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

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 26tCz-TRZBPI, including a fast radiative decay rate (kR) of 1.0x107 s-1 and a swift reverse intersystem crossing rate (kRISC) of 1.8x106 s-1, complemented by a slow non-radiative decay rate (kNR) of 6.0x103 s-1. Consequently, 26tCz-TRZBPI facilitates the fabrication of high-performance narrowband pure-blue TADF-sensitized fluorescence OLEDs (TSF-OLEDs) with a maximum external quantum efficiency (EQEmax) of 24.3 % and low efficiency roll-off even at a high brightness level of 10000 cd m-2 (EQE10000: 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. We have created an efficient sky-blue thermally activated delayed fluorescence (TADF) sensitizer by aligning heptagonal diimide and triazine acceptors. This innovative hetero-acceptor design strategy has enabled us to fabricate narrowband pure-blue TADF sensitized fluorescence organic light-emitting diodes (TSF-OLEDs) with a record-breaking external quantum efficiency at a high luminance level of 10000 cd m-2.image