Synthesis and photo-physics of red emitting europium complexes: An estimation of the role of ancillary ligand by chemical partition of radiative decay rate

Three highly luminescent Eu(III) complexes of the type [Eu(β-diket.)₃(tm-phen)] where β-diket. = hexafluoroacetylacetone (hfaa), btfa; 4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa), 2-thenoyltrifluoroacetone (tta) and tm-phen = 3,4,7,8-tetramethyl-1,10-phenanthroline have been synthesized and characterized. The detailed photophysical properties of the complexes were analyzed both theoretically and experimentally. The theoretical photophysical properties calculated using the LUMPAC program [1] are in excellent agreement with experimental results. The Judd-Ofelt (J-O) parameters (Ω₂ and Ω₄), radiative (A_rad) and non-radiative (A_nrad) decay rates were calculated and discussed. Chemical partition of the radiative decay rates (A_rad) of electric-dipole transitions revealed that β-diketone contributes 77 to 84% while tm-phen contributes 22 to 16% towards the A_rad. The intramolecular energy transfer (W_ET) and back-energy transfer (W_BT) rate are predicted from the singlet (S₁) and triplet (T₁) levels to the emissive ⁵D₁ and ⁵D₀ states for Eu(III) and follow the path S₀ → S₁ → T₁ → ⁵D₁ → ⁵D₀ → ⁷F_0,4. The complexes emit typical Eu(III) red emission with long luminescence lifetime (0.75–0.88 ms), show high intrinsic quantum yield (ϕ_Ln) (66–70%) and may be used as one of the red components in light emitting devices.