Effect of Imidazole on the Electrochemistry of Zinc Porphyrins: An Electrochemical and Computational Study.

In this study, the electrochemical behavior of zinc meso-substituted porphyrins in the presence of imidazole is examined by using both cyclic voltammetry (CV) and density functional theory (DFT) methods. The results show that the first half-wave oxidation potentials (1st E1/2) of zinc porphyrins complexed with imidazole all move to the negative side, while the second ones (2nd E1/2) move to the positive side, resulting in larger half-wave oxidation potential splittings of the two oxidation states (ΔE = second E1/2 - first E1/2) comparing with the zinc porphyrins. By employing DFT calculations, we have found that both sterically controlled inter π-conjugation between porphyrin rings and meso-substituted phenyl groups and deformation of porphyrin rings do play important roles in contributing to the half-wave oxidation potentials. Imidazole exhibits strong effects on the deformation of porphyrin rings which is dominant in determining the first E1/2 while the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups mainly contributes to the second E1/2. Without imidazole, the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups is the only important criterion which effects both first E1/2 and second E1/2 of zinc porphyrins.

Electrochemical Behavior of meso-Substituted Porphyrins: The Role of Cation Radicals to the Half-Wave Oxidation Potential Splitting.

In this study, the electrochemical behavior of free base and zinc meso-substituted porphyrins is examined by cyclic voltammetry (CV) and density functional theory (DFT). The results show that the half-wave oxidation potential splitting of the two oxidation states (ΔE= second E1/2 - first E1/2) of tetraphenylporphyrin (H2TPP) and its zinc complex (ZnTPP) are higher than those of porphyrins and their zinc complexes with meso-substituted five-membered heterocylic rings. The ΔE values follow the trend of TPP > T(3'-thienyl)P > T(3'-furyl)P > T(2'-thienyl)P for both meso-porphyrins and their respective zinc complexes. By employing DFT calculations, we have found that the trend of ΔE values is consistent with that of highest spin density (HSD) distribution and HOMO-LUMO energy gaps of cationic radicals as well as the π-conjugation between central porphyrin and meso-substituted rings. Also, they exhibit the better resonance between the porphyrin ring with meso-substituted rings as moving from porphyrins and their zinc complexes with phenyl rings to five-membered heterocyclic rings. A good agreement between calculated and experimental results indicates that cationic radicals, especially their spin density distribution, do play an important role in half-wave oxidation potential splitting of meso-porphyrins and their zinc complexes.