Anisotropic Mixed Conduction of Electrons and Ions in Liquid Crystalline Supramolecular Complexes of Polythiophene and Imidazolium-Based Ionic Liquids

Mixed ionic-electronic conducting polymers (MIECPs) show promise for next-generation electrochemical devices due to the unique ability to simultaneously conduct both ions and electrons. However, there is a trade-off between electronic and ionic conduction because of their opposite morphology dependence. Here, we report simultaneous high electron-conduction and high ion-conduction in thin films of supramolecular MIECPs through the liquid crystalline (LC) assembly pathway from solution to the solid state. The supramolecular MIECPs are prepared via non-covalent bonding between carboxylated poly(3-alkyl thiophene)s and an imidazolium-based ionic liquid (IL) surfactant and characterized by means of UV-visible (UV-vis) absorption spectroscopy, polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS). After complexation, the system displays colorimetric transitions and optoelectronic changes with the IL surfactant mole ratio. At equal stoichiometry, the polymer displays a rodlike conformation with planarization of the conjugated backbone. The hydrogel of the equimolar solution exhibits a typical LC polydomain texture with strong birefringence under POM, which is identified as a smectic LC mesophase with lamellar periodicity using SAXS. Defect-free LC monodomains containing unidirectional alignment are obtained in the hydrogel through mechanical shearing. After complete solvent evaporation, the LC monodomain structures are retained in the solid-state film, resulting in simultaneously high electronic (10^-2- 10^-1 mS/cm) and ionic (10^-3 - 10^-2 mS/cm) conductivities at ambient temperature. Generally, the block copolymers of MIECPs show high ionic conductivities (10^-2 - 1 mS/cm) but low electronic conductivities (10^-5 - 10^-2 mS/cm). While the homopolymers of MIECPs display high electronic conductivity (10^-6 - 10² mS/cm) but low ionic conductivity (10^-6 - 10^-2 mS/cm). In addition, aligned solid-state films show a significant anisotropy in both electronic (anisotropic ratio of ∼6) and ionic (anisotropic ratio of ∼117) conductivities, with faster charge transport along the shear direction than the perpendicular direction. It is believed that aligned conjugated backbones along the shear direction provide channels for fast band-like transport of electronic charge carriers and aligned imidazolium moieties in the lamellar layers form constrained channels for ion motion.