Liquid calamitic

Thennotropic liquid crystal phases are fonned by anisotropic molecules witli long-range orientational order and in many types of stmcture witli some degree of translational order. The main types of mesogen are Arose tlrat are rodlike or calamitic and Arose Arat are disclike or discotic. 

Liquid crystals may be divided into two broad categories, thermotropic and lyotropic, according to the principal means of breaking down the complete order of the soHd state. Thermotropic Hquid crystals result from the melting of mesogenic soHds due to an increase in temperature. Both pure substances and mixtures form thermotropic Hquid crystals. In order for a mixture to be a thermotropic Hquid crystal, the different components must be completely miscible. Table 1 contains a few examples of the many Hquid crystal forming compounds (2). Much more is known about calamitic (rod-Hke) Hquid crystals then discotic (disk-like) Hquid crystals, since the latter were discovered only recendy. Therefore, most of this section deals exclusively with calamities, with brief coverage of discotics at the end. 

Fig. 2. The generic molecular structure of calamitic liquid crystals illustrating the semi-rigid core fragments, the positions of the end-groups (C and A), linking groups (B) and, possibly, laterally substituted groups (L)...

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

Here the vector rj represents the centre of mass position, and D is usually averaged over several time origins to to improve statistics. Values for D can be resolved parallel and perpendicular to the director to give two components (D//, Dj ), and actual values are summarised for a range of studies in Table 3 of [45]. Most studies have found diffusion coefficients in the 10 m s range with the ratio D///Dj between 1.59 and 3.73 for calamitic liquid crystals. Yakovenko and co-workers have carried out a detailed study of the reorientational motion in the molecule PCH5 [101]. Their results show that conformational molecular flexibility plays an important role in the dynamics of the molecule. They also show that cage models can be used to fit the reorientational correlation functions of the molecule. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

It is not possible to predict from the related crystal structure alone whether the compound will melt to a liquid crystalline phase or not, because the anisotropic molecules (calamitic and discotic ones) form in favourable anisotropic packing. As a rule long shaped rod-like molecules quite often possess a layered arrangement in the solid state regardless of whether the compound is mesogenic or not. 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

Coco, S Espinet, P Marb n-Alvarez, J.M. and Levelut, A.M. (1997) Effects of Isonitrile Substituents on Mesogenic Properties of Halogoldiisonitrile Complexes Calamitic and Discotic Liquid Crystals. Journal of Materials Chemistry, 7, 19-23. 

There are now three major shape classifications of low molar mass liquid crystals - rod-like (calamitic), disc-like (discotic) and bent-core. The last of these is the most recent, and while examples of bent mesogens have been known for some years, it is only since the mid-1990s that the area has attracted widespread attention [2],... 

Calamitic liquid crystals generally contain two or more rings, at least one flexible chain and often a small, polar function in some cases, small, lateral groups (normally fluorine) are attached. Some will contain metal... 

The liquid crystal phases of calamitic mesogens fall into two types - nematic (N) and smectic (Sm). The nematic phase is the most disordered of the liquid crystal phases and possesses only orientational order, so that the long axes of the molecules are correlated in one direction (known as the director, n) while being positioned randomly (Fig. 2A). There are several smectic phases and these differ from the nematic phase in possessing partial posi-... 

Based on a known synthesis of spiro compounds with six-membered rings [63] we succeeded in the stepwise assembly of terminal substituted mono-[64] and dispiranes [65]. Hereby the cyclisation of the dibromide 35 with TOSMIC (Iter. 1) is the keystep of the reaction sequence as the initial ketone functional group (36) is recovered. Four subsequent reactions led to the spirodibromide 40 (Iter. 2). Final spirocyclisation afforded the dispirane 41, representing a precursor for new calamitic liquid crystals (Fig. 16) [66]. 

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... 

M Funahashi and JI Hanna, Fast hole transport in a new calamitic liquid crystal of 2-(4 -Heptyloxyphenyl)-6-dodecylthiobenzothiazole, Phys. Rev. Lett., 78 2184-2187, 1997. 

Thermotropic liquid crystalline (LC) phases or mesophases are usually formed by rod-like (calamitic) or disk-like (discotic) molecules. Spheroidal dendrimers are therefore incapable of forming mesophases unless they are flexible, because this would allow them to deform and subsequently line up in a common orientation. However, poly(ethyleneimine) dendrimers were reported to exhibit lyotropic liquid crystalline properties as early as 1988 [123],... 

FIG. 2. Schematic representation of different calamitic and discotic thermotropic liquid crystals (a) nematic, (b) cholesteric, (c-e) smectic, (f) columnar hexagonal, (g) columnar hexagonal tilted a-e adapted from Demus, D., and Richter, L., Textures of Liquid Crystals, Verlag Chemie, Weinheim, Germany, 1978 f,g adapted from Eidenschenk, R., Flussige Kristalle, Chem. Unserer Zeit, 18, 168-176 (1978). 

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.

In many cases, these polymer chains take on a rod-like (calamitic LCPs) or even disc-like (discotic LCPs) conformation, but this does not affect the overall structural classification scheme. There are many organic compounds, though not polymeric in nature, that exhibit liquid crystallinity and play important roles in biological processes. For example, arteriosclerosis is possibly caused by the formation of a cholesterol containing liquid crystal in the arteries of the heart. Similarly, cell wall membranes are generally considered to have liquid crystalline properties. As interesting as these examples of liquid crystallinity in small, organic compounds are, we must limit the current discussion to polymers only. 

Figure 7.1 Schematic representation of calamitic and discotic liquid crystals. A = alkynyl, CH2, C(0)0, N=N,. .

Most polymeric liquid crystals are based on stiff rod-like molecular units which are called calamitic mesogens. There are some unusual polymers (which are not discussed here) that contain flat disk-like molecular units called discotic mesogens in which the disks form columnar arrays like stacks of poker chips. 

Calamitic liquid crystals consist of rod-like molecules, i.e. a pronounced shape anisotropy with one extended principal axis and two short perpendicular ones. Three main classes can be described as in Figure 2. 

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