Symmetrical dimer liquid crystals derived from benzoxazoles

A series of dimeric twin liquid crystals derived from symmetric heterocyclic benzoxazoles 1a–1f exhibiting single smectic C phase are reported. All benzoxazoles were prepared by condensation of 2-aminophenols with benzaldehydes and then followed by subsequent intramolecular cyclization. The mesomorphic properties were investigated by DSC and POM, and the structure of the mesophases was confirmed as smectic C phases by powder XRD. The formation of mesophases was attributed to the weak dipole force induced by donor–acceptor interaction (D→A). A monolayer conformation was proposed based on the powder XRD data. A value of aspect ratio d/l=0.80–0.92 was calculated from d-spacings and the molecular lengths. By contrast, the odd–even effects by a value of entropy changes ΔSSmC→I/R=2.00–6.50 showing odd–even effect for the transition of SmC→I phases were also associated in twin system.