Intrinsic Fluorescence–Spin Crossover Synergy in a 3D Fe(II) Hofmann-Type Framework Built from 1,6-dipyridylpyrene and [MI(CN)₂]⁻ (MI = Ag, Au) bridging ligands

Abstract

We report the synthesis, structural characterization, and photophysical investigation of two isostructural 3D Hofmann-type coordination polymers, {FeII(1,6-dipypyr)[MI(CN)2]2)}·(toluene) (1,6-dipypyr = 1,6-dipyridylpyrene, MI = Ag (1Ag), Au (1Au)), incorporating a pyrene based fluorescent ligand as pillars of the framework. Single crystal X-ray diffraction reveals a primitive cubic (pcu) topology formed by square-grid FeII-[MI(CN)2] layers pillared by the 1,6-dipyridylpyrene ligands, with two interpenetrated frameworks stabilized by metallophilic interactions and hosting a disordered toluene molecule. Magnetic and calorimetric measurements demonstrate a complete, reversible two-step spin crossover (SCO) for both compounds. Variable-temperature fluorescence studies show a structured green-yellow emission centered at ca. 560–570 nm, bathochromically shifted relative to the free ligand, whose intensity is modulated by the spin-state population. Structural analysis indicates a slipped, edge-to-face arrangement of pyrene units, precluding excimer formation, confirming that the emission is essentially monomeric and directly affected by the coordination environment and spin-state switching. Overall, this work reinforces the concept that intrinsic fluorescence in SCO frameworks provides a robust strategy to design materials with a synergistic fluorescence–SCO response, offering a platform for the rational development of multifunctional spin-switchable systems for sensing and optoelectronic applications.