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

Abstract

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.