Mesomorphic behaviour and photoisomerisation of phenylene and biphenylene-substituted azobenzenes

Abstract

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.

Keywords:

• azobenzene
• liquid crystal
• smectic: photoisomerisation

Acknowledgements

We are grateful for the financial support provided by the National Council of Science and Technology (CONACYT) of Mexico under the project 61773-R. We also thank Dr D. Guillon from the IPCMS (Strasbourg, France) for the XRD facilities, and Guadalupe Méndez and Maricela García from CIQA for their helpful assistance in the DSC and NMR experiments.