Electric-field-induced transition between the polarization-modulated and ferroelectric smectic- C(S) P(F)* liquid crystalline states studied using microbeam x-ray diffraction

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.

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.

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.

Tension of freely suspended fluid filaments

Stable fluid filaments with diameters of several micrometers and slenderness ratios well above 1000 are unique objects formed by some liquid crystalline phases of bent-core mesogens. We present a technique to determine filament tensions from their deflection under defined loads. A strong temperature dependence is observed, with a minimum near the clearing temperature. Both the nonlinear relation between filament tension and diameter and the substantial increase of the tension with lower temperatures indicate contributions of volume terms, in addition to surface capillary forces. We discuss a model that relates these bulk terms to elastic forces, originating from the undulated smectic layer structure. This model can explain the origin of the filament stability.

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

We report a generalized scenario for the formation of modulated smectic phases of bent-core molecules based on locally ferroelectric layering and spontaneous splay of the polarization. Twelve phases are proposed, distinguished by neighboring splay stripes with either syn- or antiorder of the polarization and undulation slope, in addition to layer continuity versus layer discontinuity at the intervening defects. We outline the experimental techniques necessary to differentiate among the phases and interpret previous results in the present context, using high resolution x-ray scattering diffraction and block and undulation models of the layer organization to distinguish among the three 2D lattice types which emerge.

Quantitative analysis of the x-ray diffraction intensities of undulated smectic phases in bent-core liquid crystals

X-ray diffraction diagrams of undulated smectic phases in bent-core liquid crystals have been theoretically studied. The intensities of the reflections have been obtained for different layer modulations, and a general expression has been deduced for orthogonal cells in terms of the different harmonics of the distortion. The case of sinusoidal modulation is especially simple and has been studied also in oblique cells. High-quality x-ray measurements of three compounds reported in the literature have been analyzed as examples. In all cases it has been deduced that the modulation is sinusoidal and its amplitude has been easily obtained by fitting the experimental intensities. Equatorial (h0) reflections have been also considered to obtain information about the structure of defects at the maxima and minima of the undulation.

Modulated structures in bent-core liquid crystals: two faces of one phase

The Landau-de Gennes type continuous model is used to describe the 2D modulated structures made of bent-core liquid crystals. It is shown that modulation is a direct consequence of the chiral symmetry breaking. Structures with synclinic and anticlinic layer tilt are found to coexist in the polarization modulated and layer undulated (PM-LU) liquid crystal phase. The synclinic PM-LU structure corresponds to the columnar B(1RevTilted) phase. Experimentally obtained electron density maps are included, which support theoretical findings.

Several Types of Bilayer Smectic Liquid Crystals with Ferroelectric and Antiferroelectric Properties in Binary Mixture of Dimeric Compounds

The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, alpha,omega-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentane (mOAM5AMOm), by optical microscopy, X-ray diffraction, polarization switching, and second-harmonic generation measurements. One compound is 4OAM5AMO4 with a short terminal alkyl chain that forms a single-layer smectic phase (SmCAs) with a random mixing of spacer and tail groups. Another compound is 16OAM5AMO16 with a long terminal alkyl chain that forms a chiral, anticlinic, and antiferroelectric bilayer phase (SmCAb) with the bent molecules tilted to the bilayer. By mixing these two compounds, the SmCAs phase of 4OAM5AMO4 is easily destabilized, leading to the wide content region of the bilayer phases. In the bilayer regime, three other smectic phases are newly induced. Two of them are antiferroelectric and ferroelectric phases in which the molecules lie perpendicularly with respect to the layer. The other shows no polar response to an external electric field and behaves like a smectic A. The new appearance of these bilayer phases is discussed as a mixing effect of long and short tail groups.

Interpretation of the odd-even behavior for the emergence of ferroelectricity and antiferroelectricity in bent-core mesogens

Simple theoretical interpretation has been made on the previously reported odd-even behavior of the emergence of ferroelectricity and antiferroelectricity in homologous series of bent-core mesogens with the same chiral end chains (S,S) ; compounds with even and odd carbon numbers (including oxygen) exhibit ferroelectric and antiferroelectric B2 phases, respectively. The odd-even behavior was confirmed in newly synthesized racemic compounds with chiral end chains of R and S forms (R,S) together with (R,R) and (S,S) forms. According to our theoretical interpretation, ferroelectricity should be more stable in (R,S) compounds than in (S,S) compounds. Actually, the transition temperature from the isotropic phase to the Sm-CP phase was higher and the temperature range of the B2 phase was broader in the (R,S) compound than in the (S,S) compound.

Distinct layered structure with density modulation in solid phase formed from B2 phase of banana molecules

A unique two-dimensional (2D) long-range structure has been observed in a low-temperature phase X1 for a banana molecule having bromine atom substituted on the central core using synchrotron radiation (SR) x-ray scattering measurements. The X1 phase is formed from the B2 phase with the SmC(A)P(A) structure upon cooling and then shows multiple reflections around the first layer line, which are interpreted as a peculiar frustrated structure with long-range layer modulation order. Furthermore, the observation of a well-defined (100) reflection with a spacing of 171A means that there is the electron density modulation along the layers. By coupling these reflections, a 2D lattice with a=173 A, c=53.4A, and beta=81.1 degrees, is determined where the a axis is parallel to the original layer of the phase. This unique structure with modulation can be interpreted as an undulated layer structure and suggested to be the result from deformation with polarization splay defects periodically occurring along the layer. The angle, beta=81.1 degrees, between a and c axes indicates that the position of splay defects in one layer is staggered from that in the neighboring layer. In other words, the splay defect lines run in a direction tilted by roughly 80 degrees with respect to the a axis.

Structure characterization of free-standing filaments drawn in the liquid crystal state

Stable free-standing liquid filaments formed by some layered mesophases of bent-core mesogens are unique structures. Some of their physical properties have been analyzed in recent studies, but their microscopic structure and conditions for stability have still been unclear. We explore details of filament shapes and surface profiles of filaments drawn in liquid crystal phases of bent-core mesogens by AFM and SEM measurements, and we present a microscopic structure model. Conclusions on the stabilizing mechanisms are drawn. Qualitative differences in mechanical properties are found for different mesophases, even though the macroscopic appearance of the filaments is very similar.

Polarization modulation instability in liquid crystals with spontaneous chiral symmetry breaking

We present a theoretical model which describes the polarization-modulated and layer-undulated structure of the B7 phase and gives the phase transition from the synclinic ferroelectric smectic-C(S)P(F) phase to the B7 phase as observed experimentally. The system is driven into the modulated phase due to the coupling between the polarization splay and the tilt of the molecules with respect to the smectic layer normal. The modulation wavelength and the width of the wall between two domains of opposite chirality are estimated.