Stilbenes mesomorphism

TABLE 5. Mesomorphic properties of simple stilbene liquid crystals... 

Stilbenoid-like dendrimers possessing an analogous molecular structure were reported by Meier to be mesomorphic too [273]. The five generations of stilbene-based dendrimers (Fig. 57) [274,275] and the first two gener-... 

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

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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