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(HL)(HL)]·HO (HL = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, = F or Cl, = 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 of the L ligand. Elevating 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^IIIN₂O₂S₂ 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.