Oxidative electrochemistry of electropolymerized metalloprotoporphyrin films

Abstract

The course of electrochemical oxidation has been monitored by cyclic voltammetry, ring-disk voltammetry, and absorption spectroscopy for metalloporphyrin electrode films prepared by electropolymerization of Zn II and Fe III protoporphyrin IX (PP) complexes. In CH 2 Cl 2 both films show stable cyclic voltammograms (CV's) over the first oxidation wave, assigned to porphyrin radical cation formation. Addition of 2% H 2 O to the electrolyte is without effect on the first oxidation of Zn II PP, but contact with aqueous buffer (pH 6.9) enhances the anodic current six-fold, decreases the cathodic current and abolishes subsequent electroactivity; the inactivated film contains modified porphyrin species, as shown by its absorption spectrum on SnO 2 . It is suggested that oxidation proceeds via nucleophilic attack of H 2 O on the Zn II PP + radical cations, leading eventually to the six-electron product dioxoporphomethene. The same product is indicated for the Fe III PP film which also shows a six-fold anodic current enhancement in contact with aqueous buffer, and subsequent inactivation. But in this case a large negative shift in the anodic peak potential suggests (Fe IV =0)PP formation, and the inactivation pathway is suggested to involve O atom transfer to the porphyrin rings. For the Fe III PP film 2% H 2 O already produces a significant negative shift and broadening of the anodic peak, and a decrease in the cathodic current. Ring-disk voltammetry demonstrates some O 2 production accompanying oxidation of the Fe III PP film, but not the Zn II PP film, in contact with aqueous buffer; the yield accounts for only 2% of the oxidation current. Addition of (ClPh) 3 P, intended to intercept the O atoms of the (Fe IV =O)PP units, results only in accelerated film inactivated, probably via a radical process.