Measurement of the physical properties of the nematic phases formed by bent-core molecules has proven rather chal- lenging because of the high temperatures at which the nemat- ic phase often occurs (> 150 8C) and because it has been diffi- cult to obtain the high-quality monodomain alignment neces- sary for robust measurements. Nonetheless, reports of the elas- tic constants, dielectric behaviour, flexoelectric coefficients and unusual electro-optic behaviour are growing. Herein, we review the physical properties of nematic phases formed by bent-core liquid crystals. The oxadiazole-based materials are used as exemplars, though some other systems are also de- scribed. The paper is organised as follows. Section 2 describes the influence of molecular structure on the nematic-phase range in bent-core liquid crystals. As has already been men- tioned, a key issue has been to reduce the temperatures at which the nematic phase is exhibited and we describe struc- tural modifications that result in significantly lower tempera- ture nematic phases. Section 3 describes the optical properties, dielectric behaviour, elasticity and flexoelectricity in bent-core nematic liquid crystals. Finally, Section 4 considers some of the unusual electro-optic behaviour reported for these com- pounds.
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In recent years the new inhomogeneous nematic phases have been observed [1–13], and the study of the detailed structure of these phases is currently the most topical issue in liquid crystals research. The experiments for oligomers and bent–core systems indicate that there are at least two types of modulated nematic structures with one-dimensional periodicity. One of them is the so called twist–bend nematic phase (N TB ) in which the director is assumed to precess on the cone  at
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As in all LC phases the arrangement of the mole- cules is of paramount importance if molecular struc- tural properties are to be translated into macroscopic effects. In this respect SHG in homogeneous uniaxial nematic phases has been shown to be a weak effect (with typical SH intensities of the order of 10 –5 –10 –6 times that of quartz) due to such phases being centro- symmetric and thus having a macroscopic, second- order non-linear susceptibility, χ (2) equal to zero. The weak SH intensity observed in the nematic phase arises from the local breaking of the inversion symmetry so that locally the second-order non-linear susceptibility is non-zero. The local breaking of the inversion symmetry is partly caused by surface interactions [17,18] but, in the main, by nematic director fluctua- tions in the bulk LC which induce a flexoelectric polar- isation, leading to the generation of SH light. As the flexoelectric polarisation varies in accordance with the thermal director fluctuations, the SH light is emitted omni-directionally (i.e. is scattered and is
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the sign of ε (Ref. ). These effects were explained by a strain-induced anisotropy of the exchange interaction, which preferentially orients the nematic density modulation along either 01¯1 or 011 depending on the sign of ε. Koduvayur et al. argued that, since GaAs is a piezoelectric material, an out-of-plane electric field results in an in-plane strain, so the orientation of the nematic phases in unstrained devices could be related to asymmetries in the confining potential . The experiments of Ref.  showed no effect of the confining potential asymmetry on the nematic orientation in GaAs 2DESs. However, the predicted strain-induced anisotropy of the Harteee-Fock energy is two orders of magnitude larger for holes than electrons . In our samples the anisotropy is maximized for average electric fields |E ⊥ | < 10 4 V/cm,
with a large number of conformers with similar energies (e.g. disk-like, rod-like, t- shaped structures). Polymesomorphism was observed in these structures where short chain (e.g. ethyl and butyl) derivatives exhibited discotic nematic phases and longer chains (e.g. dodecyl) gave smectic A and columnar lamellar phases due to increased microphase segregations between the cores and chains of neighboring molecules. Triazines possessing an intermediate chain length (e.g. octyl) exhibited the all three nematic, smectic and columnar phases. Clearly, the layered packing of rod-like conformers were preferred over columnar packing of disk-like conformers. In addition, the polymesomorphism of these structures allowed us to propose that discotic nematic phases are almost in all cases exhibited by molecules whose core volumes are equal to or larger than the volume of the peripheral side-chains. Finally, in addition to the unique phase sequence, these molecules were found to have very low LUMO energies (-2.97 eV) and exhibit aggregation induced emission between 450 and 700 nm in the crystalline and liquid crystalline state.
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the presence of additional transverse dipole of benzoate. Further the length of flexible spacer determines the nature of mesophase with long spacers favouring smectic phases and shor t spacers favouring nematic phases. In the present case the mesogen is attached to the back bone of the substituent that is supporting the formation of crystal G phase. The benzoate family of the present series is attributed to the formation of crystal phase by reducing transition temperatures low enough to maintain layered arrangement and also by inverse stability of hydrogen bonding vs chain length. This intermolecular hydrogen bonded carboxylic acids aid the formation of mixtures that exhibit liquid crystalline nature due to dynamic nature of non covalent interaction. This provides possible explanation for the for mation of cr ystal G phaseThermal study shows smooth multi colored mosaic texture of crystal G phase in the complexes. There is simultaneous quenching of the nematic phase and smectic C phase in homologous series. In comparison with the family of benzoates the narrow thermal span results in the increase in the chain of benzoates. Further increase in the chain of benzoate loose the formation of crystal G phases. The significant contribution is due to the intermolecular hydrogen bonding between the C=O, C-O and OH of acid along with the C-O and OH of benzoate. These contributions significantly reflect the strong intermolecular hydrogen bonding between the two polar groups. The thermodynamic behavior illustrates the formation of crystal G phase, which is supported by the increased enthalpies. Hydrogen bonding by IR measurements and thermodynamic behaviour aid the formation crystal G phase.
A system of soft ellipsoid molecules confined between two planar walls is studied using classical density-functional theory. Both the isotropic and nematic phases are considered. The excess free energy is evaluated using two different Ansa¨tze and the intermolecular interaction is incorporated using two different direct correlation functions 共 DCF’s 兲 . The first is a numerical DCF obtained from simulations of bulk soft ellipsoid fluids and the second is taken from the Parsons–Lee theory. In both the isotropic and nematic phases the numerical DCF gives density and order parameter profiles in reasonable agreement with simulation. The Parsons–Lee DCF also gives reasonable agreement in the isotropic phase but poor agreement in the nematic phase. © 2004 American Institute of Physics. 关 DOI: 10.1063/1.1703522 兴
We have successfully synthesized and characterized benzothiazole based Schiff’s base ligands and their copper complexes. The chemical constitution of the ligands and their molecular structure resembles a rod like molecule exhibiting enantiotropic nematic phases. This could be due to the butyl substituent at terminal position, similar to the terminal methoxy group showing nematic phases as reported by S T Ha et.al 9 and A K prajapathi et.al.
the phase diagrams shown in Figs. 4 and 5, respectively. The qualitative structure of the phase diagrams is similar to the one obtained by the previous authors 共see, for ex- ample, 关20,24兴兲 using a phenomenological quadrupole poten- tial. In all cases the biaxial nematic phase separates two dif- ferent uniaxial nematic phases which can be called calamitic and discotic nematic phases, respectively. There exists also a “triple” point where all three different nematic phases are in equilibrium with each other and with the isotropic phase. Numerical calculations indicate that in a rather broad vicinity of the triple point the discontinuity of the isotropic-uniaxial nematic phase transition is very small or virtually absent. This apparently originates from the growing molecular biaxi- ality associated with the increasing order parameter D. It should also be noted that at the triple point the intermolecu- lar interaction potential is characterized by the higher D 4h
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1. Introduction. Consider a thin layer of nematic liquid crystalline ﬂuid sandwiched between two parallel glass plates separated by a gap of width 2d. Suppose it is subjected to a large magnetic ﬁeld aligned in the direction normal to the plates. The dynamics of the solution is then essentially one dimensional , and is well described by the director angle θ(z, t), which is the average angle a rod-like nematic liquid crystal molecule forms with the plane of the plates, and by the ﬂow speed v(z, t) parallel to the plates. Here z ∈ (−d, d) is the coordinate in the direction of the normal. We assume that the
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IULFWLRQ LH YLVFRVLW\ H[SHULHQFHG E\ DQG φ motions of the probe. Oxazine 4 is seen to correlate strongly with the order of the liquid crystal host. Under these circumstances, the rotational diffusion of the probe should exhibit a similar asymmetry to that of the nematic host in which the bulk viscosity is highly anisotropic 19 . A slowing of the molecular spinning rate with increasing temperature has been inferred from changes in Raman band shapes for the nematic phase of pure OET 20 . This behaviour was qualitatively attributed to a breakdown in local cylindrical symmetry in the vicinity of the nematic-isotropic phase transition. The breakdown in cylindrical symmetry giving rise to so-called “biaxial fluctuations” due to the uncorrelated motion of different segments (tails) of individual nematogens whose interference acts to increase the friction (viscosity) experienced for φ diffusion. Evidence for an increase in viscosity in the approach to T NI is provided by recent measurements of translational (mass) diffusion in pure
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character dispersing from - ¯ M—the holelike dispersions in Fig. 2(e). We suggest that both bands contribute to the measurement at 60 K in the tetragonal phase in Fig. 2(b)(i), but they are unresolved since their separation is less than the energy resolution; the complex line shape of the EDC observed in Fig. 2(g) with a “shoulder” at low binding energy hints that this is likely to be the case. Then, in the nematic phase, the one-ellipse structure kicks in, and correspondingly the dispersion at lower binding energy is observed along - ¯ M X and the deeper dispersion is observed along - ¯ M Y .
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on one side with the embossed photopolymer grating. Cell gaps were measured using reflection spectrometry. Each completed cell was filled with the liquid crystal 4-cyano-4Õpentyl biphenyl (5CB), chosen for the wealth of characterisation data available through literature. The use of an opposing homeotropic surface gives the cell hybrid-aligned nematic (HAN) and vertically- aligned nematic (VAN) as the bistable configurations for the D and C states, respectively, figure 1. The two states are readily discriminated using polarising optical microscopy, with the grating aligned 45¡ to the crossed polarisers to give a bright D state and dark C state. Each cell was capillary filled at room temperature in the direction parallel to the grating grooves. The area of the grating that spontaneously formed the D and C states was recorded, before heating into the isotropic phase and cooling back into the nematic phase to form the uniform D state.
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For confirming the validity and the accuracy of the system, we first make measurements on a prototype calamitic liquid crystal 5CB. The contribution of surface polarization from both surfaces of the HAN cell is different. The pyroelectric signal is independent of the direction of irradiation . The pyroelectric coefficient is measured by altering temperature by T , a small step in temperature of the cell as a result of heating from the light source. The temperature of a cell is varied over the entire temperature range of the nematic phase. We find the magnitude of P f as ∼0.04 nC/cm 2 at
One way to avoid the divergence in the free-energy is to excise from the system a small volume including the singularity, and then derive the static and dynamic properties of the disclinations by taking the limit as the excised volume goes to zero . Nevertheless, the complete physical description of small regions around the singularities requires an extension of Frank-Oseen-Zocher theory [28, 36, 17], obtained by replacing the director order parameter with the tensor of second- order moments of the local probability distribution of nematic molecules [12, 14]. As a first generalization of the classical theory, it has been shown that the structure and dynamical properties of point  and line [31, 20] defects are deeply influenced by the reduction of the degree of orientation, even if one assumes that the nematic remains everywhere uniaxial, and that it becomes isotropic on the defect. Furthermore, numerical and theoretical studies [23, 35, 6, 25, 26] of the core structure have proved that inside the core of a disclination the nematic not only decreases its degree of orientation, but it actually abandons the uniaxial phase, by becoming biaxial in a small, but finite, region surrounding the defect. Recently, tremendous interest in the structure and properties of liquid crystal disclinations has arisen, partly because they can be thought of as laboratory analogues of cosmological structures [11, 8], partly because despite their experimental visibility they are nevertheless extremely complex to describe.
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Colloidal dispersions of small particles in nematic liq- uid crystals are a novel, interesting type of soft matter. The difference from ordinary colloids arises from the ori- entational ordering of the liquid crystal molecules and the resulting structure in the colloid. Topological defects [1, 2, 3, 4] and additional long-range forces between the colloidal particles  are immediate consequences of this ordering. The nematic-induced interparticle interaction brings a new range of effects to the system: supermolec- ular structures [6, 7, 8, 9], cellular structures [10, 11], and even a soft solid  can be observed. Colloidal dispersions in liquid crystals also have a wide variety of potential applications .
structure were were characterized by 1 H-NMR and IR as well as elemental analysis . The mesomorphic properties of these compounds were investigated via differential scanning calometry and polarizing microscopy . . The thermal data indicate that all of these compounds exhibit mesomorphic properties (Nematic phase).The group efficiency order for the Nematic phase thermal stability can be derived for the compounds as : NO 2 >OCH 3 >CH 3 >Cl, With
Ever since Brown discovered the motion of inanimate pollen grains, material scientists have been fascinated by the vivid, life-like motion of colloidal particles. Indeed, the study of colloidal interactions has led to the discovery of new physics and has fueled the design of functional materials [22, 69, 116]. External applied fields provide important additional de- grees of freedom, and allow microparticles to be moved along energy gradients with exquisite control. In this context, nematic liquid crystals (NLCs) provide unique opportunities . Within these fluids, rod-like molecules co-orient, defining the nematic director field . Gradients in the director field are energetically costly; by deliberately imposing such gradi- ents, elastic energy fields can be defined to control colloid motion. Since NLCs are sensitive to the anchoring conditions on bounding surfaces [8, 87], reorient in electro-magnetic fields [10, 29], have temperature-dependent elastic constants  and can be reoriented under illumination using optically-active dopants [57, 70], such energy landscapes can be imposed and reconfigured by a number of routes.
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to a non-zero angle θ(x, z, t) with the x-direction. This occurs because the position of the fringes are displaced when the optical path length in one arm of the interferometer changes. The material E7 is a positive uniaxial medium and so this reorientation of the n-director reduces the refractive index experienced by an electromagnetic wave travelling in the z- direction and polarized in the x-direction. A fringe is displaced in position by one fringe spacing when the total optical path length through the nematic liquid crystal layer k⋅d is equal to 2π radians. The wavevector is given by k = 2πn eff (x)/λ O where n eff equals the average refractive index for a z-section through the layer at the position x.
To form a liquid crystalline (LC) gel that retains the ability to respond rapidly to applied fields, it is necessary to work with low polymer concentrations. In turn, to form a dilute polymer network it is necessary to use very long polymers that are soluble in the small molecule LC. This research focuses on the synthesis of ultra-long side-group liquid crystalline polymers (SGLCPs), their properties when dissolved in nematic hosts, and the self-assembly of a nematic gel using an ABA triblock with an SGLCP midblock and LC- phobic end-blocks. Typically, LCs are made from small molecules that can be quickly reoriented. In applications such as artificial muscles, flexible displays, or compensating films, a more robust LC gel is desired. Prior routes to LC gels, typically using in situ polymerization, suffer from director misorientation, lack of control over cross-link density, polymer network inhomogeneity, undesired phase separation, and slow responses to applied fields. The present research (at the intersection of block copolymers, gels, and LCs) has demonstrated that an optically uniform LC gel with fast reorientational response can be achieved using a self-assembling ABA triblock copolymer.
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