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 H2O 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.
Bibliographical noteFunding Information:
This work was supported by DOE grant DE-AC02-81ER10861.
All Science Journal Classification (ASJC) codes
- Analytical Chemistry
- Chemical Engineering(all)