Study of Photoinduced N-Hydroxy-arylnitroxide Radicals (ArNO•OH) by Time-Resolved EPR

Photoreduction of 2-methyl-1-nitro-9,10-anthraquinone in the presence of 1-phenylethanol

The photoreactions of 1-nitro-9,10-anthraquinone (N1) and 2-methyl-1-nitro-9,10-anthraquinone (N2) were studied in benzene and acetonitrile in the presence of 1-phenylethanol. For N2, a short-lived 10 ns transient observed upon flash photolysis is attributed to a triplet state, which can be intercepted by 1-phenylethanol to form a monohydro radical of N2 and a spectroscopically not detectable donor-derived radical. The decay of radicals yields the corresponding nitroso compound and eventually 1-amino-2-methyl-9,10-anthraquinone (A2) as photoproduct. The final reduction step requires participation of the anthraquinone carbonyl groups. The yield of radicals and the quantum yield (Phi(NH(2))) of conversion to A2 are small in inert solvents and increase with the donor concentration, approaching Phi(NH(2)) = 0.2. No triplet state was observed in the flash photolysis of N1, but a N1-derived radical and 1-amino-9,10-anthraquinone (A1) as final products were found. Various mechanistic aspects of complete photoreduction of nitroarenes to aminoarenes are discussed.