Toward Narrow-Band, Stable Blue MR-TADF Emission via Combined Heterocyclic Analogues Marching B.T.2020

The development of efficient and stable ultra-narrowband pure-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters is critical for next-generation wide-gamut OLED displays. Herein, we present a molecular design strategy that enhances emitter stability and efficiency by reinforcing the weak C─N bonds through selective incorporation of heterocyclic carbazole (Cz) units into the MR framework. Two proof-of-concept emitters, m-Cz-DABNA and tBu-Cz-DABNA, were synthesized via high-yield, lithium-free borylation. These emitters exhibit pure-blue emissions at 453 and 463 nm with narrow full-width at half-maximums (FWHMs) of 24 and 21 nm in solution and high photoluminescence quantum yields (PLQYs) of 88% and 95%, respectively. OLED devices based on m-Cz-DABNA and tBu-Cz-DABNA show emissions at 456 nm (FWHM 24 nm, EQE_max 23%, CIEy 0.06) and 467 nm (FWHM 24 nm, EQE_max 26%, CIEy 0.10), respectively, holding among the highest efficiencies for blue OLEDs without sensitizers. Notably, the tBu-Cz-DABNA-based device shows an LT₉₀ of ∼81 h at 1000 cd m^{- 2} (EQE 8.95%), representing one of the longest operational lifetimes for blue fluorescent OLEDs with CIEy ≤ 0.10. These results demonstrate a versatile and scalable molecular design strategy for the realization of high-efficiency, long-lifetime blue OLEDs approaching the BT.2020 color standard.