Post‐Synthetic Modification of Zr‐based Metal‐Organic Frameworks with Imidazole: Variable Optical Behavior and Sensing

Abstract

AbstractUiO‐66‐NH2‐IM, a fluorescent metal‐organic framework (MOF), was synthesized by post‐synthetic modification of UiO‐66‐NH2 with 2‐imidazole carboxaldehyde via a Schiff base reaction. It was examined using various characterization techniques (PXRD, FTIR, NMR, SEM, TGA, UV‐Vis DRS, and photoluminescence spectroscopy). The emissive feature of UiO‐66‐NH2‐IM was utilized to detect volatile organic compounds (VOCs), metal ions, and anions, such as acetone, Fe3+, and carbonate (CO32−). Acetone turns off the high luminescence of UiO‐66‐NH2‐IM in DMSO, with the limit of detection (LOD) being 3.6 ppm. Similarly, Fe3+ in an aqueous medium is detected at LOD=0.67 μM (0.04 ppm) via quenching. On the contrary, CO32− in an aqueous medium significantly enhances the luminescence of UiO‐66‐NH2‐IM, which is detected with extremely high sensitivity (LOD=1.16 μM, i. e., 0.07 ppm). Large Stern‐Volmer constant, Ksv, and low LOD values indicate excellent sensitivity of the post‐synthetic MOF. Experimental data supported by density functional theory (DFT) calculations discern photo‐induced electron transfer (PET), resonance energy transfer (RET), inner filter effect (IFE), or proton abstraction as putative sensing mechanisms. NMR and computational studies propose a proton abstraction mechanism for luminescence enhancement with CO32−. Moreover, the optical behavior of the post‐synthetic material toward analytes is recyclable.