Chlorine‐Enabled Double Borylation Strategy for π‐Extended MR‐TADF Emitters: From Synthetic Challenge to Benchmark OLEDs

Thermally activated delayed fluorescence (TADF) materials are promising emitters for organic light-emitting diodes (OLEDs) owing to their ability to harvest both singlet and triplet excitons. However, realizing pure-green, narrowband emission with simultaneously high efficiency and stability remains a long-standing challenge for display applications. In this study, we report a concise one-shot double borylation strategy uniquely enabled by chlorine substituents acting as both directing and functional groups. This dual role not only ensures regioselective synthesis of a π-extended multiple resonance (MR) framework-previously considered synthetically inaccessible-but also provides versatile handles for late-stage diversification. The resulting key intermediate, W-DABNA-Cl, was obtained in 48% yield and readily converted into a series of structurally diverse MR-TADF emitters through single-step derivatizations. These emitters exhibited narrowband emissions from deep green to yellow-green (FWHM 27-34 nm) with tunable photophysical properties. Notably, TADF-sensitized fluorescence devices based on W-DABNA-Cz achieved an excellent maximum external quantum efficiency (EQE) of 36.1%. More importantly, the device maintained record-high efficiencies of 34.3% and 29.5% at luminance levels of 1000 and 10 000 cd m^-2, respectively, representing the best efficiency-brightness trade-off among reported green sensitized OLEDs. This work not only overcomes synthetic bottlenecks in accessing π-extended MR-TADF frameworks but also establishes a versatile platform for color-tunable, high-performance OLEDs.