Study of the spectroscopic and electrochemical properties of tetraruthenated porphyrins by theoretical-experimental approach

The spectroscopic and electrochemical properties of two isomeric forms of the supramolecular species [μ-(H₂TPyP){Ru(bpy)₂Cl}₄]⁴⁺ (H₂TPyP = 5,10,15,20-tetra(3- or 4-pyridyl)porphyrin, bpy = 2,2′-bipyridine) have been compared and consistently interpreted with the aid of molecular orbital calculations. In these complexes, the HOMO and LUMO levels are predominantly localized in the ruthenium complexes and porphyrin ring, respectively. There is an extensive mixing of the wave functions of both components in other MOs, however, and their contributions are reflected in the spectroelectrochemical and spectroscopic behavior. For example, the electronic mixing is enough to allow the energy-transfer from the peripheral complexes to the porphyrin ring, as well as the appearance of a Ru^II(dπ) → H₄P(pπ*) charge-transfer band at 700 nm in the bis-protonated [μ-(H₄TPyP){Ru(bpy)₂Cl}₄]⁴⁺ species, showing the strong stabilization of the porphyrin LUMO levels.