EFFECTS OF THE SOLVENT DYNAMICS AND VIBRATIONAL MOTIONS IN ELECTRON-TRANSFER

Recent theoretical and experimental progress concerning electron trans fer (ET) in solution is reviewed by focusing on the mechanism of ET, w hich occurs much faster than solvation dynamics, Theories of ET in sol ution are briefly reviewed with particular emphasis placed on the rela tion to solvent dynamics. Experimental methods to investigate solvent polarization relaxation are described. Ultrafast intramolecular ET, wh ich is found in back ET from the photo-induced charge-transfer state t o the ground state, is described concerning highly polar betaines and mixed-valence compounds. Ultrafast intermolecular ET has been observed for the systems of various dyes in electron-donating solvents. A non- exponential process with a significant temperature dependence was obse rved in aniline. A faster ET with a single exponential decay as fast a s 10(13) s(-1) was observed with no temperature dependence in a system of oxazine 1 in N,N-dimethylaniline. The ET rate constants of excited coumarins in electron-donating solvents drastically depend on the sub stituent groups of the coumarin. Relatively small Stokes shifts in ste ady-state fluorescence spectra of ultrafast reacting molecules in solu tion are evidence of a ''chemical timing'' effect; namely, the reactio n occurs in a non-equilibrium configuration of the solvent. These expe rimental observations are explained in terms of the extended Sumi-Marc us theory, in which the effect of solvation dynamics and low- and high -frequency vibrational modes are taken into account.