Efficient photoinduced electron transfer between c₆₀ and tetrathiafulvalenes studied by nanosecond laser photolysis

Photoinduced electron transfer between C₆₀ and tetrathiafulvalene (TTF) or bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in polar and non-polar solvents and their mixtures was investigated by nanosecond laser photolysis/transient absorption spectroscopy in the visible and near-IR regions. The transient absorption bands of the C₆₀ triplet (^TC*₆₀) observed in polar solvents decay on addition of TTF (or BEDT-TTF), accompanied by the appearance of the transient absorption bands of C^.-₆₀. The yields of C^.-₆₀ are decreased on addition of O₂, revealing that about 10% of C^.-₆₀ is produced via the singlet state at donor concentrations of less than 5 × 10^-3 M. In benzene, the quenching of ^TC*₆₀ without the appearance of C^.-₆₀ within the nanosecond laser pulse is attributed to collisional quenching. The quantum efficiencies of C^.-₆₀ formation via ^TC*₆₀, which were evaluated from the initial [^TC*₆₀] and maximal [C^.-₆₀] values, vary with the donor ability and solvent polarity. The forward electron transfer rate constants were evaluated from the decay rates of ^TC*₆₀ (or the rise rates of C^.-₆₀) after multiplying by the quantum efficiencies. The back electron transfer rates are slowed down in polar solvents.