Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

Abstract

Synthetic strategies were developed to prepare l-tyrosine-based ionic liquid crystals with structural variations at the carboxylic and phenolic OH groups as well as the amino functionality. Salt metathesis additionally led to counterion variation. The liquid-crystalline properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (WAXS, SAXS). The symmetrical ILC chlorides bearing the same alkyl chain at both the ester and ether but either an acyclic or cyclic guanidinium group displayed enantiotropic SmA mesophases with phase widths of 31-88 K irrespective of the head group. It was particularly the replacement of chloride in the acyclic guanidinium ILC by hexafluorophosphate that induced a phase change from SmA to Col . This phase change was attributed to a higher curvature of the interface due to the larger anion, which increased the effective head group cross-sectional area of the amphiphilic ILC. The unsymmetrical acyclic guanidinium chlorides, bearing a constant C ester and variable alkyl chains on the phenolic position, formed enantiotropic SmA phases. The derivative with the largest difference in chain lengths, however, displayed a Col phase, resulting from discoid aggregates of the cone-shaped guanidinium chloride. The results are discussed in terms of the packing parameters, which indicate that the phase behaviour of the thermotropic tyrosine-based ILCs shows analogies to those of lyotropic liquid crystals.

Dicyanamide Salts that Adopt Smectic, Columnar, or Bicontinuous Cubic Liquid‐Crystalline Mesophases

“…[44] Although 1,3-bis(ndodecyl)-1,2,3-triazolium dicyanamide shows as mecticp hase between 82 and 102 8C, [7] and the 1,3-bis(n-dodecyl)imidazolium salts with I À ,[ BF 4 ] À ,o r[ ClO 4 ] À counterionse xhibit SmA phases, [92,93] 1,3-bis(n-dodecyl)imidazolium dicyanamide does not exhibit LC properties. [1] We report here that salt 8,w ith a slightly longera lkyl chain than [C 14 (Table 3), indicatest hat not only the size of the counterion [94] but also its shape should be considered in the designo fa nI LC. [1] We report here that salt 8,w ith a slightly longera lkyl chain than [C 14 (Table 3), indicatest hat not only the size of the counterion [94] but also its shape should be considered in the designo fa nI LC.…”

“…We report here that salt 8 , with a slightly longer alkyl chain than [C 14 mim][N(CN) 2 ], shows a SmA phase. The fact that mesomorphism was observed for a chain length of 14 carbon atoms in the series [C n mim][PF 6 ] and that a longer alkyl chain is required in the series [C n mim][N(CN) 2 ], despite the smaller volume of the [N(CN) 2 ] − anion (Table ), indicates that not only the size of the counterion but also its shape should be considered in the design of an ILC.…”

Insight into Out‐of‐Layer Fluctuations in the Smectic A Stability of 3,5‐Diarylisoxazole Liquid Crystals

“…The consequence of the decrease of the level of less linear and polar populations is the longitudinal diffusion of the bromine atom in the opposite direction relative to the last carbon atom of the neighboring alkyl chain. Therefore, interdigitation appears naturally as a consequence of the non‐directionality of the polarization that the bromine atom exerts on the neighboring carbon atoms in the W‐shape of alkyl chain and the predominance of gauche conformation to avoid the void volume . On the other hand, the extended alkyl chain acts as sponge between the polar ends of the spacer group by attenuating the electrostatic repulsions between the bromine atoms at the interface.…”

Current Topics in Ionic Liquid Crystals

“…Since the amphiphilic molecular structure of many rod‐shaped ILC mesogens is similar to ionic surfactant molecules, the occurrence of columnar and smectic ILC phases might be understood and predicted by the same packing parameter concept as in micellar lyotropics and indeed the relevance of this concept was proven at least once [30] . Here, a switching from smectic to columnar hexagonal ILC phases was observed in case of larger counter ions, which is explained in terms of packing parameters: Increasing anion radii increase the effective head group area, decrease the packing parameter and thus lead to a transition from planar bilayers via postulated undulated smectic bilayers to the curved columnar aggregates of the hexagonal phase.…”

“…Grey spheres: guanidinium cation; orange spheres: anion; grey ovoids: phenyl group. The figure was taken from ref [30] . with permission from © 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.…”