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 (C-3(60)*) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although C-3(60)* was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (Phi(et)) for the generation of the radical anion Of C-60 (C-60(center dot-)) via C-3(60)* were quite low even in polar solvents; nevertheless, the free-energy changes (Delta G(et)) of electron transfer from Fc to C-3(60)* are sufficiently negative. In benzonitrile (BN), the Phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from C-3(60)* to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of C-3(60)*. Therefore, energy transfer from C-3(60)* to ferrocene decreases the electron-transfer process from ferrocene to C-3(60)*. To increase the participation of electron transfer, introduction of electrondonor substituents to Fc (Phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (Phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.