Mechanism of the photocycloaddition of 1-aminoanthraquinones to olefins by visible light irradiation; oxetan formation via an exciplex

The mechanisms of the photocycloadditions of 1-aminoanthraquinone (1) and 1-amino-2,4-dibromoanthraquinone (2) to olefins by visible light irradiation have been investigated. Fluorescence of compound (1) was quenched by olefins without the appearance of a new emission and the Stern–Volmer constants accorded well with those obtained from the double reciprocal plots of the quantum yield against the concentration of olefin. The rate constant for the reaction was larger for olefins with lower ionization potentials, and was one order of magnitude larger in ethanol than in benzene. Exciplex quenching effects caused by amines such as triethylamine, butylamine, piperidine, and pyridine were also observed. All the results indicate that the reaction proceeds via exciplex formation between compound (1) in its excited singlet charge-transfer state and the olefin. The photocycloaddition of compound (2) in benzene was quenched by oxygen and gave a linear Stern–Volmer plot; the reaction in ethanol was only partially quenched. Detailed kinetic studies indicated that, in benzene, compound (2) had a single mode of reaction via the excited triplet charge-transfer state, while in ethanol it had a dual mode of reaction via both the singlet and triplet charge-transfer states.