Electron transfer in the redox reaction of cobalt tetraphenylporphyrin incorporated in a Nafion film

Abstract

Potential step chronoamperospectrometry (PSCAS) was carried out to analyze electron transfer in the redox reaction processes of 5,10,15,20-tetraphenylporphyrinatocobalt(II) ( Co II TPP (-2)) incorporated in a Nafion film. The reactions of Co II TPP (-2) to [ Co III TPP (-2)]+ and of [ Co III TPP (-2)]+ to Co II TPP (-2) took place through a diffusion mechanism, as confirmed by the first-order initial reaction rate with respect to the complex concentration in the matrix. However, the reaction of [ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+ occurred by an electron-hopping mechanism, as confirmed by the second-order initial reaction rate with respect to the complex concentration. The fraction of electroactive complex (Rct) increased with the sample time after the potential step until it reached saturation. In the reactions of Co II TPP (-2) to [ Co III TPP (-2)]+ and of [ Co III TPP (-2)]+ to Co II TPP (-2), Rct approached 1.0, while in the reaction of [ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+, only about 0.3 was reached. The apparent diffusion coefficient (Dapp) decreased in the order of [ Co III TPP (-2)]+ to Co II TPP (-2) > Co II TPP (-2) to [ Co III TPP (-2)]+>[ Co III TPP (-2)]+ to [ Co III TPP (-1)]2+. The different behavior of these redox reactions was ascribed to the microenvironment of the redox species in the matrix, interaction of the redox centers, especially the product with the framework, and counter ion migration.