Confined Assembly Chemistry within Single-Walled Carbon Nanotubes

ConspectusReducing bulk crystals to single-unit chains generally yields unique structures and properties, such as the simplest periodicity, high surface-to-volume ratios, and quantum confinement effects, but their synthesis and stabilization under working conditions remain challenging. Single-walled carbon nanotubes (SWCNTs), with well-defined 1-2 nm channels, provide an ideal platform for confined synthesis and studying atomic-scale molecular transport. Additionally, the conductive single-atomic-layer graphene enables the transmission of properties of substances across monolayer graphene, thus enabling surface property modulation and application. Solution-phase confined assembly in SWCNTs presents a versatile alternative to high-temperature vapor transport that is applicable only when the guests are thermally stable and sublimated, although it is rarely achieved. Unveiling molecular transport at the nanoscale is crucial for an in-depth understanding of the driving forces for assembly and, in turn, controlling confined crystallization of metastable crystals with tailored properties. This Account summarizes our recent progress in liquid-phase confined assembly: (i) assembly methodology and mechanistic insights; (ii) controlled synthesis of one-dimensional (1D) metastable crystals and chains; (iii) emerging properties and applications of single-unit-cell chains. In the end, we summarize the present achievements and future challenges.We employ SWCNTs and atomically precise clusters as models to study liquid-phase confined assembly and its mechanism from a real-space perspective. We developed a scalable solution-phase strategy for assembling diverse clusters within SWCNTs, irrespective of their electronic properties or solubility, which was driven by nanoconfinement and electrostatic interactions. Advanced electron microscopy combined with synchrotron X-ray absorption spectroscopy revealed close-packed ordered single-cluster chains, rather than disordered aggregates, within SWCNTs. Inspired by the confined assembly phenomena, we demonstrated efficient capture of heavy elements, including uranium and iodine, within SWCNTs in liquids, with iodine forming ordered single-atom chains. Confined assembly also facilitated the synthesis of 1D crystals with a finite unit-cell thickness and metastable structures. We developed a SWCNT-enabled two-solvent-phase extraction strategy to synthesize single-unit-cell perovskite chains. The resulting chains exhibited unconventional stoichiometries (e.g., [Cs₄PbI₅]⁺) due to dimensionality reduction, stabilized by charge balance with SWCNT^-, thus forming strong "cation-π" interactions. These single-unit-cell perovskites within the SWCNT exhibited exceptional performance in X-ray detectors due to suppressed ion migration. SWCNT-confined ordered single-unit-chain catalysts, including single-cluster and single-atom chains, with well-defined structures showed enhanced activity in redox and coupling reactions compared to their isolated counterparts. These identical single-unit chains maintained intimate contact with the conductive monolayer graphene of SWCNTs, enabling charge delocalization on the nanotube surfaces and therefore increasing the density of active sites and reducing activation barriers. These nanotube-confined single-unit chains demonstrated outstanding structural and operational stability. Our atomic-scale insights into confined assembly advance the design of 1D heterostructures with tailored functionalities and deepen the understanding of the structure-activity relationship.