Mesogenic compounds

During the last two decades, the number of publications dealing with X-ray structure analyses of mesogens in the solid state has increased enormously. A comprehensive review of the crystal structures of mesogenic compounds is presented here. 

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. 

The crystal structures of the mesogenic compounds CBn were reported by different authors [45-51]. Selected crystallographic and molecular data of the investigated compounds CBn are summarised in Table 1. Only the crystal structures of some compounds will be discussed here. 

The crystal structure of the mesogenic compound 4 -(4-hydroxy-1-n-butoxy)-4-cyanobiphenyl (CB0(CH2)40H) was described by Gehring et al. [59]. In... 

In 1989, Walz et al. [80] reported on the crystal and molecular structures of four mesogenic compounds of the series trans, tran5-4,4 -m-n-dialkyl-(la,l -bicyclohexyl)-4)S-carbonitrile (CCNnm). The chemical structure of the mesogenic CCNnm is presented in Fig. 11. 

In 1995, Iki and Hori [98] presented the crystal structures of some mesogenic compounds of the two isomeric series CPnOB and nOPCB (n = 6,... 

In this section, we will report on the X-ray structure analyses of some mesogenic compounds containing an ethylene bridge. The crystal data and the phase sequences of the investigated compounds are summarised in Table 17. 

Table 17. Crystal data and phase sequences of mesogenic compounds containing an ethylene bridge and some derivatives...

In this section, we will present the crystal structures of chiral mesogenic compounds exhibiting ferroelectric liquid crystalline phases which are listed in Table 18 [152-166]. Moreover, four compounds of the list show antiferroelectric properties and two compounds form only orthogonal smectic phases. The general chemical structures of the investigated chiral compounds are shown in Fig. 27. 

X-ray measurements have shown that the positional and orientational ordering in highly ordered crystalline smectic phases is more comparable with the ordering in the solid state than the structures of the fluid smectic phases and the nematic phase. Therefore, a comparison of the structures of crystals and highly ordered smectic phases is possible because of their closer similarities. Up to now, only the crystal structures of few mesogenic compounds exhibiting highly ordered smectic phases have been reported in... 

Many of the investigated mesogenic compounds show solid state polymorphism. In order to obtain useful information about the arrangement of the molecules in the mesophase from the X-ray data of the single crystals, it is important to investigate the crystal structure of those solid phase which transforms into the liquid crystalline phase. For instance, only the crystal structures of the low temperature solid phases of the compounds MBBA [138, 139], MHPOBC [159], and T15 [81] could be determined, but the... 

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

While the existence of the anticlinic minimum in conventional SmC materials has been suggested by experiments with DOBAMBC,17 by far the majority of tilted smectics only exhibit the synclinic structure of the SmC or SmC under normal conditions. When the mesogenic compound is unichiral (i.e., enantiomerically pure) or enantiomerically enriched, a macroscopic polarization with the same orientation in each layer is produced, as discussed above. In the SmCA phase of MHPOBC, however, the anticlinic structure is the global minimum structure, as shown at the bottom of Figure 8.9. In this case, given that the chirality of the molecules is fixed, the polarization must alternate... 

The mesogenic structures of glycolipids are due to the occurrence, on the same molecule, of a hydrophilic and a hydrophobic moiety often referred to as head and tail respectively. As a result, glycolipids are able to self-organize into a large variety of mesophases also called liquid crystals (Fig. 2) [ 10]. Supramolecular assemblies of mesogenic compounds can be caused by a rise in temperature (thermotropic liquid crystals) or by the addition of water (lyotropic liquid crystals) they result from different responses of the carbohydrate and the alkyl chain to temperature or solvent (water), respectively. 

When they are heated, mesogenic compounds do not melt directly from the highly ordered crystalline state to an isotropic liquid. They form instead, intermediate phases in which the molecules are orientated in a parallel direction and referred to as smectic (centers of the molecules organized in layers) or nematic (centers of the molecules distributed at random). Smectic and nematic mesophases are in turn divided into a variety of subgroups of thermotropic liquid crystals which will not be dealt with in detail in the present article. 

The latter involves the synthesis of monomers containing LC (mesogenic) groups, with the subsequent homopolymerization or copolymerization with mesogenic or non-mesogenic compounds (Fig. 1), or the attachment of low-molecular crystal... 

A modified method was proposed in 32) and consisted in that an original mesogenic olefin monomer was used as a mesogenic compound ... 

An example of a special kind of mesogenic compound is provided by A,A -bis (4-alkoxybenzoylhydrazines) (e.g., compound 132a,) known to form bicontinuous cubic phases [247-249]. It is interesting that fluorination of only one chain of these compounds (compound 132b) removes the cubic phase which is restored if the second alkyl chain is also fluorinated (compound 132c, see Fig. 35a) [122]. 

Fig. 42 Effect of chain fluorination on the LC phases of peri-hexabenzocoronene based mesogens (compounds 152-155) [274—276] and the fused metalloporphyrin dimer 156 (7/° C) G = glassy state, Co/ortho = orthorhombic phase [277]...

Vaughn-Spickers [5] prepared chiral photoisomerizable mesogenic compounds, (VII), that retained their chirality upon photoinitation and were used in optical and electrooptical devices. 

A hydroxyl-terminated mesogenic compound. Scheme 11.10, showed limited miscibility with l-ethyl-3-methylimidazolium tetrafluorobrate, but anisotropic ion... 

Figure 25.6 Molecular structures of hydroxyl-terminated mesogenic compounds 13-15, ionic liquid 16, and mesogenic compound 17 without functional groups.

Figure 25.7 Schematic illustration of the microphase-segregated layered structure for the mixtures of hydroxyl-terminated mesogenic compounds 13 and 15 with ionic liquid 16.

Figure 25.11 Anisotropic ionic conductivities of the complexes composed of equimolar amounts of mesogenic compounds 13,14 and ionic liquid 16 as a function of temperature ( ) parallel and (M) perpendicular to the smectic layer for the material based on 13 (O) parallel and (n) perpendicular to the smectic layer for the material based on 14.

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