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Photoinduced electron transfer (PET) plays a crucial role in a wide array of biological pathways. These electron transfer reactions happen from or to the excited state of a chromophore upon absorption of light. Hence understanding the properties of excited states is necessary in elucidating the details of such pathways. The work presented in this thesis deals with PET in two systems: Fluorescent Nucleobase Analogues (FBAs) and DNA photolyase. The introductory chapter (Chapter 1) presents some background information about the two systems and sets up the stage for the reasoning behind the problems addressed in this thesis. FBAs are fluorescent analogues of naturally occurring, weakly fluorescent native nucleic acid bases. When incorporated into single stranded (ss) or double stranded (ds) DNA, the FBA fluorescence is significantly quenched. PET has been implicated to be the cause for the observed quenching. Here we have presented our attempt to correlate the quenching behavior of free FBA: nucleic acid monophosphate (NMP) pairs with the free energies associated with excited state electron transfer delta GET. Based on the delta GET values, we have tried to assign the direction of electron transfer. The quenching behavior of the FBA:NMP pairs were studied through Stern-Volmer (SV) quenching and time-resolved fluorescence studies. The above described analysis has been applied on FBAs: 4-amino-6-methyl-8-(2'-deoxy-beta-D-ribofuranosyl)-7(8H)-pteridone (6MAP), 4-amino - 2, 6 - dimethyl - 8 - (2'-deoxy-beta-d-ribofuranosyl) -7(8H) - pteridone (DMAP), 3-methyl-8-(2'-deoxy-beta-D-ribofuranosyl) isoxanthopterin (3MI) and 6-Methyl-8-(2'-deoxy-β-D-ribofuranosyl) isoxanthopterin (6MI) (Chapter 3), 2-Aminopurine (2AP) (Chapter 4), 8-Vinyl Adenosine (8VA) (Chapter 5). The final part of this thesis (Chapter 6) is on understanding the mechanistic details of a DNA repair process that is due to photoinduced electron transfer in DNA photolyase, a flavoprotein. Before the electron reaches the damaged site in the DNA, the initial electron acceptor in this repair process has been speculated to be the adenine of the flavin adenine dinucleotide (FAD). We have tested this hypothesis by measuring and comparing the various kinetic parameters associated with this process by reconstituting into apo-photolyase the natural cofactor of photolyase (FAD) and an adenine modified flavin (Etheno FAD, epsilon FAD). / Chemistry

Ultrafast charge and electron transfer, primary events in artificial photosynthesis, are key in solar energy harvesting. This dissertation provides insight into photo-induced charge and electron transfer in the donor and acceptor constructs built using a range of donor and acceptor entities, including transition metal dichalcogenides (TMDs, molybdenum disulfide (MoS2), and tungsten disulfide (WS2)), N-doped graphene, diketopyrrolopyrrol (DPP), boron-dipyrromethene (BODIPY), benzothiadiazole (BTD), free base and metal porphyrins, zinc phthalocyanine (ZnPc), phenothiazine (PTZ), triphenylamine (TPA), ferrocene (Fc), fullerene (C60), tetracyanobutadiene (TCBD), and dicyanoquinodimethane (DCNQ). The carefully built geometries and configurations of the donor and (D), acceptor (A), with a spacer in these constructs promote intramolecular charge transfer, and intervalence charge transfer to enhance charge and electron transfer efficiencies. Steady-state UV-visible absorption spectroscopy, fluorescence and phosphorescence spectroscopies, electrochemistry (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)), spectroelectrochemistry (absorption spectroscopy under controlled potential electrolysis), transient absorption spectroscopy, and quantum mechanical calculations (density functional theory, DFT) are used to probe ground and the excited state events as well as excited state charge separation resulting in cation and anion species. The current findings are useful for the increased reliance on renewable energy resources, especially solar energy.

The mechanism and the oxidation potential of the dissociative single electron transfer for tetra-n-butylammonium acetate has been investigated via conventional (cyclic voltammetry) and convolution voltammetry. The oxidation potential for tetra-n-butylammonium acetate was determined to be 0.60 ± 0.10 (vs. Ag/ (0.1 M) AgNO₃) in anhydrous acetonitrile. The results also indicated the mechanism of oxidation was concerted dissociative electron transfer (cDET), rather than stepwise as was previously reported. To further investigate the mechanism, a series of aliphatic and aromatic tetra-n butylammonium carboxylates were synthesized and investigated via convolution and conventional methods under anhydrous conditions (propionate, pivalate, phenyl acetate, and benzoate). The reported results showed high reproducibility and consistency with a concerted dissociative electron transfer for aliphatic carboxylates with a systematic shift in the oxidation potentials (0.60 ± 0.09 V for acetate, 0.47 ± 0.05 V for propionate, and 0.40 ± 0.05 V for pivalate) within the series which is expected trend based on radical stabilization energies of the alkyl groups on the aliphatic carboxylates. Hydrogen bonding was investigated as a possible source for the discrepancy between our results and the reported mechanism of the dissociative electron transfer. Because of the extreme hygroscopic nature of carboxylate salts, it was hypothesized that the presence of small amounts of water might alter the reaction mechanism. Deionized water and deuterium oxide additions to anhydrous acetonitrile were performed to test this hypothesis. The mechanism was noted to shift towards a stepwise mechanism as water was added. In addition, the derived oxidation potentials became more positive with increasing concentrations of water. Several explanations are presented with regards to water effects on the shift in the electron transfer mechanism. Indirect electrolysis (homogeneous redox catalysis) was also employed as an alternative and independent approach to quantify the oxidation potentials of carboxylates. A series of substituted ferrocenes were investigated as mediators for the oxidation of tetra-n-butylammonium acetate. Preliminary data showed redox catalysis was feasible for these systems. Further analyses of the electrochemical results suggested a follow-up chemical step (addition to mediator) that competes with the redox catalysis mechanism. As predicted from theoretical working curves, a plateau region in the i_p/i_pd plots (where no meaningful kinetic information could be obtained) was observed. Products mixture analyses verified the consumption of the mediator upon electrolysis, but no further information with regards to the nature of the mechanism was deduced. In a related study the effects of hydrogen bonding and ions on the reactivity of neutral free radicals were examined by laser flash photolysis. The rate of the β-scission of the cumyloxyl radical is influenced by cations (Li⁺ > Mg²⁺ ≈ Na⁺ > ⁿBu₄N⁺) due to stabilizing ion-dipole interactions in the transition state of the developing carbonyl group. Experimental findings are in a good agreement with theoretical work suggesting metal ion complexation can cause radical clocks to run fast with a more significant effect if there is an increase in dipole moment going from the reactant to the transition state. / Ph. D. / Our work focuses on employing electrochemical techniques to investigate single electron transfer processes, which lead to unstable organic species that contain an odd number of electrons called radicals and radical ions. Many essential biological and environmental pathways are found to occur via radicals, i.e. photosynthesis, atmospheric degradation, enzyme catalyzed reactions in biology, autooxidation, DNA mutations, and more. Electrochemical techniques permit us to investigate the scientific fundamentals of radical processes by generating radicals and radical ions in a controlled manner with a higher efficiency. We have combined electrochemical techniques with established physical organic theories of electron transfer to allow us to determine of the rate and mechanism of electron transfer for a selective group of chemical compounds, specifically anions derived from carboxylic acids (carboxylates). A fundamental understanding of single electron transfer processes for carboxylates allows for a prediction of chemical behavior and the future design of novel chemical compounds for alternative chemical functionality. Our findings are the first to report experimental evidence for a so-called concerted dissociative electron transfer mechanism for carboxylates, where the transfer of an electron is accompanied by the simultaneous breakage of a carbon-carbon bond yielding a radical and carbon dioxide. The mechanism has been shown to proceed in a stepwise fashion only in the presence of water. Our work highlights the environmental effects on radical stability such as water and metals ions.