Nematic phase discotic mesogen

This formation of nematic, lamellar and columnar phases within a single class of molecule leads to the idea that they can be considered as a missing link between calamitic and discotic mesogens. 

The system represents a discotic mesogen of an immense diameter (1.6 nm), with six flexible side chains. The crystalline phase is transformed at 115 °C to a broad nematic phase Nj) in which two discs have a parallel arrangement with an average distance of... 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

Of all liquid crystalline phases, the nematic phase is the phase with the highest symmetry, i.e. Dooh, and the least order. As shown in Fig. 3.3a, b, the mesogens solely possess orientational order. Positional order of the mass centers does not occur in this phase. Nematic phases are usually built up by either rod-like or disc-like mesogens. For thermotropic liquid crystals these mesogens are therefore calamitic or discotic molecules, respectively. In both cases the phase is simply denoted with the abbreviation N. For lyotropics, the notation typically distinguishes between nematic phases Nc, which are formed by rod-like micelles, and nematic phases Np, which are composed of disc-like micelles. 

Fig. 3.3 Structural sketches of nematic phases composed of a calamitic and b discotic mesogens with indicated direction of the director n. In the sketches of the corresponding cholesteric phases of c calamitic and d disoctic mesogens, only the local director njoeai is drawn in...

Liquid crystals. A. Generic description of select phases. A rod-shaped mesogen can form a smectic or a nematic phase. A disk-shaped m can form a columnar/discotic phase. B. Illu.stration of how the dirertor of a cholesteric phase rotates as one moves through the material, giving rise to a helical pitch. Note that the oval in this part of the figure is not a sinci mesogenic molecule, but rather represents the director of a thin section of the cholesteric phase. 

The nematic phase of nonlinear mesogens may be biaxial - a translationally disordered fluid phase with two directors n and o specifying the orientational order (Fig. 5.3). Biaxial order in a nematic is predicted to occur [29] if the shape anisotropy of the idealized molecule (discoid) representing the nonlinear meso-gen is appropriately intermediate between the prolate shape of calamities and the oblate shape of discotics. Discoid-shaped mesogens lend themselves to a variety of stratified phases. Ferroelectric (Sapp) and antiferroelectric (Sapa) layer motifs in the normal smectic phases of discoid-shaped mesogens are readily envisioned (Fig. 5.4), but less obvious is the possibility of generating chiral supramolecular structures from such achiral discoid-shaped mesogens (Fig. 5.5) [30]. 

In the case of copolymer LCs there is speculation that the alternating primary structure. .. core-spacer-core-spacer... restricts inter-core translations and thereby stabilizes smectic phases. While we remain cognizant of the potentially important role that primary and secondary molecular structure of mesogens can play, in the remainder of this chapter, however, we will focus on aspects of mesomorphism that, for the most part, can he described in terms of the idealized prolate (oblate) shape of calamitic (discotic) mesogens (Fig. 5.1). We consider the nature and implications of the local molecular order in the nematic state after briefly reviewing the molecular crystal and the isotropic liquid. 

This is the simplest liquid crystal phase. It is formed by calamitic or discotic mesogens, typical examples of the former being shown in Fig. 5.1 (and the latter in Fig. 5.7). The molecules have no long-range translational order, just as in a normal isotropic liquid. However, they do possess long-range orientational order, unlike in a liquid. The nematic phase can thus be considered to be an anisotropic liquid. It is denoted N, and an illustration of its structure is included in Fig. 5.2. The direction of average molecular orientation is called the director, usually denoted by a unit vector, fi. 

Columnar phases are formed by discotic mesogens, examples of which are shown in Fig. 5.7. Discotic molecules can form a nematic phase (termed Nd) just like calamitic mesogens. In addition, several types of columnar... 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

---