Low-Symmetry Crown Ether Design for Coexisting Ferroelectricity and Ferromagnetism in a Two-Dimensional Molecular Crystal.

Multiferroics combining ferroelectricity and ferromagnetism within a single phase are promising materials for magnetoelectric (ME) functionalities. Compared with oxide-based systems, molecular materials offer greater structural tunability for realizing coupled ferroic behavior. Here we report a low-symmetry crown ether design for constructing a layered molecular crystal of [(o-fluoroanilinium+)(benzo[18]crown-6)][MnIICrIII(oxalate)3]-, in which supramolecular ferroelectricity coexists with two-dimensional ferromagnetic layers. This compound undergoes a reversible, single-crystal-to-single-crystal phase transition driven by the reorientation and conformational change of the polar benzo[18]crown-6 molecule, involving a 180° in-plane rotation and transformation of a polar V-shaped conformation into a planar configuration. Single-crystal X-ray diffraction, dielectric measurements, pyroelectric current, P-E hysteresis loops, and magnetic susceptibility measurements support the coexistence of ferroelectricity and ferromagnetism. Density functional theory (DFT) calculations and structural analysis identify the noncollinear supramolecular geometry of benzo[18]crown-6 units, their tilted assemblies, and the bistable motion as the origin of spontaneous polarization. This work demonstrates the effectiveness of the low-symmetry crown ether in designing molecular multiferroics.