Undulations of Smectic A Layers in Achiral Liquid Crystals Manifested as Stripe Textures.

Self-assembly of organic molecules represents a fascinating playground to create various liquid crystalline nanostructures. In this Letter, we study layer undulations on micrometer scale in smectic A phases for achiral compounds, experimentally demonstrated as regular stripe patterns induced by thermal treatment. Undulations, including their anharmonic properties, are evaluated by means of polarimetric imaging and light diffraction experiments in cells with various thicknesses. The key role in stripe formation is played by high negative values of the thermal expansion coefficient.

Large Thermally Irreversible Photoinduced Shift of Selective Light Reflection in Hydrazone-Containing Cholesteric Polymer Systems.

Stimuli responsive liquid crystalline polymers are a unique class of so-called "smart" materials demonstrating various types of mesomorphic structures easily controlled by external fields, including light. In the present work we synthesized and studied a comb-shaped hydrazone-containing copolyacrylate exhibited cholesteric liquid crystalline properties with the pitch length of the helix being tuned under irradiation with light. In the cholesteric phase selective light reflection in the near IR spectral range (1650 nm) was measured and a large blue shift of the reflection peak from 1650 nm to 500 nm was found under blue light (428 or 457 nm) irradiation. This shift is related to the Z-E isomerization of photochromic hydrazone-containing groups and it is photochemically reversible. The improved and faster photo-optical response was found after copolymer doping with 10 wt % of low-molar-mass liquid crystal. It is noteworthy that both, the E and Z isomers of hydrazone photochromic group are thermally stable that enable to achieve a pure photoinduced switch without any dark relaxation at any temperatures. The large photoinduced shift of the selective light reflection, together with thermal bistability, makes such systems promising for applications in photonics.

Chiral, Magnetic, and Photosensitive Liquid Crystalline Nanocomposites Based on Multifunctional Nanoparticles and Achiral Liquid Crystals.

Nanoparticles serving as a multifunctional and multiaddressable dopant to modify the properties of liquid crystalline matrices are developed by combining cobalt ferrite nanocrystals with organic ligands featuring a robust photosensitive unit and a source of chirality from the natural pool. These nanoparticles provide a stable nanocomposite when dispersed in achiral liquid crystals, giving rise to chiral supramolecular structures that can respond to UV-light illumination, and, at the same time, the formed nanocomposite possesses strong magnetic response. We report on a nanocomposite that shows three additional functionalities (chirality and responsiveness to UV light and magnetic field) upon the introduction of a single dopant into achiral liquid crystals.

Ultra-short helix pitch and spiral ordering in cholesteric liquid crystal revealed by resonant soft X-ray scattering.

The spontaneous formation of chiral structures offers a variety of liquid crystals (LC) phases that could be further tailored for practical applications. In our work, the characteristic features of spiral ordering in the cholesteric phase of EZL10/10 LC were evaluated. To disclose resonant reflections related to a nanoscale helix pitch, resonant soft X-ray scattering at the carbon K edge was employed. The angular positions of the observed element-specific scattering peaks reveal a half-pitch of the spiral ordering p/2 ≈ 52 nm indicating the full pitch of about 104 nm at room temperature. The broadening of the peaks points to a presence of coherently scattering finite-size domains formed by cholesteric spirals with lengths of about five pitches. No scattering peaks were detectable in the EZL10/10 isotropic phase at higher temperatures. The characteristic lengths extracted from the resonant soft X-ray scattering experiment agree well with the periodicity of the surface "fingerprint" pattern observed in the EZL10/10 cholesteric phase by means of atomic force microscopy. The stability of LC molecules under the incident beam was proven by X-ray absorption spectroscopy in transmission geometry.

Defect Structures of Magnetic Nanoparticles in Smectic A Liquid Crystals.

Topological defects in anisotropic fluids like liquid crystals serve as a playground for the research of various effects. In this study, we concentrated on a hybrid system of chiral rod-like molecules doped by magnetic nanoparticles. In textures of the smectic A phase, we observed linear defects and found that clusters of nanoparticles promote nucleation of smectic layer defects just at the phase transition from the isotropic to the smectic A (SmA) phase. In different geometries, we studied and analysed creation of defects which can be explained by attractive elastic forces between nanoparticles in the SmA phase. On cooling the studied hybrid system, clusters grow up to the critical dimension, and the smectic texture is stabilised. The presented effects are theoretically described and explained if we consider the elastic interaction of two point defects and stabilisation of prismatic dislocation loops due to the presence of nanoparticles.

Silver Nanoparticles with Liquid Crystalline Ligands Based on Lactic Acid Derivatives.

We have prepared and studied silver nanoparticles functionalized with ligands based on lactic acid derivatives. Several types of hybrid systems that differed in the size of silver nanoparticles as well as the length of surface ligands were analyzed. Transmission electron microscopy (TEM) observation provided information about the size and shape of nanoparticles and proved good homogeneity of studied systems. By dynamic light scattering (DLS) measurements, we have measured the size distribution of nanoparticle systems. Plasmonic resonance was detected at around 450 nm. For two hybrid systems, the mesomorphic behaviour has been demonstrated by x-ray measurements. The observed thermotropic liquid crystalline phases reveal lamellar character. We have proposed a model based on self-assembly of intercalated liquid crystalline ligands.

Organic nanotubes created from mesogenic derivatives.

A facile route to prepare nanotubes from rod-like mesogens dissolved in typical organic solvents is reported. For selected types of chiral rod-like molecules, nanotubes were formed from both enantiomers and racemic mixtures by slow evaporation from solution, regardless of the solvent, concentration or deposition type. The obtained supramolecular assemblies were studied using AFM, TEM and SEM techniques, and other experimental techniques (IR, UV-Vis spectroscopy and X-ray diffraction) were also applied. The difference in the surface tension at opposite crystallite surfaces is suggested as a possible mechanism for nanotube nucleation. We propose a quite new rolling-up mechanism related to the surface tension difference at opposite crystallite surfaces.

Bent-core dimers with top-to-bottom linkage between central units.

A structurally new type of core-to-core dimer with a top-to-bottom linkage between the central cores of bent molecules is presented. The structure of the bent-core molecules was modified by changing the length of the spacer between the central cores and terminal chains, by changing the type of linkage groups in the bent-core arms and by variation of the terminal chain polarity - introducing polyfluoroalkyl and oligosiloxane units. Additionally, the outer phenyl ring was substituted with three alkyl chains to mimic a polycatenar system. For all compounds the mesophase behaviour and structural parameters were established to gain information about the molecular structure-mesomorphic property relationship.

(1)H NMR Relaxometric Study of Molecular Dynamics in a "de Vries" Liquid Crystal.

Liquid crystals that exhibit de Vries smectic A phases are promising materials for new generations of ferroelectric liquid crystal displays and other electro-optical devices. We investigated the molecular dynamic properties of a rod-like de Vries liquid crystal compound, namely the 9HL, a (S)-hexyl lactate derivative, in the whole mesophasic range. This is the first molecular dynamics' investigation on a de Vries phase, and the interest of this system is related to the understanding of the structural and supramolecular organization of de Vries SmA phases, which has been a subject of a broad scientific debate in the last years. Proton NMR spin-lattice relaxation times, T1, were measured in the range 100 MHz to 5 kHz, thus covering a very wide range of motional regimes. (1)H NMR dispersion curves and temperature-dependent relaxation rates were analyzed with a global minimum target fitting approach and the main molecular motions, namely reorientational diffusion, translational self-diffusion, layer undulation, and tilting director fluctuations were fully characterized. The molecular dynamics' behavior observed across the SmA-SmC* phase transitions of 9HL was consistent with the proposed cluster diffuse cone model for the de Vries SmA phase.

Filaments in the twist-grain-boundary smectic-A phase.

A model of filaments of the twist-grain-boundary smectic-A phase (TGBA) arising from the homeotropic smectic-A phase and nucleating on the sample surface is proposed. The model is based on the concept of finite blocks of parallel smectic layers forming a helical structure. The blocks are surrounded by dislocation loops. The model describes the filament structure near the sample surface and the observed inclination of the filament axis with respect to the easy direction of the molecular anchoring on the surface. The model is based on the observations of filament textures of the TGBA phase in a chiral liquid crystalline compound, but can be applied for forming of TGBA filaments in any compound. The compression modulus of the compound has been estimated using such parameters as anchoring energy, estimated from the field necessary to transform the structure into the homeotropic smectic-A.

Unique effect of an electric field on a new liquid crystalline lactic acid derivative.

A new chiral lactic acid derivative is presented, exhibiting a frustrated liquid crystalline phase, namely the orthogonal twist grain boundary TGBA phase in a broad temperature interval. A unique effect is observed that the applied electric field reversibly transforms the planar TGBA texture to the homeotropic one, homogeneously dark in crossed polarizers. The transformation is analogous to the Frederiks transition known in nematics, in which switching under electric field is driven by the positive dielectric anisotropy. A similar effect is established also in the SmA phase of the racemic mixture, where the field induced transformation is irreversible. A positive dielectric anisotropy in both the chiral compound and the racemic mixture is detected up to the frequency of about 10 kHz, above this frequency the anisotropy is negative. The unusual behavior of the TGBA phase under the electric field can be explained by the specific packing of molecules within the smectic layers, resulting in a relatively high layer compressibility which lowers the energy of the structural defects and thus facilitates the structure transformation. The perfectly dark state of the studied compounds, induced by the electric field, either stable or reversible, is appealing for specific applications. The change of the sign of the dielectric anisotropy, known in nematics as the dual frequency effect, might be important for photonics such as adaptive or diffractive optics.

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, SmCG, 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 SmCG phase providing new information about the molecular dynamics.

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.

Reentrant orthogonal smectic-A phase below a tilted smectic-C phase in a chiral compound.

A reentrant orthogonal smectic-A (SmA) phase below the tilted smectic-C phase is established in a chiral liquid crystalline compound. The temperature evolution of the layer spacing confirms monolayer structure in both SmA phases, the upper SmA as well as the reentrant SmA phase. The reentrancy of the SmA phase is explained on the basis of the mean field free energy taking into account nonmonotoneous temperature dependence of the lowest term coefficient.

Stripe and line textures in the B2 phase of bent-shape molecules in samples with polar surface anchoring.

In the B2 phase formed by bent-shaped molecules a dense line texture is frequently observed. For the texture description a model is proposed consisting of a periodic system of anticlinic antiferroelectric bulk domains with opposite chiralities separated by π walls in which polarization rotates. The bulk domains are situated between layers of synclinic ferroelectric phase near the upper and lower surfaces. In the surface layers induced by polar anchoring domains of opposite chirality are separated by defect lines. Under the electric field the ferroelectric layer near one sample surface is growing against the antiferroelectric structure in the sample bulk and the stripe texture is fading out, but still in the saturated field surface lines or walls persist separating ferroelectric domains of opposite chirality. The proposed model is adapted also for a case when the anticlinic antiferroelectric structure in the sample bulk is sandwiched between anticlinic ferroelectric layers near the surfaces. In that case the applied electric field eliminates all the lines because the resulting anticlinic ferroelectric structure is racemic, with no chiral domains.

Binary mixtures of liquid crystalline compounds with a reentrant smectic-A* phase.

Binary mixtures of chiral liquid crystalline homologs have been studied. One compound designated 9ZBL exhibited reentrancy of a paraelectric smectic-A* phase, SmA*(RE), below the ferroelectric SmC* phase in the SmA*-SmC*-SmA*RE phase sequence. Stabilization of the SmA(RE) phase is established from studying binary mixtures of 9ZBL with its neighboring homologs 8ZBL and 10ZBL. Compound 8ZBL exhibits only SmA* phase in a wide temperature range and for 10ZBL the SmA*-SmC* phase sequence is observed on cooling. X-ray studies, dielectric spectroscopy, polarization, and tilt angle measurements have been carried out to characterize studied materials. For binary mixtures 9ZBL-10ZBL the reentrant SmA_(RE*) phase is observed for all studied concentrations. For binary mixtures 9ZBL-8ZBL a very small amount of 8ZBL (up to 0.5 mole %) causes disappearance of the SmC* phase. Nevertheless, a small anomaly in the temperature dependencies of the layer spacing, d(T), accompanied by a significant decrease in diffracted x-ray intensity occurs within the SmA* phase for mixtures containing up to 20 mole % of 8ZBL. This anomaly is evidence of the existence of a boundary between the SmA* and SmA(RE*) phases, thus proving their different nature.

Communication: Experimental proof of symmetry breaking in tilted smectics composed of molecules with axial chirality.

For the first time, domains with twisted structures have been established in planar samples of achiral compounds in tilted smectic C phase. This evidences separation of molecular conformers differing in the sense of axial chirality and confirms polar C(2) symmetry of these domains. A simple model considering polar surface anchoring energy and bulk energy of the twist can account for this finding. Conditions for coexistence of twisted and homogeneous domains are discussed.

Twist grain boundary liquid-crystalline phases under the effect of the magnetic field: a complete 2H and 13C NMR study.

A liquid crystal (HZL 7/*) containing an (S)-2-methylbutyl-(S)-lactate unit in the chiral chain, is investigated by means of (2)H and (13)C NMR spectroscopy in order to obtain information on its orientational order, its molecular structure and the effect of external magnetic fields on the supramolecular structure of its phases. This mesogen presents very peculiar mesomorphic properties and exhibits frustrated TGBA* and TGBC* phases in a wide temperature range up to 60 degrees C, as well as an additional phase transition from TGBC(1)* to TGBC(2)*. (2)H NMR measurements show, for the first time, a peculiar magnetic field effect in unwinding the supramolecular structure of both the TGBA* and TGBC* phases. This effect is particularly evident at higher magnetic fields, while different behaviour is observed at lower magnetic fields. This indicates that the supramolecular structure is very sensitive to magnetic fields of the order of 1 Tesla. Moreover, the analysis of the (2)H and (13)C NMR spectra of HZL 7/* allow us to obtain several structural properties, such as the tilt angle of the TGBC* phases and the local orientational order parameters referred to the phenyl and biphenyl fragments. This is the first structural characterization of the frustrated phases of these complexes by means of NMR.