B─O-Bond-Mediated π-Extension Enables Concurrent High Efficiency and Spectral Purity Toward BT.2020-Standard...

Deep-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are crucial for next-generation ultra-high-definition OLED displays, yet the concurrent realization of high color purity, high quantum efficiency, and a fast reverse intersystem crossing (RISC) rate remains elusive. Here, we present a B─O-bond-mediated π-extension design that harmonizes these conflicting performance metrics. In this approach, π-extension effectively narrows the emission bandwidth and reduces the singlet-triplet energy gap (ΔE_ST), while the incorporation of B─O bond mitigates the bathochromic shift typically induced by π-extension, thereby preserving high emission energy. The resulting doubly and triply borylated emitters, accessible via a lithium-free one-shot borylation on a gram scale, exhibit deep-blue emissions with nearly BT.2020 chromaticity, close-to-unity quantum yields, and an order-of-magnitude enhancements in their RISC rate constants relative to the parent emitter. Correspondingly, the non-sensitized OLEDs deliver maximum external quantum efficiencies (EQE_max) of up to 34.7%, blue index values of up to 394, and mitigated efficiency roll-off. Meanwhile, in a more stable device configuration, LT₉₀ lifetimes (time to decay to 90% of the initial luminance) of up to 114.6 h at 500 cd m^-2 are achieved. This work establishes a molecular design paradigm for constructing efficient, spectrally pure, and durable deep-blue MR-TADF emitters, advancing the pathway toward next-generation display technologies.