Nitrogen-Mediated Orbital-Compatible π-Extension: Balancing Excited-State Components and Suppressing Vibrational Broadening Toward Redshifted Narrowband MR-TADF Emitters

BN-based multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters face a critical bottleneck in achieving redshifted narrowband emission complying with the BT.2020 standard. The widely used π-extension strategy typically induces severe spectral broadening due to imbalanced, short-range charge transfer (SRCT) and long-range charge transfer (LRCT) components arising from non-MR aromatic units. Herein, we establish a nitrogen-mediated orbital-compatible π-extension strategy by incorporating sp²-hybridized nitrogen atoms at specific sites to construct BN-core-compatible MR π-segments, thereby preserving the persistent MR character across the entire skeleton. Starting from BN-TP, three derivatives (BNCz-BQ, BNCz-PQ, BNCz-PMQ) were synthesized and theoretically screened. Notably, BNCz-PQ exhibits balanced HOMO/LUMO contributions of 9.00%/8.20% on its π-extended fused segment, the lowest LRCT fraction of 17.34%, and a reorganization energy of 408.6 cm⁻¹, markedly outperforming BNCz-BQ and BNCz-PMQ. Such site-specific N-doping effectively suppresses low-frequency vibrations and out-of-plane torsions that dominate broadening in BN-TP. Consequently, BNCz-PQ emits intense green light with an ultranarrow FWHM of 24 nm, 10 nm narrower than that of BN-TP. The corresponding OLEDs achieve a maximum EQE of 27.7% with greatly suppressed roll-off, retaining 27.1% and 23.6% at 1000 and 10 000 cd m⁻². This work establishes a general orbital-guided principle for designing high-performance long-wavelength narrowband MR-TADF emitters.