Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

• Published in: Liquid crystals Vol. 47; no. 2; pp. 219 - 230
• Main Authors: Al-shargabi, Arwa, Yeap, Guan-Yeow, Mahmood, Wan Ahmad Kamil, Han, Chun-Chieh, Lin, Hong-Cheu, Ito, Masato M
• Format: Journal Article
• Language: English
• Published: Abingdon Taylor & Francis 26-01-2020; Taylor & Francis Ltd
• Subjects: Alkanes; Benzoates; Carbonyl compounds; Carbonyls; Chemical synthesis; Chiral; Cholesterol; click chemistry; Crystals; Dimers; Grain boundaries; Hybrid core; Inspection; Liquid crystals; Mesophase; Molecular structure; Molecular theory; TGB phase; Transition temperatures; Triazole; Triazoles; X-ray diffraction
• ISSN: 0267-8292; 1366-5855
• DOI: 10.1080/02678292.2019.1641637

New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (T
SmC*-SmA*
/T
SmA*-I
) is found to be 0.95, which indicate the second order transition according to the McMillan's molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.