Highly Efficient and Air‐Stable Lanthanide EuII Complex: New Emitter in Organic Light Emitting Diodes

Luminescent Eu II complexes with a characteristic 5d–4f transition have potential applications in many fields. However, their instability in ambient conditions impedes further exploration and application. Herein, we report two new Eu II complexes, bis[hydrotris(3‐trifluoromethylpyrazolyl)borate]europium(II) ( Eu‐1 ) and bis[hydrotris(3‐methylpyrazolyl)borate]europium(II) ( Eu‐2 ). Intriguingly, the blue emissive Eu‐1 showed high air stability arising from fluorine protection and close molecular packing, as maintaining a photoluminescence quantum yield (PLQY) of 91 % (initial 96 %) upon exposure to air over 2200 hours. While the orange emissive Eu‐2 showed a maximum luminance of 30620 cd m −2 , and a maximum external quantum efficiency (EQE) of 6.5 %, corresponding to an exciton utilization efficiency around 100 % in organic light‐emitting diodes (OLEDs). These results could inspire further research on stable and efficient Eu II complexes and their application in OLEDs.