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