Efficient Red Organic Light Emitting Diodes of Nona Coordinate Europium Tris(β-Diketonato) Complexes Bearing 4′-Phenyl-2,2′:6′,2′′-Terpyridine

Two novel nona-coordinated Eu(III) complexes [Eu(btfa)₃(Ph-TerPyr)] (Eu-1) and [Eu(NTA)₃(Ph-TerPyr)] (Eu-2) have been synthesized and characterized. The structure of the complexes was elucidated by density functional theory (DFT) methods. The experimental photophysical properties of the complexes were investigated and complemented with theoretical calculations. Effective energy transfer (ET) pathways for the sensitized red luminescence is discussed. The complexes were tested as emitting layers (EML) in organic light emitting diodes (OLEDs). At the optimum doping concentration of 4 wt.%, the double-EML OLEDs of Eu-1 exhibited red electroluminescence (EL) with an EQE of 4.0 % and maximum brightness (B)=1179 cd/m², maximum current efficiency (η_c)=5.64 cd/A, and maximum power efficiency (η_p)=4.78 lm/W at the current density (J) of 10 mA/cm². Interestingly, the double-EML OLEDs of Eu-2 at the optimum concentration of 3 wt.%, displayed an outstanding EL performance with EQE of 7.32 % and B=838 cd/m², η_c=10.19 cd/A and η_p=10.33 lm/W at J=10 mA/cm². The EL performance of this device is among the best reported for devices incorporating a europium complex as a red emitter.