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.