Photophysical Study of Charge Transport and Intermolecular Energy Transfer Using Femtosecond Stimulated Raman Spectroscopy

Understanding fundamental energy and electronic transfer processes in bio-inspired, artificial photosynthetic mimics is necessary to achieve these processes with high efficiency. The focus of this dissertation is characterization of the primary electronic interactions and structural dynamics required to achieve efficient light absorption, energy transport, and charge migration.;The importance of excitonic coupling and excimer formation in aggregated systems is discussed in chapters 3-6. We begin with a mechanistic investigation of the primary excimer formation steps in a series of perylene-3,4:9,10-bis(dicarboximide) (PDI) dimers. Using near-infrared transient absorption spectroscopy (Chapter 3) we identify a unique low-energy excimer transition which can be used to unequivocally determine the self-trapping rate in molecular aggregates. Femtosecond stimulated Raman spectroscopy (FSRS) is then used to characterize the Franck-Condon state of monomeric perylene dyes (Chapter 4), building to an investigation of this state in covalent H- and J-aggregate mimics (Chapter 5, 6). Quenching of excited state vibrations indicates strong electronic perturbation upon aggregation.;Chare transfer kinetics are discussed in Chapters 7 -- 9, first exploring charge separation in a perylene-PDI dyad (Chapter 7). Quantitative charge separation reveals a unique vibrational signature of the PDI anion which is used in subsequent studies to differentiate between multiple photophysical pathways.;We finish with a study of hole transport through double-stranded DNA, using a chemically-modified guanine nucleobase, 4'-(8-phenylethynylguanosine), GE, which has a strong visible absorption spectrum and high Raman activity. Direct observation of GE as an intermediate hole acceptor in a donor-acceptor DNA architecture demonstrates its utility as a site-specific probe of hole migration and charge separation (Chapter 8). Incorporation of multiple GEs into DNA strands provides evidence of the superexchange hole transport mechanism in these structures (Chapter 9).;Overall, the work discussed here sets the stage for continued mechanistic evaluation of the primary photophysical processes encountered in the development of energy transport materials. Utilization of high-order nonlinear spectroscopies will provide necessary information on exciton generation, charge mobility, and self-trapping.