Mesophases of calamitic mesogens

The mesophases of calamitic mesogens are classified in two groups nematic and smectic. The nematic mesophase (N) is characterized by an orientational order of the molecules that are aligned along a preferred direction (defined by a director n) (Figure 8.2). The molecules can slide and move in the nematic mesophase (while roughly keeping their molecular orientation) and rotate around their main axis. This is the less ordered mesophase and it is usually very fluid. 

The true liquid-crystal mesophases of calamitic mesogens are divided into two classes—the nematic and smectic phases and these will be described in turn. There is, in addition, a series of crystal smectic phases that are not really liquid-crystal phases but which for many years were classified as such these will be mentioned briefly below. 

In the section on calamitic materials, we discussed how, by appropriate consideration of the molecular shape of bis( -diketonato)copper(II) complexes, either nematic and lamellar, or columnar mesophases could be realized. Furthermore, increasing the number of chains on the periphery of calamitic mesogens can lead to molecules which demonstrate both types of mesomorphism, e.g. the phasmids of Malthete... 

Figure 2 Representative structure of the arrangement of calamitic mesogens in the nematic (N), smectic A (SmA), and smectic C (SmC) mesophases, with the director (n) shown in...

Note 5 If the mesogenic side-groups are rod-like (calamitic) in nature, the resulting polymer may, depending upon its detailed structure, exhibit any of the common types of calamitic mesophases nematic, chiral nematic or smectic. Side-on fixed SGPLC, however, are predominantly nematic or chiral nematic in character. Similarly, disc-shaped side-groups tend to promote discotic nematic or columnar mesophases while amphiphilic side-chains tend to promote amphiphilic or lyotropic mesophases. 

The introduction of suitable mesogenic building blocks into macrocycles will generally have an enormous effect on the stability of mesophases. The reason results from the restricted flexibility of the linked calamitic units. 

Calamitic mesogens in general form two main types of mesophase the nematic phase (N) and the smectic (S) phases. The nematic phase is the most disordered of the mesophase tjpes in it, the molecules possess one-dimensional orientational order and no positional order (Fig. 22). 

Based on previous work which showed that backbone rigidity correlates with mesophase behavior for SCLCPs bearing calamitic mesogens [64], the double bonds of the main chain in polybutadienes were hydrogenated, leading to pure saturated alkane main chains, poly-(XXIX-5)a and poly-(XXIX-10)a in Table 18. But in contrast to the studies of calamitic SCLCPs, no dependency of backbone rigidity on mesophase behavior could be estabhshed. 

Some examples of typical calamitic mesogens are given in Figure 4. Cyanobiphenyls are included in this list due to their special position in the development of LCDs, but it should be noted that in many ways they are not typical as their liquid-crystal mesophases arise from antiparallel molecular correlations. 

Hydrosilylation reactions have been performed with PFSs containing Si-H functionalities to attach mesogenic azobenzene groups. This has allowed the preparation of calamitic thermotropic side-chain liquid crystalline materials that display a nematic mesophase between ca. 53 and 250... 

As this supramolecular mesogen does not form a simple rod or disc shape, the molecular arrangements of calamitic and columnar mesophases can be tuned when appropriate substituents are introduced into either of the components [61, 63]. Triazines and carboxylic acids are complexed to form doubly H-bonded mesogens which display columnar mesophases, e.g. 22 [64, 65]. 

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. 

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. 

In thermotropic liquid crystals, the formation of particular mesophases depends mainly on the temperature. On cooling, the structure of the mesophases becomes more and more ordered and thus less symmetric. For thermotropic mesophases formed by calamitic mesogens a fixed sequence was found [37, 38] ... 

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