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

Abstract

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.

Keywords: asymmetrical heterohelicene; expanded helicene; multiple resonance; narrowband emission; spectra broadening.