Novel porphyrins and a chlorin as efficient singlet oxygen photosensitizers for photooxidation of naphthols or phenols to quinones

Four porphyrins and one chlorin having aromatic groups at the meso position have been synthesized and tested as potential sensitizers for the photooxidation of phenols. The yields of conversion of 1,5-dihydroxynaphthalene and 2,3,5-trimethylphenol to their corresponding quinones have been determined following photolysis in air-saturated acetonitrile–dichloromethane solutions in the presence of these compounds, and are higher than those obtained using other photosensitizers. The reactions are shown to proceed via formation of singlet oxygen (1Δg), followed by its addition to the phenol. A mechanism is presented. Rates of reaction of singlet oxygen with the substrates were determined by studying the decay of the 1O2* emission. Singlet oxygen quantum yields and formation efficiencies for the photosensitizers have been determined by laser flash photolysis and photoacoustic calorimetry. In addition, for two of the porphyrins, these were also measured using time-resolved luminescence. In all cases, highly efficient singlet oxygen formation is observed. The stabilities of the sensitizers to prolonged photolysis were studied. The compound 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin was found to be a particularly efficient photosensitizer for oxidation of phenols. It is suggested that this is due both to its high singlet oxygen yield and to its good photostability.