Some new azobenzene liquid crystals involving chalcone and ester linkages

Five new series of azobenzene derivatives containing thiophene, naphthalene or ferrocene with chalcone and ester linkages have been synthesized and characterized. The photosensitive azobenzene group underwent photoisomerization under UV light, which was monitored by UV-visible spectroscopy. The cyclic voltammograms of compounds containing ferrocene showed a quasi-reversible and diffusion-controlled redox process. These compounds displayed high thermal stability according to the thermogravimetry measurements. According to differential scanning calorimetry, thermal polarizing microscopy and powder X-ray diffraction studies, the compounds containing ferrocene and the compounds with three or no terminal alkoxy chains on the side of the ester group all showed no liquid crystal behaviour. However, the compounds with a terminal alkoxy chain on the side of the ester group or a terminal alkoxy chain at both ends of the molecule exhibited enantiotropic mesophases, but the latter had a narrow mesogenic domain. The increase of the length of the mesogenic unit by replacement of the benzene ring with a naphthalene ring resulted in both an increase in the clearing point and in an increase in the mesophase domain.