Probing the molecular basis of solvent reorganization in electron-transfer reactions

Optical electron transfer in the mixed-valence dimer [(NH₃)₅Ru^II-4,4′-bpy-Ru ^III(NH₃)₅]⁵⁺ has been investigated in mixed solvents (acetonitrile + dimethyl sulfoxide) in order to probe molecular aspects of solvent reorganization. The basis for extracting information at the molecular level lies in the phenomenon of selective solvation and the resulting ability to vary the composition of the dimer's primary solvation layer largely independently of the predominant bulk solvent composition. This enables one to probe the first molecular layer of solvent separately from the rest. From the optical electron-transfer data, corrected for unsymmetrical selective solvation effects, we find that nearly all of the solvent reorganizational energy originates from reorientations occurring within the first molecular solvent layer. The reorganization energy per solvent molecule is fairly large in the first layer (ca. 125-150 cm^-1), indicating extreme librational (or other) excitation. Somewhat surprisingly, the molecular picture which emerges from our work is broadly consistent with the predictions of a simple solvent dielectric continuum theory.