Exploring the Intense Lanthanide Luminescence and High Thermal Stability in a New Mixed Eu³⁺/Tb³⁺ Organic Framework Series for Marking in Gunshot Residues

Since their discovery, metal–organic frameworks (MOFs) have attracted increasing interest due to the different applications of these materials. One class of these materials is the mixed lanthanide-organic frameworks (m-LnOFs) formed by more than one type of lanthanide ion. Such compounds have been extensively studied, mainly as luminescent thermometers. However, another very interesting application is the use of mixed LnOFs as markers in gunshot residues (GSR), allowing for the creation of a barcode prototype for each type of ammunition. This work reports the hydrothermal synthesis, complete structural characterization, and photophysical study of nine new mixed LnOFs, of general formula [EuxTb1-x(Hbtec)]n (0.1 ≤ x ≤ 0.9), and with Hbtec3- being the anion from 1,2,4,5-benzenetetracarboxylic acid (H4btec). The mixed compound [Eu0.1Tb0.9(Hbtec)]n 1 has excellent thermal stability and interesting optical properties, such as more pronounced dual emission and a characteristic Tb3+ → Eu3+ energy transfer, culminating in high purity of its red emission, when compared to its isostructural homometallic [Eu(Hbtec)]n, previously synthesized. Thus, the compound [Eu0.1Tb0.9(Hbtec)]n 1 was selected to act as a potential ammunition marker, more precisely in .380 pistol and 38 caliber revolver, demonstrating satisfactory results and being able to provide support to public safety in the future.