Biforked 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. 

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. 

Biforked mesogens 2mp-2mp Biforked mesogens are the richest family and exhibit all the types of mesophases lamellar, columnar, cubic, and nematic. They correspond to the general formula 16 [6, 24-27]. Several examples corresponding to different values of W, X, Y, Z and A (Fig. 1), as well as various chain lengths, are given in Table 5. 

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. 

O-Cyano Chains. The fo-cyano chains associate the flexibility of the paraffins with the polarity of the cyano group. They are used in order to obtain orthogonal lamellar mesophases such as an SmA phase instead of tilted SmC phases. In fact, the biforked mesogens 42a displayed an SmA phase... 

The nematic phase observed with polycat-enars has the optical properties of the classical bicatenar nematic. Nevertheless, in some cases, as with biforked mesogens [6], observations on nematic free droplets reveal, on heating above the isotropic transition, a Schlieren texture with quite a weak birefringence, but it is difficult to decide whether this originates in Brownian motion or cybotactic fluctuations. Moreover, in spite of the strong geometrical anisotropy of some tetracatenars such as 15b (Sec. 4.1.3 of this chapter), possible biaxial properties have not been confirmed. 

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... 

Many of the mesogenic molecules are stericaUy asymmetric, which is determined by the fractures and bending of the molecular core as well as by the presence of the tail chains of different nature, including the branched, biforked or polyphilic moieties (Fig. 2c-f). In terms of the multipole model of molecular asymmetry introduced by Petrov and Derzhanski [34], we can speak about longitudinal or transverse steric dipoles or multipoles (Fig. 3). 

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