SPECTROELECTROCHEMICAL STUDIES OF TETRASULFONATED METALLOPHTHALOCYANINES ADSORBED ON THE BASAL-PLANE OF GRAPHITE IN THE PRESENCE OF CYSTEINE

Cobalt and iron tetrasulfonated phthalocyanines (M-TSPc) have been stu died by electroreflectance spectro!;copy in the adsorbed state on the basal plane of HOPG in a berate buffer (pH 9). Since both complexes ex hibit electrocatalytic activity for the electro-oxidation of cysteine, we looked at the changes of the visible spectra of the adsorbed compl exes upon adding millimolar amounts of cysteine to the electrolyte. Un der zero-current conditions (open circuit) the addition of cysteine ca uses large changes in the reflectance spectra of both Co-TSPc and Fe-T SPc. In the case of Co-TSPc the spectra resemble that elf Co(I)TSPc wh ereas for Fe-TSPc bands at 424 and 626 nm are observed. The open circu it potential shifts to values close to the standard potential of the M (II)/M(I) couple. We attribute these changes, particularly in the case of Fe-TSPc, to the formation of a radical adduct between the adsorbed phthalocyanine and the cysteine molecule, with partial electron-trans fer from the cysteine to the metal center. Evidences for these adducts is not found when polarizing the electrode, which shows that they beh ave as very unstable intermediates in the catalytic process. (C) 1998 Elsevier Science S.A. All rights reserved.