Elucidating the Electroreduction Mechanism of the Monoprotonated Octaethylporphyrin. A Comparative Study with the Diprotonated Octaethyl- andmeso-Tetraphenyl-porphyrins

The electrochemical reduction mechanisms of diprotonated tetraphenylporphyrin (H2TPP) and mono- and diprotonated octaethylporphyrin (H2OEP) were studied in tetrabutylammonium perchlorate/benzonitrile. The diprotonated forms of both porphyrins undergo two one-electron reversible reduction processes forming isophlorin. Contrastingly, monoprotonated H2OEP is reduced in a single process involving a two-electron one-proton transfer that yields two types of short-lived intermediates, isophlorin and neutral phlorin. The existence of intermolecular proton transfer reactions, from the parent protonated porphyrin to the isophlorin or neutral phlorin, to form phlorin cation species (isophlorin protonated at themeso-position) was demonstrated. In-situ UV-vis spectroelectrochemical experiments allowed us to identify the absorption of the isophlorin species of H2TPP but not of H2OEP. These results show that the lack of phenyl substituents increases the rate of protonation at themeso-position. Finally, it was demonstrated that the protonation of the porphyrin macrocycle not only lowers the reduction potentials but also increases the reactivity of the electrogenerated species.