Liquid crystalline banana-shaped monomers derived from 2,7-naphthalene: synthesis and properties

Three banana-shaped monomers, i.e. 2,7-naphthalene bis[4-(4-allyloxyphenylazo)-benzoate], 2,7-naphthalene bis[4-(4-allyloxy-3-fluorophenylazo)benzoate] and 2,7-naphthalene bis{4-[4-(10-undecenyloxy)phenylazo]benzoate}, containing azobenzene as side arms, 2,7-dihydroxynaphthalene as central units and terminal double bonds as polymerisable functional groups, were synthesised and their mesophase behaviour investigated. Polarizing optical microscopy and DSC measurements reveal that all compounds exhibit nematic mesophases. The absorption spectrum of the trans-azobenzene groups displays a high-intensity -* transition at about 365 nm and a low-intensity n-* transition at around 450 nm for all compounds. Hence, photochromism can be achieved by the introduction of the azo linkage to banana-shaped liquid crystals molecules.