C3- and C13-substituent effects on electronic absorption spectra of linear tetrapyrroles produced by photooxidation of zinc chlorophyll derivatives

Zinc chlorophyll derivatives possessing various substituents at the 3- and 13-positions were cleaved at the C19–C20 bond upon irradiation of an oxygen-saturated dichloromethane solution with visible light. The resulting photooxidation products were zinc complexes of linear tetrapyrroles which showed longer wavelength absorption bands in the near-infrared region than the visible Qy bands of the corresponding cyclic tetrapyrroles as the starting materials. Electron-withdrawing C13-substituents of the linear tetrapyrroles shifted the longest absorption maxima bathochromically, as observed in the cyclic tetrapyrroles. In contrast, more electron-withdrawing C3-substituents of the linear tetrapyrroles gave more hypsochromically shifted redmost maxima, which was the reverse of the substitution effect of the cyclic tetrapyrroles.