N-doping strategy and peripheral substituent engineering enable Indolo[3,2,1-jk]carbazole harvesting multi-resonance TADF feature with deep-blue narrowband emission

Current multi-resonance thermally activated delayed fluorescence (MR-TADF) frameworks are largely limited to B/N- or carbonyl/N-based systems, which rarely achieve deep-blue emission approaching the Rec.2020 standard. Herein, we propose a dual nitrogen-atom doping strategy to construct a novel MR-TADF scaffold. By embedding electron-deficient nitrogen (sp²) atoms into indolo[3,2,1-jk]carbazole (ICz) in a meta-position fashion, a new multiple resonance framework (NICz) is obtained. Combined with peripheral substituent engineering, five derivatives are developed. The N-doping narrows the singlet–triplet splitting (ΔE_ST) from 0.49 eV (ICz) to 0.38 eV (NICz), and the derived emitters show further reduced ΔE_ST values of 0.17–0.28 eV. Meanwhile, the four compounds exhibit narrowband MR-TADF emission with wavelength of 426–440 nm and FWHM of 32–45 nm while the tCz-NICz shows donor-acceptor featured blue TADF emission (λ_em = 466 nm, FWHM = 45 nm). In OLEDs, PhCz-NICz achieves a deep-blue emission at 436 nm with CIE (0.154, 0.050) and an EQE of 11.1%, whereas tCz-NICz reaches an EQE of 20.9% at 464 nm. This work demonstrates a promising only N-based MR-TADF platform for efficient deep-blue luminescence.