Reduction of the polynuclear quinonoiddyes16,17-dihydroxy- and dimethoxyviolanthrone with photogenerated radicals

Abstract

The radical-induced reduction of 16,17-dihydroxy- (M1O₂) 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.