Intrinsic Fluorescence–Spin Crossover Interplay in the 3D FeII Hofmann-Type MOF {FeII(2,6-di(pyridin-4-yl)naphthalene)[MI(CN)2]2}·x(PhNO2) (MI = Ag, Au)

We report the synthesis, characterization, and photophysical properties of two isoreticular 3D Hofmann-type Metal-Organic Frameworks (MOFs), {FeII(2,6-di(pyridin-4-yl)naphthalene)[MI(CN)2]2}·x(PhNO2) (M = Ag (1Ag), Au (1Au)), incorporating a naphthalene-based fluorescent ligand as bridging pillars. Single-crystal X-ray diffraction analysis of 1Au reveals a doubly interpenetrated primitive cubic (pcu) framework constructed from {FeII4[MI(CN)2]4}n grids pillared by the organic linker, with nitrobenzene molecules occupying the accessible voids and engaged in multiple host–guest interactions. Magnetic and calorimetric studies show a thermally induced and complete spin crossover (SCO) in both compounds, with similar transition temperatures but distinct cooperativity, including a multistep behavior pronounced in the Ag derivative. Variable-temperature fluorescence measurements of 1Ag and 1Au display an emission band centered at ca. 585 nm, whose intensity decreases upon cooling in correlation with the LS state population. This behavior is attributed to a reabsorption process consistent with an inner filter effect, highlighting the role of the spin-state-dependent optical properties of the framework. Structural analysis suggests that differences in the metal–cyanide linkage influence host–guest interactions and lattice response, thereby modulating the SCO profile. These results provide insights into the interplay between intrinsic fluorescence and SCO in porous frameworks and contribute to the rational design of multifunctional SCO-based materials.