Quinoxaline and Pyrido[x,y‐b]pyrazine‐Based Emitters: Tuning Normal Fluorescence to Thermally Activated Delayed Fluorescence and Emitting Color over the Entire Visible‐Light Range

Quinoxaline (Q), pyrido[2,3‐b]pyrazine (PP) and pyrido[3,4‐b]pyrazine (iPP) are used as electron acceptors (A) to design a series of D–π–A‐type light‐emitting materials with different donor (D) groups. By adjusting the molecular torsion angles through changing D from carbazole (Cz) to 10‐dimethylacridine (DMAC) or 10H‐phenoxazine (PXZ) for a fixed A, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF). By gradually enhancing the intramolecular charge‐transfer extent through combining different D and A, the emission color is continuously and regularly tuned from pure blue to orange–red. Organic light‐emitting diodes (OLEDs) containing these compounds as doped emitters exhibit bright electroluminescence with emission colors covering the entire visible‐light range. An external quantum efficiency (ηext) of 1.2 % with excellent color coordinates of (0.16, 0.07) is obtained for the pure‐blue OLED of Q‐Cz. High ηext values of 12.9 (35.9) to 16.7 % (51.9 cd A−1) are realized in the green, yellow, and orange–red TADF OLEDs. All PP‐ and iPP‐based TADF emitters exhibit superior efficiency stabilities to that of analogues of Q. This provides a practical strategy to tune the emission color of Q, PP, and iPP derivatives with the same molecular skeletons over the entire visible‐light range. Full rainbow of emission: Pyrido[2,3‐b]pyrazine, pyrido[3,4‐b]pyrazine, and quinoxaline are utilized to develop new light‐emitting compounds. Through an appropriate combination of donor and acceptor, it is possible to tune the normal fluorescence into thermally activated delayed fluorescence, and tune the light emission from pure blue to green, yellow, and orange–red (see figure).