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) x 10(-10) cm(2) s(-1) and the standard heterogeneous rate co nstant at the metal I film interface as (2.2 +/- 0.2) x 10(-5) cm s(-1). Th e width of the Warburg region measured by impedance spectroscopy showed a d istinct minimum at the formal potential, suggesting that the transference n umber of counterions was considerably larger than that of electrons. Thus c harge transport in the polymeric film is probably dominated by the rate of electron self-exchange between neighboring redox sites. Because of strong e lectrostatic binding, IrCl63- was highly concentrated in the polycalionic f ilm from its very dilute solution: the partition coefficient amounted to as much as similar to 10(4). The resulting shift in the half-wave potential o f viologen groups indicated a weak ion-pairing interaction between the dica tionic viologens and the anionic complex. The incorporated metal complex wa s released from the polymeric film in a controlled quantity in response to negative potential steps. (C) 2000 Elsevier Science S.A. All rights reserve d.