Narrow‐Emitting Multiple‐Resonance Fluorescent Emitters with Deactivated Thermally Activated Delayed Fluorescence Mechanism Using Benzo[b]thiophene as a Triplet Energy Managing Unit

Abstract Multiple‐resonance (MR) fluorescent emitters incorporating a benzo[b]thiophene unit are developed to suppress the thermally activated delayed fluorescence (TADF) mechanism by effectively decreasing the triplet energy while preserving the intrinsic MR character. The molecular design strategy resulted in pure fluorescence emitters with high photoluminescence quantum yields of 99%, and external quantum efficiency of up to 11.3% and narrow full‐width at half‐maximum values down to 22 nm in the fluorescent organic light‐emitting diodes. When employed as terminal emitters in TADF‐sensitized fluorescence devices, they achieved maximum external quantum efficiency of up to 21.3%, full width at half maximum of 25 nm, and color coordinate of (0.136, 0.117). These results highlight the potential of benzo[b]thiophene‐based MR emitters as promising candidates for high‐color‐purity blue devices, offering a viable design guideline for next‐generation blue fluorescent materials.