Chiral nematic phases columnar

The prime requirement for the formation of a thermotropic liquid crystal is an anisotropy in the molecular shape. It is to be expected, therefore, that disc-like molecules as well as rod-like molecules should exhibit liquid crystal behaviour. Indeed this possibility was appreciated many years ago by Vorlander [56] although it was not until relatively recently that the first examples of discotic liquid crystals were reported by Chandrasekhar et al. [57]. It is now recognised that discotic molecules can form a variety of columnar mesophases as well as nematic and chiral nematic phases [58]. 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

Recently Ribeiro et al. [65] reported on the synthesis and characterization of a variety of tolanes that had optically active sulfinate groups. Some of these compounds, see 23, were found to possess a phase that exhibited oily-streak textures typical of chiral nematic phases and also defect pattern associated with columnar phases (the earlier photomicrograph Plate 6 for 14P1M7 is similar). 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

Doubly stranded DNA is densely packed in cell nuclei, sperm heads, bacteria, and viruses, with an estimated concentration of up to 800 mg/ml. At lower concentrations in saline solutions, a cholesteric (chiral nematic) phase forms, but at hundreds of mg/ml, a typical columnar texture can be observed (Figure 11.16). PBLG (poly-y-benzyl-L-glutamate), also a double-helix polymer, exhibits similar textures. X-ray diffraction and freeze-fracture investigations prove that the high concentration phase is indeed columnar, not smectic, with the molecular double helices packed into a hexagonal pattern (Figure 11.17). When the concentration is increased from the cholesteric to... 

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. 

A further development of such complexes is the synthesis of the electron donor-acceptor twins 6 of which seven representatives with three different kinds of bridging groups XY (one is chiral, see 6c, 6d and 6c, TABLE S) have ncrt only been prepared but show desired nematic prc )arties. Their columnar nematic phases have a much higher viscosity at low tonpaatures, in smne cases consulerably below room temperature. Three of these interesting twin compounds (6c, 6d and 6e) exhibit chiral nematic phases. 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

TABLE 4 Transition temperatures ( C, DSC data) of intermolecular donor-acceptor complexes formed spontaneously when heating equimolar mixtures of electron donor molecules (e.g. radial pentaynes of type 3b, R = H, not liquid crystalline, or the 2,3,6,7,10,11-triphenylene hexaether 4) and the non-liquid crystalline multi(nitro)fluorenone derivatives 5a, 5b or 5c as electron acceptors yielding preferably by induction the nematic or chiral nematic columnar (Ncoi or N coh respectively) phases. 

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