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.

为阐明分子组装体的动态分子行为与相变行为，研究人员针对两类新型棒状分子（rod-like molecules）1和2展开系统表征：二者均通过两个酰胺键（-CONH-）连接三个芳香环，分别记为分子1与分子2。两类分子的结构差异在于：分子1的中心芳香环为苯基（-C6H4-），分子2的中心芳香环为吡啶基（-C5NH3-）；由于分子2不具备反演中心（center of inversion）且存在永久偶极矩（permanent dipole moment），其相变行为受到显著影响。尽管二者的晶体结构互为同构体（isostructural），但分子2的晶体熔点较分子1低约43 K，且在分子2的晶体中可可逆观测到近晶A介晶相（smectic A mesophase）。差示扫描量热法（differential scanning calorimetry, DSC）曲线显示，分子2的晶体在升温过程中出现宽化的吸热热异常现象，该现象源于热激活的偶极涨落，这一结果与频率及温度依赖性的介电弛豫（dielectric relaxations）实验结果一致。密度泛函理论（density functional theory, DFT）计算结果表明，苯基（-C6H4-）与吡啶基（-C5NH3-）环的旋转势能曲线（rotational potential energy curves）分别呈现双势阱型与单势阱型特征。旋转对称性的差异对晶体晶格能及介晶相的出现均产生了显著影响。