Manipulating guest-responsive spin transition to achieve switchable fluorescence in a Hofmann-type framework

The change of fluorescence emission manipulated by spin state transition attracts considerable attention owing to its potential applications in magneto-optical switching devices. Herein, we report two two-dimensional (2D) Hofmann-type spin crossover (SCO) metal-organic frameworks (MOFs) [Fe^II(PNI)₂{Ag^I(CN)₂}₂]·CHCl₃ (3Ag·CHCl₃) and [Fe^II(PNI)₂{Au^I(CN)₂}₂]·CHCl₃ (3Au·CHCl₃) based on the fluorescent ligand N-(4-pyridylmethyl)-1,8-naphthalimide (PNI). Both complexes exhibit interesting SCO behaviors switched by guest solvent molecules, namely three-step transitions for the solvated complexes and complete one-step hysteretic SCO for the desolvated ones, verified by temperature-dependent magnetic susceptibility measurements, Mossbauer spectra, structural analyses, and differential scanning calorimetry measurements. Correspondingly, temperature-dependent fluorescence spectra exhibit double peaks (monomer and excimer emission) with both emission peaks change consistent with the change in SCO properties during the solvent molecule removal. In this study, we integrated guest-responsive SCO behavior into MOFs to manipulate the multistability of spin state and fluorescence switching, providing a rational strategy for the development of stimuli-responsive multifunctional materials.