Adamantane-Bridged Boron-Nitrogen-based Multi-Resonance TADF Emitters for Narrowband Green OLEDs with Extended Operating Lifetime

ABSTRACT For existing multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules, expanding aromatic planes via intramolecular cyclization have become general strategies to modify structures, yet enhancing molecular stability remains challenging. Herein, we report the first pure‐green MR emitter incorporating an adamantane moiety as an intramolecular cyclization locker, not only modulating emission colors but also achieving remarkable operation stability alongside high efficiency in organic light‐emitting diodes (OLEDs). Ada‐ICzBN obtains narrowband emission at 526 nm with full width at half maximum (FWHM) of 26 nm in toluene, which could be maintained in highly doped films (16 wt%) due to the 3D steric hindrance of adamantane. Corresponding OLED achieved maximum external quantum efficiency of 31.9% with negligible efficiency roll‐off (29.0% at 1000 cd/m 2 , 26.1% at 10 000 cd/m 2 ), an ultrahigh maximum brightness of 3.6 × 10 5 cd/m 2 , and high color‐purity with a chromaticity y‐coordinate of 0.70. Notably, an exceptionally long operational lifetime of 769 h to 90% of its initial 1000 cd/m 2 is obtained, over sixfold longer than the reference emitter with a commonly adopted spiro‐fluorene. Theoretical investigations attribute this to the effective confinement of frontier molecular orbitals within the core MR skeleton, facilitated by the saturated sp 3 carbon atoms of the adamantane participating minimally in orbital delocalization.