Oxidation Potentials and Absorption Spectra of Phenothiazine Derivatives I. Benzophenothiazines and Triphenodithiazine

The homogeneous electrochemical oxidations of four phenothiazine derivatives, three isomers of benzophenothiazine and triphenodithiazine, were studied in acetonitrile solution together with phenothiazine itself. Their first and second redox potentials were determined by means of cyclic voltammetry. In all the materials examined, reversible one-electron oxidations were characterized. The electrogenerated cation radicals, except for triphenodithiazine, underwent a second one-electron oxidation and, after a subsequent chemical transformation, formed the corresponding benzophenothiazinium cations. For triphenodithiazine, only the first oxidation potential was observable under our experimental conditions. The electronic states of both the intermediates and the final products of electrolytic oxidation were identified by their electronic absorption spectra and electron spin resonance absorption spectra. The values of the first oxidation potential of the benzophenothiazines were proportional to the ionization energies, which were given by molecular orbital calculations.