Photoinduced electron-transfer processes of fullerene (C₆₀) with amine donors: Excited triplet route vs excited singlet route

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 mmoll 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 mmoldm ^-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 ³C ₆₀*.