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

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.