Tuning Spin Crossover Properties in Hofmann‐Type Framework by Guest‐Adaptive Deformation

Comprehensive Summary Three three‐dimensional Hofmann‐type metal‐organic frameworks (MOFs) [Fe(bpn){Ag(CN) 2 } 2 ]·Ph 2 S ( 1·Ph 2 S , bpn = 1,4‐di(pyridin‐4‐yl)naphthalene, Ph 2 S = diphenylsulfide), [Fe(bpn){Ag(CN) 2 } 2 ]·Ph 2 SO ( 1·Ph 2 SO , Ph 2 SO = diphenylsulfoxide) and [Fe(bpn){Ag(CN) 2 } 2 ]·Ph 2 SO 2 ( 1·Ph 2 SO 2 , Ph 2 SO 2 = diphenylsulfone) were synthesized by employing sulfur‐containing aromatic guests varying in oxidation states. 1·Ph 2 S performed a complete four‐step spin crossover (SCO) behavior with the sequence of HS↔~LS 1/3 HS 2/3 ↔~LS 1/2 HS 1/2 ↔ ~LS 2/3 HS 1/3 ↔LS, while an incomplete two‐step SCO profile with the sequence of HS↔~LS 1/3 HS 2/3 ↔~LS 2/3 HS 1/3 and a faint SCO behavior at low temperature for 1·Ph 2 SO and 1·Ph 2 SO₂ . Photomagnetic experiments indicate the light‐induced excited spin‐state trapping (LIESST) effect in 1·Ph 2 S and the bi‐directional LIESST effect for 1·Ph 2 SO and 1·Ph 2 SO₂ . Variable‐temperature structural analyses reveal the evolution of host‐guest synergy and highlight the mechanism of adaptive deformation of guests mediated by phenyl rotation amid spin transition. As the oxidation state of sulfur‐containing guests increases, the host‐guest cooperation within the lattice is limited by the steric effect, which stabilizes the high‐spin state and consequently diminishes the SCO capability in this system. These results demonstrated herein open a new perspective on host‐guest chemistry within SCO frameworks.