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.