Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

https://iopscience.iop.org/article/10.1143/JJAP.45.597/pdf

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

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.

Liquid crystals of the twenty-first century – nematic phase of bent-core molecules

Considerable progress has been achieved in understanding the fascinating structure and physical properties of the ferroelectric liquid crystalline phases formed by bent-core liquid crystals (BLC). In this review, we discuss a manifold of polar structures and phases found in BLCs such as orthogonal and tilted ferro-/antiferroelectric phases, smectic phases, switchable columnar phases, modulated structures and phases stabilised by a periodic lattice of defects such as dark conglomerate and nanofilament phases. We review the theoretical aspects of ferroelectricity in BLCs including existing microscopic theories and computer simulations, polarity and chirality phenomena. The last part of the paper is devoted to the peculiarities of the behavior of BLCs in a restricted geometry (freely suspended films and filament) and the perspective technological applications.

Polar bent-shape liquid crystals – from molecular bend to layer splay and chirality

https://link.aps.org/accepted/10.1103/RevModPhys.90.045004

Physics of liquid crystals of bent-shaped molecules

Bent-shaped molecules based on the oxadiazole central core with various side wings and terminal chain groups have been synthesized, and their liquid-crystalline behavior was investigated by optical microscopic, X-ray, and electrooptic measurements. These molecules exhibit liquid-crystal polymorphism including both the calamitic and banana phases. Such a characteristic polymorphism is attributable to the larger bend angle of the oxadiazole core compared to that of the resorcinol core used in conventional banana molecules. Only one type of banana phase, designated as the Bx phase, is formed. It appears upon cooling from the nematic and smectic liquid crystals and exhibits chiral domains with a very weak birefringence (apparently optically isotropic). By applying an electric field, the Bx phase is altered to a high-birefringence B2 phase with a homochiral SmCAPA structure that exhibits an antiferroelectric response. From detailed analyses of the optical texture and X-ray patterns through the transformation f...

Low-birefringent, chiral banana phase below calamitic nematic and/or smectic C phases in oxadiazole derivatives.

A series of 1,2-phenylene bis[4-(4-alkoxybenzylideneamino)benzoates has been prepared in order to establish that the nonlinear molecular structure is not necessarily incompatible with mesophase formation. The phases appearing in the methoxy and ethoxy derivatives are classical nematic. While the propoxy to octyloxy derivatives display a smectic A phase, as well as a nematic phase, the higher homologous members are purely smectogenic. Although the introduction of a methyl group to the 3-position of the central phenylene ring eliminates the nematic phase, the thermal stability of the smectic A phase is not much affected. The 4-methylation promotes both the nematic and smectic A phases. A metastable smectic B phase is observed in many members of these three series. The above-mentioned behavior closely resembles that given by the corresponding methyl derivative of phenyl 4-(4-alkoxybenzylideneamino)benzoate.

M. Kuboshita

Papers

Mesomorphic Behavior of 1,2-Phenylene Bis[4-(4-alkoxybenzylideneamino)benzoates