Designing thermally activated delayed fluorescence (TADF) materials with an efficient reverse intersystem crossing (RISC) process is regarded as the key to actualize efficient organic light-emitting diodes (OLEDs) with low efficiency roll-off. Herein, a novel molecular design strategy is reported where a typical TADF material 10-phenyl-10H, 10′H-spiro[acridine-9, 9′-anthracen]-10′-one (ACRSA) is utilized as a functional electron donor to design TADF materials of 2,4,6-triphenyl-1,3,5-triazine(TRZ)-p-ACRSA and TRZ-m-ACRSA. It is unique that the intramolecular charge transfer of the ACRSA moiety and the intramolecular and through-space intermolecular charge transfer between the TRZ and ACRSA moieties, provide a "multichannel" effect to enhance the rate of the reverse intersystem crossing process (k risc ) exceeding 10 −6 s −1 . TADF OLEDs based on TRZ-p-ACRSA as an emitter show a maximum external quantum efficiency (EQE) of 28% with reduced efficiency roll-off (EQEs of 27.5% and 22.1% at 100 and 1000 cd m −2 , respectively). Yellow phosphorescent OLEDs utilizing TRZ-p-ACRSA as a host material show record-high EQE of 25.5% and power efficiency of 115 lm W −1 , while phosphorescent OLEDs based on TRZ-m-ACRSA show further lower efficiency roll-off with EQEs of 25.2%, 24.3%, and 21.5% at 100, 1000, and 10 000 cd m −2 , respectively.being consistent with the El-Sayed rule of intersystem crossing (ISC) process, when the natures of 1 CT and 3 CT are different, the SOC between 1 CT and 3 CT might be efficient as well. [2] More importantly, avoiding dual emission phenomenon, which caused by conformational transforming brought by flexible moieties, is considered as the first priority of TADF molecular design strategy. [9] Conformational transforming not only results in energy loss, but also limits the triplet exciton utilization because not all possible conformations have potential to achieve TADF phenomenon. [10] On the basis of this understanding, the key to design TADF

A fast radiative rate, highly suppressed nonradiation, and a short exciton lifetime are key elements for achieving efficient thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) with reduced efficiency roll-off at a high current density. Herein, four representative TADF emitters are designed and synthesized based on the combination of benzophenone (BP) or 3-benzoylpyridine (BPy3) acceptors, with dendritic 3,3″,6,6″-tetra-tert-butyl-9′H-9,3′:6′,9″-tercarbazole (CDTC) or 10H-spiro­(acridine-9,9′-thioxanthene) (TXDMAc) donors, respectively. Density functional theory simulation and X-ray diffraction analysis validated the formation of CH···N intramolecular hydrogen bonds regarding the BPy3-CDTC and BPy3-TXDMAc compounds. Notably, the construction of intramolecular hydrogen bonding within TADF emitters significantly enhances the intramolecular charge transfer (ICT) strength while reducing the donor–acceptor (D-A) dihedral angle, resulting in accelerated radiative and suppressed nonradiative processes. With short TADF exciton lifetimes (τTADF) and high photoluminescence quantum yields (ϕPL), OLEDs employing BPy3-CDTC and BPy3-TXDMAc dopants realized maximum external quantum efficiencies (EQEs) up to 18.9 and 25.6%, respectively. Moreover, the nondoped device based on BPy3-TXDMAc exhibited a maximum EQE of 18.7%, accompanied by an extremely small efficiency loss of only 4.1% at the luminance of 1000 cd m–2. In particular, the operational lifetime of the sky-blue BPy3-CDTC-based device was greatly extended by 10 times in contrast to the BP-CDTC-based counterpart, verifying the idea that the in-built intramolecular hydrogen bonding strategy was promising for the realization of efficient and stable TADF-OLEDs.

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