Large Polarization Change Induced by Spin Crossover-Driven Fe(II) Ion Shuttling within a Tripodal Ligand

The integration of spin crossover (SCO) magnetic switching and electric polarization properties can engender intriguing correlated magnetic and electric phenomena. However, achieving substantial SCO-induced polarization change through rational molecular design remains a formidable challenge. Herein, we present a polar Fe(II) compound that exhibits substantial polarization change in response to a thermally regulated low-spin ↔ high-spin transition. This large polarization change is realized by harnessing an unusual SCO-actuated large displacement of the Fe(II) ion, encapsulated within a cage-like tripodal ligand. Owing to the uniaxially aligned polar molecular structures within the lattice, alterations in the molecular dipole moment translate to notable polarization change of the single crystal with a value of 1.9 μC cm^-2. This value is 2.4 times the highest value reported for SCO compounds. The large polarization change and small dielectric constant result in an outstanding pyroelectric response in this compound, with figures of merit comparable to those of typical pyroelectric materials. The intrinsic large displacement of the Fe(II) ion provides a new strategy to effectively modulate the electric polarization via SCO magnetic switching, and the ion shuttling within a cage structure may find applications in next-generation single-molecule magnetoelectric devices.