Combining Carbazole Building Blocks and ν‐DABNA Heteroatom Alignment for a Double Boron‐Embedded MR‐TADF Emitter with Improved Performance

Abstract

Building blocks and heteroatom alignments are two determining factors in designing multiple resonance (MR)-type thermally activated delayed fluorescence (TADF) emitters. Carbazole-fused MR emitters, represented by CzBN derivatives, and the heteroatom alignments of ν-DABNA are two star series of MR-TADF emitters that show impressive performances from the aspects of building blocks and heteroatom alignments, respectively. Herein, a novel CzBN analog, Π-CzBN, featuring ν-DABNA heteroatom alignment is developed via facile one-shot lithium-free borylation. Π-CzBN exhibits superior photophysical properties with a photoluminescence quantum yield close to 100 % and narrowband sky blue emission with a full width at half maximum (FWHM) of 16 nm/85 meV. It also gives efficient TADF properties with a small singlet-triplet energy offset of 40 meV and a fast reverse intersystem crossing rate of 2.9 × 10 5 s À 1 . The optimized OLED using Π-CzBN as the emitter achieves an exceptional external quantum efficiency of 39.3 % with a low efficiency roll-off of 20 % at 1000 cd m À 2 and a narrowband emission at 495 nm with FWHM of 21 nm/ 106 meV, making it one of the best reported devices based on MR emitters with comprehensive performance.

Elevating the upconversion performance of a multiple resonance thermally activated delayed fluorescence emitter via an embedded azepine approach

“…Based on prior MR molecular designs, some successful strategies have emerged to enhance RISC rates ( k RISC ) and mitigate the efficiency roll-off in OLEDs. For example, extended MR frameworks 18–23 can effectively lower the value of Δ E ST . With incorporating heavy atoms, 24–27 MR emitters enhance the strength of spin–orbit coupling (SOC), accelerating k RISC .…”