The liquid crystal properties of symmetric and non-symmetric dimers based on the azobenzene mesogenic group

A novel system of symmetric and non-symmetric dimers containing azobenzene groups has been synthesized and studied in an attempt to understand further the molecular origins of the intercalated smectic phases. For the non-symm etric dimers, the lack of symmetry was derived solely from the differences in length of the two terminal alkyl chains. Both the spacer and terminal ch ain lengths were varied. The spacer length was found to exert a profound in fluence on the clearing temperatures of these materials and a large odd-eve n effect was observed for the series. The smectic A phase stability was obs erved to increase with the terminal chain length, yet decrease with increas ing spacer length. X-ray diffraction has revealed the structure of the smec tic A phase of both the symmetric and non-symmetric azobenzene dimers to be of the monolayer type and not intercalated. The existence of the intercala ted phase has previously been explained in terms of either a charge-transfe r interaction, or by an electrostatic quadrupolar interaction. However, it has been thought that it may also be the result of an excluded volume or sp ace filling constraint. For the non-symmetric liquid crystal dimers describ ed here, a charge-transfer interaction should be minimal, as should the sta bilization from the quadrupolar interaction between the two mesogens. Howev er, it appears that some sort of specific interaction is required to stabil ize the intercalated structure.