Use of Luminescence Modulation in a New Series of Mixed Lanthanide Metal–Organic Frameworks for Selective Firearm Ammunition Marking

Metal-organic frameworks (MOFs) are coordination polymers that can generally be described by secondary building units (SBUs). These include lanthanide MOFs, specifically mixed lanthanide MOFs (m-LnMOFs), encompassing coordination polymers formed by two or more different Ln³⁺ ions. These compounds have been the subject of study by inorganic chemists worldwide, mainly due to the possibility of obtaining long-awaited tunable luminescence, i.e., compounds that emit luminescence in various visible spectrum regions. A wide range of emission color scan be obtained by inserting different Eu³⁺, Tb³⁺, and Gd³⁺ ion molar fractions in m-LnMOFs, which can also be adjusted by choosing the Ln³⁺ ion or its amount in each network. This study presents a new series of m-LnMOFs supported by 1,2,4,5-benzenetetracarboxylic acid (H₄btec). These compounds were completely characterized in their solid state through different analytical and spectroscopic techniques, and an in-depth photophysical study was conducted. Due to their high thermal stability and multicolored emissions, these complexes were applied, for the first time, as selective ammunition markers, allowing for the unequivocal identification of the type of weapon used in forensic scenarios and the promotion of stricter ammunition trade control.