Synthesis and characterization of two highly luminescent Eu3+ tris-β-diketonates with the δ-valerolactam ligand: Theoretical, experimental spectroscopy studies and viability as luminescent gunshot residues

Abstract

The luminescence exhibited by lanthanide ions has been increasingly applied in different areas of human knowledge. Among these metal ions, Eu³⁺ is certainly one of the most studied, due to the relative simplicity of its electronic structure and because it exhibits intense emission in the red region. An appropriate choice of ligands can result in lanthanide coordination compounds with high quantum efficiencies. The Eu³⁺ β-diketonates are intensively explored and of great interest in different applications. One of these applications is quite new, where Eu³⁺ β-diketonates are used as markers for firearm ammunition being found in the composition of gunshot residues (GSR). These GSR's are formed by lanthanide coordination compounds and can be detected in various weapon compartments, targets and spent cartridges, as well as on the clothing, hands, arms and face of the shooter by employing a simple black light lamp. In this context, Eu³⁺ β-diketonates exhibit high emission intensities and relative thermal stability, despite being completely outnumbered by lanthanide metal-organic frameworks (LnMOFs), widely employed as markers in GSR's. This study portrays an exhaustive work, from synthesis, structural characterization and photophysical studies to numerous forensic ballistics tests employing two new β-diketonates, [Eu(tta)₃(δ-val) (H₂O)] 1 and [Eu(btfa)₃(δ-val) (H₂O)] 2 (where δ-val = δ-valerolactam or 2-piperidinone), which can bring great advances for criminal experts and public security policy. These compounds were primarily tested as markers using a Taurus® G2C 9 mm pistol, a Glock® 23 Gen4 0.40 S&W pistol and a Taurus® T4 5.56 mm rifle. The influence of projectile velocity on the marked ammunition and potential physical or chemical interactions of these markers with the gunpowder were assessed and a luminescent gunshot residues (LGSRs) impregnation test in cotton fabric (T-shirt) and in animal tissue was carried out. Furthermore, the LGSR of compound [Eu(btfa)₃(δ-val) (H₂O)] 2 were collected and analyzed through emission spectrum.