Features of the Quenching of the Triplet States of Water-Soluble Porphyrins by Molecular Oxygen

Using the methods of time-resolved absorption spectroscopy, we have investigated the features of quenching, by molecular oxygen, of the excited triplet states of water-soluble 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphyrin (H₂TMPyP) and 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrin (H₂TSPP) in water–ethanol solutions. It has been revealed that for both compounds the rate constant of quenching of the triplet states increases with increasing viscosity of the medium. Quenching of the excited triplet states of the dissociated (in water) and undissociated (in ethanol) forms of water-soluble porphyrins occurs with a different efficiency, and the rate constant of quenching the triplet states by molecular oxygen k _T thereby is higher for the dissociated form. It has been shown by means of mathematical modeling that the experimental results obtained can be described in terms of the change in the rate constants of intracomplex transitions in the porphyrin–oxygen collisional complex at varied solution viscosity and their difference for the dissociated and undissociated forms of water-soluble porphyrin.