Biaxial liquid crystals smectic

A survey of recent studies of biaxial liquid crystals, in the context of the reported biaxial nematic phase in low molecular mass bent-core mesogens, with emphasize on the use of NMR spectroscopy is available. A discussion of orientational order parameters particularly in smectic-C and biaxial nematic phases and their determination by various techniques including NMR has been presented. The relation between molecular dynamics and biaxiality of nematic polymers and elastomers has been studied by H NMR spectroscopy. "" It was observed that the side-on attachment of the mesogens hinder the rotation of the molecules around their long axes and that in nematic polymers and elastomers phase biaxiality... 

Shimbo Y, Gorecka E, Pociecha D, Araoka F, Goto M, Takanishi Y, Ishikawa K, Mieczkowski J, Gomola K, Takezoe H (2006) Electric-field-induced polar biaxial order in a nontilted smectic phase of an asymmetric bent-core liquid crystal. Phys Rev Lett 97 113901... 

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

A recent observation has been that not all porphyrin LC phases are discotic. Thus 5,15-meso-substituted porphyrins have been prepared which show a variety of smectic phases that are not discotic. The porphyrin macrocycle could be acting to impose biaxial symmetry within the meso-phase, which makes the new porphyrins of great interest as new nematic liquid-crystal materials. 

Y. Wang, H. Yoon, H.K. Bisoyi, S. Kumar, Q. Li, Hybrid rod-like and bent-core liquid crystal dimers exhibiting biaxial smectic-A and nematic phases. J. Mater. Chem. 22, 20363-20367 (2012)... 

Semmler, K. J. Dingemans, T. J. Samulski, E. T. Biaxial smectic phases in non-linear optical properties and phase behaviour of an oxadiazole liquid crystal. Liq. Cryst. 1998, 24, 799-803. 

Sadashiva, B. K. Reddy, R. A. Pratibha, R. Madhusudana, N. V. Biaxial smectic A liquid crystal in a pure compound. Chem. Commun. 2001, 2140-2141. 

The Gay-Berne potential has successfully been used for many liquid crystal simulations, and (depending on the parameterisation used and the state points studied) can be used to simulate nematic, smectic-A and smectic-B phases. Variants of the GB potential have also been used to study the biaxial nematic phase (biaxial GB potential) [21] and the smectic C phase (GB with quadrupole) [22]. The GB model has been used also to provide predictions for key material properties, such as elastic constants [23] and rotational viscosities [24], which have an important role in determining how a nematic liquid crystal responds in a liquid crystal display (LCD). 

In terms of elastic or electromagnetic properties, if two of the three directions in a material are equivalent, the material is said to be uniaxial. The nematic and smectic A phases of liquid crystals are uniaxial, since all directions perpendicular to the director are equivalent and different from the direction of preferred orientational order. Solids with hexagonal, tetragonal, and trigonal symmetry are also uniaxial. If all three directions in a material are inequivalent, then the material is biaxial. The liquid crystalline smectic C phase is biaxial because one direction perpendictrlar to the director is in the plane of the layers while the other direction perpendictrlar to the director makes an angle equal to the tilt angle with the layers. Solids of orthorhombic, monoclinic, and triclinic symmetry are also biaxial. 

Liquid crystals can be composed both of polar and apolar molecules. An important fact in connection with polar substances is that in uniaxial phases there is no polar ordering of the molecules. In average the dipole moments aligned in a given direction are compensated by those aligned in the opposite direction. As a consequence no spontaneous macroscopic polarization develops. More generally one can state that rotation of the director by n does not affect the physical state of the liquid crystal. In biaxial phases built of chiral molecules, such as the chiral smectic C phase, the situation is different. In these systems the compensation of the dipole moments is not perfect, a macroscopic polarization appears in the direction perpendicular both to the layer normal and the director. These phases are therefore ferroelectric. Ferroelectric liquid crystals are currently perhaps the... 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

The symmetry of liquid crystalline phases can be categorized in terms of their orientational and translational degrees of freedom. Thus nematic, smectic and columnar phase types have respectively three, two, and one degrees of translational freedom, and within each type there can be different phases depending on the orientational or point group symmetry. Their optics (uniaxial, biaxial, optically active) are determined by the point group symmetries, which are listed in Table 1 for common liquid crystal phases the optical symmetries of variants of these phases can usually be established directly from their structures. 

The principal elastic constants for a nematic liquid crystal have already been defined in Sec. 5.1 as splay (A , j), twist(/ 22) and bend(fc33). In this section we shall outline the statistical theory of elastic constants, and show how they depend on molecular properties. The approach follows that of the generalised van der Waals theory developed by Gelbart and Ben-Shaul [40], which itself embraces a number of earlier models for the elasticity of nematic liquid crystals. Corresponding theories for smectic, columnar and biaxial phases have yet to be developed. 

Metal-containing smectic materials are of considerable interest because, as they carry metal atoms or ions, they can be effectively studied by X-ray diffraction techniques. In addition, their unique shapes and physical properties make them ideal candidates for the study of biaxiality and conductivity in smectic liquid crystals. One class of materials that stands out are the copper(II) complexes of )3-diketones. These materials can exhibit columnar as well as smectic mesophases. 

It is well known that a smectic C liquid crystal which has a monoclinic symmetry is optically biaxial [3]. A biaxial nematic (N ) phase in which the molecules are oriented along the three directions in space, i.e., they have three directors which are perpendicular to one another, is possible. Thus the shape of molecules exhibiting the Nb phase should be different from those exhibiting the uniaxial nematic phase. 

Recent optical experiments have probed the effect of flexible polymeric solutes in both lyotropic [36] and thermotropic liquid-crystal [37, 38] environments. Long-chain solutes feel an average effective anisotropy, and are found to exhibit much stronger ordering than smaller solutes. The synthesis of the bent-core mesogens ( banana molecules) has resulted in a novel biaxial smectic-A (both pure phase [39] and solute-induced [40]) as well as nematic phases [41,42]. 

J.C. Jones and E.P. Raynes, Measurement of the biaxial permittivities for several smectic C host materials used in ferroelectric liquid crystal devices, Liq. Cryst, 11, 199-217 (1992). 

Subclass 02, biparallel structures. This class, with nematic structures, was obtained by Hessel and Finkelmann and called biaxial. While the word biaxial is quite appropriate, it is also used in the liquid crystal field in connection with structures of smectic C phases, hence a new name was proposed. A low-molecular-mass 02 compound was synthesized by Malthete and his colleagues. Properties of a polysiloxane with phenyl benzoate mesogens obtained by Finkelmann were studied by Hotz and Strobl. " ... 

Intermicellar forces are generally of a repulsive nature (i.e., charged amphiphiles) and a reduction of such repulsion accompanies the transformation from spherical to cylindrical micelles. Further increase of concentration results in the formation of linear assemblies and liquid crystalline lyotropic mesophases (cf. Section ni.B). Not only nematic (Nc and Nd from rodlike or disklike shapes, respectively), hexagonal, and smectic phases, but also biaxial (mixtures of Nc and Nd) and complex cubic phases (bicontinuous networks or plastic crystals)... 

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