Why europium acylpyrazolonates are bad phosphors: lessons learned from the study of ionic and polymer complexes [MLnQ4], M = NEt4+, Ag+

Our series of works on acylpyrazolonates of lanthanides has shown that although these beta-diketonate ligands are effective in sensitizing the luminescence of Tb3+, Dy3+, and Sm3+ ions, and the IR-emitting Yb3+ ion, there is typically a sharp drop in efficiency in the case of Eu3+. To investigate this phenomenon, three series of complexes of samarium, europium, and gadolinium with the ligand 4-benzoyl-5-methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one (HQPh) were synthesized, namely [LnQ3(H2O)(dmf)] (Ln-1) (dmf = N,N-dimethylformamide), [NEt4][LnQ4] (Ln-2), and {AgLnQ4}n (Ln-3). The structures of these compounds were determined by single-crystal X-ray diffraction analysis, which showed that series Ln-1 consists of molecular complexes held together by hydrogen bonds, series Ln-2 consists of ionic crystals, and series Ln-3 consists of coordination polymers in which silver ions are coordinated by nitrogen atoms of the pyrazole rings, forming infinite chains. The study of optical properties revealed that for europium ions, efficient quenching occurs through an LMCT state, with the highest efficiency observed for the complex Eu-1. The transition from the ammonium salts Sm-2, Eu-2 to the heterometallic complexes Sm-3 and Eu-3 is also accompanied by some unexpected luminescence quenching.