Entanglement of spin transition and structural adaptability: manipulating the slow spin equilibrium by the guest-mediated fine-tuning of elastic frustration

Abstract

A comprehensive analysis of physical and chemical properties using the same family of spin-crossover complexes is crucial for understanding and designing structure–property relationships. However, finding an appropriate system remains challenging. Here, a series of guest-saturated states based on the 2D Hofmann-type framework [Fe^II(prentrz)₂Pd^II(CN)₄]·guest (prentrz = (1E,2E)-3-phenyl-N-(4H-1,2,4-triazol-4-yl)prop-2-en-1-imine; 1·guest) is reported, exhibiting a guest-manipulated slow dynamic effect on spin equilibrium in an incomplete two-step spin-crossover (SCO) process. Using a full-sealed method by modulating the mixing ratios and types of CH₃OH, H₂O, and D₂O, the stable maintenance of guest-saturated states allows fine-tuning of the elastic frustration (ξ) of the framework to realize SCO behaviors in the unexplored region between one-step incomplete (HS_0.5LS_0.5 ↔ HS) and two-step complete (LS ↔ HS_0.5LS_0.5 ↔ HS) processes. In the semi-sealed method, guest molecules gradually escape from the material until the guest-saturated state disappears. This continuous loss shifts the slow spin equilibrium from a state that is difficult to switch to one that fully completes the transition. The study demonstrates that guest molecule modulation is more controllable than structural deformation effects on elastic frustration, offering a pathway to discover hidden types of SCO materials and develop new stimulus-responsive materials.