Bent-core molecules different phases

Polar Columnar (ColP) Phases In calamitic rod-shaped LCs, the frustration occurring in the layer organization of molecules due to steric and/or polar effects leads to form a variety of 2D density structures such as undulated layers, modulated layers (ribbons), and Cols. The situation for bent-core molecules with an ability to form macroscopic polar order is much more complex, and different types of modulated smectic and Col phases form. Since their 2D X-ray patterns, structural models, and nomenclature have been in great detail described in the two previous reviews [29, 32], in this section, the field-induced switching properties of polar columnar (ColP) phases are focused on. 

There are many different types of bent-core molecules known, and most of exotic bent-core phase structures have been well elucidated. The future research and development on bent-core LCs shall be focused on the exploration of a variety of applications. Certainly, these applications shall not be limited to today s known bent-core molecules, and the development of new types of bent-core molecules incorporating varied functionalities based on new ideas or concepts is critically important. For instance, linking bent-core molecules with nanoscience and photoisomerization chemistry is a good step. Further exploration in organic field effect transistors (OFETs) and organic photovoltaics (OPVs) could be very fmitful. 

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). 

List of Different Phases of Bent-Core Molecules... 

As you can imagine, when bent-core molecules pack together, a variety of different phases may form distinct from the more conventional calamitic phases formed from rod-like molecules. These different phases result from the additional complexity added to packing bent molecules instead... 

So far we have discussed 2D density modulated phases that are formed by deformation or breaking of the layers. However, there are also 2D phases with more subtle electron density modulations. In some cases additional peaks observed in the XRD pattern (Fig. 10) are related to a double layer periodicity in the structure. As double layer periodicity was observed in the bent-core liquid crystals formed by the asymmetric as well as symmetric molecules [22-25] it should be assumed that the mechanism leading to bilayers must be different from that of the pairing of longitudinal dipole moments of molecules from the neighboring layers, which is valid for smectic antiphases made by asymmetric rod-like molecules. 

Let us consider an N phase doped with BSMs, where BSMs interact with chiral host molecules. The interaction energy between left-handed bent-core conformation and the chiral host molecule f/LH is different from that between right-handed bent-core conformation and the same chiral host molecule t/RH- The nonzero difference AU = f/LH Crh induces finite ee in BSMs, resulting in increased chiral molecules in the system. At the same time, we have to consider the dilution effect. 

Finally, the difference of chirality enhancement in the N and SmC phases should be mentioned. As shown in Sect. 2.1, enhancement rate in SmC is about one order of magnitude larger than that in N. In the SmC chirality enhancement is attributed to two effects (1) the interaction between bent-core and chiral host molecules and (2) the coupling between ee, tilt, and spontaneous polarization. The latter effect is absent in the N phase and is an additional effect in SmC. Moreover, the chiral discrimination parameter AU is expected to be larger in SmC than in N because of a confined geometry, i.e., smectic layer. 

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. 

The thermotropic liquid crystal, 4,4 -diheptylazoxybenzene (HAS), exhibiting isotropic, nematic and smectic phases, has been studied through e NMR. The temperature dependence of e chemical shifts and spin-lattice relaxation times of the Xe gas dissolved in HAS showed clear signatures of the phase transitions. Theoretical models have been used to understand the influence of the different phases on the isotropic and anisotropic parts of the chemical shielding. From the studies it is also inferred that in the smectic phase, Xe atoms preferentially occupy interlayer spacings rather than the interiors. Bent-core or banana-shaped molecules display an array of novel chiral liquid crystalline phases. NMR studies on two of the banana core moieties have been analyzed using ab initio structure calculations and the steric inertial frame model. ... 

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