Stepwise Motion in a Multivalent [2](3)Catenane

Dynamic control of circumrotation of a [2]catenane by acid-base switching

Dynamic control of the motion in a catenane remains a big challenge as it requires precise design and sophisticated well-organized structures. This paper reports the design and synthesis of a donor-acceptor [2]catenane through mechanical interlocking, employing a crown ether featuring two dibenzylammonium salts on its side arms as the host and a cyclobis(paraquat-p-phenylene) (CBPQT ⋅ 4PF6) ring as the guest molecule. By addition of external acid or base, the catenane can form self-complexed or decomplexed compounds to alter the cavity size of the crown ether ring, consequently affecting circumrotation rate of CBPQT ⋅ 4PF6 ring of the catenane. This study offers insights for the design and exploration of artificial molecular machines with intricate cascading responsive mechanisms.

Template Assisted One-Pot Synthesis of [2], Linear [3], and Radial [4]Catenane via Click Reaction

Design and synthesis of higher order catenane are unexpectedly complex and involve precise cooperation among the precursors overcoming competing and opposing interactions. We achieved synthesis of [2], linear [3], radial [4] in a one-pot reaction by consecutive ring closing through click reactions. This synthesis gave three isolable products due to two, four, and six-click reactions between suitable coupling partners. Yields of the isolate templated-catenane decrease from lower to higher-ordered catenane (40 %, 12 %, and 4 %), probably due to the bite angle as well as the flexibility of the reacting partners. Removal of templating cobalt(III) ion leads to the formation of fully organic [2], linear [3], and radial [4]catenane. These synthesized catenanes were purified by column chromatography and characterized by 1H-NMR, 13C-NMR, and ESI-MS spectroscopy. The synthesized catenanes have free binding sites suitable for post-functionalization and may be used for the synthesis of higher-ordered catenane.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

BP23C7: high-yield synthesis and application in constructing [3]rotaxanes and responsive pseudo[2]rotaxanes

A biphenyl-23-crown-7 ether (BP23C7) is synthesized in 86% yield from commercially available starting materials. BP23C7 forms pseudo[2]rotaxane with a dibenzylammonium ion (DBA+), exhibiting a good association constant value (ka = 1 × 103 M-1). Subsequently, fluorophoric properties of BP23C7 and anthracene terminated axles are blended to create responsive pseudo[2]rotaxanes. The "turn-on" fluorescence response of BP23C7 due to the addition of fluoride and chloride anions to pseudo[2]rotaxane systems has been investigated. Concomitant fluorescence quenching of the anthracene moiety of corresponding axles due to ion-pair formation has been addressed. Furthermore, two variants of [23]crown ethers, i.e. BP23C7 and o-xylene-23-crown-7 ether (X23C7), are applied for constructing homo[3]rotaxane architectures. A half-axle comprising of DBA+ moiety and a terminal olefin is mixed separately with two [23]crown ethers and subjected to self-metathesis using Grubbs' first-generation catalyst.

From construction to application of a new generation of interlocked molecules composed of heteroditopic wheels

Over the last few decades, research on mechanically interlocked molecules has significantly evolved owing to their unique structural features and interesting properties. A substantial percentage of the reported works have focused on the synthetic strategies, leading to the preparation of functional MIMs for their applications in the chemical, materials, and biomedical sciences. Importantly, various macrocyclic wheels with specific heteroditopicity (including phenanthroline, amide, amine, oxy-ether, isophthalamide, calixarene and triazole) and threading axles (bipyridine, phenanthroline, pyridinium, triazolium, etc.) have been designed to synthesize targeted multifunctional mononuclear/multinuclear pseudorotaxanes, rotaxanes and catenanes. The structural uniqueness of these interlocked systems is advantageous owing to the presence of mechanical bonds with specific three-dimensional cavities. Furthermore, their multi-functionalities and preorganised structural entities exhibit a high potential for versatile applications, like switching, shuttling, dynamic properties, recognition and sensing. In this feature article, we describe some of the most recent advances in the construction and chemical behaviour of a new generation of interlocked molecules, primarily focusing on heteroditopic wheels and their applications in different directions of the modern research area. Furthermore, we outline the future prospects and significant perspectives of the new generation heteroditopic wheel based interlocked molecules in different emerging areas of science.

Topological Entanglement of Linear Catenanes: Knots and Threadings

We used molecular dynamics simulations to investigate the self-entanglements of the collapsed linear catenanes. We found two different types of topologically complex states. First, we observed numerous long-lived knotting events of the catenane backbone. However, comparison with conventional polymers reveals that knots are suppressed in catenanes. Next, we observed topologically complex states with no analogue in polymers, where a concatenated ring was threaded by other near or distal rings sliding through it. Differently from knots, these threaded states can disentangle by becoming fully tightened. A detailed thermodynamic and microscopic analysis is employed to rationalize the persistence of threaded states, which can survive significant internal reorganizations of the entire catenane. We finally discuss the broader implications of these previously unreported types of entanglements for other systems, such as noncollapsed and interacting catenanes.

Synergistic Covalent-and-Supramolecular Polymers with an Interwoven Topology

Network topologies, especially some high-order topologies, are able to furnish cross-linked polymer materials with enhanced properties without altering their chemical composition. However, the fabrication of such topologically intriguing architectures at the macromolecular level and in-depth insights into their structure-property relationship remain a significant challenge. Herein, we relied on synergistic covalent-and-supramolecular polymers (CSPs) as a platform to prepare a range of polymer networks with an interwoven topology. Specifically, through the sequential supramolecular self-assemblies, the covalent polymers (CPs) and metallosupramolecular polymers (MSPs) could be interwoven in our CSPs by [2]pseudorotaxane cross-links. As a result, the obtained CSPs possessed a topological network that could not only promote the synergistic effect between CPs and MSPs to afford mechanically robust yet dynamic materials but also vest polymers with some functions, as manifested by force-induced hierarchical dissociations of supramolecular interactions and superior thermomechanical stability compared to our previously reported CSP systems. Furthermore, our CSPs exhibited tunable mechanical performance toward multiple stimuli including K⁺ and PPh₃, demonstrating abundant stimuli-responsive properties. We hope that these findings could provide novel opportunities toward achieving topological structures at the macromolecular level and also motivate further explorations of polymeric materials via the way of controlling their topological structures.

Multistate Structural Switching of [3]Catenanes with Cyclic Porphyrin Dimers by Complexation with Amine Ligands

Catenanes with multistate switchable properties are promising components for next-generation molecular machines and supramolecular materials. Herein, we report a ligand-controlled switching method, a novel method for the multistate switching of catenanes controlled by complexation with added amine ligands. To verify this method, a [3]catenane comprising cyclic porphyrin dimers with a rigid π-system has been synthesized. Owing to the rigidity, the relative positions among the cyclic components of the [3]catenane can be precisely controlled by complexation with various amine ligands. Moreover, ligand-controlled multistate switching affects the optical properties of the [3]catenanes: the emission intensity can be tuned by modulating the sizes and coordination numbers of integrated amine ligands. This work shows the utility of using organic ligands for the structural switching of catenanes, and will contribute to the further development of multistate switchable mechanically interlocked molecules.

Enhanced molecular binding affinity toward aromatic dications by anthracene-derived crown ethers in water

The pursuit of high molecular binding affinity using conventional crown ethers in water remains a challenging task in the field of supramolecular chemistry and may hold great promise in the creation of advanced biocompatible nanoconstructs. In this work, the molecular binding strength toward a series of structurally relevant cationic guests has been greatly enhanced by tetrasulfonated 1,5-dianthracenyl-42-crown-10 and as investigated by means of ¹H NMR, UV-vis, and fluorescence spectroscopy, the host-guest association constants can reach up to 10⁸ M^-1 order of magnitude in aqueous solution. X-ray crystal diffraction analysis further demonstrates that the aromatic dication can be tightly encapsulated in the ring of anthracene-derived crown ether via multiple π-stacking and electrostatic interactions. Meanwhile, the obtained association constants are remarkably higher than the ones in the cases of the known benzene- and naphthalene-derived sulfonated crown ethers, substantiating that the appropriate extension of π-conjugation in the molecular skeleton of crown ether is a feasible method in attaining a highly affiliative host-guest complex. Taken together, our results indicate that the anthracene-based sulfonated crown ether can be developed as a new family of water-soluble macrocyclic receptors in the fabrication of functional nanoarchitectures.

A three dimensional graphdiyne-like porous triptycene network for gas adsorption and separation

Graphdiyne, an emerging carbon allotrope, has attracted many researchers devoted to the study of its synthesis and application. The utilization of graphdiyne in gas adsorption and separation has been predicted by computer simulation with many examples. In this work, the triangular basic unit of graphdiyne was introduced into a triptycene-based porous organic polymer to obtain a three dimensional graphdiyne-like porous triptycene network named G-PTN. With high surface area and a microporous structure, G-PTN exhibited convincing application potential for the storage of gas molecules, especially for the selective adsorption of acetylene over ethylene. Computational simulation proved the importance of the triptycene units and three dimensional structure to the selectivity, as well as the potential of graphdiyne units as selective binding sites, suggesting that through judicious design, new three-dimensional porous graphdiyne could be acquired with better gas adsorption and separation performance.

Mechanically interlocked molecular handcuffs

The field of mechanically interlocked molecules that employ a handcuff component are reviewed. The variety of rotaxane and catenane structures that use the handcuff motif to interlock different components are discussed and a new nomenclature, distilling diverse terminologies to a single approach, is proposed. By unifying the interpretation of this class of molecules we identify new opportunities for employing this structural unit for new architectures.

Mechanically interlocked molecules that employ a handcuff component provide a pathway to highly unusual structures, a new nomenclature is proposed which helps to identify opportunities for employing this structural unit for new architectures.

Construction of Nanomotors with Replaceable Engines by Supramolecular Machine-Based Host-Guest Assembly and Disassembly

Micro/nanomotors (MNMs) are miniaturized devices capable of performing self-propelled motion and on-demand tasks, which have brought revolutionary renovations in nanomedicine, environmental remediation, biochemical sensing, etc. Numerous methods of either chemical synthesis or physical fabrications have been extensively investigated to prepare MNMs of various shapes and functions. However, MNMs with replaceable engines that can be flexibly assembled and disassembled, resembling that of a macroscopic machine, have not been achieved. Here, for the first time, we report a demonstration of control over the engine replacement of self-propelled nanomotors based on hollow mesoporous silica nanoparticles (HMSNPs) via supramolecular machine-based host-guest assembly and disassembly between azobenzene (Azo) and β-cyclodextrin (β-CD). Nanomotors with different driving mechanisms can be rapidly constructed by selecting corresponding β-CD-modified nanoengines of urease, Pt, or Fe₃O₄, to assemble with the azobenzene-modified HMSNPs (HMSNPs-Azo). In virtue of photoresponsive cis/trans isomer conversion of azobenzene molecules, engine switching can be accomplished by remote light triggered host-guest assembly or disassembly between HMSNPs-Azo and β-CD-modified engines. Moreover, this method can quickly include multiple engines on the surface of the HMSNPs-Azo to prepare a hybrid MNM with enhanced motion capability. This strategy not only is cost-effective for the rapid and convenient preparation of nanomotors with different propulsion mechanism but also paves a new path to future multiple functionalization of MNMs for on-demand task assignment.

Triptycene-based Chiral Porous Polyimides for Enantioselective Membrane Separation

Enantiomers of 2, 6-diaminotriptycene (R, R-1 and S, S-1) are split by chiral-phase HPLC and their absolute configurations are identified by single-crystal X-ray diffraction technology. Using the enantiomers as monomers, a couple of chiral porous polyimides (R-FTPI and S-FTPI) are prepared by polycondensation reactions and display good heat stability, high BET surface area and good solubility in organic solvents. Moreover, both of R-FTPI and S-FTPI can be cast into robust, free-standing films suitable for enantioselective separation with symmetrical chiral selectivity.

Rapid and High-Yield Synthesis of [23]Crown Ether: Applied as a Wheel Component in the Formation of Pseudo[2]rotaxane and Synthesis of [2]Catenane with a Dibenzylammonium Dumbbell

A facile, rapid, and high yield synthesis of [23]crown ether (X23C7) has been developed from commercially available starting materials, in one step with good to excellent yield. The reaction is completed in 6 h under room temperature conditions, with the highest yield being 81%. The X23C7 macrocycle formed pseudo[2]rotaxane with a dibenzylammonium ion (DBA⁺) dumbbell, exhibiting strong association (K_a = 2.61 × 10³ M^-1). Consequently, a [2]catenane was synthesized from a DBA⁺-based diolefin terminated salt and X23C7 in 81% yield, using a threading-followed-ring-closing-metathesis approach.

Overtemperature-protection intelligent molecular chiroptical photoswitches

Stimuli-responsive intelligent molecular machines/devices are of current research interest due to their potential application in minimized devices. Constructing molecular machines/devices capable of accomplishing complex missions is challenging, demanding coalescence of various functions into one molecule. Here we report the construction of intelligent molecular chiroptical photoswitches based on azobenzene-fused bicyclic pillar[n]arene derivatives, which we defined as molecular universal joints (MUJs). The Z/E photoisomerization of the azobenzene moiety of MUJs induces rolling in/out conformational switching of the azobenzene-bearing side-ring and consequently leads to planar chirality switching of MUJs. Meanwhile, temperature variation was demonstrated to also cause conformational/chiroptical inversion due to the significant entropy change during the ring-flipping. As a result, photo-induced chiroptical switching could be prohibited when the temperature exceeded an upper limit, demonstrating an intelligent molecular photoswitch having over-temperature protection function, which is in stark contrast to the low-temperature-gating effect commonly encountered.

Realizing overtemperature protection with a molecular device is challenging. Here, the authors demonstrate an overtemperature protection function by integrating thermo- and photoresponsive functions into a pillar[6]arene based pseudocatanene.

An Asymmetric Mechanism in a Symmetric Molecular Machine

The design of molecular architectures exhibiting functional motions is a promising area for disruptive technological development. Toward this goal, rotaxanes and catenanes, which undergo relative motions of their subunits in response to external stimuli, are prime candidates. Here, we report on the computational analysis of the contraction/extension of a bistable [c2]daisy chain rotaxane. Using free-energy calculations and transition path optimizations, we explore the free-energy landscape governing the functional motions of a prototypical molecular machine with atomic resolution. The calculations reveal a sequential mechanism in which the asynchronous gliding of each ring is preferred over the concerted movement. Analysis of the underlying free-energy surface indicates that the formation of partially rearranged intermediates entails crossing of much smaller barriers. Our findings illustrate an important design principle for molecular machines, namely that efficient exploitation of thermal fluctuations may be realized by breaking down the large-scale functional motions into smaller steps.

A hydrogen-bonded polymer constructed from mechanically interlocked, suit[1]ane monomers

A T-shaped 2,4,7-substituted benzimidazolium “axle” with two ester functionalities and a 24-membered crown ether “wheel” with appendages containing terminal olefin groups were threaded — axle through wheel — to form a [2]pseudorotaxane. Grubbs’ ring-closing metathesis (RCM) was then used to form a third loop and create a bicyclic cage that fully encapsulates the axle and permanently interlocks the two molecular components creating a suit[1]ane. There are no bulky groups on the axle to prevent unthreading, but the axle is trapped due to the cage-like nature of the newly created polyether host. After hydrolysis of the esters groups to carboxylic acids, this novel mechanically interlocked molecule (MIM) polymerizes in the solid state. The structure of the resulting supramolecular polymer was determined by single-crystal X-ray diffraction and contains linear one-dimensional tapes of suit[1]ane monomers linked by intermolecular hydrogen bonding between the carboxylic acid groups.

Insights into directional movement in molecular machines from free-energy calculations

A rotaxane composed of a symmetrical axle containing three binding stations and a cone-like macrocycle containing two secondary amines has been investigated at the atomic level. At high pH, the macrocycle binds to the intermediate di(quaternary ammonium) site, while at low pH, the protonated macrocycle selectively moves along the axle to one of the two symmetrical phenyl triazole binding sites facing its upper rim, but does not shuttle backward. The determined free-energy profile characterizing the translocation of the macrocycle indicates that the selected binding site is energetically more favorable than the one facing the lower rim of the macrocycle and the free-energy barrier against translocation to the former site is lower than to the latter one, rationalizing the directional movement. This selectivity mainly stems from the asymmetry of the macrocycle shape. The strong electrostatic repulsion between the ring and the axle is found to constitute the driving force for the shuttling of the ring and also the resistance for its reverse motion. Moreover, the effect of the solvent on the shuttling has been examined, suggesting that increasing the solvent polarity may weaken the directional preference of shuttling, due to the shielding effect of polar solvents on electrostatic interactions. Our study provides a theoretical framework for tuning the selectivity of directional movement in molecular machines.

Switching porosity of stable triptycene-based cage via solution-state assembly processes

It is a great challenge to tune the porosity of porous materials. As most porous organic cages are soluble, solution processability can be a possible way to regulate the porosity of such materials. Herein, a triptycene-based cage (TC) is demonstrated to be stable in acid, base or boiling water. Meanwhile, its porosity can be tuned by adjusting the solution-state assembly processes. TC molecules crystallized slowly from solution exhibit nearly no porosity to nitrogen (off-state). While, after rapid precipitating from methanol/dichloromethane solution, the obtained TC (TC-rp) is in a porous state and exhibit a high BET surface area of 653 m² g⁻¹ (on-state).

Here, a kind of triptycene-based cage is demonstrated to have good chemical stability in acid, base and boiling water. Moreover, its porosity can be tuned by varying the solution-state assembly processes.

1,2,3-Triazolium macrocycles in supramolecular chemistry

In this short review, we describe different pathways for synthesizing 1,2,3-triazolium macrocycles and focus on their application in different areas of supramolecular chemistry. The synthesis is mostly relying on the well-known "click reaction" (CuAAC) leading to 1,4-disubstituted 1,2,3-triazoles that then can be quaternized. Applications of triazolium macrocycles thus prepared include receptors for molecular recognition of anionic species, pH sensors, mechanically interlocked molecules, molecular machines, and molecular reactors.

Chemical On/Off Switching of Mechanically Planar Chirality and Chiral Anion Recognition in a [2]Rotaxane Molecular Shuttle

We exploit a reversible acid–base triggered molecular shuttling process to switch an appropriately designed rotaxane between prochiral and mechanically planar chiral forms. The mechanically planar enantiomers and their interconversion, arising from ring shuttling, have been characterized by NMR spectroscopy. We also show that the supramolecular interaction of the positively charged rotaxane with optically active anions causes an imbalance in the population of the two enantiomeric coconformations. This result represents an unprecedented example of chiral molecular recognition and can disclose innovative approaches to enantioselective sensing and catalysis.

Pumping a Ring-Sliding Molecular Motion by a Light-Powered Molecular Motor

Designing artificial molecular machines to execute complex mechanical tasks, like coupling rotation and translation to accomplish transmission of motion, continues to provide important challenges. Herein, we demonstrated a novel molecular machine comprising a second-generation light-driven molecular motor and a bistable [1]rotaxane unit. The molecular motor can rotate successfully even in an interlocked [1]rotaxane system through a photoinduced cis-to -trans isomerization and a thermal helix inversion, resulting in concomitant transitional motion of the [1]rotaxane. The transmission process was elucidated via ¹H NMR, ¹H-¹H COSY, HMQC, HMBC, and 2D ROESY NMR spectroscopies, UV-visible absorption spectrum, and density functional theory calculations. This is the first demonstration of a molecular motor to rotate against the appreciably noncovalent interactions between dibenzo-24-crown-8 and N-methyltriazolium moieties comprising the rotaxane unit, showing operational capabilities of molecular motors to perform more complex tasks.

Construction and heterogeneous photooxidization reactivity of a cyclodextrin/porphyrin polyrotaxane network

A supramolecular polyrotaxane network, based on a CD-based inclusion complex and porphyrin derivative, was synthesized by imine condensation reactions. Then it could be applied as a heterogeneous catalyst for generating singlet oxygen under photoirradiation, which showed its good ability to photooxidize anthracene derivatives.

Triptycene-Derived Macrocyclic Arenes: From Calixarenes to Helicarenes

The development of new types of synthetic macrocyclic hosts is always one of the most important and attractive topics in macrocyclic and supramolecular chemistry. Calixarenes, resorcinarenes, cyclotriveratrylenes, pillararenes, and their analogues are all composed of hydroxy-substituted aromatic rings bridged by methylene or methenyl groups and thus can be considered a type of macrocyclic arenes. Because of their unique structural features, easy functionalization, and wide applications in many research areas, such macrocyclic arenes have become some of the most important and studied synthetic macrocyclic hosts during the last decades. Triptycene and its derivatives are a class of organic molecules having unique three-dimensional rigid structures, and they have proved to be useful building blocks for constructing new synthetic macrocyclic hosts with specific structures and properties. Dihydroxy-substituted triptycene derivatives are readily available compounds, which encouraged us to conduct studies of triptycene-derived macrocyclic arenes about 10 years ago. Consequently, a series of triptycene-derived calixarenes and analogues containing 1,8-dihydroxy-substituted triptycene subunits were conveniently synthesized. With 2,7-dihydroxy-substituted triptycene as a precursor, new types of calixarene, oxacalixarene, and homooxacalixarene analogues were also obtained. These triptycene-derived macrocyclic hosts all showed fixed conformations in solution and exhibited expanded cavities compared with the corresponding typical calixarenes and analogues. The special structural features also make these triptycene-derived macrocycles show wide potential applications in molecular recognition and self-assembly. In particular, it was found that the threading direction and the orientation based on macrocycles with nonsymmetric structures could be finely controlled by adjusting the electrostatic and steric effects of the guests, which could form the oriented [2]rotaxane by unidirectional threading. We recently developed a new kind of chiral macrocyclic arenes named helicarenes that are composed of chiral 2,6-dihydroxy-substituted triptycene subunits bridged by methylene groups. It was found that the helicarenes not only exhibited convenient synthesis, high stability, good solubility, fixed conformations, and easy functionalization but also showed complexation abilities with various chiral and achiral organic guests. In particular, the switchable complexation based on these macrocycles could be efficiently controlled by multiple stimuli, including acid/base, redox, anion, or light stimuli under a photoacid. Moreover, the helicarenes have also found applications in the construction of interlocked molecules and molecular machines. This Account summarizes our recent research results on the synthesis and structures of the triptycene-derived macrocyclic arenes and analogues and their applications in host-guest chemistry and molecular assembly. We believe that these macrocyclic arenes, especially helicarenes, could be utilized as new synthetic hosts and find wide potential applications in macrocyclic and supramolecular chemistry.

Electrochemically Triggered Co-Conformational Switching in a [2]catenane Comprising a Non-Symmetric Calix[6]arene Wheel and a Two-Station Oriented Macrocycle

Catenanes with desymmetrized ring components can undergo co-conformational rearrangements upon external stimulation and can form the basis for the development of molecular rotary motors. We describe the design, synthesis and properties of a [2]catenane consisting of a macrocycle-the 'track' ring-endowed with two distinct recognition sites (a bipyridinium and an ammonium) for a calix[6]arene-the 'shuttle' ring. By exploiting the ability of the calixarene to thread appropriate non-symmetric axles with directional selectivity, we assembled an oriented pseudorotaxane and converted it into the corresponding oriented catenane by intramolecular ring closing metathesis. Cyclic voltammetric experiments indicate that the calixarene wheel initially surrounds the bipyridinium site, moves away from it when it is reduced, and returns in the original position upon reoxidation. A comparison with appropriate model compounds shows that the presence of the ammonium station is necessary for the calixarene to leave the reduced bipyridinium site.

Higher-generation type III-B rotaxane dendrimers with controlling particle size in three-dimensional molecular switching

Type III-B rotaxane dendrimers (T3B-RDs) are hyperbranched macromolecules with mechanical bonds on every branching unit. Here we demonstrate the design, synthesis, and characterization of first to third (G1–G3), and up to the fourth (G4) generation (MW > 22,000 Da) of pure organic T3B-RDs and dendrons through the copper-catalyzed alkyne–azide cycloaddition (CuAAC) reaction. By utilizing multiple molecular shuttling of the mechanical bonds within the sphere-like macromolecule, a collective three-dimensional contract-extend molecular motion is demonstrated by diffusion ordered spectroscopy (DOSY) and atomic force microscopy (AFM). The discrete T3B-RDs are further observed and characterized by AFM, dynamic light scattering (DLS), and mass spectrometry (MS). The binding of chlorambucil and pH-triggered switching of the T3B-RDs are also characterized by ¹H-NMR spectroscopy.

The complexity of rotaxane dendrimers poses a great synthetic challenge and the synthesis of higher generation rotaxane dendrimers has therefore rarely been reported. Here the authors report the synthesis of acid-base switchable rotaxane dendrimers up to generation 4 and demonstrate the uptake and release of guest molecules.

Efficient control of movement in non-photoresponsive molecular machines by a photo-induced proton-transfer strategy

The first successful application of a PIPT strategy executed by the photoacid 1-MEH in controlling the switch of MIM systems has been demonstrated.

How Secondary and Tertiary Amide Moieties are Molecular Stations for Dibenzo-24-crown-8 in [2]Rotaxane Molecular Shuttles?

Interlocked molecular machines like [2]rotaxanes are intriguing aesthetic molecules. The control of the localization of the macrocycle, which surrounds a molecular axle, along the thread leads to translational isomers of very different properties. Although many moieties have been used as sites of interactions for crown ethers, the very straightforwardly obtained amide motif has more rarely been envisaged as molecular station. In this article, we report the use of secondary and tertiary amide moieties as efficient secondary molecular station in pH-sensitive molecular shuttles. Depending on the N-substitution of the amide station, and on deprotonation or deprotonation-carbamoylation, the actuation of the molecular machinery differs accordingly to very distinct interactions between the axle and the DB24C8.

Complexation Between (O-Methyl)6 -2,6-Helic[6]arene and Tertiary Ammonium Salts: Acid/Base- or Chloride-Ion-Responsive Host-Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O-methyl)₆ -2,6-helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹ H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base- and chloride-ion-responsive host-guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6-helic[6]arene-based [2]rotaxane was also synthesized from the condensation between the host-guest complex and isocyanate.

Fluorescent Phthalocyanine Assembly Distinguishes Chiral Isomers of Different Types of Amino Acids and Sugars

The functions of some natural supramolecular architectures, such as ribosomes, are dependent on the recognition of different types of chiral biomolecules. However, the recognition of different types of chiral molecules (multiobject chiral recognition), such as amino acids and sugars, by independent and identically artificial supramolecular assembly, was rarely achieved. In this article, simple amphiphilic achiral phthalocyanine was found to form supramolecular chiral assemblies with charged water-soluble polymers upon host-guest interactions at the air/water interface. Among these systems, one identical phthalocyanine/poly(l-lysine) assembly not only can distinguish enantiomers of different amino acids but also can recognize several epimers of monose. The chiral recognitions were achieved by comparing either the steady-state fluorescence intensity or fluorescence quenching rate of phthalocyanine/poly(l-lysine) assemblies, before and after interaction with different small chiral molecules. It was demonstrated that the interactions between poly(l-lysine) and different small chiral molecules could change the aggregation of phthalocyanines. And the sensitivity of fluorescence and the excellent multiobject chiral recognition properties of the phthalocyanine/poly(l-lysine) assembly are dependent on the subtle molecular packing mode and the cooperation of different noncovalent interactions.