A photophysical study of some purpurins

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Abstract
Ground-state, fluorescence, triplet-state and singlet-oxygen luminescence studies are reported for two purpurins, a metallo (SnIV) purpurin and a porphyrin tautomer, NTW, containing an unsaturated meso-substituent. Fluorescence lifetimes of the purpurins are short (ca. 2 ns) and the fluorescence yields are low (ϕT and singlet-oxygen quantum yields ϕΔ are high for the purpurins with ϕΔ/ϕT≈ 0.8 and triplet lifetimes in deoxygenated solvent are ca. 100 µs. The porphyrin NTW, however, has ϕT < 0.1 and a triplet lifetime of 18 µs. The dominant deactivation pathway (ϕ1C > 0.9) for NTW is suggested to be free rotation/isomerisation of the unsaturated meso-substituent. These results are discussed with respect to the possible role of purpurins, the tautomeric porphyrin, NTW, and the photo-oxidation products as sensitisers for photodynamic therapy of cancer.