Alignment of Heptagonal Diimide and Triazine Enables Narrowband Pure‐Blue Organic Light‐Emitting Diodes with Low Efficiency Roll‐Off

Abstract The endeavor to develop high‐performance narrowband blue organic light‐emitting diodes (OLEDs) with low efficiency roll‐off represents an attractive challenge. Herein, we introduce a hetero‐acceptor design strategy centered around the heptagonal diimide (BPI) building block to create an efficient thermally activated delayed fluorescence (TADF) sensitizer. The alignment of a twisted BPI unit and a planar diphenyltriazine (TRZ) fragment imparts remarkable exciton dynamic properties to 26 t Cz‐TRZBPI, including a fast radiative decay rate ( k R ) of 1.0×10 7 s −1 and a swift reverse intersystem crossing rate ( k RISC ) of 1.8×10 6 s −1 , complemented by a slow non‐radiative decay rate ( k NR ) of 6.0×10 3 s −1 . Consequently, 26 t Cz‐TRZBPI facilitates the fabrication of high‐performance narrowband pure‐blue TADF‐sensitized fluorescence OLEDs (TSF‐OLEDs) with a maximum external quantum efficiency (EQE max ) of 24.3 % and low efficiency roll‐off even at a high brightness level of 10000 cd m −2 (EQE 10000 : 16.8 %). This showcases a record‐breaking external quantum efficiency at a high luminance level of 10000 cd m −2 for narrowband blue TSF‐OLEDs.