Phenyl substituted mesogens

Unlike to the mesophase behaviour of laterally alkyl substituted mesogens, smectic phases can be stabilized depending on the substituents at the lateral phenyl ring. Figure 6 shows the polymorphism of a homologous series bearing a lateral 2-(4-n-alkan-... 

Homopolymers. Phenylesters of benzoic acid were chosen as mesogenic groups for the synthesis of suitable monomers. The acryloyl- and the methacryloyl moieties were used as polymerizable groups. The flexible spacer was an alkyl chain or an alkyloxy chain of varying length. Thus a homologous series of p-"(2-methylpropenoyloxyalkyoxy) benzoic acid -p substituted phenyl esters 1 was prepared. The monomers were synthesized by the standard methods as follows ... 

Table 8. Laterally aryl substituted four-ring mesogens in which the lateral phenyl group is attached by a two-unit spacer Z.

According to the results reported above the basic mesogens must possess a minimum of three aromatic rings if the corresponding laterally substituted derivatives are to exhibit liquid crystalline properties. Recently, Hohmuth et al. [82] provided evidence that segments incorporating a phenyl... 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

The corresponding nickel complex is not mesogenic neither is the complex in which the phenyl is substituted in the 3,5 positions. Similar complexes were studied by Ohta [67] who found discotic mesophases for alkoxy (R = OCgH(7) chains and no mesophase for alkyl tails (R = CgH27). 

When the central linkage does not provide a linear molecule, however, mesophase stability suffers. Mesophases exhibited by derivatives of phenyl benzoate, for example, are generally some 35° less stable than those of correspondingly substituted azobenzenes. The effect of linearity, or at least planarity, of molecules on mesophase stability is further evidenced on comparison of tmns-4-alkoxycinnamic acids and trans-stilbenes with their corresponding cis isomers, which demonstrates that severely nonlinear molecules show no mesomorphic properties. Again, derivatives of epicholestanol and caprostanol (Fig. 5) are not known to be mesogenic. 

Fig. 2.1. Typical molecular structures, (a) Rod-like mesogens (i) p-azoxyanisole (ii) p -n-hexyloxybenzylidene-p-n-hexyl oxyaniline. (b) Discshaped mesogens (i) hexa-substituted esters of benzene (ii) hexa-substituted esters or ethers of triphenylene " (iii) hexa-n-alkanoates of truxene. (c) Phasm-like mesogens bis-(3,4,5-trialkoxybenzoyloxy)4 -phenyl terephthalates. (d) Pyramid-like mesogens (i) hexa-substituted tribenzocyclononanef (ii) octa-substituted tetrabenzocyclododecatetraene. ...

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