A ferroelectrically switchable columnar liquid crystal phase with achiral molecules: Superstructures and properties of liquid crystalline ureas

Novel columnar liquid crystalline compounds N,N'-bis(3,4,5-trialkoxylphenyl)ureas 1a-c (R = n-C8H17, n-C12H25, and n-C16H33) were synthesized, and their phase transitions were measured by differential scanning calorimetery. The superstructures were investigated by X-ray diffraction, polarized light optical microscopy, and IR spectroscopy. The compounds exhibited both rectangular and hexagonal columnar phases in which the urea molecules in each column were stacked in one direction with strong hydrogen bonds. To confirm the ferroelectric switching, optoelectronic experiments were carried out, and the hexagonal columnar phases of 1b and 1c gave a sharp peak of spontaneous polarization in response to an applied triangular wave electric field (0.1-18 Hz). This is the first example of ferroelectrically switchable columnar liquid crystal phases generated by achiral molecules.