Femtosecond studies of the influence of solvent on chemical reaction dynamics

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In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Stephen Bradforth of the University of Southern California and his students will elucidate pathways for ultrafast ionization, charge transfer and spin crossover in solution phase small molecules. A combination of broadband transient absorption and liquid-microjet time-resolved photoelectron spectroscopy will be used to understand the role that solvent plays in controlling excited state dynamics. The solvent is expected to modulate solute electronic structure, stabilize charge-separated states and modify the approach to conical intersections that control photochemical branching. In this framework, a mechanistic understanding for the photooxidation of aqueous guanine and adenine will be sought by mapping and correlating the onset for ionization of these DNA bases with the dissociative reactivity exhibited in non-polar solvents, the gas phase and theory. In addition, the cascade of intermediate states in excited-state spin crossover organo-iron complexes will be unraveled through a combination of time-resolved optical and photoelectron probes, the latter being uniquely sensitive to ligand field states. These specific chemical studies will complemented by construction of a source of extreme UV femtosecond pulses to expand the electron binding energy range for both steady-state and transient photoelectron spectroscopy. The fundamental mechanisms revealed in this project will impact efforts in the photobiology community to correlate UV wavelength and lesion formation in cellular DNA. In addition, the new spectroscopic tools applied here to spin-crossovers systems should help to optimize organometallic chromophores utilizing earth-abundant metals for solar energy conversion. Training of PhD, postdoctoral and undergraduate students in the PI?s lab will prepare a new generation of physical scientists in designing and analyzing experiments that use contemporary spectroscopic probes of matter. Students will have abundant opportunities to work with international scientists both here and abroad through a network of collaborations. A broadening participation program with proven success will be continued whereby Bradforth offers Los Angeles county community college students 8-week summer internships themed on light-to-energy conversion research. Participation of diverse groups and extensive collaboration will be emphasized at all levels of involvement in the project.