Central mesogen linkages

Polymers with methyl substitution on the central linkage in the stilbene mesogenic group have been prepared, Polymers 4 and 5, and show strictly nematic mesophases . The methyl group seems to act mostly to inhibit the existence of smectic order, but otherwise does not influence the mesophase formation or thermal stability very much. 

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. 

Fig. 3—Common central linkages of mesogenic compounds of Structure 1.

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. 

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