Key Developments in Ionic Liquid Crystals

Abstract

Ionic liquid crystals are materials that combine the classes of liquid crystals and ionic liquids. The first one is based on the multi-billion-dollar flat panel display industry, whilst the latter quickly developed in the past decades into a family of highly-tunable non-volatile solvents. The combination yields materials with a unique set of properties, but also with many challenges ahead. In this review, we provide an overview of the key concepts in ionic liquid crystals, particularly from a molecular perspective. What are the important molecular parameters that determine the phase behavior? How should they be introduced into the molecules? Finally, which other tools does one have to realize specific properties in the material?

Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

“…The attachment of two long alkyl chains in the 4-and 5-positions of imidazolium or thiazolium rings is unusual because it requires a different synthetic approach than simply alkylating commercially available precursors, such as 1-methylimidazole, imidazole, or thiazole. While 1,3-dimethyl-4,5-bis(n-heptyl)imidazolium iodide (1-7) is non-mesomorphic, 1,3-dimethyl-4,5-bis(n-undecyl)imidazolium iodide (1-11), 1,3-dimethyl-4,5bis(n-pentadecyl)imidazolium iodide (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15) and the analogue with an additional methyl group in the 2-position (2) were found to be liquid-crystalline; they adopt SmA phases upon heating albeit over relatively narrow temperature ranges. For comparison, 3-methyl-4,5-bis(n-pentadecyl)thiazolium iodide (3) or 2-amino-4,5-bis(n-pentadecyl)imidazolium chloride (4) did not display liquid-crystalline properties under the conditions explored.…”

“…Examination by POM revealed that the sample became plastic after the first transition upon heating. 7 Peak temperature. 8 The transition to the LC mesophase upon heating was preceded by multiple transitions ( Figure S4).…”

“…Attachment of relatively long alkyl or fluoroalkyl chains to the nitrogen atoms of heterocyclic cations, such as imidazolium moieties, typically bestows thermotropic liquid-crystalline (LC) properties due to an enhancement in amphiphilicity [1][2][3][4]. Ionic liquid crystals (ILCs) are of high interest because they hold potential for use in a variety of applications [5][6][7][8][9]. For instance, it has already been demonstrated that ILCs may be used as non-volatile electrolytes in dye-sensitized solar cells [10][11][12], as organized reaction media [13], and as active components in electrochromic devices that do not require additional electrolytes [14][15][16].…”

“…Although imidazolium moieties are among the most widely used cationic cores to obtain ILCs, the majority of studies have focused on N-substituted derivatives due to the ease of their synthesis [5][6][7][28][29][30].…”

“…Collectively, the results that will be described below show how the disposition of long alkyl chains around five-membered, heteroaromatic, cationic cores affects the thermal characteristics of the resulting salts. Although imidazolium moieties are among the most widely used cationic cores to obtain ILCs, the majority of studies have focused on N-substituted derivatives due to the ease of their synthesis [5][6][7][28][29][30]. The influence of installing alkyl or aryl substituents in the 2-position of imidazolium-based ILCs on thermal phase characteristics has also been investigated [31][32][33][34][35][36][37][38][39][40].…”