Spontaneous helix formation in non-chiral bent-core liquid crystals with fast linear electro-optic effect

Spontaneous symmetry breaking in polar fluids

Spontaneous symmetry breaking and emergent polar order are each of fundamental importance to a range of scientific disciplines, as well as generating rich phase behaviour in liquid crystals (LCs). Here, we show the union of these phenomena to lead to two previously undiscovered polar liquid states of matter. Both phases have a lamellar structure with an inherent polar ordering of their constituent molecules. The first of these phases is characterised by polar order and a local tilted structure; the tilt direction processes about a helix orthogonal to the layer normal, the period of which is such that we observe selective reflection of light. The second new phase type is anti-ferroelectric, with the constituent molecules aligning orthogonally to the layer normal. This has led us to term the phases the Sm C P H and SmAAF phases, respectively. Further to this, we obtain room temperature ferroelectric nematic (NF) and Sm C P H phases via binary mixture formulation of the novel materials described here with a standard NF compound (DIO), with the resultant materials having melting points (and/or glass transitions) which are significantly below ambient temperature. The new soft matter phase types discovered herein can be considered as electrical analogues of topological structures of magnetic spins in hard matter.

Liquid Crystal Dimers and the Twist-Bend Phases: Non-Symmetric Dimers Consisting of Mesogenic Units of Differing Lengths

The syntheses and characterisation of the 4-[{[4-({n-[4-(4-cyanophenyl)phenyl]-n-yl}oxy)phenyl]-methylidene}amino]phenyl-4-alkoxybenzoates (CBnOIBeOm) are reported with n=8 and 10 and m=1-10. The two series display fascinating liquid crystal polymorphism. All twenty reported homologues display an enantiotropic nematic (N) phase at high temperature. When the length of the spacer (n) is greater than that of the terminal chain (m), the twist-bend nematic (NTB) phase is observed at temperatures below the N phase. As the length of the terminal chain is increased and extends beyond the length of the spacer up to three smectic phases are observed on cooling the N phase. One of these smectic phases has been assigned as the rare twist-bend smectic C subphase, the SmCTB-α phase. In all the smectic phases, a monolayer packing arrangement is seen, and this is attributed to the anti-parallel associations of the like mesogenic units.

Glassy relaxation in a de Vries smectic liquid crystal consisting of bent-core molecules

We report experimental investigations of a liquid crystal comprising thiophene-based achiral bent-core banana shaped molecules. The compound exhibits the following phase sequence on cooling: Isotropic (517.4 K), N (514.9 K), de Vries SmA (402 K), SmC. Practically no layer contraction was observed across the SmA to SmC transition, confirming the "de Vries" nature of the SmA phase. Interestingly, the crystallization does not occur on cooling the sample, unlike most other liquid crystals. Instead, the SmC phase undergoes a glass transition at 271 K even at a slow cooling rate. The dielectric spectroscopy studies carried out on the sample reveal the presence of a dielectric mode whose relaxation process is of the Cole-Cole type. The relaxation frequency of the mode was found to drop rapidly with decreasing temperature, confirming the glassy behavior. The variation of relaxation frequency with temperature follows the Vogel-Fulcher-Tammann equation, indicating the fragile glassy nature of the sample. This report identifies a bent-core liquid crystal exhibiting a "de Vries" SmA phase and glassy behavior at lower temperatures.

Organic chiral nano- and microfilaments: types, formation, and template applications

Organic chiral nanofilaments are part of an important class of nanoscale chiral materials that has recently been receiving significant attention largely due to their potential use in applications such as optics, photonics, metameterials, and potentially a range of medical as well as sensing applications. This review will focus on key examples of the formation of such nano- and micro-filaments based on carbon nanofibers, polymers, synthetic oligo- and polypeptides, self-assembled organic molecules, and one prominent class of liquid crystals. The most critical aspects discussed here are the underlying driving forces for chiral filament formation, potentially answering why specific sizes and shapes are formed, what molecular design strategies are working equally well or rather differently among these materials classes, and what uses and applications are driving research in this fascinating field of materials science.

Spontaneous mirror symmetry breaking and chiral segregation in the achiral ferronematic compound DIO

An achiral compound, DIO, known to exhibit three nematic phases namely N, N_X and N_F, is studied by polarizing microscopy and electro-optics for different surface conditions in confinement. The high temperature N phase assigned initially as a conventional nematic phase, shows two additional unusual features: the optical activity and the linear electro-optic response related to the polar nature of this phase. An appearance of chiral domains is explained by the spontaneous symmetry breaking arising from the saddle-splay elasticity and followed by the formation of helical domains of the opposite chirality. This is the first example of helical segregation observed in calamitic non-chiral molecules in the nematic phase. As reported previously, the ferronematic N_F shows strong polar azimuthal surface interaction energy which stabilizes a homogeneous structure in planar aligned LC cells rubbed parallel and exhibits a twisted structure in cells with antiparallel buffing. The transmission spectra are simulated using Berreman's 4 × 4 matrix method. The observed agreement between the experimental and the simulated spectra quantitatively confirms the presence of twisted structures in antiparallel rubbed cells.

New patterns of twist-bend liquid crystal phase behaviour: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4'-yl)-10-(4-alkylaniline-benzylidene-4'-oxy)decanes (CB10O·m)

The synthesis and characterisation of the 1-(4-cyanobiphenyl-4'-yl)-10-(4-alkylanilinebenzylidene-4'-oxy)decanes (CB10O·m) are reported. This series shows a rich liquid crystal polymorphism including twist-bend nematic and smectic phases. All the homologues reported exhibit an enantiotropic conventional nematic phase. For the homologues with m ≤ 10, the local packing in the nematic phases and the layer spacing in the smectic phases indicates an intercalated arrangement of the molecules. An intercalated smectic C_A phase is observed if m/11 ≈ 0.5. Either side of this condition, the twist-bend nematic phase is observed, a novel pattern of behaviour for a series on increasing a terminal chain length. For longer chain lengths, m = 12, 14, 16 and 18, two twist-bend smectic C (SmC_TB) phases are observed, and the packing of the molecules is now of a bilayer-type. The higher temperature variant is termed SmC_TB-SH in which SH (single helix) refers to the presence of a short, distorted clock-type helix. In the lower temperature SmC_TB-DH phase, an additional longer helix is superimposed on the short one, and DH denotes double helix.

Observation of “de Vries-like” properties in bent-core molecules

“de Vries” liquid crystals, defined by a maximum layer shrinkage of ≤1% from the smectic A to C phase transition, are an integral component of ferroelectric liquid crystal (FLC) displays. Bona fide de Vries materials described in the literature are primarily perfluorinated, polysiloxane and polysilane-terminated rod-like (or calamitic) LCs. Herein, for the first time, we report a series of newly designed achiral unsymmetrical bent-core molecules with terminal alkoxy chains exhibiting similar properties to “de Vries” LCs. The new molecular structure is based on the systematic distribution of four phenyl rings attached via ester and imine linkers having 3-amino-2-methylbenzoic acid as the central core with a bent angle of 147°. Detailed microscopic investigations in differently aligned (planar as well as homeotropic) cells along with SAXS/WAXS studies revealed that the materials exhibited a SmA–SmC phase sequence along with the appearance of the nematic phase at higher temperatures. SAXS measurements divulged the layer spacings (d-spacings) and hence, the layer shrinkage was calculated ranging from 0.19% to 0.68% just below the SmA–SmC transition. The variation of the calculated molecular tilt angle (α) derived from the temperature-dependent SAXS data, followed the power law with exponent values 0.29 ± 0.01 and 0.25 ± 0.01 for compounds 1/10 and 1/12, respectively. The experimental values obtained were very close to the theoretically predicted values for the materials with de Vries-like properties. The analysis of temperature-dependent birefringence studies based on the prediction of the Landau theory, showed a dip across the SmA–SmC phase transition typical of compounds exhibiting the de Vries characteristics. The collective results obtained suggest “de Vries” SmA as a probable model for this bent-core system which may find applications in displays.

A simple molecular design of unsymmetrical bent-core molecules exhibiting low layer shrinkage and a dip in the birefringence at the SmA–SmC phase transition, typical characteristics of “de Vries” liquid crystals.

Photonic Bandgap in Achiral Liquid Crystals-A Twist on a Twist

Achiral mesogenic molecules are shown to be able to spontaneously assemble into liquid crystalline smectic phases having either simple or double-helical structures. At the transition between these phases, the double-helical structure unwinds. As a consequence, in some temperature range, the pitch of the helix becomes comparable to the wavelength of visible light and the selective reflection of light in the visible range is observed. The photonic bandgap phenomenon is reported for achiral liquid crystals.

A New Class of Chiral Nematic Phase with Helical Polar Order

A novel chiral nematic phase with a polar helical order is realized via the introduction of helical twisting power into a polar nematogen. The properties of the induced polar nematic (polar cholesteric: Np*) phase differ from those of the conventional cholesteric (N*) phases existing thus far. Np*, which is a new class of N* structures, is characterized not only by its helically twisted nematic director, but also by a continuously twisted polarization. Transmission spectroscopy and helical pitch measurements in a wedge cell revealed that the half-helical pitch in the Np* phase vanished because of the polar response in the Np* helix. The inner polar director in the Np* phase is confirmed in dielectric and second-harmonic-generation studies. Furthermore, this unique Np*LC, which corresponds to a half-/full-pitch helix, enables ultrafast electro-optic switching (τ < 20 µs), and proposes new potential applications for electrically interchangeable photonic bandgaps.

Molecular Conformation of Bent-Core Molecules Affected by Chiral Side Chains Dictates Polymorphism and Chirality in Organic Nano- and Microfilaments

The coupling between molecular conformation and chirality is a cornerstone in the construction of supramolecular helical structures of small molecules across various length scales. Inspired by biological systems, conformational preselection and control in artificial helical molecules, polymers, and aggregates has guided various applications in optics, photonics, and chiral sorting among others, which are frequently based on an inherent chirality amplification through processes such as templating and self-assembly. The so-called B4 nano- or microfilament phase formed by some bent-shaped molecules is an exemplary case for such chirality amplification across length scales, best illustrated by the formation of distinct nano- or microscopic chiral morphologies controlled by molecular conformation. Introduction of one or more chiral centers in the aliphatic side chains led to the discovery of homochiral helical nanofilament, helical microfilament, and heliconical-layered nanocylinder morphologies. Herein, we demonstrate how a priori calculations of the molecular conformation affected by chiral side chains are used to design bent-shaped molecules that self-assemble into chiral nano- and microfilament as well as nanocylinder conglomerates despite the homochiral nature of the molecules. Furthermore, relocation of the chiral center leads to formation of helical as well as flat nanoribbons. Self-consistent data sets from polarized optical as well as scanning and transmission electron microscopy, thin-film and solution circular dichroism spectropolarimetry, and synchrotron-based X-ray diffraction experiments support the progressive and predictable change in morphology controlled by structural changes in the chiral side chains. The formation of these morphologies is discussed in light of the diminishing effects of molecular chirality as the chain length increases or as the chiral center is moved away from the core-chain juncture. The type of phase (B1-columnar or B4) and morphology of the nano- or microfilaments generated can further be controlled by sample treatment conditions such as by the cooling rate from the isotropic melt or by the presence of an organic solvent in the ensuing colloidal dispersions. We show that these nanoscale morphologies can then organize into a wealth of two- and three-dimensional shapes and structures ranging from flower blossoms to fiber mats formed by intersecting flat nanoribbons.

Twist-Bend Nematic Glasses: The Synthesis and Characterisation of Pyrene-based Nonsymmetric Dimers

A selection of pyrene-based liquid crystal dimers have been prepared, containing either methylene-ether or diether linked spacers of varying length and parity. All the diether linked materials, CBOnO.Py (n=5, 6, 11, 12), exhibit conventional nematic and smectic A phases, with the exception of CBO11O.Py which is exclusively nematic. The methylene-ether linked dimer, CBnO.Py, with an even-membered spacer (n=5) was solely nematogenic, but odd-members (n=6, 8, 10) exhibited both nematic and twist-bend nematic phases. Replacement of the cyanobiphenyl fragment by cyanoterphenyl giving CT6O.Py, gave elevated melting and nematic-isotropic transition temperatures, and SmA and SmC_A phases were observed on cooling the nematic phase. Intermolecular face-to-face associations of the pyrene moieties drive glass formation, and all these materials have a glass transition temperature at or above room temperature. The stability of the glassy twist-bend nematic phase allowed for its study using AFM, and the helical pitch length, P_TB , was measured as 6.3 and 6.7 nm for CB6O.Py and CB8O.Py, respectively. These values are comparable to the shortest pitch of a twist-bend nematic phase measured to date.

Experimental and Computational Study of a Liquid Crystalline Dimesogen Exhibiting Nematic, Twist-Bend Nematic, Intercalated Smectic, and Soft Crystalline Mesophases

Liquid crystalline dimers and dimesogens have attracted significant attention due to their tendency to exhibit twist-bend modulated nematic (NTB) phases. While the features that give rise to NTB phase formation are now somewhat understood, a comparable structure-property relationship governing the formation of layered (smectic) phases from the NTB phase is absent. In this present work, we find that by selecting mesogenic units with differing polarities and aspect ratios and selecting an appropriately bent central spacer we obtain a material that exhibits both NTB and intercalated smectic phases. The higher temperature smectic phase is assigned as SmCA based on its optical textures and X-ray scattering patterns. A detailed study of the lower temperature smectic ''X'' phase by optical microscopy and SAXS/WAXS demonstrates this phase to be smectic, with an in-plane orthorhombic or monoclinic packing and long (>100 nm) out of plane correlation lengths. This phase, which has been observed in a handful of materials to date, is a soft-crystal phase with an anticlinic layer organisation. We suggest that mismatching the polarities, conjugation and aspect ratios of mesogenic units is a useful method for generating smectic forming dimesogens.

Controlling Mirror Symmetry Breaking and Network Formation in Liquid Crystalline Cubic, Isotropic Liquid and Crystalline Phases of Benzil-Based Polycatenars

Spontaneous development of chirality in systems composed of achiral molecules is important for new routes to asymmetric synthesis, chiral superstructures and materials, as well as for the understanding of the mechanisms of emergence of prebiotic chirality. Herein, it is shown that the 4,4'-diphenylbenzil unit is a universal transiently chiral bent building block for the design of multi-chained (polycatenar) rod-like molecules capable of forming a wide variety of helically twisted network structures in the liquid, the liquid crystalline (LC) and the crystalline state. Single polar substituents at the apex of tricatenar molecules support the formation of the achiral (racemic) cubic double network phase with Ia 3 ‾ d symmetry and relatively small twist along the networks. The combination of an alkyl chain with fluorine substitution leads to the homogeneously chiral triple network phase with I23 space group, and in addition, provides a mirror symmetry broken liquid. Replacing F by Cl or Br further increases the twist, leading to a short pitch double gyroid Ia 3 ‾ d phase, which is achiral again. The effects of the structural variations on the network structures, either leading to achiral phases or chiral conglomerates are analyzed.

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

Intensive research on chiral liquid crystals (LCs) has been fueled by their actively tunable physicochemical properties and structural complexity, comparable to those of sophisticated natural materials. Herein, recent progress in the discovery of new classes of chiral LCs, enabled by a combination of nano- and macroscale investigations is reviewed. First, an overview is provided of liquid crystalline phases, made of chiral and achiral low-weight molecules, that exhibit chiral structure and/or chiral morphology. Then, recent progress in the discovery of new classes of chiral LCs, particularly enabled by the application of resonant X-ray scattering is described. It is shown that the method is sensitive to modulations of molecular orientation and therefore provides information hardly accessible by means of other techniques, such as the sense of helical structures or chirality transfer across length scales. Finally, a perspective is presented on the future scope, opportunities, and challenges in the field of chiral LCs, in particular related to nanocomposites.

Inverse Helical Nanofilament Networks Serving as a Chiral Nanotemplate

Herein, an epoch-making method based on bottom-up templating is proposed for the fabrication of a chiral nanoporous film that provides a chiral environment in which to confine nematic liquid crystals. A helical nanofilamental network of bent-core molecules was utilized as a three-dimensional mold, and thus the fabricated chiral nanoporous film has an inverse nanohelical structure. The presence of a chiral superstructure was confirmed by the observation of circular dichroism signals. Upon refilling this chiral nanoporous film with an achiral nematic liquid crystal, distinct circular dichroism signals appeared due to the transfer of chirality from the inverse helical nanofilaments to the achiral nematic liquid crystal. The circular dichroism signals can be readily modulated by external stimuli, such as the application of heat or an electric field. In addition, by refilling the chiral nanoporous film with a nematic liquid crystal doped with fluorescent dye, it exhibits stimuli-responsive circularly polarized luminescence. The proposed approach has huge potential for practical applications, such as for chiroptical modulators and switches and biological sensors.

Viscoelastic properties of a thioether-based heliconical twist-bend nematogen

The twist-bend nematic (N_TB) phase is one of the new types of nematics found recently, which possesses local nematic order with a heliconical orientational modulation at the nanoscale. Herein, we quantitatively determined, for the first time, the temperature-dependent elastic and viscosity properties in both the nematic (N) and N_TB phases using a thioether-linked cyanobiphenyl dimer CBS7SCB exhibiting a broad temperature range of the N_TB phase which is stable down to room temperature. In the N phase, the fundamental elastic moduli: splay and bend elastic moduli (K₁₁ and K₃₃, respectively) are found to be in the order of 10^-12 N, and the effective rotational viscosity (γ₁) is determined to be in the range of 5-200 mPa s. Meanwhile, the N_TB phase is found to exhibit a compressive elastic modulus B in the order of several tens of kilopascals, the effective K₁₁ in the order of 10^-10-10^-8 N, and a considerably large γ₁ value of ∼68.7 Pa s right below the N-N_TB phase transition. The present study provides insights into the comprehensive viscoelastic properties based on comparison of the obtained experimental data with not only the existing theoretical prediction but also the preceding experimental works.

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

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.

Novel elastic response in twist-bend nematic models

Bent-shaped liquid crystals have attracted significant attention recently due to their novel mesostructure and the intriguing behavior of their elastic constants, which are strongly anisotropic and have an unusual temperature dependence. Though theories explain the onset of the twist-bend nematic phase (NTB) through spontaneous symmetry breaking concomitant with transition to a negative bend (K3) elastic constant, this has not been observed as yet in experiments. There, the small bend elastic constant has a strongly non-monotonic temperature dependence, which first increases after crossing the isotropic (I)-nematic (N) transition, then dips near the nematic (N)-twist-bend (NTB) transition before it increases again as the transition is crossed. The molecular mechanisms responsible for this exotic behavior are unclear. Here, we utilize density of states algorithms in Monte Carlo simulation applied to a variant of the Lebwohl-Lasher model which includes bent-shaped-like interactions to analyze the mechanism behind elastic response in this novel mesostructure and understand the temperature dependence of its Frank-Oseen elastic constants.

Structural Understanding, Photoswitchability, and Supergelation of a New Class of Four Ring-Based Bent-Shaped Liquid Crystal

Herein, we report a new type of azobenzene-based unsymmetrical bent-core molecules exhibiting photoswitchability in the liquid crystalline state, solid state, and solution state and in mixture upon UV irradiation and intense visible light. The compounds exhibited solid-state photochromism upon exposure to UV light, whereas in liquid crystalline state, reversible phase transitions were observed via both UV irradiation and intense visible light exposure. Crystal structure analysis reveals the basic structural understanding such as nonplanar bent molecular shape, antiparallel arrangement of the polar bent molecules, intra- and intermolecular hydrogen bonding, and different π-π interactions and interdigitation of long alkyl chains. The compounds are also found to act as supergelator toward various organic solvents. Hence, this is an excellent example of such potential bent-shaped liquid crystals that promise an immense perspective for device applications such as optical storage, molecular switches, etc.

Multi-level chirality in liquid crystals formed by achiral molecules

Complex materials often exhibit a hierarchical structure with an intriguing mechanism responsible for the ‘propagation’ of order from the molecular to the nano- or micro-scale level. In particular, the chirality of biological molecules such as nucleic acids and amino acids is responsible for the helical structure of DNA and proteins, which in turn leads to the lack of mirror symmetry of macro-bio-objects. To fully understand mechanisms of cross-level order transfer there is an intensive search for simpler artificial structures exhibiting hierarchical arrangement. Here we present complex systems built of achiral molecules that show four levels of structural chirality: layer chirality, helicity of a basic repeating unit, mesoscopic helix and helical filaments. The structures are identified by a combination of hard and soft x-ray diffraction measurements, optical studies and theoretical modelling. Similarly to many biological systems, the studied materials exhibit a coupling between chirality at different levels.

It was previously shown that chiral structures can be formed from achiral bent-shaped mesogens. Here the authors observe hierarchical chiral structures with coupling of chirality at different levels in a system with achiral constituents.

Molecular curvature, specific intermolecular interactions and the twist-bend nematic phase: the synthesis and characterisation of the 1-(4-cyanobiphenyl-4'-yl)-6-(4-alkylanilinebenzylidene-4'-oxy)hexanes (CB6O.m)

The syntheses and characterisation of the first ten homologues of the 1-(4-cyanobiphenyl-4'-yl)-6-(4-alkylanilinebenzylidene-4'-oxy)hexanes (CB6O.m) are reported. All ten members of the series exhibit an enantiotropic nematic, N, phase, and a monotropic twist-bend nematic, NTB, phase. Only CB6O.10 shows a smectic phase. The assignment of both nematic phases was confirmed using X-ray diffraction. For short chain lengths (m = 1-6) the local packing in both nematic phases is an intercalated arrangement, for intermediate chain lengths a frustrated local structure is seen and for the longest chain length, a bilayer arrangement is observed. This change in the local structure on increasing m has no apparent effect on the stability of either nematic phase, and TNTBN and TNI show a regular dependence on m. Specifically, TNTBN and TNI decrease on increasing m and superimposed upon this is a weak odd-even effect in which the odd members show the higher values. TNI decreases more rapidly than TNTBN on increasing m such that the ratio TNTBN/TNI increases. The lower temperature liquid crystal phase shown by 1-(4-cyanobiphenyl-4'-yloxy)-5-(4-butylanilinebenzylidene-4'-oxy)pentane (CBO5O.4) is reassigned as a twist-bend nematic phase. The transitional properties of the CB6O.m, CB6O.Om and CBO5O.m series are compared.

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

An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a nonpolar, achiral de Vries smectic A which transitions to a chiral, ferroelectric state in applied electric fields above a threshold. At lower temperature, a chiral, ferrielectric phase with a periodic, supermolecular modulation of the tilt azimuth, indicated by a Bragg peak in carbon-edge resonant soft x-ray scattering, is observed. The absence of a corresponding resonant umklapp peak identifies the superlayer structure as a twist-bend-like helix that is only weakly modulated by the smectic layering.

Formation and development of nanometer-sized cybotactic clusters in bent-core nematic liquid crystalline compounds

Two homologue achiral bent-core liquid crystals (LCs), BCN66 and BCN84, in their nematic phases are studied by dielectric spectroscopy in the frequency range 10 Hz-10 MHz. In each of these compounds, two relaxation processes are identified and assigned to (i) collective dynamics of molecules in nanometer-sized cybotactic clusters and (ii) individual molecular relaxations, in the ascending order of frequency of the probe field. The temperature and the bias electric field dependence of the dielectric strength and relaxation frequency for these processes are shown to give rise to sharpness in cluster boundaries, increased size and volume fraction in the LC nematic phase. The effect of the bias field on the LC cell is similar to reducing its temperature; both variables increase the cluster size and volume fraction and give rise to sharp cluster boundaries. The findings confirm that dielectric spectroscopy is a powerful and an extremely useful technique to provide a deeper understanding of the mechanism of cybotactic cluster formation in the isotropic liquid and the nematic phase of LCs as a function of temperature and the bias field.

Emergence of polar order and tilt in terephthalate based bent-core liquid crystals

The shown compound forms ferroelectric domains in a SmA phase, which adopt antipolar correlations before condensing into a weakly tilted antiferroelectric smectic phase.

Effects of molecular chirality on self-assembly and switching in liquid crystals at the cross-over between rod-like and bent shapes

A bent-core compound derived from a 4-cyanoresorcinol core unit with two terephthalate based rod-like wings and carrying chiral 3,7-dimethyloctyloxy side chains has been synthesized in racemic and enantiomerically pure form and characterized by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical investigations to study the influence of molecular chirality on the superstructural chirality and polar order in lamellar liquid crystalline phases. Herein we demonstrate that the coupling of molecular chirality with superstructural layer chirality in SmCsPF domain phases (forming energetically distinct diastereomeric pairs) can fix the tilt direction and thus stabilize synpolar order, leading to bistable ferroelectric switching in the SmC* phases of the (S)-enantiomer, whereas tristable modes determine the switching of the racemate. Moreover, the mechanism of electric field induced molecular reorganization changes from a rotation around the molecular long axis in the racemate to a rotation on the tilt-cone for the (S)-enantiomer. At high temperature the enantiomer behaves like a rod-like molecule with a chirality induced ferroelectric SmC* phase and an electroclinic effect in the SmA'* phase. At reduced temperature sterically induced polarization, due to the bent molecular shape, becomes dominating, leading to much higher polarization values, thus providing access to high polarization ferroelectric materials with weakly bent compounds having only "weakly chiral" stereogenic units. Moreover, the field induced alignment of the SmCsPF(()*()) domains gives rise to a special kind of electroclinic effect appearing even in the absence of molecular chirality. Comparison with related compounds indicates that the strongest effects of chirality appear for weakly bent molecules with a relatively short coherence length of polar order, whereas for smectic phases with long range polar order the effects of the interlayer interfaces can override the chirality effects.