Sym‐ and Asym‐Expanded Heterohelicene Isomers Featuring Extended Multi‐Resonance Skeleton for Narrowband Deep‐Blue Fluorescence

Abstract Expanded heterohelicenes composed of alternating linearly and angularly fused multi‐resonance (MR) skeletons have gained wide interest owing to their promising narrowband emissions. Herein, a pair of sym ‐ and asym ‐expanded heterohelicene isomers was obtained by merging boron/oxygen (B/O)‐embedded MR triangulene and indolo3,2,1‐ jk carbazole units via one‐pot synthesis. Owing to their fully resonating extended helical skeleton, the target heterohelicenes exhibit a significantly narrowed spectra bandwidth while emission red‐shifting, thus affording deep‐blue narrowband emission with a peak at approximately 460 nm, full‐width‐at‐half‐maximum (FWHM) of only 18 nm, and near‐unity photoluminescence quantum yields. In comparison to the symmetrical structure, the asym ‐expanded heterohelicene displays suppressed aggregation within doped films, thereby showing superior narrowband electroluminescence in devices with CIE coordinates of (0.12, 0.18) and a high external quantum efficiency of up to 25 %, which retains 18.1 % even at a high luminance of 10,000 cd m −1 . Overall, this work provides a novel paradigm for the development of narrowband expanded heterohelicenes.