A time-resolved spectroscopic study of photoinitiated electron transfer reactions in solution

Christopher James Galli, University of Pennsylvania


Photoinitiated electron transfer reactions may be considered within the context of two categories: (1) direct electron transfer, as observed in charge transfer molecular complexes. The photoexcitation couples the ground and charge transfer potential surfaces and (2) indirect electron transfer, as observed in covalently bonded donor (D) and acceptor (A) moieties. The optical preparation of an eigenstate of the donor or acceptor is followed by the evolution of the system to the charge transfer state. This thesis presents a study of pico- and subpicosecond polarization and vibrational relaxations accompanying these two classes of condensed phase electron transfer reactions. The thesis presents optical ultrafast pump-probe measurements on the electron donor-acceptor complex tetracyanoethylene-hexamethylbenzene in polar and non-polar solvents. The experimental electron transfer rates are compared with nonadiabatic and adiabatic electron-transfer theories using a previously published analysis of all the vibrational modes active of the reaction. As the experimental electron transfer rates are competitive with and in same cases faster than the polarization relaxation time of the solvent, it is necessary to simulate the Smoluchowski diffusion of the reacting system over an equilibrating reaction coordinate. Regarding the coupling of the reactant and product electronic surfaces, it is shown that the nuclear kinetic operator can give rise to the coupling responsible for the electron transfer reaction. This non-Born-Oppenheimer matrix element is estimated using information obtained from the absorption and Raman spectra. Using this coupling, good agreement is found between the experimentally observed and theoretically predicted rates. This thesis also presents an optical time resolved spectroscopic study of the indirect electron transfer reaction in the model system magnesium triphenylporphyrinquinone in a range of solvent environments. These molecules have long served as model compounds for the ultrafast dynamics in photosynthetic complexes. In view of the recently observed coherent relaxation dynamics of Mg-tetraphenylporphyrin, these multicomponent electron transfer kinetic data are analyzed, focussing on the role of low frequency coherences in electron transfer processes.

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Recommended Citation

Galli, Christopher James, "A time-resolved spectroscopic study of photoinitiated electron transfer reactions in solution" (1993). Dissertations available from ProQuest. AAI9413833.