Solvent effect on photoinduced electron transfer between C 60 and 3,3′,5,5′-tetramethylbenzidine studied by nanosecond laser photolysis

Photoinduced electron transfer from 3,3′,5,5′-tetramethylbenzidine (TMB) to C60 in polar and non-polar solvents has been investigated by nanosecond (ns) laser photolysis in the visible and near-IR regions. The transient absorption bands of triplet C60(TC60*), the C60 radical anion (C60˙–) and the TMB radical cation (TMB˙+) were observed in both non-polar and polar solvents. The rate constants of electron transfer from TC60* to TMB, which were evaluated from the decay of TC60* and growths of TMB˙+ and C60˙–, decrease with increasing solvent polarity. In non-polar solvents, TMB˙+ and C60˙– seem to be produced immediately after the ns laser excitation at the same time as the formation of TC60*, suggesting a weak interaction between C60 and TMB, before electron transfer. On decreasing the solvent polarity, the kinetics of the back-electron-transfer reaction from C60˙– to TMB˙+ change from second-order to first-order, indicating that C60˙– and TMB˙+ exist as ion pairs in non-polar and less-polar solvents. The back-electron-transfer rates also tend to decrease with the solvent polarity.