Short bent-core molecules: X-ray, polarization, dielectricity, texture and electro-optics investigations.

Bent-core liquid crystals based on 1,2,4-oxadiazole as a central unit have been the first mesogens to exhibit a ferroelectric response in the nematic phase. This behavior has been widely recognized as due to the presence of smectic-like polar cybotactic clusters permeating the nematic phase. Unfortunately, these compounds exhibited rather high melting points, about 120 °C, due to the presence of four benzene rings in the molecules. Here we describe the synthesis and physical characterization of a new series of BC mesogens, featuring the same bent core as the previous compounds but shorter outer substituents. By keeping only two benzene rings, we were able to lower the melting points to about 70 °C. However, while X-ray diffraction and dielectric spectroscopy measurements confirm the cybotactic nature of the nematic phase of these compounds, polarization and electro-optical measurements ascribe their polar response to flexoelectricity rather than to spontaneous polarization. Finally, texture investigation suggests the biaxiality of the nematic phase, which is indicated also by conoscopic measurements. These results are important for recognizing size and rigidity limitations in designing bent-core liquid crystal molecules suitable for applications.

Phase behavior and characterization of heptamethyltrisiloxane-based de Vries smectic liquid crystal by electro-optics, x rays, and dielectric spectroscopy.

A heptamethyltrisiloxane liquid crystal (LC) exhibiting I-SmA^{*}-SmC^{*} phases has been characterized by calorimetry, polarizing microscopy, x-ray diffraction, electro-optics, and dielectric spectroscopy. Observations of a large electroclinic effect, a large increase in the birefringence (Δn) with electric field, a low shrinkage in the layer thickness (∼1.75%) at 20 °C below the SmA^{*}-SmC^{*} transition, and low values of the reduction factor (∼0.40) suggest that the SmA^{*} phase in this material is of the de Vries type. The reduction factor is a measure of the layer shrinkage in the SmC^{*} phase and it should be zero for an ideal de Vries. Moreover, a decrease in the magnitude of Δn with decreasing temperature indicates the presence of the temperature-dependent tilt angle in the SmA^{*} phase. The electro-optic behavior is explained by the generalized Langevin-Debye model as given by Shen et al. [Y. Shen et al., Phys. Rev. E 88, 062504 (2013)10.1103/PhysRevE.88.062504]. The soft-mode dielectric relaxation strength shows a critical behavior when the system goes from the SmA^{*} to the SmC^{*} phase.

Design and investigation of de Vries liquid crystals based on 5-phenyl-pyrimidine and (R,R)-2,3-epoxyhexoxy backbone.

Calamitic liquid crystals based on 5-phenyl-pyrimidine derivatives have been designed, synthesized, and characterized. The 5-phenyl pyrimidine core was functionalized with a chiral (R,R)-2,3-epoxyhexoxy chain on one side and either siloxane or perfluoro terminated chains on the opposite side. The one involving a perfluorinated chain shows SmA^{*} phase over a wide temperature range of 82 °C, whereas the siloxane analog exhibits both SmA^{*} and SmC^{*} phases over a broad range of temperatures, and a weak first-order SmA^{*}-SmC^{*} transition is observed. For the siloxane analog, the reduction factor for the layer shrinkage R (relative to its thickness at the SmA^{*}-SmC^{*} transition temperature, T_{AC}) is ∼0.373, and layer shrinkage is 1.7% at a temperature of 13 °C below the T_{AC}. This compound is considered to have "de Vries smectic" characteristics with the de Vries coefficient C_{deVries} of ∼0.86 on the scale of zero (maximum-layer shrinkage) to 1 (zero-layer shrinkage). A three-parameter mean-field model is introduced for the orientational distribution function (ODF) to reproduce the electro-optic properties. This model explains the experimental results and leads to the ODF, which exhibits a crossover from the sugar-loaf to diffuse-cone ODF some 3 °C above T_{AC}.

Flexoelectric polarization studies in bent-core nematic liquid crystals.

The flexoelectric polarization (Pf) of four bent-core nematic liquid crystals (LCs) has been measured using the pyroelectric effect. Hybrid aligned nematic cells are fabricated for measuring the pyroelectric response over the entire range of the nematic phase. It is found that the magnitude of flexoelectric polarization Pf and the sum of the flexoelectric coefficients |e1+e3| for the bent-core LCs studied here are three to six times higher than for the calamitics. Pf is found to depend on the transverse dipole moment of LC molecules. However, |e1+e3| values are by no means giant as |e3| alone had been reported for a bent-core nematic system previously. The dependence of the sum of "splay and bend flexoelectric coefficients" is discussed in terms of the shape of the molecule and of the dipole moment directed normal to the molecular axis.

Spontaneous periodic deformations in nonchiral planar-aligned bimesogens with a nematic-nematic transition and a negative elastic constant.

Hydrocarbon linked mesogenic dimers are found to exhibit an additional nematic phase below the conventional uniaxial nematic phase as confirmed by x-ray diffraction. The phase produces unusual periodic stripe domains in planar cells. The stripes are found to be parallel to the rubbing direction (in rubbed cells) with a well-defined period equal to double the cell gap. The stripes appear without external electromagnetic field, temperature or thickness gradients, rubbing or hybrid alignment treatments. Simple modeling proposes a negative sign for at least one of the two elastic constants: splay and twist, as a necessary condition for the observed pattern.

Experimental study of de Vries properties in antiferroelectric smectic liquid crystals.

Results of the experimental study on different antiferroelectric liquid crystal (AFLC) materials are presented using a number of techniques such as the optical birefringence, electro-optics and the measurements of optical thickness of free-standing films. Despite differences in the molecular structures of the various AFLC materials studied, these are found to exhibit a de Vries type of smecticA (SmA) properties in a temperature range higher than SmC. This correlation leads to the conclusion that these two classes of liquid crystals are related to each other. Furthermore, we suggest that these arise from the same physical mechanism, namely the existence of the weak synclinic (or reduced anticlinic) correlations between the neighbouring molecular tilt directions.

Experimental study of high-temperature smectic- C_{FI2}{ *} phase in chiral smectic liquid crystals that exhibit phase-sequence reversal.

We report the results of an experimental study of a recently observed phase sequence reversal of smectic-C_{FI2}{ *} [ SmC;{ *}(q_{T}=1/2); a four layer antiferroelectric] phase appearing in the temperature range above the smectic-C{ *} (SmC;{ *}) phase from the results of optical birefringence, spontaneous polarization, selective reflection, conoscopy, and dielectric spectroscopy. The SmC_{FI2}{ *} phase is observed in an antiferroelectric liquid crystalline compound, 10OHF, in a temperature range above that of SmC{ *} phase and is found to be thermodynamically monotropic, i.e., it appears only upon cooling from SmC_{alpha}{ *} phase. This is also unstable as if it is once transformed to SmC{ *} by the application of the bias, it does not return to its original state unless the sample is heated and cooled again in the absence of the bias. Nevertheless this phase is stabilized by the addition of a chiral smectic compound 9OTBBB1M7 (abbreviated as C9), having a wide temperature range of the SmC_{FI2}{ *} phase. The temperature range of the low temperature SmC{ *} decreases with increase in the concentration of C9 and for a concentration of 55 wt. %, SmC{ *} disappears and the transition takes place directly from SmC_{FI2}{ *} to the crystalline phase on cooling. The existence of such a high-temperature SmC_{FI2}{ *} phase is also supported by a phenomenological model.

Electro-optic and dielectric study of the de Vries-type smectic-A* phase exhibiting transitions to smectic-C*A and smectic-C* phases.

Mixtures of different compositions of an antiferroelectric liquid crystal compound that exhibits direct smectic-A*(Sm-A*)-smectic-C*(A) (Sm-C*(A)) transition with a ferroelectric liquid crystal compound that exhibits Sm-A*-smectic-C*(Sm-C*) transition are studied using electro-optics and dielectric spectroscopy. The results of optical texture, birefringence, and the tilt angle suggest that a part of the Sm-A* phase is of de Vries type, since an increase in the tilt angle with decreasing temperature results in a reduction in the value of the birefringence in the Sm-A* phase, whereas the birefringence at Sm-A* to Sm-C* transition goes up by 12.7%. The soft mode relaxation strength, the Landau coefficient of the temperature dependent term, and the other related parameters of the de Vries-type Sm-A-Sm-C*(A) and Sm-A*-Sm-C* transitions are determined using the Landau theory of the second-order phase transition. For the Sm-A*-Sm-C* transition, we find that the soft mode relaxation strength decreases, the Landau coefficient increases, and the Curie-Weiss temperature range decreases with an increased ferroelectric composition in the mixture. These observations can be explained by assuming that with increased ferroelectric composition in the mixture, the layer shrinkage at the de Vries Sm-A*-Sm-C* transition increases. On comparing the results of de Vries-type Sm-A* to Sm-C*(A) and Sm-C* transitions, we find that the soft mode dielectric strength and the other related Landau parameters of the de Vries Sm-A* phase are of the same order of magnitude for transitions from Sm-A* to Sm-C* and to Sm-C*(A) except for the composition of the mixture where both Sm-C* and Sm-C*(A) transitions are stable and the phase diagram shows phase sequence Sm-A* to Sm-C* to Sm-C*(A).

Discontinuous change in the smectic layer thickness in ferrielectric liquid crystals.

The temperature dependence of the thickness of thick free-standing films is studied using a high-resolution film thickness measurement technique. A small discontinuity in the temperature dependence of the smectic layer thickness at every phase transition between ferro-, ferri-, and antiferroelectric phases is observed. We show that the major contribution to it arises from a change in the smectic tilt angle.

Comparison of the characteristics of the chiral analog of the de Vries type of smectic-A* phase.

In this paper we compare the results of three ferroelectric materials that exhibit unusual smectic-A (SmA) phases. These phases have been assigned to chiral analogs of the de Vries SmA phase. Several experimental techniques have been employed for this investigation. The possible molecular structures of de Vries SmA* phases are discussed.