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 C-60 to p-chloranil occurs to produce C-60 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 C-60 (C-3(60)*) 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 C-60/p-chloranil/Sc3+ system, the k(et) value for electron transfer from C-3(60)* 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 (TPP2- = 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 C-60, C-70, 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.