Quantum Yield and Photoluminescence Intensity Enhancement Effects of a Diphosphine Dioxide Ligand on a 6-Coordinate Eu(III)-β-Diketonate Complex with Low Luminescence

[Image: see text] Tris{6,6,7,7,8,8,8-heptafluoro-1-[2-(9,9-dimethylfluorenyl)]-1,3-octanedionate} europium(III) (Eu(III)(hfod)(3)1) was synthesized, which was designed to have low luminescence and a large absorption coefficient in order to elucidate the coordination effects of phosphine oxide ligands. The quantum yield (Φ(TOT)) and photoluminescence intensity of complex 1 were dramatically enhanced by coordinating a diphenyl-4-(dibutylphosphinyl)butyl phosphine oxide (DPDB) ligand, thanks to the increased intrinsic photoluminescence quantum yield of the lanthanide (Φ(Ln)) and the increased energy transfer efficiency (Φ(ET)) in the solution and solid states. In the solid state, there was no energy dissipation by solvent molecules. This excluded the steric shielding effects of the DPDB ligand and allowed the effects of the ligand field environment to be extracted. Φ(Ln) and Φ(ET) of complex 2 were much larger in the solid state than those in the solution state, resulting in larger Φ(TOT) (solution state: Φ(Ln) 0.50, Φ(ET) 0.42, and Φ(TOT) 0.21 and solid state: Φ(Ln) 0.74, Φ(ET) 0.47, and Φ(TOT) 0.35). Larger asymmetry ratios (ratio R) of Eu(III)(hfod)(3)(DPDB) 2 than those of complex 1 in the solution and solid states indicate that the ligand field of the Eu(III) ion becomes more asymmetric by coordination of the DPDB ligand. Density functional theory calculations showed that Φ(Ln) and Φ(ET) increased when the ligand field around the Eu(III) ion became more asymmetric. Based on these results, we propose a hypothesis on the enhancement of the photoluminescence intensity of 6-coordinated Eu(III)-β-diketonate by a DPDB ligand. When a DPDB ligand coordinates to a Eu(III) ion, the positions of the nearest oxygen atoms around the Eu(III) ion are shifted by steric repulsion and the relative positions of the nearest oxygen atoms are distorted. The distorted coordination environment induces asymmetry in the ligand field, increasing Φ(Ln) and Φ(ET). Φ(TOT) is enhanced by the DPDB ligand because it is the product of Φ(Ln) and Φ(ET). Photoluminescence intensity increases because of the enhanced Φ(TOT).