Electrochemical investigation of modified photosynthetic pigments

Bacteriochlorophyll a (BChla), bacteriopheophytin a (BPhea), and pheophytin a (Phea) derivatives with a modified substituent pattern of the macrocyclic system were investigated by means of cyclic voltammetry. The more electron-withdrawing beta-substituents shifted the redox potentials toward more negative values; the reduction being much more affected than the oxidation. Substituents at C-13(2) or C-17 have only a minor influence on the redox potentials, with the exception of the strongly electron-withdrawing fluoro group at position C-13(2). The trianions of the Phea derivatives were observed for the first time, providing information of the energy of the LUMO + 1.