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

Abstract

ABSTRACT 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 sp 2 ‐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.