Electrostatic binding and release: incorporation of IrCl63− into polypyrrole containing viologen groups

The coupled diffusion coefficient of electrons and counterions in poly[N,N′-bis(3-pyrrol-1-ylpropyl)-4,4′-bipyridinium]chloride films was evaluated as (1.0±0.2)×10−10 cm2 s−1 and the standard heterogeneous rate constant at the metal ∣ film interface as (2.2±0.2)×10−5 cm s−1. The width of the Warburg region measured by impedance spectroscopy showed a distinct minimum at the formal potential, suggesting that the transference number of counterions was considerably larger than that of electrons. Thus charge transport in the polymeric film is probably dominated by the rate of electron self-exchange between neighboring redox sites. Because of strong electrostatic binding, IrCl63− was highly concentrated in the polycationic film from its very dilute solution: the partition coefficient amounted to as much as ∼104. The resulting shift in the half-wave potential of viologen groups indicated a weak ion-pairing interaction between the dicationic viologens and the anionic complex. The incorporated metal complex was released from the polymeric film in a controlled quantity in response to negative potential steps.