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.