Formation of Singlet Oxygen by Photo–excitation of Substituted Aromatic Azo Compounds in Oxygenated Solution

Irradiation into the visible adsorption band of four trans–4–dialkylamino–4′-nitroazobenzenes not subject to azo–hydra–zone tautomerism leads to conversion of triplet oxygen to singlet oxygen. Evidence for formation of singlet oxygen is provided by characterization of cis–dibenzoylstilbene as the predominant oxidation product of tetraphenylcyclopenta–dienone and by inhibition of the oxidation by DABCO and other singlet oxygen quenchers. Addition of the same concentration of 2, 6–di–t–butyl–phenolhas essentially no effect. The rates of disappearance of the singlet oxygen scavengers bilirubin and 1, 3–diphenylisobenzofuran are also reduced in the presence of DABCO. Quantum yields of singlet oxygen formation between 0. 03–2 × W^-3 were obtained for the compounds examined in di–n–butylterephthalate. The singlet oxygen yield parallels the triplet yield obtained from the same azo compounds by laser flash photolysis.