It Matters Where the Heavy Atom Is Placed: Optimizing the Spin–Orbital Coupling in Multiresonant TADF (MR‐TADF) Emitters and Its Impact on OLED Performance

Abstract This study explores the impact of the regioisomerism of a heavy chalcogen atom on the photophysical properties of multi‐resonant thermally activated delayed fluorescence (MR‐TADF) materials. Two pairs of isomeric MR‐TADF emitters containing different benzothienocarbazole moieties, tDPABT1B/tDPABT2B and tCzBT1B/tCzBT2B, are synthesized. Theoretical calculations indicate that tDPABT2B and tCzBT2B possess higher spin–orbital coupling values (0.27 and 0.60 cm⁻¹) compared to their respective isomers. The photophysical study reveals that tDPABT2B and tCzBT2B have twofold faster reverse intersystem crossing rate constants of 0.5 × 10⁵ and 2.7 × 10⁵ s⁻¹, respectively, than their isomeric counterparts. The sensitizer‐free organic light‐emitting diodes (OLEDs) with tCzBT1B and tCzBT2B exhibit green emissions CIE coordinates of (0.12, 0.54) and show high maximum external quantum efficiencies (EQE max ) of 34.9 and 34.3%, respectively. Notably, the device with tCzBT2B demonstrates a reduced efficiency roll‐off (34% decrease at 1000 cd cm⁻ 2 ) compared to that with tCzBT1B (48% decrease at 1000 cd cm⁻ 2 ), highlighting the distinct benefits and importance of the regiochemistry of the heavy atom in contributing to an enhancing device performance.