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

Abstract

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.