Photoreduction of Anthraquinone by Triethylamine in Ethanol and Toluene Studied by Steady-State Photolysis and Laser Photolysis

Abstract

Abstract Photoreduction of anthraquinone (AQ) by triethylamine (TEA) in toluene and ethanol was studied by means of steady-state photolysis and laser photolysis. In toluene, addition of TEA enhanced the quantum yield of photoreduction of AQ, and the reaction was found to originate via the formation of an exciplex between ground-state TEA and the lowest triplet state of AQ: This exciplex changed to a contact ion pair followed by proton transfer, generating anthrasemiquinone radical and triethylamine radical, and 9,10-anthracenediol was obtained as the final photoproduct. Although the mechanism of the photoreduction of AQ in ethanol/TEA was essentially the same as that in toluene/TEA and the final products in ethanol/TEA were 9,10-anthracenediol and its anion, the quantum yield of photoreduction decreased with increase in TEA concentration. It was proposed that the exciplex in ethanol was quenched by ground-state TEA via the formation of a triplex of a radical anion of AQ and a radical cation of TEA dimer bonded by the three σ-electrons on two nitrogen atoms, based on the suggestion made by Hub et al. (J. Am. Chem. Soc., 106, 701 (1984); 106, 708 (1984)).