One-Shot Synthesis of Expanded Heterohelicene Exhibiting Narrowband Thermally Activated Delayed Fluorescence.

An expanded heterohelicene consisting of three BN-embedded [4]helicene subunits () has been synthesized by one-shot triple borylation. The key to success is the excessive use of boron tribromide in an autoclave. Based on the multiple resonance effect of three boron and six nitrogen atoms, exhibited a narrowband sky-blue thermally activated delayed fluorescence with a full width at half-maximum of 16 nm. The resonating π-extension minimized the singlet-triplet energy gap and enabled rapid reverse intersystem crossing with a rate constant of 4.4 × 10 s. The solution-processed organic light-emitting diode device, employed as an emitter, exhibited a narrowband emission at 480 nm with a high external quantum efficiency of 22.9%.