The radical-induced reduction of 16,17-dihydroxy- (MlO2) and 16,17-dimethoxyviolanthrone (M2O2) in argon-saturated 1-phenylethanol or mixtures of dimethyl sulfoxide and acetonitrile or glycerol triacetate was studied by photochemical methods. The photoproduct of MiO2 (i = 1, 2) in the presence of a sensitizer, such as acetone, acetophenone or benzophenone, and a donor, e.g. an alcohol or triethylamine, is the fluorescing dihydroquinone, Mi(OH)2 which undergoes back-oxidation. Flash photolysis reveals ketyl radicals after the pulse and indicates radical formation in the 1 ms range en route to Mi(OH)2, while the putative donor and semiquinone radicals are undetectable intermediates. The concentrations of quinonoid dyes and donors were varied to explore the phenomena and to provide data for a mechanistic interpretation.
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