Time-resolved luminescence—pulse radiolysis determination of the fractions of porphyrin photosensitizer triplets giving rise to singlet excited oxygen on quenching by ground state triplet oxygen

Abstract

By exploiting the different modes of excitation by ionizing radiation as opposed to light, a group of five porphyrin photosensitizer triplet excited states were formed free of the corresponding singlet excited states. Measurements were made of the corresponding triplet-induced singlet oxygen yields S Δ, with the aid of a recently developed technique for measuring the weak IR luminescence of singlet oxygen at 1270 nm generated by pulse radiolysis. The S Δ values obtained in C 6D 6 were 0.74, 0.75, 0.76, 0.88 and 0.92 for coproporphyrin III tetramethyl ester, deuteroporphyrin IX dimethyl ester, haematoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester and chlorophyll b respectively. This method for accurately measuring S Δ is generally applicable to triplet states with lifetimes of 10 μs or longer, provided that the parent photosensitizers react with singlet oxygen with rate constants of less than 10 8 M −1 s −1.