Drastic Changes in the Lifetime and Electron Transfer and Energy Transfer Reactivity of the Triplet Excited State of p-Benzoquinone by Complex Formation with Scandium Ion Salts

Metal-ion salts such as scandium triflate form strong complexes with the triplet excited state of p-benzoquinone, which have extremely long lifetimes as compared with the lifetime of the uncomplexed quinone triplet, whereas no complex is formed with metal-ion salts in the ground state. The absorption maxima (λmax) of triplet−triplet absorption of the metal-ion salt complexes are red-shifted from the λmax value of the uncomplexed quinone triplet and vary depending on the Lewis acidity of the metal-ion salts. The rate of the complexation of the triplet excited state with scandium triflate is determined, obeying pseudo-first-order kinetics. The pseudo-first-order rate constant increases linearly with increasing metal-ion concentration. The transient absorption spectra of donor radical cations produced by photoinduced electron transfer from N,N-dimethylanilines to the triplet excited state of p-benzoquinone are detected by laser-flash photolysis measurements, whereas no such radical cations are detected in the presence of scandium triflate under otherwise the same experimental conditions. Thus, the electron transfer reactivity of the triplet excited state complex of p-benzoquinone with scandium triflate toward electron donors is diminished significantly as compared with that of the uncomplexed quinone triplet. In contrast, the energy transfer from the triplet excited state to oxygen is enhanced by complex formation with scandium triflate.