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

Abstract

The photophysical and photochemical properties of violanthrone (V1) and two 16,17-R derivatives with R = OC₈H₁₇ and OCOC₁₇H₃₅ (V2 and V3, respectively) in solution were examined. The fluorescence quantum yield of V3 is Φ_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 (VH₂s), which fluoresce strongly at 560–600 nm, Φ_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 VH₂s in the 0–1 ms range.