Polycatenar liquid crystals

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. 

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. 

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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. 

However, in 1986, Chandrasekhar [70] published a derivative (Figure 35) which he claimed to show a biaxial nematic phase. This report was interesting because the biaxial nematic phase (Nb), demonstrated in lyotropic systems [71], had been long sought after in thermotropic materials. Further, the molecules were described as bridging the gap between rod- and disc-like materials (a reference perhaps better reserved for polycatenar liquid crystals—vide infra)... 

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. 

The division of the subject matter concentrates primarily on the type of liquid crystal phase formed and, to a large degree, this is a reflection of molecular shape, although as the reader will see this is not such a clear-cut distinction where polycatenar liquid crystals are concerned. However, within these broader classifications, material is then organized paying attention to the ligand types used to form the complexes. This is not a unique classification, but it does provide some sort of framework within which the field may be viewed. 

FIGURE 9 General structure of polycatenar liquid crystals. In this figure a hexacatenar liquid crystal is shown. 

Fig. 2 Left. The basic 2D structure in the rod-like liquid crystals made by strongly polar molecules (the dipole moment directions are represented by arrows). The asymmetric rod-like molecules tend to form double layer structures in order to compensate dipole moments. Since the thickness of a double layer is smaller than twice the thickness of a single layer the resulting stress in the system is relieved by the in-layer modulation. Right. 2D structures in polycatenar rod-like liquid crystals. The ribbon like structure is formed by the asymmetric polycatenar molecules. To accommodate the additional alkyl chains layers become slightly bent. Both structures possess the body centered crystallographic unit cell...

Ferrocene-containing co-polyesters, preparation, 12, 350 Ferrocene-containing liquid crystals dendrimers, 12, 235 disubstituted ferrocenes, 12, 227 hydrogen-bonded ferrocene derivatives, 12, 234 monosubstituted ferrocenes, 12, 222 polycatenar ferrocenes, 12, 234 properties, 12, 221... 

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 cubic phase is still a rather rare observation in thermotropic liquid crystal systems, and is foimd in conventional calamitic materials, as well as in polycatenar materials vide infra). It is a viscous, optically isotropic phase whose kinetics of... 

The same type of columnar structure is also proposed for other polycatenar materials, such as tetrone derivatives [51], phasmidic macrocyclic liquid crystals [52], silver (I) complexes [53], or 2,2 -bipyridine derivatives [54]. 

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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