Molecular Ladders of Lanthanide-3-phenyl-4-benzoyl-5-isoxazolonate and Bis(2-(diphenylphosphino)phenyl) Ether Oxide Complexes: The Role of the Ancillary Ligand in the Sensitization of Eu<sup>3+</sup> and Tb<sup>3+</sup> Luminescence

Three new lanthanide heterocyclic β-diketonate complexes having the general formula Ln(PBI)<sub>3</sub>(DPEPO) [HPBI = 3-phenyl-4-benzoyl-5-isoxazolone, DPEPO = bis(2-(diphenylphosphino)phenyl) ether oxide and Ln<sup>3+</sup> = Eu<sup>3+</sup> (<b>1</b>), Tb<sup>3+</sup> (<b>2</b>), and Gd<sup>3+</sup> (<b>3</b>)] have been synthesized. Compounds <b>1</b> and <b>2</b> were characterized by single-crystal X-ray diffraction. The single-crystal X-ray diffraction analyses of <b>1</b> and <b>2</b> revealed that these complexes are mononuclear, and that the central Ln<sup>3+</sup> ion is coordinated by six oxygen atoms furnished by three bidentate β-diketonate ligands and two oxygen atoms from the bidentate DPEPO ligand. The overall molecular geometry is distorted square prismatic. Examination of the packing diagrams for <b>1</b> and <b>2</b> revealed the presence of interesting molecular ladder structures held together by π···π and intermolecular hydrogen-bonding interactions. The effect of the chelating ancillary phosphine oxide ligand DPEPO on the sensitized luminescence of Eu<sup>3+</sup> and Tb<sup>3+</sup> luminescence in these heterocyclic β-diketonate complexes has been investigated. The ancillary ligand increases considerably the luminescence efficiency of [Eu(PBI)<sub>3</sub>(DPEPO)] (overall quantum yield 2−30%; <sup>5</sup>D<sub>0</sub> lifetime 250−1056 μs) compared to Eu(PBI)<sub>3</sub>(C<sub>2</sub>H<sub>5</sub>OH)(H<sub>2</sub>O), through the formation of intraligand states, while it is detrimental to Tb<sup>3+</sup> luminescence in [Tb(PBI)<sub>3</sub>(DPEPO)] (overall quantum yield 11 to 0.5%; <sup>5</sup>D<sub>4</sub> lifetime 400 to 168 μs).