Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium trifiate [Sc(OTf)₃] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)₃. The rate constants of the Sc(OTf)₃-promoted photoinduced electron-transfer reactions (k_et) of acridine to afford the complex between acridine radical anion and Sc(OTf)₃ remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)₃. In contrast to the case of acridine, the ket value of the Sc(OTf)₃-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)₃ to exhibit first-order dependence on [Sc(OTf)₃] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k_et on [Sc(OTf)₃] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)₃. The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)₃ as compared to those in the absence of Sc(OTf)₃ have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)₃-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O ₂-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)₃. The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.