Irreversible processes in the electrochemical reduction of polythiophenes. Chemical modifications of the polymer and charge-trapping phenomena

Irreversible changes occurring in the electrochemical reduction of some polythiophenes in acetonitrile have been investigated by cyclic voltammetry (CV), UV-Vis and IR spectroscopy and the electrochemical quartz crystal microbalance (EQCM). Reduction in the presence of alkali metal cations (Li+, Na+, K+ and Cs+) does not result in n-doping but in hydrogenation of the alkene moieties. The promotion of double-bond saturation by alkali metal cations following the order Li+, Na+ > K+ > Cs+ is attributed to their charge-pinning action which decreases as the ion radius is increased. Reversible reduction in tetralkylammonium electrolyte (n-doping) is accompanied by production of hydroxide ions within the polymer which are thereby inserted by subsequent oxidation. The resulting quinone-like groups are responsible for the CV pre-peaks usually observed in conducting polymers after n-doping and previously assigned to trapped charges.