Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal–Organic Frameworks with Open Metal Sites

We show that fluorescent molecules incorporated as ligands in rigid, porous metal-organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC(2-) = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest-framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.