Synthesis and Chiroptical Activity of π-Expanded Electron-Rich Heterohelicenes Based on the 1,4-Dihydropyrrolo[3,2-b]pyrrole Core

Herein, we report the synthesis and chiroptical characteristics of the first (double) helicenes possessing the 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) moiety as their central core. We have developed a three-step synthesis with 6π-electrocyclization accompanied by HBr elimination as its key step. We found that, whereas for smaller peripheral arms double 6π-electrocyclization occurs smoothly forming a double helicene, in the case of longer polycyclic aromatic hydrocarbons the reaction becomes less efficient and mono-helicenes are the only products. The electron density distribution analysis of LUMO explains the different regioselectivity of 6π-electrocyclization. The synthesized heterohelicenes are characterized by greenish-blue emission, distinct solvatofluorochromism and good fluorescence quantum yields (up to 42 %). Moreover, the chiroptical measurements reveal that unsymmetrical double heterohelicene exhibits excellent circularly polarized luminescence brightness (BCPL) reaching 30 M-1 cm-1. The combined experimental and computational study has revealed that a charge-transfer state is responsible for the observed emission (hence the solvatochromism), while low-energy absorption derives from multiple transitions.

European Union's Horizon2020 research and innovation programme under the Marie Skłodowska-Curie grant agreements No 101007804 (Micro4Nano) and No 860762; Polish National Science Centre, Poland (OPUS 2020/37/B/ST4/00017); COMP-HUB and COMP-R initiatives, funded by the “Departments of Excellence” program of the Italian Ministry for University and Research (MIUR, 2018-2022 and MUR, 2023-2027).