Scandium ion-promoted photoinduced electron-transfer oxidation of fullerenes and derivatives by p-chloranil and p-benzoquinone

In the presence of scandium triflate, an efficient photoinduced electron transfer from the triplet excited state of C₆₀ to p-chloranil occurs to produce C₆₀ radical cation which has a diagnostic NIR (near-infrared) absorption band at 980 nm, whereas no photoinduced electron transfer occurs from the triplet excited state of C₆₀ (³C₆₀*) to p-chloranil in the absence of scandium ion in benzonitrile. The electron-transfer rate obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing p-chloranil concentration. The observed second-order rate constant of electron transfer (k_et) increases linearly with increasing scandium ion concentration. In contrast to the case of the C₆₀/p-chloranil/Sc³⁺ system, the k_et value for electron transfer from ³C₆₀* to p-benzoquinone increases with an increase in Sc³⁺ concentration ([Sc³⁺]) to exhibit a first-order dependence on [Sc³⁺], changing to a second-order dependence at the high concentrations. Such a mixture of first-order and second-order dependence on [Sc³⁺] is also observed for a Sc³⁺-promoted electron transfer from CoTPP (TPP^2- = tetraphenylporphyrin dianion) to p-benzoquinone. This is ascribed to formation of 1:1 and 1:2 complexes between the generated semiquinone radical anion and Sc³⁺ at the low and high concentrations of Sc³⁺, respectively. The transient absorption spectra of the radical cations of various fullerene derivatives were detected by laser flash photolysis of the fullerene/p-chloranil/ Sc³⁺ systems. The ESR spectra of the fullerene radical cations were also detected in frozen PhCN at 193 K under photoirradiation of the fullerene/p-chloranil/Sc³⁺ systems. The Sc³⁺-promoted electron-transfer rate constants were determined for photoinduced electron transfer from the triplet excited states of C₆₀, C₇₀, and their derivatives to p-chloranil and the values are compared with the HOMO (highest occupied molecular orbital) levels of the fullerenes and their derivatives.