Neutral and Dicationic [5]helicene-embedded Cycloparaphenylene Nanohoops with Möbius Topology and Local/In-Plane Aromaticity

The relationship between Möbius topology and aromaticity 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 constructed Möbius systems. In this work, we report 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 analyses reveal 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. Chiroptical studies revealed that [5]H-[7,8]CPPs displayed an obvious Cotton effect in circular dichroism and bright circularly polarized luminescence, indicating that the chirality of [5]helicene was efficiently transferred to the overall carbon nanohoops. Importantly, theoretical investigations reveal that, though possessing a Möbius topology and a 4n π-electron array in the neutral state, such all-carbon nanohoops fundamentally exhibit local Hückel aromaticity, while their dications, with a 4n+2 π-electron in the conjugation circuits, show Hückel in-plane global aromaticity, deviating from the Heilbronner prediction. The results may help us to better understand the complicated relationship between Möbius topology and aromaticity.