Covalent Modification by Click Mechanochemistry: Systematic Installation of Pendant OH Groups in a MOF for Rigidity Control and Luminescence-Based Water Detection

Covalent linker transformations in metal-organicframeworks(MOFs) enable their functionalization but often suffer from low conversionsor require harsh conditions, including heating, corrosive reactantsand solvents, or catalysts. In this work, using solvent-free mechanochemistryfor the first time for such conversions, we demonstrate the systematicMOF pore modification with pendant hydroxyl groups and the resultingeffects on the network rigidity, its luminescent properties, as wellas adsorption of CO2 and vapors of methanol, ethanol, isopropanol,D2O, and H2O. A new zinc-based heterolinkerMOF (JUK-20) containing both protic luminescent units and reactivetetrazine cores was used as a model and subjected to an inverse electron-demandDiels-Alder (iEDDA) click reaction with a series of dienophiles(x) of different lengths having OH groups. From the obtained seriesof JUK-20-(Zn)-x MOFs, a flexible material capable of luminescent humiditysensing was identified, and the influence of water on the luminescenceof the material was explained by analogy with the excited-state intramolecularproton transfer (ESIPT) model. In general, our results provide guidancefor designing and tuning MOFs for luminescence-based detection usinga stepwise synthetic approach.

Automated summary

In this study, the authors demonstrate the solvent-free mechanochemical transformation of metal-organic frameworks (MOFs) for the functionalization of their pores. The modified MOFs with pendant hydroxyl groups show changes in network rigidity, luminescent properties, and adsorption capacity for various gases and liquids. The authors also identify a flexible MOF with luminescent humidity sensing capabilities and explain the influence of water on its luminescence using the excited-state intramolecular proton transfer (ESIPT) model. Overall, this work provides guidance for designing and tuning MOFs for luminescence-based detection.