Isoporphyrin formation upon electrochemical oxidation of zinc tetrakis(4-sulfonato-phenyl)porphine in aqueous solution

Abstract

Zinc tetrakis(4-sulfonato-phenyl)porphine, (ZnTSPP) in pH 4 aqueous buffer is converted cleanly to an isoporphyrin upon electrooxidation at potentials corresponding to mono- and dication formation (+0.55 and +0.80 V vs. SCE). This is demonstrated by optical thin-layer spectroelectrochemistry, which shows a characteristic absorption band at 778 nm for the oxidation product; the spectrum is the same as that reported previously for the isoporphyrin of zinc tetraphenylporphine formed in methanol. It is suggested that the ZnTPP isoporphyrin is formed by attack of H 2O on a meso carbon atom of the dication, which is available via disproportionation of the monocation. Electroreduction at 0.0 V returns the isoporphyrin to ZnTSPP. This process is seen in isoporphyrin cyclic voltammograms as a well-formed but irreversible cathodic wave at −0.2 V at a glassy carbon electrode, or a drawn-out wave at 0.0 V at a gold electrode. The isoporphyrin is remarkably stable at pH 4.0; three cycles of isoporphyrin formation and ZnTSPP regeneration were carried out with less than 5% loss in the original absorbance. When electrooxidation is carried out at pH 7.0, however, a different species, with an absorption peak at 513 nm is formed initially; this peak then decays, leaving a featureless absorption spectrum. ZnTSPP cannot be regenerated by electroreduction of this solution. Evidently the pH elevation induces further oxidation reactions, leading to rapid degradation of the porphyrin rings.