Nematic mesophase discotic

The discotic mesophases are classified in two types columnar, and nematic discotic. The structure of the nematic discotic mesophase (Np, Figure 8.3, left) is similar to that of rod-like molecules, but constituted by disk-like units. In columnar mesophases, the molecules are stacked in a columnar disposition and, depending on the type of columnar arrangement, several columnar mesophases are known. The most common lattices of the columnar phases are nematic discotic (No), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rectangular (Col,) mesophases. 

Figure 8.3 Schematic representation of the molecular arrangement in the nematic discotic (Nq), columnar nematic (Ncoi), columnar hexagonal (Coin), and columnar rentangular (Coir) mesophases. A tablet represents a molecule with disk-like shape.

After the discovery of the nematic discotic (No) mesophase,8,9 it was soon... 

Fig. 10. Schematic representation of nematic, smectic and discotic mesophases...

Table 2. Examples of Heats and Entropies of Transition of Nematic, Smectic, and Discotic Mesophases...

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

Earlier, H-shaped orthometallated palladium(II) complexes were discussed which displayed nematic and smectic phases. However, Praefcke has reported related complexes where the number of chains on the periphery of the molecule is increased leading to columnar mesophases. The complex in Figure 91 displays a monotropic nematic discotic phase (No) and the structure has been confirmed as that in the diagram by single-crystal X-ray crystallography [170]. 

Perhaps one of the most important applications of chiral induction is in the area of liquid crystals. Upon addition of a wide range of appropriate chiral compounds, the achiral nematic, smectic C, and discotic phases are converted into the chiral cholesteric (or twisted nematic), the ferroelectric smectic C and the chiral discotic phases. As a first example, we take the induction of chirality in the columns of aromatic chromophores present in some liquid-crystalline polymers. " The polymers, achiral polyesters incorporating triphenylene moieties, display discotic mesophases, which upon doping with chiral electron acceptors based on tetranitro-9-fluorene, form chiral discotic phases in which the chirality is determined by the dopant. These conclusions were reached on the basis of CD spectra in which strong Cotton effects were observed. Interestingly, the chiral dopants were unable to dramatically influence the chiral winding of triphenylene polymers that already incorporated ste-reogenic centers. 

Cyclic hydrocarbons with six equivalent substituent chains emanating from several carbon atoms about the ring can again adopt conformations in which the molecular shape is disk-like, but the aspect ratio is much larger. If the chains are somewhat flexible and the molecules possess an appropriate balance between order and disorder and attractive and repulsive intermolecular forces, the creation of new mesophase types can be envisioned - and several have been found [102,104]. The two general classes of aggregation in liquid crystalline phases of discotic molecules are lenticular nematic (N ) and columnar discotic (D) [100-104]. Carbonaceous pitch mesophases, discussed in section 1.4.5, resemble phases. Only those discotic mesophases with benzene, cyclohexane and shape-related cores having primarily alkyl chains as substituents will be discussed here. 

Figure 1 General structures of calamitic and discotic thermotropic liquid crystals. (A) Layered calamitic smectic liquid crystal. The structures of the various types depend on the local packing of the molecules, the extent of the packing, and the orientation of the long axes with respect to the layers. (B) Calamitic nematic liquid crystal. The molecules have no long-range order, and are only orientationally ordered. (C) Ordered columnar discotic liquid crystal. Disk-like molecules form ordered or disordered columns different column packings give rise to various mesophase structures. (D) Nematic-discotic liquid crystal phase. The disk-like molecules are only orientationally ordered.

During the last 17 years an enormous amount of research into the synthesis of disc-like materials and into the stmcmral characterisation of the discotic mesophases generated has been performed by many research groups throughout the world. Two basic types of discotic mesophases have been widely recognised, these arc columnar and nematic. 

The three different shapes (rodlike, disclike, boardlike) and two different mesophases (nematic and smectic) together form six different kinds of crystals. They are rodlike nematic, discotic nematic, boardlike nematic, rodlike smectic, discotic smectic, and boardlike smectic. Figure 6.3 shows these six combinations. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

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). 

The dinuclearpallado compounds, 59, are the first (monotropic) cases of metallome-sogens exhibiting the nematic discotic phase. On doping with a strong electron acceptor, the stabilization and/or induction of mesophases was observed [139]. 

TNF-CT complexes of 2,3,6,7,10,11-hexakispentyloxytriphenylene or related amphiphilic derivatives are also able to form Langmuir-Blodgett (LB) films [57]. Polarized UV/VIS and infrared spectra of those of TNF complexes of hexakisalkyloxytri-phenylenes discussed here, for example, indicate a preferred orientation of the columnar axes parallel to the surface and to the dipping direction [57 a]. An amphiphilic triphenylene derivative with a pair of co-OH chains and four pentyloxy groups, forming a lamellar mesophase in the neat state, a nematic discotic main chain polymer derived from the aforementioned monomer both exhibit induced columnar hexagonal (Col j,) mesophases with TNF in the bulk state [57b]. The X-ray diffraction pattern of the... 

Figure 4. Schematic phase diagrams of members of the radial hexayne series 1,2, and 3 as electron donors with TNF as the electron acceptor [26] Cr crystalline, Nd nematic discotic, Col, columnar rectangular, and Col), columnar hexagonal mesophase (o intracolumnar ordered). Iso isotropic liquid (see footnote 1 in Sec. 3 commenting on the nomenclature of columnar mesophases).

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