Columnar 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. 

Gorecka E, Pociecha D, Mieczkowski J, Matraszek J, Guillon D, Donnio B (2004) Axially polar columnar phase made of polycatenar bent-shaped molecules. J Am Chem Soc 126 15946-15947... 

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. ... 

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. 

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 study was extended to polycatenar derivatives 7 and the phase diagram for the behavior of the 3,4-tetracatenar materials (7 R = H) is shown in Figure 26. Thus, the addition of the ReBr(CO)4 moiety has suppressed the SmC phase and, due to the increase in the core volume, has ensured that the columnar phases (and now a cubic phase, too, with Ia3d symmetry ) occurred to longer chain length. In common with many other systems, it is assumed that the phase indicated Col is, in fact, Colh, while that indicated Col is one of the Col phases. All complexes of the hexacatenar derivative (7 R = OC Fl2 +i) showed a columnar phase. 

If it is straightforward to understand why columnar mesophases are easily obtained with disk-like shaped molecules (the columns are formed by the piling of the flat cores of the molecules, the molten aliphatic chains filling the space between the columnar cores), it may appear rather surprising, at first sight, to observe columnar mesophases with polycatenar calamitic molecules. 

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]. 

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...

R 553 A. C. Ribeiro, P. J. Sebastiao and C. Cruz, Contribution of Proton NMR Relaxation to the Investigation of Molecular Dynamics in Columnar Mesophases of Discotic and Polycatenar Molecules , Pramana, 2003,61, 205... 

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... 

---