Highly efficient deep-red organic light emitting diodes based on acenaphthopyrazine derivatives via π-bridge with thermally activated delayed fluorescence†

Deep-red emitters have been promising in applications such as organic light-emitting diodes (OLEDs), night vision, telecommunications, bioimaging and photodynamic therapy. However, these red fluorescent molecules, according to the energy gap law, generally suffer from large non-radiative internal conversion rates. Therefore, organic fluorescent materials with an emission wavelength beyond 640 nm remain highly limited till date. Herein, two thermally activated delayed fluorescence (TADF) emitters are designed and synthesized by attaching electron donors such as 9,9-diphenyl-9,10-dihydroacridine (DPAC) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) to the electron acceptor of acenaphthopyrazine (AP). The high molecular rigidity of donors and acceptor suppresses energy loss via non-radiative internal conversion, and the introduction of bridging benzene increases the HOMO and the LUMO overlap. Thus, the combinations of DMAC–AP and DPAC–AP manifest a small ΔE_ST and fast reverse intersystem crossing. With the delicate optimization of OLEDs, they both exhibit deep-red emission at 640 nm with the maximum external quantum efficiency over 14%. This work provides an effective strategy to obtain efficient deep-red emitters and also enriches the insights on TADF materials.