On the Origin of the “Giant” Electroclinic Effect in a “De Vries”‐Type Ferroelectric Liquid Crystal Material for Chirality Sensing Applications

W415 is a chiral smectic compound with a remarkably weak temperature dependence of its giant electroclinic effect in the liquid crystalline smectic A* phase. Furthermore it possesses a high spontaneous polarization in the smectic C* phase. The origin of this striking electroclinic effect is the co‐occurrence of a de Vries‐type ordering with a weak first‐order tilting transition (see the synchroton X‐ray scattering profiles).

     

Intrinsically Chiral Twist-Bend Nematogens: Interplay of Molecular and Structural Chirality in the NTB Phase

The front cover artwork is provided by Dr Rebecca Walker of the Liquid Crystals Group at the University of Aberdeen. The image is a cartoon depiction of the formation of the heliconical chiral twist-bend nematic phase (N*_TB) from its constituent bent molecules. The presence of a single enantiomer of the chiral, lactate-based liquid crystal dimers biases the formation of helices with only one handedness, unlike in the conventional N_TB phase, observed for achiral molecules, for which the left- and right-handed helices are doubly degenerate. Read the full text of the Research Article at 10.1002/cphc.202200807 .     

Current topics in smectic liquid crystal research.

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).     

Thermal‐ and Photo‐Induced Phase‐Transition Behaviors of a Tapered Dendritic Liquid Crystal with Photochromic Azobenzene Mesogens and a Bicyclic Chiral Center

A ribbon‐shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ₃DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure‐sensitive diffraction techniques. Based on the experimental results, it was found that the AZ₃DLC molecule exhibited the low‐ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high‐temperature phase region. AZ₃DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiral‐inducing agent, it exhibited a good solubility, a high helical‐twisting power, and a large change in the helical‐twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly “remote‐control” the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.     

Fluctuation Transmission Electron Microscopy: Detecting Nanoscale Order in Disordered Structures

The structures of many disordered materials are not ideally random, but contain structural order on the scale of 1–3 nm. However, such nanoscale order, called medium‐range order, cannot be detected by conventional diffraction methods in most cases. Fluctuation transmission electron microscopy (FTEM) has the capability to detect medium‐range order in disordered materials based on statistical analysis of nanodiffraction patterns or dark‐field images from TEM. FTEM has been successful in demonstrating the theoretically predicted development of nanoscale nuclei in amorphous chalcogenides, as well as in revealing the subtle effect of different preparation routes on the medium‐range order in amorphous semiconductors and metals. The fluctuation principle can also be applied to study structural order on longer length scales in polymers and other disordered materials using X‐rays or visible light. Further advances in theory and practice of FTEM will greatly increase our understanding of amorphous structures and nucleation phenomena.     

Possible Influence of Layer Deformation and Chiral Segregation on Dielectric Modes in the Dark Conglomerate Liquid Crystal

Dielectric spectroscopy is used to investigate the structure, molecular dynamics, and relaxation phenomena in electric‐field‐induced switchable dark conglomerate (DC) phases in a bent‐core liquid crystal. The DC phases are obtained by applying a high‐frequency ac electric field in the B_1rev phase or by cooling under a dc or an ac field from the isotropic phase. Although the DC phases exhibit good electro‐optic switching properties, the dielectric parameters are different from those observed in typical lamellar SmCP phases and similar to those obtained in a non‐switchable DC phase. We therefore propose that the dielectric response and reduced intensity of the relaxation modes may be a general feature in DC phases and may owe its origin to the deformed layer structure in which certain molecular motions are impeded. Further, we find that in the field‐induced DC phases derived from the isotropic phase, the dielectric modes are affected by chiral segregation promoted by the applied field.     

The Solution of the Puzzle of Smectic‐Q: The Phase Structure and the Origin of Spontaneous Chirality

The so‐called smectic‐Q (SmQ) liquid crystal phase was discovered in 1983 in rod‐like molecules, but its structure remain unclear in spite of numerous attempts to solve it. Herein, we report what we believe to be the solution: A unique bicontinuous phase that is non‐cubic and is made up of orthogonal twisted columns with planar 4‐way junctions. While SmQ had only been observed in chiral compounds, we show that this chiral phase forms also in achiral materials through spontaneous symmetry breaking. The results strongly support the idea of a helical substructure of bicontinuous phases and long‐range homochirality being sustained by helicity‐matching at network junctions. The model also explains the triangular shape of double‐gyroid domains growing within a SmQ environment. SmQ‐forming materials hold potential for applications such as circularly polarized light emitters that require no alignment or asymmetric synthesis.     

A tensor model of liquid crystalline polymers: application to basic disclination processes

One of the features of liquid crystalline polymers (LCPs) is their strong elastic anisotropy, which means they have unequal elastic constants. Elastic anisotropy plays a crucial role in the microstructure and macroscopic properties of LCPs. In this paper, the effect of unequal elastic constants on microstructure is investigated without an external field by using a deterministic tensorial approach. In this model, the evolution of the director field can be viewed as a process driven towards the state of zero elastic torque. A tensor expression of the elastic torque is used so that the nematic symmetry is automatically conserved. In simulations of bulk samples, disclination lines of strength half and escaped integer disclinations are observed. The distortion fields around the disclinations are found to depend on elastic anisotropy. If the twist constant is the lowest, as is the case for main chain liquid crystalline polymers, the disclination lines are predominantly of the twist type.     

Enantiotropic Nematics From Cross‐Like 1,2,4,5‐Tetrakis(4′‐alkyl‐4‐ethynylbiphenyl)benzenes and Their Biaxiality Studies

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross‐shaped compound 3 , which showed enantiomeric nematic phase behavior. This cross‐like core structure could alternatively be viewed as two fused V‐shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface‐induced biaxiality was evidenced by optical studies. Cluster‐size analysis within the nematic phase of compound 3 revealed the formation of meta‐cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small‐angle 2D X‐ray diffraction patterns of magnetic‐field‐aligned samples indicated that the nematic phase was composed of small smectic C‐like clusters with the tilting of molecules within the clusters. The wide‐temperature‐range enantiomeric nematic phase of cross‐like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent‐rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher‐order liquid‐crystalline phases and crystallization, for future biaxial investigations.