Theoretical Estimation of Important Ferroelectricity-Related Parameters for Molecular Design of Host–Guest Compounds, Substituted Anilinium Tetrafluoroborate 18-Crown‑6

Through rational chemical modification, a series of host–guest complexes, substituted anilinium tetrafluoroborate 18-crown-6, are constructed to achieve excellent ferroelectricity in terms of large spontaneous polarization, high phase transition temperature, and small coercive field. In order to evaluate our new design, computational methods are developed to estimate the ferroelectricity-related parameters, namely, spontaneous polarization, phase transition temperature, and coercive field, in sufficient consideration of structural and symmetrical characteristics of this series of host–guest complexes at the molecular level. In each host–guest crystal, the substituted anilinium acts as a rotator anchoring in the cavity of a stator, 18-crown-6, with three N–H···O hydrogen bonds. This rotator does the pendulum motion of the phenyl ring with its substituent to switch ferroelectric-paraelectric phase transition. High-accuracy quantum chemistry calculations are carried out to obtain complete energy and polarization information along this pendulum-motion-related reaction coordinate. Then the Landau–Devonshire phase transition theory and the Boltzmann statistics are employed to create a bridge between the above ferroelectric molecular behavior and the crystalline macroscopic properties and estimate the phase transition temperatures of these studied host–guest complexes. By virtue of applying an external electric field in our quantum chemistry calculations, the relative magnitude of the coercive field is approximately measured to exhibit the electric-field-responsive behavior of these host–guest self-assemblies. Finally, two candidates, namely, (4-CN-Ani⁺)­(18-crown-6)­BF₄^– and (4-CHO-Ani⁺)­(18-crown-6)­BF₄^–, are designed by us with large excellent ferroelectricity compared to the model system (4-MeO-Ani⁺)­(18-crown-6)­BF₄^–. They would wait for the future experimental confirmation. Our molecular design of these ammonium-crown ether-based host–guest compounds would provide insights into the structure–property relationship of these rotator-stator-type ferroelectrics to help both theoretical and experimental specialists design or/and synthesize new targeted compounds. Our developed computational methods to estimate ferroelectricity-related parameters would also supply ideas for computational scientists to produce new similar methods for some ferroelectrics with specific phase-transition-related molecular dynamics.