Divalent europium 5d-4ftransition has aroused great attention in many fields, in a way of doping Eu²⁺ions into inorganic solids. However, molecular Eu²⁺complexes with 5d-4ftransition 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-N₈complexes 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-N₈as 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 Eu²⁺complexes and could inspire further research on application in OLEDs.

Compared to red and green organic light-emitting diodes (OLEDs), blue OLEDs are still the bottleneck due to the lack of efficient emitters with simultaneous high exciton utilization efficiency (EUE) and short excited-state lifetime. Different from the fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), and organic radical materials traditionally used in OLEDs, we demonstrate herein a new type of emitter, cerium(III) complex Ce-1 with spin-allowed and parity-allowed d–f transition of the centre Ce³⁺ ion. The compound exhibits a high EUE up to 100% in OLEDs and a short excited-state lifetime of 42 ns, which is considerably faster than that achieved in efficient phosphorescence and TADF emitters. The optimized OLEDs show an average maximum external quantum efficiency (EQE) of 12.4% and Commission Internationale de L’Eclairage (CIE) coordinates of (0.146, 0.078).

In the field of RGB diodes, development of a blue organic light-emitting diode (OLED) is a challenge because of the lack of an emitter which simultaneously has a short excited state lifetime and a high theoretical external quantum efficiency (EQE). We demonstrate herein a blue emissive rare earth cerium(III) complex Ce-2 showing a high photoluminescence quantum yield of 95% and a short excited state lifetime of 52.0 ns in doped film, which is considerably faster than that achieved in typical efficient phosphorescence or thermally activated delayed fluorescence emitters (typical lifetimes >1 μs). The corresponding OLED shows a maximum EQE up to 20.8% and a still high EQE of 18.2% at 1000 cd m⁻², as well as an operation lifetime 70 times longer than that of a classic phosphorescence OLED. The excellent performance indicates that cerium(III) complex could be a candidate for efficient and stable blue OLEDs because of its spin- and parity-allowed d−f transition from the Ce³⁺ ion.