Field-induced switching between states of opposite chirality in a liquid-crystalline phase

Chiral Superstructure Mesophases of Achiral Bent-Shaped Molecules - Hierarchical Chirality Amplification and Physical Properties

Chiral mesophases in achiral bent-shaped molecules have attracted particular attention since their discovery in the middle 1990s, not only because of their homochirality and polarity, but also due to their unique physical/physicochemical properties. Here, the most intriguing results in the studies of such symmetry-broken states, mainly helical-nanofilament (HNF) and dark-conglomerate (DC) phases, are reviewed. Firstly, basic information on the typical appearance and optical activity in these phases is introduced. In the following section, the formation of mesoscopic chiral superstructures in the HNF and DC phases is discussed in terms of hierarchical chirality. Nanoscale phase segregation in mixture systems and gelation ability in the HNF phase are also described. In addition, some other related chiral phases of bent-shaped molecules are shown. Recent attempts to control such mesoscopic chiral structure and the alignment/confinement of HNFs are also discussed, along with several examples of their fascinating advanced physical properties, i.e. huge enhancement of circular dichroism, electro- and photo-tunable optical activities, chirality-induced nonlinear optics (second-harmonic-generation circular difference and electrogyration effect), enhanced hydrophobicity through the dual-scale surface morphological modulation, and photoconductivity in the HNF/fullerene binary system. Future prospects from basic science and application viewpoints are also indicated in the concluding section.

Chiral conglomerates observed for a binary mixture of a nematic liquid crystal trimer and 6OCB

Dark conglomerates of domains with opposite handedness, which are designated as dark conglomerate phases (DC phases), have attracted much attention. After designing an achiral liquid crystal trimer, 4,4′-bis{7-[4-(5-octyloxypyrimidin-2-yl)phenyloxy]heptyloxy}biphenyl (1), which exhibits only a nematic phase, we prepared binary mixtures with some typical rod-like nematic liquid crystals, i.e., 4′-hexyloxy-4-cyanobiphenyl (6OCB), 2-(4-hexyloxyphenyl)-5-pentyloxypyrimidine (PPY), or 4-methyloxyphenyl 4-hexyloxycyclohexanecarboxylate (PCA), and investigated their phase transition behaviour. The binary mixtures containing 55–90 mol% of 6OCB were found to exhibit a nematic phase and a DC phase of chiral domains with opposite handedness. However, neither PPY nor PCA induced such a chiral conglomerate phase in the mixture with trimer 1. We discuss how core–core interactions contribute to produce such a chiral conglomerate phase.

Chiral symmetry breaking dictated by electric-field-driven shape transitions of nucleating conglomerate domains in a bent-core liquid crystal

Generating and controlling chiral symmetry breaking and enantiomeric excess is not only interesting from a fundamental perspective but can also lead to novel functional materials. In this work, we show how the dark conglomerate (DC) liquid crystalline phase characterized by macroscopic chiral domains offers such a possibility if formed under an electric field. In addition the chiral domains are electro-optically switchable. The chiral segregation in the DC phase can be tuned by using dc or ac fields at different frequencies. Consequently, the enantioselectivity, dielectric parameters and switching polarization in the DC phase become tunable. Another interesting aspect is that the nucleating conglomerate domains formed under ac fields exhibit frequency dependent shape transitions which have a striking resemblance to domain shape changes observed in two-dimensional monolayers. This can therefore be used as a model experimental system to get a physical insight into the effects of chiral and electrostatic interactions, under external fields, on domain growth and interface structures. The domain shape transitions can also be used to investigate the role of growth morphology in coarsening and scaling hypotheses. From a technological point of view this opens up the possibility of obtaining chiral thin films with preferential sense of chirality which can be useful in chiroptical and nonlinear optical applications.

Model of dark conglomerate structure in the B2 phase of bent-shape molecules

Texture observations in B2 phase of bent-shape molecules showed a coexistence of so-called dark conglomerate (DC) structure with fan-shaped texture composed of focal conic domains (FCDs). A model of DC structure based on grains of dimensions lower than visible wavelengths is proposed and used to compare the energies of DC and FCD structures. The comparison of energies of both structures enables the estimation of approximate model parameters. Reorientation of smectic layers in grains under an electric field and transformation into the system of FCD structure are discussed.

Ambidextrous bend patterns in free-standing polar smectic- CP{F} films

We report an unusual behavior of a ferroelectric smectic-CP{F} film formed by bent-shaped molecules. The ground state of the c -director in such film is not uniform but forms a striped pattern with alternating bend deformation. We found that the sense of the alternating bend is not related to an alternating handedness defined by the mutual orientation of the tilt ( c director) and the bow ( p director) of the molecules. Despite its similarity to a previously described twist-bend instability [J. Pang and N. A. Clark, Phys. Rev. Lett. 73, 2332 (1994)], this pattern cannot be explained in terms of spontaneous chiral symmetry breaking with continuous variation of the chirality order parameter, since the synclinic order of the polar molecules predefines the chirality of the film. We discuss possible models describing the spontaneous formation of an ambidextrous bend pattern of the c director.

Experimental test of curvature-driven dynamics in the phase ordering of a two dimensional liquid crystal

We study electric field driven deracemization in an achiral liquid crystal through the formation and coarsening of chiral domains. It is proposed that deracemization in this system is a curvature-driven process. We test this prediction using the recently obtained exact result for the distribution of hull-enclosed areas in two-dimensional coarsening with nonconserved scalar order parameter dynamics [J. J. Arenzon et al., Phys. Rev. Lett. 98, 145701 (2007)]. The experimental data are in very good agreement with the theory. We thus demonstrate that deracemization in such bent-core liquid crystals belongs to the Allen-Cahn universality class, and that the exact formula, which gives us the statistics of domain sizes during coarsening, can also be used as a strict test for this dynamic universality class.

Field-induced texture transitions in a bent-core nematic liquid crystal

It is demonstrated in electro-optic experiments that an external electric field of the order of 10;{5} V/m induces persisting texture transitions in a nematic phase formed by bent-core mesogens. The field-induced metastable state is identified by its optical and electric properties. After the field is switched off, the original and induced states can coexist in domains for about one hour in planar sandwich cells. During this time, the induced domains gradually shrink but they can be stabilized in moderate electric fields. The occurrence of similar domains in homeotropic cells suggests that the transition into a metastable biaxial state is observed. In the field-free planar ground state, the formation of inversion walls is observed inside the metastable domains.

Chirality Transfer between Weakly Birefringent and Electric-Field-Induced Highly Birefringent B2 Phases in a Bent-Core Mesogen

Electric-field-induced transition was observed for the weakly birefringent chiral B2 phase, which is formed from the banana molecule based on the naphthalene bent core. This phase is considered to possess the twisted grain boundary (TGB)-like helical structure. When an electric field is applied, the TGB-like helix unwinds. The resulting large domain of the SmC(A)P(A) phase shows the high birefringence and simultaneously the antiferroelectric switching between SmC(A)P(A) and SmC(S)P(F) states. Through this field-induced transformation, two interesting features are obtained. First, the initially formed chiral domains are preserved even after the field-induced transformation to the unwound SmC(A)P(A) phase. This indicates the close correlation between the TGB-like helix and the layer chirality in such a way that the helical sense of the TGB-like helix is memorized as the layer chirality of the homochiral SmC(A)P(A) phase. Second, there is a critical temperature, above which the helicoidal structure is stable against the electric field. There is a competition between winding into a TGB-like structure and unwinding due to the electric field, and at higher temperatures, the helicoidal power is too strong to surpass the effect of the electric field.

Influence of the core structure on the development of polar order and superstructural chirality in liquid crystalline phases formed by silylated bent-core molecules: lateral substituents

The influence of lateral substituents was studied for a new series of bent-core liquid crystals with heptamethyltrisiloxane units at both ends of the aliphatic side chains. These materials were investigated by polarized light microscopy, differential scanning calorimetry, X-ray scattering and electro-optical methods. An antiferroelectric switching columnar phase was found for the non-substituted compound. Different types of non-switching columnar phases were obtained for molecules having substituents (CH, NO) in the bay position, whereas a temperature-dependent transition from polar to non-polar smectic phases was found for molecules having substituents (Cl, CN) adjacent to one of the ester linking units. For the CN-substituted compound a new phase sequence SmCP-SmC-Cub-Iso was observed. The occurrence of the cubic phase is associated with a steric frustration due to the bulky silyl groups. The special rectangular columnar mesophase of the nitro-substituted compound is discussed with respect to its relation to B7-type mesophases.

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle approximately 45 degrees). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35 V microm(-1)) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, 13C- and 1H-NMR investigations, dielectric and electro-optical measurements.

Some aspects about the structure of the optically isotropic phase in a bent-core liquid crystal: chiral, polar, or steric origin

We have studied a bent-core liquid crystal where two different optically isotropic phases can be induced by a strong electric field. Depending on the field treatment the phases can present optical rotation or be optically inactive. The switching dynamics of the phases is studied by means of electrooptic and optical second-harmonic generation measurements. It is found that the ground state of the phases is locally antiferroelectric. The structure of the phases is consistent with a disordered version of the kind of structures recently proposed for the smectic blue phase: layered systems with high Gaussian curvature. The origin of the smectic layer distortion is discussed. It is concluded that the direct reason for the curvature of planes is of steric nature.

Low-Birefringent, Chiral Banana Phase below Calamitic Nematic and/or Smectic C Phases in Oxadiazole Derivatives

Bent-shaped molecules based on the oxadiazole central core with various side wings and terminal chain groups have been synthesized, and their liquid-crystalline behavior was investigated by optical microscopic, X-ray, and electrooptic measurements. These molecules exhibit liquid-crystal polymorphism including both the calamitic and banana phases. Such a characteristic polymorphism is attributable to the larger bend angle of the oxadiazole core compared to that of the resorcinol core used in conventional banana molecules. Only one type of banana phase, designated as the Bx phase, is formed. It appears upon cooling from the nematic and smectic liquid crystals and exhibits chiral domains with a very weak birefringence (apparently optically isotropic). By applying an electric field, the Bx phase is altered to a high-birefringence B2 phase with a homochiral SmC(A)P(A) structure that exhibits an antiferroelectric response. From detailed analyses of the optical texture and X-ray patterns through the transformation from well-oriented calamitic phases, the Bx phase was found to exhibit a helical structure, which arises as a frustration from the ground-state B2 phase in such a manner that the blocks of B2 layers are twisted with respect to each other in a direction parallel to the layer plane similarly to the twisted grain boundary (TGB) phase.

Induced and spontaneous deracemization in bent-core liquid crystal phases and in other phases doped with bent-core molecules

Recently discovered chiral properties of several bent-core smectic liquid crystal phases are summarized and discussed in detail under the assumption that typical bent-core molecules may exist in chiral conformational states and are achiral only on average. Results of atomistic computer simulations are presented which indicate the existence of strongly chiral conformational states for typical bent-core mesogens. A theory is developed which describes a possible shift of equilibrium between left- and right-handed conformations in a macroscopically chiral phase. The theory describes a chirality induction in the B2 bent-core phase and a reduction of the helical pitch in cholesteric and chiral SmC* phases doped with bent-core molecules. Finally, the possibility of spontaneous deracemization in bent-core smectic phases is discussed in detail.

Polarization-modulated smectic liquid crystal phases

Any polar-ordered material with a spatially uniform polarization field is internally frustrated: The symmetry-required local preference for polarization is to be nonuniform, i.e., to be locally bouquet-like or "splayed." However, it is impossible to achieve splay of a preferred sign everywhere in space unless appropriate defects are introduced into the field. Typically, in materials like ferroelectric crystals or liquid crystals, such defects are not thermally stable, so that the local preference is globally frustrated and the polarization field remains uniform. Here, we report a class of fluid polar smectic liquid crystals in which local splay prevails in the form of periodic supermolecular-scale polarization modulation stripes coupled to layer undulation waves. The polar domains are locally chiral, and organized into patterns of alternating handedness and polarity. The fluid-layer undulations enable an extraordinary menagerie of filament and planar structures that identify such phases.

Interpretation of unusual textures in the B2 phase of a liquid crystal composed of bent-core molecules

A liquid crystal material of bent-core molecules is investigated by means of optical texture observations, x-ray measurements, and miscibility studies. While the x-ray and miscibility data point towards a B2 phase, the texture is however unusual, showing optical isotropy and segregation in two domains with opposite gyrations. It is shown that the texture can be interpreted successfully in terms of a smectic-C(A)P(A) structure in small domains with random orientations. The optical activity data are also explained semiquantitatively.