C60 as Photosensitizing Electron-Transfer Mediator for Ion-Pair Charge-Transfer Complexes between Borate Anions and Methyl Viologen Dication

Abstract

Photochemical reactions between the excited triplet state of C60 and the ion-pair charge-transfer complex which consists of electron-acceptor cation (methyl viologen (MV2+)) and electron-donor anion (organo borates (-BPh3R, where R = Ph or Bu)) have been investigated by both steady-state and laser-flash photolysis. By photoirradiation of C60 in the presence of MV2+(-BPh3R)2 in organic solvents, the amount of MV+• increases with irradiation time, and persists for a long time even in the dark. From the laser-flash photolysis, it is found that electron transfer proceeds via 3C60* from -BPh3R in the complex, yielding •BPh3R and C60-•. Although generated C60-• does not decay in the absence of MV2+ because of the rapid dissociation of •BPh3R, in the presence of MV2+, C60-• decays quickly by transferring an electron to MV2+ yielding MV+•. Thus, it is proved that C60 acts as a photosensitizer for pumping up an electron from -BPh3R as well as an electron mediator to MV2+. In these systems, MV+• persists even in air-saturated solution for more than an hour, suggesting that the electron transfer from MV+• to O2 is retarded.