Spiral Intramolecular Charge Transfer and Large First Hyperpolarizability in Möbius Cyclacenes: New Insight into the Localized π Electrons

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Abstract
Much effort has been devoted to investigating the unusual properties of the π electrons in Möbius cyclacenes, which are localized in a special region. However, the localized π electrons are a disadvantage for applications in optoelectronics, because intramolecular charge transfer is limited. This raises the question of how the intramolecular charge transfer of a Möbius cyclacene with clearly localized π electrons can be enhanced. To this end, [8]Möbius cyclacene ([8]MC) is used as a conjugated bridge in a donor–π‐conjugated bridge–acceptor (D–π–A) system, and NH2‐6‐[8]MC‐10‐NO2 exhibits a fascinating spiral charge‐transfer transition character that results in a significant difference in dipole moments Δμ between the ground state and the crucial excited state. The Δμ value of 6.832 D for NH2‐6‐[8]MC‐10‐NO2 is clearly larger than that of 0.209 D for [8]MC. Correspondingly, the first hyperpolarizability of NH2‐6‐[8]MC‐10‐NO2 of 12 467 a.u. is dramatically larger than that of 261 a.u. for [8]MC. Thus, constructing a D–π–A framework is an effective strategy to induce greater spiral intramolecular charge transfer in MC although the π electrons are localized in a special region. This new insight into the properties of π electrons in Möbius cyclacenes may provide valuable information for their applications in optoelectronics.

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