Organic light emitting diodes based on thermally activated delayed fluorescent (TADF) emitters with both high external quantum efficiency (EQE) and low efficiency roll-off are under urgent pursuing. Three TADF emitters based on fluorene-spiro-acridine derivatives as donors are developed. Composing of rigid donor and acceptor, all emitters exhibit high PLQYs of 96-99%. Direct conjugation attached on acridine offers an enhanced reverse intersystem crossing (k RISC ) in IA-TRZ, while the extension via spiro-junction accelerates the radiative decay rate (k r ) in 2S-TRZ and IT-TRZ. Remarkably, all emitters enable high electroluminescent performance with EQE of 35.6% for 2S-TRZ, 36.1% for IT-TRZ and 32.0% for IA-TRZ, respectively, and very high luminance of up to 100 000 cd m −2 . Surprisingly, 2S-TRZ and IT-TRZ that hold larger k r and smaller k RISC than that of IA-TRZ exhibit more relieved efficiency roll-off, indicating the significance of k r . More importantly, IT-TRZ with a moderate k r and k RISC among these emitters shows the lowest roll-off which demonstrates that the comprehensive consideration of all photophysical processes is important to attain excellent devices. It also emphasizes the molecular modulation method through side group that can induce elaborate control on decay channels for finely optimizing emitters. This study can provide a useful perspective in designing practical emitters.

A fast radiative decay process for long-wavelength molecular light-emitters is vital to achieving a high emission efficiency by outcompeting the nonradiative decay imposed by the energy-gap law. An ensuing short emission lifetime is also beneficial for fabricating high-performance organic light-emitting diodes. Herein a series of half-lantern dinuclear platinum(II) complexes is reported, which shows high-efficiency deep red phosphorescence (𝝀 em > 660 nm). The molecules are designed to have a cofacially aligned structure featuring short Pt-Pt distances of 2.80-2.83 Å by using 10H-pyrido[3,2-b][1,4]benzoxazine (PyXZ) as the rigid bridges, which are revealed by single crystal X-ray diffraction analysis. Together with the strong electron-donating property of PyXZ bridges, the metallophilic interaction endows low-energy triplet excited states with mixed metal-metal-to-ligand charge-transfer ( 3 MMLCT) and ligand-to-ligand charge-transfer ( 3 LLCT) characters. The deep red devices based on the diplatinum(II) complexes show maximum external quantum efficiencies (EQEs) up to 21.8%. The EQE of 19.4% and operational lifetime (LT 85 ) of 334 h (the operational time after which the luminance drops to 85% of the initial value) at a luminance of 1000 cd m −2 promise the pratical use of these complexes.

Achieving high efficiency and low efficiency roll-off simultaneously is of great significance for further application of thermally activated delayed fluorescent (TADF) emitters. A balance between radiative decay and reversed intersystem crossing must be carefully established. Herein, we propose a qunolino-acridine (QAc) donor composing two acridine with both planar (pAc) and bended (bAc) geometries. Combining with triazine, a TADF emitter QAc-TRZ is assembled. The pAc provides a well interaction with triazine which ensures a decent TADF behavior, while the bAc offers a delocalization of highest occupied molecular orbital (HOMO) which guarantees an enhancement of radiative decay. Remarkably, QAc-TRZ enables a highly efficient organic light emitting diode (OLED) with maximum external quantum efficiency (EQE) of 37.3 %. More importantly, the efficiencies under 100/1000 cd m À 2 stay 36.3 % and 31.7 %, respectively, and remain 21.5 % even under 10 000 cd m À 2 .

Increasing the radiative decay rates of triplet excited states to overcome the energy-gap law is important for the development of high-efficiency and short-lived phosphorescent metal complexes in the red to...

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A new acridine donor with trispiro junctions is developed for assembling highly effcient thermally activated delayed fluorescent emitter. The multispiro junctions ensure a rigid geometry that leads to a well-suppressed...

The development of blue thermally activated delayed fluorescence (TADF) emitters that can simultaneously achieve narrowband emission, high efficiency, and low efficiency roll-off in organic light-emitting diodes (OLEDs) remains a significant...