A Systematic Study on the Mesomorphic Behavior of Asymmetrical 1-Alkyl-3-dodecylimidazolium Bromides

Abstract

To determine the essential parameters for mesophase formation in imidazolium-based ionic liquids (ILs), a library of 1-alkyl-3-dodecylimidazolium bromides was synthesized, abbreviated as CnC12, where 0 ≤ n ≤ 13, as the general notion is that a dodecyl side chain would guarantee the formation of an ionic liquid crystal (ILC). All salts were fully characterized by NMR spectroscopy and mass spectrometry. Their thermal properties were recorded, and mesophase formation was assessed. An odd-even effect is observed for 5 ≤ n ≤ 10 in the temperatures of melting transitions. While the majority of this series, as expected, formed mesophases, surprisingly compounds C2C12 and C6C12 could not be classified as ILCs, the latter being a room temperature IL, while C2C12 is a crystalline solid with melting point at 37 °C. The single crystal structure of compound 1-ethyl-3-dodecylimidazolium bromide (C2C12) was successfully obtained. Remarkably, the arrangement of imidazolium cores in the structure is very complicated due to multiple nonclassical hydrogen bonds between bromide anions and imidazolium head groups. In this arrangement, neighboring imidazolium rings are forced by hydrogen bonds to form a "face-to-face" conformation. This seems to be responsible for the elimination of a mesophase. To conclude, the general view of a dodecyl chain being a functional group to generate a mesophase is not entirely valid. Disciplines Ceramic Materials | Other Chemical Engineering | Other Materials Science and Engineering CommentsReprinted with permission from Cryst. Growth Des., 2014, 14 (4) ABSTRACT: To determine the essential parameters for mesophase formation in imidazolium-based ionic liquids (ILs), a library of 1-alkyl-3-dodecylimidazolium bromides was synthesized, abbreviated as CnC12, where 0 ≤ n ≤ 13, as the general notion is that a dodecyl side chain would guarantee the formation of an ionic liquid crystal (ILC). All salts were fully characterized by NMR spectroscopy and mass spectrometry. Their thermal properties were recorded, and mesophase formation was assessed. An odd−even effect is observed for 5 ≤ n ≤ 10 in the temperatures of melting transitions. While the majority of this series, as expected, formed mesophases, surprisingly compounds C2C12 and C6C12 could not be classified as ILCs, the latter being a room temperature IL, while C2C12 is a crystalline solid with melting point at 37°C. The single crystal structure of compound 1-ethyl-3-dodecylimidazolium bromide (C2C12) was successfully obtained. Remarkably, the arrangement of imidazolium cores in the structure is very complicated due to multiple nonclassical hydrogen bonds between bromide anions and imidazolium head groups. In this arrangement, neighboring imidazolium rings are forced by hydrogen bonds to form a "face-to-face" conformation. This seems to be responsible for the elimination of a mesophase. To conclude, the general view of a dodecyl chain being a functional group to generate a mesophase is not entirely valid.

Influence of the Counteranion on the Ability of 1-Dodecyl-3-methyltriazolium Ionic Liquids to Form Mesophases

“…Nevertheless, it appears that mesophase formation is favored for triazolium salts compared to imidazolium salts, an observation which we already have made for symmetric 1,3-dodecyl salts. 19 ■ EXPERIMENTAL SECTION Synthesis. Dodecylazide was synthesized according to a common literature method.…”

NMR Spectroscopic Studies of Cation Dynamics in Symmetrically-Substituted Imidazolium-Based Ionic Liquid Crystals

“…The imidazolium derivatives are the most popular cations for IL synthesis [ 9 ]. While the majority of studies have explored single-chain asymmetrically substituted imidazolium cores, there have been a number of recent reports on the investigation of imidazolium-based ILs with two symmetrically attached long alkyl chains [ 3 , 10 , 11 , 12 , 13 , 14 , 15 ]. Relocation of the polar and apolar domains provides a route to tune the physicochemical properties of ionic liquids while the enhanced rod-shape character of a symmetrically-substituted cation offers additional opportunities for self-assembly [ 13 , 16 ].…”

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

“…The observation of SmA mesophases for imidazolium salts 1-n (n = 11, 15) and 2 was not unexpected, since structurally related 1,3-bis(n-alkyl)imidazolium salts with long alkyl chains are known to adopt thermotropic SmA phases [54][55][56][57][58][59]. However, inspection of the literature data, some of which are collected in Table 3, shows that the clearing points of the 4,5-disubstituted ILCs are lower than those of 1,3-analogues.…”

“…From the library of compounds, we selected six 4,5-bis(n-alkyl)azolium salts (1-n (n = 7, 11, 15) and 2-4, Scheme 1) and investigated their thermal characteristics. The data were compared with those reported for analogous 1,3-bis(n-alkyl)imidazolium and 2-[3,4-bis(n-alkyloxy)phenyl]-1,3-dimethylimidazolium ILCs (for example, 5-n and 6-n in Scheme 1) [38,[54][55][56][57][58][59]. 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.…”