Molecule calamitic

It was quickly recognized that chirality would play an important role in discotic liquid crystals, not only for the possibility of creating cholesteric and ferroelectric liquid crystals but also as a tool for studying the self-assembly of these molecules as a whole, both in solution and in the solid state. However, initial studies revealed that expression of chirality in discotic liquid crystals was not as straightforward as for liquid crystals derived from calamitic molecules. More recently, with the increase in interest in self-assembly and molecular recognition, considerably more attention has been directed to the study of chiral discotics and their assemblies in solution. The objective of this chapter is... 

It is now instructive to ask why the achiral calamitic SmC a (or SmC) is not antiferroelectric. Cladis and Brand propose a possible ferroelectric state of such a phase in which the tails on both sides of the core tilt in the same direction, with the cores along the layer normal. Empirically this type of conformational ferroelectric minimum on the free-energy hypersurface does not exist in known calamitic LCs. Another type of ferroelectric structure deriving from the SmCA is indicated in Figure 8.13. Suppose the calamitic molecules in the phase were able to bend in the middle to a collective free-energy minimum structure with C2v symmetry. In this ferroelectric state the polar axis is in the plane of the page. 

Liquid crystals are materials that exist in a state that is intermediate between a liquid state and a solid. They are formed by anisotropic molecules, known as calamitic molecules, which are long and narrow, i.e. have a large length-to-breadth ratio. These rod shaped molecules orientate themselves in different ways as they change from the crystalline to the liquid state at different temperatures, existing in the smectic and nematic phases, as shown in Figure 5.1. 

Figure 5.1 Liquid crystal phases formed by rod-like (calamitic) molecules.

Fig. 16 2D-cartoons of quasi-spherical nanoparticles protected with mesogenic or pro-mesogenic capping agents giving rise to liquid crystalline quasi-spherical nanoparticles. The three major approaches include the decoration with calamitic molecules in an end-on fashion (7), with dendrons featuring calamitic or polycatenar moieties at the termini (8), and with laterally substituted calamitic molecules in a side-on fashion (9). The concept shown for the quasi-spherical nanoparticle 7 was also successfully used for spindle-like nanoparticles [533, 534]... 

The average orientational direction of the molecules is denoted by the director, n,with the degree of orientational correlation evaluated by the order parameter, S, expressed as in Eq. (8), where 0 is the angle between a molecule and the director. It is the nematic phase of calamitic molecules that is used in liquid crystal displays. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

Through 5,15-substitution about a zinc(II) porphyrin, it has been shown that calamitic molecules may be realized [64, 65]. These complexes (Figure 33) may... 

As discussed earlier, structural anisotropy is paramount in importance when considering the intermolecular interactions necessary to promote mesomorphism. The structural anisotropy is related to the molecular shape and we shall now endeavour to investigate its effect upon the mesomorphic properties of several complexes. In several instances we have discussed N, Sa and Sc phases arising from calamitic molecules, and in later sections we shall discuss disc-like molecules exhibiting columnar mesophases. Unfortunately such rigid definitions of molecular... 

Molecules of a disc-like shape form either nematic or columnar phases. Structurally the most simple mesophase is the discotic nematic phase (Nd). Just as in the case of the nematic phase formed by calamitic molecules, the molecules possess orientational order, but no positional order (Figure 2.8). The columnar nematic phase (Nc) consists of short columns of a few molecules that act like the rod-like molecules in the nematic calamitic phase (Figure 2.9). There is no organisation of the columns in a two-dimensional lattice. 

Copolymeric systems with amylose are therefore systems in which at least one component is based on a conformationally rigid segment, which are generally referred to as rod-coil systems." By combining rod-like and coillike polymers a novel class of self-assembling materials can be produced since the molecules share certain general characteristics typical of diblock molecules and thermotropic calamitic molecules. The difference in chain rigidity of rod-... 

If it is straightforward to understand why columnar mesophases are easily obtained with disk-like shaped molecules (the columns are formed by the piling of the flat cores of the molecules, the molten aliphatic chains filling the space between the columnar cores), it may appear rather surprising, at first sight, to observe columnar mesophases with polycatenar calamitic molecules. 

Similarly to the molecular engineering of calamitic molecules to produce ferroelectric smectic C phases [129], disk-like molecules with chiral peripheral chains tilted with respect to the columnar axis were predicted to lead to ferroelectric columnar mesophases [130]. Indeed, as it is the case with all flat disk-shaped mesogenic molecules, the tilt is mainly associated with the flat rigid aromatic cores of the molecules, the side-chains being in a disordered state around the columnar core. Thus, the nearest part of the chains from the cores makes an angle with the plane of the tilted aromatic part of the molecules. If the chiral centre and the dipole moment are located close to the core, then each column possesses a non-zero time averaged dipole moment, and therefore a spontaneous polarization. For reasons of symmetry, this polarization must be, on average, perpendicular to both the columnar axis and to the tilt direction in other words, the polarization is parallel to the axis about which the disk-shaped molecules rotate when they tilt as shown in Fig. 29. 

The mesophases of calamitic molecules mentioned above have a well-defined thermod3mamic order when a hypothetical calamitic mesomorphic compound exhibiting almost all types of mesophases (enantiotropic with respect to the melting point and disregarding the various chiral phases) is heated, the mesophases will occur in a certain defined sequence ... 

S. Tanaka, H. Takezoe, N. Eber, K. Fodor-Csorba, A. Vajda and A. Buka, Electroconvection in nematic mixtures of bent-core and calamitic molecules, Phys. Rev. E 80(2), 021702/1-8, (2009). doi 10.1103/PhysRevE.80.021702... 

Despite 1,2,4-oxadiazoles being highly efficient in promoting mesomorphic behavior, they are generally achiral calamitic molecules. To achieve the noncentrosymmetry necessary for FE behavior, a chiral alkoxy chain derived from (R)-2-octanol has been introduced into the structure of calamitic LCs derived from 1,3,4- and 1,2,4-oxadiazole, resulting in formation of the first chiral oxadiazole-based LC exhibiting FE properties (2008LCl251). 

There is also a group of the so-called lyotropic nematics. They are intermediate between the isotropic micellar phase and structured (lamellar or hexagonal) phases and can be formed by both discotic and calamitic molecules. The lyotropic nematics can be aligned by an electric or magnetic field and show Schlieren texture as thermotropic nematics. The building blocks of these mesophases are vesicles or similar mesoscopic objects. From the symmetry point of view the nematic phases can be uniaxial or biaxial, as shown in Fig. 4.22. In fact, the biaxial nematics have been found unequivocally only in the lyotropic systems [13]. 

Rod-shaped molecules forming liquid crystalline phases are called calamitic . A prominent example of such a calamitic molecule is terephthal-bis-(p-butylaniline) (TBBA) [2]. Its chemical structure is shown in the upper left comer of Fig. 3.1. The molecule possesses a rigid aromatic core as well as flexible alkyl chains. The aromatic core favors a parallel packing of the molecules, while the flexible chains keep them from crystallizing. These intermolecular interactions, as well as entropic effects and steric interactions between the mesogens, promote the formation of mesophases, as discussed by Onsager [3]. The mesophases formed by calamitic mesogens frequently possess a layered structure, but different phase types are also possible. 

Fig. 5.11. Schematic representations of the molecular crystal of calamitic molecules and the associated idealized diffraction pattern for incident X-rays parallel to b. The intermolecular distances L (the approximate length of the molecule) and D (the lateral spacing) correspond to diffraction spots on the meridian and equator, respectively there is no appreciable azimuthal spread of the diffraction along/.

IQENS from the Localized Motion of Calamitic Molecules... 

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