Order–disorder structural phase transition and dielectric–ferroelectric properties of a host-guest methionine–crown ether inclusion complex

Designing crown-ether inclusion complexes for molecular ferroelectrics is a current research focus. In this study, a novel amino acid-based host–guest supramolecular system, the crown-ether inclusion complex [L-Met]2+[(18-crown-6)3(H2O)2][FeCl4]2−([L-Met] = l-methionine), was synthesized through evaporation. The complex belongs to the polar ferroelectric space group P1. From 100 K to 293 K, the 18-crown-6 molecule and l-Met+ cation undergo order–disorder structural phase transitions, and the l-Met+ cation forms a two-dimensional network structure through hydrogen bonding, inducing changes in the thermal and electrical properties of the complex. The differential scanning calorimetry curve shows irreversible and reversible phase transitions at 207 K and 242.5 K, respectively, causing a reversible dielectric anomaly with a novel ON/OFF function and clearly induces ferroelectricity near 233 K. Meanwhile, the cyclic voltammetry curves reveal good electrocatalytic performance. These findings indicate that the complex acts as a multifunctional phase-change material with ferroelectric and electrocatalytic properties and a reversible dielectric response.