Liquid crystals polycatenar mesogens

While rods and disks represent extremes of shape anisotropy that give rise to mesophases, one class of compoimds appears to bridge the gap between these extremes. These materials are termed polycatenar liquid crystals (30), where polycatenar simply means many tails. The mesogens are characterized by their rod-like shape, with a rather extended core, and by possession of three to six terminal chains. There are several isomeric possibihties for such systems, two of which sae considered in this review. 

Having synthesized a number of complexes of monoalkoxystilbazoles, we explored the effect of using various poly(alkoxy)stilbazoles, given the interesting mesomorphism found in polycatenar mesogens (Section VI,A,3). This work is discussed in detail elsewhere 24). Here, we give an overview of the work to enable the reader to see the overall pattern of liquid-crystal behavior and the issues that arise. 

While it is in discotic liquid crystals that the formation of columnar phases is most readily recognized supra), there exists a family of non-disk-like mesogens, the polycatenar liquid crystals, where these mesophases are also formed extensively. As will be seen from their shape, this observation is of some interest, but it is the fact that certain polycatenar materials can, within a homologous series, show mesomorphism characteristic of both rod-like and disklike mesogens that makes them particularly interesting. 

While it is in discotic liquid crystals that the formation of columnar phases is most readily recognized (vide supra), there exists a family of non-disk-like mesogens, the polycatenar liquid... 

Results were also described from studies which introduced lateral substituents into these complexes and in one paper the effect of introducing lateral alkanoate groups was reported (113). " The use of lateral alkanoates had been described previously by Maitlis and co-workers in palladium complexes of 4-alkoxystilbazoles (Section 7.9.14.3.6(iv)) and had been shown to induce the formation of liquid crystal nematic phases. In the present situation where the complex was polycatenar, it was not clear whether the chains would act to increase the volume of the core of the polycatenar mesogen or simply as extra chains, so rendering the complex more classically discotic. In fact, the answer was neither and at all chain lengths (n and m) and degrees of substitution (R = H or OC H2 +i) where mesomorphism was observed, a nematic phase was seen at or near room temperature. This behavior was somewhat puzzling and is so far without explanation. 

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)... 

Phasmids and polycatenar mesogens are just ten years old. Their intermediate shape between classical rod-like and disc-like liquid crystals allows practically all the mesomorphic symmetries (0, Cub,... 

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