Synthesis of Highly Fluorescent Cationic Chlorophyll-a Derivatives Possessing a p-Aminopyridinio Group at the 31-Position

p-Aminopyridines were oxidatively substituted at the C31-position of methyl pyropheophorbide-a, a chlorophyll-a derivative. The electron-donating p-dimethylamino groups induced silver metallation during the substitution and inhibited the electron-transfer quenching of fluorescence emission in the metal-free C31-cationic pyridinio adducts prepared by the acidic demetallation of the initially isolated Ag-chlorins.