Ratiometric recognition of humidity by a europium-organic framework equipped with quasi-open metal site

Open metal site (OMS) seated in a luminescent lanthanide (Ln) metal center offers an opportunity for rationally tuning the spectroscopic behavior of lanthanide-organic frameworks aiming for a wide range of sensing applications. However, given the spherical nature of common coordination geometries of trivalent lanthanides and the generally strong Ln-O bonds, the lanthanide based OMS is rarely reported and difficult to be functionalized. We report here a unique europium-organic framework containing abundant quasi-OMS that is protected by an abnormal weak Eu-O bond. These quasi-OMSs offer reversible direct binding sites for water molecules probed by X-ray crystallography, leading to sensitive, visible, and ratiometric luminescent sensing toward humidity and water content in organic solvents. The specific recognition of water based on quasi-luminescent-OMSs gives rise to a superior water detection limit down to 0.0003% v/v, which is one order of magnitude lower than that of Karl Fischer method.