Photoreactions of biacetyl with tetramethyl-1,3-dioxole and 1,1-diethoxyethene: solvent dependence of the product quantum yield as an indication of the competition between product formation and ionic photodissociation

Abstract

The product quantum yields of the photoreactions between singlet excited biacetyl ( 1) and tetramethyl-1,3-dioxole ( 2) and between triplet excited 1 and 1,1-diethoxyethene ( 3) decrease as the dielectric constant ϵ of the solvent increases. This solvent effect is explained in terms of the competition between product formation and ionic photodissociation. The dependence of the calculated free enthalpies of the solvated ion pairs ( 1 s −⋯ 2 s +) and the dissociated ion radicals ( 1 s − + 2 s +) on ϵ clearly demonstrates that ionic photodissociation becomes exothermic in solvents with ϵ ≳ 5.