Biforked compounds

For example, for the biforked compound in Fig, 7, the values of Ub obtained are close to 4.5 and are weakly temperature dependent. This number (=4,5 molecules in each elementary cluster of the column) would correspond to an average paraffinic crown formed by 18 alkyl chains. Now, considering a disc of diameter of 30 A equal to the length of the rigid core, its circumference corresponds precisely to 18 times the average distance between molten aliphatic chains (about 5 A). Thus, the circumference of the columnar rigid core seems to be an essential parameter to control with respect with the number of terminal chains in each molecule in order to fully embed the core in... 

Fig. 7. Chemical structure of a biforked compound and its corresponding thermotropic behaviour. K is the crystalline phase. Sc the lamellar smectic C phase, 4>h the colmnnar mesophase and 1 the isotropic phase...

Such molecules exhibit columnar mesophases and smectic C phases. In contrast to the corresponding tetracatenar compounds described above, the columnar phase is, in most of these swallow-tailed compounds, the low temperature phase with respect to the smectic C phase [58]. This inversion of thermotropic sequence with respect to the biforked compounds described just above has not found any explanation in relation with molecular parameters. The symmetry of the columnar two-dimensional lattice is found to be hexagonal, oblique and even rectangular [59] in some cases. For the latter case, the model proposed involves several molecules packed side by side to form the lattice unit, the different moieties being packed alternately in order to form a two-dimensional centred cell as shown in Fig. 11. 

To date, the crystal structures of more than 200 mesogenic compounds are known. In this review, we wish to present a general overview of the crystal structures of mesogenic compounds up to the end of 1997. Unfortunately, it is not possible to consider the crystal structure determinations of carbohydrate liquid crystals [13, 14], metallomesogens [15-18], phasmid and biforked mesogens [19-22], perfluorinated mesogenic compounds [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], cinnamate compounds [8, 36-40], and discotic liquid crystals [41-43] due to the lack of space. 

Compounds 5 were generally recrystallized from absolute ethanol. For different symmetrical phasmids and biforked mesogens, the synthetic route is similar with protected and deprotected phenols or anilines. The central ring moiety is often terephthal-ic acid, 1,4-cyclohexanedicarboxylic acid, or terephthaldehyde. With non-symmetric polycatenar mesogens, the different intermediate compounds are protected or deprotected phenols, anilines and acids or aldehydes. The same method is repeated to add a benzoate. The last step is often an esterification reaction (Scheme 2) unlike the imine derivatives, compounds with ester linkages can be easily purified by chromatography on silica gel with dichloromethane as eluent. 

SmA Phase In contrast to polar compounds 41 exhibiting a partially bilayer S. phase (Sec. 4.2.2 of this chapter), the lamellar mesophase of biforked (O-cyano chain compounds 42a (also Sec. 4.2.2 of this chapter) is a monolayer S -... 

SmC Phase In the immense majority of cases, the lamellar mesophases displayed by these mesogens are Sc (Fig. 2b), except for non-polar penta- and hexacatenars, which do not exhibit any lamellar mesomorphism. For example, the X-ray pattern of magnetically aligned samples of biforked mesogens 16 (typically Table 5, compound with n = l,... 

Up to now, only biforked mesogens have provided single crystals and they have invariably presented a lamellar crystalline structure, whatever the mesophase observed on heating. For example, this is the case for compounds 24 ( = 12) with the flexible group CH2-CH2-COO in the core and the unsaturated homologue (CH=CH-COO, n = 10), which do not display lamellar and columnar mesophases, respectively. The... 

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