Synthesis, Crystal Structure, and Luminescent Properties of Novel Eu3+ Heterocyclic β-Diketonate Complexes with Bidentate Nitrogen Donors

New tris(heterocyclic β-diketonato)europium(III) complexes of the general formula Eu(PBI)3·L [where HPBI = 3-phenyl-4-benzoyl-5-isoxazolone and L = H2O, 2,2‘-bipyridine (bpy), 4,4‘-dimethoxy-2,2‘-bipyridine (dmbpy), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (bath)] were synthesized and characterized by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), 1H NMR, high-resolution mass spectrometry, thermogravimetric analysis, and photoluminescence (PL) spectroscopy. Single-crystal X-ray structures have been determined for the complexes Eu(PBI)3·H2O·EtOH and Eu(PBI)3·phen. The complex Eu(PBI)3·H2O·EtOH is mononuclear, and the central Eu3+ ion is coordinated by eight oxygen atoms to form a bicapped trigonal prism coordination polyhedron. Six oxygens are from the three bidentate HPBI ligands, one is from a water molecule, and another is from an ethanol molecule. On the other hand, the crystal structure of Eu(PBI)3·phen reveals a distorted square antiprismatic geometry around the europium atom. The room-temperature PL spectra of the europium(III) complexes are composed of the typical Eu3+ red emission, assigned to transitions between the first excited state (5D0) and the multiplet (7F0 - 4). The results demonstrate that the substitution of solvent molecules by bidentate nitrogen ligands in Eu(PBI)3·H2O·EtOH richly enhances the quantum yield and lifetime values. To elucidate the energy transfer process of the europium complexes, the energy levels of the relevant electronic states have been estimated. Judd−Ofelt intensity parameters (Ω2 and Ω4) were determined from the emission spectra for Eu3+ ion based on the 5D0 → 7F2 and 5D0 → 7F4 electronic transitions, respectively, and the 5D0 → 7F1 magnetic dipole allowed transition was taken as the reference. The high values obtained for the 4f−4f intensity parameter Ω2 for europium complexes suggest that the dynamic coupling mechanism is quite operative in these compounds.