Investigations of the electrocatalytic activity of mononuclear and binuclear cobalt phthalocyanines for the reduction of molecularoxygen established that the latter compounds are more effective catalysts than the corresponding mononuclear species.A correlation between the half-bandwidths for the Q band of these species and their catalytic efficiency, expressed as a kinetically-limited current, suggests that the increase in activity may be attributed to electronic coupling between the phthalocyanine rings.Mrcnner R. Her*apstpnp, A. B. P. Leven et Cllppotto C. LtzNopr. Can. J. Chem. 65,2671Chem. 65, (1987 (22)(23)(24)(25)(26)(27)(28) and Chang and co-workers (29)(30)(31)(32)(33)(34)(35) has revealed that certain binuclear porphyrins are much more effective. Similarly, mononuclear phthalocyanines of iron and cobalt are effective (36-50), but some ill-characterised "dimeric" and polymeric phthalocyanines appear more effective (51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61)(62). Savy and co-workers (54-56) performed ESCA, esr, Mdssbauer and optical spectroscopic sfudies to establish correlations between the spectroscopic properties of polymeric and monomeric iron phthalocyanines and the eiectrocatalytic activity of these species towards oxygen reduction. We consider here a correlation between the optical spectra of a series of well-characterized mono-and binuciear cobalt phthalocyanines and their electrocatalytic activity. for about an hour before scanning the working electrode at a rate of 100 mV/s over the range 0.00 to *1.00 V versus the silver wire.Adsorbed layers on OPG or SAPG electrodes were obtained by cycling until the anodic and cathodic peak currents associated with the adsorbed species remained constant over 5 min ofcontinuous scanning. When this condition was satisfied, the adsorbed layer was assumed to have reached a steady coverage. Scans at various rates were then recorded to determine the surface coverage (vide infra).The catalytic efficiency of the various phthalocyanine compounds was investigated with the adsorbed species on the OPG electrode. This modified electrode was removed from the phthalocyanine solution, washed with ethanol, then with water, and finally placed in the cell for the aqueous studies. The 0.l0 M NaOH solution in this cell had been purged with oxygen for several hours to ensure saturation. The electrode was then scanned over the range 0.00 to -1.00 V versus sce at a rate of l0 mV/s for rotation rates ranging from 400 to l0 0O0 rpm.After each scan. the electrode was held at 0.00 V for I min to oermit the rotation rate to be changed for the next scan. The theory of intramolecular coupling in these binuclear phthalocyanines has been presented elsewhere (66). In solution, these binuclear species are expected to exist in a dynamic equilibrium between various conformations, dependent upon the nature of the bridge since, in some cases, steric effects may provide barriers to free rotation. Therefore, the electronic absorption spectrum of a solution of one of these binuclear species wo...
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