A Double Helicene with Quadruple Indole-Fused Skeleton and Double NBN-Type Cove-Edged Structure†

Construction of helicenes by inducing heteroatoms has regarded as an effective strategy to enhance the chiroptical properties. We report a facile synthesis of a multiple helicene based on four indoles with two electron-rich NBN-coved edges. The structure of the NBN-coved multiple helicene was confirmed by single-crystal X-ray diffraction analysis, revealing a twisted double [5]helicene motif with a saddle-shaped skeleton. This NBN-coved helicene showed strong green fluorescence with good photoluminescence quantum yield. Both theoretical calculations and experimental investigation have been exploited to probe the impact of electron-rich NBN units on aromaticity and photophysical properties of the helicene.