Fluorescent Sensing of SO2 by MFM-300(M) Metal-Organic Frameworks: Influence of Semi-Open Metal Centres.

The MFM-300(M) series (M = Al(III), Sc(III), Cr(III), and In(III)) have previously demonstrated excellent sulfur dioxide (SO2) adsorption capabilities, however, their potential as fluorescent SO2 sensors remains unexplored. Here, this work presents a comparative study of their fluorescence response upon SO2 exposure, with a particular focus on the role of the metal centers. MFM-300(Al) exhibits the strongest emission and highest quenching upon SO2 exposure, attributed to localized interactions with µ2-OH functional groups and aromatic sites. In contrast, Sc(III) and In(III) analogues show moderate quenching via charge transfer at the semi-open metal sites, while Cr(III) remains weakly emissive. Density Functional Theory (DFT) calculations employing periodic boundary conditions are conducted to characterize the electronic structure of MFM-300(M), to elucidate the role of metal centers in SO2 retention and to assess the semiconducting nature of these metal- organic frameworks (MOFs).