Specific photoreactions of flavanones typical of n,π* and π,π* characters in lowest triplet states

Abstract

On irradiation with u.v. light, flavanones undergo photochemical opening of the dihydropyranone ring (φ₂ < 0.2) and/or bimolecular photoreduction (φ₃ < 0.8). The relative occurrence of these processes markedly depends on the substituents as well as on the solvent. Triplet mechanisms are implied for both reactions by the results of quenching experiments, as well as by fluoroescence and phosphorescence spectra. A Hammett p value of ca.–1.2 suggests the intermediacy of radicals in the ring-opening reaction. The marked variations in φ₂, φ₃, and φ_f with substituents are explicable in terms of relative contributions of n,π* and π,π* character in lowest triplet states.