Rectangular columnar discotics

Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

Spontaneous polarisation can also be observed for chiral discotic metallomesogens forming columnar mesophases, when the chiral molecules are tilted with respect to the column axis. The tilt induces a dipole moment within the plane of the molecule. A net macroscopic polarisation can be obtained for rectangular columnar phases with Cz or P2i symmetry. Serrano and Sierra reported on ferroelectric switching in the columnar mesophase for chiral /3-diketonate complexes (Figure 2.52). ... 

H. Monobe, N. Terasawa, K. Kiyohara, Y. Shimizu, H. Azehara, A. Nakasa, M. Fujihara, Alignment behavior of discotic nematic and rectangular columnar mesophases on self-assembled monolares of alkanethiols and asymmetrical disulphides. Mol. Cryst. Liq. Cryst. 412, 229-236 (2004)... 

Even though no tabular data are available for truxene derivatives, inclusion in this Datareview is important because these materials exhibit discotic nematic phases. The truxene phase data presented by Busine and co-workers [11] are for the transition of the discotic nematic phase to the disordered rectangular columnar phase (TABLE 18). Over the phase range studied which was up to 900 bar, the relationship between T and p is most conveniently represented as linear. Once again the magnitudes... 

TABLE 18 Fitting equati(Hi parameto-s fw discotic nematic-disra-dered rectangular columnar [vessure-temperature phase transition lines for hmnologous truxenes. Fitting equation is T/K = a + b(p/bar). 

Figure C2.2.7. Schematic illustrating tire classification and nomenclature of discotic liquid crystal phases. For tire columnar phases, tire subscripts are usually used in combination witli each otlier. For example, denotes a rectangular lattice of columns in which tire molecules are stacked in a disordered manner (after [33])...

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 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

Discotic liquid crystals arise from disk-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to the respective ealamitie mesophases, that is, the normals of the disks are oriented parallel. Instead of the smectic mesophases, diseotie columnar liquid crystals arise from eonnecting the disks to each other. The columns of the discotic columnar mesophase form a two-dimensional lattice whieh is in a hexagonal or rectangular modification. In addition, the columns may be tilted (Fig. 2f,g). 

Fig. 4.1 Typical LC phases, a nematic (N), b smecitc A (SmA), c smectic C (SmC), d discotic nematic, e columnar nematic, f columnar hexagnal (Colji), g columnar rectangular (Coir), h columnar oblique (Colob)...

FIGURE 3.1 Schematic representations of commonly observed discotic liquid crystalline phases. Discotic nematic (a) columnar nematic (b) columnar hexagonal (c) columnar rectangular (d). 

Figure 28. Structures and symmetries of various columnar phases of discotic mesogen (a) hexagonal Colhd (b) rectangular Cobd (P2i/a) (c) rectangular Colfd (C2/m) (d) the 4-sublattice triangular phase (P321). (From [392]).

Figure 1. Schematic diagram illustrating the basis for the classification and nomenclature of the discotic me-sophases. For the columnar phases the subscripts are often used in combination with each other. Hence refers to a (two dimensional) rectangular lattice of columns within which the discs are stacked in a disordered manner...

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