Key Takeaways & Executive Findings
- •• • M1 macrocycle self-assembles into quadruple-stranded right-handed (P) helices via charge-transfer and CH···π interactions, with single-crystal X-ray diffraction confirming the helical sense and packing mode. • • Binding of electron-deficient NDI guests triggers a transformation to left-handed (M) double helices, demonstrating a rare crystalline-state multiple-helix conversion with helicity inversion. • • The macrocycle's adaptive cavity and interstitial voids enable host-guest charge-transfer interactions, which alter molecular geometry and packing modes, leading to opposite chiroptical signals as measured by circular dichroism. • • This template-free methodology provides a new strategy for constructing multi-stranded π-helices with controlled handedness, potentially enabling applications in chiral optics and supramolecular materials.
Abstract
Controlled fabrication of artificial multiple-stranded helices is central to deciphering chirality complexity and hierarchical self-assembly processes. Inspired by biological helical nanostructures, we designed a twisted figure-of-eight chiral macrocycle (M1) from pyrene and benzene diimide subcomponents to direct hierarchical assembly of double- and quadruple-stranded superhelices. Single-crystal X-ray diffraction reveals that M1 undergoes charge-transfer and CH···π interactions-driven helical wrapping, forming right-handed (P) single strands that intertwine into quadruple π-helical superstructures. Crucially, the macrocycle's adaptive cavity and interstitial voids could bind electron-deficient naphthalene diimide (NDI) guests through charge transfer interactions, triggering transformation to left-handed (M) double helices. This structural shift induces helicity inversion and optical anisotropy changes, demonstrating a rare case of crystalline-state multiple-helix conversion with supramolecular chirality inversion. This work establishes a template-free methodology for synthesizing multiple-stranded π-helices and controlling their transformations through supramolecular engineering.
1. Introduction
Helical structures are ubiquitous in nature, from protein α-helices to DNA double-helices, and their hierarchical assembly has inspired extensive research into artificial helical systems. While coordination-driven helicates have achieved higher-order strands, precise assembly of small organic molecules into multi-stranded helices with controlled handedness without metal templates remains a significant challenge. Existing systems often lack chirality control, limiting their utility in advanced materials.
Here, we address this bottleneck by designing a twisted figure-of-eight chiral macrocycle (M1) that pre-organizes subcomponents into a cyclic architecture with a preferred helical sense. This macrocycle self-assembles into quadruple and double helices with opposite handedness in the crystalline state, driven by charge-transfer and CH···π interactions. The ability to switch between helical structures via guest binding offers a new pathway for controlling supramolecular chirality, potentially enabling applications in chiroptical devices and responsive materials.
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DU Cong, ZHU Xuefeng, WANG Hanxiao, LIANG Tongling, FAN Huahua, OUYANG Guanghui, LIU Minghua (2026). Helical wrapping and charge-transfer driven multi-stranded crystalline helices from a twisted figure-of-eight macrocycle. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3830-6
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Frequently Asked Questions
What is the role of charge-transfer interactions in the formation of the quadruple helix, and how does the binding of NDI guests trigger the transformation to double helices?
Charge-transfer interactions between the pyrene and benzene diimide subcomponents of M1 drive the initial helical wrapping, leading to the formation of right-handed quadruple helices. When electron-deficient NDI guests are introduced, they bind within the macrocycle's adaptive cavity and interstitial voids via charge-transfer interactions. This binding alters the molecular geometry of M1, changing its packing mode and resulting in a transformation to left-handed double helices, as confirmed by single-crystal X-ray diffraction and circular dichroism measurements.
How does the helicity inversion observed in this system compare to other reported multi-stranded helical systems, and what are the potential applications?
This system demonstrates a rare crystalline-state conversion from quadruple to double helices with opposite helicity, which is not commonly observed in other multi-stranded helical systems. The ability to switch helicity and optical properties through guest binding could be exploited for chiroptical switches, sensors, or responsive materials. The template-free methodology also offers a new approach for designing chiral supramolecular architectures with controlled handedness.
What is the significance of using a figure-of-eight macrocycle compared to linear or other cyclic building blocks for constructing multi-stranded helices?
The figure-of-eight conformation provides a pre-organized, twisted geometry that promotes helical wrapping and facilitates the formation of multi-stranded superstructures. This design allows for precise control over the helical sense and strand number without the need for metal templates. The macrocycle's adaptive cavity also enables host-guest chemistry, providing a handle for post-assembly modification and transformation.
What are the limitations of this approach in terms of scalability and practical applications?
The current study focuses on crystalline-state self-assembly, which may limit scalability for bulk applications. However, the principles demonstrated could be extended to solution-phase assemblies or thin films. Further research is needed to optimize conditions for large-scale synthesis and to explore the stability and processability of these helical structures in practical devices.
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