Reduction of isoviolanthrone by reaction with photogenerated radicals.

The properties of isoviolanthrone (V(1)) and its corresponding dihydro form (V(1)-H(2)) in argon-saturated 1-phenylethanol in the presence of acetophenone were examined by photophysical and photochemical methods. Upon photolysis of the sensitizer, a radical-induced reduction of V(1) to V(1)-H(2) occurs. The quantum yield of this reduction is close to unity. The fluorescence of V(1) has a quantum yield of 0.08, for V(1)-H(2)Φ(f) is strongly enhanced and the emission maximum is blue-shifted. The ketyl radical, the semiquinone radical and V(1)-H(2) were observed in this sequence by flash photolysis. V(1)-H(2) is unstable and undergoes oxidation to V(1) on admission of oxygen.

Properties of 16,17-disubstituted dihydroviolanthrones formed by reaction of violanthrones with photogenerated radicals.

The photophysical and photochemical properties of violanthrone (V1) and two 16,17-R derivatives with R = OC(8)H(17) and OCOC(17)H(35) (V2 and V3, respectively) in solution were examined. The fluorescence quantum yield of V3 is Phi(f) = 0.2, while that of V1 is low (0.01) and V2 even lower. A radical-induced reduction takes place in the presence of a photosensitizer, such as acetone, acetophenone or benzophenone, and an alcohol as donor. The reduction quantum yield in an argon-saturated solution, e.g. in 1-phenylethanol and glycerol triacetate, is close to unity. The products are the corresponding dihydroquinones (VH(2)s), which fluoresce strongly at 560-600 nm, Phi(f) = 0.6-1, and undergo back-oxidation on the admission of oxygen. The flash photolysis of violanthrones in the presence of both sensitizer and donor reveals ketyl radicals after the pulse, the intermediacy of the semiquinone radicals (VH*) at 650-750 nm and the formation of VH(2)s in the 0-1 ms range.

Reduction of the polynuclear quinonoid dyes 16,17-dihydroxy- and dimethoxyviolanthrone with photogenerated radicals.

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.

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.