Calamitic compounds

Calamitic compounds which exhibit a smectic and/or nematic phase usually consist of a relatively rigid central core containing co-linear six-membered rings, either aromatic rings, such as 1,4-disubstituted-phenylene, 2,5-disubstituted-pyridine, 2,5-disubstituted-pyrimidine, 3,6-disubstituted-pyridazine, and alicyc-lic rings, such as /ra j-l,4-disubstituted-cyclohexane, 1,4-disubstituted-bicy-clo[2.2.2]octane, 2,5-disubstituted-dioxane. Heteroaromatic rings tend to lead to the formation of smectic phases rather than the nematic phase unless combined with a polar terminal function, such as a cyano group. The dependence of the liquid crystalline transition temperatures on the nature of... 

Fig. 7.2 Qualitative temperature dependences of the principal components of diamagnetic (negative) and paramagnetic (positive) susceptibility for calamitic compounds in the isotropic, nematic and SmA phases...

Cadiot-Chodkiewicz coupling 99 Cahn nomenclature 117 calamitic compounds 134 f... 

Calamitic compounds with high charge carrier mobilities. 

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. 

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. 

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]. 

Note 4 The situation for thermotropic calamitic systems is less clear and for some compounds claimed to form a Nb, detailed investigations have found the mesophase to be of type Nu. 

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. 

The liquid crystalline properties of 43a-d were interesting as all derivatives showed very stable mesophases with phase widths of 57-109 K. It is remarkable that Schiff bases 43c,d are stable at temperatures above 300 °C. It also strikes that, in contrast to conventional calamitic liquid crystals, the Schiff bases 43c,d possess significantly higher phase transition temperatures compared to azo linked 43a,b. Additionally, the trans compounds tend to possess higher clearing temperatures due to the elongated shape of the molecules. 

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... 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

Some of the simplest systems reported raise a possible point of semantics as to whether these systems are, in fact, polycatenar in nature or perhaps ought to be described as discotic. The first of these 77 was based simply on the coordination of 3,4,5-trialkoxybenzonitriles to Pd(ll), following a motif for the formation of simple, calamitic systems "" described some years earlier. Only two compounds with different chain lengths were studied ( =10, 18). The ligands were themselves not mesomorphic, but on complexation, a Col), phase was observed over small temperature ranges, i.e., 73-91 °C for the complex with six tridecyloxy chains, and between 58 and 80 °C for that with six octadecyloxy chains. 

Another example where the amphipathic feature of the molecules plays an important role is the case of some thermotropic ionic calamitic liquid crystals such as ditholium salts [64]. In spite of their rod-like shape, these compounds exhibit columnar mesophases with supramolecular organization similar to that of amphiphilic molecules. In this case, it is clear that the driving force for the supramolecular organization is the amphipathic character of the molecules... 

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