Columnar nematic phase

Cobalt(II) nitrate induces a columnar nematic phase in the 1 2 complex with 98 (Table 7) as deduced from X-ray scattering which is stable for 30 K. Also, nickel(II) nitrate turned 98 into a liquid crystal with an unknown phase. The complex between 99 and copper(II) nitrate showed the same unknown mesophase. The phase range was 140 K but the samples decompose before clearing. A possible reason for the induction of a mesophase by complexation is the stiffening of the crown and the adjacent flexibly linked groups [112]. 

Molecules of a disc-like shape form either nematic or columnar phases. Structurally the most simple mesophase is the discotic nematic phase (Nd). Just as in the case of the nematic phase formed by calamitic molecules, the molecules possess orientational order, but no positional order (Figure 2.8). The columnar nematic phase (Nc) consists of short columns of a few molecules that act like the rod-like molecules in the nematic calamitic phase (Figure 2.9). There is no organisation of the columns in a two-dimensional lattice. 

Fig. 1.1.10. Mesophases of discotic polymers (a) the hexagonal columnar phase. The diagram illustrates intercolumnar binding as well as intracolumnar back-folding of the main chain (Herrmann-Schonherr, Wendorff and Ringsdorf ) (b) the sanidic nematic phase composed of boards stacked parallel to one another (Herrmann-Schonherr et a/. ) (c) the columnar nematic phase (Ringsdorf...

A second noteworthy series is that of the dirhodium tetraalkanaotes and tetrabenzoates which have been studied extensively as thermotropic mesogens (vide supra). In solution in alkane, columnar nematic phases were formed as evidenced by the schlieren textures observed. 

FIGURE 6 Schematic representation of the discotic nematic phase, Nd (left) and of the columnar nematic phase, Njoi (right). 

Figure 13. Schematic diagram of the structure of the columnar nematic phase induced hy charge transfer interactions between an electron donor, pentakisfphe-nylelhynyl)phenyialkyl ether (in hlack), and an electron acceptor, 2,4,7-trinitrofluorenone (hatched) [37-39].

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

The two compositions of the hexaether 4 which exhibits a columnar phase with the non-mesomorphic fluorenone derivative 5c are the known examples [18] for reduced order phase inductions on heating of low molar mass mesogens resulting not in the complete destruction of the previous highly ordered mesophase, but the formation of the columnar nematic phase with its reduced degree of order. Apparently, this process arises from a combination of steric and electron donor-acceptor interactions within the complexes. 

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. 

TABLE 6 Transition temperatures (°C) of eight disc-shaped metallorganic compounds of the molecular structures 7 or 8 of whidi 7a-7e contain a super-disc core with the greatest diameter of 1.5 nm, two platinum or two palladium atoms, and different bridging units. Included are the binary mixtures of 7d and 7e with 2,4,7-hinitrofluorenone (5a) each exhibiting by induction die columnar nematic phase. 

The most mobile and least-ordered mesophase formed by discotic molecules is the Nd phase, sketched in Fig. 1.12. This is the discotic analogue of the calamitic nematic phase. The planes of the molecules lie more or less parallel, but there is no positional order in any direction. In the columnar nematic phase, Ncoi there is some degree of aggregation, with short stacks of molecules floating in a nematic array. Both of these phases are relatively rare and there only are few known examples. 

Fig. 1.12 The nematic discotic, Nd phase, where individual molecules have lateral freedom of movement, and the columnar nematic phase, Ncoi where the molecules are aggregated in short stacks of stacks of three or four molecules...

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