Ultralow Roll-off\nThermally Activated Delayed Fluorescent\nLight-Emitting Diodes Based on Furo[2,3‑<i>b</i>]quinoxaline\nEmitters

Herein, a Y-type compound (<b>67dMeOTPA-FQ</b>)\nand a T-type\ncompound (<b>58dMeOTPA-FQ</b>) based on furo[2,3-<i>b</i>]quinoxaline were synthesized. The theory calculation shows the S<sub>1</sub> and T<sub>1</sub> of both compounds own a charge-transfer\nfeature while their T<sub>2</sub> states have a local excitation feature.\nThe calculated <i>k</i><sub>RISC(T2‑S1)</sub> is\none to 2 orders of magnitude larger than <i>k</i><sub>RISC(T1‑S1)</sub>. Thus, the nonadiabatic spin-vibronic mechanism involved in the\nT<sub>2</sub> state is suggested to be responsible for the thermally\nactivated delayed fluorescence (TADF) feature. Meanwhile, when 2-methyl-9,10-bis(naphthalen-2-yl)anthracene\nis selected as host, the maximum luminance of the device based on <b>67dMeOTPA-FQ</b> is up to 104215 cd·m<sup>–2</sup>, and the external quantum efficiency (EQE) keeps in the 8.2–8.0%\nrange with the luminance changed from 55.0 cd·m<sup>–2</sup> to 90000 cd·m<sup>–2</sup>, only 2.4% efficiency roll-off.\nAs for <b>58dMeOTPA-FQ</b>, a slightly lower EQE of 7.1–6.7%\nwith the luminance range of 1–40000 cd·m<sup>–2</sup> was achieved for orange-red emission. Both the reverse intersystem\ncrossing (RISC) and triplet-triplet annihilation mechanisms are supposed\nto concurrently contribute to the utilization of triplet excitons\nand suppress the notorious efficiency roll-off observed in TADF-based\ndevices.