The Electrochemistry Group Medal Lecture. Electron self-exchange dynamics between redox sites in polymers

Abstract

Using microelectrochemical techniques, our laboratory has explored self-exchange-based electron transport in a variety of mixed-valent polymeric media. The transport rate is measured as the electron diffusion coefficient, De, or the self-exchange rate constant kex. The basic variables for electron transport in mixed-valent polymer materials include: (a) the physical mobility of the counterions of the polymer that migrate due to electroneutrality requirements, (b) the physical diffusion coefficient, Dphys, of the monomeric or polymeric oxidized and reduced molecular sites or ions relative to the rate of electron hopping or tunnelling between donor/acceptor pairs, (c) the observational timescale relative to these mobilities which provides the distinction between transient and steady-state experiments, and (d) the chemical environment of the polymer, whether dry and solvent-free or contacted by solvent vapour or liquid. Experimental strategies and results are presented for the measurement of rates of ion diffusion, Di, in N2-dry and solvent-wetted mixed valent polymers. In a dry, mixed-valent osmium complex polymer, the electron-transport rate measured under steady-state conditions, where no ion transport occurs concurrently, is much faster than the diffusion rate of the ion as estimated in a transient electrolysis experiment. In a solvent-wetted osmium complex polymer, the electron-transport rate measured under transient conditions is much slower than that of the ion which was measured under steady-state conditions. These circumstances allow isolation of individual processes and are interpreted as giving electron-transport rates not strongly influenced by macroscopic ion-transport rates. Cyclic voltammetry of [Co(bpy)3]2+ and of Li+TCNQ– in dry poly(ethylene oxide) polymer electrolyte solvents exhibits differing measured diffusion coefficients, Dapp, for the oxidation vs. the reduction of each compound, reflecting the coupling of physical diffusion and electron self-exchange transport. Microdisc electrode voltammetry of solutions of a synthesized ferrocene mono-tagged poly(ethylene oxide) in a polymer solvent of comparable molecular weight gives Dapp values smaller than those for ferrocene monomer dissolved in the same polymer solvent. The Dapp in the former case measures the self-diffusion rate of a linear chain polymer within a linear chain polymer solvent. Measurability of this rate has implications for assumptions about diffusive mobility of redox molecules attached to polymer chains.