UV-vis spectroscopic study of Co(II)/Co(III) oxidation in poly[M-protoporphyrins] films and their interaction with axial ligands.

A bathochromic shift for both Soret and Q bands in the polyCo(III)PP were indicative of Co(III) oxidation state in film. The presence of an isosbestic point indicates a chemical equilibrium between polyCo(III)PP (band I) in polyCo(III)PP with water as axial neutral ligand (band II). Concentration levels of iodide of 10(-1) M showed irreversible broadening of Soret band with a maximum shift from 400 nm to 380 nm attributed to film reduction. The thiocyanate anion shows a remarkable effect on polyCo(III)PP spectra. The degree of configuration interaction for Q and B transitions is nearly constant in air and water for Ni(II)PP, Cu(II)PP and Zn(II)PP films. The poly[Co(III)-protoporphyrin IX] showed strong deviation from the pattern. This result indicates that the Co atom does not present a planar conformation in polyCo(III)PP which is consistent with the less packed structure of this film. The apparent diffusion coefficients (D') were calculated for electroactive species using the polyNi(II)PP chemically modified electrode, with an experiment short enough to avoid preconcentration. D' was compared with D (diffusion coefficient), obtained with the bare working electrode. Apparent diffusion coefficients (D') changed regularly with molecular volume indicating certain molecular sieving effect.