Columnar phases, discotic mesogen

A few words of clarification about the nomenclature are relevant here. Columnar phases have been known for many years they were evident, for example, in the work of Spegt and Skoulios (28) on metal soaps, although these are not classical disk-shaped molecules. In 1977, however, a hexasubstituted benzene derivative was reported (29), this derivative was the first example of a properly disk-like mesogen, and the term discotic was coined to describe the mesophases it formed. Thus, for example, the discotic hexagonal phase was labeled Dh- The introduction of this nomenclature has actually caused confusion as disk-like molecules are not alone in their capacity to form columnar phases (indeed, some... 

Recent work focuses on non-classical mesogenes which are built up by self-assembly. One example is a family of polymers containing disk-like groups which form no liquid crystalline phase, but ean act as an electron acceptor or donor. Charge transfer complexation with a complementary low molecular mass compound induces nematic or columnar discotic liquid crystalline order [153,154]. Figure 13 demonstrates this with the example of a polyester, bearing electron-rich tetra(alkoxy)tri-phenylene-units in the main chain, mixed with the electron deficient aromatic 2,4,7-trinitro-9-fluorenone (TNF). While the pure polymer shows a non-ordered isotropic melt, a columnar phase appears on addition of TNF. 

This formation of nematic, lamellar and columnar phases within a single class of molecule leads to the idea that they can be considered as a missing link between calamitic and discotic mesogens. 

Understanding the organization of disc-shaped mesogens in columnar phases is of practical interest because of their useful optoelectronic applications [168-170]. It will be instructive to explore the role of fluctuations in influencing transport properties of thermotropic discotic liquid crystals—in particular, in the columnar phase. 

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. 

Double and triple hydrogen bonds can also be used to design the supramolecular mesogens. Complexes 33 [49] and 34 [21] have shapes between rodlike and dis-cotic mesogens. Complex 33 exhibits monotropic calamitic or discotic phases. Triple hydrogen bonded complex 34 is a discotic mesogen with a columnar phase from 200 °C to room temperature on cooling. 

The most obvious family of compounds able to induce columnar phases is that constituted by the macrocyclic compounds which often possess a discoid molecular shape. The ability of macrocyclic systems to stack and to self-assemble into columnar mesophases confers on these systems the properties required for potential utilization in molecular devices based on photonics, electronics, or low-dimensional ionic transport. The search for new discotic mesogens is thus still of current interest, in order to improve their chemical, thermal, and photophysical stabihty, and new families of discogenic materials appear regularly. 

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... 

The vast majority of discotic mesogens with cyclohexane cores and six substituent chains are based on ui- and s-IS. Pertinent to the discussion above concerning the relationship between the direction of projection of chains on 1,2,3,4,5,6-hexasubstituted cyclohexanes, the hexa(n-hexyl) ether of m-IS with one axial hexyloxy (6-O-m-IS) has been shown not to be liquid crystalline the corresponding s-IS hexaether with a more disk-like shape (6-O-s-IS) forms an enantiotropic columnar discotic phase... 

Columnar phases are typically formed by either thermotropic liquid crystals made up of discotic mesogens stacking upon each other to form columns or by lyotropic liquid crystals composed of rod-like micelles. These columns or rods arrange into two-dimensionaUy correlated structures. Alongside the long axis of the columns... 

Nematic liquid crystalline phases can be thought of as smectic phases absent the layering effect (Figure 13.7 A). There is clearly still a preferred orientation, a director, but that is the only type of order. Another type of phase termed the columnar or discotic liquid crystalline phase can form when the mesogen is more disk-shaped rather than cigar-shaped (Figure 13.7 A). 

Columnar phases are formed by discotic mesogens, examples of which are shown in Fig. 5.7. Discotic molecules can form a nematic phase (termed Nd) just like calamitic mesogens. In addition, several types of columnar... 

Vorlander also had the idea to look for mesogenic properties in the cases of star-like or cross-like molecules, however, his coworker did not have any success in this area [5, 6]. About 1977, Chandrasekhar et al. [7] and Billard et al. [8] were, independently, able to prove that disk-like molecules can in fact form mesophases, called columnar phases. Since then, several hundred discotic compounds have been synthesized [9-14]. 

Figure 28. Structures and symmetries of various columnar phases of discotic mesogen (a) hexagonal Colhd (b) rectangular Cobd (P2i/a) (c) rectangular Colfd (C2/m) (d) the 4-sublattice triangular phase (P321). (From [392]).

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