Electrochemical studies of phenothiazine-iodine charge-transfer complexes

Abstract

The conductivity of the freshly prepared yellow adduct of phenothiazine and iodine in acetonitrile is due in about equal parts to 2:3 and 1:2 complexes at 20°C. In CH 3CN the complexes are completely dissociated. The equivalent conductivity remains constant, independent of concentration, at 130 mho. cm 2/eq which is of the same order as that of strong electrolytes. The mean mobility of the carriers is 6·2 × 10 −4 cm 2/V.s. Transport numbers were obtained from electrolysis in a Hittorf-type cell as t + = 0·17 and t ± = 0·83, yielding for the mobilities μ − = 1·0 × 10 −3 and μ + = 2·1 × 10 −4 cm 2/V.s. The anions, identified as I 3 −, are thus the majority carriers. It is estimated that the mean radius of the cations is about 2 to 4 times that of a hypothetical benzene ion in the same solvent. Further electrolysis experiments show that the complex formation-dissociation equilibria are reversible. Ageing and/or illumination, or addition of water, causes the colour of the adduct to change first to green, then to blue and eventually to violet. A new absorption initially at 650 nm and considered as a charge-transfer band shifts upon ageing to 625 nm for the green and to 550 nm for the violet adduct. The conductivity of the complex dissolved in acetonitrile rises with temperature at a rate considerably less than that of the pure solvent, the apparent activation energies being 0·003 and 0·06 eV respectively; this is considered as indicating the probability of recombination to rise with increasing temperature. The ground state of the complex in acetonitrile solution is shown to be predominantly ionic and its electronic structure to resemble that of the excited state of the isolated complex molecule.