Structures 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. 

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. 

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...

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. 

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. ... 

So what about the cubic phase In polycatenar systems, it is possible to rationalize the formation of cubic phases on the basis of surface curvature alone, which will be considered in subsequent sections. However, it can be argued that, for calamitic systems, these arguments do not hold—at least on their own—and that other factors are important. For example, if cubic-phase formation is due to surface curvature, it is not possible to explain why an Sa phase (lamellar and with no surface curvature) is seen at higher temperatures. An important factor is the presence of specific intermolecular interactions and in the case of the silver systems, these are the intermolecular electrostatic interactions resulting from the presence of formally ionic groups. This is consistent with the observation of cubic phases in the biphenylcarboxylic acids and hydrazines (Fig. 29), as well as with other materials. However, it is also evident that this is not the only factor, as no cubic phase is seen with anion chains shorter than DOS, while other studies with fluorinated alkoxystilbazoles showed that the position of fluorine substitution could determine the presence or absence of the mesophase observed in the unsubstituted derivatives (56). Thus, structural factors are clearly not negligible. 

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]. 

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]. 

Fig. 5.30 Development of mesophase strueture including vesicle-like columnar and micellar cubic phases depending on the length and number of the C-terminal chains and the length of the N-terminal chains and the structure of the linking group X of polycatenar imidazolium salts 90. Reproduced from Ref. [198] by permission of The Royal Society of Chemistry...

In all the two ring-systems discussed above, each elementary repeat unit is constituted of one molecule only, similar to pure discotic materials, and as such differ from true polycatenar materials. Indeed, for the latter, the internal structure of the columns is completely changed upon the elongation of the rigid cores, and the repeat unit is constituted of several molecular species which aggregate together into clusters, these clusters stacking into columns. These systems will be discussed below. 

Dianionic platinum(II) complexes ((119) n =10, 14, 18) with sulfur-rich dithiolato ligands based on tetrathiafulvalene have been reported.These potentially interesting complexes for their unique electronic properties, having a polycatenar structure unfortunately did not form a mesophase. The absence of mesomorphism may be simply due to the bulkiness of the counter ion. 

Other 5,15-disubstituted porphyrins were described by Ohta and co-workers, but most do not fit the label polycatenar and are described in the section on discotic materials (Section 1.9.12). However, one derivative did possess a more conventional tetracatenar structure (131) and was reported as showing the so-called discotic lamellar phase. While it does not contain a central metal, it is included here for completeness. 

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

FIGURE 6.15 Polycatenar bent-core molecules 7 and their structural organization (a-c) in a ColhPAF phase. Reproduced from [162] with permission from the American Chemical Society. 

FIGURE 6.16 Polycatenar bent-core-like molecules 8 and their structural organization in a ColhPpE phase. The dipole points from the negative to the positive, as defined in physics. The pictures in (b) and (c) are taken with minor modifications from [166]. Reprinted from [166] by permission of the American Association for the Advancement of Science. 

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). 

As far as miscibility studies are concerned, no example of complete miscibility between the cubic phases of substances with different chemical structures could be observed. The binary mixture of compounds 2a-l and la-2 [8] shows a very restricted miscibility in the cubic phase (smaller than 5 mol% of the second component). The cubic phase of the polycatenar substance 7e-2 is able to incorporate about 80% of the rodlike component la-4 [20]. In the binary systems ll/la-4 [21], 10-3/la-4, 10-3/ 2a-3, and 10-3/11 [22] the cubic phases are separated only by a small heterogeneous region. 

---