Asymmetric Coordination Designed Heterocrown Ether Host–Guest Ferroelectrics with Ultrahigh <i>T</i>_c

Crown ether compounds have garnered significant research interest for decades due to their unique host-guest supramolecular structure and various properties, such as ferroelectricity. However, although many crown ether derivatives have been developed, only the classical crown ether represented by 18-crown-6 was used as the host for crown ether host-guest-type ferroelectrics, and the heterocrown ether-based ferroelectrics have never been reported. Here, for the first time, we propose the strategy of asymmetric coordination through heterocrown ether as a host molecule to unlock ferroelectricity in metal-crown host-guest assemblies. Compared to the symmetrical coordination of the parent M(NO₃)₂(18-crown-6) (M = Pb²⁺ or Cd²⁺), an asymmetric coordination environment for the M²⁺ ion was created in M(NO₃)₂(1-aza-18-crown-6), enabling precise symmetry breaking and inducing ferroelectricity. Remarkably, Pb(NO₃)₂(1-aza-18-crown-6) exhibits an ultrahigh <i>T</i>_c of 470 K, which represents the maximum value reported to date for crown ether-based ferroelectrics. Such high <i>T</i>_c originates from the synergistic effect of disordered rotation commonly observed in the host-guest compounds and the unique out-of-plane flipping motion of the -NH- group in aza-crown ether. To the best of our knowledge, M(NO₃)₂(1-aza-18-crown-6) represents the birth of the first heterocrown ether ferroelectric family. Our findings provide a new way to design crown ether host-guest ferroelectrics with high performance and greatly stimulate the further exploration of heterocrown ether ferroelectrics.