AN ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL INVESTIGATION OF METAL-SUBSTITUTED BACTERIOCHLOROPHYLL-ALPHA

A series of transmetalated bacteriochlorophyll a, [M]-BChla (M = Mn, Zn, Cd, Co, Ni, Cu, Pd), and the corresponding 13(2)-hydroxy derivatives, [M]-OH-BChla, were investigated by low-temperature cyclic voltammetry and by spectroelectrochemistry in the vis/near-IR range. This is the first systematical investigation of bacteriochlorin macrocycles with electrochemical methods. In the cyclic voltammetry measurements, we were able to generate the dianions and dications of all species. Furthermore, we have observed the trianions for the Ni and Cu derivatives as well as the tetraanions of the Co derivatives. With the exception of the tri- and tetraanions, the redox potentials exhibit a linear relationship with the E(N)/r(i) values (E(N) = electronegativities, r(i) = radius of the divalent metal ion). Deviations are observed for the Ni, Co, and Cd derivatives and are interpreted in terms of structural deformations. Kinetic electrochemical measurements show that the native BChla has the highest rate constant for the heterogeneous charge transfer. Using VIS/near-IR spectroelectrochemistry, we were able to distinguish between metal-centered and ring-centered redox processes. For the Co and Mn derivatives, metal-centered redox reactions are observed. A ''pi-anion-state-marker'' band positioned between 927 and 974 nm is proposed.