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 (kRISC), which is itself influenced by both the magnitude of the singlet-triplet energy gap (ΔEST) and spin-orbit coupling (SOC) matrix elements. Most MR-TADF emitters possess relatively large ΔEST and small SOC. With the goal of designing an MR-TADF compound having a small ΔEST 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 λPL of 438 nm in toluene with a narrow full width at half maximum (FWHM) of 33 nm. DDOBDiKTa has a ΦPL of 70% and a ΔEST of 0.05 eV in 5 wt% doped films in 26DCzPPy. Remarkably, without any heavy atoms, this emitter shows fast prompt, τp, and delayed, τd, fluorescence lifetimes of 2.3 ns and 1.48 µs, respectively, leading to a kRISC of 1.98 ×106 s-1 in 26DCzPPy. This is one of the fastest kRISC values among reported MR-TADF emitters that have no heavy atoms. The OLED showed EQEmax/EQE10000 of 23.0%/10%, demonstrating unusually mild efficiency roll-off for a blue device.