Mesophase behavior, discotics

The rigid nature of the mesophase pitch molecules creates a strong relationship between flow and orientation. In this regard, mesophase pitch may be considered to be a discotic nematic liquid crystal. The flow behavior of liquid crystals of the nematic type has been described by a continuum theory proposed by Leslie [36] and Ericksen [37]. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Dihydroxycyclohexyl compounds 70 and 71 containing two and three alkyl chains, respectively, have been shown to exhibit columnar mesophases [157, 158], In the liquid-crystalline behavior for the diol mesogens, intermolecular hydrogen bonding plays a determining factor for the formation of mesomorphic hexagonal discotic aggregates. 

H. Monobe, N. Terasawa, K. Kiyohara, Y. Shimizu, H. Azehara, A. Nakasa, M. Fujihara, Alignment behavior of discotic nematic and rectangular columnar mesophases on self-assembled monolares of alkanethiols and asymmetrical disulphides. Mol. Cryst. Liq. Cryst. 412, 229-236 (2004)... 

While borazine is analogous in shape and structure to benzene, the behavior observed here differs sharply from that of benzene. During the pyrolysis of benzene at temperatures in excess of 500 C, linear biphenyl, terphenyl and polyphenyl units are formed which do not exhibit liquid crystallinity [22]. In contrast, when naphthalene or anthracene are heated, they will enter a liquid crystalline mesophase and will produce graphitizable carbon upon continued pyrolysis (Figure 2.3). This mesophase consists of a wide range of planar, polynuclear species which have a tendency to stack, thereby forming a discotic phase. The mesophase is also observed to form when a 50 50 mixture of benzene and naphthalene is heated [23]. 

The advantage of this method is that it permits the selective synthesis of substituted HBCs. Without any functional groups, HBCs are insoluble in most organic solvents. The addition of solubilizing chains facilitated the characterization of those molecules. In addition, the processability of such HBCs allows for their application as functional materials. For example, alkyl substimted HBCs form discotic mesophases, which show liquid crystalline behavior [33]. 

The monofunctional P-U complex (supermolecule 19, Table 3) has been shown to form disklike dimers that can self-assemble into columns displaying thermotropic behavior. Each disk has a thickness/diameter ratio of 0.1 [155], The formation of columnar mesophases by similar discotic supermolecules is described in the previous section when cases in which liquid crystallinity and growth are hierarchically related, coupled, or totally uncoupled were considered. Lack of data on the equilibrium constants, or DP, prevents a definite assessment of the assembling mechanism of discotics based on supermolecule 19. 

The orthogonal arrangement of the disc-like molecules in the columns of and D id phases makes these phases uniaxial, while the tilted phases (Drd and Doh.d and Dt) are optically biaxial. There are two additional columnar phases labeled as and that have not yet been classified. The columnar phases were discovered before the observation of a nematic phase for disc-like molecules. Both chiral nematic phases and the re-entrant behavior have now been observed in discotics. The phase diagram and molecular structure of a typical discotic liquid crystal are shown in Fig. 1.11. Finally, it is noted that another classification scheme for the discotic mesophases has been used [1.26], which is based on the notation used for the conventional smectics. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

The quadruply bonded molybdenum carb-oxylates [Mo2(02C H2 +i)4] present a similar behavior to that observed for M=Cu, Rh, Ru. When a side chain is branched on the complex the transition temperature is lowered to about 37 °C. Substitution of fluorine increases the temperatures but an enan-tiotropic hexagonal discotic mesophase is still found [54]. 

A particular attribute of octasubstituted phthalocyanines is their liquid crystalline behavior. Phthalocyanines substituted in peripheral positions by long-chain substituents form, depending on the structure (alkyl or alkoxy), liquid discotic (from the disclike shape of the molecules) crystalline phases [68-71 ] or columnar meso-phases [72]. Liquid crystalline phases are found for side chains longer than C4 or Q (depending on the metals and substituents used) [73]. In these mesophases the phthalocyanine moieties stack in columns with arrangements of different symmetry (Fig. 15.8), depending on the nature of the side chains [71,74]. Only chains connected with a heteroatom X (X = O, S) to the aromatic macrocycle show an arrangement with the molecular planes perpendicular to the column axis. 

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