Diketonylpyridinium Cations as a Support of New Ionic Liquid Crystals and Ion-Conductive Materials: Analysis of Counter-Ion Effects

Abstract

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 [HOO^R(n)pyH]⁺ and BF₄^-, ReO₄^-, NO₃^-, CF₃SO₃^-, CuCl₄^2- 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 [HOO^R(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 CuCl₄^2- 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 CuCl₄^2- 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.

Scheme 1

Synthetic route to obtain the ionic salts. The numbering scheme used in the ¹H-NMR assignments is also denoted.

Figure 1

Crystal structure of [HOO^R(12)pyH][ReO₄] (5): Layer-type distribution showing interdigitated chains. The inset shows the ORTEP plot with 40% probability.

Figure 2

POM photomicrographs of: (a) BF₄-12 at 114 °C on cooling; (b) ReO₄-12 at 104 °C on cooling; (c) CF₃SO₃-14 at 148 °C on cooling; (d) NO₃-12 at 112 °C on cooling; (e) CuCl₄-12 at 158 °C on heating; (f) CuCl₄-18 at 111 °C on cooling.

Figure 3

Phase transition temperatures of the different salts as dependent on the anion and the cation length.

Figure 4

X-ray diffractograms of ReO₄-16 (7) at (a) 25 °C; (b) 120 °C and (c) 140 °C on heating. The insets show a magnification of the XRD trace at the mesophase temperature (a) showing the (002) peak and (b) the broad halo corresponding to the liquid-like order of the molten alkyl chains.

Figure 5

Schematic representation of the structural features: (a) [HOO^R(16)pyH][ReO₄] at the solid state based on the structure of ReO₄-12; (b) proposal for [HOO^R(16)pyH][ReO₄] at the SmA mesophase; and (c) proposal for [HOO^R(12)pyH]₂[CuCl₄] at the SmC mesophase.

Figure 6

(a–c) Z″ vs. Z′ and (d) C′ vs. f plots for the Cl⁻, CuCl₄²⁻ and ReO₄⁻ salts; (e) σ′ vs. f plots for the favorable ReO₄⁻ salt.

Figure 7

(a–c) C′ vs. T and (d–f) σ′ vs. f plots for the Cl⁻, CuCl₄²⁻ and ReO₄⁻- based compounds.