Photoluminescent Lanthanide(III) Complexes Based on 2-[((4-Chlorophenyl)amino)methylene]-5,5-dimethylcyclohexane-1,3-dione

Five coordination compounds of the general formula [LnL₂(NO₃)₃]_n (Ln³⁺ = Eu (I), Sm (II), Tb (III), Dy (IV), and Gd (V)) based on 2-[((4-chlorophenyl)amino)methylene]-5,5-dimethylcyclohexane-1,3-dione (L) are synthesized. The crystal structures of the ligand and complex III are determined by X-ray diffraction (XRD) analysis of single crystals (CIF files CCDC no. 2298715 (L) and 2298716 (III)). Complex III is polymeric due to the bidentate-bridging coordination of the ligand by the oxygen atoms of the cyclohexanedione fragment, and the coordination number of the central atom is ten. According to the powder XRD data, all synthesized polycrystalline compounds are isostructural to the single crystals of complex III. The photoluminescence properties of the ligand and coordination compounds in the polycrystalline state are studied. The energy transfer from the ligand to lanthanide(III) ion is shown to proceed via the “antenna” mechanism in the case of the europium(III), samarium(III), and terbium(III) compounds. Among the series of the complexes, the highest quantum yield is observed for compound I (21.9%), and the sensitization efficiency of the europium(III) complex is 43.5%.