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

Luminescent EuII 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 EuII 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 EuII complexes and their application in OLEDs.