Photoinduced electron-transfer reactions of C60 and p-substituted anilines

Abstract

Photoinduced electron transfers between triplet excited state of C₆₀ (^TC₆₀^*) and p-substituted anilines in polar solvents [benzonitrile, benzonitrile–benzene (1+2) and o-dichlorobenzene] were investigated using laser flash photolysis by observing transient absorption spectra in the visible and near-IR regions. All anilines donated electrons to ^TC₆₀^*, generating the radical anion of C₆₀ and radical cations of anilines. Quenching rate constants of ^TC₆₀^* (k_q^T) and quantum yields (Φ_et^T) for electron transfer via^TC₆₀^* were determined; the Φ_et^T values increased with increases in the k_q^T values. In highly polar solvents, the Φ_et^T values were larger than those in less polar solvents, suggesting fast dissociation of the triplet exciplex to an ion pair in polar solvents. Rate constants for the electron transfer (k_et) in polar solvents followed Rehm–Weller relationship, whereas only a poor correlation was obtained in less polar solvents. The linear relationship between the k_et values and Hammett substituent constants showed a negative slope indicating high electrophilicity of ^TC₆₀^*. In the Hammett plot of the Φ_et^T values, the slope in polar solvents was more negative than that in less polar solvents, suggesting that the transition state of the electron transfer becomes polar with increase in solvent polarity. In the case of the photoinduced reaction of p-ethynyl- or p-ethenylaniline, C₆₀ adduct formation was confirmed in addition to oligomerization of these anilines.