Developing red and NIR thermally activated delayed fluorescence emitters based on 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. However, the performance of long wavelength TADF emitters is inherently limited by the energy gap law, which reduces their photoluminescence quantum yields (ΦPL), making a robust design strategy challenging. Herein, we designed and synthesized a series of red/NIR TADF emitters. The emission wavelengths were rationally tuned by modulating the strength of the electron-donating groups, DMAC, PXZ, and DPACz, that are coupled to the same electron-accepting PyBP (dibenzo [f, h] pyrido [2,3-b]quinoxaline) moiety. These compounds emit with photoluminescence maxima (λPL) 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 (ΔEST) are < 0.1 eV. DMACPyBP, with the weakest donor, has the highest ΦPL of 62.3%, the smallest ΔEST of 0.03 eV, and a fast reverse intersystem crossing rate constant (kRISC) of 0.5 × 105 s-1 in a 1 wt% doped film in CBP. In contrast, PXZPyBP, containing the strongest donor, has a lower ΦPL (21.2%), a relatively larger ΔEST (0.10 eV), and a slower kRISC (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 PL in PyBP-based donor-acceptor emitters.