Red Emissive Double Aza[7]helicenes with Antiaromaticity / Aromaticity Switching via the Redox‐Induced Radical Cation and Dication Species

A pyridazine core structure with antiaromatic character has been utilized to synthesize a double aza[7]helicene with rarely accessible chiroptical properties in the red region as well as redox-active behaviour. This chiral polyaromatic hydrocarbon system shows oxidation-state-dependent aromaticity/antiaromaticity switching via oxidation-induced cation species.

Abstract

We herein present the synthetic approach to a new antiaromatic double aza[7]helicene C that features NN-embedded polycyclic aromatic hydrocarbons (PAHs). This heteroatom-doped helicene showed a rarely obtained long-wavelength emission and far-red circularly polarized luminescence (CPL) in the solid state. These optical and chiroptical properties could be ascribed to both the NN-PAH core structure and the further extension through angular ring fusions. Such a unique electronic structure also culminated in facile chemical oxidations of neutral C to the positively charged chiral radical (C⋅⁺) and dication species (C ²⁺). Interestingly, DFT computations revealed that the pyridazine central core showed an antiaromaticity-to-aromaticity switching, in contrast to the inversed transition for the helical periphery in cationic states. The reported approaches are anticipated to lead to the development of further redox-active chiral systems for potential applications in chiroptoelectronics, spintronics as well as fluorescent bioimaging.

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