Liquid Crystalline Phase Induced by Molecular Rotator and Dipole Fluctuation

The thermal properties, crystal structures, dielectric relaxations, and rotational potential energy curves were examined for new rod-like molecules 1 and 2 bearing three aromatic rings connected by two -CONH- linkage groups to clarify the dynamic molecular behavior and phase transition behavior of the molecular assemblies. The molecular structures of 1 and 2 differed in that the central aromatic ring was phenyl (-C6H4-) in 1 and pyridyl (-C5NH3-) in 2, which affected the phase transition behavior owing to the permanent dipole moment without the center of inversion in molecule 2. Although the crystal structures of 1 and 2 were isostructural, the melting point of crystal 2 was approximately 43 K lower than that of crystal 1, and a smectic A mesophase was reversibly observed in crystal 2. A broad endothermic thermal anomaly of crystal 2 was observed in the heating process on the differential scanning calorimetry chart because of thermally activated dipole fluctuation, which was consistent with the frequency- and temperature-dependent dielectric relaxations. Double- and single-minimum-type potential energy curves were observed in the rotations of -C6H4- and -C5NH3- rings, respectively, from density functional theory calculations. The difference in rotational symmetry affected the crystal lattice energy and appearance of the mesophase.