Organic light-emitting diodes (OLEDs) have had commercial success in displays and lighting. Compared to red and green OLEDs, blue OLEDs are still the bottleneck because the high-energy and long-lived triplet exciton in traditional blue OLEDs causes the short operational lifetime of the device. As a new type emitter, lanthanide complexes with a 5d–4f transition could have short excited-state lifetimes on the order of nanoseconds. To achieve a high-efficiency 5d–4f transition, we systematically tuned the steric and electronic effects of tripodal tris­(pyrazolyl)­borate ligands and drew a full picture of their Ce­(III) complexes. Intriguingly, all of these complexes show bright blue emission with high photoluminescence quantum yields exceeding 95% and short decay lifetimes of 35–73 ns both in the solid powder and in dichloromethane solutions. Using the Ce­(III) complex emitter, we show a blue OLED with a maximum external quantum efficiency of 14.1% and a maximum luminance of 33,160 cd m–2, and the specific electroluminescence mechanism of direct exciton formation on the Ce­(III) ion with a near-unity exciton utilization efficiency is also confirmed. The discovered photoluminescence and electroluminescence property–structure relationships may shed new light on the rational design of highly efficient lanthanide-based blue emitters and their optoelectronic devices such as OLEDs.

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(3methylpyrazolyl)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 À2 , 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.

Compared with red and green organic light-emitting diodes (OLEDs), blue is the bottleneck that restricts the wide development of OLEDs from being the next-generation technology for displays and lighting. As a new type of emitter, a Ce­(III) complex shows many satisfactory advantages, such as a short excited-state lifetime, 100% theoretical exciton utilization efficiency, and tunable emission color. Herein we synthesized three heteroleptic Ce­(III) complexes Ce­(TpMe2)2(dtfpz), Ce­(TpMe2)2(dmpz), and Ce­(TpMe2)2(dppz) with the hydrotris­(3,5-dimethylpyrazolyl)­borate (TpMe2) main ligand and different substituted pyrazole ancillary ligands, namely, 3,5-di­(trifluomethyl)­pyrazolyl (dtfpz), 3,5-dimethylpyrazolyl (dmpz), and 3,5-diphenylpyrazolyl (dppz), and studied their structures and luminescence properties. All the Ce­(III) complexes exhibited a near-unity photoluminescence quantum yield both in solution and as a powder with maximum emission wavelengths in the range of 450–486 nm. The OLED employing Ce­(TpMe2)2(dppz) as the emitter showed the best performance, including a turn-on voltage, maximum luminance, and external quantum efficiency of 3.2 V, 29 200 cd m–2, and 12.5%, respectively.

We successfully manipulate organic triplet harvesting in a series of N-benzoyl-carbazole microcrystals simply via changing the substitution positions.

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(3methylpyrazolyl)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 À2 , 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.

White organic light-emitting diodes (WOLEDs) is a new generation of lighting technology and has stimulated wide-ranging studies. Despite the advantage of simple device structure, single-emitting-layer WOLEDs (SEL-WOLEDs) still face the challenges of difficult material screening and fine energy level regulation. Herein, we report efficient SEL-WOLEDs with a sky-blue emitting cerium(III) complex Ce-TBO2Et and an orange-red emitting europium(II) complex Eu(Tp2Et)2 as the emitters, showing a maximum external quantum efficiency of 15.9% and Commission Internationale de l’Eclairage coordinates of (0.33, 0.39) at various luminances. Most importantly, the electroluminescence mechanism of direct hole capture and hindered energy transfer between the two emitters facilitate a manageable weight doping concentration of 5% for Eu(Tp2Et)2, avoiding the low concentration (<1%) of the low-energy emitter in typical SEL-WOLEDs. Our results indicate that d-f transition emitters may circumvent fine energy level regulation and provide development potential for SEL-WOLEDs.

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Doublet emission from open‐shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open‐shell molecules are far less than that of closed‐shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4‐(9H‐carbozol‐9‐yl)phenyl‐tris(pyrazolyl)borate complex Ce(CzPhTp)3 with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal‐centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open‐shell molecules.