Directional Intermolecular Interactions for Precise Molecular Design of a High-<i>T</i>_c Multiaxial Molecular Ferroelectric

Abstract

Quasi-spherical molecules have recently been developed as promising building blocks for constructing high-performance molecular ferroelectrics. However, although the modification of spherical molecules into quasi-spherical ones can efficiently lower the crystal symmetry, it is still a challenge to precisely arouse a low-symmetric polar crystal structure. Here, by introducing directional hydrogen-bonding interactions in the molecular modification, we successfully reduced the cubic centrosymmetric Pm3̅ m space group of [quinuclidinium]ClO₄ at room temperature to the orthorhombic polar Pna2₁ space group of [3-oxoquinuclidinium]ClO₄. Different from the substituent groups of -OH, -CH₃, and ═CH₂, the addition of a ═O group with H-acceptor to [quinuclidinium]⁺ forms directionally N-H···O═C hydrogen-bonded chains, which plays a critical role in the generation of polar structure in [3-oxoquinuclidinium]ClO₄. Systematic characterization indicates that [3-oxoquinuclidinium]ClO₄ is an excellent molecular ferroelectric with a high Curie temperature of 457 K, a large saturate polarization of 6.7 μC/cm², and a multiaxial feature of 6 equiv ferroelectric axes. This work demonstrates that the strategy of combining quasi-spherical molecule building blocks with directional intermolecular interactions provides an efficient route to precisely design new eminent molecular ferroelectrics.