Irreversible transformation of polypyrrole-bound viologen with two-electron reduction in acidic aqueous solutions

Abstract This report discusses the electrochemical behavior of polymeric N , N ′-bis(3-pyrrol-1-ylpropyl)-4,4′-bipyridinium dichloride in acidic aqueous media and the chemical reactions of its doubly reduced form with protons and water molecules. The first redox reaction at the polymer-modified electrode was limited by charge diffusion in the film, whereas the second was limited by charge transfer at the electrode ∣ film interface. The electron diffusion coefficient increased more than 10-fold as the polymer was singly reduced, probably because the resulting film deswelling raised the redox site concentration. The second reduction process in acidic solutions involved an irreversible chemical reaction following charge transfer. At weakly acidic pHs, the chemical reaction occurred via slow hydration of the protonated quinoid form. The doubly reduced polymer raised the oxidation current of 2,5-dihydroxybenzenesulfonate electrostatically incorporated into the polymer. The catalytic effect implied that the product of two-electron reduction of the polymer was able to donate both electrons and protons to the oxidized electroactive anion. These results lead to the conclusion that the hydrogenated quinoid form reduces the oxidized electroactive anion to the original hydroquinone and reverts to the dicationic state.