One‐Shot Synthesis of Sym‐ and Asym‐Expanded Heterohelicene Isomers Exhibiting Narrowband Deep‐Blue Fluorescence

Expanded heterohelicene composing of alternating linearly and angularly fused multi‐resonance (MR) skeleton has garnered wide interest for their promising narrowband emission. Herein, a pair of sym‐ and asym‐expanded heterohelicene isomers are firstly developed by merging boron/oxygen (B/O)‐embedded MR triangulene and indolo[3,2,1‐jk]carbazole units via one‐shot synthesis. Owing to the fully resonating extended helical skeleton, the target heterohelicenes exhibit significantly narrowed spectra bandwidth while emission red‐shifting, thus affording deep‐blue narrowband emission with peak at around 460 nm, full‐width‐at‐half‐maximum (FWHM) of merely 18 nm and near‐unity photoluminescence quantum yields. In comparison with symmetrical one, the asym‐expanded heterohelicene displays suppressed aggregation in the doped films, thereby showcasing a superior narrowband electroluminescence in devices with CIE coordinates of (0.12, 0.18) and high external quantum efficiency up to 25%, which retains 18.1% even at high luminance of 10,000 cd m‐1. This is the first work achieving FWHM < 20 nm in B/O‐embedded [4]helicene MR derivatives, and provides a novel paradigm for the development of narrowband expanded heterohelicence.