Sulfide, Sulfoxide, and Sulfone Substitution: Electron Structure Modulation from Para‐D‐π‐B to Para‐A‐π‐B in Multiple‐Resonance Emitters with Narrowband Emission and High Color Purity

Abstract Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have been studied to address the issue of the broadband emission in organic light‐emitting diodes (OLEDs). Herein, the authors have systematically investigated the effect of electron‐donating or ‐withdrawing units in the para position of B atom on the optoelectronic emission modulation BCz‐BN MR‐TADF emitters. Due to the enhanced spin‐orbit coupling (SOC) effect, BN(p)SCH₃ with electron structure of para‐D‐π‐B is synthesized by introducing a heavy S atom into the para position of B atom of BCz‐BN. By oxidizing BN(p)SCH₃, BN(p)SOCH₃ and BN(p)SO₂CH₃ with electron structure of para‐A‐π‐B have been synthesized. The quantum simulations and photophysical studies have illustrated BN(p)SCH₃ can exhibit large reverse intersystem crossing rate constant (k_RISC) of 6.4 × 10⁴ s⁻¹ due to the large SOC constants and small singlet‐triplet energy splitting (ΔE_ST) of 0.12 eV. BN(p)SOCH₃ and BN(p)SO₂CH₃ with electron structure of para‐A‐π‐B displayed red‐shift emissions with smaller full‐width at half‐maximum (FWHM) values of ≈21 nm and k_RISC values owing to enhanced ΔE_ST and the low emission contribution of the triplet excitons in contrast to those of BN(p)SCH₃ with electron structure of para‐D‐π‐B. Consequently, BN(p)SCH₃‐based OLEDs show highly efficient blue emission with an external quantum efficiency (EQE) of 26.2% and excellent color purity.