FLUORESCENCE QUENCHING OF WATER-SOLUBLE PORPHYRINS BY MOLECULAR OXYGEN

Quenching by molecular oxygen of excited states of water-soluble anionic 5,10,15,20-tetarkis-(4-sulfonatophenyl)-porphyrin (H2TSPP) and cationic 5,10,15,20-tetrakis-(4-N-methylpyridyl)-porphyrin (H2TMPyP) in aqueous ethanol was investigated. It was found that fluorescence quenching of dissociated (in water) and undissociated (in ethanol) forms of H2TSPP was diffusion-controlled and occurred at distances close to contact ones (0.5-0.8 nm). Fluorescence of the dissociated form of H2TSPP was quenched with rate constant k(S) that was 1.7 times greater than that of the undissociated form. It was proposed that this was due to a decrease in the porphyrin molecule oxidation potential on going from the undissociated to the dissociated form. It was shown that the most probable reason for the dramatic increase in the rate constant of the fluorescence quenching of H2TMPyP in water compared with that of H2TSPP was the low-lying intramolecular charge-transfer state typical of H2TMPyP.