A Blue Emissive Heavy Atom‐Free Multiresonant Thermally Activated Delayed Fluorescent Emitter Shows Ultra‐Fast Reverse Intersystem Crossing

One of the key factors affecting the stability and efficiency roll‐off of organic light‐emitting diodes is the rate constant of reverse intersystem crossing ( k RISC ), which is itself influenced by both the magnitude of the singlet‐triplet energy gap (Δ E ST ) and spin‐orbit coupling (SOC) matrix elements. Most MR‐TADF emitters possess relatively large Δ E ST and small SOC. With the goal of designing an MR‐TADF compound having a small Δ E ST and strong SOC that emits in the deep blue region, here we report a heavy atom‐free boron‐nitrogen‐carbonyl hybrid MR‐TADF emitter, DDOBDiKTa , which emits at λ PL of 438 nm in toluene with a narrow full width at half maximum (FWHM) of 33 nm. DDOBDiKTa has a Φ PL of 70% and a Δ E ST of 0.05 eV in 5 wt% doped films in 26DCzPPy. Remarkably, without any heavy atoms, this emitter shows fast prompt, τ p , and delayed, τ d , fluorescence lifetimes of 2.3 ns and 1.48 µs, respectively, leading to a k RISC of 1.98 ×10 6 s −1 in 26DCzPPy. This is one of the fastest k RISC values among reported MR‐TADF emitters that have no heavy atoms. The OLED showed EQE max /EQE 10000 of 23.0%/10%, demonstrating unusually mild efficiency roll‐off for a blue device.