Polycatenar mesogens

Fig. 36 Examples of molecular [262] and supramolecular [246] polycatenar mesogens with two fluorinated ends (138, 139) and combining a fluorinated with a nonfluorinated end (137) (77° C)...

LC phases usually corresponds to at least the number of attached chains [258]. Moreover, no SmA phases were observed for non-fluorinated polycatenar mesogens, whereas chain fluorination obviously enables the formation of SmA phases, which are most probably of the de Vries type. It seems that non-fluorinated polycatenars are mainly stabilized by core-core interactions, whereas the mesophase stabilization by the fluorophobic effect retains the layer structure even if core-core interactions are relatively weak. 

Donnio, B., Heinrich, B., Allouchi, H., et al., Generalized model for the molecular arrangement in the columnar mesophases of polycatenar mesogens. Crystal and molecular structure of two hexacatenar mesogens. J. Am. Chem. Soc. 2004, 126, 15258-15268. 

Note that, for some time, the columnar phase of these polycatenar mesogens was given the symbol 0. However, following the arguments presented above, the use of Col is more appropriate and is now vddely accepted. 

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. 

The ( -arene)tricarbonylchromium complexes 55 were reported by De-schenaux et al. [117]. The uncomplexed ligands showed a mesomorphism characteristic of polycatenar mesogens, progressing from N and Sc phases at shorter chain lengths to Colob and Colh phases at longer chain lengths. 

Fig. 9a,b. Possible arrangements of polycatenar mesogens in the coliunnar phase according to... 

Figure 52 Schematic representation of polycatenar mesogens showing different potential terminal ring...

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. 

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)... 

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