Layer Frustration, Polar Order and Chirality in Liquid Crystalline Phases of Silyl‐Terminated Achiral Bent‐Core Molecules

Keith, Christina; Reddy, R. Amaranatha; Prehm, Marko; Baumeister, Ute; Kresse, H.; Chao, Jessie Lorenzo; Hahn, Hans-Jürgen; Lang, Heinrich; Tschierske, Carsten

doi:10.1002/chem.200600876

Cited by 56 publications

(54 citation statements)

References 119 publications

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“…There are many studies which suggest that achiral bent-core molecules may exist in chiral conformational states and under suitable conditions an enantiomeric excess (defined by e=(n L ˗n R )/(n L +n R ) where n L and n R are the number densities of left and right handed domains respectively) may appear spontaneously. Thus the optical activity in the DC phase of achiral bent-core mesogens is attributed to the intrinsic layer chirality or to the coupling of molecular conformational chirality to the layer chirality [21][22][23][24][25][26][27].…”

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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“…Both computer simulations and experimental investigations have established that BCLCs exhibit chiral conformations [19][20][21]. Spontaneous chiral symmetry breaking in lamellar mesophases has been attributed to either the intrinsic layer chirality or to the coupling of molecular conformational chirality to the layer chirality [20,[23][24][25].…”

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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“…Considering the temperature-dependent relations of f L and f H , we referred the slow relaxation of f L to a molecular assembly of the phase transition and attributed the rapid relaxation of f H to the responses of molecular rotation around the long axis. 55 Comparably, the f L curve of S1 in Figure 9b strongly reflects the ferroelectric SmCG-type phase sequence, i.e., SmC̃o b G 2 P F SmC̃rG 2 P F SmCG 2 P F , but the f L curves of P1 and C4 in Figures 9c and 9d monotonically drop with decreasing temperature, featuring a single LC phase upon cooling. Monotonically decreasing f H curve upon cooling is matched with no antiferroelectric-ferroelectric switching in the three complexes (to be shown next).…”

Section: Chemistry Of Materialsmentioning

confidence: 92%

Shape and Confinement Effects of Various Terminal Siloxane Groups on Supramolecular Interactions of Hydrogen-Bonded Bent-Core Liquid Crystals

Chiang

Chuang

et al. 2015

Chem. Mater.

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To investigate the shape and confinement effects of the terminal siloxane groups on the self-assembled behavior of molecular arrangements in hydrogen-bonded (Hbonded) bent-core complexes, four H-bonded bent-core complexes S1, P1, C4, and P8 with string-, ring-, and cagelike siloxane termini (i.e., linear siloxane unit −Si−O−Si−O− Si−, cyclic siloxane unit (Si−O) 4 , and silsesquioxane unit POSS, respectively) were synthesized and investigated. By Xray diffraction measurements, different types of SmCG (B8) phases and leaning angles were controlled by the shape effect of the string-and cage-like siloxane termini for S1 and P1 (with only one arm of H-bonded bent-core), respectively. In addition, the confinement effect of P1, C4, and P8 (accompanied by increasing the numbers of attached H-bonded bent-core arms) resulted in higher transition temperatures and the diminishing of mesophasic ranges (even the disappearance of mesophase in P8). Moreover, AFM images showed the bilayer smectic CG phases of S1 and P1 were aligned to reveal highly ordered thread-like structures by a DC field. By spontaneous polarization measurements within the mesophasic ranges, S1 and P1 showed ferroelectric transitions but C4 displayed antiferroelectricity. Finally, the electro-optical performance of B8 phases could be optimized through binary mixtures of S1 and P1, and a well aligned modulated ribbon phase could be formed via specific molar ratios of the binary mixtures. ■ INTRODUCTIONBent-core or banana-shaped liquid crystals (LC) were first synthesized and investigated by Vorlander et al. in 1932. 1,2 Since the discovery of their special electro-optical properties by Niori et al., 3 many scientists became interested in the complicated mesophasic behavior, molecular biaxiality, layer polarity, and supramolecular chirality of achiral bent-core molecules. 4−7 These excellent electro-optical properties and structural characteristics might enable many applications, e.g., fast responsive LC displays, 8,9 light-scattering LC displays, 10 storage devices, 11 nonlinear optical appliances, 12 biosensors, 13 piezoelectric objects, 14 charge-generation interfaces, 15 bluephase materials, 16,17 supramolecular complexes, 18−20 biaxial mesogens, 21,22 and tunable chromophores. 23 On the basis of texture features and the results of X-ray diffractions, eight mesophases with smectic or columnar or three-dimensional orders, namely B1, B2, ..., B8 in chronological order, have been found in bent-core LCs with the effects of their configurations, including the zenithal constituents, the shapes of the rigid cores, the lengths of the flexible chains, and the roles of substituents at selected positions of rigid cores, on the properties of LCs. 6,7,24−27 As a result of the splay of polarization in smectic layers, 28 the modulation induces the breaking layers or ribbons to undergo long-range ordering in two-dimensional (2D) lattices of oblique (Col ob ), rectangular (Col r ), or hexagonal (Col h ) packing, classified as the B1 phase. 29 Depending upon the directio...

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Layer Frustration, Polar Order and Chirality in Liquid Crystalline Phases of Silyl‐Terminated Achiral Bent‐Core Molecules

Cited by 56 publications

References 119 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

Shape and Confinement Effects of Various Terminal Siloxane Groups on Supramolecular Interactions of Hydrogen-Bonded Bent-Core Liquid Crystals

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