Optically Biaxial, Re-entrant and Frustrated Mesophases in Chiral, Non-symmetric Liquid Crystal Dimers and Binary Mixtures.
Sixteen optically active, non-symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)-2-octyloxy or (S)-2-octyloxy chain attached to salicylaldimine core, was held constant, the number of methylene units in the spacer was varied from 3 to 10 affording eight pairs of (R & S) enantiomers. They were probed for their thermal properties with the aid of orthoscopy, conoscopy, differential scanning calorimetry and X-ray powder diffraction. In addition, the binary mixture study was carried out using chiral and achiral dimers with the intensions of stabilizing optically biaxial phase/s, re-entrant phases and important phase sequences. Notably, one of the chiral dimers as well as some mixtures exhibited a biaxial smectic A (SmAb) phase appearing between a uniaxial SmA and a re-entrant uniaxial SmA phases. The mesophases such as chiral nematic (N*) and frustrated phases viz., blue phases (BPs) and twist grain boundary (TGB) phases, were also found to occur in most of the dimers and mixtures. X-ray diffraction studies revealed that the dimers possessing oxybutoxy and oxypentoxy spacers show interdigitated (SmAd) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmAc) phase formed by a dimer having oxydecoxy spacer the periodicity was found to be approximately half the molecular length. The handedness of the helical structure of the N* phases formed by two enantiomers was examined with the aid of CD measurements; as expected, these enantiomers showed optical activities of equal magnitudes but with opposite signs. Overall, it appears that the chiral dimers and mixtures presented herein may serve as model systems in design and developing novel materials exhibiting the apolar SmAbphase possessing D2hsymmetry and nematic-type biaxiality.
Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India.|Department of Organic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India.
Vediappen
http://orcid.org/0000-0003-1979-4819
V
Padmini
Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India.
Palakurthy Nani
http://orcid.org/0000-0003-2315-5382
PN
Babu
Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India.
Geetha G
http://orcid.org/0000-0003-1006-3315
GG
Nair
Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India.
D S Shankar
http://orcid.org/0000-0002-9643-6604
DS
Rao
Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013, India. yelamaggad@cens.res.in, yelamaggad@gmail.com.
Channabasaveshwar V
http://orcid.org/0000-0003-3098-8358
CV
Yelamaggad
chiral oligomers; enantiomers; liquid crystals; phase transitions; structure-property relationship