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-hydrazone 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 tetraphenylcyclopentadienone and by inhibition of the oxidation by DABCO and other singlet oxygen quenchers. Addition of the same concentration of 2,6-di-t-butyl-phenol has 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 X 10e3 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