Synthesis, characterization and theoretical considerations of a novel class of banana-shaped compounds with liquid crystal properties
TL;DR: In this paper, two bis(acylated) resorcinol derivatives incorporating two and four additional benzene rings, respectively, were synthesized, and valuable liquid crystalline properties were detected.
Abstract: As analogues of banana-shaped compounds, two novel series of bis(acylated) resorcinol derivatives incorporating two and four additional benzene rings, respectively, were synthesized. In the first series of compounds resorcinols were acylated with 4-alkoxycinnamoyl chlorides, whereas in the second series, 1,3-bis(4-hydroxybenzoyloxy)-4-chlorobenzene was prepared first, and subsequently, 4-alkoxycinnamoyl groups were introduced. In the cases of five-ring compounds 34 – 39 , valuable liquid crystalline properties were detected. Nematic phases could be observed within a wide range of temperatures. Moreover, of these compounds, the hexadecyloxy derivative displayed a short monotropic smectic phase with a latticed texture. Molecular modeling studies have indicated that compounds of five-ring structures of type 34 – 39 may have energy minima in which the three planar building moieties of the central bisacylated resorcinol unit are not necessarily coplanar.
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25 Jun 2013
TL;DR: A comprehensive review of the main research directions and results can be found in this article, where the authors describe the properties of smectic nano clusters often observed in BCNs, and discuss rheological properties that reveal unusually large viscosities and small twist and bend elastic constants.
Abstract: Bent-core (BC) materials that form nematic liquid crystals (BC nematics (BCNs)) are known only since 2000, but since then they have been intensively studied. In this review we give a comprehensive summary of the main research directions and results. After discussing representative examples of molecules making BCNs, we review theoretical predictions and experimental findings about macroscopic symmetries, such as chirality of achiral and chiral molecules, tetrahedratic symmetry and phase biaxiality versus uniaxiality. Then we describe the properties of smectic nano clusters often observed in BCNs, and discuss rheological properties that reveal unusually large viscosities and small twist and bend elastic constants. Finally, we focus on the electric properties of BCNs, such as dielectric and ferroelectric properties, electroconvection (EC) and electromechanical effects.
148 citations
TL;DR: Three nonstandard electrohydrodynamic instabilities in nematic liquid crystals composed of bent-core molecules that create a pattern with stripes parallel to the initial director orientation and are distinguished by different threshold behavior are characterized.
Abstract: We characterize three nonstandard electrohydrodynamic instabilities in nematic liquid crystals composed of bent-core molecules In addition to their shape, another important attribute of this material is that the anisotropy in the electrical conductivity changes sign as the frequency of the applied electric field changes These instabilities do not appear to fit within the standard model for electroconvection The first instability creates a pattern with stripes parallel to the initial director orientation, with a wavelength about equal to the separation of the cell plates The next is the previously reported prewavy instability The third instability is optically and dynamically identical to the prewavy instability, but is distinguished by different threshold behavior
122 citations
TL;DR: The dielectric fluctuations in the uniaxial nematic phase of a bent-core liquid crystal have been studied by dynamic light scattering, suggesting that the bent core nematogenic units may be microscopic smectic domains ("cybotactic" clusters).
Abstract: The dielectric fluctuations in the uniaxial nematic phase of a bent-core liquid crystal have been studied by dynamic light scattering. Polarization selection of the scattering cross-section reveals one mode due to ordinary director fluctuations and, in the lower part of the nematic phase, a second mode attributable to fluctuations of the biaxial order parameter. The director fluctuations are approximately 100 times slower than observed in typical nematics based on straight-core molecules, suggesting that the bent core nematogenic units may be microscopic smectic domains ("cybotactic" clusters).
89 citations
TL;DR: In this paper, it was shown that bent-core liquid crystalline materials exhibit non-Newtonian flow in their optically isotropic liquid phase, and that this behavior is due to the existence of nanostructured, fluctuating clusters composed of a few smectic-like layers.
Abstract: We show that bent-core liquid crystalline materials exhibit non-Newtonian flow in their optically isotropic liquid phase. We conjecture that this behavior is due to the existence of nanostructured, fluctuating clusters composed of a few smectic-like layers. Shear alignment of these clusters explains the shear thinning observed in bent-core liquid crystals having either a nematic phase or non-modulated smectic phase. By contrast, smectogens having a modulated smectic phase do not shear thin at low shear rates, but even show a slight shear thickening which may be due to entanglements of wormlike and/or helical clusters.
69 citations
TL;DR: Homogeneously aligned samples of an oxazole heterocyclic ring in the central core are found to behave more like a conventional straight-core nematic than a previously investigated ester-based class of bent-core molecules that have a benzene ring as the core linkage between the two arms of the bow-shaped molecule.
Abstract: Various optical and dynamic light scattering studies have been conducted on the nematic phase of an oxazole-derived bent-core liquid crystal. At optical length scales and in the absence of applied fields, homogeneously aligned samples of this material, which has an oxazole heterocyclic ring in the central core, are found to behave more like a conventional straight-core nematic than a previously investigated ester-based class of bent-core molecules that have a benzene ring as the core linkage between the two arms of the bow-shaped molecule. In particular, the nematic refractive indices of the oxazole compound combine in the standard way @i.e., A(2no 1ne )/3] to match the isotropic value throughout the nematic range, and the observed director fluctuation modes have relaxation rates comparable to those of the usual thermotropics. However, polarized light scattering data reveal evidence of weak biaxial fluctuations, and indications of electric-field-induced biaxiality are observed in the refractive index measurements.
67 citations
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TL;DR: Ferroelectric smectic C (FSC) liquid crystals are used in a simple new geometry that allows the spontaneous formation of either of two surface-stabilized smective C monodomains of opposite ferroelectric polarization as discussed by the authors.
Abstract: Ferroelectric smectic C (FSC) liquid crystals are used in a simple new geometry that allows the spontaneous formation of either of two surface‐stabilized smectic C monodomains of opposite ferroelectric polarization. These domains are separated by well‐defined walls which may be manipulated with an applied electric field. The resulting electro‐optic effects exhibit a unique combination of properties: microsecond dynamics, threshold behavior, symmetric bistability, and a large electro‐optic response.
2,312 citations
TL;DR: In this article, the synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer.
Abstract: The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the C2v symmetry of the molecular packing.
1,173 citations
TL;DR: A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt.
Abstract: A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. These instabilities combine to form a chiral layer structure with a handedness that depends on the sign of the tilt. The bulk states are either antiferroelectric-racemic, with the layer polar direction and handedness alternating in sign from layer to layer, or antiferroelectric-chiral, which is of uniform layer handedness. Both states exhibit an electric field-induced transition from antiferroelectric to ferroelectric.
1,041 citations
839 citations
TL;DR: In this paper, anomalies of vitesse observee ne peut etre totalement expliquee par la theorie des dislocations were investigated. André et al. effectue entre 100 and 273 K des mesures d'attenuation and de vitesness d'ondes ultrasonores de frequence 5 MHz sur des echantillons monocristallins de glace.
Abstract: Nous avons effectue entre 100 et 273 K des mesures d'attenuation et de vitesse d'ondes ultrasonores de frequence 5 MHz sur des echantillons monocristallins de glace ayant subi une deformation plastique. Des traitements thermiques a 271 K restaurent une partie des caracteristiques de propagation. L'anomalie de vitesse observee ne peut etre totalement expliquee par la theorie des dislocations.
149 citations