Solvent effect on photoinduced electron transfer between C60 and 3,3′,5,5′-tetramethylbenzidine studied by nanosecond laser photolysis

Abstract

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.