Singlet-oxygen reactions in cyanoaromatic-sensitized photo-oxidations

Abstract

Cyanoaromatic-sensitized photo-oxidations of substrates (α-pinene, β-pinene, 3,4-dihydro-2H-pyran, 1,4-diphenyl-1,3-butadiene, limonene and cyclohexadiene) which react readily with singlet oxygen were studied. It was proved that a singlet-oxygen reaction takes place in each of the above photo-oxidations. The solvent polarity and solvent viscosity have the same influence on both the quantum yields of photo-oxidations and the fluorescence quenching rate constants. All the experiments support the suggestion that electron transfer occurs between the sensitizer excited singlet state and the substrate. The singlet oxygen formed then acts as a reactive intermediate in the subsequent process through back electron transfer. In carbon tetrachloride, the products of all the above substrates are the same as in the dyesensitized photo-oxidations and singlet oxygen may be generated through intersystem crossing which leads to the triplet-excited reaction.