Antiferroelectric liquid crystal from a banana-shaped achiral molecule

A new banana-shaped achiral compound, 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxyphenyliminomethyl)benzoate] (PBFOB) was synthesized and its antiferroelectric liquid crystallinity determined. The PBFOB was characterized by differential scanning calorimetry, X-ray diffractometry in the small and wide angle regions, and polarizing optical microscopy; its polarization and antiferroelectric properties were also investigated. The presence of a lateral fluoro-substituent in the banana-shaped achiral molecules containing a Schiff's base mesogen induced a decrease in melting temperature and formation of the switchable smectic B₂ phase in the melt. The spontaneous polarization for PBFOB was about 250 nC cm^-2 and the polarization of this phase switched on the reversal of an applied electric field.     

Helical filamentary growth in liquid crystals consisting of banana-shaped molecules

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.     

Helical filamentary growth in liquid crystals consisting of banana-shaped molecules

The formation of coils is common in nature when achiral symmetry breaking occurs. Here we describe spectacular examples of single, double and triple coils observed in smectic liquid crystal phases of achiral banana-shaped molecules. Such molecules form chiral smectic phases due to two symmetry-breaking instabilities: polar molecular packing, and molecular tilt. The appearance of helical filaments at the isotropic-smectic transition is therefore a direct indication of the achiral symmetry-breaking of the smectic structures. The number of observed left- and right-handed domains is equal, reflecting the achiral nature of the constituent molecules. Our studies indicate that the helical filaments consist of concentric smectic layers. The coiling stabilizes the growth process and suppresses the penetration of molecules from the isotropic phase, leading to moving of the tip with constant speed.     

Antiferroelectric liquid crystal from a banana-shaped achiral molecule

A new banana-shaped achiral compound, 1,3-phenylene bis[4-(3-fluoro-4-n-octyloxyphenyliminomethyl)benzoate] (PBFOB) was synthesized and its antiferroelectric liquid crystallinity determined. The PBFOB was characterized by differential scanning calorimetry, X-ray diffractometry in the small and wide angle regions, and polarizing optical microscopy; its polarization and antiferroelectric properties were also investigated. The presence of a lateral fluoro-substituent in the banana-shaped achiral molecules containing a Schiff's base mesogen induced a decrease in melting temperature and formation of the switchable smectic B₂ phase in the melt. The spontaneous polarization for PBFOB was about 250 nC cm^-2 and the polarization of this phase switched on the reversal of an applied electric field.     

Structural and electro-optical investigations of the smectic phase of chlorine-substituted banana-shaped compounds

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an XB2 phase behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by 13C NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the XB2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers. which exhibits antiferroelectric switching     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^-2 on reversal of an applied electric field.     

Textural transformations of a smectic phase with asymmetric banana-shaped molecules

Materials consisting of asymmetric banana-shaped molecules may form a smectic C_G phase having C₁ symmetry. We have studied textural transformations in a smectic phase of an asymmetric chlorine-substituted banana-shaped material under electric and mechanical fields. We observed two novel features that have not been observed so far on corresponding materials with symmetric banana-shaped molecules. These observations, however, could be explained by the same arguments as were used for the symmetric molecules. Although our studies do not exclude the possibility that the material has C₁ symmetry, we suggest that the chlorine molecules are positioned arbitrarily and the bulk has C₂ symmetry.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^?2 on reversal of an applied electric field.     

Ferroelectric smectic meso-phase formed by banana-shaped achiral liquid crystals

A new banana-shaped achiral molecule, 1,3-phenylene bis[4-(3-chloro-4-n-octyloxyphenyliminomethyl)benzoate] (PBCOB) has been synthesized, and its ferroelectric properties and homeotropic alignment investigated. The presence of a lateral chloro-substituent in the Schiff 's base moiety prevents the regular stacking of molecules and results in lowering the transition temperature and the degree of crystallinity of the switchable banana phase. Their smectic mesophases, including a switchable banana phase B₇, were characterized by differential scanning calorimetry, X-ray scattering and polarizing optical microscopy. Both the left- and right-handed helical domains are spontaneously formed upon cooling from the isotropic liquid to the switchable banana phase B₇. By X-ray study, the smectic phases showed a layer spacing of 38.1 Å, compatible with the end-to-end distance of the molecule with a bent conformation. Significantly, the smectic B7 phase exhibited a periodicity of 292 Å that corresponds to a helical structure with periodicity about 7.5 times 38.1 Å. The spontaneous polarization for PBCOB is about 50 nCcm^-2 and shows a temperature dependence. The ferroelectric lyomesophase of PBCOB showed a ferroelectric electro-optical switching range extending more than 50°C, switchable at room temperature.     

Compression induced chiral symmetry breaking of monolayers comprised of banana-shaped achiral molecules at an air-water interface: Williams-Bragg approach

Chirality of monolayers comprised of banana-shaped achiral molecules at an air-water interface was investigated theoretically, and a forming mechanism of chiral structure as an assembly of achiral molecules was argued. A model of such monolayers was constructed taking into account the short-range repulsive interaction between constituent banana-shaped achiral molecules, and the free energy density functional of the model was derived as a generalization of Williams-Bragg approach. It was predicted that chiral symmetry breaking occurs by monolayer compression, where two-dimensional characteristics of monolayers at an interface plays an important role in the formation of chiral structure by banana-shaped achiral molecules.     

An unusual chevron structure in an achiral smectic C liquid crystal device

The layer structure that occurs in an achiral smectic C liquid crystal device has been investigated as a function of temperature using the small angle X-ray scattering facility at the Synchrotron Radiation Source, Daresbury UK. The material studied shows a direct phase transition from the nematic to the smectic C phase. The layer structure proposed on the basis of the diffraction data is relatively complex, containing regions with chevron, quasi-bookshelf and curved structures. A rationale for the formation of the structure is presented, relying on both the phase transition characteristics of the system and the anisotropic layer elasticity in the smectic C phase. Qualitative analysis indicates that the layer constant A is greater than A 21, i.e. layer flexing is easier perpendicular to the plane of the director than parallel to it. It is also demonstrated that the surface chevron angle is several degrees different from the tilt angle of the smectic C phase at temperatures well below the smectic C to nematic phase transition. 12     

Computer simulation of chiral liquid crystal phases. I. The polymorphism of the chiral Gay-Berne fluid

We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in the NVT ensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parameter c describing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constant c, we obtained a phase region showing characteristics of the recently discovered helical smectic A* phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small system size of N = 256 molecules and the use of periodic boundary conditions.     

Structural characterization of the new polymorphic mesophases formed by bent-core molecules

A new achiral banana-shaped five-ring 2-methylresorcinol derivative fluorinated on the outer rings has been synthesized. This compound exhibits four antiferroelectric smectic phases characteristic for bent-core molecules (B 2 , B' 2 , B' 2 , B 5 ). In addition, a highly viscous solid-like phase has been detected which also shows electro-optical switching. This is the first switchable highly ordered B phase. The characterization of the mesophases was made by X-ray diffraction, electro-optical measurements and dielectric spectroscopy.     

Enhancement of twisting power in the chiral nematic phase by introducing achiral banana-shaped molecules

Achiral banana-shaped molecules with dodecyloxy tail groups, P-12-O-PIMB, N-12-O-PIMB, and S-12-O-PIMB, have exhibited unusual smectic phases which possess chiral and helical structures. In this work, we mixed these banana-shaped molecules with the chiral molecule forming a chiral nematic liquid crystal and found an exclusive effect of the achiral dopant that the twisting power of the chiral nematic phase in the mixtures is significantly increased with the increase of the content of achiral banana-shaped molecules. This characteristic effect in the chiral nematic field seems to offer the rational evidence for the twist conformation of such banana-shaped molecules, since the chirality should be included intrinsically within each chain. The asymmetric twist conformation in the ester linkage group connecting the central core with the side wings is likely to be the origin of enhanced twisting power.     

A microscopic model considering the effect of molecular biaxiality on ferroelectric ordering of liquid crystalline smectic C* phases

On the basis of a mean-field approach, a microscopic model is presented to explain some ferroelectric properties of induced smectic C* phases of chiral dipolar guest molecules in an achiral smectic C host phase. The molecular biaxiality of the chiral dopants has been taken into account, resulting in a polar and quadrupolar ordering of the molecular short axes in the rotational distribution function of the guest molecules. This model explains the dependence of the magnitude and sign of the spontaneous polarization on the molecular structure of a series of cyclo-hexanone derivatives used as chiral dopants, as well as the effect of a local field at higher dopant concentrations.     

Computer simulation studies of anisotropic systems. XIX. Mesophases formed by the Gay-Berne model mesogen

We report the results of a molecular dynamics computer simulation of particles interacting via the Gay-Berne potential with parameters selected to approximate those of mesogenic molecules. The system was found to form a variety of mesophases as the temperature was lowered. We have characterized these phases with the aid of computer graphics techniques to visualize the molecular organization within configurations taken from the production stage of the simulations. The phases have been identified, on the basis of such images, as isotropic, nematic, smectic A, smectic B and crystal.     

Ferroelectric properties of smectic C liquid crystals with induced helical structure

Ferroelectricity has been observed in liquid crystalline mixtures of chiral smectic C type with a helical structure induced by chiral guest molecules (pitch p ≈ 100 μm). Samples of thickness d?p form monodomains with finite zero-field electric polarization because the spontaneous polarization of the smectic layers is not compensated by the helical twist. The temperature dependence of the spontaneous polarization has been studied near the phase transition smectic C→ smectic A.     

Computer Simulation Studies of Anisotropic Systems XVI. The Smectic E-Smectic B Transition

We have investigated the smectic E-smectic B transition with the aid of a model smectogen whose properties have been calculated using the Monte Carlo technique of computer simulation. The lath-like mesogenic molecules are defined to lie in a plane with their centres on a triangular lattice and with their long axes orthogonal to the smectic layer. The quadrupolar interaction, restricted to nearest neighbours, is assumed to be responsible for the herring-bone arrangement of the molecular short axes, characteristic of the smectic E phase. The computer simulations have been employed to evaluate both thermodynamic and structural parameters as a function of temperature. The model smectogen is found to exhibit a continuous transition at which the long range herring-bone structure of the smectic E phase is destroyed only to be replaced by the analogous short range structure of the smectic B. Where possible the results simulated for the model smectogen are compared with the behaviour of real mesogens and the predictions of molecular field theories for the transition. In particular, contact is made with X-ray diffraction studies of the two phases by using optical techniques to generate the diffraction patterns associated with configurations produced by the simulation. The model is found to be in good accord with experiment but the molecular field prediction of the smectic E-smectic B transition temperature is shown to be poor.     

Polar Order,Mirror Symmetry Breaking,and Photoswitching of Chirality and Polarity in Functional Bent-Core Mesogens

In recent years, liquid crystals (LCs) responding to light or electrical fields have gained significant importance as multifunctional materials. Herein, two new series of photoswitchable bent-core liquid crystals (BCLCs) derived from 4-cyanoresorcinol as the central core connected to an azobenzene based wing and a phenyl benzoate wing are reported. The self-assembly of these molecules was characterized by differential scanning calorimetry (DSC), polarizing light microscopy (POM), electro-optical, dielectric, second harmonic generation (SHG) studies, and XRD. Depending on the direction of the COO group in the phenyl benzoate wing, core-fluorination, temperature, and the terminal alkyl chain length, cybotactic nematic and lamellar (smectic) LC phases were observed. The coherence length of the ferroelectric fluctuations increases continuously with decreasing temperature and adopts antipolar correlation upon the condensation into superparaelectric states of the paraelectric smectic phases. Finally, long-range polar order develops at distinct phase transitions; first leading to polarization modulated and then to nonmodulated antiferroelectric smectic phases. Conglomerates of chiral domains were observed in the high permittivity ranges of the synclinic tilted paraelectric smectic phases of these achiral molecules, indicating mirror symmetry breaking. Fine-tuning of the molecular structure leads to photoresponsive bent-core (BC)LCs exhibiting a fast and reversible photoinduced change of the mode of the switching between ferroelectric- and antiferroelectric-like as well as a light-induced switching between an achiral and a spontaneous mirror-symmetry-broken LC phase.