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 tetracyanoqu inodimethane (TCNQ), chloranil (CA) and o-dicarbollyl iron complex (o- dCFe) at a poly-3-methylthiophene-coated glassy carbon electrode in ac etonitrile has been studied. Values of the formal potentials of these substances are shifted in the negative direction with respect to the p olymer film by 0.54 V, 0.74 V and 1.10 V respectively, corresponding t o a decrease in the mobile charge carrier concentration in the polymer film. Reduction currents for both TCNQ and CA are lower than the diff usion-limiting currents at a bare electrode. They are only slightly lo wer for TCNQ, but for CA they are considerably less than the diffusion currents (a ''kinetic limiting current'' regime). The behaviour of th e polymer film was interpreted in the framework of a p-type semiconduc tor model including equilibrium and non-equilibrium potential distribu tions 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 insu lator.