Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K.

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures ( T c ) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [Fe III (H 0.5 L Cl ) 2 - 2 x (H 0.5 L F ) 2 x ]·H 2 O (H 0.5 L X = 5- X -2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the T c of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the L F ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the Fe III N 2 O 2 S 2 ligand field and corresponding SCO energy barrier, consequently resulting in increased T c . This work provides a new pathway for tuning the T c of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.