Photoinduced electron transfer competitive with energy transfer of the excited triplet state of [60]fullerene to ferrocene derivatives revealed by combination of transient absorption and thermal lens measurements

The quenching processes of the exited triplet state of fullerene ( ³C₆₀*) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although ³C₆₀* was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (φ_et) for the generation of the radical anion of C₆₀ (C₆₀) via ³C₆₀^* were quite low even in polar solvents; nevertheless, the free-energy changes (ΔG _et) of electron transfer from Fc to ³C₆₀* are sufficiently negative. In benzonitrile (BN), the φ_et value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from ³C₆₀* to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of ³C₆₀*. Therefore, energy transfer from ³C₆₀* to ferrocene decreases the electron-transfer process from ferrocene to ³C₆₀*. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (φ_et = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (φ_et = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.