Neutral Möbius [5]helicene-embedded Cycloparaphenylene Nanohoops: Synthesis, [4n]Möbius Topology and Hückel Aromaticity

The relationship between Möbius topology and aromaticity and topological chirality is still elusive to date, which is, to a large extent, due to the related synthetic challenges and, further, the scarcity in both the quantity and the diversity of the con-structed Möbius systems. In this work, we reported the synthesis of [4n]Möbius conjugated all-carbon nanohoops ([5]H-[7,8]CPPs) by utilizing a [5]helicene unit as a hidden writhe and a masked aromatic unit to overcome the strain inherited from Möbius topology. X-ray analysis revealed that [5]H-[7,8]CPPs contain a [5]helicene moiety and an oligoparaphenylene unit, and display a Möbius topology. Photophysical investigations demonstrated that [5]H-[7,8]CPPs exhibited moderately high fluorescence quantum yields, which are significantly higher than those of pristine [5]helicene and [7,8]CPPs. Chiropti-cal studies revealed that [5]H-[7,8]CPPs displayed an obvious Cotton effect in circular dichroism and bright circularly polar-ized luminescence, indicating that the chirality of [5]helicene was efficiently transferred to the overall carbon nanohoops. Importantly, theoretical investigations reveal that, though possessing a 4n π-electron array, such all-carbon nanohoops are fully conjugated systems with Hückel aromaticity. The results may help us to better understand the relationship between Möbius topology and aromaticity.