Photoinduced Electron-Transfer Processes of Fullerene (C60) with Amine Donors: Excited Triplet Route vs Excited Singlet Route

Abstract

Dynamic quenching processes of the excited states of C60 with amine donors have been studied by changing donor strength, donor concentration, and solvent polarity using steady-state and transient absorption and fluorescence spectroscopic techniques. Fluorescence quenching of C60 by amines was observed, which suggests a dynamic quenching process via the excited singlet state of C60 (1C60*) in polar and nonpolar solvents. In polar solvents, electron transfer via the excited triplet state of C60 (3C60*) producing radical ions (C60•− and amine•+) was observed by the nanosecond laser flash photolysis (ca. 6 ns laser pulse). When the donor concentrations are below 10–20 mmol dm−3, the concentrations of the radical ions increased with the donor concentrations. With further increase of the concentrations of the donors up to 100 mmol dm−3, on the other hand, the yields of the radical ions generated via 3C60* decreased. These findings indicate that dynamic quenching of 1C60* by amines increases with donor concentrations, resulting in a decrease of the intersystem crossing path to 3C60*; thus, the yields of the radical ions generated via 3C60* decreased even in the highly polar solvent. This implies that the radical ions generated by the dynamic quenching of 1C60* may be quite short-lived, even in polar solvents, compared with the radical ions via 3C60*.