Construction of Hetero[n]rotaxanes by Use of Polyfunctional Rotaxane Frameworks

Feeding a Molecular Squid: A Pliable Nanocarbon Receptor for Electron-Poor Aromatics

A hybrid nanocarbon receptor consisting of a calix[4]arene and a bent oligophenylene loop ("molecular squid"), was obtained in an efficient, scalable synthesis. The system contains an electron-rich cavity with an adaptable shape, which can serve as a host for electron deficient guests, such as diquat, 10-methylacridinium, and anthraquinone. The new receptor forms inclusion complexes in the solid state and in solution, showing a dependence of the observed binding strength on the shape of the guest species and its charge. The interaction with the methylacridinium cation in solution was interpreted in terms of a 2:1 binding model, with K₁₁ = 5.92(7) × 10³ M^-1. The solid receptor is porous to gases and vapors, yielding an uptake of ca. 4 mmol/g for methanol at 293 K. In solution, the receptor shows cyan fluorescence (λ_max^em = 485 nm, Φ_F = 33%), which is partly quenched upon binding of guests. Methylacridinium and anthraquinone adducts show red-shifted emission in the solid state, attributable to the charge-transfer character of these inclusion complexes.

Type III-C rotaxane dendrimers: synthesis, dual size modulation and in vivo evaluation

Type III-C rotaxane dendrimers were synthesized by a divergent approach. Dual shuttling behavior and size modulation were observed from non-methylated/methylated rotaxane dendrimers under the same external stimuli. The biological distribution of dendrimers in C57BL/6J mice determined by MALDI-TOF-MS shows predominant accumulation in the spleen and liver. Drug encapsulations with chlorambucil and lithocholic acid were demonstrated.

Heterorotaxanes

Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attention during the past few decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, a survey of the successful synthesis of heterorotaxanes will be provided based on the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, active metal template approach, etc.

Synthesis of rotaxanes and catenanes using an imine clipping reaction

Supramolecular chemistry and self-assembly provide a valuable chance to understand the complicated topological structures on a molecular level. Two types of classical mechanically interlocked molecules, rotaxanes and catenanes, possess non-covalent mechanical bonds and have attracted more attention not only in supramolecular chemistry but also in the fields of materials science, nanotechnology and bioscience. In the past decades, the template-directed clipping reaction based on imine chemistry has become one of the most efficient methods for the construction of functionalized rotaxanes and catenanes. In this review, we outlined the main progress of rotaxanes and catenanes using the template-directed clipping approach of imine chemistry. The review contains the novel topological structures of rotaxanes and catenanes, functions and applications.

One-pot synthesis of hetero[6]rotaxane bearing three different kinds of macrocycle through a self-sorting process

A hetero[6]rotaxane bearing three different kinds of macrocycle is designed and successfully synthesized through a one-pot “click” reaction by employing a facile and efficient integrative self-sorting principle.

In this article, a six-component self-sorting process that involves three types of crown ether macrocycle and three types of cation guest molecule was carefully and thoroughly investigated. The six components include three kinds of crown ether, namely bis(p-phenylene-34-crown-10) (BPP34C10), dibenzo-24-crown-8 (DB24C8) and benzo-21-crown-7 (B21C7), and their corresponding cation guest molecules, namely a 4,4′-bipyridine dication (BPY²⁺) and dibenzylammonium (DBA) and benzylalkylammonium (BAA) ions, respectively. Based on this well-established highly selective six-component self-sorting process, a hetero[6]rotaxane bearing three different kinds of crown ether macrocycle was designed and successfully synthesized through a facile and efficient one-pot “click” stoppering strategy. Such work is proposed to be a significant advance in the construction of mechanically interlocked molecules with high structural complexity, as well as a good supplement in the areas of multi-component self-sorting and noncovalent self-assembly.

Insights into the Difference Between Rotaxane and Pseudorotaxane

Rotaxane and pseudorotaxane are two types of mechanically interlocked molecular architectures, and there is a clear topological difference and boundary between them. In this work, a "suggested [2]rotaxane 1⊂α-CD" was constructed based on axle molecule 1 bearing two terminal ferrocene groups and a wheel component α-cyclodextrin (α-CD), but the result obtained indicated that the ferrocene group cannot prevent α-CD dethreading under UV irradiation. That is, 1⊂α-CD is just a pseudo[2]rotaxane. Furthermore, the two ferrocene groups in 1⊂α-CD were encapsulated by two cucurbit[7]uril (CB[7]) units to obtain a heteropseudo[4]rotaxane 1⊂α-CD⋅2CB[7]. This heteropseudo[4]rotaxane displayed high stability towards harsh temperatures and the isomerization of azobenzene in 1, so it can be regarded as a [2]rotaxane. In this [2]rotaxane, the stoppers are not the bulky groups covalently bonded to the axle, but the cyclic CB[7] units connected through noncovalent interactions.

Our Expedition in Linear Neutral Platinum-Acetylide Complexes: The Preparation of Micro/nanostructure Materials, Complicated Topologies, and Dye-Sensitized Solar Cells

During the past few decades, the construction of various kinds of platinum-acetylide complexes has attracted considerable attention, because of their wide applications in photovoltaic cells, non-linear optics, and bio-imaging materials. Among these platinum-acetylide complexes, the linear neutral platinum-acetylide complexes, due to their attractive properties, such as well-defined linear geometry, synthetic accessibility, and intriguing photoproperties, have emerged as a rising star in this field. In this personal account, we will discuss how we entered the field of linear neutral platinum-acetylide chemistry and what we found in this field. The preparation of various types of linear neutral platinum-acetylide complexes and their applications in the areas of micro/nanostructure materials, complicated topologies, and dye-sensitized solar cells will be summarized in this account.

Synthesis of [3]Rotaxanes by the Combination of Copper-Mediated Coupling Reaction and Metal-Template Approach

[3]Rotaxanes with two axle components and one ring component were synthesized by the combination of a coupling reaction using a transition-metal catalyst and a metal-template approach. Thus, [2]rotaxanes were prepared by the oxidative dimerization of alkyne promoted by macrocyclic phenanthroline-CuI complexes. The [2]rotaxane was reacted with a Cu(I) salt and an acyclic ligand to generate a tetrahedral Cu(I) complex. Metal-free [3]rotaxane was isolated by the end-capping reaction of the acyclic ligand, followed by the removal of Cu(I) ion. The stability of the tetrahedral Cu(I) complexes depended on the size of both the ring component and the acyclic ligand, which was correlated with the yield of the corresponding [3]rotaxane.

Formation of a hetero[3]rotaxane by a dynamic component-swapping strategy

Acid-catalysed scrambling of the mechanically interlocked components between two different homo[3]rotaxanes, constituted of dumbbells containing two secondary dialkylammonium ion recognition sites encircled by two [24]crown-8 rings, each containing a couple of imine bonds, affords a statistical mixture of a hetero[3]rotaxane along with the two homo[3]rotaxanes, indicating that neither selectivity nor cooperativity is operating during the assembly process.

Two Stepwise Synthetic Routes toward a Hetero[4]rotaxane

Heterorotaxanes have been emerging as an important class of mechanically interlocked molecules and have attracted much attention in recent years. Driven by the distinguishable host-guest interactions between crown ether macrocycles and ammonium with different sizes, a novel hetero[4]rotaxane was successfully prepared by employing the combination of copper-catalyzed "click" reaction and P(n-Bu)3-catalyzed esterification reaction as stoppering reactions. The hetero[4]rotaxane contains an interlocked species in which a dibenzo[24]crown-8 ring threaded by a dibenzylammonium-containing component with two benzo[21]crown-7 macrocycles at both ends to act as stoppers, and each of the two benzo[21]crown-7 rings is also threaded with a benzylalkylammonium unit to form the second interlocked species. The hetero[4]rotaxane was prepared through two different stepwise synthetic routes, and the complicated chemical structure of the hetero[4]rotaxane was well-characterized by (1)H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry. The investigation shows that the construction of complicated topological heterorotaxane can be achieved via distinct approaches with high efficiencies, which may provide a foundation for the construction of more sophisticated heterorotaxane systems or functional supermolecules.

A [2]rota[2]catenane, constructed from a pillar[5]arene-crown ether fused double-cavity macrocycle: synthesis and structural characterization

A [2]rota[2]catenane, based on a pillar[5]arene-crown ether fused bicycle, was constructed and its structure was determined crystallographically.

Construction of photoswitchable rotaxanes and catenanes containing dithienylethene fragments

Mechanically interlocked structures such as rotaxanes and catenanes provide a novel backbone for constructing functional materials with unique structural characteristics.

Tetraphenylethene modified [n]rotaxanes: synthesis, characterization and aggregation-induced emission behavior

A series of novel [n]rotaxanes based on a tetraphenylethene (TPE) backbone were constructed by a template-directed clipping approach and their structures were well-characterized.

Aggregation-induced emission behavior of a pH-controlled molecular shuttle based on a tetraphenylethene moiety

A TPE-based molecular shuttle having amide and amine units has been synthesized. The shuttling motion of the macrocycle component can adjust its AIE behaviour.

Two switchable star-shaped [1](n)rotaxanes with different multibranched cores

Two novel star-shaped [1](n)rotaxanes with three and four identical [1]rotaxane arms but different multibranched cores were designed, synthesized, and well-characterized. In the two systems, external base-acid stimuli result in the uniform relative mechanical movement of the macrocyclic rings and threads of their [1]rotaxane arms. The energy-minimized structures of the two rotaxanes in different states were obtained using molecular dynamics simulations in acetone solution, suggesting the construction of more sophisticated molecular machines mimicking the extension and contraction motions.

Switchable host-guest systems on surfaces

CONSPECTUS: For device miniaturization, nanotechnology follows either the "top-down" approach scaling down existing larger-scale devices or the "bottom-up' approach assembling the smallest possible building blocks to functional nanoscale entities. For synthetic nanodevices, self-assembly on surfaces is a superb method to achieve useful functions and enable their interactions with the surrounding world. Consequently, adaptability and responsiveness to external stimuli are other prerequisites for their successful operation. Mechanically interlocked molecules such as rotaxanes and catenanes, and their precursors, that is, molecular switches and supramolecular switches including pseudorotaxanes, are molecular machines or prototypes of machines capable of mechanical motion induced by chemical signals, biological inputs, light or redox processes as the external stimuli. Switching of these functional host-guest systems on surfaces becomes a fundamental requirement for artificial molecular machines to work, mimicking the molecular machines in nature, such as proteins and their assemblies operating at dynamic interfaces such as the surfaces of cell membranes. Current research endeavors in material science and technology are focused on developing either a new class of materials or materials with novel/multiple functionalities by shifting host-guest chemistry from solution phase to surfaces. In this Account, we present our most recent attempts of building monolayers of rotaxanes/pseudorotaxanes on surfaces, providing stimuli-induced macroscopic effects and further understanding on the switchable host-guest systems at interfaces. Biocompatible versions of molecular machines based on synthetic macrocycles, such as cucurbiturils, pillararenes, calixarenes, and cyclodextrins, have been employed to form self-assembled monolayers of gates on the surfaces of mesoporous silica nanoparticles to regulate the controlled release of cargo/drug molecules under a range of external stimuli, such as light, pH variations, competitive binding, and enzyme. Rotaxanes have also been assembled onto the surfaces of gold nanodisks and microcantilevers to realize active molecular plasmonics and synthetic molecular actuators for device fabrication and function. Pillararenes have been successfully used to control and aid the synthesis of gold nanoparticles, semiconducting quantum dots, and magnetic nanoparticles. The resulting organic-inorganic hydrid nanomaterials have been successfully used for controlled self-assembly, herbicide sensing and detection, pesticide removal, and so forth, taking advantage of the selective binding of pillarenes toward target molecules. Cyclodextrins have also been successfully functionalized onto the surface of gold nanoparticles to serve as recycling extractors for C60. Many interesting prototypes of nanodevices based on synthetic macrocycles and their host-guest chemistry have been constructed and served for different potential applications. This Account will be a summary of the efforts made mainly by us, and others, on the host-guest chemistry of synthetic macrocyclic compounds on the surfaces of different solid supports.

A novel pentiptycene bis(crown ether)-based [2](2)rotaxane whose two DB24C8 rings act as flapping wings of a butterfly

A novel [2](2)rotaxane based on pentiptycene-derived bis(crown ether) can be efficiently synthesized via a "click chemistry" method and the subsequent N-methylation. Due to the different affinities of DB24C8 with the ammonium and triazolium stations, the wing-flapping movement of the DB24C8 "wings" in the [2](2)rotaxane can be easily achieved by acid/base stimulus.

Construction of rotacatenanes using rotaxane and catenane frameworks

The construction of novel mechanically interlocked structures has become a topic of great current interest due to the requirements of topology and their potential application in molecular machines and devices.

Photo-responsive [2]catenanes: synthesis and properties

A series of novel dithienylethene-based macrocycles containing ammonium moieties has been synthesized.

Dithienylethene-based rotaxanes: synthesis, characterization and properties

The photochromic materials have been widely applied in many fields.