Enhancing Spin–Orbit Coupling in an Indolocarbazole Multiresonance Emitter by a Sulfur‐Containing Peripheral Substituent for a Fast Reverse Intersystem Crossing

Abstract A fast reverse intersystem crossing (RISC) remains an ongoing pursuit for multiresonance (MR) emitters but faces formidable challenges, particularly for indolocarbazole (ICz) derived ones. Here, heavy‐atom effect is introduced first to construct ICz‐MR emitter using a sulfur‐containing substitute, simultaneously enhancing both spin–orbit and spin–vibronic coupling to afford a fast RISC with a rate of 1.2 × 10 5 s −1 , nearly one order of magnitude higher than previous maximum values. The emitter also exhibits an extremely narrow deep‐blue emission peaking at 456 nm with full‐width at half‐maxima of merely 12 nm and a photoluminescence quantum yield of 92%. Benefiting from its efficient triplet upconversion capability, this emitter achieves not only a high maximum external quantum efficiency (EQE) of 31.1% in organic light‐emitting diodes but also greatly alleviates efficiency roll‐off, affording record‐high EQEs of 29.9% at 1000 cd m −2 and 18.7% at 5000 cd m −2 among devices with ICz‐MR emitters.