Thermotropic columnar phases, liquid crystalline

An inorganic compound, diisobutylsilanediol 79, shows a columnar mesophase due to the formation of hydrogen-bonded columnar networks based on the dimer [165, 166]. The temperature range of the mesophase is between 88 and 99 °C. Although this compound was synthesized in 1952, the liquid-crystalline behavior of the compound was not established until later. Analogous compounds, dihydroxytetraalkyldisiloxanes, show thermotropic columnar phases [167]. 

Thermotropic liquid-crystalline properties of different metal alkanesulfonates are studied by microscopy and X-ray diffraction [59]. Sodium soaps show smectic polymorphism of smectic A and smectic B phases. Ammonium soaps only show smectic A phases but polymorphism in the crystalline state. Calcium soaps show columnar mesophases. In Figs. 32 and 33 some textures and x-ray diffraction patterns are depicted. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

Columnar phases could also be obtained by reacting disk-shaped molecules such as crown ethers with lanthanide salts. BiinzU and coworkers reported, for example, the preparation of thermotropic hexagonal columnar phases from nonmesogenic crown ethers and several different ions of the lanthanide family [76], An additional property of these compounds is a metal-centered emission, in the case of the liquid-crystalline phases containing Eu and Tb, making them attractive for the design of luminescent liquid-crystalline materials [76],... 

Liquid crystalline phases can show not only long-range orientational order as nematic phases do but also long-range positional order. When this positional order is one-dimensional, the mesophase is called lamellar or smectic when it is two-dimensional, it is called columnar. The latter case is often found with thermotropic liquid-crystal disk-like molecules. Such molecules stack in columns that assemble on a 2-D lattice of hexagonal, rectangular, or oblique symmetry. The molecules in a given column only show 1-D liquid-Hke order and the uncorrelated columns are free to slide past each other, which ensures the mesophase fluidity [73]. 

The molecular organization in thermotropic liquid-crystal line phases is associated predominantly with a rigid anisometric architecture of the constituent single molecules. The triazines 13 were the first examples of electron donors that fotm columnar phases, which give rise to the induction of smectic liquid crystalline structures through donor-acceptor interactions (Figure 11). 

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

The absence of mesomorphism in these compounds was explained on the basis of space-filling requirements. Thus, the intercalation of pyrazine between the binuclear units creates free volume which needs to be filled to obtain a stable, condensed phase when the carboxylates bear only one chain, the interdimeric space is likely filled by the aliphatic chains belonging to a different polymeric chain, giving rise to a crossed structure which prevents the formation of a columnar mesophase. However, as will be seen later, liquid-crystalline behavior was induced in the case of mixed-valence diruthenium(II,III) carboxylate complexes with bulky equatorial Kgands bearing two and three aliphatic chains as with such ligands, it was possible to fill the interdimeric space and thus to induce a thermotropic columnar mesophase. Very recently, the synthesis, characterization, and mesomorphic properties of pyrazine-polymerized divalent rhodium benzoates have also been reported (99). " Most of these compounds exhibit columnar (Colh, Coir, CoIn) and cubic mesophases with melting transition temperatures close to, or even below, room temperature. 

Low molecular mass semiconductors are particularly appealing for computational scientists because the timescales of their rearrangements are relatively fast and often accessible to atomistic simulations. It is then becoming possible to reproduce both the equilibrium structure and the self-assembly process of crystalline, liquid crystalline, and amorphous organic semiconductors. Among those, thermotropic LCs are certainly the most fascinating and challenging materials from the structural point of view because of the variety of different phases (nematic, smectic, columnar, and many more [124]) they exhibit in relatively small temperature windows. 

The cubic phases of thermotropic liquid crystals display an independent class of liquid crystalline phases besides the smectic and columnar ones. The great variety of chemical structure leads to different cubic phases which can be expected to be constituted of molecular aggregates of different character. 

The liquid crystalline phase of dinuclear copper stearate has been known since 1938, and was reinvestigated in 1964 [38] but it was not until 1984 [39] that this mesophase was definitely characterized by Giroud-Godquin et al. as columnar discotic by X-ray diffraction. This was the first example of a thermotropic hexagonal discotic meso-... 

This book does not intend to cover the whole field of thermotropic liquid crystalline (TLC) materials as it is extremely difficult to cover within a single book. Instead it presents a collection of Chapters written by experts on various exciting topics in the field. Properties of recently developed TLCs (such as banana-type, thiophene-based, and columnar TLCs), phase biaxiaUty, and novel polymeric TLCs are discussed In detail. Solid-state NMR studies to obtain atomistic-level structural and geometrical information of TLCs are presented. Synthesis of liquid crystalUne conjugated polymers, fast switching of nematic materials by an electric field, and photoconducting discotic systems are also presented. 

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