Effect of Chirality on the Structural Behaviour of Hydrogen‐Bonded n‐Alkylammonium Pyroglutamates in the Crystalline and Smectic State

Abstract

A set of optically active and racemic n-alkylammonium pyroglutamates from dodecyl to octadecyl were synthesized and characterised. Their thermotropic polymorphism was investigated by polarizing optical microscopy, differential scanning calorimetry and dilatometry. Their structure in the crystalline and smectic state was analysed by X-ray diffraction. The hydrogen bonding of the molecules in the crystalline and smectic layers was examined by infrared spectroscopy. The chirality control over the supramolecular self-assembly of the molecules along with the homochiral and heterochiral architecture of the self-assembled dimers are briefly discussed.

Chiral ionic liquid crystals with a bulky rigid core from renewable camphorsulfonic acid

“…This result indicates the layer distance at the liquid crystal phase increased approximately linearly with the increment of chain length. However, the layer distance of the salts at the crystal phase displayed a robust increase when the chain length increased from 14 to 16 carbons, which was contrary to the linear increment displayed by the salts with straight chains of 12 to 18 carbons 10 and a smaller ve-membered pyroglutamic acid rigid core 24 at crystal and liquid crystal phases. This result suggested that D-and L-C n ACS exhibited extended arrangements when the chain lengths were short but became bended when the alkyl chains were long.…”

“…This result implies the heating of optically active compounds at high temperatures induced racemization. 24 Therefore, special care must be taken to avoid unnecessarily prolonged heating at high temperatures for optical active compounds. 24 Table 2 gives the phase transition temperatures, transition enthalpies and mesophase ranges of D-and L-C n ACS determined from the rst heating.…”

“…24 Therefore, special care must be taken to avoid unnecessarily prolonged heating at high temperatures for optical active compounds. 24 Table 2 gives the phase transition temperatures, transition enthalpies and mesophase ranges of D-and L-C n ACS determined from the rst heating. The dodecylammonium salts of D-and L-camphorsulfonic acids had low transition temperatures at 41.6 and 41.8 C at the rst heating, and 34.3 and 34.0 C at the second heating, respectively.…”

“…The salts with shorter alkyl chain lengths exhibited lower transition temperatures from crystal to liquid crystal phase with wider mesophase ranges, which was contrary to that of the salts with a smaller ve-membered pyroglutamic acid rigid core. 24 It is also essential to note that there was a clear increase in crystal-liquid crystal phase transition temperature with increase in the alkyl chain length (n), indicating that more stable crystal structure occurred in the crystalline phase with increasing alkyl chain length. However, the melting temperature of the liquid crystal-isotropic phase transition only increased slightly with chain length, indicating the chain length had a minimum inuence on the clearing process.…”

“…21 As recently reported, bulky rigid moiety is an important factor to induce and stabilize the mesophase, and rigid cores are known to have the ability to enhance the thermal stability of organic molecules. 22 Various rigid core structures, such as benzenesulfonic acid, pyridine-3-sulfonic acid, naphthalene sulfonic acid, hydroxyl nathalene sulfonic acid 23 and pyroglutamic acid, 24 with long chain alkylamines can form thermotropic liquid crystals (LCs). However, the salts of naphthalene sulfonic acid and long chain alkylamine only exhibits a mesophase range of 30 C. 23 Long chain derivatives of guanine and cytosine or their equimolar mixtures are also liquid-crystaline.…”