Electrochemical reduction of substituted cobalt phthalocyanines adsorbed on graphite

Unsubstituted phthalocyaninatocobaIt(II) [PcCo] and the cobalt complexes of the substituted ligands octabutoxyphthalocyanine [Pc(OBu)(8)Co], octacyanophthalocyanine [Pc(CN)(8)Co], phthalocyaninetetracarboxylic acid [Pc(COOH)(4)Co] and tetrapyridotetraazaporphyrin [TPyTAPCo] were absorbed on the basal plane of pyrolytic graphite (BPG) from solutions in organic solvents to form ultrathin films. The films were studied by cyclic voltammetry in the presence of an inert aqueous electrolyte to yield the redox characteristics of the films. The films undergo reduction and reoxidation at the ligand inner si system as well as at the metal centre with a linear dependence of peak currents on scan rate that is typical for surface-confined species. The kinetics of the two reactions are further discussed on the basis of the analysis of peak shapes and charge found in the different waves in the cyclic voltammograms. The redox potentials of both reactions an shifted according to the electron donating or withdrawing character of the ligand's periphery as seen by comparison to molecular orbital (MO) calculations and ultraviolet photoelectron spectroscopy (UPS) results. During the reduction step, protons from the electrolyte are intel acting with the films and are desorbed during reoxidation, leading to a strong pH dependence of the redox potential. The acid-base properties of the ligands determine a threshold pH above which the potential of the metal-centred first reduction is no longer dependent on pH. The ligand-centred second reduction, however, led to interaction with protons over the whole accessible pH range. Both electrode reactions are discussed in view of chemical interactions of the protons with the various ligand systems. (C) 1998 Elsevier Science S.A.