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The distance dependence of electron transfer in a random ensemble has been probed by time resolving the emission from a donor (tris(3,4,7,8-tetramethylphenanthroline)ruthenium(II)) in the presence of an electron acceptor (methyl viologen). The emission line shape is well described by an exponential dependence of the electron transfer rate on distance, as k = (constant) x exp(-1.4R).Theoreti~all-~ and experimenta1"l' interest in electron transfer reactions has increasingly centered on how electron transfer rates depend on distance.One approach to this question has been to study electron transfer rates between donors and acceptors which are (1) J. A correction for the Coulombic repulsion between Ru(bpy)," and Mv2+ is also necessary. This correction increases the calculated value of Ro from ca. 16.2 A to ca. 17.3 A. (9) J. Miller, J. Peeples, M. Schmitt, and G. L. Close, J. Am. Chem. SOC., 104,6488 (1982). (IO) J. Miller, K. Hadman, and S. Morash, J. Am. Chem. SOC., 104, 4296 (1982).

Nonadiabatic electron transfer has been studied in glycerol in which the solvent relaxation time (rL) is varied (by temperature) from 10~s to 10'1 s. A strong dependence of rate on is observed with k « (tl)~°•6. A qualitative rationale suggests that the actual dependence for a nonadiabatic process can range from (rL)°t o (tl)-1 depending on whether the electronic coupling strength or solvent polarization determines the frequency factor for reaction. Such intermediate cases may be significant in a variety of condensed-phase electron-transfer processes.

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