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

Abstract

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.