Adsorption and reduction of iron and cobalt phthalocyanines in DMSO at glassy carbon and pyrolytic graphite electrodes

Abstract

Voltammetry of iron and cobalt phthalocyanines [Fe(II)Pc and Co(II)Pc] at carbon electrodes differs in some features because of more extensive aggregation and stronger adsorption of Co(II)Pc. Whereas Fe(II)Pc (2.3 m M) exists mainly as dimers and trimers in the 0.1 M LiCl + DMSO medium, visible spectrophotometry suggested that Co(II)Pc (3 m M) exists as higher oligomers. Electrochemical data gave an apparent diffusion coefficient for Co(II)Pc about 5-fold smaller than that of Fe(II)Pc, in agreement with larger aggregates for Co(II)Pc. The rate of monomerization of both complexes was too small to influence voltammetry. Electrochemistry of FePc is explained mainly by quasi-reversible electron transfer and weak adsorption of reactants. Adsorption is more apparent on PGE than on GCE. Strong adsorption of Co(II)Pc at GCE and PGE gives rise to a separate cathodic peak by cyclic and square-wave voltammetry. Adsorption equilibrium is achieved in < 5 min. Surface concentrations are consistent with an edge orientation of face-to-face macrocyclic rings separated by DMSO molecules as axial ligands. Efficient electron transfer was demonstrated between electrogenerated Co(I)Pc − and Fe(II)Pc.