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

Abstract

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 donor from carbazole (Cz) to 10-dimethyl-acridine (DMAC) or 10H-phenoxazine (PXZ) for a fixed acceptor, 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 emitter exhibit bright electroluminescence with emission color 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 ηexts of 12.9% (35.9 cd A-1) to 16.7% (51.9 cd A-1) are realized in the green, yellow and orange-red TADF-OLEDs. All the PP and iPP based TADF emitters exhibit superior efficiency stabilities to the quinoxaline analogues. It provides a practical strategy to tune the emission color of quinoxaline and pyrido[x,y-b]pyrazine derivatives with same molecular skeletons over the entire visible light range.