Formation of ferryl porphyrin by electrochemical reduction of iron porphyrin in aqueous solution

The formation of ferryl porphyrin, induced by electrochemical reduction of FeIIITMPyP, was examined by measuring luminol chemiluminescence and recording the absorption spectrum using a flow-injection method. Both the anodic oxidation and the cathodic reduction of iron(III) porphyrin induced the formation of a ferryl species, which is widely accepted to be responsible for stimulating luminol chemiluminescence. Emission was observed both below the reduction potential of FeIIITMPyP (−0.08 V at pH 11, −0.02 V at pH 7 and 0.15 V at pH 3) and above the oxidation potential (0.6 V at pH 11, 0.75 V at pH 7 and 1.1 V at pH 3). The emission was relatively greater below the reduction potential than in the oxidative region. It was significantly higher especially at lower pH (<pH 4). The cathodic emission was largely dependent on the concentration of dissolved molecular oxygen while the anodic emission was not. Emission on both sides was inhibited significantly by the addition of alkene solutions downstream of the working electrode. The spectra on both the positive and negative potential sides were shifted to longer wavelengths (>424 nm) compared to the original spectrum of FeIIITMPyP (422 nm). Computer analysis also showed that the spectra on both sides were perfectly reproduced by a combination of two spectra, those of FeIIITMPyP and OFeIVTMPyP (436 nm in pH 11). We therefore believed that the ferryl species, formed by the autoxidation reaction of electro-generated ferrous porphyrin, was responsible for the emission. Although, ferryl formation by anodic oxidation has been reported, this is the first time, to our knowledge, that the cathodic formation has been reported.