Polarization splay as the origin of modulation in the B1 and B7 smectic phases of bent-core molecules

Leaning induced layer undulated tilted smectic phase of asymmetric bent-core liquid crystals

We report the synthesis and experimental studies on the liquid crystalline phase of a homologous series of compounds comprised of strongly asymmetric bent-core banana shaped molecules. Our x-ray diffraction studies clearly indicate that the compounds exhibit a frustrated tilted smectic phase with layer undulated structure. The low value of the dielectric constant as well as switching current measurements indicate the absence of polarization in this layer undulated phase. In spite of the absence of polarization, a planar aligned sample can be transformed irreversibly to a higher birefringent texture on the application of a high electric field. The zero field texture can only be retrieved by heating the sample to the isotropic phase and cooling it subsequently to the mesophase. We propose a double tilted smectic structure having layer undulation to account for the experimental observations with the layer undulation arising due to the leaning of the molecules in the layer.

Some geometrical aspects of packing of bent-core molecules in the triclinic smectic-C_{g} liquid crystals and simple models of the modulated SmC_{gmod} phase

Organic compounds with bent-core (BC) molecules usually form the layered smectic liquid crystals with tilted molecules and polarization (P) which lies in the plane of the layer. A few such compounds have been found in which P itself tilts out of the plane of the layer, and the medium with general tilt (SmC_{g}) of the molecules has the low chiral triclinic symmetry. We discuss the geometric constraints of molecular packing in this structure to show that projecting groups attached to one of the arms of the BC molecules favors the formation of the SmC_{g} phase. We also extend our model for the modulated phases exhibited by BC molecules to show that the stripe structure made of bilayers shown by a few BC compounds, which is a signature of SmC_{g} layers, is preferentially formed by rotation of the BC molecules about their long axes, rather than about the layer normal. The theoretical results based on the unified model which describes all the modulated phases exhibited by the BC molecules broadly reflect experimental trends.

A two-state model for the modulated B7 liquid crystalline phase exhibited by bent-core molecules

Liquid crystals (LC) made of bent-core (BC) organic molecules have been intensively studied over the past two decades. The B₇ LC consists of smectic layers in which tilted molecules have an in-plane polar packing, the polarization vector ( P ) having a splay distortion forming stripes of tens of nanometers in width, and undulated layers giving rise to 2D rectangular or oblique lattices. The prevailing phenomenological theory attributes this structure to a strong coupling between molecular tilt in the layers and div P . Based on our recent studies on other phases exhibited by BC molecules, we propose a new model in which the physical origin of the stripes arises from a minority (~10%) of the BC molecules having less bent excited state (ES) conformers which can freely rotate about their long axes, and aggregate to form smectic C type walls, in which the tilt angle is relatively small. The more bent ground state (GS) conformers with polar packing in turn form the splayed structure between such walls to lower the free energy. The mismatch in the layer spacing between the domains with ES and GS conformers generates the structures in the B₇ phase described above. The model predicts a weak first order transition from B₇ to the uniform B₂ phase as the temperature is lowered, as experimentally observed in some compounds. We show that the observed slow increase of the stripe width in the LC to micrometer dimensions in free standing films can be attributed to changes in the physical parameters by chemical degradation and absorption of ions by the exposed polarized layers. We also describe different possible structures of the undulated layers, including a stacking of racemic pairs of SmC_aP_F layers within the stripes. Some possible methods of testing the model are also indicated.

Simple model for the stripe phase in compounds with bent-core molecules which exhibit a lower-temperature ferroelectric smectic-A phase

Bent-core (BC) molecules usually exhibit polar packing in smectic layers in which the long or bow axes tilt with respect to the layer normal. In many compounds, the tilt angle goes to zero, and typically the polarization (P) of neighboring layers has an antiferroelectric order (SmAP_{AF}). A careful molecular engineering has led to the discovery of the ferroelectric SmAP_{F} phase in a few BC compounds. Detailed experimental studies [Zhu et al., J. Am. Chem. Soc. 134, 9681 (2012)10.1021/ja3009314] have shown that one of the compounds undergoes a weak first order transition to a striped phase (SmAP_{Fmod}), in which the repolarization switching occurs at a threshold electric field, the latter increasing with temperature. The main optical birefringence of the SmAP_{Fmod} phase increases rapidly with the field at lower temperatures of its range. A small (<10%) fraction of the BC molecules can be expected to have less bent conformations in excited states (ESs), and we argue that the ES conformers rotate freely about the bow axes, and aggregate to form regions without any polar order to gain rotational entropy. In turn, a favorable divP term leads to the formation of stripes in the polarized regions made of ground state conformers. Based on this physical picture, we develop a simple phenomenological model to reproduce all the experimental trends qualitatively. In sample cells with insulating layers neighboring stripes prefer to have an antiparallel orientation of the mean P direction. The dielectric constant of the stripe phase increases with a dc bias electric field, a trend which is opposite to that in other ferroelectric liquid crystals.

Polarization-Modulated Bent-Core Liquid Crystal Thin Films without Layer Undulation

Spatial confinement is known to affect molecular organizations of soft matter. We present an important manifestation of this statement for thin films of bent-core smectic liquid crystals. Prior freeze-fracture transmission electron microscopy (FFTEM) studies carried out on nitro-substituted bent-core mesogens (n-OPIMB-NO_{2}) revealed an undulated smectic layer structure with an undulation periodicity of ∼8  nm. We report cryogenic TEM measurements on ∼100  nm thick 8-OPIMB-NO_{2} films. In contrast to FFTEM results, our studies show only density modulation with periodicity b=16.2  nm, and no smectic layer undulation. We show that the discrepancy between the FFTEM and cryogenic transmission electron microscopy (cryo-TEM) results can be attributed to the different sample thicknesses used in the experiments. FFTEM monitors cracked surfaces of a relatively thick (5-10  μm) frozen sample, whereas cryo-TEM visualizes the volume of a thin (0.1  μm) film that was quenched from its partially fluid phase. These results have importance in possible photovoltaics and organic electronics applications where submicron thin films are used.

Azo-containing asymmetric bent-core liquid crystals with modulated smectic phases

We present the synthesis and characterization of ten asymmetric bent-core liquid crystals with enantiotropic modulated smectic (B₇ type) phases. Their relatively low and wide photosensitive mesomorphic temperature range offers potential applications.

Thermotropic liquid crystals from biomacromolecules

Complexation of biomacromolecules (e.g., nucleic acids, proteins, or viruses) with surfactants containing flexible alkyl tails, followed by dehydration, is shown to be a simple generic method for the production of thermotropic liquid crystals. The anhydrous smectic phases that result exhibit biomacromolecular sublayers intercalated between aliphatic hydrocarbon sublayers at or near room temperature. Both this and low transition temperatures to other phases enable the study and application of thermotropic liquid crystal phase behavior without thermal degradation of the biomolecular components.

Bent molecules with a 60° central core angle that form B7 and B2 phases

Small-angle bent-core liquid-crystalline (LC) molecules based on a 1,2-bis(phenylethynyl)benzene central core have been synthesized that form banana smectic phases with a ferroelectric B7-antiferroelectric B2 phase sequence upon cooling. The formation of polar, switchable ferro-/antiferroelectric banana phases indicates that, despite the low core bend angle of approximately 60°, banana smectic phases are still formed with the bend direction parallel to the layer. This study offers significant evidence that shows bent-core molecules with a 60° bend angle can form the well-known B2 and B7 banana phases. Consequently, it may lead to the preparation of a wide variety of novel bent molecules with low bend angles that spontaneously form an LC phase with both polarization and chirality.

Unusual polymorphism in new bent-shaped liquid crystals based on biphenyl as a central molecular core

Bent-shaped mesogens possessing a biphenyl as a central core have been synthesized and the role of the terminal chain and the orientation of the ester as a linkage group have been investigated. For the studied molecular core we have established that both parameters play an important role for the mesomorphic properties. The polyfluoroalkyl terminal chain supports the formation of mesophases, and the introduction of a chiral lactate terminal chain destabilizes mesophases for the first type of mutual orientation of ester groups, attached to the central core. On the contrary, for the opposite orientation of esters, the terminal chain has no effect on the mesomorphic properties, and columnar phases have been found for all compounds. A unique phase sequence has been found for the mesogen with the fluorinated chain. A generalized tilted smectics, SmC_G, have been observed in a temperature interval between two different lamellar SmCP phases and characterized by X-ray and dielectric measurements. The dielectric spectroscopy data are unique and presented for the first time in the SmC_G phase providing new information about the molecular dynamics.

Labyrinthine instability in freely suspended films of a polarization-modulated smectic phase

We report on fingering and labyrinthine instabilities of the layer dislocation lines in freely suspended polar liquid-crystalline films. These polar fingerlike and labyrinth structures reversibly form upon a transition into a modulated phase. External electric fields of several kV/m applied in the film plane can reversibly influence the formation of the finger textures. We show that the labyrinthine pattern is intrinsically related to regular splay deformations of the polarization.

Transitions between paraelectric and ferroelectric phases of bent-core smectic liquid crystals in the bulk and in thin freely suspended films

We report on the contrasting phase behavior of a bent-core liquid crystal with a large opening angle between the mesogenic units in the bulk and in freely suspended films. Second-harmonic generation experiments and direct observation of director inversion walls in films in an applied electric field reveal that the nonpolar smectic C phase observed in bulk samples becomes a ferroelectric "banana" phase in films, showing that a mesogen with a small steric moment can give a phase with polar order in freely suspended films even when the corresponding bulk phase is paraelectric.

Pattern-stabilized decorated polar liquid-crystal fibers

Geometric frustration gives rise to new fundamental phenomena and is known to yield the formation of exotic states of matter, such as incommensurate crystals, modulated liquid-crystalline phases, and phases stabilized by defects. In this Letter, we present a detailed study of polar structure of freely suspended fluid filaments in a polarization modulated liquid-crystal phase. We show that a periodic pattern of polarization-splay stripes separated by defect boundaries and decorating smectic layers can stabilize the structure of fluid fibers against the Rayleigh-Plateau instability. The instability is suppressed by the resistance of the defect structure to a radial compression of the cylindrical fibers. Our results provide direct experimental observation of a link between the stability of the liquid fibers, internal polar order, and geometrical constraints. They open a new perspective on a wide range of fluid polar fiber materials.

Polar structures of bent-core liquid crystals with tetrathiafulvalene units

We report on a series of three bent-core liquid crystals containing tetrathiafulvalene units. Their characterization consisted of electro-optical observations, second-harmonic generation (SHG) analysis, and x-ray diffraction measurements. Compound I exhibits a lamellar optically isotropic phase on cooling from the isotropic liquid and undergoes a nonreversible field-induced transition to a birefringent state. The two others present two-dimensional structural periodicities, being only compound II switchable. Additionally, the electron density maps of compounds II and III have been obtained based on the analysis of the x-ray diffraction intensities. According to SHG measurements the ground state is antiferroelectric in the three compounds. A quite good SHG performance is found in compounds I and II.