A crossover from rod-shaped to bent-shaped in symmetric isoflavone liquid crystal trimers exhibiting unusual mesomorphic behaviour

Abstract

A new series of symmetric isoflavone-based liquid crystalline trimers, 1,3-bis{[3-(4′-decyloxyphenyl)-4H-1-benzopyran-4-one-7-oxy]alkyloxy}benzenes with 3–12 methylene units in both lateral spacers have been synthesized and characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. As expected, the phase transition temperatures strongly alternate as the number of methylene units in the spacers changes. An unusual mesomorphic behaviour was observed wherein short and long trimers exhibited smectic A (SmA) and/or smectic C (SmC) phase(s) while homologues with intermediate spacer length were purely nematogenic (N) in a broad temperature range. Formation and stabilization of the uniaxial nematic phase is caused by change of molecular geometry from rod-shaped for short homologues to strongly bent-shaped for long homologues.