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

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. We synthesized two pairs of isomeric MR-TADF emitters containing different benzothienocarbazole moieties, tDPABT1B/tDPABT2B and tCzBT1B/tCzBT2B. 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 exhibited green emissions [CIE coordinates of (0.12, 0.54)] and showed high maximum external quantum efficiencies (EQEmax) of 34.9% and 34.3%, respectively. Notably, the device with tCzBT2B demonstrated 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.