Driving force dependence of intermolecular electron-transfer reactions of fullerenes

Pulse-radiolytic studies were performed to determine the rate constants of intermolecular electron transfer (k_et) from fullerenes (C₆₀, C₇₆, and C₇₈) to a series of arene radical cations in dichloromethane. The one-electron oxidation potentials of the employed arenes - corresponding to the one-electron reduction potentials of arene π-radical cations - were determined in dichloromethane to evaluate the driving forces of electron-transfer oxidation of fullerenes with arene π-radical cations. The driving force dependence of log k_et shows a pronounced decrease towards the highly exothermic region, representing the first definitive confirmation of the existence of the Marcus inverted region in a truly intermolecular electron transfer. Electron-transfer reduction of fullerenes with anthracene radical anion was also examined by laser flash photolysis in benzonitrile. The anthracene radical anion was produced by photoinduced electron transfer from 10,10′-dimethyl-9,9′,10,10′-tetrahydro-9,9′- biacridine [(AcrH)₂] to the singlet excited state of anthracene in benzonitrile. The rate constants of electron transfer (k_et) from anthracene radical anion to C₆₀, C₇₀, and a C₆₀ derivative were determined from the decay of anthracene radical anion in the presence of various concentrations of the fullerene. Importantly, a significant decrease in the k_et value was observed at large driving forces (1.50 eV) as compared to the diffusion-limited value seen at smaller driving forces (0-96 eV). In conclusion, our study presents clear evidence for the Marcus inverted region in both the electron-transfer reduction and oxidation of fullerenes.