A new thermotropic reentrant behaviour in a chiral liquid crystal dimer: the occurrence of SmA–SmAb–SmA phase sequence

Yelamaggad, C. V.; Tamilenthi, V. Padmini; Rao, D. S. Shankar; Nair, Geetha G.; Prasad, S. Krishna

doi:10.1039/b906315c

“…The simplest oligomers, termed liquid crystal dimers, contain just two liquid crystal units interconnected by a single spacer and these have been by far the most widely studied oligomers [2,3] although higher oligomers such as trimers and tetramers have been reported (for recent examples, see [5][6][7][8][9]). The intense interest in liquid crystal dimers has arisen because they exhibit very different properties to conventional low molar mass liquid crystals and recent investigations have included the observation of nematic-nematic transitions [10][11][12][13]; siloxanebased dimers [14]; studies of their light emitting [15] and flexoelectric properties [16,17]; the effects of spacer length and linking groups [18,19]; bent corecalamitic dimers [20,21]; anthracene-based [22] and discotic dimers [23]; H-shaped [24,25] and cyclic dimers [26]; intercalated [27,28] and biaxial smectic phases [29]. and compared their properties to those of the corresponding dimers containing a conventional alkyl spacer [31,32]:…”

Section: Introductionmentioning

confidence: 99%

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Lee¹,

Lü²,

Henderson³

et al. 2012

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Seven members of the homologous series of the liquid crystal dimers, the bis(ω-(cholesteryloxycarbonyl)alkyl) disulfides, which contain a sulfur−sulfur link in the flexible spacer have been synthesised and their liquid crystal properties characterised. The dimers are referred to using the acronym Chol-n-SS-n-Chol in which n denotes the number of carbon atoms linking the cholesteryl-based groups and the sulfur atoms, and was varied between 3, 5, 6, 8, 10, 11 and 12. All seven homologues exhibit a chiral nematic phase and for the longest three members a smectic A phase was also observed. An odd−even effect is apparent in both the transition temperatures and the values of the entropy change associated with the chiral nematic−isotropic transition, S N * I /R, in which dimers with even values of n show the higher values. This is interpreted in terms of the average molecular shapes in which the C−S−S−C dihedral angle is around 90 • . The values of S N * I /R shown by these dimers are very small for liquid crystal dimers and this is attributed to the increased molecular biaxiality arising from the C−S−S−C dihedral angle. The smectic A phase exhibited by the homologues with n = 10, 11 and 12 is proposed to consist of an intercalated arrangement of the dimers which is driven by the mismatch in cross-sectional areas of the cholesteryl-based groups and alkyl chains.

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“…The ( S )‐enantiomer I( S )‐7 with an oxyheptoxy (7) spacer exhibits, analogous to the ( R )‐enantiomer I( R )‐7 , a very rich phase sequence. It shows BP, N*, TGB, SmA, SmA b and SmA phases with the uniaxial smectic A phase showing re‐occurrence.…”

Section: Resultsmentioning

confidence: 99%

“… Molecular structures of fifteen optically active, nonsymmetric dimers synthesized and characterized (a) and the known dimers, I( R )‐7 and I‐5 used for binary mixture studies (b).…”

Section: Introductionmentioning

confidence: 99%

Optically Biaxial, Re‐entrant and Frustrated Mesophases in Chiral, Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Padmini

¹

,

Babu

²

,

Nair

³

et al. 2016

Chemistry An Asian Journal

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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 (SmA ) 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 (SmA ) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmA ) 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 SmA phase possessing D symmetry and nematic-type biaxiality.

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“…Numerous good examples of this kind of system exist, ranging from (S)-2-methylbutyl derived Schiff's base materials (130)(131)(132)(133)(134) [179,180], 1-methylheptyl derived biphenyl ester systems (135)(136)(137) [181,182], 1-methylheptyl derived salicylaldimines (138) [183] to cyanostilbene-derived 2-methylbutyloxy systems (139) [184]. These are shown below along with typical properties of their chiral nematic phases.…”

Section: Type Iib Materials Using Achiral Flexible Spacers and Chiralmentioning

confidence: 99%

Design and Synthesis of Chiral Nematic Liquid Crystals

Taugerbeck

¹

,

Booth

²

2014

Handbook of Liquid Crystals

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The article contains sections titled: Introduction to the Chiral Nematic Phase and Its Properties Formulation and Application of Chiral Nematic Liquid Crystal Mixtures Applications of Chiral Nematic Phases Classification of Chiral Nematic Liquid‐Crystalline Compounds Aspects of Molecular Symmetry for Chiral Nematic Phases Type I Chiral Nematic Liquid Crystals Azobenzenes and Related Mesogens Azomethine ( S chiff's Base) Mesogens Stable Phenyl, Biphenyl, Terphenyl, and Phenylethylbiphenyl Mesogens Ester Mesogens Derivatives of Chiral Alcohols Type II Chiral Nematic Liquid Crystals Type IIa Twin Systems Using Chiral Flexible Spacers Type IIb Materials Using Achiral Flexible Spacers and Chiral Terminal Groups Type IIc and d Materials Using One or Two Chiral Cores Type III Chiral Nematic Liquid Crystals Cholesteryl Esters Chiral Mesogens Derived from Cyclohexane Chiral Heterocyclic Mesogens Axially Chiral Liquid Crystals Axially Chiral Alkenes and Overcrowded Alkenes Allenes Tricyclo[4.4.0.0 3,8 ]decane or Twistane Derived Mesogens Sterically Hindered Biphenyl Derivatives Spirobiindane Derivatives Concluding Remarks

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A new thermotropic reentrant behaviour in a chiral liquid crystal dimer: the occurrence of SmA–SmAb–SmA phase sequence

Cited by 28 publications

References 21 publications

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Cholesteryl-based liquid crystal dimers containing a sulfur–sulfur link in the flexible spacer

Optically Biaxial, Re‐entrant and Frustrated Mesophases in Chiral, Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Design and Synthesis of Chiral Nematic Liquid Crystals

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