Hybrid rod-like and bent-core liquid crystal dimers exhibiting biaxial smectic A and nematic phases

Liquid crystal hybrid dimers involving bent-core units and rod-like units were designed, synthesized and investigated by a combination of polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction studies. All the four unsymmetrical dimers 1-4 exhibited stable nematic phases over a wide temperature range. Moreover, dimer 1 exhibited the biaxial smectic A (SmA) phase and dimer 2 exhibited a highly ordered mesophase below the nematic (N) phase. Two rod-like and bent-core nematogenic units were covalently linked in dimers 3 and 4 to furnish exclusively the nematic phase. The observation of the biaxial SmA phase underlying the nematic phase in one of the bent-core-rod dimers 1 would be a significant step forward towards achieving the biaxial nematic phase and resolving controversies related to the presence of cybotactic groups. Hybrid rod-like and bent-core liquid crystal dimers have been designed and synthesized which exhibit biaxial smectic A and nematic phases.