Photophysical properties and photodynamic activity of octacationic oxotitanium(iv) phthalocyanines

Abstract

The photophysical and photosensitizing properties of two octacationic oxotitanium phthalocyanines (TiOPcs), bearing pyridiniomethyl or cholinyl substituents, have been studied in aqueous and alcohol solutions. In water, both compounds were monomeric with the high quantum yields of fluorescence (Φ_F = 0.17–0.19) and singlet oxygen formation (Φ_Δ = 0.4–0.5). The Φ_F and Φ_Δ of both phthalocyanines decreased with the increase of solvent hydrophobicity from water to ethanol. This effect was much stronger in alcohol solutions of the pyridiniomethyl-substituted phthalocyanine and probably results from aggregation of TiOPc molecules caused by association of chloride anions with phthalocyanine cationic groups. Evidence is presented that under illumination aqueous TiOPc solutions also produce hydroxyl radicals, which probably appear owing to photocleavage of water molecules. The quantum yield of OH formation was (3–5) × 10⁻⁵ after argon purging and twice as much in the presence of air. It is shown that irradiation of TiOPc solutions causes photobleaching of TiOPcs. The photobleaching quantum yield in water was found to be about 1 × 10⁻⁴. The data suggest that photobleaching occurs owing to the reactivity of hydroxyl radicals, though singlet oxygen is generated by TiOPcs much more efficiently. The phototoxicity of the tested TiOPcs toward bacteria has been revealed. It is proposed that both OH and ¹O₂ might be responsible for the observed bactericidal effects.