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) (H2DHBDC2– = 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.