Ionic liquid crystals from β-diketonyl containing pyridinium cations and tetrachlorozincate anions

In this work, an approach towards ionic liquid crystals is strategically designed. A pyridine group has been attached to an 1,3-diketone containing an alkyloxyphenyl R substituent (R = C6H4OCnH2n+1; n = 10–18) and protonated with hydrochloric acid towards the formation of the chloride salts of 2-[3-(4-n-alkyloxyphenyl)propane-1,3-dion-1-yl]pyridinium cations [OOR(n)py]Cl (I), which have been proved to be non-mesomorphic. Reaction of these organic–inorganic compounds (I) with ZnCl2 yields to the ionic liquid crystals of the type bis{2-[3-(4-n-alkyloxyphenyl)propane-1,3-dion-1-yl]pyridinium}tetrachlorozincate [OOR(n)py]2[ZnCl4] (II).The crystalline structure of compounds [OOR(12)py]Cl and [OOR(10)py]2[ZnCl4] as representative examples of both kinds of salts have been solved and discussed. Both compounds exhibit layered structures containing cationic and anionic sublayers. In addition for the tetrachlorozincate salt [OOR(16)py]2[ZnCl4] structural relationships between the mesophases and the crystalline phase are proposed on the basis of the variable-temperature small-angle X-ray diffraction studies.

Graphical abstractBis{2-[3-(4-n-alkyloxyphenyl)propane-1,3-dion-1-yl]pyridinium}tetrachlorozincates [OOR(n)py]2[ZnCl4] have been synthesized and proved as ionic liquid crystals exhibiting thermotropic polymorphism and smectic enantiotropic behaviour. Layered structures, containing cationic and anionic sublayers, of the solid phase, a ordered smectic phase and a smectic A phase are established on the basis of the X-ray studies (crystal structure and variable temperature small-angles X-ray diffraction) to explain the polymorphism of those compounds.Figure optionsDownload full-size imageDownload as PowerPoint slide