Discotic nematic phase

If the molecules are chiral or if a chiral dopant is added to a discotic Hquid crystal, a chiral nematic discotic phase can form. The director configuration ia this phase is just like the director configuration ia the chiral nematic phase formed by elongated molecules (12). Recendy, discotic blue phases have been observed. 

Turning now to those molecules whose shape can be approximated by oblate spheroids, one arrives at the discotic phase. Here the average of the normals to planes of the molecules corresponds to the director. A fluid phase in which these normals point in roughly the same direction over a macroscopic distance is said to be discotic. If this factor is the only degree of order, the material is said to be in the nematic discotic phase. If, in addition, the discs stack in regular columns, the material is said to be in the columnar discotic phase. Such structures have been discussed in Section 4.5.1. 

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]. 

The simplest discotic liquid crystal phase is the nematic discotic phase, Nd, in which the normals of the molecular discs tend to align with respect to a preferred direction, i.e., the director, but the mass centers of molecules do not have any positional order. The discs in Figure 1.11 represent the disclike molecules, the molecules are packed in the way a pile of coins is packed randomly. The discotic nematic phase has its chiral counterpart, i.e., the... 

III. The simplest discotic LC phase is the nematic discotic phase, in which the normal of the discs tends to align to the director. The mass centers of the molecules do not have any positional order. A phase similar to rod-like smectic LC phase is the column phase where molecules are packed in columns parallel to each other (Figure 10). Columns are arranged in a hexagonal or rectangular array. 

Compound 18 exhibits solely a chiral nematic discotic phase (N ) phase because the steric effect of the branched chains at the chiral centre disrupts the ability of the molecules to pack in columns. The large size of the planar aromatic core ensures a high clearing point, but the liquid crystal tendency depends critically on the type of chiral peripheral chain. Hexa-substituted phenylacetylenes were discussed in chapter 3 and exhibited the Np phase. Perhaps not surprisingly, when one of the peripheral acetylene units is chiral, the N phase is exhibited (compound 19). 

In the discotic phase, disclike molecules form liquid crystal phases in which the axis perpendicular to the planes of the molecules, orients along a specific direction. The nematic discotic phase has orientational order but no positional order. In the columnar discotic phase, the disclike molecules form columns and therefore exhibit orientational and positional order. In a chiral discotic liquid crystal, the director rotates in a helical path throughout the system. 

The formation of charge transfer (EDA) complexes between suitable components may induce columnar [327-329] or nematic discotic phases [329-331]. Also the formation of intramolecular EDA complexes in twin molecules can influence the mesomorphic properties substantially [310,332]. 

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]. 

By using the chiral electron acceptor (-)-TAPA, even the cholesteric versions of the nematic discotic phase Ng or of the nematic columnar one Ncoi can be obtained, for example, by doping a nematic discotic pentayne ether with up to 30 mol% (-)-TA-PA or by inducing the N oi phase in a ternary mixture composed of pentayne ether (4) (n = 16) with TNF and (-)-TAPA the latter electron acceptor does not itself lead to the induction of a columnar mesophase with such pentayne donors [26, 59 d]. 

Dinuclear palladium organyls with a discoid molecular shape constitute a class of electron donor compounds displaying an unusual and complex phase behavior with TNF as a strong electron acceptor [64] (see Fig. 7). The mesomorphism of these meso-gens, exhibiting monotropic nematic discotic phases in their pure forms, in TNF-CT... 

When the disks have no long-range translational order, an analog of the nematic phase, identified as N, is formed. Very few compounds exhibit an Ng phase. It is not entirely miscible with the nematic phase formed by calamitic systems. If a chiral center is incorporated into a nematic discogen, the material usually exhibits a chiral nematic discotic phase. Figure 2.12 shows disk-shaped molecules exhibiting discotic liquid crystal phases between the solid and isotropic liquid phases. 

Fig. 16.3 Schematic representation of (a) a discotic nematic, (b) a columnar phase (Kimiar2006) 16.4.1.2.1 Nematic Discotic Phase...

An alternative way [25] to obtain a chiral nematic discotic phase is simply the use of a chiral organic solvent, e.g. the hydrocarbon R-(+)-Iimonene, in which, for example, an achiral, slightly folded disclike di-palladium organyl is dissolved. 

A similar scheme for discotic systems is shown in Fig. 1.33. The diffraction pattern of the nematic discotic phase is analogous to that of the calamitic nematic phase and contains only diffuse reflections, indicating the liquid-like distribution of face-to-face and edge-to-edge separations. For the columnar phases, the high-angle... 

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