Phasmidic mesogen

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. 

Fig, 18a,b, The polymorphic behaviour of the phasmidic-like mesogens a molecular model with six terminal chains b phase diagram for the binary mixture of double swallow-tailed compound (I) with conventional rod-like mesogen (II) (adapted from Letko et al. [123])... 

In the section on calamitic materials, we discussed how, by appropriate consideration of the molecular shape of bis( -diketonato)copper(II) complexes, either nematic and lamellar, or columnar mesophases could be realized. Furthermore, increasing the number of chains on the periphery of calamitic mesogens can lead to molecules which demonstrate both types of mesomorphism, e.g. the phasmids of Malthete... 

Molecules which combine the features of the rod and the disc may be expected to form new types of mesophases. An example is the biaxial nematic phase reported in thermotropic systems (see 6.6). Malthete et a/. have prepared an interesting series of mesogens shaped like stick insects called phasmids (fig. 6.1.5(n)). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown schematically in fig. 6.1.5( >). 

Malthete and coworkers [6,29] have prepared a series of mesogens shaped like stick insects called phasmids (8). Some of them form columnar mesophases the structure proposed for the hexagonal phase is shown in Figure 9. 

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. 

Up to now, in most cases, polycatenars such as phasmids and biforked mesogens have been symmetrical and have not contained any polar substituents such as Br, CN, or NO2. In order to study the influence of a lateral polarity on the mesomorphic properties, some new polar and non-synunetrical poly-catenar mesogens have been prepared. 

Phasmids and polycatenar mesogens are just ten years old. Their intermediate shape between classical rod-like and disc-like liquid crystals allows practically all the mesomorphic symmetries (0, Cub,... 

Lin C, Rigsdorf H, Ebert M, Kleppinger R, Wraidraff JH (1989) Structural variations of liquid crystalline polymras with phasmidic-type mesogens. Liq Cryst 5(6) 1841—1847. doi 10.1080/ 02678298908045692... 

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