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 (ΔE_ST) are <0.1 eV. DMACPyBP, with the weakest donor, has the highest Φ_PL of 62.3 %, the smallest ΔE_ST of 0.03 eV, and a fast reverse intersystem crossing rate constant (k_RISC) of 0.5×10⁵ 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 ΔE_ST (0.10 eV), and a slower k_RISC (0.04×10⁵ 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.