Linker-Locking Strategy for the Rigidification of Flexible Metal-Organic Framework: Structural Stabilization and Controlled ESIPT for H2O/D2O Sensing.

Controlling the photodynamics of excited-state intramolecular proton transfer (ESIPT) fluorophores is crucial for sensing, due to their large Stokes shifts and dual emission. However, structural flexibility and solvent-induced degradation often hinder isolating a single tautomeric emission in water. Here, we introduce a linker-locking strategy that rigidifies the flexible metal-organic framework (MOF) MIL-53-(OH)2 through cooperative stabilization by encapsulated ligands and structural water, forming a stable phase, rigid L@MIL-53-(OH)2. The hydrogen-bonding network among structural water, encapsulated ligands, and the framework stabilizes the pore structure while preserving reversible interconversion between flexible and rigid phases via ligand and water removal or post-encapsulation. This rigidification suppresses particle fragmentation in aqueous media and enables controlled ESIPT behavior. While flexible MIL-53-(OH)2 exhibits mixed enol and keto emissions due to size-dependent heterogeneity, rigid L@MIL-53-(OH)2 displays a single keto emission with prolonged excited-state lifetimes (>14 ns in D2O vs <5 ns in H2O) and distinct optical responses in H2O and D2O. Consequently, the pronounced kinetic isotope effect allows rigid L@MIL-53-(OH)2 to sensitively detect trace H2O in D2O with high reusability over at least five cycles. This work establishes linker locking as a new strategy for stabilizing flexible MOFs and controlling excited-state processes for aqueous sensing.