Room-temperature polymorphism of a benzene-1,4-dicarboxamide liquid crystal: Hydrogen-bonded polar columnar assembly

Abstract

Polymorphism is a fascinating research subject as it enables diverse functionalities to be achieved from a single molecule. In this study, we present the synthesis and polymorphic behavior of a liquid crystalline (LC) benzene-1,4-dicarboxamide (BDA) derivative (1). Depending on specific thermal treatments, two distinct polymorphs can be obtained at room temperature. These include a metastable glassy columnar (Colgla) and thermodynamically stable monoclinic crystalline (Cry) phases. Through the electric analysis, the Colgla polymorph exhibits a significant piezoelectric response, while the Cry polymorph shows a negligible piezoelectric signal. Notably, the BDA molecule exhibits an 81-helix columnar assembly (Colhel) in the LC phase, comprising syn-conformers with non-zero dipole moments. According to density functional theory (DFT) calculations, the amide group is tilted by 25° with respect to the central benzene ring. The piezoelectric response of the Colhel is comparable to that of the Colgla polymorph. Therefore, it can be inferred that the polar character of the Colgla polymorph is transferred from the Colhel LC phase during the cooling process. Simulation and spectroscopic analyses further reveal that the axial polarizations of both the Colgla and Colhel are driven by the hydrogen-bonding interactions between adjacent amide units in head-to-tail. These findings shed light on the underlying mechanisms of the polymorphic behavior and piezoelectric properties exhibited by the BDA molecule.