Preparação de redes metalo-orgânicas mistas de lantanídeos para emprego como marcadores luminescentes de resíduos de tiros em armas de fogo

Abstract

The coordination polymers which possibly present empty spaces, as porous or channels, are called metal-organic frameworks (MOF). When the MOF is synthesized with lanthanide ions (Ln3+) the polymer is the so-called lanthanide metal-organic framework (LnMOF). These LnMOFs might be formed by a mixture of different lanthanide ions, giving rise to the mixed lanthanide metal-organic frameworks (m-LnMOFs). These compounds are applied in the different areas and subareas of the chemistry due to their excellent thermal properties, high surface area and, especially, their high luminescent emissions, when excited under ultraviolet light sources. This work aims to synthesize, characterize and study photophysical properties of new m-LnMOFs derivatives of 1,2,4,5-benzenetetracarboxylic acid (H4btec). The more promising compound (the one with higher thermal stability and higher emission intensity) was used as marker for ammunition of firearms, to generate the so-called Gunshot Residues (GSR). Therefore, nine unpublished m-LnMOFs were synthetized, using as metal center a range of fractions of Eu3+ and Tb3+ and the H4btec as ligand. Such compounds have the general formula [EuxTb1-x(Hbtec)]n, with x=0,1;0,2;0,3;0,4;0,5;0,6;0,7;0,8;0,9. The synthesized compounds were characterized through analytical and spectroscopic techniques, such as elemental analysis (%C and %H), Raman and infrared vibrational spectroscopy and powder X-ray diffraction (XRD). The powder XRD technique showed that the m-LnMOFs are isostructural to [Eu(Hbtec)]n and [Tb(Hbtec)]n, previously synthesized by our group. According to the crystallographic data, the unit cell volume of m-LnMOF followed a trend consistent with the different fractions of Eu3+ and Tb3+ added to the compounds. With increasing concentration of Eu3+ in the structure, the cell volume tends to increase, given that its ionic radius is greater. The results of thermogravimetric analysis (TG/dTG) showed that all m-LnMOFs are very thermally stable since dTG curves revealed the first mass loss between 446-458 °C. The emission spectra of the [Gd(Hbtec)]n was acquired to inspect the triplet state energy of the ligand in the m-LnMOFs. Thus, the ligand Hbtec3- has been shown to be an excellent sensitizer for both Eu3+ and Tb3+ since the triplet state energy is approximately 25.675 cm-1. In addition to DRX, the luminescent study corroborated the incorporation of the fractions of the Tb3+ and Eu3+ in the m-LnMOFs when comparing the emission spectra of the m-LnMOFs with the physical mix between [Eu(Hbtec)]n and [Tb(Hbtec)]n. Once the emitted color by m-LnMOFs is nearest of red, only [Eu0,1Tb0,9(Hbtec)]n was chosen to act as marker in firearms ammunition (38 caliber and .380 pistol), since it showed the 5D4 →7F5 transition with high intensity. The insertion of [Eu0,1Tb0,9(Hbtec)]n as marker resulted in luminescent gunshot residues in hand, target, floor, cartridge and in firearm. Hence, [Eu0,1Tb0,9(Hbtec)]n was considered the most feasible firearm marker, which generated a single emission spectra and exhibited a typical color emission among the Eu3+/Tb3+ ratios used.