ELECTROCHEMICAL AND SPECTROSCOPIC PROPERTIES OF DIMERIC COFACIAL PORPHYRINS WITH NONELECTROACTIVE METAL CENTERS - DELOCALIZATION PROCESSES IN THE PORPHYRIN PI-CATION-RADICAL SYSTEMS

The proximity of two or more porphyrins has been shown to be crucial in numerous biological processes such as electron transfer or oxygen activation. We report the electrochemical and spectroscopic (UV-vis, EPR) behavior of two families of dimeric cofacial diporphyrins, either covalently linked by two amide bridges (FTFn family) or monolinked by a polyaromatic bridge ("Pacman" DPX family). The derivatives with nonelectroactive centers (H2, Zn, Cu) are investigated; thus the redox systems are centered on the pi-ring systems only. When the two rings are sufficiently far apart, the dimer nearly behaves as the juxtaposition of two monomers. A two-electron oxidation process formally gives rise to a pi-cation diradical, and in one case a triplet-state spectrum can be observed. When the two rings are very close, strong pi-pi interactions generate mixed-valence behavior: the first oxidation process is split into two one-electron steps. The first oxidation product is a totally delocalized pi-radical, while in the second the two delocalized electrons of the rings become spin-paired, giving rise to a kind of nonclassical pi-pi bond. Analysis of the results and rationalization with the literature demonstrate that from a redox standpoint these molecules have to be considered as a single redox entity. A qualitative molecular orbital diagram is proposed which accounts for the enhanced electronic properties of these dimers.