Light-induced stepped thermal relaxation in a Hofmann-type metal-organic framework

Spin-crossover (SCO) materials with light-induced multistability have potential applications in multi-addressable opto-magnetic memory devices and multiple switches. Here, we reported a three-dimensional Hofmann-type metal-organic framework (MOF) [Fe(bpy){Pt(CN)₄}] coumarin (1·coumarin, bpy = 4,4′-bipyridine), which exhibits an incomplete and hysteretic SCO behavior. Most importantly, the light-induced stepped thermal relaxation (LISTR) with obvious magnetic plateau for a hidden intermediate spin state is firstly realized. This hidden spin state is also observed for the first time in the thermal relaxation after light-assisted spin state annealing (LASSA). Meanwhile, an extremely rare bi-directional light-induced excited spin-state trapping (LIESST) with the highest reverse-LIESST temperature is observed. Therefore, five spin states can be captured in 1·coumarin and the tristability can be modulated below 102 K through the excitation-relaxation mechanism. Hence, the light- and temperature-induced multistability provides a new strategy for developing multistable materials.