New luminescent metal-organic frameworks

Abstract

The present master thesis entitled New Luminescent Metal-Organic Frameworks focuses on the synthesis and characterization of several new metal– organic frameworks (MOFs), which are a kind of organic-inorganic hybrid compounds with one-, two-, or three-dimensional structural topologies consisting of inorganic metal ions/clusters and organic ligands. The relevance of the new MOFs synthesized dwells in the luminescence properties that they exhibit. The construction of new MOFs has been rapidly developed in recent years, not only due to their interesting molecular topologies and crystal packing motifs but also due to their potential applications as functional materials. MOFs can be readily synthesized from the self-assembly of metal ion nodes with organic ligands. Multi-carboxylic acids have been widely used to synthesize high dimensional frameworks. In particular, 1,2,4,5- benzenetetracarboxylate is a versatile ligand that leads to numerous different coordination polymers with lanthanide ions. Among the wide variety of properties of interest that a given material can exhibit, luminescence is attracting an increasing attention due to its potential application in optical devices for lighting equipment and optical storage, optical switching, and sensing. At this respect, luminescent metal–organic frameworks (LMOFs) are a quickly growing class of crystalline materials with great potential for use across a broad range of applications. The lanthanide elements, which possess partially filled 4f orbitals, can form coordination polymers that exhibit various coordination geometries even with the same ligand. An extensive work has been carried out using 1,2,4,5-benzenetetracarboxylic acid (H4bta) as ligand and 1,2-di(4-pyridyl)-ethylene (bpe), 1,2,4- triazole (tz), imidazole (im) and 4,4’-azobis(pyridine) (azpy) in order to perform cation exchanges. Several lanthanide coordination polymers with good luminescence properties have been synthesized by hydrothermal method, and an investigation on the formation of lanthanide coordination compounds based on the use of cation exchange has also been performed. The seven LMOFs prepared are, specifically: [Ln(bta)][(CH2)2NH2] (Ln=Tb (1), Ln=Pr (2)), [Ln(bta)]2(bpe) (Ln= Tb (3), Ln=Pr (4)), [Tb(bta)](im) (5), [Tb(bta)]2 (azpy) (6), and [Tb(bta)](tz) (7). Single-crystal X-ray diffraction reveals that complex 1 crystallizes in the to orthorhombic space group P212121, while complexes 2 and 3 in the triclinic space group P-1. The Pr-complexes obtained are isostructural as deduced by means of single crystal and powder X-ray diffraction analysis, unlike Tb-complexes, which are found to possess different crystal structures. Luminescence measurements reveal that the Tb3+ compounds exhibit several strong characteristic emission bands for isolated Tb3+ ions in the visible region when excited between 320 and 375 nm; similarly, the Pr3+ compounds display the characteristic emission bands for isolated Pr3+ ions.