Conducting polymer 'nanogates' – Controllable diffusivities in thin films of novel tether-containing sulfonated polythiophenes

We describe a nanoscale gating effect by an atypical tether-containing polythiophene, [poly(thiophene-3-carboxylic acid 2-(2-(2-(2-ethoxy-ethoxymethyl sulfonate)-ethoxy)-ethoxy)-amide], referred to as poly(TP-OEG-SO4). Cyclic voltammetry data permitted us to calculate liquid-phase ion diffusivities in the polymer as a function of its oxidation state. Diffusivities are more than 350 times higher in the oxidized state versus the reduced state. As a control, poly 3′,4′-dimethyl-[2,2′;5′,2″] terthiophene (DMPT) was synthesized and characterized. On comparing both polymers in the reduced state, we find diffusivities in poly(TP-OEG-SO4) to be more than 4500 times lower than in poly(DMPT). To explain this behavior, we propose a model that features a charge-balancing mechanism by the sulfonate tethers in poly(TP-OEG-SO4), which causes nanoporous regions around the polymer main chains to be opened and closed, leading to the large observed differences in diffusivities. These data suggest that the polymer poly(TP-OEG-SO4) is evidently able to act as a reversible “nanogate” with an open pore structure when oxidized, and a closed one when reduced. Keywords: Nanogate, Self-doping, Tether, Oligoethylene glycol, Polythiophene, Diffusion