Red and Near‐Infrared Thermally Activated Delayed Fluorescence Emitters Based on a Dibenzo[f,h]pyrido[2,3‐b]quinoxaline Acceptor

Recently, there has been growing interest in deep red (DR) and near‐infrared (NIR) thermally activated delayed fluorescence (TADF) emitters due to their potential use in applications in bioimaging and night‐vision displays. Herein, we designed and synthesized a series of red/NIR TADF emitters, DMAC , PXZ , and DPACz , that all contain the same electron‐accepting PyBP (dibenzo [f, h] pyrido [2,3‐b]quinoxaline) moiety. These compounds emit at 643 nm for DMACPyBP , 722 nm for DPACz PyBP , and 743 nm for PXZPyBP in toluene solution, while their thin films singlet‐triplet energy gaps (Δ E ST ) are <0.1 eV. DMACPyBP , with the weakest donor, has the highest Φ PL of 62.3 %, the smallest Δ E ST of 0.03 eV, and a fast reverse intersystem crossing rate constant ( k RISC ) of 0.5×10 5 s −1 in a 1 wt% doped film in CBP. In contrast, PXZPyBP , containing the strongest donor, has a lower Φ PL (21.2 %), a relatively larger Δ E ST (0.10 eV), and a slower k RISC (0.04×10 5 s −1 ). Thus, our work highlights the molecular design challenges involved in pushing emission into the NIR region while maintaing both TADF and high Φ PL in PyBP‐based donor‐acceptor emitters.