Counter-Anion-Dependent Optical Properties of Cationic N22-Methylated Chlorophyll-a Derivatives

Methyl N22-methylpyropheophorbides-a with chloride or hexafluorophosphate were prepared by chemically modifying chlorophyll-a. The resulting product composed of cationic N-methylated chlorin and chloride anion exhibited visible absorption and fluorescence emission maxima in chloroform at longer wavelengths than those with hexafluorophosphate. Both the absorption and emission spectra of the former were hypsochromically shifted by change of the solvent from chloroform to methanol to give almost the same corresponding spectra of the latter independent of solvents. The apparent counter-anion dependency in chloroform and the specific solvent dependency in the N-methyl-chlorin with chloride are ascribable to the weak solvation of a hard chloride anion over soft hexafluorophosphate in chloroform and strong electrostatic interaction of the cationic chlorin with a chloride anion over hexafluorophosphate in chloroform as well as well solvation of both the anions in methanol. In addition, less emission of N-methyl-chlorin with chloride in chloroform would be due to partial fluorescence quenching based on the heavy atom effect of the adjacent chloride anion.