Ion and solvent transfer accompanying polybithiophene doping and undoping

The electrochemical quartz crystal microbalance (EQCM) has been used to monitor mass changes accompanying polybithiophene (PBT) doping and undoping in acetonitrile solutions containing tetraalkylammonium salts. The observed mass changes are close, but not identical to those anticipated if counter ion (anion) were the only transferred species. More detailed consideration shows that the data can be rationalised if there is also salt and solvent transfer. The motions of these two species are in opposite directions, resulting in nearly compensatory mass changes. Comparison of the mass and charge fluxes show that the two are correlated during PBT oxidation, but not during reduction. Reduction of doped PBT occurs in two stages. In the first stage, at more positive potentials, the mass change per unit charge is less than the average value for the overall redox process. In the second stage, at more negative potentials, it is larger. This could not be accommodated within a model involving a single mobile species. Interpretation of the overall mass change must be consistent with mass and charge fluxes observed throughout the redox switching process.