Spin Crossover Materials: Progress, Challenges, and Future Prospects

Spin-crossover (SCO) materials have attracted significant interest in recent decades due to their potential applications in various technologies. Iron(II) and iron(III) SCO chemistry has seen significant advances, with many room temperature SCO systems exhibiting hysteresis over 30 K. Fe(II) complexes exhibit sharp transitions of spin states and large thermally induced hysteresis widths, while Fe(III) systems often show smoother and more intricate spin state profiles due to the necessity to deal with two different paramagnetic states during the spin transition. Supramolecular interactions between SCO molecules play a crucial role in promoting room temperature spin transitions. However, the performance of SCO systems is significantly affected by their local environment, including factors such as the substituent of the ligand, counterbalancing ion, crystalline water content, crystal stacking, and pressure. Recently, researchers have focused on neutral complexes exhibiting high temperature transformations, enabling the coupling of material and device properties. SCO materials have potential applications in micromechanical actuators, electronic devices, and resistance switching memories. However, challenges remain, including the need to expand the library of room temperature SCO transition materials and the preparation of controllable high-quality SCO nanoparticles and thin films. Continued research and development efforts are required to fully explore the potential of SCO materials in various fields.