Slow magnetic relaxation and luminescence properties in β-diketonate lanthanide(iii) complexes. Preparation of Eu(iii) and Yb(iii) OLED devices

Abstract

The reaction of [Ln(btfa)₃(H₂O)₂] (btfa⁻ = 4,4,4-trifluoro-1-phenyl-1,3-butanedionate) with 4,4′-dinonyl-2,2′-bipyridyl (4,4′-dinonylbipy) in ethanol allows isolation of mononuclear complexes [Ln(btfa)₃(4,4′-dinonylbipy)] Ln = Sm (1-Sm), Eu (2-Eu), Tb (3-Tb), Dy (4-Dy), Er (5-Er) and Yb (6-Yb). The solid state luminescence emission in the visible region for 1-Sm, 2-Eu, 3-Tb and 4-Dy and in the NIR region for 1-Sm, 5-Er and 6-Yb shows efficient energy transfer from the 4,4,4-trifluoro-1-phenyl-1,3-butanedionate ligands to the central Ln³⁺ ion for all the compounds. Finally, complexes 2-Eu and 6-Yb were successfully used as emitters in multilayer vacuum-deposited OLEDs. The electroluminescence quantum efficiency (EQE) of the corresponding devices reached 2.1% and ∼0.1–0.2% for 2-Eu (λ_EL = 614 nm) and 6-Yb (λ_EL = 977 nm), respectively. Maximum radiant emittance recorded for the Ln-associated emission achieved 135 μW cm⁻² for 2-Eu and 121 μW cm⁻² for 6-Yb. These values for efficiency and radiant emittance are unusually high for such type of emitters. Moreover, magnetic studies were performed on all compounds. Alternating current (AC) dynamic measurements indicated Single Molecular Magnet (SMM) behaviour for 4-Dy and field-induced slow relaxation of the magnetization for complexes 3-Tb, 5-Er and 6-Yb. The anisotropy energy barriers and pre-exponential factors are 91.1 cm⁻¹, τ₀ = 7.2 × 10⁻⁹ s (under zero magnetic field) and ΔE = 109.3 cm⁻¹, τ₀ = 9.3 × 10⁻¹⁰ s under 0.1 T magnetic field for 4-Dy and ΔE = 24.6 cm⁻¹, τ₀ = 8.7 × 10⁻⁸ s (under 0.07 T) for 5-Er. Besides, we observe that for compounds 3-Tb and 6-Yb the relaxation of the magnetization does not occur through the Orbach process.