Electropolymerization of phenothiazine, phenoxazine and phenazine derivatives: Characterization of the polymers by UV-visible difference spectroelectrochemistry and Fourier transform IR spectroscopy

Abstract

The discrepancies observed between the redox potential values of poly(phenothiazines or phenoxazines) obtained from cyclic voltammetry and from spectroelectrochemical measurements (curves of Δ A vs. E ) suggests that the polymer films are not homogeneous, but include some amount of monomeric species which tend to detach from the electrode surface after electrochemical scanning. The polymerization of the phenazine neutral red, however, gave rise to a homogeneous film. On the contrary, the stability of the polymer film was found to be strongly dependent on the nature of the parent monomer. A model for the structure of poly(phenothiazines or phenoxazines) is proposed on the basis of their optical and vibrational spectra. In every case, the mechanism of electropolymerization seems to involve the formation of a cation-radical species after release of one hydrogen atom from the monomer at high positive potentials. When the monomer has a primary or secondary amino group as ring substituent, the polymer may be composed of phenothiazine units linked through one secondary or tertiary amine moiety. However, when the monomer has only tertiary amino groups as ring substituents (as methylene blue), the cation-radical species are only formed at extremely positive potentials, close to the potential of oxygen evolution, and it would be possible that at least one of the tertiary amino groups could be oxidized before the formation of the cation-radical yielding an oxime derivative of the monomer and formaldehyde as subproduct. However, the great similarity of the spectra of poly(thionine), poly(azur A) and poly(methylene blue) suggests that the kind of linkage between monomer units is the same in all these polymers.