Lanthanide Antenna Amplifier Multiplies the Optical Sensing Efficiency in Phototautomeric Metal–Organic Frameworks

Metal–organic frameworks (MOFs) incorporating phototautomeric ligands have shown significant potential for new-generation optical sensing devices. It is challenging to enlarge the energy difference of tautomers, which is crucial for improving the sensing efficiency. Herein, we report a novel tactic to amplify the excited-state intramolecular proton transfer (ESIPT) effect via the “antenna effect” in lanthanide sensitization. Specifically, by leveraging the enol (E)–keto (K) phototautomerization process, we achieve precise manipulation of the energy gap between the lowest ligand triplet state and the Eu(III) emitting level through fine-tuning. As a result, the small wavelength shift (10 nm) between the E* and K* emissions is amplified into an approximately 65-fold intensity change in Eu(III) emission. This enhancement is accompanied by a significantly lowered limit of detection (LOD: 0.53 μg/L) and improved sensitivity (0.6955% RH –1 ) in real-time humidity monitoring across a broad linear detection range (0–95.0% RH). Computational simulations and single-crystal analyses demonstrate two key mechanisms: (1) the appropriate hydrophilic/hydrophobic distribution in the MOF cavity facilitates rapid hydration/dehydration and (2) H 2 O desorption/adsorption-dependent ESIPT switching governs the deactivation/activation of the Eu(III) 4f excited state. This work presents an optimization approach to enhance energy utilization in MOF-based optical sensing.