Highly efficient and air-stable Eu(II)-containing azacryptates ready for organic light-emitting diodes

Divalent europium 5d-4f transition has aroused great attention in many fields, in a way of doping Eu2+ ions into inorganic solids. However, molecular Eu2+ complexes with 5d-4f transition are thought to be too air-unstable to explore their applications. In this work, we synthesized four Eu2+-containing azacryptates EuX2-N-n (X=Br, I, n=4, 8) and systematically studied the photophysical properties in crystalline samples and solutions. Intriguingly, the EuX2-N-8 complexes exhibit near-unity photoluminescence quantum yield, good air-/thermal-stability and mechanochromic property (X=I). Furthermore, we proved the application of Eu2+ complexes in organic light-emitting diodes (OLEDs) with high efficiency and luminance. The optimized device employing EuI2-N-8 as emitter has the best performance as the maximum luminance, current efficiency, and external quantum efficiency up to 25470cdm(-2), 62.4cdA(-1), and 17.7%, respectively. Our work deepens the understanding of structure-property relationship in molecular Eu2+ complexes and could inspire further research on application in OLEDs.