Impact of the Addition of Redox-Active Salts on the Charge Transport Ability of Radical Polymer Thin Films

Radical polymers (i.e., macromolecules composed of a nonconjugated polymer backbone and with stable radical sites present on the side chains of the repeat units) can transport charge in the solid state through oxidation–reduction (redox) reactions that occur between the electronically localized open-shell pendant groups. As such, pristine (i.e., not doped) thin films of these functional macromolecules have electrical conductivity values on the same order of magnitude as some common electronically active conjugated polymers. However, unlike the heavily evaluated regime of conjugated polymer semiconductors, the impact of molecular dopants on the optical, electrochemical, and solid-state electronic properties of radical polymers has not been established. Here, we combine a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), with a small molecule redox-active salt, 4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (TEMPOnium), in order to elucidate the effect of molecular doping on this emerging class of functional macromolecular thin films. Note that the TEMPOnium salt was specifically selected because the cation in the salt has a very similar molecular architecture to that of an oxidized repeat unit of the PTMA polymer. Importantly, we demonstrate that the addition of the TEMPOnium salt simultaneously alters the electrochemical environment of the thin film without quenching the number of open-shell sites present in the PTMA-based composite thin film. This environmental alteration changes the chemical signature of the PTMA thin films in a manner that modifies the electrical conductivity of the radical polymer-based composites. By decoupling the ionic and electronic contributions of the observed current passed through the PTMA-based thin films, we are able to establish how the presence of the redox-active TEMPOnium salts affects both the transient and steady-state transport abilities of doped radical polymer thin films. Additionally, at an optimal loading (i.e., doping density) of the redox-active salt, the electrical conductivity of PTMA increased by a factor of 5 relative to that of pristine PTMA. Therefore, these data establish an underlying mechanism of doping in electronically active radical polymers, and they provide a template by which to guide the design of next-generation radical polymer composites.