Thermally Activated Delayed Fluorescence in Phenothiazine–Phenoxazine–Quinoline Conjugates and Electroluminescence

Efficient organic thermally activated delayed fluorescent (TADF) emitters emerge when the donor and acceptor are finely balanced, as raising the reverse intersystem crossing (kRISC) rate can markedly enhance the performance of organic light-emitting diodes (OLEDs). In this study, we introduce a new design strategy that uses two hetero donors to strengthen TADF efficiency. We synthesize two donor–acceptor conjugates, PXPTQ and PXPTMQ, where phenoxazine and phenothiazine donors bond to quinoline-based acceptors at distinct positions through C–N single bonds. This structure promotes dense triplet states near the excited singlet state. Both experiments and quantum chemical calculations verify the strong TADF activity of both isomers. They show remarkable kRISC values between 2.6 and 4.4 × 107 s–1. The high efficiency stems from RISC pathways that connect locally excited or hybrid triplet states to charge transfer singlets. Vibronic coupling with higher-lying triplet states, alongside small energy gaps (S1–T1 = 0.07–0.11 eV; T1–T2 = 0.31–0.34 eV), further supports this process. The compounds deliver photoluminescence quantum yields of 63–70% and enable OLEDs with external quantum efficiencies near 20% and luminance levels above 21,000 cd/m2. These findings highlight how carefully tuned donor strengths can unlock next-generation OLED designs.