Benzo[b]Thiophene‐Derived Low‐T ₁ Multi‐Resonance Emitter with Suppressed Thermally Activated Delayed Fluorescence for Extending Organic Light‐Emitting Diode Lifetime by Managing Triplet‐Related Bimolecular Interactions

We report a narrowband blue multi‐resonance (MR) fluorescent emitter, BN‐TPh‐TPA, designed to suppress thermally activated delayed fluorescence (TADF) to extend organic light‐emitting diode (OLED) operational lifetime. By introducing a 5‐phenylbenzo[b]thiophene unit into the DABNA core, BN‐TPh‐TPA achieves an enlarged singlet‐triplet energy gap (∆ E ST ) that renders reverse intersystem crossing (rISC) energetically unfavorable while retaining deep‐blue, narrow emission. In sensitizer‐free OLEDs, using an exciplex host, BN‐TPh‐TPA delivers a substantially extended operational lifetime at 10 mA/cm 2 (LT 50 = 90 h), representing an approximately threefold improvement over the TADF‐active MR emitter, t‐DABNA (LT 50 = 27 h). In phosphor‐sensitized fluorescence (PSF) OLEDs operated at the same current density, BN‐TPh‐TPA likewise exhibits an improved LT 50 of 108 h compared with 80 h for t‐DABNA. Transient photoluminescence and electroluminescence (TrEL/TrPL) measurements confirm the absence of an rISC‐mediated delayed fluorescence component and a markedly shorter decay time for BN‐TPh‐TPA. Magneto‐electroluminescence (MEL) analysis further indicates that TADF activity influences the dominant triplet‐related bimolecular interaction pathway under electrical driving, providing mechanistic insight into the enhanced device stability.