Mechanism of cathodic reactions at electron-conducting polymer films: electroreduction of chloranil and tetracyanoquinodimethane at a poly-3-methylthiophene-coated glassy carbon electrode

The mechanism of the reversible one-electron reduction of tetracyanoquinodimethane (TCNQ), chloranil (CA) and o-dicarbollyl iron complex (o-dCFe) at a poly-3-methylthiophene-coated glassy carbon electrode in acetonitrile has been studied. Values of the formal potentials of these substances are shifted in the negative direction with respect to the polymer film by 0.54 V, 0.74 V and 1.10 V respectively, corresponding to a decrease in the mobile charge carrier concentration in the polymer film. Reduction currents for both TCNQ and CA are lower than the diffusion-limiting currents at a bare electrode. They are only slightly lower for TCNQ, but for CA they are considerably less than the diffusion currents (a "kinetic limiting current" regime). The behaviour of the polymer film was interpreted in the framework of a p-type semiconductor model including equilibrium and non-equilibrium potential distributions across the metal + semiconductor + solution system. Reduction of o-dCFe was not observed at all because of its highly negative formal potential; hence its electrode behaviour is similar to that of an insulator.