Photophysical and photochemical studies of zinc(II) phthalocyanine derivatives - Effects of substituents and solvents

The effects of substituents and solvents on the photophysical and photochemical parameters of zinc(II) phthalocyanines are reported. The complexes studied are zinc phthalocyanine (ZnPc), zinc tetra(tert-butylphenoxy) phthalocyanine [ZnPc(TBPh)4], zinc octa(methylphenoxy)phthalocyanine [ZnPc(MPh)8], zinc tetranitrophthalocyanine [ZnPc(NO 2)4], zinc octachlorophthalocyanine (ZnPcCl8), zinc tetrasulfophthalocyanine [ZnPc(SO3-)4], a mixture of zinc mono-, di-, tri- and tetrasulfophthalocyanine [ZnPc(SO 3-)mix] and zinc naphthalocyanine (ZnNPc). It was found that the presence of peripheral substituents on the macrocycle enhances the yield of the triplet state. Among the different substituents, the sulfonated derivative, ZnPc(SO3-)mix, has the longest triplet lifetime (τT) and the highest singlet oxygen quantum yield (φΔ). The near infra-red absorptions of the solvents reveal that solvents that absorb around 1100 nm (triplet energy level) and around 1270 nm (singlet oxygen energy level), quench the triplet state of the ZnPc derivative as well as singlet oxygen. Although water has a high singlet oxygen quenching effect, the φΔ value for ZnPc(SO 3-)mix in water is still reasonably high at 0.48, which may provide an explanation for the efficient photosensitization by this molecule in photodynamic studies.