Solvent polarity effect on photoinduced electron transfer between C₆₀ and tetramethylbenzidine studied by laser flash photolysis

The photoinduced electron transfer between C₆₀ and N,N,N′,N′-tetramethylbenzidine (NTMB) in polar and nonpolar solvents and their mixtures has been investigated by nanosecond laser flash photolysis/transient absorption spectroscopy in the visible and near-IR regions. The transient absorption bands of the C₆₀ triplet state (^TC₆₀*) and the NTMB radical cation were observed in both nonpolar and polar solvents. In polar solvents such as benzonitrile, the reaction rate constants were determined from the decay of ^TC₆₀* at 740 nm, which were consistent with the rate constants evaluated from the growth of the NTMB radical cation, suggesting that direct electron transfer occurs from ground-state NTMB to ^TC₆₀*. In nonpolar solvents such as benzene, the NTMB radical cation and C₆₀ radical anion were produced immediately after the nanosecond laser pulse, simultaneous with the formation of ^TC₆₀*, indicating that a different electron transfer mechanism exists in nonpolar solvents. In the solvent mixtures, two simultaneous reaction routes are present for both forward and back electron transfer reactions. Pronounced solvent effects found for electron transfer reaction kinetics and rates are characteristic of the photoinduced electron transfer reactions between C₆₀ and some kinds of aromatic amines.