Bent-core mesophase

Since most of bent-core mesophases (e.g., Bl, B2, B5, B7, B7-like, and B8) possess intrinsic polar order, they are responsive upon applying an external stimulus such as an E-field, exhibiting either AF or EE switching or both. In this section, the author will focus on polar SmCP phases including B2 and DC, SmAP, and polar Col phases and review their field-induced switching, chirality transformation, and structural transformation. 

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

Gorecka E, Vaupotic N, Pociecha D, Cepic M, Mieczkowski J (2005) Switching mechanisms in polar columnar mesophases made of bent-core molecules. Chemphyschem 6 1087-1093... 

Finally, our group reported on gold nanoparticles decorated with bent-core liquid crystals showing pattern formation on TEM grids after slow solvent evaporation (18 in Fig. 22). These particles showed interesting self-assembly effects in different bent-core liquid crystal hosts (SmCPA and Colr) and slightly improved electro-optic effects such as shorter response time, x, and unaltered spontaneous polarization in the SmCPA host, but no mesophase formation [547]. 

Supramolecular chemistry can be used to create the bent cores that give rise to the symmetry breaking in this family of liquid crystals [139]. The formation of a complex between a calamitic benzoic acid derivative and a bent core terminated with a pyridyl group—neither of which display mesomorphic behaviour—gave rise to a material which displayed SmCP mesophases. The achiral bent cores can also give rise to symmetry breaking when they are attached to flexible polymeric chains, such as poly(siloxane) [140]. 

Usually, it is possible to observe chiral mesophases exhibiting ferroelec-tricity in mesogens made from optically active molecules, but in some particular cases, achiral bent-core molecules can form polar ordered smectic phases... 

The first examples of end-to-end connected bent-core—rod couples and rod—bent-core—rod trimesogens incorporating a bent 3,5-diphenyl-l,2,4-oxadiazole core (Figure 12) have been synthesized and their LC phase behaviors studied (2012BJO472). AH compounds form nematic phases over wide temperature ranges, in some cases accompanied by additional nontHted (CybA) or tilted (SmC) mesophases at lower temperatures. 

A variety of systems exhibit liquid crystalline properties and a rich variety of phases that have been investigated by NMR have been reported. Particularly, molecules exhibiting unusual topologies different from the usual rod like structure such as bent-core or hockey stick-shape have been of significant interest. The biaxial nature of some of these systems has also attracted much attention. Here, reports of studies on the above systems as well as studies on molecules exhibiting nematic, smectic, columnar or lyotropic mesophases and the study of orientational order in such systems have been included. 

The present research and development on LC display technology is conducted primarily in industrial labs. Academic research focuses mainly on more exciting and explorative topics that can not only stimulate fundamental scientific interest, but offer tremendous potential for innovative applications beyond the realm of displays, for example, new materials and attractive properties, and new uses in optics, nano/micromanipulation, novel composites, and biotechnology [7]. Future applications depend on the increase of complexity and functionality in LC materials and phases. The past three decades have seen the discovery of complex LC molecules with a variety of new shapes for instance, disc shape (Fig. 6.1b) [8], bent-core shape (Fig. 6.1c) [9], H shape (Fig. 6.1d) [10-13], board shape (Fig. 6.1e) [14,15], T shape (Fig. 6.1f) [16], cone shape (Fig. 6.1g) [17], and semicircular shape (Fig. 6.1h) [18]. The shapes of the molecules are not exactly associated with the types of mesophases formed. Like rod-shaped molecules, each complex shape is likely to organize a nematic, Sm, Col, and 3D-ordered mesophases [19,20]. The incorporation of functionality, amphiphilicity, and nano-segregation into these molecular shapes offers different ways to increase the complexity of LC phases. 

In summary, the permutation and combination of these units central BUs, aromatic rings, linking groups, lateral substituents, and tail termini, will generate an infinite number of bent-core molecules which may give rise to many interesting mesophases... 

Tschierske, C. Dantlgraber, G. From antiferroelectricity to ferroelectricity in smectic mesophases formed by bent-core molecules. Pramana — J. Phys. 2003, 61, 455-481. 

Murthy, H. N. S. Sadashiva, B. K. Observation of a transition from non-switchable B7 mesophase to an antiferroelectric sub-phase in strongly polar bent-core compounds. J. Mater. Chem. 2003,13, 2863-2869. 

Weissflog, W. Naumann, G. Kosata, B. Schroder, M. W. Eremin, A. Diele, S. Vakhovskaya, Z. Kresse, H. Friedemaim, R. Krishnan, S. A. R. Pelzl, G. Ten isomeric five-ring bent-core mesogens The influence of the direction of the carboxyl connecting groups on the mesophase behavior. J. Mater. Chem. 2005,15, 4328-4337. 

Reddy, R. A. Raghunathan, V. A. Sadashiva, B. K. Novel ferroelectric and antiferroelec-tric smectic and columnar mesophases in fluorinated symmetrical bent-core compounds. Chem. Mater. 2005,17, 274—283. 

Prasad, V. Kang, S.-W. Qi, X. Kumar, S. Photo-responsive and electrically switchable mesophases in a novel class of achiral bent-core azo compounds. J. Mater. Chem. 2004, 14, 1495-1502. 

Finally here, it is important to note that the supramolecular nature of liquid crystal mesophases, in conjunction with polarity, can also lead to the induction of chirality in nonchiral materials. Current interest dates back to 2006 when Niori et a/. reported the observation of ferroelectric switching in some achiral, bent-core liquid crystals. The molecules are shown in Figure 6 and are unusual inasmuch as convention suggests that liquid-crystalline molecules should be highly anisotropic. Matsnnaga eta/. had prepared these materials in the 1990s and had noted that they did indeed form a liquid crystal phase. However, what Niori et al. showed was that the symmetry of the liquid crystal phases must be broken in order to observe a ferroelectric response and further that chiral domains could be observed. ... 

Table 2 demonstrates the various types of the most well known and widespread mesophases consisting of rod-, board (or lath)-, disk-, and banana-shaped molecules. Depending on the orientational and positional organization of the molecules, these mesophases can roughly be divided into nematic, smectic, columnar, and bent-core (banana) LC phases. 

It is important to note that also nonchiral molecules are capable of forming chiral mesophases. In particular, molecules with a bent core ( bananashaped molecules) can build polar, and even chiral liquid crystal structures [75]-[78]. Bent-core molecules form a variety of new phases (B1-B7, Table 1.3) which differ from the usual smectic and columnar phases (see also Chapter 8). As a consequence of the polar arrangement, antiferroelectric-like switching was observed in the B2 phase formed by bent-core molecules, and second harmonic generation was found in both the B2 phase and the B4 phase. The latter phase is probably a solid crystal. It consists of two domains showing selective reflection with opposite handedness. In the liquid crystalline B2 phase, the effective nonlinear susceptibility can be modulated by an external dc field [79] (Figure 1.15). 

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