Compounds polycatenars

For polycatenar hydrogen bonded complexes with fluorinated chains at both ends (e.g., 138,139, see Fig. 36) formation of columnar phases was observed [246]. However, compound 137, having a branched Rp-chain at one end and three RH-chains at the other has a sequence of three distinct phases in the unusual sequence Cub-Col-SmA-Iso. For the SmA phase of compound 137 a structure with intercalated aromatic cores and RF-chains and separated layers of the hydrocarbon chains was proposed. At lower temperature, when incompatibility rises and the aromatics and Rp-chains disintegrate, all three components form their own layers. However, this produces interface curvature and a columnar phase with square lattice is formed. On further cooling a transition to a cubic phase with Im3m lattice takes place which is most likely of the bicontinuous type [262]. This leads to the unusual phase sequence Cubv-Col-SmA where the positions of the Cubv and Col phases are exchanged with respect to the usually observed phase sequences. The Col-Cub transition at lower temperature could be the result of the decreased conformational disorder of the terminal chains which reduces the steric frustration and hence reduces the interface curvature. 

Polycatenars form a broken-layer type 2D density modulated phases because of the mismatch between the cross-section areas required for the terminal chains and for the mesogenic core. For such compounds layers become locally bent to provide more space for tails and, since the bending of layers is finite, this leads to... 

Polycatenar compounds, with more than one terminal chain at each end of a rod-like molecule can also exhibit columnar phases, although they possess a linear molecular structure. Three linear molecules aggregate together to form a self-assembled disc. These discs then stack up to form a fluid columnar structure. Non-dispersive charge-carrier mobility in such polycatenar compounds has also been found. ... 

Some of the simplest systems reported raise a possible point of semantics as to whether these systems are, in fact, polycatenar in nature or perhaps ought to be described as discotic. The first of these 77 was based simply on the coordination of 3,4,5-trialkoxybenzonitriles to Pd(ll), following a motif for the formation of simple, calamitic systems "" described some years earlier. Only two compounds with different chain lengths were studied ( =10, 18). The ligands were themselves not mesomorphic, but on complexation, a Col), phase was observed over small temperature ranges, i.e., 73-91 °C for the complex with six tridecyloxy chains, and between 58 and 80 °C for that with six octadecyloxy chains. 

Even if they are not, strictly speaking, polycatenar molecules, double swallow-tailed compounds with long branches can be considered as a special case of tetracatenar mesogens, where the branches are not directly attached to the terminal rings [57], as shown in Fig. 10. 

There are, in fact, rather few such compounds. We have recently reported the thermal behaviour of several series of polycatenar metal complexes, 40-45, and observed very interesting relationships between the structure of the metal complexes and their thermal behaviour [81]. 

This series of compounds did not strictly behave as true polycatenar mesogens in the sense that no columnar mesomorphism was observed, despite their relatively low aspect ratio. Increasing the number of aromatic rings (144) to elongate the mesogenic core modified the thermal behavior quite drastically. Thus, complex (144) now displayed a CoIq phase at high temperature (Cr 198 Colo 215... 

Thus, supermolecular liquid crystals with a cyclotriphosphazene dendritic core and polycatenar mesogenic units (144) were obtained in three steps by the conventional sequence of substitution (i), derivatization (ii and iii) methods from [N3P3CI6] (Scheme 9). Due to the microsegregation of the alkyl chains and the aromatic central cores and the space-filling properties, compounds (144) adopt a discotic conformation assembled in a columnar mesophase and illustrate the possibilities of using cyclotriphosphazenes for the design of columnar assemblies at room temperature, in the mesophase or in a vitrified solid state with interest for applications in material science. Similarly, the new family of solution processable, photoluminescent, monodisperse nanocomposite dendrimers (145) (Tg > 165 °C, > 465 °C)... 

Polycatenar compounds possess two to six flexible chains attached at the terminal rings of large rod-like cores, and are called bi-, tri-, tetra-, penta- and hexacatenar compounds, respectively. Malthete et al. [21] have published a good review that explains many details about this substance class. 

Table 42. Phase behaviour of lateral long-chain substituted, swallow-tailed and polycatenar compounds.

The simultaneous occurrence of nematic, cubic, layer and columnar structures in pure mesogens is of great importance to bridging the gap between calamitic and discotic molecules and the corresponding mesophases. Smectic, cubic and columnar phases have also been described in polycatenar compounds, and thus double-swallow-tailed mesogens can also be considered to be a special case of tetracatenar compounds (see Chap. XII of this volume). 

Compounds 5 were generally recrystallized from absolute ethanol. For different symmetrical phasmids and biforked mesogens, the synthetic route is similar with protected and deprotected phenols or anilines. The central ring moiety is often terephthal-ic acid, 1,4-cyclohexanedicarboxylic acid, or terephthaldehyde. With non-symmetric polycatenar mesogens, the different intermediate compounds are protected or deprotected phenols, anilines and acids or aldehydes. The same method is repeated to add a benzoate. The last step is often an esterification reaction (Scheme 2) unlike the imine derivatives, compounds with ester linkages can be easily purified by chromatography on silica gel with dichloromethane as eluent. 

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