A Water-Stable Cationic SIFSIX MOF for Luminescent Probing of Cr2O72- via Single-Crystal to Single-Crystal Transformation

The sensing and monitoring of toxic oxo-anion contaminants in water are of significant importance to biological and environmental systems. A rare hydro-stable SIFSIX metal-organic framework, SiF6@MOF-1, {[Cu(L)(2)(H2O)(2)]& BULL;(SiF6)(H2O)}(n), with exchangeable SiF62- anion in its pore is strategically designed and synthesized, exhibiting selective detection of toxic Cr2O72- oxo-anion in an aqueous medium having high sensitivity, selectivity, and recyclability through fluorescence quenching phenomena. More importantly, the recognition and ion exchange mechanism is unveiled through the rarely explored single-crystal-to-single crystal (SC-SC) fashion with well-resolved structures. A thorough SC-SC study with interfering anions (Cl-, F-, I-, NO3-, HCO3-, SO42-, SCN-, IO3-) revealed no such transformations to take place, as per line with quenching studies. Density functional theory calculations revealed that despite a lesser binding affinity, Cr2O72- shows strong orbital mixing and large driving forces for electron transfer than SiF62-, and thus enlightens the fluorescence quenching mechanism. This work inaugurates the usage of a SIFSIX MOF toward sensing application domain under aqueous medium where hydrolytic stability is a prime concern for their plausible implementation as sensor materials.