Tilted columns, discotics

Figure 1. Schematic representation of columnar structures of discotic mesogens (a) upright columns, (b) tilted columns.

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). 

In discotic phases the orientation of the molecules is perpendicular to the molecular plane. Here, the columns can be arranged in a nematic or columnar manner. In the nematic phase the molecules possess a centre of gravity randomly ordered, but with the short molecular axis of each molecule more or less parallel. In the columnar phase, beside the preferable orientation of the short molecular axes, the disc-like molecules are ordered forming columns. Depending on the correlation strength between he columns these phases can be subdivided into ordered or disordered. A third possibility is to have a thermodynamically preferable position of the columns in the mesophase, like in a hexagonal cell. Additionally, a tilt of the columns is also possible. 

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). ... 

Nd phase. A phase similar to the rod-like smectic liquid crystal phase is the column phase in which molecules are packed as columns parallel to each other. The columns are arranged in a hexatic or rectangular array. Inside the columns, the spacings of the molecules are either constant or random. The axes of the columns may be tilted with respect to the normal of discotic molecules. Figure 1.12 sketches the chemical formulae of one example of a discotic liquid crystal molecule. 

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. 

Discotic Ferroelectrics. Columnar meso-phases with chiral disk-shaped molecules whose normals are tilted with respect to the column axis also manifest ferroelectric switching [217]. In such phases the direction of the spontaneous polarization is perpendicular to both the column axis and to the normal to the molecular disks. The tilt of the disk forms a helix with its axis oriented along the columns. In an amphiphilic... 

Columnar phases can be ferroelectric if the discotic molecules are chiral and tilted with respect to the column axis [74]. From sym-... 

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