Quantitative Photoswitching of Spin States in o ‐Fluoroazobenzene‐Loaded Metal–Organic Frameworks

ABSTRACT The integration of photochromic guest molecules into spin‐crossover (SCO) metal‐organic frameworks (MOFs) offers a promising approach for optically regulating magnetic properties. Herein, we report a crystalline material, [Fe(bpn){Ag(CN) 2 } 2 ]· E ‐F 4 AB ( 1‐ E ) (bpn = 1,4‐bis(4‐pyridyl)naphthalene and E ‐F 4 AB = E ‐ o ‐tetrafluoroazobenzene), which displays an asymmetric three‐/four‐step SCO behavior. At room temperature, the material undergoes efficient (90%) and reversible E / Z photoisomerization of guest molecules upon alternating irradiation with green (530 nm) and blue (410 nm) light. Following green‐light irradiation, the crystal of 1‐ Z , containing 90% F 4 AB in the Z configuration, was obtained via a light‐induced single‐crystal‐to‐single‐crystal transformation. The E ‐to‐ Z isomerization of F 4 AB introduces steric constraints within the pores, effectively locking the host framework into the high‐spin (HS) state. Furthermore, by controlling the proportion of the Z isomer through variation of irradiation time, we achieve continuous and quantitative modulation of the HS fraction. This work provides the first direct structural evidence of guest‐driven light‐induced spin change (GD‐LISC), establishing a robust strategy for designing photoswitchable materials.