A Blue Emissive Heavy Atom-Free Multiresonant Thermally Acti-vated Delayed Fluorescent Emitter Shows Ultra-fast Reverse In-tersystem Crossing

One of the key factors affecting the stability and efficiency roll-off of organic light-emitting di-odes is the rate constant of reverse intersystem crossing (kRISC), which is itself influenced by the magnitude of the singlet-triplet energy gap (ΔEST) and spin-orbit coupling (SOC). Most MR-TADF emitters possess relatively large ΔEST and small SOC. Here we report a boron-carbonyl hybrid MR-TADF emitter DDOBDiKTa, emitting at 438 nm in toluene with a narrow full width at half maximum (FWHM) of 33 nm. DDOBDiKTa has a high ΦPL of 70% and a small ΔEST of 0.05 eV in 5 wt% doped films in 26DCzPPy. Remarkably, without any heavy atoms, this emitter shows fast decay kinetics, with prompt, p, and delayed, d, fluorescence lifetimes of 2.3 ns and 1.48 μs, respectively. A remarkably fast kRISC of 1.98 ×106 s-1 was determined for DDOBDiKTa in 26DCzPPy, which is one of the fastest kRISC values among reported MR-TADF emitters that do not contain any heavy atoms. The OLED using 26DCzPPy as the host showed an EQEmax of 23.0% and an EQE of 10.0% at 10,000 cd m-2, demonstrating unusually mild efficiency roll-off for a blue device.