2015): Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases, Liquid Crystals,We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.
In order to study what role the direction of the whole mesogenic groups play on their mesomorphic behavior we synthesized a series of bent-shaped dimers in which the mesogens differ in their relative orientation along the long molecular axis of the dimers. A tilted anticlinic smectic mesophase is presented exclusively by the dimer where the mesogens are tethered in a head-to-head fashion. The head-to-tail and tail-to-tail organizations of the mesogens preclude any kind of mesophase formation. The mesomorphic behavior of dimers was characterized by a combination of polarizing optical microscopy, differential scanning calorimetry and computational techniques.
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