Mesomorphic behaviour and photoisomerisation of phenylene and biphenylene-substituted azobenzenes

Azobenzene derivatives substituted with phenylene and biphenylene groups were synthesised and their liquid crystalline (LC) properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The effect of extra phenylene groups on the LC properties of the azobenzene was mainly reflected in the stability of mesophases as well as in the transition temperatures - both associated with the increasing cohesive forces between molecules as the conjugation length increases. Nematic and monolayered smectic-type mesophases with tilted molecules were observed for these compounds. Highly ordered mesophases (CrG and/or CrK/CrH) were particularly observed for the azocompounds bearing extra phenylene groups. The calculated layer spacing was correlated either to a parallel or to an anti-parallel arrangement of molecules in the mesophase, depending on the symmetry of the rigid core. On the other hand, the photoisomerisation was characterised (dilute solution) as part of a preliminary study of future investigations on photo-induced properties of these new azocompounds. High rates of photoisomerisation as well as trans-cis conversions higher than 80% in the photostationary state were determined for these compounds.