Discotic mesophases columnar hexagonal

Figure 6. Schematic phase diagram of a radial twin-pentayne ether as electron donor with TNF as the electron acceptor [26] Cr crystalline, Nj, nematic discotic, Col(,(, columnar hexagonal mesophase (o intracolumnar ordered), Iso isotropic liquid (see footnote 1 in Sec. 3 commenting on the nomenclature of columnar mesophases).

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.

There are several polyols that exhibit discotic mesophases. Diisobutylsilanediol, which has been known since 1955 [342] but was not classified as hexagonal columnar until 1980 [343], associates in dimers [343] or oligomers [344] in order to form disk-like units (42). 

Figure 33. Mesophases of discotic polymers (a) hexagonal columnar phase (b) the board-like, or sanidic, nematic phase (c) the columnar nematic phase.

All the complexes showed a discotic hexagonal disordered columnar mesophase (D jj). The mesophases were identified by X-ray diffraction. Platinum complexes exhibit a very clear thermochromism the color of the liquid crystalline discotic mesophase turns from green to red with increasing temperature from room temperature. 

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... 

In the mesophase, the molecules exist in a discotic hexagonal columnar ordered stmcture, schematically shown in Figure 4. 

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. 

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

In contrast to calamitic mesogens, discotic liquid crystals are built from disk-like molecules that can arrange into different structures, such as the discotic nematic mesophase, the discotic columnar mesophase, or the discotic hexagonal meso-phase. 

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