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

A series of transmetalated bacteriochlorophyll a, [M]-BChla (M = Mn, Z n, Cd, Co, Ni, Cu, Pd), and the corresponding 13(2)-hydroxy derivative s, [M]-OH-BChla, were investigated by low-temperature cyclic voltammet ry and by spectroelectrochemistry in the vis/near-IR range. This is th e first systematical investigation of bacteriochlorin macrocycles with electrochemical methods. In the cyclic voltammetry measurements, we w ere able to generate the dianions and dications of all species. Furthe rmore, 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 relati onship with the E(N)/r(i) values (E(N) = electronegativities, r(i) = r adius of the divalent metal ion). Deviations are observed for the Ni, Co, and Cd derivatives and are interpreted in terms of structural defo rmations. Kinetic electrochemical measurements show that the native BC hla has the highest rate constant for the heterogeneous charge transfe r. Using VIS/near-IR spectroelectrochemistry, we were able to distingu ish 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 pr oposed.