Utilization of Ternary Europium Complex for Organic Electroluminescent Devices and as a Sensitizer to Improve Electroluminescence of Red-Emitting Iridium Complex

Abstract

Two new lanthanide complexes [Ln(hfaa)₃(Py-Im)] [hfaa = hexafluoroacetylacetone, Py-Im = 2-(2-pyridyl)benzimidazole and Ln = Eu(III) (1) and Tb(III) (2)] were synthesized and characterized. An X-ray crystal structure determination confirms that complex 1 is eight-coordinate with a distorted trigonal dodecahedral geometry. It shows typical vivid red Eu(III) emission in the solid state, in solution, and in a polymer matrix. The observed lifetime (τ_obs) of complex 1 in the solid state, in dichloromethane (DCM) solution, and in thin films is 833.01, 837.95, and 626.16-715.69 μs, respectively, with a photoluminescence quantum yield Q_Eu^L ≈ 33% in DCM solution. Complex 2 displays a yellowish-green emission in the solid state (τ_obs ≈ 36.99 μs), while a near white-light emission in solution (x; 0.2574: y; 0.3371) and in thin films. Therefore, it is a potential candidate for generating single-component white light-emitting materials for solid-state lighting applications. The kinetic scheme for modeling energy-transfer processes shows that the main donor state for 1 is the ligand triplet state (T₁) and that energy transfer occurs to both the ⁵D₁ (56.55%) and ⁵D₀ (40.58%) levels. We fabricated a series of single- and double-layer organic light-emitting devices using complex 1. The luminance of the optimized double-layer electroluminescence (EL) device was 373 cd/m² with very low turn-on voltage of ∼4.2 V. Complex 1 was further utilized as a sensitizer to improve the EL of a red-emitting iridium complex PQ₂Ir(dpm) (PQ = phenylquinoly-N,C2', dpm = dipivaloylmethane). The codoped device achieved a maximum brightness and maximum current efficiency (η_c) of 93 224 cd/m² and 36.38 cd/A, respectively.