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Photophysics of Single Chain Poly(Arylene Ethynylene)s

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PAGE 1

PHOTOPHYSICS OF SINGLE CHAIN POLY(ARYLENE ETHYNYLENE)S By ERIC ELY SILVERMAN 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 2005

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Copyright 2005 by Eric Ely Silverman

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For Jen.

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iv ACKNOWLEDGMENTS My work at the University of Florida would not have been possible without the advice, support, and friendship of the many pe ople I have interacted with during my time here. First, I thank my advisor, Kirk Schanze. His advice, support, and seemingly unending patience have been invaluable to my achievements over the past five years. I further acknowledge the c ontribution of many people w ho have contributed both to the work I have accomplished and to making my time in graduate school more enjoyable. Ksenija HaskinsGlusac, Boris Kristal, and Kye-Young Kim have all been both great friends and coworkers. I thank the many current and former coworkers with whom I have shared conversation and insights ov er the years. I would like to specifically point out Mauricio Pinto and Xiaoming Zhao fo r their contributions to my work. I also acknowledge the contributions of my collabo rators at Brookhaven National Laboratories, John Miller and Alison Funston. More thanks are due to my friends Aleksa and Ilka Jovanavic and Cris Dancel. A sp ecial thank you goes to Janice Young. Finally, I am grateful to my family for a ll of their love. My parents have offered me unending support throughout my graduate sch ool years. I thank my grandmother for her kindness and wisdom, and also my grandfat her, even though he is no longer here to share in my completion of this work. I am gr ateful for all of the love shared with my sisters Rochelle and Lisa. La st, I thank my brother Adam wh o never ceases to share his love, understanding, advice, and support.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES...........................................................................................................viii LIST OF FIGURES...........................................................................................................ix ABSTRACT......................................................................................................................x ii CHAPTER 1 INTRODUCTION........................................................................................................1 Conjugated Polymers: Background..............................................................................1 Poly(Phenylene Ethynylene)s.......................................................................................2 Applications of Poly(phenylene ethynylene)s.......................................................2 Synthesis of Poly(paraphenylene ethynylene)s.....................................................6 Platinum Containing Po ly(phenylene ethynylene)s: Platinum Polyynes............10 Meta Linked Poly(phenylene ethynylene)s.........................................................12 Conjugated Polymers as Molecular Wires.................................................................15 Single Chain Conjugated Polymers.....................................................................15 Charge Transport.................................................................................................18 Charge Injection..................................................................................................21 Description of the Present Study................................................................................22 2 RADICAL TRANSPORT IN END-CAPPED POLY(ARYLENE ETHYNYLENE)S......................................................................................................25 Introduction.................................................................................................................25 Results and Discussion...............................................................................................26 Synthesis and Structural Characterization of Po ly(arylene ethynene)s...............26 Absorption and Fluorescence Spectroscopy........................................................35 Phenylene based polymers...........................................................................35 Biphenylene based polymers........................................................................40 Radiolytic production of ca tion and anion radicals......................................41 Electron and hole transport to polymers following radiolysis.....................43 Semi-empirical calculations: spectr oscopy of PPE-based ion radicals........46 Visible/Near-IR Spectroscopy of Charged PAEs................................................47 Radical anions..............................................................................................47

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vi Radical cations.............................................................................................50 Bimolecular Hole Transfer Reactions: Thermodynamics of Intrachain PPE T3 Hole Transfer..............................................................................................53 Dynamics of Interchain Hole Transfer................................................................57 Conclusions.................................................................................................................57 Experimental Section..................................................................................................59 Materials and General Synthesis.........................................................................59 Synthetic Procedures...........................................................................................59 Photophysical Methods.......................................................................................64 Generation of the T3 and Ph-T3 Radical Cations...............................................65 Radiation Techniques..........................................................................................65 3 MECHANISM AND DYNAMICS OF TRIPLET TRANSPORT IN PLATINUM CONTAINING POLY (PHENY LENE ETHYNYLENE)S.......................................67 Introduction.................................................................................................................67 Results and Discussion...............................................................................................69 Polymer Synthesis and Stru ctural Characterization............................................69 Photophysical Mesurements................................................................................74 Absorption and emission spectroscopy........................................................74 Phosphorescence quenching.........................................................................79 Transient absorption spectroscopy...............................................................81 Time resolved emission................................................................................82 General Discussion..............................................................................................85 Electronic model..........................................................................................85 Kinetics and mechanism of energy transport...............................................90 Conclusions.................................................................................................................92 Experimental Section..................................................................................................93 General Synthesis and Materials.........................................................................93 Synthetic Procedures...........................................................................................94 Photophysical Methods.......................................................................................97 4 META LINKED PLATINUM CONTAINING POLY(PHENYLENE ETHYNYLENE)S......................................................................................................99 Introduction.................................................................................................................99 Results and Discussion.............................................................................................101 Synthesis and Characterization..................................................................101 Solvent Induced Conformational Effects...................................................106 Chiral Induction..........................................................................................112 Intercalator Binding....................................................................................114 Conclusions...............................................................................................................120 Experimental Section................................................................................................121 Materials and General Synthesis................................................................121 Synthetic Procedures..................................................................................122 Photophysical Methods..............................................................................126

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vii 5 CONCLUSIONS......................................................................................................128 Carrier Transport......................................................................................................128 Helical Self-Assembly..............................................................................................129 Future Outlook..........................................................................................................129 REFERENCES AND NOTES.........................................................................................131 BIOGRAPHICAL SKETCH...........................................................................................139

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viii LIST OF TABLES Table page 2-1 Conditions used in end-capping polymerizations....................................................34 2-2 Selected photophysical data for PAEs....................................................................35 2-3 Reaction rate constants of poly-(aryle ne ethynylene)s (PAEs) and terthiophene end-capped PAEs with positive and negative charge carriers in solution................45 2-3 Bimolecular rate and thermodynamic data for reactions of PAEs with thiophene oligomers..................................................................................................................54 3-1 Summarized absorption and photoluminescence data.............................................75

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ix LIST OF FIGURES Figure page 1-1. Examples of conjugate d polymer structures..............................................................2 1-2. Schematic of Swagers pentiptycene containing PPE................................................3 1-3. LCD displaysconstructed with a PPE as the luminescent layer, and a commercially available display..................................................................................4 1-4. Chemical sensors functioning independently.............................................................4 1-5. Chemical sensors functioning in series......................................................................5 1-6. Flourescence assays for enzyme activity using a PPE as the fluorophore. ...............6 1-7. Mechanism of the Sonogashira Reaction...................................................................8 1-8. General structure of a platinum containing PPE......................................................10 1-9. Absorption and emission spectra of o ligo platinum PPEs with increasing chain length........................................................................................................................1 1 1-10. Oligomeric m-phenylene ethynylenes......................................................................12 1-11. A segment of the meta-para PPE used by Tan.........................................................13 1-12. Chiral induction in a helical polymer.......................................................................14 1-13. A conjugated oligomer in use as a molecular wire..................................................16 1-14. Anthracene end-capped polymer.............................................................................17 1-15. A metal binding conjugated polymer.......................................................................18 1-16. MEH-PPV with satura ted conjugation breaks.........................................................20 2-1. Structures of polymers and model compounds........................................................27 2-2. Synthesis of polymers and model.............................................................................28 2-3. NMR of T3PPE13 with 2-ethylhexylo xy side groups...............................................32

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x 2-4. NMR of T3 end-capped PPE with undecyloxy side groups.....................................32 2-5. Expansion of the aromatic region of 2-4..................................................................33 2-6. Absorption and photoluminescence spect ra of poly(phenylene ethynylene)s and end group model compounds.............................................................................36 2-8. Absorption and emission of BpE21 and T3BpE12.....................................................41 2-9. Schematic representation of the LEAF facility........................................................42 2-10. Transition energies computed for radical cations of phenylene ethynylene oligomers as a function of length.............................................................................47 2-11. Spectra of cations and ani ons of poly(phenylene ethynylenes)...............................48 2-12. Differential absorbance spectra of PhT3 + and T3 +.............................................51 2-13. A segment of a PPE featuring seven phenyl ethynyl rings at the left and a terthiophene end-cap at the right..............................................................................52 2-14. Schematic of the oxidation potential s of various PAEs and thiophene oligomers..................................................................................................................55 2-15. Absorbance as a function of time after an electron pulse for solutions of PPE164 and varied concentrations of terthiophene...............................................................56 3-1. Structure of polymers fe atured in this chapter.........................................................68 3-2. Synthesis of intermediate 3 and P0T100....................................................................70 3-3. Synthesis of platinum contai ning phenylene-thiophene copolymers.......................71 3-4. Aromatic region of the polymers 1H NMR spectra..................................................73 3-5. Absorption spectra of polymers in THF solution....................................................75 3-6. Photoluminescence spectra of polymers..................................................................77 3-7. Stern-Volmer quenching of P95T5 in THF solution..................................................80 3-8. Triplet-state transi ent absorption spectra.................................................................82 3-9. Time resolved emission spectrum of P95T5..............................................................83 3-10. Normalized emission decay of P95T5 at 78 K...........................................................84 3-11. Schematic representation of platinum acetylide polymers.......................................87

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xi 3-12. Jablonki diagrams representing the alltriplet and all-sing let mechanisms for energy transfer..........................................................................................................91 4-1. Transoid and cisoid conformations of a tetrameric meta linked OPE...................100 4-2. Polymers and models featured in this study...........................................................101 4-3. Synthesis of monomer 3 .........................................................................................102 4-4. Synthesis of P-O.....................................................................................................104 4-5. Mass spectrum of M-W..........................................................................................106 4-6. UV-Vis absorption and photoluminescence spectra of P-O in mixtures of DCM and Hex........................................................................................................107 4-7. Absorbance spectra of P-W in mi xtures of methanol and water............................109 4-8. Variable pH absorption spectra of P-W.................................................................110 4-9. Photluminescence spectra of P-W in methanol:water mixtures.............................111 4-10. CD spectra of P-W with varying concentrations of (-)pinene...........................113 4-11. Time dependant CD spectra of P-W with added (-)pinene................................114 4-12. A twelve monomer segment of P-W in a helical conformation.............................115 4-13. Structures of Rudppz and AA................................................................................116 4-14. Absorption spectra of AA with added P-W...........................................................117 4-15. Decay of AA triplet absorpti on in free and intercalated AA.................................118 4-16. Titration of Rudppz with P-W................................................................................119 4-17. Photoluminescence augmentation of Rudppz with addition of P-W.....................120

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xii 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 PHOTOPHYSICS OF SINGLE CHAI N POLY (ARYLENE ETHYNYLENE)S By Eric Ely Silverman May 2005 Chair: Kirk S. Schanze Major Department: Chemistry This dissertation presents the results of an investig ation into the photophysical properties of single conjugated polymers. The study focuses on the properties of polymers as single-chain molecular wires rather than as bulk materials. The dissertation focuses on the design and synthesis of these materials and the use of a variety of techniques to probe their unique properties. The prim ary focus of the photophysical investigations is the use of optical techniques, although radiol ytic techniques were also applied where appropriate. All of the polymers under investigation were demonstrated to show rapid and efficient transport of singlet, poleron, and triplet carriers. In order to study these properties, polymers with incorporated carrier traps were synthesized. These traps were either randomly distributed th roughout the polymer chain or appended to the polymers as end-caps.

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xiii Helical, organometallic polymers were al so synthesized and investigated. A hydrophobic polymer with ester pendant groups was subjected to post-polymerization functionalization to give a hydrophilic materi al with pendant acid groups. This polymer has many similarities to DNA in that it is a polyanion that readily adopts a helical conformation characterized by face-to-face stack ing of aromatic moieties. This polymer accepts many well-known DNA intercalators whic h show characteristic spectral changes upon binding. Additionally, binding of the polymer to a chiral guest induces the preferential formation of one of the two po ssible enantiomeric helices, as evidenced by circular dichroism spectroscopy.

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1 CHAPTER 1 INTRODUCTION Conjugated Polymers: Background Conjugated polymers are unsaturated mate rials in which the entire polymer backbone consists of atoms with a continuous orbital system. In 1977, it was reported that polyacetylene can exhib it significant conductivity when it is oxidized or reduced.1 The field was brought to the forefront of scie ntific and public atte ntion in 2000, when the principal discoverers of this property, Heeger, MacDiarmid, and Shirakawa, were awarded the Nobel Prize in Chemistry. From the original communication of this extraordinary property in 1977 until the present day, conjugated polymers have found a wide variety of applications, often in device fabrication. Devices as varied as light emitting diodes, light emitting electrochemical cells, plastic lasers, and thin-film transist ors have featured conjugated polymers as functional components.2-9 A wide variety of conjugated polymer stru ctures have been reported, and several of these are depicted in Figure 1-1. With th e exception of polyacetylene, these materials feature aromatic or heteroaromatic rings th at are connected either directly or via conjugated vinyl or ethynyl linkers. Poly(phenylene vinylene) (PPV)s, as a clas s, have perhaps found the most uses to date, and at least one common member of th is class poly(methyl ethylhexyl phenylene vinylene), or MEH-PPV, is curr ently commercially available.10 The fact that PPVs have achieved such success despite their moderate stability, cisbackbone defects, and the

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2 rather delicate processing conditions needed in order to obtain hi gh-quality materials, strongly suggests that investigations into other conjugated polymer architectures may allow for the fabrication of improved materials and devices.11 n R R R R n n S R n N H n Polyacetylene Poly(3-alkyl)thiophene Polyaniline Polyp -phenylene Poly(phenylene vinylene) Poly(phenylene ethynylene) n R R Figure 1-1. Examples of c onjugated polymer structures. Poly(Phenylene Ethynylene)s Applications of Poly(phenylene ethynylene)s One of the most promising classes of conjugated polymers is poly(arylene ethynylene)s, or PAEs. PAEs feature aroma tic or heteroaromatic rings connected by ethynyl linkages.12 By far the most common type of PAEs are poly(phenylene ethynylene)s (PPEs), where the aromatic group is a benzene ring. PPEs have a tremendous advantage in that they may bear a great variety of functional group substitutents. PPEs with alkyl,13 alkoxy,14 carboxylate,15 phosphonate,16 and other pendant groups12,17 have been studied. In addition to varying the pendant group, it is possible to dope the polymer chains backbones with other moieties, including organic12,14 or metal-containing18 co-monomer units.

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3 Recent investigations have shown that PPEs have an astounding variety of practical uses. Many of these applications are made possible by the relative ease in which structural diversity can be built in to PPEs. For example, by incorporating pentipticyl groups into the backbone, as in Figure 1-2, Yang and Swager utilized a PPE as an effective sensor for trinitrotoluene, a common explosive.19 Figure 1-2. Schematic of Swagers pentiptycene containing PPE, an effective TNT sensor. Picture from Swager [19]. In 1998, Weder and co-workers used a PPE with a well-designed substitution pattern to fabricate a liq uid crystalline display.20 They note that the particular PPE used in their device features an ideal set of properties for the preparation of polarized photoluminescent layers.21 Notably, their device shows ma rkedly better contrast than common commercially available LCD di splays, as shown in Figure 1-3. Even more recently, PPEs have found applic ation in sensing biological activity. Because PPEs are effectively chromophores wire d in series they have the potential to act as amplifiers for chemical signaling info rmation. Swager calls this the molecular wire approach to si gnal amplification.17 This approach takes advantage of the electronic communication between chromophores in a PPE.

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4 Figure 1-3. LCD displays (A) constructed with a PPE as the luminescent layer, and (B), a commercially available displa y. Photograph from Weder [21]. Figure 1-4 and 1-5 are schematic diagrams explaining this e ffect. Figure 1-4 depicts independently functioning chemical sensors. Note that each molecule of analyte can cause a response in only one sensory molecu le. The system is therefore limited by its 1:1 stoichiometry. The ability of such a system to detect, for example, a single molecule of analyte would depend on the ability to elicit a measurable response from a single receptor molecule. Figure 1-4. Chemical sensors functioning i ndependently. Adapted from Swager [17]. This limitation can be overcome by wiring sens ors in series, as is the case of the chromophores in a PPE. In this situation, de picted in Figure 1-5, a binding event at a single receptor causes a response in every molecu le in the wire. Considering that modern techniques can easily produce polymers with several hundred repeat units, this method

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5 should be able to produce a signa l amplification of two or mo re orders of magnitude over independently functioning receptors. Figure 1-5. Chemical sensors functioning in series. Adapted from Swager [17]. The Whitten lab used this effect to develop a sensor for the protein avidin.22 This design takes advantage of avidins extraordin ary high binding affinity for the molecule biotin. The sensor used an anionic, wate r soluble PPV and a cationic quencher, methyl viologen (MV+), which was covalently bound to bio tin. When the biotin bound quencher is introduced to a polymer solution the quencher binds to the polymer due to electrostatics, and the polymer fluorescence is quenched. Upon subsequent addition of avidin, tight binding between avidin and bi otin moves the quencher away from the polymer, resulting in a strongly fluorescent polymer solution. After this initial work with a PPV, PPEs with anionic pendant groups were used by Pinto and Schanze to develop fluorescence assays that monitor the activity of proteases in real time.23 The assays are somewhat alike to Whittens in that they depend upon the amplified quenching PPEs by quenchers tethered to biomolecules. Figure 1-6 depicts the two types of assays developed by this appr oach. The first, a t urn on assay, features a quencher bound to a positively charged anchor via a biomolecule tether. The polymer is initially non-emissive due to th e proximity of the quencher. Enzymatic cleavage of the tether releases the quench er giving rise to the typical bright photoluminescence of the PPE. The turn off appr oach utilizes a caged quencher that is

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6 initially insulated from the polymer backbone Enzymatic cleavage of the cage leaves the quencher in close proximity to the pol ymer where it effects emission quenching. Figure 1-6. Flourescence assays for enzyme activity using a PPE as the fluorophore. Figure from Pinto [23]. Synthesis of Poly(paraphenylene ethynylene)s The most common method of synthesizi ng PPEs is the S onogashira coupling reaction.12,17 This method, involving the use of copper and palladium to couple terminal acetylenes to aryl halides, wa s developed by Sonogashira in 1975.24 Other examples of this transformation appeared in the literature at about the same time, 25,26 however, Sonogashiras method, which requires less forcing conditions, is tolera nt of a wide range of functional groups, and does not involve prep aration of copper acetylides, is the only one that has provided access to PPEs.

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7 In its most common form, the Sonogashir a reaction utilizes readily available bis(triphenylphosphine)palladium(II) chloride and copper (I) iodide in the presence of an amine to effect cross coupling of an aryl i odide or bromide with a terminal acetylene under anaerobic conditions. The accepted mech anism for this reaction is depicted in Figure 1-7. When a palladium (II) catalyst is used, the in itial step involves substitution of the chlorides by two terminal acetylides, followed by reductive elimination to yield the active catalyst, a palladium (0) species, and an acetylene dimer. This byproduct is not typically a problem for reactions of small molecules. However, for synthesis of polymers where exact stoichiometric balance is required, this side reaction imbalances the stoichiometry and results in reduced polymer molecular we ights. Swagger suggested using a slight excess of acetylide monomer to alleviate this problem17. However, a bette r solution is to substitute a palladium (0) species for the pall adium (II), to circumvent the initial catalyst activation step. Tetrakis(tri phenylphospine) palladium is an excellent choice as an alternative catalyst. The next step in the reaction is insertion of the active ca talyst into the aryl-halide bond, followed by transmetallation of the halide by a Cu (I) acetylide. Reductive elimination of the aryl acetylid e product regenerates the cataly st. In order to prevent the formation of an inactive complex betw een palladium and oxygen, the reaction is completed under anaerobic conditions. The reactivity of aryl halides in Sonoga shira conditions depends on the aryl-halide bond dissociation energy. Thus, in general, aryl iodides are more reactive than aryl bromides, and aryl chlorides are typically unreactive. The difference between the

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8 reactivity of iodides and bromides is suffici ently great that aryl iodides are known to undergo reaction to the nearly complete excl usion of a competing reaction with aryl bromides.27 In addition, reactivity of aryl iodi des is typically fast even at room temperature, while aryl bromides often requi re higher temperatures and longer reaction times. (PPh3)2PdCl2R H CuI, NR'3NR'3H+Cl-(PPh3)2PdR 2 (PPh3)2Pd0RR (PPh3)2Pd Ar-X Ar X (PPh3)2Pd Ar R R H CuI, NR'3NR'3H+Cl-RAr Figure 1-7. Mechanism of the Sonogashira Reaction. Recent advances in catalysis have made possible the use of aqueous conditions and inorganic bases,28 the use of aerobic condi tions without copper,29 and coupling to alkyl chlorides.30 However, because these methodologies require the use of harsh conditions, catalysts that are not readily available, or both, they are not often used to access PPEs.

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9 Molybdenum and tungsten based catalysts have also been used to synthesize PPEs by alkyne metathesis reactions. This methodol ogy, developed primarily in the Bunz and Moore laboratories, was the subject of a review article in 2001.31 Bunz coined the term acyclic diyne metathesis (ADIMET) to desc ribe this condensati on polymerization, which formally condenses a molecule of hydrogen while forming a new bond between two terminal acetylenes. The ADIMET methodology addresses some of the problems associated with Sonogashira polymerizations, including ill-def ined endgroups due to dehalogenation and phosphonium salt formation,32 and the difficulty in producin g very high molecular weight materials.12 In addition, ADIMET polymerizations require only a single type of functional group, thus eliminating the severa l practical difficulties. Specifically, the problems associated with the strict stoichio metric balance required for two component AA, B-B type Sonogashira polymerizations betw een aryl diiodides a nd aryl diethynes, or the synthetic difficulties often associated with the synthesis of ultra-high purity A-B type iodo-ethynyl substituted arenas. In its earliest incarnations, ADIMET had se veral major drawbacks. These include the need for harsh conditions, and intoleran ce of a wide degree of functionality. However, more recently, the Moore gr oup developed a simple synthesis of a molybdenum alkylidine catalyst that show s good activity in ADIM ET under reasonable reaction conditions.33 This catalyst also shows impr oved functional group tolerance, and yields higher molecular weight pol ymers than the Sonogashira methodology.34 Despite these improvements, ADIMET remains the le ss popular of the two methods, probably because the effective ADIMET catalysts are not commercially available, whereas the

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10 palladium catalysts used for Sonogashira po lymerizations can be purchased relatively inexpensively. Platinum Containing Poly(phenylene ethynylene)s: Platinum Polyynes Another significant class of PPEs are thos e which contain platinum in the polymer backbone. These materials are of particul ar interest because their photophysics are dominated by long lived, phosphorescent 3 excited states.35-39 This property makes them useful candidate materials for the fabrication of electro luminescent devices,39,40 and for applications in optical limiting.41 The general structure of a platinum cont aining PPE is depicted in Figure 1-8. These materials consist of sp hybridized alkyne carbons linking bis(trialkylphosphine)platinum (II) to phenylene rings. When the aromatic rings are not substituted, good solubility in organic solven ts can be achieved by using butyl or longer alkyl groups. Pt PR3PR3n Figure 1-8. General structure of a platinum containing PPE. Platinum containing PPEs are synthesi zed utilizing the Hagihara coupling reaction, which was discovered in 1978.42 The reaction involves the Cu (I) catalyzed substitution of chlorine on a bis(trialkylphos phine)platinum (II) chloride in the presence of an amine. Although the platinum ch loride may be present as a mixture of cis and trans isomers its stereochemistry is in practice irrelevant, because the cis complex rapidly isomerizes to trans in the presence of a secondary or tertiary amine.

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11 The amine serves to deprotonate the acetylene, which presumably forms a copper acetylide upon reaction with the catalyst. The activated acetylide then substitutes chloride forming the platinum-carbon bond. Th e reaction can also be carried out in the absence of catalyst, but under these conditions the product is only mono-substituted, with one chloride ligand rema ining on the platinum.43 The catalyst-free variation is therefore not useful for making polymers, but it has prove d useful in the synthesis of oligomeric platinum containing PPEs.44 Figure 1-9. Absorption (a) and emission (b) spectra of oligo platinum PPEs with increasing chain length. In the emission spectra, fluorescence is denoted by F and phosphorescence is denoted by P. No te that the fluorescence intensity scale is magnified 100x with respect to phosphorescence. Figure from Liu [44]. Interestingly, several studies have show n that conjugation th rough the metal center in platinum containing PPEs is not efficient, and therefore these materials have a rather low conjugation length. For example, Beljonne used calculations to conclude that the triplet state of platinum containing PAEs in general is spatially confined.45 Later, Liu and coworkers synthesized a series of m onodisperse oligo-Pt PPEs and studied their photophysical characteristics.44 The absorption and emission spectra of these polymers, reprinted in Figure 1-9, are quite informative. The increase in molar absortivity and red-

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12 shift in the fluorescence with increasing chain length indicat e that the singlet ground state and first singlet excited state are delocali zed across the oligomers. However, the invariance of the phosphorescence with respect to increasing chain le ngth indicates that the triplet state in platinum PPEs is spatially confined over a relatively small distance of about five monomer units. Meta Linked Poly(phenylene ethynylene)s While poly( para phenylene ethynylene)s have been known for some time, it was not until 1997 that Lavastre and co-workers reported a method to efficiently access their ortho and meta analogues.46 Soon after, researchers in Moores group reported that oligo( meta phenylene ethynylene)s have the abil ity to undergo dramatic conformational self assembly in solution.47,48 Moores oligomers, featuri ng a solubulizing oligo(ethylene oxide)ester pendant group, ar e depicted in Figure 1-10. SiMe3OO O O O n n = 2, 4, 6, 8, 10, 12, 16, 18 Figure 1-10. Oligomeric m-phenylene ethynylenes studied by Moores group. When they are of sufficient length, Moor es phenylene ethynylene oligomers have the ability to adopt a helical tertiary struct ure. This conformational effect is largely solvent dependant. In methylene chloride, th e oligomers are well-solvated, and exist as a random coil. In acetonitrile, however, the ol igomers fold into a helical conformation. This change can be monitored by using abso rption and emission spectroscopy in solvents of different composition. Specifically, an incr ease in the mole fraction of acetonitrile in

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13 the solvent results in a hypochr omic effect in the absorption, with a good isobestic point. Also, eximer-like fluorescence is observed in with increasing amount s of acetonirile in the solvent. These results imply a cooperative transition. Later, Tan and coworkers used a PPE w ith sulfonate pendant groups and a metapara linkage pattern to study thes e conformational changes in water.49 They found that the polymer they studied was well solvated by methanol and poorly solvated by water. This polymer exhibited properties quite simila r to that of Moores oligomers when the composition of the solvent was varied from pure methanol to pure water. The conformation changes in this polym er are depicted in Figure 1-11. Figure 1-11. A segment of the meta-par a PPE used by Tan adopts a random coil conformation in methanol (left), and a he lical conformation in water (right). The side chains have been left off for ease of viewing. More recently, the Tew laboratory studied polymeric meta phenylene ethynylenes ( m -PPEs).50,51 These studied compared poly elect rolytes with and without bulky side groups. Their studies showed that if, for ster ic reasons, a large pendant group must orient itself inside of the helix, then self-assembly is restricted even in the presence of a poor solvent. Additionally, the cavity inside the helix of m -PPEs can be used to bind guest molecules. For example Moores group used an oligo m -PPE to bind a chiral

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14 monoterpene guest.52 This guest induces chirality in th e helical conformation of the m -PPE oligomer, and results in a circular di chroism (CD) signal. The induced chirality originates from a preferential bi nding of one enantiomer of th e helix to the guest, and is made possible because the association constant between the chiral guest and the polymer is different for each of the enantiomeric helices This process is depicted in Figure 1-12. Figure 1-12. Chiral inducti on in a helical polymer by pref erential binding to a chiral guest. Meta-PPEs may have other interactions with small molecules besides this hostguest chemistry. For example, Tans polymer, described earlier, is physically similar to DNA in that it adopts a helical conformation w ith face-to-face stacking of aromatic rings. These similarities allow it to bind to molecu les in a fashion similar to DNA. An example is binding to the complex Ru(bpy)2(dppz), where bpy is 2,2 bipyridine and dppz is dipyrido[3,2-a:2,3-c]phenazine. In a solution of pure water, Ru(bpy)2(dppz) is non emissive because hydrogen bonding between the phenazine ligands and the solvent allow the emission to couple to th e vibrational modes of water and effectively quench the compounds luminescence.53 However, upon intercalation into DNA, Ru(bpy)2(dppz) is shielded from this solvent intera ction and becomes strongly luminescent.53,54 This

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15 phenomenon is often known as the light stick e ffect. That this effect is also observed when Tans polymer, in its helical confor mation, is added to a solution of Ru(bpy)2(dppz) strongly suggests that the metal comple x intercalates into the polymer. Conjugated Polymers as Molecular Wires Single Chain Conjugated Polymers Much of the recent interest in conjug ated polymers stems from their potential applications in electronic, elec tro-optical and optical devices.55 One major factor which makes conjugated polymers excellent candidate s for these applications is that they provide exceptional transport pr operties for charges and exciti ons. As such, a great deal of effort has been expended to understand the carrier transport properties of these materials. The vast majority of the work in this area has focused on bulk properties of the polymers in films.37,56-59 This is understandable, as ther e is a significant interest in using polymer films as active layers in optio-electronics.60 These investigations have offered considerable insight into structure-pr operty relationships of polymers in bulk. However, there is another paradigm for exploiting the intriguing properties of conjugated polymers. It is al so conceived that these materials may serve as transport materials as single-chain molecular wires. The molecular wire concept entails single molecules acting as carriers in one-dimension only. Thus, a molecular wire would have to have an appropriate molecular geomet ry and be able to adopt a predictable conformation in addition to having excellent transport properties al ong its long axis. In addition to the requirements that they be excel lent charge carriers ove r long distances, the energetics of a good molecular wire must not vary drastically with length.61

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16 Several examples of conjugated oligomers or polymers being used as molecular wires are known. For instance, in 2000, T our attached a linear terphenyl to gold electrodes by using thiol substi tuents as alligator clips.62 Upon applying a potential difference across the electrodes, the terphe nyl undergoes a conformational change and completes the circuit between the electr odes with a measurable conductivity and resistivity. This experiment is depicted in Figure 1-13. Figure 1-13. A conjugated oligomer in use as a molecular wire. Figure adapted from Tour [62]. The study of molecular wires is particular ly important in the context of nano-scale electronic devices, where it is e nvisioned that the charge or ex citon carriers will be single polymer or oligomer chains.61,63 Many conjugated polymers, especially poly(paraarylene ethynylene)s (PAEs) fit the requirement s for molecular wires. First, PAEs are locked in a linear geometry along the di rection of conjugation. Second, although the primary mode of transport thr ough PPEs in the solid state is interchain hopping, the extended conjugation in these materials seems intuitively ideal for intra -chain transport properties along the direc tion of conjugation.

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17 Indeed, there has been a great deal of r ecent interest in studying the transport of excitons along single chain c onjugated polymers. This is somewhat of a challenge, because PPEs are known to aggregate in so lution, and the photophysics of the aggregates are differ significantly from that of single-chain polymers.64 The most common approach to these investigations is to study the polymers in dilute solutions, where interchain interactions are minimized.65 Several techniques have been used to explore exciton transport in conjugated polymers. One method uses co-polymerization to incorporate low-energy trap sites for excitons in the polymer chain.66-68 For example, Swager, Gil, and Wrighton synthesized a PPE end-capped with anthracene linked th rough the 9-position, as shown in Figure 114.66 Upon photoexcitation of the polymer main chain, the emission of the polymer is quenched and replaced with anthracene fluorescence. OC16H33C16H33O Ph Ph Figure 1-14. Anthracene end-capped polymer studied by Swager, Gil, and Wrighton. Alternatively, the polymer may be used to bind a small molecule quencher, which serves to accept either ener gy or an electron from the excited state of the polymer.17,69 In one particularly intriguing study, Wang and Wasielewski used th e PPV derivative in Figure 1-15, which features pyridine comonomers in the main-chain, to bind metal ions. Upon binding, metal ions such as Ni2+, Cu2+, and Mn2+ effectively quench the polymer emission.70 This effect is not observed with a physical mixture of a pristine polymer

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18 (without bipyridine) and bipyrid ine. The highly effective quenching indicates that the exciton created by initial phot oexcitation diffuses rapidly to the metal trap site. Figure 1-15. A metal binding conjugated polymer. Figure adapted from Wang [62]. In related studies, cyclophane monomers have been used to bind viologen quenchers,70-72 and ionic side chains have been used to bind oppositely charged metals and molecular ion traps.72-76 All of these studies demons trate that energy transfer in single chain conjugated polymers is extr emely rapid and highly efficient. Charge Transport Charge or electron transpor t (ET) in solution can typi cally be viewed as an interaction between an electron donor (D) linked to an electron acceptor (A) through some bridging species (B).77 This DBA scheme is understood fairly well when the bridge is molecular.78-80 Indeed, detailed comparisons between theoretical and experimental results have been essentia l in unifying this field.80 In this work, the gap between the donor-HOMO and bridge-LUMO for a variet y of systems was tabulated. In general, there are two mechanisms fo r ET in DBA systems. Superexchange is observed when ET occurs by a tunneling pro cess and the electron never resides on the

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19 bridge.81 Alternatively, charge transport may proceed by a hopping mechanism via oxidation or reduction of the bridge by the donor, followed by ET from the bridge to the acceptor.82 These two processes may both be op erative in the same system, as per equation 1-1, where ktun is the rate of ET by superexchange and khop is the rate of ET by hopping.61 kET = ktun + khop (1-1) In conjugated polymers, as in typical mo lecular systems, the superexchange model is expected to be highly dependant on the le ngth of the polymer. The dependence of the rate of superexchange on th e degree of polymerization (Dp) is expressed by equation 1-2. kET = ktun = k0exp(Dp) (1-2) In this equation, N is dependant on the length of the monomeric unit (whi ch, in the case of PAEs, may contain one, tw o, or more aromatic rings), an d the natural logarithm of the ratio of the coupling energy between the monomeric units, HBB, and the energy gap between the initial state (DBA ) and the mediating state, EDB. The mediating state is D+B-A for electron transport, or DB+Afor hole transport. Therefore it follows that a small value of is desirable for long-range ET, thus pointing to the desire to minimize the energy of the mediating state. It is important to note that in the superexchange mechanism the mediating state is only a virtua l state, and that the electron or hole never actually resides on the bridge. In the hopping mechanism, the mediating stat es are real, and the bridge is oxidized and then reduced (or reduced and then oxidized ) during the ET process. The rate of this process is Ohmic, that is, it is inversely proportional to the distance between D and A.83

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20 In the case of a conjugated polymer, this means that as Dp increases, the rate of charge transport by hopping should decrease. Several interesting phenomena are pred icted when both mechanisms are applied in parallel. One particular ly interesting result of th e mathematics is a phenomenon known as resonant tunneling, which occurs when the energy gap is zero. In this scheme, the overall rate of ET should not vary greatly with increasing Dp.84 O O O O x y Figure 1-16. MEH-PPV with saturated conjugation breaks. ET theory therefore indicates that under standing the nature of the oxidized or reduced state of conjugated polymers is of key importance, since the energetics of the charged state and the diffusiveness of the charge are key factors in explaining the transport abilities of conjugated polymers used as molecular wires. A study by Candeias and coworkers was the first to provide this information. Candeias examined a series of MEH-PPV derivatives in which varying amounts of the vinyl linkages had been hydrogenated, as in Figure 1-16.65 The hydrogenated olefin s effectively break the conjugation in the polymer backbone. Pulse radiolysis was used to perform a one-electron oxidation on the conjugated polymer backbone, and the energy of the polymer cation radical was measured by absorption spectroscopy. The study showed that breaks in the conjugation length

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21 increase the energy of the cation radical, thus unambiguous ly demonstrating that the charged species is stabilized by co njugation through the polymer chain. Charge Injection In Candeiass work and in some of the wo rk described in this dissertation, charge is injected into a conjugated polymer by producing a solvated hole or electron which migrates to the analyte. This process can be modeled using existing theory, and understanding this theory is a useful precursor to the studies presented herein. Theory has provided solutions to the pr oblem of a mobile point charge diffusing to a number of spherical monomer units jo ined together in a one, two or three dimensional array. The solution for a one dime ntional array is an appropriate model for PAEs, which are essentially one dimens ional rigid rods. Traytaks soution85,86 and its recent test by Grozema65 employ an effective radius, Reff defined in terms of the reaction radius Rm, for one monomer unit and the number of units in the polymer chain, n, as in equation 1-3. Reff nRm1 Rma ln( n ) (1-3) The diffusion-controlled rate of reaction with a chain of length n, is then given by equation 1-4, which is identical to Smoluc howskis classic solution for a reaction of spherical particles where D is the sum of th e diffusion coefficients of the reactants. k(t) = 4 ReffD(1 + Reff/( Dt)) (1-4) The second term in equation 1-4 is often called the transient te rm, and contributes less than 10% to the overall rate when larger than 10 R2/ D For small species that diffuse rapidly in solution, this time is ge nerally less than a few nanoseconds. However,

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22 Grozema showed that large polymers have much larger effective radii and consequently a much larger and long-lived tr ansient term. While the stea dy-state, diffusion controlled rate constant per repeat unit falls with incr easing degree of polymerization, some of that loss is recovered by the larger transient term.85,86 Thus, the bimolecular rate constant for transfer of an solvated electron (e-) or hole (h+) to a PPE will initially decrease with an increasing chain length and then, after re aching an minimum, increase again. Description of the Present Study The present study aims to gain an incr eased understanding of the properties of single chain PPEs. Careful design and synthesi s of polymers with appropriate charge or energy traps, or with the capability to under go predictable conformational changes, is the primary means to these goals. Well-desi gned polymers allow for the monitoring of specific spectral changes associated with th e injection of energy or charge, or those associated with conformational changes, t hus making it possible to gain an increased understanding of these properties. The experiments described in this disser tation were performed in dilute solution, except for a few cases where higher concentr ations or films were used in order to demonstrate the differences between single-chain and aggr egated or bulk PPEs. Although modern microscopic methods allo w for the collection of a variety of photophysical data on single-chain conjugated po lymers in the solid state, there are severe practical difficulties in performi ng many of the time-resolved experiments described in this study under those conditions. Chapter 2 of this dissertation is concerned primarily with charge transport in PPEs. PPEs with two different molecular architectures were synthesized bo th in their pristine

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23 form and with low oxidation potential moieties affixed to the termini. These traps were designed to act as low-energy traps for cation radicals (holes ) in the polymer backbone. The unique laser electron accelerator f acility (LEAF) at Brookhaven National Laboratories was employed to inject char ges into the polymers and measure their spectroscopy in a time-resolved fashion. By carefully choosing a trap that exhibits spectral characteristics that are different from those of the parent polymer, it was hoped that the dynamics of charge migration in PPEs could be measured quantitatively. Although the limits of even the most mode rn instrumentation prevent quantitative measurement of these rates, this system enabled many other useful spectral and thermodynamic properties of a charged PPE to be elucidated. Chapters 3 and 4 deal with platinum c ontaining PPEs. Chapter 3 describes the synthesis and characterization of a series of platinum containing PPEs where low energy traps are incorporated in to the main chain. The trap concentrati on was varied throughout the series. These materials were used to gain insight into the mechanism of energy transport in platinum containi ng PPEs. Use of a variety of steady-state and time-resolved methods reveal that although the photophys ics of platinum containing PPEs are dominated by their long-lived trip let states, the results in this chapter show that their short and often optically inactive si nglet excited states do play an important in inter-chain charge transport. Chapter 4 deals with a platinum cont aining PPE that is linked through the meta position. This polymer is able to self assemb le into a helical conf ormation that accepts guest molecules, much in the way that organic m -PPEs or DNA do. This work represents

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24 the first careful study of a water soluble organometallic polymer that undergoes these types of conformational changes.

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25 CHAPTER 2 RADICAL TRANSPORT IN END-CAPPE D POLY(ARYLENE ETHYNYLENE)S Introduction Very little effort has been made to examin e the transport of charges (radical cation or radical anions) in sing le chain poly(arylene ethynylene)s, or to understand the spectroscopy or thermodynamics of these charged species.65 This is unfortunate, because charged single chain conjugated polymers are envisioned to be the active species in a variety of applications, especially molecula r electronics. In fact a need to develop further understanding into th e nature of charged states in conjugated polymers was recently highlighted by several se nior researchers in the field.61 The work presented in this chapter is an important step towards fulfilling this need.87 Pulse radiolysis experiments were used to investigate the tr ansport dynamics of radical cations (holes) in single chain conjugated polymer s. This problem was approached by using conjugated polymers th at are end-capped with functional groups that serve as traps for radical cations ge nerated on the main polymer chain. The conditions for an end-cap to serve as a tr ap is the end-cap have a lower oxidation potential than that of th e main chain, that is, E0 ox (T) < E0 ox (polymer), where T represents the radical cation trapping moiety. In th is chapter, the resu lts of a study using poly(arylene ethynylene)s with 2,2:5,2 terthiophene (T3) end-caps is reported. PAEs were chosen because they posses a linear m olecular wire structure, and are easily synthesized via Pd mediated Sonoga shira condensation polymerizations.12 T3 was chosen as an end-cap because it was anticipated to meet the requi rements for a radical cation

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26 trap, eg. E0 ox (T3) < E0 ox (polymer). In addition, the spectroscopy of oligothiophene cation radicals has been invest igated extensively, and it was th erefore anticipated that it would be possible to distinguish between a cation radical localized on the end-group (ie. T3 +.) from a polymer-based cation radical (ie. PAE+.). The objectives of this study are twofold. The first objective is to study experimentally the spectroscopic properties of charged single-chain PAEs. The second is to determine the rate of hole transfer (kHT) for a radical cation localized on a the polymer main chain to the end-cap, as in equation 2-1. h+ kHT T3-(PAE)-T3 T3-(PAE+ )-T3 T3-(PAE)-T3 + (2-1) Results and Discussion Synthesis and Structural Characteriza tion of Poly(arylene ethynene)s The chemical structures and acronyms for the polymers featured in this study are shown in Figure 2-1. The numerical subscrip t in each polymers name represents the number average degree of polymerization, nX, as calculated from the GPC derived number average molecular weight, Mn. For the end-capped polymers, nX includes only the units of the polymer main chain and not the end-groups. Furthermore, it is important to note that PPE type polymer s contain two phenylene rings per repeat unit while PBpE type polymers contain three pheny lene rings per repeat unit. In addition to the polymers, the model compound Ph-T3 was synthesized by coupling 2-terthophene and p -iodo toluene. This compound was designed to model the

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27 end-capped termini of the polymers. The s ynthesis of the polymers and monomers used in this study is out lined in Figure 2-2. nn S S S S S OR RO OR RO OR RO n S S S S S n RO OR S SPPEnPBpE21T3PBpE12T3PPE13R = 2-ethylhexylS S SPh-T3n=24: PPE24 Mn=10,770 Mw =32,388 PDI=3.0 n=164: PPE164 Mn=96,253 Mw=182,880 PDI=1.9 Mn = 11321 Mw = 18,117 PDI = 1.6 Mn = 7,200 Mw = 18,155 PDI = 2.5 Mn = 6,454 Mw = 13,623 PDI = 2.1 Figure 2-1. Structures of the polymers and model compounds used in this study. The monomers used in these polymeriza tions, 2,5-bis(2-ethylhexyl)-1,4-diiodobenzene and 1,4-diethynyl benzene, and 4, 4 -diethynyl biphenyl were synthesized by modified literature procedures.14 In particular, the synthe sis of the diiodo monomer was substantially improved. D iiodination of 1,4 dimethoxybenzene was accomplished with one equivalent of molecula r iodine and 0.7 equivalents of potassium periodate in refluxing acidic media. Using this ratio of reagents affords substantially better yields (85-90%) than use of an exce ss of iodine and potassium periodate (about 40% in our hands).14 Additionally, since iodine is present in a stoichiometric amount, the reaction can be monitored by watching for the disappearance of the dark-brown iodine color. It is important to note that iodine may sublime in the condenser during the reaction, and it must occasionally be pushed back into the reaction mixture using a glass rod to prevent

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28 clogging. If the condenser becomes complete ly clogged, the build up of pressure may result in an explosion. Addition of a small amount of chloroform to the reaction mixture can help minimize this problem, but may reduce the yield somewhat. H H O O II + PPE16 or PPE166PBpE21O O II + H H H H O O II + + S I H 3 T3BpE12O O II + + S I H 3 T3PPE13H H S I H 3 H +PhT3 Figure 2-2. Synthesis of polymers and model compounds used in this study.

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29 Demethylation of the alkoxy groups was pr eformed using boron tribromide in dry dichloromethane under anaerobic conditions. The resulting hydroquinone is recovered by evaporation of most of the solvent, a nd precipitation from methanol-water. Upon workup of this reaction, it is important to add a few spatula tips of sodium sulfite (Na2SO3) to prevent air-oxidation of the hydr oquinone in solution. The hydroquinone was then alkylated using 1-bromo-2-ethyl hexa ne in refluxing methyl ethyl ketone with potassium carbonate under anaerobic conditions. Again, addition of sodium sulfite upon workup is important to prevent oxidation of the product upon exposure to air in a basic medium. The monomer is obtained as a colorless liquid. In an effort to bypass the use of boron tribromide, which is both expensive and highly toxic, another synthesis of this monomer was attempted. In this effort, hydroquinone was first alkylated with 1-brom o-2-ethylhexane and then iodinated as described above. However, the monomer resulting from this synthetic sequence is a redbrown oil, instead of the colorless liquid obtai ned by the original synthesis. The color, likely due to complexation of the product with trace amounts of iodine, cannot be removed by treatment with activated carbon or by column chromatography. It is probable that, in the original synthetic seque nce, any remaining traces of iodine are removed by recrystalization of the intermediates (bot h 2,5-diiodo-1,4-dimethoxybenzene and 2,5-diodo-1,4-dihydroquinone are highly cr ystalline). However, there are no crystalline intermediates in this alternate sy nthesis. Importantly, use of the colored monomer (from the alternate synthesis) in polymerizations gives only low molecular weight polymer, even under optimized polymeri zation conditions. Thus, this synthesis is

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30 not a suitable method to produce the hi gh-quality monomer needed for successful polymerizations. The alkoxy side groups are used to help impa rt solubility on the final polymers. A racemate of the chiral 2-ethylhexyl group was chosen because this group gives a mixture of stereoisomers in the resulting monomer. Specifically, the di-iodo monomer is present in the polymer as a pair of diasteromers, one of which is meso and the other which exists as a pair of enantiomers. The resulting st ereo-irregularity in th e polymer backbone was designed to discourage inter-ch ain aggregation and improve so lubility of the polymers. In fact, the biphenylene based polymers, with and without end-caps, proved to be quite soluble in many common organic solven ts, including THF, chloroform, and dichloromethane. The phenylene based polym ers also exhibited good solubility in these solvents, however, upon complete evaporati on of solvent the phenylene based polymer forms a film which does not completely redisso lve even after severa l days of continued stirring and gentle heating. Thus, in order to obtain the 1H NMR spectrum of this polymer, it was necessary to dissolve it in a mixture of dichloromethane, acetone, and hexanes. Because hexanes and acetone are poor solvents for this polymer, the film formed by evaporation of this polymer solu tion can be redissolved in a deuterated solvent. However, even using this method, if the solvent is comple tely removed the film becomes insoluble. Therefore the NMR, show n in Figure 2-3, is partially obscured by solvent peaks. These problems made it desireable to fi nd an alternative to the 2-ethylhexyloxy side group that would show im proved solubility characteris tics. Use of the undecyloxy side group alleviated many of the problems, and allowed a simpler synthesis. It was

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31 expected that 2,5-diundecyl oxy-1,4-diiodobenzene would be crystalline, and it was therefore hoped that the altern ate synthesis that failed to produce useful monomer with 2ethylhexyloxy side chains would be useful. Indeed, alkylation of hydroquinone with 1bromoundecane followed by iodination as descri bed above yields a product that can be easily purified by recrystalizati on from methanol-water giving a white solid that can be used to make reasonable molecular weight polymers. The films that result from evaporati on of solutions of the phenylene-based polymers are somewhat more soluble than those from similar polymers with the 2ethylhexyloxy side group, and it is possible to obtain prot on NMR spectra that clearly show the end-group protons Figure 2-4 shows the 1H NMR spectra of the phenylene based polymer with undecyloxy si de groups. Figure 2-5 is an expansion of the aromatic region of the phenylene polymer with undecyl oxy side groups which has been scaled to clearly show the thienyl protons The doublet at 7.19 ppm integr ates to one relative to the other thiophene protons, and can be assigned to the prot on in the 5 position on the terminal thiophene. Three multiplets at 6.84, 7.11, and 7.66 ppm each integrate to two, and each is assigned to the pairs of protons in the 3 and 4 positions on each of the respective thiophene rings. Despite the somewhat improved physical properties of the polymers resulting from 2,5-diundecyloxy-1,4-di iodobenzene, all of the photophysical and radiolytic experiments described here were preformed on the polymers shown in Figure 2-1, which feature 2-ethylhexyloxy side groups, and all discussion henceforth will refer to those polymers unless specifically noted.

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32 Figure 2-3. NMR of T3PPE13 with 2-ethylhexy loxy side groups. Figure 2-4. NMR of T3 end-capped PPE with undecyloxy side groups.

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33 Figure 2-5. Expansion of the aromatic region of Figure 2-4. Note that the large off-scale peaks are correspond to phenylene pr otons, and the integrated peaks correspond to thiophene protons. The final polymers were synthesized by AA, BB condensation polymerizations of aryl iodides and aryl ethynylen es using the Sonogashira reaction.12,17,66,69 For the polymers without end-caps (the parent polymer s) an equimolar ratio of aryl iodide to aryl ethynylene groups were used in the pol ymerizations. For the synthesis of T3PPE13 numorous experiments were conducted to de termine the optimal stoichiometry for endcapping, and the results are outlined in Table 2-1. In this table, I-Ar-I refers to 2,5diundecyloxy-1,4-diiodo benzene. These expe riments indicated that a small excess of iodine functionality was re quired to give moderate Mn values. Additional experiments using the optimized ratio of end-cap to dii odo arene in which the reaction time was varied showed little change in the Mn of the resu lting polymer for reaction times of sixteen hours or longer.

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34 Table 2-1. Conditions used in end-capping polymerizations. Trial Equivalentsa Time/ hrs Mnb T3 I-Ar-I 1 1 99 12 3,978 2 5 95 12 3,065 3 10 90 12 761 4 5 95 10 2,900 5 5 95 24 1,188 6 5 95 48 3,707 7 10 100 16 6,500a all trials used 100 equivale nts of the diethynyl monomer. b The Mn is reported from GPC relative to polystyrene standards. Each of the polymers was analyzed by gel permeation chromatography relative to polystyrene standards, and 1H NMR spectroscopy. 13C NMR was not performed because of the solubility limits of the polymers. Even in the ethylhexyloxy substituted polymers, which are not completely free of solvent, NMR can be used to give an independent estimation of nX in the end-capped polymers. In order to estimate the degree of polymerization, it is useful to take advant age of a stoichiometric imbalance of the monomers due to end-capping. Careful insp ection of the structur e of the end-capped polymers reveals that the ratio of phenyl ene (or biphenylene) to dialkoxyphenylene monomers is (n+1):n, where n is the chain lengt h. By comparing the peak ratios of the phenylene (or biphenylene) and alkoxyphenylen e monomers it is thus possible to calculate nX=7 for T3PPE, and nX= 8 for T3BpPE. These NMR derived values are somewhat smaller than those derived from GP C, which is consistent with accepted notion that GPC, especially relative to standard r andom coil polymers, te nds to exaggerate the nXof conjugated rigid-rod polymers.13,88,89 Regardless of these difficulties, GPC remains the most common method for determin ing the molecular weights of conjugated polymers, and polystyrene or other random coil polymers are the most commonly used standards.

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35 Absorption and Fluorescence Spectroscopy Phenylene based polymers The UV/visible absorption spectra of PPE164, T3PPE13, T3, and PhT3 are shown in Figure 2-6. The spectra of PPE24 are identical to those of the longer molecular weight sample, and are therefore excluded. Molar ab sortivity and selected fluorescence lifetime data is provided in Table 2-2. The absorption and emission spectra of the parent polymers are alike to those of other, structurally similar PPEs.12,66,90 The absorption spectra are dominated by the allowed, long-axis polarized transition, while the fluores cence features a sharp, narrow band which is dominated by the zero phonon transition The absorption spectrum of T3PPE13 is quite similar to that of the parent PPE164, except for the presence of a long wavelength tail (note arrow in Figure 2-5), which correlates to a low-energy moiety in the end-capped polymer, presumably the T3 end-cap. The small intensity of this tail is unsurprisi ng, considering that the end-caps are present at very low concentration compared with the main chain of the polymers. Table 2-2. Selected phot ophysical data for PAEs 450 nm 550 nm Polymer max ab s / nm max / M-1 cm -1 max flr / nm em flr fl / ns ( ) a fl / ns ( ) a PPE164 411 5.6 x 104 452 0.33 0.5 (0.97) 3.9 (0.03) 0.6 (0.80) 2.5 (0.20) T3PPE13 414 6.5 x 104 453, 496 0.44 0.5 (0.94) 3.4 (0.06) 0.6 (0.89) 3.2 (0.11) a Fluorescence decay lifetimes from biexponential fits. and values are, respectively, the li fetimes and normalized amplitudes of the individual decay components.

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36 Figure 2-6. Absorption (left) and photolumine scence (right) spectra of poly(phenylene ethynylene)s and end group model compounds. (____): T3; (__ __ __): Ph-T3; (__ __ ): PPE164; (__ . __ . ): T3PPE13. The difference in fluorescence between th e parent and end-capped polymers is quite noticeable, even upon simple examin ation by eye. Upon irradiation with longwavelength ultraviolet light, the parent polym er emits a bright blue fluorescence, while the fluorescence of the end-capped polymer appears green. Initial examination of the data suggest that the fluorescence of T3PPE13 is a combination of emission from two different exitons: one on the polymer main-chain (PPE*-T3), and one localized on the T3 end-group (PPE-T3 *). However, examination of the small molecule models shows that the situation is actually much more comp lex. The absorption and fluorescence of T3 is considerably blue-shifted from that of both PPE164 and T3PPE13. This is a clear indicator that an excited state localized on T3 is at a significantly higher energy than the PPE* 300350400450 N o r m a l i z e d A b s o r b a n c e 0.0 0.2 0.4 0.6 0.8 1.0 W a v e l e n g t h / n m 400450500550600650 N o r m a l i z e d I n t e n s i t y 0.0 0.2 0.4 0.6 0.8 1.0

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37 exciton. A clearer understanding can be ga ined by noting that the absorption and fluorescence of Ph-T3 is significantly red-sh ifted from that of T3, indicating that there is a substantial delocalization of the T3 excited state into the adj acent phenylene unit. Indeed, excited state molecular orbital calculations (outlined below) clearly indicate that although both the HOMO and LUMO are mostly localized on the T3 end-cap, orbital mixing between the end-cap and the polymer main-c hain extends the conjugation through several phenylene rings. Thus, the spectra of T3PPE13 derives from an excite d state that is mostly contained on the T3 end-cap, with some contribution from the neighboring phenylenes in the chain. The fact that the excited state of T3PPE13 is a composite of phenylene and thiophene mixing is supported by th e fact that the fluorescence T3PPE13 is independent of excitation wavelength, even when the excitati on is at the long-wavel ength tail of the absorption which is due primarily to the end-cap s. It is also impor tant to recognize that these results are also not consistent with a physical mixture of end-capped and non-endcapped polymers, thus lending additional support to the structural characterization of the polymers. The notion that there is considerable mixing between the end-cap and the polymer main chain is also supported by fluorescence lifetime measurements (Table 2-2). The decay profiles of PPE164 and T3PPE13 are quite similar. Both are dominated by a very fast ( 0.5 ns) component. Furthermore, the emission lifetimes are wavelength independent, even when measured in the gr een band associated with the end-cap. The fast decay of PAEs is largely associated with their extended conjugation. The fast, wavelength independent decay of T3PPE13 thus supports the idea that the end-cap is significantly delocalized into the polymer chain. If this were not the case, the decay

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38 profile of T3PPE13 would be expected to vary sign ificantly with wavelength; the longwavelength emission, if it were due to a moiety with only a small degree of delocalization, (ie. T3, with only three aryl rings) would be expected to exhibit a much slower decay than the emission associated w ith the polymer main-chain which contains an average of 26 aryl rings. The spectroscopic behavior of the T3 end-capped polymers present some contrast to previous work where a structurally sim ilar PPE was end-capped with anthracene linked through the 9-position.66 In the 9-anthracenyl end-capped polymer, the PPE-based fluorescence was completely quenched and replac ed with a red-shifted emission that was assigned to the anthracene end-caps. This begs the question of why the fluorescence is quenched when an anthracene end-cap is used, whereas when a T3 end-cap is used a mixed excited state is the result, even though the energy difference between PPE and anthracene is similar to that for PPE and T3. The answer likely lie s in the considerably different molecular orbital and excited stat e symmetries of the two end-groups. The symmetry of the T3 end-group is the same as that of the polymer, allowing for good excited state mixing. However, the 9-anthr acenyl end-group is polarized orthogonally to that of the polymer. The result of this is that mixing between the 9-anthracenyl endgroup and the polymer chain is low, and thus the polymer chain is not well conjugated into the 9-anthracenyl end-group. It is important for the hole transfer experiments described below that the endgroup used is in a symmetry-allowed conjugati on with the polymer chain, as the ultimate objective of this study is to measure the ra te of charge transport through a conjugated system. If the end-group orbitals did not mi x well with those of the polymer main chain,

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39 then the rate limiting step in charge tr ansport would be hopping across the nonconjugated bridge to the endcap, and it would be impossible in principle to measure the rate of transport through the conjugated portion of the polym er. Thus, the 9-anthracenyl end-capped polymers would have been a poor choice for this study. To confirm the idea that energy interchain energy transport is significantly different than intrachain transport, the fluorescence of th e polymers were also measured in the solid state. The results are shown in Figure 2-7. When comparing the fluorescence of the PPE164 as a film to the solution-state fluores cence, it is clear that the emission from the film is broader with a lo ss of structure and re d-shifted when compared to the emission in dilute solution. These effect s are associated with inter-ch ain aggregation in the solid. Wavelength / nm 450500550600650700 Normalized Intensity 0.0 0.2 0.4 0.6 0.8 1.0 Figure 2-7. Thin f ilm fluorescence of PPE164(_ _) and T3PPE13 (____)

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40 The change in emission properties of T3PPE13 when going from solution to the solid is quite dramatic. Whereas the solu tion emission shows a si gnificant amount of PPE-like fluorescence, this main emission band is completely quenched in the solid state and replaced by a broad band with little structure and max = 523 nm. The reason for this difference lies in the different mode of exciton transport in the solid state as compared to in solution. In a film, the pr imary mode of transport in the film is inter chain exciton hopping. The distan ce between an end-cap trap and any exciton is much less in the solid than in solution because exc itions may migrate between chains instead of just walking down one of them. Thus, exci ton migration in films is expected to be more efficient than in solution, and this is borne out by experimental evidence. Biphenylene based polymers The biphenylene based polymers show ed essentially id entical photophysical properties as their phenylene ba sed analogues. The absorption and emission spectra of these polymers is shown in Figure 2-8. The primary feature of in the absorption of both BpE21 and T3BpE12 is a long-axis polarized band, which in both cases is blueshifted somewhat with respect to the phenyl ene based polymer. The end-capped polymer also exhibits a tail at the low-energy e nd of the spectrum, which is explained by conjugation with the low-energy T3 end-group, by analogy to the all phenylene polymer. The emission spectra of both copolymers are dominated by the zero-phonon transition. The main difference between th e phenylene and biphe nylene based polymers is in the green end-cap based component of the emission. This additional band, with a maxiumum at 490 nm, is due to emission from the low energy end-cap. As in the phenylene based polymer, the end-cap emission the result of orbital mixing between the thiophene-based end-groups and adjacent phenylene units. The end-caps contribution to

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41 the overall emission spectrum is noticeably weaker in T3BpE12 than in T3PPE13. This indicates that energy transport in biphenylene based PPEs is somewhat less efficient than in the phenylene based PPEs, an effect th at may be related to the fact that the biphenylene-based polymers can more readily adopt a non-planar twisted conformation than the all-phenylene based polymers. 450500550600 Normalized Intensity 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Wavelength / nm 300350400450500 Normalized Absorption 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Figure 2-8. Absorption and emission of BpE21 (_ _) and T3BpE12 (____) Radiolytic production of cation and anion radicals Radical cations and radical anions were produced on PAEs in solution by pulse radiolysis. These experiments were conducted at the Laser Electron Accelerator Facility (LEAF) at Brookhaven National Laboratories. This facility has been described previously in detail,91,92 so only a brief description will be provided here. Figure 2-9 is a

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42 schematic representation of the LEAF. LEAF is essentially a pulse-radiolysis facility. However, unlike traditional radiolysis inst rumentation, which can only produce electrons beams with long pulse width, LEAF is able to produce high energy el ectrons with pulse widths as small as 5 ps. This is accomp lished by using a photocathode as the electron source, and exciting it with a fast pulsed laser. The short puls e width of the laser translates into a short pu lse width of electrons. Figure 2-9. Schematic representation of the LEAF facility. The generation of PAE-based ion radicals involves a series of reactions in which high energy electron pulses are converted into strongly oxidi zing solvent cation radicals or holes (h+) or electrons (e-), which are then transferred to PAE chains. At the LEAF, the electrons involved in ra diolysis have an energy of 10 MeV and a pulse width <120 ps. The high energy electr ons pass completely through the solution and exit the spectrophotometric cell, howev er, each electron produces roughly 104 ionizations RF Feede h Time delay Laser Electron Gun 5 ps 5 ps 9 MeV

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43 that result in thermalized solvated electrons and solvent radical cations in concentrations of a few M The precise nature of the reactions depends strongly on the solvent. 1,2 dichloroethane (DCE) for example, captures th e electrons immediately to form radicals and Clions.93,94 In tetrahydrofuran (THF) the so lvent cation radicals decompose to radicals and solvated protons. Thus the net effect of the electron pulses is the production of thermalized solvent cation radicals (h+) in DCE and thermalized solvated electrons (e-) in THF. Because holes in DCE and electrons in THF attach to the desired analyte only after diffusion, formation of the desired radica l cation or anion species is a bimolecular process. Since the analyte is always pres ent in excess in these experiments, these reactions are pseudo-first order, and the observed rates of h+ or etransfer to analyte depends upon analyte concentra tion and the bimolecular rate constant for reaction of h+ or ewith the analyte. A fraction of h+ in DCE and ein THF are lost by reactions with counter ions, however, these react ions are measured and taken in to account in the results. Electron and hole transport to polymers following radiolysis A series of experiments were carried out in order to determine the bimolecular rate constants for h+ and etrapping reactions, i.e. k2-2 and k2-3, PAE + (PhMe)2 + PAE + + 2 PhMe (2-2) DCE/PhMe PAE + es PAE + 2 PhMe (2-3) THF Reactions 2-2 and 2-3 are both strongly exoergic ( G0 < -0.5 eV), and occur with diffusion controlled rates. It is important to note that the rate consta nts reported in this discussion are not in fact cons tant, although they do not vary significantly with in the time

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44 of the reactions under study. The experiment ally observed pseudo-first order decays are therefore nearly exponential. Consequently, the rate c onstants reported herein are effective or average rate constants which repr esent best fits to the observed pseudo-first order kinetic traces for the rise of the absorption due to the creation or PAE radical ions. If the range of concentrations of polymer was significantly higher or lower, these constants would have somewhat different values. The species produed by radiolysis of PPE164 in DCE/toluene reacts with TMPD, a good hole scavenger, with a bimolecular rate k = 2.8 x 109 M-1 s-1 to produce TMPD +. This is confirmed by the observation of the characteristic absorbances of TMPD + at 570 nm and 620 630 nm.95 Also, the species produced as a result of radiolysis of PPE164 in THF reacts with tetracyanoethylen e (TCNE) and chloranil to give species with the characteristic absorbance bands of TCNE and chloranil -.95 On this basis, the product of PAE radiolysis in DCE/toluene can be assi gned as the radical cation and the product of PAE radiolysis in THF can be assigned as the radical anion. The effective rate constants for the bimolecular reactions of h+ (in DCE/toluene) and e(in THF) formed by pulse radiolysis w ith PAEs to produce radical cations and anions (respectively) of the polymers is pr esented in Table 2-3. The bimolecular rate constants are reported with respect to both pol ymer and repeat unit concentration. There are several points worth noting with respect to this data. First, the experimentally determined rate constants for growth of th e PAE radical ions, as expressed per monomer repeat unit, is significantly lower than th e expected diffusion cont rolled reaction between two small molecules. Second, in general th e rates for electron transfer to PAE are

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45 considerably greater than the rates for hole tran sfer. This difference is due to the faster diffusion rate of es.96-99 Table 2-3. Reaction rate constants of poly-(arylene ethynylene)s (PAEs) and terthiophene end-capped PAEs with pos itive and negative charge carriers in solution. Polymer N k (M-1 s-1) k(monomer) (M-1 s-1) a PPE164 + (Me )2 + 164 2.1 x 1011 1.3 x 109 PPE24 + (Me )2 + 24 4.8 x 1010 2.0 x 109 PBpE21 + (Me )2 + 21 5.1 x 1010 2.4 x 109 T3PPE13 + (Me )2 + 13 7.1 x 1010 5.5 x 109 T3PbpE12 + (Me )2 + 12 3.0 x 1010 2.4 x 109 T2 + (Me )2 + 1.1 x 1010 5.3 x 109 T3 + (Me )2 + 1.7 x 1010 5.5 x 109 T3 + [Me :Cl] 5.0 x 109 1.7 x 109 T4 + (Me )2 + b b T4 + [Me :Cl] 4.1 x 109 1.0 x 109 PPE164 + eTHF 164 4.7 x 1012 2.8 x 1010 PBpE21 + eTHF 21 1.3 x 1012 6.3 x 1010 T3PPE13 + eTHF 13 7.1 x 1011 5.5 x 1010 T3PbpE12 + eTHF 12 3.5 x 1011 2.8 x 1010 a For comparison to small molecules the average ra te constant per repeat unit is reported. For T3-containing polymers the T3 was not included. b Could not be measured due to absorption of the product cation at 1000 nm. Poly(arylene ethynylene)s ar e known to aggregate in solution. The degree of aggregation is dependant on both the solven t system and the concentration of the polymer.22,50 The pseudo-first order growth of PPE164 + as a function of polymer concentration is linear when the c oncentration is less than 1.35 x 10-3 M in repeat units. This suggests that aggregation is not an impor tant factor in DCE/toluene solution at the

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46 concentration ranges used in these studies. Since THF is a better solvent for PAEs than DCE/toluene, it is unlikely that aggregation is a factor in that medium either. Semi-empirical calculations: spectr oscopy of PPE-based ion radicals Before examining the spectra of PAE ion ra dicals, it is useful to use theory to attempt to provide insight into the expected electronic transitions. Thus, semi-empirical calculations (ZINDO/S, CI = 6,6, AM1u geom etries) on a series of PPE-type model oligomers, [Ph-(-C C-Ph-)n-H] + (structure in Figure 2-10), were carried out.100 The Zindo/S calculations predict two optical transitions with energies that decrease with increasing oligomer length, n. With the excepti on of the shortest member of the series (i.e., diphenylacetylene, n = 1) the lowest ener gy transitions in the ra dical cations derive primarily from a strongly allowed, one-electr on transition from the HOMO-1 to the halfoccupied HOMO (SOMO) as indicated sche matically in Figure 2-9. (Here HOMO and LUMO refer to the orbitals that are the hi ghest occupied and lowe st unoccupied in the neutral parent molecule.) This low-energy tran sition is predicted to occur in the near IR region, and its energy is expected to vary strongly with the length of the PPE segment. The calculations predict that a second, highe r energy transition will occur in the visible region. This transition is primarily of HOMO (SOMO) LUMO character. Furthermore, the calculations predict that the energy of this transition varies only weakly with oligomer length. Similar results are obt ained for the anion radicals, however, in this case the near IR bands derive primarily from the LUMO (SOMO) LUMO + 1 transition (see Figure 2-10).

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47 15 10 5 0 ~ (cm-1) x103 10 8 6 4 2n 2000 1000 (nm) n +h (anion IR) h (cation IR) h (vis) h (anion IR) h (cation IR) h (anion IR) h (cation IR) h (vis) Figure 2-10. Transition energies computed for radical cations of phenylene ethynylene oligomers as a function of length, n. The energy of the lowest allowed transition (o) is very sensitive to le ngth, while the energy of the strong transition in the visible (+ ) changes little with n. The simple MO diagram at right indicates the nature of the tr ansitions, determined from the CI calculation. Calculations were ZINDO/ S for at AM1u geometries of the cations. Visible/Near-IR Spectrosc opy of Charged PAEs Radical anions By monitoring the wavelength dependence of the transient absorption of PAE solutions at a fixed delay time following the epulse it is possible to construct the visiblenear IR absorption spectra of the PAE-based radical ions. The delay time used is dependent upon the concentra tion of polymer and the bimo lecular rate constant for charge transfer from the solvent to the polym er. Generally in these experiments the delay times used were of the order of 0.1-1.0 s. The time was chosen to follow the growth of the radical ion species under observation and pr ecede its decay. It is possible to estimate

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48 the molar absorptivity ( M-1cm-1) of the PAE radical ions ba sed on radiolysis yields of (PhMe)2 + in DCE/toluene and es in THF. The resulting spectra for a variety of PAEbased radical ions, along w ith the spectra of the T3 oligomer based radical ions, are illustrated in Figure 2-11. Figure 2-11. Spectra of cations and anions of poly(phenylene ethynylenes) ( ), endcapped poly(phenylene ethynylenes) ( ) and T3 ( ) where a) is the spectrum of PBpE21 -, T3PBpE12 and T3 in THF, b) is the spectrum of PPE164-, T3PPE13 and T3 in THF, c) is the spectrum of PBpE21 + and T3PBpE12 + in DCE/toluene and T3 + in DCE and d) is the spectrum of PPE164+ and T3PPE13 + in DCE/toluene and T3 + in DCE. The polymer concentrations were in the range 0.6 2.8 mM in repeat units and delay times were in the range 0.1 1.0 s.

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49 Figure 2-11a shows the spectra of the radical anions of T3 and the biphenyl-based PAEs, PBpE21 and T3PBpE12 -. All of these anion radicals feature two absorption bands: one relatively sharp band in the visi ble region and a second broader band in the near-IR. These two bands are intense, with molar absortivities ( greater than 50,000 M-1cm-1. The observed spectra are qualitativel y consistent with the predictions of Zindo/S calculations, which supports their assi gnment to PBpE-based radicals. The experimentally observed absorption bands occur at max 600 and 1100 nm in T3 -, while in PBpE21 and T3PBpE21 max 625 and > 1600 nm. It is noteworthy that the lower energy band of PBpE anion radi cal is considerably broader and red-shifted than the band for the T3 anion radicals. This indicates that the electr on is considerably more delocalized in the former. Even more significantly, the spectra of PBpE21 and T3PBpE21 are essentially identical, indicating that T3 end-caps do not influence the spectrum of T3PBpE21 -. Thus, the electron is not localized on the T3 end-groups in the end-ca pped polymer; it is instead delocalized on the PBpE main chain. This is constant with expectation because T3 is more electron rich than PBpE, and its LUMO is therefore likely to be higher in energy than that of PBpE. Figure 2-11b shows the spect ra of the radical anions of the phenylene based polymers. Like in the biphenylene-based polymers, the T3 end-cap has no effect on the spectra, which exhibit max 625 and > 1600 nm for both PPE164 and T3PPE13 -. This result is expected, and indicates that there radical anion is again located on the polymer main chain and not on the end-caps.

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50 Radical cations Figure 2-11 also depicts the absorption spectra of the T3PBpE12 +, PBpE21 +, and T3 + radical cations. All of the spectra feature two strong ab sorption bands, one in the visible and a second in the near -IR, consistent with the Zindo/S predictions. The bands in T3 + are observed at max 550 and 850 nm, in good agreement with the previously reported spectrum of this species. The two bands appear at max 600 and 1150 nm in PBpE21 + and T3PBpE12 +. With respect to the biphenylene based polymers, the most significant feature of these two sp ectra is that they are essentia lly identical. This strongly suggests that in T3PBpE12 + the hole resides on the PBpE ch ain and is not trapped by or localized on the T3 end-caps. This result, while initi ally surprising, is confirmed on the basis of oxidation potentials estimated by bimolecular hole transfer equilibrium experiments (see below). From this data, it is evident that the Eox of T3 is somewhat greater than that of PBpE. Thus at equilibrium PBpE + is favored over T3 + (i.e. the equilibrium in equation 24 lies to the left). T3-(PBpE )-T3T3-(PBpE)-T3(2-4) Figure 2-11d compares the spectra of PPE164 + and T3PPE13 +. The spectra of these species are noticeably different. While PPE164 + features two bands at max 600 and 1950 nm, the visible band is red-shifted to max 640 nm and the near-IR band is blue shifted to max 1350 nm in T3PPE13 +. The spectrum of PPE24 +(not shown), was the same as that of PPE164 +. This result indicates that the differences in the spectra of PPE164 + and T3PPE13 + are not simply due to the differences in the degrees of polymerization of the two polymers. The diffe rences in the spectra strongly imply that

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51 the hole in the end-capped polymer is trapped on the T3 end group. However, although the presence of the end group does have an e ffect on the spectrum of the cation radical, the spectrum of the hole on the end-capped polyme r is distinctly different from that of the end group cation radical, T3 +. Specifically, the low energy transition in T3PPE13 + is red shifted and broader compared to the analogous transition in T3 +. Some insight into this observation can be garnered from the spectrum of the cation radical of the end-group model, PhT3 +, which is shown in Figure 2-12. This spectrum features narrow bands at max 620 and 1050 nm. That these bands are at wavelengths intermediate between the observed bands in T3 + and T3PPE13 + suggests that the cation radical is delocalized onto the phenylene ad jacent to the terthienyl end-cap. Figure 2-12. Differential abso rbance spectra of PhT3+ (solid line) and T3+ (dashed line). The spectra were obtained by photo induced elec tron transfer to methyl viologen, and recorded 400 ns af ter the laser pulse. Note that this T3+ spectra is identical to the radiolyticaly generated spectra.

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52 This idea is further supported by calcul ations. AM1 level molecular orbital calculations indicate that while the sp ecies HOMO resides predominantly on the T3, there is a significant orbital density on several adjacent phenylene units. Figure 2-13 shows this graphically. These result s indicate that the cation radical T3PPE13 + is not localized on the end-cap. Instead, it is somewhat delocalized into the polymer main chain. Figure 2-13. A segment of a PPE featuring se ven phenyl ethynyl rings at the left and a terthiophene end-cap at the right. AM1 calculated orbital densities indicate that the HOMO lies predominantly on the T3, but also has significant orbital density on the PPE main chain. This provides an explanation as to why the absorption spectrum of T3PPE13 + is not a simple linear combination of spectra for T3 + and PPE164 +. Additionally, the low energy band in T3PPE13 + is considerably red-shifted and much broader than in T3 +. These facts support the notion that the hole in the end-cap ped polymers is delocalized significantly more than in the terthiophene e nd-cap alone. It is not eworthy that there is likely to be an equilibrium established involving hole-transf er from the end-group to the main chain, as in equation 2-5. This equili brium may also have an effect on the observed spectrum. T3-(PPE )-T3T3-(PPE)-T3(2-5)

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53 Bimolecular Hole Transfer Reactions: Thermodynamics of Intrachain PPE T3 Hole Transfer In order to explain the surprising result that holes are not trapped by the T3 endgroups in T3PBpE12 +, experiments involving bimolecular ho le transfer from a variety of 2,5-oligothienyls (Tn, n = 1, 2, 3, 4) were performed. These experiments were designed to determine the energetics of intra-chain hole transfer by measur ing the position of the equilibria, and the rate constants, for the following series of hole-transfer reactions. PAE(2-6) + T2PAE+ T2PAE(2-7) + T3PAE+ T3PAE(2-8) + T4PAE+ T4 Using the reported oxida tion potentials for the Tn 0/ + couples,102 and the experimentally determined equilibrium cons tants for each of the above reactions, the redox potentials (Eo ox) for the polymers were determined. Cyclic voltammetry, (CV) both in the solid state and in solution, was attempted to confirm these results, however, CV experiments proved unsuccessful, giving onl y broad, irreversible waves at 1.1 1.3 eV. Table 2-4 is a compilation of experiment ally determined rate and equilibrium constant data, and Figure 2-14 provi des a redox scale summarizing the Eo ox values. An example of the data used in these measur ements is provided in Figure 2-15. These measurements unambiguously show that the ox idation potentials for all of the polymers are bracketed between the potentials for T2 and T4 (1.25 V > Eo ox > 0.80 V), while equilibria are established with T3.

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54 Table 2-3. Bimolecular rate and thermodyna mic data for reactions of PAEs with thiophene oligomers. a Estimated uncertainties in G or Eox are (+ 10, -40 mV) for reactions with T3 and T4 due to uncertain contributions from a second oxidizing species and to formation of an unknown species at longer times (see text). b Oxidation potentials for the polymers based on reported potentials Eo ox = 1.25 (T2), 0.95 (T3) and 0.80 (T4) in V vs. Ag/AgCl103 were converted to SCE reference by subtracting 44 mV to give 1.21, 0.91 and 0.76. See note a. c The sign indicates that this is the reverse reaction. Only the reverse reaction (a transfer of charge from T2 + to PPE) was observed. The experimentally determined equilibriu m constants provide an explanation for the fact that the T3 end-caps capture holes in the case of the phenylene polymer, but not in the biphenylene polymer. Eo ox for the biphenylene polymer is approximately 100 mV lower than in the phenylene analogue. This difference is signif icant enough that the equilibrium constant for the hol e-transfer reaction between PBpE21 and T3 strongly favors placing the hole on the polymer whereas in the case of the phenylene the equilibrium favors the hole residing on the end-cap. Anothe r significant result of these experiments is that hole injection intoPPE164 is thermodynamically favor ed over injection into PPE24 by a factor of 8 2.5. This is nearly identical to the ratio of their chain lengths (6.8), and Reaction n K (M-1 s-1) Keq G (mV) aEox (V) vs. SCE PPE164 + + T2 164 -(2.5 1.5) x 1011 c (1.0 0.5) x 10-3 177 40 1.03 0.04 PPE164 + + T3 164 4.5 x 109 0.82 5.2 0.91 a PPE24 + + T3 24 3.6 x 109 6.0 2 -46 0.96 a PPE164 + + T4 164 5.6 x 109 > 1.7 < -14 PBpE21 + + T2 21 < 4.1 x 10-2 > 82 <1.13 PBpE21 + + T3 21 0.12 55 0.86 a PBpE21 + + T4 21 4.4 x 109 > 6.3 < -47 >0.81 a T3PPE13 + + T3 13 1.1 -1.3 0.91 a T3PPE13 + + T4 13 3.8 x 109 > 45 < -98 >0.86 a

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55 very likely reflects entropic factors which favor the hole on th e longer chain. If L is the delocalization length of a charge carrie r, in repeat units, then there are Dp-L ways to place a charge in a polymer with a given degree of polymerization. Thus the ratio of the equilibrium constants for two polymers whose degrees of polymerization are Dp1 and Dp2 should be K(Dp1)/K(Dp2) = (Dp1-L)/ (Dp2-L). As a consequence, these results imply that the delocalization length of a hole, L, is slightly less than 10 repeat units, or twenty phenylene rings. Furthermore, Keq for the reaction between T3PPE13 + and T3 is about 1, leading to the surprising conclusion that th e charge on the end-capped polymer is not stabilized due to the delocali zation of the end groups. This likely reflects a balance between the destabilizing electron wit hdrawing effects of the PAE main-chain104 and the stabilization associated with delocalization of the T3 HOMO into the polymer chain. Figure 2-14. Schematic of the oxidation pot entials of various PAEs and thiophene oligomers. The potentials are vs. SCE. Eox T3, T3PPE13 PPE164 PBpE21 PPE24 54 mV 47 mV 0.85V 0.96 V 0.9 1 V 2T 1 21 V 4T 0.76V {

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56 20 15 10 5 0A (1000 nm) x103 15 10 5 0Time s 0.39 mM T3 8.2 M PPE 8.2 M PPE + 15.5 M T3 8.2 M PPE + 46 M T3 8.2 M PPE + 78 M T3 8.2 M PPE + 216 M T3 Figure 2-15. Absorbance (1000 nm) as a f unction of time after an electron pulse for solutions of PPE164 (PPE) and varied concentr ations of terthiophene (T3) in 1,2-dichloroethane + 1.6 M toluene. At this wavelength, the absorbance is due almost entirely to the polymer cation radical. The rate of charge transfer and the equilibrium are evident from this data, along with the complication that an additional species is formed at long times, especial ly at the highest concentration of T3, 216 M. At short times, (<0.5 s) the strong absorption of the dimeric toluene cation radical (MePh2+) is evident, most noticeably in the trace of the experiment with only 0.39 mM T3. The use of electron transfer equilibria to obtain oxidation potentials typically involves errors of only a few mV. Howeve r, the uncertainties in the equilibrium constants and free energy changes reported here are reduced due to complexities inherent in the system. The Cl-toluene -charge transfer complex (Cl:PhMe) oxidizes PAEs only slowly if at all, but it does oxidize T3 and T4 to their radical cations. This fact, and the limited lifetime of Cl:PhMe means that th e number of ions entering into equilibria change with the concentration of T3 and T4. Another complexity arises relating to the slow formation of an unknown species, possible a hetero-dimer between T3 and PAE +

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57 that causes limitations in the ability to wo rk at higher reactant concentrations. These effects contribute to the uncer tainties reported for the free energy changes in Table 2-3. Dynamics of Interchain Hole Transfer As noted in the introduction to this chapter, one goal of this work is to determine the dynamics of intrachain hole transfer th rough the conjugated PPE. When holes are injected into T3PPE13 more than 80% are likely to be captured by the long PPE segment, which consists of an average of 26 phenylene ri ngs. This preference arises because of the high exothermicity of hole tran sfer from the primary donor, (PhMe)2 +, and the fact that there is a much higher concentration of phe nylene moieties than thiophene end-caps. However, at the earliest times accessible afte r radiolysis, the only observable product in T3PPE13 is a species where the hole already re sides on the end-cap. This means that intramolecular hole transfer is faster than the bimolecular hole capture reaction between the solvent and the polymer. With a saturated solution of T3PPE13 ([PRU] = 1.6 mM), the rate constant for hole attachment is 3.6 x 1010 M-1s-1. Thus, while an accurate figure for kHT cannot be reported at the present time, it is possible to ascertain a lower limit of 1 x 108 s-1. Conclusions In summary, PPE molecular wires with two different architectures one based on phenylene and the other on biphenylene, were sy nthesized both in th eir prestine form and with terthiophene end-caps. The ul traviolet and visible absorption and photoluminescence spectroscopy of these materials indicate that their excited state is not completely localized on the oligo thio phene endgroup, but is instead somewhat delocalized into the polyme r main chain. This idea was confirmed by studying model compounds and by Zindo/S calculations.

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58 The cation and anion radicals of the polymer s were generated by pulse radiolysis at the LEAF at Brookhaven National Laboratories. Both radicals show transitions in the visible and IR that agree with the reported theoretical predicti ons for species of this type. The anion radicals of the end-capped species are not trapped by terthiophene, which is the expected result consideri ng that terthiophene is more el ectron rich than the polymer backbone. The cation radicals of the phenyl ene based polymer are trapped by the endcap, although as in the case of the exciton, the charge is so mewhat delocalized into the polymer main chain. On the other hand, in the case of the biphenylene based polymer, cation radicals are not trapped by the end gr oups and remain on the polymer main chain. This result is explained by bimolecular hole transfer experiments which show that the oxidation potential of the biphenylene based polym er is somewhat higher than that of the end-group. These same bimolecular hole transf er experiments are used to determine the oxidation potentials of the pa rent and end-capped polymers. It was hoped that this inves tigation would allow for the determination of the rate constant associated with the transfer of a ra dical cation from the main chain of a PPE to the terthiophene end cap. Unfortunately, this pr ocess is much faster than capture of holes by the polymer, indicating that the rate of hole transfer is greater than 1 x 108 s-1. This result is expected from the absorption spectra of the parent polymers, which indicate that the charges are highly delocalized in the polymer chain. It is likely that charge transport is much faster than the limit observed in these experiments, pointing to the desirability of developing new and more eff ective methodologies for charge injection on ultrafast timescales.

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59 Experimental Section Materials and General Synthesis 1,2-Dichloroethane (Aldrich) was HPLC gr ade and dried over type 4 molecular sieves prior to use. Anhydrous 99.8% tolu ene (Aldrich) was used as received. Tetrahydrofuran (Aldrich, anhydrou s) was distilled from LiAlH4 and then from sodium/benzophenone and stored in an environment of argon. 2,2-Bithiophene, 2,2:5,2-terthiophene (T3), 2,2:5,2:5,2 -quaterthiophene (T4), tetramethylphenylenediamine (TMPD) tetracyanoethylene (TCNE), 2,3,5,6tetrachlorobenzoquinone (chloranil), 1,4iodobenzene, 1,4-dimethoxybenzene, Pd(PPh3)4 and CuI were purchased from Aldrich or Acros. Quaterthiophene was recrystallized from toluene and 2,2-bithiophene, TMPD, TCNE a nd chloranil were sublimed prior to use and the other compounds were used as rece ived. 2-Iodo-5:2,5:2-terthiophene (I-T3) was prepared by a literature procedure.106 Chromatography was carried out using silica gel (Merk, 230-400 mesh). NMR spectra we re obtained on Varian Gemini or VXR spectrophotometers operating at 300 MHZ or an Inova spectrophotometer operating at 500 MHZ. Synthetic Procedures 2,5-Diiodo-1,4-dimethoxybenzene. This compound was prepared by a modified literature procedure.105 A solution of potassium perioda te (8.0 g, 35 mmol) in water (30 mL) was added to a round bottom flask charged with 1,4-dimethoxybenzene (6.9 g, 50.0 mmol) and iodine (12.7 g, 50.0 mmol) and acet ic acid (120 mL). The flask was fitted with a condenser, and the solution was heated to a gentle reflux with stirring. After attaining reflux, a solution of sulfuric acid (3.0 mL in 15 mL water) was added slowly through the condenser. An exothermic reactio n was observed after addition of the first

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60 few drops of acid solution, and the heating wa s suspended temporarily until addition of the sulfuric acid was complete. The reac tion was then returned to reflux. After approximately 30 minutes, the brown iodine co lor of the solution appeared significantly lighter. (Caution: While the solution is reflux ing, crystals of iodine sublime into the condenser, and it is necessary to use a long glass rod to push th ese crystals b ack into the solution. If the condenser becomes complete ly clogged with iodine, an explosion may result due to the buildup of gas pressure in the system.) When 1.5 hours had elapsed after the addition of sulfuric acid, heat was removed from the reaction and it was allowed to cool to room temperature. The soluti on was then treated with a saturated aqueous solution of Na2S2O4 until no iodine color was noticea ble. The reaction mixture was diluted with water to double the original vol ume to induce precipitation of the product as yellow crystals. The solid is collected by vacuum filtration, washed with water, and recrystallized from hot acetone/w ater giving the desired product as a white, crystalline solid, yield 17.4 g (89%). 2,5-Diodo-1,4-dihydroquinone. This compound was prepared by a modified literature procedure.105 A 500 mL Erlenmeyer flask with a ground-glass top was charged with 2,5-diiodo-1,4-dimethoxybenzene (30.0 g, 76.9 mmol) and 350 mL methylene chloride (previously dried over P2O5). The resulting solution was cooled in a bath of ethanol and liquid nitrogen, and fitted with a condenser. Neat BBr3 (38.5 g, 14.5 mL, 153.8 mmol) was added slowly through the conde nser. After the addition was complete, a septum cap was affixed to the top of the condenser, and a gentle flow of N2 gas was introduced to the reaction thr ough a needle. The mixture was allowed to warm to room temperature, then heated to reflux and stirred for 48 hours. After this time, small portions

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61 of methanol and then water were added to the solution. Methylene chloride was evaporated by rotory evaporation and the residue was diluted with methanol-water (80:20). A few spatula tips of sodium sulfite (Na2SO3) was then added. The solution was then placed in the freezer overnight, after whic h fine off-white crystals of product formed and were collected by vacuum filtration, yield 24.9 g (89%). 2,5-Bis-(2-ethylhexylox y)-1,4-diiodobenzene. This compound was prepared by a modification of a literature procedure.105 To a solution of 2,5-diiodohydroquinone (4.24 g, 11 mmol) in methyl ethyl ke tone (80 mL) in an Erlenmey er flask with a ground-glass top was added potassium carbonate (15.2 g, 110 mmol). The system was fitted with a condenser, and a nitrogen inlet and outlet. The mixture was gently heated for 20 min with stirring to dissolve mo st of the potassium carbonate. Then 1-bromo-2-ethylhexane (2.5 mL, 44 mmol) was added via syringe. The system was then heated to reflux for 16 hours. Substantial amounts of salt precipitate d over the course of the reaction. After reflux, 1 g of sodium sulfite (Na2SO3) was added to the mixture, and the solution was then neutralized by slow addition of 1 N a queous HCl. The reaction mixture remained under positive nitrogen pressure, with an outle t to allow for the escape of gasses, during the entire process. After neutralization th e mixture was further diluted with 100 mL of warm (40 C) water, and stirred for one hour. The mixture was then extracted with pentanes (3 x 40 mL) and the or ganic layer was dried over MgSO4 and concentrated to a volume of 10 mL. The concentrated pentan e solution was passed through a short (3 length, 2diameter) column of silica by elut ion with 200 mL of pentanes, followed by evaporation of the solvent under reduced pressu re afforded the desired product as a clear

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62 liquid, yield 4.07g, 63%. 1H NMR (CDCl3): 0.9-1.8 (complex multiplet, 26H), 3.8 (t, 4H), 7.1 (s, 2H). 1,4-Diethynylbenzene. This compound was prepar ed by a modified literature procedure.105 1,4-diiodobenzene (18.0 g, 54.6 mmol), Pd(PPh3)2Cl2 (0.4 g, 0.57 mmol) and CuI (0.1 g, 0.525 mmol) were dissolved in a mixture of THF and diisopropylamine (200 mL, 8:2) which has been previously de oxygenated by 30 min of bubbling with N2. This solution was cooled with an ice ba th whereupon trimethylsilylacetylene (20 mL, 13.9 g, 0.142 mmol) was added dropwise with s tirring and under positive pressure of nitrogen. After a small volume (< 5 mL ) was added, an endothermic reaction was observed with formation of a thick precipitate of white powder. After the addition was complete, the reaction was allowed to warm to room temperature and it was stirred an additional twenty-four hours. After this time, the reaction mixture was a dark color. The solvents were removed by rotory evaporati on and the remaining black solid was washed several times with water and a water-metha nol (40:60) mixture. The solid was then dissolved in hot hexane, decolorized with active charcoal, and passed through a short column of alumina. The resulting white so lid was recrystallized from acetone/water to afford 1,4-bis-(trimethylsilylethynyl)benzene as shiny white crystalline flakes. The 1,4bis-(trimethylsilylethynyl)ben zene crystals were dissolved in a mixture of THF and methanol (80:20). To the resulting soluti on was added a solution of KF (10.0 g) and a spatula-tip of tetrabutylammonium bromide in water (80 mL). The resulting suspension was vigorously stirred at room temperature under N2 for 12 hours, and then diluted with water until the product precipitated as a fine white powder. The solid was collected by

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63 vacuum filtration, washed with water and recr ystallized from methanol/water to afford 1,4-diethynylbenzene as a white solid, yield 5.6 g (81%). 4,4-Diethynylbiphenyl. This compound was prepared by the same procedure as described above for 1,4-diethynylbenzene, exce pt that 4,4-diiodobiphenyl was used in place of 1,4-diethynylbenzene. The product was obtained as a white solid, yield for the two steps 75%. 1,4-Diundecyloxy benzene. A mixture of potassium carbonate (22 g, 0.16 mol) in methyl ethyl ketone (40 mL) in an erlenm eyer flask with a ground glass top was heated gently and stirred to dissolve most of th e potassium carbonate. The solution was then spurged with argon for 15 minutes, after wh ich time hydroquinone (5 g, 0.045 mol) and 1-bromo undecane (26 g, 0.10 mol) was added. The reaction mixtur e was fitted with a condenser and heated to reflux for 12 hours. A large volume of salt precipitated during the course of the reaction. After this time, the solution was cooled to room temperature and neutralized by slow addition of 1 N HC l. The reaction mixture was kept under a positive pressure of argon, with a vent to allow for escape of gasses, during this entire process. A few spatula tips of sodium sulfite were then added to the reaction mixture. The crude product was precipitated by pouring into a large quantity of ice-cold water, and the recovered by filtration as a tan solid. A portion of the product used for analysis was purified by recrystalization from methanol-water but the crude material was used in the next step. The crude yield was 14.8 g (78%). 1H NMR (CDCl3): 0.08 (t, 6 H), 1.6 (br, 32 H), 1.8 (m, 4 H), 3.9 (t, 4 H), 7.0 (s, 4 H). 2,5-Diundecyloxy-1,4-diiodo benzene. This product was obtained by the same procedure used to pepare 2,5-dimethoxy-1,4diiodo benzene, except the amounts of

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64 reagents used were: 1,4-diundecyloxy benzene (10 g, 0.024 mol), iodine ( 6.1 g, 0.024 mol) and potassium periodate (3.6 g, 0.016 mo l). The product was purified by elution through a short plug of silica with hexanes, treatment with activated carbon and recrystalization from methanol-water. 1H NMR (CDCl3): 0.08 (t, 6 H), 1.6 (br, 32 H), 1.8 (m, 4 H), 3.9 (t, 4 H), 7.0 (s, 4 H). Photophysical Methods UV-visible absorption spectra were obtained on a Varian Cary 100 spectrophotometer. Corrected steady-state emission spectra were recorded on a SPEX F112 fluorescence spectrophotometer. Samples were contained in 1 cm x 1 cm quartz cuvettes, and the optical density was adjust ed to approximately 0.1 at the excitation wavelength. Emission quantum yields are reported relative to perylene ( em = 0.94)107 and appropriate correction was applied for the difference in refractive indices in the sample and actinometer solvent.108 Time-resolved emission decays were obtained by time-correlated single photon c ounting on an instrument that was constructed in-house. Excitation was effected by using a violet diode laser (IBH instruments, Edinburgh, 405 nm, pulse width 800 ps). The time-resolved emission was collected using a red-sensitive, photon counting PMT (Hamamatsu, R928) and the light was filtered using 10 nm bandpass interference filters. Lifetimes were de termined from the observed decays with the DAS6 deconvolution software (IBH Instrume nts, Edinburgh, Scotland). GPC was preformed using a Rainin Dynamax model SD-200 solvent delivery system equipped with two PL-Gel 5 micron Mixed D columns (P olymer Laboratories, Inc., Amherst, MA) connected in series, and a UV de tector set at a wavelength where the polymer absorbs.

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65 Molecular weight information was calculate d from the chromatograms using Polymer Laboratories software. Generation of the T3 and Ph-T3 Radical Cations These species were generated by photo-indu ced electron transfer from the neutral molecules to methyl violegen (MV2+). To a 15 mM solution of MV2+ in THF/acetonitrile (2:1) T3 or Ph-T3 were added until the absorbance of the analyte at 355 nm was 0.7. The solution was subjected to 10 mJ, 355 nm laser pul ses to generate the cation radical. The differential absorption spectrum of the cation radical was recorded on an instrument built in-house. Radiation Techniques This work was carried out at the Broo khaven National Laboratory Laser-Electron Accelerator Facility (LEAF). The fa cility has been described elsewhere.109,110 The electron pulse ( 120 ps duration) was focussed into a quartz cell with an optical path length of 20 mm containing the solution of interest. For the polymer solutions, the concentration of repeat units used was typica lly 0.2 2 mM. The monitoring light source was a 75 W Osram xenon arc lamp pulsed to a few hundred times its normal intensity. Wavelengths were selected using either 40 nm or 10 nm band pass inte rference filters. Transient absorption signals were det ected with either FND-100Q silicon ( 1000 nm) or GAP-500L InGaAs ( 1100 nm) diodes and digitized with a Tektronix TDS-680B oscilloscope. The transmission/time data were analysed with Igor Pro software (Wavemetrics). Reaction rate constants were determined us ing a non-linear least squares fitting procedure described previously.112 This procedure accounts for geminate recombination, which is encountered on the time scales investigated. Bimolecular rate constants were determined us ing the linearity of the observed pseudo-first order growth

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66 of the product with respect to the solute c oncentration. Where not stated, uncertainties are +/15%. Bimolecular reactions of PAE ions with charge acceptors often went to completion with no detectable PAE+ remaining. But in others including most reactions with terthiophene, the kinetics proceeded to equilibria in which substantial fractions of the PAE+ remained. The equilibrium constant s could be calculated using this information. Molar extinction coefficients of the radical cations were calculated using G(DCE+ ) = 0.68,113 where G is the radi ation chemical yield (molecules produced/100 eV). For the anions the reported Gvalues for the electron in THF has varied greatly. The value used was G(eTHF) = 0.53, the average of a number of reported values.114-119 The total dose per pulse was determined before each species of experiments by measuring the change in absorbance of th e electron in water. The dose received was calculated using 700 nm, eaq) = 18,830 M-1 cm-1 and G(eaq) = 2.97. The dose was corrected for the difference in electron density of the organic solvents used compared to that of water. Radiolytic doses of 5 18 Gy were employed. For the DCE/toluene solutions, dissolved oxygen was removed by purging with argon gas for at least 10 minutes, and subsequently sealing the cells w ith septa and parafilm. Solutions in THF were prepared in an argon environment and sealed under argon with Teflon vacuum stoppers. Samples were prepared immediately prior to use. During irradiation, samples were exposed to as little UV light as possible to avoid photodecomposition, although no evidence of this occurring wa s found within the time fram es monitored. Measurements were carried out at 21 C.

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67 CHAPTER 3 MECHANISM AND DYNAMICS OF TRIPLET TRANSPORT IN PLATINUM CONTAINING POLY (PHENYLENE ETHYNYLENE)S Introduction A great deal of effort has been placed into investigating the carrier properties of singlet excitons in single chain conjugated polymers.17,66,71,73-75,87,120,121 However, despite these efforts, relatively little is known about th e analogous properties of triplet excitons in these systems. This chapter describes a st udy aimed at investigating the properties and dynamics of triplet transport in platinum containing PPEs, often called Pt-acetylides.43,122 The major aim of the study is to determine wh ether or not intrachain energy transfer in Pt-acetylides is an efficient pr ocess. Another goal is to es tablish a model for transport in these materials that explains why this is or is not so. The polymers used in this study are depict ed in Figure 3-1. Th ese polymers feature a Pt-acetylide backbone based on 1,4-diethynyl benzene as the acetylide unit. The 2,5diethynyl thiophene unit was incorporated into the polymers at various loadings to serve as a trap for phenylene based triplets. Pla tinum acetylides were chosen because their triplet states are highly phosphorescent, and are thus easily monitored by photoluminescence spectroscopy.37-39 Furthermore, they posses a rigid rod molecular wire structure, and are easil y synthesized by use of copper iodide catalyzed Hagihara condensation polymerization.44,122-124 In order to study the tran sport in these materials, thiophene was chosen as an energy trap. The conditions for a triplet energy trap are that

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68 the triplet state of the trap must have a lowe r energy than the triplet state of the polymer main chain. In this case, the energy of the thiophene triplet is know n to be 2.0 eV, while that of the phenylene triplet is 2.4 eV.36 In addition, varying the amount of thiophene in the reaction feed allows for st raightforward synthesis of a se ries of copolymers in which the ratio of co-monomers varies. Pt S Pt PBu3 PBu3 PBu3 PBu3 x y n x = 1, y = 0 P100T0x = 0.95, y = 0.05 P95T5x = 0.85, y = 0.15 P85T15x = 0.75, y = 0.25 P75T25x = 0, y = 1 P0T100 Figure 3-1. Structure of polymers featured in this chapter. Unlike the polymers described in Chapte r 2, which feature trap sites only at the end of the polymer chain, the polymers described in this chapter have trap sites dispersed throughout their backbone. There are several reasons for this change in approach. First, the synthetic challenges associated with end-capping conjugated polymers proved to be considerable. However, synthesis of th e co-monomers used to make the polymers depicted in Figure 3-1 is fa irly straightforward, and th e associated polymerization reactions are also fairly simple and do not require a great deal of optimization. Also, although the end-capping approach ma y seem somewhat more elegant than a random copolymerization, there is no difference in the resulting systems with respect to carrier transport. Migrat ion along the copolymer chain is a one dimensional randomwalk, meaning that a carrier does not move dir ectly to the nearest tr ap. Instead, a carrier on a monomer unit that is far from a trap site has an equal probability of migrating to

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69 either adjacent monomer, regardless of weathe r this brings the carri er nearer or further from the trap. Therefore, the number of monomer units th at a carrier migrates through before reaching a trap site doe not directly depe nd on the carriers distance from that site. In addition, the distance between a carrier a nd a trap is not well defined even in an end-capped polymer because of the polydisper sity inherent in synthetic polymers. Polydispersity relates to the di stribution of molecular wieghts, and thus chain lengths, in a polymer sample. The higher the polydispersit y, the greater the variety in chain lengths. For reference, any well-defined molecule has a polydispersity equal to 1. All conjugated polymers are synthesized by step condensat ion polymerizations, and the theoretical polydispersity of such processes appro aches 2 as the functional group conversion approaches 100%.125,126 Practical polydispersities in conjugated polymers are often much higher because functional group conversion is less than quantitative. While fractionation of the polymer can help the situation somewhat, conjugated polymers are nonetheless quite polydisperse. Thus, the dist ance between an excit on and a trap is illdefined regardless of whether the traps are inco rporated into the polym er as end-caps or randomly in the backbone. Since this is s o, the end-capping strategy does not offer any significant advantages over randomly incorpor ated traps. Theref ore the latter method, which is much easier syntheticall y, was utilized in this study. Results and Discussion Polymer Synthesis and Stru ctural Characterization The chemical structures of the polymers which are the focus of this work are presented in Figure 3-1. The nomenclat ure adopted for the polymers is PxTy where x and y are the mole fraction of phenylene and thi ophene monomers, respectively, used in the polymerization reactions.

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70 It was originally envisioned that the pol ymers in this study would be prepared by the CuI catalyzed coupling of appropriate amounts of 2,5-diethynyl thiophene and 2,4diethynyl benzene with bis(tributyl phosphine)palladium dichloride.121-124 However, 2,5diethynyl thiophene proved, in our hands, to be unstable to the polymer ization conditions. Since bis(trimethylsilylethynyl) thiophene exhibits excellent stability under the polymerization conditions, it was deprotected in situ using tetrabutyl ammonium fluoride in the presence of copper (I) iodide and one equivalent of Pt(PBu3)Cl2 under Hagihara conditions to afford P0T100. This synthetic route is detailed in Figure 3-2. S I I S TMS TMS a 1 2 b S Pt PBu3PBu3n P0T100Reagents and conditions: (a) Pd(PPh3)4, CuI, TMSAc, R.T., 24 hrs. (b) Pt(PBu3)2Cl2 (1 eq.), Bu4NF, toluene piperidine, R.T., 24 hrs. (c) Pt(PBu3)2Cl2 (3.5 eq), Bu4NF, toluene, piperidine, 48 hrs.S PtPt ClCl PBu3PBu3PBu3PBu3c 3 Figure 3-2. Synthesis of intermediate 3 and P0T100. The synthesis of the copolymers P75T25, P85T15, and P95T5 proved to be somewhat more challenging, as in situ deprotection of 2,5-bis(trim ethylsilylethynyl) thiophene (2) in the presence of copper iodide and tetrab utylammonium fluoride with appropriate amounts of bis(tributylphosphine) platinum (II) chloride and 1,4-diethynylbenzene gave only low molecular weight material. In previous work, platinum containing PPE oligomers were synthesized using diplat inated diethynylbenzene and the resulting acetylides, featuring monochlorinated platinum end-groups, were then used in further Hagihara coupling reactions.44 This methodology was extended to thiophene based

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71 monomers by deprotecting 2 in situ with tetrabutyl ammonium fluoride in the presence of CuI and an excess of Pt(PBu3)2Cl2 to afford compound 3 (see Figure 3-2), which is very stable and can be easily purified by chromatography. Thus 3 was used in a three component A-A + A-A + B-B polymeri zation with appropriate amounts of 1,4diethynyl benzene, Pt(PBu3)Cl2 and CuI gives copolymers P95T5, P85T15, and P75T25, as shown in Figure 3-3. Note that unlike the end-capping polymeri zations discussed in Chapter 2, these polymerizations were preformed under condi tions where the end-group stoichiometry was balanced. That is, each polymerizat ion featured an equal molar quantity of chloroplatinum and terminal acetylene functi onalities. Any stoichiometric imbalances give only low molecular weight s, as predicted by the cl assic equation of Carothers.125,126 x 3 + (1-x)Pt PBu3PBu3ClCl +CuI, toluene/DIPA R.T. 24 hrsP95T5 (x = 0.05, y = 0.95) P85T15 (x = 0.15, y = 0.85) P75T25 (x = 0.25, y = 0.75) Figure 3-3. Synthesis of platinum co ntaining phenylene-thiophene copolymers. The copolymers feature a platinum-phenylen eacetylide backbone in which varying amounts of thiophene are substituted for phenyl ene. Because the thiophene moieties in the backbone orient their substituents at the 2 and 5 position at angles somewhat less than 1800, these polymers are not expected to be completely linear, however, they should nonetheless adopt a rather extended conforma tion in solution. The synthetic design, where thiophene is introduced only via the diplatinated compound 3 prevents blocks of adjacent thiophene units in the copoly mer, since the re active termini of 3 cannot couple with one another. The only possible reaction of 3, in this system, is coupling with diethynyl benzene. Thus ther e is no possibility that a ny of the photophys ical results

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72 presented herein are the product of very low energy trap sites involving multiple adjacent thiophene units. The molecular weights of these polymers vary greatly. However, this is unimportant for the purposes of this study whic h is not concerned w ith behaviors of the bulk materials. Furthermore, the level of thiophene incorporation does not depend on the molecular weight of the polymers, but on the thiophene loading. T hus, carrier transport from the phenylene main-chain to a thiophene tr ap-site, the property of interest here, does not depend on molecular weight. 1H NMR is particularly instructive in elucidating the structure of these polymers. The aromatic region of the polymers prot on NMR is presented in Figure 3-4. The phenylene moieties in P100T0 show resonances at 7.1 ppm, a nd are clearly distinct from the thiophene resonances in P0T100, which appear at 6.5 ppm. On this basis, it is possible assign the aromatic resonan ces in the copolymers and co mpare the integration of thiophene to phenylene protons in order to determine the exact composition of the polymer.127 The ratio of thiophe ne to phenylene in P95T5, P85T15, and P75T25 is calculated to be 1:30, 1:13, and 1:8, as compared to the theoretical values of 1:19, 1:6 and 1:4, respectively. Thus, approximately half the am ount of thiophene in the feed is actually incorporated into the polymers. Th is suggests that thiophene monomer 3 is somewhat less reactive to Hagihara coupling than Pt(PBu3)2Cl2. This observation may also explain why the copolymers exhibit lower mol ecular weights in comparison to the homopolymers.

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73 Figure 3-4. Aromatic region of the polymers 1H NMR spectra. From top to bottom, the spectra are P100T0, P95T5, P85T15, P75T25, and P0T100. The resonance at 7.26 ppm corresponds to the solvent (chloroform). It is important to note that the thienyl pr otons give relatively sharp resonances in the NMR. This indicates that all thiophene moieties are in essentially identical magnetic environments. Thus, the thiophene com onomers are randomly distributed along the

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74 polymer backbone and the evidence shows no si gn of blocks or regions on the polymer chains with a high localized c oncentration of thiophene units. Photophysical Mesurements Absorption and emission spectroscopy The absorption spectra of the polymers used in this study are presented in Figure 3-5, and their photophysical characteristics are summarized in Table 3-1. First the spectra of the homopolymers, P100T0 and P0T100, is considered. E ach of these spectra exhibit one broad, featureless band with max at 341 nm and 402 nm, respectively. This band is assigned to the long-axis po larized absorption of each polymer. The copolymers show more complex spectra. The absorption of P95T5 is dominated by a broad band with max at 347 nm. This band is at ne arly identical in shape and wavelength to the absorption of the all-phenyl ene polymer, and as such it is assigned to absoption of a phenylene based chromophore. In addition to the main band, a shoulder at 402 nm appears in this spectrum. Because its energy is similar to that of the thiophene homopolymer, this band is assigned to absopt ion of a thiophene based chromophore. The spectrum of P85T15 shows the same two bands, except th at the relative intensity of the thiophene-based (402 nm) absorption is greater for this po lymer than in P95T5. This corresponds with expectation, given the incr eased level of thio phene loading in P85T15. A broad band centered at 375 nm is the majo r feature in the abso rption spectrum of P75T25. This band is red-shifted with respect to the dominant absoptions of P95T5 and P85T15. A shoulder at 402 nm is also present in this spectrum. However, the dominant band ( max = 375) is so broad as to encomp ass nearly the entire shoulder.

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75 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Wavelength / nm 300350400450500Normalized Absorbances 0.2 0.4 0.6 0.8 1.0 P0T100P75T25 0.2 0.4 0.6 0.8 1.0 P85T15 0.2 0.4 0.6 0.8 1.0 P95T5 P100T0 Figure 3-5. Absorption spectra of polymers in THF solution. Table 3-1. Summarized absorp tion and photoluminescence data. Photoluminescence max. / nm em. Polymer Abs. max. / nm max. F max. P F P P100T0 341 ---514 ---0.045 P95T5 347 420 604 0.002 0.071 P85T15 347 420 604 0.063 0.058 P75T25 375 420 604 0.053 0.043 P0T100 402 420 604 0.057 0.031

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76 The photoluminescence spectra of the homoand copolymers are presented in Figure 3-6, and the results are also tabulated in Table 3-1. Photoluminescence lifetimes appear in Table 3-2. Each band is labled as P or T, corresponding respectively to phenylene or thiophene based luminescence. Additionally, the subscripts F or P designate fluorescence and phos phorescence, respectively. The room temperature spectra, begi nning with the homopolymers, will be considered first. P100T0 exhibits very weak emission between 400 500 nm which is assigned as fluorescence on the basi s of previous investigations.44,128 In addition, this polymer shows moderate emission centered at 514 nm. This band exhibits a very long lifetime (Table 3-2), and is completely que nched in the presence of oxygen. On this basis, and because of its similarity to previ ously published spectra of structurally similar oligomers,44 the emission centered at 514 nm is assigned as phosphorescence. P0T100 also shows two emission bands, one centered at 420 nm, and the other centered at 604 nm. The 604 nm band is co mpletely quenched in an air-saturated solution, and has a lifetime of several micros econds (Table 3-2). This band is therefore assigned as phosphorescence. The band at 420 nm is not quenched by oxygen and has a lifetime less than 5 ns. The 420 nm band is therefore assigned as fluorescence. These assignments agree with previously reported data on similar materials.36

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77 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 P 100 T 0 P 95 T 5 0.2 0.4 0.6 0.8 1.0 P 85 T 15 P 75 T 25 Wavelength / nm 400500600700 Normalized PL Intensities 0.2 0.4 0.6 0.8 1.0 P 0 T 100PFPPTFTPPPPPTFTFTFTPTPTP Figure 3-6. Photoluminescence spectra of polym ers in argon saturated solutions of THF (room temperature) and 2-me thyl THF (78 K). Solid lines: room temperature, dashed lines: 78 K. Note that the ro om temperature spectra are normalized to the fluorescence band when applicable, while the low-temperature spectra are normalized to the strongest phosphorescence band.

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78 Table 3-2. Photoluminescence ra te and lifetime information. 298 K 78 K 514 nm 605 nm 514 nm 605 nm Polymer k / s-1 / s k / s-1 / s k1 / s-1 (rel. amp.) k2 / s-1 (rel. amp.) k1 / s-1 (rel. amp.) k2 / s-1 (rel. amp.) P100T0 5.4 x 104 18 ------6.0 x 105 ---------P95T5 1.7 x 105 5.3 1.1 x 105 9.1 1.4 x 106 (0.62) 2.1 x 104 (0.38) 8.8 x 104 (0.89) 2.8 x 104 (0.11) P85T15 3.0 x 105 3.3 3.1 x 105 7.7 3.6 x 106 (0.48) 4.2 x 104 (0.52) 9.0 x 104 (0.91) 2.6 x 104 (0.09) P75T25 7.1 x 105 1.4 1.4 x 105 7.1 5.7 x 105 ---4.4 x 105 ---P0T100 ------1.8 x 105 5.6 ------6.4 x 105 ---Now turning to the copolymers, P95T5 exhibits a very weak band at 420 nm that is not quenched by oxygen. This band is assigned to thiophene-based fluorescence. A band at 514 nm, corresponding to phenylene base d phosphorescence, is also apparent. However, this band is quite weak (< 5% of the total phosphorescence). Instead, a band centered at 605 nm, corresponding to thi ophene phosphorescence, dominates the spectrum. The emission spectra of P85T15 and P75T25 both feature a strong band at 420 nm, which, based on comparison to the all-th iophene polymer, corresponds to thiophenebased fluorescence. No emission centere d at 514 nm, corresponding to phenylene phosphorescence, is readily apparent in either of these polymers. Time resloved studies (see below) show that the phe nylene-based phosphorescnce at 514 nm is present, but is so weak as to be buried under the tail of the fluorescence. In fact, the only phosphorescence that is readily apparent in these copolymers is centered at 605 nm, corresponding to emission from a thiophene-based luminophore. These results are quite remarkable. The lack of phenylene based (514 nm) phosphorescence in P85T15 and P75T25 indicates efficient energy transfer from the phenylene main-chain to the thiophene t raps along the chain. Even when the

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79 concentration of thiophene is less than 5%, (P95T5), energy transfer is more than 95% efficient, based on the relati ve intensities of the phenyl ene and thiophene emission. Trends in the phosphorescence lifetimes of the copolymers support this idea. At room temperature, P100T0 exhibits a phosphorescence lifetime ( ) of 18 s, whereas P0T100 displays = 5.6 s. The copolymer lifetimes (T able 2) at room temperature show a distinct trend. As the thi ophene concentration increases, of both the phenylene and thiophene components of the emission decrease s. If the two excited states were not coupled in equilibrium, but were instead re lated to two independent chromophores, the lifetimes would not be expected to show any significant change with increased loading of thiophene. The photoluminescence spectra of the c opolymers changes significantly at low temperature. Whereas thi ophene-based phosphorescence domi nates the triplet-emission at room temperature for all of the co polymers, at 78 K the phosphorescence of P95T5 and P85T15 is dominated by phenylene-based phos phorescence. Thus, decreasing the temperature dramatically decreases the effici ency of energy transfer in polymers with lower thiophene loadings. This effect is not seen at highe r loadings of thiophene, as evidenced by the low temperature emission of P75T25. In this polymer, the spectrum is dominated by thiophene-based emi ssion even at low temperatures. Phosphorescence quenching The phosphorescence of P100T0 and P0T100 are quenched by addition of methyl viologen (N,N dimethyl-4,4-bipyridinium, MV2+). Stern-Volmer analysis of these polymers gives quenching constants (Ksv) of 3.9 x 105 for P100T0 and 6.7 x 105 for P0T100. Considering the phosphorescence lifetimes, these constants correspond to a purely diffusion controlled (dynamic) quenching process.

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80 The quenching behavior of P95T5 was also explored, and the results are presented in Figure 3-7. P95T5 was used for these experiments be cause it is the only copolymer for which both phenylene and thiophene based phos phorescence is readily observable at room temperature. These experiments s how quenching constants for the phenylene and thiophene components of the phosphorescence that are essentially identi cal to those in the corresponding homopolymers. Wavelength / nm 500520540560580600620640 Photoluminescence Intensity / arb. units Conc. MV 2+ (uM) 01234 (I 0 /I)-1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 K sv(P) = 3.2 x 10 5 K sv(T) = 6.2 x 10 5 Figure 3-7. Stern-Volm er quenching of P95T5 in THF solution. The uppermost spectrum has [MV2+] = 0 M, and the concentration of MV2+ increases by 1 M in each subsequent spectrum. The insert shows the linear por tion of the SternVolmer plots for the phenylene (530 nm circles) and thiophene (614 nm, triangles) portions of the spectra. These results again imply th at the phenylene and thi ophene based triplets are coupled in a dynamic equilibrium. If this were not the case, and the phosphorescence of P95T5 was a result of thiophene and phenyl ene based triplet states that did not interconvert, then the results would be expected to be much different. Recall that results from quenching experiments with the homopolymers show that MV2+ can effectively

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81 quench both phenylene and thiophene phosphores cence, and both processes are diffusion controlled. Thus, in the absence of an equili brium system, it would be expected that the thiophene-based emission would be quenche d preferentially to the phenylene-based emission because, as evidenced by the photoluminescence spectrum, there is a much larger concentration of thiophene triplets th an phenylene triplets in the polymer chain. Transient absorption spectroscopy Figure 3-8 shows the time-resolved transient absorption spectra of the polymers. In the homopolymers, the major feature is a broa d transient absorption centered at 6100 nm in P100T0, and around 640 nm in P0T100. These bands are assigned to the triplet-triplet (T1 Tn) absorption of the polymers. The transient absorption of the three copolymers are quite similar. All are dominated by a broad absorption centered at 630 nm, intermediate between the allphenylene and all-thiophene homopolymers. The shape the bands are essentially identical for all three of the copolymers. However, there is a more subtle, yet noteworthy, component of the transient ab sorption spectra. Specifically, close examination of the negative intensity ba nd due to ground state bleaching reveals an interesting trend. This f eature does not mirror the ground state absorption, but instead shows a continuous red-shift w ith increasing thiophe ne concentration across the series.

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82 Wavelength / nm 300400500600700 -0.010 -0.005 0.000 0.005 0.010 0.015 -0.04 -0.02 0.00 0.02 P 100 T 0 P 95 T 5 Delta Absorbance -0.06 -0.04 -0.02 0.00 0.02 0.04 P 85 T 15 -0.15 -0.10 -0.05 0.00 0.05 P 75 T 25 Wavelength / nm 300400500600700 -0.04 -0.02 0.00 0.02 P 0 T 100 Figure 3-8. Triplet-state transient absorption spectra. The most intense spectrum were obtained immediately after photoexcitation with a 10 mJ 355 nm laser with a 10 ns pulse width. Each subsequent spectrum was obtained 1 s after the preceding spectrum. The dashed vertical line is at 350 nm. Time resolved emission Time resolved photoluminescence spectroscopy was applied to P95T5 in order to gather information about energy tr ansfer in this polymer. Th e results, as shown in Figure 3-9, show that while the phenyl ene phosphorescence (514 nm) is complete within about 5 s, the thiophene (605 nm) phosphorescence is still present after more than 30 s. This

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83 observation supports the notion that energy is being transferred fr om a phenylene-based state to a thiophene-based state. In order to gain any information about energy transfer, it would be necessary to deconvolute the decay curves into two components: one corresponding to phosphorescence and the second corresponding to energy transfer. However, at room temperature, the decay at both 514 nm and 605 nm is described by a single exponential equation. This precludes any such analysis. Figure 3-9. Time resolved emission spectrum of P95T5 in THF solution at room temperature, excitation wavelength 355 nm Inset: Normalized emission decay at 514 nm (circles) and 605 nm (triangles) The large changes that were observed in the phosphorescence spectra at low temperatures indicated the possibility that at 78 K energy transfer might be slowed

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84 sufficiently as to allow for observation of the process. Indeed, at low temperatures, two decay components were observed in the d ecay of both the phenylene and thiophene phosphorescence of P95T5 and P85T15. Figure 3-10 presents the normalized phenylene (514 nm) and thiophene (605 nm) decays in P95T5. This data is further summarized in Table 3-2. Time / s 051015 Normalized PL 0.0 0.2 0.4 0.6 0.8 1.0 Figure 3-10. Normalized emission decay of P95T5 at 78 K. The dashed line represents phenylene (514 nm) decay and the solid line represents thiophene phosphorescence decay (605 nm). The phenylene-based phophoresence decay in P95T5 is dominated by a fast componenet (k = 1.4 x 106 s-1). This decay is far too fa st to relate to phophorescence, based on comparison with the homopolymers. It must therefore be associated with energy transfer to thiophene. If th is is true, increasing the loadi ng of thiophene should increase

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85 the rate of the short decay component. This prove to be the case, as demonstrated by the results on P85T15, the 514 nm emission decay of whic h features an energy transfer component that is faster than in P95T5. However, the phenylene-based phosphorescence decay in P75T25 is unlike that in the copolymers with lower concentrations of thiophene in that it is monoexponential. This implies that transport in this polymer is too fast to be measured. This phenomenon is discussed in detail below. Note that the thiophene-based (605 nm ) phosphorescence is also biexponential in both P95T5 and P85T15. The minor component of both of these decays corresponds well to the phophorescence decay of the phenylene-based emission (514 nm). Careful inspection of the steady-state spectra reveal that there is some residual phenylene-based phosphorescence even at 605 nm. This residual phenylene p hosphorescence likely accounts for the minor component of the 605 nm emission decay. General Discussion Electronic model In addition to providing information about the structure of the polymers, the 1H NMR spectra provide useful information a bout the influence of thiophene doping on the phenylene-based polymer chains (Figure 3-4). When thiophe ne is introduced into the polymer chain new resonances corresponding to thiophene protons appear in the NMR. In addition a change is observed in the res onances associated with phenylene protons. Specifically, a new resonance at 7.1 ppm, which is not present in P100T0, appears in the 1H NMR spectra of the copolymers. The rati o of the relative integration of this new resonance and the thiophene resonance is 4:1. Noting that each thiophene has two protons and each phenylene four, the resona nce at 7.1 ppm can be assigned to the phenylene moieties adjacent to thiophene. This assignment makes sense, considering that

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86 the electron-releasing thiophene ring would be expected to cause an upfield shift on nearby groups. Importantly, this result shows that even in the gr ound state, the electronreleasing effects of thiophene perturb nearby phenylene moieties. This information, considered in light of previous work showi ng that the singlet excited state of Pt containing PAEs is de localized over approximately five to seven arylene units allows for the c onstruction of a model to explai n the excited states of these polymers. 44 The cartoon in Figure 3-10A provides an illustration of the effects of a single thiophene trap in a Pt-acetylide ch ain composed of phenylene-based. For the purposes of this discussion, the delocalization of the singlet excited states is taken to be five repeat units.44 In this depiction, the dark circle represents thiophene and the light circles represent phenylene. The grey circ les represent phenylene moieties that are within one chromophore length of a thiophene moiety. Note a single thiophene unit perturbs its nearest neighbor and second-nearest neighbor. In this case, it would be expected that an initial exci tion created within one chrom ophore length of a thiophene unit (ie. on the black or grey circles in 3-10A ), would undergo ultra-fast transport in the singlet to the thiophene trap.66 An exciton created elsewhere on the chain (the white circles in 3-10A) may migrates to the trap either as a singlet or a triplet (vide infra). Figure 3-10B considers the case where the c oncentration of thiophene is somewhat higher, in this case one in five. Although the individual ch romophores in this case are identical to those in a polymer with more dilute thiophene loadi ng, here every phenylene is either a nearest neighbor or second-n earest neighbor to a th iophene. Thus, every phenylene unit is within one chromophore leng th of a thiophene. In this case, any exciton on the chain is with one chromophore length of a thiophene trap. Thus, it would

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87 be expected that the system can be described by ultra-fast singlet transport within a single chromophore.66 Importantly, because transport is mu ch faster in the singlet state as compared to the triplet state, triplet transport is not expected to play a role in this scenario. Figure 3-11. Schematic representation of pl atinum acetylide polymers with low (A) and high (B) levels of thiophene incorpor ation. The dark circles represent thiophene monomers, and the light ci rcles represent phenylene monomers. The grey circles represent phenylene un its that are within one chromophore length of a thiophene moiety. The absorption spectra (Figures 3-5) are ea sily understood in light of this model. The shoulder in the ab sorption spectra of P95T5 and P85T15, previously assigned to thiophene-based absorption, accounts for a di sproportionately high amount of the total absorption in these cases if only the concen tration of thiophene in the backbone is considered. Recall that P95T5 contains only 1 thiophene in about every 30 repeat units, or about 3% of the total number of repeat units. However, the shoulder centered at 402 nm, previously assigned to thiophene based abso rption, accounts for roughly 10% of the total absorption in this polymer. Taking into account that each thiophene is actually part of a A B

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88 chromophore that contains four nearby phenylen e units, then this apparent contradiction becomes clear, since the thi ophene-based chromophores actu ally include not one but 4 out of every 30 (or about 10%) of the arylen e units in the polymer. The spectrum of P85T15 can be explained in the same fashion. Here, the thiophene-based absorption accounts for about 30% of the total absoption of the polymer. This is consistent with expectation considering that the thiophene-to-phenylene ratio is about three times higher than in P95T15. The model also accounts for redshift in the absorption of P75T25. This polymer, featuring the highest loading of thiophene among the copolymers, is best explained by considering that the concentra tion of thiophene is high enough that it is approaching the regime in depicted in Figure 3-10B. In th is polymer, nearly every phenylene is perturbed by a nearby thiophene. Thus, the major absorp tion band is neither phenylene-based nor is it completely thiophene-based. Careful examination of the triplet-triplet transient absorption se pctra (Figure 3-8) is useful in explaining the trip let state of these materials. Note that the copolymers all feature a single, broad triplet-triplet absorpti on centered at 610 nm, intermediate between the absorptions of the all-phe nylene and all-thiophene homop olymers. This means that the lowest energy triplet cannot be describe d as being localized on thiophene alone. A more reasonable description of the lowest energy triplet state in the copolymers is a thiophene-based chromophore that is perturbe d the nearby phenylene moieties. That the absorption is essentially identical for all of the copolymers, regardless of thiophene loading, implies that the lowest energy triple t states of all of th e copolymers are also identical. In other words, the lowest energy triplet is mostly localized on thiophene, but it

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89 leaks slightly onto the adjacent phenylene units. This is consistent with previous work that showing that the triplet state is highly localized.44 More subtly, the negative intensity bleac h of the ground states in the transient absorption does not mirror the absorption spectr a. In fact, while all of the ground states show strong absorption at 350 nm, in the transien t absorption (dashed line), there is little to no ground state bleaching at this waveleng th. In fact, the ground state bleaching in the transients are focused at the low energy end of the ground state absorption. This indicates that the triplet state is dominated by thiophene-based chromophores, even when the concentration of thiophene in the polymer backbone is low. The photoluminescence data (Figure 3-6) are consistent with a model where the singlet is somewhat delocalized and the triple t is highly localized. The room temperature photoluminescence features highly efficient energy transfer from the phenylene-based main chain to the thiophene-based trap sites. At low temperature, a significant portion of energy transfer can be eliminated, thus enab ling observation of the situation when energy transfer between chromophores is incomplete. Thus at 78 K P95T5 and P85T15 each show a mixture of phenylene and thiophene based phosphorescence. This observation fits with expectations, considering Figure 3-10A. On the other hand, the photoluminescence of P75T25 does not change significantly at low temperature. This is expected if P75T25 can be modeled by Figure 3-10B, where every chromophore contains a thi ophene. In this case, all of the energy transfer would be expected to occur within a si ngle chromophore in the singlet st ate, and thus be fast even at low temperatures. This bears out in the results; P75T25 does not exhibit any significant phenylene based emission, even at low temperatures.

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90 Kinetics and mechanism of energy transport Having reached an understanding of the na ture of the excited state, it is also possible to examine the kinetics of ener gy transport. Time resolved emission experiments are useful to this end. Transport at room te mperature proved too fast to observe, thus the most interesting experiment s focused on studying the polymers at 78 K. Furthermore, it is only expected that triple t transport could be obs erved in polymers with a dilute loading of thiophene, P95T5 and P85T15. Transport in P75T25 is expected to occur only in the singlet state fo r reasons explained above. At 78 K, the phenylene phosphorescence decay (Figure 3-9) has two components. The rate of the fast component (Table 3-2) is too slow for singlet energy transfer, which is expected to occur on the picosecond timescale,61 but is clearly much faster than the rate of phosphorescence based on comparison w ith the phosphorescence lifetime of P100T0. It is therefore reasonable to conclude that this fast component of the decay corresponds to energy transfer in the triplet state. If the assignment of the fast decay com ponent at 514 nm to triplet transfer is correct, then this rate constant should increase as the load ing of thiophene increases. This is indeed the case. P85T15 features a thiophene concentr ation 2.3 times greater than P95T5, based on 1H NMR integration. The fast com ponent of the 514 nm emission decay, which corresponds to triplet energy transfer, is 2.6 times faster in P85T15 than in P95T5, which agrees remarkably well with expectati on. When the loading of thiophene is high, as in P75T25, no triplet transport is observed. This is consistent with the notion that transport in this system occurs only in th e singlet for copolymers with high thiophene loadings.

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91 It is important to note th at the slow components of the phenylene emission in P95T5 and P85T15 do not correspond to the actual ra te of phenylene phosphorescence as observed in P100T0. This indicates that while the obs erved rate constants are useful as approximate indicators of the energy transfer rates, they do not correspond to the actual rates of triplet energy transfer.129 Zimmerman et. al. showed that in a well-defined donor-bridge-acceptor system it is possible to obtain the exac t rate constants by mathematical manipulation of the observed rate data.129 However, these polymers are illdefined, and it is therefore impossi ble to treat these results in this fashion. Nonetheless, the observed results are excellent guidelines for obtaining a general understanding of the kinetics of this system. Figure 3-12. Jablonki diagrams representi ng the all-triplet (A) and all-singlet (B) mechanisms for energy transfer. The subscripts P and T indicate phenylene and thiophene based excited states, respectively. Energy transfer in platinum acetylide polymers can occur by two limiting mechanisms. These are depicted in Jablons ki diagrams in Figure 3-11. Figure 3-11A depicts the all-triplet li mit. In this case, th e initial photoexcited st ate intersystem crosses 0 SP 1 SP 0 TP 1 ST 0 TT 0 ST 0 SP 1 S 0 T 1 ST 0 TT 0 ST A B

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92 to a triplet very rapidly with respect to the ra te of energy transport. The resulting triplet then migrates along the chain to a thiophene trap. In Figure 3-11B, the all-singlet mechanism is depicted. In this limit, the initia l photoexcited state migr ates to the trap site forming a thiophene singlet. The singlet then intersystem crosses at thiophene, resulting in a thiophene triplet. The results seem to indicate that both m echanisms are operational. The fact that the P85T15 and P75T25 both show considerable thiophe ne-based fluorescence is a clear indicator that singlet state transport play s an important role. Importantly, this fluorescence is strong even when the excitati on is at a wavelength where thiophene is not expected to absorb. For example, th e photoluminescence specra in Figure 3-6 were all obtained with a 350 nm excitation wave length, where the absorption is dominated by phenylene-based chromophores. This obser vation supports the notion that singletstate energy transfer occurs from an init ially excited phenylene-based chromophore to the thiophene trap sites. However, at 78 K the observed energy tran sfer clearly occurs in the triplet, as discussed above. This leads to the some what surprising conclusion that at room temperature transport occurs primarily in the singlet state, despite the fact that the readily observable optical properties of platinum acetylides are dominated by triplet state photophysics. At 78 K the singlet-st ate pathway is less important, and transport occurs primarily in the triplet state. Conclusions A series of platinum containing PPEs with varying loadings of thiophene were synthesized by the Hagihara coupling methodology. The polymers were characterized by GPC and 1H NMR. 1H NMR confirmed both the relative phenylene and thiophene

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93 content of the polymers, and provided some ea rly hints as to the effects of the electron releasing thiophene groups on nearby phenylene moieties. The triplet excited state of these polymer s is delocalized over approximatly five arylene rings. Rapid and efficient energy tr ansfer from the phenylene based main-chain to the thiophene trap sites results in polymers where th e vast majority of phosphorescence is thiophene-like, even when the thiophene lo ading is less than 5%. This energy transfer is slowed considerably at low temperatur es, resulting in a mixture of phenylene and thiophene based emission in polymers w ith a low level of thiophene doping. Furthermore, the low-temperature phe nylene-based phosphorescence decay can be deconvoluted into two components. The fast component relate to the rates of energy transfer, while the slow component relates to phosphorescence decay. As expected, the rate of energy transfer increases with increased thiophene loading. It is impossible to directly correlate th e experimentally observed decay rates to the actual energy transfer rates.129 However, it is clear that energy transport in the triplet state is important in platinum acetylide polym ers, and that this process is competitive with phosphorescence. Nonetheles s, it is reasonable to concl ude that energy transfer in platinum acetylide polymers occurs in both th e singlet and triplet ex cited states. Energy transfer is fast in the singlet state, and some what slower in the triplet state. The triplet excition diffuses more slowly due to its spat ially confined nature, and because coupling of the chromophore segments by exchange interactions is comparatively weak. Experimental Section General Synthesis and Materials trans-Bis(tributylphosph ine)palladium dichloride (Pd(PPh3)2Cl2) and tetrakis(tributylphosphine)palladium were pur chased from Strem Chemicals and used as

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94 received. THF and 2-metyl tetrahydrofuran were purchased from Acros and distilled from sodium/benzophenone under argon before use. All other reagents and solvents were purchased from either Acros or Aldrich and used as received unless otherwise noted. 1H and 13C spectra were recorded on a Vari an 300 MHz spectrometer. Column chromatography was preformed using silica gel (Merk, 230 400 mesh). Gel permeation chromatography (GPC) was preformed using a Rainin Dynamex model SD-200 solvent delivery system equipped with two PL-Gel 5 Mixed D columns (Polymer Laboratories, Inc., Amherst, MA) and a UV detector set at a wavelength where the polymer under investigation was known to absorb. Poly mer molecular weight information was calculated using Polymer Laboratories software and is reported relative to polystyrene standards. The synthesis and characte rization of 1,4-diethynylbenzene and P100T0 has been discussed previously.87,128 Synthetic Procedures 2,5-Diiodothiophene (1) To a flask containing a mixture of acetic acid and chloroform (14 mL, 3:4) was adde d thiophene (1.00 g, 11.9 mmol) and Niodosuccinamide (NIS) (5.84 g, 24.4 mmol). Th e flask was covered, and the mixture stirred at room temperature for 16 hours. The reaction mixture was then washed with an aqueous solution of sodium thiosulfate (10% ), and pure water. The organic phase was dried over MgSO4 and the solvent was removed under reduced pressure. Column chromatography with hexanes afforded the desired product as a re ddish solid. (yield 2.80g, 70%) 1H NMR (CDCl3, 300MHz) 6.90(s, 2H); 13C NMR (CDCl3, 300 MHz) 76.36, 138.65.

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95 Compound 2 To a flask charged with THF and diiospropylamine (DIPA) (8 mL, 1:1) was added 2,5-diodothiophene (668 m g, 2.0 mmol). The resulting solution was degassed for 30 min by spurging with argon. To this solution was added trimethylsilylacetylene (491 mg, 5 mmol), Pd(PPh3)4 (12 mg, 1.6 mmol), and copper iodide (4 mg, 2 mmol). The reaction was then stirred at room temperature for 24 hours. After this time the solvent was removed under reduced pressure. Flash chromatography of the residue with hexanes gave the desired product as a yellow solid. (yield 440 mg, 80%). 1H NMR (CDCl3, 300MHz) 0.24 (s, 18H), 7.04 (s, 2H); 13C NMR (CDCl3, 300 MHz) -0.23, 96.86, 99.90, 124.47, 132.29. Compound 3 To a flask charged with Pt(PBu3)2Cl2 (501 mg, 0.75 mmol) and 2 (69 mg, 0.25 mmol) in a mixture of toluene and pi peridine (12 mL, 3:1) was added CuI (3 mg) and tetrabutylammonium fluoride ( 0.51 mL, 0.51 mmol). The reaction mixture was deoxygenated with argon spurging for 15 mi n. After stirring for 2 days at room temperature, the solvent was removed under re duced pressure. The resulting residue was stirred in dichloromethane, and the resulti ng suspension filtered. The soluble portion was collected and purifed by flash chromatogra phy with hexanes as the eluent to yield 3 as a yellow solid. (yield 224 mg, 64%). 1H NMR (CDCl3, 300MHz) 0.93 (t, J = 7.20Hz, 36H), 1.38-1.62 (m, 48H), 1.90-2.80 (m, 24H); 13C NMR (CDCl3, 300 MHz) 13.76, 21.90(t, J = 66.30 Hz), 24.24(t, J = 27.9 Hz), 26.01, 88.26, 93.77, 126.84. 31P NMR (CDCl3, 300 MHz) -1.68, 8.04, 17.77. P95T5 An ampoule charged with a mixture of diisopropylamine and toluene (3 mL, 1:5) with a small magnetic stirring bar was f itted with a septum and spurged with argon for five minutes. The ampoule was then subj ected to vacuum for one minute by inserting

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96 a needle connected by hoses to a water aspi rator into the septum. This process was repeated twice, ending with argon-spurgi ng. To the ampoule were rapidly added 3 (14 mg, 0.01 mmol) 1,4-diethynylbenzene (24 mg, 0 .19 mmol) and CuI (2 mg, 0.01 mmol). The solution was degassed by three more argon-vacuum cycles, sealed under vacuum, and stirred at room temperature for 24 hour s. After this time, the ampuole was broken and the solution was poured into 50 mL of methanol to induce precipitation of the polymer as a yellow solid. The desired polymer was collected by filtration and washed with methanol and then water. This so lid was dissolved in a minimal amount of chloroform, precipitated again into methanol and collected by filtration to yield the desired polymer as a yellow solid (yield 96 mg, 70%). 1H NMR (300 MHz, CDCl3) 0.77-1.03 (br, 72H), 1.30-1.72 (br, 96H), 1.86-2.25 (br, 48H), 6.57-6.61 (s, 0.48H), 7.037.11 (d, 3.15H), 7.14-7.32 (m, 11.19H). GPC: Mn = 4,570; Mw = 12,843, PDI= 2.81. P85T15 The same procedure used for P95T5 was employed except the reagents used were as follows: 3 (47 mg, 0.03mmol), Pt(PBu3)2Cl2 (105 mg, 0.16mmol), 1,4diethynylbenzene (24 mg, 0.19mmol), and CuI (2 mg, 0.01 mmol). P85T15 was obtained as a yellow solid (yield 99.10mg, 72%) 1H NMR (300 MHz, CDCl3) 0.76-1.02 (m, 72H), 1.32-1.75 (m, 96H), 1.84-2.25 (m, 48H), 6.58-6.61 (s, 0.67H), 7.04-7.12 (m, 2.53H), 7.18-7.27 (m, 6.22H). GPC: Mn = 3,374; Mw = 11,830; PDI = 3.51. P75T25The same procedure used for P95T5 was employed except the amounts of reagents used were as follows: 3 (70 mg, 0.05 mmol), Pt(PBu3)2Cl2 (67 mg, 0.10 mmol), 1,4-bisethynylbenzene (18 mg, 0.15 mmol) and CuI (2 mg, 0.01 mmol). P75T25 was obtained as a yellow soli d (yield 85 mg, 78%). 1H NMR (300 MHz, CDCl3) 0.81-1.04

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97 (m, 72H), 1.32-1.74 (m, 96H), 1.85-2.24 (m, 48H), 6.58-6.02 (s, 1.53H), 7.04-7.15 (br, 7.22H), 7.18-7.29 (m, 4.96H). GPC: Mn = 28,873; Mw = 105,885; PDI = 3.67. P0T100 The same procedure used for P95T5 was employed except the amounts of reagents used were as follows: 2 (69 mg, 0.25 mmol), Pt(PBu3)2Cl2 (166.67mg, 0.25 mmol), tetrabutylammonium fluoride (1 M solution in THF 0.51 mL) and CuI (1 mg, 0.005 mmol). P0T100 was obtained as a yellow solid (145 mg, 80%) 1H NMR (300 MHz, CDCl3) 0.09 (m, 18 H), 1.50 (m, 27 H), 2.1 (br 12 H), 6.48 (s, 2H) GPC: Mn = 39,132; Mw = 81,187; PDI = 2.08. Photophysical Methods UV-Visable absorption spectra were obtained in 1 cm quartz cuvettes using a Perkin-Elmer 100 Spectrophotometer. Correct ed, steady-state photoluminescence spectra were recorded on a SPEX F-112 fluorescen ce spectrophotometer. Samples were contained in 1 cm x 1 cm quartz cuvettes, and the optical density was adjusted to approximately 0.1 at the excitation waveleng th. All photophysical measurements were carried out in argon degassed te trahydrofuran solutions exce pt for those conducted at 78 K, which were carried out in argon satu rated 2-methyltetrahydrofuran, or where otherwise noted. Emission quantum yiel ds are reported relative to Ru(bpy)3 in water for which = 0.055130 and an appropriate correction wa s applied for the difference in refractive indices of actinometer and sample solvent.108 Fluorescence quenching was carried out by performing a micro-scal e titration in a fluorescence cuvette. Time resoled emission and transient ab sorption experiments were performed by using the third harmonic of a Nd:YAG laser (355 nm, 10 ns fwhm, 10 mJ pulse) as the

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98 excitation source. Regression analysis of the rate data was preformed using SigmaPlot graphing software using built-in functions.

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99 CHAPTER 4 META LINKED PLATINUM CONTAI NING POLY(PHENYLEN E ETHYNYLENE)S Introduction Because many biopolymers, including DNA and proteins, may adopt a helical conformation in solution there has been a sign ificant effort to develop synthetic materials that can mimic the folding and unfolding processe s of natural materials. In some of the first work on conjugated helical systems, the Moore group synthesized a series of phenylene ethynylene oligomers that self assemble into a helix in a poor solvent.124 However, the utility of these materials may be somewhat limited because of their hydrophobicity. Addressing this problem, Ta n and coworkers synthesized two meta linked PPE with ionic pendant groups, making the polymers highly water soluble.49 These materials indeed proved to be have many of the properties of DNA, including helical self-assembly and the ability to accept intercalators. In addition, one of the interesting properties noted in both reports is that wh en meta-linked phenylene ethylnylenes coil into a helical shape, their photoluminescenc e properties cease to correspond to those of single-chain polymers and instead resemble polymer aggregates. This is evidenced by solvent dependant changes to the optical properties of the materials. The ability of meta linked PPEs to fold and unfold is related to their ability to adopt different conformations by rotation about carbon-carbon triple bonds. A four ring segment of a meta-linked oligo(phenylen e ethynylene) (OPE) may adopt two limiting conformations. These are illust rated in Figure 4-1. In 4-1A the oligomer adopts a translike (transoid) conformation, while in 4-1B the conformation is cis-like (cisoid).

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100 Extrapolating from these depictions, it is clear that a polymer that consists of mainly transoid linkages will be essentially a random coil-like material, whereas a polymer consisting primarily of cisoid linkages will collapse into a helical superstructure. Figure 4-1. Transoid (A) and cisoid (B) conf ormations of a tetrameric meta linked OPE. The work described in this chapter explores the effect s of introducing platinum into the meta linkages of a m-PPE. The study aims to determine whether the conformational changes illustrated in Figure 4-1 can still take place when platinum is introduced. Additionally the effects of binding to small molecules will be explored. Finally, because incorporation of platinum allo ws access to the triplet manifold of arylene ethynylenes, it was hoped that a le vel of control over the anti cipated aggregation effects could be exerted in order to allow for the observation of triplet excimer emission. In addition to polymers, small molecule model compounds were considered for comparative purposes. The polymers and m odel compounds under consideration in this study, and their abbreviated nomenclature, are depicted in Figures 4-2. The

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101 nomenclature adopted for these materials is that P or M represent a polymer or model, and O or W signify solubility in organic or water based solvents. Pt O OOC12H25PMe3Me3P n Pt Pt O OOC12H25PMe3Me3P Me3P PMe3P-O M-OPt O OOH PMe3Me3P n Pt Pt O OOH PMe3Me3P Me3P PMe3P-W M-W( ) ) ( Figure 4-2. Polymers and mode ls featured in this study. Results and Discussion Synthesis and Characterization The polymers and model compounds were de signed so that the organic-soluble materials feature ester linkage s that can easily be hydrolyzed to give the water soluble materials. The monomeric base unit in P-O and M-O is compound 3, which was sythesized from 2,5 diiodophenol as depicted in Figure 4-3.

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102 II OH II O O HO II O O C12H25O O O C12H25O H H a bc Reagents and conditions: a) 2-bromoacetic acid, Na2CO3, DMF, reflux 16 hrs. (b) 1. Na2CO3, Water. 2. 1-bromododecane, DMF, reflux. (c) 1. Trimethylsilylacetylene, Pd(PPh3)4, CuI, THF, TEA, RT, 3 hrs. 2. TBAF, dioxane/acetic acid, RT 15 min. 1 2 3 Figure 4-3. Synthesis of monomer 3. The first step of this synthesis involve s substitution of bromine on 2-bromoacetic acid. Care must be taken to exclude air from this reaction to prev ent oxidation of the phenol. Purification of the intermediate acid 1, is accomplished by dissolving the crude material in a minimal amount of aqueous s odium hydroxide and trea ting the solution with activated carbon. After removal of the charco al, the pure product is recovered by adding hydrochloric acid to indu ce precipitation of the desired ma terial as a pure, white solid. Esterification of 1 can be accomplished by heating in dodecanol with catalytic acid and a Dean-Starke trap. However, this method proved inefficient, giving low quality, highly colored material and only moderate yields. It is likely that either 1 or its dodecyl ester decompose at the high temperatures (>1500 C) required for that reaction. To circumvent this problem, the carboxyla te salt was prepared by dissolving 1 in a small amount of water with a slight excess of sodium bicarbonate. After removal of the water by rotory evaporation, the r ecovered sodium carboxylate wa s allowed to react with 1bromododecane in refluxing DMF. The reac tion proceeds remarkably cleanly, with no apparent impurities by TLC analysis. The resu lting ester is particularly susceptible to hydrolysis, an issue that must be taken into account in the remaining synthetic steps.

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103 The final steps in the monomer synt hesis are a Sonogashira coupling of 2 with trimethylsilylacetylyne followed by deprotec tion with tetrabutylammonium fluoride. This Sonogashira reaction is run with triethylamine in stead of the more common diisopropylamine in order to prevent aminolysis of the es ter. Amine catalyzed ester hydrolysis is a serious issue with this r eaction unless strictly anhydrous conditions are enforced. Therefore the amine is mixed with THF prior to the r eaction, and the mixture is freshly distilled from sodium hydride imme diately before use. During the deprotection step this issue is addressed by adding a drop of acetic acid to the reaction mixture. The acid serves to immediately prot onate the acetylide anions th at are created when fluoride acts on the trimethylsilyl protecting group, pr eventing the establishment of a basic environment conducive to ester hydrolysis. Trimethylphosphine was chosen as the liga nd for platinum for this system for a variety of reasons. First, it is small compared to the tri butylphosphine used with the polymers described in Chapter 3. This is important because a stericaly bulky ligand would be anticipated to interfere w ith helix formation in the polymers.51 Additionally, because a hydrophilic polymer was a major part of the objective of this study, using the smallest alkyl groups possible seemed desi rable to minimize the hydrophobicity of the final product. Unlike the polymers in Chapte r 3, which relied on alkylphosphine ligands to provide solubility, the mono mers described in this chap ter have pendant groups on the aryl rings that are sufficient to solublize the final polymers. The model compounds were synthesized as shown in Figure 4-4. Compound 3 was reacted with a mono substituted platinum chlo ride under Hagihara conditions in triethyl amine. As in the synthesis of the monome r, the solvent was distilled from sodium

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104 hydride immediately before use to prevent pr emature hydrolysis. M-O is easily purified by column chromatography. Hydrolysis of M-O can be effected cleanly by sodium hydroxide in 2-methoxyethanol. The hydrolysis product, M-W, is recovered as a pure substance by precip itation in ether. PtCl PMe3+ O O C12H25O H H 2.2 eq Hagihara ConditionsM-O 4PMe3 Figure 4-4. Synthesis of P-O. The specifi c conditions appear in the text and the experimental section. Mass spectroscopy (MS) proved quite valuab le in the characterization of M-W. The spectrum of the positive mode EI MS app ears in Figure 4-5. HRMS showed that MW is isolated as a mixture of the protic acid and the sodium carboxylate salt. In fact, the MS matrix is sodiated upon incorporation of the M-W, as evidenced by a sodiated matrix peak at m/z = 571. Therefore, m/z peaks corr esponding to fragments of both the acid and the sodium carboxylate appear, and as a furthe r complication, many peaks appear as both [M+H] and as [M+Na]. Nevert heless, careful interpretation of the spectrum provides an excellent basis for characterizing the molecule. For example, the peaks centered at m/z = 1139 and 1117 correspond to the sodium carboxylate plus Na+ and H+, respectively, while the peak at m/z = 1094 corresponds to the protonated acid + H+. The large fragmentation peak at m/z = 1072 corres ponds to the decarboxylated acid plus Na+ (or the decarbxylated sodium salt plus H+) while the peak at 1043 corresponds to the fragment resulting from loss of the entire pendant group (-OCH2COOH) + Na+. A final logical fragment appears at m/z = 602. This frag ment corresponds to a molecule that has

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105 undergone decarboxylation and cleavage of one of the aryl-ethynyl bonds. (see Figure 45). In addition, a very small peak co rresponding to trace amounts of potassium carboxylate was also found. All of these as signments are confir med by high resolution mass spectrometry (HRMS), where their isotopic distributions match th e expected results for the assigned fragments. M-W was also characterized by 1H NMR, and this data appears in the experimental section of this chapter. The polymerization to produce P-O was pe rformed by a Hagihara coupling reaction using bis(trimethylphosphine)platinum (II) chloride Again, in order to prevent unwanted hydrolysis or aminolysis, the solvent, a mixt ure of dichloromethane and triethyl amine, was freshly distilled from sodium hydride im mediately before use. The polymer was purified by repeated precipitation in methanol Post-polymerizati on functionalization of P-O afforded P-W. The labile ester groups pr oved a significant adva ntage, as hydrolysis was affected by subjecting the polymer to a solution of tetrabutyl ammonium hydroxide in a dioxane-water mixture ove rnight at room temperature. The resulting polymer was purified by dialysis against millipure water fo r 36 hours. It was stored in an aqueous buffer solution of pH 6.5 to pr event irreversib le aggregation.

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106 Figure 4-5. Mass spectrum of M-W. Solvent Induced Conformational Effects The folding and unfolding of the polymer s used in this study is observable by photophysical methods. Specifically, UV/Vi sible absorption and photoluminescence spectroscopy are useful tools in monitoring this process. Thus, the absorption and photoluminescence spectra of P-O were recorded in mixtures of dichoromethane, a good solvent for the polymer, and hexanes, a poor solvent. It is useful to precede discussion of th e polymers under investig ation here with a brief account of some previous work. M oores group made m-OPEs which showed two bands in their absorption spectra.47,48 One band was assigned to the polymers helical form and the other to the random-coil form. In that work, changing from a good solvent to a poor solvent caused an increase of the os cillator strength of the band associated with the helical conformation, and a concomitant de crease in oscillator strength of the band

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107 associated with the random coil conforma tion. It was also observed that the photoluminescence spectra of these polymers showed less single chain type emission and more excimer or aggregate-like emissi on as the amount of poor solvent increased. Analogous results were also observed in similar systems in the Schanze and Tew groups.49,51 The absorption and photoluminescence results for P-O are shown in Figure 4-6. In pure DCM, the P-O shows a strong band with max = 345 nm with a high-energy (blue) shoulder at 327 nm. As the amount of hexane s increases, the oscillator strength of both bands decreases, and the absorption spectra blue-shift. 300400 Normalized Absorption 400500600700 Normalized PL Intensity P s P s Figure 4-6. UV-Vis absorption (l eft) and photoluminescence (right) spectra of P-O in argon degassed mixtures of DCM and Hex. Solid line 100% DCM; dotted line 75% DCM, 25% Hex; dash-dot lin e 50% DCM, 50% Hex; dashed line 25% DCM, 75% Hex. The photoluminescence spectra also show pronounced changes as the solvent varies. In pure DCM, P-O exhibits two emission bands. The lower energy band is centered at 550 nm. This band is strongl y quenched by oxygen, and has a lifetime of 19 s. On this basis, it is assigned as phos phorescence. The other emission band centered at 400 nm corresponds to fluorescence. As the am ount of hexanes in the solvent mixture

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108 increases, the fluorescence band decreases dram atically in intensity. However, only a slight red-shift is observed, and the fluores cence remains relatively narrow, even when hexanes is the dominant component of the solven t. This is unlike the situation in either Moore or Schanzes materials, which featur ed dramatic spectral broadening and redshifts in the fluorescence with in creasing fraction of poor solvent.49,,51,132 Similar changes are observed in the phosphorescence (5 50 nm) band. As the amount of hexanes in the solvent increases, the intensity, quantum yield, and lifetime of the phosphorescence band all decrease. However, there is no conc omitant change in the shape or wavelength of the phosphorescence band. The results from the absorption spectra are somewhat ambiguous because the size of the spectral changes associated with vari ations in solvent composition is small. However, the changes in phosphorescence are consistent with t hose resulting from aggregation of linear platinum acetylide polymers.133 On this basis, the observed spectral changes are assigned to be the result of an aggregation effect. Based on previous work with meta-linked OPEs and PPEs, it is reasona ble to presume that these results are not due to interchain aggregates, but instead intrachain aggregates caused by the polymer adopting a helical conformation which brings the phenylene rings in close proximity to one another. If this is so, then the polymer adopts its maximum extended conformation in pure DCM, and its most helical confor mation in a mixture of 25% DCM and 75% hexanes. Note that the polymer is insoluble in higher volume fractions of hexanes. Also, it is impossible to determine from this information what fraction of the polymer is in a helical domain and what fraction is in a random coil domain in any particular composition of the solvent.

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109 Similar results are observed for P-W. Abso rption spectra of this polymer appear in Figure 4-7. In this experiment, the solven t composition was varied between methanol and water. Here, the highest os cillator strength is observed when the solvent is an 80:20 mixture of methanol and water. In this so lvent mixture, the absorbance maximum is 325 nm, and there is a blue shoulder at 312 nm As the amount of water in the solvent increases, the oscillator stre ngth of both bands decreases and the spectrum undergoes a modest blue-shift. Additiona lly, the 312 nm shoulder increases in intensity as the amount of water increases. For example, in 80:20 methanol:water, the shoulders intensity is about 75% of that of the main band, while in 20:80 methanol:wat er, the shoulder and main band are of nearly equal intensity. 300400 Normalized Absorbance 100:0 MeOH:Water 80:20 MeOH:Water 6040 MeOH:Water 40:60 MeOH:Water 20:80 MeOH:Water Figure 4-7. Absorbance spectra of P-W in mixtures of methanol and water. In 100% water, the absorption of P-W in wa ter is dependant on pH, as illustrated in Figure 4-8. The spectra in wate r all closely resemble that of the polymer in a mixture of 20% water and 80% methanol (Figure 4-7) in terms of their shape and absorption

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110 maxima, which agrees with expectation if the pol ymer is mostly helical in this solvent. The absorption does not change significantly wh en pH > 7. This reflects that the acid side-groups are deprotonated under these c onditions. As the solvent media becomes more acidic, the intensity of the absorp tion decreases. This change relates to conformation changes induced by protonati on of the acid pendant groups. Wavelength / nm 250300350400450 Normalized Absorption pH = 10 pH = 9 pH = 8 pH = 7 pH = 6 pH = 5 Figure 4-8. Variable pH ab sorption spectra of P-W. These results show that in water the polymer adopts a mo re helical conformation at high pH, when the acid pendant groups are prot onated. This is logical, because when the side groups are deprotonated, columbic repulsion between th e carboxylate anions should tend to force the polymer to adopt a mo re extended conformation. Conversely, protonated acid side groups tend to aggr egate to maximize favorable hydrogen bonding interactions, which would favor a more heli cal conformation. Cons idering all of the solvent effects, P-W adopts a maximum ex tended conformation in a mixture of 80% methanol and 20% water, and its helic ity is maximized in water at low pH.

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111 Figure 4-9 shows photoluminescence spect ra of P-W. The polymer is not phosphorescent, but it does exhibit a fluorescen ce band centered about 415 nm, the exact maximum being somewhat dependant on solvent composition. The most intense fluorescence is observed when the solvent mixtur e is 80% methanol and 20% water. This is consistent with the notion that the polym er adopts its most extended conformation in this solvent mixture. P-W is not emissive in water, regardless of pH. This supports the notion that the polymer is in a mostly helical conformation in water. 500600700 Normalized PL Intensity 80:20 90:10 70:30 100:0 60:40 Figure 4-9. Photluminescence spectra of P-W in methanol:water mixtures. Notably, none of these effects appear when the model compounds M-O and M-W are treated under the same conditions. The models are non-emissive, and their absorption spectra do not show a significant dependence on solvent composition. This observation lends support to the idea that th ese spectral changes are truly th e result of solvent induced conformation effects, and not si mple interchain aggregation.

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112 Chiral Induction Many natural biological polymers, incl uding nucleic acids and proteins are optically active and adopt only one of two possible helical enantiomers. Many examples exist where chirality is induced into an ach iral synthetic polymer by taking advantage of interactions with chiral guest molecules.135-138 A variety of binding interactions including host-guest,135 acid-base,136 hydrogen-bonding,138 and electrostatics137 may be taken advantage of for this purpose. In two separate studies, the Moore132 and Schanze49,75 groups used (-)pinene as a chiral guest to i nduce chirality in helical conjugated oligomers and polymers. In these studies ci rcular dichroism (CD) spectroscopy was used to detect the interaction between the chiral guest and the host molecule. Addition of the chiral guest to the helical form of a conjugate d material results in a significant CD Cotton effect. This indicates that there is preferential binding of the guest to one enantiomeric form of the helix. Figure 4-10 shows the CD spectra of P-W in a solution composed of 60% water and 40% acetonitrile with increasing (-)pinene concentration. Note that the polymer is expected to be in a primarily helical conformation in this solv ent mixture. In the absence of (-)pinene, the achiral polymer gives no CD signal. As the amount of (-)pinene is increased, an induced CD signal is observed, and the strength of the signal increases with increasing concentration of (-)pinene. There are several features of the spectra in Figure 4-10 that are worth addressing. First, the CD signal does not appear in the ch aracteristic bis-signet shape as observed in other studies with similar materials and in DNA.132 This may be because the transition moment for this Pt-containing PAE is not expe cted to be the same as in either DNA or the analogous all-organic m-PAEs. There are examples of other chiral helical materials

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113 that give a similar si gnal with one-sign only.139 The chromophore in this case is likely to have a transition dipole moment that is more alike to that of P-W than to DNA. Wavelength / nm 280300320340360380400 Delta Abs. -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 Figure 4-10. CD spectra of P-W in 60% methanol 40% acetonitrile with varying concentrations of (-)pinene. The concentration of (-)pinene varies from 0 to 6 mM, and the spectra were obtai ned 24 hours after ad dition of pinene. Additionally, note that the op tical density of the polymer solutions used in these experiments were approximately 0.8. Thus, the la rge CD signal implies that nearly all of the polymer chains in the sample adopt one heli cal enantiomer preferentially to the other. One possible explanation for this is that th e difference in binding constants for the two enantiomeric helices to (-)pinene may be very large. If this is so, a solution of P-W and (-)pinene at equilibrium will have a large excess of one helical enantiomer. Watanabe and coworkers demonstrated that helix folding and unfolding in polymers like P-W is often a slow process.139 In order to test this idea, CD spectra of polymer samples were taken ten minutes a nd twenty four hours af ter addition of (-)

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114 pinene to a polymer sample. The results, which appear in Figur e 4-11 show that the intensity of the CD signal is more than 10 times greater when 24 hours has elapsed as compared to the intensity 10 minutes after adding the guest. A dditional waiting did not result in any further increase in the CD signa l, indicating that the system has completely equilibrated after 24 hours. Wavelength / nm 260280300320340360380400 mdeg -80 -60 -40 -20 0 20 40 24 Hours 10 Minutes Figure 4-11. CD spectra of P-W in 60% methanol 40% water with 6 mM added (-)pinene. Spectra were obtained 10 minutes and 24 hours after addition of (-)pinene. Intercalator Binding The CD results clearly show that the pol ymer adopts a helical conformation in poor solvents such as water. Small aromatic and heteroaromatic chromophores are known to intercalate into the helical superstructure of DNA. Figure 4-12 is a depiction P-W in its helical conformation. Careful consideration of the structure of DNA and that of P-W reveals several interesting similarities. Both form helices that include face-to-face staking of aromatic rings, and both have pe ndant anionic groups. Because of these

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115 similarities, it was anticipated that DNA interc alators might also bind to P-W. The fact that Tan et. al. have previously shown that binding of DNA intercal ators to an organic conjugated polyelectrolyte is pos sible in a previous study made this line of investigation seem particularly promising.49 Figure 4-12. A twelve monomer segment of P-W in a helical conformation. Hydrogen atoms and some pendant P-alkyl groups have been omitted for clarity. Ru(bpy)2(dppz)2+, where bpy = 2,2-bipyridine a nd dppz = dipyrido[3,2-a:2c]phenazine (Rudppz) and 9-amino acridine hydrochloride (AA) were used in this study. The structures of these materials appear in Figure 4-12. Rudppz has been extensively used as a DNA probe because its photoluminescence is negligible in water due to quenching associated with hydrogen bonding of wa ter to the phenazine ligand. However, upon binding to DNA, the phenazine nitrogens ar e shielded from solvent interactions and strong photoluminescence results.140,141 This phenomenon is popularly referred to as the light switch effect.

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116 Ru NN N N N N N N Rudppz N NH3 Cl AA Figure 4-13. Structures of Rudppz and AA. It was initially anticipated that 9-anth rylmethylammonium chloride (AMA) would be used as an intercalator for this study. AMA is a well known DNA intercalator that exhibits significant changes in its absorption and phot oluminescence characteristics upon binding to DNA.142 However, the absorption spectrum of AA overlaps with that of P-W, making it difficult to use optical spectroscopy to monitor its intercalation. AA was used instead because it is similar to AMA in both charge and structure, but features lower energy (red-shifted) absorption, which was expe cted to facilitate the use of spectroscopy to study this system. The absorption of AA with added P-W is shown in Figure 4-13. In water (A), where the polymer adopts a primarily helical conformation, AA shows a hypochromic effect as the concentration of polymer increases This effect saturates when the ratio of polymer repeat unit to AA is 10:1, and is si milar to the effect observed when AMA is added to DNA.142 In a mixture of 80% methanol and 20% water (B), addition of the

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117 polymer has no significant eff ect on the spectrum of AA. 400420440 400420440 Normalized Absorption 0 M polymer 20 M polymer 40 M polymer 60 M polymer 80 M polymer 100 M polymer A B Figure 4-14. Absorption spectrum of 10 M AA with added P-W in (A) water (pH = 7, 0.5 mM tris buffer) and (B) methanol/water (80:20). The triplet excited state of AA is acce ssible by laser excitation, and it can be monitored by laser flash photolysis. The effe ct of P-W on the AA triplet decay is shown in Figure 4-14. In the absence of polymer, the observed triplet lifetime of AA in water is 80 s. In the presence of an excess of polymer (PRU:AA >10: 1), the lifetime increases to 100 s. This increase in lifetime represents a shielding of AA from the solvent, which can vibronically couple to the AA excite d state thereby provid ing an additional nonradiative decay pathway. This is an expected result, and it conforms to the behavior of this type of system upon binding to DNA.142,143 As expected, addition of AA to the model compound M-W does not result in any spectral changes.

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118 Time / s 0100200300400 Relative Delta Abs. -2 -1 0 1 = 80 s = 100 s Figure 4-15. Decay of AA triplet absorption in free (dotted line) and intercalated (solid line) AA. The AA triplet-triplet ab sorption was monitored at 450 nm. Intercalation of Rudppz is easily monito red by following its photoluminescence. Figure 4-15 shows that as a solution of R udppz in water is titrated by P-W, its photoluminescence intensity increases. Figur e 4-16 is a plot of the increase in photoluminescence intensity verses the ratio of polymer repeat units to Rudppz in both water and 80:20 methanol:water. This figure sh ows that the light switch effect is not observed when the solvent is an 80:20 mixture of methanol and water. Adding M-W to a Rudppz solution also fails to induce any fl uorescence enhancement. In water, the photoluminescence turn-on effect saturates when the PRU:Ru ratio is 10:1. That the same ratio was found for Rudppz and AA indicates that the generalized PRU:intercalator binding ratio for this polymer is 10:1.

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119 Wavelength / nm 500550600650700750800 Emission Intensity / counts 0 5e+4 1e+5 2e+5 2e+5 Figure 4-16. Titration of Rudppz with P-W. The concentration of Rudppz was 10 M and the polymer concentration varied from 0 M 100 M. The solvent was water with 0.5 mM tris buffer at pH = 7. The 10:1 PRU:intercalator ratio is notable, because one turn of the polymer helix consists of only 6 PRUs. Thus, the polymer binds less than on inte rcalator per turn. More insight into this is given in Figure 4-16. The plot in figure 4-16 which corresponds to the titration in water has two distinct regi ons. At low ratios of [PRU]:[Ru], the graph is linear, with a moderate slope. At highe r ratios, the slope of the graph increases dramatically, until leveling off at 10:1 [PRU]: [Ru]. The inflection point occurs at 6:1 [PRU]:[Ru], corresponding to one biding site pe r turn of the helix. Thus, there are likely two modes of binding in this system. One m ode likely corresponds to the situation when there is insufficient polymer to bind all of th e intercalator. When the number of polymer turns is in excess of the amount of intercalat or, a different binding mode is in effect.

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120 [PRU]/[Ru] 02468101214 Emission Augmentation / per cent 0 50 100 150 200 250 300 Figure 4-17. Photoluminescence augmentation of Rudppz at 650 nm with addition of P-W in water (circles) and 80:20 methanol:water (triangles). It is also notable that ne ither Rudppz nor AA show evidence of intercalation when the solvent is 80:20 methanol:water. This is a strong indication that in this solvent mixture the polymer adopts a completely random-coil conformation, with no helical domains. However, it is not possible to determine if the polymers conformation in water consists of completely helical domain s or if it includes some random coil domains as well. Conclusions Two polymers, P-O and P-W, featuring me ta linked Pt-acetylide backbones, were synthesized and characterized. Solvent dependant absorption and photoluminescence spectroscopy revealed that the polymers may adopt eith er a helical or random coil conformation, depending on the solvent composition.

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121 Because of its helical shape and hydrophi licity, P-W is able to bind to small hetroaromatic molecules that are known to intercalate into helices of DNA, specifically Rudppz and AA. Because the spectral ch anges associated with binding of these molecules to P-W are identical to those associ ated with their intercalation into DNA, it is reasonable to conclude that AA and Rudppz bind to P-W by intercalation as well. The binding stoichiometry is greater than one intercalator per turn of the helix. That these effects are not observed when the model comp ound M-W is substituted for P-W confirms that the spectroscopic changes are the result of binding to the polymer helix and not a simple interaction with individual monomeric units. Additionally, P-W exhibits strong indu ced chirality upon binding to the (-)pinene. This is evidenced by a strong induced CD signal of the bound polymer. Binding is a slow process, and the samples used in these experiments required at 24 hours to equilibrate and give the maximu m possible induced CD signal. Experimental Section Materials and General Synthesis THF was purchased from Acros and distil led over sodium/benzophenone prior to use. Tetrakis(triphenylphosphine)palladium was purchased from Strem and used as received. All other materials were purchased from either Acros or Aldrich and used as received except where noted. The synthesis of 4 and 3,5-diiodophenol have been described previously.44,144 Chromatography was carried out using silica gel (Merk, 230400 mesh). NMR spectra were obtained on a Varian Gemini, VXR, or Mercury spectrometer operating at 300 MHZ, or an Inova spectrometer operating at 500 MHZ. GPC was preformed using a Rainin Dynama x model SD-200 solvent delivery system equipped wth two PL-Gel 5 micron Mixed D columns (Polymer Laboratories, Inc.,

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122 Amherst, MA) and a UV detector set at a wavelength where the polymer absorbs. Molecular weight information was calculate d from the chromatograms using Polymer Laboratories software. Synthetic Procedures Compound 1. To an erlenmeyer flask with a gr ound glass top and a magnetic stir bar was added sodium carbonate (7.6 g, 72 mmol), and -bromoacetic acid (1.8 g, 13 mmol) and DMF (40 mL). A condenser was fitt ed to the top of the flask, and the system was heated gently to dissolve most of th e sodium carbonate, and then sealed with a septum on top of the condenser and de gassed by argon spurging for 20 minutes. 3,5 Diiodophenol (5 g, 15 mmol) was added quick ly, and the solution was degassed by spurging for another 15 minutes. The soluti on was heated to a gentle reflux under a positive pressure of argon with strong stirri ng for 12 hours. After this time, the solution was cooled to room temperature and the remaining carbonate ne utralized by careful addition of dilute hydrochloric acid. The re sulting solution was then poured into 400 mL of water, and precipitation of the crude product was induced by dropwise addition of 6 N hydrochloric acid. The tan solid was collected by filtration, dried, and redissolved in a minimal amount of 1 N sodium hydroxide. Th is yellowish alkaline solution was treated with activated carbon and filtered. The clea r supernatant was treated with dropwise addition of 6N hydrochloric acid to induce precipitation of the pure product as a white solid (yield, 2.45 g, 43 %). 1H NMR (DMSO, d6) 7.53 (s, 1H), 7.25 (s, 2H), 4.78 (s, 2H). 13C NMR 170.4, 159.5, 137.8, 123.8, 96.7, 65.3. Compound 2. To a 100 mL round bottom flask was added 1 (2 g, 5 mmol), sodium carbonate (600 mg, 612 mmol) and wate r (25 mL). The mixture was heated gently until the acid was completely dissolv ed. The water was then evaporated under

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123 reduced pressure, and the residue was redi ssolved in DMF (40 mL). 1-Bromododecane (1.6 g, 6 mmol) was added and the solution was heated to reflux. The solution was maintained at reflux until TLC indicated that the acid was completely consumed (the acid remains on the baseline of our silica TLC pl ates when the eluent is 1:1 hexane to dichloromethane). The solution was then c ooled to room temper ature and poured into 300 mL of ice-water to induce precipitation of the product as an off-white solid. The product was collected by filtration, dried under high vacuum at 400 C to remove any remaining alkyl bromide, and recrystalized from methanol-water to give the desired product as a white solid (yield, 3.1 g, 91%). 1H NMR (CDCl3) 7.67 (s, 1 H), 7.24 (s, 2 H), 4.49 (s, 2 H), 4.21 (t, 2 H), 1.54 (br, 2 H), 1.21 (br, 18 H), 0.80 (t, 3 H). Compound 3. To a freshly distilled, anhydr ous solution of THF (10 mL) and diiospropylamine (5 mL) was added 2 (1.5 g, 2.6 mmol). The resulting solution was sealed with a septum and spurged with argon for twenty minutes. Excess trimethylsilyl acetylene (3 mL), and catalytic amounts of te tra(tributylphosphine) palladium (2 mg) and copper (I) iodide (2 mg) were added quickl y. A voluminous white precipitate formed almost immediately. The solution was spurge d with argon for another five minutes, and then kept under a positive pr essure of argon. The reaction mixture was stirred at room temperature for three hours. During this ti me, the mixture turned black. After three hours, the reaction mixture was filtered, solids discarded, and the supernatant was removed by evaporation. The residue was th en dissolved in hot hexanes (30 mL) and eluted through a short (approx. 7 cm x 3 cm) pl ug of silica with diethyl ether. The solvent was then removed, leaving a yellow oil. Th e oil was dissolved in dioxane (10 mL) and 1 mL of 50% aqueous acetic acid was added. This solution was spurged with argon for ten

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124 minutes, and then a solution of tetrabutyl am monium fluoride in THF (1 M, 5 mL), was added through the septum. The resulting solu tion was stirred at room temperature for fifteen minutes, and then poured into ice cold water to preci pitate the crude product as an off white solid. The product required fu rther purification by column chromatography with a mixture of hexanes and diethyl ether ( 15:1) as the eluent to give the pure product as a white solid (yield, 660 mg, 63%). 1H NMR (CDCl3) 7.25 (s, 1 H) 7.05 (s, 2 H), 4.62 (s, 2 H), 4.20 (t, 3 H), 3.05 (s, 2 H), 1.63 (br, 2 H), 1.25 (br, 18 H), 0.84 (t, 3 H). 13C NMR 168.7, 158.6, 129.5, 123.8, 119.1, 82.5, 78.3, 65.9, 65.6, 32.1, 29.9, 29.9, 29.8, 29.7, 29.6, 29.5, 29.4, 28.7, 26.0, 22.9, 14.4. HRMS (EI, positive mode) calc. [M+H] 368.2351, found [M+H] 368.2351. P-O. To a solution of dichloromethane and diisopropyl amine (2 mL, 3:1) that was freshly distilled over sodium hydride was added 3 (150 mg, 0.4 mmol) and bis(trimethylphosphine) platinum (II) chloride (170 mg, 0.4 mmol). The reaction mixture was spurged for 15 minutes with argon, and le ft under a positive pressure of argon while a catalytic amount (ca. 1 mg) of copper (I) iodide was added. The reaction mixture was heterogeneous at this point. After stirring at room temperature for thirty minutes, the reaction mixture became homogenous. The reacti on was allowed to proceed for a total of five hours before it was quenched by precip itating the polymer in a large volume of methanol to which a few drops of acetic acid had been added. The product was recovered by filtration, redissolved in dich loromethane before it dried completely, and precipitated again in methanol with a few dr ops of acetic acid. The solid, pale yellow polymer was recovered by filtration (y ield, 205 mg, 77%) GPC Mn = 9,400, Mw = 12,800 PDI = 1.36. 1H NMR (CDCl3) 6.90 (s, 1 H), 6.83 (s, 2 H), 4.58 (s, 2 H), 4.11

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125 (t, 2 H ), 1.89 (br, 18 H), 1.22 (br, 18 H), 0.83 (t, 3 H). 13C NMR (CDCl3) 169.2, 157.4, 128.8, 127.9, 65.7, 65.6, 32.1, 29.9, 29.8, 29.7, 29.6, 29.4, 28.7, 26.0, 22.9, 19.4, 16.3, 15.9, 15.7, 15.3, 14.4. P-W. To 30 mL of dioxane was added P-O (150 mg). To this solution was added tetrabutylammonium hydroxide (3 mL, 1 M solution in methanol). The solution was stirred for thirty minutes, after which it became inhomogeneous. Water (2 mL) was added to redissolve the precipitate, and th e solution was stirred overnight at room temperature. After this time, the solution was poured into 300 mL of cold diethyl ether and stirred vigorously. Methanol was adde d dropwise to induce precipitation of the polymer as a waxy, yellow solid. The solid polymer was collected by filtration and redissoled in 50 mL of millipure water in a dialysis bag (10 kD, cellulose). The polymer solution was dialyzed for thirty six hours against millipure water, and the water was changed every twelve hours during the dialysis After dialysis, th e pH of the solution was adjusted to approximately 6.5 by adding a few drops of a dilute aqueous sodium bicarbonate solution. The polymer solution was then filtered through a 0.45 filter, and stored in solution. The yield of the reacti on was determined by diluting the final solution to a known volume, and determining the con centration of the solution by gravimetric analysis after drying a liquots of known volume (yield, 95 mg, 90 %). M-O. A solution of THF and triethylamine (5 mL, 3:1) was freshly distilled from sodium hydride. To this solution was added 1 (30 mg, 0.083 mmol) and 3 (100 mg, 0.1 mmol). The resulting solution was spurged with argon for 20 minutes, and a catalytic amount of CuI (ca. 2 mg) was added. The solution was the spurged with argon for an additional 10 minutes. After this time, th e solution became homogeneous and turned a

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126 greenish color. The reaction was stirred at room temperature for 6 hours, after which time the solvent was removed by rotory-evaporation. Chromatography with hexanes:ethyl acetate (4:1) as the eluent afforded the pure product as a white solid. (yield, 45 mg, 43%). 1H NMR (CDCl3) 7.31 (d, 4 H), 7.22 (t, 4 H), 7.15 (d, 2 H), 6.97 (s, 1 H), 6.70 (s, 2 H), 4.55 (s, 2 H), 4.18 (t, 2 H), 1.67 (br, 36 H) 1.25 (br, 20 H) 0.08 (t, 3 H). M-W. To a mixture of 2-methoxy methanol (3 mL) and two drops of millipure water was added M-O (45 mg, 0.035 mmol ) and NaOH (4 mg, 0.1 mmol). The heterogeneous reaction mixture was stirred at room temperature under positive pressure of argon for 30 min. After this time, anothe r 1 mg of NaOH was added, and the reaction was allowed to stir for an additional 30 mi n. The reaction mixture was homogeneous at this point. Diethyl ether (20 mL) was then a dded to induce precipitat ion of the product as a fine white powder. The mixture was cooled to 00 C for three hours and the product was recovered as a fine white pow der after centerfugation of th e mixture and decanting the solvent (yield, 37 mg, 96%). 1H NMR (CD3OD) 7.31 (m, 10 H), 6.82 (s, 1 H), 6.75 (s, 1 H), 4.30 (s, 2 H), 1.78 (t, 36 H). HRMS (EI, positive mode) calc. [M+H] 1117.2912, [M+H] 1117.2912. Photophysical Methods The water used in the experiments was prepared in a Millipure Milli-Q plus purification system and had a re sistivity of not less than 18 M Experiments in water were carried out in 0.5 mM tris(hydroxymethyl )aminomethane (tris) buffer adjusted to the specified pH by addition of appropriate am ounts of acetic acid or sodium carbonate. When not specified the pH was 7. UV-Visib le absorption spectra were obtained on a Varian Carey 100 spectrophotometer. Correct ed steady-state emission spectra were

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127 recorded on a SPEX F-112 flourescence spectr ophotometer. Samples were contained in 1 cm x 1 cm quartz cuvettes, and the optical density was adjusted to approximately 0.1 at the excitation wavelength. Em ission quantum yields are reported relative to Ru(bpy)3 in water for which = 0.055130, and an appropriate correcti on was applied to account for the differences in refractive indices of the actinometer and sample solvents.131 Circular dichroism (CD) experiments were obtai ned on an AVIV-202 circular dichroism spectrometer. Samples were prepared in a 1:1 mixture of methanol and acetonitrile, and the optical density of the polymer was adju sted to approximately 0.8 at its absorption maximum. An appropriate amount of (-)pinene was added, and the solution was stirred vigorously for 10 minutes and then allowed to sit for 24 hours before measurements, except when otherwise noted.

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128 CHAPTER 5 CONCLUSIONS In the preceding chapters, the synthe sis, characterization, and photophysical behavior of several PAEs was described. Organic and organometallic PAEs were studied, and the investigations included both linear rigid-r od materials and polymers that are able to vary their conformation in solution. Optical spectroscopy, including UV/Visible absorption, steadystate and time-resolved photoluminescence, and timeresolved transient absorption were used to probe the properties of these materials. Ultrafast pulse radiolysis at Brookhaven National Laborat orys LEAF proved a useful supplement to these techniques. Carrier Transport Single chain poly(arylene ethynylene)s live up to their popular moniker molecular wires. These materials exhibit extraordinar ily fast and efficient intrachain transport properties with respect to electr ons (or holes), singlet and triple t excitions. This transport is largely due to the extensive orbital mixi ng between adjacent monomer units in PAEs. As a consequence, moieties designed to trap holes or excitons have a tendency to interact with the polymer main chain forming delocaliz ed low energy regions instead of localized trap-sites. In addition, the polydispersit y inherent in PAEs furthe r complicate attempts to measure carrier transport rates in these materials. Nonetheless, it was determined that the rate of hole transport in a single chai n conjugated polymer is at least 1 x 108 s-1. Triplet transport is much slower, occurring on timescales competitive with phosphorescence,

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129 with rates of the order 106 s-1. Importantly, even when th e observable optical properties of a PAE are dominated by the triplet manifold, energy transfer in the singlet state is still an important process. Helical Self-Assembly Two meta-linked PAEs featuring platinum in their backbone were designed and synthesized. In good solvents, P-O and P-W ex ist as random coils, but in poor solvents they self-assemble into helices. The helical conformation is confirmed by using a chiral guest molecule to bind to P-W and induce pr eferential formation of one of two possible enantiomeric helices. Because it is hydrophili c, P-W has the ability to bind to small heteroaromatic molecules much in the sa me way that DNA does. Binding of AA and Rudppz only occurs in solvent systems wh ere P-W adopts a helical conformation, and results in predictable spectral changes. Additionally, P-W can bind to the chiral guest molecule (-)pinene. This interacti on results in P-W preferenti ally adopting one of the two possible enantiomeric helices, as ev idenced by an induced CD signal. Future Outlook Conjugated polymers in general and PAEs in particular have a bright future as materials for a variety of applications. Many of the fundamental properties of these materials have been reported in this dissert ation. However, there is still work to be accomplished if scientists are to gain a more complete understanding of these systems. For example, direct observation of hole transport in PAEs is still impossible because the rate of hole inject ion in invariably slower than the rate of hole migration. This points to the need for new methods of ch arge injection that are significantly faster than those currently available.

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130 With regards to triplet transport, it is impossible to directly attribute the rate constants determined experimentally to the carrier migration rates. Because the distribution of thiophene in the polymers is random, and the polymers are not monodisperse, the distance between the exciton and the trap is not identical for every exciton. The experiments therefore represent th e average of several different distances of energy migration. One possible solution to this problem might be th e synthesis of donorbridge-acceptor type molecules where the dono rs triplet energy is higher than that of diethynylbenzene, the bridge is a phenylene-based Pt-acetylide, and the acceptor is thiophene or another moiety with a low-energy tr iplet state. If a series of these molecules with different bridge lengths were synt hesized, time resolved emission experiments would likely yield defini tive rate information. Finally, the helical, Pt-acetyl ide polymer seems to be so mewhat less ordered than corresponding all-organic analogues. This is li kely due to steric e ffects related to the large trialkyl phospine ligands on the metal. Development of a new ligand system that avoids the use of bulky groups might lead to materials with more ordered conformational transitions.

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131 REFERENCES AND NOTES 1. Shirakawa, H.; Louis, E. J.; MacDiarmi d, A. G.; Chiang, C. K.; Heeger, A. J. Chem. Commun. 1977, 578. 2. Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 402. 3. Yang, Y.; Pei, Q. B. App. Phys. Lett. 1996, 68, 2608. 4. Brabee, C. J.; Sariciftci, N. S.; Humm elen, J. C. Adv. Funct. Mater. 2001, 15. 5. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270. 6. Halls, J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. V. Nature 1995, 376, 498. 7. McGehee, M. D.; Heeger, A. J. Adv. Mater. 2000, 12, 1655. 8. Reynolds, J. R.; Epstein, A. J. Adv. Mater. 2000, 12, 1565. 9. Redidinger, J. L.; Reynolds, J. R. Advances in Polymer Science 1999, 145, 57. 10. Scherf, U. Top. Curr. Chem. 1999, 201, 163. 11. Wang, S.; Oldham, W. J.; Hudack, R. A.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 5695. 12. Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. 13. Huang, S. L.; Tour, J. M. J. Am. Chem. Soc. 1999, 121, 4908. 14. Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. 15. Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446. 16. Pinto, M. R.; Kristal, B. M.; Schanze, K. S. Langmuir 2003, 19. 17. Swager, T. M. Acc. Chem. Res. 1998, 31, 201. 18. Ley, K. D.; Li, Y.; Johnson, J. V.; Powell, D. H.; Schanze, K. S. J. Chem. Soc. Chem. Commun. 1999, 2777. 19. Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.

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139 BIOGRAPHICAL SKETCH Eric Ely Silverman was born in Bronx, New York, and grew up in Thiells and Pomona, two small hamlets in New Yorks lo wer Hudson River Valley. After graduation from North Rockland High School in 1996, Er ic moved to Pittsburgh, Pennsylvania, where he received a Bachelor of Science in chemistry from Carnegie Mellon University in 2000. After receiving his undergraduate degree, he continued immediately on to graduate work at the University of Florida, where he began the pursuit a doctorate in chemistry. After graduation in May, 2005, Eric will begin his profe ssional career as a Patent Examiner in the United States Pa tent and Trademark Office in Alexandria, Virginia.


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PHOTOPHYSICS OF SINGLE CHAIN POLY(ARYLENE ETHYNYLENE)S


By

ERIC ELY SILVERMAN
















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005




























Copyright 2005

by

Eric Ely Silverman




































For Jen.















ACKNOWLEDGMENTS

My work at the University of Florida would not have been possible without the

advice, support, and friendship of the many people I have interacted with during my time

here. First, I thank my advisor, Kirk Schanze. His advice, support, and seemingly

unending patience have been invaluable to my achievements over the past five years.

I further acknowledge the contribution of many people who have contributed both

to the work I have accomplished and to making my time in graduate school more

enjoyable. Ksenija Haskins-Glusac, Boris Kristal, and Kye-Young Kim have all been

both great friends and coworkers. I thank the many current and former coworkers with

whom I have shared conversation and insights over the years. I would like to specifically

point out Mauricio Pinto and Xiaoming Zhao for their contributions to my work. I also

acknowledge the contributions of my collaborators at Brookhaven National Laboratories,

John Miller and Alison Funston. More thanks are due to my friends Aleksa and Ilka

Jovanavic and Cris Dancel. A special thank you goes to Janice Young.

Finally, I am grateful to my family for all of their love. My parents have offered

me unending support throughout my graduate school years. I thank my grandmother for

her kindness and wisdom, and also my grandfather, even though he is no longer here to

share in my completion of this work. I am grateful for all of the love shared with my

sisters Rochelle and Lisa. Last, I thank my brother Adam who never ceases to share his

love, understanding, advice, and support.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TA BLE S ...... .... .................... ........ ..... .. .. ...... ........ ....... viii

LIST OF FIGURES ......... ............................... ........ ............ ix

ABSTRACT ........ ........................... .. ...... .......... .......... xii

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

C onjugated Polym ers: B background ................................... ................... ...................
Poly(Phenylene Ethynylene)s.................................................. 2
Applications of Poly(phenylene ethynylene)s........................... ..............2
Synthesis of Poly(paraphenylene ethynylene)s........................... ...................6
Platinum Containing Poly(phenylene ethynylene)s: Platinum Polyynes ............10
Meta Linked Poly(phenylene ethynylene)s ...............................................12
Conjugated Polymers as M olecular W ires ...................................... ............... 15
Single Chain Conjugated Polym ers ............... ............................................. 15
C charge Transport ........................................................ .. ............ 18
C charge Injection ....................................................................2 1
D description of the Present Study .................................................................... ...... 22

2 RADICAL TRANSPORT IN END-CAPPED POLY(ARYLENE
ETHYNYLENE)S ........................................................................... ...... 25

In tro d u ctio n .......................................................................................2 5
R results and D discussion ..... ........... .............. .......... ........... ............ .... .. ......... .. 26
Synthesis and Structural Characterization ofPoly(arylene ethynene)s..............26
Absorption and Fluorescence Spectroscopy ................ ................ .................. 35
Phenylene based polym ers ........................................ ........ ............... 35
Biphenylene based polymers................. ... ............................40
Radiolytic production of cation and anion radicals.............................. 41
Electron and hole transport to polymers following radiolysis ...................43
Semi-empirical calculations: spectroscopy of PPE-based ion radicals........46
Visible/Near-IR Spectroscopy of Charged PAEs....... ...............................47
R radical an ion s ............................................... ................ 4 7









R radical cations ............. .... .... ........... ........ ...... .... .. ... .. .. ............ 50
Bimolecular Hole Transfer Reactions: Thermodynamics of Intrachain PPE -
T 3 H ole T transfer ................. .............. ................................ .... .......... 53
Dynamics of Interchain Hole Transfer..... .......... ...................................... 57
C o n c lu sio n s........................................................................................................... 5 7
E xperim ental Section ........... ....... .................................... .............. .. .... ...... 59
M materials and G general Synthesis ........................................ ...... ............... 59
Synthetic P procedures ........................ .. ....................... .... .. ........... 59
Photophysical M methods ................................................... ........................ 64
Generation of the T3 and Ph-T3 Radical Cations ............................................65
R radiation Techniques ........................ .................... .............. .. ........... 65

3 MECHANISM AND DYNAMICS OF TRIPLET TRANSPORT IN PLATINUM
CONTAINING POLY (PHENYLENE ETHYNYLENE)S.................................67

Introdu action ............... ................ .. ......... ................ ................. 67
R results and D discussion ............. ................................................................... 69
Polymer Synthesis and Structural Characterization ........................................69
Photophysical M esurem ents...................................................... ..... .......... 74
Absorption and emission spectroscopy ................................. ............... 74
Phosphorescence quenching..................................... ......... ............... 79
Transient absorption spectroscopy ............................................................81
Tim e resolved em ission.......................................... ........... ............... 82
G general D discussion .................. .............................. ............. ... 85
Electronic m odel ............ .................... .......... .......... .... ........ ......85
Kinetics and mechanism of energy transport.............................................90
C o n c lu sio n s........................................................................................................... 9 2
Experimental Section................. .. ....................................... 93
G general Synthesis and M materials ........................................ ...... ............... 93
Synthetic P procedures ........................ .. ....................... .... .. ........... 94
Photophysical M ethods ............................................... ............................ 97

4 META LINKED PLATINUM CONTAINING POLY(PHENYLENE
ETHYNYLENE)S .................................... .. .......... .. ............99

Introdu action ...................................... ................................................. 99
Results and D discussion ............................................... ..... ..... .......... .... 101
Synthesis and Characterization ...................................... ............... 101
Solvent Induced Conformational Effects .............................106
Chiral Induction .................. ...................... ............ ...... ........ .. 112
Intercalator B inding................................................ ......... .. ........ .. 114
C o n c lu sio n s......................................................................................................... 12 0
E xperim mental Section ...................................................... ............... ............... 12 1
Materials and General Synthesis .............. ..................... ..........121
Synthetic Procedures ....................................................... ................... 122
Photophysical M ethods ........................................ ......................... 126









5 C O N C L U SIO N S ................................ ........................ .............. ..... .......... 128

Carrier Transport .......................... ...... .... .. .. .. ........ ....... .. 128
H elical Self-A ssem bly .............................................................. .. .................. 129
Future O utlook................................................. 129

REFERENCES AND NOTES ........................................................... ............... 131

BIOGRAPHICAL SKETCH ............................................................ ........139
















LIST OF TABLES


Table page

2-1 Conditions used in end-capping polymerizations. ................................................34

2-2 Selected photophysical data for PAE's ........................................ ............... 35

2-3 Reaction rate constants of poly-(arylene ethynylene)s (PAEs) and terthiophene
end-capped PAEs with positive and negative charge carriers in solution ...............45

2-3 Bimolecular rate and thermodynamic data for reactions of PAE's with thiophene
o lig o m e rs ......................................................................... 5 4

3-1 Summarized absorption and photoluminescence data. .........................................75
















LIST OF FIGURES


Figure p

1-1. Examples of conjugated polymer structures. ........................................ ..................2

1-2. Schematic of Swager's pentiptycene containing PPE......................... .............3

1-3. LCD displaysconstructed with a PPE as the luminescent layer, and a
com m ercially available display.......................................... ........... ............... 4

1-4. Chemical sensors functioning independently ......................................................4

1-5. Chemical sensors functioning in series. ........................................ ............... 5

1-6. Flourescence assays for enzyme activity using a PPE as the fluorophore. .............

1-7. Mechanism of the Sonogashira Reaction. .........................................................8

1-8. General structure of a platinum containing PPE. ................................ ............... 10

1-9. Absorption and emission spectra of oligo platinum PPEs with increasing chain
length ......................................................................... ............... 11

1-10. Oligomeric m-phenylene ethynylenes......................... ......................... 12

1-11. A segment of the meta-para PPE used by Tan................................. ... ............... 13

1-12. Chiral induction in a helical polymer ............................ ..... ... ............... 14

1-13. A conjugated oligomer in use as a molecular wire ...............................................16

1-14. Anthracene end-capped polym er................................................ ............... 17

1-15. A metal binding conjugated polym er.............................. ........ ..... ................ 18

1-16. M EH-PPV with saturated conjugation breaks. .............................. ......... .......20

2-1. Structures of polymers and model compounds ................................ ............... 27

2-2. Synthesis of polymers and model.................. .............. ... ............... 28

2-3. NMR of T3PPE13 with 2-ethylhexyloxy side groups. ............................................32









2-4. NMR of T3 end-capped PPE with undecyloxy side groups. ...................................32

2-5. Expansion of the aromatic region of 2-4..............................................33

2-6. Absorption and photoluminescence spectra of poly(phenylene ethynylene)s
and end group m odel com pounds....................................... .......................... 36

2-8. Absorption and emission of BpE21 and T3BpE12 ......................................... 41

2-9. Schematic representation of the LEAF facility. ...................................................42

2-10. Transition energies computed for radical cations of phenylene ethynylene
oligom ers as a function of length ........................................ ......................... 47

2-11. Spectra of cations and anions of poly(phenylene ethynylenes) .............................48

2-12. Differential absorbance spectra ofPh- T3*+ and T3.+....................................51

2-13. A segment of a PPE featuring seven phenyl ethynyl rings at the left and a
terthiophene end-cap at the right. ........................................ ......................... 52

2-14. Schematic of the oxidation potentials of various PAE's and thiophene
oligomers ............... ............ ...... .................................... 55

2-15. Absorbance as a function of time after an electron pulse for solutions of PPE164
and varied concentrations of terthiophene .................................... ......... ......... 56

3-1. Structure of polymers featured in this chapter. .....................................................68

3-2. Synthesis of intermediate 3 and PoT oo. ...................................... ............... 70

3-3. Synthesis of platinum containing phenylene-thiophene copolymers....................71

3-4. Aromatic region of the polymers H NMR spectra .............................................73

3-5. Absorption spectra of polymers in THF solution............... .................. ...........75

3-6. Photoluminescence spectra of polymers ............................................ .............77

3-7. Stern-Volmer quenching of P95T5 in THF solution ............................................ 80

3-8. Triplet-state transient absorption spectra ...................................... ............... 82

3-9. Time resolved emission spectrum of P95T5 .................................... ................... 83

3-10. Normalized emission decay of P95T5 at 78 K .................. .............................. 84

3-11. Schematic representation of platinum acetylide polymers ...................................... 87









3-12. Jablonki diagrams representing the all-triplet and all-singlet mechanisms for
energy transfer. ....................................................................... 91

4-1. Transoid and cisoid conformations of a tetrameric meta linked OPE.................00

4-2. Polym ers and m odels featured in this study......................................................... 101

4-3. Synthesis of monomer 3. ............. ...... ... .................................. 102

4-4. Synthesis of P-O .......... ........................................................... 104

4-5. M ass spectrum of M -W ........................................................................... ...... 106

4-6. UV-Vis absorption and photoluminescence spectra of P-O in mixtures of
D CM and H ex. ......................................................................107

4-7. Absorbance spectra of P-W in mixtures of methanol and water............................109

4-8. Variable pH absorption spectra of P-W .............................................................. 110

4-9. Photluminescence spectra of P-W in methanol:water mixtures............................ 111

4-10. CD spectra of P-W with varying concentrations of (-)-a pinene. ........................ 13

4-11. Time dependant CD spectra of P-W with added (-)-a pinene.......................... 114

4-12. A twelve monomer segment of P-W in a helical conformation............................. 115

4-13. Structures of Rudppz and AA ............... ....... .............................................. 116

4-14. Absorption spectra of AA with added P-W ...................................................117

4-15. Decay of AA triplet absorption in free and intercalated AA ..............................18

4-16. Titration of R udppz w ith P-W ......................................................... .....................119

4-17. Photoluminescence augmentation of Rudppz with addition of P-W .....................120















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

PHOTOPHYSICS OF SINGLE CHAIN POLY (ARYLENE ETHYNYLENE)S


By

Eric Ely Silverman

May 2005

Chair: Kirk S. Schanze
Major Department: Chemistry

This dissertation presents the results of an investigation into the photophysical

properties of single conjugated polymers. The study focuses on the properties of

polymers as single-chain "molecular wires" rather than as bulk materials. The

dissertation focuses on the design and synthesis of these materials and the use of a variety

of techniques to probe their unique properties. The primary focus of the photophysical

investigations is the use of optical techniques, although radiolytic techniques were also

applied where appropriate.

All of the polymers under investigation were demonstrated to show rapid and

efficient transport of singlet, poleron, and triplet carriers. In order to study these

properties, polymers with incorporated carrier traps were synthesized. These traps were

either randomly distributed throughout the polymer chain or appended to the polymers as

end-caps.









Helical, organometallic polymers were also synthesized and investigated. A

hydrophobic polymer with ester pendant groups was subjected to post-polymerization

functionalization to give a hydrophilic material with pendant acid groups. This polymer

has many similarities to DNA in that it is a polyanion that readily adopts a helical

conformation characterized by face-to-face stacking of aromatic moieties. This polymer

accepts many well-known DNA intercalators which show characteristic spectral changes

upon binding. Additionally, binding of the polymer to a chiral guest induces the

preferential formation of one of the two possible enantiomeric helices, as evidenced by

circular dichroism spectroscopy.














CHAPTER 1
INTRODUCTION

Conjugated Polymers: Background

Conjugated polymers are unsaturated materials in which the entire polymer

backbone consists of atoms with a continuous 7t orbital system. In 1977, it was reported

that polyacetylene can exhibit significant conductivity when it is oxidized or reduced.1

The field was brought to the forefront of scientific and public attention in 2000, when the

principal discoverers of this property, Heeger, MacDiarmid, and Shirakawa, were

awarded the Nobel Prize in Chemistry.

From the original communication of this extraordinary property in 1977 until the

present day, conjugated polymers have found a wide variety of applications, often in

device fabrication. Devices as varied as light emitting diodes, light emitting

electrochemical cells, plastic lasers, and thin-film transistors have featured conjugated

polymers as functional components.2-9

A wide variety of conjugated polymer structures have been reported, and several of

these are depicted in Figure 1-1. With the exception of polyacetylene, these materials

feature aromatic or heteroaromatic rings that are connected either directly or via

conjugated vinyl or ethynyl linkers.

Poly(phenylene vinylene) (PPV)'s, as a class, have perhaps found the most uses to

date, and at least one common member of this class poly(methyl ethylhexyl phenylene

vinylene), or MEH-PPV, is currently commercially available.10 The fact that PPV's have

achieved such success despite their moderate stability, cis- backbone defects, and the









rather delicate processing conditions needed in order to obtain high-quality materials,

strongly suggests that investigations into other conjugated polymer architectures may

allow for the fabrication of improved materials and devices.11

R


n n n

Polyacetylene Poly(3-alkyl)thiophene Polyaniline



R R R


n n n
R R R

Poly-p-phenylene Poly(phenylene vinylene) Poly(phenylene ethynylene)

Figure 1-1. Examples of conjugated polymer structures.


Poly(Phenylene Ethynylene)s

Applications of Poly(phenylene ethynylene)s

One of the most promising classes of conjugated polymers is poly(arylene

ethynylene)s, or PAEs. PAEs feature aromatic or heteroaromatic rings connected by

ethynyl linkages.12 By far the most common type of PAEs are poly(phenylene

ethynylene)s (PPEs), where the aromatic group is a benzene ring. PPEs have a

tremendous advantage in that they may bear a great variety of functional group

substitutents. PPEs with alkyl,13 alkoxy,14 carboxylate,15 phosphonate,16 and other

pendant groups12'17 have been studied. In addition to varying the pendant group, it is

possible to "dope" the polymer chains' backbones with other moieties, including

organic12,14 or metal-containing's co-monomer units.









Recent investigations have shown that PPE's have an astounding variety of

practical uses. Many of these applications are made possible by the relative ease in

which structural diversity can be built into PPE's. For example, by incorporating

pentipticyl groups into the backbone, as in Figure 1-2, Yang and Swager utilized a PPE

as an effective sensor for trinitrotoluene, a common explosive.19

-- -772'








peniiipiyicere group cainjugated polymer backbone

Figure 1-2. Schematic of Swager's pentiptycene containing PPE, an effective TNT
sensor. Picture from Swager [19].

In 1998, Weder and co-workers used a PPE with a well-designed substitution

pattern to fabricate a liquid crystalline display.20 They note that the particular PPE used

in their device features an ideal set of properties for the preparation of polarized

photoluminescent layers.21 Notably, their device shows markedly better contrast than

common commercially available LCD displays, as shown in Figure 1-3.

Even more recently, PPE's have found application in sensing biological activity.

Because PPE's are effectively chromophores wired "in series" they have the potential to

act as amplifiers for chemical signaling information. Swager calls this the "molecular

wire approach to signal amplification.""17 This approach takes advantage of the

electronic communication between chromophores in a PPE.












F 1
B





Figure 1-3. LCD displays (A) constructed with a PPE as the luminescent layer, and (B),
a commercially available display. Photograph from Weder [21].
Figure 1-4 and 1-5 are schematic diagrams explaining this effect. Figure 1-4
depicts independently functioning chemical sensors. Note that each molecule of analyte
can cause a response in only one sensory molecule. The system is therefore limited by its
1:1 stoichiometry. The ability of such a system to detect, for example, a single molecule
of analyte would depend on the ability to elicit a measurable response from a single
receptor molecule.




Figure 1-4. Chemical sensors functioning independently. Adapted from Swager [17].
This limitation can be overcome by wiring sensors in series, as is the case of the
chromophores in a PPE. In this situation, depicted in Figure 1-5, a binding event at a
single receptor causes a response in every molecule in the wire. Considering that modern
techniques can easily produce polymers with several hundred repeat units, this method









should be able to produce a signal amplification of two or more orders of magnitude over

independently functioning receptors.






Figure 1-5. Chemical sensors functioning in series. Adapted from Swager [17].

The Whitten lab used this effect to develop a sensor for the protein avidin.22 This

design takes advantage of avidin's extraordinary high binding affinity for the molecule

biotin. The sensor used an anionic, water soluble PPV and a cationic quencher, methyl

viologen (MV ), which was covalently bound to biotin. When the biotin bound quencher

is introduced to a polymer solution the quencher binds to the polymer due to

electrostatics, and the polymer fluorescence is quenched. Upon subsequent addition of

avidin, tight binding between avidin and biotin moves the quencher away from the

polymer, resulting in a strongly fluorescent polymer solution.

After this initial work with a PPV, PPEs with anionic pendant groups were used by

Pinto and Schanze to develop fluorescence assays that monitor the activity of proteases in

real time.23 The assays are somewhat alike to Whitten's in that they depend upon the

amplified quenching PPE's by quenchers tethered to biomolecules. Figure 1-6 depicts

the two types of assays developed by this approach. The first, a "turn on" assay, features

a quencher bound to a positively charged "anchor" via a biomolecule tether. The

polymer is initially non-emissive due to the proximity of the quencher. Enzymatic

cleavage of the tether releases the quencher giving rise to the typical bright

photoluminescence of the PPE. The turn off approach utilizes a "caged" quencher that is









initially insulated from the polymer backbone. Enzymatic cleavage of the "cage" leaves

the quencher in close proximity to the polymer where it effects emission quenching.

Turne-ot Approach +







/.Z

Turn-off Approch
t I
L /






C* 0'.. \ '



Figure 1-6. Flourescence assays for enzyme activity using a PPE as the fluorophore.
Figure from Pinto [23].



Synthesis of Poly(paraphenylene ethynylene)s

The most common method of synthesizing PPE's is the Sonogashira coupling

reaction.12,17 This method, involving the use of copper and palladium to couple terminal

acetylenes to aryl halides, was developed by Sonogashira in 1975.24 Other examples of

this transformation appeared in the literature at about the same time, 25,26 however,

Sonogashira's method, which requires less forcing conditions, is tolerant of a wide range

of functional groups, and does not involve preparation of copper acetylides, is the only

one that has provided access to PPE's.









In its most common form, the Sonogashira reaction utilizes readily available

bis(triphenylphosphine)palladium(II) chloride and copper (I) iodide in the presence of an

amine to effect cross coupling of an aryl iodide or bromide with a terminal acetylene

under anaerobic conditions. The accepted mechanism for this reaction is depicted in

Figure 1-7.

When a palladium (II) catalyst is used, the initial step involves substitution of the

chlorides by two terminal acetylides, followed by reductive elimination to yield the active

catalyst, a palladium (0) species, and an acetylene dimer. This byproduct is not typically

a problem for reactions of small molecules. However, for synthesis of polymers where

exact stoichiometric balance is required, this side reaction imbalances the stoichiometry

and results in reduced polymer molecular weights. Swagger suggested using a slight

excess of acetylide monomer to alleviate this problem17. However, a better solution is to

substitute a palladium (0) species for the palladium (II), to circumvent the initial catalyst

activation step. Tetrakis(triphenylphospine) palladium is an excellent choice as an

alternative catalyst.

The next step in the reaction is insertion of the active catalyst into the aryl-halide

bond, followed by transmetallation of the halide by a Cu (I) acetylide. Reductive

elimination of the aryl acetylide product regenerates the catalyst. In order to prevent the

formation of an inactive complex between palladium and oxygen, the reaction is

completed under anaerobic conditions.

The reactivity of aryl halides in Sonogashira conditions depends on the aryl-halide

bond dissociation energy. Thus, in general, aryl iodides are more reactive than aryl

bromides, and aryl chlorides are typically unreactive. The difference between the










reactivity of iodides and bromides is sufficiently great that aryl iodides are known to

undergo reaction to the nearly complete exclusion of a competing reaction with aryl

bromides.27 In addition, reactivity of aryl iodides is typically fast even at room

temperature, while aryl bromides often require higher temperatures and longer reaction

times.

(PPh3)2PdCI2

SH RCul, NR'3


V NR'3H+CIl

(PPh3)2Pd- R)2



-R R

(PPh3)2Pdo





Ar
/ Ar
(PPh3)2Pd
x


R Ar H -RCul, NR'3

/Ar NR'3H+CI-
(PPh3)2Pd<

R

Figure 1-7. Mechanism of the Sonogashira Reaction.

Recent advances in catalysis have made possible the use of aqueous conditions and

inorganic bases,28 the use of aerobic conditions without copper,29 and coupling to alkyl

chlorides.30 However, because these methodologies require the use of harsh conditions,

catalysts that are not readily available, or both, they are not often used to access PPE's.









Molybdenum and tungsten based catalysts have also been used to synthesize PPE's

by alkyne metathesis reactions. This methodology, developed primarily in the Bunz and

Moore laboratories, was the subject of a review article in 2001.31 Bunz coined the term

acyclic diyne metathesis (ADIMET) to describe this condensation polymerization, which

formally condenses a molecule of hydrogen while forming a new bond between two

terminal acetylenes.

The ADIMET methodology addresses some of the problems associated with

Sonogashira polymerizations, including ill-defined endgroups due to dehalogenation and

phosphonium salt formation,32 and the difficulty in producing very high molecular weight

materials.12 In addition, ADIMET polymerizations require only a single type of

functional group, thus eliminating the several practical difficulties. Specifically, the

problems associated with the strict stoichiometric balance required for two component A-

A, B-B type Sonogashira polymerizations between aryl diiodides and aryl diethynes, or

the synthetic difficulties often associated with the synthesis of ultra-high purity A-B type

iodo-ethynyl substituted arenas.

In its earliest incarnations, ADIMET had several major drawbacks. These include

the need for harsh conditions, and intolerance of a wide degree of functionality.

However, more recently, the Moore group developed a simple synthesis of a

molybdenum alkylidine catalyst that shows good activity in ADIMET under reasonable

reaction conditions.33 This catalyst also shows improved functional group tolerance, and

yields higher molecular weight polymers than the Sonogashira methodology.34 Despite

these improvements, ADIMET remains the less popular of the two methods, probably

because the effective ADIMET catalysts are not commercially available, whereas the









palladium catalysts used for Sonogashira polymerizations can be purchased relatively

inexpensively.

Platinum Containing Poly(phenylene ethynylene)s: Platinum Polyynes

Another significant class of PPEs are those which contain platinum in the polymer

backbone. These materials are of particular interest because their photophysics are

dominated by long lived, phosphorescent 37,7r* excited states.35-39 This property makes

them useful candidate materials for the fabrication of electroluminescent devices,39'40 and

for applications in optical limiting.41

The general structure of a platinum containing PPE is depicted in Figure 1-8.

These materials consist of sp hybridized alkyne carbons linking

bis(trialkylphosphine)platinum (II) to phenylene rings. When the aromatic rings are not

substituted, good solubility in organic solvents can be achieved by using butyl or longer

alkyl groups.

PR3
Pt
PR3 n

Figure 1-8. General structure of a platinum containing PPE.

Platinum containing PPE's are synthesized utilizing the Hagihara coupling

reaction, which was discovered in 1978.42 The reaction involves the Cu (I) catalyzed

substitution of chlorine on a bis(trialkylphosphine)platinum (II) chloride in the presence

of an amine. Although the platinum chloride may be present as a mixture of cis and

trans isomers its stereochemistry is in practice irrelevant, because the cis complex rapidly

isomerizes to trans in the presence of a secondary or tertiary amine.










The amine serves to deprotonate the acetylene, which presumably forms a copper

acetylide upon reaction with the catalyst. The activated acetylide then substitutes

chloride forming the platinum-carbon bond. The reaction can also be carried out in the

absence of catalyst, but under these conditions the product is only mono-substituted, with

one chloride ligand remaining on the platinum.43 The catalyst-free variation is therefore

not useful for making polymers, but it has proved useful in the synthesis of oligomeric

platinum containing PPEs.44


m a P b t






04
IE Ii! II,..
." ^ \ i \ X
0.0

0 M3W WO 40a0 4W? 500 600 ?D0
Wavelength I nm

Figure 1-9. Absorption (a) and emission (b) spectra of oligo platinum PPEs with
increasing chain length. In the emission spectra, fluorescence is denoted by F
and phosphorescence is denoted by P. Note that the fluorescence intensity
scale is magnified 100x with respect to phosphorescence. Figure from Liu
[44].

Interestingly, several studies have shown that conjugation through the metal center

in platinum containing PPE's is not efficient, and therefore these materials have a rather

low conjugation length. For example, Beljonne used calculations to conclude that the

triplet state of platinum containing PAE's in general is spatially confined.45 Later, Liu

and coworkers synthesized a series of monodisperse oligo-Pt PPEs and studied their

photophysical characteristics.44 The absorption and emission spectra of these polymers,

reprinted in Figure 1-9, are quite informative. The increase in molar absortivity and red-









shift in the fluorescence with increasing chain length indicate that the singlet ground state

and first singlet excited state are delocalized across the oligomers. However, the

invariance of the phosphorescence with respect to increasing chain length indicates that

the triplet state in platinum PPEs is spatially confined over a relatively small distance of

about five monomer units.

Meta Linked Poly(phenylene ethynylene)s

While poly(para phenylene ethynylene)s have been known for some time, it was

not until 1997 that Lavastre and co-workers reported a method to efficiently access their

ortho and meta analogues.46 Soon after, researchers in Moore's group reported that

oligo(meta phenylene ethynylene)s have the ability to undergo dramatic conformational

self assembly in solution.47'48 Moore's oligomers, featuring a solubulizing oligo(ethylene

oxide)ester pendant group, are depicted in Figure 1-10.

0 0




SiMe3
n = 2, 4, 6, 8, 10, 12, 16, 18

Figure 1-10. Oligomeric m-phenylene ethynylenes studied by Moore's group.

When they are of sufficient length, Moore's phenylene ethynylene oligomers have

the ability to adopt a helical tertiary structure. This conformational effect is largely

solvent dependant. In methylene chloride, the oligomers are well-solvated, and exist as a

random coil. In acetonitrile, however, the oligomers fold into a helical conformation.

This change can be monitored by using absorption and emission spectroscopy in solvents

of different composition. Specifically, an increase in the mole fraction of acetonitrile in









the solvent results in a hypochromic effect in the absorption, with a good isobestic point.

Also, eximer-like fluorescence is observed in with increasing amounts of acetonirile in

the solvent. These results imply a cooperative transition.

Later, Tan and coworkers used a PPE with sulfonate pendant groups and a meta-

para linkage pattern to study these conformational changes in water.49 They found that

the polymer they studied was well solvated by methanol and poorly solvated by water.

This polymer exhibited properties quite similar to that of Moore's oligomers when the

composition of the solvent was varied from pure methanol to pure water. The

conformation changes in this polymer are depicted in Figure 1-11.













Figure 1-11. A segment of the meta-para PPE used by Tan adopts a random coil
conformation in methanol (left), and a helical conformation in water (right).
The side chains have been left off for ease of viewing.

More recently, the Tew laboratory studied polymeric meta phenylene ethynylenes

(m-PPEs).50'51 These studied compared poly electrolytes with and without bulky side

groups. Their studies showed that if, for steric reasons, a large pendant group must orient

itself inside of the helix, then self-assembly is restricted even in the presence of a poor

solvent.

Additionally, the cavity inside the helix of m-PPEs can be used to bind guest

molecules. For example Moore's group used an oligo m-PPE to bind a chiral









monoterpene guest.52 This guest induces chirality in the helical conformation of the

m-PPE oligomer, and results in a circular dichroism (CD) signal. The induced chirality

originates from a preferential binding of one enantiomer of the helix to the guest, and is

made possible because the association constant between the chiral guest and the polymer

is different for each of the enantiomeric helices. This process is depicted in Figure 1-12.

chiral
guest


M _KM*
IL

chiral



P -Kp*

Figure 1-12. Chiral induction in a helical polymer by preferential binding to a chiral
guest.

Meta-PPEs may have other interactions with small molecules besides this host-

guest chemistry. For example, Tan's polymer, described earlier, is physically similar to

DNA in that it adopts a helical conformation with face-to-face stacking of aromatic rings.

These similarities allow it to bind to molecules in a fashion similar to DNA. An example

is binding to the complex Ru(bpy)2(dppz), where bpy is 2,2' bipyridine and dppz is

dipyrido[3,2-a:2',3'-c]phenazine. In a solution of pure water, Ru(bpy)2(dppz) is non

emissive because hydrogen bonding between the phenazine ligands and the solvent allow

the emission to couple to the vibrational modes of water and effectively quench the

compound's luminescence.53 However, upon intercalation into DNA, Ru(bpy)2(dppz) is

"shielded" from this solvent interaction and becomes strongly luminescent.53'54 This









phenomenon is often known as the "light stick effect". That this effect is also observed

when Tan's polymer, in its helical conformation, is added to a solution of Ru(bpy)2(dppz)

strongly suggests that the metal complex intercalates into the polymer.

Conjugated Polymers as Molecular Wires

Single Chain Conjugated Polymers

Much of the recent interest in conjugated polymers stems from their potential

applications in electronic, electro-optical and optical devices.5 One major factor which

makes conjugated polymers excellent candidates for these applications is that they

provide exceptional transport properties for charges and excitions. As such, a great deal

of effort has been expended to understand the carrier transport properties of these

materials.

The vast majority of the work in this area has focused on bulk properties of the

polymers in films.37,56-59 This is understandable, as there is a significant interest in using

polymer films as active layers in optio-electronics.60 These investigations have offered

considerable insight into structure-property relationships of polymers in bulk.

However, there is another paradigm for exploiting the intriguing properties of

conjugated polymers. It is also conceived that these materials may serve as transport

materials as single-chain "molecular wires." The molecular wire concept entails single

molecules acting as carriers in one-dimension only. Thus, a molecular wire would have

to have an appropriate molecular geometry and be able to adopt a predictable

conformation in addition to having excellent transport properties along its long axis. In

addition to the requirements that they be excellent charge carriers over long distances, the

energetic of a good molecular wire must not vary drastically with length.61










Several examples of conjugated oligomers or polymers being used as molecular

wires are known. For instance, in 2000, Tour attached a linear terphenyl to gold

electrodes by using thiol substituents as "alligator clips."62 Upon applying a potential

difference across the electrodes, the terphenyl undergoes a conformational change and

completes the circuit between the electrodes with a measurable conductivity and

resistivity. This experiment is depicted in Figure 1-13.

I COHTFCL

GDLD


CNtGIEH


H 1- F W

SULFUR


,g1-L [, .*~F EIIT



Figure 1-13. A conjugated oligomer in use as a molecular wire. Figure adapted from
Tour [62].

The study of molecular wires is particularly important in the context of nano-scale

electronic devices, where it is envisioned that the charge or exciton carriers will be single

polymer or oligomer chains.61'63 Many conjugated polymers, especially poly(para-

arylene ethynylene)s (PAEs) fit the requirements for molecular wires. First, PAE's are

locked in a linear geometry along the direction of conjugation. Second, although the

primary mode of transport through PPE's in the solid state is inter-chain hopping, the

extended conjugation in these materials seems intuitively ideal for intra-chain transport

properties along the direction of conjugation.









Indeed, there has been a great deal of recent interest in studying the transport of

excitons along single chain conjugated polymers. This is somewhat of a challenge,

because PPE's are known to aggregate in solution, and the photophysics of the aggregates

are differ significantly from that of single-chain polymers.64 The most common approach

to these investigations is to study the polymers in dilute solutions, where interchain

interactions are minimized.65

Several techniques have been used to explore exciton transport in conjugated

polymers. One method uses co-polymerization to incorporate low-energy trap sites for

excitons in the polymer chain.66-68 For example, Swager, Gil, and Wrighton synthesized

a PPE end-capped with anthracene linked through the 9-position, as shown in Figure 1-

14.66 Upon photoexcitation of the polymer main chain, the emission of the polymer is

quenched and replaced with anthracene fluorescence.


Ph OC16H33
Ph Ph

C16H330


Figure 1-14. Anthracene end-capped polymer studied by Swager, Gil, and Wrighton.

Alternatively, the polymer may be used to bind a small molecule quencher, which

serves to accept either energy or an electron from the excited state of the polymer.17'69 In

one particularly intriguing study, Wang and Wasielewski used the PPV derivative in

Figure 1-15, which features pyridine comonomers in the main-chain, to bind metal ions.

Upon binding, metal ions such as Ni2+, Cu2+, and Mn2+ effectively quench the polymer

emission.70 This effect is not observed with a physical mixture of a pristine polymer









(without bipyridine) and bipyridine. The highly effective quenching indicates that the

exciton created by initial photoexcitation diffuses rapidly to the metal trap site.










ORt






Figure 1-15. A metal binding conjugated polymer. Figure adapted from Wang [62].

In related studies, cyclophane monomers have been used to bind viologen

quenchers,70-72 and ionic side chains have been used to bind oppositely charged metals

and molecular ion traps.72-76 All of these studies demonstrate that energy transfer in

single chain conjugated polymers is extremely rapid and highly efficient.

Charge Transport

Charge or electron transport (ET) in solution can typically be viewed as an

interaction between an electron donor (D) linked to an electron acceptor (A) through

some bridging species (B).77 This DBA scheme is understood fairly well when the bridge

is molecular.78-8 Indeed, detailed comparisons between theoretical and experimental

results have been essential in unifying this field.80 In this work, the gap between the

donor-HOMO and bridge-LUMO for a variety of systems was tabulated.

In general, there are two mechanisms for ET in DBA systems. Superexchange is

observed when ET occurs by a tunneling process and the electron never resides on the









bridge.81 Alternatively, charge transport may proceed by a hopping mechanism via

oxidation or reduction of the bridge by the donor, followed by ET from the bridge to the

acceptor.82 These two processes may both be operative in the same system, as per

equation 1-1, where ktun is the rate of ET by superexchange and khop is the rate of ET by

hopping.61

kET ktun + khop (1-1)

In conjugated polymers, as in typical molecular systems, the superexchange model

is expected to be highly dependant on the length of the polymer. The dependence of the

rate of superexchange on the degree of polymerization (Dp) is expressed by equation 1-2.

kET kun = koexp(-pDp) (1-2)

In this equation, 3N is dependant on the length of the monomeric unit (which, in the case

of PAE's, may contain one, two, or more aromatic rings), and the natural logarithm of the

ratio of the coupling energy between the monomeric units, HBB, and the energy gap

between the initial state (DBA) and the mediating state, AEDB. The mediating state is

D+B-A for electron transport, or DB+A- for hole transport. Therefore it follows that a

small value of 3 is desirable for long-range ET, thus pointing to the desire to minimize

the energy of the mediating state. It is important to note that in the superexchange

mechanism the mediating state is only a virtual state, and that the electron or hole never

actually resides on the bridge.

In the hopping mechanism, the mediating states are real, and the bridge is oxidized

and then reduced (or reduced and then oxidized) during the ET process. The rate of this

process is Ohmic, that is, it is inversely proportional to the distance between D and A.83









In the case of a conjugated polymer, this means that as Dp increases, the rate of charge

transport by hopping should decrease.

Several interesting phenomena are predicted when both mechanisms are applied

in parallel. One particularly interesting result of the mathematics is a phenomenon

known as resonant tunneling, which occurs when the energy gap is zero. In this scheme,

the overall rate of ET should not vary greatly with increasing Dp.84




0

0- O\

0 Y


Figure 1-16. MEH-PPV with saturated conjugation breaks.

ET theory therefore indicates that understanding the nature of the oxidized or

reduced state of conjugated polymers is of key importance, since the energetic of the

charged state and the diffusiveness of the charge are key factors in explaining the

transport abilities of conjugated polymers used as molecular wires. A study by Candeias

and coworkers was the first to provide this information. Candeias examined a series of

MEH-PPV derivatives in which varying amounts of the vinyl linkages had been

hydrogenated, as in Figure 1-16.65 The hydrogenated olefins effectively break the

conjugation in the polymer backbone.

Pulse radiolysis was used to perform a one-electron oxidation on the conjugated

polymer backbone, and the energy of the polymer cation radical was measured by

absorption spectroscopy. The study showed that breaks in the conjugation length









increase the energy of the cation radical, thus unambiguously demonstrating that the

charged species is stabilized by conjugation through the polymer chain.

Charge Injection

In Candeias's work and in some of the work described in this dissertation, charge

is injected into a conjugated polymer by producing a solvated hole or electron which

migrates to the analyte. This process can be modeled using existing theory, and

understanding this theory is a useful precursor to the studies presented herein.

Theory has provided solutions to the problem of a mobile point charge diffusing

to a number of spherical monomer units joined together in a one, two or three

dimensional array. The solution for a one dimentional array is an appropriate model for

PAEs, which are essentially one dimensional rigid rods. Traytak's soution85'86 and its

recent test by Grozema65 employ an effective radius, Reff defined in terms of the reaction

radius Rm, for one monomer unit and the number of units in the polymer chain, n, as in

equation 1-3.

nR
Reff= R -m (1-3)
1 + In(n)
a

The diffusion-controlled rate of reaction with a chain of length n, is then given by

equation 1-4, which is identical to Smoluchowski's classic solution for a reaction of

spherical particles where D is the sum of the diffusion coefficients of the reactants.

k(t) = 47ReffD(1 + Ref/(7Dt)1) (1-4)

The second term in equation 1-4 is often called the transient term, and contributes

less than 10% to the overall rate when larger than 10R2/TrD. For small species that

diffuse rapidly in solution, this time is generally less than a few nanoseconds. However,









Grozema showed that large polymers have much larger effective radii and consequently a

much larger and long-lived transient term. While the steady-state, diffusion controlled

rate constant per repeat unit falls with increasing degree of polymerization, some of that

loss is recovered by the larger transient term.85,86 Thus, the bimolecular rate constant for

transfer of an solvated electron (e-) or hole (h+) to a PPE will initially decrease with an

increasing chain length and then, after reaching an minimum, increase again.

Description of the Present Study

The present study aims to gain an increased understanding of the properties of

single chain PPEs. Careful design and synthesis of polymers with appropriate charge or

energy traps, or with the capability to undergo predictable conformational changes, is the

primary means to these goals. Well-designed polymers allow for the monitoring of

specific spectral changes associated with the injection of energy or charge, or those

associated with conformational changes, thus making it possible to gain an increased

understanding of these properties.

The experiments described in this dissertation were performed in dilute solution,

except for a few cases where higher concentrations or films were used in order to

demonstrate the differences between single-chain and aggregated or bulk PPEs.

Although modern microscopic methods allow for the collection of a variety of

photophysical data on single-chain conjugated polymers in the solid state, there are

severe practical difficulties in performing many of the time-resolved experiments

described in this study under those conditions.

Chapter 2 of this dissertation is concerned primarily with charge transport in PPEs.

PPEs with two different molecular architectures were synthesized both in their pristine









form and with low oxidation potential moieties affixed to the termini. These traps were

designed to act as low-energy traps for cation radicals (holes) in the polymer backbone.

The unique laser electron accelerator facility (LEAF) at Brookhaven National

Laboratories was employed to inject charges into the polymers and measure their

spectroscopy in a time-resolved fashion. By carefully choosing a trap that exhibits

spectral characteristics that are different from those of the parent polymer, it was hoped

that the dynamics of charge migration in PPEs could be measured quantitatively.

Although the limits of even the most modem instrumentation prevent quantitative

measurement of these rates, this system enabled many other useful spectral and

thermodynamic properties of a charged PPE to be elucidated.

Chapters 3 and 4 deal with platinum containing PPEs. Chapter 3 describes the

synthesis and characterization of a series of platinum containing PPEs where low energy

traps are incorporated in to the main chain. The trap concentration was varied throughout

the series. These materials were used to gain insight into the mechanism of energy

transport in platinum containing PPEs. Use of a variety of steady-state and time-resolved

methods reveal that although the photophysics of platinum containing PPEs are

dominated by their long-lived triplet states, the results in this chapter show that their short

and often optically inactive singlet excited states do play an important in inter-chain

charge transport.

Chapter 4 deals with a platinum containing PPE that is linked through the meta

position. This polymer is able to self assemble into a helical conformation that accepts

guest molecules, much in the way that organic m-PPEs or DNA do. This work represents






24


the first careful study of a water soluble organometallic polymer that undergoes these

types of conformational changes.














CHAPTER 2
RADICAL TRANSPORT IN END-CAPPED POLY(ARYLENE ETHYNYLENE)S

Introduction

Very little effort has been made to examine the transport of charges (radical cation

or radical anions) in single chain poly(arylene ethynylene)s, or to understand the

spectroscopy or thermodynamics of these charged species.65 This is unfortunate, because

charged single chain conjugated polymers are envisioned to be the active species in a

variety of applications, especially molecular electronics. In fact, a need to develop

further understanding into the nature of charged states in conjugated polymers was

recently highlighted by several senior researchers in the field.61 The work presented in

this chapter is an important step towards fulfilling this need.87

Pulse radiolysis experiments were used to investigate the transport dynamics of

radical cations (holes) in single chain conjugated polymers. This problem was

approached by using conjugated polymers that are end-capped with functional groups

that serve as traps for radical cations generated on the main polymer chain. The

conditions for an end-cap to serve as a trap is the end-cap have a lower oxidation

potential than that of the main chain, that is, Eox (T) < Eox (polymer), where T represents

the radical cation trapping moiety. In this chapter, the results of a study using

poly(arylene ethynylene)'s with 2,2':5',2" terthiophene (T3) end-caps is reported. PAE's

were chosen because they posses a linear "molecular wire" structure, and are easily

synthesized via Pd mediated Sonogashira condensation polymerizations.12 T3 was chosen

as an end-cap because it was anticipated to meet the requirements for a radical cation









trap, eg. Eox (T3) < Eox (polymer). In addition, the spectroscopy of oligothiophene

cation radicals has been investigated extensively, and it was therefore anticipated that it

would be possible to distinguish between a cation radical localized on the end-group (ie.

T3 ) from a polymer-based cation radical (ie. PAE+').

The objectives of this study are two-fold. The first objective is to study

experimentally the spectroscopic properties of charged single-chain PAE's. The second

is to determine the rate of hole transfer (kHT) for a radical cation localized on a the

polymer main chain to the end-cap, as in equation 2-1.



h+ kHT
T3-(PAE)-T3 T3-(PAE+)-T3 T3-(PAE)-T3+ (2-1)


Results and Discussion

Synthesis and Structural Characterization of Poly(arylene ethynene)s

The chemical structures and acronyms for the polymers featured in this study are

shown in Figure 2-1. The numerical subscript in each polymer's name represents the

number average degree of polymerization, Xn, as calculated from the GPC derived

number average molecular weight, Mn. For the end-capped polymers, Xn includes only

the units of the polymer main chain and not the end-groups. Furthermore, it is important

to note that PPE type polymers contain two phenylene rings per repeat unit while PBpE

type polymers contain three phenylene rings per repeat unit.

In addition to the polymers, the model compound Ph-T3 was synthesized by

coupling 2-terthophene andp-iodo toluene. This compound was designed to model the











end-capped termini of the polymers. The synthesis of the polymers and monomers used


in this study is outlined in Figure 2-2.

OR OR


R PPEn RO PBpE21 Ph-T3
n=24 PPE24Mn=10,770 M,=32,388 PDI=30 Mn= 11321 M,= 18,117 PDI= 1 6
n=164 PPE164 Mn=96,253 M,=182,880 PDI=1 9


OR

Z S n
S S S
RO
T3PBpE12
Mn 7,200 M = 18,155 PDI= 25

OR
S SS
/ n
RO
T3PPE13
R = 2-ethylhexyl
Mn= 6,454 M,= 13,623 PDI =2 1


Figure 2-1. Structures of the polymers and model compounds used in this study.

The monomers used in these polymerizations, 2,5-bis(2-ethylhexyl)-1,4-diiodo-


benzene and 1,4-diethynyl benzene, and 4, 4'-diethynyl biphenyl were synthesized by


modified literature procedures.14 In particular, the synthesis of the diiodo monomer was


substantially improved. Diiodination of 1,4 dimethoxybenzene was accomplished with


one equivalent of molecular iodine and 0.7 equivalents of potassium periodate in


refluxing acidic media. Using this ratio of reagents affords substantially better yields


(85-90%) than use of an excess of iodine and potassium periodate (about 40% in our


hands).14 Additionally, since iodine is present in a stoichiometric amount, the reaction


can be monitored by watching for the disappearance of the dark-brown iodine color. It is


important to note that iodine may sublime in the condenser during the reaction, and it


must occasionally be pushed back into the reaction mixture using a glass rod to prevent









clogging. If the condenser becomes completely clogged, the build up of pressure may

result in an explosion. Addition of a small amount of chloroform to the reaction mixture

can help minimize this problem, but may reduce the yield somewhat.


H H +


PBpE21


H H + 1I 1

0


0
H H+ 1 ~-H + H






H H + 1 I H S I
-0- W/ 3
/ 0


).- PPE16 or PPE166






S--- T3BpE12


T3PPE13


H I + H
U 3


)W. PhT3


Figure 2-2. Synthesis of polymers and model compounds used in this study.









Demethylation of the alkoxy groups was preformed using boron tribromide in dry

dichloromethane under anaerobic conditions. The resulting hydroquinone is recovered

by evaporation of most of the solvent, and precipitation from methanol-water. Upon

workup of this reaction, it is important to add a few spatula tips of sodium sulfite

(Na2SO3) to prevent air-oxidation of the hydroquinone in solution. The hydroquinone

was then alkylated using 1-bromo-2-ethyl hexane in refluxing methyl ethyl ketone with

potassium carbonate under anaerobic conditions. Again, addition of sodium sulfite upon

workup is important to prevent oxidation of the product upon exposure to air in a basic

medium. The monomer is obtained as a colorless liquid.

In an effort to bypass the use of boron tribromide, which is both expensive and

highly toxic, another synthesis of this monomer was attempted. In this effort,

hydroquinone was first alkylated with 1-bromo-2-ethylhexane and then iodinated as

described above. However, the monomer resulting from this synthetic sequence is a red-

brown oil, instead of the colorless liquid obtained by the original synthesis. The color,

likely due to complexation of the product with trace amounts of iodine, cannot be

removed by treatment with activated carbon or by column chromatography. It is

probable that, in the original synthetic sequence, any remaining traces of iodine are

removed by recrystalization of the intermediates (both 2,5-diiodo-l,4-dimethoxybenzene

and 2,5-diodo-1,4-dihydroquinone are highly crystalline). However, there are no

crystalline intermediates in this alternate synthesis. Importantly, use of the colored

monomer (from the alternate synthesis) in polymerizations gives only low molecular

weight polymer, even under optimized polymerization conditions. Thus, this synthesis is









not a suitable method to produce the high-quality monomer needed for successful

polymerizations.

The alkoxy side groups are used to help impart solubility on the final polymers. A

racemate of the chiral 2-ethylhexyl group was chosen because this group gives a mixture

of stereoisomers in the resulting monomer. Specifically, the di-iodo monomer is present

in the polymer as a pair of diasteromers, one of which is meso, and the other which exists

as a pair of enantiomers. The resulting stereo-irregularity in the polymer backbone was

designed to discourage inter-chain aggregation and improve solubility of the polymers.

In fact, the biphenylene based polymers, with and without end-caps, proved to be quite

soluble in many common organic solvents, including THF, chloroform, and

dichloromethane. The phenylene based polymers also exhibited good solubility in these

solvents, however, upon complete evaporation of solvent the phenylene based polymer

forms a film which does not completely redissolve even after several days of continued

stirring and gentle heating. Thus, in order to obtain the 1H NMR spectrum of this

polymer, it was necessary to dissolve it in a mixture of dichloromethane, acetone, and

hexanes. Because hexanes and acetone are poor solvents for this polymer, the film

formed by evaporation of this polymer solution can be redissolved in a deuterated

solvent. However, even using this method, if the solvent is completely removed the film

becomes insoluble. Therefore the NMR, shown in Figure 2-3, is partially obscured by

solvent peaks.

These problems made it desirable to find an alternative to the 2-ethylhexyloxy

side group that would show improved solubility characteristics. Use of the undecyloxy

side group alleviated many of the problems, and allowed a simpler synthesis. It was









expected that 2,5-diundecyloxy-1,4-diiodobenzene would be crystalline, and it was

therefore hoped that the alternate synthesis that failed to produce useful monomer with 2-

ethylhexyloxy side chains would be useful. Indeed, alkylation of hydroquinone with 1-

bromoundecane followed by iodination as described above yields a product that can be

easily purified by recrystalization from methanol-water giving a white solid that can be

used to make reasonable molecular weight polymers.

The films that result from evaporation of solutions of the phenylene-based

polymers are somewhat more soluble than those from similar polymers with the 2-

ethylhexyloxy side group, and it is possible to obtain proton NMR spectra that clearly

show the end-group protons. Figure 2-4 shows the 1H NMR spectra of the phenylene

based polymer with undecyloxy side groups. Figure 2-5 is an expansion of the aromatic

region of the phenylene polymer with undecyloxy side groups which has been scaled to

clearly show the thienyl protons. The doublet at 7.19 ppm integrates to one relative to the

other thiophene protons, and can be assigned to the proton in the 5 position on the

terminal thiophene. Three multiplets at 6.84, 7.11, and 7.66 ppm each integrate to two,

and each is assigned to the pairs of protons in the 3 and 4 positions on each of the

respective thiophene rings. Despite the somewhat improved physical properties of the

polymers resulting from 2,5-diundecyloxy-1,4-diiodobenzene, all of the photophysical

and radiolytic experiments described here were preformed on the polymers shown in

Figure 2-1, which feature 2-ethylhexyloxy side groups, and all discussion henceforth will

refer to those polymers unless specifically noted.
















PtNh. dq$.ci: i I


5 i 4 je rPPE
A I I i 1* pp.
..31 n2.t!LI
I .* is~ 41.)


Figure 2-3. NMR of T3PPE13 with 2-ethylhexyloxy side groups.


H,


-n.J N'.


- 1
~Js


Jr


i. i. -I ,p,,
., .'


Figure 2-4. NMR of T3 end-capped PPE with undecyloxy side groups.










Nih 5nrJrnl lp'A



















r,7 FIG r. 7.4 7.3 .Z 7..l .l PV M


Figure 2-5. Expansion of the aromatic region of Figure 2-4. Note that the large off-scale
peaks are correspond to phenylene protons, and the integrated peaks
correspond to thiophene protons.


The final polymers were synthesized by AA, BB condensation polymerizations of

aryl iodides and aryl ethynylenes using the Sonogashira reaction.12 17'66'69 For the

polymers without end-caps (the "parent" polymers) an equimolar ratio of aryl iodide to

aryl ethynylene groups were used in the polymerizations. For the synthesis of T3PPE13

numerous experiments were conducted to determine the optimal stoichiometry for end-

capping, and the results are outlined in Table 2-1. In this table, I-Ar-I refers to 2,5-

diundecyloxy-1,4-diiodo benzene. These experiments indicated that a small excess of

iodine functionality was required to give moderate Mn values. Additional experiments

using the optimized ratio of end-cap to diiodo arene in which the reaction time was varied

showed little change in the Mn of the resulting polymer for reaction times of sixteen

hours or longer.









Table 2-1. Conditions used in end-capping polymerizations.

Trial Equivalentsa Time/ hrs Mnb
T3 I-Ar-I
1 1 99 12 3,978
2 5 95 12 3,065
3 10 90 12 761
4 5 95 10 2,900
5 5 95 24 1,188
6 5 95 48 3,707
7 10 100 16 6,500
a -all trials used 100 equivalents of the diethynyl monomer.
b -The Mn is reported from GPC relative to polystyrene standards.

Each of the polymers was analyzed by gel permeation chromatography relative to

polystyrene standards, and H NMR spectroscopy. 13C NMR was not performed because

of the solubility limits of the polymers. Even in the ethylhexyloxy substituted polymers,

which are not completely free of solvent, NMR can be used to give an independent

estimation of Xn in the end-capped polymers. In order to estimate the degree of

polymerization, it is useful to take advantage of a stoichiometric imbalance of the

monomers due to end-capping. Careful inspection of the structure of the end-capped

polymers reveals that the ratio of phenylene (or biphenylene) to dialkoxyphenylene

monomers is (n+l):n, where n is the chain length. By comparing the peak ratios of the

phenylene (or biphenylene) and alkoxyphenylene monomers it is thus possible to

calculate Xn =7 for T3PPE, and X. = 8 for T3BpPE. These NMR derived values are

somewhat smaller than those derived from GPC, which is consistent with accepted notion

that GPC, especially relative to standard "random coil" polymers, tends to exaggerate the

Xn of conjugated rigid-rod polymers.13,88,89 Regardless of these difficulties, GPC

remains the most common method for determining the molecular weights of conjugated

polymers, and polystyrene or other random coil polymers are the most commonly used

standards.









Absorption and Fluorescence Spectroscopy

Phenylene based polymers

The UV/visible absorption spectra ofPPE164, T3PPE13, T3, and PhT3 are shown in

Figure 2-6. The spectra of PPE24 are identical to those of the longer molecular weight

sample, and are therefore excluded. Molar absortivity and selected fluorescence lifetime

data is provided in Table 2-2.

The absorption and emission spectra of the parent polymers are alike to those of

other, structurally similar PPE's.12,66,90 The absorption spectra are dominated by the

allowed, long-axis polarized n --* n* transition, while the fluorescence features a sharp,

narrow band which is dominated by the zero phonon transition.

The absorption spectrum of T3PPE13 is quite similar to that of the parent PPE164,

except for the presence of a long wavelength tail (note arrow in Figure 2-5), which

correlates to a low-energy moiety in the end-capped polymer, presumably the T3 end-cap.

The small intensity of this tail is unsurprising, considering that the end-caps are present at

very low concentration compared with the main chain of the polymers.

Table 2-2. Selected photophysical data for PAE's
450 nm 550 nm
Polymer axabs/ max axflr emr n / ns (a) a / ns () a
nm M1 cm nm
PPE164 411 5.6 x 104 452 0.33 0.5 (0.97) 0.6 (0.80)
3.9 (0.03) 2.5 (0.20)
T3PPE13 414 6.5 x 104 453,496 0.44 0.5 (0.94) 0.6 (0.89)
3.4(0.06) 3.2(0.11)
" Fluorescence decay lifetimes from biexponential fits. T and c values are, respectively, the lifetimes and normalized amplitudes of
the individual decay components.













0 I l I 0.Io

oe !ngh \ /

1. 0.6 1 0.6

ehs an e g md \u () T ( -) -T
( .

N 0.4 0.4 _

E .\

o 0.2 /\ \ \ 0.2 <
S.\ v "
-3





0.0 0.0
300 350 400 450 400 450 500 550 600 650
Wavelength / nm
Figure 2-6. Absorption (left) and photoluminescence (right) spectra of poly(phenylene
ethynylene)s and end group model compounds. ( ): T3; ( ): Ph-T3;
( ): PPE164; ( ): T3PPE13.
The difference in fluorescence between the parent and end-capped polymers is

quite noticeable, even upon simple examination by eye. Upon irradiation with long-

wavelength ultraviolet light, the parent polymer emits a bright blue fluorescence, while

the fluorescence of the end-capped polymer appears green. Initial examination of the

data suggest that the fluorescence of T3PPE13 is a combination of emission from two

different exitons: one on the polymer main-chain (PPE*-T3), and one localized on the T3

end-group (PPE-T3*). However, examination of the small molecule models shows that

the situation is actually much more complex. The absorption and fluorescence of T3 is

considerably blue-shifted from that of both PPE164 and T3PPE13. This is a clear indicator

that an excited state localized on T3 is at a significantly higher energy than the PPE*









exciton. A clearer understanding can be gained by noting that the absorption and

fluorescence of Ph-T3 is significantly red-shifted from that of T3, indicating that there is a

substantial delocalization of the T3 excited state into the adjacent phenylene unit. Indeed,

excited state molecular orbital calculations (outlined below) clearly indicate that although

both the HOMO and LUMO are mostly localized on the T3 end-cap, orbital mixing

between the end-cap and the polymer main-chain extends the conjugation through several

phenylene rings. Thus, the spectra of T3PPE13 derives from an excited state that is mostly

contained on the T3 end-cap, with some contribution from the neighboring phenylenes in

the chain. The fact that the excited state of T3PPE13 is a composite of phenylene and

thiophene mixing is supported by the fact that the fluorescence T3PPE13 is independent of

excitation wavelength, even when the excitation is at the long-wavelength "tail" of the

absorption which is due primarily to the end-caps. It is also important to recognize that

these results are also not consistent with a physical mixture of end-capped and non-end-

capped polymers, thus lending additional support to the structural characterization of the

polymers.

The notion that there is considerable mixing between the end-cap and the polymer

main chain is also supported by fluorescence lifetime measurements (Table 2-2). The

decay profiles ofPPE164 and T3PPE13 are quite similar. Both are dominated by a very

fast (Tz 0.5 ns) component. Furthermore, the emission lifetimes are wavelength

independent, even when measured in the "green" band associated with the end-cap. The

fast decay of PAE's is largely associated with their extended conjugation. The fast,

wavelength independent decay of T3PPE13 thus supports the idea that the end-cap is

significantly delocalized into the polymer chain. If this were not the case, the decay









profile of T3PPE13 would be expected to vary significantly with wavelength; the long-

wavelength emission, if it were due to a moiety with only a small degree of

delocalization, (ie. T3, with only three aryl rings) would be expected to exhibit a much

slower decay than the emission associated with the polymer main-chain which contains

an average of 26 aryl rings.

The spectroscopic behavior of the T3 end-capped polymers present some contrast

to previous work where a structurally similar PPE was end-capped with anthracene linked

through the 9-position.66 In the 9-anthracenyl end-capped polymer, the PPE-based

fluorescence was completely quenched and replaced with a red-shifted emission that was

assigned to the anthracene end-caps. This begs the question of why the fluorescence is

quenched when an anthracene end-cap is used, whereas when a T3 end-cap is used a

mixed excited state is the result, even though the energy difference between PPE and

anthracene is similar to that for PPE and T3. The answer likely lies in the considerably

different molecular orbital and excited state symmetries of the two end-groups. The

symmetry of the T3 end-group is the same as that of the polymer, allowing for good

excited state mixing. However, the 9-anthracenyl end-group is polarized orthogonally to

that of the polymer. The result of this is that mixing between the 9-anthracenyl end-

group and the polymer chain is low, and thus the polymer chain is not well conjugated

into the 9-anthracenyl end-group.

It is important for the hole transfer experiments described below that the end-

group used is in a symmetry-allowed conjugation with the polymer chain, as the ultimate

objective of this study is to measure the rate of charge transport through a conjugated

system. If the end-group orbitals did not mix well with those of the polymer main chain,









then the rate limiting step in charge transport would be "hopping" across the non-

conjugated bridge to the end-cap, and it would be impossible in principle to measure the

rate of transport through the conjugated portion of the polymer. Thus, the 9-anthracenyl

end-capped polymers would have been a poor choice for this study.

To confirm the idea that energy interchain energy transport is significantly

different than intrachain transport, the fluorescence of the polymers were also measured

in the solid state. The results are shown in Figure 2-7. When comparing the fluorescence

of the PPE164 as a film to the solution-state fluorescence, it is clear that the emission from

the film is broader with a loss of structure and red-shifted when compared to the emission

in dilute solution. These effects are associated with inter-chain aggregation in the solid.





1.0 -

/ \
S0.8 -\


S0.6 -

O L
E 0.4 -
z

0.2


0.0
450 500 550 600 650 700
Wavelength / nm

Figure 2-7. Thin film fluorescence ofPPE164( -) and T3PPE13 ( )









The change in emission properties of T3PPE13 when going from solution to the

solid is quite dramatic. Whereas the solution emission shows a significant amount of

"PPE-like" fluorescence, this main emission band is completely quenched in the solid

state and replaced by a broad band with little structure and max = 523 nm. The reason for

this difference lies in the different mode of exciton transport in the solid state as

compared to in solution. In a film, the primary mode of transport in the film is inter

chain exciton hopping. The distance between an end-cap "trap" and any exciton is much

less in the solid than in solution because excitions may migrate between chains instead of

just "walking down" one of them. Thus, exciton migration in films is expected to be

more efficient than in solution, and this is borne out by experimental evidence.

Biphenylene based polymers

The biphenylene based polymers showed essentially identical photophysical

properties as their phenylene based analogues. The absorption and emission spectra of

these polymers is shown in Figure 2-8. The primary feature of in the absorption of both

BpE21 and T3BpE12 is a long-axis polarized 7r -- band, which in both cases is blue-

shifted somewhat with respect to the phenylene based polymer. The end-capped polymer

also exhibits a tail at the low-energy end of the spectrum, which is explained by

conjugation with the low-energy T3 end-group, by analogy to the all phenylene polymer.

The emission spectra of both copolymers are dominated by the zero-phonon

transition. The main difference between the phenylene and biphenylene based polymers

is in the "green" end-cap based component of the emission. This additional band, with a

maximum at 490 nm, is due to emission from the low energy end-cap. As in the

phenylene based polymer, the end-cap emission the result of orbital mixing between the

thiophene-based end-groups and adjacent phenylene units. The end-cap's contribution to









the overall emission spectrum is noticeably weaker in T3BpE12 than in T3PPE13. This

indicates that energy transport in biphenylene based PPE's is somewhat less efficient than

in the phenylene based PPE's, an effect that may be related to the fact that the

biphenylene-based polymers can more readily adopt a non-planar "twisted" conformation

than the all-phenylene based polymers.




1.2 / 1.2


1.0 1.0


S0.8 0.8
o / \
< _0
o 0.6 /0.6 (
(N
\ -
S0.4 0.4 0


0.2 0.2


0.0 -0.0
300 350 400 450 500 450 500 550 600

Wavelength / nm

Figure 2-8. Absorption and emission ofBpE21 ( ) and T3BpE12 ()

Radiolytic production of cation and anion radicals

Radical cations and radical anions were produced on PAE's in solution by pulse

radiolysis. These experiments were conducted at the Laser Electron Accelerator Facility

(LEAF) at Brookhaven National Laboratories. This facility has been described

previously in detail,91'92 so only a brief description will be provided here. Figure 2-9 is a








schematic representation of the LEAF. LEAF is essentially a pulse-radiolysis facility.

However, unlike traditional radiolysis instrumentation, which can only produce electrons

beams with long pulse width, LEAF is able to produce high energy electrons with pulse

widths as small as 5 ps. This is accomplished by using a photocathode as the electron

source, and exciting it with a fast pulsed laser. The short pulse width of the laser

translates into a short pulse width of electrons.






1 hv ,
Laser hV
5 ps Time delay




Electron e/ e [-_ ]
Gun 5 ps

RF Feed 9 MeV

Figure 2-9. Schematic representation of the LEAF facility.

The generation of PAE-based ion radicals involves a series of reactions in which

high energy electron pulses are converted into strongly oxidizing solvent cation radicals

or holes (h ) or electrons (e-), which are then transferred to PAE chains.

At the LEAF, the electrons involved in radiolysis have an energy of 10 MeV and a

pulse width <120 ps. The high energy electrons pass completely through the solution and

exit the spectrophotometric cell, however, each electron produces roughly 104 ionizations









that result in thermalized solvated electrons and solvent radical cations in concentrations

of a few tM .

The precise nature of the reactions depends strongly on the solvent. 1,2

dichloroethane (DCE) for example, captures the electrons immediately to form radicals

and C1- ions.93'94 In tetrahydrofuran (THF) the solvent cation radicals decompose to

radicals and solvated protons. Thus the net effect of the electron pulses is the production

of thermalized solvent cation radicals (h+) in DCE and thermalized solvated electrons (e)

in THF. Because holes in DCE and electrons in THF attach to the desired analyte only

after diffusion, formation of the desired radical cation or anion species is a bimolecular

process. Since the analyte is always present in excess in these experiments, these

reactions are pseudo-first order, and the observed rates of h or e- transfer to analyte

depends upon analyte concentration and the bimolecular rate constant for reaction of h

or e- with the analyte. A fraction of h+ in DCE and e- in THF are lost by reactions with

counter ions, however, these reactions are measured and taken into account in the results.

Electron and hole transport to polymers following radiolysis

A series of experiments were carried out in order to determine the bimolecular rate

constants for h+ and e- trapping reactions, i.e. k2-2 and k2-3,




PAE + (PhMe)2* PAE* + 2 PhMe (2-2)
DCE/PhMe

PAE + e-s PAE'* + 2 PhMe (2-3)
THF

Reactions 2-2 and 2-3 are both strongly exoergic (AGo < -0.5 eV), and occur with

diffusion controlled rates. It is important to note that the "rate constants" reported in this

discussion are not in fact constant, although they do not vary significantly within the time









of the reactions under study. The experimentally observed pseudo-first order decays are

therefore nearly exponential. Consequently, the rate constants reported herein are

effective or "average" rate constants which represent best fits to the observed pseudo-first

order kinetic traces for the rise of the absorption due to the creation or PAE radical ions.

If the range of concentrations of polymer was significantly higher or lower, these

constants would have somewhat different values.

The species produced by radiolysis of PPE164 in DCE/toluene reacts with TMPD, a

good hole scavenger, with a bimolecular rate k = 2.8 x 109 M 1 s-1 to produce TMPD'

This is confirmed by the observation of the characteristic absorbances of TMPD+ at 570

nm and 620 630 nm.95 Also, the species produced as a result of radiolysis of PPE164 in

THF reacts with tetracyanoethylene (TCNE) and chloranil to give species with the

characteristic absorbance bands of TCNE'- and chloranil'.95 On this basis, the product of

PAE radiolysis in DCE/toluene can be assigned as the radical cation and the product of

PAE radiolysis in THF can be assigned as the radical anion.

The effective rate constants for the bimolecular reactions of h (in DCE/toluene)

and e- (in THF) formed by pulse radiolysis with PAE's to produce radical cations and

anions (respectively) of the polymers is presented in Table 2-3. The bimolecular rate

constants are reported with respect to both polymer and repeat unit concentration. There

are several points worth noting with respect to this data. First, the experimentally

determined rate constants for growth of the PAE radical ions, as expressed per monomer

repeat unit, is significantly lower than the expected diffusion controlled reaction between

two small molecules. Second, in general the rates for electron transfer to PAE are









considerably greater than the rates for hole transfer. This difference is due to the faster

diffusion rate of e-s.96-99

Table 2-3. Reaction rate constants of poly-(arylene ethynylene)s (PAEs) and
terthiophene end-capped PAEs with positive and negative charge carriers in
solution.
Polymer N k (M' s-) k(monomer) (M'1 s')a

PPE164 + (MeD)2+ 164 2.1 x 1011 1.3 x 109
PPE24 + (Me(D)2+ 24 4.8 x 1010 2.0 x 109

PBpE21 + (Met)2*+ 21 5.1 x 1010 2.4 x 109
T3PPE13 + (Me()2+ 13 7.1 x 1010 5.5 x 109
T3PbpE12 + (Me(D)2+ 12 3.0 x 1010 2.4 x 109
T2 + (Me()2+ 1.1 x 1010 5.3 x 109

T3 + (MeO)2*+ 1.7 x 1010 5.5 x 109
T3 + [Me(':C1] 5.0x 109 1.7 x 109

T4 + (MeD)2 b b
T4 + [Me(':C1] 4.1 x 109 1.0 x 109
PPE164+ e-THF 164 4.7 x 1012 2.8 x 1010
PBpE21 +eTHF 21 1.3 x 1012 6.3 x 1010
T3PPE13 + e-THF 13 7.1 x 1011 5.5 x 1010
T3PbpE12 + eTHF 12 3.5 x 1011 2.8 x 1010
a For comparison to small molecules the average rate constant per repeat unit is reported. For T3-containing
polymers the T3 was not included.
b Could not be measured due to absorption of the product cation at 1000 nm.

Poly(arylene ethynylene)s are known to aggregate in solution. The degree of

aggregation is dependant on both the solvent system and the concentration of the

polymer.22'50 The pseudo-first order growth of PPE164*+ as a function of polymer

concentration is linear when the concentration is less than 1.35 x 10-3 M in repeat units.

This suggests that aggregation is not an important factor in DCE/toluene solution at the









concentration ranges used in these studies. Since THF is a better solvent for PAE's than

DCE/toluene, it is unlikely that aggregation is a factor in that medium either.

Semi-empirical calculations: spectroscopy of PPE-based ion radicals

Before examining the spectra of PAE ion radicals, it is useful to use theory to

attempt to provide insight into the expected electronic transitions. Thus, semi-empirical

calculations (ZINDO/S, CI = 6,6, AMlu geometries) on a series of PPE-type model

oligomers, [Ph-(-C-C-Ph-)n-H]+ (structure in Figure 2-10), were carried out.100 The

Zindo/S calculations predict two optical transitions with energies that decrease with

increasing oligomer length, n. With the exception of the shortest member of the series

(i.e., diphenylacetylene, n = 1) the lowest energy transitions in the radical cations derive

primarily from a strongly allowed, one-electron transition from the HOMO-1 to the half-

occupied HOMO (SOMO) as indicated schematically in Figure 2-9. (Here HOMO and

LUMO refer to the orbitals that are the highest occupied and lowest unoccupied in the

neutral parent molecule.) This low-energy transition is predicted to occur in the near IR

region, and its energy is expected to vary strongly with the length of the PPE segment.

The calculations predict that a second, higher energy transition will occur in the visible

region. This transition is primarily of HOMO (SOMO) LUMO character.

Furthermore, the calculations predict that the energy of this transition varies only weakly

with oligomer length. Similar results are obtained for the anion radicals, however, in this

case the near IR bands derive primarily from the LUMO (SOMO) LUMO + 1

transition (see Figure 2-10).















15 I I I I
+ + +-

Shv(anionIR)
10 -
;f^ (nm) hv (vis)

1 1000 y hv(cation R)
1> 5-
2000

0 I I I I I
2 4 6 8 10
n


Figure 2-10. Transition energies computed for radical cations of phenylene ethynylene
oligomers as a function of length, n. The energy of the lowest allowed
transition (o) is very sensitive to length, while the energy of the strong
transition in the visible (+) changes little with n. The simple MO diagram at
right indicates the nature of the transitions, determined from the CI
calculation. Calculations were ZINDO/S for at AMlu geometries of the
cations.


Visible/Near-IR Spectroscopy of Charged PAEs

Radical anions

By monitoring the wavelength dependence of the transient absorption of PAE

solutions at a fixed delay time following the e- pulse it is possible to construct the visible-

near IR absorption spectra of the PAE-based radical ions. The delay time used is

dependent upon the concentration of polymer and the bimolecular rate constant for

charge transfer from the solvent to the polymer. Generally in these experiments the delay

times used were of the order of 0.1-1.0 hts. The time was chosen to follow the growth of

the radical ion species under observation and precede its decay. It is possible to estimate









the molar absorptivity (s, M-cm-1) of the PAE radical ions based on radiolysis yields of

(PhMe)2*+ in DCE/toluene and es in THF. The resulting spectra for a variety of PAE-

based radical ions, along with the spectra of the T3 oligomer based radical ions, are

illustrated in Figure 2-11.


1200 1600
X (nm)


Figure 2-11. Spectra of cations and anions of poly(phenylene ethynylenes) (+),
endcapped poly(phenylene ethynylenes) (A) and T3 (e) where a) is the
spectrum of PBpE21z-, T3PBpE12*- and T3*- in THF, b) is the spectrum of
PPE164*-, T3PPE13*- and T3*- in THF, c) is the spectrum of PBpE21i+ and
T3PBpE12*+ in DCE/toluene and T3*+ in DCE and d) is the spectrum of
PPE164*+ and T3PPE13*+ in DCE/toluene and T30+ in DCE. The polymer
concentrations were in the range 0.6 2.8 mM in repeat units and delay times
were in the range 0.1 1.0 [ts.









Figure 2-1 la shows the spectra of the radical anions of T3*- and the biphenyl-based

PAEs, PBpE21'- and T3PBpE12*. All of these anion radicals feature two absorption

bands: one relatively sharp band in the visible region and a second broader band in the

near-IR. These two bands are intense, with molar absortivities (s) greater than 50,000

M^cm-1. The observed spectra are qualitatively consistent with the predictions of

Zindo/S calculations, which supports their assignment to PBpE-based radicals. The

experimentally observed absorption bands occur at Xmax w 600 and 1100 nm in T3*-, while

in PBpE21'- and T3PBpE21i'- max = 625 and > 1600 nm. It is noteworthy that the lower

energy band of PBpE anion radical is considerably broader and red-shifted than the band

for the T3 anion radicals. This indicates that the electron is considerably more

delocalized in the former.


Even more significantly, the spectra of PBpE21'- and T3PBpE21i' are essentially

identical, indicating that T3 end-caps do not influence the spectrum of T3PBpE21i'. Thus,

the electron is not localized on the T3 end-groups in the end-capped polymer; it is instead

delocalized on the PBpE main chain. This is constant with expectation because T3 is

more electron rich than PBpE, and its LUMO is therefore likely to be higher in energy

than that of PBpE.


Figure 2-1 lb shows the spectra of the radical anions of the phenylene based

polymers. Like in the biphenylene-based polymers, the T3 end-cap has no effect on the

spectra, which exhibit ,max = 625 and > 1600 nm for both PPE164*- and T3PPE13-. This

result is expected, and indicates that there radical anion is again located on the polymer

main chain and not on the end-caps.









Radical cations

Figure 2-11 also depicts the absorption spectra of the T3PBpE12+, PBpE21i, and

T3+ radical cations. All of the spectra feature two strong absorption bands, one in the

visible and a second in the near-IR, consistent with the Zindo/S predictions. The bands

in T3*+ are observed at Xmax & 550 and 850 nm, in good agreement with the previously

reported spectrum of this species.101 The two bands appear at Xmax 600 and 1150 nm in

PBpE21+ and T3PBpE12+. With respect to the biphenylene based polymers, the most

significant feature of these two spectra is that they are essentially identical. This strongly

suggests that in T3PBpE12+ the hole resides on the PBpE chain and is not trapped by or

localized on the T3 end-caps. This result, while initially surprising, is confirmed on the

basis of oxidation potentials estimated by bimolecular hole transfer equilibrium

experiments (see below). From this data, it is evident that the Eox of T3 is somewhat

greater than that of PBpE. Thus at equilibrium PBpE* is favored over T3*+ (i.e. the

equilibrium in equation 2-4 lies to the left).

T3-(PBpE +)-T3 -0- T3-(PBpE)-T3+ (2-4)

Figure 2-1 Id compares the spectra of PPE164*+ and T3PPE13*. The spectra of these

species are noticeably different. While PPE164*+ features two bands at Xmax 600 and

1950 nm, the visible band is red-shifted to Xmax 640 nm and the near-IR band is blue

shifted to Xmax 1350 nm in T3PPE13*. The spectrum of PPE24 *(not shown), was the

same as that of PPE164^. This result indicates that the differences in the spectra of

PPE164*+ and T3PPE13*+ are not simply due to the differences in the degrees of

polymerization of the two polymers. The differences in the spectra strongly imply that










the hole in the end-capped polymer is trapped on the T3 end group. However, although

the presence of the end group does have an effect on the spectrum of the cation radical,

the spectrum of the hole on the end-capped polymer is distinctly different from that of the

end group cation radical, T3*. Specifically, the low energy transition in T3PPE13*+ is red

shifted and broader compared to the analogous transition in T3*

Some insight into this observation can be garnered from the spectrum of the cation

radical of the end-group model, Ph- T3*, which is shown in Figure 2-12. This spectrum

features narrow bands at max 620 and 1050 nm. That these bands are at wavelengths

intermediate between the observed bands in T3*+ and T3PPE13+ suggests that the cation

radical is delocalized onto the phenylene adjacent to the terthienyl end-cap.



0.4


0.3 -


0.2


S.1 / \


0.0 --------------------

-0.1 -



-0.2
400 600 800 1000 1200 1400 1600
Wavelength nm


Figure 2-12. Differential absorbance spectra ofPh- T3*+ (solid line) and T3*+ (dashed
line). The spectra were obtained by photo induced electron transfer to
methyl viologen, and recorded 400 ns after the laser pulse. Note that this
T3*+ spectra is identical to the radiolyticaly generated spectra.









This idea is further supported by calculations. AM1 level molecular orbital

calculations indicate that while the species' HOMO resides predominantly on the T3,

there is a significant orbital density on several adjacent phenylene units. Figure 2-13

shows this graphically. These results indicate that the cation radical T3PPE13i+ is not

localized on the end-cap. Instead, it is somewhat delocalized into the polymer main

chain.











Figure 2-13. A segment of a PPE featuring seven phenyl ethynyl rings at the left and a
terthiophene end-cap at the right. AM1 calculated orbital densities indicate
that the HOMO lies predominantly on the T3, but also has significant orbital
density on the PPE main chain.

This provides an explanation as to why the absorption spectrum of T3PPE13+ is not

a simple linear combination of spectra for T3+ and PPE164+. Additionally, the low

energy band in T3PPE13+ is considerably red-shifted and much broader than in T3

These facts support the notion that the hole in the end-capped polymers is delocalized

significantly more than in the terthiophene end-cap alone. It is noteworthy that there is

likely to be an equilibrium established involving hole-transfer from the end-group to the

main chain, as in equation 2-5. This equilibrium may also have an effect on the observed

spectrum.


O T3-(PPE)-T3'+


T3-(PPE'f )-T3


(2-5)









Bimolecular Hole Transfer Reactions: Thermodynamics of Intrachain PPE -> T3
Hole Transfer

In order to explain the surprising result that holes are not trapped by the T3 end-

groups in T3PBpE12*, experiments involving bimolecular hole transfer from a variety of

2,5-oligothienyls (Tn, n = 1, 2, 3, 4) were performed. These experiments were designed

to determine the energetic of intra-chain hole transfer by measuring the position of the

equilibria, and the rate constants, for the following series of hole-transfer reactions.

PAE' + T2 O PAE + T2+ (2-6)

PAE+ + T3 O PAE + T3+ (2-7)

PAE+ + T4 O PAE + T4+ (2-8)

Using the reported oxidation potentials for the TnO/+ couples,102 and the

experimentally determined equilibrium constants for each of the above reactions, the

redox potentials (Eox) for the polymers were determined. Cyclic voltammetry, (CV)

both in the solid state and in solution, was attempted to confirm these results, however,

CV experiments proved unsuccessful, giving only broad, irreversible waves at 1.1 1.3

eV.

Table 2-4 is a compilation of experimentally determined rate and equilibrium

constant data, and Figure 2-14 provides a redox scale summarizing the Eox values. An

example of the data used in these measurements is provided in Figure 2-15. These

measurements unambiguously show that the oxidation potentials for all of the polymers

are bracketed between the potentials for T2 and T4 (1.25 V > Eox > 0.80 V), while

equilibria are established with T3.









Table 2-3. Bimolecular rate and thermodynamic data for reactions of PAE's with
thiophene oligomers.


Reaction n K (M1 s) Keq AG (mV) Eox (V)
vs. SCE

PPE164+ + T2 164 -(2.5 + 1.5) x 1011c (1.0 + 0.5) x 10-3 177 + 40 1.03 + 0.04

PPE164+ + T3 164 4.5 x 109 0.82 5.2 0.91 a

PPE24+ + T3 24 3.6 x 109 6.0 2 -46 0.96 a

PPE164+ + T4 164 5.6 x 109 > 1.7 <-14

PBpE21+ + T2 21 < 4.1 x 10-2 > 82 <1.13

PBpE21+ + T3 21 -0.12 55 0.86 a

PBpE21+ + T4 21 4.4 x 109 > 6.3 < -47 >0.81 a

T3PPE13+ + T3 13 -1.1 -1.3 0.91 a

T3PPE13+ + T4 13 3.8 x 109 > 45 < -98 >0.86 a
a Estimated uncertainties in -AG or Eox are (+ 10, -40 mV) for reactions with T3 and T4 due to uncertain
contributions from a second oxidizing species and to formation of an unknown species at longer times (see
text).
b Oxidation potentials for the polymers based on reported potentials Eox = 1.25 (T2), 0.95 (T3) and 0.80
(T4) in Vvs. Ag/AgC1103 were converted to SCE reference by subtracting 44 mV to give 1.21, 0.91 and
0.76. See note a.
The sign indicates that this is the reverse reaction. Only the reverse reaction (a transfer of charge from
T2+ to PPE) was observed.

The experimentally determined equilibrium constants provide an explanation for

the fact that the T3 end-caps capture holes in the case of the phenylene polymer, but not

in the biphenylene polymer. E0ox for the biphenylene polymer is approximately 100 mV

lower than in the phenylene analogue. This difference is significant enough that the

equilibrium constant for the hole-transfer reaction between PBpE21 and T3 strongly favors

placing the hole on the polymer whereas in the case of the phenylene the equilibrium

favors the hole residing on the end-cap. Another significant result of these experiments is

that hole injection intoPPE164 is thermodynamically favored over injection into PPE24 by

a factor of 8 + 2.5. This is nearly identical to the ratio of their chain lengths (6.8), and










very likely reflects entropic factors which favor the hole on the longer chain. IfL is the

delocalization length of a charge carrier, in repeat units, then there are Dp-L ways to place

a charge in a polymer with a given degree of polymerization. Thus the ratio of the

equilibrium constants for two polymers whose degrees of polymerization are Dp, and Dp2

should be K(Dpl)/K(Dp2) = (Dpi-L)/ (Dp2-L). As a consequence, these results imply that

the delocalization length of a hole, L, is slightly less than 10 repeat units, or twenty

phenylene rings. Furthermore, Keq for the reaction between T3PPE13*+ and T3 is about 1,

leading to the surprising conclusion that the charge on the end-capped polymer is not

stabilized due to the delocalization of the end groups. This likely reflects a balance

between the destabilizing electron withdrawing effects of the PAE main-chain104 and the

stabilization associated with delocalization of the T3 HOMO into the polymer chain.


1.21 V 2T

S 0.96V PPE24
47 mV
T3, T3PPE13
-0.91 PPE
PPE164

54 mV
0.85 V PBpE21





0.76 V 4T





Figure 2-14. Schematic of the oxidation potentials of various PAE's and thiophene
oligomers. The potentials are vs. SCE.











20 8.2 MM PPE
8.2 |M PPE + 15.5 6M T3
O
1- 15
x

0 58.2 TM PPE




S46M ee is T3
C 10

8.2 M PPE + 78 pM T3












due almost entirely to the polymer cation radical. The rate of charge transfer
and the equilibrium are evident from this data, along with the complication
that an additional species is formed at long times, especially at the highest
concentration of8.2 M PPE +216 short times, (.5 s) the strong absorption








of the dimeric toluene cation radical (MePh2.+) is evident, most noticeably in
the trace of the experiment with only 0.39 mM T3
0 5 Time ps 10 15








Figure use of electron transfer equilibria function of time after an eletoxidation potentials typically
involves errors of only a few mV. However, the uncertainties in terthiophene (T3) equilibrium

1,2-constants and free energy change + 1.6 M toluene. At this wavelength, to complexitiesorbance inherent
indue almost entirely to the systempolymer cation radical. The rate-toluene of charge transfer complex (CPhMe) oxidizes PA
and the equilibrium are evident from this data, along with the complication
that an additional species is formed at long times, especially at the highest
concentration of T3, 216 [M. At short times, (<0.5 [s) the strong absorption






only slowly if at all, but it does oxidize T and T to their radical (MePh2+) is evident, most noticeably in
the trace of the experiment with only 0.39 mM T3.

The use of electron transfer equilibria to obtain oxidation potentials typically

involves errors of only a few mV. However, the uncertainties in the equilibrium

constants and free energy changes reported here are reduced due to complexities inherent

in the system. The Cl'-toluene 7r-charge transfer complex (CI:PhMe) oxidizes PAE's

only slowly if at all, but it does oxidize T3 and T4 to their radical cations. This fact, and

the limited lifetime of Cl:PhMe means that the number of ions entering into equilibria

change with the concentration of T3 and T4. Another complexity arises relating to the

slow formation of an unknown species, possible a hetero-dimer between T3 and PAE'+









that causes limitations in the ability to work at higher reactant concentrations. These

effects contribute to the uncertainties reported for the free energy changes in Table 2-3.

Dynamics of Interchain Hole Transfer

As noted in the introduction to this chapter, one goal of this work is to determine

the dynamics of intrachain hole transfer through the conjugated PPE. When holes are

injected into T3PPE13 more than 80% are likely to be captured by the long PPE segment,

which consists of an average of 26 phenylene rings. This preference arises because of the

high exothermicity of hole transfer from the primary donor, (PhMe)2", and the fact that

there is a much higher concentration of phenylene moieties than thiophene end-caps.

However, at the earliest times accessible after radiolysis, the only observable product in

T3PPE13 is a species where the hole already resides on the end-cap. This means that

intramolecular hole transfer is faster than the bimolecular hole capture reaction between

the solvent and the polymer. With a saturated solution of T3PPE13 ([PRU] = 1.6 mM),

the rate constant for hole attachment is 3.6 x 1010 M^s-1. Thus, while an accurate figure

for kHT cannot be reported at the present time, it is possible to ascertain a lower limit of

1 x 108 s-

Conclusions

In summary, PPE molecular wires with two different architectures one based on

phenylene and the other on biphenylene, were synthesized both in their prestine form and

with terthiophene end-caps. The ultraviolet and visible absorption and

photoluminescence spectroscopy of these materials indicate that their excited state is not

completely localized on the oligo thiophene endgroup, but is instead somewhat

delocalized into the polymer main chain. This idea was confirmed by studying model

compounds and by Zindo/S calculations.









The cation and anion radicals of the polymers were generated by pulse radiolysis at

the LEAF at Brookhaven National Laboratories. Both radicals show transitions in the

visible and IR that agree with the reported theoretical predictions for species of this type.

The anion radicals of the end-capped species are not trapped by terthiophene, which is

the expected result considering that terthiophene is more electron rich than the polymer

backbone. The cation radicals of the phenylene based polymer are trapped by the end-

cap, although as in the case of the exciton, the charge is somewhat delocalized into the

polymer main chain. On the other hand, in the case of the biphenylene based polymer,

cation radicals are not trapped by the end groups and remain on the polymer main chain.

This result is explained by bimolecular hole transfer experiments which show that the

oxidation potential of the biphenylene based polymer is somewhat higher than that of the

end-group. These same bimolecular hole transfer experiments are used to determine the

oxidation potentials of the parent and end-capped polymers.

It was hoped that this investigation would allow for the determination of the rate

constant associated with the transfer of a radical cation from the main chain of a PPE to

the terthiophene end cap. Unfortunately, this process is much faster than capture of holes

by the polymer, indicating that the rate of hole transfer is greater than 1 x 108 s. This

result is expected from the absorption spectra of the parent polymers, which indicate that

the charges are highly delocalized in the polymer chain. It is likely that charge transport

is much faster than the limit observed in these experiments, pointing to the desirability of

developing new and more effective methodologies for charge injection on ultrafast

timescales.









Experimental Section

Materials and General Synthesis

1,2-Dichloroethane (Aldrich) was HPLC grade and dried over type 4 A molecular

sieves prior to use. Anhydrous 99.8% toluene (Aldrich) was used as received.

Tetrahydrofuran (Aldrich, anhydrous) was distilled from LiA1H4 and then from

sodium/benzophenone and stored in an environment of argon. 2,2'-Bithiophene,

2,2':5',2"-terthiophene (T3), 2,2':5',2":5",2" '-quaterthiophene (T4),

tetramethylphenylenediamine (TMPD), tetracyanoethylene (TCNE), 2,3,5,6-

tetrachlorobenzoquinone (chloranil), 1,4-iodobenzene, 1,4-dimethoxybenzene, Pd(PPh3)4

and Cul were purchased from Aldrich or Acros. Quaterthiophene was recrystallized from

toluene and 2,2'-bithiophene, TMPD, TCNE and chloranil were sublimed prior to use

and the other compounds were used as received. 2-Iodo-5:2',5':2"-terthiophene (I-T3)

was prepared by a literature procedure.106 Chromatography was carried out using silica

gel (Merk, 230-400 mesh). NMR spectra were obtained on Varian Gemini or VXR

spectrophotometers operating at 300 MHZ or an Inova spectrophotometer operating at

500 MHZ.

Synthetic Procedures

2,5-Diiodo-1,4-dimethoxybenzene. This compound was prepared by a modified

literature procedure.105 A solution of potassium periodate (8.0 g, 35 mmol) in water (30

mL) was added to a round bottom flask charged with 1,4-dimethoxybenzene (6.9 g, 50.0

mmol) and iodine (12.7 g, 50.0 mmol) and acetic acid (120 mL). The flask was fitted

with a condenser, and the solution was heated to a gentle reflux with stirring. After

attaining reflux, a solution of sulfuric acid (3.0 mL in 15 mL water) was added slowly

through the condenser. An exothermic reaction was observed after addition of the first









few drops of acid solution, and the heating was suspended temporarily until addition of

the sulfuric acid was complete. The reaction was then returned to reflux. After

approximately 30 minutes, the brown iodine color of the solution appeared significantly

lighter. (Caution: While the solution is refluxing, crystals of iodine sublime into the

condenser, and it is necessary to use a long glass rod to push these crystals back into the

solution. If the condenser becomes completely clogged with iodine, an explosion may

result due to the buildup of gas pressure in the system.) When 1.5 hours had elapsed

after the addition of sulfuric acid, heat was removed from the reaction and it was allowed

to cool to room temperature. The solution was then treated with a saturated aqueous

solution of Na2S204 until no iodine color was noticeable. The reaction mixture was

diluted with water to double the original volume to induce precipitation of the product as

yellow crystals. The solid is collected by vacuum filtration, washed with water, and

recrystallized from hot acetone/water giving the desired product as a white, crystalline

solid, yield 17.4 g (89%).

2,5-Diodo-1,4-dihydroquinone. This compound was prepared by a modified

literature procedure.105 A 500 mL Erlenmeyer flask with a ground-glass top was charged

with 2,5-diiodo-l,4-dimethoxybenzene (30.0 g, 76.9 mmol) and 350 mL methylene

chloride (previously dried over P205). The resulting solution was cooled in a bath of

ethanol and liquid nitrogen, and fitted with a condenser. Neat BBr3 (38.5 g, 14.5 mL,

153.8 mmol) was added slowly through the condenser. After the addition was complete,

a septum cap was affixed to the top of the condenser, and a gentle flow of N2 gas was

introduced to the reaction through a needle. The mixture was allowed to warm to room

temperature, then heated to reflux and stirred for 48 hours. After this time, small portions









of methanol and then water were added to the solution. Methylene chloride was

evaporated by rotory evaporation and the residue was diluted with methanol-water

(80:20). A few spatula tips of sodium sulfite (Na2SO3) was then added. The solution was

then placed in the freezer overnight, after which fine off-white crystals of product formed

and were collected by vacuum filtration, yield 24.9 g (89%).

2,5-Bis-(2-ethylhexyloxy)-1,4-diiodobenzene. This compound was prepared by a

modification of a literature procedure.105 To a solution of 2,5-diiodohydroquinone (4.24

g, 11 mmol) in methyl ethyl ketone (80 mL) in an Erlenmeyer flask with a ground-glass

top was added potassium carbonate (15.2 g, 110 mmol). The system was fitted with a

condenser, and a nitrogen inlet and outlet. The mixture was gently heated for 20 min

with stirring to dissolve most of the potassium carbonate. Then 1-bromo-2-ethylhexane

(2.5 mL, 44 mmol) was added via syringe. The system was then heated to reflux for 16

hours. Substantial amounts of salt precipitated over the course of the reaction. After

reflux, 1 g of sodium sulfite (Na2SO3) was added to the mixture, and the solution was

then neutralized by slow addition of 1 N aqueous HC1. The reaction mixture remained

under positive nitrogen pressure, with an outlet to allow for the escape of gasses, during

the entire process. After neutralization the mixture was further diluted with 100 mL of

warm (400 C) water, and stirred for one hour. The mixture was then extracted with

pentanes (3 x 40 mL) and the organic layer was dried over MgSO4 and concentrated to a

volume of 10 mL. The concentrated pentane solution was passed through a short (3"

length, 2"diameter) column of silica by elution with 200 mL of pentanes, followed by

evaporation of the solvent under reduced pressure afforded the desired product as a clear









liquid, yield 4.07g, 63%. 1H NMR (CDC13): 6 0.9-1.8 (complex multiple, 26H), 3.8 (t,

4H), 7.1 (s, 2H).

1,4-Diethynylbenzene. This compound was prepared by a modified literature

procedure.105 1,4-diiodobenzene (18.0 g, 54.6 mmol), Pd(PPh3)2C12 (0.4 g, 0.57 mmol)

and Cul (0.1 g, 0.525 mmol) were dissolved in a mixture of THF and diisopropylamine

(200 mL, 8:2) which has been previously deoxygenated by 30 min of bubbling with N2.

This solution was cooled with an ice bath whereupon trimethylsilylacetylene (20 mL,

13.9 g, 0.142 mmol) was added dropwise with stirring and under positive pressure of

nitrogen. After a small volume (< 5 mL) was added, an endothermic reaction was

observed with formation of a thick precipitate of white powder. After the addition was

complete, the reaction was allowed to warm to room temperature and it was stirred an

additional twenty-four hours. After this time, the reaction mixture was a dark color. The

solvents were removed by rotory evaporation and the remaining black solid was washed

several times with water and a water-methanol (40:60) mixture. The solid was then

dissolved in hot hexane, decolorized with active charcoal, and passed through a short

column of alumina. The resulting white solid was recrystallized from acetone/water to

afford 1,4-bis-(trimethylsilylethynyl)benzene as shiny white crystalline flakes. The 1,4-

bis-(trimethylsilylethynyl)benzene crystals were dissolved in a mixture of THF and

methanol (80:20). To the resulting solution was added a solution of KF (10.0 g) and a

spatula-tip of tetrabutylammonium bromide in water (80 mL). The resulting suspension

was vigorously stirred at room temperature under N2 for 12 hours, and then diluted with

water until the product precipitated as a fine white powder. The solid was collected by









vacuum filtration, washed with water and recrystallized from methanol/water to afford

1,4-diethynylbenzene as a white solid, yield 5.6 g (81%).

4,4'-Diethynylbiphenyl. This compound was prepared by the same procedure as

described above for 1,4-diethynylbenzene, except that 4,4'-diiodobiphenyl was used in

place of 1,4-diethynylbenzene. The product was obtained as a white solid, yield for the

two steps 75%.

1,4-Diundecyloxy benzene. A mixture of potassium carbonate (22 g, 0.16 mol)

in methyl ethyl ketone (40 mL) in an erlenmeyer flask with a ground glass top was heated

gently and stirred to dissolve most of the potassium carbonate. The solution was then

spurged with argon for 15 minutes, after which time hydroquinone (5 g, 0.045 mol) and

1-bromo undecane (26 g, 0.10 mol) was added. The reaction mixture was fitted with a

condenser and heated to reflux for 12 hours. A large volume of salt precipitated during

the course of the reaction. After this time, the solution was cooled to room temperature

and neutralized by slow addition of 1 N HC1. The reaction mixture was kept under a

positive pressure of argon, with a vent to allow for escape of gasses, during this entire

process. A few spatula tips of sodium sulfite were then added to the reaction mixture.

The crude product was precipitated by pouring into a large quantity of ice-cold water, and

the recovered by filtration as a tan solid. A portion of the product used for analysis was

purified by recrystalization from methanol-water, but the crude material was used in the

next step. The crude yield was 14.8 g (78%). 1H NMR (CDC13): 6 0.08 (t, 6 H), 1.6 (br,

32 H), 1.8 (m, 4 H), 3.9 (t, 4 H), 7.0 (s, 4 H).

2,5-Diundecyloxy-l,4-diiodo benzene. This product was obtained by the same

procedure used to pepare 2,5-dimethoxy-1,4-diiodo benzene, except the amounts of









reagents used were: 1,4-diundecyloxy benzene (10 g, 0.024 mol), iodine ( 6.1 g, 0.024

mol) and potassium periodate (3.6 g, 0.016 mol). The product was purified by elution

through a short plug of silica with hexanes, treatment with activated carbon and

recrystalization from methanol-water. 1H NMR (CDC13): 6 0.08 (t, 6 H), 1.6 (br, 32 H),

1.8 (m, 4 H), 3.9 (t, 4 H), 7.0 (s, 4 H).

Photophysical Methods

UV-visible absorption spectra were obtained on a Varian Cary 100

spectrophotometer. Corrected steady-state emission spectra were recorded on a SPEX F-

112 fluorescence spectrophotometer. Samples were contained in 1 cm x 1 cm quartz

cuvettes, and the optical density was adjusted to approximately 0.1 at the excitation

wavelength. Emission quantum yields are reported relative to perylene (Oem= 0.94)107

and appropriate correction was applied for the difference in refractive indices in the

sample and actinometer solvent.108 Time-resolved emission decays were obtained by

time-correlated single photon counting on an instrument that was constructed in-house.

Excitation was effected by using a violet diode laser (IBH instruments, Edinburgh, 405

nm, pulse width 800 ps). The time-resolved emission was collected using a red-sensitive,

photon counting PMT (Hamamatsu, R928) and the light was filtered using 10 nm band-

pass interference filters. Lifetimes were determined from the observed decays with the

DAS6 deconvolution software (IBH Instruments, Edinburgh, Scotland). GPC was

preformed using a Rainin Dynamax model SD-200 solvent delivery system equipped

with two PL-Gel 5 micron Mixed D columns (Polymer Laboratories, Inc., Amherst, MA)

connected in series, and a UV detector set at a wavelength where the polymer absorbs.









Molecular weight information was calculated from the chromatograms using Polymer

Laboratories software.

Generation of the T3 and Ph-T3 Radical Cations

These species were generated by photo-induced electron transfer from the neutral

molecules to methyl violegen (MV2+). To a 15 mM solution of MV2+ in THF/acetonitrile

(2:1) T3 or Ph-T3 were added until the absorbance of the analyte at 355 nm was 0.7. The

solution was subjected to 10 mJ, 355 nm laser pulses to generate the cation radical. The

differential absorption spectrum of the cation radical was recorded on an instrument built

in-house.

Radiation Techniques

This work was carried out at the Brookhaven National Laboratory Laser-Electron

Accelerator Facility (LEAF). The facility has been described elsewhere.109,110 The

electron pulse (< 120 ps duration) was focused into a quartz cell with an optical path

length of 20 mm containing the solution of interest. For the polymer solutions, the

concentration of repeat units used was typically 0.2 2 mM. The monitoring light source

was a 75 W Osram xenon arc lamp pulsed to a few hundred times its normal intensity.

Wavelengths were selected using either 40 nm or 10 nm band pass interference filters.

Transient absorption signals were detected with either FND-100Q silicon (< 1000 nm) or

GAP-500L InGaAs (> 1100 nm) diodes and digitized with a Tektronix TDS-680B

oscilloscope. The transmission/time data were analysed with Igor Pro software

(Wavemetrics). Reaction rate constants were determined using a non-linear least squares

fitting procedure described previously.112 This procedure accounts for geminate

recombination, which is encountered on the time scales investigated. Bimolecular rate

constants were determined using the linearity of the observed pseudo-first order growth









of the product with respect to the solute concentration. Where not stated, uncertainties

are +/- 15%. Bimolecular reactions of PAE ions with charge acceptors often went to

completion with no detectable PAE+* remaining. But in others, including most reactions

with terthiophene, the kinetics proceeded to equilibria in which substantial fractions of

the PAE+ remained. The equilibrium constants could be calculated using this

information. Molar extinction coefficients of the radical cations were calculated using

G(DCE+*) = 0.68,113 where G is the radiation chemical yield (molecules produced/100

eV). For the anions the reported G-values for the electron in THF has varied greatly.

The value used was G(e-THF) = 0.53, the average of a number of reported values.114-119

The total dose per pulse was determined before each species of experiments by

measuring the change in absorbance of the electron in water. The dose received was

calculated using ; (700 nm, eaq) = 18,830 M-1 cm1 and G(e-aq) = 2.97. The dose was

corrected for the difference in electron density of the organic solvents used compared to

that of water. Radiolytic doses of 5 18 Gy were employed. For the DCE/toluene

solutions, dissolved oxygen was removed by purging with argon gas for at least 10

minutes, and subsequently sealing the cells with septa and parafilm. Solutions in THF

were prepared in an argon environment and sealed under argon with Teflon vacuum

stoppers. Samples were prepared immediately prior to use. During irradiation, samples

were exposed to as little UV light as possible to avoid photodecomposition, although no

evidence of this occurring was found within the time frames monitored. Measurements

were carried out at 210 C.














CHAPTER 3
MECHANISM AND DYNAMICS OF TRIPLET TRANSPORT IN PLATINUM
CONTAINING POLY (PHENYLENE ETHYNYLENE)S



Introduction

A great deal of effort has been placed into investigating the carrier properties of

singlet excitons in single chain conjugated polymers.17,66,71,73-75,87,120,121 However, despite

these efforts, relatively little is known about the analogous properties of triplet excitons in

these systems. This chapter describes a study aimed at investigating the properties and

dynamics of triplet transport in platinum containing PPEs, often called Pt-acetylides.43'122

The major aim of the study is to determine whether or not intrachain energy transfer in

Pt-acetylides is an efficient process. Another goal is to establish a model for transport in

these materials that explains why this is or is not so.

The polymers used in this study are depicted in Figure 3-1. These polymers feature

a Pt-acetylide backbone based on 1,4-diethynyl benzene as the acetylide unit. The 2,5-

diethynyl thiophene unit was incorporated into the polymers at various loadings to serve

as a trap for phenylene based triplets. Platinum acetylides were chosen because their

triplet states are highly phosphorescent, and are thus easily monitored by

photoluminescence spectroscopy.37-39 Furthermore, they posses a rigid rod "molecular

wire" structure, and are easily synthesized by use of copper iodide catalyzed Hagihara

condensation polymerization.44'122-124 In order to study the transport in these materials,

thiophene was chosen as an energy trap. The conditions for a triplet energy trap are that









the triplet state of the trap must have a lower energy than the triplet state of the polymer

main chain. In this case, the energy of the thiophene triplet is known to be 2.0 eV, while

that of the phenylene triplet is 2.4 eV.36 In addition, varying the amount of thiophene in

the reaction feed allows for straightforward synthesis of a series of copolymers in which

the ratio of co-monomers varies.


PBu3 PBu3

tPBU3 X PBu3
n
x = 1, y = 0 PooTo
x = 0.95, y = 0.05 P95T5
x= 0.85, y = 0.15 P85T15
x = 0.75, y = 0.25 P75T25
x= 0, y= 1 PoTloo

Figure 3-1. Structure of polymers featured in this chapter.

Unlike the polymers described in Chapter 2, which feature trap sites only at the

end of the polymer chain, the polymers described in this chapter have trap sites dispersed

throughout their backbone. There are several reasons for this change in approach. First,

the synthetic challenges associated with end-capping conjugated polymers proved to be

considerable. However, synthesis of the co-monomers used to make the polymers

depicted in Figure 3-1 is fairly straightforward, and the associated polymerization

reactions are also fairly simple and do not require a great deal of optimization.

Also, although the end-capping approach may seem somewhat more elegant than a

random copolymerization, there is no difference in the resulting systems with respect to

carrier transport. Migration along the copolymer chain is a one dimensional random-

walk, meaning that a carrier does not move directly to the nearest trap. Instead, a carrier

on a monomer unit that is far from a trap site has an equal probability of migrating to









either adjacent monomer, regardless of weather this brings the carrier nearer or further

from the trap. Therefore, the number of monomer units that a carrier migrates through

before reaching a trap site doe not directly depend on the carrier's distance from that site.

In addition, the distance between a carrier and a trap is not well defined even in an

end-capped polymer because of the polydispersity inherent in synthetic polymers.

Polydispersity relates to the distribution of molecular wieghts, and thus chain lengths, in

a polymer sample. The higher the polydispersity, the greater the variety in chain lengths.

For reference, any well-defined molecule has a polydispersity equal to 1. All conjugated

polymers are synthesized by step condensation polymerizations, and the theoretical

polydispersity of such processes approaches 2 as the functional group conversion

approaches 100%.125,126 Practical polydispersities in conjugated polymers are often

much higher because functional group conversion is less than quantitative. While

fractionation of the polymer can help the situation somewhat, conjugated polymers are

nonetheless quite polydisperse. Thus, the distance between an exciton and a trap is ill-

defined regardless of whether the traps are incorporated into the polymer as end-caps or

randomly in the backbone. Since this is so, the end-capping strategy does not offer any

significant advantages over randomly incorporated traps. Therefore the latter method,

which is much easier synthetically, was utilized in this study.

Results and Discussion

Polymer Synthesis and Structural Characterization

The chemical structures of the polymers which are the focus of this work are

presented in Figure 3-1. The nomenclature adopted for the polymers is PxTy where x and

y are the mole fraction of phenylene and thiophene monomers, respectively, used in the

polymerization reactions.










It was originally envisioned that the polymers in this study would be prepared by

the Cul catalyzed coupling of appropriate amounts of 2,5-diethynyl thiophene and 2,4-

diethynyl benzene with bis(tributylphosphine)palladium dichloride.121-124 However, 2,5-

diethynyl thiophene proved, in our hands, to be unstable to the polymerization conditions.

Since bis(trimethylsilylethynyl) thiophene exhibits excellent stability under the

polymerization conditions, it was deprotected in situ using tetrabutyl ammonium fluoride

in the presence of copper (I) iodide and one equivalent of Pt(PBu3)C12 under Hagihara

conditions to afford PoT1oo. This synthetic route is detailed in Figure 3-2.

TMS- TMS
I S a b Bu3

PBu3/
1 2 PoToo



PBu3 PBu3
CI t Pt-CI
PBu3 PBu3
3
Reagents and conditions: (a) Pd(PPh3)4, Cul, TMSAc, R.T., 24 hrs. (b) Pt(PBu3)2Cl2 (1 eq.), Bu4NF, toluene piperidine, R.T., 24 hrs. (c)
Pt(PBu3)2Cl2 (3.5 eq), Bu4NF, toluene, piperidine, 48 hrs.
Figure 3-2. Synthesis of intermediate 3 and PoT1oo.

The synthesis of the copolymers P75T25, P85T15, and P95T5 proved to be somewhat

more challenging, as in situ deprotection of 2,5-bis(trimethylsilylethynyl) thiophene (2)

in the presence of copper iodide and tetrabutylammonium fluoride with appropriate

amounts of bis(tributylphosphine) platinum (II) chloride and 1,4-diethynylbenzene gave

only low molecular weight material. In previous work, platinum containing PPE

oligomers were synthesized using diplatinated diethynylbenzene and the resulting

acetylides, featuring monochlorinated platinum end-groups, were then used in further

Hagihara coupling reactions.44 This methodology was extended to thiophene based









monomers by deprotecting 2 in situ with tetrabutyl ammonium fluoride in the presence of

Cul and an excess of Pt(PBu3)2C12 to afford compound 3 (see Figure 3-2), which is very

stable and can be easily purified by chromatography. Thus 3 was used in a three

component A-A + A'-A' + B-B polymerization with appropriate amounts of 1,4-

diethynyl benzene, Pt(PBu3)Cl2 and Cul gives copolymers P95T5, P85T15, and P75T25, as

shown in Figure 3-3.

Note that unlike the end-capping polymerizations discussed in Chapter 2, these

polymerizations were preformed under conditions where the end-group stoichiometry

was balanced. That is, each polymerization featured an equal molar quantity of

chloroplatinum and terminal acetylene functionalities. Any stoichiometric imbalances

give only low molecular weights, as predicted by the classic equation of Carothers.125,126

PBu3 P95T5 (x = 0.05, y = 0.95)
x 3 + (1-x) CI-Pt-CI + Cultouene/DPs P85T15 (x = 0.15, y = 0.85)
PBu3 R.T. 24 hrs P75T25 (x = 0.25, y = 0.75)
Figure 3-3. Synthesis of platinum containing phenylene-thiophene copolymers.

The copolymers feature a platinum-phenyleneacetylide backbone in which varying

amounts of thiophene are substituted for phenylene. Because the thiophene moieties in

the backbone orient their substituents at the 2 and 5 position at angles somewhat less than

1800, these polymers are not expected to be completely linear, however, they should

nonetheless adopt a rather extended conformation in solution. The synthetic design,

where thiophene is introduced only via the diplatinated compound 3 prevents "blocks" of

adjacent thiophene units in the copolymer, since the reactive termini of 3 cannot couple

with one another. The only possible reaction of 3, in this system, is coupling with

diethynyl benzene. Thus there is no possibility that any of the photophysical results









presented herein are the product of very low energy trap sites involving multiple adjacent

thiophene units.

The molecular weights of these polymers vary greatly. However, this is

unimportant for the purposes of this study which is not concerned with behaviors of the

bulk materials. Furthermore, the level of thiophene incorporation does not depend on the

molecular weight of the polymers, but on the thiophene loading. Thus, carrier transport

from the phenylene main-chain to a thiophene trap-site, the property of interest here, does

not depend on molecular weight.

1H NMR is particularly instructive in elucidating the structure of these polymers.

The aromatic region of the polymer's proton NMR is presented in Figure 3-4. The

phenylene moieties in P1ooTo show resonances at 7.1 ppm, and are clearly distinct from

the thiophene resonances in PoToo00, which appear at 6.5 ppm. On this basis, it is possible

assign the aromatic resonances in the copolymers and compare the integration of

thiophene to phenylene protons in order to determine the exact composition of the

polymer.127 The ratio of thiophene to phenylene in P95T5, P85T15, and P75T25 is calculated

to be 1:30, 1:13, and 1:8, as compared to the theoretical values of 1:19, 1:6 and 1:4,

respectively. Thus, approximately half the amount of thiophene in the feed is actually

incorporated into the polymers. This suggests that thiophene monomer 3 is somewhat

less reactive to Hagihara coupling than Pt(PBu3)2C12. This observation may also explain

why the copolymers exhibit lower molecular weights in comparison to the

homopolymers.














I 1 I 1 I 1 I 1 I r 1 1





I .1 I




i, _
i I




r -r I I -I


8
8


7 6


Figure 3-4. Aromatic region of the polymers 1H NMR spectra. From top to bottom, the
spectra are P1ooTo, P95T5, P85T15, P75T25, and PoT1oo. The resonance at 7.26
ppm corresponds to the solvent (chloroform).
It is important to note that the thienyl protons give relatively sharp resonances in

the NMR. This indicates that all thiophene moieties are in essentially identical magnetic

environments. Thus, the thiophene comonomers are randomly distributed along the









polymer backbone and the evidence shows no sign of blocks or regions on the polymer

chains with a high localized concentration of thiophene units.

Photophysical Mesurements

Absorption and emission spectroscopy

The absorption spectra of the polymers used in this study are presented in Figure

3-5, and their photophysical characteristics are summarized in Table 3-1. First the

spectra of the homopolymers, P1ooTo and PoT100, is considered. Each of these spectra

exhibit one broad, featureless band with Xmax at 341 nm and 402 nm, respectively. This

band is assigned to the long-axis polarized absorption of each polymer.

The copolymers show more complex spectra. The absorption of P95T5 is dominated

by a broad band with Xmax at 347 nm. This band is at nearly identical in shape and

wavelength to the absorption of the all-phenylene polymer, and as such it is assigned to

absoption of a phenylene based chromophore. In addition to the main band, a shoulder at

402 nm appears in this spectrum. Because its energy is similar to that of the thiophene

homopolymer, this band is assigned to absoption of a thiophene based chromophore. The

spectrum of P85T15 shows the same two bands, except that the relative intensity of the

thiophene-based (402 nm) absorption is greater for this polymer than in P95T5. This

corresponds with expectation, given the increased level of thiophene loading in P85T15.

A broad band centered at 375 nm is the major feature in the absorption spectrum of

P75T25. This band is red-shifted with respect to the dominant absoptions of P95T5 and

P85T15. A shoulder at 402 nm is also present in this spectrum. However, the dominant

band (Xax = 375) is so broad as to encompass nearly the entire shoulder.












































1 0 PoTloo
08
06
04
02

300 350 400 450 500

Wavelength / nm

Figure 3-5. Absorption spectra of polymers in THF solution.

Table 3-1. Summarized absorption and photoluminescence data.


Abs. max. / nm

341
347
347
375
402


Photoluminescence ,max / nm
SF P


&max.

420
420
420
420


Amax.
514
604
604
604
604


Polymer


Dem.

0.045
0.071
0.058
0.043
0.031


0.002
0.063
0.053
0.057










The photoluminescence spectra of the homo- and copolymers are presented in

Figure 3-6, and the results are also tabulated in Table 3-1. Photoluminescence lifetimes

appear in Table 3-2. Each band is labled as P or T, corresponding respectively to

phenylene or thiophene based luminescence. Additionally, the subscripts F or P

designate fluorescence and phosphorescence, respectively.

The room temperature spectra, beginning with the homopolymers, will be

considered first. P1ooTo exhibits very weak emission between 400 500 nm which is

assigned as fluorescence on the basis of previous investigations.44'128 In addition, this

polymer shows moderate emission centered at 514 nm. This band exhibits a very long

lifetime (Table 3-2), and is completely quenched in the presence of oxygen. On this

basis, and because of its similarity to previously published spectra of structurally similar

oligomers,44 the emission centered at 514 nm is assigned as phosphorescence.

PoToo00 also shows two emission bands, one centered at 420 nm, and the other

centered at 604 nm. The 604 nm band is completely quenched in an air-saturated

solution, and has a lifetime of several microseconds (Table 3-2). This band is therefore

assigned as phosphorescence. The band at 420 nm is not quenched by oxygen and has a

lifetime less than 5 ns. The 420 nm band is therefore assigned as fluorescence. These

assignments agree with previously reported data on similar materials.36








77





1 0 P 0IooT
08
0 6 P
04
0 2 PF

1 0 -\ P P95T5
0 8 -
06 I T
04
0 2 TF y

c) 10 Pp P85T15
S /TF II
0 o8 0I
j 06 0
-a
0 0 4
N--

0 2
Z 1 0 TF ^ P75T25


08 -
06 \ T
04 -












400 500 600 700
06


0 4
0 2

400 500 600 700

Wavelength / nm







Figure 3-6. Photoluminescence spectra of polymers in argon saturated solutions of THF
(room temperature) and 2-methyl THF (78 K). Solid lines: room temperature,
dashed lines: 78 K. Note that the room temperature spectra are normalized to
the fluorescence band when applicable, while the low-temperature spectra are
normalized to the strongest phosphorescence band.









Table 3-2. Photoluminescence rate and lifetime information.

298 K 78 K
514 nm 605 nm 514 nm 605 nm
Polymer k / s- / s k / s-1 / s kl / s k2 / s-1 kl / s-1 k2 / s-
(rel. amp.) (rel. amp.) (rel. amp.) (rel. amp.)
P1ooTo 5.4 x 104 18 ---- ---- 6.0 x 105 -- -
P95T5 1.7 x 105 5.3 1.1 x 105 9.1 1.4 x 106 2.1 x 104 8.8 x 104 2.8 x 104
(0.62) (0.38) (0.89) (0.11)
P85T15 3.0 x 05 3.3 3.1 x 105 7.7 3.6 x 106 4.2 x 104 9.0 x 104 2.6 x 104
(0.48) (0.52) (0.91) (0.09)
P75T25 7.1 x 105 1.4 1.4 x 105 7.1 5.7 x 105 ---- 4.4 x 105 -
PoTioo ---- ---- 1.8 x 105 5.6 ---- ---- 6.4 x 105 ----


Now turning to the copolymers, P95T5 exhibits a very weak band at 420 nm that is

not quenched by oxygen. This band is assigned to thiophene-based fluorescence. A band

at 514 nm, corresponding to phenylene based phosphorescence, is also apparent.

However, this band is quite weak (< 5% of the total phosphorescence). Instead, a band

centered at 605 nm, corresponding to thiophene phosphorescence, dominates the

spectrum. The emission spectra of P85T15 and P75T25 both feature a strong band at 420

nm, which, based on comparison to the all-thiophene polymer, corresponds to thiophene-

based fluorescence. No emission centered at 514 nm, corresponding to phenylene

phosphorescence, is readily apparent in either of these polymers. Time resloved studies

(see below) show that the phenylene-based phosphorescnce at 514 nm is present, but is so

weak as to be buried under the "tail" of the fluorescence. In fact, the only

phosphorescence that is readily apparent in these copolymers is centered at 605 nm,

corresponding to emission from a thiophene-based luminophore.

These results are quite remarkable. The lack of phenylene based (514 nm)

phosphorescence in P85T15 and P75T25 indicates efficient energy transfer from the

phenylene main-chain to the thiophene "traps" along the chain. Even when the









concentration of thiophene is less than 5%, (P95T5), energy transfer is more than 95%

efficient, based on the relative intensities of the phenylene and thiophene emission.

Trends in the phosphorescence lifetimes of the copolymers support this idea. At

room temperature, P1ooTo exhibits a phosphorescence lifetime (z) of 18 [as, whereas

PoT100 displays T = 5.6 as. The copolymer lifetimes (Table 2) at room temperature show

a distinct trend. As the thiophene concentration increases, T of both the phenylene and

thiophene components of the emission decreases. If the two excited states were not

coupled in equilibrium, but were instead related to two independent chromophores, the

lifetimes would not be expected to show any significant change with increased loading of

thiophene.

The photoluminescence spectra of the copolymers changes significantly at low

temperature. Whereas thiophene-based phosphorescence dominates the triplet-emission

at room temperature for all of the copolymers, at 78 K the phosphorescence of P95T5 and

P85T15 is dominated by phenylene-based phosphorescence. Thus, decreasing the

temperature dramatically decreases the efficiency of energy transfer in polymers with

lower thiophene loadings. This effect is not seen at higher loadings of thiophene, as

evidenced by the low temperature emission of P75T25. In this polymer, the spectrum is

dominated by thiophene-based emission even at low temperatures.

Phosphorescence quenching

The phosphorescence of PlooTo and PoTloo are quenched by addition of methyl

viologen (N,N' dimethyl-4,4'-bipyridinium, MV2+). Stem-Volmer analysis of these

polymers gives quenching constants (Ksv) of 3.9 x 105 for PlooTo and 6.7 x 105 for PoTloo.

Considering the phosphorescence lifetimes, these constants correspond to a purely

diffusion controlled (dynamic) quenching process.










The quenching behavior of P95T5 was also explored, and the results are presented in

Figure 3-7. P95T5 was used for these experiments because it is the only copolymer for

which both phenylene and thiophene based phosphorescence is readily observable at

room temperature. These experiments show quenching constants for the phenylene and

thiophene components of the phosphorescence that are essentially identical to those in the

corresponding homopolymers.



g l 3 0 i--------------------------


-, Ksv(T) = 6.2 x 105





SCone MV2+ (M)


0
a_

500 520 540 560 580 600 620 640
Wavelength Inm

Figure 3-7. Stern-Volmer quenching of P95T5 in THF solution. The uppermost spectrum
has [MV2+] = 0 aM, and the concentration of MV2+ increases by 1 aM in
each subsequent spectrum. The insert shows the linear portion of the Stem-
Volmer plots for the phenylene (530 nm, circles) and thiophene (614 nm,
triangles) portions of the spectra.

These results again imply that the phenylene and thiophene based triplets are

coupled in a dynamic equilibrium. If this were not the case, and the phosphorescence of

P95T5 was a result of thiophene and phenylene based triplet states that did not

interconvert, then the results would be expected to be much different. Recall that results

from quenching experiments with the homopolymers show that MV2+ can effectively









quench both phenylene and thiophene phosphorescence, and both processes are diffusion

controlled. Thus, in the absence of an equilibrium system, it would be expected that the

thiophene-based emission would be quenched preferentially to the phenylene-based

emission because, as evidenced by the photoluminescence spectrum, there is a much

larger concentration of thiophene triplets than phenylene triplets in the polymer chain.

Transient absorption spectroscopy

Figure 3-8 shows the time-resolved transient absorption spectra of the polymers. In

the homopolymers, the major feature is a broad transient absorption centered at 6100 nm

in P1ooTo, and around 640 nm in PoT00o. These bands are assigned to the triplet-triplet

(Ti--T,) absorption of the polymers.

The transient absorption of the three copolymers are quite similar. All are

dominated by a broad absorption centered at 630 nm, intermediate between the all-

phenylene and all-thiophene homopolymers. The shape the bands are essentially

identical for all three of the copolymers. However, there is a more subtle, yet

noteworthy, component of the transient absorption spectra. Specifically, close

examination of the negative intensity band due to ground state bleaching reveals an

interesting trend. This feature does not mirror the ground state absorption, but instead

shows a continuous red-shift with increasing thiophene concentration across the series.










W a ve le n g th / n m


W ave le n gth / n m

Figure 3-8. Triplet-state transient absorption spectra. The most intense spectrum were
obtained immediately after photoexcitation with a 10 mJ, 355 nm laser with a
10 ns pulse width. Each subsequent spectrum was obtained 1 uts after the
preceding spectrum. The dashed vertical line is at 350 nm.

Time resolved emission

Time resolved photoluminescence spectroscopy was applied to P95T5 in order to

gather information about energy transfer in this polymer. The results, as shown in Figure

3-9, show that while the phenylene phosphorescence (514 nm) is complete within about 5

jis, the thiophene (605 nm) phosphorescence is still present after more than 30 jis. This









observation supports the notion that energy is being transferred from a phenylene-based

state to a thiophene-based state.

In order to gain any information about energy transfer, it would be necessary to

deconvolute the decay curves into two components: one corresponding to

phosphorescence and the second corresponding to energy transfer. However, at room

temperature, the decay at both 514 nm and 605 nm is described by a single exponential

equation. This precludes any such analysis.


















2000

1000 L.

0
10"


0'. 600



Figure 3-9. Time resolved emission spectrum of P95T5 in THF solution at room
temperature, excitation wavelength 355 nm. Inset: Normalized emission
decay at 514 nm (circles) and 605 nm (triangles)

The large changes that were observed in the phosphorescence spectra at low

temperatures indicated the possibility that at 78 K energy transfer might be slowed






84


sufficiently as to allow for observation of the process. Indeed, at low temperatures, two

decay components were observed in the decay of both the phenylene and thiophene

phosphorescence of P95T5 and P85T15. Figure 3-10 presents the normalized phenylene

(514 nm) and thiophene (605 nm) decays in P95T5. This data is further summarized in

Table 3-2.


1.0 -


0.8 -


0.6 -


0.4 -


0.2 -


0.0 -


0 5 10 15


Time / [s


Figure 3-10. Normalized emission decay of P95T5 at 78 K. The dashed line represents
phenylene (514 nm) decay and the solid line represents thiophene
phosphorescence decay (605 nm).

The phenylene-based phophoresence decay in P95T5 is dominated by a fast

component (k = 1.4 x 106 S-1). This decay is far too fast to relate to phophorescence,

based on comparison with the homopolymers. It must therefore be associated with energy

transfer to thiophene. If this is true, increasing the loading of thiophene should increase









the rate of the short decay component. This prove to be the case, as demonstrated by the

results on P85T15, the 514 nm emission decay of which features an energy transfer

component that is faster than in P95T5. However, the phenylene-based phosphorescence

decay in P75T25 is unlike that in the copolymers with lower concentrations of thiophene in

that it is monoexponential. This implies that transport in this polymer is too fast to be

measured. This phenomenon is discussed in detail below.

Note that the thiophene-based (605 nm) phosphorescence is also biexponential in

both P95T5 and P85T15. The minor component of both of these decays corresponds well to

the phophorescence decay of the phenylene-based emission (514 nm). Careful inspection

of the steady-state spectra reveal that there is some residual phenylene-based

phosphorescence even at 605 nm. This residual phenylene phosphorescence likely

accounts for the minor component of the 605 nm emission decay.

General Discussion

Electronic model

In addition to providing information about the structure of the polymers, the 1H

NMR spectra provide useful information about the influence of thiophene doping on the

phenylene-based polymer chains (Figure 3-4). When thiophene is introduced into the

polymer chain new resonances corresponding to thiophene protons appear in the NMR.

In addition a change is observed in the resonances associated with phenylene protons.

Specifically, a new resonance at 7.1 ppm, which is not present in PiooTo, appears in the

1H NMR spectra of the copolymers. The ratio of the relative integration of this new

resonance and the thiophene resonance is 4:1. Noting that each thiophene has two

protons and each phenylene four, the resonance at 7.1 ppm can be assigned to the

phenylene moieties adjacent to thiophene. This assignment makes sense, considering that









the electron-releasing thiophene ring would be expected to cause an upfield shift on

nearby groups. Importantly, this result shows that even in the ground state, the electron-

releasing effects of thiophene perturb nearby phenylene moieties.

This information, considered in light of previous work showing that the singlet

excited state of Pt containing PAE's is delocalized over approximately five to seven

arylene units allows for the construction of a model to explain the excited states of these

polymers. 44 The cartoon in Figure 3-10A provides an illustration of the effects of a

single thiophene trap in a Pt-acetylide chain composed of phenylene-based. For the

purposes of this discussion, the delocalization of the singlet excited states is taken to be

five repeat units.44 In this depiction, the dark circle represents thiophene and the light

circles represent phenylene. The grey circles represent phenylene moieties that are

within one chromophore length of a thiophene moiety. Note a single thiophene unit

perturbs its "nearest neighbor" and "second-nearest neighbor". In this case, it would be

expected that an initial excition created within one chromophore length of a thiophene

unit (ie. on the black or grey circles in 3-10A), would undergo ultra-fast transport in the

singlet to the thiophene trap.66 An exciton created elsewhere on the chain (the white

circles in 3-10A) may migrates to the trap either as a singlet or a triplet videe infra).

Figure 3-10B considers the case where the concentration of thiophene is somewhat

higher, in this case one in five. Although the individual chromophores in this case are

identical to those in a polymer with more dilute thiophene loading, here every phenylene

is either a "nearest neighbor" or "second-nearest neighbor" to a thiophene. Thus, every

phenylene unit is within one chromophore length of a thiophene. In this case, any

exciton on the chain is with one chromophore length of a thiophene trap. Thus, it would









be expected that the system can be described by ultra-fast singlet transport within a single

chromophore.66 Importantly, because transport is much faster in the singlet state as

compared to the triplet state, triplet transport is not expected to play a role in this

scenario.


A








B






Figure 3-11. Schematic representation of platinum acetylide polymers with low (A) and
high (B) levels of thiophene incorporation. The dark circles represent
thiophene monomers, and the light circles represent phenylene monomers.
The grey circles represent phenylene units that are within one chromophore
length of a thiophene moiety.

The absorption spectra (Figures 3-5) are easily understood in light of this model.

The shoulder in the absorption spectra of P95T5 and P85T15, previously assigned to

thiophene-based absorption, accounts for a disproportionately high amount of the total

absorption in these cases if only the concentration of thiophene in the backbone is

considered. Recall that P95T5 contains only 1 thiophene in about every 30 repeat units, or

about 3% of the total number of repeat units. However, the shoulder centered at 402 nm,

previously assigned to thiophene based absorption, accounts for roughly 10% of the total

absorption in this polymer. Taking into account that each thiophene is actually part of a