Linker‐Locking Strategy for the Rigidification of Flexible Metal–Organic Framework: Structural Stabilization and Controlled ESIPT for H 2 O/D 2 O Se
Amitosh Sharma, Seonghwan Lee, Seo I Park, Hak‐Won Nho, Junmo Seong, Eunseo Lee, Oh‐Hoon Kwon, Myoung Soo Lah, Jaewoong LimABSTRACT
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 D 2 O vs <5 ns in H 2 O) and distinct optical responses in H 2 O and D 2 O. Consequently, the pronounced kinetic isotope effect allows rigid L@MIL‐53‐(OH) 2 to sensitively detect trace H 2 O in D 2 O 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.