Electroreflectance spectroscopic study of a dicobalt cofacial porphyrin, Co2FTF4, adsorbed on gold and graphite electrodes

Electroreflectance spectroscopy (ER) is a sensitive method which allows, when an alternating electrical potential is applied, the detection of minor changes in the reflectivity of an electrode on whose surface molecules are adsorbed. This technique was applied to a dicobalt bisporphyrin, Co2FTF4, a cofacial dimer adsorbed on gold and graphite electrodes in contact with aqueous solutions and also with dichloromethane. The aim of this study was to obtain information on the redox properties of the adsorbed molecule under the experimental conditions where it is used as an efficient catalyst for the reduction of dioxygen. The cofacial dimeric porphyrin was adsorbed as a monolayer on the surface of the electrode. Its redox properties were different from what was expected after the electrochemical investigation in non-aqueous solutions: one of the two cobalt centres was first oxidized, the second electron-transfer step being the abstraction of one electron from a porphyrin ring. The electrode material, as well as the nature of the solution in contact with the electrode, greatly influences the ER spectra of the adsorbed molecule.