Towards room temperature ionic liquid crystals: linear versus bent imidazolium phenylpyrimidines

Abstract

The synthesis and the mesomorphic properties of novel imidazolium salts with mesogenic 2-phenylpyrimidine or 2-alkylpyrimidinecarboxylic acid central cores are reported. The mesogenic units are connected to the imidazolium head groups via an alkoxy spacer. In order to adjust the mesomorphic properties the length of the alkoxy spacer and the terminal alkyl group, the counter ion, the substitution pattern of the imidazolium head group and the molecular geometry (linear vs. bent) are varied and the corresponding compounds were investigated in detail using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray scattering (WAXS, SAXS). Whereas SmA phases with monolayer orientation were observed for imidazolium salts with short N-substituents (R = CH₃, C₄H₉) at the imidazolium head group, the corresponding derivatives with longer N-substituents (R = C₁₂H₂₅, C₁₂H₂₅OC₆H₄) displayed SmA phases with bilayer orientation irrespective of a linear or bent geometry. For two derivatives, p-5(10,8) and p-5(12,8), a SmC phase was observed. Indeed, bending of the mesogenic core led to a shift of the mesophases towards lower temperatures. Several of the meta-2-phenylpyrimidine derivatives as well as 2-pyrimidine carboxylates displayed melting points below 50 °C. For 2-pyrimidine carboxylates replacement of a bromide anion by triflate resulted in a further decrease of the melting transition close to ambient temperature.