Ionic liquid crystals (ILCs) allow the combination of the high ionic conductivity of ionic liquids (ILs) with the supramolecular organization of liquid crystals (LCs). ILCs salts were obtained by the assembly of long-chained diketonylpyridinium cations of the type [HOOR(n)pyH]⁺ and BF₄-, ReO₄-, NO₃-, CF₃SO₃-, CuCl₄2- counter-ions. We have studied the thermal behavior of five series of compounds by differential scanning calorimetry (DSC) and hot stage polarized light optical microscopy (POM). All materials show thermotropic mesomorphism as well as crystalline polymorphism. X-ray diffraction of the [HOOR(12)pyH][ReO₄] crystal reveals a layered structure with alternating polar and apolar sublayers. The mesophases also exhibit a lamellar arrangement detected by variable temperature powder X-ray diffraction. The CuCl₄2- salts exhibit the best LC properties followed by the ReO₄- ones due to low melting temperature and wide range of existence. The conductivity was probed for the mesophases in one species each from the ReO₄-, and CuCl₄2- families, and for the solid phase in one of the non-mesomorphic Cl- salts. The highest ionic conductivity was found for the smectic mesophase of the ReO₄- containing salt, whereas the solid phases of all salts were dominated by electronic contributions. The ionic conductivity may be favored by the mesophase lamellar structure.
Keywords: ionic conductivity; ionic liquid crystals; ionic salts; liquid crystals; polymorphism; smectic mesophase.