ELECTRON-TRANSFER DYNAMICS IN HIGHLY REDUCED STATES OF SIMPLE AND SUPERSTRUCTURED METALLOPORPHYRINS

The standard rate constants of the Fe(1)-/Fe("0")2- couple in a series of four simple and basket-handle superstructured porphyrins have been measured by means of fast cyclic voltammetry at mercury and gold ultramicroelectrodes. Analysis of the experimental data by the Marcus-Hush model revealed that the main rate-controlling factor of these very fast electron-transfer reactions is solvent reorganization. The presence of secondary amide groups borne by the basket-handle structure and located in the close vicinity of the metalloporphyrin center largely facilitates the reaction from a thermodynamic viewpoint. This facilitation of the reaction is not counterbalanced by any significant contribution of the fluctuational reorganization of the NHCO dipoles thanks to their attachment to the basket-handle chains. A few complementary experiments were carried out with zinc and copper porphyrins where the same general trends were observed.