Singlet oxygen yields and radical contributions in the dye-sensitised photo-oxidation in methanol of esters of polyunsaturated fatty acids (oleic, linoleic, linolenic and arachidonic).

Abstract

Abstract— The triplet state characteristics (spectrum, lifetime and quantum yield) for four dye sensi tisers [methylene blue (MB), erythrosin (ER), haematoporphyrin (HP) and riboflavin (RF)] were determined in methanol by laser flash photolysis and singlet oxygen yields (0.60 to 0.48) from time‐resolved measurements of the 1270 nm near infrared emission. The reaction of singlet oxygen with four long chain unsaturated phenyl esters [oleate (18: 1), linoleate (18: 2), linolenate (18: 3) and arachidonate (20: 4)] was followed quantitatively using the singlet oxygen luminescence technique and also, after continuous420–700 nm irradiation, by HPLC and other analysis of the isomeric product monohydroperoxides. The overall quantum yield of photooxidation (∼10‐2) was shown to be consistent with the observed singlet oxygen quenching constants(2–12 times 104 dm3 mol‐1 s‐1) for the four esters studied and the singlet oxygen lifetime in methanol (τ∼ 9 μs). The isomer product distribution was interpreted in terms of a dual singlet oxygen and radical mechanism, the radical contribution increasing with sensitiser in the order ER = MB < HP ≪ RF, but also showing some dependence on substrate unsaturation. Evidence is presented for singlet oxygen quenching by MB and RF (kO= 1.6 and 6.0 times 107 dm3 mol‐1 s‐1) and for the accelerated photobleaching of the dye sensitisers in the presence of the unsaturated esters.