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 (ΦTOT) 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 (ΦLn) and the increased
energy transfer efficiency (ΦET) 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. ΦLn and ΦET of
complex 2 were much larger in the solid state than those
in the solution state, resulting in larger ΦTOT (solution
state: ΦLn 0.50, ΦET 0.42, and ΦTOT 0.21 and solid state: ΦLn 0.74, ΦET 0.47, and ΦTOT 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 ΦLn and ΦET 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 ΦLn and ΦET. ΦTOT is enhanced
by the DPDB ligand because it is the product of ΦLn and ΦET. Photoluminescence intensity increases
because of the enhanced ΦTOT.