Self‐Assembly of Imidazolium‐Based Rodlike Ionic Liquid Crystals: Transition from Lamellar to Micellar Organization

Abstract

By using aryl-amination chemistry, a series of rodlike 1-phenyl-1H-imidazole-based liquid crystals (LCs) and related imidazolium-based ionic liquid crystals (ILCs) has been prepared. The number and length of the C-terminal chains (at the noncharged end of the rodlike core) and the length of the N-terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self-assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. For the single-chain imidazole derivatives nematic phases (N) and bilayer SmA2 phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C-terminal chains and in this series it leads to the phase sequence SmA-columnar (Col)-micellar cubic (CubI /Pm3n). Elongation of the N-terminal chain gives the reversed sequence. Short N-terminal chains prefer an end-to-end packing of the mesogens in which these chains are separated from the C-terminal chains. Elongation of the N-terminal chain leads to a mixing of N- and C-terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA2) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end-to-end packing leads to core-shell aggregates. In this case, elongation of the N-terminal chains distorts core-shell formation and removes CubI and Col phases in favor of single-layer SmA phases. Hence, by tailoring the length of the N-terminal chain, a crossover from taper-shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self-assembly in ILCs.

T‐Shaped Ionic Liquid Crystals Based on the Imidazolium Motif: Exploring Substitution of the C‐2 Imidazolium Carbon Atom

“…The only other examples of such behaviour for imidazolium ionic liquid crystals have only very recently been reported. [23,29] A transition from lamellar mesophases (with flat interfaces) to columnar mesophases (with curved interfaces), via an intermediate cubic structure, is typically found for tetracatenar liquid crystals. [46][47][48][49][50][51] This is explained by the increase in the aliphatic volume fraction with temperature and/or chain length.…”

“…www.chemeurj.org 6) Finally, Tschierske et al reported four rod-like imidazolium bromide compounds containing three tetradecyl or hexadecyl chains that exhibited micellar Cub Pm3 n phases. [29] The following sections describe the X-ray diffraction data that were obtained for our 2-arylsubstituted imidazolium salts. The crystalline nature of the solid phases at room temperature was confirmed by the presence of several sharp Bragg reflections at both small and wide angles.…”

“…[19][20][21][22][23][24][25] Kouwer and Swager recognised that the imidazolium fragment is a polarisable rigid group that is suitable as part of a mesogenic core rather than being placed at one end of a flexible alkyl chain. [26] However, the mesomorphism of ionic mesogens, wherein the imidazolium group is an integral part of the rigid core, remained virtually unexplored until very recently when reports appeared in the literature about 1-aryl-3-alkylimidazolium salts [26][27][28][29] and 1,3-diarylimidazolium salts. [30,31] However, no examples exist of 2-arylsubstituted imidazolium ionic liquid crystals (i.e., with a substituted aryl group directly connected to the carbon atom in the 2-position of the imidazolium ring).…”

Ionic Liquid Crystals Based on Pyridinium Salts

“…Moreover, the nematic phase is commonly used in electrooptical devices. The nematic phase is quite rare in the case of ILCs, several examples have been reported so far for ammonium [43,44], imidazolium [45][46][47][48], pyridinium-based ILC [49], or miscellaneous type of ILC [50][51][52][53][54]. Generally, as the smectic phases are the most common phases for ILCs, especially due to electrostatic interactions, the nematic phase can be seen rather as an exception.…”

Fluorocarbon and Hydrocarbon N‐Heterocyclic (C5–C7) Imidazole‐Based Liquid Crystals

“…[11] Imidazolium-based ILCs including symmetrical and unsymmetrical molecular frameworks have been generated and most of them show smectic phases. [12] Recently, mesomorphic imidazolium salts were also used as new vectors for small interfering RNA (siRNA) transfection. [13] Most of the organic compounds known for optoelectronic applications still have some major limitations such as low electron conductivity or luminescence efficiency.…”