Photochemical reactions of duroquinone. Part 2.—Flash photolytic investigations

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Abstract
Flash photolysis studies have established that the mechanism for inhibition of the photoreduction of duroquinone in isopropanol by durohydroquinone and diduroquinone is due to chemical reaction of the reactive triplet state, Q*(T1), with these inhibitors rather than with isopropanol. Water also inhibits the reaction but this is probably due to quenching of the triplet state by a physical process. Quantitative measurements on the semiquinone transient demonstrate that the quantum yield of triplet state formation of duroquinone is unity, and auto-inhibition by durohydroquinone QH2 is due to the reaction sequence, Q*(T1)+QH2→ 2QH·→ Q+QH2. Kinetic measurements confirm that the durosemiquinone radicals and the 2-hydroxyprop-2-yl radicals formed when triplet duroquinone reacts with isopropanol disproportionate independently. Rate constant ratios determined by flash photolytic studies are in good agreement with those obtained from continuous irradiation experiments as determined in part 1. The hydrogen-atom abstraction by triplet duroquinone from ethanol has an appreciable activation energy. In polymethyl methacrylate flash photolysis of duroquinone gives a single transient which decays by a combination of temperature-dependent and temperature-independent processes and is most probably the triplet state.

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