Non‐Fused Carbonyl/Nitrogen Multi‐Resonance Emitters Enable Efficient Pure‐Red Narrowband Electroluminescence

Abstract Multi‐resonance (MR) narrowband emitters are attractive for ultrahigh‐definition displays, but their development is hindered by scarcity of red‐emitting molecular skeletons. Here, we propose a new design strategy for efficient pure‐red carbonyl/nitrogen (C═O/N) MR emitters, by integrating electron‐deficient carbonyl (C═O) and electron‐rich nitrogen (N) into a non‐fused conjugated diene skeleton. Unlike the counterparts having fused polycyclic skeletons with only one C═C bond between adjacent C═O and N atoms, the non‐fused MR emitters allow insertion of two conjugated C═C bonds between them, which not only lowers energy gap (1.97 eV) to redshift emission, but also retains short‐range charge transfer with atomically alternating highest occupied molecular orbitals (HOMO)/lowest unoccupied molecular orbitals (LUMO) distributions along the diene framework to achieve narrowband emission and thermally activated delayed fluorescence effect. Solution‐processed organic light‐emitting diodes based on the emitters exhibit pure‐red electroluminescence at 628 nm with small full‐width at half maxima (FWHM EL ) of 37 nm, together with high external quantum efficiencies of 24.2% at maximum and 23.8% at 1000 cd m −2 , which to our best knowledge, makes them the first example of pure‐red C═O/N MR emitters with FWHM EL below 40 nm. The results provide new insight for developing long‐wavelength MR emitters by expanding their material systems from fused polycyclic frameworks to non‐fused conjugated polyene skeletons.