Influence of halogenation and aggregation on photosensitizing properties of zinc phthalocyanine (ZnPC)

The effects of halogenation and aggregation on photosensitizing properties of zinc phthalocyanine (ZnPC) have been examined by photophysical methods. Halogenation decreases the fluorescence yield and lifetime, increases triplet formation and shortens the triplet lifetime. These effects, which increase in the order Cl < Br < l, can be explained in terms of spin–orbit coupling theory. The aggregation behaviour for halogenated ZnPC in dimethyl sulfoxide (DMSO) was observed by absorption spectroscopy. Assuming that no higher aggregates than dimers are formed, dimerization equilibrium constants were calculated using a modified non-linear least-squares fitting method. Quantitative analysis for the influence of dimerization on apparent molar absorption coefficient, fluorescence quantum yield and the quantum yield of singlet oxygen generation was given. Factors that govern the photosensitized production of singlet oxygen were also discussed quantitatively in order to explain the ability of singlet oxygen production by halogenated ZnPC. The enhanced intersystem crossing process between the triplet state of sensitizer and oxygen was not affected by the nature of the halogen atom.