Time-Resolved Fluorescence Spectroscopy of a Poly(phenylene-ethynylene) Polymer and Polyelectrolyte

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Time-Resolved Fluorescence Spectroscopy of a Poly(phenylene-ethynylene) Polymer and Polyelectrolyte
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english
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Backus, Timothy W
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Master's ( M.S.)
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University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
KLEIMAN,VALERIA DANA
Committee Co-Chair:
BOWERS,CLIFFORD RUSSELL
Committee Members:
OMENETTO,NICOLO
HAGEN,STEPHEN JAMES
WEI,WEI

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energy -- fluorescence -- polyelectrolyte -- polymer -- quench -- transfer
Chemistry -- Dissertations, Academic -- UF
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Chemistry thesis, M.S.
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theses   ( marcgt )
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Abstract:
Conjugated polymers with pi-conjugated backbones have recently been given considerable attention in the scientific community because of their interesting photo properties and wide range of applications.Conjugated polyelectrolytes are of particular interest for their exceptional energy, electron transfer properties, and the ability to easily fabricate them into films for use in applications. Their high quantum yields, strong absorptions, and solubility in polar solvents make them exceptional candidates for use in such things as photovoltaics, light emitting diodes, and photosensors. In order to tailor polymers for such applications, the understanding of their underlying photophysics is necessary.   In an effort to better understand the photophysics, the photoinduced energy transfer processes was explored. The use of steady-state spectroscopy and time-resolved spectroscopy was utilized to study the energy transfer mechanism both with and without quencher molecules. Because of the large affinity of polymers to aggregate in polar solvents, polymers with a high charge density were used. With the minimization of aggregation, the role of interchain mechanisms was minimized so that intrachain mechanisms can be studied without complications. A better understanding of the intrachain mechanisms will aid in the design of better conjugated polyelectrolytes for future applications.
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by Timothy W Backus.
Thesis:
Thesis (M.S.)--University of Florida, 2013.
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Adviser: KLEIMAN,VALERIA DANA.
Local:
Co-adviser: BOWERS,CLIFFORD RUSSELL.

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1 TIME RESOLVED FLUORESCENC E SPECTROSCOPY OF A POLY ( PHENYLENE ETHYNYLENE ) POLYMER AND POLYELEC TROLYTE By TIMOTHY W. BACKUS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF TH E REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2013

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2 2013 Timothy W. Backus

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3 To my family, who h ave supported me in everything I have done

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4 ACKNOWLEDGMENTS I would first like t o thank all of the teachers and advisors throughout my education al career Without everyone I would not have been able to get this far. So many opportunities have been opened up to me through them that have lead to my love of math and science. I would also like to thank all of my friends who have filled my life with happiness, making the hard times more bearable and the easy times more enjoyable. Specifically, I would like to thank Kathy Ortiz for all of the good time s and also for helping me with editing even with short time restraints. There are so many people at the University of Florida whom I would like to thank. I would like to thank my committee members who gave me many constructive criticisms and help throughout my gradu ate career. I would like to thank everyone in the Kleiman research group who were always there to help me with answering any questions I ha d and for listening to my presentations in order to help me improve them. I would like to specifically thank Sevnur K mrl Keceli for all of her help in learning the steady state and upconversion experiments. I would especially like to thank my advisor, Dr. Valeria Kleiman. Without her help, guidance and encouragement I would not have been able to finish my master s deg ree Finally, I would like to thank my family. Thank you for all of the love and support, for always being there to talk and visiting whenever possible.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 6 LIST OF FIGURES ................................ ................................ ................................ .......... 7 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 BACKGROUND AND SIGNIFICANCE ................................ ................................ ....... 11 Conjugated Polyelectrolytes ................................ ................................ ................... 11 Specific Aims ................................ ................................ ................................ .......... 14 2 EXPERIMENT AND THEORY ................................ ................................ ................... 15 Experimental Setup ................................ ................................ ................................ 15 Up Conversion ................................ ................................ ................................ ........ 16 Solution Preparation ................................ ................................ ............................... 17 3 RESULTS AND DISCUSSION ................................ ................................ ................... 19 PPE E ................................ ................................ ................................ ..................... 19 PPE S ................................ ................................ ................................ ..................... 27 PPE S Quenching Experiment ................................ ................................ ................ 31 4 CONCLUSIONS ................................ ................................ ................................ ......... 34 LIST OF REFER ENCES ................................ ................................ ............................... 37 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 38

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6 LIST OF TABLES Table page 3 1 Fluorescence lifetime time constants and fractional intensities for PPE E in chloroform for various wavelengths. ................................ ................................ ... 23 3 2 Fluorescence lifetime time constants and fractional intensities for PPE E in 7:3 paraffin oil:chlorofo rm for various wavelengths. ................................ ........... 26 3 3 Fluorescence lifetime decay constants and fractional intensities for PPE S in pH 8.0 H 2 O for various wavelengths. ................................ ................................ .. 30

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7 LIST OF FIGURES Figure page 1 1 Structures of the polymer PPE E (left) and polyelectrolyte PPE S (right) studied. ................................ ................................ ................................ ............... 12 2 1 A diagr am representing the up conversion setup. ................................ .............. 16 3 1 Normalized absorption and emission spectra of PPE E in pure CHCl 3 and a 7:3 paraffin oil:chloroform mixture. ................................ ................................ ..... 19 3 2 Fluorescence decay of PPE E in chloroform detected at 410 nm following excitation at 370 nm. ................................ ................................ .......................... 22 3 3 Fluorescence decay of PPE E in chloroform detected at 424 nm f ollowing excitation at 370 nm. ................................ ................................ .......................... 22 3 4 Fluorescence decay of PPE E in chloroform detected at 450 nm following excitation at 370 nm. ................................ ................................ .......................... 23 3 5 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 410 nm following excitation at 370 nm. ................................ ................................ ...... 25 3 6 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 424 nm following excitation at 370 nm. ................................ ................................ ...... 25 3 7 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 450 nm following excitation at 370 nm. ................................ ................................ ...... 26 3 8 Normalized absorption and emission spectra of PPE S in pH 8.0 H 2 O. ............. 27 3 9 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 410 nm following excitation at 370 nm. ................................ ................................ .......................... 28 3 10 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 424 nm following excitation at 370 nm. ................................ ................................ .......................... 29 3 11 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 440 nm following excitation at 370 nm. ................................ ................................ .......................... 29 3 12 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 450 nm following excitation at 370 nm. ................................ ................................ .......................... 30 3 13 Steady state emission spectra for PPE S with various concentrations of MV quencher. ................................ ................................ ................................ ........... 31 3 14 Photoluminescence intensity (PL) of PPE S quenched solutions divided by PL 0 for the non quenched PPE S time resolved fluorescence signals. .............. 32

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8 LIST OF ABBREVIATIONS BBO barium Borate CP Conjugated Polymer CPE Conjugated Polyelectrolyte FI Fractional Intensity IRF Instrument Response Function LED Light Emitting Diode MV Methyl Viologen OPA Optical Parametric Amplifier PMT Photomultiplier Tube PPE E P oly(phenylen e ethynylene) Ester protected polymer PPE S P oly(phenylene ethynylene) S al t de protected polyelectrolyte UV Ultraviolet

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the R equirements for the Degree of Master of Science T IME RESOLVED FLUORESCENCE SPECTROSCOPY OF A POLY ( PHENYLENE ETHYNYLENE ) POLYMER AND POLYELECTROLYTE By Timothy W. Backus December 2013 Chair: Valeria Kleiman Major: Chemistry Conjugated polymers with conjugated backbones have recently been given considerable attention in the scientific community because of their interesting photo properties and wide range of applications. Conjugated polyelectrolytes are of particular interest for their exceptional en ergy electron transfer properties and the ability to easily fabricate them into films for use in applications. Their high quantum yields, strong absorptions, and solubility in polar solvents make them exceptional candidates for use in such things as phot ovoltaics, light emitting diodes, and photo sensors. In order to tailor polymers for such applications, the understanding of their underlying photophysics is necessary. In an effort to better understand the photophysics, the photoinduced energy transfer p rocesses was explored. The use of steady state spectroscopy and time resolved spectroscopy was utilized to study the energy transfer mechanism both with and without quencher molecules. Because of the large affinity of polymers to aggregate in polar solvent s, polymers with a high charge density were used. With the minimization of aggregation, the role of interchain mechanisms was minimized so that intrachain mechanisms can be studied without complications. A better understanding of the

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10 intrachain mechanisms will aid in the design of better conjugated polyelectrolytes for future applications.

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11 CHAPTER 1 BACKGROUND AND SIGNIFICANCE Conjugated Polyelectrolytes Conjugated polyelectrolytes (CPEs) are a type of conjugated polymer (CP), that have been functionali zed with ionic side chains. Because of their conjugated backbones, they exhibit interesting photophysical properties such as energy and charge transfer as well as exciton and polaron transport. 1 CPs also exhibit strong absorption and high emission quantum yields making them ideal candidates for use in photovoltaic cells, light emitting diodes (LEDs) and photo sensors. Their extended conjugation lengths make CPs good candidates for use in photovoltaic cells where charge and energy separation and transfer are essential over large distances. 2 Since CPs are built from monomer units, their absorption and emission wavelengths can be tailored by modifying the backbone or side chain structures, which makes them especially useful in the manufacturing of LEDs. They can also exhibit "amplified quenching," which makes them good candidates for photo sensors. 3,4 With the addition of ionic side chains, CPEs become soluble in polar solvents which causes them to be easily processed into films, usually performed in aqueous solutions. However, since CPEs also have a lo ng organic backbone, they are amphiphilic and thus can form aggregates in polar solvents. This sometimes leads to complicated photophysics as aggregates can have interchain transfer mechanisms. The CPEs used in this investigation will have a large number o f charged side chains (high charge density) in an effort to limit aggregation by charge repulsion. 5,6,9 Since CPEs are synthesized by step growth polymerization they often have poly dispersed chain lengths making them heterogeneous mixtures. The heterogen eous

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12 mixture of chain lengths can cause their excited states to be more complex and difficult to interpret. 6 Figure 1 1. Str uctures of the polymer PPE E (left) and polyelectrolyte PPE S (right) studied. The polymers stud ied have a poly(phenylene ethynylene) (PPE) backbone and have carboxylate groups on their side chains protected by a long saturated carbon chain or ester protected side chains. These groups are removed in order to expose the carboxylate (CO 2 ) groups so th ey can be soluble in polar solvents. The structures of the PPEs studied are given in Figure 1 1 and will be referred to as PPE E for the ester protected polymer and PPE S for the de protected carboxylate salt. Aggregates have a tendency to cause the emiss ion spectrum to have a broad, featureless band. The emission spectrum for these polymers shows a clear vibration structure so it is believed to not aggregate to a great extent. This was further tested by collecting the excitation spectra by varying the ex citation wavelength and observing the resulting emission spectra at various emission wavelengths in order to gauge whether any aggregation occurs in these solutions. Since the excitation spectra of these polymers shows no discernable differences to the abs orption spectra when detecting at the three major emission wavelengths it is concluded that very little, if any, aggregation occurs at the concentrations used. 5 If aggregation did occur we would see a difference in the excitation spectrum at longer wavel engths as we would be detecting the

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13 emission from aggregate species which would show a different spectrum from the absorption spectrum of the non aggregate species. The effects of electron transfer were then be studied in relation to "amplified quenching with the polymer CPEs. "Amplified quenching" refers to the enhanced quenching that most CPs undergo where the unusually high quenching (high Stern Volmer constants) cannot be explained by the diffusion controlled limit. This can be explained by the molec ular wire effect where energy can be absorbed at one part of the polyelectrolyte but then be quenched at a different part of the chain by exciton transport or energy transfer through the chain. 13 The mechanism for amplified quenching consists of two main parts: (1) the formation of ion pair complexes allow very rapid energy transfer between the CP and quencher molecule and (2) energy can then transport via intrachain (along the polymer) and interchain (within aggregates) mechanisms. 4 Since aggregation cau ses delocalization of the electron density among many different chains, the energy can be transferred between these chains via the interchain mechanism. 7 This is a quick process and can considerably complicate the interpretation of the energy transfer mechanisms. 8 In order to s tudy this mechanism, the quencher molecules of methyl viologen ( MV + ) were used with the PPE S poly electrolyte The steady state quenching experiments were carried out as well as time resolved spectroscopy experiments in order to study the dynamics.

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14 Specif ic Aims CPEs are noteworthy to the scientific community, because of their wide use in applications in photo active devices. As such, the understanding of the underlying physics that give rise to their interesting photochemistry can lead to the design of b etter CPEs for their applications. However, the study of their dynamics can be quite complex and difficult to interpret because of aggregation and their poly dispersity. The use of carboxyl groups as the side chains in these polymers cause a high charge de nsity and will help limit the aggregation that many CPEs undergo in polar solvents. Possible energy transfer mechanisms are studied through the use of steady state absorption and emission spectroscopy as well as up conversion measurements of the fluorescen ce decay.

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15 CHAPTER 2 EXPERIMENT AND THEORY Experimental Setup A commercially available Ti Sapphire chirped pulse amplifier (Spectra Physics) is used to generate ~50 fs FWHM pulses centered at 790 nm with average power of ~420 mW at a repetition rate of 1 kHz. This is fed into an optical parametric amplifier ( Spectra Physics OPA 800 C ) where it is amplified and then passed through two non linear crystals where the second harmonic and fourth harmonic of the signal can be adjusted to obtain the necessary wav elength for sample excitation. The excitation pulse is then compressed through two UV grade isosceles fused silic a prisms (CVI, IB 12.4 69.1 UV) and the polarizatio n is adjusted to the magic angle (54.7 ) with a Berek polarization compensator in order to e liminate the depolarization effects on the fluorescence decays. An excitation wavelength of 370 nm with pulse energies of around 20 60 nJ/pulse at the sample position was used in the experiments performed. The pump pulse excites the sample and two off axi s parabolic mirrors collect the resulting fluorescence. The fluorescence is then focused onto a BBO type I crystal where it goes through sum frequency generation with a temporally and spatially overlapping 790 nm gate beam from the fundamental laser. The gate beam's temporal overlap is controlled by a delay stage which can be scanned to obtain the fluorescence decay. The up conversion (sum frequency) signal is filtered through a UG 11 filter and dispersed by a monochromator (Oriel Cornerstone 260 m) in order avoid detection of stray light and multi harmonics of the gate beam. The signal is then detected by a photomultiplier tube ( PMT Hamamatsu R928 ) and integrated by a boxcar (SR 250, Stanford Systems) A diagram of the s etup can be seen in Figure 2 1.

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16 Figure 2 1. A diagram representing the up conversion setup. Up Conversion Energy transfer in CPEs and CPs involves ultrafast processes on the time scale of 10 12 10 1 4 s. In order to investigate dynamics on these time scales the time resolution of the technique used must also be on these time scales. Time resolved fluorescence spectroscopic techniques that can be used include time correlated single photon counting, pump probe streak camera detection and u p conversion experiments. Since time correlated single photon counting methods use s electronic devices they have a limitation to their time resolution based on the time response of those electronic devices. While these devices can be used to measure time resolved flu orescence, if time resolutions faster than 10 10 s are desired the use of up conversion should be used This is because the time resolution of the up conversion technique i s limited

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17 mainly by the gate pulse duration. Since up conversion techniques tend to have instrument response functions (IRFs) of 100 200 fs for ultrafast gating lasers, the up conversion technique is ideal for the study of energy transfer in CPEs. 11,12 Up conversion uses the sum frequency generation of two spatially and temporally overl apped beams within a non linear crystal to obtain signal. The fluorescence decay is collected by scanning the gate pulse in the time domain through the use of a delay stage. The sum frequency signal is the convolution of the two pulse intensities (fluoresc ence and gate beam) and is given by E quation 2 1 : (2 1) The time resolution therefore depends on the gate pulse duration and the cross correlation between the gate pulse and the fluorescence With the setup used a typical time response function is 200 300 fs. Since the gate beam intensity does not change during the fluorescence decay time, the sum frequency signal is proportional to the fluorescence and thus the decay can be measured. The de cay can be expressed as a sum of exponentials in E quation 2 2 : (2 2) where i is the pre exponential factor denoting the contribution to the total time resolved decay of the component with lifetime i which is directly related to the excited state population of that com ponent 11,12 Solution Preparation S olutions were prepared using spectra grade Fischer Scientific solvents. PPE E and PPE S were provided by Dr. Kirk Schanze group in solid form and then dissolved in chloroform and pH 8.0 deionized H 2 O respectfully. pH 8. 0 H 2 O was prepared by

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18 measuring the pH of the deionized water used with a pH meter and then adding NaOH as necessary to reach a pH 8.0. The higher viscosity experiments were prepared by first dissolving the PPE E in chloroform and then mixing with paraffin oil in a 7:3 paraffin oil:chloroform ratio. All solutions prepared were diluted until an optical density of 0.1 0.3 was reached at the absorbance maximum measured in a 2 mm UV grade quartz cuvette. The solutions were then stirred during the experiments us ing a magnetic stirring rod and magnetic stirrer in order to avoid photo bleaching and photo degradation.

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19 CHAPTER 3 RESULTS AND DISCUSSION PPE E The steady state absorption and emission spectra for PPE E in CHCl 3 as well as a higher viscosity mixture of CHCl 3 and paraffin oil are presented in Figure 3 1. The absorption spectrum in CHCl 3 shows a broad featureless band with a maximum at 387 significant differences. The emission s pectrum in CHCl 3 shows two distinct bands with maxima of 424 and 450 nm as well as a third extremely broad band at around 500 nm. The emission spectrum for the higher viscosity solvent also does not show significant differences. Figure 3 1. Normalized absorption and emission spectra of PPE E in pure CHCl 3 and a 7:3 paraff in oil:chloroform mixture Polymers comprised of long conjugated backbones can have a large degree of structural disorder when in solution. If the structural disorder causes bends or torsions

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20 along the chain then conjugation can break. Because of this, there can be a wide range of ground state conjugation lengths and thus cause the absorption spectrum to be broad and featureless. 13 ,14 ,16 This can be seen in P PE E in F igure 3 1 as the absorption spectra are broad and featureless due to the many accessible conjugation lengths for the polymer. Since the excited state tends to favor a more planar orientation this gives the emission spectrum vibration al band struc ture 13 ,16 If the structural relaxation process occurs on a time scale comparable to the natural fluorescence time scale then it can possibly show up on the steady state data. 15 This will be explained further in the discussion of the time resolved fluores cence decays. Time resolved fluorescence decays were then collected exciting the sample at 370 nm and then detecting the up conversion signal at various wavelengths of the emission. The emission signal was usually collected for a wavelength on the blue si de of the maximum, at the maximum, on the red side of the maximum, and at the second maxima of the fluorescence. The time resolved fluorescence traces of PPE E in CHCl 3 for different wavelengths as well as their exponential fits are given in F igures 3 2 t o 3 4. In all cases, the decays were fit with either a double or triple exponential function which was convoluted with the IRF. In order to do these fits, a M ATLAB program was used. The program uses a function, A 1 *exp( ( t t 0 )/ 1 )+A 2 *exp( ( t t 0 )/ 2 )+A 3 *exp( ( t t 0 )/ 3 ) with parameters t 0 's given initial values The program then fits a Gaussian function to a given IRF data and then convolutes the exponential function with the normalized Gaussian function. After normal izing and baseline correcting the fluorescence data given, the program then subtracts the convoluted fitting function from

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21 the normalized fluorescence data ; these are the residual data points. Finally, using the residuals, it calculates the sum of the resi duals squared and minimizes that sum using a Nelder Mead simplex search. Since the fitting function is always convoluted with an IRF, then any time constants that are less than 1 ps will still be valid in the fitting as any rise time due to the excitation pulse is already taken into account. In all of the T ables showing the time constants and fractional intensities of the experimental fits the errors of the time constants are given in the parenthesis. For the long time component there were errors of betwe en 20 30 ps, shorter time components had errors between 5 20 ps and extremely fast time components had errors between 0.05 0.1 ps. These errors were arrived at by varying the time constants one at a time and seeing how well the new times fit the data (look ing at residual plots). In all experiments, the major source of error arose from the ability to detect very weak up conversion signal intensities. The signal is obtained by a second order process in the non linear crystal from interactions between the gate pulse and the fluorescence. This process is extremely inefficient and thus the up converted signal is often very weak. Since the signal is so weak, a large voltage (700 1200 V) had to be applied to the PMT which often caused a larger amount of electrical noise from that device as well. These two factors also explain why the 410 and 450 nm signals had larger errors since they were less intense signals than the 424 nm signal at the maximum of the fluorescence. The decay constants and fractional intensities for each wavelength are given in Table 3 1. The fractional intensities (FI) were calculated using the formula FI n = A n n / (A 1 t 1 + A 2 2 n n ), where A n is the amplitude and n is the time constant of the nth component. Since the fractional intensities are weighted with the time constants, for

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22 very short time constants, even a small percent FI will still be a signifi cant contributor to the fluorescence signal. A B Figure 3 2. Fluorescence decay of PPE E in chloroform detected at 410 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot B) A n expanded time plot for the shorter component. A B Figure 3 3. Fluorescence decay of PPE E in chloroform detected at 424 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals ar e in red. A) Long time plot. B) An expanded time plot for the shorter component.

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23 A B Figure 3 4 Fluorescence decay of PPE E in chloroform detected at 450 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component. Table 3 1: Fluorescence lifetime time constants and fractional intensities for PPE E in chloroform for various wavelengths. nm) FI 1 (%) 1 (ps) FI 2 (%) 2 (ps) FI 3 (%) 3 (ps) 410 94 2 80 ( 30) 5 3 0 ( 10) 1 0. 3 ( 0.05) 424 98 310 ( 20) 1 30 ( 5 ) 1 0.3 ( 0.05) 450 9 9 3 20( 20) 1 8 ( 4) Note: Rise times are shown with a red FI component. The fluorescence responses at all waveleng ths show a long time component with a decay of between 250 350 ps. This can be ascribed to the natural fluorescence from the S 1 state of the polymer and most likely has a range of decay constants due to the different conjugated lengths possessing different environments around them (s o lvation shells, proximity of other segments, etc.). 15 The signal detected at 410 nm has an additional decay component of around 30 ps with the signal detected at 424 nm having an additional rise component of around 30 ps. These two components are thought to be linked and most likely are due to structural relaxation of the polymer, more specifically

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24 the torsional relaxation from shorter conjugation lengths to longer ones. 13 ,15 This will be explored further in the analysis of the higher viscosity experiments. The signals detected at 410 and 424 nm also have an additional quick rise time component of around 300 fs. This quick rise is thought to be from energy transfer from shorter conjugation segments (higher energy) to the longer c onjugation segments (lower energy). Time resolved fluorescence was then collected for PPE E in a higher viscosity solvent. Chloroform has a dynamic viscosity of 0.569 cP while paraffin oil is ~1000 cP. Since it was difficult to dissolve PPE E in pure paraf fin oil I first dissolved it in chloroform which I then mixed with paraffin oil. In order to find out a good ratio of paraffin oil to chloroform I collected the absorbance and emission spectra for various different ratios and compared them to pure chloro form. As long as no additional absorbance or emission bands were seen it would be a good ratio to use as the structure of the PPE E in the solution most likely would not have changed (no aggregation occurred). From those measurements I arrived at a ratio of 7:3 paraffin oil:chloroform where the absorbance and emission spectra can be seen in Figure 3 1. Looking at how this higher viscosity solution affects the time components in the time resolved fluorescence we may be able to gain insight as to the causes behind those time components. If structural relaxation is the cause of the time component then we would expect the component to have a larger time constant or possibly contribute less (smaller FI) to the overall signal. 15

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25 A B Figure 3 5 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 410 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter compone nt. A B Figure 3 6 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 424 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component.

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26 A B Figure 3 7 Fluorescence decay of PPE E in 7:3 paraffin oil:chloroform detected at 450 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component. Table 3 2: Fluorescence lifetime time constants and fractional intensities for PPE E in 7:3 paraffin oil:chloroform for various wavelengths. nm) FI 1 (%) 1 (ps) FI 2 (%) 2 (ps) FI 3 (%) 3 (ps) 410 96 2 70 ( 30) 3 3 0( 10) 1 0.1 ( 0.05) 424 98 3 30( 20) 1 40( 5 ) 1 0.2 ( 0.05) 450 99 3 30( 20) 1 2 ( 0.5) Note: Rise times are shown with a red FI component. Just like with PPE E in CHCl 3 the PPE E in the higher viscosity solvent has a long decay of between 2 5 0 350 ps ascribed to the natural decay from the S 1 state. In a higher viscosity solvent there may be less torsional disorder as well as we would expect the torsional relaxation to take longer. 15 The additional rise component at 42 4 nm has a longer time constant of closer to 40 ps which is consistent with this prediction. The additional fast sub ps rise components for 410 and 424 nm are again thought to be from energy transfer along the polymer chain. 13

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27 PPE S The PPE E polymer had its saturated carbon chains removed from the side chains to have the ionic carboxylate groups exposed in order to make the polyelectolyte PPE S. The steady state absorption and emission spectra as well as the time resolved fluorescence were collected for PPE S and analyzed. The steady state absorption and emission spectra for PPE S in pH 8.0 H 2 O are presented in Figure 3 8. The absorption spectrum shows a broad fea tureless band with a maximum around 390 nm. The emission spectrum shows two distinct bands wi th maxima of 423 and 447 nm. Figure 3 8 Normalized absorption and emission spectra of PPE S in pH 8.0 H 2 O. The absorbance spectrum is shown in black and the emission spectrum exciting at the maximum absorbance of 390 nm is sho wn in blue. The broad featureless absorption spectrum for PPE S is again due to a wide range of conjugation lengths from structural disorder. 13 The emission spectrum also

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28 shows a vibration al structure, but in contrast to PPE the emission spec trum of PPE S appears to be less well defined. This is most likely caused by less structural disorder since the long carbon chains were removed and the ionic carboxylate side chains are exposed. Without the long carbon chains on the side chains the polyme r could also possibly start to exhibit more stacking structures of the conjugated backbones. The time resolved fluorescence traces of PPE S in pH 8.0 H 2 O for different wavelengths as well as their exponential fits are given in Figure 3 9 to 3 12. In all c ases, the decays were fit with either a double or triple exponential function which was convoluted with the IRF. The decay constants for each wavelength are given in Table 3 3. A B Figure 3 9 Fluorescence decay of PPE S in pH 8.0 H 2 O detect ed at 410 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component.

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29 A B Figure 3 10 Fluorescence decay of PPE S in pH 8.0 H 2 O det ected at 424 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component. A B Figure 3 11 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 440 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component.

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30 A B Figure 3 12 Fluorescence decay of PPE S in pH 8.0 H 2 O detected at 450 nm following excitation at 370 nm Data is shown in blue, exponential fit in green and the residuals are in red. A) Long time plot. B) An expanded time plot for the shorter component. Table 3 3: Fluorescence lifetime decay constants and fractional intensities for PPE S in pH 8.0 H 2 O for various wavelengths. nm) FI 1 (%) 1 (ps) FI 2 (%) 2 (ps) FI 3 (%) 3 (ps) 410 97 64 0( 40) 2 90( 20) 1 3 ( 1) 424 99 43 0( 30) 1 1 ( 1) <1 0. 6 ( 0.1) 440 99 360 ( 30) 1 2 ( 1) 450 99 38 0( 30) 1 2 ( 1) Note: Rise times are shown with a red FI component. Just like in PPE E, PP E S has a long time component of 200 450 ps again attributed to the natural fluorescence decay from the S 1 state. A wider range of natural lifetime could possibly be due to there being more accessible structural interactions with other segments of the poly mer since there are no longer long carbon chains in the way. A short decay component of around 80 ps for 410 nm is again seen and most likely because of structural relaxations. This structural relaxation however is not seen as a rise time at 424 nm in PPE S as it was in PPE E. This is most likely caused by it being obscured by the extremely fast decay component. An extremely fast decay component

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31 of around 2 ps is seen at all wavelengths and is thought to be the result of energy transfer along the polymer ch ain. 13 PPE S Quenching Experiment Quenching experiments were performed on PPE S in pH 8.0 H 2 O using methyl viologen (MV) as a quencher molecule. The fluorescence emission steady state measurements of PPE S with different concentrations of MV are given in Figure 3 13. The ratios of the time resolved fluorescence intensity of the PPE S with quencher molecules over the intensity without quencher molecules is given in Figure 3 14. Figure 3 13 Steady state emission spectra for PPE S with various concentrations of MV quencher. Emission spectrum without quencher (green), 32% quenched (black), 52% quenched (red) and 78% quenched (blue) concentrations of PPE S with MV. The percent quenched for steady state was calculated using the integr ated area under each quenched spectra divided by the integrated area under the non quenched spectrum.

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32 Figure 3 14 Photoluminescence intensity (PL) of PPE S quenched solutions divided by PL 0 for the non quenched PPE S time resol ved fluorescence signals. The ratios represent the amount of quenching at a given time. At t=0 the ratio represents the amount statically quenched. The colors for the ratios correspond to the same solutions as in Figure 3 13. As explained in the backgroun d section, charged quencher molecules can form ion pairs with CPs. MV is an ionic quencher molecule which uses electron transfer to quench the fluorescence of PPE S. When this happens the section of the CP that is in the direct vicinity of the quencher mo lecule is quenched in a direct and prompt way. This can be observed in Figure 3 14 as the initial percent quenched at time zero. However there is a dynamic component seen in the figure as a fast decay on the ps time scale. This dynamic component is further evidence that there is energy t ransfer along the polymer chain. The dynamic component would only be able to be described by either diffusion of the quencher or exciton transport in the polyelectrolyte chain. Since the time scale is sub ps, it would have t o be a result of exciton transport as diffusional motions occur on the ns time scale. When a section of the polyelectrolyte is

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33 excited that is further down the chain from the quencher molecule the energy or the exciton can be transferred through the chain until it arrives at a section that is in close proximity to the quencher molecule where it is then quenched. 4,7

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34 CHAPTER 4 CONCLUSIONS Steady state absorbance and emission was studied for a conjugated polymer and a conjugated polyelectrolyte. The absorbanc e spectra of both PPE E and PPE S studied show a broad and featureless band with maximum of 390 nm. The emission spectra show vibration al bands with maxima of 424 and 450 nm These differences are thought to be caused from the ground state of the polymer a nd polyelectrolyte having more structural disorder in solution with twists and turns along the backbone and the excited state having a more planar configuration. The time resolved fluorescence was then studied using the up conversion technique. The fluores cence decay for PPE E in chloroform was obtained for wavelengths of 410, 424 and 450 nm corresponding to the blue side of the emission maximum, the maximum and secondary maxim a respect ively When interpreting the fluorescence signals for the various wavele ngths, an additional time component for 410 nm and rise for 424 nm of 30 40 ps is thought to be a result of structural relaxations of the torsional disorder along the backbone. This conclusion is supported by the time resolved data of PPE E in a higher vis cosity solvent mixture of 7:3 paraffin oil:chloroform as the rise component for the 424 nm signal is longer The additional rise components of 0.3 ps are thought to be caused by energy transfer through the polymer backbone. The fluorescence decay for PPE S in pH 8.0 water was also obtained for wavelengths of 410, 424, 440 and 450 nm corresponding to the blue side of the emission maximum, the maximum, the local minimum and secondary maxima respectfully. When interpreting the fluorescence signals for the vari ous wavelengths,

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35 the signals at 440 nm and 450 nm both show additional time components of 2 ps. The additional time component of 10 ps for 410 nm is again thought to be a result of structural relaxation where the rise component which would appear in the 4 24 nm decay is being obscured by the additional fast 2 ps decay. The additional 2 ps decays in all the wavelengths are thought to be caused by energy transfer along the polyelectrolyte backbone. This conclusion is supported by the time resolved quenching e xperiments which show a decay on that time scale. Future Work: So far there is little evidence to support the conclusion that the 30 40 ps decay/rise time components are coming from structural relaxations As such, there are a few different experiments that might be able to be performed. First, time resolved anisotropy experiments could be performed. Anisotropy experiments are a way as a result of photo selection. The experiment involves exciting the sample with polarized light and then measuring the change of polarization from the fluorescence by measuring the difference in perpendicularly polarized fluorescence to parallel polarized fluorescence. This e xperiment can often times elucidate structural changes in molecules from the changes in polarization of the fluorescence. If a change in anisotropy has a time component on the same time scale of 10 40 ps it could be further evidence that the decay/rise is caused by structural relaxation. Second, the time resolved transient absorption spectrum could be measured. If the excited state absorption band(s) is looked at one could look for narrowing or broadening of the band(s) over time. If the excited state abso rption band is initially broader and less structured, but becomes more vibration al structured similar to the

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36 steady state over time then it could be further evidence that structural relaxation is occurring. If there is a large amount of structural disorder at the time of absorption, then we would expect the excited state absorption to have a broad and featureless band. When this structural disorder relaxes to more planar configurations then the absorption would start to show more vibrational structure.

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37 LIS T OF REFERENCES ( 1 ) Andrew, T. L.; Swager, T. M. J. Polym. Sci., Part B: Polym. Phys. 2011 49, 476 498. ( 2 )Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007 107, 1324. ( 3 ) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Ch em. Soc. 2000 122, 8561 ( 4 ) Tan, C.; Atas, E.; Muller, J. G.; Pinto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004 126, 13685 13694. ( 5 ) Fakis, M.; Anestopoulos, D.; Giannetas, V.; Persephonis, P. J. Phys. Chem. B 2006 110, 24897 24902. ( 6 ) Kaur, P.; Yue, H. J.; Wu, M. Y.; Liu, M.; Treece, J.; Waldeck, D. H.; Xue, C. H.; Liu, H. Y. J. Phys. Chem. B 2007 111, 8589 8596. ( 7 ) Muller, J. G.; Atas, E.; Tan, C.; Schanze, K. S.; Kleiman, V. D. J. Am. Chem. Soc. 2006 128, 4007 4016. ( 8 ) Hardison, L. M.; Zhao, X.; Jiang, H.; Schanze, K. S.; Kleiman, V. D. J. Phys. Chem. C 2008 112, 16140. ( 9 ) Wu, M.; Kaur, P.; Yue, H.; Clemmens, A. M.; Waldeck, D. H.; Xue, C.; Liu, H. J. Phys. Chem. B 2008 112, 3300. (10) Jiang, H.; Zhao, X. Y.; Schanze, K. S. La ngmuir 2006 22 5541 5543 ( 11 ) Bright, F. V.; Munson, C. A. Anal. Chim. Acta 2003 500, 71. (12) Valeur, B. Molecular Fluorescence ; WILEY VCH, 2002 ( 13 ) Scholes, G. D.; Hwang, I. Chem. Mater. 2011 23, 610 620 (14) Kauffmann, H. F. ; Brunner, K.; Tor tschanoff, A.; Warmuth, Ch.; Bassler, H. J. Phys. Chem. B 2000, 104, 3781 3790 (15) Vauthey, E.; Duvanel, G.; Grilj, J.; Schuwey, A.; Gossauer, A. Photochem. Photobiol. Sci. 2007, 6 956 963 (16) Berg, M.; Sluch, M.; Godt, A.; Bunz, U. J. Am. Chem. So c. 2001, 123, 6447 6448

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38 BIOGRAPHICAL SKETCH Tim othy William Backus was born in Fairfax, Virginia in 1985 to Barbara and Ray Backus. Before the age of one, his family moved to Colorado Springs, Colorado where he lived until second grade. His family then moved to Palm Harbor, Florida where he lived until college. It was in middle and high school where his love for math and science was first realized and pursued. While in high school he was given the great opportunity to attend a summer program at Florida State University that exposed him to a great deal of higher math and science, as well as the research lab experience. After graduating from Tarpon Springs High School sum m a cum l a ude he wen t on to pursue his b degree at Florida State University It was there where he was able to receive more experience in a physical chemistry research lab In 2007 he graduated with a b achelor degree in p hysics and a minor in chemistry. He then acquired a job coding randomizations for online resources for colleg e books while pursuing his second b achelor degree in c hemistry, for two years until his acceptance to the University of Florida graduate program in 2009. He pursuing a Master of Science in p hysical c hemi stry