A Multiferroic Spin‐Crossover Molecular Crystal

Abstract Spin‐crossover (SCO) ferroelectrics with dual‐function switches have attracted great attention for significant magnetoelectric application prospects. However, the multiferroic crystals with SCO features have rarely been reported. Herein, a molecular multiferroic Fe(II) crystalline complex [Fe II (C 8 ‐F‐pbh) 2 ] ( 1‐F , C 8 ‐F‐pbh = (1 Z,N'E )‐3‐F‐4‐(octyloxy)‐ N’ ‐(pyridin‐2‐ylmethylene)‐benzo‐hydrazonate) showing the coexistence of ferroelectricity, ferroelasticity, and SCO behavior is presented for the first time. By H/F substitution, the low phase transition temperature (270 K) of the non‐fluorinated parent compound is significantly increased to 318 K in 1‐F , which exhibits a spatial symmetry breaking 222F2 type ferroelectric phase transition with clear room‐temperature ferroelectricity. Besides, 1‐F also displays a spin transition between high‐ and low‐spin states, accompanied by the d ‐orbital breaking within the t 2g 4 e g 2 and t 2g 6 e g ° configuration change of octahedrally coordinated Fe II center. Moreover, the 222F2 type ferroelectric phase transition is also a ferroelastic one, verified by the ferroelectric domains reversal and the evolution of ferroelastic domains. To the knowledge, 1‐F is the first multiferroic SCO molecular crystal. This unprecedented finding sheds light on the exploration of molecular multistability materials for future smart devices.