Guest-Anion-induced rotation-restricted emission in UiO-66-NH₂and advanced structure elucidation

We report the guest-anion-induced photoluminescence enhancement of metal-organic frameworks (UiO-66-NH2), first based upon diffraction and computational evidence. We found that only limited anions, namely, carbonate and fluoride, can lead to a significant enhancement in photoluminescence, whereas their related anions, such as acetate and chloride, cannot. The optimized crystal structures reveal that the guest carbonate and fluoride ions interact with four framework amino functional groups through hydrogen bonding (ca. 1.6-1.7 Å) that ultimately forms a quaternary (-N(H))4···X- molecular bridge around the nodal center. Hence, the hydrogen-bonded molecular bridge not only restricts the intermolecular C-C rotation of the linker molecules but also greatly perturbs the electronic densities between the guest anions and the framework amino groups.