Azobenzenes and Related Mesogens

Typical examples of chiral azobenzenes are (/f)-4-(2-methylbutyl)-4 -substituted azobenzenes (9-13) which appeared with the emergence of synthetic methods which allowed the preparation of chiral precursors, such as (-i-)-2-methylbutylbenzene [51-54]. In the case of compound 12, the presence of two chiral terminal moieties was found to increase the helical twisting power ()8) by a factor of two [51]. Both the compounds 9 and 10 show enantiotropic chiral nematic phases which have relatively low clearing points, unlike compounds 11 and 12 which are not liquid crystalline. 

The azo group may be oxidized further, usually with a peracid such as 3-chloroper-benzoic acid to give azoxy linked materials examples include compounds 14-16 which are shown below [51, 54, 55], but only compounds 14 and 16 show chiral nematic phases. 

The use of the azo-linking group (-N = N -) in liquid crystal chemistry is now well documented, and hundreds of compounds of this class have been cited in the chemical literature [47] the linkage is formed by an azo coupling reaction between a substituted aryl diazonium salt and a suitably activated substituted benzenoid compound [50]. 

Although early commercially available nematic mixtures for use in electro-optic displays were based on this class of material, they proved short lived because of their inherent strong yellow colour and because of their susceptibility to photochemical isomerization/degradation [18], 

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