SPECTROELECTROCHEMISTRY AND CYCLIC VOLTAMMETRY OF THE EE MECHANISM IN A PORPHYRIN DIACID REDUCTION

The electrochemical reduction of mesotetra(4-N-methyl-pyridyl)porphine diacid at a gold electrode has been investigated by cyclic voltammetry and spectroelectrochemistry. The reduction proceeds in two closeiy-spaced one-electron steps having E0' values of -0.016 and -0.100 V vs. SCE, respectively. Spectra obtained from potential-step chrono-absorptometry experiments suggest the first electron transfer step to involve formation of a cation radical (P(-I)H4+.) and the second a porphodimethene (P(-II)Hδ2+). The latter species is unstable and decays to a phlorin monocation (P-(-II)H5+). The rate constant for this step is 0.25 ± 0.02 s-1 (cyclic voltammetry) and 0.3 ± 0.05 s-1 (spectroelectrochemistry). Methods for multiparameter estimation in both cyclic voltammetry and spectroelectrochemistry are presented. © 1979, American Chemical Society. All rights reserved.