[5]Helicene‐Embedded Cycloparaphenylene Nanohoops with Möbius Topology: Synthesis, Photophysical Properties, and Aromaticity

Chiral [5]helicene-imbedded [7,8]CPPs are constructed. The Möbius nanohoops exhibit a significantly higher fluorescence quantum yields than pristine [5]helicene and [7,8]CPPs and, meanwhile, display moderately obvious Cotton effect and bright circularly polarized luminescence. While the hoops in the neutral state exhibit a local Hückel aromaticity, their dications show Hückel in-plane global aromaticity according to the theoretical investigations.

Abstract

The relationship between Möbius topology and aromaticity still remains elusive, which is largely 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, clearly exhibiting Möbius topology. Photophysical investigations demonstrated that [5]H-[7,8]CPPs exhibited moderately high fluorescence quantum yields, 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 efficient transfer of chirality from the [5]helicene 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, [5]H-[7,8]CPPs 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.

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