Chiral Incommensurate Helical Phase in a Smectic of Achiral Bent-Core Mesogens

Green, Adam A. S.; Tuchband, Michael R.; Shao, Renfan; Shen, Yongqiang; Visvanathan, Rayshan; Duncan, A. E.; Lehmann, Anne; Tschierske, Carsten; Carlson, Eric D.; Guzman, Edward; Kolber, Maria; Walba, David M.; Park, Cheol S.; Glaser, Matthew A.; Maclennan, Joseph E.; Clark, Noel A.

doi:10.1103/physrevlett.122.107801

Cited by 21 publications

(29 citation statements)

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“…Helix formation was further confirmed for compound 1/16 by the observation of the so‐called deformed helical ferroelectric (DHF) effect in planar cells, indicating a helix deformation under an electric field similar to that previously known for helical SmC* and SmC α * phases of permanently chiral molecules . In addition, the helical superstructure with a pitch of only 14 nm was visualized by AFM of freeze fractured samples and finally unambiguously proven by RSoXS (see Figure a), confirming a pitch of 15 nm for compound 1/14 . The pitch of almost three times the layer distance is not exactly commensurate with a three‐layer helix (see Figures , 13b, c and Table ).…”

Section: Resultssupporting

confidence: 74%

“…XRD of aligned samples also provides no clear indication of tilt (see Figures S4, S15 and S28). Only in few cases of compounds 1/12–1/16 , electro‐optical investigations uncover the actually tilted organization with an optical tilt (tilt of the aromatic cores) of 15–20° . For compounds 1/18–1/22 with longer chains the d / L mol ratio becomes smaller (0.80, Figure b), in line with an enhanced tendency to form synclinic tilted SmC s phases (Table ), though the tilt angle itself appears to be not increased.…”

Section: Resultsmentioning

confidence: 89%

“…[c] This phase was designated as SmC a P F in ref. , the reasons for using SmC a P A are given in the section describing this helix‐free low temperature SmC a P A phase. [d] This phase was designated as SmAP A in ref.…”

Section: Resultsmentioning

confidence: 99%

“…Associated with this phase transition a speckled texture develops (Figure b→e) which is unique for the anticlinic SmC a phases (SmC a P A ) of the compounds under investigation. It is assumed to be due to two different modes of planar alignments of the bent molecules, either with the bow plane parallel (low Δ n , green) or perpendicular to the substrate surfaces (high Δ n , yellow), see Figure f, views B and A, respectively …”

Section: Resultsmentioning

confidence: 99%

“…a) Development of the XRD data (SAXS and RSoXS) in the LC phases of 1/14 depending on temperature; b) shows a computer generated model of the helical structure with twist of 110 and 140° bent‐core angle and c) shows a simplified models; the helical pitch P in terms of a number of smectic layers is incommensurate and smaller than 3 times the thickness of a layer; a, b) were reprinted with permission from refs. and , respectively, copyright (2019) by the American Physical Society (https://doi.org/10.1103/PhysRevLett.122.107801; https://doi.org/10.1103/PhysRevMaterials.3.045603).…”

Section: Resultsmentioning

confidence: 99%

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Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

Lehmann

Alaasar

Poppe

et al. 2020

Chemistry A European J

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A series of bent‐shaped 4‐cyanoresorcinol bisterephthalates is reported. Some of these achiral compounds spontaneously form a short‐pitch heliconical lamellar liquid‐crystalline phase with incommensurate 3‐layer pitch and the helix axis parallel to the layer normal. It is observed at the paraelectric‐(anti)ferroelectric transition, if it coincides with the transition from random to uniform tilt and with the transition from anticlinic to synclinic tilt correlation of the molecules in the layers of the developing tilted smectic phase. For compounds with long chains the heliconical phase is only field‐induced, but once formed it is stable in a distinct temperature range, even after switching off the field. The presence of the helix changes the phase properties and the switching mechanism from the naturally preferred rotation around the molecular long axis, which reverses the chirality, to a precession on a cone, which retains the chirality. These observations are explained by diastereomeric relations between two coexisting modes of superstructural chirality. One is the layer chirality, resulting from the combination of tilt and polar order, and the other one is the helical twist evolving between the layers. At lower temperature the helical structure is replaced by a non‐tilted and ferreoelectric switching lamellar phase, providing an alternative non‐chiral way for the transition from anticlinic to synclinic tilt.

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Section: Resultssupporting

confidence: 74%

Section: Resultsmentioning

confidence: 89%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

Lehmann

Alaasar

Poppe

et al. 2020

Chemistry A European J

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Chirality of Liquid Crystals Formed from Achiral Molecules Revealed by Resonant X‐Ray Scattering

et al. 2020

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effect is also responsible for some "biological recognition" processes, including, for example, our physiological response to odor or taste of opposite enantiomeric species. Although chiral interactions have been broadly studied, the mechanism that leads to single handedness in our world is still under discussion. For example, Viedma [1] showed that deracemization in the solid state, coupled with flexibility of transforming one enantiomeric form into another in the liquid state, may lead to chirality synchronization, that is, in a system that starts from a mixture of racemic crystals, a homochiral end state can be obtained. Similarly, crystallization combined with reversible organic reactions, have been reported to transform initially achiral reactants into an enantiopure solid. [2] Whether the spontaneous formation of a homochiral state from a racemic mixture or achiral molecules can be observed in a fluid phase, is still uncertain. In general, in racemic systems lacking long range positional correlations, the chirodiastaltic forces are probably too weak to achieve spontaneous deracemization and (through amplification) enantiomeric excess. Also, in the liquid crystal (LC) state with only orientational order (nematic phase, N), spontaneous deracemization has not been observed so far, since rod-like molecules forming the N phase have freedom to rotate around their long axes, and thus the chiral discrimination effect is reduced. [3] Only few examples have been reported in which chiral discrimination was found to strongly affect nematic phase stability, but not lead to deracemization. [4] There is, however, a particularly interesting group of achiral mesogenic molecules with (constitutional or conformational) bent geometry, for which rotation is strongly hindered and therefore local interactions between neighboring molecules are expected to be much stronger than those in rod-like molecules. In fact, bent molecules have been shown to spontaneously form structurally chiral domains through chirality synchronization in the isotropic liquid [5] and LC [6,7] phases. Thus, further exploration of chirality in soft systems, such as LCs, may have high relevance toward better understanding the origin of life and be transformative for various technologies relying on chirality in soft matter systems. Testing chirality in soft matter systems (liquid crystals and solid crystals) is not always trivial. The most commonly used tools are optical methods focusing on measurements of optical activity (i.e., the ability of matter to rotate the polarization

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Nematic Phase of Polar Unsymmetrical Bent‐Core Molecules

et al. 2023

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In liquid crystalline systems, the presence of polar groups at lateral or terminal positions is fundamentally and technologically important. Bent-core nematics composed of polar molecules with short rigid cores usually exhibit highly disordered mesomorphism with some ordered clusters that favourably nucleate within. Herein, we have systematically designed and synthesized two new series of highly polar bent-core compounds comprised of two unsymmetrical wings, highly electronegative À CN and À NO 2 groups at one end, and flexible alkyl chains at the other end. All the compounds showed a wide range of nematic phases composed of cybotactic clusters of smectic-type (N cyb ). The birefringent microscopic textures of the nematic phase were accompanied by dark regions. Further, the cybotactic clustering in the nematic phase was characterized via temperature-dependent XRD studies and dielectric spectroscopy. Besides, the birefringence measurements demonstrated the ordering of the molecules in the cybotactic clusters upon lowering the temperature. DFT calculations illustrated the favourable antiparallel arrangement of these polar bent-core molecules as it minimizes the large net dipole moment of the system.

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Chiral Incommensurate Helical Phase in a Smectic of Achiral Bent-Core Mesogens

Cited by 21 publications

References 35 publications

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

Chirality of Liquid Crystals Formed from Achiral Molecules Revealed by Resonant X‐Ray Scattering

Nematic Phase of Polar Unsymmetrical Bent‐Core Molecules

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