Nitrogen/Carbonyl Multiple Resonance Emitters Based on Nitrogen Doped 5/6/7-Membered Rings

Abstract

The development of high-performance multi-resonance (MR) emitters has been booming vigorously in recent years, driven by the enormous demand for organic light-emitting diodes (OLEDs). Typically, MR emitters incorporate electron-withdrawing groups (EWGs) and electron-donating groups (EDGs), which are conducive to reducing the energy gap between the excited triplet (T₁) and singlet (S₁) states. Among the diverse family of MR emitters, quinolino[3,2,1-de]acridine-5,9-dione (QAO) cores have garnered increasing attention due to their excellent molecular modularity and tunable electronic properties. However, most reported MR emitters based on the QAO core feature benzenoid six-membered rings. Five- and seven-membered ring-containing QAO-based MR emitters remain scarce, primarily due to the synthetic challenges involved, despite their potential to exhibit intriguing anti-aromatic properties. Herein, three novel nitrogen/carbonyl-based MR emitters (NE-1, NE-2, and NE-3) with nitrogen-doped 5/6/7-membered rings were successfully synthesized and fully characterized via thermal analysis, cyclic voltammetry, theoretical calculations, and OLED device tests. Notably, the seven-membered nitrogen/carbonyl rings in these emitters were found to possess strong anti-aromaticity, as evidenced by large positive Nucleus-Independent Chemical Shift (NICS) values: 15.09 for NE-1, 7.61 for NE-2, and 18.05 for NE-3. In contrast, the six-membered nitrogen/carbonyl rings exhibited much weaker anti-aromaticity. All three emitters emitted green light in OLED devices, with electroluminescent (EL) peaks at 491 nm (NE-1), 484 nm (NE-2), and 530 nm (NE-3), respectively. Among them, NE-3 achieved the optimal device performance, with a maximum current efficiency (CE_mₐₓ) of 8.5 cd·A^–1, a maximum power efficiency (PE_mₐₓ) of 8.8 lm·W^–1, a maximum external quantum efficiency (EQE_mₐₓ) of 2.2%, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.30, 0.65).

Keywords

Multiple resonance / Emitters / Nitrogen/carbonyl system / Nitrogen doped 5/6/7-membered rings / OLEDs

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