Counterion Effects on the Mesomorphic Properties of Chiral Imidazolium and Pyridinium Ionic Liquids

Abstract

Novel calamitic N‐methylimidazolium and pyridinium salts with modified citronellyl side‐chains and various counterions 3–14 have been synthesised by utilising the commercially available (R)‐citronellol (15) as the starting material. Differential scanning calorimetry (DSC), polarising optical microscopy (POM) and temperature‐dependent X‐ray diffraction studies revealed smectic A (SmA) mesophases for all the series. 1H NMR studies of the N‐methylimidazolium salts have shown that there is a correlation between the mesophase dependence on the counterion and the counterion‐dependent chemical shifts of the acidic 2‐H proton.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Effect of counterion on the mesomorphic behavior and optical properties of columnar pyridinium ionic liquid crystals derived from 4-hydroxypyridine

“…In comparison with the bromide salt 3 (d = 9.19 ppm), upfield shifts were observed for all the other counterions (up to 8.48 ppm for 6, X À ¼ PF À 6 ), and thus, the chemical shifts for pyridinium salts 3-6 were found to decrease in the following order: Br À > NO À 3 > BF À 4 > PF À 6 . Such a trend follows the one that was observed for other different series of simple pyridinium salts or imidazolium salts [3].…”

“…Clearly, the formation of LC phases is driven by the segregation between the ionic pyridinium moieties and hydrophobic aliphatic chains and the counterion has an important influence on mesophase stabilization, similar to other reported pyridinium-based ILC [3,7,13]. The TG curves were recorded in the 25-550°C temperature range.…”

“…Several reviews covering this topic were published in the recent years [1,2]. Amongst them, pyridinium-based ILC, whose mesomorphism has been known since long time, show similar properties with the related imidazolium based ILC [3].…”

Ionic Liquid Crystals Based on Pyridinium Salts

“…Recently, a series of N-alkylated pyridinium salts containing a chiral center in the four positions with respect to nitrogen atoms have been prepared and investigated by Laschat et al [20,21]. Their thermal properties were compared to their imidazolium counterparts.…”

Ionic Liquid Crystals Derived from Amino Acids

“…The insertion of a stereogenic centre leads to chiral ILCs, which are interesting compounds due to their promising applications for asymmetric synthesis, stereoselective polymerisation, chiral chromatography, transport membranes for CO 2 and NMR chiral discrimination 2. Recently, we synthesised chiral imidazolium and pyridinium ILCs starting from ( R )‐citronellol as a natural precursor 3. Due to the price of the starting material and difficulties with the upscaling, we wanted to use cheaper chiral building blocks, which could be efficiently converted into chiral ILCs.…”