Mesophases columns

It has been found [42] that the barrier for the CTV ring inversion in 68 in isotropic solution at 100 °C is AG 26.9 kcal mol-1, a figure which does not differ from that of other cydotriveratrylenes. In the mesophase at 100 °C, the inversion rate is slower by two orders of magnitude, with AG 30.3 kcal mol-1. Thus the tight embedding of the CTV units in the mesophase columns increases the inversion barrier by 3.4 kcal mol-1. 

The mesophases and the transition temperatures are also listed in TABLE 1. As die transition temperature is more sensitive to the purity of the materials, a wider dispersion among the literature values can be seen compared to the dielectric properties, and the dispersion is represented by (xx - yy) in the mesophase column, and mesophase transitions given in parentheses rqiresent a monotropic phase. 

The mesophases of thermotropic liquid crystals are described as calamitic if the constituent molecules are rod-like and columnar, if the constituent molecules, which often have a disc like shape(discotic), stack into columns. 

Fig. 2. Schematic representation of the supramolecular cylinders of the dendrimer derived from macromonomer 9 (R=OC12H25,n=3) in the Qh mesophase atop view of a cylinder containing six repeat units in a stratum with the alkyl tails melted to match the average column radius determined by X-ray scattering experiments b side view of a cylinder containing 30 repeat units of the polymer assembled with melted alkyl tails. Reproduced with permission from references 5 a...

To understand how chirality is expressed, it is important to first describe the different thermotropic mesophase assemblies which can be formed by chiral discotics. Even though expression of chirality has been observed in thermotropic mesophases, the chiral expression occurs in a rather uncontrolled manner, and systems which are suitable for applications, for example, easily switchable columns/ferroelectric discotic liquid crystals, consequently have not yet been developed. Hence, the assembly of discotics in solution has received considerable attention. Supramolecular assemblies of discotic molecules in solution are still in their infancy and have not yet found commercial application, but they are of fundamental importance since they allow a detailed and focused investigation of the specific interactions that are required to express chirality at higher levels of organization. As such, the fundamental knowledge acquired from supramolecular assemblies in solution might formulate the design criteria for thermotropic chiral discotic mesophases and provide the necessary tools for the creation of functional systems. 

Trzaska and co-workers showed a similar propeller mechanism for the formation of helical columns from disclike metallomesogens (29-31).34 These metallomesogens also have C3 symmetry and 30 and 31 are provided with chiral side chains. In the hexagonal columnar mesophase these chiral side chains induce a Cotton effect in the chromophore of the helically arranged core. Heating the mesophase to the isotropic liquid results in the disappearance of the Cotton effect because of the loss of helical order. This effect illustrates the need for the molecules to be positionally ordered in order for the side-chain chirality to be transferred to the supramolecular column. 

A helical arrangement within columns was also found for other metal 3-diketonate complexes provided with chiral side chains (32) by Serrano and co-workers.35,36 These compounds form rectangular columnar mesophases with helical order within the columns. A spin-coated sample of 32 showed a positive exciton-splitted signal in the CD spectra, which was interpreted as a left-handed (M) helix. Annealing of the film resulted in much higher optical activities and a shift of the absorption maxima. The observed optical changes clearly point to a chiral organization of the columns in the mesophase. 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

Van Nostrum and co-workers first published evidence for a chiral superstructure in the discotic mesophase of chiral phthalocyanines in 1993.44 Homochiral phthalocyanine 43 was compared with its racemic analog 44. Whereas 44 showed a normal Colh<) phase and did not form helical columns, the homochiral side chains of 43 gave rise to a helical packing of the discotics... 

Discotic liquid crystals based on carbohydrates are intrinsically chiral due to the chiral nature of their core.49,50 Numerous discotic liquid crystals derived from these mesogens have been reported however, the expression of chirality in the subsequent mesophases has only rarely been seen. An example of the latter is the a-anomer of penta- O -decanoylglucopyranosc (47), which forms a discotic mesophase in which the molecules pack helically in the 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). 

X-ray spacing along the column axis per repeat imit of the main chain as determined for the columnar mesophase in bulk. 

Mesophase in which disc-shaped molecules, the disc-shaped moieties of macromolecules or wedge-shaped molecules assemble themselves in columns packed parallel on a two-dimensional lattice, but without long-range positional correlations along the columns. 

Note Depending on the order in the molecular stacking in the columns and the two-dimensional lattice symmetry of the columnar packing, the columnar mesophases may be classified into three major classes hexagonal, rectangular and oblique (see Definitions 3.2.2.1. to 3.2.2.3). 

Columnar mesophase characterised by a hexagonal packing of the molecular columns. 

Note 4 The lyotropic equivalent of a columnar hexagonal mesophase is known as a hexagonal mesophase in it, columns of amphiphilic molecules are surrounded by the solvent, normally water, or an oil in an inverse hexagonal mesophase. 

Columnar mesophase characterised by a liquid-like molecular order along the columns, in which the columns are arranged in a rectangular packing. 

Note 4 There also exist chiral columnar rectangular mesophases, with the molecular discs tilted periodically in the columns and with the tilt directions changing regularly down the columns. 

Fig. 15. Plan views of the two-dimensional lattice of the columns in columnar rectangular (a) to (c) and oblique (d) mesophases. Ovals indicate the planes of the molecular discs.

The X-ray diffraction patterns show that the materials obtained from the various configurational isomers of tartaric acid have different architectures. Both are hexagonal columnar mesophases, but, whereas the data for (LP2, LU2) are consistent with columns formed by three polymeric strands having a triple helix superstructure (Figure 41), those for the (MP2, MU2) mixture fit a model built on three strands in a zig-zag conformation. The LD mixture has another arrangement again. 

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