Synthesis and mesophase behaviour of ionic liquid crystals

Abstract

N-Alkyl-pyridinium derivatives 1–3 and N-alkyl-stilbazolium halides 4–6 with hydroxy, methoxy and hydrogen 4′-substituents and long non-branched alkyl chains (n = 14, 16, 18, 20, 22) were synthesized and characterized by polarizing microscopy, differential scanning calorimetry and X-ray measurements. The compounds exhibit liquid crystalline SmA phases when heated above their melting point. Phenyl substitution in the 3- and 4-position of the pyridinium ring causes a large tendency to decrease the clearing temperature in comparison to the 4-methyl substituted N-alkyl-pyridinium salts. By elongation of the pyridinium ring to the 4′-substituted stilbazolium unit the clearing points of the new compounds 4–6 increase drastically up to temperatures ≥200 °C in which partial decomposition of the compounds sets in. The length of the alkyl chains and the type of counter ions have large influences on the stability of the mesophase. Elongation of the alkyl chain length n increases the temperature range of the liquid crystalline phase. The counter ions increase the stability of the SmA phase in order Cl⁻ > Br⁻ > I⁻ > BPh₄⁻. When CH₃–C₆H₅SO₃⁻ is introduced as an anion no liquid crystalline phase can be observed. UV/Vis measurements indicate the presence of a charge-transfer complex between the pyridinium cation and the iodide anion. Differences in the liquid crystalline behaviour of N-alkyl-4′-substituted stilbazolium halides 4–6 compared with N-alkyl-3- and -4-substituted-pyridinium derivatives 1–3 may be explained by additional intramolecular charge-transfer and resulting strong dipole–dipole interactions between stilbazolium compounds.