Enantioseparation of liquid crystals and their utilization as enantiodiscrimination materials

Synthesis and Properties of Highly Tilted Antiferroelectric Liquid Crystalline (R) Enantiomers

This work reports the synthesis method and various properties of four rod-like antiferroelectric (R) laterally substituted enantiomers, with or without fluorine atoms used as substituents in the benzene ring. The influence of fluorine substitution on the mesophase temperature range was determined. The synthesized compounds are three-ring rod-like smectics with a chiral center based on (R)-(-)-2-octanol. Their chemical and optical purity was checked using high-performance liquid chromatography (HPLC). Two newly synthesized enantiomers and three previously reported (R) enantiomers were used to formulate two antiferroelectric mixtures. The mesomorphic behavior was characterized by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD). The helical pitch and tilt angle measurements were done using the selective light reflection phenomenon and the electro-optical method, respectively. All the enantiomers exhibit a wide temperature range of the antiferroelectric phase, with a high tilt angle. Furthermore, the enantiomer with lateral fluorine substitution in the ortho position has a very long helical pitch (more than 2.0 µm), relatively low enthalpy of melting point, and a tilt angle close to 45 degrees. The designed (R) enantiomers can be useful for formulating eutectic mixtures for further use in various devices, including photonics and optoelectronics.

Advantages of polar organic solvent chromatography for enantioseparation of chiral liquid crystals

Three sets of fluorinated chiral liquid crystals were used to explore the polar organic solvent chromatography mode for their enantioseparation. The materials include a set of newly synthesized compounds with chiral center derived from 2-hexanol and two sets of compounds with chiral center derived from 2-/3-octanol. Baseline enantioseparation of all materials was achieved using binary mobile phases without additives. For some of the compounds exceedingly high values of enantioresolution (> 20) and enantioselectivity (> 4) were found. The chromatographic behavior of the sample set was studied on three different polysaccharide-based chiral columns - Chiralpak IA-U, IG-U and IB-U. Comparison of results from Chiralpak IA-U and IB-U shows the effect of amylose vs. cellulose polysaccharide backbone while comparison of Chiralpak IA-U and IG-U reveals the effect of 3,5-dimethylphenylcarbamate vs. 3‑chloro-5-methylphenylcarbamate substituent. The mobile phases tested included whole range of acetonitrile/methanol mixtures to demonstrate that acetonitrile-rich and alcohol-rich mobile phases offer different enantiorecognition mechanisms and can provide complementarity to some extent. The effect of temperature on enantioseparation was investigated on Chiralpak IA-U by constructing van't Hoff plots for selected liquid crystals in pure acetonitrile and pure methanol as mobile phases.