olyzing these organic PPEs containing dodecyl carboxylic esters. The dependence of the basic photophysical

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46 properties, such as molar extinction coefficient, absorption maximum, was studied in methanol and water. Also, the fluorescence quenching with different quenchers was studied in methanol. The relationship between the Stern-Volmer c onstant and chain length was discussed. In Chapter 5, poly(arylene ethynylene)s with variable band gaps were synthesized by copolymerizing diiodoarenes and ethynylated hete roarenes. The photophysical properties of the PAE series with carboxylate groups were carefully studied in diffe rent solvents. And aggregation was found to have a detrimental effect on the em ission of red-emitting polymers. To prevent the aggregation-induced fluorescen ce quenching, two dendritic ioni c side groups were designed. Photophysical properties of these PAEs containi ng dendritic solubilizing groups were also studied in aqueous solution at va rious conditions. Also, the applic ation of these PAEs containing carboxylate groups in the conventio nal Grzel-type sola r cell without ruthenium complexes was explored.

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47 CHAPTER 2 SELF-ASSEMBLY OF META -LINKED POLY(PHENYLENE ETHYNYLENE) Introduction Soon after the self-ass embly properties of meta -linked phenylene ethynylene oligomers were revealed,49,50,94 Tew95 and our group96 independently synthesized and studied meta -linked poly(phenylene ethynylene)s (PPE s) carrying ionic side groups In a series of studies,97-100 Tew and co-workers synthesized a group of meta -linked PPEs functionalized with various non-polar alkyl and ammonium ionic side chai ns (for an example, see Figure 2-1). It was shown that these amphiphilic PPEs could self-assemble into either ordered bilayers or helical conformations in aqueous solution dependent on the substitution pattern s. In the meantime, we have synthesized two meta -linked PPEs with sulfonate a nd carbonate side chains, as shown in Figure 2-1. Studies of their optical properties (abs orption and fluorescence) in meth anol and water suggested that these polymers exist in a random-coil conforma tion in methanol and fold into a helical conformation in water. It was also shown that common DNA intercalators, such as 9aminomethylanthracene and [Ru(bpy)2(dppz)]2+, could interact with th e helical conformation of these polymers in the same manner as with DNA.96 Figure 2-1. Structures of meta -linked PPE reported in the literature. One of the most useful techniques to study the chiral structures is ci rcular dichroism (CD) spectroscopy. The appearance of a CD signal requires e ither an enantiomeric excess of chiral molecules or the existence of a chiral environmen t. When an achiral ol igomer or polymer folds

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48 into a helical structure, a racemic mixture of the rightand left-handed forms (the P and M forms) would be expected. Therefore, no CD signal can be observed. In order to probe the helical structure using CD spectroscopy, chir ality induction is required. To produce an enantiomeric excess in the helical conformation of meta -linked oligomers, Moore and coworkers used several different strategies, in cluding incorporation of chiral side chains,101 chiral binaphthylene groups in the oligomer backbone,102 and host-guest binding of (-)-pinene.103 Arnt and Tew also observed a weak CD signal when they mixed m PPE-NH3 + (Figure 2-1) with chiral D-mandelic acid, suggesting an excess of the helix with one-handedness results.98 In the work described in the present ch apter, we designed and synthesized a new meta linked PPE featuring chiral and optically active ionic side gro ups based on L-alanine. The spectroscopic properties in different solvent syst ems were studied and th e results demonstrated that the polymer undergoes a simila r solvent driven folding like m PPE-SO3 -. Circular dichroism spectroscopy provided direct evid ence for the helix formation. Also, due to the similarity between our helical polymer and DNA, the intera ction of the polymer with a well-known DNA intercalator, [Ru(bpy)2(dppz)]2+, was studied. And most in terestingly, induced helical aggregation of cyanine dyes, such as 1,1 ,3,3,3 ,3 -hexamethylindodicarbocyanine iodide (HMIDC) and 3,3 -diethylthiadicarbocyanine (DiSC2(5)) via groove binding to the helical polymer in water was also observed. Results and Discussion Synthesis Figure 2-2 illustrates the synthe tic scheme for the monomers. The synthesis started with commercially available 3,5-diaminobenzoic acid 1 which was converted to 3,5-diiodobenzoic acid 2 in 65% yield through the in-situ formation of diazonium salt and subsequent reaction with

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49 KI (Sandmeyer reaction).104 Compound 2 was then reacted with benzyl ester protected L-alanine using carbodiimide-based condensation105 to give compound 3 in a yield of 67%. The latter was then subjected to Sonogashira reac tion with 2 eq. of trimethylsily acetylene (TMSA) to give the protected acetylene derivative 4 in 74% yield. After desilylation with TBAF in THF, the desired monomer 5 was obtained in 86% yield. In another route, tert -butyl substituted 3,5diiodobenzamide 7 was prepared via a two step process from 2 which was converted to 3,5diiodobenzoyl chloride using thionyl chloride and isolated as yellow crystal in 77% yield. The latter was then reacted with tert -butyl protected L-alanine in the presence of 2 eq. of 4dimethyaminopyridine in dry methylene chloride to give compound 7 in an excellent 94% yield. Figure 2-2. Synthesis of monomers. The synthesis of the model compound that co ntains the base chromophore resembling the repeat unit structure of the polymer is shown in Figure 2-3. Starting from either the benzyl ( 3 ) or tert -butyl ( 7 ) protected monomers, Sonogashira coupling with phenylacetylene gave the desired products in excellent yields. Th en the benzyl ester was cleaved using 1 N NaOH in DMF to give

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50 the water soluble model compound 9 in 90% yield; while the tertbutyl group was removed using a 1:1 mixture of trifluoroacetic acid (TFA)/CH2Cl2, further treatment with saturated aqueous Na2CO3 afforded the same compound in 92% yield. Figure 2-3. Synthesis of the model compound 9 The polymerization was first carried out between monomer 3 (AA) and monomer 5 (BB) to prepare the meta -linked homopolymer P1 (Figure 2-4). Unfortunately even after optimizing the reaction solvents, temperatur e, amount of catalysts, the poly merization only led to a mixture of oligomeric species with an average length of 5 determined by gel permeation chromatography (GPC). Considering the mech anism of Sonogashira coupling,24 we suspected that the presence of substituted side chains on each repeat unit in the oligomeric species during the reaction process raises the energy barrier for the trans/cis isomerization before the reductive elimination and thus prevents the efficient chain growth to give high molecular weight polymer. In response to the assumption, we carried out the polymerization between 3 (benzyl protected) and unsubstituted 1,4-diethynylbenzene ( 12 ), as well as the polymerization between 7 ( tert -butyl protected) and 1,4-diethylbenzen e. The latter compound was pr epared from commercially available 1,4-diiodobenzene following the procedure reported in the literature.106

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51 Figure 2-4. Synthesis of the homopolymer. Figure 2-5. Synthesis of co-polymers. Under the same conditions, both polymerizations afforded the alternating meta/ paraphenylene copolymers with considerably higher mol ecular weight than that of the homopolymer P1 as illustrated in Figure 2-5. The number-average molecular weight (Mn) of the polymer carrying benzyl protected ester is 12 kD, corresp onding to 31 repeat units. Simply changing the protecting group from benzyl to tert -butyl, a polymer with av erage 106 repeat units (Mn = 39 kD) was obtained. Such an increase of molecular wei ght is most possibly due to the better solvation of the polymer with t -butyl side chains in THF. Beside s THF, both polymers were found to be also soluble in chlorinated solvents such as chloroform and methylene chloride. The organic polymers ( P2 and P3 ) were hydrolyzed using the conditions which were used to hydrolyze the model compounds ( 8a and 8b ), respectively. After the hydrolysis both polymers were isolated as yellow solids and could be dissolved in wa ter and other polar solv ents such as DMF and

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52 DMSO. Final purification of the water-soluble polymers was done by dialysis of the aqueous solutions of the polymers ag ainst ultrapure water (18.2 M ) using a 12-14 kD molecular weight cut-off (MWCO) cellulose membrane. Model Compound (9) Figure 2-6 shows the absorp tion (a) and emission (b) sp ectra of the model compound 9 (Figure 2-3) in methanol (MeOH) and aqueous solutions, respectivel y. In general, the spectra do not vary much from methanol to aqueous soluti on. The absorption spectrum is characterized by two bands arising from the different vibronic mo des of planar and twis ted backbone structures, respectively. The absorption at 300 nm is assigned to the transition of the planar structure due to its longer conjugati on length. The emission feat ures a strong band with max = 352 nm for MeOH solution and max = 361 nm for aqueous solution. The 9 nm red-shift of the emission maximum might be due to the change of polarity from methanol to water. 8 6 4 2 0 Intensity 550 500 450 400 350 Wavelength (nm) (b) 0.4 0.3 0.2 0.1 0.0 Absorbance 350 325 300 275 250 Wavelength (nm) (a) Figure 2-6. Absorption (a) and emission (b) spectra of model compound 9 in methanol () and water (---). [ 9 ] = 10 M.

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53 Solvatochromic Properties of Meta -Linked PPEs UV-Vis absorption spectroscopy Despite the difference in the molecular weight between wP2 and wP3 they exhibit similar photophysical properties. In the following text, we will only present the results for wP2 Figure 2-7 illustrates the UV region absorption spectra of wP2 in MeOH, in water, and in mixtures of these two solvents at various compos itions. First, the absorption of the polymer in MeOH is characterized by a broad maximum at max 326 nm and a second band that appears as a shoulder at 350 nm. These bands have been prev iously assigned as arising from the cisoid and transoid conformations of the m PPE backbone.50,96 Interestingly, as the amount of water in the solvent mixture increases, the 328 nm absorp tion band blue-shifts (< 10 nm) and decreases in intensity (hypochromic effect), whereas the 350 nm shoulder red-shifts (< 5 nm) and decreases in intensity. The relative change in the A350/A328 absorption ratio indicat es a high population of cisoid conformation in the polymer helix (Figur e 2-7, inset). The spectral changes for wP2 that are induced by the change of solv ent from methanol to water are very similar, but slightly attenuated, relative to those obs erved for the sulfonate-substituted meta -linked PPE polymer ( m PPE-SO3 -).96 By analogy to the previously studied system, the change in absorption that accompanies an increase in the volume fraction of wa ter in the solvent is believed to arise due to an increase in the fraction of the polymer which exists in the helical conformation. Specifically, the absorption spectrum of wP2 exhibits a hypochromic effect with increasing water in the solvent; this effect is likely caused by -stacking of the aromatic ch romophores in the folded helical conformation. The solven t effects on the absorption of wP2 are attenuated relative to those reported previously for m PPE-SO3 -,96 presumably because in wP2 a significant fraction of the polymer exists in a helical conf ormation even in pure MeOH solution ( vide infra ).

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54 0.6 0.4 0.2 0.0 Absorbance 450 400 350 300 250 Wavelength (nm) 0.64 0.60 0.56 A350/A326 1.0 0.8 0.6 0.4 0.2 0.0 Volume% water in methanol Figure 2-7. UV-Visi ble absorption of w -P2 (c = 10 M) in methanol, water and methanol/water mixtures. The arrow shows the direction of change with increasing volume % water. Inset shows the ratio of intensities of UV maxima at 350 nm and 326 nm (A350/A328) versus percent of water in MeOH (by volume). Steady state fluorescence spectroscopy Fluorescence spectra (Figure 2-8) of w -P2 obtained in MeOH, water and MeOH/water mixtures provide additional information regard ing the influence of solvent composition on the polymers conformation. First, in pure MeOH, the fluorescence of wP2 is dominated by a structured band with a 0-0 transition at max = 353 nm with weaker vibronic bands in the 360 400 nm region. In addition, the polymer also e xhibits a weak and very broad emission band centered at 500 nm. The structured near-UV emi ssion is believed to emanate from the random coil conformation of the polymer, whereas the broad, structureless excimer like visible emission band likely arises from the folded, helical conformation of w -P2 .96 The fact that the structured emission dominates in MeOH so lution, suggests that in this solvent w -P2 exists

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55 predominantly in the random coil conformation in this medium; however, the appearance of the broad excimer like emission indicates th at even in MeOH a small fraction of w -P2 is folded so that stacking of adjacent chains is possible. 12 8 4 0 I353/I500 1.0 0.8 0.6 0.4 0.2 0.0 Volume% water in methanol 1.5 1.0 0.5 0.0 Intensity 600 550 500 450 400 350 Wavelength (nm) Figure 2-8. Fluorescence spectra ( exc = 320 nm) of w -P2 (c = 10 M polymer repeat units) in MeOH, water and MeOH/water mixtures. Th e arrow shows the di rection of change with increasing volume % water. The spectra were obtained under the same conditions, so the spectral intensities refl ect the approximate change in fluorescence quantum efficiency with solv ent composition. Inset shows th e ratio of intensities of emission at 363 nm and 500 nm (A363/A500) versus percent of water in MeOH (by volume). Interestingly, as the volume fr action of water in the solvent increases, the intensity of the structured near-UV emission attributed to the random coil conformation d ecreases significantly (Figure 2-8 inset). For solutions that contain 60% water, the structured emission is completely quenched, and the only emission observed is the broad, visible region fl uorescence. This observation is consistent with the conclusions based on the solvent-induced shift in the absorption spectrum, i.e., in MeOH w -P2 exists mainly in a random coil conformation, whereas

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56 in solvent mixtures that contain 60% water the polymer is mostly folded into the helical conformation. Circular dichroism spectroscopy The alanine-derived side group in w -P2 is chiral and optically active, and we anticipated that its presence would induce an enantiomeri c excess in the helical conformation of the m PPE backbone. In order to probe this effect, CD sp ectroscopy was used to study the optical activity of w -P2 and the model compound 9 in solutions of methanol, water and methanol/water mixtures. Figure 2-9 shows the CD spectra of w -P2 in a series of solvent mixtures along with the spectrum of 9 in water. First, it is quite evident that the model compound 9 is CD inactive. This finding is not surprisi ng in view of the fact that the conjugated chromophore in w -P2 is inherently achiral. It clearly shows that the perturbation provided by the chiral center in the alanine side group alone is in sufficient to induce a measurab le chiroptical effect on the conjugated chromophore. By contrast to the model, w -P2 exhibits a strong bisignate CD spectrum in methanol, water as well as in mixtures of the tw o solvents. Since the model compound 9 is CD-inactive, the bisignate CD signal observed for w -P2 clearly arises because the conjugated m PPE backbone of the polymer is in a chiral conformation (i.e., a helix) and the chiral side group induces an enantiomeric excess in one of the helical conformers. The longer wavelength Cotton effect is negative ( 368 nm), whereas the short wa velength couplet is positive ( 325 nm). This relationship is termed negative chirality, and suggests that the left-handed helical conformation (i.e., an M-helix) of the m PPE backbone is in excess.107

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57 -4 -3 -2 -1 [ ] 365 1.0 0.8 0.6 0.4 0.2 0.0 Volume % water in methanol -6 -4 -2 0 2 4 6[ ] (105 deg cm2 dmol-1) 400 375 350 325 300 Wavelength (nm) Figure 2-9. Circular dichroism spectra of model compound 9 in water (c = 15 M, solid black line) and w -P2 (c = 15 M polymer repeat units) in methanol, water and in methanol/water mixtures. Arrows show the direction of change with increasing volume % water. Interestingly, the amplitude of the CD signal for w -P2 varies with solvent composition. In particular, the spectrum is relatively weak in Me OH, and it increases approximately 8-fold with increasing water content, reaching a maximum at 60 volume % water. Further addition of water results in a slight decrease ( ca. 20%) in the amplitude of the CD si gnal. First, the observation of a CD signal for w -P2 in MeOH indicates that in this solvent the polymer adopts a helical conformation to some extent. This fact is cons istent with the fluoresce nce of the polymer in MeOH, where the excimer-like emi ssion attributed to the helical conformation is observed. The fact that w -P2 exists in part in a helical conformati on in pure MeOH contrast s with the behavior of m PPE-SO3 -,96 which on the basis of fluorescence and absorption spectroscopy is believed to

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58 exist exclusively in a random coil conforma tion in MeOH. The helical conformation in w -P2 may be stabilized in MeOH by hydrogen bond forma tion between proximal amide units in the helix. The increase in CD intensity with increasi ng water content in the solvent indicates that the hydrophobic effect is important in inducing the po lymer to adopt the helical conformation to a greater extent. The loss in CD intensity for vol ume fractions of water > 60% may signal a loss of helical conformation, or more likely it is due to a change in the structure of the helix (e.g., a decrease in the helical pitch) which leads to a change in the strength of the exciton coupling along the meta -linked -system. Similar loss of helicity has been observed by Swager in the aggregate of conjugated polymer s carrying chiral side groups.108 Guest Binding with Meta -Linked PPEs Given the qualitative similarity between the secondary structure of the helical conformation of anionic, m PPEs and double-helical DNA (i.e., bot h are helical polyanions which feature aromatic units that are -stacked along the helical axis), we have an interest in exploring whether cationic dyes will interact with m PPEs in a comparative manner. Interaction of dyes with DNA has been intensively employed to probe the DNA structure.109 There are three well-known binding m odes for DNA helix, i.e., electrostatic binding, intercalation and groove binding. Electrostatic bind ing is simply the association between DNA (negatively charged) and positively charged small molecules via electrostatic interaction. This is the most common way that DNA interacts with metal ions. Intercalation is a binding mode that small molecules insert their pl anar aromatic functionality between the base pair of DNA helix. Thus, th e binding is stabilized by stacking. Many organic dyes that contain aromatic or hetero-aromatic rings ha ve been proven to in teract with DNA through intercalation, such as ethidium bromide,110 acridine orange,111 (9-anthrylmethyl)ammonium

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59 chloride,112 etc.113,114 Metal complexes that contain fused aromatic ligands, such as dipyridophenazine (dppz),115,116 9,10-phenanthrenequinone diimine,117 etc.118 have also been widely studied as DNA intercalators. One well-known example is a ruthenium complex, [Ru(bpy)2(dppz)]2+, which becomes luminescent in water wh en intercalated in to double-helical DNA.119 Groove binding involves the assembly of guest molecules in the minor or major grooves of DNA. The binding is stabilized by a combination of many forces, such as hydrogen bonding, hydrophobic and electros tatic interactions. The most successful class of minor groove binding molecules is th e hairpin polyamides120 which mimic the structure of natural minor groove binder, distamycin.121 For organic dyes, Armitage and co-workers122-125 have shown that cationic cyanine dyes can assemble into the minor groove as dimers, which are further aligned in an end-to-end fashion to form helical aggregates. This work is important because it not only showed that cyanine dyes could be effective photocleavage agents for DNA but also present a good example of supramolecular helical assembly of achiral molecules. In a previous report96 we demonstrated that the complex [Ru(bpy)2(dppz)]2+ binds to the helical conformation of m PPE-SO3 via intercalati on of the dppz ligand into the -stack of the adjacent phenylene ethynylene units Intercalation of [Ru(bpy)2(dppz)]2+ within the polymer helix is signaled by the appearan ce of the strong orange-red phot oluminescence characteristic of the intercalated form of the complex. In the present investigation we have explored the interaction of wP2 with [Ru(bpy)2(dppz)]2+ and the cationic cyanine dye HMIDC (Fi gure 2-10) in order to determine if these dyes will interact with the polymer via inte rcalation and/or groove-bi nding, and if so, if the chiral environment of the polymer helix will give rise to a CD transition for the intercalated dye.

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60 Figure 2-10. Chemical struct ures of the organic dyes. Intercalation of [Ru(bpy)2(dppz)]2+ First, we examined the effect of addition of w -P2 on the photoluminescence of the cationic metallo-intercalator [Ru(bpy)2(dppz)]2+. As shown in Figure 2-11, in aqueous solution alone [Ru(bpy)2(dppz)]2+ (c = 15 M) does not display any photoluminescence. The intensity of the photoluminescence increases very sharply over the polymer concentration range 0 50 M, and then it increases more slow ly until it saturates at [ w -P2 ] 150 (concentrations in polymer repeat units). This stoichiometry suggests that the binding site size fo r the intercalated metal complex is approximately three w -P2 repeat units; moreover, since one turn of the helix requires ca 6 polymer repeat units, this corresponds to two metal complexes bound per turn of the helix (e.g., one on either side as shown in Figur e 2-12). Addition of polymer above the 50 M level leads to an additional, but less pronounced increase in the [Ru(bpy)2(dppz)]2+ photoluminescence. The additional increase in em ission intensity is likely due to a small increase in the number of bound complexes, but more importantly due to ti ghter intercalation of the complexes as they are able to bind less densely on the w -P2 helix, as illustrated in Figure 2-12.

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61 I640 200 150 100 50 0 [PRU]/ M 12 9 6 3 0 Intensity 800 700 600 500 Wavelength (nm) Figure 2-11. Emission spectra of [Ru(bpy)2(dppz)]2+ in the absence (solid black line) and presence of wP2 (concentration of wP2 ranges from 0 120 M polymer repeat units, excitation wavelength is 450 nm). Solutions are deoxygenated by argon bubbling. The inset illustrate s the variation of [Ru(bpy)2(dppz)]2+ emission intensity at 640 nm with increasing concentration of wP2 Figure 2-12. Schematic representa tion of the interaction between w -P2 and [Ru(bpy)2(dppz)]2+. An aqueous solution that contained 100 M w -P2 and 15 M [Ru(bpy)2(dppz)]2+ was explored by using circular dichroism spectro scopy. This solution exhibited the near-UV

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62 bisignate CD signal characteristic of the heli cal polymer, however, no CD was observed in the 400 500 nm region which corresponds to the me tal-to-ligand charge transfer absorption (MLCT) of the metal complex. This finding indi cates that the chiral environment provided by the helix is insufficient to induce a CD signal in the MLCT transition localized on the metal complex chromophore. Groove binding of cyanine dyes UV-Vis absorption In a previous research,126 we have shown that a para -linked CPE, PPE-SO3 could template the formation of J -aggregate of HMIDC when the polymer existed in the aggregate state. Here, we explored the effect of addition of meta -linked CPE on the optical spectroscopy of two cationic cyanine dyes HMIDC and DiSC2(5) (Figure 2-10). In general, similar results were obtained for both dyes, so only results for the former dye are presented herein. Data for DISC5(5) interacting with w -P2 is provided in the Appendix. In aqueous solution HMIDC features an absorption band due to the transition of the polymethine chromophore at max 635 nm. As shown in Figure 2-13, addition of w -P2 to the aqueous solution of HMIDC induces significant changes in the dyes absorption. In particular, as w -P2 is added, the 635 nm absorption decreases, and it is replaced by a redshifted absorption at 660 nm. The red-shift of the HMIDC absorption upon addition of w -P2 clearly signals that the dye binds to the polymer helix. A plot of the HMIDC absorbance at 660 nm as a function of the polymer:dye molar ratio reveals that dye binding is 80% co mplete at a 1:1 stoichoimetry a nd it is saturated at a ratio of ca. two w -P2 repeat units per HMIDC (Figure 2-13 inset). The nearly stoichiometric binding ratio suggests that the dye-polymer inte raction is driven by electrosta tics (ion-pairing), and supports the notion that HMIDC binds to the periphery of the w -P2 helix (i.e., groove binding).

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63 A660 2.0 1.5 1.0 0.5 0.0 [PRU]/[HMIDC] 1.2 0.9 0.6 0.3 0.0 Absorbance 800 700 600 500 Wavelength (nm) Figure 2-13. UV-Visible absorpti on of HMIDC in water (c = 5 M) titrated with w -P2 Concentration range of w -P2 is 0 5.0 M polymer repeat units, in 0.5 M increments. Arrows show direction of ch ange of spectrum with increasing polymer concentration. Fluorescence spectroscopy Addition of w -P2 to HMIDC also elicits changes in the dyes fluorescence. As shown in Figure 2-14, addition of w -P2 to an aqueous solution of HMIDC leads to a decrease in the fluorescence from the free dye ( em = 667 nm), with concomitant a ppearance of a red-shifted fluorescence with max 685 nm. Interestingly, the fluoresce nce of the free dye is completely quenched at relatively low polymer:dye ratio. Th is suggests that most of the HMIDC is bound to the polymer even at low polymer concentration, and that energy transfer occurs to the fully groove-bound dyes which emit at longer wavelength.

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64 2.4 1.8 1.2 0.6 0.0 Intensity 800 750 700 650 Wavelength (nm) Figure 2-14. Fluorescence spectra of HMID C (c = 5.0 M) in water titrated with w -P2 excitation wavelength is 610 nm Concentration range of w -P2 is 0 5.0 M polymer repeat units, in 0.5 M increments. Arrows show direction of change of spectrum with increasing polymer concentration. Circular dichroism spectroscopy Figure 2-15 illustrates the circular dichroism spectrum obtained for solutions that contain HMIDC only (c = 7 M) or HMIDC and w -P2 (c = 7 M and 10 M, respectively). As expected, HMIDC alone features no CD signal; ho wever, the polymer-HMIDC mixture features a distinct unsymmetrical, bisignate CD signal in the region corresponding to the absorption of the polymer-bound dye. Although the bi signate signal is unsymmetrical, we believe that it arises due to exciton coupling within a chiral, dye-chr omophore aggregate that is formed as the dye molecules are oriented by the helical w -P2 template. Figure 2-16 shows an idealized model for this structure. Individual HMIDC molecules bound within the w -P2 groove are oriented by the helical turn of the polymer. The individual HMIDC molecules are thus oriented relative to

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65 one another such that the dipole-dipole coupli ng between them will follow a helical path, which affords the condition necessary for the exci ton-coupled CD signal to be observed. -8 -6 -4 -2 0 2 4 [ deg cm2 dmol-1) 800 750 700 650 600 550 500 450 Wavelength (nm) HMIDC HMIDC+w-P2 Figure 2-15. Circular dichroism spectra of HMIDC (c = 7.0 M) alone and HMIDC with w -P2 (c = 7 M and 10.0 M, respectively) in aqueous solution. Figure 2-16. Schematic representa tion of the interaction between w -P2 and HMIDC.

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66 The importance of polymer helix to template the formation of HMIDC aggregates was further confirmed by the results of titration of wP2 into the methanol solution containing HMIDC. As seen from Figure 2-17, there is only a slight change of intensity in the absorption and emission spectrum of HM IDC with the addition of w -P2 indicating that the polymer helical conformation is crucial to the g roove binding of HMIDC to form the chiral dye aggregates. 4 3 2 1 0 Intensity 800 750 700 650 Wavelength (nm) (b) 1.4 1.1 0.7 0.4 Absorbance 800 700 600 500 Wavelength (nm) (a) Figure 2-17. UV-Vis absorpti on (a) and fluorescence emission spectra (b) of HMIDC in methanol with titrations of w -P2 Fluoresence emission spectra were measured with excitation wavelength at 610 nm. [HMIDC] = 5.5 M. w -P2 solution was added in 0.5 M aliquots, ranging from 0 to 6 M. Experimental Materials Palladium catalysts were used as received from Strem Chemical Co. Triethylamine and THF and CH2Cl2 were purified by distillati on over sodium hydride. N,N -Diisopropylcarbodiimide (DIPC) was bought from Aldrich Chemical Co. Anhydrous DMF was used as supplied by Acros Chemical Company. The cyanine dyes HMIDC, DiSC2(5) were

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67 obtained from Acros and they were used as received. 4-(Dimethylamino)pyridinium 4toluenesulfonate (DPTS),105 1,4-Diethynylbenzene were synthesi zed following the literature.106 The ruthenium (II) complex [Ru(bpy)2(dppz)]2+ was synthesized from RuCl3xH2O in two steps.127,128 All other chemicals were purchased from either Acors or Aldrich chemical Company and used as received. Instrumentation NMR spectra were recorded on a Varian VXR-300 FT-NMR, operating at 300 MHz for 1H-NMR and at 75.4 MHz for 13C-NMR. Chemical shifts were reported in ppm using CHCl3 or C2HD5SO as internal reference. FTIR spectra we re taken on a Perkin-Elmer 1600 spectrometer. Gel permeation chromatography (GPC) analyses we re carried out on a system comprised of a Rainin Dynamax SD-200 pump and a Beckma n Instruments Spectroflow 757 absorbance detector. UV-visible absorption spectra we re obtained on Varian Cary 100 dual beam spectrophotometer, with a scan rate of 300 nm/min. Steady-st ate fluorescence spectra were recorded on a SPEX Fluorolog-2 or a SPEX Fluorolog-3 fluorescence spectrometer. Emission spectra were collected at 90 C relative to exc itation. 1 cm square quart z cuvette were used for both absorption and emission measurements. CD spectra were recorded on an Aviv model 202 CD spectrometer, with temperature set at 25 C using 1-cm rectangular quartz cells. General Methods For optical measurements on polymer solutions, the reported concentrations refer to the concentration of polymer repeat units. Soluti ons for spectroscopic studies were prepared by dilution of stock solutions. Titrations were carried out by adding microliter aliquots of stock solutions to the sample solution that was contai ned in a quartz optical cuvette. Luminescence studies on [Ru(bpy)2(dppz)]2+ were carried out on sample s that had been deoxygenated by bubbling with argon for 20 minutes.

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68 Synthetic Procedures 3,5-Diiodobenzoic acid (2).104 To an ice-cooled, stirred suspension of 3,5-diaminobenzoic acid (6.0 g, 39.5 mmol) in 90 ml of sulfuric acid a nd water (v/v = 2/1) mixture, sodium nitrite powder (6.5 g, 94 mmol) was added with the temperat ure controlled between -5 C 0 C. After 1 h, urea (0.50 g, 8.3 mmol) was added, and then a cold solution of Potassium Iodide (65.6 g, 395 mmol) in 60 ml water was added drop by drop. An additional 3 hr was allowed to react before the reaction mixture was heated up. Th en the warm suspension was poured into 300 ml ice-cold water and the brown precipitate was collected by vacuum filtration, dried and extracted using diethyl ether. The obt ained dark brown diethyl ethe r solution was washed with Na2S2O3 water solution until a pale yellow color was observed. After diethyl ether was removed in vacuo the resulting yellow solid was purified by recrysta llization from toluene. Yield: 9.6 g (65%). 1H NMR (DMSO-d6): = 8.30 (s, 1H, 4-H), 8.17 (s, 2H, 2, 6-H). 13C NMR (DMSO-d6): 165.35, 148.88, 137.68, 134.99, 96.84. (S)-Benzyl 2-(3,5-diiodobenz amido)propanoate (3). 3, 5-diiodobenzoic acid (1.3 g, 3.5 mmol) was dissolved in 10 ml DMF, then 1.2 g DPTS (4.1 mmol) was added. Heat was applied to dissolve all the mate rials, and during the dissolution, 5 ml more DMF was added to the mixture. 0.59 g L-alanine benzyl ester (3.3 mmol ) was added to the clear solution. Then DIPC (0.57 g, 4.6 mmol) was added, and the reaction was a llowed to stir for 12 hr. The reaction was diluted with CH2Cl2, washed three times with H2O and concentrated in vacuo Flash chromatograph using CH2Cl2/hexane (v/v = 7:3) as the elue nt afforded the pure product as a white solid. Yield: 1.2 g (64%), mp 143-144 C. 1H NMR (CDCl3, ppm): 1.52 (d, 3H), 4.80 (q, 1H), 5.23 (d, 2H), 6.79 (d, 1H), 7.38 (m, 5H), 8.05 (s, 2H), 8.20 (s, 1H). 13C NMR (CDCl3, ppm): 18.68, 48.99, 67.73, 95.03, 128.42, 128.83, 128.93, 135.30, 135.55, 137.39, 148.32, 164.01, 173.01. HR-MS (EI): Calcd for C17H16I2NO3 [M+H+] 535.9214, found

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69 535.9229. Elemental analysis: Calcd for C17H15I2NO3: C, 38.16; H, 2.83; N, 2.62. Found: C, 37.97; H, 2.51; N, 2.61. FT-IR ( max, KBr pellet): 3258, 3056, 2980, 1760, 1732, 1637, 1627, 1533, 1450, 1384, 1348, 1278, 1207, 1140, 1126, 1049, 959, 912, 861, 752, 696, 667. ( S)-Benzyl 2-(3,5-bis((trimethylsilyl) ethynyl)benzamido)propanoate (4). A solution of compound 3 (1.3 g, 2.4 mmol) in 20 ml dry THF/Et3N (v/v = 2/1) was degassed for 15 minutes using argon. Then 50.5 mg Pd(PPh3)2Cl2 (72 mol) and 27.4 mg of CuI (144 mol) was added into the solution unde r the protection of argon, followe d by the addtion of 3 ml of trimethlysilylacetylene (21.3 mmol). The resulting mixture was stir red at room temperature for 3 hr. The brown suspension was filtrated and the filtrate was concentrated in vacuo The residue was loaded on a silica column and eluted with a mixture of CH2Cl2/hexane (3/2) affording a yellow oil (yield: 0.85 g, 74%). 1H NMR (CDCl3, ppm): 7.79 (d, 2H), 7.68 (t, 1H), 7.38 (s, 5H), 6.68 (d, 1H), 5.22 (d, 2H), 4.82 (q, 5H), 1.53 (d, 3H), 0.25 (s, 18H). (S)-Benzyl 2-(3,5-diethynylb enzamido)propanoate (5). The yellow oil obtained as above (0.85 g, 1.8 mmol) was dissolved in 20 ml THF. The solution was acidified by adding 0.5 ml acetic acid before adding tetrabutylammonium fluoride (1M in THF, 5 ml). After stirred at room temperature for 30 minutes, the reaction mixture was diluted w ith 40 ml diethyl ether, then washed with 1 M HCl (60 ml 1), aqueous NaHCO3 (60 ml 1) and water (60 ml 1), then dried with MgSO4. After the solvent was removed in vacuo the crude product was purified by flash chromatography (silica gel, eluent: CH2Cl2/hexane = 9:1) to afford 0.51 g of 5 (yield: 86%). 1H NMR (CDCl3, ppm): 7.86(d, 2H), 7.71(t, 1H), 7.39 (s, 5H), 6.79 (d, 1H), 5.21 (d, 2H), 4.82 (q, 1H), 3.14 (s, 2H), 1.53 (d, 2H). 3,5-Diiodobenzoyl chloride (6). Compound 2 (2.0 g, 5.3 mmol) and 5 ml thionyl chloride were placed in a 50 ml round bottom flask f itted with a condenser with a bubbler. The

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70 suspension was heated at reflux for 30 minutes at which time no more gas evolution was observed. Then the excess SOCl2 was removed by vacuum distil lation, the resulting residue was purified by crystallization from hexane. A white powder was obtained (yield: 1.6 g, 77%). 1H NMR (CDCl3, ppm): 8.38 (d, 2H), 8.36 (t, 1H). (S)tert -Butyl 2-(3,5-diiodobenzamido)propanoate (7). A solution of L-alanine benzyl esterHCl (1.04 g, 4.8 mmol) in 20 ml CH2Cl2 was added 0.62 g of 4-dimethylaminopyridine (5.0 mmol). Then 942 mg of 6 (2.4 mmol) was added. After 30 minutes, the solvent was removed in vacuo The crude product was purified by flas h chromatography (silica gel, eluent: CH2Cl2) to afford 1.13 g white wax-like product (yield: 94%). 1H NMR (CDCl3, ppm): 8.18 (t, 1H), 8.07 (d, 2H), 6.73(d, 1H), 4.63 (q, 1H), 1.40 (s, 9H), 1.35 (d, 3H). 13C NMR (CDCl3, ppm): 172.67, 163.94, 148.40, 137.82, 135.72, 95.21, 83.09, 49.67, 28.47, 19.18. LR-MS: Calcd for C14H17I2NO3 [M+] 501 found 501. (S)-Benzyl 2-(3,5-bis(phenylethyny l)benzamido)propanoate (8a). A solution containing 1 g of 3 (1.8 mmol), 0.95 g of phenylacetylene (93 mmol), 62 mg of tetrakis(triphenylphosphine)Palladium (54 mol), 12 mg of copper( ) iodide (32 mol) in 10 ml of a mixture of a THF/Et3N was stirred at room temperature for 6 hr under argon. Then the solution was filtrated, the solvent was removed under reduced pressure, and the remaining solid was purified by flash chromatography. The product was isolated as a white solid, 0.79 g (91%), mp 180-181 C. 1H NMR (CDCl3, ppm): 7.91 (d, 2H), 7.83 (t, 1H), 7.56 (m, 4H), 7.39 (m, 11H), 6.80 (d, 1H), 5.15 (t, 2H), 4.87 (q, 1H), 1.56 (d, 3H). 13C NMR (CDCl3): 173.15, 165.64, 137.33, 135.44, 134.80, 131.97, 129.89, 128.97, 128.92, 128.78, 128.68, 128.43, 124.57, 122.84, 91.26, 87.85, 67.65, 48.97, 18.84. FT-IR ( max, KBr pellet): 3435, 3278, 3056, 2932, 2210, 1736,

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71 1645, 1599, 1543, 1490, 1452, 1409, 1387, 1348, 1312, 1224, 1151, 1109, 1071, 1027, 1109, 1071, 1027, 953, 910, 878, 754, 687. (S)tert -Butyl 2-(3,5-bis(phenylethynyl)benzamido)propanoate (8b). Compound 7 (0.6 g, 1.2 mmol) was dissolved in 12 ml of dry THF/Et3N (3/1) and degassed with argon for 15 minutes. Then Pd(PPh3)4 (83 mg) and CuI (27 mg) were added to the solution, followed by the addition of 0.6 ml of phenylacetylene (5.5 mmol). The resulting mixture was slowly heated to 65 C and stirred for 13 hr. After filtration, the solvent was removed in vacuo The crude product was purified by flash chroma tography (silica gel, eluent: CH2Cl2/hexane = 4/1). 1H NMR (CDCl3, ppm): 7.92 (d, 2H), 7.82 (t, 1H), 7.55 (m, 4H), 7.39 (m, 6H), 4.69 (q, 1H), 1.52 (m, 12H). LR-MS: Calcd for C30H27NO3 [M+] 449 found 449. Sodium (S)-2-(3,5-bis(phenylethynyl)benzamido)propanoate (9). Method 1: A solution of 0.2 g of 8a in 5 ml DMF was added 5 ml 1 M NaOH so lution, then left at room temperature under stirring for 15 minutes. Then the solvent was removed under reduced pressure, the residue was washed with CH2Cl2, acetone, cold water. Further pu rification could be applied by recrystallization from H2O/MeOH (v/v = 7/3). 0.15 g white solid was obtained (yield: 87%). Method 2: Compound 8b (0.5 g, 1 mmol) was dissolved in 5 ml CH2Cl2 and cooled down to 0 C. Trifluoroacetic acid (TFA) (5 ml) was added drop-wise in to the solution. After finishing the addition, the reaction mixture was allowed to warm to room temperature and stirred further for 2 hr. The solvent was removed in vacuo The residue was dissolved in 20 ml DMF/H2O (v/v = 1/1) and added 106 mg of Na2CO3. The solution was stirred at room temperature for 5hrand then precipitated into 200 ml of acetone. The product was isolated as a white solid. 1H NMR (DMSO-d6, ppm): 8.27 (d, 1H), 7.99 (d, 2H), 7.86 (t, 1H), 7.61 (m, 4H), 7.45 (m, 6H), 4.05 (q, 1H), 1.30 (d, 3H). 13C NMR (DMSO-d6, ppm): 175.29, 163.63, 136.89, 136.36, 132.26,

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72 130.47, 129.88, 129.50, 123.83, 122.48, 91.27, 88.48, 51.18, 19.68. FT-IR ( max, KBr pellet): 3411, 3307, 3080, 3064, 2989, 2939, 2212, 1624, 1599, 1586, 1532, 1491, 1456, 1442, 1407, 1365, 1284, 1174, 1154, 1101, 1070, 1027, 958, 913, 886, 840, 757, 691, 670. 4-(Dimethylamino)pyridinium 4-Toluenesulfonate (DPTS) (10).105 A 50 ml solution of 4-dimethylaminopyridine (2 N) was added slowly into a 50 ml solution of p-toluenesulfonic acid monohydrate (2 N) at room temperature under st irring. The resulting white precipitate was collected, washed with 100 ml of THF and dried under vacuum. 1H NMR (DMSO-d6): 8.22 (d, 2H), 7.50 (d, 2H), 7.14(d, 2H), 7.00 (d, 2H), 3.18 (d, 6H), 2.29 (s, 3H). 1,4-Bis((trimethylsilyl)ethynyl)benzene (11).106 1,4-Diiodobenzene (18.0 g, 54.6 mmol) was dissolved in 200 ml of THF/HN(i-Pr)2 (4/1) in a Schlenk flask and degassed with argon for 30 minutes. The solution was cooled down to 5 C using an ice/water ba th. Then 400 mg of Pd(PPh)2Cl2 (0.57 mmol) and 100 mg of CuI (0.53 mmol ) were added, followed by the addition of 20 ml of trimethylsilylacetylene (141.8 mmol) via a syringe. A thick precipitate formed immediately (ensure efficient stir ring!). After half an hour, the reaction mixture was allowed to warm to room temperature and stirred overnight After filtering through a bed of celite, the solvent was removed in vacuo The brown crude product was di ssolved in a large amount of hexane and filtrated through a shor t plug of silica. Recrystalliz ation from ethanol afforded a white flake crystalline pr oduct (yield: 10.4 g, 74%). 1H NMR (CDCl3, ppm): 7.40 (s, 4H), 0.25 (s, 18H). 1,4-Diethynylbenzene (12). Compound 11 (2.0 g, 7.4 mmol) was dissolved in a minimum of dioxane (~ 20 ml) in a 250 ml beaker and acid ified with 1 ml acetic acid. Then 18 ml of tetrabutylammonium fluoride (18 mmol) was a dded drop-wise. After 30 minutes, the volume of the solution was reduced to around 20 ml by flowing nitrogen above the surface. Then 200 ml of

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73 H2O/MeOH (9/1) was added to the reaction mixture. The product precipitated as a white solid and was collected on a bchnel funnel, washed with 50 ml of H2O/MeOH (v/v = 9/1), dried in a desiccator loaded with 10 g of anhydrous CaCl2. The dry product is usually pure enough for polymerization, but further purification could be done by dissolving in hexane and passing through a short plug of s ilica to improve the qual ity of the product. 1H NMR (CDCl3, ppm): 7.45 (s, 4H), 3.18 (s, 2H). [Ru(Bpy)2]Cl2 (13).127 Commercial RuCl3xH2O (2.45 g, 11.8 mmol), bipyridine (3.75 g, 24 mmol) and LiCl (2.6 g, 61.5 mmol) were heated at reflux in 15 ml reagent grade DMF for 8 h. The reaction mixture was allowed to cool to room temperature, and then 100 ml cold acetone was added, the resulting suspension was kept in a refrigerator overnight The purple solid was collected by vacuum filtr ation, washed with H2O (20 ml 3), cold acetone (20 ml 3), diethyl ether (20 ml 3), and then dried on the bchne l funnel by pulling with an aspirator. The obtained green-black solid was us ed for next step reaction wi thout further purification. [Ru(bpy)2(dppz)](BF4)2 (14).128 A mixture of [Ru(bpy)2]Cl2 (200 mg, 0.39 mmol) and dipyridophenazine (130 mg, 0.46 mmol) in 50 ml of MeOH/H2O (v/v = 1/2) was heated at reflux for 4 hr. At that time, the original dark he terogeneous solution became red and clear. This solution was evaporated to 10% of its orig inal volume then diluted with 30 ml H2O and boiled for 10 minutes. After the solution was slowly co oled down using an ice-water bath, the formed pale yellow precipitate was removed by filtration. The clear red filtrate was treated with 5 ml 10% NaBF4 solution. The orange precipitate was colle cted by vacuum filtra tion. After dried on the bchnel funnel for 2 hr, the precipitate was crystallized from absolute ethanol. 1H NMR (Acetone-d6, ppm): 9.79 (d, 2H), 8.89 (t, 4H), 8.51 8.58 (m, 4H), 8.09 8.31 (m, 12H), 7.67 (t, 2H), 7.43 (t, 2H).

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74 Polymerization Procedures P1. (S)-benzyl 2-(3,5-diiodobenzamido)propanoate ( 3 ) (323 mg, 0.60 mmol) and (S) benzyl 2-(3,5-diethynyl benzamido)propanoate ( 5 ) (200 mg, 0.60 mmol) were dissolved in 1 ml DMF/Et3N (v/v = 3/1) and degassed with argon fo r 30 minutes. Another degassed solution containing 34.6 mg of Pd(PPh3)4 (30 mol) and 5.7 mg of CuI (30 mol) in 1 ml DMF/Et3N (3/1) was added to the former solution via a syri nge. The resulting mixture was heated to 70 C and stirred for 24 hr. GPC (THF, polystyrene standards): Mw = 1,680, Mn = 1,550, PDI = 1.10. P2. (S)-benzyl 2-(3,5-diiodobenzamido)propanoate (3) (535 mg, 1mmol) and 1,4diethynyl-benzene (12) (126 mg, 1mmol) were dissolv ed in 10 ml of dry THF/Et3N (v/v = 3:1) solution using a Schlenk flask and dega ssed for 30 minutes. Then 57.7 mg Pd(PPh3)4 (50 mol) and 9.5 mg CuI (50 mol) powder were added under the protection of Argon. The resulting solution was stirred at room temperature for 24 hr, and then poured in to a large volume of methanol. The polymer precipitated as a yell ow fine powder and it was further purified by several steps of dissolution in DMF followed wi th precipitation from me thanol and acetone. Yield: 71%. GPC (THF, Polystyrene standards): Mw = 23,660, Mn = 12,710, PDI = 1.80. 1H NMR (DMSO-d6, ppm): 9.11 (br, d, 1H), 8.10 (br, m, 2H), 7.95 (br, m, 1H), 7.68 (br, m, 4H), 7.36 (br, m, 6H), 5.16 (br, s, 2H), 4.58 (br, m, 1H), 1.45 (br, d, 3H). P3. (S)tert -butyl 2-(3,5-diiodobenzamido)propanoate (7) (501 mg, 1 mmol) and 1,4diethynyl-benzene ( 12 ) (126 mg, 1 mmol) were dissolved in 5 ml dry THF/Et3N (v/v = 3/1) mixture. The resulting solution was degassed with argon for 15 minutes. Another degassed solution containing 69 mg of Pd(PPh3)4 (60 mol) and 23 mg of CuI (120 mol) in THF/Et3N (v/v = 3/1) was added to the former solution via a syringe. The resulting mixture was heated to 70 C and stirred for 5 hr. The polymer was isol ated as a yellow solid by precipitating into 400

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75 ml of methanol and further purif ied by 2 cycles of dissolution in THF followed with precipitation from methanol. Yield: 72%. GP C (THF, polystyrene standards): Mw = 125,490, Mn = 39,510, PDI = 3.10. Hydrolysis of the Neutral Polymers Method 1 : P2 (250 mg, 0.6 mmol) was dissolved in 30 ml of DMF, and then 10 ml of 1 N NaOH solution was added drop-wise. The resulting solution was stirred at room temperature. During the course of the polymer hydrolysis, a few drops of water were systematically added in order to keep the polymer in solution. After 30 minutes, the mixture wa s poured into 500 ml of methanol/acetone/ether (v/v/v = 1:4:5) mixture. The polymer precipitated as a light yellow solid. Method 2 : P3 (260 mg, 0.7 mmol) was dissolved in 30 ml CH2Cl2 and cooled down in an ice/water bath. 30 ml of trifluoroacetic acid (T FA) was added to the polymer solution drop-wise. Upon the completion of the addition, the react ion mixture was allowed to warm to room temperature and stirred for another 16 hr. The excess TFA and the solvent were removed in vacuo The residue was treated with aqueous Na2CO3 solution (10 ml, 20%). After 6 hr, the solution was poured into 200 ml of acetone; the po lymer precipitated as a yellow solid and was isolated by centrifuge. Final purification of the polymer was accomplished by dialysis of aqueous solution of the polymer against ultrapur e water (Millipore Simplicity Water System) and using a 12 kD molecular weight cut-off (MWCO) cel lulose membrane (Fisher Scientific). After dialysis, the polymer solution was filtered through a nylon membrane (pore size: 0.8 m). The polymer was stored in this format and dilu ted as appropriate for spectroscopic studies. 1H NMR (DMSO-d6, ppm): 8.05-8.10 (br, m, 1H), 8.00 (s, 2H), 7.75 (s, 4H), 4.0-4.08 (br, m, 1H), 1.24 (d, 3H).

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76 CHAPTER 3 POLY(PHENYLENE ETHYNYLENE) CARBOXYLATE Introduction In a previous work,71,126 we reported the synthesis of poly(phenylene ethynylene) (PPE) carrying sulfonate solubilizing group s. The polymer was synthesized in a direct fashion that the monomer carrying sulfonate and the other organic monomer were dissolved in an aqueous mixture of DMF/H2O/i-Pr2NH (3/2/1) and polymerized in the presence of a catalytic amount of Pd(PPh3)4 and CuI. Polymers prepared following this strategy are soluble in polar solvents, such as water, DMF and DMSO. Using techniques such as viscometry, ultraf iltration and pulsed field gradient NMR, the molecular weights of these polymers were estimated.96,126 However, characterization of the molecular weights of such-prepared polymers using gel permeation chromatography (GPC) was unsuccessful, probably due to the strong aggregation tendency of this type of polymers. In addition to su ch a direct approach to prepare conjugated polyelectrolytes, another method involves the pol ymerization of monomers in which the ionic groups are protected; the ionic groups are produced in a subsequent reaction that is carried out on the polymer. 53,129 The precursor polymers obtained by th is method are typically soluble in organic solvents such as THF, CHCl3, which enables the determination of the molecular weights of these polymers by GPC. As one of the most common ionic groups, car boxylate could be prot ected as carboxylic acid ester in various forms, such as methyl ester, ethyl ester, oxazoline ester, etc.130 The ester groups can be cleaved quantitatively using either acid or base methodologies. Most interestingly, carboxylate group show s the ability to selectively bi nd some dior trivalent metal ions to form coordination complexes. C onjugated polymers contai ning pendant carboxylate groups have been reported. For example, McCu llough and co-workers synthesized regioregular

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77 polythiophenes with carboxylate side groups an d studied their ionchr omatic sensing in water.81,131 Also the association be tween carboxylate group and Ga3+ was used to develop highly sensitive assays for kinase and phosphatas e based on conjugated polyelectrolyte-coated polystyrene microspheres.132 In the present chapter, we report the synt hesis of monomers ca rrying carboxylic dodecyl ester groups. Two water-soluble PPEs were pr epared by organic phase polymerization and subsequent base-promoted hydrolysis. These two PPEs share the same backbone structure (phenylene-ethynylene) but differ in the carboxylate s ubstitution pattern. One contains two carboxylate side chains per repeat unit (one phenylene-ethynylene) and the other contains two carboxylate side chains for every two repeat units. The photophysic al properties of these two CPEs were compared to each other and to those of PPE-SO3 -. In the second part, the interaction between the latter polymer and 9 different divalent metal ions wa s studied in methanol (MeOH) and 4-(2-hydroxyethyl)-1-piperazine-1-ethanesul fonic acid (HEPES) buffer solution. Quenching of the polymer fluorescence by organic ions, such as methyl viologen (MV2+) in MeOH was also studied. To the end, a fluorescence turn-on sensor that is highly selective and sensitive to pyrophosphate (PPi) was developed using a CPE-metal ion system. Results and Discussion Synthesis The monomer that carries carboxylic dod ecyl esters, didodecyl-2,2'-(2,5-diiodo-1,4phenylene)bis(oxy)diacetate ( 8 ) was synthesized via two different routes, as shown in Figure 3-1. In route A, the monomer was synthesized fr om commercially available 1,4-dimethoxybenzene ( 1 ) in 4 steps with an overall yield of 33%. The route involves the preparation of 2,5diiodohydroquinone ( 3 ) with procedures adapted from the literature.27 Substitution reaction of 1 equivalent of 2,4-diiodohydroquinone ( 3 ) to 2 equivalents of bromoacetic acid afforded

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78 O O O O I I OH HO I I O O I I CO2H HO2C iiiiii 53%87%84% OH OH O O CO2H HO2C O O CO2C12H25 C12H25O2C O O CO2C12H25 C12H25O2C I I iv85%RouteA RouteBiiiivv 67%83%90% 1234 5678 O O CO2C12H25 C12H25O2C Si Si O O CO2C12H25 C12H25O2C vi vii 66%(2steps) 109i.KIO3,I2,H2SO4,CH3CO2H,heat;ii.BBr3,CH2Cl2,-78oC0oC;iii.1)NaOH, BrCH2CO2H,heat;2)6NHCl;iv.H3PO4,C12H25OH;v.PhI(CO2CF3)2,I2,CCl4,RT; vi.Trimethylsilylacetylene,Pd(PPh3)2Cl2,CuI,THF/Et3N,heat;vii.TBAF,THF,RT. Figure 3-1. Synthesis of monomers 8 and 10 compound 8 in 84% yield. Subsequent Fisc her esterificati on between compound 4 and 1-dodecanol in the presence of a catalytic amount of H3PO4 gave the desired product 8 which was isolated as a white crystalline solid in 85% yield. Following this procedure monomer 8 was prepared in a 10-20 g scale. However, the sy nthesis of 2,5-diiodohydroquinone involved the use

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79 of boron tribromide in the second step. Although the demethylation works in a high yield (87%), the handling of boron tribromide needs special attent ion because of its high toxicity. In an effort to avoid using boron tribrimide, a 3-step proced ure was developed to sy nthesize the monomers. In route B, 1,4-hydroquinone ( 5 ) was first reacted with brom oacetic acid to afford 2,2'(1,4-phenylenebis(oxy))diacetic acid ( 6 ) in 67% yield, which was then reacted with 1-dodecanol under Fischer esterification conditions. The produc t was then iodinated using 1.2 equivalents of a trivalent iodine compound, [bis (trifluoroacetoxy)iodo]benzene toge ther with 1 equivalent of I2 in an excellent yield of 90%. The overa ll yield of route B to produce monomer 8 is 50% with the same quality as a crystalline white solid. The diacetylene monomer 10 was prepared from monomer 8 following a standard Pd-catalyzed Sonogash ira coupling and subs equent removal of the trimethylsilyl groups by tetrabutylammonium fluoride (TBAF). The yield for the two-step process is 66%; and monomer 10 was isolated as a slightly yellow powder. Figure 3-2. Synthesis of the neutral polymer precursors.

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80 The polymers studied in this chapter were synthesized using pol ycondensation between bifunctional monomers based on Sonogashira coupling, as illustrated in Figure 3-2. Polymerization between 8 and 10 in THF/Et3N (v/v = 3/2) afforded the homopolymer P4 which carries the ester groups on each repeat uni t. And, the copolymerization between 8 and 1,4-diethynylbenzene produced the polymer P5 with carboxylic dodecyl ester groups every two repeat units. Both polymers are soluble in THF, CH2Cl2, CHCl3, but not soluble in polar solvents, such as methanol, DMSO and DMF. The GPC chromatograms of P4 and P5 are shown in Figure 3-3, indicating a mono-modal distribution of the molecular weights for both polymers. 1.2 0.9 0.6 0.3 Response 30 25 20 15 10 5 0 Minutes P5P4 Figure 3-3. GPC chromatograms of P4 and P5 The number-average molecular weights determin ed against the polystyrene standard for P4 and P5 are 12 kD and 127 kD, respectively. The de grees of polymerization (DP or n) were calculated based on the number-average molecular weights. The polydispersity indices (PDI) for P4 and P5 were calculated to be 2.3 and 2.2 respectively, which are cl ose to the theoretical value of 2.0 for ideal step-g rowth polycondensation.133 The large difference in the molecular weights obtained for the homopolymer ( P4 ) and copolymer ( P5 ) is possibly due to the sterically

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81 favorable transition state for the copolymer durin g the palladium-catalyzed polymerization. To date, the copolymer P5 is one of the alkoxy-substituted PPEs with highest molecular weight that have been reported in the literature.10 The water-soluble PPEs were prepared by hydr olyzing the correspond ing neutral polymers in dioxane using tetrabutylammonium hydroxide (TBAH) (1 M in methanol) with subsequent counter cation excha nge by aqueous NaClO4 solution, as shown in Figure 3-4. As confirmed by 1H NMR and FT-IR, the base cleaved the carboxy lic dodecyl ester group with a conversion yield higher than 95%. Since the conj ugated backbone remained intact during the hydrolysis, thus the water-soluble polymers should have the same chain length as that of th eir neutral precursor polymers. Characterizations of P4 and w -P4 were reported previously by our group,134 in the following context, we presented the characterization of the copolymer P5 and the water-soluble polymer w -P5 Figure 3-4. Hydrolysis of th e neutral polymers to prepare the water-soluble polymers, w -P4 and w -P5

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82 Polymer Characterization 1H NMR spectra Figure 3-5 shows the 1H NMR spectra of monomer 8 P5 and w -P5 Due to its high molecular weight and strong tendency to aggregat e in the solid state, the organic polymer, P5 becomes barely soluble after complete drying, even in its good solvents, such as chloroform and THF (solubility is less than 2 mg/ml). Although in the 1H NMR spectra of P5 the aromatic protons are hardly seen, the protons in the aliphatic area derived from monomer 8 can be clearly seen. Figure 3-5. 1H NMR spectra of monomer 8 organic polymer P5 and the water-soluble polymer wP5 9 8 7 6 5 4 3 2 1 ppm monomer 8 P5 w-P5 CDCl3 CDCl3 D2O DMSO-d6

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83 After hydrolysis, the polyelectrolyte, w -P5 is soluble in water (> 5 mg/ml). However, the 1H NMR spectrum of w -P5 in D2O shows very broad signals, s uggesting that the polymer is strongly aggregated in concentrated aqueous solutions (~ 5 mg/ml). DMSO-d6 is a good solvent for polyelectrolytes and was used to obtain NMR spectra for many conjugated polyelectrolytes,135 but the current polymer ( w -P5 ) only shows limited solubility in DMSO (< 1 mg/ml). Using a mixture of D2O/DMSO-d6 (v/v = 1:1), 1H NMR spectrum of w -P5 was finally obtained after intensive scanning (1024 scans) at 70 C. As show n in Figure 3-5, the singlet at = 4.37 ppm is assigned to the methylene groups (O CH2CO2Na ), which shifts upfield compared to the signals in the spectra of th e monomer and the precursor polymer. In the aromatic region, two singlets are observed. The singlet at = 7.54 ppm is attri buted to the 1,4phenylene. And the other singlet at = 6.97 ppm is from the 2,5-disubstituted-1,4-phenylene protons on the polymer bac kbone derived from monomer 8 Infrared spectra The hydrolysis was also confirmed by FT-IR spectra, as shown in Figure 3-6. The absorption at 1730 cm-1 in the IR spectra of the monomer 8 is assigned to the C=O stretching of the ester groups. For the organic polymer P5 the backbone C C stretching appears as a weak signal at 2208 cm-1. The absorption at 1734 cm-1 is due to the ester groups. And the absorption at 1762 cm-1 that is absent in the monomer spectrum probably results from the Fermi resonance between the C=O stretching band and the first overtone band near 839 cm-1.136 After hydrolysis, the ester band at 1734 cm-1 disappears and two new bands appear at 1610 cm-1 and 1445 cm-1, which are assigned to the asymmetric and symmetric CO2 stretching, respectively. The absorptions near 1200 cm-1 for all the samples in the spectra probably represent the asymmetric C-O-C stretching.

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84 2000 1500 1000 Wavenumber (cm-1) monomer 8 P5 w-P5 1730 1734 1762 1778 1610 1445 1196 880 1188 839 828 1221 2208 2191 Figure 3-6. FT-IR spectra of monomer 8 organic polymer P5 and the water-soluble polymer w -P5 In these experiments, KBr pellets c ontaining the samples were prepared and used. Photophysical Characterization Previously, we have reported the photophysic al properties of a structure similar PPE, PPE-SO3 in different solvent mixtures.71 The studies revealed that the absorption and fluorescence of the polymer are strongly solvent-de pendent, which is due to aggregation of the polymer chains in different solven t environments. After hydrolysis, w -P4 and w -P5 bearing carboxylate groups have the same bac kbone structure as that of PPE-SO3 -. As noted before, w -P4 has two carboxylate side gr oups per repeat unit while w -P5 has two carboxylate groups for every two repeat units, which is more like PPE-SO3 -. These polymers show similar solubility in water and other polar solvents, such as Me OH, DMF and DMSO. Since aggregation is a

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85 cooperative process driven by a comb ination of many forces, such as stacking, hydrophobic and electrostatic interaction, the density of ionic groups on the hydrophobic backbone will play a key role in changing the polymer conformation and stabilizing/destab ilizing the aggregate structure. In order to study this effect, the absorption and fl uorescence spectra in different solvents were carefully examined for w -P4 and w -P5 1.2 0.9 0.6 0.3 0.0 Absorbance 700 600 500 400 300 Wavelength (nm) 12 9 6 3 0Fluorescence Figure 3-7. Normalized absorption (left) and fluorescence (r ight) spectra of w -P4 in MeOH ( ), (1:1) H2O-MeOH () and H2O ( -). Fluorescence spectra were measured with excitation at 400 nm and are intensity-normalized accordi ng to respective quantum yield. As shown in Figure 3-7, in MeOH, w -P4 has a major absorption band centering at 439 nm, which blue-shifts (<5 nm) with increasing volume of water in the mixture. In the fluorescence spectra, w -P4 shows a structured blue emission at 462 nm (0-0 band) in MeOH and the emission intensity is continually quenched with increa sing water. However, there is no observable emerging new band. The polymer maintains its blue emission even in water. By contrast, a new

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86 red-shifted and narrow band appears in the absorption spectra of PPE-SO3 as the volume of water in the MeOH/water mixture increases. In the fluorescence spectra, the blue emission band observed in MeOH is completely quenched in water and a new broa d and red-shifted band appears.71 Compared to w -P4 w -P5 is structurally more close to PPE-SO3 except they have different ionic groups. As expected, w -P5 shows more similar spectral change as PPE-SO3 when its absorption and emission properties were studied in different solvent mixtures. Figure 3-8 illustrates the absorption spectra of w -P5 in different methanol/water mixtures along with the spectrum of its neutral precursor ( P5 ) in THF. THF is a good solvent for the neutral polymer, in which the aggregation of P5 is expected to be minimal. In the absorption spectrum of P5 (Figure 3-8 a), there is a main abso rption band with a maximum at 415 nm, which is assigned to the transition of the un-aggregated conjugated polymer chains. As shown in Figure 3-8 (b), the absorption spectrum of w -P5 in methanol shows a main absorption band at 415 nm (un-aggregated po lymer chains) and a shoulder peak at 433 nm (aggregate band), which arises from the planarization of the conjug ated backbone in the a ggregate. As the volume fraction of H2O increases, the absorbance of the maxima at the longer wavelength continually increases and becomes more pronounced. There is also a stepwise red-shift (< 7 nm) of the absorption maximum. The water-driven aggregati on trend is clearly show n by plotting the ratio of the absorbance at the maximum divided by the absorbance at 415 nm to the volume of water in methanol (Figure 3-8 c).

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87 1.5 1.4 1.3 1.2 1.1 Amax/A415 1.0 0.8 0.6 0.4 0.2 0.0 H2O% (c) 1.0 0.8 0.5 0.3 0.0 (a) 1.6 1.2 0.8 0.4 0.0 600 500 400 300 Wavelength (nm) (b) Figure 3-8. (a) Normalized absorption spectrum of P5 in THF solution; (b) Normalized absorption spectra of w -P5 in MeOH/H2O mixtures, the arrow shows the direction of change with increasing volume % H2O (c) Variation of Amax/A415 in the absorption spectra of w -P5 with the volume % H2O.

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88 As expected, w -P5 and PPE-SO3 that have the same backbon e feature but different ionic groups show similar solvatochromic properties. However, the shoulder peak (433 nm) in the spectrum of w -P5 in MeOH indicates that even in metha nol there is already a small fraction of aggregated chains, which differs from PPE-SO3 -. The latter polymer exhibits a major absorption at 425 nm without the apparent aggregate band in MeOH, which is similar to those of their organic analogues in good solvents (THF or CHCl3).10,73 This different behavior of w -P5 and PPE-SO3 in methanol is related to the acid-base equilibrium of both materials in the neutral water in which the stock solutions were prepared and stored. Sulfonate (SO3 -) is a weaker base with pKa (R-SO3H) 2.6,137 thus PPE-SO3 exists exclusively in its base form in the stock solution. Compared to sulfonate, carboxylate (CO2 -) is a much stronger base, pKa (R-CO2H) 4.8).137 Thus in its stock solution (pH = 5 6), a considerable fraction of the R-CO2 groups on the w -P5 chains is protonated, which decreases the in trachain or interchain electrostatic repulsion and favors the aggregate formation. This a ssumption was further proven by a pH control experiment. When the pH of the stock soluti on was higher than pH = 8, it was found that the shoulder peak (433 nm) in the absorption spectra of wP5 in MeOH was obviated. In the fluorescence spectra (Figure 3-9), in me thanol, at 10 M polymer repeat unit (PRU) concentration, the polymer features a structured fluorescence spectrum with a maximum intensity at 465 nm and a quantum yield of 0.24. At lower concentrations (less than 5 M), the 0-0 emission band at 436 is actually the dominan t peak, which is attenuated in concentrated solutions because of the self-abs orption of the polymer (typically a small Stokes shift of 3 nm was observed for these polymer solutions). As ca n be seen in Figure 3-9, with increasing amount of water, a continuous decrease of the fluorescence intensity in the blue region is observed and finally in the water solution, a red-shifted and broad emission at 524 nm becomes the dominant

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89 peak with a quantum yield of 7.5%. These re sults are quite simila r to those of PPE-SO3 -, implying that the electronic properties of thes e two polymers are predominantly determined by the conjugated backbones while th e ionic side groups play a mi nor role. And the pronounced excimer-like band existing in both polymers indi cates a strong excimer interaction in the aggregates due to the optimized stacking between the polymer chains. 5.0 3.7 2.5 1.3 0.0 Fluorescence 700 600 500 400 Wavelength (nm) H2O% 0 20 40 60 80 100 Figure 3-9. Fluorescence spectra ( exc = 380 nm) of w -P5 (c = 10 M polymer repeat units) in MeOH, water and MeOH/water mixtures. Th e spectra were obtained under the same conditions, so the spectral intensity re flects the approximate change in the fluorescence quantum efficiency with solv ent composition. The data used in this graph was obtained by Hui Jiang in Schanze group. Figure 3-10 (a) shows a sche matic representation of stacking of w -P5 in the aggregate. Because of the combination of different forces including stacking, hydrophobic and electrostatic interactions, a faceto-face stacking of the phenyl rings is realized and the ionic side chains are extended into the solvent. In such a way, th e contact between the hydrophobic aromatic backbones with the aqueous environment is minimized. This proposed model is

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90 consistent with some of the experimental result s from Swagers research group. In a series of work, they studied the Langmuir mo nolayer of some amphiphilic PPEs. Their results reveal that the cofacial spacing of PPE chains less than a bout 4.3 4.5 leads to a dominant excimer-like emission band in the fluorescence spectra.138-141 While in the aqueous solution of w -P4 because of the existence of carboxylate side chains on each repeat unit, the electro nic repulsion (interchain and intrachain) betw een the ionic side groups preven ts such a co-facial stacking. However, in the water-rich solutions of w -P4 agglomeration of polymer chains still occurs and generates some less ordered low-energy traps, wh ich leads to the reduced quantum efficiency. This was also proven by the observa tion of pH-induced aggregation of w -P4 .134 In acidic conditions, the carboxylate groups are protonated, which enables the co-facial packing mode, thus similar absorption and fluor escence spectra as those of w -P5 and PPE-SO3 were observed. Figure 3-10. Schematic representation of the stacking interaction in CPE aggregates. Fluorescence Quenching Properties Previously, we have studied the quenching of PPE-SO3 and w -P4 by cationic electron acceptors with different charges and/or some cationic cyanine dyes, which can quench the polymer fluorescence via an energy transfer mechanism.71,126,134 PPE-SO3 is quenched very efficiently by these molecules either in methanol or water in an amplified quenching process. The Stern-Volmer quenching constants ( Ksv) are usually in the range of 106 M-1 to 107 M-1 in water. w -P4 is found to be quenched less efficiently than PPE-SO3 by methyl viologen (MV2+).

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91 Ksv values are in the order of 105 M-1 in water. The less efficient quenching of w -P4 is probably due to the lack of aggregate fo rmation in water and/or the shor t chain length of the polymer.55,71 However, the carboxylate groups of w -P4 and w -P5 provide the receptor sites for metal ions, which allows us to look at the perturbation of polymer fluorescence by different metal ions in either MeOH or water. Independently, Kim a nd Bunz recently also reported the results of fluorescence quenching of a carboxylate-substitu ted PPE with the same structure of w -P4 by different metal ions.142-144 Quenching with metal ions in MeOH The interaction of w -P5 with divalent metal ions was first studied in methanol. This includes a series of 9 different metal ions, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+. Addition of 1 equivalent of metal ions i nduced similar changes in the absorption and emission spectra of the polymer ([PRU] = 5M) for all the metal ions except Cu2+. Figure 3-11 shows the absorption (a ) and emission (b) spectra of w -P5 at various concentrations of Ca2+. Interestingly, addition of small amounts of Ca2+ induces spectroscopic changes that resemble those when the solvent is changed from MeOH to water. In the absorption spectra, the intensity of the shoulder peak (430 nm) increases gradually with the addition of Ca2+ and in the emission spectra, the strong structured emission at 436 nm decreases and is finally replaced by a red-shifted, stru ctureless and broad emission band at 510 nm (aggregation band). Such observations indicates that Ca2+ and other metal ions (Mn2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+ and Pb2+) quenches the polymer fluorescence in metha nol via an aggregation-inducing process. Since most divalent metal ions could coordinate up to 2 or 3 carboxylate groups, thus they can bridge different polymer ch ains and cause the aggregat ion of the polymer. For Cu2+, although a similar change as the other metal ions is obser ved in the absorption spect ra (Figure 3-11 c), the change in the fluorescence spectrum is different. As shown in Figure 3-11 (d), addition of Cu2+

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92 to the methanol solution of w -P5 leads to the strong quenching of fluorescence intensity without appearance of the new emission band at 510 nm, which indicates that Cu2+ can quench the polymer fluorescence via a different mechanism, possibly a photo-indu ced electron transfer process (PET) or energy transfer (ET) process.145 1.6 1.2 0.8 0.4 0.0 700 600 500 400 (b) 1.2 0.9 0.6 0.3 0.0 500 400 300 (a) 1.2 0.9 0.6 0.3 0.0 500 400 300 Wavelength (nm) (c) 1.2 0.9 0.6 0.3 0.0 700 600 500 Wavelength (nm) (d) Absorbance Fluorescence Figure 3-11. Absorption (a) a nd fluorescence (b) spectra of w -P5 in MeOH with added Ca2+, Ca2+ was added in 1 M aliquots, [Ca2+] ranges from 0 5 M. Absorption (c) and fluorescence (d) spectra of w -P5 in MeOH with added Cu2+, Cu2+ was added in 1 M aliquots, [Cu2+] ranges from 0 5 M. [ w -P5 ] = 5 M. The arrow shows the direction of change with incr easing amount of metal ions.

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93 Quenching with metal io ns in the HEPES buffer We have demonstrated that the fluorescence of w -P5 in methanol could be quenched by different metals via either indu ced aggregation (less efficient) process or PET/ET mechanisms (more efficient). In order to develop sensors for biological applications, it is more useful that all the experiments can be carried out in a buffer solution. He re we use 4-(2-hydroxyethyl)-1piperazine-1-ethanesulfonic acid (HEPES) because it does not bind to metal ions and thus will not interfere with the interac tion between the polymer and the metal ions. From the study of solvent effect on the optical properties of w -P5 we know that in water the polymer is aggregated. Figure 3-12 shows the absorpti on (left) and emission (right) spectra of w -P5 in HEPES buffer (pH = 7.5), which are essentially the same as thos e of the polymer in pure water, indicating the polymer is al so strongly aggregated in the HEPES buffer solution. 0.8 0.6 0.4 0.2 Absorbance 700 600 500 400 300 Wavelength (nm) 1.6 1.2 0.8 0.4 0.0Fluorescence Figure 3-12. Absorption (Left) and emission (right) spectra of w -P5 in 10 mM HEPES buffer solution. [ w -P5 ] = 5 M. Excitation at 380 nm. In the HEPES buffer, quenching experiments of w -P5 with different metal ions were carried out. For all 9 metal ions, the absorption spectra show negligible change, indicating the

PAGE 94

94 addition of metal ions does not lead to furthe r aggregation or confor mation changes of the polymer in the buffer solution. Figure 3-13 shows the absorption a nd emission spectra of w -P5 in 10 mM HEPES buffer with the addition of either Ca2+ or Cu2+. 1.6 1.2 0.8 0.4 0.0 700 600 500 400 Wavelength (nm) (d) 1.6 1.2 0.8 0.4 0.0 700 600 500 400 (b) 0.8 0.6 0.4 0.2 0.0 600 500 400 300 (a) 0.8 0.6 0.4 0.2 600 500 400 300 Wavelength (nm) (c) Absorbance Fluorescence Figure 3-13. Absorption (a) a nd fluorescence (b) spectra of w -P5 in the HEPES buffer (pH = 7.5) with added Ca2+, Ca2+ was added in 1 M aliquots, [Ca2+] ranges from 0 5 M. Absorption (c) and fluorescence (d) spectra of w -P5 in the HEPES buffer with added Cu2+, Cu2+ was added in 1 M aliquots, [Cu2+] ranges from 0 5 M. [ w -P5 ] = 5 M. The arrow shows the direction of change with increasing am ount of metal ions. In contrast to the strong quenching of th e polymer fluorescence in methanol by Ca2+, in the HEPES buffer where the polymer is alre ady aggregated, adding 1 equivalent Ca2+ (5 M) only quenches 15% of the original polymer fl uorescence (Figure 3-13 b). While for Cu2+, which can

PAGE 95

95 quench the polymer fluorescence via PET/ET mechanisms, the addition of 5 M Cu2+ (1 equivalent) quenches 90% of the or iginal fluorescence (Figure 3-13 d). 4.0x106 3.0 2.0 1.0 0.0 Ksv 0 10 20 30 40 50 60 70 80 90 40 30 20 10 0 I0/I 10 8 6 4 2 0 [M2+]/ M Ca2+ Mn2+ Co2+ Ni2+ Cu2+ Zn2+ Cd2+ Hg2+ Pb2+ Ca2+ Mn2+ Co2+ Ni2+ Cu2+ Zn2+ Cd2+ Hg2+ Pb2+ PPE Ca2+Mn2+Co2+Ni2+Cu2+Zn2+Cd2+Hg2+Pb2+PPE Ca2+Mn2+Co2+Ni2+Cu2+Zn2+Cd2+Hg2+Pb2+PPE Figure 3-14. Stern-Volmer plots (upper) of w -P5 emission quenching by different metal ions in 10 mM HEPES buffer and comparison of Ksv values for different metal ions (lower). The inset shows a photograph of w -P5 /M2+ (5 M/10 M) solutions illuminated with a UV lamp. The Stern-Volmer (SV) plots for all the 9 meta l ions are shown in Figure 3-14 (upper). In the low concentration range (0-5 M), a linear co rrelation is observed for all the metal ions except Cu2+, for which an upward curvature occurs a bove the concentratio n of 3 M. Stern-

PAGE 96

96 Volmer quenching constants ( Ksv) for all the metal ions were extrapolated by fitting the linear regions of their SV plots and are compared in the bar graph shown in Figure 3-14 (lower). For most of the metal ions (Ca2+, Mn2+, Ni2+, Zn2+, Cd2+, Hg2+), Ksv values in the range of 104 M-1 105 M-1 are observed. For Co2+ and Pb2+, relatively larger Ksv values of 3.8 105 M-1 and 3.9 105 M-1 are obtained, respectively. The largest Ksv value is obtained for Cu2+ and is in the order of 106, which is comparable to the quenching of w -P5 by MV2. The inset in Figure 3-14 shows a photograph of the fluorescence from the w -P5 /M2+ solutions acquired under UV-illumination. Clearly, the polymer fluorescen ce is strongly quenched by Cu2+, but only slightly quenched by Co2+ and Pb2+ and all the other mixtures remain highly fluorescent. Quenching by methyl viologen (MV2+) in MeOH mediated by Ca2+: an explanation of superlinear quenching behavior for CPE-quencher system. Typically, in most of the CPEquencher systems, the SV plot s exhibit a lin ear correlation at low quencher concentration ra nge and a superlinear behavior (upward curvature) at higher quencher concentrations. Several phenomena have been proposed to acc ount for the superlinear SV quenching behavior, includi ng ion-pair complex formation between the polymer and the quencher,57,61,146 efficient singlet exciton migration w ithin the polymer,57,60 efficient long-range Frster energy transfer between the polymer and quencher,146 and aggregation of polymer chains induced by the quencher.57,71,126 As shown in previous sections, the fluorescence of w-P5 is quenched by Ca2+ in methanol due to the metal ion i nduced aggregation of polymer chains. Consistent with the mechanism, in water where the polymer is already aggregated, there is not much quenching in the fluorescence intensity with the addition of Ca2+. In view of this fact, in order to probe the effect of induced polymer aggregation on the quenching efficiency by oppositely charged quenchers, we carried out the experiments of fluorescence quenching by

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97 MV2+ in methanol, in which th e aggregation state of w -P5 is systematically varied by adding different amount of Ca2+. 40 30 20 10 0I0/I 5 4 3 2 1 0 [MV2+] / M Figure 3-15. Stern-Volmer quenching of 10 M w -P5 emission by MV2+ in water ( ) and in MeOH with 0 M ( ), 2.5 M ( ), 5.0 M ( ), 7.5 M ( ) 10 M ( ) CaCl2. These experiments were carried out by Hui Jiang in Schanze group. Figure 3-15 shows the SV plots of w -P5 emission quenching by MV2+ in pure water, pure methanol and in methanol with added Ca2+ at different concentration. In all cases, a linear relationship is observed at low quencher concentration range, and an upward curvature occurs at certain quencher concentration. Si milar to the quenching of PPE-SO3 with MV2+, quenching of w -P5 is more efficient in water than in methanol. For the aggregated polymer in water, the SV plot is superlinear even at very low quencher concentration (c < 1 M). By contrast, in methanol where the polymer is less aggregated, the SV plot features a distinct i nduction region wherein the correlation is nearly linear and the slope ( KSV) is significantly less than at higher MV2+ concentration ( c > 3 M) where the correlation becomes supe rlinear. As also shown in Figure

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98 3-15, addition of Ca2+ has a pronounced effect on the efficiency by which MV2+ quenches the w -P5 fluorescence. In particular, the range of MV2+ concentrations corresponding to the induction range in the SV plot is compressed in the presence of Ca2+. Remarkably, for methanol solutions that contain >7.5 M Ca2+ (corresponding to appr oximately 1 equiv of Ca2+ per polymer repeat unit), the i nduction range in the SV plots is virtually eliminated, and MV2+ quenches w -P5 emission with efficiency comparable to that seen in aqueous solution. Taken together, these results clearly demons trate the connection between cation-induced CPE aggregation and the superlin ear quenching response typical of CPE-quencher systems. The current results also indicate that the sphere-of-action quenching model,61 which has been applied to analyze the SV quenching data in CP E-quencher systems, is likely not valid with polyvalent quencher ions because the state of the system (i.e., effective exciton radius, effective CPE-quencher ion-pair binding cons tant) is strongly dependent on the relative concentrations of the CPE and quencher. Analysis of the quenc hing efficiency and dyna mics by using a model that incorporates the possibility of 3-dimensional exciton diffu sion within a polymer aggregate that contains many individual CPE chains is clearly more appropriate for the situation. Application to Pyrophosphate (PPi) Sensing Pyrophosphate (P2O7 4-, PPi) is an inorganic anion that is involved in many important biological processes such as cellular si gnal transduction and protein synthesis.147 Fluorogenic sensors for detecting PPi in aque ous solution have been the focus of considerable research. Fluorophore-spacer-receptor sensors based on signal transduction mechanisms such as photoinduced electron transfer (PET)148-152 or monomer-excimer formation151 have been reported. Turn-on sensors that rely on indi cator displacement have also been designed.153-155 However, in these systems small molecule dyes were used as the indicators, therefore the

PAGE 99

99 sensitivity to PPi is limited due to the comparat ively inefficient quenching of the indicators by the receptors. Because of their signal amplification property, conjugated polyelectrolytes have been used for sensing a variety of analyt es, including heavy metal ions,142,143 DNA,18,58,59,78 RNA18 and some proteins.18,58,156 However, examples of using CPE to specifically sense pyrophosphate (PPi) have not yet been reported. Since many of the small molecule sensors for PPi were designed utilizing the binding of PPi to the me tal-complex, we anticipated that the current CPE/metal ion system could provide a new pl atform to develop efficient sensors for pyrophosphate. Based on the results describe d above, we chose a composite system of w -P5 and Cu2+, in which the concentration ratio between the polymer and Cu2+ is 1:2 (with respect to carboxylate groups which are th e actual binding sites for metal, th e ratio is 1:1). At this ratio ([PRU]/[Cu2+] = 5 M/10 M) the intensity of polym ers fluorescence is quenched by more than 98% at max = 530 nm compared to the intensity of a solution containing only polymer ([PRU] = 5 M). Figure 3-16 (a) illustrates the fluorescence intensity change of w -P5 /Cu2+ in the HEPES buffer at pH 7.5 with the addition of various amount of PPi. Titra tion of PPi into the w -P5 /Cu2+ (5 M/10 M) solution results in a continuous re covery of the polymers fluorescence intensity, and at 10 M of added PPi (1 equivalent relative to [Cu2+]) a 17-fold enhancement of fluorescence intensity is observed. As shown in Figure 3-16 (b), th e full titration curve of adding 0-100 M PPi displays a sigmoidal progression indi cating that multiple equilibria are involved in the process.157 These equilibria likely involve dissociation of the w -P5 /Cu2+ complex and formation of the Cu2+/PPi complex. The most significant fluorescence increase is seen with addition of PPi up to 20 M (30-fold enhancem ent). Above that con centration the increase

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100 occurs more gradually, and at [PPi] = 50 M (5 equivalents added) approximately 85% of the polymers initial fluorescence intensity is reco vered (38-fold enhancem ent) (nearly100% of the fluorescence intensity is recovere d at 100 M [PPi]). During the en tire PPi titration, there is no shift of the emission maximum, and UV-Vis absorpti on exhibits only a small blue shift (< 5 nm). This finding suggests that PPi acts mainly by sequestering Cu2+, and that there ar e not significant changes in the polymer aggregation st ate occur during the anion titration. Figure 3-16. (a) Fluorescen ce spectra of a solution of w -P5 /Cu2+ (5 M/10 M) titrated with PPi in 10 mM HEPES buffer at pH 7.5, 25 C. [PPi] ranges from 0 10 M. (b) Intensity enhancement (I/I0) at 530 nm titrated with PPi (0-100 M). Excitation at 380 nm. [PPi]/M 020406080100 I/I0 0 10 20 30 40 Wavelength (nm) 500600700 FL intensity (x10 5 ) 2.0 4.0 6.0 8.0 0 M 10 M

PAGE 101

101 Selectivity The selectivity of the w -P5 /Cu2+ sensor for PPi was evaluated by carrying out the same fluorescence titration with other anio ns including monovalent anions (F-, Cl-, Br-, I-, HSO4 -, NO3 -, HCO3 -, H2PO4 -, CH3CO2 -) and divalent anions (SO4 2-, CO3 2-, HPO4 2-). Addition of 50 M any of these anions induced only a small change of the w -P5 fluorescence intensity (typical change was less than 2%). In another experime nt, two separate solutions were prepared; one contained 50 M each anion (totally 12 anions) except PPi, the other contained all 13 anions. Only the latter solution showed fluorescence, dem onstrating that PPi was specifically detected. Figure 3-17. Fluor escence response of w -P5 /Cu2+ (5 M/10 M) to various anions at 50 M concentration in 10 mM HEPES bu ffer at pH 7.5, 25 C (1, F-; 2, Cl-; 3, Br-; 4, I-; 5, HPO4 2-; 6, H2PO4 -; 7, P2O7 4-; 8, CH3CO2 -; 9, HSO4 -; 10, NO3 -; 11, HCO3 -; 12, SO4 2-; 13, CO3 2-). Inset shows a photograph of the sens or/anion solutions illuminated with a UV-lamp. Figure 3-17 compares the response of w -P5 /Cu2+ to the addition of 50 M of 12 different anions. This presentation shows that the sensor is highly selective to PPi with an intensity enhancement of 40-fold, compared with all of the other the anions te sted, including phosphate 1 2 3 4 5 6 7 8 9 10 11 12 13 I / I0 0 10 20 30 40 50 60 1 2 3 4 5 6 7 8 9 10 11 12 13

PAGE 102

102 [PPi]/uM 024681012 I/I0 0 2 4 6 8 10 12 14 16 18 20 0.00.20.40.60.8 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 (Pi: H2PO4 and HPO4 2-). The inset shows a photogra ph of the fluorescence from the w -P5 /Cu2+/anion solutions acquired under UV-illumina tion. The photograph clearly shows that the fluorescence of the polymer is very strong in the presence of PPi, whereas it is very weak in the presence of the other anions. Sensitivity As mentioned, one of the advantages of CPEbased sensors over small organic dye-based sensors is the signal amplification imparted by the conjugated polymer because of the molecular wire effect. To date, the sensitivity for most PPi sensors achieved by small organic dyes in aqueous solution is still in the micromolar range The sensitivity of the current sensor was determined by carrying out the titration of w -P5 /Cu2+ with PPi in a concentration range of 0.1 10 M. Figure 3-18. Intensity enhancement (I/I0) at 530 nm of w -P5 /Cu2+ (5 M/10 M) titrated with PPi (0-10 M). The inset shows the emission enhancement at lower PPi concentrations.

PAGE 103

103 As shown in Figure 3-18, an upward curvature occurs after adding ve ry low concentration of PPi, which is consistent with the sigmoidal progression in the full ti tration curve (Figure 314). However, in the concentration range of 0.1 0.7 M, a nearly linear relationship is obtained (Inset). The analytical detection limit is dete rmined to be 80 nM using the following equation: D.L. = 3 bk/m ( bk standard deviation of the bla nk; m, slope of the curve).158 This is the first CPE-based sensor for PPi with sensitivity higher than that of most PPi sensors based on organic dyes or small molecules and with a high selectiv ity over many other inorga nic anions, including H2PO4 and HPO4 2-. Sensing mechanism Based on the above studies of the sensing respon se to various anions, the selectiv ity of the current sensor for PPi is believed to aris e from the ability of PPi to chelate Cu2+ via the diphosphate moiety.159 The proposed sensing mechanism is shown in Figure 3-19. Figure 3-19. Schematic represen tation of the sensing of PPi by w -P5 /Cu2+ system. Initially the fluorescence of the polymer is quenched by Cu2+ through the formation of w P5 /Cu2+ weak complex by binding to the carboxylate gr oups, and then the fluorescence is turned on by the addition of PPi because the formation of PPi/Cu2+ complex (2/1 complex, most possibly) disrupts the binding of Cu2+ with the polymer carboxylat e groups. The hypothesis that the diphosphate unit acts by binding to and sequestering Cu2+ is further supported by the observation that the addition of 10 M (1 equivalent) of the strong chelator ethylenediamine

PAGE 104

104 tetraacetic acid (EDTA) to the w -P5 /Cu2+/HEPES solution leads to the complete recovery of the polymers fluorescence intensity. As the last point, since the selective se nsing of PPi in the presence of adenosine triphosphate (ATP), adenosine diphosphate (ADP) is important for some biosensor applications, the response of the current se nsor to these two organic an ions was also checked. Not surprisingly, addition of ATP e licits a very similar fluorescen ce response compared to PPi. However, the fluorescence response to ADP is mark edly lower than that of PPi; at saturation, addition of ADP leads to only 25% recovery of the initial fluorescence intensity whereas as noted above addition of PPi lead s to > 95% fluorescence recovery. Design of sensors based on CPEs that can discriminate PPi, ATP and ADP is the goal of future research. Experimental Materials Pd(PPh3)2Cl2 and Pd(PPh3)4 were purchased from Strem Ch emical Company and used as received. 1,4-diiodo-2,5-dimethoxybenzene, bromo acetic acid, trimethylsilylacetylene (98%), 1dodecanol, bis(trifluoroacetoxy)phen yliodine, tetrabutylammonium fluoride (1 M in THF) and tetrabutylammonium hydroxide (1 M in methanol) were purchased from Acros Chemical Company. Et3N and THF used in the polymerization were purified by dist illation over CaH2. For all the metal ions, their chloride salts were us ed. For all the anions, their sodium salts were used. ATP and ADP were purchased from Aldrich chemical Co. and kept in a refrigerator. All the other chemicals were supplied by either Acros or Aldrich Chemi cal Company and used without further purification. Instrumentation NMR spectra were recorded on a Varian VXR-300 or Gemin-300 FT-NMR, operating at 300 MHz for 1H-NMR and at 75.4 MHz for 13C-NMR. High temperature NMR spectra were

PAGE 105

105 recorded on a Varian Mercury 300 FT-NMR. Chem ical shifts were reported in ppm using CHCl3 or C2HD5SO as internal reference. FTIR spectra were taken on a Perkin-Elmer 1600 spectrometer. Gel permeation chromatography (G PC) analyses were carried out on a system comprised of a Rainin Dynamax SD-200 pump and a Beckman Instruments Spectroflow 757 absorbance detector. UV-visible absorption sp ectra were obtained on Varian Cary 100 dual beam spectrophotometer, with a scan rate of 300 nm/min. Steady-state fluorescence spectra were recorded on a PTi fluorescence spectrometer. General Methods Fluorescence quenching and absorption titrati on experiments were carried out by microtitration in a fluorescence cuvette. In a typica l titration experiment, 2 mL of a polymer solution was placed in a 1 cm quartz fluor escence cell with a small magnetic stir bar. The UV-Visible absorption and fluorescence spectra were record ed at room temperature. Then fluorescence and/or absorption spectra were repeatedly acquired subsequent to the additi on of L aliquots of a concentrated solution that contained the quencher. Quencher solution aliquots were delivered by using a calibrated Eppendorf mi cro liter pipetter. For ani on sensing experiments, the w -P5 /Cu2+ (1/2, concentration ratio) stock solution was prepared one nigh t before the anion titration experiments. The photograph was taken using a Pe ntax Digital Camera (5 Megapixel) with a bandpass filter (LG 450). Synthetic Procedures 1,4-Diiodo-2,5-dimethoxybenzene (2). In a 1000 ml three-neck ed flask fitted with a condenser and a mechanical stirring was charged with a mixtur e of glacial acetic acid (200 ml), H2SO4 (3 ml), H2O (7 ml), KIO3 (7.8 g, 36.5 mmol) and I2 (19.3 g, 76.0 mmol). 1,4Dimethoxybenzene (10.0 g, 72.4 mmol) was added to the mixture and the dark purple mixture was heated at 120 C and stirred for 15 h. Then the purple solution was po ured into 200 ml of

PAGE 106

106 NaHSO3 (10%). The grey precipitate was collected on a Bchnel f unnel, washed with 200 ml of cold water and dried overnight in the hood. Th e solid was dissolved in 200 ml of THF/CHCl3 (1/1) and treated with activated carbon. After filteri ng through a bed of celite, a nice yellow crystalline solid was obtained after the solven t was removed under reduced pressure (Yield: 53%). 1H NMR (CDCl3, ppm): 7.17 (s, 2H), 3.81 (s, 6H). 2, 5-Diiodohydroquinone (3).27 This compound was synthesized using a literature procedure modified for ea sier work-up. Compound 2 (10.0 g, 38 mmol) was dissolved in 300 ml of dry CH2Cl2 in a 1000 ml round bottom flask fitted with a condenser and th e resulting mixture was cooled to -78 C (dry ice/acetone bath). Boron tribromide (14 ml, 148 mmol) was added slowly through the condenser in 1 hr. Upon the completion of the addition, the yellow suspension was stirred at -78 C for 2 h and then allowed to warm to room temperature. The brown solution was further stirred for 48 h. Then the reaction was quenched by adding 15 ml of cold water (Violent gas evoluti on!). After the gas evolution subsided, 300 ml of water was poured into the flask. With the addition of 100 ml acetone, the white prec ipitate formed during the addition of water was re-dissolved in the solution. The bottom laye r was collected and the solvent was removed in vacuo Dissolution of the crude product in a minimum of CH2Cl2 followed with precipitation into hexane afforded a slight yellow powder (yield: 12.0 g, 87%). 1H NMR (DMSO-d6, ppm): 9.78 (s, 2H), 7.14 (s, 2H). 2,2'-(2,5-Diiodo-1,4-phenylene)bis (oxy)diacetic acid (4). In a 100 ml of three-necked round bottom flask purged with argon, a solution of 2.6 g of NaOH (66 mmol) in 10 ml of water was added drop-wise to a st irred solution of 5.4 g of 3 and 5.0 g of bromoacetic acid (36 mmol) in 25 ml of water. The reaction mixture was heat ed at reflux for 4 h. At which time, there was a lot of white precipitate formed. The reaction flas k was then placed in a refrigerator overnight.

PAGE 107

107 The settled precipitate was collect ed by vacuum filtration, then re -dissolved in 60 ml of water and acidified with 20 ml of 6 N HCl. The white solid obtained was collected by vacuum filtration, dried under the vacuum. Yield: 6.0 g, 84%. 1H NMR (DMSO-d6, ppm): 13.07 (s, br, 2H), 7.25 (s, 2H), 4.74 (s, 4H). 13C NMR (DMSO-d6, ppm): 169.76, 151.78, 122.28, 86.13, 66.03. 2,2'-(1,4-Phenylenebis(oxy ))diacetic acid (6). A solution of 5.0 g (125 mmol) of sodium hydroxide in 20 ml of water was added drop-wise to a stirred so lution of 3.3 g (30 mmol) of hydroquinone and 9.2 g (66 mmol) of bromoacetic aci d in 30 ml water. The resulting solution was refluxed for 4 hr, then cooled to 2-5 C whereupon the sodium salt of 6 settled at the bottom of the flask. The white solid was collected on a Bchnel funnel, washed with 60 ml of water, 60 ml of methanol, dried and dissolv ed in 30 ml of hot water, then acidified with 6 N HCl. The white precipitate was collected by vacuum filtration, rinsed w ith cold water and dried under vacuum. Yield: 4.5 g, 67%. 1H NMR (DMSO-d6, ppm): 12.94 (s, br, 2H), 6.83 (s, 4H), 4.60 (s, 4H). 13C NMR (DMSO-d6, ppm): 140.32, 122.06, 85.25, 34.96. Didodecyl 2,2'-(1,4-phenylenebis(oxy))diacetate (7). A mixture of 1.5 g (6.6 mol) of 6 25 g (134 mmol) of dodecyl alcohol and 1 ml of 85% phosphoric acid was he ated at 150 C in a two-necked round bottom flask with a magnetic stirring bar. The water formed during the esterification was removed by flowing nitrogen sl owly through the top of the reaction flask. After 6 hr, the clear solution was poured into 300 ml of hot hexane. The resulting mixture was cooled in a refrigerator overnight. The obtaine d white solid was collected by vacuum filtration; further recrystallization from isopropanol affo rding a white crystall ine product (yield: 3.1 g, 84%). 1H NMR (CDCl3, ppm): 6.86 (s, 4H), 4.57 (s, 4H), 4.20 (t, 4H). 1.65 (m, 4H), 1.27 (m,

PAGE 108

108 36H), 0.89 (t, 6H). 13C NMR (CDCl3, ppm): 169.12, 115.80, 66.17, 65.46, 31.93, 29.65, 29.36, 19.21, 28.54, 25.80, 22.71, 14.14. Didodecyl 2,2'-(2,5-diiodo-1,4-phe nylene)bis(oxy)diacetate (8). Route A: A mixture of 5.0 g (10.5 mmol) of 4 37.2 g (0.2 mol) of dodecyl alcohol and 0.5 g of phosphoric acid was heated at 150 C in a two-necked round bottom flask with magnetic stirring for 6 h. The water formed during the esterification was rem oved by flowing nitrogen slowly through the top of the reaction flask. After 6 hr, the clear solution was poured in to 300 ml of hot hexane. The resulting mixture was cooled in a refrigerator overnight. Th e obtained white solid wa s collected by vacuum filtration; further purification was done by recrystallization from iso-propanol (yiled: 85%). Route B: compound 7 (5.2 g, 9.2 mmol) iodine (2.3 g, 9.2 mmol) and bis(trifluoroacetoxy)phenyliodine (4.3 g, 9.0 mmol) were mixed in 25 ml CCl4. The purple suspension was stirred at room temperatur e for 6 h, and then diluted with 75 ml CH2Cl2, washed with 5% NaHSO3 till the purple color disappeared. The solution was then dried with anhydrous MgSO4 and the solvent was removed under reduced pressure. The obtained yellow solid was further purified by recrystalliz ation from 25 ml isopropanol, wh ite crystal product was obtained (yield: 6.8 g, 90%). 1H NMR (CDCl3, ppm): 7.16 (s, 2H); 4.62 (s, 4 H); 4.22 (t, 4H); 1.67 (m, 4H), 1.27 (m, 36 H); 0.89 (t, 6H). 13C NMR (CDCl3, ppm): 168.53, 153.23, 124.03, 86.61, 67.75, 32.37, 30.09, 29.97, 29.81, 29.67, 28.99, 26.28, 23.15, 14.59. Didodecyl 2,2'-(2,5-bis((trimethylsilyl)eth ynyl)-1,4-phenylene)bis(oxy)diacetate (9). Compound of 8 (814 mg, 1 mmol) was dissolved in 10 ml of dry THF/Et3N (3/1, v/v) in a Schlenk flask and degassed with argon for 15 minutes. Then 21 mg of Pd(PPh3)2Cl2 (30 mol) and 12 mg of CuI (63 mol) were added, followed with the addition of 0.7 ml of trimethylsilylacetylene (5 mmol). The reaction was stirred at room temperat ure for 22 hr. After

PAGE 109

109 filtration through a bed of celite (~ 10 cm), the solvent was removed in vacuo The crude product was used for next step without further purification. Didodecyl 2,2'-(2,5-diethynyl-1,4-ph enylene)bis(oxy)diacetate (10). The crude product obtained as above was dissolved in 10 ml of THF. Tetrabutylamm onium fluoride (3 ml of a 1 M solution in THF) was added and the resulting mixt ure was stirred at room temperature for 1 hr. Then the solution was diluted with 20 ml of et hyl ether, poured into a seperatory funnel and washed with water (30 ml 1). The organi c layer was collected a nd the water layer was extracted with ethyl ether (30 ml 1). The orga nic solutions were combined and removal of the solvent afforded a brownish crude product. It was dissolved in a minimum of CH2Cl2 and loaded on the top of silica column. The colu mn was first eluted with 300 ml CH2Cl2/hexane (v/v = 1/2), then with a mixture of CH2Cl2/hexane (v/v = 3/1). A slight yellow solid was obtained (yield: 400 mg, 66% for 2 steps). 1H NMR (CDCl3, ppm): 6.95 (s, 2H), 4.66 (s, 4H), 4.18 (s, 4H), 3.37 (s, 2H), 1.56 (m, 4H), 1.26 (s, 36H), 0.87 (t, 6H). 13C NMR (CDCl3, ppm): 168.44, 153.69, 118.65, 114.01, 83.53, 78.95, 66.82, 65.64, 31.97, 29.69, 29.68, 29.62, 29.55, 29.40, 29.25, 28.57, 25.85, 22.74, 14.16. General polymerization procedure for the preparation of P4 and P5. A solution of 2,5-bis(dodecyloxycarbonylmethoxy)-1,4diiodobenzene (204 mg, 0.25 mmol) ( 8 ) and didodecyl 2,2'-(2,5-diethynyl-1,4-phe nylene)bis(oxy)diacetat e (153 mg, 0.25 mmol) ( 10 ) or 1,4diethynylbenzene (32 mg, 0.25 mmol) in 10 mL of dry THF/Et3N (v/v = 2/1) was degassed with argon for 15 minutes. Then 8.7 mg of Pd(PPh3)4 ( 7.5 mol) and 4 mg of CuI (7.5 mol) were added under argon. The reaction was stirred at 60 C for 18 hr. The obtained viscous suspension was poured into 150 mL of methanol, resulting in the precipitation of the ester precursor polymers ( P4 or P5 ) as orange powders and light yellow fi bers, respectively. The polymers were

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110 further purified by two repeated cy cles of dissolution in THF and pr ecipitation into methanol. A small amount (~30 mg) of each polymer was dried completely under vacuum and used for NMR study immediately (note that after complete drying these polymers are barely soluble in their good solvents, such as THF and CHCl3 and the solubility becomes worse with storing). The rest of the polymer was kept as a TH F solution till the next step. Hydrolysis To a solution of P4 (117 mg, 0.20 mmol) or P5 (135 mg, 0.20 mmol) in 30 mL of dioxane/THF (v/v = 5/1) was added 1.5 mL of 1 M (n-Bu)4NOH in methanol, and the resulting mixture was stirred at room temperatur e for 24 hr. During the course of the hydrolysis reaction, 2 mL of water were systematically added in order to keep the so lution clear. Then a solution of 0.20 g of NaClO4 (1.6 mmol) in 3 mL of water was added to the hydrolyzed polymer solution, and the resulting mixtur e was poured into 400 mL of co ld acetone, resulting in the precipitation of w -P4 and w -P5 as fine orange and yellow powd ers, respectively. The polymer ( w -P4 or w -P5 ) was then dissolved in 50 mL of dei onized water (several drops of 1 N NaOH solution were added) and was purified by dialysis against deionized wate r using a regenerated cellulose membrane (12 kD molecu lar weight cut-off). After di alysis, the polymer solution was filtered through 1.0 m glass fiber filter (Fisher Scientific) a nd stored as the stock solution in the refrigerator. P4. GPC (THF, polystyrene standard): Mw = 26, 790, Mn = 12, 420, PDI = 2.10. w -P4 1H NMR (D2O, ppm): 7.22 (br, s, 2H), 4.56 (br,s, 4H). P5 1H NMR(CDCl3, ppm): 7.56 (br, s, 4H), 7.02 (br, s, 2H), 4.74 (br, s, 4H), 4.24 (br, m, 4H), 1.68 (br, m, 4H), 1.25 (br, m, 36H), 0.88 (br, t, 6 H). GPC (THF, polystyrene standard): Mw = 285, 390; Mn = 126, 680, PDI = 2.30. FT-IR ( max, KBr pellet): 3117, 3039, 2924, 2853, 2208, 1763, 1732, 1598, 1518, 1489, 1467, 1440, 1411, 1282, 1185, 1079, 836, 721, 600, 544.

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111 w -P5 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.53 (br, s, 4H), 6.97 (br, s, 2H), 4.37 (s, 4H). FT-IR ( max, KBr pellet): 3332, 2924, 2191, 1778, 1610, 1445, 1323, 1282, 1196, 1047, 941, 880, 837, 779, 700, 668, 600, 544.

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112 CHAPTER 4 VARIABLE CHAIN LENGTH POLY(PHENYLENE ETHYNYLENE) CARBOXYLATE Introduction The relationship between the length of c onjugated molecules and their optical and electronic properties has b een studied intensively.43,160-162 Most of the research has been focused on the synthesis of monodisperse oligomers w ith precisely defined length to obtain such information. Unfortunately, preparing these o ligomers often requires many synthetic steps and tedious separation and purification.163-165 Although conjugated oligomers provide excellent model systems for interpreting the electronic and op tical properties of th eir polymeric analogues, study of the chain length dependence in polymeric systems could be more useful in a practical point of view. Molecular weight of conjugated polymer has been shown to play an important role in determining the chain conformation, solid state morphology and thus optical and electronic properties of c onjugated polymer thin films. For example, Koynov et al.166 found that the refractive index and birefringence of pol y(2-methoxy-5-(2-ethyl-h exyloxy)-1,4-phenylene vinylene) thin films are strongly dependent on the molecular weight of the polymers. Goh et al.167 also reported that the hole mobility of regioregular poly(3-hexyl-thiophene) in fieldeffect transitors increases almost 4 orders of magnitude when the molecular weight is varied from 3.2 kD to 36.5 kD. As mentioned earlier in Chapter 1, amplif ied fluorescence quenching is an inherent property of conjugated polymers due to the efficient exciton migr ation along the polymer chain. Thus the signal enhancement of the polymer relative to the small model compound will be a factor that depends on the chain length of the polymer. Swager and co-workers55 compared the quenching constants of their receptor poly(pheny lene vinylene)s with 3 different molecular weights by methyl viologen and poi nted out that the increase in molecular weight can produce

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113 greater enhancement only when the chain length of polymer is smaller than the diffusion length of the exciton. For conjugated polyelectrolytes, several research ers have also pointed out that chain length has a big effect on the quenching effi ciency of CPEs, but their results were based on comparing only two different molecular weight sa mples, one extremely low, one substantially high.60,168 In this chapter, we present a systematic study of the relationships between photophysical properties, quenching behaviors of CPEs and thei r molecular weights. Th e successful synthesis of anionic poly(p-phenylene ethynyl ene) (PPE)s with 5 different chain lengths was realized via the precursor route.129 The molecular weight of the orga nic soluble polymer precursors was varied in a well-controlled manner by cha nging the loading amount of a monofunctional monomer, 1-iodo-4-(trifluoromethyl)benzene, in the mixture of bifunctional monomers, 2,5bis(dodecyloxycarbonylmethoxy)1,4-diiodobenze (AA-type) a nd 1,4-diethynylbenzene (BBtype). The molecular weight of each polymer was determined by gel permeation chromatography (GPC), as well as NMR end-gro up analysis. The series of water-soluble conjugated PPEs with the same chain length as derived from their or ganic-soluble analogues were obtained by base-catalyzed hydrolysis. Then we carried out a systematic photophysical investigation of the series in methanol and wate r. To the end, the electron transfer quenching behavior of the series of CPEs with methyl viologen (MV+, MV2+) and heptyl viologen (HV+ and HV2+) were explored in methanol. Results and Discussion Synthesis The structures of the monomers and polymer s used in the current work are shown in Figure 4-1. 2,5-Bis(dodecyloxycar bonylmethoxy)-1,4-diiodobenze ( 1 ) and 1,4-diethynylbenzene ( 2 ) were synthesized following the procedures re ported in the previous chapters. 1-Iodo-4-

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114 (trifluoromethyl)benzene ( 3 ) was purchased from Aldrich and used as the monofunctional monomer. Its reaction with the growing polymer can generate chain ends devoid of the reactive functional group (the iodide group) making the chain incapable of further reaction. Thus the monofunctional monomer is often referred as the e nd-capper for the polymer chains. In such a polymerization, the amount of the monofunctiona l monomer in the init ial reaction mixture controls and limits the polymerization of bifunc tional monomers, which leads to polymers with varied molecular weights. In order to minimi ze the influence of the polymerization by other factors, such as catalyst loading, monomer conc entration, etc., all the polymerizations were carried out in the mixture of THF/Et3N (v/v, 3/2) with monomer concentrations for the bifunctional monomers kept at 25 mM in the presence of Pd(PPh3)4 and CuI catalysts. Figure 4-1. The synthetic route for conjugated polyelectrolytes with different chain lengths Since polycondensation based on Sonogashir a coupling follows the step-growth polymerization mechanism, the resulting degree of polymerization (DP) is related inversely to the extent of the reaction ( p ) by the equation:133 DP = 1/(1p ) (4-1)

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115 if the two reactive monomers are present in a stoich iometric balance. In view of this, first we carried out a polymerizat ion using a stoichiometric amount of monomer 1 and 2 in the absence of monofunctional monomer 3 The polymerization process was monitored using GPC and indeed we observed typical GPC traces characteristic of the step-growth polymerization. The numberaverage molecular weight of the final polymer is 127 kD corresponding to a DP of 187, as determined by GPC using polystyrene as the standa rd in THF. The extent of polymerization ( p ) is estimated to be 0.995 by a revers e calculation using the equation of p = 1-1/DP. Assuming a similar p for all the reactions under the same polymer ization conditions, the expected molecular weights of the other polymers by polymerizing 1 and 2 in the presence of varying amounts of 3 (from 25% to 1%) were predicte d using the Carothers equation133 and listed as column 3 in Table 4-1. Table 4-1. GPC and 1H NMR end-group analysis of the precursor polymers. Note: a molar percent; bDP calculated using Carothers equation:133 DP = (1+ r )/(1+ r -2 rp ); r stoichiometric imbalance; p extent of reaction. Polymer Characterization Gel permeation chromatography The molecular weights (MWs) of all the polymers were measured in THF solutions using GPC with a UV detector. The GPC chromatogram s of all the polymers are shown in Figure 4-2, GPC NMR End-capper (%)a [DP]b Mn DP PDI DP PPE-CO2R-7 25 9 5,390 7 1.80 7 PPE-CO2R-13 15 14 9,040 13 2.30 12 PPE-CO2R-35 5 33 24,390 35 2.30 21 PPE-CO2R-49 3 51 33,760 49 2.30 28 PPE-CO2R-108 1 100 74,350 108 2.70 85 PPE-CO2R-187 0 127,950 187 2.90

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116 indicating monomodal distributio ns of molecular weights for all the polymers. The numberaverage molecular weights (Mn) obtained from GPC (column 4 in Table 4-1) correspond well with the predicted Mn, suggesting that all th e polymers with contro lled chain length are functionalized at both ends with the end-capper. The polydispersi ty indices (PDIs) of all the polymers are ~ 2, which is in agreement with Fl ory-Schulz distribution of the molecular weight for ideal step-growth polycondensation.169 For clarity, all the polymers are named in this format, PPE-CO2X-DP (X = R for organic precursors and X = Na for CPEs). 1.2 0.9 0.6 0.3 0.0Response 18 15 12 9 6Minutes Increasing MW Figure 4-2. GPC chromatograms of the precursor polymers, PPE-CO2R-7 ( ), PPE-CO2R-13 ( ), PPE-CO2R-35 ( ), PPE-CO2R-49 ( ), PPE-CO2R-108 ( ) and PPE-CO2R-187 ( ). Symbols were added to delineate different curves. NMR spectra Each polymer was characterized by 1H NMR spectroscopy in order to examine the purity and prove the existence of the e nd groups. As an example, the 1H NMR spectrum of PPECO2R-7 is shown in Figure 4-3. The singlet at = 4.73 ppm (d) is assigned to the methylene group protons ( O CH2CO2C12H25 ) and the other signals in the aliphatic area (e, f, g, h) are

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117 caused by the dodecyl groups. The inset shows th e expansion of the aromatic region. The aromatic protons of the end groups (trifluor omethylbenzene) appear as a singlet at = 7.64 ppm (a)170 and the aromatic protons of the 1,4-phenyl ene and 2,5-disubstituted-1,4-phenylene on the backbone appear at =7.56 ppm (b) and =7.02 ppm (c), respectively. 8 7 6 5 4 3 2 1 0 ppm a b c d e f g h CDCl3 7.8 7.6 7.4 7.2 7.0 ppm a b CDCl3*c Figure 4-3. 1H NMR spectrum of PPE-CO2R-7 in CDCl3. The starred signal at 7.17 ppm is from octafluoro[2.2]paracyclophane adde d as the internal standard for 19F NMR study. Analysis of the molecular weight using 19F NMR was unsuccessful due to the distinct relaxation behavior of the polymer and the small fluorinated compound. End group analysis by NMR The GPC method used to determine the molecula r weights of our polymers is sensitive to the employed calibration standards.169 It has been proven that using the polystyrene calibration to evaluate the GPC of rigid-rod polymers (lik e ours) overestimates the molecular weight by a factor of 1.4 ~ 2.0.10 Since the aromatic protons from the end group phenyl (a) and backbone phenyl (b and c) can be discriminated in the 1H NMR spectra, the average number of repeat units

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118 (n or DP) can be determined by comparing the in tegrations of proton signals from the end group and the main chain 7.8 7.6 7.4 7.2 7.0 ppm a b c n=7 n=13 n=34 n=49 n=108 Figure 4-4. The aromatic region in the 1H NMR spectra of the precurs or polymers with variable chain length in CDCl3. As shown in Figure 4-4, with the increase of the molecular weight, the signal at =7.56 ppm (b) corresponding to the 1,4-phenylene on th e backbone becomes broader and begins to overlap the signal for the end groups (a, = 7.64 ppm). By contrast another signal from the polymer backbone protons at =7.02 ppm (c, 2,5-disubstituted-1,4phenylene) is well separated from the end group signal. Thus the DPs of th e polymers were extrapolated by comparing the integration at = 7.64 ppm (a) to the integration at =7.02 ppm (c) and the results are listed in

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119 Table 4-1. The values obtained by 1H NMR are close to the values determined by GPC for lowMW samples (PPE-CO2Na-7 and PPE-CO2Na-13). For medium-MW samples (PPE-CO2Na-35 and PPE-CO2Na-49), the values calculated by 1H NMR differ by a factor of 1.7-1.8 compared to the values by GPC. And a smaller factor of 1.3 is resulted for high-MW sample, PPE-CO2Na108. These results are consistent with other obser vations made for rigid-rod oligomers and highMW PPEs.10,46,171 Since GPC is an indirect measure of the molecular weight based on the hydrodynamic volume,169 the rigidity of the PPE chains has a strong eff ect on the accuracy of the molecular weight obtained by GP C using randomly coil polystyrene as the standard. In the lowMW region, prior to significant coiling of polys tyrene chains, GPC gives accurate measurement of molecular weight for rigidrod like PPEs. For medium-MW PPEs, the molecular weights by GPC are largely exaggerated because of the signif icant coiling of polystyrene chains. However, for high-MW PPEs, the polymer chains become considerably more flexible, which makes polystyrene a better standard again. Structural characterization of water-soluble polymers The water-soluble polyelectrolytes of different chain length were prepared following the procedure used in Chapter 3 for hydrolyzing the dodecyl ester groups. After hydrolysis, the polymers were characterized by 1H NMR and IR, as shown in Figure 4-5. The 1H NMR spectra of all the polymers in the D2O/DMSO-d6 mixture (v/v, 1/1) show only one signal in the aliphatic area at = 4.35 ppm corresponding to the methylene groups ( O CH2CO2Na ), which confirms the complete cleavage (> 95%) of the ester groups. In the aromatic region, the protons from the polymer backbone are observed. Similar to the changes in the 1H NMR spectra of their organic precursor polymers, with the increase of the mo lecular weight, the signal for the end groups becomes weak and is barely detectable for high-MW polymers.

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120 9 8 7 6 5 4 3 2 1 0 ppm b c d n=7 n=13 n=34 n=49 n=108 (a) 2000 1500 1000 Wavenumber (cm-1) n=7 n=13 n=34 n=49 n=108 (b) Figure 4-5. 1H NMR (a) and IR (b) spectra of the wate r-soluble polymers with different chain length. The starred signals at 4.05 ppm and 2.50 ppm in the 1H NMR spectra are due to DMSO-d6 and D2O, respectively. Infrared spectra for all the water-soluble polymer s are shown in Figure 4-5 (b). As there is no structural difference between these polymers ex cept for the molecular weight, IR spectra of these polymers are very similar to ea ch other. The absorption at 1600 cm-1 and 1400 cm-1 can be attributed to the symmetrical and unsymme trical vibration of the carboxylate (CO2-) group, respectively. Another st rongly absorption at 1205 cm-1 for all the polymers arises most possibly from the unsymmetrical stretching of C-O-C bond. Photophysical Characterization In methanol Figure 4-6 (a) illustrates the absorption and emission spectra of the five CPEs in MeOH. Across the series, the absorption maximum ( *) shifts from 404 nm (PPE-CO2Na-7),

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121 6.0 4.5 3.0 1.5 0.0 M-1cm-1) 500 400 300 (a) 8.0 6.0 4.0 2.0 max Chain Length 0.8 0.6 0.4 0.2 0.0FL 7 13 3449108187 (c) 0.6 0.4 0.2 0.0 Intensity (A.U.) 700 600 500 400 Wavelength (nm) (b) Figure 4-6. Absorption (a) and emission spectr a (b) of the series in methanol, PPE-CO2Na-7 ( ), PPE-CO2Na-13 ( ), PPE-CO2Na-35 ( ), PPE-CO2Na-49 ( ), PPE-CO2Na-108 ( ). Symbols were added to de lineate different curves. (c) Dependence of molar extinction coefficient ( ) and fluorescence quantum yield ( ) on the polymer chain length. Arrows show the direction of change with increasing chain length.

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122 411 nm (PPE-CO2Na-13) to 417 nm (PPE-CO2Na-49) due to the fact that conjugation length increases when chain length increases. This e ffect has been well docum ented by the intensive studies of well-defined oligomers.43,160 For the current series, the extension of conjugation saturates at this point, furthe r increase of molecular weight of the polymer leads to the appearance and the growing of the shoulder absorption (432 nm ) and eventually the shoulder becomes the maximum absorption peak (PPE-CO2Na-108). Based on our studies on PPEs with sulfonate71 and carboxylate side groups172 and other peoples work,140 this peak at 432 nm is assigned to planarization of PPE chains due to aggregation. Besides the change of wavelength for samples with different chain lengths, the tre nd of increasing molar extin ction coefficient from 3.6 104 M-1cm-1 (PPE-CO2Na-7) to 5.8 104 M-1cm-1 (PPE-CO2Na-35) and 6.9 104 M-1cm1 (PPE-CO2Na-187) is also evident. The less-than -expected molar extinction coefficients for PPE-CO2Na-49 and PPE-CO2Na-108 might be due to the emerging new band because of aggregation. In contrast to the clear red-shift of the ab sorption maximum dependi ng on the chain length, all the polymers emit at the same wavelength of 436 nm (0-0 band), as shown in Figure 4-6 (b). This can be explained by the fact that these polymers are poly-dispersed (PDI 2) and efficient energy transfer could occur among segments of different conjugation length for each polymer sample. Eventually, the emission is from th e segments with the longest conjugation length (lowest energy). As also seen in Figure 4-6 (b) in which the emission spectra are normalized according to respective quantum yield, it is obvi ous that the quantum efficiency decreases dramatically when the polymer chain lengt h increases from low-MW polymers (PPE-CO2Na-7 and PPE-CO2Na-13) to medium-MW polymers (PPE-CO2Na-35 and PPE-CO2Na-49), and eventually to the larg e-MW polymer (PPE-CO2Na-108). This effect is due to two possible

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123 reasons. First, the chain length increase adds more freedom of conformational, vibrational and rotational changes to the polymer chain, therefore increases th e possibility of non-radiative decay. Second, aggregate formation in high-MW polymers creates the excimer-like state, which competes with the radiative decay proce ss from the excited state of the free polymer chains. The molar extinction coefficients and quantum yi elds of the series ar e plotted versus the chain length together with those of PPE-CO2Na-187, as shown in Figure 4-6 (c). The trends for decreasing molar extinction coefficients and incr easing quantum yields with the chain length as described above are clearly evident. The little increase of quantum yield of PPE-CO2Na-187 compared to PPE-CO2Na-108 might arise from the absence of the end groups (a potential electron acceptor) on the polymer chains of PPE-CO2Na-187. In water In comparison with their photophysical prop erties in methanol, the absorption and emission spectra of these CPEs in water are pred ominantly determined by the aggregation of the polymer chains. As shown in Figur e 4-7 (a), the absorption of PPE-CO2Na-7 shows a major absorption band centered at 409 nm and a shoul der band at 437 nm. Further increase of molecular weight leads to a negligible wavelengt h shift of the absorption band at 409 nm, rather a continuous increase of the shoulde r intensity is observed. For PPE-CO2Na-35, PPE-CO2Na-49 and PPE-CO2Na-108, the band at 437 nm becomes the major absorption. The molar extinction coefficients of the series in water show an incr easing trend with the incr ease of molecular weight (7 < 13 < 35 < 49, 108 < 187).

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124 0.24 0.18 0.12 0.06 0.00Relative Intensity (A.U.) 700 600 500 400 Wavelength (nm) (b) 8 6 4 2 0 cm-1) 500 400 300 Wavelength (nm) (a) Figure 4-7. (a) Absorption and (b) emission spectra of the series in water, PPE-CO2Na-7 ( ), PPE-CO2Na-13 ( ), PPE-CO2Na-35 ( ), PPE-CO2Na-49 ( ), PPE-CO2Na-108 ( ). Arrows shows the change di rection with increasing ch ain length. Symbols were added to delineate different curves. Figure 4-7 (b) illustrates the fluorescence sp ectra in water for polymers with different molecular weights. Similar to the earlier resu lts for PPE with pendant carboxylate side chains (chapter 3) without any special end groups the fluorescence spectra of the current CPEs with different chain length in water show a broad, red-shifted and st ructureless band at 520 nm with significantly lower quantum yiel d (4% 18%), which confirms the aggregate formation. Table 4-2 summarizes the photophysical data of all the polymers in methanol and water. Three trends are clearly evident from these data. Firs t, all the polymers featur e a blue emission at 436 in methanol and a green emission at 520 nm in wa ter regardless of their chain length; second, the molar extinction coefficient increases with the increasing chain length in both methanol and water; third, the quantum yield decreases with the increasing chain leng th in both solvents.

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125 Table 4-2. Photophysical properties of the CPEs with different chain lengths in methanol and water. a Coumarin 102 in EtOH as standard, FL = 0.95.173 b This is the polymer obtained when no end-capper was added in the reaction mixture. Fluorescence Quenching with Electron Acceptors Previously, we and others have studied th e fluorescence quenching of anionic conjugated polyelectrolytes with methyl vi ologen carrying different charges.126,134 It was found that Ksv increases when the charge on the quencher is increased, which is ascribed to the stronger complexation and/or induced-aggregation for quenchers with greater charger.61,126,134 Figure 4-8. Molecular structures of the quenchers used in the current study. Here, we studied the quenching of the PPE series with different chain lengths by 4 different quenchers, incl uding methyl viologen (MV+, MV2+) and heptyl viologen (HV+ and HV2+). Their structures are shown in Figure 4-8. In order to exam ine the effect of chain length In methanol In water max ab (nm) max (4) M-1cm-1 max em (nm) FL a max a b (nm) max (4) M-1cm-1 max em (nm) FL a PPE-CO2Na-7 404 3.6 0.64 408 3.3 0.14 PPE-CO2Na-13 411 5.1 0.60 436 5.1 0.18 PPE-CO2Na-35 417 5.8 0.32 438 6.5 0.09 PPE-CO2Na-49 419 4.6 0.31 439 7.0 0.1 PPE-CO2Na-108 432 5.1 0.15 440 6.8 0.04 PPE-CO2Na-187b 433 6.9 436 0.23 441 9.1 520 0.07

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126 2.0 1.6 1.2 I0/I 0.8 0.6 0.4 0.2 0.0[MV2+]/ M Ksv = 0.8*106 16 12 8 4 0Fluorescence 700 600 500 400 Wavelength (nm) (d) 0.24 0.18 0.12 0.06 0.00Absorbance 500 400 300 (b) 0.24 0.18 0.12 0.06 0.00Absorbance 500 400 300 (a) 1.2 1.1 1.0 I0/I 3.0 2.0 1.0 0.0 [MV+]/ M ksv= 0.5*105 16 12 8 4 0Intensity 700 600 500 400Wavelength (nm) (c)on the quenching constants, all the experiments were carried out in methanol, in which the PPEs are believed to predominantly exist in a molecularly dissolved state. Quenching of PPE-CO2Na-7 by MV+ and MV2+ First we compared the quenching of PPE-CO2Na-7 in methanol by MV+ and MV2+. The polyelectrolyte concentra tions were kept at 5.0 M (polymer repeat unit) to prevent possible concentration-dependent aggregation. Figure 4-9. Absorption (a) and emission (c) spectra of PPE-CO2Na-7 with the addition of MV+, [MV+] ranges from 0.4 M to 3.2 M. Absorption (b) and emission (d) spectra of PPE-CO2Na-7 with the addition of MV2+, [MV2+] ranges from 0.1 M to 0.8 M. Emission intensity decreases with incr easing quencher concentration. [PPECO2Na-7] = 5 M. Emission spectra were meas ured with excitation at 380 nm. Insets of c and d show the SV-plots of each quencher and the linear fits.

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127 Figure 4-9 illustrates the absorpti on and emission spectra of PPE-CO2Na-7 upon the addition of MV+ and MV2+. The insets in both fluorescence spectra (c and d) are the SternVolmer (SV) plots obtained by plotting the fluor escence intensity ratio (monitored at 436 nm) as a function of quencher concentra tion. From the spectral comparis on, two trends are evident. First, the addition of even very small am ount of the divalent cationic quencher (MV2+) induces continuous absorption intensity decr eases and red-shifts of the absorption spectra (Figure 4-9 b), indicating the aggrega tion of the polymer chains. Second, quenching of the polymer fluorescence is more efficient for MV2+ than MV+. The Stern-Volmer constants ( Ksv) for both quenchers were extrapolated by fitting th e linear region of the SV plot. The Ksv value for MV2+ is 0.8 106 M-1, which is 10 times larger than the value for MV+, which is 0.5 105 M-1. Based on our earlier results and other peoples work,61,172 we attribute the enhanced quenching by MV2+ to the ability of the divalent cation to indu ce the aggregation of the polymer chains. Quenching of the series by monovalent cationic quenchers. In order to explore the chai n length effect on the amplif ied fluorescence quenching, first the quenching of the series by two monovalent cationic methyl viologen derivatives (MV+ and HV+) was investigated. Similar to PPE-CO2Na-7 (Figure 4-9 a), there is also no spectral shift in the absorption spectra of PPE-CO2Na-13, PPE-CO2Na-35 and PPE-CO2Na-49 with the addition of quencher (MV+ or HV+), indicating that the quencher-indu ced aggregation of polymer chains is a minor process when monovalent ca tionic quenchers are used. For PPE-CO2Na-108 which shows a strong tendency to aggregate even in MeOH, adding monovalent cationic quenchers (MV+ or HV+) helps to break the polymer aggregate, which will be discussed in the following section. The quenching effects we re all evaluated using the Ster n-Volmer (SV) equation with the fluorescence intensity monitored at 436 nm. As shown in Figure 4-10 (a), the SV quenching plots for the series with MV+ exhibit nearly linear correlations within the quencher concentration

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128 range from 0 M to 3.2 M. The Ksv values for the series of different chain lengths were extrapolated by fitting the linear region of the SV plots. It increases from 0.5 105 M-1 (PPECO2Na-7) to 2.1 105 M-1 (PPE-CO2Na-49), then drops down to 1.4 105 M-1 (PPE-CO2Na108). Figure 4-10. SV Plots of th e series in MeOH quenching by MV+ (a) and HV+ (b), respectively. [Polymer] = 5 M. MV+ or HV+ was added in 0.4 M aliquots, ranging from 0 M to 3.2 M. The emission intensity was monitored at 436 nm. Figure 4-10 (b) illustrates the SV quenching plot of the series by HV+. Similar dependence of quenching efficiency on pol ymer chain length is observed. The Ksv values were also obtained from the slopes of the linear fits of their SV plots, ranging from 0.9 105 M-1 (PPE-CO2Na-7) to 2.2 105 M-1 (PPE-CO2Na-49). For PPE-CO2Na-108, a smaller value of 1.3 105 M-1 is obtained for Ksv. In general, for the same polymer, HV+ shows a higher quenching efficiency than MV+ (larger Ksv values), which is due to the more hydrophobic nature of HV+ compared to MV+. 2.0 1.8 1.5 1.3 1.0I0/I 2.8 2.1 1.4 0.7 0.0 [ MV+]/ (a) n = 7 n = 13 n = 35 n = 49 n = 108 2.0 1.8 1.5 1.3 1.0I0/I 3.2 2.6 2.0 1.3 0.7 0.0 [HV+]/ M (b) n = 7 n = 13 n = 35 n = 49 n = 108

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129 2.1 1.8 1.5 1.2 I0/I 0.8 0.6 0.4 0.2 0.0[MV2+]/ M (a) n = 7 n =13 n = 35 n = 49 n =108 4.0 3.2 2.4 1.6 I0/I 0.6 0.4 0.3 0.1 0.0 [ HV2+ ] / M (b) n = 7 n = 13 n = 35 n = 49 n = 108Quenching of the series by divalent cationic quenchers In the second line of the investigation, we carried out the fluorescence quenching of the series by two different divalent cationic quenchers, MV2+ and HV2+. Figure 4-11. SV plots of the series quenching in MeOH by MV2+ (a) and HV2+ (b), respectively. [Polymer] = 5 M. MV2+ or HV2+ was added in 0.1 M aliquots, ranging from 0 M to 0.8 M. The emission intensity was monitored at 436 nm. As shown earlier in the quenching of PPE-CO2Na-7 by MV2+, the divalent cationic quencher shows higher quenching e fficiency compared to the monovalent cationic quencher, MV+. The enhanced quenching efficiency is attri buted to the ability of the divalent cationic quencher to induce the aggregat ion of the polymer chains.71,172 During the fluorescence quenching experiments by MV2+ and HV2+ of the series, we monitored the UV-Vis absorption spectra of each polymer in the course of quenc her titration. It was obser ved that bathochromic shift is a common feature for all the polymers, i ndicating that the aggregation is induced by the divalent cationic quenchers. Figure 4-11 shows the SV quenching plots of the series by MV2+

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130 (Figure 4-11 a) and HV2+ (Figure 4-11 b). In the concentrat ion range of 0 0.4 M, a linear SV relationship is observed for the fluor escence quenching by both quenchers. The Ksv values obtained for the polymers by either MV2+ or HV2+ are all in the order of 106 M-1 and follow the following order: PPE-CO2Na-49 > PPE-CO2Na-7, PPE-CO2Na-13 > PPE-CO2Na-35, PPECO2Na-108. Quenching of PPE-CO2Na-108 by MV+ and MV2+ As noted earlier in the study of absorption and emission spectra of the series in different solvents, it was observed that for PPE-CO2Na-108, there is a pronounced contribution from the aggregated polymer chains to both spectra even in methanol. When the quenching experiments by MV+ or MV2+ were carried out, different spectral changes were observed in the absorption spectra. Figure 4-12. UV-visible ab sorption spectra of PPE-CO2Na-108 in MeOH with the addition of MV+ (a) and MV2+ (b). [PPE-CO2Na-108] = 5 M. MV+ was added in 0.4 M aliquots, [MV+] ranges from 0 M 3.2 M. MV2+ was added in 0.1 M aliquots, [MV2+] ranges from 0 M -0.5 M. Arrows s how the direction of change with the addition of quenchers. As shown in Figure 4-12 (a a nd b), with the addition of MV+, the absorption intensity of the shoulder (432 nm) is attenuated in a stepwise manner; while a continu ous intensity decrease and red-shift of the absorption are resulted with increasing MV2+. Because the shoulder peak 0.32 0.24 0.16 0.08 0.00Absorbance 500 400 300Wavelength (nm) (a) 0.32 0.24 0.16 0.08 0.00 Absorbance 500 400 300 Wavelength (nm) (b)

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131 3.0 2.0 1.0 I0/I 3.0 2.0 1.0 0.0 [MV+] / M 520 nm 436 nm (c) 3.2 2.4 1.6 0.8 0.0Fluorescence 700 600 500 400Wavelength (nm) 436 nm 520 nm (a) 2.5 2.0 1.5 1.0 I0/I 0.8 0.6 0.4 0.2 0.0 [MV2+]/ M 520 nm 436 nm (d) 3.2 2.4 1.6 0.8 0.0 Fluorescence 700 600 500 400 Wavelength (nm) 436 nm 520 nm (b)arises from the planarization of PPE backbone in the aggregate, the decrease of the shoulder intensity signals that some of the aggregated chains are de-aggregated after forming the weak complex with MV+. In contrast, during the titration of MV2+, the bathochromic shift suggests that more chains are aggregated with the addi tion of divalent cationic quencher as observed for other polymers with sh orter chain lengths. Figure 4-13. Emission spectra (a) and SV-plot (c) of PPE-CO2Na-108 in MeOH with the addition of MV+. [PPE-CO2Na-108] = 5 M. MV+ was added in 0.4 M aliquots, [MV+] ranges from 0 M 3.2 M. Emission spectra (b) and SV -plot (d) of PPECO2Na-108 with the addition of MV2+. MV2+ was added in 0.1 M aliquots, [MV2+] ranges from 0 M 0.8 M. As shown in Figure 4-13 (a, b), although the emission at 436 nm is still the dominant band in the fluorescence spectrum of PPE-CO2Na-108 in methanol, the band at 520 nm due to the aggregate can be readily seen (upper) With the addition of either MV+ or MV2+, the emission

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132 band at 520 nm (aggregate) decreases in intens ity prior to the band at 436 nm (free polymer chain). The difference in the quenching efficiency for the two bands is clearly revealed by the slope of the SV plots obtained by monitoring th e emission intensity at different wavelengths (Figure 4-13 c, d). The Ksv values calculated for MV+ are 7.9 105 M-1 (aggregate) and 1.4 105 M-1 (free polymer chain), respectively. For MV2+, Ksv values of 1.7 106 M-1 and 1.3 105 M-1 are obtained for the aggregates and free polymer chains, respectively. One conclusion that can be drawn from the result is that there is a competing quenching by the quencher molecule between the free polymer chains and the aggregates of PPE-CO2Na-108 in MeOH. And the emission from the aggregates is quenched more efficiently by the quencher molecules compared to the emission from the free chains. Chain length dependence of Ksv Taken together, the current research shows th at there is a correlation between the SV quenching constant ( Ksv) and the chain length of the PPEs. In our study, using the monovalent cationic methyl viologen derivatives (MV+ and HV+) as the quenchers, we were able to clarify the influence of the chain length on the quenching constant. For the tw o monovalent quenchers, the SV constants obtained from the quenching expe riments in methanol are plotted versus the chain length as a bar graph, whic h is shown in Figure 4-14. For MV+, the Ksv value is tripled when the chain le ngth is increased from 8 to 35, and further increase of the chain length (n = 49) only results in a slight improvement (15%) indicating that the chain length e ffect saturates around this point. Since each polymer repeat unit contains two phenylene ethynylene mo ieties; the exciton diffusion le ngth of the current polymers can be estimated to be about 98 phenylene ethynylene units based on GPC or 56 phenylene ethynylene units based on NMR. The drop of Ksv values for PPE-CO2Na-108 is most probably due to the fact that the aggregat es existing in the polymer soluti on compete with the free polymer

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133 chains for the quencher molecu les (as shown in Figure 4-13 c) Thus the actual number of quencher molecules that can quench the fluorescen ce of the free polymer chain (intensity at 436 nm) is much smaller than the number of quencher molecules added. Since the Ksv value is calculated based on the total concentration of th e quencher molecules, it underestimates the Ksv value for PPE-CO2Na-108. 0.25 0.20 0.15 0.10 0.05 0.00 Ksv (106 M-1) Chain Length 713 35 49 108 MV+ HV+ Figure 4-14. Comparison of the Ksv values obtained from SV plots for the series quenching with MV+ and HV+ in MeOH. Another conclusion that can be drawn from the present study is that th e aggregation of the polymer chains plays a key role in determin ing the quenching efficiency of CPEs and it overwhelms the effect of chain length on Ksv whenever aggregation occurs in the polymer solution. It has been proven that interchain exciton migration takes pl ace on a time scale of a few picoseconds, while intrachain exciton migr ation occurs on a time scale of a few hundred picoseconds.174 Because of the fast kinetics of interchain exciton migration, any polymer aggregation or quencher-indu ced polymer aggregation will have a strong effect on the Ksv. Then

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134 the evaluation of chain length effect on Ksv will have to discriminate the contributions from both intrachain and interchain exciton migrations. Otherwise there will be no clear correlation between chain length and Ksv in such circumstances, as observed in our experiments when divalent quenchers were used. Experimental Materials Pd(PPh3)4 catalyst was used as received from Strem Chemical Company. Triethylamine and THF were purified by distil lation over sodium hydride. 1-I odo-4-(trifluoromethyl)benzene, coumarin 102, N,N-dimethyl-4,4'-bipyridinium chloride (MV2+), N,N-heptyl-4,4'-bipyridinium bromide (HV2+) were purchased from Sigma-Aldrich a nd used without further purification. NHeptyl-4,4'-bipyridinium bromide (HV+) was purchased from Sigma-Aldrich and further purified by recrystallization from acetone.175 N-Methyl-4,4'-bipyr idinium iodide (MV+) was synthesized following the literature procedure.176 Octafluoro[2.2]paracyclopha ne was kindly provided by Prof. William R. Dolbier. Instrumentation NMR spectra were obtained with a Varian Mecury-300 or a Varian Gemini-300 and chemical shifts were reported in ppm using CHCl3 or C2HD5SO as internal reference. GPC analyses were carried out on a system comprised of a Rain in Dynamax SD-200 pump and a Beckman Instruments Spectroflow 757 absorban ce detector. UV-Visibl e absorption spectra were recorded using a Lambda 25 spectropho tometer from PerkinElmer. Steady-state fluorescence spectra were obtained with a Fluoro log-3 spectrofluorometer from Jobin Yvon. A 1-cm square quartz cuvette was used for all spectral measurements.

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135 General Methods Stern-Volmer quenching A polymer solution (2 ml, c = 5 M) was placed in a rectangular quartz cell and titrate d with different quenchers. The absorption and fluorescence spectra were measured after each addition of the quencher. Fluorescence peak intensities were used for the construction of the Stern-Volmer (SV) plots if not specifically mentioned. The ratio of initial fluorescence to observed fluorescence was plotted versus the quencher concentration. By fitting of the linear region of SV plots, Ksv values were obtained. Quantum yield measurements Quantum yields of the series were measured by a relative method using Coumarin 102 as the standard (QY = 0.95 in EtOH solution)173 and were corrected for solvent refractive index. The absorbance at the excitation wavelength ( = 380 nm) were kept between 0.1 0.12 to reduce the inner filter effect. Synthetic Procedures General procedure for polymerization. Monomer 1 (407.3 mg, 0.50 mmol), monomer 2 (63.1 mg, 0.50 mmol) and a certain amount of 3 (1-iodo-4-(trifluoromethyl)benzene, varying from 1% to 25%, molar ratio) were dissolved in 20 ml of dry THF/Et3N (v/v = 3/2) mixture in a Schlenk flask. The solution was deoxygenated with argon for 15 minutes at 55 C. Then 16.2 mg (0.014 mmol) Pd(PPh3)4 and 5.0 mg (0.026 mmol) of CuI were added into the solution under the protection of argon. The resu lting mixture was heated to 60 C and stirred for 24 hr. The obtained yellow solution was poured into 200 ml of methanol, which cause d the precipitation of the polymer. The precipitate was collected by vacuum filtration and further purified by two repeated cycles of dissolution in THF and precip itation into a large volume of methanol. All these polymers can be dissolved in THF or CHCl3 after complete drying under vacuum. However, with storing, the polymers with l onger chain length (n = 108 or n = 187) became barely soluble. Typical reaction yield is 75%-85%.

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136 Hydrolysis A small amount (~ 30 mg) of each polymer was taken from the obtained organic soluble polymers and used for GPC and NM R characterization. The rest of the polymers were dissolved in 40 ml of 1,4-dioxane. To the solution, 4 ml of n-Bu4NOH (1 M in MeOH) was added. The solution became turbid immediately upon the addition of the base. Water (4 ml) was added at this stage; the mixture became clear and was stirred at room temperature for 24 hr. Then a solution of sodium perchlorate (480 mg, 4 mmol) dissolved in 1 ml of water was added to the hydrolyzed polymer solution and the result ing mixture was poured into 500 ml of cold acetone, whereupon the water soluble polymer preci pitated as a fine yellow powder. Final purification of the polymers was accomplished by dialysis of the aqueous solution of the polymers against nanopure water (Millipore Simplic ity water system). Regenerated cellulose membranes (Fisher Scientific) with 12 kD molecu lar weight cut-off (MWCO) were used for all the polymer samples including PPE-CO2Na-7 and PPE-CO2Na-13. During the dialysis, several drops of NaOH (1 N) was added to the polymer solution to prevent the precipitation of the polymer. After dialysis, the solution was filtered through a 1.0 m glass fiber membrane and the concentration of the aqueous solution was calib rated using gravimetric analysis. The polymers were stored in this format and diluted as appropr iate for spectroscopic studies. Typical reaction yield is 85%-95%. The polymer samples for NMR studies were pr epared in the following way: the polymer was isolated by precipitating its a queous solution into acetone, dried in an oven (heated at 100 120 C) and then dissolved in 1 ml of DMSO-d6/D2O mixture (v/v = 1/1). The NMR spectra were acquired at 70 C with at least 128 scans. PPE-CO2R-7 GPC (THF, polystyrene standard): Mw = 9,500, Mn = 5,390, PDI = 1.80. 1H NMR (CDCl3, ppm): 7.64 (end groups), 7.56 (br, m, 4H), 7.02 (s, 2H), 4.74 (s, 4H), 4.24 (t, 4H), 1.68 (m, 4H), 1.31-1.25 (m, 36H), 0.90 (t, 6H). 19F NMR (CDCl3, ppm): -63.22, -118.63

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137 (internal standard). FT-IR ( max, KBr pellet): 3041, 2925, 2854, 2208, 1762, 1733, 1614, 1520, 1489, 1468, 1439, 1412, 1323, 1282, 1188, 1081,839, 722, 598, 544. PPE-CO2Na-7 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.66 (end groups), 7.54 (s, 4H), 7.02 (s, 2H), 4.35 (s, 4H). FT-IR ( max, KBr pellet): 3383, 2204, 1599, 1404, 1324, 1282, 1208, 1125, 1104, 1051, 943, 840, 765, 677, 597, 547. PPE-CO2R-13 GPC (THF, polystyrene standard): Mw = 20,640, Mn = 9,040, PDI = 2.30. 1H NMR (CDCl3, ppm): 7.64 (end groups), 7.56 (br, m, 4H), 7.02 (s, 2H), 4.74 (s, 4H), 4.24 (t, 4H), 1.68 (m, 4H), 1.31-1.26 (m, 36H), 0.90 (t, 6H). 19F NMR (CDCl3, ppm): -63.22, -118.63 (internal standard). FT-IR ( max, KBr pellet): 3040, 2925, 2854, 2207, 1761, 1738, 1615, 1520, 1490, 1467, 1440, 1412, 1377, 1323, 1282, 1188, 1130, 1082, 1017, 837, 722, 544. PPE-CO2Na-13 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.68 (end groups), 7.55 (s, 4H), 6.96 (s, 2H), 4.34 (s, 4H). FT-IR ( max, KBr pellet): 3401, 2927, 2255, 2205, 2127, 1607, 1515, 1488, 1404, 1323, 1281, 1205, 1126, 1104, 1049, 1025, 882, 838, 763, 703, 591, 541. PPE-CO2R-35 GPC (THF, polystyrene standard): Mw = 56,980, Mn = 24,390, PDI = 2.30. 1H NMR (CDCl3, ppm): 7.64 (end groups), 7.56 (br, m, 4H), 7.02 (s, 2H), 4.74 (s, 4H), 4.24 (t, 4H), 1.68 (m, 4H), 1.31-1.26 (m, 36H), 0.90 (t, 6H). 19F NMR (CDCl3, ppm): -63.22, 118.63 (internal standard). PPE-CO2Na-35 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.66 (end groups), 7.54 (s, 4H), 6.98 (s, 2H), 4.39 (s, 4H). FT-IR ( max, KBr pellet): 3394, 2929, 2256, 2205, 2128, 1607, 1515, 1489, 1404, 1323, 1281, 1206, 1105, 1051, 1026, 1006, 882, 840, 825, 763, 721, 703, 614, 596, 546 PPE-CO2R-49 GPC (THF, polystyrene standard): Mw = 78,750, Mn = 33,760, PDI = 2.30. 1H NMR (CDCl3, ppm): 7.56 (br, m, 4H), 7.02 (s, 2H), 4.74 (s, 4H), 4.24 (t, 4H), 1.68 (m,

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138 4H), 1.31-1.26 (m, 36H), 0.90 (t, 6H). 19F NMR (CDCl3, ppm): -63.22, -118.63 (internal standard). PPE-CO2Na-49 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.54 (s, 4H), 6.97 (s, 2H), 4.37 (s, 4H). FT-IR ( max, KBr pellet): 3384, 2934, 2255, 2204, 2127, 1604, 1515, 1488, 1404, 1323, 1281, 1207, 1104, 1050, 1026, 1005, 884, 840, 825, 763, 720, 704, 629, 595, 545. PPE-CO2R-108 GPC (THF, polystyrene standard): Mw = 202,890, Mn = 74,350, PDI = 2.70. 1H NMR (CDCl3, ppm): 7.55 (br, m, 4H), 7.02 (s, 2H), 4.74 (s, 4H), 4.24 (t, 4H), 1.68 (m, 4H), 1.31-1.26 (m, 36H), 0.90 (t, 6H). 19F NMR (CDCl3, ppm): -63.22, -118.63 (internal standard). PPE-CO2Na-108 1H NMR (D2O/DMSO-d6 = 1/1, ppm): 7.55 (s, 4H), 6.96 (s, 2H), 4.34 (s, 4H). FT-IR ( max, KBr pellet): 3398, 2930, 2256, 2205, 2128 1602, 1515, 1489, 1406, 1323, 1281, 1198, 1094, 1051, 1026, 1004, 885, 841, 825, 764, 721, 700, 631, 617, 544.

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139 CHAPTER 5 VARIABLE BAND GAP POLY(ARYLENE ETHYNYLENE)S Introduction Reduction of the band gap (Eg) of conjugated polymers (CPs ) has been the objective of polymer chemists since the discovery of conductivity of doped polyacetylene.177,178 The decrease of Eg can lead to intrinsic conducting polymers which do not need additional chemical or electrical doping. And th e continuous reduction of Eg will give CPs that can continuously absorb and emit light in the visible and near-IR sp ectral range. These materi als will be useful for the fabrication of multicolor LEDs, sensing arrays and solar cells.179-181 Two general approaches have been widely used to reduce the Eg of conjugated polymers, i.e., minimization of bondlength alternation and alternat ion of electron-donor and electro n-acceptor in the main chain.178 As the most common type of poly(arylene et hynylene)s (PAEs), poly( phenylene ethynylene) (PPE)s have a optical band gap of ~ 2.4 eV.10 Because the frequently used Sonogashira coupling is compatible to a wide variety of functionalitie s, numerous PAEs with variable band gaps that are soluble in organic solvents have been s ynthesized using the donoracceptor repeating unit strategy.10 We reported the first synthesis of a ho mologous family of PAE-based CPEs that differ widely in HOMO-LUMO band gap.135 In that work, the band gap was tuned by copolymerizing diiodophenylene carrying sulfonate (SO3 -) or alkyl ammonium groups (NR3 +) with 5 different arylene ethynylen es in DMF/water mixtures. In the present chapter, we report the synthe sis of variable band gap PAEs that feature carboxylate (CO2 -) and ammonium (NH3 +) side groups. For PAEs th at carry linear carboxylate (-OCH2CO2 -Na) side chains, four polymers that in corporate 1,4-phenyl, 2,5-thienyl (TH), 2,5(3,4-ethylenedioxy)thienyl (EDOT), and 1,4-benzo[ 2,1,3]thiodiazole (BTD) in the repeat unit were synthesized. To provide conjugated pol yelectrolytes that can retain high quantum

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140 efficiency in water and solid state films, two dend ritic side chains that can sterically hinder the aggregation of the polymer chains were design ed and synthesized. C onjugated polymers that incorporate these dendritic ionic side chains were prepared an d their photophysical properties were studied in different solvents. Effects of acidity and ionic strength on the polymer optical properties were also studied in an aqueous environm ent. To the end, the pot ential applications of these PAEs that have linear car boxylate side groups in Grtzel t ype solar cells were discussed. Results and Discussion Synthesis of PAEs with Linear Side Chains The tuning of the PAE band gap wa s realized by copolymerizing 2,5bis(dodecyloxycarbonylme thoxy)-1,4-diiodobenze 1 with different trimethylsilyl (TMS) protected diacetylene compounds. The synthetic routes used to prepare the polymers are shown in Figure 5-1. 2,5-Bis((trimethylsilyl) ethynyl)thiophene (TH) 3 was prepared by coupling trimethysilylacetylene (TMSA) to commercia lly available 2,5-dibromothiophene under Sonogashira conditions in 95% yield. 2,5Diiodo-3,4-ethylenedi oxythiophene (EDOT)135 and 4,7-dibromobenzo[c][1,2,5]thiadiazole (BTD)163 were prepared following the procedures reported in the literature. They were then react ed with TMSA respectivel y to give the protected diacetylene monomers in moderate yields. Although the deprotection of the TMS group for all the monomers was successful, we found that thes e deprotected monomers are extremely unstable and change their color quickly if stored even for relatively short periods of time (from white or yellow color to brownish). Thus for making these PAEs containing heteroaromatic rings, a different strategy was used. In a typical pol ymerization, a stoichiometric quantity of 2,5bis(dodecyloxycarbonylme thoxy)-1,4-diiodobenze 1 and the stable TMS-protected monomer was dissolved in a THF/Et3N mixture (v/v, 3/1) and carefu lly degassed with argon. Then tetrabutyl ammonium fluoride (TBAF) was added to the mixture following with the addition of a

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141 catalytic amount of Pd(PPh3)4 and CuI. The polymerization usually required 20 24 hours at 60 C. The number-average molecular weights of PPE, TH-PPE EDOT-PPE and BTD-PPE determined by GPC are 36 kD, 30 kD, 11 kD and 16 kD, respectively. These polymers are soluble in most non-polar solvents, such as CH2Cl2, CHCl3 and THF. After confirming the structure of these polymers by 1H NMR in CDCl3, they were subjected to the base-promoted hydrolysis that was used to prepare water-s oluble PPEs in the earlier chapters. Using tetran -butyl ammonium hydroxide (TBAH), we were able to obtain the water-soluble polymers from PPE TH-PPE and EDOT-PPE (Note that PPE-CO2Na has the same chemical structure of the polymer that we studied in chapter 3 but with lower molecular weight). However, when we tried to hydrolyze the BTD-PPE a material with the maximum absorption at 322 nm was resulted. The big blue shift (~ 180 nm) comparing the unknown material to BTD-PPE indicates that the backbone of the polymer was destroyed probably by the attack of BTD ring by the base. Through a different route, the water-soluble counterpart of BTD-PPE BTD-PPE-CO2Na was prepared from the hydrolyzed bis(dod ecyloxycarbonylmethoxy)-1,4-diiodobenze and 4,7bis((trimethylsilyl)ethynyl )benzo[c][1,2,5]thiadiazole 8 in a DMF/water/diisopropylamine mixture (9/6/2) under the same polymerization cond itions. The solubility of these polymers in water is about 3 5 mg/mL. The same as what we found for PPE-SO3 -, these polymers can not be re-dissolved even in water or other polar solvents, such as DMF and DMSO after complete drying, which is probably due to the strong tendenc y of these polymers to aggregate in the solid state. Thus these polymers were stored in th eir water solutions after final purification using membrane dialysis. To prevent the precipitation during the storage, the pH of the stock solution was kept between pH = 8 10.

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142 N S N S O O S I I O O CO2R RO2C + O O CO2R RO2C S I I O O CO2R RO2C + S O O O O CO2R RO2C I I O O CO2R RO2C + R=C12H25, TH-PPE R=Na+, TH-PPE-CO2Na R=C12H25, EDOT-PPE R=Na+, EDOT-PPE-CO2Na i ii i ii i N S N O O CO2R RO2C R=C12H25, BTD-PPE R=Na+ ii N S N I I O O CO2Na NaO2C + iii N S N O O CO2Na NaO2C i.Pd(PPh3)4,CuI,THF/Et3N,60oC,TBAF,24hr;ii.1)(n-Bu)4NOH,dioxane,ambient,12hr; 2)a.q.NaClO4;iii.Pd(PPh3)4,CuI,DMF/H2O/i-Pr2NH,60oC,24hr.TMS TMS TMS TMS TMS TMS TMS TMS BTD-PPE-CO2Na I I O O CO2R RO2C + i O O CO2R RO2C R=C12H25, PPE R=Na+,PPE-CO2Na ii TMS TMS n n n n n 12 13 15 17 7 Figure 5-1. Synthesis of variable band gap PAEs with linear carboxylate side chains.

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143 Synthesis of PAEs with Dendritic Side Chains In a series of studies,64,71,126,172 we have investigated the effect of aggregation on the photophysical properties of PAE-ba sed conjugated polyelectrolytes. And aggregation has also been shown to strongly influence the optical properties of other types of conjugated polyelectrolytes in aqueous so lution, such as poly(2-methoxy5-propyloxy sulfonate phenylene vinylene) (MPS-PPV),57 poly[9,9-bis(6-N,N,N-trimethyla mmoniumhexyl)fluorene diiodide]75 and etc.182 Attaching dendritic side groups to rigid conjugated polym er chains to prevent their aggregation in thin films have b een populated in the literature.183-185 However, there were only two examples of using dendritic side chains to disrupt the aggregation of CPEs in aqueous solution.186,187 Here we present our results of synt hesizing two new dendritic side chains and PAEs carrying the first generati on of these dendritic groups. Monomer synthesis Figure 5-2 shows the synthe tic routes of monomers 19 20 that carry the dendritic side chains. A convergent strategy wa s used. The triester compound 11 was prepared from tert -butyl acrylate in two steps with an overall yi eld of 63%. In parallel, compound 16 was synthesized in 4 steps. Starting from comme rcially available 4-(2-carboxyet hyl)-4-nitroheptan edioic acid 12 it was converted to 4-(3-chloro-3-oxopr opyl)-4-nitroheptanedioyl dichloride 13 in nearly quantitative yield by treating w ith thionyl chloride (SOCl2). The latter was then reacted with trimethylsilyl azide to give 3-(2-aminoe thyl)-3-nitropentane1,5-diamineHCl salt 14 in 81% yield. After reacting wi th 3 equivalents ditert -butyl dicarbonate, compound 15 was obtained in 85% yield. The nitro group in compound 15 was then hydrogenated (60 C, 100 psi) in the presence of T-1 Raney nickel188 to give compound 16 in 93% yield. De ndritic monomers 19 and 20 were prepared by the coupling of either 2,2'-(2,5-diiodo-1,4-phenylene)bis(oxy)diacetic acid 17 or its chloride 18 with 11 or 16 However, the dire ct reaction between 17 and 11 using

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144 4-(dimethylamino)pyridinium 4-toluenesulfonate (DPTS)105 and diisopropylcarbodiimde (DIPC) as coupling reagents only gave the desired monomer 19 in 38% yield. To improve the yield, compound 17 was converted to 18 by reacting with SOCl2. Compound 18 was found to be fairly stable and purified by recrystallization. Then reactions of 18 with 11 or 16 afforded monomer 19 and 20 in 76% and 50% yield, respectively. Th e purity of these monomers was proven by 1H and 13C NMR spectroscopy, elemental an alysis and mass spectroscopy. Figure 5-2. Synthesis of monomers ( 19 20 ) carrying dendritic side chains.

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145 Synthesis of PAEs carrying anionic side chains The synthetic procedures used to prepare the PA Es that carry anionic dendritic side chains are shown in Figure 5-3. O O I I COR RO2C O O COR RO2C R=NHC(CH2CH2CO2 tBu)3, PPE-R1R=NHC(CH2CH2CO2Na)3, PPE-dCO2Na n O O I I COR RO2C O O COR RO2C n N S N R=NHC(CH2CH2CO2 tBu)3, BTD-PPE-R1R=NHC(CH2CH2CO2Na)3, BTD-PPE-dCO2Na N S N i ii ii ii.Pd(PPh3)4,CuI,THF/Et3N,60oC,24hr;ii)1).TFA/CH2Cl2(1/1),ambient,24hr;2).a.q.Na2CO3,24hr.19 198 Figure 5-3. Synthesis of PAEs carry ing dendritic anion side groups. The Sonogashira polycondensation of an stoichiometric amount of monomer 19 and 1,4diethynylbenzene was carried out in a mixture of THF/Et3N (3:1) with Pd(PPh3)4/CuI as catalysts. After the reaction was stir red at 60 C for 24 h, the polymer ( PPE-R1) was isolated as a yellow solid and purified by repe ated precipitation from THF solutions into methanol. The number-average molecular weight of PPE-R1 determined by GPC against polystyrene standards is 33 kD (PDI = 3.1), corresponding to 29 repeat units. The ester groups of PPE-R1 was hydrolyzed using a CH2Cl2/trifluoacetic acid (TFA) mixture (v/v = 1/1) at room temperature. After removal of the solvent, the residue was treated with saturated aqueous Na2CO3 and then dialyzed against deionized (DI) water using 12 kD molecular-weight-cut off (MWCO) dialysis

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146 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1) (a) (b) (c)cellulose membranes. The water-soluble polymer ( PPE-dCO2Na ) was isolated as a pale yellow solid by precipitation from the aqueous solution into acetone, and then dried in a vacuum oven for at least 2 days. Figure 5-4. 1H NMR (left) and FT-IR (right) spectra of monomer 19 (a), PPE-R1 (b) and PPE-dCO2Na (c). The 1H NMR and FT-IR spectra of the dendritic monomer 19 the neutral polymer PPE-R1 and the water-soluble polymer PPE-dCO2Na are shown in Figure 5-4. Comparison between the spectra of monomer 19 and PPE-R1 reveals that there is only one new resonance peak at = 7.58 ppm, which is assigned to the aromatic prot ons of 1,4-phenylene on the polymer backbone. The tert -butyl protons appear as a strong singlet at = 1.42 ppm in the sp ectra of monomer 19 and PPE-R1. After hydrolysis, the 1H NMR spectroscopy of PPE-dCO2Na was accomplished in a mixture of DMSO-d6/D2O (v/v = 1/1). There are no si gnals in the 1.4 1.5 ppm range, indicating that the tert -butyl groups were cleaved with an excellent yield (> 95%). 9 8 7 6 5 4 3 2 1 0 ppm (a) (b) (c)CDCl3 CDCl3 D2O DMSO-d6

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147 Figure 5-4 (right) show s the IR spectra of PPE-R1 and PPE-dCO2Na the spectrum of monomer 19 is also shown for the purpose of comp arison. The IR spectrum of the neutral polymer ( PPE-R1) is very similar to the spectrum of monomer 19 except that a very weak absorption at 2205 cm-1 due to C C stretching is seen for the pol ymer. The absorption bands, which can be attributed to the ester and amide groups in the spectra of 19 and PPE-R1, occur at 1728 cm-1 and 1688 cm-1, respectively. After hydrolysis, the absorption at 1728 cm-1 disappears, indicating the removal of the tert -butyl ester groups. The absorp tion bands associated with the carboxylate groups (CO2 -) appear at 1566 cm-1 and 1403 cm-1. The strong broad absorption ranging from 3000 cm-1 to 3500 cm-1 implies the existence of H2O in the sample, due to the strong hydrophilicity of PPE-dCO2Na Using a similar procedure, one PAE that incorporates the st rong electron acceptor, benzo[c][1,2,5]thiadiazole (BTD) into the backbone, was also synthesized. The number-average molecular weight determined by GPC for the neutral polymer ( BTD-PPE-R1) is 12 kD (PDI = 1.4). The water-soluble polymer, BTD-PPE-dCO2Na was obtained using the same hydrolysis condition used for PPE-dCO2Na Characterizations of BTD-PPE-R1 and BTD-PPE-dCO2Na by NMR spectroscopy and IR spect roscopy are consistent with their propos ed structures. Synthesis of PAEs carrying cationic side chains Figure 5-5 shows the synthetic routes to make PAEs that carry dendritic charged amino groups. The polymerizations of monomer 20 with 1,4-diethynylbenzene or 4,7diethynylbenzo[c][1,2,5]thiadiazole 8 were carried out under the Sonogashira coupling conditions. But because of the ri chness of amide functionality in the polymers (2 amide and 6 carbamate groups per repeat unit) these polymers are more polar than the previous polymers ( PPE-R1 and BTD-PPE-R1) and were purified by repeated precipitation from THF solutions into hexane, instead of methanol. PPE-R2 was isolated as a yellow fiber and has a number-

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148 average molecular weight of 24 kD (PDI = 4.3). Deprot ection of the Boc group was accomplished using 4 N HCl/dioxane. The polymer was obtained as a pale yellow solid after precipitation from acetone in a quantitative yield. After washing with a large amount of cold hexane, the polymer was dried in a vacuum oven. Figure 5-5. Synthesis of PAEs carry ing dendritic cationic side groups. The structures of the neutral polymer PPE-R2 and the water-soluble polymer PPE-dNH3Cl were confirmed by 1H NMR and FT-IR spectroscopy, as show n in Figure 5-6. For the purpose of comparison, the 1H NMR and IR spectra of monomer 20 are also shown. In the 1H NMR spectrum of monomer 20 the phenylene protons appear as a singlet at = 7.14 ppm. Two broad singlets at = 6.71 and = 4.80 ppm are assigned to the amide protons of OCH2CO NH and -CH2CH2NH COOtBu, respectively. The singlet signal at = 4.34 ppm is due to the methylene group of O CH2CO NH Signals for the methylene groups of -C( CH2CH2NHtBoc)3 appear at = 2.02 ppm and = 3.20 ppm, respectively. The tert -butyl protons a ppear as a strong

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149 singlet at = 1.40 ppm. The 1H NMR spectrum of the neutral polymer ( PPE-R2) is very similar to that of monomer 20 except that there is one new signal at = 7.60 ppm due to the protons of 1,4-phenylene on the polymer backbone. As can be seen from the spectrum of water-soluble polymer ( PPE-dNH3Cl ) in DMSO-d6/D2O (1/1) after deprot ection, the peaks at = 4.80 and = 1.40 ppm disappear, indicating the removal of the tBoc group. The occurrence of signals for all the other protons confirms that th ere were no other side reactions. Figure 5-6. 1H NMR (left) and FT-IR (right) spectra of monomer 20 (a), PPE-R2 (b) and PPE-dNH3Cl (c). The starred signals are due to H2O in the solvent or the sample. The IR spectra of monomer 20 and the neutral polymer are also very similar. The absorption at 1692 cm-1 is due to the C=O stretching of carbamate groups, which overlaps the absorption of C=O stretching of the amide groups. In the spectrum of water-soluble polymer, BTD-PPE-dNH3Cl three new absorptions associated with NH3 + groups appear at 1607 cm-1, 2008 cm-1 and 3015 cm-1, which can be assigned to the be nding vibration, combination and stretching bands of NH3 +, respectively. The absorption at 1692 cm-1 due to carbamate groups 4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1) (a) (b) (c) 9 8 7 6 5 4 3 2 1 0 ppm (a) (b) (c)* CDCl3 CDCl3 DMSO-d6 D2O

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150 disappears, the absorption band of amide at 1672 cm-1 is observed. In all the spectra, the absorption bands around 3400 cm-1 are assigned to the N-H stretching of the amide groups. The other organic polymer, BTD-PPE-R2 was isolated as a red solid and has a numberaverage molecular weight of 12 kD (PDI = 3.6). The water-sol uble polymer was obtained after hydrolyzing BTD-PPE-R2 with 4 N HCl. The structures of both polymers were confirmed by 1H NMR and FT-IR spectroscopy. Optical Properties of PAEs with Linear Carboxylate Side Chains In our previous studies of PPE-SO3 and PPE-CO2 -,71,172 we found that the optical properties of these polymers in solution are strong ly dependent on the solvents. In methanol, the absorption and emission spectra of these conjug ated polyelectrolytes correspond well with the spectra of their structurally analogous polymers th at contain alkyl or al koxy-solubilizing groups in good solvents. In water, the spectra of the CPEs display spectral changes that are characteristic of aggregate formation as seen in the solid films of the structurally similar organic polymers.189 Here, we present our studies of th e absorption and fluor escence spectra of TH-PPE-CO2Na EDOT-PPE-CO2Na and BTD-PPE-CO2Na (the spectra of PPE-CO2Na are also shown for comparison) in methanol and water. Figure 5-7 illustrates the absorp tion and fluorescence spectra of the series in MeOH. The wavelength maxima of the absorption and fluores cence spectra systematica lly red-shift in the order of phenyl < TH < EDOT < BTD. Across th e entire series, the absorption maximum shifts from 417 nm ( PPE-CO2Na ) to 520 nm ( BTD-PPE-CO2Na ), whereas the fluorescence maximum shifts from 430 nm ( PPE-CO2Na ) to 665 nm ( BTD-PPE-CO2Na ). The origin of the red-shifts in the absorption and fluorescence likely arise from severa l factors that vary with the structure of the heterocyclic ring in the PAE backbone. Specifi cally, the red-shift for the 2,5thienyl(TH) and 3,4-ethylenedioxy-2,5-thienyl (EDOT) polymers likely arises because the

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151 HOMO level of the polymer is increased because of the electron-donor char acteristic of the TH and EDOT units.190 The increase in the HOMO energy reduces the HOMO-LUMO gap, thereby reducing the energy of the lowest singlet excite d state. In contrast to the TH and EDOT polymers, in the 2,1,3-benzothiad iazole (BTD) polymer the nitrogen-containing heterocycles likely act as electron acceptors thereby reducing the LUMO level of the polymer.191,192 In effect, the dialkoxybenzene units act as donors and the BD T groups act as electron acceptors giving rise to charge transfer character in th e lowest excited state. The existe nce of charge transfer character in the singlet state of BTD-PPE-CO2Na polymer is evidenced by the relatively large Stokes shift ( ca. 150 nm) and combined with the broad, structurel ess band shape of the fluorescence spectra. 1.2 0.9 0.6 0.3 0.0Photoluminescence 800 700 600 500 400 Wavelength (nm) 1.2 0.9 0.6 0.3 0.0 Absorbance 700 600 500 400 300 Wavelength (nm) Figure 5-7. Absorption (a) and Emission (b) of PAEs containing linear carboxylate side chains in MeOH. PPE-CO2Na ( ), TH-PPE-CO2Na ( ), EDOT-PPE-CO2Na ( ), BTD-PPE-CO2Na ( ) Similar to PPE-CO2Na these polymers also show a str ong tendency to aggregate in water (spectra are not shown here). Aggregation of these polymers is signaled by a red-shift and narrowing of the polymers absorption spectrum, a significant red-shift and broadening of the fluorescence spectrum, and a decrease in PL.

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152 Table 5-1. UV-Vis absorption and photoluminescent properties of PAEs containing linear carboxylate side chains. a Coumarin 102 in EtOH as the standard, FL = 0.95.173 b Ru(bpy)3Cl2 in H2O as the standard, PL = 0.036.193,194 Table 5-1 compiles the optical data, incl uding the maximum absorption and emission wavelength, PL quantum efficiency ( PL) of these CPEs in MeOH and H2O. There are several trends that are clearly evident in the data. First, the absorption and emission maxima shift to longer wavelength when the solven t is changed from MeOH to H2O. The origin of this effect is attributed to the aggregati on-induced planariza tion of polymer backbone, which leads to increased conjugation length. S econd, across the series, the fl values systematically decrease in the sequence Ph > Th > EDOT > BDT. Interesti ngly, this ordering correla tes approximately with the emission energy, i.e., fl generally decreases with decr easing emission energy. The third effect is that the fl values are dramatically lower for all of the polymers in H2O solution. This effect has been previously observed for PPE-SO3 and PPE-CO2Na and is attributed to aggregation-induced quenching of the intrachain singlet exciton.71,172 Optical Properties of PAEs Featuring Dendritic Side Chains Compared with the PAEs containing linear carboxylate side chains these polymers having either dendritic carboxylate or charged amino groups exhibit much better solubility (> 8 mg/ml in water). After complete drying under vacuum, the solid materials are able to be re-dissolved in Polymers solvent max, abs (nm) max, em (nm) PL MeOH 417 437 0.23a PPE-CO2Na H2O 435 520 0.07a MeOH 438 476 0.15aTH-PPE-CO2Na H2O 447 512 0.06aMeOH 457 498 0.09aEDOT-PPE-CO2Na H2O 468 527 0.04aMeOH 504 650 0.002 b BTD-PPE-CO2Na H2O 519 668 N/A

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153 water, indicating the charged dendritic groups reduce the intermolecula r interaction of the polymer chains in the solid state. To fully expl ore the potential of these dendritic side groups in preparing a new generation of water-soluble conjugate polymers, a comprehensive study was carried out in which the absorp tion and fluorescence spectra of the four CPEs were measured when the solution conditions, such as solven t, pH and ionic stre ngth were varied. Absorption and fluorescence properties Figure 5-8 illustrates the absorption and fl uorescence spectra of these CPEs carrying dendritic ionic side chains in MeOH. In gene ral there is a very close correspondence between the wavelength maxima and band shape for the corresponding pairs of an ionic and cationic PAEtype CPEs. This is not surprising in view of the fact that the polymer s optical properties are mainly determined by the structure of the -conjugated backbone. In comparison with the absorption of PPE-CO2Na the absorption of PPE-dCO2Na and PPE-dNH3Cl that have the same -conjugated backbone blue-shifts 15 nm; while the absorption of BTD-PPE-dCO2Na and BTD-PPE-dNH3Cl also blue-shifts 13 nm co mpared to the absorption of BTD-PPE-CO2Na The conjugated length of PPE-type polymers is determined to be 10 ~ 12 phenylene ethynylene units.160 Since the degrees of polymer ization (DP) for all these polym ers are larger than 10 (~ 20 arylene ethynylene units), we believe that this effect (blue-shift of absorption maximum) for PAEs with dendritic side groups is due to the more twisted backbone conformation in the methanol solutions of these polymers because of the increased electroni c repulsion between the charged dendritic groups. In the fluorescence sp ectra, for the blue-emitting polymers, there is a negligible shift in the maximum emission wavelength (< 3 nm) comparing PPE-dCO2Na or PPE-dNH3Cl to PPE-CO2Na However, for the red-emitting polymers, the broad emission spectra of BTD-PPE-dCO2Na and BTD-PPE-dNH3Cl center at 600 nm, which blue-shifts about

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154 50 nm comparing to the emission of BTD-PPE-CO2Na One possible reason for this effect is the decrease of contact between methanol and th e conjugated backbone of PAEs with dendritic side groups. 0.9 0.6 0.3 0.0 600 500 400 300 Wavelength (nm) (c) 1.2 0.9 0.6 0.3 0.0 (a) 1.2 0.9 0.6 0.3 0.0 (b) 0.9 0.6 0.3 0.0 800 700 600 500 400 Wavelength (nm) (d) Absorbance Photoluminescence Figure 5-8. Normalized absorption (a) and fluorescence (b) spectra of PPE-dCO2Na (solid line) and BTD-PPE-dCO2Na (dotted line) in MeOH. Normalized absorption (c) and fluorescence (d) spectra of PPE-dNH3Cl (solid line) and BTD-PPE-dNH3Cl (dotted line) in MeOH. Solvent effects on abso rption and fluorescence In order to explore how effectively thes e dendritic side groups can disrupt the intermolecular interaction of the polymer chains, a series of experiments were carried out in

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155 which the absorption and fluorescence spectra of all of the CPEs were studied in MeOH/H2O mixtures of varying composition. 1.2 0.9 0.6 0.3 (a) 1.2 0.9 0.6 0.3 (b) 1.2 0.9 0.6 0.3 0.0 600 500 400 300 Wavelength (nm) (c) 1.2 0.9 0.6 0.3 0.0 800 700 600 500 400 Wavelength (nm) (d) Absorbance Photoluminescence Figure 5-9. Absorption (a) a nd fluorescence (b) spectra of PPE-dCO2Na (solid line) and BTD-PPE-dCO2Na (dotted line). Absorption (c ) and fluorescence (d) spectra of PPE-dNH3Cl (solid line) and BTD-PPE-dNH3Cl (dotted line). 0% H2O ( ), 50% H2O ( ) and 100% H2O ( ). All the spectra were colle cted continuously and symbols were added to delineate different curves. Figure 5-9 illustrates the absorption and fluor escence spectra of th e anionic PAEs (upper) and cationic PAEs (lower) in MeOH/H2O mixtures. As the volume fraction of H2O in the solvent increases, the absorption spectra of these PAEs show little changes; while for the PAEs with linear carboxylate groups, a si gnificant red-shift and narrowing of the absorption spectra is generally resulted. The fluorescence of PPE-dCO2Na or PPE-dNH3Cl decreases in intensity but

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156 maintains their blue emission (~ 435 nm) as the volume fraction of H2O increases. By contrast, for PPE-CO2Na the structured blue emission band is replaced by a broad and red-shifted fluorescence band as shown in chapter 3. For the red-emitting polymer ( BTD-PPEdCO2Na and BTD-PPEdNH3Cl ), their fluorescence also decrease significantly in intensity with increasing volume fraction of H2O. But in comparison with BTD-PPECO2Na their fluorescence remains relatively efficient even in water. Taken together, the interchain aggregation in the aqueous so lutions of these polymers ( PAE-dCO2Na and PAE-dNH3Cl ) is attenuated by attaching these bulky ionic groups, as evidenced by relative high quantum efficiencies in their H2O solutions (Table 5-2). And the current results also suggest that to fully disrupt the interchain aggregat ion, synthesis of PAEs carrying higher generation dendritic side groups is needed. Table 5-2. UV-Vis absorption a nd photoluminescent properties of PAEs containing dendritic carboxylate side groups. a Coumarin 102 in EtOH as the standard, FL = 0.95.173 b Ru(bpy)3Cl2 in H2O as the standard, PL = 0.036.193,194 Acidity effects on the absorption and fluorescence Figure 5-10 shows the absorpti on and emission spectra of PPE-dCO2Na and PPE-dNH3Cl in water as a function of pH. The pH of th e polymer-water solution was adjusted with HCl MeOH H2O max abs (nm) max em (nm) PL max abs (nm) max em (nm) PL PPE-dCO2Na 403 433 0.31a 404 432 0.12a BTD-PPE-dCO2Na 491 530 (sh) 605 0.04b 490 530(sh) 623 0.007b PPE-dNH3Cl 402 432 0.45a 405 432 0.13a BTD-PPE-dNH3Cl 487 604 0.04b 489 530 (sh) 620 0.003b

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157 and/or NaOH using a pH meter. In general, a strong dependence of ab sorption and emission on the pH is observed for both polymers. 0.6 0.4 0.3 0.1 0.0 pH = 10.5 pH = 4.5(a) 4 3 2 1 0 pH = 4.5 pH = 10.5 (b) 4 3 2 1 0 700 600 500 400 Wavelength (nm) pH = 10.5 pH = 4.5 (d) 0.6 0.4 0.3 0.1 0.0 500 400 300 Wavelength (nm) pH = 4.5 pH = 10.5 (c) Absorbance Photoluminescence Figure 5-10. Absorption (a) and emission spectra (b) of PPE-dCO2Na in aqueous solutions as a function of pH. [ PPE-dCO2Na ] = 5 M. Absorption (c) and emission spectra (d) of PPE-dNH3Cl in aqueous solutions as a function of pH. [ PPE-dNH3Cl ] = 5 M. pH range from 4.5 to 10.5 in 0.5 pH unit intervals. In the basic solution of PPE-dCO2Na (pH = 10.5), the absorption shows one band centered at 404 nm and the emission shows a structured ba nd at 435 nm. When the pH is adjusted from 10.5 to 4.5, a continuous growing of the shoulder ba nd at 435 nm in the absorption spectra is observed. The largest change in the absorption is visible at pH = 4.5, in line with the p Ka value of propionic acid, with p Ka = 4.88.137 At this point, approxim ately 70% of the carboxylate

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158 groups on the polymer chains are protonated. Ba sed on our previous res earch on several other CPEs and other groups study,71,135,140,172 we believe that the red-shift of the absorption spectra arises from the conformation change of the pol ymer backbone. When the pH is above 7.5, the neighboring phenylene ethynylene groups are tw isted in the polymer to minimize the electrostatic repulsion between the negatively charged carbox ylate side groups. As the carboxylate is protonated (decreasin g pH), the decreased repulsion between the side groups and the desolvation of the neutra l and hydrophobic polymer lead to the planarization of the conjugated backbone. It is also ve ry important to note that the series of absorption spectra define an isosbestic point at 410 nm, suggesting that the pH-induced change is between two distinct types of chromophores, which are probably the neut ral and charged segments. Concomitant with the absorption changes, the fluorescence of PPE-dCO2Na undergoes a quenching of the blue emission band as the pH is decreased. At pH = 4.5, the blue emission is completely replaced by a red-shifted and broad band at 415 nm. Simila r to the solvatochromism effect observed for PPE-CO2Na ,172 we believe that this low-en ergy band also arises from the stacking of the polymer chains. A similar change of absorption a nd emission spectra is observed for PPE-dNH3Cl when the pH was varied from 4.5 to 10.5. The conforma tion change of the polymer backbone and the aggregation of the polymer chains are tuned by the protonation and depr otonation of the amino groups. Although ethylamine, an analogue of the PPE-dNH3Cl side chains, has a p Ka of 10.7.137 the polymer began to precipitate out of the so lution when the pH was brought up to 8.5. As shown in Figure 5-10 (c), the oscillator strength can be compared relatively by integrating the absorption band. A significant decrease occurs when the pH increases from 8.5 to 9.5, probably indicating that at this po int, precipitation of the polymer chains occurs.

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159 2.4 1.8 1.2 0.6 0.0 pH = 4.5 pH = 10.5 (b) 0.6 0.4 0.2 0.0 pH = 4.5 pH = 10.5 (a) 2.4 1.8 1.2 0.6 0.0 800 700 600 500 Wavelength (nm) pH = 10.5 pH = 4.5(d) 0.6 0.4 0.2 0.0 600 500 400 300 Wavelength (nm) pH = 10.5 pH = 4.5(c) Absorbance Photoluminescence Figure 5-11. Absorption (a) and emission spectra (b) of BTD-PPE-dCO2Na in aqueous solutions as a function of pH. [ BTD-PPE-dCO2Na ] = 5 M. Absorption (c) and emission spectra (d) of BTD-PPE-dNH3Cl in aqueous solutions as a function of pH. [ BTD-PPE-dNH3Cl ] = 5 M. pH range from 4.5 to 10.5 in 0.5 pH unit intervals. Figure 5-11 illustrates the absorption and emission spectra of BTD-PPE-dCO2Na and BTD-PPE-dNH3Cl in water when the pH is varied. In the absorption spectra, similar changes are observed as in the spectra of the blue-emitting polymers ( PPE-dCO2Na and PPE-dNH3Cl ). Interestingly, a distinct different behavior is observed in the emission spectra of BTD-PPE-dCO2Na and BTD-PPE-dNH3Cl compared to those of the blue-emitting polymers. For BTD-PPE-dCO2Na at pH 8.5, there is little change of th e emission spectra. When the pH is further decreased, the emission band narrows a nd increases gradually in intensity. At pH = 4.5, a 3-fold increase of the emission intensity is resu lted. We believe that this aggregation-enhanced

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160 emission effect is due to the shielding of BTD unit from water within the polymer aggregates. Recently, a similar effect was obs erved by Bazan and co-workers.195 The possibility that protonation of the nitrogen atom in the BTD un its causes the emission enhancement is ruled out by observing the same effect when PPE-dNH3Cl aggregate is formed by de-protonation of the amino groups. Salt Effects on the absorption and fluorescence The pH-induced absorption and fluorescence cha nges show that the charge density on the polymer side chains is very important to regulate the optical properties of conjugated polyelectrolytes in solution. Anothe r important factor in the aqueous solution is the ionic strength and its effect on the photophysical properties of several different conjugated polyelectrolytes has been studied by several research groups.66,196 Our polymers with dendr itic ionic side groups show relative high quantum efficiencies in wa ter compared to other PAE-based conjugated polyelectrolytes reported in the literature.53 A detailed study of the e ffect of ionic strength on their optical properties is nece ssary for a full understanding of their behavior in the aqueous environment and will be potentially important for their future biosensor applications. Figure 5-12 illustrates the absorption and fluorescence spectra of PPE-dCO2Na and PPE-dNH3Cl at varied concentrations of NaCl in water. The results clearly show that aggregation of the polymer chains takes place for both polymers wh en the NaCl concentration is increased, which is consistent with ma ny observations made for other conjugated polyelectrolytes.66,196 Since the polymers retain their charged groups, the aggregation of the polymers chains is enabled by the effective scre ening of the electrostatic repulsion between the pendant charged groups by the Na+ and/or Clions in solution. Cons equently, after the barrier for planarization is overcome, a ggregation is facilitated in order to minimize the contact between

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161 the hydrophobic backbone and water. For BTD-PPE-dCO2Na and BTDPPE-dNH3Cl similar effects were observed. 4.0 3.0 2.0 1.0 0.0 700 600 500 400 Wavelength (nm) [NaCl] 0 150 mM(d) 3.2 2.4 1.6 0.8 0.0 [NaCl] 0 150 mM(b) 0.6 0.4 0.2 0.0 500 400 300 Wavelength (nm) 0 150 mM [NaCl](c) 0.6 0.4 0.2 0.0 [NaCl] 0 150 mM(a) Absorbance Photoluminescence Figure 5-12. Absorption (a) and emission spectra (b) of PPE-dCO2Na in aqueous solutions as a function of NaCl concentration, [ PPE-dCO2Na ] = 5 M. Absorption (c) and emission spectra (d) of PPE-dNH3Cl in aqueous solutions as a function of NaCl concentration, [ PPE-dNH3Cl ] = 5 M. [NaCl] = 0, 1, 5, 10, 100, 150 mM. The arrows show the direction of change with increasing NaCl concentration. Applications of PAEs containing Linear Carboxyl ate Groups in Dye-sens itized Solar Cells. One of the many applications of conjugated po lyelectrolytes (CPEs) is to use them to replace their organic analogues to fabricate opto-electronic devices. There are several advantages for conjugated polyelectrolytes. First, CPEs exhibit similar electronic properties as their organic analogues but distinct solubility behavior, which enables the construction of heterostructures simply using spin coating; s econd, their solubility in water and alcohols could

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162 eventually allow the processing of these materials in environment-friendly solvents; third, the existence of ionic side groups in CPEs allows the control of film structur e in the molecular level by layer-by-layer (LbL) film deposition method. In a previous report, we described the fabrication of photovoltaic devices in which PA Es with pendant sulfonate groups and a watersoluble C60 derivative were assembled in a LbL fashion.197 As mentioned in chapter 1, CPEs carrying carboxylate groups can be used as the sensitizer to construct TiO2 based solar cells in the absence of the ruthenium complexes. Using one of the polymers synthesized in this chapter, PPE-CO2Na combined with a commer cial polythiophene (PT-CO2 -, Rieke Metals, Inc.), we have demonstrated that cell performance can be enhanced by broadening the absorption spectra of the polymer sensitizer (a dua l-polymer system in this case).198 As our continuing investigati ons, we constructed DSSCs usi ng the series of PAEs with linear carboxylate side chains and studied the correlation between cell performance and polymer band gap. As shown by many research groups, ca rboxylic acid functiona lity is critical for anchoring sensitizer onto th e porous nanocrystalline TiO2 surface.87,199 To ensure the adsorption of the polymer to the TiO2, the carboxylate groups in the poly mer chains were protonated using 3 N HCl. The current series of PAEs with carbox ylic acid groups were completely dried in a vacuum oven at 60 C. They were then dissolved in dry DMF (solubility is less than 0.2 mg/ml due to the strong aggregation of the polymers in the solid stat e) and used for the immersion deposition into porous TiO2 films. Usually, it takes 12 to 14 hours for the polymer adsorption process to saturate as proven by absorption measur ements of the films. At saturation, the optical densities of the films for all the polymers are clos e to 1.0. Then the films were used to fabricate the solar cells, of which the cell perf ormances were tested immediately.

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163 60 40 20 0IPCE (%) 600 500 400Wavelength (nm) (a) PPE-CO2H TH-PPE-CO2H EDOT-PPE-CO2H BTD-PPE-CO2H 4.0 3.0 2.0 1.0 0.0 J (mA/cm2) 0.5 0.4 0.3 0.2 0.1 Potential (V) (b) Figure 5-13. IPCE spectra (a) and Current-potential (I-V) curves (b) for CPE-sensitized solar cells under AM 1.5 condition. These experime nts were carried out by Hui Jiang in Schanze group. Figure 5-13 (a) shows the incident photo to cu rrent efficiency (IPCE) of the CPE sensitized solar cells as a function of wavelength. The IPCE values at the maxima for PPE-CO2H THPPE-CO2H and EDOT-PPE-CO2H are in the range of 42% to 47% and match very well with their absorption spectra. Since all the films for different polymer s have very close absorbance at the absorption maxima, the IPCE value at the maximum for BTD-PPE-CO2H is expected to be

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164 in the same range. However, the IPCE values in the full range of the polymers absorption are lower than 25%. The current density voltage ( J V ) characteristics for the CP E sensitized cells are shown in Figure 5-13 (b). These cells performa nce in terms of short circuit current ( Jsc), open circuit voltage ( Voc), fill factor ( FF ), and power conversion efficiency () are summarized in Table 5-3, along with the maximum IPCE values. Consiste nt with the IPCE re sults, the overall power conversion efficiencies show the order of PPE-CO2H < TH-PPE-CO2H < EDOT-PPE-CO2H which can be ascribed to the enhanced solar ener gy harvesting due to the continuous red-shift of their absorption spectra. Howeve r, the performance of the cell ut ilizing the most red-absorbing polymer, BTD-PPE-CO2H in the series is only comparab le to that of the cell based on PPE-CO2H Table 5-3. Summary of solar cell performance. Note: these data were obtained by Hui Jiang in Schanze group. To further understand the corr elation between the cell performance and the electronic properties of current polymers, we measured the oxidation potentials of th e excited state of the series in solution by cyclic voltammetry (CV) an d differential pulse voltammetry (DPV). Due to the solubility issue, the dodecyl ester protected precursors ( PPE TH-PPE and EDOT-PPE ) in dichloromethane and BTD-PPE-CO2H in DMSO were used. Ba sed on the optical band gap energy extracted from the cross point of ab sorption and fluorescence spectra, the oxidation Device IPCE (%) Jsc (mA/cm2) Voc (V) FF (%) (%) PPE-CO2H 42.7 @420 nm 1.46 0.47 49.2 0.34 TH-PPE-CO2H 45.8 @440 nm 2.60 0.47 43.4 0.53 EDOT-PPE-CO2H 47.4 @460 nm 3.12 0.50 37.2 0.58 BTD-PPE-CO2H 24.7 @480 nm 2.52 0.43 30.5 0.33

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165 potentials of the ground state of the CPEs are al so estimated. The results are summarized in Table 5-4. These results clearly show that the energies of the excited state and the ground state of all the polymers match ve ry well with those of TiO2 conduction band and redox pair I-/I3 -, indicating favorable driving forces for charge in jections. Another reason that might explain the poor cell performance for BTD-PPE-CO2H is that the charge injec tion occurs from the charge transfer band, while the other polymers in ject electrons to th e conduction band of TiO2 from their excited states. Thus a competing intramolecula r/intermolecular charge transfer process takes place when the polymers are adsorbed on the surface of TiO2, which decreases the efficiency of charge injection to the TiO2 and thus the cell performance. Table 5-4. Redox potentials of studied c onjugated polyelectrolytes. a Measured using ferrocene as the internal standard. b Optical band gap based on cross point of absorption and emission spectra. Th ese data were obtained by Katsu Ogawa in Schanze group. Experimental Materials Pd(PPh3)2Cl2 and Pd(PPh3)4 were purchased from Strem Ch emical Company and used as received. Triethyamine and THF used in the polymerization were purif ied by distillation over CaH2. Iodine monochloride-pyridine complex and 4-(2-carboxyethyl)-4-nitr oheptanedioic acid were purchased from Aldrich Chemical Compan y. All the other chemicals were supplied by Excited State Oxidation Potential a (V vs. SCE) Optical Band Gap b (eV) Ground State Oxidation Potential (V vs. SCE) PPE -1.12 2.89 1.77 TH-PPE -1.12 2.65 1.53 EDOT-PPE -0.87 2.53 1.66 BTD-PPE-CO2H -1.02 2.19 1.17

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166 either Acros or Aldrich Chemi cal Company and used without further purification. Fluorinedoped tin oxide (FTO, TEC 8) glass substrates were purchased from Harford Glass, and nanocrystalline TiO2 paste (12% wt., average particle size about 13 nm) was purchased from a Switzerland based company, Solaronix SA. Instrumentation NMR spectra were recorded on a Varian VXR-300 or Gemin-300 FT-NMR, operating at 300 MHz for 1H-NMR and at 75.4 MHz for 13C-NMR. High temperature NMR spectra were recorded on a Varian Mercury 300 FT-NMR. Chem ical shifts were reported in ppm using CHCl3 or C2HD5SO as internal reference. FT-IR spectra were taken on a Perkin-Elmer 1600 spectrometer. Gel permeation chromatography (G PC) analyses were carried out on a system comprised of a Rainin Dynamax SD-200 pump and a Beckman Instruments Spectroflow 757 absorbance detector. UV-Visible absorpti on spectra were recorded with a Lambda 25 spectrophotometer from PerkinElmer. Steady-st ate fluorescence spectra were obtained with a Fluorolog-3 spectrofluorometer from Jobin Yvon. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed on a Bioanalytical Systems CV50 electrochemical analyzer at a scan rate of 50 mV/s. Measurem ents were carried out in nitrogendegassed solutions with 0.1 M tetrabut ylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte and ferro cene as the internal standa rd (0.43 V vs SCE in dry dichloromethane). General Methods Photovoltaic device fabrication and characterization. All the solar cells were fabricated and characterized by Hui Jiang in Schanze group The DSSCs were fabricated by following the published procedures. Briefly, TiO2 paste was spread out on a FTO substrate and sintered at 450 C for 30 min. The TiO2 film thickness was about 5 m. The film was immersed

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167 in a polymer solution for about 14 hours for adsorption. To assemble the cell, 5 nm Pt coated FTO was used as the counter electrode a nd a propylene carbonate solution of 0.05 M I2 / 0.5 M LiI was applied as the electrolyte. Th e active solar cell area was about 1.0 cm2. The currentvoltage characteristics were m easured with a Keithley SMU 2400 source meter under the illumination of AM 1.5 (100 mW/cm2) using a 150 W Xe arc lamp (Oriel instruments). For IPCE measurements, the device was illumina ted by monochromatic light from an Oriel Cornerstone 130 1/8 m spectrometer, and the current response under short circuit condition was recorded at 10 nm intervals using a Keithley SMU 2400 source meter. The intensity of the light source at each wavelength was determined using an energy meter (S350, UDT Instruments) equipped with a calibrated silicon de tector (Model 221, UDT Instruments). Synthetic Procedures 2,5-Bis((trimethylsilyl)ethynyl)thiophene (3) 2,5-Dibromothiophe ne (1.2 g, 5 mmol) was dissolved in a mixture of 25 ml of THF/(i-Pr)2NH (v/v, 4/1) and degassed with argon for 15 minutes. Then under the protection of argon, 70 mg of Pd(PPh3)2Cl2 (10 mmol)and 20 mg (10 mmol) of CuI were added, followed by the add ition of 2.1 mg of trim ethylsilyacetylene (15 mmol). The resulting mixture was stirred at room temperature for 5 hr, then filtrated. After removal of the solvent, the brown oil was purif ied by chromatography on s ilica gel eluting with hexane to afford a yellow solid (yield: 1.5 g, 95%). 1H NMR (CDCl3, ppm): 7.04 (s, 2H), 0.25 (s, 18H). 2,5-Diiodo-3,4-ethylenedioxythiophene (4). A solution of 5.6 g (40.0 mmol) of 3,4ethylenedioxythiophene and 23.0 g (90.0 mmol) of iodine monochl oride-pyridine complex in 150 mL of dry dichloromethane wa s stirred at room temperature for 1 hr. A precipitate of pyridinium salt that was produced during the reac tion was removed by vacuum filtration, and the filtrate was evaporated. The remained solid residue was rinsed with a mixture of water/methanol

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168 (1:1) and it was recrystallized tw ice from acetone. The product was obtained as greenish white crystals (yield: 14.8 g, 93%). 1H NMR (DMSO-d6; ppm from TMS): 4.25 (s, 4H). 13C NMR (DMSO-d6; ppm): 143.84, 64.83, 54.08. 2,5-Bis(trimethylsilylethynyl)-3,4 -ethylenedioxythiophene (5). A deoxygenated solution of 3.93 g (10.0 mmol) of 2 6 mL (42.6 mmol) of trimethyl silylacetylene, 580 mg (0.5 mmol) of Pd(PPh3)4 and 95 mg (0.5 mmol) of CuI in 20 mL of THF/Et3N (2/1) mixture was stirred at room temperature under argon for 2 hr The triethylammonium salt which precipitated during the reaction was removed by filtra tion and the filtrate was evaporated in vacuo The remaining dark oil was eluted through a silica gel column with hexanes. The pale yellow hexane solution was evaporated and the solid obtained was recrystallized two times from 90% methanol. The product was obtained as a fine light yellow solid (yield: 1.27 g, 38%). 1H NMR (CDCl3, ppm): 4.26 (s, 4H), 0.24 (s, 18H). 13C NMR (CDCl3, ppm): 143.79, 103.12, 99.91, 94.39, 64.97, 0.12. 2,5-Diethynyl-3,4-ethylenedioxythiophene (6). A solution of 1.0 g (3.0 mmol) of 3 in 20 mL of dioxane was acidified with 2 mL of 50% acetic acid and was deoxygenated with purging argon for 10 minutes. Then 25 mL of a solution of n -Bu4NF in THF (c = 1 M) was added and the resulting mixture was stirre d for 30 minutes at room temperature. Half of the solution volume was evaporated and the resulting soluti on was poured into 300 mL of ice water. The precipitated solid was collected by vacuum filtra tion, dissolved in 50 mL of hexanes and eluted through a short plug of silica gel. The solvent was evaporated a nd the solid obtained was stored in refrigerator for a few hours prior to use. 1H NMR (CDCl3; ppm): 4.30 (s, 4H), 3.50 (s, 2H),. 13C-NMR (CDCl3; ppm): 65.04, 73.96, 85.06, 98.70, 144.37.

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169 4,7-Bis((trimethylsilyl)ethynyl) benzo[c][1,2,5]thiadiazole (7). 4,7Dibromobenzo[c][1,2,5]thiadiazole (1.5 g, 5.1 mmol) was dissolved in a mixture of 50 ml of THF / (i-Pr)2NH (v/v, 3/1) and degassed by argon for 15 minutes. Then 1.5 g of trimethylsilyacetylene (15.3 mmol) was added usi ng a syringe and followed by the addition of 107 mg (0.16 mmol) of Pd(PPh3)2Cl2 and 29 mg (0.15 mmol) of CuI. The resulting mixture was heated at 55 C for 12 h, then filt rated. After the removal of the solvent, the residue was purified by chromatography on silica gel eluting with CH2Cl2/hexane (v/v, 1/2) to afford a yellow solid (yield: 1.65 g, 98%). 1H NMR (CDCl3, ppm): 7.71 (s, 2H), 0.34 (s, 18 H). 4,7-Diethynylbenzo[c][1,2,5]thiadiazole (8). Compound 7 (164 mg, 0.5 mmol) was dissolved in 10 ml THF/MeOH (v/v, 4/1). To th e solution, 62 mg of K OH (1.1 mmol) in 0.5 ml of H2O was added. The reaction mixture was stirre d at room temperature for 1 hr and then poured into 60 ml of H2O. A crystalline yellow solid was obtained and washed with cold MeOH/H2O (1/1, 20 ml). After drying under vacuum, th e solid was stored in the refrigerator for a very short period before using (yield: 81 mg, 87%). 1H NMR (CDCl3, ppm): 7.76 (s, 2H), 3.68 (s, 2H). Ditertbutyl 4-Nitro-4-(3-tert-butoxy3-oxopropyl)heptanedioate (10) .200 A stirred solution of MeNO2 (6.1 g, 100 mmol), Triton B (benzylt rimethylammonium hydroxide, 40% in MeOH; 1.0 ml) in dimethoxyethane (DME, 20 ml) wa s heated to 65-70 C. To the mixture, tert butyl acrylate (39.7 g, 310 mmol) was added drop -wise. Additional tr iton B (1.0 ml ) was added after 15 minutes and the reaction was kept at 70-75 C for 1 hr. After concentrated in vacuo the residue was dissolved in CHCl3 (200 ml), washed with 10% HCl (100 ml 1) and brine (100 ml 1), and dried with anhydrous MgSO4. After removal of the solvent, the crude product was purified by crystallizing from ethanol (95%), which afforded a white crystalline

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170 solid (yield: 70%). 1H NMR (CDCl3, ppm): 2.18 (s, 12H), 1.42 (s, 27H). 13C NMR (CDCl3, ppm): 171.04, 92.17, 81.16, 30.34, 29.80, 28.04. Ditert -butyl 4-Amino-[2-( tert -butoxycarbonyl)ethyl]-heptanedioate (11).200 A solution of nitro ester (10) (4.46 g, 10 mmol) in absolute ethanol (100 ml) with T-1 Raney Ni (4.0 g) was hydrogenated at room temperature with a balloon filled with hydrogen for 48 hr. The catalyst was cautiously filtered away through a bed of celite. The solvent was removed in vacuo affording a viscous liquid, which was purifie d by column chromatograph (silica gel, EtOAc/hexane (v/v, 1/1)) to give a clear yellow o il (4.0 g, 93%). The oil solidified after standing in the hood overnight, affording a white solid. 1H NMR (CDCl3, ppm): 2.21 (t, 6H); 1.58 (t, 6H), 1.41 (s, 27H). 13C NMR (CDCl3, ppm): 172.97, 80.26, 52.26, 34.33, 29.92, 28.01. 4-(3-Chloro-3-oxopropyl)-4-nitrohe ptanedioyl dichloride (13). A 100 ml round bottom flask was charged with 4.44 g of 4-(2-carboxyethy l)-4-nitroheptanedioic acid (16 mmol) and 30 ml thionyl chloride (SOCl2). Two drops of DMF was added to the suspension and the mixture was then slowly heated up to reflux. After 1 hr, the solution became clear and there was no more gas evolution. The excess SOCl2 was removed by vacuum distillation. The yellow residue solidified after flushing with nitrogen a nd used without further purification. 1H NMR (CDCl3, ppm): 2.96 (t, 6H), 2.30 (t, 6H). 13C NMR (CDCl3, ppm): 172.46, 89.91, 41.29, 30.16. 3-(2-Aminoethyl)-3-nitropentan e-1,5-diamineHCl salt (14). The acid chloride obtained from last step was dissolved in 30 ml dioxane in a three-necked r ound bottom flask protected with argon. Trimethylsilyl azide (6.3 ml, 48 mmol) was added to the solution at room temperature. The solution was then slowly h eated up to 80 C. When there was no more gas evolution, the reaction mixture was allowed to cool down to 45 C and then added 20 ml of acetone. Concentrated HCl (12 ml) was added to the mixture drop-wise. White precipitate

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171 formed immediately following the addition. After 1 hr, the reaction mixture was allowed to cool down to room temperature. The white precipitat e was collected by vacuum filtration and washed with 200 ml cold acetone. Af ter drying in the hood overnight a slightly yellow solid was obtained (yield: 3.86 g, 81%). 1H NMR (DMSO-d6): 8.39 (s, 9H), 2.81 (t, 6H), 2.33 (t, 6H). Compound (15). 3-(2-Aminoethyl)-3-nitrope ntane-1,5-diamineHCl salt ( 14 ) (3.0 g, 10.0 mmol)was dissolved in 100 ml of Et3N/CH3CN (v/v, 1/3). Then 10.2 g of ditert -butyl dicarbonate (46.8 mmol) was added. The mixture was heated at reflux for 7 hr and then diluted with 150 ml of ethyl acetate. The mixture was washed with H2O (250 ml 1). Then the aqueous phase was extracted with ethyl acetate (150 ml 1). The organic phase was combined and dried with anhydrous MgSO4. After the filtration, the solvent was removed in vacuo affording a yellow oil which solidifi ed under vacuum (yield: 4.2 g, 86%). 1H NMR (CDCl3, ppm): 4.81 (s, 3H), 3.13 (m, 6H), 2.16 (t, 6H), 1.40 (s, 27H). 13C NMR (CDCl3, ppm): 155.84, 90.59, 79.63, 35.79, 35.59, 28.34. LR-MS: calcd for C22H42N4O8[M+H] = 491.6, found 491. Elemental analysis: Calcd for C22H42N4O8: C, 57.86; H, 8.63; N, 11.42. Found: C, 53.78; H, 9.03; N, 11.24. Compound (16) .201 A solution of 3.1 g of compound 15 (6.3 mmol) in 200 ml of ethanol with T1 Raney Nickel (3.0 g) was hydrogenated at 100 psi and 70 C for 36 hr. The catalyst was removed by filtering the reaction mixture through a bed of celite. The solvent was removed in vacuo affording a slightly yellow oil, which soli dified as a fluffy white solid under vacuum (yield: 90%). 1H NMR (CDCl3, ppm): 5.06 (s, 3H), 3.18 (m, 6H), 1.78 (s, 2H), 1.56 (t, 6H), 1.41 (s, 27H). 13C NMR (CDCl3, ppm): 155.99, 79.22, 52.96, 39.37, 36.14, 28.41. LR-MS: calcd for C22H44N4O6 [M+H] = 461.6, found 461.

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172 2,2'-(2,5-Diiodo-1,4-phenylene)bis( oxy)diacetyl chloride (18). 2,2'-(2,5-Diiodo-1,4phenylene)bis(oxy)diacetic ac id (5.8 g, 12 mmol) was suspended in 30 ml of SOCl2. After adding 2 drops of DMF, the reaction mixture was h eated up and stirred at reflux for 2 hr. then the excess SOCl2 was removed by vacuum distillation and the resulting off-white solid was crystallized from 200 ml of hept ane/toluene (v/v, 10/1), affordi ng a slightly yellow crystalline solid (yield: 5.0 g, 80%). 1H NMR (CDCl3, ppm): 7.15 (s, 2H), 4.92 (s, 4H). 13C NMR (CDCl3, ppm): 152.58, 124.41, 113.62, 86.44, 74.05. Compound (19). Ditert -butyl 4-Amino-[2-( tert -butoxycarbonyl)ethyl]-heptanedioate ( 11 ) (1.2 g, 2.9 mmol), 0.4 ml of Et3N and 25 ml of dry CH2Cl2 were placed in a 50 ml round bottom flask and cooled with an ice/water bath. Then 0.67 mg of 2,2'-(2,5-diiodo-1,4phenylene)bis(oxy)dia cetyl chloride ( 9 ) (1.3 mmol) in 15 ml of CH2Cl2 was added via a syringe. After 2 hr, the reaction mixture wa s allowed to warm to room temperature and further stirred for 24 hr. The solvent was removed in vacuo the crude product was purified by flash chromatography (silica gel, EtOAc/hexane (1/3)) to give a white solid (yield: 1.4 g, 78%). 1H NMR (CDCl3, 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). 13C NMR (CDCl3, ppm): 172.19, 165.73, 151.57, 122.63, 86.29, 80.63, 68.83, 57.80, 30.17, 29.74, 28.09. HR-MS: calcd. for C54H86I2N2O16 [M+Na] = 1295.3959, found 1295.3966. Compound (20). Compound 16 (1.5 g, 3.3 mmol), 0.45 ml of Et3N (3.2 mmol) and 30 ml of dry CH2Cl2 were placed in a 50 ml round bottom flask, which was cooled in an ice/water bath. To the mixture, a solution of 0.76 g of 2,2'-(2,5-diiodo-1,4-phenyl ene)bis(oxy)diacetyl chloride ( 9 ) (1.5 mmol) was added via a syringe. After 2 hr the reaction mixture was allowed to warm to room temperature and further stirred for 24 hr. The solvent was removed in vacuo the crude product was purified by flash chromatography on s ilica gel with EtOAc/hexane (1/1) to give a

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173 colorless oil, which solidifie d as a white solid under vac uum (yield: 1.0 g, 50%). The 1H NMR (CDCl3, ppm): 7.15 (s, 2H), 6.71 (s, 2H), 4.80 (s, 6H ), 4.36 (s, 4H), 3.19 (m, 12H), 2.03 (m, 12H), 1.42 (s, 27H). 13C NMR (CDCl3, ppm): 166.38, 155.94, 151.61. 122.72, 86.43, 79.33, 68.71, 59.92, 35.76, 35.60, 28.39. HR-MS: calcd for C54H92I2N8O16 [M+Na] = 1385.4613, found 1385.4613. General polymerization procedure for PPE with linear side chains 2,5Bis(dodecyloxycarbonylmethoxy)-1,4diiodobenze (814.6 mg, 1mmol) and 1 mmol of the other monomer (1,4-bis((trimethylsilyl) ethynyl)benzene or 2,5-bis((tri methylsilyl)ethynyl)thiophene or 2,5-bis(trimethylsilyleth ynyl)-3,4-ethylenedioxythiophene) were dissolved in 30 ml of THF/Et3N (v/v. 3/2) in a Schlenk flask sealed with a septum. The resulting solution was deoxygenated by purging with argon for 15 minutes. Then 0.25 ml of TBAF (1 M in THF) was added to the solution via a syringe, followed by the addition of Pd(PPh3)4 (34.8 mg, 0.03 mmol) and CuI (12.6 mg, 0.06 mmol). The reactio n mixture was then heated to 60 C and stirred for 24 hr. Then the solution was poured in to 300 ml methanol. The prec ipitate was collected by vacuum filtration and further purified by dissolution in THF and precipitation into methanol. Typical yields of these polymerization are 80% 90%. A small amount (~30 mg) of each polymer was dried completely under vacuum and used for NM R study immediately. The rest of the polymer was hydrolyzed following the proced ure described in Chapter 3. PH-PPE. GPC (THF, polystyrene standard): Mw = 131, 560, Mn = 36, 820, PDI = 3.60. TH-PPE. 1H NMR (CDCl3, ppm): 7.22 (s, 2H), 6.97 (s, 2H), 4.72 (s, 4H), 4.24 (t, 4H), 1.68 (m, 4H), 1.30-1.25 (m, 36H), 0.88 (t, 6H). GPC (THF, polystyrene standard): Mw = 108, 980, Mn = 30, 300, PDI = 3.60.

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174 TH-PPE-CO2Na 1H NMR (D2O/DMSO-d6 = 1/1): 7.28 (s, 2H), 6.92 (s, 2H), 4.34 (s, 4H). EDOT-PPE. 1H NMR (CDCl3, ppm): 6.96 (s, 2H), 4.70 (s, 4H), 4.34 (s, 4H), 4.22 (t, 4H), 1.67 (m, 4H), 1.30-1.25 (m, 36H), 0.88 (t, 6H). GPC (THF, polystyrene standard): Mw = 26, 380, Mn = 10, 570, PDI = 2.50. EDOT-PPE-CO2Na. 1H NMR (DMSO-d6): 7.05 (s, 2H), 4.78 (s, 4H), 4.38 (s, 4H). BTD-PPE-CO2Na. A solution of sodium 2,2'-(2,5-diiodo-1,4phenylene)bis(oxy)diacetate ( 521.9 mg, 1.0 mmol) and compound 7 (328.6 mg, 1.0 mmol) in 85 ml of DMF/H2O/i-Pr2NH (v/v/v = 9/6/2) was deoxygenated with argon for 30 minutes. Then 0.25 ml of TBAF (1 M in THF) was added under argon, followed by the addition of Pd(PPh3)4 (70 mg, 0.06 mmol) and CuI (22 m g, 0.12 mmol). The resulting brow nish solution was heated at 60 C for 24 hr. The reaction mixture was then poured into 600 ml of acetone. The resulting reddish precipitate was collected by vacuum filtration and further purified by two repeated cycles of dissolution in H2O and precipitation into acetone. Then the solid was dissolved in 75 ml of deionized H2O and dialyzed against a large amount of water. After dialysis, the polymer was stored as its aqueous solution. During the storage, the solution pH was cont rolled in th e range of 8-10. 1H NMR (DMSO-d6): 7.88 (br, s, 2H), 7.22 (b r, s, 2H), 4.83 (s, 4H). General polymerization procedure for PAEs with dendritic anionic side chains. Monomer 19 (318.3 mg, 0.25 mmol) and 0.25 mmol of the other monomer (1,4diethynylbenzene or 4,7-diethynyl benzo[c][1,2,5]thiadiazole) we re dissolved in 16 ml of THF/Et3N (v/v, 3/1). The resulting solution was de oxygenated with argon for 15 minutes. Then 17.3 mg of Pd(PPh3)4 (15 M) and 5.7 mg of CuI (30 M) were added to the stirred solution under the protection of argon. The reaction mixtur e was then heated up to 60 C 65 C and

PAGE 175

175 stirred for 24 hr. The viscous solution was then poured into 200 ml of meth anol. The precipitate was collected by vacuum filtration and washed with methanol (200 ml). After dried under vacuum, the polymer was stored as a solid. T ypical reaction yields fo r the polymerization are 80% 90%. For the hydrolysis, the orga nic polymer was disso lved in 20 ml CH2Cl2 and cooled in an ice/water bath. 20 ml of trifluoroacetic acid (TFA) wa s added to the polymer solution drop-wise. Upon the completion of the addition, the reaction mixture was allowed to warm to room temperature and stirred for another 12 hr The excess of TFA and the solvent were removed in vacuo The residue was treated with saturated aqueous Na2CO3 solution (10 ml) and stirred at room temperature for 3 hr. The soluti on was then poured into 200 ml of acetone. The polymer precipitate was then dissolved in wa ter and purified by dialys is using 12 kD MWCO regenerated cellulose membranes (yield: 90% 100%). The water-soluble polymers could be either stored as aqueous solu tions or as solid powders. PPE-R1. 1H NMR (CDCl3, ppm): 7.57(br, s, 4H), 7.04 (s, 2H), 6.39 (s, 2H), 4.47 (s, 4H), 2.13 (br, m, 12H), 1.96 (br, s, 12H), 1.39 (s, 54 H). GPC (THF, polystyrene standard): Mw = 33, 230, Mn = 101, 210, PDI = 3.00. FT-IR ( max, KBr pellet): 3403, 2978, 2935, 2205, 1731, 1692, 1532, 1512, 1484, 1456, 1410, 1393, 1368, 1312, 1282, 1256, 1214, 1154, 1101, 1051, 954, 891, 848, 758. PPE-dCO2Na 1H NMR (D2O/DMSO-d6 = 1/1): 7.58 (br, 4H), 7.16 (s. 2H). 5.25 (s, 4H). FT-IR ( max, KBr pellet): 3391, 2937, 2202, 1665, 1564, 1404, 1283, 1208, 1099, 1053, 892, 847, 675. BTD-PPE-R1. 1H NMR (CDCl3, ppm): 7.91 (br, s, 2H), 7.19 (s, 2H), 6.51 (s, 2H), 4.59 (s, 4H), 2.12 (br, m, 12H), 1.94 (br, s, 12H), 1.39 (br, s, 54H). GPC (THF, polystyrene standard): Mw = 16, 250, Mn = 11, 690, PDI = 1.40. FT-IR ( max, KBr pellet): 3405, 2978, 2936, 2679,

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176 2494, 2204, 1731, 1693, 1519, 1486, 1457, 1393, 1368, 1312, 1281, 1256, 1213, 1154, 1101, 1056, 954, 847, 758, 721. BTD-PPE-dCO2Na 1H NMR (D2O/DMSO-d6 = 1/1): 7.88 (br, s, 2H), 7.22 (br, s, 2H), 4.83 (s, 4H). FT-IR ( max, KBr pellet): 3391, 2951, 2204, 1667, 1566, 1403, 1283, 1207, 1097, 1061, 838, 778, 721, 667. General polymerization procedure for PAEs with dendritic cationic side chains. Monomer 20 (340.8 mg, 0.25 mmol) and 0.25 mmol of the other monomer (1,4diethynylbenzene or 4,7-diethynyl benzo[c][1,2,5]thiadiazole) we re dissolved in 25 ml of THF/Et3N (v/v, 4/1). The resulting solution was de oxygenated with argon for 15 minutes. Then 17.3 mg of Pd(PPh3)4 (15 M) and 5.7 mg of CuI (30 M) were added to the stirred solution under the protection of argon. The reaction mixtur e was then heated up to 55 C 60 C and stirred for 20 hr. The viscous solution was then poured into 200 ml of he xane. The precipitate was collected by vacuum filtration and washed with hexane (200 ml). After dried under vacuum, the polymer was stored as a solid. Typical reac tion yields for the polymerization are 80% 90%. For the hydrolysis, the organic polymer was disso lved in 20 ml of dioxane. The polymer solution was then cooled to 0 C 5 C using an ice/water bath. Concen trated HCl (7 ml, 12 N) was added to the stirred solution drop-wise. Upon the completion of the addition, the reaction mixture was allowed to warm to room temperat ure and stirred for another 12 hr. The polymer was then precipitated by pouring the solution in to a large amount of acetone (200 ml). The precipitate was collecte d, washed with acetone (100 ml) and finally dried under vacuum (yield: 90% 100%). No further purification was done on th ese polymers and they were stored as solid powders in a desiccator and can be re-dissolved in water easily.

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177 PPE-R2. 1H NMR (CDCl3, ppm):7.61 (br, s, 4H), 7.05 (s, 2H), 6.60 (br, s, 2H), 4.92 (s, 6H), 4.46 (s, 4H), 3.09 (br, s, 12H), 1.93 (br, 2, 12H), 1.39 (s, 54H). GPC (THF, polystyrene standard): Mw = 105, 640, Mn = 24, 080, PDI = 4. 40). FT-IR ( max, KBr pellet): 3393, 2977, 1691, 1517, 1457, 1392, 1367, 1274, 1252, 1170, 1046, 866, 839, 781, 637, 601. PPE-dNH3Cl 1H NMR (D2O/DMSO-d6 = 1/1): 7.61 (br, s, 4H), 7.18 (s, 2H), 4.66 (s, 4H), 2.92 (br, s, 12H), 2.05 (br, 2, 12H). FT-IR ( max, KBr pellet): 3392, 3031, 2202, 2002, 1672, 1607, 1516, 1489, 1407, 1281, 1191, 1063, 1017, 966, 906, 842, 786, 721, 548. BTD-PPE-R2. 1H NMR (CDCl3, ppm): 7.94 (br, s, 2H), 7.25 (b r,s, 2H), 4.97 (s, 6H), 4.58 (s, 4H), 3.09 (s, 12H), 1.94 (s 12H), 1.40 (s, 54H). GPC (THF, polystyrene standard): Mw = 44, 700, Mn = 12, 320, PDI = 3.60). FT-IR ( max, KBr pellet): 3350, 2977, 2939, 2679, 2490, 2203, 1693, 1570, 1458, 1392, 1366, 1279, 1252, 1171, 1041, 966, 892, 866, 780, 634, 564. BTD-PPE-dNH3Cl 1H NMR (D2O/DMSO-d6 = 1/1): 8.01 (br, s, 2H), 7.36 (s, 2H), 4.78 (s, 4H), 2.94 (s, 12H), 2.07 (s, 12H). FT-IR ( max, KBr pellet): 3394, 3035, 2202, 2011, 1672, 1610, 1542, 1509, 1409, 1342, 1281, 1191, 1067, 1020, 965, 893, 852, 786, 632, 563, 509.

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178 CHAPTER 6 CONCLUSIONS In the previous chapters, th e synthesis, characterizati on and photophysical properties (absorption, emission and fluorescence quenching) of poly(arylene ethynylene)-based conjugated polyelectrolytes (CPEs) were described. In chapter 2, meta -linked poly(phenylene ethynylene)s (PPEs) carrying chiral side groups were synthesi zed and their self-assembly behavior in solution was investigated. In chapter 3, two types of para -linked poly(phenylene ethynylene) carrying carboxylate side groups were synt hesized and their pho tophysical properties were studied. Based on the results of fluoresce nce quenching by divalent meta l ions, a highly selective and sensitive sensor was developed for pyrophosphate, a biologically important inorganic anion. In chapter 4, a group of anionic poly( p -phenylene ethynylene)s with 5 different chain lengths was synthesized and characterized. The relationship between phot ophysical properties and chain length was investigated. In chapter 5, in the fi rst part, the band gap of poly(arylene ethynylene)s (PAEs) with linear carboxylate si de chains was tuned by incorpor ating different heteroaromatic units; in the second part, to disr upt the interchain interaction, PAEs with dendritic anionic and cationic side groups were synthesized. The phot ophysical properties of these PAEs in solution were systematically studied and the application of PAEs with linear carbox ylate side chains in dye-sensitized solar cells was also explored. Helical Self-assembly Meta -linked poly(phenylene ethynylene )s carrying ionic side groups can fold into a helical conformation in water, which is stabilized by favorable stacking and hydrophobic interactions. To probe the helical folding process, meta -linked PPEs featuring carboxylate groups based on L -alanine ( w -P2 and w -P3 ) were synthesized. The molecular weight of the polymer was varied by using diffe rent protecting groups for carboxyl ic acid functionalities and

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179 showed a negligible effect on the polymer fold ing process. The presence of the chiral and optically active ionic side groups induced the formation of left-handed helix of w -P2 in enantiomeric excess, as evidenced by the ci rcular dichroism spect roscopy. The helical conformation of w -P2 bears some structural similarity to double-strand DNA. Specifically, the stacks in the polymer helical conformation resemble the -stacked base pairs in DNA. And their helical structures are both stabilized by negatively charged si de groups that are extended to the surrounding environment. Interactions of sm all dye molecules with the helical conformation of the polymer could take pl ace via different binding mode s. DNA metallointercalator, [Ru(bpy)2(dppz)]2+, interacts with the w -P2 in a similar mode compared to DNA, leading to the turn on of the emission from the metal comp lex. Cationic cyanine dyes assemble into the grooves of the helical polymer and form chiral and optically active supr amolecular aggregates induced by the polymer template. Aggregation & Chain Length Rigid rod-like para -linked poly(phenylene ethynylene)s car rying ionic groups exist in an aggregated state in water and a random monomeric state in MeOH. The aggregation is believed to occur through the plan arization of the conjugated backbone and the formation of cofacial stacking of phenylene rings. It is characterized by the appearance of red-shifted band in the absorption spectrum and also a red-shifted broad excimer-like band with less quantum efficiency in the emission spectrum. Two types of PPEs which have the same backbone structure but different substitution patterns of carboxylate grou ps were synthesized a nd studied. It was found that the carboxylate-substitution pattern has a stro ng effect on the aggregate formation. For the polymer that carries carboxylate per repeat unit (one phenylene ethynylene), wP4 the aggregation is suppressed by the incr eased electronic repulsion in the stacks. While for the

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180 polymer that carries carboxylat e for every two repeat units, w -P5 the stacks are formed between carboxylated 2,5-disubs tituted-1,4-phenylene and 1,4-phenylene, which minimizes the electronic repulsion. Fluorescence quenching studies of w -P5 by MV2+ in water and in methanol in the presence of different amount of Ca2+ reveal that the superlin ear quenching behavior in many CPE-quencher systems arises from the aggr egation and/or quenche r-induced aggregation of conjugated polyelectrolyte chains. A careful study of the influence of molecular weight of w P5 on the photophysical properties of the polymer indicates that high-MW polymers tend to form aggregates even in methanol. A maximum of 4fold enhancement of the Stern-Volmer constant is observed for high-MW polymers compared to low-MW samples when mono-valent viologen quenchers were used. Whenever aggregation or quencher-induced aggregation occurs, the influence of aggregation on the que nching efficiencies overwhelms th e influence of chain length. These results indicate that the best sensing re sponse could be attained by designing CPEs that can retain high quantum efficiencies in their ag gregate state, which at the same time still facilitates the exciton diffusion. Anion Sensing The fluorescence of w -P5 is most efficiently que nched by cupric ion (Cu2+) both in methanol and in 4-(2-hydroxyethyl)-1-piper aziethanesulfonic acid (HEPES) buffer solution among 9 different divalent metal ions that were ex amined. In methanol, a ll the other metal ions induced the aggregation of w -P5, evidenced from the red-shifted band in the absorption spectra and the red-shifted broad excimer-like band in the emission spectra. But for Cu2+, a complete quenching of the polymer fluorescence was obs erved, which indicates that the quenching by Cu2+ follows different mechanisms. In the HEPES buffer, Cu2+ was the most efficient quenching metal ion. The Ksv value for Cu2+ is in the order of 106 M-1, which is comparable to the

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181 quenching of w -P5 by MV2+. Taking advantage of the effi cient fluorescence quenching of w -P5 by Cu2+, a system consisting of w -P5 /Cu2+ (5 M/10 M) was used as a fluorescence turn-on sensing platform and shows high selectivity for pyrophosphate over 12 other anions. The selectivity is believed to arise from the ability of pyrophosphate to form a complex by chelation of the diphophate moiety to Cu2+. The analytical detection limit is determined to be ~ 80 nM. The current research presented the first exampl e of CPE-based sensor for PPi with sensitivity higher than that of most PPi sensors which are based on organic dyes or small molecules. Dye-sensitized Solar Cells The application of poly(arylen e ethynylene)s with carboxylic acid functionalities in TiO2 based dye-sensitized solar cells was also ex plored. These polymers feature a backbone consisting of a carboxylated bis(alkoxy)phenylene1,4-ethynylene unit alternating with a second arylene ethynylene moiety. For nanocrystalline TiO2 solar cells, the IPCE (maximum ~ 50%) and power conversion efficiency increase with the reduc tion of band gap of the polymers except that for the most red-absorbing polymer, BTD-PPE-CO2H, a lower cell performance results. This polymer exhibits a donor-a ccepter charge-transfer band, which is evidenced by the relatively large Stokes shift ( ca. 150 nm) and the broad, struct ureless band shape of the fluorescence spectra. The poor cell performance for BTD-PPE-CO2H is likely due to the strong competition of intramolecular/intermolecular charge transfer process with the desired charge injection to TiO2. Although the overall power conversion e fficiencies of the cells using these PAEs are relatively low, these results indicate that the usage of donor-acceptor interaction to reduce the band gap of conjugated polymer for applic ations in solar cells might have to consider its negative effect, i.e, the competition between the intramolecular/intermolecular charge transfer and the charge injection from the polymer to the electron acceptor.

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182 APPENDIX A HELICAL FOLDING OF META -LINKED POLY(PHENYLENE ETHYNYLENES) 0.28 0.21 0.14 0.07 0.00 Absorbance 500 400 300 Wavelength (nm) 5.0 4.0 3.0 2.0 1.0 Intensity 600 500 400 Wavelength (nm) H2O % 0 20 40 60 80 100 Figure A-1. Absorption (left) and fluorescence (right) of w -P3 (DP = 39 kD) (c = 10 M polymer repeat units, path length 1 cm) in methanol, water and methanol/water mixtures. 100x103 80 60 40 20 0 Intensity 800 700 600 500 Wavelength (nm) Figure A-2. Emission spectra of [Ru(bpy)2(dppz)]2+ in the absence (solid black line) and presence of w -P3 (concentration of w -P3 ranges from 0 120 M polymer repeat units, excitation wavelength is 450 nm). Solutions are deoxygenated by argon bubbling.

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183 -8 -6 -4 -2 0 2 CD / mdeg 800 700 600 500 Wavelength (nm) DiSC2(5) DiSC2(5) + w-P2 0.8 0.6 0.4 0.2 0.0 Absorbance Figure A-3. TOP: UV-visi ble absorption of DiSC2(5) in water titrated with w -P2 [DiSC2(5)] = 5.0 M and w -P2 was added in 0.5 M aliquots. BOTTOM: Circular dichroism spectra of DiSC2(5) in water (c = 7.0 M) and DiSC2(5) in water (c = 7.0 M) mixed with w -P2 (c = 10.0 M).

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184 0.5 0.4 0.3 0.2 A675 1.6 1.2 0.8 0.4 0.0 PRU/[DiSC2(5)] Figure A-4. Plot of absorbance at 675 nm (A675) vs. [PRU]/[DiSC2(5)] ratio, where [PRU] is the repeat unit concentration of w -P2 added to the DiSC2(5) solution in water. 5x106 4 3 2 1 0 Intensity 800 700 Wavelength (nm) Figure A-5. Fluorescence spectra of DiSC2(5) ([DiSC2(5)] = 5.0 M) in water titrated with w -P2 recorded with excitation at 610 nm. w -P2 was added in 0.5 M aliquots.

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185 APPENDIX B NMR SPECTRA Figure B-1. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 3 (chapter 2). Figure B-2. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 3 (chapter 2).

PAGE 186

186 Figure B-3. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 7 (chapter 2). Figure B-4. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 7 (chapter 2).

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187 Figure B-5. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 8 (chapter 3). Figure B-6. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 8 (chapter 3).

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188 Figure B-7. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 19 (chapter 5). Figure B-8. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 19 (chapter 5).

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189 Figure B-9. 1H NMR (300 M Hz, CDCl3) spectrum of monomer 20 (chapter 5). Figure B-10. 13C NMR (75 MHz, CDCl3) sepctrum of monomer 20 (chapter 5).

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190 LIST OF REFERENCES (1) Hall, N. Chem. Commun. 2003 1-4. (2) Shirakawa, H.; Louis, E. J.; Macdiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977 578-580. (3) Shirakawa, H. Angew. Chem., Int. Ed. 2001 40 2575-2580. (4) Heeger, A. J. Angew. Chem., Int. Ed. 2001 40 2591-2611. (5) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988 88 183-200. (6) Grimsdale, A. C.; Mullen, K. Emiss. Mater., Nanomater. 2006 199 1-82. (7) Perepichka, I. F.; Perepi chka, D. F.; Meng, H.; Wudl, F. Adv. Mater. 2005 17 2281-2305. (8) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001 40 2581-2590. (9) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998 37 402-428. (10) Bunz, U. H. F. Chem. Rev. 2000 100 1605-1644. (11) Scherf, U.; List, E. J. W. Adv. Mater. 2002 14 477-487. (12) Burroughes, J. H.; Bradley, D. D. C.; Br own, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990 347 539-541. (13) Heeger, A. J. Solid State Commun. 1998 107 673-679. (14) Hide, F.; DiazGarcia, M. A. ; Schwartz, B. J.; Heeger, A. J. Acc. Chem. Res. 1997 30 430436. (15) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007 107 1324-1338. (16) Sirringhaus, H. Adv. Mater. 2005 17 2411-2425. (17) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000 100 2537-2574. (18) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007 107 1339-1386. (19) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998 120 11864-11873. (20) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; D unbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996 271 1705-1707. (21) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, G.; Smith, P. Science 1998 279 835-837.

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191 (22) Montali, A.; Bastiaansen, G.; Smith, P.; Weder, C. Nature 1998 392 261-264. (23) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975 4467-4470. (24) Chinchilla, R.; Najera, C. Chem. Rev. 2007 107 874-922. (25) Dieck, H. A.; Heck, F. R. J. Organomet. Chem. 1975 93 259-263. (26) Cassar, L. J. Organomet. Chem. 1975 93 253-257. (27) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995 117 12593-12602. (28) Bunz, U. H. F. Acc. Chem. Res. 2001 34 998-1010. (29) Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997 119 12441-12453. (30) Furstner, A.; Davies, P. W. Chem. Commun. 2005 2307-2320. (31) Mortreux, A.; Blanchar.M J. Chem. Soc., Chem. Commun. 1974 786-787. (32) Schrock, R. R. Acc. Chem. Res. 1986 19 342-348. (33) Weiss, K.; Michel, A.; Auth, E. M.; Bunz, U. H. F.; Mangel, T.; Mullen, K. Angew. Chem., Int. Ed. 1997 36 506-509. (34) Brizius, G.; Kroth, S.; Bunz, U. H. F. Macromolecules 2002 35 5317-5319. (35) Zhang, W.; Kraft, S.; Moore, J. S. Chem. Commun. 2003 832-833. (36) Zhang, W.; Kraft, S.; Moore, J. S. J. Am. Chem. Soc. 2004 126 329-335. (37) Zhang, W.; Moore, J. S. Macromolecules 2004 37 3973-3975. (38) Zhang, W.; Moore, J. S. Adv. Synth. Catal. 2007 349 93-120. (39) Mori, A.; Kondo, T.; Kato, T.; Nishihara, Y. Chem. Lett. 2001 286-287. (40) Wang, Y. F.; Watson, M. D. J. Am. Chem. Soc. 2006 128 2536-2537. (41) Woody, K. B.; Bullock, J. E.; Parkin, S. R.; Watson, M. D. Macromolecules 2007 40 4470-4473. (42) Moroni, M.; Lemoigne, J.; Luzzati, S. Macromolecules 1994 27 562-571. (43) Tour, J. M. Chem. Rev. 1996 96 537-553.

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192 (44) Cravino, A.; Zerza, G.; Neugebauer, H. ; Maggini, M.; Bucella, S.; Menna, E.; Svensson, M.; Andersson, M. R.; Brabec, C. J.; Sariciftci, N. S. J. Phys. Chem. B 2002 106 70-76. (45) Cotts, P. M.; Swager, T. M.; Zhou, Q. Macromolecules 1996 29 7323-7328. (46) Ricks, H. L.; Choudry, U. H.; Marshall, A. R.; Bunz, U. H. F. Macromolecules 2003 36 1424-1425. (47) Teraoka, I. Polymer Solutions: An Introduction to Physical Properties ; John Wiley & Sons, Inc.: New York, 2002. (48) Samori, P.; Francke, V.; Mullen, K.; Rabe, J. P. Chem.Eur. J. 1999 5 2312-2317. (49) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997 277 1793-1796. (50) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem. Soc. 1999 121 3114-3121. (51) Blatchly, R. A.; Tew, G. N. J. Org. Chem. 2003 68 8780-8785. (52) Jones, T. V.; Slutsky, M. M.; Laos R.; de Greef, T. F. A.; Tew, G. N. J. Am. Chem. Soc. 2005 127 17235-17240. (53) Pinto, M. R.; Schanze, K. S. Synthesis-Stuttgart 2002 1293-1309. (54) Lakowicz, J. R. Principles of Fluorescence Spectroscopy ; 2nd ed.; Kluwer Academic/Plenum Publishers, 1999. (55) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995 117 7017-7018. (56) Swager, T. M. Acc. Chem. Res. 1998 31 201-207. (57) Chen, L. H.; McBranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U. S. A. 1999 96 12287-12292. (58) Achyuthan, K. E.; Bergstedt, T. S.; Chen, L.; Jones, R. M.; Kumaraswamy, S.; Kushon, S. A.; Ley, K. D.; Lu, L.; McBranch, D.; Mukundan, H.; Rininsland, F.; Shi, X.; Xia, W.; Whitten, D. G. J. Mater. Chem. 2005 15 2648-2656. (59) Ho, H. A.; Bera-Aberem, M.; Leclerc, M. Chem.Eur. J. 2005 11 1718-1724. (60) Gaylord, B. S.; Wang, S. J.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2001 123 6417-6418. (61) Wang, D. L.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001 17 12621266.

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193 (62) Fan, C. H.; Hirasa, T.; Plaxco, K. W.; Heeger, A. J. Langmuir 2003 19 3554-3556. (63) Jiang, H.; Zhao, X. Y.; Schanze, K. S. Langmuir 2007 23 9481-9486. (64) Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003 19 6523-6533. (65) Fan, Q. L.; Zhou, Y.; Lu, X. M.; Hou, X. Y.; Huang, W. Macromolecules 2005 38 29272936. (66) Wang, J.; Wang, D. L.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A. J. Macromolecules 2000 33 5153-5158. (67) Chen, L. H.; McBranch, D.; Wang, R.; Whitten, D. Chem. Phys. Lett. 2000 330 27-33. (68) Chen, L. H.; Xu, S.; McBranch, D.; Whitten, D. J. Am. Chem. Soc. 2000 122 9302-9303. (69) Dalvi-Malhotra, J.; Chen, L. H. J. Phys. Chem. B 2005 109 3873-3878. (70) Abe, S.; Chen, L. H. J. Polym. Sci. Pol. Phys. 2003 41 1676-1679. (71) Tan, C. Y.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002 446-447. (72) Muller, J. G.; Atas, E.; Tan, C.; Schanze, K. S.; Kleiman, V. D. J. Am. Chem. Soc. 2006 128 4007-4016. (73) Halkyard, C. E.; Rampey, M. E.; Kloppenbu rg, L.; Studer-Martinez, S. L.; Bunz, U. H. F. Macromolecules 1998 31 8655-8659. (74) Birks, J. B. Photophysics of Aromatic Molecules ; Wiley-Inter-Science: London, 1970. (75) Wang, S.; Bazan, G. C. Chem. Commun. 2004 2508-2509. (76) Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2004 126 1942-1943. (77) Dore, K.; Dubus, S.; Ho, H. A.; Levesque I.; Brunette, M.; Corbei l, G.; Boissinot, M.; Boivin, G.; Bergeron, M. G.; Boudreau, D.; Leclerc, M. J. Am. Chem. Soc. 2004 126 4240-4244. (78) Liu, B.; Bazan, G. C. Chem. Mater. 2004 16 4467-4476. (79) Liu, B.; Bazan, G. C. Proc. Natl. Acad. Sci. U. S. A. 2005 102 589-593. (80) Leclerc, M. Adv. Mater. 1999 11 1491-1498. (81) McCullough, R. D.; Ewbank, P. C.; Loewe, R. S. J. Am. Chem. Soc. 1997 119 633-634.

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194 (82) Ho, H. A.; Boissinot, M.; Bergeron, M. G. ; Corbeil, G.; Dore, K.; Boudreau, D.; Leclerc, M. Angew. Chem., Int. Ed. 2002 41 1548-1551. (83) Karlsson, K. F.; Asberg, P.; Nilsson, K. P. R.; Inganas, O. Chem. Mater. 2005 17 42044211. (84) Herland, A.; Nilsson, K. P. R.; Olsson, J. D. M.; Hammarstrom, P.; Konradsson, P.; Inganas, O. J. Am. Chem. Soc. 2005 127 2317-2323. (85) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Proc. Natl. Acad. Sci. U. S. A. 2002 99 1095410957. (86) ORegan, B.; Gratzel, M. Nature 1991 353 737-740. (87) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000 33 269-277. (88) Gratzel, M. Inorg. Chem. 2005 44 6841-6851. (89) Koumura, N.; Wang, Z. S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. J. Am. Chem. Soc. 2006 128 14256-14257. (90) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004 16 4533-4542. (91) Kim, Y.-G.; Walker, J. ; Samuleson, L. A.; Kumer, J. Nano Lett. 2003 3 523-525. (92) Yanagida, S.; Senadeera, G. K. R.; Nakamura, K.; Kitamura, T.; Wada, Y. J. Photochem. Photobio., A 2004 166 75-80. (93) Mwaura, J. K.; Zhao, X.; Jiang, H.; Schanze, K. S.; Reynolds, J. R. Chem. Mater. 2006 18 6109-6111. (94) Mio, M. J.; Prince, R. B.; Moor e, J. S.; Kuebel, C.; Martin, D. C. J. Am. Chem. Soc. 2000, 122 6134-6135. (95) Arnt, L.; Tew, G. N. J. Am. Chem. Soc. 2002 124 7664-7665. (96) Tan, C. Y.; Pinto, M. R.; Kose, M. E.; Ghiviriga, I.; Schanze, K. S. Adv. Mater. 2004 16 1208-1211. (97) Arnt, L.; Tew, G. N. Langmuir 2003 19 2404-2408. (98) Arnt, L.; Tew, G. N. Macromolecules 2004 37 1283-1288. (99) Breitenkamp, R. B.; Arnt, L.; Tew, G. N. Polym. Adv. Technol. 2005 16 189-194. (100) Kim, T.; Arnt, L.; Atkins, E.; Tew, G. N. Chem.Eur. J. 2006 12 2423-2427.

PAGE 195

195 (101) Prince, R. B.; Brunsveld, L. ; Meijer, E. W.; Moore, J. S. Angew. Chem., Int. Ed. 2000 39 228-230. (102) Gin, M. S.; Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999 121 26432644. (103) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000 122 2758-2762. (104) Endres, A.; Maas, G. Tetrahedron 2002 58 3999-4005. (105) Moore, J. S.; Stupp, S. I. Macromolecules 1990 23 65-70. (106) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis-Stuttgart 1980 627-630. (107) Berova, N.; Nakanishi, K.; Woody, R. Circular Dichroism: Principles and Applications ; 2nd ed.; Wiley-VCH: New York, 2000. (108) Zahn, S.; Swager, T. M. Angew. Chem., Int. Ed. 2002 41 4225-4230. (109) Erkkila, K. E.; Odom, D. T.; Barton, J. K. Chem. Rev. 1999 99 2777-2795. (110) Cuniberti, C.; Guenza, M. Biophys. Chem. 1990 38 11-22. (111) Powers, J. C.; Peticola.Wl J. Phys. Chem. 1967 71 3191-3195. (112) Kumar, C. V.; Asuncion, E. H. J. Am. Chem. Soc. 1993 115 8547-8553. (113) Becker, H. C.; Norden, B. J. Am. Chem. Soc. 2000 122 8344-8349. (114) Brun, A. M.; Harriman, A. J. Am. Chem. Soc. 1991 113 8153-8159. (115) Jenkins, Y.; Friedman, A. E.; Turro, N. J.; Barton, J. K. Biochemistry 1992 31 1080910816. (116) Holmlin, R. E.; Yao, J. A.; Barton, J. K. Inorg. Chem. 1999 38 174-189. (117) Pyle, A. M.; Chiang, M. Y.; Barton, J. K. Inorg. Chem. 1990 29 4487-4495. (118) Kirschbaum, T.; Briehn, C. A.; Bauerle, P. J. Chem. Soc., Perkin Trans. 1 2000 8 12111216. (119) Friedman, A. E.; Chambron, J. C.; Sauvage, J. P.; Turro, N. J.; Barton, J. K. J. Am. Chem. Soc. 1990 112 4960-4962. (120) Dervan, P. B.; Burli, R. W. Curr. Opin. Chem. Biol. 1999 3 688-693.

PAGE 196

196 (121) Pelton, J. G.; Wemmer, D. E. Proc. Natl. Acad. Sci. U. S. A. 1989 86 5723-5727. (122) Seifert, J. L.; Connor, R. E.; Kushon, S. A.; Wang, M.; Armitage, B. A. J. Am. Chem. Soc. 1999 121 2987-2995. (123) Wang, M. M.; Silva, G. L.; Armitage, B. A. J. Am. Chem. Soc. 2000 122 9977-9986. (124) Garoff, R. A.; Litzinger, E. A.; C onnor, R. E.; Fishman, I.; Armitage, B. A. Langmuir 2002 18 6330-6337. (125) Hannah, K. C.; Armitage, B. A. Acc. Chem. Res. 2004 37 845-853. (126) Tan, C. Y.; Alas, E.; Muller, J. G.; Pi nto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004 126 13685-13694. (127) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978 17 3334-3341. (128) Amouyal, E.; Homsi, A.; Chambron, J. C.; Sauvage, J. P. J. Chem. Soc., Dalton Trans. 1990 1841-1845. (129) Schanze, K. S.; Zhao, X. Y. In Handbook of Conducting Polymers ; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: Boca Raton, 2007; Vol. 3, p 1-29. (130) Greene, T. W.; Wuts, P. G. M. Protective groups in Organic Synthesis ; 3rd ed.; Wiley: New York, 1999. (131) Ewbank, P. C.; Loewe, R. S.; Zhai, L. ; Reddinger, J.; Sauve, G.; McCullough, R. D. Tetrahedron 2004 60 11269-11275. (132) Rininsland, F.; Xia, W. S.; Wittenburg, S.; Shi, X. B.; Stankewicz, C.; Achyuthan, K.; McBranch, D.; Whitten, D. Proc. Natl. Acad. Sci. U. S. A. 2004 101 15295-15300. (133) Odian, G. G. Principles of Polymerization ; 4th ed.; John Wiley & Sons, Inc.: Hoboken, N.J., 2004. (134) Haskins-Glusac, K.; Pinto, M. R.; Tan, C. Y.; Schanze, K. S. J. Am. Chem. Soc. 2004 126 14964-14971. (135) Zhao, X. Y.; Pinto, M. R.; Hardison, L. M. ; Mwaura, J.; Muller, J. ; Jiang, H.; Witker, D.; Kleiman, V. D.; Reynolds, J. R.; Schanze, K. S. Macromolecules 2006 39 6355-6366. (136) Mohan, J. Organic spectroscopy: Principles and Applications ; 1st ed.; CRC Press: New York, 2000. (137) Evans, D. A. In http://daecr1.harvard.edu/ pdf/evans_pKa_table.pdf

PAGE 197

197 (138) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000 122 1238912390. (139) Levitsky, I. A.; Kim, J.; Swager, T. M. Macromolecules 2001 34 2315-2319. (140) Kim, J.; Swager, T. M. Nature 2001 411 1030-1034. (141) Kim, J.; Levitsky, I. A.; McQuade, D. T.; Swager, T. M. J. Am. Chem. Soc. 2002 124 7710-7718. (142) Kim, I. B.; Dunkhorst, A.; Gilbert, J.; Bunz, U. H. F. Macromolecules 2005 38 45604562. (143) Kim, I. B.; Bunz, U. H. F. J. Am. Chem. Soc. 2006 128 2818-2819. (144) Kim, I. B.; Phillips, R.; Bunz, U. H. F. Macromolecules 2007 40 814-817. (145) Fan, L. J.; Jones, W. E. J. Am. Chem. Soc. 2006 128 6784-6785. (146) Fan, C. H.; Wang, S.; Hong, J. W.; B azan, G. C.; Plaxco, K. W.; Heeger, A. J. Proc. Natl. Acad. Sci. U. S. A. 2003 100 6297-6301. (147) Nelson, D. L.; Cox, M. M. Lehninger Principle of Biochemistry ; 3rd ed.; Worth Publishers: New York, NY, 2000. (148) Lee, D. H.; Kim, S. Y.; Hong, J. I. Angew. Chem., Int. Ed. 2004 43 4777-4780. (149) Vance, D. H.; Czarnik, A. W. J. Am. Chem. Soc. 1994 116 9397-9398. (150) Lee, H. N.; Swamy, K. M. K.; Kim, S. K. ; Kwon, J. Y.; Kim, Y.; Kim, S. J.; Yoon, Y. J.; Yoon, J. Org. Lett. 2007 9 243-246. (151) Lee, H. N.; Xu, Z.; Kim, S. K.; Swam y, K. M. K.; Kim, Y.; Kim, S. J.; Yoon, J. J. Am. Chem. Soc. 2007 129 3828-3829. (152) Jang, Y. J.; Jun, E. J.; Lee, Y. J.; Kim, Y. S.; Kim, J. S.; Yoon, J. J. Org. Chem. 2005 70 9603-9606. (153) Fabbrizzi, L.; Marcotte, N.; Stomeo, F.; Taglietti, A. Angew. Chem., Int. Ed. 2002 41 3811-3814. (154) Hanshaw, R. G.; Hilkert, S. M.; Hua, J.; Smith, B. D. Tetrahedron Lett. 2004 45 87218724. (155) McDonough, M. J.; Reynolds, A. J.; Lee, W. Y. G.; Jolliffe, K. A. Chem. Commun. 2006 2971-2973.

PAGE 198

198 (156) Ambade, A. V.; Sandanaraj, B. S.; Klaikherd, A.; Thayumanavan, S. Polym. Int. 2007 56 474-481. (157) Aldakov, D.; Palacios, M. A.; Anzenbacher, P. Chem. Mater. 2005 17 5238-5241. (158) Valcarcel, M. Principles of Analytical Chemistry: A textbook ; Springer-Verlag: New York, 2000. (159) Lee, D. H.; Im, J. H.; Son, S. U.; Chung, Y. K.; Hong, J. I. J. Am. Chem. Soc. 2003 125 7752-7753. (160) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999 38 1350-1377. (161) Moore, J. S. Acc. Chem. Res. 1997 30 402-413. (162) Roncali, J. Acc. Chem. Res. 2000 33 147-156. (163) Khan, M. S.; Al-Mandhary, M. R. A.; Al-S uti, M. K.; Ahrens, B.; Mahon, M. F.; Male, L.; Raithby, P. R.; Boothby, C. E.; Kohler, A. Dalton Trans. 2003 74-84. (164) Meier, H.; Ickenroth, D.; Stalmach, U.; Koynov, K.; Bahtiar, A.; Bubeck, C. Eur. J. Org. Chem. 2001 4431-4443. (165) Jorgensen, M.; Krebs, F. C. J. Org. Chem. 2005 70 6004-6017. (166) Koynov, K.; Bahtiar, A.; Ahn, T.; Cordeiro, R. M.; Horhold, H. H.; Bubeck, C. Macromolecules 2006 39 8692-8698. (167) Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J. Appl. Phys. Lett. 2005 86 122110-122113. (168) Cabarcos, E. L.; Carter, S. A. Macromolecules 2005 38 10537-10541. (169) Sperling, L. H. Introduction to Physical Polymer Science ; John Wiley & Sons, Inc.: New York, 2001. (170) Erdelyi, M.; Gogoll, A. J. Org. Chem. 2001, 66 4165-4169. (171) Pearson, D. L.; Schumm, J. S.; Tour, J. M. Macromolecules 1994 27 2348-2350. (172) Jiang, H.; Zhao, X.; Schanze, K. S. Langmuir 2006 22 5541-5543. (173) Jones, G.; Jackson, W. R.; Choi, C.; Bergmark, W. R. J. Phys. Chem. 1985 89 294-300. (174) Nguyen, T. Q.; Wu, J. J.; Doan, V.; Schwartz, B. J.; Tolbert, S. H. Science 2000 288 652-656.

PAGE 199

199 (175) Barltrop, J. A.; Jackson, A. C. J. Chem. Soc., Perkin Trans. 2 1984 367-371. (176) Feng, D. J.; Li, X. Q.; Wang, X. Z.; Jiang, X. K.; Li, Z. T. Tetrahedron 2004 60 61376144. (177) Roncali, J. Chem. Rev. 1997 97 173-205. (178) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W. Mat. Sci. Eng. R. 2001 32 1-40. (179) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000 10 1471-1507. (180) Wright, A. T.; Anslyn, E. V. Chem. Soc. Rev. 2006 35 14-28. (181) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004 14 1077-1086. (182) Nilsson, K. P. R.; Rydber g, J.; Baltzer, L.; Inganas, O. Proc. Natl. Acad. Sci. U. S. A. 2003 100 10170-10174. (183) Marsitzky, D.; Vestberg, R.; Blainey, P. ; Tang, B. T.; Hawker, C. J.; Carter, K. R. J. Am. Chem. Soc. 2001 123 6965-6972. (184) Setayesh, S.; Grimsdale, A. C.; Weil, T. ; Enkelmann, V.; Mullen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001 123 946-953. (185) Chou, C. H.; Shu, C. F. Macromolecules 2002 35 9673-9677. (186) Jiang, D. L.; Choi, C. K.; Honda, K.; Li, W. S.; Yuzawa, T.; Aida, T. J. Am. Chem. Soc. 2004 126 12084-12089. (187) Zhu, B.; Han, Y.; Sun, M. H.; Bo, Z. S. Macromolecules 2007 40 4494-4500. (188) Dominguez, X.; Lopez, I. C.; Franco, R. J. Org. Chem. 1961 26 1625-&. (189) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F. Macromolecules 2000 33 652-654. (190) Hou, J.; Yang, C.; Qiao, J.; Li, Y. Synth. Met. 2005 150 297. (191) Yamamoto, T.; Fang, Q.; Morikita, T. Macromolecules 2003 36 4262-4267. (192) Yasuda, T.; Imase, T.; Nakamura, Y.; Yamamoto, T. Macromolecules 2005 38 46874697. (193) Harriman, A. J. Chem. Soc., Chem. Commun. 1977 777-778.

PAGE 200

200 (194) Thornton, N. B. Ph. D. Disse rtation, University of Florida, 1995. (195) Wang, F. K.; Bazan, G. C. J. Am. Chem. Soc. 2006 128 15786-15792. (196) Fan, C. H.; Plaxco, K. W.; Heeger, A. J. Trends In Biotechnology 2005 23 186-192. (197) Mwaura, J. K.; Pinto, M. R.; Witker, D.; Ananthakrishnan, N.; Schanze, K. S.; Reynolds, J. R. Langmuir 2005 21 10119-10126. (198) Mwaura, J. K.; Zhao, X. Y.; Jiang, H.; Schanze, K. S.; Reynolds, J. R. Chem. Mater. 2006 18 6109-6111. (199) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1994 33 5741-5749. (200) Newkome, G. R.; Behera, R. K.; Moorefield, C. N.; Baker, G. R. J. Org. Chem. 1991 56 7162-7167. (201) Joester, D.; Losson, M.; Pugin, R.; Hein zelmann, H.; Walter, E.; Me rkle, H. P.; Diederich, F. Angew. Chem., Int. Ed. 2003 42 1486-1490.

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201 BIOGRAPHICAL SKETCH Xiaoyong Zhao was born in Shannxi province, Ch ina. He spent his delightful childhood in the countryside and then moved with his parent s in 1985 to a small town named Fugu, which lies at the west side of the Yellow River. There, he went to the primary sc hool and high school. In 1996, he left his home town and went to Changchun, a city locating in the far Northeast of China to attend the college. After getting his bachelo rs degree in 2000 from J ilin University, he was admitted to the Institute of Chemistry, Chinese Academy of Sciences in Beijing as a graduate student. In the first year of his graduate study, he decided to come to the United States of America to pursue his Ph. D. In August 2003, one month after he obtained his masters degree; he came to the University of Florida and started his training here at Gainesvill e. In the past four years, he did research in the area of water-s oluble conjugated polymers under the supervision of Dr. Kirk S. Schanze. He and his wife Huime ng, who is also pursuing Ph. D. in the same department, were married in 2004 and had thei r first daughter, Duoduo, in the winter of 2005. After his Ph. D., Xiaoyong will spend two years doi ng postdoctoral research in the group of Dr. Jean M. J. Frchet at the Univer sity of California, Berkeley.