Spin Crossover OFF/ON Triggered by Ligand Chemical Doping in an Fe(III) Solid Solution†

Spin crossover (SCO), characterized by distinct high-spin (HS) and low-spin (LS) states, has potential applications in memory, electronic, and electroluminescent devices. The OFF/ON switching of SCO is crucial for obtaining bistable magnetic properties. However, there are few strategies for achieving this switching. Herein, based on a ligand chemical doping strategy, we report an Fe(III) solid solution that can be prepared using a ligand chemical doping strategy, enabling not only the OFF/ON switching of SCO but also the fine-tuning of the spin transition temperature (T_c) within a 45 K range near room temperature. The experimental results show that when the polar ligand doping ratio reaches 20%, SCO behavior is triggered, and the crystal phase transforms significantly, becoming loose and flexible. Furthermore, T_c can be continuously regulated as the ligand-doping ratio increases. Density functional theory (DFT) calculations reveal that solid packing-induced molecular distortion blocks SCO, whereas loosely flexible packing triggers SCO via fluorinated ligand chemical doping.