Open AccessElucidating the Electroreduction Mechanism of the Monoprotonated Octaethylporphyrin. A Comparative Study with the Diprotonated Octaethyl- and meso-Tetraphenyl-porphyrins
Author(s) -
Hiram M. Castro-Cruz,
Leslie R. Arias-Aranda,
Norberto Farfán,
Elba Xochitiotzi-Flores,
Norma A. Macı́as-Ruvalcaba
Publication year - 2020
Publication title -
journal of the electrochemical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.258
H-Index - 271
eISSN - 1945-7111
pISSN - 0013-4651
DOI - 10.1149/1945-7111/abaae4
Subject(s) - protonation , chemistry , porphyrin , photochemistry , benzonitrile , electron transfer , tetraphenylporphyrin , reactivity (psychology) , electrochemistry , perchlorate , proton , proton coupled electron transfer , medicinal chemistry , organic chemistry , ion , electrode , medicine , alternative medicine , pathology , physics , quantum mechanics
The electrochemical reduction mechanisms of diprotonated tetraphenylporphyrin (H 2 TPP) and mono- and diprotonated octaethylporphyrin (H 2 OEP) were studied in tetrabutylammonium perchlorate/benzonitrile. The diprotonated forms of both porphyrins undergo two one-electron reversible reduction processes forming isophlorin. Contrastingly, monoprotonated H 2 OEP 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 the meso -position) was demonstrated. In-situ UV–vis spectroelectrochemical experiments allowed us to identify the absorption of the isophlorin species of H 2 TPP but not of H 2 OEP. These results show that the lack of phenyl substituents increases the rate of protonation at the meso -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.