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 temperatureinduced multistability provides a new strategy for developing multistable materials.