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 C-60 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 C-60 by amines was observed, which suggests a dynamic quenching process via the excited singlet state of C-60 (C-1(60)*) in polar and nonpolar solvents. In polar solvents, electron transfer via the excited triplet state of C-60 (C-3(60)*) producing radical ions (C-60(.-) 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 C-3(60)* decreased. These findings indicate that dynamic quenching of C-1(60)* by amines increases with donor concentrations, resulting in a decrease of the intersystem crossing path to C-3(60)*; thus, the yields of the radical ions generated via C-60* decreased even in the highly polar solvent. This implies that the radical ions generated by the dynamic quenching of C-1(60)* may be quite short-lived, even in polar solvents, compared with the radical ions via C-3(60)*.