Effects of molecular chirality on superstructural chirality in liquid crystalline dark conglomerate phases

Ocak, Hale; Bilgin‐Eran, Belkız; Prehm, Marko; Tschierske, Carsten

doi:10.1039/c2sm25763g

Cited by 50 publications

(39 citation statements)

References 72 publications

(35 reference statements)

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“…During this process a simultaneous reversal in the polarization and the tilt direction takes place giving rise to a switching from SmC s P A to SmC a P S and SmC a P A to SmC s P S . Bent-core mesogens are well known for the optically isotropic phases they exhibit, which include the B4 phase [7][8][9], the dark conglomerate (DC) [10][11][12][13][14][15][16][17][18][19] and the dark enantiomeric (DE) phases [20]. The DC phase, which is the topic of interest of this paper, is usually found to occur below the isotropic phase.…”

Section: Introductionmentioning

confidence: 99%

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

et al. 2014

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Unusual behaviour of the dark conglomerate (DC) phase seen in an oxadiazole-based achiral bent-core liquid crystal, which has not previously been reported for the DC phase of other liquid crystals, is described. Under polarizing optical microscopy, we see no domains of opposite handedness in the ground state of the DC phase. However, it shows unusual transformations when an electric field is applied to the system: On increasing the electric field, at first the domains of opposite handedness become visible and then they grow in size and slowly the sample transforms to a monochiral or single handed form which is followed by a nonchiral state at very high fields. The threshold electric fields required to achieve these changes are temperature dependent and the transformations are seen irrespective of the frequency of the applied electric field (100 Hz to 5 kHz), type of the waveform (sine, square and triangular) and the thickness (1.5 µm to 15 µm) or the geometry (planar and twisted) of the device used. Further, there is no field-induced high birefringence texture observed even though sufficiently large electric field (~22 V/µm) has been applied across the devices. The nature of the behaviour is investigated by various techniques such as optical microscopy, conoscopy, circular dichroic and Raman spectroscopies, electro-optics and dielectric spectroscopy. The possible physical phenomena behind these changes are discussed in detail.

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Section: Introductionmentioning

confidence: 99%

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

et al. 2014

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“…One such set of phases exhibited by bent-core liquid crystals (BCLCs) are optically isotropic, including the recently discovered B4 and the dark conglomerate (DC) phases. The discovery of these phases and the subsequent understanding of their internal structures are two of the major developments in the field of BCLCs [4][5][6][7][8][9][10][11][12][13][14][15][16]. A characteristic common to the DC and the B4 phases is spontaneous breaking of chiral symmetry that occurs even though the constituent molecules are achiral.…”

Section: Introductionmentioning

confidence: 99%

Understanding the unusual reorganization of the nanostructure of a dark conglomerate phase

et al. 2015

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The dark conglomerate phase exhibited by a bent-core liquid crystal material shows remarkable properties including an electric-field tuneable chiral domain structure and a large (0.045) reduction of refractive index, while maintaining an optically dark texture when observed under crossed polarisers. A detailed investigation of the system is presented, leading to a model that is fully consistent with the experimental observations. It reports the observation of two distinct regimes in the DC phase: a higher temperature regime in which the periodicity measured by small angle X-ray scattering decreases slightly (0.5%) and a lower temperature regime where it increases considerably (16%). Also, the paper discusses the unusual electric field-induced transformations observed in both the regimes. These changes have threshold fields that are both temperature and frequency dependent, though the phenomena are observed irrespective of device thickness, geometry and the alignment layer. The electro-optic behaviour in the DC phase correspond to a number of structural changes leading to unusual changes in physical properties including small (1%) increase in periodicity and a doubling of the average dielectric permittivity. We propose a model of the DC phase where in the ground state the nanostructure of phase exhibits an anticlinic antiferroelectric organization. Under an electric field, it undergoes a molecular rearrangement without any gross structural changes leading to an anticlinic ferroelectric order while keeping the overall sponge-like structure of the DC phase intact.2

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“…Induction of globally homochiral, bent-core LC domains has been demonstrated by several methods, including controlled growth in a twisted director field 48 , using biomolecular adsorbates 49 and by mixing chiral dopants into the fluid B2 phase 50 . Homochiral samples can also be achieved by using structurally chiral bent-core mesogens 51,52 . To this end, we investigated in our experiments the properties of left-and right-HNF domains in binary mixtures of the prototypical achiral, HNF-forming bentcore mesogen NOBOW 53 mixed with a variety of chiral dopants.…”

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confidence: 99%

Diastereomeric liquid crystal domains at the mesoscale

et al. 2015

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In many technologies used to achieve separation of enantiomers, chiral selectors are designed to display differential affinity for the two enantiomers of a chiral compound. Such complexes are diastereomeric, differing in structure and free energy for the two enantiomers and enabling chiral discrimination. Here we present evidence for strong diastereomeric interaction effects at the mesoscale, manifested in chiral liquid crystal guest materials confined in a chiral, nanoporous network of semi-crystalline helical nanofilaments. The nanoporous host is itself an assembly of achiral, bent-core liquid crystal molecules that phase-separate into a conglomerate of 100 micron-scale, helical nanofilament domains that differ in structure only in the handedness of their homogeneous chirality. With the inclusion of a homochiral guest liquid crystal, these enantiomeric domains become diastereomeric, exhibiting unexpected and markedly different mesoscale structures and orientation transitions producing optical effects in which chirality has a dominant role.

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Effects of molecular chirality on superstructural chirality in liquid crystalline dark conglomerate phases

Cited by 50 publications

References 72 publications

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

Understanding the unusual reorganization of the nanostructure of a dark conglomerate phase

Diastereomeric liquid crystal domains at the mesoscale

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