Photochemical reactions of duroquinone. Part 1 .—Auto-inhibition and inhibition of the photoreduction of duroquinone in isopropanol
Irradiation of concentrated solutions of duroquinone in isopropanol yields five products. These include durohydroquinone and diduroquinone and three other products produced in very low yields under all conditions. At concentrations of duroquinone ⩽10–3 M, products other than durohydroquinone are negligible and the quantum yield of photoreduction of duroquinone is 0.39±0.02. This value is independent of concentration, intensity and wavelength but the reaction shows autoinhibition due to quenching of the triplet state of duroquinone by the product durohydroquinone with almost unit collisional efficiency. Water and diduroquinone also inhibit the reaction. The experimental results are in quantitative agreement with the general reaction scheme for quinone photoreduction presented. Quenching of the relatively long-lived triplet state of duroquinone is responsible for the inhibition in each case. Estimates of the rate constants of various steps in the mechanism are given.