The Impact of Ancillary Ligand in Tris(β-Diketonato) Europium(III) Complexes on the Electroluminescence Color and Organic Light-Emitting Diodes Performance

Two new octacoordinated europium coordination complexes (EuCCs) with the general formula [Eu(DPTB)₃(H₂O)₂] (EuCC₁) and [Eu(DPTB)₃BathPhen] (EuCC₂) are developed by incorporating a push-pull primary antenna β-diketone 1-(4-(Diphenylamino)phenyl)-4,4,4-trifluorobutane-1,3-dione (HDPTB) and a hole or exciton-blocking N^N Bathophenanthroline (BathPhen) ancillary ligand. Experimental photophysical properties of EuCC₁ and EuCC₂ in tandem with the computational methods are analyzed and discussed in detail to understand the role of ligands (HDPTB and BathPhen) on the photoluminescence (PL) processes. To further establish the impact of the ligands on the electrical properties and the emitted color, both EuCCs are finally employed as an emitting layer (EML) in a multilayered device to fabricate organic light-emitting diodes (OLEDs). Interestingly, the single-EML and double-EML EuCC₁-based devices exhibit warm and cool white light emission with (CIE)_x,y ≈ 0.37,0.28, color correlated temperature (CCT) ≈ 3307 Kelvin (K) and maximum external quantum efficiency (EQE_max) = 1.59%, and (CIE)_x,y ≈ 0.36, 0.30, CCT ≈ 4045 K and EQE_max = 1.54%, respectively. Moreover, double-EML EuCC₂-based device exhibits highly monochromatic red emission with turn-on voltage (V_turn-on) = 3.5 V, maximum brightness (B) = 291 cd m⁻², maximum current efficiency (η_c) = 4.01 cd A⁻¹, maximum current efficiency (η_p) = 3.6 lm W⁻¹, EQE_max = 3.50% and (CIE)_x,y ≈ 0.61, 0.33 which are almost twofold higher than the single-EML EuCC₂-based device.