Porous Metallophilic Frameworks Incorporating Metal–Organic Chains as Humidity Sensors Exploring Uranyl Photoluminescence

Abstract

Luminescent coordination polymers, including metal–organic frameworks, serve as a platform for optical sensing of physical and chemical stimuli, including temperature or pressure for the former, gases, solvent vapors, explosives, toxic species, etc. for the latter. The related materials built of uranyl cations, UO₂²⁺, remain unexplored, despite their tunable photoluminescence. While recently, bimetallic coordination and supramolecular systems built of cyanido metal complexes have proven a successful pathway toward stimuli-responsive materials, their combination with uranyl species is limited. Here, the construction of novel uranyl-based systems demonstrating a distinct sensitivity of emission to solvent vapors is reported, which is achieved by generating metal–organic, i.e, uranyl–2,4′-bipyridine-N,N’-dioxide, chains that are arranged into a porous supramolecular framework by metallophilic interactions between attached tetracyanidometallates(II), [M^II(CN)₄]²⁻ (M^II = Pd, 1; Pt, 2). Both resulting materials reveal reversible single-crystal-to-single-crystal transformation upon removal of crystallization solvent molecules, which is due to the flexibility of metallophilic interactions forming a porous metallophilic framework resembling MOF-74. The presence of infinite channels introduces sensing capabilities to water vapors while the emissive character leads to the humidity-variable emission characteristics, including the strong variation in emission intensity and lifetime. The obtained systems exhibit repeatable adsorption and emission characteristics, meeting the criteria for luminescent sensors.