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[Display omitted] • "Asymmetric D-A-wA" strategy is demonstrated for high-efficiency red TADF emitters. • Diphenylphosphine oxide as the second acceptor markedly improves TADF properties. • Strong red emission peaked at 645 nm with a high PLQY of 94.7 % is realized. • The maxima of 19360 cd m−2 for luminance and 11.4 % for EQE at 632 nm are achieved. A feasible strategy of asymmetric donor–acceptor-second acceptor (D-A-A) structure is proposed for improving the photoluminecense (PL) and electroluminescence (EL) performance of red thermally activated delayed fluorescence (TADF) emitters. As a concept, four newly TADF emitters are designed and synthesized, D-A-A structure o TPAPO-DTPZ and p TPAPO-DTPZ and D-A-D type o DTPA-DTPZ and p DTPA-DTPZ, by using triphenylamine (TPA), dithieno[2,3- a :3′,2′- c ]phenazine (DTPZ) and diphenylphosphine oxide (DPPO) groups as the donor, acceptor and second acceptor, respectively. It is demonstrated that abound intra- and inter-molecular C H···O hydrogen bonds are forming between TPA donor and DPPO acceptor for the asymmetric compounds, which effectively promote the intra- and inter-molecular charge transfer. Moreover, the steric hindrance of DPPO group availably suppresses the concentration quenching. As a consequence, the asymmetric D-A-A structured o TPAPO-DTPZ and p TPAPO-DTPZ exhibit relatively higher PL quantum yields and lower nonradiation rates, and thus better EL performance. In particular, device based on p TPAPO-DTPZ achieves the maximum luminance of 19360 cd m−2 and external quantum efficiency of 11.4 % at 632 nm, which are 2.3 and 1.5 times the values of p DTPA-DTPZ based device. [ABSTRACT FROM AUTHOR]

[Display omitted] • Near-infrared TADF emission from phenanthroline based molecule is demonstrated. • NIR emission and radiation efficiency are balanced in CNPP-TPA. • Strong NIR emission peaked at 701 nm with PLQY of 96.5% is realized. • A maximum EQE (8.69%) of vacuum-evaporated device peaked at 684 nm is achieved. • A maximum EQE (2.02%) of spin-coated device peaked at 700 nm is achieved. Efficient thermally activated delayed fluorescence (TADF) emitters are of practical significance for realizing high-performance organic light emitting diodes (OLEDs), which remains the great challenge for near-infrared (NIR) emitters because of their narrow bandgap worsened non-radiative transition. Herein, a NIR TADF molecule (CNPP-TPA) is reported, composed of 2,3-dicyanopyrazino phenanthroline (CNPP) as acceptor and triphenylamine as donor. The strong electron-withdrawing effect of CNPP and intramolecular hydrogen bond endow CNPP-TPA with efficient NIR photoluminescence emission peaked at 701 nm with a high photoluminescence quantum yield of 96.5%, owing to rapid singlet radiation and effective reverse intersystem crossing. Vacuum -evaporated and spin-coated organic light-emitting diodes exhibit the NIR emissions respectively peaked at 684 nm and 700 nm, corresponding to Commission International de L'Eclairage (CIE) coordinates of (0.68, 0.31) and (0.69, 0.30), respectively. Significantly, these two devices achieved the state-of-the-art external quantum efficiencies of 8.69% and 2.02%. [ABSTRACT FROM AUTHOR]