Tuning the Structural and Luminescent Properties of MIL-53(Al) MOF via VOCs Interactions Paving the Way for Selective Sensing Applications

Metal-organic frameworks (MOFs) are a new class of crystalline microporous materials with a high surface area and customizable functions that have a significant impact on green and renewable energy. In fact, they are used as media for gas storage, luminescent sensors, and drug delivery systems. While the structural integrity of MOFs is ensured by the coordination bonds between organic moieties and metallic nodes, an important class of these materials has a flexible structure that can cause the breathing or gate-opening phenomenon between large pore (LP) and narrow pore (NP). The MIL-53(M) series, with M = Al, Sc, Cr, Fe, and Ga, is one of the main families of flexible MOFs that show a reversible structural transition from LP to NP upon hydration-dehydration and guest adsorption. This transition is proposed to occur through two different mechanisms in the absence of van der Waals force interactions between the adsorbent and the adsorbate: twisted benzene groups of benzenedicarboxylate ligands, also known as "π-flipping; distortion mode from the corner-sharing octahedral MO6 (M = Al, Cr) clusters [1]. Moreover, MIL-53(Al) displays varying emission characteristics based on its structural arrangement and encapsulated guest molecules in its pores, which has great interest in exploring its potential for sensing applications, particularly in detecting volatile organic compounds (VOCs) and water vapor [2]. This study investigates the adsorption mechanism of MIL-53(Al) with different VOCs, by several techniques (X-ray diffraction, Raman spectroscopy and thermogravimetric analysis) to evidence the structural transitions, and photoluminescence properties. It is observed that MIL-53(Al) undergoes structural transformations due to the adsorption of VOCs, and its photoluminescence properties selectivity change in response to different guest molecules. These results help to advance the understanding of MOF-guest interactions and contribute to the development of innovative MOF-based sensing platforms. Bibliography: [1] Alhamami M, Doan H, Cheng C.H. “A Review on Breathing Behaviors of Metal- Organic-Frameworks (MOFs) for Gas Adsorption”. Materials,7,3198-3250 (2014) [2] Shen Y, Tissot A, Serre C. “Recent progress on MOF-based optical sensors for VOC sensing”. Chemical Science,13,13978-4007 (2022)