ABSTRACT
DiketonatesMost complexes used as emitting materials in OLEDs are based on β-diketonates of europium or other lanthanide salts. Their preparation is relatively simple and follows a common procedure established in the 1950s [2]. Posterior modifications were introduced, aiming at reducing the toxicity of the solvents and reactants employed or deeming to meet the solubility requirements of more specific ligands. Thus, and using europium as a model, the preparation of lanthanide diketonates can be made by treating an aqueous solution of the corresponding salt (in this case, EuCl3) and the desired diketonate (dket) with an alkalizing agent, typically sodium hydroxide; for each mole of salt, three equivalents of base are required, plus the ligand in a slight excess, that is, around 3.3 moles. The resulting product can be recrystallized for further purification. For ligands that are less soluble in water, as for instance 1,1,1-trifluoro-4-phenyl-1,3-butanedione (HBTA), a 50:50 mixed solution of water and ethanol can be used as the reaction medium [3]; other co-solvents may also be employed, depending on the specific solubilities of the ligands. This procedure leads to the isolation of a europium tris-β-diketonate with two water molecules completing the coordination sphere, i.e. Eu(dket)3(H2O)2.The presence of coordination water molecules in an emitting complex (EC), however, and as mentioned in the previous chapters, is highly undesirable due to the occurrence of non-irradiative relaxation processes. The lanthanide acqua complexes can thus be further modified by replacing the waters in the coordination sphere with a non-charged, or neutral, ligand.
