Synthesis of Cationic Pyridinium–Chlorin Conjugates with Various Counter Anions and Effects of the Anions on Their Photophysical Properties

Abstract

Abstract Cationic pyridinium pending chlorophyll-a derivatives with several counter anions were synthesized by Ag(I)-induced oxidation reactions of methyl pyropheophorbide-a possessing the 3-vinyl group with pyridine. The high water-solubility of the synthetic 31-pyridinio-3-vinyl-chlorin cation with a chloride anion resulted in efficient anion exchange reactions conducted in a biphasic system of H2O and CH2Cl2. Using either or both of the two synthetic procedures, ten salts were successfully synthesized with SbF6−, PF6−, BF4−, ClO4−, (CF3SO2)2N−, CF3SO3−, NO3−, I−, Br−, and Cl−. The effects of the counter anions on the physical properties of the cationic chlorophyll-a derivatives were discussed using their 1H NMR, visible absorption, and fluorescence emission data as well as theoretical model calculations. Although the visible absorption and fluorescence emission spectra were changed faintly, the fluorescence quantum yields and lifetimes were dependent on the counter anions, showing that the anions affected the pyridinium group acting as an electron-acceptor and the fluorescence quenching by intramolecular charge transfer.