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

Abstract

Covalent linker transformations in metal-organic frameworks (MOFs) enable their functionalization but often suffer from low conversions or require harsh conditions, including heating, corrosive reactants and solvents, or catalysts. In this work, using solvent-free mechanochemistry for the first time for such conversions, we demonstrate the systematic MOF pore modification with pendant hydroxyl groups and the resulting effects on the network rigidity, its luminescent properties, as well as adsorption of CO2 and vapors of methanol, ethanol, isopropanol, D2O, and H2O. A new zinc-based heterolinker MOF (JUK-20) containing both protic luminescent units and reactive tetrazine cores was used as a model and subjected to an inverse electron-demand Diels-Alder (iEDDA) click reaction with a series of dienophiles (x) of different lengths having OH groups. From the obtained series of JUK-20(Zn)-x MOFs, a flexible material capable of luminescent humidity sensing was identified, and the influence of water on the luminescence of the material was explained by analogy with the excited-state intramolecular proton transfer (ESIPT) model. In general, our results provide guidance for designing and tuning MOFs for luminescence-based detection using a stepwise synthetic approach.