Discotic mesophases cholesteric

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. 

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. 2.16 Schematic representation of the four main types of mesophases. Smectic with ordered (a) and unordered (a ) arrangement of the molecules in layers b) nematic c) cholesteric and d) discotic (from Plate and Shibaev (1987) Courtesy Plenum Press). 

Figure 2.19 Orderings in LC mesophases. (a) nematic (b) smectic (c) cholesteric (d) discotic-nematic (e) discotic-columnar.

Discotic liquid crystals arise from disc-shaped molecules as nematic or cholesteric mesophases. Their structural characteristics are similar to that of their respective calamitic mesophases, that is, the normals... 

Lyotropic liquid crystals occur abundantly in nature, being ubiquitous in living systems.Their structures are quite complex and are only just beginning to be elucidated. However, in this monograph we shall be confining our attention mainly to the physics of low molecular weight thermotropic liquid crystals and do not propose to discuss polymer and lyotropic systems in any further detail. In chapters 2-5, we deal with the nematic, cholesteric and smectic mesophases of rod-like molecules and in chapter 6 discotic systems. 

The term mesophase also includes ordered liquids (nematic, smectic, cholesteric and discotics), which present long-range orientational order like in a solid, but positional disorder like in a liquid [2]. In these materials, large-scale molecular motion is possible, which is a characteristic of the liquid state rather than of the solid state. The term liquid crystals is conventionally used to address them. This sub-class of mesophases will not be treated in this context. 

The limits of deformed LC networks are the usual completely ordered (crystalline) and completely disordered (isotropic) phases. The nematic LC phase is a mesophase between them. The oriented parts are the sequences along the chain backbone. In this case, the disorder is the already-mentioned sliding of segment relative to one another to place them out of register. There are also a variety of smectic LC phases, in which layers of molecules or chain sequences occur in layers that are disordered relative to one another. In contrast, cholesteric phases have layers of nematic arrangements that are stacked in rotated arrangements, and a similar stacking occurs in the case of the discotics. ... 

FriedeP was the first who distinguished three main classes of liquid crystals, according to the different kind of orders in the mesophases nematic, smectic, and cholesteric. From the point of view of the geometrical shape of molecules, we divide the thermotropic LCs into calamitic phases (when the molecules are rodlike), sanidic phases (when the molecules are bricklike), and the discotic phases (when the molecules are disklike)... 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

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]. 

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