Conducting polymers containing in-chain metal centers: Homogeneous charge transport through a quaterthienyl-bridged Os(tpy)(2) polymer

Electron transport in electrochemically polymerized films of a bithienyl-substituted Os(tpy)(2) complex has been studied. The voltammetric response associated with the Os2+/3+ process is unusually ideal under both semi-infinite linear and finite, diffusion conditions. Significantly, ac impedance results indicate that the rate-limiting step for charge transport is electron hopping at overpotentials more negative than -50 mV or more positive than +100 mV. In contrast, at smaller overpotentials the rates of electron hopping and counterion diffusion are similar. The electron hopping diffusion coefficient determined using dual electrode voltammetry is (2.6 +/- 0.5) x 10(-1) cm(2) s(-1). Significantly, this rate of charge transport is more than 2 orders of magnitude larger than those found for comparable metallopolymers in which the osmium centers are linked by nonconjugated bridges. This result is discussed in terms of the effect of bridge conjugation and the relatively close proximity in energy of the redox center and bridge states.