Development of Pseudorotaxanes and Rotaxanes: From Synthesis to

Review pubs.acs.org/CR

Development of Pseudorotaxanes and Rotaxanes: From Synthesis to Stimuli-Responsive Motions to Applications Min Xue,† Yong Yang,‡ Xiaodong Chi,† Xuzhou Yan,† and Feihe Huang*,† †

State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China 2.11. Rotaxane-Related Host−Guest Complexes Based on Molecular Tweezers and Clips 3. Stimuli-Responsive Molecular Mobility of Pseudorotaxanes 3.1. Association/Dissociation Mobility 3.1.1. Solvent Effects on Association Constants 3.1.2. Temperature Effect: Slippage or Deslippage? 3.1.3. External Stimuli 3.2. Shuttling Mobility 3.3. Functions of Pseudorotaxanes 3.3.1. Fluorescent Sensors 3.3.2. Molecular Logic Gates 3.3.3. Molecular Switches 3.3.4. Other Molecular Machines 4. Stimuli-Responsive Molecular Mobility of Rotaxanes 4.1. Rotational Mobility 4.2. Shuttling Mobility 4.2.1. Observation of Shuttling and Its Dynamic Control 4.2.2. Stimuli-Responsive Molecular Shuttles 4.2.3. Modulating Properties and Functions of Molecular Shuttles 4.3. Mobility of [c2]Daisy Chains 5. Applications of Pseudorotaxanes and Rotaxanes 5.1. Protection of Encapsulated Molecules 5.1.1. Unstable Species 5.1.2. Dyes 5.2. Stimuli-Responsive Supramolecular Polymeric Materials 5.3. Applications of Rotaxane-Based Molecular Machines 5.3.1. Molecular Electronic Devices Based on the Langmuir−Blodgett Monolayers 5.3.2. Rotaxane-Based Molecular Machines on Surfaces 5.4. Metal−Organic Frameworks 6. Conclusions and Future Perspectives Author Information Corresponding Author Notes

CONTENTS 1. Introduction 2. Synthesis of Rotaxanes Based on Various Macrocycles 2.1. Crown Ethers 2.1.1. Bis(m-phenylene)-32-crown-10 and Crown Ethers with Larger Sizes 2.1.2. Dibenzo-24-crown-8 2.1.3. Benzo-21-crown-7 2.1.4. Crown Ether-Based Cryptands 2.2. Cyclodextrins 2.3. Cucurbiturils 2.3.1. Cucurbit[6]uril 2.3.2. Cucurbit[7]uril 2.3.3. Cucurbit[8]uril 2.4. Calixarenes 2.4.1. Calix[4]arenes as Linkers or Stoppers 2.4.2. Calix[5]arenes and Calix[6]arenes as Wheels 2.4.3. Heterocalix[n]arenes or Calix[n]heteroarenes 2.5. Pillararenes 2.5.1. Organic Solvent-Soluble Pillararenes 2.5.2. Water-Soluble Pillararenes 2.6. Tetracationic Cyclophanes 2.7. Amide-Based Macrocycles 2.7.1. Tetralactam Macrocycles with Diarylmethane Motif 2.7.2. Tetralactam Macrocycles with Benzylamide Motif 2.7.3. Other Lactam Macrocycles 2.8. Coordination-Based Macrocycles 2.9. Synthesis of Oligorotaxanes 2.10. Isomeric Rotaxanes

© XXXX American Chemical Society

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Special Issue: 2015 Supramolecular Chemistry Received: October 10, 2014

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DOI: 10.1021/cr5005869 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews Biographies Acknowledgments References

Review

Furthermore, the most popular and effective “click chemistry” has also been widely exploited for the rotaxane synthesis.15 This breakthrough in synthesis makes these compounds more accessible, and allows the introduction of various functional groups into their backbones and more complexity in their structures. System complexity ranges from simple [1]rotaxanes16 or [2]rotaxanes17 to daisy chains,18 oligorotaxanes,19 and even to polyrotaxanes (Figure 2).20 With the unique encapsulated structural features, rotaxanes have found applications in the modification of physical and chemical properties of guest molecules,21 and protection of substrates such as dyes from chemical attack and aggregation.22 The interest in these supramolecular architectures came first from their beautiful interlocked structures, but was further developed due to their unique mobile features, which resulted from the high degree of freedom among the relatively independent rod or linear components. Control of mobilities can be achieved by some chemical or physical stimuli, such as acid/base, anion, cation, photochemistry, and electrochemistry. For pseudorotaxanes, there mainly exists the mobility of association and dissociation, as well as the shuttling motion of the wheels on unstoppered linear units in a few examples. On the basis of the dynamic property of association and dissociation, pseudorotaxanes can be endowed with the functions of control/release and lock/key. It allows them to be applied in the construction of various molecular devices such as switches,23,24 logic gates,25 sensors,26 and nanovalves.27 For rotaxanes, two types of motions, rotating and shuttling of the macrocycles around the axles, are involved. Since the mobility of rotaxanes was observed, intensive investigations from controlling the mobile rate,28 or even stopping its behavior,29 to manipulating its motion at will via external stimuli30 have been devoted to this field. Stimuli-responsive molecular shuttles provide rotaxanes with a promising basis for artificial molecular machines, such as nanomotors,31 elevators,32 or molecular rachets.33 Among different types of rotaxanes, daisy chains, because of their unique muscle-like mobility, have gained special attention.18 Besides the various forms of motions in solution, rotaxanes have also been prepared in the solid state,34 on surfaces,35 or as a part of metal−organic frameworks (MOFs).36 The transfer of molecular-machine technology into solid substrates is a key step for the development of many potential applications of rotaxanes as smart materials with switchable properties in molecular electronics and other types of molecular machinery.37 Specially rotaxane-incorporated metal−organic frameworks as a new perspective for rotaxanes, giving up their disadvantage of random motion in solution, have achieved a higher level of molecular organization and provided a platform for coherent switching between their mounted components.38 By incorporating pseudorotaxane or rotaxane moieties into polymers, polypseudorotaxanes and polyrotaxanes can be constructed in various types.39 On the other hand, pseudorotaxanes or rotaxanes can also be employed to self-assemble into supramolecular polymers through noncovalent interactions.40,41 Because of their convenient environmental-responsiveness and reversible nature, these supramolecular systems have novel applications in the fabrication of polymeric materials such as hydrogels, molecular muscles, nanoparticles, and micelles.42 The stimuli-responsive supramolecular polymeric materials have been investigated for biomedical and pharmaceutical applications in recent years, which have a strong impact on materials science accordingly.43,44

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1. INTRODUCTION The history of isomeric types of organic molecules has passed through several important stages: (1) Molecules that differ from each other with different numbers and types of atoms; (2) constitutional isomers with typical sequence and nature of bonding between these atoms; (3) stereoisomers that are distinguished by the spatial arrangement around an atom or center; and (4) topological isomers with distinct topological geometries. It is the last form of isomerism that leads to mechanically interlocked molecules (MIMs), such as catenanes, rotaxanes, and knots, which gain their inspiration from various natural objects and architectures.1−5 Chemists have paid attention to making MIMs for more than 50 years, not only because of their aesthetic beauty but also due to their potential applications as molecular devices for information storage and processing, etc.6 As one type of interlocked molecules, the rotaxane architecture is considered as a versatile platform to construct functional artificial nanomachines.7 “Rotaxane” is derived from the Latin words for “wheel” and “axle”, and describes a compound that consists of a linear species (sometimes called the rodlike part or guest) and cyclic species (sometimes called the beadlike part or host) bound together in a threaded structure by noncovalent forces. “Pseudo” means false, so “pseudorotaxane” without bulky stoppers at the end of the axle means false rotaxane, which is actually a supramolecular complex but not a compound. Sometimes a pseudorotaxane with only one stopper is called semirotaxane. Cartoon representations of pseudorotaxane, semirotaxane, and rotaxane are shown in Figure 1. The gray balls here represent stoppers, which are bulky groups and can prevent dethreading of the cyclic component.

Figure 1. Cartoon representations of pseudorotaxane, semirotaxane, and rotaxane.

The strategies for the synthesis of rotaxanes developed from statistical threading to directed template synthesis.8,9 The statistical threading method (Scheme 1) was originally introduced by Harrison and co-workers in 1967.8 The formation of pseudorotaxanes and rotaxanes by this method is based on a purely statistical progress without any apparent attractive force between the linear species and the cyclic molecules. In the course of the past 30 years, rotaxanes have been easily accessed via template methods9 derived from π−π stacking interactions,10 hydrogen bonding,11 hydrophobic interactions,12 metal ion coordination,13 and anion templates.14 B


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Scheme 1. First Synthesis of a [2]Rotaxane by the Statistical Threading Method

Scheme 2. General Approaches to the Construction of Rotaxanes

Figure 2. Cartoon representations of different types of rotaxanes.

Up to now, rotaxane-based chemistry has been developed to a giant research area with various fine branches, such as template synthesis of rotaxanes,9,45 porphyrin rotaxanes,46 dendrimer rotaxanes,47 metal-directed synthesis of rotaxanes,48 pH-driven rotaxanes,49 rotaxane-based molecular machines,50 and so on. Each branch can be treated as a topic of study. In this Review, we will mostly describe the chemistry of rotaxanes in the last 10 years in a historical development order. In the first section, the synthesis of rotaxanes will be presented. These rotaxanes will be divided into different types based on the macrocycles incorporated in them, such as crown ethers, cyclodextrins, cucurbiturils, calixarenes, pillararenes, cyclobis(paraquat-p-phenylene), amide macrocycles, and coordinationbased macrocycles. The mobility and external stimuliresponsive properties of pseudorotaxanes and rotaxanes then will be discussed, respectively, followed by potential applications and perspectives on these mobile molecules. Polypseudorotaxanes and polyrotaxanes will not be included here. Up to now, more than 6000 papers have been published on pseudorotaxanes and/or rotaxanes. It is almost impossible for us to cite all of them in this Review. We must say “sorry” to those authors whose papers on pseudorotaxanes and/or rotaxanes are not cited here.

capping the thread with bulky groups to prevent it from dethreading.52 A similar route is snapping, in which the axle containing an end group allows threading by the cyclic bead (yielding a semirotaxane), and then bears another end group.53 This method is often used to prepare nonsymmetric rotaxanes. When the macrocycle is formed around the linear part of the dumbbell-like unit, it is called clipping.54 However, in a slipping route, the cyclic molecule can thread over a blocking group of the dumbbell-like unit at elevated temperatures.55 In Scheme 2e, “active metal template” strategy is described, in which the metal plays a dual role in gathering and positioning of the ligands as well as catalyzing covalent bond formation that captures the threaded structure.48 New strategies for preparing rotaxanes have emerged recently (Scheme 3). In a threading-

2. SYNTHESIS OF ROTAXANES BASED ON VARIOUS MACROCYCLES In this section, we focus on rotaxanes formed by templatedirected synthetic methodologies.51 Five types of typical mechanisms (Scheme 2) for the construction of rotaxanes can be identified. Scheme 2a demonstrates the capping method, in which the macrocycle first encircles the thread to form a socalled pseudorotaxane and a [2]rotaxane is then formed by end-

followed-by-shrinking strategy,56,57 the free space within the macrocycle shrinks through coordination after the threading process, while in a threading-followed-by-swelling strategy,58 the terminal group enlarges after the threading process. Pseudorotaxanes and rotaxanes can be classified according to different standards. On the basis of the difference in the main driving forces for the threading process, supramolecular interactions that induce the formation of pseudorotaxanes

Scheme 3. New Strategies for the Construction of Rotaxanes

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Figure 3. Macrocycles used in the synthesis of rotaxanes.

the early plentiful work, crown ethers have become very popular in cation complexation chemistry, and have been utilized to prepare pseudorotaxanes and rotaxanes. 2.1.1. Bis(m-phenylene)-32-crown-10 and Crown Ethers with Larger Sizes. Bis(m-phenylene)-32-crown-10 (BMP32C10) and crown ethers with larger sizes were initially tried to construct host−guest complexes. In 1987, Stoddart and co-workers demonstrated that BMP32C10 binds paraquat and diquat dications in acetone.71 The main driving forces were attributed to donor−acceptor π−π stacking interactions between the complementary aromatic units, as well as C− H···O hydrogen bonding and N+···O electrostatic interactions. On the basis of this host−guest recognition, [2]rotaxanes containing the BMP32C10 or bis(p-phenylene)-34-crown-10 (BPP34C10) macrocycle and the axle with one or more 4,4′bipyridinium units were constructed through both the clipping and the slipping methods.72−74 To investigate the stopper effects on the properties of rotaxanes, bulky dendrimers were covalently attached to rotaxane cores to afford [2]-, [3]-, and [4]rotaxanes incorporating BPP34C10 wheels as the ring components.75 Besides 4,4′-bipyridinium units, other electron-deficient aromatic derivatives like pyromellitic diimide (PmI) and naphtho-diimide (NpI) also complex with larger crown ethers containing aromatic rings through similar interactions to construct MIMs. For example, dinaphtho-38-crown-10 (DNP38C10) was used to form a [2]rotaxane with a dumbbell component containing both PmI and NpI units.76 Very recently, dimethyldiazaperopyrenium dication (MP2+), a πelectron-poor organic molecule, was used as a guest molecule to construct [2]rotaxanes and a [3]rotaxane with DNP38C10 and BPP34C10 to investigate its chameleonic homo- and

and rotaxanes can be divided into hydrogen bonding, hydrophilic−hydrophobic interaction, metal−ligand coordination, π−π stacking, and charge transfer, among others.9,59−61 On the basis of the difference in reaction type, the widely utilized chemical reactions for rotaxane synthesis are Williamson ether synthesis, amide and ester bond formation reactions, Glaser and Eglinton couplings, imine-bond formation, metal−ligand coordination, the Menschutkin alkylation of pyridines with alkyl halides, the copper(I)-catalyzed azide− alkyne cycloaddition (the CuAAC “click” reaction), and other types.62−69 Here, we will discuss the construction of rotaxanes according to the types of wheel-like macrocyclic molecules, such as crown ethers, cyclodextrins, cucurbiturils, calixarenes, pillararenes, tetracationic cyclophanes, amide-based macrocycles, and metal−ligand-based macrocycles (Figure 3). In this way, we will have a clear picture about which type of macrocycle is appropriate to prepare rotaxanes, and also the differences and advantages among various wheel-like molecules are clearly known. Because of the complicated structures and unique features, oligorotaxanes and their formations will be summarized in this subsection independently. Besides, some special phenomena in the synthetic process such as isomeric properties will also be discussed. As a special supramolecular system, rotaxane-related host−guest complexes based on molecular tweezers and clips will also be discussed in this section. 2.1. Crown Ethers

Crown ethers, which were first discovered by Charles Pedersen in 196770 and subsequently rapidly developed on a large scale, consist simply of a cyclic array of ether oxygen atoms linked by organic spacers, typically −CH2CH2− groups (Figure 3). Since D


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Scheme 4. Formation of [2]- and [3]Rotaxanes Based on the DNP38C10/MP2+ and BPP34C10/MP2+ Molecular Recognition Motifs

heterophilic recognition nature (Scheme 4).77 Complexation of MP2+ derivative 1 with DNP38C10 first forms a pseudorotaxane with a binding constant of 1.05 × 104 M−1 in acetonitrile. After copper-catalyzed [3+2] cycloaddition to stopper the complex with 2, [2]rotaxane 3·2PF6 was afforded in 22%. When BPP34C10 was used as the wheel instead, [2]rotaxane 4·2PF6 and [3]rotaxane 5·2PF6 were obtained in yields of 14% and 4%, respectively. The [3]rotaxane 5·2PF6 with two BPP34C10 rings onto a single MP2+ recognition site offers a rare example of a rotaxane in which two donor rings share one acceptor unit in the dumbbell. T h e la r g e r m a cr o cy c l e d i n a p h t h o - 5 0 - cr o w n - 1 4 (DNP50C14), containing two 1,5-dioxynaphthalene (DNP) units similar to DNP38C10, clips two cyclobis(paraquat-pphenylene) rings (CBPQT4+) to form a donor−acceptor [3]catenane 68+ with a π-electron-deficient pocket (Scheme 5).78 Such [3]catenane acted as a host and formed a 1:1 complex with the guest 1,5-bis[2-(2′-(2″-hydroxyethoxy)ethoxy)ethoxy]naphthalene (BHEEEN) 7 to afford a [2]pseudorota[3]catenane 88+. However, the binding affinity of this [2]pseudorotaxane-type complex is very weak in CH3CN, with a binding constant of 33 ± 12 M−1 measured by UV/vis spectroscopic titrations. It was shown by X-ray crystallographic analysis of the superstructure that the polyether loops in 88+ prevented the tri(ethylene glycol) chains in BHEEEN from

garnering any significant [C−H···O] interactions with 88+. Exchanging eight Cl− anions for eight PF6− counterions decreased the solubility in water. By contrast, the very stable 1:1 complex could be obtained in water based on hydrophobic interactions. 2.1.2. Dibenzo-24-crown-8. Besides the large crown ethers, dibenzo-24-crown-8 (DB24C8) is usually considered as a wheel to form pseudorotaxanes with secondary ammonium cations. Early studies found that secondary dialkylammonium salts bound DB24C8 strongly and resided within the macrocyclic cavity.79 The main driving forces are N+−H···O and C−H···O hydrogen bonds between the hydrogen atoms of the NH2+ and NCH2+ groups of the guest and the polyether oxygen atoms of the host. The association constant (Ka) of the complex based on DB24C8 and dibenzylammonium hexafluorophosphate was determined in a range of solvents (from DMSO of ∼0 M−l to CDCl3 of 27 000 M−l) by a single-point 1 H NMR spectroscopic determination performed at 298 K.80 One example of finding appropriate guests for BPP34C10 and DB24C8 in one-pot systems was reported by Huang and coworkers.81 By taking advantage of the strong complexations between DB24C8 and the dibenzylammonium moiety and between BPP34C10 and the paraquat unit, two AB-type heteroditopic monomers 9 and 10 were designed (Scheme 6). Such self-sorting organization of the two monomers indicated E


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Scheme 5. Chemical Structure and X-ray Crystal Structure of the [2]Pseudorota[3]catenane 88+ Based on DNP50C14a

a

Reproduced with permission from ref 78. Copyright 2012 American Chemical Society.

Scheme 6. Self-Sorting Organization of Heteroditopic Monomers 9 and 10a

a


based on the DB24C8/dibenzylammonium cation recognition was prepared (Scheme 7).92,93 In this synthesis process, cyclophane 12 was reacted with DCC-derived [2]pseudorotaxane 13 (DCC = N,N′-dicyclohexylcarbodiimide) to form [2]rotaxane 14. Subsequent modification of the DB24C8 component introduced arginines to the ring through DCC coupling with a catalytic amount of HOBt. Such a cyclophane-modified [2]rotaxane is required for the intracellular delivery of a series of fluoresceinated peptides. Besides secondary ammonium cations, paraquat derivatives have been considered as guests of DB24C8 and seemed not to be good candidates for recognition of the host.96 As compared to paraquat, the N+−N+ distance in 1,2-bis(pyridinium)ethane (BPE) decreases from 7.00 to 3.75 Å. This difference leads to different binding properties of the two compounds.97 Loeb and co-workers reported that BPE dications thread DB24C8 wheellike compounds stabilized by N+···O electrostatic interactions, C−H···O hydrogen bonds, and π-stacking interactions between the electron-rich catechol rings of the crown ether and the electron-poor aromatic rings of BPE.98 On the basis of this recognition motif, [2]rotaxanes containing BPE axles and 24-

the different binding abilities of DB24C8 and BPP34C10. It led to the alternating arrangement of the two monomers, which further self-organized into a linear supramolecular alternating copolymer 11. The recognition motif between dialkylammonium cations and 24-membered crown ethers has provided a very good platform for fabrication of rotaxanes. The wheels include DB24C8 or dipyrido-24-crown-8 (DP24C8), and the axles contain one or more dialkylammonium sites. Through the capping strategy, bulky stoppers can be introduced into pseudorotaxanes by the triazole formation protocol,82 thiol− ene click reaction,83 urea formation,84 triphenylphosphonium formation,85 Wittig reactions,86 alkene formation via ruthenium-catalyzed metathesis,87 and Sonogashira coupling.88 Recent studies on such rotaxane formation were focused on modifying functionalized moieties such as fullerenes, calix[4]arenes, porphyrins, other cyclophanes, and fluorescent moieties in the terminal groups of the axles or the branches of the ring components of rotaxanes to achieve some special functions.89−95 For example, with the purpose of delivery of fluoresceinated peptides across cell membranes, [2]rotaxane 16 F


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Scheme 7. Formation of Cyclophane-Modified DB24C8-Based [2]Rotaxane 16·5TFA

Figure 4. Formation of [2]rotaxanes based on the DB24C8/BPE recognition motif.

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Scheme 8. Formation of [2]Rotaxanes [22-H][BF4] and 22 Based on the DB24C8/Benzimidazolium Molecular Recognition Motif

crown-8 type wheels were constructed with either symmetrical or nonsymmetrical conformations (Figure 4).99 Because both the BPE axle and the crown ether wheel have different end groups, the resulting [2]rotaxane 19 can exist in two distinct coconformations. To expand the guest classes, cations such as N-benzylic anilinium100 and N-alkyl secondary anilinium moieties101 have been used as guests for DB24C8-type wheels. Very recently, Loeb and co-workers reported another new recognition template between DB24C8 and a benzimidazolium axle with extended aromatic substituents to form a [2]pseudorotaxane and [2]rotaxanes (Scheme 8).102 The benzimidazolium salt [20-H][BF 4 ] complexed with DB24C8 to form [2]pseudorotaxane [21-H][BF4], in which the wheel adopted a C-shape conformation. The main interactions for the complexation contain N−H···O hydrogen bonding between benzimidazolium NH groups and DB24C8 ether oxygen atoms and N+···O ion−dipole interactions as well as π-stacking of the benzimidazolium rings with the catechol rings of DB24C8. Besides, there are four additional C−H···O interactions between DB24C8 oxygen atoms and aromatic hydrogens, suggesting the significant role of the addition of the aromatic substituents to the increase in the association strength between the axle and wheel. [2]Rotaxane [22-H][BF4] was prepared through condensation of a diamine with an aldehyde followed by catalytic oxidation. Deprotonation of [22-H][BF4] produced [2]rotaxane 22. Without ion−dipole interactions and significant driving force for π-stacking, the DB24C8 mocrocycle in [2]rotaxane 22 adopts a more open conformation and shows a rapid shuttling movement between the two equivalent recognition sites on the ends of the rigid axle. Besides the introduction of functional groups on rotaxanes, the interest in DB24C8-based rotaxanes also comes from the development of synthetic methods and improvement of the reaction conditions. Hirose and co-workers reported the synthesis of a 24C8-based [2]rotaxane via aminolysis of prerotaxane compounds (Scheme 9).103 The intermediate compound 24 was obtained through esterification of a phenolic crown ether with an acid chloride. Subsequent aminolysis with

Scheme 9. Synthesis of [2]Rotaxane 25 via Aminolysis of Prerotaxane 24 and Chemical Structure of 26

an amine having a bulky group in C6H6 gave [2]rotaxane 25 in 82% yield. This reaction also had a high [2]rotaxane selectivity without the possible compound 26 (Scheme 9), which might result from the participation of the crown ether ring for the acceleration of aminolysis. It was confirmed that the substituents on the axle unit of the prerotaxane exerted a significant effect on the rotaxane selectivity. To improve the reaction conditions, a solid-state reaction was tried to prepare rotaxanes and proved to be successful.104 By adopting a capping method and choosing a suitable stoppering reaction, DB24C8-based rotaxanes were obtained via an efficient solvent-free approach. Chiu and co-workers reported a solid-state ball-milling reaction to prepare both [2]and [4]rotaxanes in high yields. As described in Scheme 10, concentrating a solution of DB24C8 and trisammonium salt 27-H3·3PF6 in CH3CN afforded a solid. Upon mixing with diamine 28, the corresponding [4]rotaxane 29-H3·3PF6 was isolated in 78% yield. This approach was convenient, waste-free, and complementary to the rotaxane preparation methods in solution. H


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Scheme 10. Solid-State Synthesis of [4]Rotaxane 29-H3·3PF6a

a

Reproduced with permission from ref 104. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

2.1.3. Benzo-21-crown-7. Huang and co-workers demonstrated that secondary dialkylammonium salts thread the cavity of benzo-21-crown-7 (B21C7) (Scheme 11).105 The related binding constants (527−1062 M−1 in acetone) were higher than the corresponding values (135−261 M−1 in acetone) of the analogous complexes with the traditionally used host DB24C8. On the basis of this B21C7/secondary dialkylammonium salt recognition motif, [2]rotaxane 32·PF6 with two small

phenyl groups as the stoppers was successfully prepared in 74% yield. It was confirmed by X-ray crystallographic analysis that all four N-methylene hydrogens and two N-H hydrogens of the dumbbell-shaped component 30·PF6 were involved in nine hydrogen bonds (Scheme 11) with the oxygen atoms of B21C7, indicating the good size fit between the host and guest. Furthermore, pyrido-21-crown-7 (P21C7) was synthesized and shown to form [2]pseudorotaxanes spontaneously with secondary dialkylammonium ions.106 The association strengths of these complexes are stronger than those of their B21C7 counterparts and much stronger than those of their DB24C8 counterparts. With the similar 21-crown-7 (21C7) macrocycle, a smaller [2]rotaxane was obtained in high yield (81%) by ballmilling of a [2]pseudorotaxane formed from dipropargylammonium tetrafluoroborate and 21C7 on SiO2 with 1,2,4,5tetrazine.107 Besides, a series of [2]rotaxanes were prepared very recently by encircling 20-, 21-, and 22-crown ethers onto the dibenzylammonium dumbbell.108 On the basis of the crown ether/dibenzylammonium host−guest recognition motif and using trifluoromethyl as stopper, the smallest [2]rotaxane reported so far containing a 20-member-crown ether macrocycle was synthesized.109 As described above, a phenyl group can not thread the cavity of B21C7, but it can easily enter the cavity of DB24C8.80 On the basis of this literature knowledge, Schalley and co-workers designed and synthesized a four-component self-sorting system consisting of B21C7, DB24C8, 30·PF6, and 33·PF6 (Scheme 12).110 It was confirmed that B21C7 forms [2]pseudorotaxane 35·PF6 with guest 30·PF6, because a phenyl group could not pass through the cavity of B21C7 under the experimental

Scheme 11. Synthetic Route to and X-ray Crystal Structure of [2]Rotaxane 32·PF6 Containing B21C7a

a Hydrogen-bond parameters: H···O distances (Å), C(N)−H···O angles (deg) A, 2.48, 152; B, 2.48, 122; C, 2.43, 150; D, 2.56, 144; E, 2.72, 144; F, 2.50, 129; G, 2.04, 154; H, 2.11, 140; I, 2.33, 139. Reproduced with permission from ref 105. Copyright 2007 American Chemical Society.

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Scheme 12. A Four-Component Self-Sorting System Constructed from an Equimolar Mixture of B21C7, DB24C8, 30·PF6, and 33·PF6

Scheme 13. Formation of Sequence-Specific Hetero[3]rotaxane 38·2PF6

workers designed and prepared a series of cryptands based on bis(m-phenylene)-32-crown-10 (BMP32C10) (Figure 5, 39),117−125 which was a type of bicyclic crown ethers containing two 1,3,5-phenylene units linked by three bridges. They not only offer much better binding affinities than the corresponding simple crown ethers, but also can be used to form pseudorotaxanes and rotaxanes with various small organic guests such as paraquat, bisparaquat, diquat, diazapyrenium, monopyridinium, bispyridium, trispyridinium, and imidazolium salts (Figure 5).117−125 The significant improvement in the complexation of crown ether-based cryptands is a result of the combination of the preorganization of the cryptand hosts and the introduction of additional and optimized binding sites. For example, the pyridyl ester cryptand 39b, which has a pyridyl nitrogen atom located at a site occupied by either a water molecule or a PF6 anion in analogous complexes, exhibited the highest association constant Ka = 5.0 × 106 M−1 with paraquat in acetone, which is ∼9000 times higher than the corresponding simple crown ether system.126 Furthermore, a supramolecular cryptand containing BMP32C10 diol, with the assistance of a trifluoroacetate anion, displayed a 6.8-fold increase in Ka.127 The following studies showed that a pseudorotaxane formed from a supramolecular cryptand that resulted from the interaction of the CH2OH moieties with a water molecule, increasing the number of N− H···O hydrogen bonds between the host and guest.128−130 Although usually the BMP32C10-based cryptand/paraquat complexes show 1:1 stoichiometry in solution, the gaseous state, and the solid state based on 1H NMR, mass spectral characterization, and X-ray analysis, complexes of 2:1

conditions, while DB24C8 thermodynamically preferred 33·PF6 over 30·PF6, which resulted in the formation of [2]pseudorotaxane 34·PF6. On the basis of the above self-sorting pattern, the sequence-specific hetero[3]pseudorotaxane 37· 2PF6 was expected to prevail in an equimolar solution of B21C7, DB24C8, and 36·2PF6 in noncompetitive solvents. Hetero[3]rotaxane 38·2PF6 (Scheme 13) then was synthesized by treating the hetero[3]pseudorotaxane 37·2PF6 with benzoic anhydride in the presence of tributyl phosphine as the catalyst in 70% yield. Subsequently, deriving from the above simple four-component self-sorting system, six ditopic key compounds were used to self-assemble multiply threaded complexes with high positional control.111,112 ESI mass spectrometry and, in particular, tandem mass spectrometric experiments were used to unravel the details of the complicated structures of these multiply threaded complexes. Considering the easy availability of B21C7 derivatives, their efficient binding ability for secondary dialkylammonium salts, and their smaller size making it easier to find stoppers for preparation of rotaxanes, various pseudorotaxanes and rotaxanes based on B21C7 were successfully prepared and used in the construction of supramolecular polymers with functions of environmental responsibility, gelation, and logic gates,113,114 which will be described in section 5.2. 2.1.4. Crown Ether-Based Cryptands. To construct rotaxanes more effectively, the improvement of association constants is important. For this purpose, crown ether-based cryptands have been explored.115 After the first dibenzo-30crown-10 (DB30C10)-based cryptand was reported by Stoddart and co-workers in 1985,116 Gibson, Huang, and coJ


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Figure 5. Chemical structures of BMP32C10-based cryptands and their guests.

Scheme 14. Synthesis of [2]Rotaxane 49·4PF6 and [3]Rotaxane 51·6PF6 Based on the Cryptand/Paraquat Recognition Motif

K


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Scheme 15. Schematic Illustration of the Formation of Linear Supramolecular Polymers by Self-Assembly of AA Monomers 52 and 53, Respectively, with BB Monomer 54a

a


Scheme 16. Synthesis of Cryptand-Based [2]Rotaxanes 58·2PF6 and 60·3PF6

cryptand 39a and paraquat in the solid state.131 On the other hand, bisparaquat guest 47 was also used in the preparation of a

stoichiometry do exist, which was evidenced by the crystal structure of a [3]pseudorotaxane-like complex based on L


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[3]pseudorotaxane with cryptand 39a, and the complexation appeared to be cooperative.132 Because of the strong BMP32C10-based cryptand/paraquat association, Huang and co-workers prepared a series of MIMs.133−135 For example, [2]rotaxane 49·4PF6133 and [3]rotaxane 51·6PF6134 (Scheme 14) were obtained with high yields (92% and 86%, respectively) by the capping strategy. Still on the basis of the BMP32C10-based cryptand/paraquat recognition motif, Gibson and co-workers designed and synthesized two biscryptand hosts 52 and 53, one bearing a terephthalic linkage and the other with a 1,10-ferrocenedicarboxylic linkage, which can be viewed as AA monomers (Scheme 15).136 A bisparaquat BB monomer 54 then self-assembled with these biscryptands, respectively, to produce AA-BB-type linear supramolecular polymers. These supramolecular polymers with relatively high molecular weights have well-defined pseudorotaxane structures, which provide a promising route to form rotaxane units by attachment of bulky stoppers to the ends of difunctional bisparaquat derivatives. Besides BMP32C10 and DB30C10, crown ether moieties such as bis(m-phenylene)-26-crown-8 (BMP26C8)137−139 and DB24C8140 were also incorporated into cryptands and supramolecular cryptands to form pseudorotaxanes and rotaxanes. For example, Liu and co-workers used pyridine cation derivatives to template the [2+3] clipping reaction between simple aldehyde 55 and amine precursor 56 through 6-fold imine bond formation (Scheme 16). When the template was dumbbell-shaped paraquat-containing 57, [2]rotaxane 58 was obtained;137 if the tricationic species 59 was used as the template, [2]rotaxane 60 formed.138 The driving forces for the formation of such [2]rotaxanes included π−π interactions, and multiple [C−H···O] and [C−H···N] hydrogen-bonding interactions. Triptycene-derived cylindrical macrotricyclic polyethers are cryptand analogues, which were developed by Chen and coworkers.141−148 Different from those macromonocycles and macrobicycles, the macrotricyclic hosts 61a−c contain one central cavity and two lateral circular cavities, which incorporate a rigid triptycene unit and link through four bridges (Figure 6).143 These specific topological features enhance their complexation abilities toward various substrates.149 It was found that host 61b forms stable 1:1 or 1:2 complexes in different complexation modes with different functional paraquat derivatives in solution and in the solid state, in which the two alkyl groups of the paraquat unit threaded the two lateral crown ether cavities of the host. Especially, the association constant for the 1:1 complexes reached 4.0 × 105 M−1 in a 1:1 CDCl3/ CD3CN solution. Besides paraquat derivatives, the cylindrical macrotricyclic host 61b also self-assembled with two dibenzylammonium salts to form a stable 1:2 [3]pseudorotaxane in solution and in the solid state, in which multiple hydrogen-bonding and π−π stacking interactions between the host and guest played an important role. Furthermore, a series of dendritic pseudorotaxanes was constructed, and the structures were studied in detail.150 When one triptycene unit in 61b was replaced by an anthracene unit, host 62 was obtained and it showed different binding properties (Figure 6). An interesting sandwich structure with the guest out of the cavity and positioned in the middle of two of the hosts in the solid state was made on the basis of the additional π−π stacking interactions between the anthracene unit and the guest molecules.151 Besides triptycene, pentiptycene derived bis(crown ether)s were also

Figure 6. Chemical structures of triptycene-derived crown ether hosts and X-ray structures of their related pseudorotaxanes. Reproduced with permission from refs 143 and 151. Copyright 2006 and 2007 American Chemical Society.

designed and synthesized, which showed good binding ability with tetracationic cyclobis(paraquat-p-phenylene).146,147 By using the capping method, potassium-ion-templated complexes containing host 61b and axle components with anthraquinone units were synthesized in yields of 78−81% (Scheme 17).152 Such complexes were also obtained by the slippage method. Because the potassium ions acted not only as templates during the stoppering reactions but also as nonslipping chocks to shrink the inner diameter of the wheel cavity, the deslipping behaviors of the complexes with different triazole stoppers by peeling off the potassium ions with 18crown-6 were further investigated. The results showed that complexes 64a and 64b could be destroyed, but under the same conditions the dumbbell and ring components of complex 65 remained interlocked. On the other hand, derived from the complexation between host 61b and dibenzylammonium salts, a [3]rotaxane and hence a linear main-chain poly[3]rotaxane were constructed by the efficient Cu+-catalyzed Huisgen 1,3dipolar cycloaddition.153 2.2. Cyclodextrins

Cyclodextrins (CDs) are a series of cyclic oligosaccharides normally consisting of six (α-CD), seven (β-CD), or eight (γCD) 1→4 α-linked D-(+)-glucopyranose units.154−187 Several other CDs are known including δ-cyclodextrin and ε-cyclodextrin (9 and 10 units, respectively). They are distinguished by the different ring sizes of homologous series (Figure 7). With a lot of hydroxyl groups, CDs are water-soluble and biocompatible. Their truncated funnel-shaped cavity endows them with M


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Scheme 17. Formation of Threaded Complexes 64a−c and 65 Based on Triptycene-Derived Cylindrical Host 61ba

a


formation of CD-based rotaxanes.155−158 In 1981, Ogino and co-workers reported the synthesis of a CD-based [2]rotaxane. 1 5 5 By coupling Co I I I complexes to [2]pseudorotaxanes consisting of α- or β-CD threaded by 1,10diaminodecane in DMSO, [2]rotaxanes were obtained in yields of 19% and 7%, respectively. Later, another two transition metal complexes RuIII and FeII units were adopted as stoppers to prepare rotaxanes in high yields.156,157 Ionic subunits between tetraphenylboron and ammounium salts represent another type of stoppers for the construction of CD-containing rotaxanes in aqueous solution.159,160 To enhance the stability of the threaded structures, covalent bonds are used to attach bulky units. In this regard, Kaifer and co-workers did pioneering work in 1991 through the snapping method.53 The ferrocene subunit was adopted as one stopper to prevent α-CD from dethreading and a carboxylic acid group attached at the other end of the chain to easily snap the [2]pseudorotaxane via standard amidation procedures. On the other hand, an aromatic nucleophilic substitution (SNAr) reaction between 2,4,6trinitrobenzenesulfonic acid sodium salt and diamino derivatives resulted in C−N bond formation to construct α-CD rotaxanes with trinitrophenyl groups as stoppers.161,162 Other ways to construct CD rotaxanes based on covalent bonds involve aqueous Suzuki coupling reactions,163 reactions of azobenzene diazonium salts with aqueous β-naphthol derivatives,164 reactions of isothiocyanate compounds with amino derivatives,165 and enzyme-catalyzed acylation.166 Satisfying the hydrophobic property and the appropriate diameter size, many linear compounds can be used as axles for the construction of CD-based rotaxanes, such as [R(CH2)nR′]m+ (m = 0−2, R or R′ = pyrazine, bipyridine, amino, or carboxylic groups),167−171 and paraquat or reduced paraquat derivatives.172 To change the optical properties or to utilize photoisomerization, fluorescent moieties including stilbene and tolan units,163,173 the cyanine group,174 and the tolidine diazonium salts175 have been encapsulated in the cavity of CDs to form rotaxanes. It was reported by Anderson and coworkers that the large macrocycle γ-CD simultaneously binds two π-systems with one stilbene and one cyanine dye to form [3]rotaxanes (Scheme 18).176 [2]Rotaxane γ-CD⊃68 was first prepared via Suzuki coupling of iodoterphenylenedicarboxylic acid 67 and stilbene diboronic acid 66 in the presence of γ-CD in 17% yield. This [2]rotaxane was confirmed to have a

Figure 7. (a) Chemical structure of α-CD, and (b) the cartoon representations of α-, β-, γ-CD with minimum internal diameters indicated.186

unique complexation ability in aqueous solution mainly through hydrophobic interactions, and/or other interactions such as hydrogen bonds. The excellent binding property makes them good candidates as wheel-like components for the construction of inclusion complexes. The most common strategy to synthesize CD-based rotaxanes is the capping approach by attaching stoppers to both ends of a linear axis molecule. Several conditions are required because the hydrophobic driving forces need an aqueous medium or highly polar solvent. First, the bulky substituents should be hydrophilic. Second, the pseudorotaxane-like structure should not be dissociated in aqueous media. Third, the coupling reactions should be carried out under mild conditions without high reaction temperatures and inert, anhydrous demand. Initially, coordination reactions with metal complexes as the stopper units were applied to the N


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[3]rotaxane γ-CD⊃(68)2 and hetero-[3]rotaxane γ-CD⊃(68· 70) were thus obtained in yields of 87% and 18%, respectively. Specifically, [3]rotaxane γ-CD⊃(68·70) was easier to synthesize than [2]rotaxane γ-CD⊃70 (in 11% yield). Recent studies found that other π-conjugated compounds containing anthracene,177 thiophene,178 tetrathiafulvalene (TTF),179 and oligoresorcinol moieties180 also seem to be good candidates for CD threading. For example, using Suzuki coupling, two β-CD-based [2]rotaxanes 73 and 74 containing anthracene177 and bithiophene threads,178 respectively, have been synthesized (Scheme 19). With the protection of β-CD rings, the fluorescence quenching and chemical attack on the anthracene moiety and the aggregation of dithiophene were prevented effectively. Bulky groups that act as stoppers for rotaxane formation play a role as important as the axle units containing the interaction sites. Harada and co-workers found that methyl-substituted pyridyl groups kinetically control the threading of α-CD onto axle molecules.181,182 One methyl group at the 2-position of the pyridinium part decreases the rate of complex formation as compared to the case with no substitution on the pyridyl unit. However, two methyl groups at the 3-,5-positions or the 2-,6positions of the pyridinium part prevent α-CD from passing. Takata and co-workers investigated size-complementary groups to end-cap the α-CD cavity (Scheme 20).183 It was found that 2-bromophenyl isocyanate but not phenyl- or 2-methylphenyl isocyanate acted as an end-cap group to react with [3]pseudorotaxane 75 to produce [3]rotaxane 76 in a high yield of 73%, which was sufficiently stable at room temperature. After heating at 100 °C in DMSO, the threaded structure 76 dissociated and both [2]rotaxane 77 and dumbbell-shaped molecule 78 formed. As the deslippage of 76 was stepwise and the deslippage rate of 77 was slower than that of 76, the [2]rotaxane 77 was isolated in 54% yield. Such a stepwise deslippage process has been rarely observed in other thermodynamic studies. Selective functionalization of CD rings provides the related rotaxanes with interesting physical properties, different motions, and additional host−guest interactions.165,184,185 The successful modification of CD-based rotaxanes with multivalent carbohydrate groups was reported as a typical example (Scheme

Scheme 18. Synthesis of γ-CD-Based [3]Rotaxane γCD⊃(68·70)

substantial cavity and then acted as a host to bind another linear compound such as 66 or cyanine dye 69. Homo-

Scheme 19. Synthesis of CD-Based [2]Rotaxanes Containing Anthracene and Bithiophene Units

O


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Scheme 20. Synthesis of the Size-Complementary α-CD-Based Rotaxanes

Scheme 21. Synthesis of the Carbohydrate Linked α-CD-Based Rotaxanes

21).185 The preparation of such interlocked molecules involved “click chemistry”. Capping 81 with N-propargyl lactosylamide gave [2]rotaxane 83 in 54% yield, and assembling of double molecules of 80 by bis-propargyl spacers 84 afforded [3]rotaxane 85 in 24% yield. Here, saccharidic ligands were introduced on rotaxanes both as biological recognition elements and as capping groups. Their ability to inhibit the binding of Arachis hypogaea agglutinin to asialofetuin was assessed by enzyme linked lectin assays. The inclusion complexes based on CDs have the ability to change the properties of the linear components inside the ring, such as their chemical stability, fluorescence efficiency, and solubility. Because of the capability of CDs on associating various guest molecules such as drugs, nucleic acids, proteins, and other biological substrates, CD-based rotaxanes have been successfully utilized in many biological fields such as fluorescence sensing, drug solubilization and delivery, controlled release, and so on.186 Furthermore, CDs are suitable macrocycles for the formation of polyrotaxanes. Such supra-

molecular polymers represent a large family, which is not included here.20,187 2.3. Cucurbiturils

Cucurbit[n]urils (CB[n]s), each of which consists of n glycoluril units with a hydrophobic cavity and two identical carbonyl-laced portals, are an important family of macrocyclic compounds prepared from the acid-catalyzed condensation reaction of glycoluril and formaldehyde (Figure 8).188 Similar to CDs, the hydrophobic interior of CBs provides a potential site for inclusion of hydrocarbon molecules. However, there also exist unique features for CBs. For example, the polar carbonyl groups at the portals allow them to bind ions and molecules through charge−dipole and hydrogen-bonding interactions. All of these make CBs attractive building blocks for the construction of interlocked molecules, in particular rotaxanes and polyrotaxanes.189 2.3.1. Cucurbit[6]uril. The cavity of cucurbit[5]uril is too small to be threaded by axle molecules. Cucurbit[6]uril (CB[6]) is the smallest member of this family useful in the P


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Figure 8. Structure and cartoon representation of CB[n].

contruction of pseudorotaxanes and rotaxanes. The initial examples of CB[6]-based [2]rotaxanes were constructed using a molecular bead and triazole or spermine as guest molecules.190−193 Spermine had high affinity (Ka = ∼107 M−1) toward CB[6], and its terminal amine groups are easily stoppered by bulky substituents (Scheme 22).191 It was revealed by the X-ray crystal structure of [2]rotaxane 88 that a CB[6] molecule was located tightly at the middle of the string by strong hydrogen bonding between the two inside protonated amine hydrogen atoms and the oxygen atoms at the CB[6] portals. On the basis of the above-described host−guest complexation, a lot of functional interlocked systems incorporating CB[6] were explored by Kim and co-workers and other groups. For example pseudorotaxane-terminated dendrimers were produced by attaching diaminobutane units at the terminals of commercially available polypropylimine dendrimers, and then threading of CB[6] onto the terminals (Figure 9).194 This investigation endowed these novel dendrimers with a mechanism for reversible encapsulation and release of guest molecules, which may find useful application in the field of drug delivery. In accordance with the recent studies, CB[6] also binds 1,4bis(alkylaminomethyl)benzene195 and bispyridinium ethylene derivatives.196 For example, the association constant for the 1:1 complexation between CB[6] and bispyridinium ethylene is (2.1 ± 0.2) × 106 M−1 in 0.05 M NaCl solution. A rotaxane derived from this complex was prepared in 38% yield (Scheme 23).196 Furthermore, a shuttling process of the wheel was observed in [2]rotaxane 91. CB[6] also associates with polymers to form polypseudorotaxanes or polyrotaxanes.197 If CB[6]-based pseudorotaxanes have two stoppers with coordination properties, they can further coordinate with metal ions to construct large, rigid coordination polymers, which will be discussed in detail in section 5.4.198 2.3.2. Cucurbit[7]uril. With larger cavity and portal sizes, cucurbit[7]uril (CB[7]) has higher association affinity toward

Figure 9. Pseudorotaxane-terminated dendrimer. Reproduced with permission from ref 194. Copyright 2001 Wiley-VCH GmbH & Co. KGaA, Weinheim.

larger guest molecules such as 2,6-bis(4,5-dihydro-1H-imidazol2-yl)naphthalene, protonated adamantanamine, dimethyl paraquat dication, ferrocene derivatives, triphenylmethane dyes, and bis(pyridinium)-1,4-xylylene derivatives, which can not be encapsulated in the cavity of CB[6] (Figure 10).199−204 For example, the binding constants of CB[7] for protonated adamantanamine in 50 mM NaO2CCD3-buffered D2O (pD 4.74) and ferrocene derivatives in water are as high as 1012 M−1. As one example of application, the exceptionally stable CB[7]− ferrocene pair serves as a replacement of the strong avidin− biotin system205 to immobilize a protein on a solid surface (Scheme 24).206 The allyloxyCB[7] was first synthesized and anchored on an alkanethiolate self-assembled monolayer on gold. Ferrocenemethylammonium units then were attached to glucose oxidase by EDC coupling. Thanks to strong association between the CB[7] unit and the ferrocene moiety, the immobilization of ferrocenylated proteins on the CB[7]anchored gold substrate was achieved and subsequently monitored by surface plasmon resonance (SPR). Recent research indicated that CB[7] recognized protonated oligoaniline 92 with high binding affinity (Ka > 2.2 × 106 M−1 in dilute aqueous acetic acid (5.9 mM HOAc, pH 3.5)).207 It was utilized to prepare [2]rotaxane 94 in 52% yield by reductive amination with aldehyde stopper 93 (Scheme 25). If the amount of stopper was increased, the yield of [2]rotaxane 94 could be raised to 85%. The radical cation of the threaded

Scheme 22. Synthesis of [2]Rotaxane 88 Based on the CB[6]/Spermine Recognition Motif

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Scheme 23. Formation of [2]Rotaxane 91 Based on Inclusion Complexation between CB[6] and a Bispyridinium Ethylene Derivative

Scheme 25. Synthesis of CB[7]-Based [2]Rotaxane 94

Figure 10. Typical guest molecules for CB[7].

CB[8] can bind two different guest molecules inside its cavity, including an electron-deficient molecule and an electron-rich molecule, to form a stable 1:1:1 ternary complex, derived from the enhanced charge-transfer (CT) interactions between the guest pair inside the hydrophobic cavity of CB[8] (Scheme 26).210,211

Scheme 24. Immobilization of a Protein on Gold Surface Based on the Ferrocene/CB[7] Interactiona

Scheme 26. Formation of [3]Pseudorotaxanes Based on CB[8]

a

The cylindrical cartoon represents CB[7]. Reproduced with permission from ref 206. Copyright 2007 American Chemical Society.

oligoaniline in the rotaxane is thermodynamically and kinetically stabilized by the presence of CB[7]. This implies that encapsulation of conjugated polymers in cucurbiturils can have more than just a passive insulating effect and can dramatically change the oxidation potential of the threaded π-system. 2.3.3. Cucurbit[8]uril. Different from CB[6] and CB[7], cucurbit[8]uril (CB[8]) exhibits remarkable host−guest properties, including the encapsulation of two aromatic guest molecules such as naphthalene derivatives inside the cavity to form stable 1:2 host−guest complexes.208 This makes CB[8] an excellent nanoreactor to improve chemical reactions such as stereospecific [2+2] cycloadditions.209 More interestingly,

Pseudorotaxanes based on CB[8] provide useful platforms for the design and synthesis of redox-driven molecular machines such as molecular loop locks, development of redox-controllable vesicles, and detection of biologically important molecules.212−214 For example, through host−guest interactions, CB[8] macrocycles were connected by a rigid linker containing an electron donor and an electron acceptor R


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thus it is impossible to synthesize rotaxanes with calix[4]arene as a wheel component. A pioneering calix[4]arene-containing rotaxane was constructed by Vögtle and co-workers in 1998 (Figure 11),218 in which two calix[4]arene units were employed as stoppers.219 Recently, the anion-templated synthesis of a [2]rotaxane with a wheel containing a calix[4]arene unit was reported by Beer and co-workers.220,221 The calix[4]arene precursor incorporated electron-rich hydroquinone groups to facilitate π−π stacking interactions with the electron-deficient pyridinium axle component. As a part of the wheel component and functionalized with terminal amine groups, compound 100 complexed with the axle 101·Cl and then underwent clipping condensation with an appropriate bis-acyl chloride derivative to form the final rotaxane 102·Cl in a yield of 11% (Scheme 29).221 Although calix[4]arene can not encircle an axle-like component in its cavity, its tetraurea derivatives are successfully used as the framework to fabricate more complicated multimacrocyclic compounds, which are helpful for further formation of 4-fold [2]rotaxanes 103 (Scheme 30).222−224 The preorganized heterodimers were first held together by a hydrogen-bonded belt between the urea groups to form socalled [2]pseudorotaxanes. Two types of [2]pseudorotaxanes were converted into rotaxanes either by attachment of bulky stoppers using the capping strategy via Diels−Alder reactions (103a),223 or by fourfold ring closing metathesis using the clipping strategy (103b).224 2.4.2. Calix[5]arenes and Calix[6]arenes as Wheels. As compared to calix[4]arenes, the slightly larger calix[5]arenes are potential wheel candidates for pseudorotaxane or rotaxane formation. The solid-state structure of a calix[5]arene/nbutylammonium endo-cavity complex225 revealed that the nitrogen atom of the included guest molecule was surrounded by the oxygen atoms of the host. This might imply that calix[5]arenes were sufficiently flexible to allow for the inclusion of secondary alkylammonium cations. This hypothesis was proved to be correct by the formation of a calix[5]arene/ di-n-alkylammonium salt-based [2]pseudorotaxane (Scheme 31).226 The kinetics of assembly and disassembly of these interpenetrated supramolecules were controlled by the length of alkyl groups on the axle, whereas the extent of their formation and their thermodynamic stability depended predominantly on the counterion of the axle.

unit on each end with a proper angle, to form a cyclic oligomer, which was considered as a molecular necklace ∼3.7 nm in diameter and ∼1.8 nm in thickness (Scheme 27).215 Such selfassembly of topologically intriguing supramolecules may afford insight into the fabrication of nanoscale objects with welldefined structures and functions. Scheme 27. Formation of a Molecular Necklace Based on CB[8] and Its Crystal Structurea

a


A good example of the use of a CB[8] rotaxane to bind another guest has been described by Urbach et al.216 The axle moiety contained a viologen core to guide host threading and promote the selective binding of a second guest. After molecular recognition, a water-soluble [2]pseudorotaxane was formed and treated with a saturated aqueous solution of KPF6 to be transferred efficiently into an organic solvent (Scheme 28). Using a “click” reaction, tetraphenylmethane stoppers were capped to the inclusion complex, and [2]rotaxane 97 was obtained in 25% yield. Hence, water-soluble [2]rotaxane 98 was also synthesized, which bound the 2,6-dihydroxynaphthalene guest more efficiently. 2.4. Calixarenes

2.4.1. Calix[4]arenes as Linkers or Stoppers. Calix[n]arenes, which contain phenolic units bridged by methylene groups at meta-positions, have been widely used in supramolecular chemistry as platforms for the synthesis of artificial receptors due to their tunable size, versatility of derivatization (both at the wide and at the narrow rims), and availability.217 However, as compared to crown ethers and cyclodextrins, they have received less attention as building blocks for the construction of interlocked supermolecules. For the calix[4]arene, the cavity is too small to be threaded by a linear guest; Scheme 28. Synthesis of CB[8]-Based [2]Rotaxanes

S


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Figure 11. Chemical structure of [2]rotaxane 99 with calix[4]arene stoppers.

Besides paraquat derivatives, calix[6]arene derivatives also form pseudorotaxanes with other cationic compounds.230,231 For example, the diazapyrenium-based axle threads into the cavity of triphenylureidocalix[6]arene in apolar solution.230 This self-assembled supramolecular species has three components, the wheel, the axle, and its counterions, that can mutually interact. The stoichiometry of the resulting supramolecular complex depends on the nature and concentration of the counterions. Recently, a head-to-tail bis-calix[6]arene having the structure of an oriented nanotube that is about 2.6 nm long and 1.6 nm wide was prepared and characterized (Scheme 33).232 This molecule acts as a wheel-type host and forms a pseudorotaxane with a paraquat derivative as the axle. 2.4.3. Heterocalix[n]arenes or Calix[n]heteroarenes. Heterocalixarenes, in which the methylene linkages of calixarenes between the aromatic units are replaced by heteroatoms such as sulfur, oxygen, and nitrogen atoms, represent a class of interesting host molecules for their peculiar supramolecular receptor properties.233−235 However, very few of them have been used as wheel-like components to construct MIMs, possibly due to the small cavities of the heterocalix[4]arenes or the flexible conformations of larger heterocalixarenes. A notable exception is the finding of Chen and co-workers (Scheme 34), who investigated [2]pseudorotaxanes and [2]rotaxanes based on triptycene-derived extended (hetero)calixarenes.236−241 By introduction of the rigid triptycene moiety and naphthyridine subunits, the new oxacalixarenes 113a and 113b showed not only large cavities but also fixed conformations in solution and the solid state. They both form 1:1 [2]pseudorotaxane-type complexes with paraquat derivatives containing different functional groups. Subsequently, a pair of isomeric [2]rotaxanes were synthesized by the capping method in yields of 56% and 25%, respectively. Compounds 113a and 113b represent the pioneering examples of oxacalixarenes acting as wheels for rotaxanes. Another novel example of a pseudorotaxane structure with an azacalix[8]pyridine as the wheel encapsulating an acetylidetetrasilver aggregate into its cavity was reported by Wang, Zhao, and co-workers (Scheme 35).242 Azacalixheteroaromatics,243,244 with nitrogen bridges adopting sp2 and/or sp3 electronic configurations, possess a good coordination ability to bind metal ions because of the strong electron-donating nature of N(R) bridging moieties.245 When the polymeric silver acetylide complex and silver triflate were mixed with azacalix[8]pyridine 114 in CH3OH/CH2Cl2, a pseudorotaxane structure formed.

Scheme 29. Formation of [2]Rotaxane 102·Cl with Calix[4]arene as Part of the Wheel

The calix[6]arene platform possesses an annulus large enough to allow a sufficiently bulky and enlongated guest to thread the ring. Early in 2000, Arduini and co-workers reported [2]pseudorotaxane- and [2]rotaxane-type threaded structures with a triphenylureidocalix[6]arene derivative as the wheel and paraquat derivatives as axles (Scheme 32).227 In this type of [2]pseudorotaxanes, it was found that the counteranions of the paraquat species played an important role in the formation of the complexes.228 The use of either hexafluorophosphate or tosylate salts for the dicationic threading species influenced the thermodynamic stability of the pseudorotaxanes and affected markedly the threading/dethreading kinetics. However, for the calix[6]arene/paraquat-based [2]rotaxanes, the strength of intercomponent interactions could be influenced by the length of the axle as well as the nature of the solvent.229 T


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Scheme 30. Formation of 4-Fold [2]Rotaxanes 103a

a

Reproduced with permission from refs 223 and 224. Copyright 2005 American Chemical Society and 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Scheme 31. Formation of a Calix[5]Arene/Di-n-alkylammonium Salt [2]Pseudorotaxanea

a


characteristics (Figure 12).256,257 First, they are highly symmetrical and rigid as compared to crown ethers and calixarenes, and this affords their selective binding to guests. Second, they are easier to functionalize with different substituents on the benzene rings than cucurbiturils. The symmetrical pillar architecture and electron-donating cavity of pillararenes are particularly intriguing, and endow them with some special host−guest properties258−260 and self-assembling applications.261−264

This structure represents a novel example of a cluster-centered organometallic rotaxane among the reported organometallic and hybrid organic−inorganic rotaxanes.246,247 2.5. Pillararenes

Pillararenes are macrocyclic molecules made up of hydroquinone units linked by methylene bridges at para positions.248−255 Although the composition of pillararenes is similar to that of calixarenes, they have different structural U


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Scheme 32. Formation of Calix[6]arene-Based [2]Rotaxane 108·2Bra

a


Scheme 33. Formation of a Pseudorotaxane Based on a Head-to-Tail Bis-calix[6]arene in the Solid Statea

a


2.5.1. Organic Solvent-Soluble Pillararenes. Pillar[5]arenes, which are composed of the electron-rich aromatic rings, exhibit interesting host−guest properties with electron-accepting molecules such as paraquat and pyridinium derivatives,248,265,266 imidazolium cations,267 bis(imidazolium) dications,268 secondary ammoniums,269,270 and n-octyltrimethylammonium hexafluorophosphate271,272 in organic media (Scheme 36). Li and co-workers investigated the binding modes and complexation stoichiometries for two types of guests in detail and found that pillar[5]arene formed 2:1 external host−guest complexes with N,N′-dialkyl-4,4′-bipyridiniums, while it formed 1:1 pseudorotaxane-type inclusion complexes with polymethylene [−(CH2)n−] linked bis(pyridinium) derivatives possessing appropriate chain lengths. On the basis of the interactions between pillar[5]arene and paraquat units, polyrotaxanes were successfully prepared with extremely high yields by capping the chain ends with adamantyl moieties.273 Very recently, Huang and co-workers reported some functional cavity-extended pillar[5]arenes containing electronrich naphthyl groups.274 It was indicated that by introduction of additional binding sites for guests with electron-deficient 4,4′-

bipyridinium units, the binding affinity could be enhanced efficiently. Besides, urea groups were also introduced into the pillar[5]arene scaffold to improve the binding ability of the host−guest systems.275,276 A bis-urea-functionalized pillar[5]arene formed [2]pseudorotaxanes with linear alkyl dicarboxylates in the highly polar solvent DMSO. The hydrogenbonding interactions between the bis-urea hydrogens and dicarboxylate oxygens played an important role in stabilizing the novel [2]pseudorotaxanes alongside C−H···π interactions. Along with charge-transfer interactions occurring between the electron-rich cavities of pillar[5]arenes and the encircled electron-deficient guest molecules, C−H···π interactions provide the main driving forces for the formation of [2]pseudorotaxane complexes, as confirmed by Huang and co-workers during the preparation of copillar[5]arenes (Figure 13).277 Interestingly, it was found that an n-hexane molecule was symmetrically included in the cavities of homopillar[5]arene 116 and copillar[5]arene 117 to form [2]pseudorotaxanes, driven by multiple C−H···π interactions. Such host−guest chemistry of pillar[5]arene derivatives with n-hexane has been used in the preparation of pseudorotaxV


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Scheme 34. Formation of [2]Rotaxanes 113a·2PF6 and 113b·2PF6 Based on Triptycene-Derived Oxacalixarenesa

a


Scheme 35. Formation of Cluster-Centered Organometallic [2]Pseudorotaxane 115 Based on an Azacalix[8]pyridine Wheela

a


concentration. Moreover, rodlike fibers were drawn from a high concentration solution and observed by SEM.282 Mainly based on C−H···π interactions, pillar[5]arene derivatives also form stable interpenetrated complexes with neutral bis(imidazole) guests.284 Stoddart and co-workers found that uncharged aliphatic amines are also encapsulated in the cavity of 1,4-dimethoxypillar[5]arene (DMP[5]). This binding motif was exploited to prepare [2]rotaxane 119 via a capping strategy using a condensation reaction between amine and aldehyde (Scheme 37).285 Another example of pillar[5]arene to bind aliphatic amines was recently reported by Wang and co-workers.286 In this way, bifunctional ureidopyrimidinones (UPy) were modified on the wheel, and pillar[5]arenebased supramolecular polypseudorotaxanes were constructed by quadruple hydrogen-bonding interactions. Although many guest molecules have been investigated for pillar[5]arenes, the pseudorotaxanes and rotaxanes based on pillar[6]arenes and pillar[7]arenes have rarely been explored. Huang and co-workers investigated the complexation properties of 1,4-diisobutoxypillar[6]arene and 1,4-bis(n-propoxy)-

Figure 12. (a) Structure of per-hydroxylated pillararenes and (b) cartoon representation of per-hydroxylated pillar[5]arene.

anes,278,279 [2]rotaxanes,280,281 and even polypseudorotaxanes.282,283 For example, an easily available copillararene monomer 118 was used as a building block to construct linear polypseudorotaxanes driven by quadruple C−H···π interactions (Figure 14).282 It was demonstrated by a combination of various techniques that the formation of this polypseudorotaxane was highly dependent on the temperature and monomer W


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Scheme 36. Structures of Pillar[5]arenes and Their Guest Moleculesa

a


chloroform, while the related pillar[5]arene and pillar[7]arene derivatives showed no or weak complexation with the same guest due to their smaller or bigger cavity sizes.256,257 The same group also investigated the host−guest complexation of perhydroxylated pillar[6]arene with a series of bipyridinium salts in solution.287 It was found that a [2]pseudorotaxane formed in acetone with an association constant of 2.2 × 102 M−1 between the per-hydroxylated pillar[6]arene and paraquat. Moreover, the complexation properties of pillar[n]arenes (n = 8−10) with n-octyltrimethylammonium were reported by Hou and coworkers.288 2.5.2. Water-Soluble Pillararenes. To investigate the binding property of pillararenes in aqueous media, watersoluble pillar[5]arenes have been prepared. In 2010, Ogoshi and co-workers synthesized a water-soluble pillar[5]arene containing 10 negative-charged carboxylate groups, which bound cationic paraquat to form a [2]pseudorotaxane in water.289 The binding behavior of the carboxylatopillar[5]arene with substituted 1,4-bis(pyridinium)butane derivatives was also investigated in aqueous phosphate buffer solution, and in some cases [2]pseudorotaxanes with large association constants (>105 M−1) formed.290 Subsequently, a water-soluble pillar[5]arene containing neutral amino groups was prepared by Hou and co-workers; it encapsulated linear diacids under neutral, alkaline, and acidic conditions and formed ion pair-stoppered [2]rotaxanes.291 Recently, a cationic pillar[5]arene 120 bearing trimethylammonium groups on both rims was synthesized by Huang and co-workers.292 It was revealed to bind sodium 1octanesulfonate in aqueous media, forming a [2]pseudorataxane mainly driven by hydrophobic and electrostatic interactions (Scheme 38). Water-soluble pillar[6]arene (WP[6]) 121 containing 12 negative-charged carboxylate groups was first prepared by Huang and co-workers.258 WP[6] strongly encapsulated organic pyridinium salt 122 in water, mainly driven by hydrophobic and electrostatic interactions (Scheme 39a). Without the host, the amphiphilic guest molecule 122 selfassembled into nanotubes in water. However, vesicles formed upon addition of WP[6] to form a [2]pseoudorotaxane. The reversible transformation between nanotubes and vesicles was easily controlled by changing the solution pH (Scheme 39b). Furthermore, by interacting with neutral guest 123, WP[6] was

Figure 13. Chemical structures of 116 and 117 and crystal structures of 116⊃n-C6H14 (a,c) and 117⊃n-C6H14 (b,d). Hydrogens are omitted for clarity; the atoms in macrocycles are red, oxygen atoms are green, and n-hexane molecules are blue. The purple dotted lines indicate C−H···π interactions. Reproduced with permission from ref 277. Copyright 2010 American Chemical Society.

Figure 14. Structure of copillar[5]arene 118 (left) and two views of the linear polypseudorotaxanes (right) in the solid state. Reproduced with permission from ref 282. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

pillar[6]arene with n-octyltrimethylammonium hexafluorophosphate and found that both formed 1:1 complexes in X


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Scheme 37. Formation of [2]Rotaxane 119 with DMP[5] as the Ring Componenta

a


Scheme 38. Formation of a [2]Pseudorotaxane from Water-Soluble Cationic Pillar[5]arene 120 and Sodium 1-Octanesulfonatea

a


cyclization of the ring to obtain the [2]rotaxane in 32% yield. The low yield might result from various conformations of the linear two-binding-site component in solution. Hence, different molecular motifs with π-electron-donating nature have been chosen as part of the dumbbell components of [2]rotaxanes, involving the p-xylyl unit, the 2,3,5trisubstituted indole unit, the tetrathiafulvalene (TTF) unit, the benzidine unit, and the 1,5-dioxynaphthalene (DNP) unit (Figure 15).295−297 The structural diversity of the rotaxanes was introduced by changing stoppers such as a benzylic alcohol, a hydrophilic or hydrophobic stopper, or a dendritic mesogen.298−300 By changing the interaction modes, the supramolecular architecture could accomplish the desired switching behavior upon electrochemical or chemical stimulation.301 Because the CBPQT4+ ring is easily attacked by reducing agents, bases, and nucleophiles, it is better to construct the rings at the last stage. In most cases, the clipping method is appropriate and effective for CBPQT4+-based rotaxane formation. Recently, the capping approach was also exploited to synthesize this type of bistable [2]rotaxane through click reactions. As described in Scheme 41, a CBPQT4+-based TTF/ DNP rotaxane was synthesized by this method.302 The first step

used in the pH-controlled reversible dispersion of multiwalled carbon nanotubes (MWNTs) in water (Scheme 39c). These new recognition motifs based on WP[6] have potential applications in many fields, including supramolecular polymers, nanoelectronics, sensors, and drug and gene delivery systems. 2.6. Tetracationic Cyclophanes

Broadly speaking, the term “cyclophane” means any macrocyclic molecule containing aromatic rings linked by aliphatic bridges. There are a lot of cyclophanes and cages that have been prepared up to now. One of the most famous cyclophanes is cyclobis(paraquat-p-phenylene) (CBPQT4+), prepared by the group of Stoddart.293 It is known as the “blue box” because of the violet blue color of the radical cation formed upon oneelectron reduction of many paraquat derivatives. This πelectron-deficient receptor binds a variety of π-electron-rich guests to construct MIMs. In 1991, a donor/acceptor [2]rotaxane based on CBPQT4+ was prepared through the clipping method (Scheme 40).294 At first, the bispyridinium dication reacted with 1,4-bis(bromomethyl)benzene to form a bipyridinium unit, which had a strong ability to bind hydroquinone sites on the dumbbell component. This supramolecular complex served as a template for the final Y


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Scheme 39. (a) Formation of [2]Pseudorotaxanes Based on Water-Soluble Pillar[6]arene 121 and Guest 122 or 123; (b) Schematic Representations of the Reversible Transformations between 122-Based Nanotubes and 121⊃122-Based Vesicles; and (c) pH-Responsive Dispersion of MWNTs in Water in the Presence of 121⊃123a

a


Scheme 40. Formation of a Donor/Acceptor [2]Rotaxane 126·4PF6 Based on CBPQT4+

Recently, Stoddart and co-workers demonstrated that paraquat radical cations form strong inclusion complexes with the tetracationic cyclophane in its reduced diradical dicationic state (CBPQT2(•+)) as a consequence of radical-pairing interactions.303 Subsequently, an unlikely [2]rotaxane 131· 6PF6 composed only of electron-deficient CBPQT4+ and paraquat units with no complementary electron-rich components has been prepared by radical templation.304 As illustrated in Scheme 42, CBPQT4+ initially had no interactions with paraquat derivative 1302+. By using visible light to activate the well-known [Ru(bpy)3]2+ reducing system (bpy = 2,2′bipyridine), the bipyridinium units in compounds 1302+ and CBPQT4+ were highly efficiently reduced through photoinduced charge transfer. Thus, a [2]pseudorotaxane formed, driven by radical dimer interactions. During the process, triethanolamine, which was employed as the sacrificial electron donor, prevented back electron transfer from the bipyridinium radical cation to the [Ru(bpy)3]3+ species. Synthesis of [2]rotaxane 131·6PF6 was achieved by the capping strategy using a copper-free azide−alkyne 1,3-dipolar cycloaddition with a yield of 35%. This synthetic strategy provides a good method for formation of MIMs that have little or no interaction between host and guest components in their ground states.

Figure 15. Building units used for the dumbbell components of [2]rotaxanes based on CBPQT4+.

was the formation of pseudorotaxane 127·4PF6, wherein the πelectron-deficient CBPQT4+ ring threaded onto a linear molecule containing two π-electron-rich recognition units. The terminal azide groups of 127·4PF6 underwent a Cu(I)catalyzed Huisgen 1,3-dipolar cycloaddition with the alkyne compound 128, finally forming rotaxane 129·4PF6 incorporating 1,2,3-triazole units in 60% yield. It provided a new effective strategy for the preparation of switchable donor/acceptor [2]rotaxanes and expanded their applications. Z


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Scheme 41. Synthesis of a CBPQT4+-Based [2]Rotaxane 129·4PF6 Using Click Chemistry

Scheme 42. Radically Promoted Synthesis of a CBPQT4+-Based [2]Rotaxane 131·6PF6a

a

The PF6− counterions were omitted for clarity.

1324+ was generated by Sessler and co-workers.305 With a facile synthetic process in high yield, this macrocycle proved to bind

Along with the rapid development of the chemistry on the CBPQT4+ macrocycle, a larger tetraimidazolium macrocycle AA


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Scheme 43. Schematic Representation of the Formation of Pseudorotaxanes Based on the Tetraimidazolium Macrocyclea

a


Scheme 44. Formation of Tetralactam Macrocycle-Based [2]Rotaxanes 136a−f

macrocycle and formed pseudorotaxanes. On the basis of the

aromatic anionic species effectively. Several pseudorotaxane and rotaxane structures have been produced by using this molecular receptor.306 For example, the complexation of this cyclophane with 4,4′-biphenyldicarboxylic acid and its anions was investigated in detail (Scheme 43).307 It was revealed that the diacid did not show any interaction with the ring. However, both its monoanion and its dianion inserted into the

Job plot and NOESY NMR analyses, both complexes had a binding stoichiometry of 2:3 (host:guest), with the excess guest molecular anion “sandwiched” between two sets of individual 1:1 host−guest complexes. AB


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Scheme 45. High-Yielding Anionic Template Synthesis of Rotaxane 139 Based on Tetralactam Macrocycle 133

Scheme 46. Formation of Tetralactam Macrocycle 140 and Its [2]Rotaxane 142

2.7. Amide-Based Macrocycles

increase the template effect, the central part of the axle was changed from a meta-phenylene to a para-phenylene unit, fivemembered heterocyclic rings, sulfonyl groups, and so on. Hence, a series of [2]rotaxanes (Scheme 44),311 as well as a [3]rotaxane,312 were prepared in yields up to 41%. In addition to the diamide guests described above, the tetralactam macrocycles also exhibit strong affinities toward small inorganic anions as well as organic anions in nonpolar solvents. For example, on the basis of the interaction between the macrocycle and a phenoxide anion, a new anionic template

2.7.1. Tetralactam Macrocycles with Diarylmethane Motif. In the early 1990s, Vögtle and Hunter independently reported tetralactam macrocycles accompanied by the formation of catenanes.308,309 The isophthaloyl diamide groups and the diarylmethane motifs on the macrocycle provide multiple hydrogen-bonding sites and π−π interaction sites for recognition. Subsequently, on the basis of these interaction sites, the early amide-based [2]rotaxane formed by the capping strategy in 11% yield (Scheme 44).310 To improve the yield and AC


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hydrogen bonds formed between the amide groups in the fumaramide thread and the isophthalamide ring without any distortion; moreover, two DMF molecules were included as a linker for the host and guest by hydrogen-bonding interactions with the linear component and π-stacking with both phenyl stoppers and isophthaloyl rings. All of the above findings indicated that in a rigid linear backbone, multiple cooperative binding sites were fixed in an ideal spatial arrangement, which allowed even poor hydrogen-bond acceptors such as esters (Scheme 47, X = O) to be used to prepare such rotaxanes, although with a relatively low yield. On the basis of this powerful strategy, a variety of rotaxanes have been designed and used to fabricate molecular machines.30 Besides various amide and ester compounds, neutral squaraine units have also been employed as the threads to construct benzylic amide macrocycle-containing [2]rotaxanes.22 Very recently, the hydrogen-bond acceptors in the thread were extended to other ionic or mesomeric groups with significant hydrogen-bond basicity such as N+−O−, S+−O−, and P+− O−.319−321 For example, the bisnitrone thread 145 with the two oxygen atoms at an opposite orientation similar to those of the amide carbonyls in a dipeptide was utilized to synthesize [2]rotaxanes 146a−c (Scheme 48).319 When isophthaloyl

synthesis of rotaxanes was introduced with a surprisingly high yield of 95% (Scheme 45).313 In the first step, the phenolate stopper inclined to form a supramolecular complex 137 with macrocycle 133 via hydrogen bonding probably with the anion positioned on top of the wheel. The intermediate phenolate− wheel complex then acted as a supramolecular nucleophile to react with semiaxle 138 to furnish the [2]rotaxane 139. Further investigation of this template motif by Schalley and co-workers has revealed that recognition of the highly reactive phenoxide anion by this tetralactam macrocycle inhibits its nucleophilic reactivity. On the basis of this discovery, a [2]rotaxane was generated by amine condensation reactions with sterically bulky acyl chloride frameworks.314 2.7.2. Tetralactam Macrocycles with Benzylamide Motif. The smaller tetralactam macrocycles incorporating benzylamide units were subsequently developed by Leigh and co-workers. Initially, the condensation of p-xylylenediamine with isophthaloyl dichloride could only form the [2]catenane, whereas macrocycle 140 was intractable from a mixture of other precipitated cyclic oligomers and polymers.315 However, such problems were overcome through satisfying hydrogen-bonding requirements with other molecules (Scheme 46).316 A linear molecule 141 acted as a hydrogen-bonding template for the macrocycle, which led to the formation of [2]rotaxane 142 in the yield of 28%. This is a nice example in that the [2]rotaxane formation is a powerful means to alter the reaction product. The isolated [2]rotaxane was disassembled via hydrolysis of the ester groups to remove the large stoppers and release the desired macrocycle. Furthermore, if a dipeptide axle incorporating glycylglycine was used as a template, which possessed excellent complementary hydrogen-bonding sites to the tetralactam macrocycle, a higher yield (62%) was obtained.317 To improve the synthesis of hydrogen-bonding-mediated rotaxanes, preorganization of the hydrogen-bonding sites on the axle by incorporating a rigid spacer was a successful strategy, which showed a notable influence on template efficiency.318 For example, with the fumaramide motif as a good hydrogen-bond acceptor, a remarkable yield of 97% for [2]rotaxane 144a was obtained (Scheme 47). The crystal structure of this [2]rotaxane showed that four bifurcated

Scheme 48. Formation of Bisnitrone [2]Rotaxanes 146a−c

Scheme 47. Templated Synthetic Route to [2]Rotaxanes 144 and X-ray Crystal Structure of [2]Rotaxane 144aa dichloride was used, [2]rotaxane 146a formed in 70% yield. Under analogous conditions, replacing isophthaloyl dichloride with 2,6- or 3,5-pyridinedioyl dichloride afforded the corresponding [2]rotaxanes 146b and 146c in 21% and 40% yields, respectively. The solid-state structures of 146a−c showed that [2]rotaxanes 146b and 146c were driven by two sets of bifurcated hydrogen bonds between the 1,3-diamide groups of the macrocycle and the nitrone oxygen atoms, while the isophthalamide macrocycle-based [2]rotaxane 146a adopted a solid-state structure different from the other two rotaxanes. The amide-nitrone hydrogen bonds presented in these systems are significantly stronger than analogous amide− ester and even amide−amide hydrogen-bonding interactions as confirmed by dynamic 1H NMR experiments. This rotaxane architecture protects the thread from chemical reduction with an external reagent and increases the contribution of the C N+(R)−O− canonical form relative to that of the C−−N+(R) O form in comparison with simple nitrones.

a

Reproduced with permission from ref 318. Copyright 2001 American Chemical Society. AD


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Scheme 49. Synthesis of Phosphinamide, Thiophosphinamide, and Selenophospinamide [2]Rotaxanes and the X-ray Crystal Structure of One of Thema

a


After the investigation on the rotaxane formation based on nitrone319 and sulfoxides groups,320 some phosphinic, thiophosphinic, selenophosphinic amides [(PX) NHR; X = O, S, Se] and phosphine oxides were employed as hydrogen-bonding templates in the rotaxane formation.321 When one amide group of the classical glycylglycine template was replaced with a phosphinamide unit (X = O, Y = NH, Z = C), a [2]rotaxane was obtained in 60% yield (Scheme 49). The weaker intercomponent hydrogen bonding was consistent with the lower yields obtained in the synthesis of the PS and PSe rotaxanes. In the cases of two such groups in the thread, the bis(phosphinamide) rotaxane (X = O, Y = NH, Z = P) was isolated in 18% yield, while no rotaxane could be detected in the reactions involving the bis(thiophosphinamide) or bis(selenophosphinamide) threads (X = S or Se, Y = NH, Z = P). X-ray crystallography of these rotaxanes showed the presence of up to four intercomponent hydrogen bonds between the amide groups of the macrocycle and phophosphinamide, thiophosphinamide, or selenophosphinamide groups on the thread. Specially, there existed remarkable interactions for the phosphine oxide−phosphinamide rotaxane (X = O, Y1 = NH, Y2 = O, Z = C) in the solid state, featuring no direct intercomponent hydrogen bonds but rather a hydrogen-bond network involving water molecules that bridge the H-bonding groups of the macrocycle and thread through bifurcated hydrogen bonds (Scheme 49). The incorporation of phosphorus-based functional groups into rotaxanes may prove useful for the development of molecular shuttles in which the macrocycle can be used to hinder or expose ligating sites for metal-based catalysts. Recently, tetralactam macrocycle 140 was used to synthesize polyrotaxanes by Gibson and co-workers.322 At first, they attempted to incorporate macrocycle 133 into a side-chain polyrotaxane by reaction of tritylphenol with poly(vinybenzyl chloride) but failed apparently due to steric hindrance. On the basis of this macrocycle, they prepared three [2]rotaxanes. However, by clipping tetralactam 140 onto a preformed polyurethane in CH2Cl2, they got a single-phase polyrotaxane with a significantly lower glass transition temperature. 2.7.3. Other Lactam Macrocycles. Parallel to the work of Vögtle and Leigh, Beer and co-workers have developed a general anion template strategy based on lactam-containing macrocycle 147 (Scheme 50).323,324 It was designed to

Scheme 50. (a) Schematic Representation for the Formation of Anion-Template [2]Rotaxane; and (b) Chemical Structure of Macrocycles 147 and 148 and [2]Pseudorotaxane 149·Xa

a


incorporate an isophthalamide anion binding unit so as to saturate the halide anion’s coordination sphere of a pyridinium ion-pair (Scheme 50). In addition, the introduction of hydroquinone groups into the macrocyclic framework allows π−π stacking interactions with the pyridinium cation unit, and the other polyether linkages are expected to aid interpenetration of the thread by providing additional hydrogenbonding interactions. These structural features make it possible to efficiently construct pseudorotaxanes with cationic thread components including pyridinium nicotinamide, imidazolium, benzimidazolium, and guanidinium functionalities.325,326 Besides, the isophthalyl unit in the macrocycle can be replaced by another halide binding cleft such as the neutral rhenium(I) bipyridyl-containing precursor (see macrocycle 148 in Scheme 50).219 [2]Rotaxanes based on anion templates have usually been prepared by the clipping strategy via ring-closing metathesis using Grubbs’ catalyst (Scheme 51). For example, an orthogonal complex was formed after the macrocycle precursor AE


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Scheme 51. Formation of Chloride-Templated [2]Rotaxanes 152·Cl and 152·PF6a

a


Scheme 52. Anion-Templated Formation of [2]Rotaxane 155·Br Based on an Iodotriazolium Axle

150 and ion-pair 151·Cl were 1:1 mixed in solution.327 The two vinyl groups in 150 were drawn together by anion coordination. The [2]rotaxane 152·Cl was subsequently obtained in 47% yield through ring-closing metathesis reaction in CH2Cl2 with the aid of Grubbs’ catalyst. It was noteworthy that if the chloride anion was replaced by bromide, iodide, or hexafluorophosphate anion, no interlocked products could be obtained. From removal of the chloride anion, the hexafluorophosphate rotaxane 152·PF6 formed, leaving a unique threedimensional anion-binding domain. Halogen bonds forming between polarized halogen atoms are considered to be formally noncovalent interactions to integrate into the interlocked binding pockets. On the basis of this host−guest recognition, Beer and co-workers reported the use of halogen bonding to facilitate the chloride aniontemplated assembly of a 2-bromo-functionalized imidazoliumthreading pseudorotaxane.328 Subsequently, a [2]rotaxane was synthesized by using such a halide anion-templated method.329 It was established that iodotriazolium halide salts were capable

of forming anion-templated interpenetrative assemblies. The synthesis of the halogen-bonding [2]rotaxane 155·Br then was undertaken by reaction of bisvinyl-functionalized derivative 154 and iodotriazolium axle 153·Br with Grubbs’ II catalyst in 15% yield (Scheme 52). It was shown by the X-ray crystal structure of [2]rotaxane 155·Br that the bromide anion was coordinated by both the triazolium iodine atom and the amide hydrogens. The incorporation of the iodine atom into the cavity significantly enhanced the rotaxane’s anion-recognition properties in comparison with the hydrogen-bonding analogue, providing unusual selectivity for iodide. Amide-based macrocycles also bind neutral urea compounds330 and secondary ammonium salts331 through intermolecular CO···H−N hydrogen bonding. For example, Li and co-workers prepared a type of amide-based two-layered capsules by using dynamic covalent chemistry (DCC).331 The binding ability of these compounds toward bisammonium cations 157 and 158 was investigated. As described in Scheme 53, when mixing the host and guest molecules in 1:1 molar AF


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Scheme 53. Formation of Pseudorotaxanes 159a and 159b Based on Two-Layered Capsulea

a


Scheme 54. Formation of Metal-Templated [2]Rotaxane 163

design of new three-dimensional architectures for molecular encapsulation or catalysis.

ratio, the ammonium guests inserted into the cavity of the capsule and formed pseudorotaxane complexes, as confirmed by 1H NMR spectra, 2D NOESY spectra, and ESI mass spectra. The association constant (Ka) of the complex 159b was determined to be 590 M−1 in CDCl3 by fluorescent titration. These pseudorotaxanes with unique coconformations from the capsule and aliphatic bisammonium ions bode well for the

2.8. Coordination-Based Macrocycles

All of the above examples involving templated syntheses of rotaxanes are derived from hydrogen bonding, π−π stacking, hydrophobic, and/or charge-transfer interactions. In the development of MIMs, metal templated synthesis is also a very powerful strategy.332−340 Macrocycle 160 containing 1,10AG


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Scheme 55. Formation of Metal-Templated [3]Rotaxane 170 and [5]Rotaxane 171

Figure 16. Formation of Fe(II)-templated [3]rotaxane 175. Reproduced with permission from ref 341. Copyright 2008 American Chemical Society.

disubstituted phenanthroline ligands was designed by Gibson and co-workers (Scheme 54).332 Metal ions such as copper ion

serve as a bridge to connect the macrocycle and a linear molecule 161 also containing 1,10-disubstituted phenanthroline AH


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Scheme 56. CuAAC Active Template Synthesis of [2]Rotaxane 179 and [3]Rotaxane 180

unit together, resulting in the formation of a [2]pseudorotaxane 162. Further alkylation of the axle component with bulky stoppers led to the formation of a copper ion coordinated rotaxane.332,337 The template metal was selectively removed from the complex by the ion-exchange with Amberlite resin to afford the metal-free rotaxane 163 as the main product. Unthreaded coupling product 164 was also obtained. When the monochelating molecular thread was replaced by a bis-chelating molecule, more complicated interlocked molecules could be designed and constructed.338,339 In an early example, a metal-templated [3]rotaxane stoppered by porphyrin subunits was obtained by Sauvage and co-workers (Scheme 55).338 A compartmental tetracopper(I)-complexed [5]rotaxane also unexpectedly formed. At first, macrocycle 160 formed a 1:1 complex with Cu(I), followed by the complex-

ation with 0.5 equiv of bisphenanthroline thread 165 to prepare a binuclear [3]pseudorotaxane 166. This complex was subsequently reacted with a mixture of (3,3′-dihexyl-4,4′dimethyl-2,2′-dipyrryl)methane (167) and 3,5-di-tert-butylbenzaldehyde (168) in the presence of CF3CO2H. After purification, (Cu)2-[3]rotaxane 170, (Cu)4-[5]rotaxane 171, and a porphyrin derivative 169 were obtained in yields of 34%, 8%, and 32%, respectively. Furthermore, coordination chemistry of the (Cu)2-[3]rotaxane was investigated in detail. In addition to the copper(I) metal center, a three bidentate chelate-coordinated octahedral ruthenium(II) complex is also an attractive template for preparing rotaxanes with larger rings.340 Especially the [Ru(bpy)3]2+ family displays very interesting electro- and photochemical properties in relation to the construction of light-driven molecular machines. AI


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template reactions have been quickly developed, such as Cumediated alkyne homocoupling348 and Cu-catalyzed alkyne− alkyne heterocoupling.349 With these effective reactions, various hetero- or homocoupled rotaxanes with excellent selectivity and good isolated yields have been achieved. Furthermore, palladium-catalyzed cross-coupling reactions were also successfully extended to synthesize rotaxanes, which greatly improved the synthetic versatility of the active metal template method. After failure using Pd(0)-catalyzed reactions, attention was turned to Pd(II)-catalyzed homocoupling of alkynes, which was proved to be effective for the formation of rotaxanes.350 Besides, Pd(II)-mediated Michael additions have also been applied to prepare rotaxanes through this active metal template method.351 The approach allows ready access to rotaxanes, in which the intercomponent interactions can be designed to enable the construction of molecular shuttles and other molecular machine structures with the fastest possible dynamics.352 The Pd(II) oxidative Heck cross-coupling has recently been applied to the active metal template synthesis of [2]rotaxanes, which shows mild, efficient, and substrate tolerant properties (Scheme 57).353 The Pd(II) complex with the

The 2,9-diphenyl-1,10-phenanthroline (dpp) fragment, due to its extended rigid backbone, has played a particularly prominent role in the construction of MIMs based on metal templates. To decrease the steric constraints and fabricate complexes with more sophisticated topologies, more chelating agents were explored for metal-templated synthesis of rotaxanes. As described in Figure 16, the 8,8-diphenylsubstituted 3,3-biisoquinoline fragment (dpbiiq), with a distance of ∼11 Å between the phenyl-rings attached to the 3,3-biisoquinoline backbone, offers a much more open coordination site than the dpp ligand (whose distance between the phenyl rings is only ∼7 Å).341,342 This chelate was shown to form octahedral complexes with Fe(II) or Ru(II) in high yields. It was indicated that macrocycle 172 could be threaded by two string-like compounds containing the dpbiiq chelate. Thus, [3]rotaxane 175 was constructed in high yield using “click” chemistry. On the basis of metal templated syntheses, a wide range of rotaxanes have been constructed and utilized as molecular machine prototypes, because metal centers are often electroactive, allowing them to induce rearrangements via a metallocalized redox signal, and thus circumventing any potential difficulty associated with the generation of organic radicals.343 In addition, the structure of some transition metal complexes can be profoundly modified by changing the pH of the medium or by generating a dissociative excited state, thus allowing some parts of the compounds to be set in motion using a chemical signal or a photonic impulse. This part will be discussed in the following section. The introduction of the metal-template strategy by Sauvage represents a very useful synthetic route to prepare rotaxanes and other MIMs. Recently, Leigh and colleagues described a rotaxane-forming protocol in which the Cu(I) atom functions as a catalyst as well as a template and turns over during the reaction, quite different from the reported approaches that require a stoichiometric quantity of template.344 In this design, a permanent coordination site is only needed on the macrocycle; the rest of the interlocked compound is assembled through functional groups that react together under catalysis by the metal, which also serves to hold the fragments in position such that their catalyzed reaction leads to the desired interlocked product. The CuAAC reaction345 with mild reaction conditions and high yield acts as a promising candidate for testing the active template rotaxane concept.346,347 Taking one of Leigh’s studies as an example, simply stirring an equimolar mixture of the pyridine macrocycle 176a, alkyne 177, azide 178, and [Cu(CH3CN)4](PF6) and then demetalation with KCN successfully afforded [2]rotaxane 179 in high yields (Scheme 56).346 The structure of the pyridine-based macrocycle was systematically varied from the monodentate 176a to bidentate 176b. However, the use of tridentate macrocycles 176c failed to produce any rotaxane, probably because of the lack of the vacant coordination sites necessary to bind both azide and alkyne for the complexes. When the macrocycle 176a to Cu(I) molar ratio was varied from 1:1 to 10:1, the rate of reaction was considerably slowed, and [3]rotaxane 180 with two macrocycles and one triazole ring was surprisingly isolated. Such [3]rotaxane arose from a bimetallic intermediate with two macrocycles bound to both Cu(I) ions during the formation of one triazole ring. Following the CuAAC reaction, which has been successfully used for the active template rotaxane synthesis, other active

Scheme 57. Pd(II)-Template Synthesis of [2]Rotaxane 182

bidentate macrocycle 176b first formed, then was transmetalated with a boronic acid, followed by substitution of the second acetate ligand by an electron-poor alkene to generate complex 181−Pd(II). Migratory insertion of the alkene into the aryl (or alkenyl)−Pd bond resulted in new C−C bond formation through the macrocyclic cavity, capturing the threaded structure. β-Hydride elimination led to poorly coordinating Pd(0) and release of the metal-free rotaxane AJ


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Very recently, Stoddart and co-workers reported two series of oligorotaxanes (up to the [20]rotaxane) based on the reaction of 2,6-pyridinedicarboxaldehyde and tetra(ethylene glycol) bis(2-aminophenyl) ether with −CH2NH2+CH2− containing dumbbell components as templates by the clipping strategy.19 The −NH2+− recognition sites in the axle were separated by trimethylene bridges or p-phenylene linkers, respectively (Scheme 59a). The distance between two −NH2+− recognition sites in the trimethylene-linked series was designed to be 3.5 Å for facilitating efficient [π···π] stacking between contiguous arene units in the rings of the rotaxanes (Scheme 59b). As a result, a series of [n]rotaxanes up to [20]rotaxane were isolated in 88−98% yields. The efficiency of the higher order oligorotaxanes was improved by the positive cooperativity resulting from the designed extended [π···π] stacking interactions. Besides the above example of rotaxanes with multiple ring components, a large ring could also encircle several linear components to form another type of oligopseudorotaxane.86,366 In the following case, the dumbbell component was replaced by a 3-fold branched component to form a branched rotaxane.223,358−362 By using the click methodology, the DNP azide 192 reacted with tris-1,3,5(4′-ethynylphenyl)benzene 193 in the presence of CBPQT4+, leading to the formation of branched [4]rotaxane 194·12PF6 in 72% isolated yield (Scheme 60).358 Because the rings and binding sites in branches are identical, this type of rotaxane could also be described as a homorotaxane. If the three terminal components are designed as macrocyclic host molecules with large central cavity, three linear molecules can be threaded by such branched multiple rings to form another type of rotaxane. On the basis of complexation of a homotritopic tris(crown ether) and a polymeric paraquat derivative, a supramolecular triarm star polymer was obtained via a supramolecular coupling method by Gibson’s group (Scheme 61).72 The axle 196 was designed by introducing a paraquat moiety at the end of the polystyrene chain, while a BMP32C10 derivative was reacted with 1,3,5-benzenetricarbonyl trichloride to form a homotritopic host 195. [4]Pseudorotaxane 197 then was prepared with 1:3 stoichiometry in solution. This work provides a new method for preparing guest-terminated macromolecular building blocks, which are very valuable in the fabrication of well-defined macromolecular architectures by self-assembly. As compared to the synthesis of homorotaxanes, the preparation of heterorotaxanes is more challenging due to their structural complexity. Self-sorting binding is generally required for selective recognition.103,367 In a pioneering work, Li and co-workers reported a hetero[3]rotaxane comprising a tetralactam macrocycle and CBPQT4+ macrocycle as ring components, driven by hydrogen-bonding and donor−acceptor interactions, respectively.368 Another type of hetero[3]rotaxane with one stilbene and one cyanine dye as axle components both threading through the same γ-CD ring was synthesized by Anderson and co-workers.176 Wu and co-workers demonstrated the synthesis of a hetero[4]rotaxane by a “threadingstoppering-followed-by-clipping” approach.363 Very recently, an efficient one-pot “click” reaction was applied for quantitative emergence of hetero[4]rotaxanes (Scheme 62).364 By mixing bispropargyl derivative 199·2Cl, azide derivative 198·Cl, β-CD or γ-CD, and CB[6] in aqueous solution at 80 °C in the molar ratio of 1:2:1:2, hetero[4]rotaxanes 200a·4Cl and 200b·4Cl were obtained very rapidly and quantitatively (>99%). Such

182. In situ reoxidation of the metal with benzoquinone and O2 afforded the rotaxane in 73% yield using only 10 mol % Pd as the catalyst. 2.9. Synthesis of Oligorotaxanes

On the basis of various wheel-like molecules, a large number of [n]rotaxanes have been prepared by diversified strategies, among which oligorotaxanes have more complicated structures and possibly require more challenging synthetic routes. Generally there are two types of oligorotaxanes: homorotaxanes that consist of only one type of macrocycles19,354−357 or axles 138,223,358−362 and binding sites, and heterorotaxanes103,363−365 that have different rings and recognition sites on the dumbbell-like thread. The [3]rotaxane 5·2PF6 described in Scheme 4 is an example of homorotaxanes. These linear interlocked molecules with multiple rings threaded onto the same axle containing several binding sites were synthesized by the capping strategy. Recently, Leigh and co-workers reported a novel method for the synthesis of homorotaxanes with multiple rings threading onto a single-binding-site thread by the clipping strategy (Scheme 58).354 First, through deprotonation of the Scheme 58. Metal Template Synthesis of Oligorotaxanes with Multiple Ringsa

a

Reproduced with permission from ref 354. Copyright 2007 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

amide groups of a tridentate 2,6-pyridinedicarboxamide derivative, the metal atom Pd(II) was bound to the ligand to form complex 184, which interacted with the thread 183 and subsequently underwent macrocyclization by ring-closing metathesis (RCM) to generate a saturated palladium [2]rotaxane in 72% yield. After demetalation, the metal-free [2]rotaxane was obtained in 62% overall yield. This [2]rotaxane further underwent the process of complexation, macrocyclization, and demetalation to afford the [3]rotaxane in 79% overall yield. Furthermore, the [4]rotaxane was also obtained under the same conditions. By using this efficient and effective strategy, both the number and the order in which macrocycles are assembled onto a thread can be controlled with unprecedented precision. AK


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Scheme 59. (a) Synthesis of Oligorotaxanes with Multiple Rings Based on Hydrogen-Bonding Template; and (b) Cartoon Representation of [20]Rotaxane 189ga

a


[2]rotaxanes investigated until now are mostly caused by the different directions of guest insertion as described above.53,370 There are also very few examples resulting from the intrinsic isomeric property of the host itself.271,371 Pioneering work on isomeric [2]rotaxanes was reported by Kaifer in 1991,53 in which two isomeric rotaxanes differed in the orientations of the CD torus with respect to the different terminal groups on the axle component. On the basis of a similar principle, isomeric phenomena have also been observed both in [2]pseudorotaxanes and in [3]pseudorotaxanes incorporating α-CD.169 From a theoretical respective, there should be three isomers in [3]pseudorotaxanes or [3]rotaxanes with two CD-rings encircling in one axle bearing two different stoppers. However, an unexpected result was found in the case of a [3]rotaxane with the azo dye derivative threading into two α-CDs.372 This azo dye [3]rotaxane 206 formed as a single stereoisomer, with the wider rim of both cyclodextrins pointing outward (Figure 17). This high selectivity of directional isomers was described as unidirectional threading. A unidirectional [2]rotaxane 207 was synthesized by Anderson and co-workers in 2001 with a yield of 96% (Figure 17).373 A space-filling representation of the energy-minimized structure supported the experimental result that the wide rim of the hexakis(2,3,6-tri-Omethyl)-α-cyclodextrin (TM-α-CD) was over the naphthalene unit, whereas the narrow rim was over the para-phenylene unit. For further investigation, Tian and co-workers developed new methods for controlling the orientation of CDs when synthesizing a [2]rotaxane-based molecular shuttle (Scheme 64).374−376 The origin for the formation of single isomeric [2]rotaxane 211 was described as follows: when α-CD was mixed with the linear stilbene moiety 208, two [2]pseudorotaxane isomers 209a and 209b formed simultaneously after 50 min. However, isomer 209a disappeared almost completely after 36 h. It was indicated that the threading rates

CD-CB[6]-promoted azide−alkyne cycloadditions opened the real possibility of being able to synthesize high molecular weight polyrotaxanes. Recently in Liu’s group, a more complicated hetero[7]rotaxane with twin axles was prepared using B21C7 and BPP34C10 as the wheel-like components (Scheme 63).365 First, a four-component self-assembly system involving two macrocyclic polyethers, BPP34C10 and B21C7, and two secondary ammonium compounds, 201·PF6 and 202·PF6, was designed. After mixing all compounds, [2]pseudorotaxane 203· PF6 and [3]pseudorotaxane 204·2PF6 with twin axles formed exclusively. Hetero[7]rotaxane 205·10PF6 then was obtained in 42% yield through CuAAC “click” reaction, in which four B21C7 rings were stoppered by the outer phenyl groups, while the central BPP34C10 ring was stoppered by the B21C7 rings. This strategy provides a good methodology for preparing more complicated functional MIMs and allows their precise positional control in the structure. 2.10. Isomeric Rotaxanes

Isomeric phenomena are a very interesting and significant topic in the area of supramolecular chemistry,369 as is so in the field of rotaxanes. In the field of pseudorotaxanes and rotaxanes, the cyclic components employed as wheels involve two types: one is symmetrical, such as crown ethers, cucurbiturils, and pillararenes; the other is nonsymmetrical, such as cyclodextrins and calix[6]arenes. In the case of symmetrical macrocycles, if the two bulky stoppers of a rotaxane are different, it may display isomeric phenomenon, which results in an unsymmetrical structure and leads to doubling of some of the 1H NMR signals.322 When a nonsymmetrical macrocycle is used as the wheel, an axle with different terminal groups can insert into the macrocycle from both rims, thus leading to two isomeric [2]pseudorotaxanes or [2]rotaxanes. Isomeric phenomena on AL


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Scheme 60. Synthesis of Branched [4]Rotaxane 194·12PF6 Using Click Methodology

on the ring component facilitated the separation of anions from ion pairs. Therefore, the cationic portions of the axles threaded almost exclusively from the upper rim. Thus, isomeric [2]pseudorotaxanes 214a and 214b were yielded in independent ways. By stoppering the other alcoholic OH group with different bulky units, [2]rotaxanes 215a and 215b were subsequently produced through the snapping method (Scheme 2). Furthermore, with the same host system, to evidence possible effects of the structural features of the axles on the threading direction into the wheel and to implement the straightforward synthesis of orientational rotaxane isomers, a family of nonsymmetrical axles with a stopper at one side and the other side with either an ammonium, a hydroxy, or a methyl headgroup were designed and synthesized by Arduini and coworkers.378 A detailed thermodynamic and kinetic study was carried out and revealed that all axles threaded the wheel from the upper rim for kinetic reasons at room temperature; conversely, the axle with an ammonium headgroup was capable of entering the cavity from the lower rim at 340 K under thermodynamic control. Besides CD-induced or calix[6]arene-induced isomeric pseudorotaxanes, a DB24C8-based cryptand also induced different spatial arrangements of host−guest complexes, as reported by Huang, Gibson, and co-workers.379 In this case,

of stilbene unit through the wide rim and through the narrow rim were not much different, but the dethreading rate of the former [2]pseudorotaxane 209a was much faster than that of the latter, giving mostly the thermodynamically stable isomer 209b. Finally, the resulting [2]pseudorotaxane 209b underwent Suzuki coupling with the boronic acid 210 to give the unidirectional [2]rotaxane 211. Not only nonsymmetrical cyclodextrins, but nonsymmetrical calix[6]arene derivatives, with a truncated cone structure, have been used to construct unidirectional [2]pseudorotaxanes and [2]rotaxanes by Arduini and co-workers.232,370,377,378 In 2003, unidirectional [2]pseudorotaxanes based on triphenylureidocalix[6]arene were initially reported.370 It was shown that the nonsymmetrical axle accessed the calixarene cavity only through the wider rim. On the basis of this discovery, unidirectional [2]rotaxanes 215a and 215b, described as isomeric compounds, could be highly selectively synthesized, respectively (Scheme 65).377 Axles 213a and 213b incorporated the paraquat unit as interactional sites, one stopper at one side (213a, diphenylacetyl unit; 213b, dicyclohexylacetyl unit) and an alcoholic OH group for further stopping at the other side. The axles existed as tight ion pairs, so it was difficult for them to thread into the cavity of calix[6]arene during the initial process of pseudorotaxane formation. The hydrogen-bonding ability of ureido NH groups AM


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Scheme 63. Preparation of a Hetero[7]rotaxane 205·10PF6a

Scheme 61. Schematic Illustration of the Formation of a Supramolecular Triarm Star Polymera

a


Scheme 62. Synthesis of Hetero[4]rotaxanes 200a·4Cl and 200b·4Cl by One-Pot “Click” Reaction

a


In the literature, isomeric phenomena of [2]rotaxanes are mostly caused by the direction of guest insertion. When the guest is symmetrical, can isomeric [2]rotaxanes still form by the effect of the host? This is also an interesting question to explore. Recently, Chen and co-workers designed and synthesized a series of squaraine-based [2]pseudorotaxanes and [2]rotaxanes incorporating triptycene-based tetralactam macrocycles.380 In particular, isomeric [2]pseudorotaxanes and [2]rotaxanes were obtained from both the linking modes of triptycene derivatives and the directions of the guest insertion (Scheme 67). Consequently, in the case of [2]rotaxanes 222, three isomers were simultaneously obtained from one reaction. It provides a new route to form isomeric [2]rotaxanes through the template synthetic process. Sequence isomerism in rotaxanes also exists when the nonsymmetrical thread is encircled by constitutionally different

how the guests threaded into the cavity of the host was determined by the terminal N-substituents. As seen from Scheme 66, cryptand 216 formed 1:1 inclusion complexes 218a−c with three paraquat derivatives 217a,b and 40. Their association constants determined in acetone by using a UV−vis titration method were 1.0 × 105 M−1 for 218a, 1.2 × 105 M−1 for 218b, and 5.0 × 103 M−1 for 218c, respectively. It was confirmed by X-ray crystal structures that methyl-substituted paraquat derivatives 217a and 217b bound cryptand with similar T-type inclusion complexation conformations, while the nonmethyl-substituted paraquat derivative 40 bound the cryptand with the pseudorotaxane-type complexation conformation. Therefore, the conformation of the host−guest complex could be controlled by simple substitution. AN


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nonsymmetrically with a pyridine group on one side and an alkane chain on the other side, two different macrocycles could be positioned on the thread with a special sequence, which was determined by the structure of the starting [2]rotaxane. When [2]rotaxane 223a was used, the ring containing an isopropyl ether on the 4-position of the pyridine group would be finally located at the pyridine side of the axle, while [3]rotaxane 225b with the contrary sequence of the rings was produced if the starting material was [2]rotaxane 223b. The rings in such [3]rotaxanes were sufficiently small and rigid that the sequence was maintained mechanically. This synthetic strategy can be extended to add multiple different rings to a rotaxane thread in any desired order, and thus enable the synthesis of single isomeric rotaxane of higher order with a predefined macrocycle sequence. 2.11. Rotaxane-Related Host−Guest Complexes Based on Molecular Tweezers and Clips

Figure 17. Representation of azo dye [3]rotaxane 206 and [2]rotaxane 207 as single stereoisomers.

Before rotaxane formation, there exist two types of supramolecular complexes considered as intermediates driven by noncovalent intermolecular interactions (Scheme 69). Through the capping or snapping methodology, a pseudorotaxane complex is initially and necessarily afforded, the coconformations and movements of which have already been widely investigated. When the clipping method is involved, the supramolecular intermediate forms with a tweezer- or clipping-like component encircling a dumbbell-like component, which we refer to as the clipping complex. Such complex also shows its unique conformation and association/dissociation or shuttling motion under external stimuli. Although various molecular clips and tweezers and their host−guest chemistry have been reported so far,382−384 only a few investigations have been focused on clipping complexes containing dumbbell-like guests.385 To study the synergistic effects generated by multiple arene− arene interactions, Klärner and co-workers designed and synthesized a series of tetramethylene-bridged molecular tweezers 226 and 227, trimethylene-bridged molecular clip 228, and dimethylene-bridged clips 229−232 (Figure 18).385 All of these structures exploit multiple π-stacking interactions in a positively cooperative manner, and demonstrate selective binding of cationic or neutral guests that bear acceptor groups. The tweezers 227 and clips 228−232 bind aromatic guest molecules preferentially via CH−π and π−π interactions, whereas tweezers 226 prefer to bind alkane chains of aliphatic guest molecules inside their cavities via CH−π interactions.386,387 Because of their belt-shape structures and energy requirement for bond angle distortions, these molecules are well preorganized but present small flexibility in tuning the cavity sizes, which endow the host−guest complexes with particularly dynamic properties. Molecular tweezer 227b, comprising one naphthalene and four benzene components bridged by four methylene units, forms a stable 1:1 complex with dendrimers 233a−d containing a bipyridinium core, both in solution and in the gas phase caused by charge-transfer interactions (Scheme 70).388,389 Such host−guest formation caused the quenching of the tweezer fluorescence. The association constants measured by fluorescence titration experiments in dichloromethane or a mixture of dichloromethane/acetonitrile were of the order of 104 M−1, which decreased dramatically in polar acetone solution. Besides, increasing dendrimer generation also resulted in a decrease of association constants. Cyclic voltammetry experiments in-

Scheme 64. Formation of Unidirectional CD-Based [2]Rotaxane 211

macrocycles.103,354 Leigh and co-workers recently reported such a pair of [3]rotaxane sequence isomers (Scheme 68).381 On the basis of the strategy described in Scheme 58,354 [3]rotaxanes 225a and 225b with two different macrocycles and one nonsymmetrical thread were obtained under the process of coordination, macrocyclization, hydrogenation, and demetalation. Because the dumbbell component was designed AO


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Scheme 65. Formation of Unidirectional Calix[6]arene-Based [2]Rotaxanes 215a and 215ba

a

(i) Toluene, reflux; (ii) Cy2CHCOCl, toluene; (iii) Ph2CHCOCl, toluene. Cy = cyclohexyl.

Scheme 66. Chemical Structures of Cryptand 216 and Paraquat Derivatives 40 and 217a,b and Cartoon Representations of Their Host−Guest Complexes 218a−ca

a


dicated that such complexes could be reversibly assembled/ disassembled by electrochemical stimulation. Further investigation concluded that shuttling of the tweezer from one pyridinium ring to another happened rapidly (ΔG⧧ < 10 kcal/ mol).390 Furthermore, the addition of Bu4NPF6 to the solutions of the complex in low-polar solvents caused an unfolding

change in dendrimer conformation, which stabilized the complex with higher association constants.391 Besides polyaromatic tweezers, TTF-side-walled molecular clip 235 was designed and allowed the shuttling motion of a pH-controllable [2]rotaxane 236 to be monitored by the naked eye (Scheme 71).392 By mixing solutions of colorless [2]rotaxane 236·2PF6 and yellow molecular clip 235, the AP


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Scheme 67. Formation of Isomeric [2]Rotaxanes from Both the Linking Modes of Triptycene Derivatives and the Directions of the Guest Insertiona

a


bonding interactions to create a stable species in solution. While chemically interesting in their own right, pseudorotaxanes are dynamic species and may dissociate when subjected to changes of solvent and/or temperature, or addition of appropriate external stimuli, such as chemical, electrochemical, and/or photochemical stimuli. The stimuli-responsiveness of the association/dissociation mobility of pseudorotaxanes will be summarized in detail. 3.1.1. Solvent Effects on Association Constants. The solvent effects in host−guest chemistry can hardly be neglected. The solvation interactions always participate in competition with the complexation between host and guest molecules in solution.393−395 Hydrogen bonding is considered as a powerful driving force to form pseudorotaxane-type complexes. However, polar solvents, particularly water, can affect hydrogenbonding interactions to a large degree.395 The increase of Ka values for this type of complexes depends on the decrease of the Gutmann donor numbers of the solvents. For example, DB24C8 forms supramolecular complexes with dialkylammonium salts in a number of different solvents.80 In CHCl3, the Ka value is the highest, whereas no complex formation is observed in DMSO. On the basis of the above phenomenon, most pseudorotaxane complexations driven by hydrogen bonding have been investigated in nonpolar solvents.286,316 In conclusion, it is essentially meaningless to discuss the magnitude of binding constants without mention of solvents and impossible to compare binding constants or even relative affinities across different solvent media.

solution turned green, suggesting the formation of [2]rotaxane/ clip complex (235·236-H)3PF6 with a charge-transfer band (λ = 709 nm). The association constant (Ka) in CD3CN/CDCl3 (5:1) was determined to be 4100 ± 400 M−1, based on a 1H NMR spectroscopic dilution experiment. This complex could be dissociated by addition of Et3N and was reformed upon introduction of CF3CO2H. Without the clip molecule, the switching of the macrocyclic unit between the NH2+ unit and the bipyridinium moiety also happened by tuning the pH of the solution with little color change. It was concluded that such a molecular clip provided a visible way for monitoring the switchable process.

3. STIMULI-RESPONSIVE MOLECULAR MOBILITY OF PSEUDOROTAXANES A pseudorotaxane is a supramolecular system composed of a thread-like species inserted through the cavity of a macrocycle. Because there are no stoppers at the ends of the thread, dissociation of the complex is easy in solution and the pseudorotaxane is always equilibrated with the “free” molecular components (Scheme 72). In addition to the main association/ dissociation mobility, shuttling of the wheel along the bistable axle is another type of mobility for pseudorotaxanes. In this section, these two types of mobilities and the functions of pseudorotaxanes based on these mobilities will be discussed. 3.1. Association/Dissociation Mobility

Generally, the threading of a linear component through the macrocycle is driven thermodynamically by noncovalent AQ


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Scheme 68. Parallel Synthesis of [3]Rotaxane Sequence Isomers 225a and 225b

Scheme 69. Complexes for the Construction of Rotaxanes: Pseudorotaxane and Clipping Complex

The hydrophobic effect is another case involving solvent influence.394 It generally relates to the exclusion of guests, usually from polar solvents, particularly water. It is of crucial importance in the binding of organic guests by cyclodextrinand cucurbituril-based hosts in water. Because these host cavities are hydrophobic, water inside the cavity does not have strong interactions with the host walls and is therefore of high energy. Organic guests, which can thread into the cavity of the host to form a complex, result in less disruption to the solvent structure and hence lower the overall free energy. 3.1.2. Temperature Effect: Slippage or Deslippage? Temperature can affect the association process not only from

Figure 18. Chemical structures of molecular tweezers 226 and 227 and molecular clips 228−232. Reproduced with permission from ref 385. Copyright 2013 American Chemical Society.

its binding constant, but also from the association/dissociation rate of its complex. The most outstanding example of the AR


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Scheme 70. Cartoon Representation of the Assembly/ Disassembly Process for the Clipping Complex 2342+a

Scheme 72. Two Types of Molecular Mobilities of Pseudorotaxanes

component to form a rotaxane. When the solution is cooled to ambient temperature, the axle cannot slip out of the ring because of the high energy barrier (Figure 19).396 To make the

a


Scheme 71. Formation of the [2]Rotaxane/Clip Complex and Corresponding pH Controllable Switchinga

Figure 19. Schematic representation of the formation of rotaxane-like entities using the slippage approach.

threaded molecules kinetically stable under the appropriate conditions, it is essential to modify the termini of the axle with appropriate stoppers: a too small stopper makes the host and guest complex as a pseudorotaxane, and a too big stopper prevents slipping after heating. In 1993, Stoddart and co-workers used this strategy to prepare rotaxanes in high yields up to 87% between π-electronrich hydroquinone-based macrocycles and π-electron-deficient bipyridinium-based dumbbells, which could not be obtained by the capping method (Scheme 73).398 A range of 4,4′bipyridinium dications 237a−d were prepared, in which the size of the stoppers varied systematically. As a result, rotaxanes 238a−c were isolated after heating at 60 °C for 10 days. However, the stoppers of 237d were too big to permit slipping to occur at a preparatively useful rate. Thus, no rotaxane formed. To gain further understanding of the mechanism and the size complementarity requirements associated with the slippage approach to rotaxanes, several host−guest systems such as πelectron-deficient bipyridinium-based macrocycles and πelectron-rich hydroquinone-based dumbbells,397 DB24C8 and secondary dialkylammonium ions,396 and the tetralactam macrocycles and their diester axles399 have been exploited for these investigations. Besides its size, complex molecular structure, and the high flexibility of the mechanical bond, the

a

Reproduced with permission from ref 392. Copyright 2006 Royal Society of Chemistry.

temperature effect is the formation of rotaxanes based on the slippage method.396−400 In this method, the macrocycle and the thread molecule are independent at room temperature without any interactions, whereas after they are heated together in solution, the high temperature will provide enough energy for the axle with appropriate stoppers to slip into the ring AS


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Scheme 73. Synthesis of Rotaxanes 238a−c Using the Slippage Approach

deslipping reaction of a rotaxane is very sensitive even to extremely small changes in the molecular architecture, and the steric demand of its components can affect the process.400 These features provide a viable and alternative synthetic procedure for the construction of larger rotaxanes and polyrotaxanes based on elevating temperature. 3.1.3. External Stimuli. Supramolecular complexes such as pseudorotaxanes are attractive to scientists not only due to their topological importance, but also because of their applications in the construction of artificial molecular machines, which respond to appropriate external stimuli. By modulating the attractive/ repulsive forces between the cyclic and linear components of pseudorotaxanes, control of the association/dissociation process can be readily and reversibly achieved. Several methods have been employed for this purpose, including chemical stimuli,401 photochemical stimuli,402 redox stimuli,403 or other control elements.404 3.1.3.1. Chemical Stimuli. The threading and dethreading of a [2]pseudorotaxane can be induced by chemical stimuli, such as pH control,405−411 anion control,306,307,412,413 cation control,414−417 or addition of other competitive host or guest molecules.418,419 It was known that the association and dissociation of the complex between DB24C8 and secondary ammonium salts could be chemically controlled by pH.79 For π-electron-rich crown ether 239 and its guest π-electrondeficient 2,7-dibenzyldiazoniapyrene dication 240, the motion of the relevant [2]pseudorotaxane could also be induced by adding acid and base reagents.420 The association constant of the complex 239⊃240 was about 3 × 105 M−1 in MeCN. The hexylamine molecule, which forms a 2:1 complex with 240 with an association constant of the order of 108 M−2, was used to unthread the [2]pseudorotaxane 241 (Scheme 74). Addition of trifluoroacetic acid (TFA) resulted in the disruption of the interactions between hexylamine and 240 by protonation of the amine and rethreading of the [2]pseudorotaxane between 239 and 240. Another example of pH-controlled complexation systems is a stable ternary complex 242 formed by the triptycene-based cylindrical macrotricyclic host 61b, electron-deficient diquat, and electron-rich benzidine guest, which is stabilized not only by a charge-transfer (CT) interaction between electron-rich and electron-deficient guests but also by the face to face π-

Scheme 74. Chemically Driven Unthreading and Rethreading of the [2]Pseudorotaxane Incorporating Crown Ether 239

stacking interactions between the host and the guests (Scheme 75).421 Upon addition of TFA, the benzidine was protonated to form a cationic diammonium ion, which dethreaded from the two lateral DB24C8 cavities of cylindrical macrotricylclic host to form another type of 1:1 complex 243. Consequently, the diquat cation was extruded and the ternary complex decomposed. Furthermore, addition of tributyl amine to the above system led to a reversible rethreading process. The manipulation of the above pseudorotaxane systems was based on changes of the guest species in response to chemical reagents. A pH-responsive example based on changes of the host species was described by Gibson and co-workers.422 Their studies showed that the complex of the pyridine-containing cryptand host 39e and the paraquat guest had a 1:1 stoichiometry in solution (Scheme 76). Addition of TFA resulted in the protonation of the pyridyl nitrogen atom on the cryptand host and subsequently the decomplexation of the cryptand host and paraquat guest. The complexation process could be recovered by addition of triethylamine. Furthermore, Gibson and co-workers also synthesized bis(m-phenylene)-32crown-10-functionalized poly(propyleneimine) dendrimers of the first and third generations and investigated their pHresponsive complexation with paraquat diol.423 Interestingly, protonation of the dendrimer skeleton resulted in enhanced binding due to expansion of the structure. The above examples are based on the pH control. Anions have also been used as a control to modulate pseudorotaxane formation. As described in Scheme 44, by coordination with AT


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Scheme 75. An Acid−Base Controlled Selective Complexation Process Based on Macrotricyclic Host 61ba

a


Scheme 76. An Acid−Base Controllable Cryptand/Paraquat [2]Pseudorotaxane via Protonation/Deprotonation of Host 39ea

Scheme 77. (a) Formation of Cryptand/Paraquat-Based Pseudorotaxane 244⊃40; and (b) A Cation Controllable Switch Based on [2]Pseudorotaxane 244⊃40a

a


a


acceptor is to add photosensitizers. The pioneering work in this respect was reported by Stoddart’s group.424 The dethreading of a pseudorotaxane based on a tetracationic cyclophane and a linear molecule containing DNP group was observed by adding 9-anthracenecarboxylic acid as a photosensitizer (Scheme 78). Upon being sensitized by light, 9-anthracenecarboxylic acid donated an electron to the CBPQT4+ ring to generate a diradical dication species in the presence of the sacrificial reagent triethanolamine, which prevented the back electron transfer process. The complexation was recovered by introduction of air into the system, which regenerated the CBPQT4+ ring and reproduced the original fluorescence spectrum. Further investigation on a “second-generation” light-driven association and dissociation process of a pseudorotaxane has been subsequently developed. In this system, the photosensitizing component was directly incorporated into the pseudorotaxane suprastructure.425 In addition to two 4,4′-bipyridinium electron-acceptor units, the cyclophane 2454+ also contained a 2,2′-bipyridine coordinating ligand, which was coordinated to [Re(CO)3Cl] to obtain the cyclophane [Re(CO)3245Cl]4+ incorporating a metal-based photosensitizer component (Scheme 79). When the wheel-like component and a DNP derivative were 1:1 mixed together in solution, a [2]-

amide NHs, chloride anion was used as a template for a lactam macrocycle-based [2]pseudorotaxane. Removal of chloride led to dissociation of the complex. In addition to anions, Gibson and co-workers developed a new cation mechanism for the control of molecular motion a few years ago.414 Formation of the cryptand/paraquat-based pseudorotaxane 244⊃40 could be switched off and on by adding K+ and 18C6, respectively (Scheme 77a). The corresponding binding constant was determined to be 1.4 × 104 M−1. Stronger binding affinity was observed for cryptand 244 and KPF6, which provided a new mechanism for the molecular motion control by K+. After addition of K+ to a solution of complex 244⊃40, K+ displaced paraquat diol 40 from the cryptand cavity, leading to a color change from yellow-orange to colorless (Scheme 77b). Furthermore, addition of 18-crown-6, which binds K + preferentially, allowed the colored cryptand−paraquat complex to reform. 3.1.3.2. Photochemical Stimuli. Pseudorotaxanes based on donor−acceptor interactions can be destabilized and even dissociated by reduction of the electron acceptor or oxidation of the electron donor. One method for reducing the electron AU


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Scheme 78. Light-Induced Dethreading of a Pseudorotaxane Based on CBPQT4+ Using 9-Anthracenecarboxylic Acid As a Photosensitizer

Scheme 80. Electrochemically Induced Molecular Motions in Pseudorotaxanes Incorporating CBPQT4+ and 1/ 5DN38C10 as Ring Components and the TTFs in Different Oxidation States as Axles

on the fact that CB[8] strongly associates the dimer of the paraquat-containing dendrimers in their one-electron reduction state (Scheme 81).429 The formation of [2]pseudorotaxanes Scheme 81. Electrochemical Control of the CB[8]-Induced Dimerization of a Paraquat-Containing Dendrimera Scheme 79. Schematic Representation of the Light-Induced Disassembly of a [2]Pseudorotaxane Incorporating the Photosensitizing Motif in the Macrocycle Ring

pseudorotaxane formed because of donor−acceptor interactions. Light excitation of the cyclophane of the pseudorotaxane caused transfer of an electron from the metal center to one of the 4,4′-bipyridinium units in the ring, thereby decreasing the noncovalent-bonding interactions responsible for the formation of the pseudorotaxane. Disassembly occurred in the presence of a sacrificial reductant. Furthermore, when oxygen was allowed to enter the irradiated solution, the reduced host was promptly reoxidized and the pseudorotaxane was obtained again. 3.1.3.3. Electrochemical Stimuli. Both the tetracationic cyclophane and the TTF unit can behave in charge-transfer complexes as a donor or an acceptor depending on their different oxidation states. Therefore, pseudorotaxanes based on these two components can be controlled by electrochemical stimuli.426,427 For TTF, there exist specifically three stable forms, TTF(0), TTF+•, and TTF2+. Neutral tetrathiafulvalene (TTF(0)) acts as a π-electron-donating (ED) guest when it complexes with the π-electron-accepting (EA) CBPQT4+. Dication TTF2+, with EA property, binds strongly within the cavity of the ED macrocyclic polyether 1,5-dinaphtho[38]crown-10 (1/5DN38C10). However, the radical cation TTF+· can not be bound by either of the hosts. On the basis of the three states, a three-pole supramolecular switch with CBPQT4+ and 1/5DN38C10 as hosts and TTF as the guest was designed and developed (Scheme 80).428 Another example of the electrochemical stimuli-induced association/dissociation mobility of pseudorotaxanes is based

a


incorporating paraquat-terminated dendrimers as the axles and CB[8] as the ring component was initially monitored by electronic absorption spectroscopy. Reduction of a solution of the dendrimers and CB[8] led to extensive dimerization. Although the increase in the size of the dendrimers may be a factor that hinders their dimerization, electrostatic interactions play a much more important role. The third-generation dendrimer 246 dimerized in a yield of 50% with their paraquat radical cations inside the cavity of the CB[8] ring. AV


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3.1.3.4. Multi-Stimuli. As the above examples exemplified, the association/dissociation mobility of the [2]pseudorotaxanes consisting of CBPQT4+ and DNP derivatives can be controlled by photochemical stimuli. Additionally, the complexation of CBPQT4+ and DNP derivatives can also be modulated by electrochemical stimuli.430 There are many other cases where the supramolecular systems can be controlled by two or more stimuli. In an example reported by Credi and co-workers, the formation of a [2]pseudorotaxane incorporating triureidocalix[6]arene as the ring component was controlled via protonation/deprotonation of the monoalkyl-substituted 4,4-bipyridine cation via addition of acid or base.431 Although this system was photoinactive, this process could be controlled by adding a photosensitive reagent (Scheme 82). Spiropyran

Figure 20. Photoinduced electron transfer in a [3]pseudorotaxane that can be assembled/disassembled by three different external inputs. Reproduced with permission from ref 432. Copyright 2002 American Chemical Society.

Scheme 82. Control of Threading/Dethreading Process in Paraquat-Containing Pseudorotaxane by Means of LightInduced Proton Exchangea

secondary dialkylammonium-ion center and a paraquat unit, were incorporated into the wire-type component 248-H3+ to thread into the DB24C8 socket of 2472+ and the cavity of 1/ 5DN38C10 239, respectively. It was confirmed that reversible association/dissociation of the two plug/socket systems 2472+· 248-H3+ and 248-H3+·239 could be controlled independently by acid/base and redox stimulation. In the fully connected three-components system, light excitation of the [Ru(bpy)3]2+ unit of component 2472+ was followed by electron transfer to the paraquat unit in component 248-H3+, which was plugged into component 239. Its operation was governed by three different types of stimuli (light, acid/base, redox), and its behavior could be monitored by changes in one electrochemical, two light-emission, and two light-absorption signals. 3.2. Shuttling Mobility

Besides the main association/dissociation mobility, shuttling of the wheel along the bistable axle is another important type of mobility for pseudorotaxanes. In 2000, Kim and co-workers reported a pseudorotaxane-based shuttling process (Scheme 83).433 The system consisted of CB[6] as the bead and Scheme 83. A Fluorescent, Reversible [2]PseudorotaxaneBased Molecular Switch a


photochrome, whose acid−base properties could be photocontrolled, was selected for this mission. Upon being exposed to visible light irradiation, the merocyanine compound underwent cyclization to release a proton, which further induced protonation of the monoalkylated 4,4-bipyridine cation axle and formation of the [2]pseudorotaxane. The reversible process was triggered by heating. Upon heating, the spiropyran skeleton absorbed a proton from the dicationic axle to undergo a ring-opening reaction. Thus, the [2]pseudorotaxane decomposed. This is a nice example wherein pseudorotaxane formation can be controlled by triple stimuli of acid/base, heat, and light. Another example of multi-stimuli-triggered systems is involved with a [3]pseudorotaxane consisting of three components. It can mimic, at a molecular level, the function of a macroscopic electrical extension cable (Figure 20).432 The [Ru(bpy)3]2+ unit on component 2472+ played the role of a power source under light excitation, and the DB24C8 motif fulfilled the function of a socket. Two moieties, including a

fluorenyltriamine as the string. When all of the nitrogen atoms of the string were protonated, CB[6] resided at the protonated diaminohexane site, because of formation of a more stable complex. Upon deprotonation of the aniline nitrogen (pKa = 6.7), CB[6] migrated to the diprotonated diaminobutane site because the binding of CB[6] with the monoprotonated diaminohexane was weaker. The process could be manipulated several times by addition of acid and base alternately. The switching of CB[6] from one site to the other in the string was easily detected by changes in color and fluorescence with high sensitivity. Besides CB[6], pH-controlled shuttling motion also AW


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happened on a [2]pseudorotaxane self-assembled from CB[7] and paraquat moiety.434 Triggered by electrochemical stimuli, a CB[7]-based [2]pseudorotaxane showed the shuttling movement of the ring along a dicationic axle from the central binding site to the terminal ferrocenyl group (Scheme 84).435 Complexation

Scheme 85. Photo-Controlled Shuttling Mobility of a [4]Pseudorotaxane Based on α-CD and CB[7]

Scheme 84. Shuttling Mobility of a CB[7]-Based [2]Pseudorotaxane Controlled by Chemical Stimulia

and showed favorable fluorescence emission in the presence of (Figure 21a).26 Another method for the formation of

D-glucose

a


between the guest 250 and CB[7] was quantitative due to the remarkably high association constant (Ka > 1012 M−1) of CB[7] with the ferrocenyl unit. Upon oxidation of the ferrocene center, the complex was significantly destabilized, and CB[7] shuttled from one of the terminal binding sites to the central binding site. The system takes advantage of hydrophobic, ion-dipole, and electrostatic forces in reversibly controlling the shuttling movement of CB[7]. Photochemically stimulated shuttling was also observed with a more complicated [4]pseudorotaxane system.436 Tian and coworkers reported a supramolecular complex based on α-CD and CB[7] and investigated the shuttling movement by induced circular dichroism (ICD) experiments. The linear compound 251 (Scheme 85) contained an azobenzene moiety and two paraquat units, providing the binding sites for both α-CD and CB[7], respectively. The E/Z photoisomerization of the azobenzene unit by irradiation of the aqueous [4]pseudorotaxane at 360 nm resulted in the shuttling of the αCD ring away from the azobenzene unit. This change was shifted back reversibly by irradiation of the solution at 430 nm. Meanwhile, thermally induced shuttling motion of the α-CD ring was also detected by variable-temperature ICD measurements.

Figure 21. Fluorescent sensors based on [2]pseudorotaxanes.

pseudorotaxane-type fluorescent receptors is to functionalize the macrocyclic unit with a fluorescent signaling element and consider it as a receptor for linear guest molecules. For example, in recently reported work, by modifying a pillar[5]arene host with a pyrenyl fluorophore via CuAAC “click” chemistry, the sensor 254 for 1,6-hexanediamine was obtained (Figure 21b).285 In this example, 1,6-hexanediamine acted both as an axle of the pseudorotaxane and as a sensor guest molecule. 3.3.2. Molecular Logic Gates. The design and construction of molecular systems that respond to chemical and/or photonic inputs, in accordance with logic-gate behavior, have attracted considerable attention.440 Stimuli-responsive pseudorotaxanes and rotaxanes are ideal candidates for molecular logic gates.420,441 YES and NOT single-input gates are the simplest logic devices. Various molecular systems can perform such operations.442,443 To perform more complex logic operations, carefully designed multicomponent chemical systems are needed. One of the successful examples is a [2]pseudorotaxane-type XOR logic gate reported by Balzani and co-workers.25 The [2]pseudorotaxane was constructed by association of 2,3-dioxynaphthalene crown ether 256 and 2,7-

3.3. Functions of Pseudorotaxanes

3.3.1. Fluorescent Sensors. Pseudorotaxanes may be viewed as prototypes of molecular machines because of their reversible assembly/disassembly and shuttling movement under external stimuli. In some cases, their photophysical properties are influenced along with the kinetic process, making them sensitive analytical tools in many areas.437−439 They can be used as chemical sensors when fluorescent signaling elements are involved. For example, a β-CD derivative bearing a phenylboronic acid residue formed a pseudorotaxane-type complex with 1-heptyl-4-(4′-dimethylaminostyryl) pyridinium, and this supramolecular complex was an excellent host for saccharides AX


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Both processes caused a strong increase of emission intensity at 343 nm. It was concluded that the chemical system showed the input/output relationships indicated by the truth table of the XOR logic gate. Therefore, the operation of the chemical input and fluorescent output can be considered as an XOR logic gate at the molecular level. 3.3.3. Molecular Switches. The threading/dethreading movements of pseudorotaxanes have been used to construct molecular switches for modulating the properties of supramolecular systems.444−448 For example, a lanthanide complex containing three DB24C8 macrocycles binds dialkylammonium derivative 258 to form tris[2]pseudorotaxane system [Tb·2573· 2583] (Scheme 87).446 The luminescence of the lanthanide coordination complex was completely quenched due to photoinduced electron transfer (PET) from the ferrocene moiety of 258 to the pyridine-2,6-dicarboxylic acid 257. Upon addition of KPF6, the dialkylammonium cation in DB24C8 was replaced by K+ and the PET process was suppressed; thus the quenched lanthanide emission was restored. To check the reversibility of this process, 18-crown-6 was added. It caused the quenching of luminescence again. This work demonstrated a new strategy for the fabrication of reversible switches through electron transfer. Recently, chemical switches were applied to retain their activities when attached to metal nanoparticles (NPs).448 A variety of metal NPs, such as Au, Pt, and Pd NPs, functionalized with linear π-electron-rich recognition sites such as TTF were prepared. On the basis of the charge-transfer interaction between TTF and CBPQT4+, the pseudorotaxane complex formed on the NP surface (Scheme 88). The CBPQT4+ ring associates or dissociates with the metal NPs triggered by electrical stimuli. Their switching movement was verified by ζ potential measurements and CV, which provided the potential application in the assembly of responsive, nanostructured materials. 3.3.4. Other Molecular Machines. Switchable [2]pseudorotaxanes play a very important role in the construction of molecular switches, logic gates, sensors, as well as other molecular machines.449,450 In 2003, Kim and co-workers successfully designed and constructed a [2]pseudorotaxanebased molecular loop.451−453 In this report, hexamethylenebridged bisparaquat 259 underwent a large reversible structural change from linear to loop by electrochemical and photo-

dibenzyldiazapyrenium dication 240, in which the electrondeficient diazapyrenium unit was sandwiched between two electron-rich 2,3-dioxynaphthalene units of 256 (Scheme 86). Scheme 86. Schematic Representation of an XOR Logic Gate Based on the Threading/Unthreading Pattern of the Pseudorotaxane 256⊃240

Upon addition of tributylamine, the [2]pseudorotaxane disassociated due to the formation of a more stable 1:2 complex between 240 and the amine. Subsequent addition of TFA unlocked the axle compound from the 240·B2 (B = tributylamine) adduct and allowed rethreading between 256 and 240 to form the [2]pseudorotaxane 256⊃240 again. Furthermore, the unthreading−rethreading cycle could also be performed by reversing the order of the two inputs. First, addition of TFA caused the complexation between crown ether 256 and H+, and further unthreading of [2]pseudorotaxane; then addition of tributylamine unlocked crown ether from 256· H+, allowing rethreading to give back the [2]pseudorotaxane.

Scheme 87. Schematic Representation of a Reversible Luminescent Lanthanide Switcha

a

Reproduced with permission from ref 446. Copyright 2008 American Chemical Society. AY


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of 259 with a linear structure was approximately 28 × 18 Å, whereas that of 259 with a loop structure was only about 15 × 18 Å. Along with this work, a molecular loop lock, which acted as a redox-driven molecular machine, was fabricated (Scheme 89b).452 Linear molecule 260 contained both a naphthalen-2yloxy unit and a paraquat unit. These two units formed a charge-transfer complex inside CB[8]. Therefore, the treatment of 260 with CB[8] in water resulted in the formation of stable 1:1 complex 261. After addition of another guest paraquat and being triggered by electrochemical stimuli, paraquat motifs in both guests became radical cations. Because CB[8] strongly associated with the dimer of the one-electron-reduced paraquat unit, a ternary [3]pseudorotaxane 262 was obtained. In this process, the paraquat molecule acted as a key, which could open the lock-like molecular loop 261 encapsulated in CB[8]. Another function of pseudorotaxanes based on the threading/dethreading process was to control molecular rotary motion.454 The design was based on the fact that a secondary dialkylammonium ion (R2NH2+) threads into the cavity of DB24C8. Under neutral conditions, the system cis-263 rotates freely (Scheme 90); after addition of acid, a self-complexing [1]pseudorotaxane molecular system cis-263-H·PF6 formed. The hydrogen-bonding interactions between the R2NH2+ unit and the DB24C8 macrocycle were strong enough to lock the structure, which prevented rotation. Further deprotonation by adding base led to the unlocked state of the motor cis-263. Thus, control of the mechanical motion at the single-molecule level was achieved. This study represents a good way to fabricate more advanced molecular devices.

Scheme 88. Reversible Transformation of the Pseudorotaxane-Type Complexes Attached to the Metal NPsa

a


chemical stimuli (Scheme 89a).451 The linear guest 259 and CB[8] initially formed stable [2]pseudorotaxane CB[8]⊃259 Scheme 89. (a) An Electrochemically Triggered Molecular Loop; and (b) Formation of a Molecular Loop Locka

4. STIMULI-RESPONSIVE MOLECULAR MOBILITY OF ROTAXANES Rotaxanes, acting as the most fundamental MIMs, envisage particularly two types of interesting molecular motions: shuttling of the ring along the axle, and rotation of the ring around the axle (Figure 22). Thus, rotaxanes are good prototypes for the construction of both shuttling and rotary molecular devices. 4.1. Rotational Mobility

Under the influence of external stimuli, the mechanical motion within the rotaxanes can be addressed, and to some extent controlled. In this section, we pay attention to the rotational mobility of the wheel around the axle. These systems with rotational motion represent a first step toward the elaboration of rotary motors455,456 at the molecular level. First, metalation−demetalation of the central coordination site is considered as an effective way to control the rotation of the wheel. On the basis of a multiporphyrinic rotaxane, Sauvage and co-workers triggered the rotation of the ring around the axle (Scheme 91).457 The [2]rotaxane 264 incorporated a gold(III) porphyrin on the ring and two zinc(II) porphyrins on the axle. Upon demetalation of Cu(I), which facilitated the formation of the [2]rotaxane as a template, free rotation of the wheel occurred. The extremely different and complementary electronic properties of the gold(III) porphyrin (electron acceptor) and zinc(II) porphyrin (electron donor) resulted in attraction of three porphyrin parts and the rearrangement of their positions from opposition to the same orientation. Still on the basis of the transition-metal-containing rotaxane system, Sauvage and co-workers rotated the ring around the threaded dumbbell by electrochemical stimuli.458,459 In their work, both 2,9-diphenyl-1,10-phenanthroline and 2,2′:6′,2″-

a


with a binding constant of 2.3 × 105 M−1 in water. CB[8] resided at the central alkyl chain site of the guest. Upon reduction, the terminal paraquat motifs became radical cation units, which rapidly underwent an intramolecular pairing process inside CB[8]. Accompanying this process, the shape of the guest changed from linear to loop. As well as the shape, its size was also changed substantially: the molecular dimension AZ


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Scheme 90. A Lockable [1]Pseudorotaxane-Type Molecular Motor and Its Threading/Dethreading Movement

Scheme 91. Rotation of the Ring around the Axle by Metallation−Demetallation of the Central Coordination Site

Figure 22. Two types of molecular mobilities of rotaxanes: (a) rotation and (b) shuttling.

terpyridine units were involved in the wheel-like component, and the molecular string contained only one 2,9-diphenyl-1,10phenanthroline unit (see [2]rotaxane 266 in Scheme 92). Varying the redox state of the copper (from CuI to CuII) led to a large rearrangement of the surrounding ligands: CuI favored two phenanthroline ligands, and CuII favored a phenanthroline ligand and a terpyridine ligand. This relaxation process of the compound induced pirouetting of the wheel around its axle, which brought the system to its new equilibrium position. The electrochemically induced motion was detected by cyclic voltammetry. Further investigation on the chemical structural influence of the rotational movement indicated that, by using long and flexible linkers between the stoppers and the central complex, ligand exchange was very fast, which led to short response times (on the millisecond time scale and below).460 Alternating-current (ac) electric fields are commonly used to interact with large-scale molecular motions, such as the backbone fluctuations of semirigid polymers.461 In 2000, Leigh, Zerbetto, and co-workers used it to address the rotation of the wheels of two hydrogen-bond-assembled rotaxanes around their axles.462 It is an original application for ac electric fields to probe the structures of rotaxanes. The chemical structures of the threads were different in [2]rotaxanes 144a and 146a, which afforded them different mobility properties (Figure 23). Addressed by oscillating electric fields, [2]rotaxane 146a showed only rotary motion, while the other interlocked molecule 144a displayed a more complex picture with a

superposition of rotation and pivoting motions. This phenomenon could also be caused by thermal stimulus as revealed by variable-temperature 1H NMR experiments. The isomerization of an olefin by light was used to trigger the rotation of a ring along an axle.463 Because of the influence of an alternating-current electric field, the rate of rotation of the above-mentioned rotaxanes was slowed by 2−3 orders of magnitude. To accelerate the rate of rotation, another broadly useful stimulus, light, was used for the olefin-based rotaxane 267 (Scheme 93). The two hydrogen-bond-accepting groups of the trans-olefin component were highly complementary to the hydrogen-bond-donating sites of the tetralactam macrocycle. Isomerization of the olefin from trans to cis by light inevitably disrupted the near-ideal hydrogen-bonding motif between the macrocycle and thread and therefore dramatically reduced the energy barrier for the macrocycle to pirouette around the axle. BA


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Scheme 92. Principle of the Electrochemically Induced Molecular Motions in a Copper Complex Rotaxane 266

Strategies for controlling rotational motion of rotaxanes have been also devised by linking the rotor to the axle, to produce new [1]rotaxanes.467 Rotaxanes 268−271 that have been designed for this investigation incorporated an α-CD as the rotor, a stilbene as the axle, and trinitrophenyl substituents as the capping groups (Figure 24). It was indicated by 2D NMR techniques that the α-CD ring of rotaxane 269 without any functional groups rotated freely around the stilbene axle; incorporation of a methoxyl group into the axle, and the succinamide link between the axle and α-CD produced rotaxane 268, which behaved analogously to a ratchet tooth and pawl, respectively. The [1]rotaxane structure restricted the rotational motion on the NMR time-scale. Alone, the effect of the succinamide link of 271 was much less, and the methoxyl group of 270 had no detectable effect.

Figure 23. Alternating-current electric field-induced rotational motion of rotaxanes 146a and 144a.

Thus, the rate of rotation of [2]rotaxanes was accelerated by >6 orders of magnitude. Rotaxanes’ rotary property can also be altered by adding new recognition sites to the interlocked molecules.464−466 A [2]rotaxane with a crown ether-containing wheel component was investigated by Smith and co-workers. It showed the capability to bind K+, which froze out a single coconformation and therefore decreased the rotation of the wheel component.464 Furthermore, the influence of anion and salt binding properties and solvation of coconformations were evaluated in detail.465,466

4.2. Shuttling Mobility

4.2.1. Observation of Shuttling and Its Dynamic Control. The shuttling movement of rotaxanes was early observed in 1991 by Stoddart and co-workers. The [2]rotaxane 126·4PF6 consisted of a tetracationic cyclophane and a polyether “thread” (Figure 25).294 The dumbbell-like molecule contains two hydroquinol units, which provide two identical stations for the wheel component. Hence, the tetracationic macrocycle shuttled back and forth between the hydroquinol

Scheme 93. Rotational Motion of the Ring of Rotaxane 267 Accelerated by Light

BB


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ring units, and the ΔGc⧧ value of ca. 13 kcal mol−1 was calculated from 1H NMR spectroscopy. A similar translocation of macrocycles between two identical stations in the threads was also achieved with the peptide rotaxane systems driven by hydrogen-bonding interactions in halogenated solvents.317,468 When the solvent was changed to methanol, which reduced the strength of hydrogen bonding and thus decreased the ΔGshuttling⧧, the shuttling rate of the macrocycle between the two peptide stations increased. However, when DMSO was used as the solvent, the macrocycle in [2]rotaxane 272 stopped shuttling and stayed on the lipophilic station due to lack of hydrogen bonding (Scheme 94a). Besides the solvent influence, other factors controlling the rate of shuttling were also investigated. Decreasing the temperature and adding steric barriers between the two stations in the threading molecules both freeze the shuttling macrocycle at a single peptide station. For example, the shuttling in the sulfur-linked rotaxane 273 was suppressed by introduction of a bulky tosyl imino group (TsNCl, Bu4NOH, CH2Cl2, 2 h, 93%) to the thread. Reduction (P4S10, CH2Cl2, 4 h, 100%) of the imine re-established the shuttling motion (Scheme 94b). The strategy of adding steric barriers to hinder the shuttling movements was also adopted by Stoddart and co-workers in a [2]rotaxane containing DB24C8 and a dumbbell with two dialkylammonium recognition sites.469 To achieve reversibility of controlling the shuttling movement on and off, several methods based on noncovalent interactions or external stimuli are tried and show very good results. For example, alternating intermolecular complexation and decomplexation processes are involved in the linker unit, which connects the two shuttling stations on the dumbbell component.470 [2]Rotaxane 275 was based on the recognition of the bipyridinium unit with BPP34C10 (Scheme 95). The linker between the two equivalent stations contained a

Figure 24. Schematic representation of a tooth and pawl restricting rotational motion in α-CD-based rotaxanes. Reproduced with permission from ref 467. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 25. Molecular shuttle 126 with two identical stations. BC


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Scheme 94. Peptide-Based Molecular Shuttles and Their Dynamic Control by (a) Solvents and (b) Steric Barriers

Scheme 95. Intermittent Molecular Shuttle as a Binary Switch

motions in [2]rotaxanes.471 Furthermore, photochemical stimuli can be used as a brake for rotaxane-based shuttles.472 Recently, a light-gated STOP-GO molecular shuttle was designed and constructed on the basis of a bistable rotaxane linked by tetrafluoro-4,4′-azobiphenyloxy units (Figure 26). When the rotaxane was in its trans configuration, the blue box moved back and forth between the two DNP units at 309 K. Furthermore, the photoisomerization of trans-276·4PF6 to cis-

bipyridine moiety, which acted as a ligand for coordination with CuI ion. When these chelating sites combined with CuI ion, the shuttling of the macrocycle was hampered. However, treatment of the complex with a suspension of ion-exchange resin led to the complete decomplexation, and the shuttling of the macrocycle happened again. It acted as a molecular switch to dominate the dynamic state of the rotaxane-based shuttle. Besides the coordination interaction involved above, electrostatic interactions have also been used to switch the shuttling BD


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Figure 26. A light-gated STOP-GO molecular shuttle based on [2]rotaxane 276·4PF6. Reproduced with permission from ref 472. Copyright 2009 American Chemical Society.

Scheme 96. Conformational Isomerism of the Peptide in [2]Rotaxane 277

external stimuli to become the latter preferential binding site of the macrocycle. 4.2.2.1. Chemical Stimuli. Shuttling motions of the macrocycles in rotaxanes can be triggered by addition of chemical reagents. In some cases, through changing solvents, the localization of macrocycles can be altered.473,474 The polarity of solvents can be a factor controlling the shuttling movement. One such example involved an environmentally sensitive peptide [2]rotaxane reported by Leigh and coworkers.474 The [2]rotaxane 277 containing sarcosylglycine unit exhibited only the E tertiary amide rotamer in apolar solvents, as the Z rotamer allowed the formation of two

276·4PF6 stopped the shuttling motion. This work provides a useful method to control molecular shuttles at will. 4.2.2. Stimuli-Responsive Molecular Shuttles. When the shuttling motion was observed in rotaxane systems, chemists began to modulate the shuttling speed from fast to slow mode or from “on” to “off” via various routes. After that, efforts were made to reversibly control the shuttling movements from one station to another under external stimuli, such as chemical, photochemical, and electrochemical stimuli. In this regard, the two recognition sites in the dumbbell components are always different from each other, one of which is the initially favored position for the wheel, and the other can be altered by BE


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Scheme 97. Protonation or Electrochemical Oxidation of the Benzidine Station in the Axle Resulting in Shuttling Motion of the Bead

Scheme 98. An Acid−Base Controlled Molecular Shuttle

favorable hydrogen bonds as compared to four in the E rotamer. In hydrogen-bond-competing solvents, such as DMSO-d6, hydrogen-bonding interactions between the peptide and macrocycle were broken, and the macrocycle preferred to be positioned over the C-terminus amino acid of the thread, which resulted in a mixture of two rotamers (Scheme 96). The most common situation for chemical stimuli is pHcontrol of shuttling movement.475 An early example for controlling the position of the macrocycle at one station or another through addition of acid or base was reported by Kaifer and co-workers in 1994.476 The [2]rotaxane 278·4PF6 that they chose for investigation contained the CBPQT4+ macrocycle and a linear molecule with benzidine and biphenol units. At 229 K, the macrocycle mainly occupied the benzidine position in the dumbbell, because the binding affinity of CBPQT4+ for the

benzidine derivative was about 10 times greater than that for the corresponding biphenol derivative. Addition of excess TFA to the system resulted in macrocyclic translation to the biphenol station (Scheme 97). Subsequently reversible behavior could be obtained by neutralization of TFA with pyridine. Another important feature of this rotaxane-based shuttle was that it could also be triggered by electrochemical means as analyzed by UV−vis spectroscopy and cyclic voltammetry. It is well-known that pseudorotaxanes containing ammonium ions are often triggered by addition of acid−base pairs to achieve their association/dissociation movements. On the basis of the formation of pseudorotaxane-type inclusion complexes between secondary dialkylammonium ions (NH2+) or paraquat and DB24C8, a bistable rotaxane 279·3PF6 was described with BF


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Scheme 99. A pH-Switchable Molecular Shuttle through Anion Recognition

Scheme 100. A pH-Switchable Palladium-Complexed Molecular Shuttle

a dumbbell-shaped component possessing both NH2+ and N,N′-dialkylated-4,4′-bipyridinium (Bpym2+) units (Scheme 98).477 The ring’s position between the two different recognition sites was switched by pH stimuli. Deprotonation of the NH2+ center was expected to induce the displacement of the ring to the Bpym2+ station. Because the Bpym2+ unit was very sensitive to nucleophilic bases, it was not easy to choose an appropriate base. Diisopropylethylamine (i-Pr2NEt) and tributylamine (Bu3N) were at last found to be ideal bases. Reprotonation could be performed by the addition of TFA or triflic acid. To easily monitor the system’s state, an anthracene moiety was chosen as a stopper due to its absorption,

luminescence, and redox properties. According to recent research, the following investigation on the binding abilities of various interaction sites for DB24C8 were carried out.478 The results indicated that the affinity of different stations for DB24C8 at room temperature was determined as follows: anilinium > monosubstituted pyridinium amide ≈ triazolium > disubstituted pyridinium amide > aniline. In another mechanism for molecular shuttles based on the change of pH strategy, the hydrogen-bonding stations for the macrocycle can be altered by anion formation.479 [2]Rotaxane 280 contained a thread that featured two potential hydrogenbonding stations (the succinamide group and cinnamate group) BG


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Scheme 101. Shuttling Motion of the Macrocycle through the Alternative Addition of Li+ and 12C4

Scheme 102. Shuttling Motion of [2]Rotaxane 283 through the Stepwise Competitive Binding of Transition-Metal Ions

Bu4NOH, t-BuOK, DBU, and phosphazine P1, the translocation of the macrocycle in the phenolate anion station was displayed. The shuttling was reversible after addition of CF3CO2H, which returned the macrocycle to the original

for the benzylic amide macrocycle (Scheme 99). In the neutral form, the macrocycle resided preferentially on the succinamide station, while after addition of bases that were capable of deprotonating the phenol, such as LiOH, NaOH, KOH, CsOH, BH


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Scheme 103. An Allosterically Regulated Molecular Shuttle

Scheme 104. Shuttling through Reversible Covalent Bond Formation

shuttle.480 During the shuttling process, a palladium-complexed macrocycle was translocated between 4,4-dimethylaminopyridine and pyridine monodentate ligand sites triggered by adding or removing TsOH (Scheme 100). In other words, protonation of the more basic heterocycle determined the position of equilibrium because the N−H bond is significantly stronger than the Pd−N bond. This metal−ligand coordination driven (and dynamic covalent chemistry) molecular shuttle could play a prominent role in the construction of artificial molecular machines. Besides the pH stimulus, some other chemical reagents such as cationic compounds76,481,482 or halogen and acetate anions483 have also been used to shuttle the macrocycles in rotaxanes. For example, Sanders, Stoddart, and co-workers developed a Li+-driven molecular shuttle based on a [2]rotaxane with DNP38C10 as the ring component and an axle

succinamide station. The anion-induced shuttling has several remarkable features. First, it is highly solvent dependent. Solvents like DMF, CH3CN, and CH3OH, in which the designed hydrogen-bonding interactions are relatively weak, are chosen for this system. Second, although the nature of the accompanying cation strongly influences the strength of anion hydrogen bonding, the counterion does not affect the coconformation adopted by anion in the rotaxane. Third, the presence of alternative anionic hydrogen-bond acceptors, such as Bu4NX (X = F−, Cl−, Br−, I−, HO−, NO3−, AcO−), does not affect the shuttling process either. On the basis of acid−base stimuli, the pyridine motifs on threads could be protonated and decomplexed from metal ions to trigger the shuttling motion of coordination-derived rotaxanes. Inspired by this mechanism, Leigh and co-workers reported a pH-switchable palladium-complexed molecular BI


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Scheme 105. A Fluorescence Switch That Can Be Independently Operated by Both Acid/Base and Metal-Ion Complexation/ Decomplexationa

a

Reproduced with permission from ref 333. Copyright 2012 John Wiley & Sons, Ltd.

Scheme 106. Photoinduced Molecular Shuttle of Rotaxane 288 through E/Z Photoisomerization of the Azobenzene Dumbbell

acetone at 298 K. After addition of CuII or CdII ions, although the terminal peptide carbonyl group chelated to the metal, the preferred position of the macrocycle remained unchanged. Translocation of the macrocycle to the succinic amide ester station was caused by subsequent deprotonation of the amide proton of the coordination part with phosphazene base P1-tBu. The stepwise shuttling process was fully reversible with the addition of NaCN to remove the metal ion and NH4Cl for reprotonation of the amide nitrogen atom. In the following example, the shuttle motion was also stimulated by adding the CdII ion, but the mechanism was completely different.482 When the bis(2-picolyl)amine (BPA) moiety was attached directly to the succinamide unit, which was considered as one hydrogen-bonding station of the thread, chelation of a metal ion used all three nitrogen atoms of the

containing a naphthaldiimide station and a pyromellitic diimide station (Scheme 101).76 The macrocycle initially resided on the naphthaldiimide station due to the better π-stacking interactions. Because two Li+ ions form a strong 2:1 complex with DNP38C10 and pyromellitic diimide station, the position of the macrocycle was changed. Subsequent addition of 12-crown4 (12C4) induced the reverse process as 12C4 was a very strong sequestering agent for Li+. Transition-metal ions such as CuII or CdII have also been involved in switching the position of macrocycles on threads.481,482 [2]Rotaxane 283 had a glycylglycine station, as well as the succinic amide ester station on the thread for binding the benzylic amide macrocycle (Scheme 102).481 Without metal ions, the occupancy of the macrocycle was approximately 90:10 in favor of the glycylglycine station in BJ


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Scheme 107. Unidirectional Photoinduced Shuttling in [2]Rotaxane 289 with a Symmetric Stilbene Dumbbell

Scheme 108. A Lockable Light-Driven Molecular Shuttle with a Fluorescent Signal

an amide unit. The molecular switch can be operated by changing pH or by adding or removing Li+ or Zn2+. Significantly, the three independent movement processes were all accompanied by multilevel expression of fluorescence. 4.2.2.2. Photochemical Stimuli. Light-driven molecular shuttles are of considerable interest because their response to a photochemical process is usually rapid and precise and can be operated remotely without generating any chemical waste. In 1997, Nakashima and co-workers reported an example of a light-driven rotaxane-based molecular shuttle.486 At room temperature, the α-CD macrocycle stayed at the azobenzene moiety. After UV irradiation, the wheel moved to the methylene spacer caused by the photoisomerization of the azobenzene unit from the trans to the cis configuration (Scheme 106). Moreover, irradiation with visible light resulted in the reverse process. It is indicated that E/Z photoisomerization of the azobenzene dumbbells is an efficient method to achieve light-driven translational motion in molecular shuttles.487 Stilbene is a molecule similar to azobenzene and can also be switched between its cis and trans conformations by light. There is an interesting feature when a symmetric stilbene component is chosen as a dumbbell and nonsymmetric CD molecule is chosen as a wheel.488 The unidirectional photoinduced shuttling in such a rotaxane was unprecedented with the cis alkene unit near the narrow rim of α-CD (Scheme 107). The α-

BPA group, which caused the pyridine arms to twist orthogonally and enter the space that was already occupied by the benzylamide macrocycle (Scheme 103). Therefore, the conformational change led to translocation of the macrocycle to the inherently weaker hydrogen-bonding succinic amide ester site 1.5 nm away. Addition of NaCN led to removal of Cd2+ and repositioning of the macrocycle. This was a negative heterotropic allosteric binding event, which provided a new strategy for metal-binding events to construct functional molecular machines. A bistable stimuli-responsive molecular shuttle can also be triggered through the formation (and breaking) of C−C bonds, using the well-established Diels−Alder and retro-Diels−Alder reactions.484 Rotaxane 285 contained a trans double bond holding the two amide carbonyls of the fumaramide station complexed with the benzylic amide macrocycle, which also opened the possibility of utilizing reversible C−C bond formation to trigger the shuttling response (Scheme 104). It provided a new way to switch the position of the macrocycles. Recently, an interesting molecular shuttle was reported, which could be independently operated by both acid/base and metal-ion complexation/decomplexation (Scheme 105).485 In this report, an appropriately designed crown ether-based macrocycle was bound mechanically onto a dumbbell possessing two different recognition sites, a NH2+ center and BK


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Scheme 109. A Photoresponsive H-Bond-Assembled Molecular Shuttle

Scheme 110. Chemical Structure of [2]Rotaxane 292 and the Proposed Mechanism for Its Excited-State Dynamicsa

a


CD ring resided on the stilbene unit initially and slid to the biphenyl unit under the irradiation at 340 nm. The reverse process could be realized by the irradiation at 265 nm, making the α-CD ring reside on the stilbene unit again. The usual methods used to monitor the position change of macrocycles in rotaxanes are based on 1H NMR, absorption, and circular dichroism spectroscopies. Changes in fluorescence are easily detected output signals for shuttling motion. One interesting example was demonstrated by Tian and coworkers.489,490 In the initial work, the strong hydrogen-bonding interaction between the α-CD macrocycle and the isophthalic acid unit prevented the shuttling motion of the ring along the two binding sites even if the system was irradiated by light and the trans-stilbene changed to the cis-stilbene (Scheme 108).489 By the addition of a base such as Na2CO3, the hydrogen bonds were destroyed. Thus, the α-CD macrocycle could shuttle back and forth upon the E/Z photoisomerization of stilbene. The motional process was operated by alternating the irradiation frequency, which was accompanied by obvious changes in the intensity of fluorescence at 530 nm. This switching system featured the convenient use of an optical input and easy reading of the optical output. Later, a novel light-driven [3]rotaxane molecular shuttle was synthesized and characterized, in which the two α-CD rings shuttled back and forth on the molecular

thread that contained an azobenzene unit, a biphenyl unit, and a stilbene unit.491 Besides azobenzene and stilbene moieties, spiropyran is also a typical photoisomerizable compound, which can be exploited for the construction of rotaxane-based shuttles.492 On all accounts, due to the remarkable difference of the two photoisomers and their high reversibility under irradiation, the photoisomerization strategy has been widely applied for the fabrication of various optical molecular shuttles. In other cases, photoinduced chemical reaction493 and photoinduced electrontransfer494 have also been included to drive molecular shuttles. For example, aryl cycloheptatrienes, which can be photochemically converted into the related tropylium ions, have been used to monitor the coconformational change of rotaxanes as one station of the linear component.493 A photosensitizer such as naphthalimide can be designed as one binding site for the macrocycle in a bistable rotaxane.494 In 2001, Leigh and co-workers reported photoinduction of fast, reversible translational motion in a molecular shuttle based on a peptide [2]rotaxane with two binding sites, a succinamide unit and a naphthalene imide unit (Scheme 109). In the general state, the succinamide site was an excellent fit for the benzylic amide macrocycle as compared to the naphthalimide unit, which was a poor H-bond acceptor. After photoreduction by an BL


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Scheme 111. Chemical Structure and Shuttling Principle of Coordination [2]Rotaxane 293

second time scale.495 The supramolecular system consisted of a tetralactam macrocycle and a dumbbell-like component containing a glycylglycine recognition unit and a bulky anthracene stopper (Scheme 110). It was confirmed by 1H NMR spectroscopy and X-ray crystallography that rotaxane 292 adopted coconformations with the macrocycle surrounding the peptide part of the thread in relatively nonpolar solvents. After optical excitation, the macrocycle was displaced from the peptide station to the carbonyl unit directly attached to the anthracene. The explanation was given that in the excited singlet state, hydrogen bonding between the macrocycle and the carbonyl group was enhanced through a considerable transfer of charge from the anthracene ring onto the carbonyl oxygen atom. It provided a new principle for macrocycle motion in molecular shuttles. 4.2.2.3. Electrochemical Stimuli. Electrochemistry is an attractive method to trigger the shuttling behavior of rotaxanes because it can be easily and rapidly turned on and off. What is more, it is also a reagent- and waste-free procedure. Another

external donor such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and a nanosecond laser pulse, naphthalimide became a radical anion, which greatly enhanced its H-bond-accepting affinity and therefore made it bind strongly to the wheel. The wheel was driven from the original succinamide station to the naphthalene imide station. This process was determined by transient changes in the optical absorption spectrum after photoexcitation. Furthermore, the macrocycle moved back to its original position after charge recombination to make the process reversible and cyclable. This molecular shuttle may be applied in the “fetching-and-carrying” of molecules or clusters of atoms between specific locations such as across membranes or the construction of photoresponsive molecular devices. Controlling shuttling motions of the above-mentioned photoactive rotaxanes requires external reagents such as sacrificial reductants and photosensitizers to be added. There also exists an unexpected photoinduced coconformational change based on a rearrangement in the pattern of hydrogen bonds between the thread and macrocycle. No external assistance is needed, and the process occurs on a subnanoBM


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Scheme 112. Shuttling Movement of CBPQT4+ between the TTF and DNP Recognition Units in [2]Rotaxane 294·4PF6

Scheme 113. An Electrochemically Driven Molecular Switch Based on [2]Rotaxane 295·4PF6 with a Rigid Axle

stay in the recognition site with bidentate chelation. Upon oxidation by electrochemical process, the system was switched from four-coordinate to five-coordinate and the process was reversible. As compared to the similar catenane species,499 the metal ion in the rotaxane was less protected from interaction with external species. Furthermore, an oxidative photochemical process could also be used as a stimulus for the motion. TTF is an excellent component for the construction of redox-driven molecular switches because it forms a strong green-color 1:1 complex (Ka = 8000 M−1 in MeCN) with CBPQT4+ and is easily oxidized to TTF+• and TTF2+ ions.428 It

advantage is that the same stimulus can simultaneously act as both effector and detector of the motion.496 The motions of coordination supramolecular systems developed by Sauvage and co-workers were mostly driven by electrochemical signals.497 The general principle is based on two markedly different coordination environments for the CuI and CuII ions. Such an electrochemically controlled rotaxanebased molecular shuttle was designed and operated successfully in 1999; it consisted of both a coordinating ring and a molecular thread containing two different coordination sites (Scheme 111).498 The stable CuI complex made the macrocycle BN


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Scheme 114. Assembly of an Electrochemically Driven Molecular Shuttle on an Au Surfacea

a


was characterized by chronoamperometric and impedance measurements. It was also demonstrated that electrochemical shuttling of the redox-active cyclophane around the molecular string controlled the hydrophobic/hydrophilic properties of the interface, implying that directional molecular mechanical motion in supramolecular systems might be translated into controlled motion of bulk liquids in appropriate surfacemodified containers. Electrochemical stimuli can have a dramatic effect on hydrogen-bonding interactions. On the basis of this mechanism, Leigh and co-workers reported a series of electrochemically switchable hydrogen-bonded molecular shuttles containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle.504 The ring initially resided on the succindiamide moiety due to the favorable hydrogenbonding interactions (Scheme 115). By reduction of the

has been used as one of the binding stations on dumbbells for the construction of molecular shuttles. In the initial research, a monopyrrolotetrathiafulvalene (MPTTF) moiety was chosen to bind CBPQT4+ in competition with a 1,5-dioxynaphthalene (DNP) unit.297 Unfortunately, there simultaneously existed two stable translational isomers in a 1:1 ratio in acetone at room temperature. To improve the situation, MPTTF unit was replaced by a disubstituted TTF.500 It was revealed by UV/vis spectroscopy that the TTF site was first encircled by the CBPQT4+ ring. Addition of Fe(ClO4)3 as an oxidant led to the formation of the TTF+• radical cation and the TTF2+ ion, which produced a charge−charge repulsion between the oxidized TTF+•/2+ unit and the CBPQT4+ ring and resulted in the movement of the ring from the TTF unit to the DNP moiety (Scheme 112).501 The reverse process was achieved by the addition of Zn powder as characterized by UV/vis and 1H NMR spectroscopies. During the process, the CBPQT4+ component moved 3.7 nm between the TTF and DNP recognition sites. In the above research, the use of the TTF unit in the dumbbell component led to the problem that the dumbbell existed as two inseparable isomers created by the facile cis/trans isomerization of TTF. Replacing the TTF unit by the isomerfree MPTTF overcame this shortcoming. However, most investigated MPTTF-containing two-station [2]rotaxanes were mixtures of two possible translational isomers in which the CBPQT4+ ring encircled two units equally. Further research involved the incorporation of a rigid spacer into the two-station [2]rotaxane 295·4PF6, which resulted in a very favorable and temperature-independent isomeric distribution of the rotaxane with the macrocycle staying at the MPTTF site (Scheme 113).502 An eletrochemical stimulus was used to trigger the shuttling motion reversibly. It was calculated from CV that the switching distance was 1.5 nm between the two states. This strategy provided a new concept that utilization of rigidity could be better to obtain well-defined structures and functions in the fabrication of molecular pistons and simple motor molecules. Not only can the binding station on the thread be triggered by electrochemical signals, but the macrocycle such as the tetracationic cyclophane can also be induced by the reduction or oxidation process, and subsequently move back and forth between the two stations on the string.427 Willner and coworkers developed a redox-active [2]rotaxane 296 as a monolayer assembly on an Au electrode.503 The [2]rotaxane comprised a CBPQT4+ cyclophane threaded onto a “molecular string” that included a π-donor diiminobenzene unit and an adamantine stopper (Scheme 114). The cyclophane initially localized on the diiminobenzene unit, and the shuttle was induced by the reduction or oxidation of the cyclophane, which

Scheme 115. An Electrochemically Switchable, HydrogenBonded Molecular Shuttle

naphthalimide moiety to the corresponding radical anion, the macrocycle-binding ability of the two binding sites was altered by over 8 orders of magnitude, which induced the shuttling motion. The reverse process was realized by the oxidation of the naphthalimide radical anion. As described in Scheme 109, the shuttling motion could be induced both by electrochemical stimuli and by photochemical stimuli. To investigate the shuttling characteristics of the above shuttle comprehensively, the naphthalimide unit on the thread was replaced by a naphthalene-1,4,5,8-diimide.505 As compared to the single imide group, the more extensively delocalized BO


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Scheme 116. Three-State Redox-Active Molecular Shuttle 297

Scheme 117. Entropy-Driven Tristable Molecular Shuttle 298a

a

(a) hν (254 nm), 20 min, CH2Cl2, 298 K, 54%, or hν (350 nm), catalytic benzophenone, 5 min, 65%; (b) hν (312 nm), 35 min, CH2Cl2, 298 K, >95%, or hν (400−670 nm), catalytic Br2, 2 min, CH2Cl2, 298 K, ∼100%; (c) CDCl3, 258 K, 85%; (d) CDCl3, 308 K, 90%; (e) hν (312 nm), 35 min, CH2Cl2, >95%, or hν (400−670 nm), catalytic Br2, 2 min, CH2Cl2, ∼100%; (f) hν (254 nm), 20 min, CDCl3, 258 K, 54%.

BP


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Scheme 118. A Multi-Stimuli-Driven Molecular Shuttle 299

part, we will mainly focus on the cooperative effects of different driving forces. Leigh, Zerbetto, and co-workers reported a light- and entropy-driven tristable molecular shuttle, in which the ring could be switched among three different positions on a thread (Scheme 117).506 The [2]rotaxane 298 consisted of the benzylic amide macrocycle with endo-pyridine units, and a thread containing a fumaramide group and a succinic amide ester unit separated by a dodecane chain. Without any stimuli, the position of the macrocycle was over the fumaramide station at room temperature. Photoisomerization of stilbene on irradiation with 254 nm light afforded the cis-[2]rotaxane, which led to displacement of the macrocycle to the succinic amide ester site on the thread at room temperature. However, the conformation of the cis-rotaxane was highly temperature dependent. At 258 K, the macrocycle was not primarily located over either of the designed stations, but positioned over the alkyl chain. To satisfy the hydrogen-bonding requirements of the macrocycle, the alkyl chain presumably adopts a folded “Sshape” conformation. It provided a useful means of controlling translational isomerism in a rotaxane by varying the temperature. Nakashima and co-workers developed a multi-stimuli-driven molecular shuttle, which contained α-CD as the ring, azobenzene as the photoactive group, paraquat as the energy barrier for slipping of the ring, and 2,4-dinitrobenzene groups as the stoppers (Scheme 118).507 The [2]rotaxane 299 showed

aromatic system had a less negative reduction potential for the formation of the radical anion, but also exhibited a second reduction process to give the dianionic state. Therefore, a threestate redox-active molecular shuttle was designed and synthesized (Scheme 116). The reversible and cyclable switching process was confirmed by cyclic voltammetry. The translational isomer ratios and shuttling dynamics for their interconversion in each state were also quantified. The reduction potential of the naphthalene diimide unit was sufficiently low (−0.68 V) to make the rotaxane compatible with operation in self-assembled monolayers on gold. This is the first time that the electrochemically induced shuttling between two states has been demonstrated for a self-assembled monolayer of amide-based molecular shuttles. It provides a significant new platform to achieve molecular machines on surfaces. 4.2.2.4. Multiple Stimuli. As described above, most external stimuli used to induce shuttling are based on the addition of chemical reagents, light, and electrochemistry. A simple temperature or solvent change can also reverse the relative binding affinity of the macrocycle for different stations. In some cases, particularly in complicated systems, molecular machines are triggered by cooperative effects of multiple stimuli to generate some specific functions.506−508 Some examples such as a pH-driven molecular shuttle triggered by redox stimulus476 as demonstrated before can be included in this category. In this BQ


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a reversible isomerization between the trans- and cis-forms of the azobenzene moiety via irradiation with UV light and visible light alternatively, and shuttled back and forth between the azobenzene and the alkylene moieties. Interestingly, the shuttling motion happened only in DMSO, but not in water. Furthermore, it was confirmed by the dynamic NMR, NOE differential, and CD spectroscopies that during the shuttling process in DMSO, the α-CD ring moved to the azobenzene moiety at temperatures below 100 °C, while after the temperatures reached above 100 °C, it was displaced between the azobenzene moiety and the propylene moieties. Thus, the shuttling distance could be controlled by temperature. This system achieved the shuttling movement driven by light, heating, and solvent polarity, respectively, and provided the opportunity to design and develop nanoscale switching devices based on multi-stimuli-driven molecular shuttles. 4.2.3. Modulating Properties and Functions of Molecular Shuttles. 4.2.3.1. New Features of Molecular Shuttles. Studies on switching the positions of macrocycles in bistable rotaxane-based molecular shuttles have developed rapidly and widely. Chemists have started to pay attention to the functions of the molecular shuttles. In 2002, Leigh, Zerbetto, and co-workers reported a system in which a translational movement of a tetralactam macrocycle along a chiral peptide thread elicited a chiral optical response driven by the polarity of the solvent.509 It was initially indicated that the chirality of the molecule could be switched “on” and “off” by controlling the interactions between mechanically interlocked submolecular components as evidenced by circular dichroism (CD). Subsequently, the same group found another method through photochemical stimuli to construct chiroptical switching in a bistable molecular shuttle (Scheme 119).510 In this system, glycyl-L-leucine with an asymmetric center acted as one

binding site on the axle for the wheel-like component. Upon photoisomerization of the olefin station on the thread, the ring moved to the glycyl- L -leucine unit from its previous fumaramide portion. The process provided a chiral optical response from the chiral station as evidenced by CD spectroscopy. In a hydrogen-bond-assembled fullerene molecular shuttle, the different locations of the macrocycle were achieved by changing solvent from dichloromethane to dimethyl sulfoxide, which led to the macrocycle being close to the fullerene spheroid or far away (Scheme 120).511 Although the proximity Scheme 120. Hydrogen-Bond-Assembled Fullerene Molecular Shuttle 301

Scheme 119. Switching “On” and “Off” the Expression of Chirality in Peptide Rotaxane 300a

of the macrocycle did not affect the fluorescence, a significant effect on the triplet−triplet spectrum of the fullerene fragment was detected by 1H NMR and time-resolved spectroscopy. In another example reported by Li and co-workers, the macrocycle translocation switched the energy transfer from the pyrene unit to the perylene moiety, which elicited a fluorescence response.512 4.2.3.2. Construction of Molecular Machines. Stimuliresponsive molecular shuttles provide a promising basis for artificial molecular machines. Various types of rotaxane structures provide good platforms to construct molecular machines, such as molecular logic gates, switches, necklaces, nanomotors, elevators, or molecular rachets. Several typical examples of the functional molecules will be described in this part. Supramolecular systems can be used to construct molecular logic gates to exploit the abilities of molecules to process information. A half adder, with distinct AND and XOR logic gates, was reported on the basis of a [2]rotaxane system 302 that was driven by photochemical stimuli and gave rise to optical outputs (Scheme 121).513 The [2]rotaxane was comprised of an α-CD macrocycle locked onto a dumbbell with two different photoswitchable binding sites, an azobenzene

a

(a) Either 254 nm, CH2Cl2, 20 min, 56%, or 254 nm, CH3CN, 20 min, 49%, or 350 nm, benzophenone, CH2Cl2, 20 min, 70%; (b) either 312 nm, CH2Cl2 or CH3CN, 20 min, 62%, or 400−670 nm, cat. Br2, CH2Cl2, 2 min, >95%, or 130 °C, C2H2Cl4, 6 days, 95%. BR


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Scheme 121. A Half Adder Based on Rotaxane 302 Driven by Photochemical Stimuli

site and a stilbene site, and was end-capped by two fluorescent units, which could be switched independently by using light at different wavelengths (UV/vis). The AND gate could be realized by the output of the changes of absorption response to the two different inputs (irradiated at 380 and 313 nm). The XOR gate could be realized by the fluorescence output in response to the same inputs. Last, the two signals were combined to construct a half adder, which is widely used in mathematics and computing. It is worth mentioning that all of the processes are reversible, indicating that the half adder can be operated repeatedly. This is an early example to mimic a half adder on a unimolecular scale, and gives us an idea to construct more sophisticated systems that involve complicated logic circuits. Molecular logic gates with superior processing capabilities may be realized to break through the physical limits of the silicon chips if the logic circuits can function in solid-state molecular electronic devices. Harada and co-workers reported a molecular abacus based on a molecular necklace of α-CD molecules threading onto poly(ethylene glycol) chains.514 The selected α-CD molecules

reversibly shuttled using a scanning tunneling microscope (STM) under mild conditions in air at room temperature (Figure 27). Three modes of transformation were manipulated and observed by STM: shuttling of a single α-CD ring, simultaneous movement of a pair of α-CDs, and a hook-shape motion formed by the synchronized repositioning of several αCDs. A more complicated system was developed by combining the three modes of transformation. It represents a very important stage in the development of molecular devices, which are the ultimate devices in terms of large-scale integration. Another abacus-like machine was operated by light stimulus, which contained BPP34C10 as a wheel, a dumbbell component with two bipyridinium units, and a photoactive stopper.515 Stoddart and co-workers developed a linear autonomous artificial nanomotor powered by sunlight without generation of waste products. The shuttling movement was undergone in an intramolecular charge-separated state.31 As described in Scheme 122, irradiation of the ruthenium−trisbipyridine complex (green structure) generated a highly reduced excited BS


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cycle−station interactions so that the macrocycle preferred to relocate to the alternative paraquat unit (pink). Remarkably, in this system, the back electron-transfer process was slow enough to allow shuttling of the ring toward the other station in approximately 10% of the molecules in the relaxed process. The system was remarkably stable under mild conditions, and could be driven at high frequency (kilohertz). Development of supramolecular architectures often goes along from the simple to the complex. By combination of three linear rotaxane-based shuttles, a nanometer-scale molecular elevator was designed and successfully operated.32 This system initially formed a pseudorotaxane-like complex 306⊃305 that had a tritopic host with three DB24C8 rings fused together by a triphenylene core, and a trifurcated guest with three dibenzylammonium ions and paraquat units linked to a central benzenoid core (Scheme 123). By functionalization of the ends of each leg of this pseudorotaxane with bulky 3,5-di-tertbutylbenzyl feet, the rotaxane-type elevator 307 formed and was characterized by 1H NMR spectroscopy, electrochemistry, and absorption and fluorescence spectroscopy. It was circa (ca.) 2.5 nm in height with a diameter of ca. 3.5 nm, and the distance traveled by the platform was about 0.7 nm. Because of the acid−base switching ability of a [2]rotaxane based on the DB24C8/secondary ammonium salt recognition motif,79 this triply threaded two-component system translated up and down on alternative addition of slightly more than 3 equiv of appropriate acid and base. During the shuttling movement from the upper to lower level, a force of up to 200 pN was generated. Furthermore, from a plot of the absorbance changes on titration of the system with base, it could be shown that there existed three quite distinct steps and the recognition sites operated not in unison but rather one after the other.

Figure 27. A molecular abacus through STM manipulation of a CD necklace. (a) Chemical structure of the CD necklace; and (b−d) shuttle manipulation by STM. Reproduced with permission from ref 514. Copyright 2000 American Chemical Society.

state. An intramolecular electron transfer then occurred between the excited metal center and the most easily reduced paraquat station (blue), on which the macrocycle resided preferentially. The result was destabilization of the macro-

Scheme 122. A Linear Autonomous Artificial Nanomotor 304 Powered by Sunlighta

a

Reproduced with permission from ref 333. Copyright 2012 John Wiley & Sons, Ltd. BT


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Scheme 123. (a) Synthesis of a Molecular Elevator and (b) Cartoon Representation of Operating It by Acid/Base Paira

a


co-worker, acted as a compartmentalized molecular machine with the behavior of ratcheting a particle energetically uphill.33 Control over the kinetics for exchange of the substrate between two sites of the machine was introduced to create artificial Brownian machines, which were more sophisticated than simple positional switches. The rotaxane-based system 309 was initially statistically balanced. After being triggered by a balance-breaking stimulus upon irradiation with 312 nm light to generate a 49:51 ± 2% E:Z photostationary state and followed by removal of the kinetic barrier, the balance was restored by biased Brownian motion of the ring toward the new equilibrium distribution. When the barrier was restored, the system became unlinked and was not in equilibrium, although statistically balanced. The last resetting step through the Z/E olefin isomerization made the system statistically unbalanced, unlinked, and not in equilibrium. This stimuli-induced irreversible molecular machine in the position of the macrocycle represented a new type of molecular shuttle in phenomenological terms.

Another way of amplifying and harnessing molecular shuttling motions is the formation of linear artificial molecular muscles. One example was based on the switchable and bistable [3]rotaxane 308 with a pair of mobile CBPQT4+ rings encircling a single dumbbell containing TTF and NAP stations (Scheme 124a).516 The locations of the two ring components of this [3]rotaxane along its dumbbell-shaped component could be controlled precisely either by chemical or by electrochemical stimuli with the inter-ring distance changing from 4.2 to 1.4 nm. At low temperatures, it was revealed by fast scan-rate cyclic voltammetry that the two rings of the [3]rotaxane moved stepwise from their respective ends of the rotaxane under redox conditions. Furthermore, disulfide tethers attached covalently to the rings were designed for the purpose of their self-assembly onto a gold surface. Because of anchoring to a nanoelectromechanical system, the device underwent controllable and reversible bending when subjected to chemical oxidants and reductants (Scheme 124b). It was indicated by control studies that the contraction and extension movements of the surface-bound molecular muscles changed 5 orders of magnitude in size. This observation gives evidence for the hypothesis that nanoscale movements of molecular muscles can be used to perform larger-scale mechanical work when attached to solid substrates. Classic stimuli-responsive molecular shuttles often act as reversible molecular switches.517 In contrast, the system based on rotaxane 309 (Figure 28), which was reported by Leigh and

4.3. Mobility of [c2]Daisy Chains

In addition to pseudorotaxanes and rotaxanes, another family of threaded structures are described as daisy chains (Figure 29), which have been constructed from self-complementary AB-type plerotopic monomers, each of which contains two units: A as the host part and B as the guest part.518−523 The self-assemblies can be either acyclic or cyclic daisy chain arrays, [a]daisy chains or [c]daisy chains, respectively. The numerical descriptor refers BU


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Scheme 124. (a) A Bistable [3]Rotaxane-Based Linear Artificial Molecular Muscle and (b) Graphical Representation of the Proposed Operation Mechanism of a Molecular Device Based on This [3]Rotaxanea

a


Figure 28. Ratcheting a particle energetically uphill with a compartmentalized molecular machine and the structure of [2]rotaxane 309. Reproduced with permission from ref 33. Copyright 2006 American Chemical Society.

applied for the preparation of [c2]daisy chains.524−528 A [c2]daisy chain prepared by Stoddart and co-workers was based on the DB24C8/secondary ammonium salt recognition

to the number of monomer units that comprise any particular superstructure. For example, a cyclic dimer is a [c2]daisy chain (Figure 29). Various host−guest recognition motifs have been BV


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bidentate chelates, corresponding to the contracted situation 311. It meant that the rotaxane dimer was capable of stretching and contracting at will under metal exchange. On the basis of the interactions between DB24C8 and a R2NH2+ unit or a Bpym2+ unit, an acid−base controllable [c2]daisy chain was designed and synthesized by Stoddart and co-workers.530 In this molecule, the two DB24C8 rings moved between the two different recognition sites, R2NH2+ and Bpym2+, under acid−base control (Scheme 126). The working mechanism of this [c2]daisy chain was similar to the above examples of molecular shuttles and could be named a molecular muscle. There were two stable states, a contracted state and an extended state during the movements of the ring. Under normal conditions, the DB24C8 resided at the NH2+ center for its better binding affinity, and this was considered as the extended state. When the ammonium center was deprotonated by adding base, the ring slid to the bipyridinium unit, and this was the contracted state. The lengths of the two states were ca. 3.1 and 2.2 nm, respectively. The synthetic strategy for this acid−base controllable muscle molecule is possible to be further functionalized and incorporated into liquid crystalline531 or polymeric532 systems. All of the above-reported artificial contraction/stretching molecules can only change their lengths stepwise caused by translocation of the ring parts between different guest moieties. To mimic the unique spring-like function of biological systems,535 Huang and co-workers prepared a molecular device 313 based on an amino-modified copillar[5]arene without different guest moieties in the thread component.528 The length of the device changed continuously as a spring when the polarity of the solvent was changed (Figure 30). On the basis of the investigation of pillar[5]arene/alkane pseudorotaxanes, it was known that a guest containing a linear alkyl chain with four methylenes usually showed a bigger binding constant than

Figure 29. Different types of daisy chains.

motif.518 Later, they applied crown ether/paraquat recognition motifs to fabricate [c2]daisy chains.519 In 2000, Sauvage and coworkers prepared a linear rotaxane dimer driven by coordination interactions.526 Very recently, Huang and coworkers reported two types of [c2]daisy chains based on the benzo-21-crown-7/secondary ammonium salt recognition motif and the pillar[5]arene/alkyl chain recognition motif, respectively.527,528 Given the intrinsic extension and contraction properties of [c2]daisy chains (Scheme 125a), they are excellent building blocks for the construction of artificial molecular muscles526−533 and application in other areas, such as chemical catalysts.534 To simulate real muscles, Sauvage and co-workers designed a mechanically interlocked system, in which two linear components glided along one another but stayed together because of the rotaxane nature of the system.526,529 The “hermaphrodite” monomer contained a coordinated macrocyclic part and a linear part incorporating simultaneously a fourcoordinate and a five-coordinate metal center. The molecular “muscle” was initially synthesized in its extended conformation 310, in which the two CuI centers were coordinated to the fourcoordinate situation of the axle (Scheme 125b). When the system was demetallized and subsequently remetalated with ZnII, the metal centers became coordinated to the three

Scheme 125. (a) Extended and Contracted Movements of a [c2]Daisy Chain and (b) a Molecular Muscle Based on the Metal Coordination Systema

a

Reproduced with permission from ref 333. Copyright 2012 John Wiley & Sons, Ltd. BW


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Scheme 126. Formation of a pH-Controllable [c2]Daisy Chain 312a

a


similar compounds with longer or shorter alkyl chains.270,536,537 It was concluded by the 1H NMR signals of the encapsulated or free protons that when a pillar[5]arene was swaying along a guest with a long linear alkyl chain, the cavity was statistically located on the four methylenes whose protons showed relatively larger upfield shifts in solution. From this method, the length of this molecular spring was calculated. In CDCl3, it was in a contracted state with a length of about 31 Å. In DMSO-d6, it was in an extended state with a length of 37 Å. This work not only provided a useful method to investigate the relative motion of a pillararene-based molecular switch, but also offered a basis for the construction of an environmentally responsive interlocked polymer that could mimic the biologic contraction/stretching process.

5. APPLICATIONS OF PSEUDOROTAXANES AND ROTAXANES Figure 30. Molecular structure of a copillar[5]arene-based rotaxane dimer 313 and schematic presentation of its vibration motion. Reproduced with permission from ref 528. Copyright 2012 Royal Society of Chemistry.

5.1. Protection of Encapsulated Molecules

5.1.1. Unstable Species. When linear molecules are encircled by macrocycles, they are laid under a specific environment provided by the wheel components, which may change the properties of the encircled molecules, such as

Scheme 127. Rotaxane-Stabilized Thiophosphonium Salt from Disulfide and Phosphine

BX


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enhancement of fluorescence signals, transformation of selfassembly morphologies, and activation and stabilization of drugs.538−542 In 1999, Vögtle and co-workers investigated the influence of the ring on the rate of chemical reactions of rotaxanes.543 It was found that, due to the steric hindrance of the wheel, the rates of hydrogenation of rotaxanes were substantially lower than those of hydrogenation of free axles. In other words, the rotaxane wheels provided noncovalent protecting groups to decrease the activity of functional groups in the central part. The rotaxane structures also have the capability of stabilizing unstable species. For example, an alkylthiophosphonium salt, which was hard to isolate, acted as a key intermediate in the desulfurization of dialkyl disulfides with phosphines.544 However, using a rotaxanated form, a stable thiophosphonium salt 315·2PF6 was isolated successfully through the reaction of a rotaxanated disulfide with hexamethylphosphorus triamide (Scheme 127). The structure of the thioposphonium salt was confirmed by spectroscopic and X-ray crystal structure analysis. An interesting contrast is the reduced acidity of secondary ammonium salts of DB24C8/sec-ammonium hexafluorophosphate (PF6)-type rotaxanes reported by Takata and coworkers.545−547 By exchanging the counteranion PF6 for the fluoride anion, they successfully isolated nonionic free secamine-type rotaxanes.547 This counteranion-exchange method for neutralization of the ammonium rotaxanes decreased the cationic character of the ammonium moiety and produced the unusual sec-amine-type rotaxanes. Kim and co-workers reported a U-shaped conformation of a bolaamphiphile embedded in CB[8] (Scheme 128).548 With

back to last century, when azo dye-based dumbbell compounds were encircled by CDs.373 As compared to nonrotaxanated dyes, the rotaxanated dyes, which were permanently protected inside the cavities of CDs, showed good solubility in most solvents, and less aggregation in water.164 Because dyes are often covalently attached to solid substrates for application, whether the encapsulation of dye molecules on surfaces by macrocycles can improve the longevity of the azo chromophore or not sounds like a very important question. The problem was addressed clearly by the synthesis of a chlorotriazine-functionalized azo dye rotaxane in 2001.373 This reactive dye rotaxane displayed enhanced stability toward reductive bleaching. Besides enhanced stability, dye rotaxanes also provide a versatile approach to control of interfacial charge transfer, which has been utilized to construct molecular-level insulation. For example, Haque’s group presented an effective strategy based on dye rotaxanes for the immobilization of azo dyes on nanocrystalline TiO2 electrodes (Figure 31).549 After the dye

Scheme 128. U-Shaped Conformation of a Bolaamphiphile Embedded in CB[8]a

Figure 31. Insulated dye on TiO2 electrodes based on rotaxane 317. Reproduced with permission from ref 549. Copyright 2004 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

molecules were encapsulated by CD molecules, the interaction between the TiO2 semiconductor surface and dye molecule was limited, allowing control of interfacial charge recombination. Such CD threaded sensitizer dye rotaxanes can not only provide a shell to increase the stability of dyes, but also offer a physical barrier to charge combination. In addition, the driving force for dye adsorption on a nanocrystalline TiO2 film was also provided by the hydrophilic outer surface of CD. Besides azo dye rotaxanes, fluorescent dyes such as cyanines,174 stilbenes,163 and acetylenes550 have also been encapsulated within CDs and behaved differently when trapped inside the macrocyclic cavity. For example, their fluorescence efficiency and photostability increased, their reversible redox processes were exhibited, their kinetic stability was enhanced, and their fluorescence quenching was reduced. For cationic rhodamine 6G, cucurbit[7]uril acted as its receptor to form an inclusion complex, which allowed the development of a “supramolecular dye laser” with environmental and safety benefits, high lasing efficiency and stability, and an impressive

a


the stable conformation of the long alkyl chain of the guest 316, a [2]pseudorotaxane complex would be expected. However, when CB[8] and the guest were mixed together in a molar ratio of 1:1, the long alkyl chain of the guest adopted a U-shaped conformation inside the CB[8] cavity along with two positively charged ammonium groups interacting with the same carbonyllaced portal of CB[8]. The discovery of the unconventional conformation of 316 in the macrocycle not only sheds light on the unusual architecture of archaebacterial membranes, but also may provide an opportunity to design molecular knots, machines, and switches. 5.1.2. Dyes. Macrocyclic molecules in rotaxanes can serve as protectors for dyes. An early example of dye rotaxanes traced BY


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beam shape.551 All of these features provide the dyes with numerous practical applications. Squaraines are a family of fluorescent near-IR dyes with specific photophysical properties for wide applications such as imaging, nonlinear optics, photovoltaics, and ion sensing.552 However, their inherent reactivity with nucleophiles and the tendency to form nonfluorescent aggregates in water limit the use of these dyes.553 To improve the chemical stability and photophysical properties of the dyes, some squaraine-derived rotaxanes have been developed by Smith and co-workers in recent years, which provided an effective approach to protect the dyes.554,555 The first type of wheel-like molecule, which was used to construct these rotaxanes, was tetralactam macrocycle (Scheme 129).554,556 With the squaraine unit as the template

based on the bifurcated hydrogen bonding between the 1,3dicarboxamide moieties and the squaraine oxygens, rotaxanes 319a and 319b were obtained through the clipping method. There were several advantages for squaraine dyes after formation of rotaxanes. The encapsulating macrocycle altered the photophysical properties of a squaraine dye by inducing red-shifts in its absorption/emission wavelengths, and stabilization of the squaraine excited state by electronic reorganization of the surrounding macrocycle.557 Meanwhile, solvent-induced quenching was reduced due to the deep encapsulation of the dye inside the macrocycle. Furthermore, the chemical stability of the squaraine was greatly increased when the rotaxane formed. For example, it prevented the dye from nucleophilic attack and aggregation and made its color remain for many weeks. By a similar strategy, other macrocycles were designed and synthesized to encapsulate squaraines.558−561 Investigations on these rotaxanes indicated that squaraine encapsulation also occurred in highly competitive media such as mixed aqueous/ organic solutions, vesicle membranes, and the organelles within living cells.558 The protection role in stabilizing the chemical and physical properties of squaraine dyes was maintained. A rotaxane with an anthracene-containing macrocycle produced more red-shift in absorption/emission wavelengths, while another tetra(iodo)-substituted squaraine rotaxane demonstrated remarkable photostability, very high resistance to photobleaching.562 Formation of rotaxanes also helps squaraine dyes to achieve some challenging tasks. For example, squaraine-rotaxanes 320a and 320b (Figure 32a) had tremendous promise as extremely stable near-infrared fluorescent probes for in vitro and in vivo optical imaging of live and fixed cells (Figure 32b) and living mice (Figure 32c). They were considered to be superior substitutes for sulfonated carbocyanine dyes in many biotechnology and imaging applications.563,564

Scheme 129. Formation of Squaraine-Based Rotaxanes 319a and 319b

Figure 32. (a) Chemical structures of squaraine-based rotaxanes 320a and 320b; (b) fluorescence-microscopy image of live mammalian cells treated with 320a; and (c) optical image of a live mouse with subcutaneous injections of S. aureus and E. coli bacteria that were prelabeled with 320b. Reproduced with permission from ref 563. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. BZ


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interactions have been used in great convenience.81,136,587−590 For example, Huang and co-workers reported dynamic supramolecular polymers that have linear or cross-linked topologies.588 The linear supramolecular polymer was first obtained by self-organizing a heteroditopic monomer 322 with a bridging ligand driven by hydrogen bonding and face-to-face π-stacking interactions between the cryptand host part and paraquat derivative guest part on the monomer. Coordinating the triazole ligand on this monomer with PdCl2(PhCN)2 then afforded a cross-linked supramolecular polymer network (Figure 34). Furthermore, the linear chains were restored

5.2. Stimuli-Responsive Supramolecular Polymeric Materials

Supramolecular materials, with components bridged by dynamic and reversible noncovalent bonds, have had a strong impact on materials science accordingly.43,44,565−568 Because of their stimuli-responsive properties, pseudorotaxanes play a very important role in the construction of supramolecular polymers,569−576 making them unique candidates for supramolecular materials. In the 1990s, Gibson and co-workers constructed a series of linear, branched, or cross-linked polypseudorotaxanes and polyrotaxanes.577−579 Using the dynamics of threading/dethreading of the macrocycles, they found a method to measure polymer molecular weights.577 They also investigated the solvent switchable properties of the supramolecular polymers.473,578,579 In the polyurethanes/crown ethers systems, the crown ethers are localized at the NH groups by hydrogen bonding in CHCl3, while they are delocalized (mobile) but removed from the NH groups in DMSO.473 A variety of noncovalent interactions can be used to bring the building blocks together to prepare supramolecular polypseudorotaxanes. Taking hydrophobic interaction, for example, CDs and CB[n] are ideal and widely used macrocyclic hosts for supramolecular polymers because of their hydrophobic cavities, which can selectively bind guest molecules.580−586 Zhang and co-workers elegantly utilized CB[8] to bind a multifunctional monomer 321 for the fabrication of supramolecular polymers (Figure 33) based on multiple host-stabilized charge-transfer

Figure 34. Chemical structure of heteroditopic monomer 322 and cartoon representation of the metal-coordination-based reversible control of the supramolecular polymer topology. Reproduced with permission from refs 40 and 588. Copyright 2012 Royal Society of Chemistry and 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

again by the addition of a competitive ligand, PPh3, to trap the cross-linker. This orthogonal noncovalent interaction strategy provides an effective method for the topological control of supramolecular polymers. On the basis of different noncovalent interactions and the structures of the building blocks, there exist various responsivenesses of the resultant supramoleclar polymeric materials to surrounding environments. First, thermoresponsiveness is one of the fundamental and most accessible properties of supramolecular polymeric materials as weak noncovalent interactions are inherently susceptible to thermal stimuli. For example, a thermosensitive supramolecular polymer hydrogel was reported recently by Scherman and co-workers, which was obtained by recognizing the pendant paraquat units on multivalent copolymer 323 and NAP moieties on copolymer 324 by the host molecule CB[8] to facilitate reversible crosslinking of the two copolymers (Figure 35a).591 The resultant supramolecular hydrogels showed intermediate mechanical properties (plateau modulus = 350−600 Pa and zero-shear viscosity = 5−55 Pa·s) based on the rheological characterization at 5 wt % of the polymers in water. Moreover, the hydrogels exhibited thermal reversibility because of the dynamic crosslinks (1:1:1 supramolecular ternary complexes of CB[8]/ paraquat/NAP), which could be qualitatively characterized by probing the hydrogel microstructures. Upon heating, the hydrogel underwent a gel-to-sol transition, which was reversed upon cooling or adding more CB[8] (Figure 35b).

Figure 33. Cartoon representation of the formation of a CB[8]-based water-soluble supramolecular polymer. Reproduced with permission from refs 40 and 586. Copyright 2012 Royal Society of Chemistry and 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

interactions in water. Such polymers could form a potassium cation-responsive supramolecular gel at a low concentration (4 mM).586 Therefore, by tuning the stimuli-responsive properties of self-complementary molecules, water-soluble potassium cation-responsive supramolecular polymeric materials driven by hydrophobic interactions can be easily obtained. To build highly complex and multifunctional supramolecular polymers, either self-sorting organization of two heteroditopic monomers or the combination of multiple noncovalent CA


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links for supramolecular polymer gels by changing the solution pH, such as in the DB24C8/dibenzylammonium salt systems.595 As a challenge to the formation of supramolecular polymer gels from low-molecular-weight molecules, Huang and co-workers prepared a heteroditopic monomer 326 comprising the reversible host−guest recognition motif between the DB24C8 unit and its complementary guest dibenzylammonium salt moiety linked by a long aliphatic chain (Figure 37).598 Selfassembly of this monomer in solution produced a linear supramolecular polymer, and then formed an entangled threedimensional fiber network, resulting in gelation. Furthermore, reversible gel−sol transitions were realized by subsequent alternation of heating and cooling, or acidification and neutralization. More importantly, the dual-responsive supramolecular polymer gel was successfully employed in the controlled release of rhodamine B. Another monomer containing a triptycene-derived polyether moiety and two dibenzylammonium salt units designed by Chen and coworkers was also used to form a dual-responsive supramolecular polymer network gel.599 These two studies demonstrated that functional smart materials can be constructed by rational design of low-molecular-weight building blocks. Besides pseudorotaxanes, the [c2]daisy chains and rotaxanes can also be incorporated into supramolecular polymeric materials, allowing regulation of mechanical properties by transforming motions of these MIMs at the molecular level.600,601 Grubbs and co-workers prepared mechanically interlocked supramolecular polymers from the polymerization of [c2]daisy chains by using ring-closing metathesis.602 Subsequently, with the aim to create muscle-like supramolecular polymeric materials, Stoddart and co-workers reported a linear, mechanically interlocked, main-chain polymer with repeated bistable [c2]daisy chains incorporated (Figure 38).522 The [c2]daisy chain molecule 327 had propargyl groups on its stoppers, which underwent reversible extension and contraction upon exposure to external stimuli. This dialkynefunctionalized [c2]daisy chain AA monomer then was subjected to step-growth polymerization with a diazide BB monomer employing click chemistry. The resultant bistable poly[c2]daisy chain 328 was analyzed by size exclusion chromatography/ multiangle light scattering, showing that this polymer had a molecular weight (Mn) of 33 kDa and a polydispersity of 1.85. This Mn value demonstrated that each polymer chain was composed of about 11 repeating units. The acid/base controlled extension and contraction behavior of both the monomer and the polymer were studied in solution by 1H NMR spectroscopy, UV/vis absorption spectroscopy, and cyclic voltammetry. The [c2]daisy chains and their polymeric derivatives underwent quantitative, efficient, and reversible extension and contraction processes in solution. The kinetics of the acid/base-induced switching processes on the basis of the measurements of stopped-flow spectrophotometry showed that the switching rates for the extension/contraction processes of polymeric [c2]daisy chain were faster than those of its monomeric counterpart. From this valuable example, we can conclude that movements at the molecular level can lead to changes in macroscopic material properties. Polyrotaxanes have become a hot topic in polymer chemistry and materials science because of their unique topologies and properties.603−609 Early examples attempted to construct crosslinked polyrotaxanes. For example, Gibson and co-workers prepared macromolecular knitting by self-assembly of poly[bis(5-methylene-1,3-phenylene)-32-crown10 sebacate] and pre-

Figure 35. (a) Chemical structures of polymers 323 and 324 and cartoon representation of the formation of a 3D network cross-linked by CB[8]; (b) illustration of the thermal reversibility of the supramolecular polymer network hydrogel demonstrated by SEM images. Reproduced with permission from refs 40 and 591. Copyright 2012 Royal Society of Chemistry and 2010 American Chemical Society.

Second, photoresponsive supramolecular polymer gels have been widely fabricated on the basis of azobenzene-type derivatives.592,593 A beautiful example was reported by Jiang and co-workers through the association/dissociation of the pseudorotaxane-containing azobenzene-type derivative 325 and α-CD.594 Initially, the self-assembly of poly(ethylene glycol) and α-CD formed a pseudopolyrotaxane hydrogel. After addition of water-soluble competitive guest 1-[p-(phenylazo)benzyl]pyridinium bromide trans-325 and ultrasonication, the gel turned into a transparent sol in a few minutes (Figure 36) due to the stronger host−guest interactions between trans-325 and α-CD. Using photoisomerization of the azobenzene moiety upon UV (365 nm) irradiation, the α-CD⊃trans-325 complex dissociated and then α-CD returned to the PEG chain, reforming the hydrogel. The reversible process was achieved by visible light irradiation. The photoresponsive gel to sol and sol to gel transitions could be repeated for several cycles without any disturbance. Third, chemoresponsiveness is still a fascinating property, which makes some applications of responsive supramolecular polymeric materials possible.595−597 Some early successful examples were based on disruption of the noncovalent crossCB


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Figure 36. Cartoon representation of photoresponsive gel−sol−gel transitions driven by competitive inclusion complexation. Reproduced with permission from refs 40 and 594. Copyright 2012 Royal Society of Chemistry and 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

cooperatively.609 This gel has good tensility, low viscosity, and large swellability in water. Recently, Takata and co-workers reported the synthesis of a type of graft polyrotaxanes (GPR) 329 by empolying the grafting-onto protocol to a polypseudorotaxane and analyzing its structural characteristics and dynamic behavior.600 Mixing poly(DB24C8) (Mn = 4000, PDI = 1.35) with an equimolar amount of secondary ammonium salt in CH2Cl2 produced the polypseudorotaxane. Polyrotaxanes 329 were prepared by subsequent addition of the polytetrahydrofuran grafting agent (Figure 39). To achieve high mobility of the graft chain GPRH2PF6, N-acetylation of the polymer was performed to give GPRAc. The GPRAc was obtained in 94% yield by treatment of GPRH2PF6 with acetic anhydride (Ac2O) and triethylamine at 60 °C for 3 days. The movement of the DB24C8 wheel to the polytetrahydrofuran moiety was confirmed by 1H NMR spectroscopy and other techniques. Because of the host− guest recognition, the secondary ammoniun salt part of the graft chain was located in the crown ether cavity of the main chain for the polymer GPRH2PF6. After N-acetylation, the graft chain length of GPRAc obviously increased because of the enhanced mobility or free translation of the graft chain. The mobile rotaxane connection decided the dynamic property of the polyrotaxanes as compared to traditional graft copolymers, which allowed them to be applied in advanced supramolecular materials. Fourth, redox-responsive supramolecular polymeric materials can be constructed and manipulated.610,611 Harada and coworkers synthesized poly(acrylic acid) (Mw = 2.5 × 105) modified with β-CD units as a host polymer 330 and with ferrocene moieties as a guest polymer 331. Using versatile host−guest interactions inherent with easily tuned switching efficiencies and functions, self-healing supramolecular polymeric materials were prepared (Figure 40).612 Hydrogelation led to an increase of the viscosity of the solution when a 1:1 mixture of a solution of host polymer 330 and guest polymer 331 in a pH = 9 boric acid/KCl/NaOH buffer solution was prepared. Further studies showed that host−guest comple-

Figure 37. (a) Cartoon representation of the formation of a linear supramolecular polymer from a heteroditopic monomer 326; and (b) illustration of dual-responsive supramolecular polymer gel−sol transitions and supramolecular polymer aggregates (glue-like viscous liquid and transparent film). Reproduced with permission from refs 40 and 598. Copyright 2012 Royal Society of Chemistry and 2011 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

formed polyurethane containing paraquat moieties in tetrahydrofuran.608 The branching process was reversible depending on the solvent and temperature. Okumura and co-workers reported a polyrotaxane gel by figure-of-eight cross-links acting like pulleys to equalize the tension of the polymer chains CC


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Figure 38. Chemical structures of [c2]daisy chain 327 and poly[c2]daisy chain 328 and cartoon representation of the acid−base switching process of 328. Reproduced with permission from refs 40 and 522. Copyright 2012 Royal Society of Chemistry and 2009 American Chemical Society.

Figure 39. Chemical structures of GPRH2PF6 and GPRAc. Reproduced with permission from ref 40. Copyright 2012 Royal Society of Chemistry.

mentarity and multipoint cross-links have cooperative roles in forming the supramolecular polymer network hydrogel. The reversible gel−sol transitions were achieved by adding oxidant (NaClO aq) and then reductant (glutathione, GSH). Moreover, electrochemical oxidation also transformed the supramolecular polymer hydrogel into the sol. Subsequent reduction by heating the sol at 50 °C recovered the hydrogel. The dynamic reversible nature of the host−guest interactions between the side chains of both polymers determines the self-healing property of the supramolecular polymer network (Figure 40a). This was proved by rheological experiments. A cube-shaped supramolecular polymer hydrogel was cut in half, and then rejoined. After standing for 24 h, the crack disappeared, and the sample hydrogel sufficiently healed to

Figure 40. (a) Chemical structures of host polymer 330 and guest polymer 331 and schematic representation of the redox-responsive supramolecular polymer network prepared from self-assembly of them; and (b) cartoon representation of the self-healing process of the supramolecular polymer network. Reproduced with permission from refs 40 and 612. Copyright 2012 Royal Society of Chemistry and 2011 Nature Publishing Group.

form one gel (Figure 40b). What is more interesting is that the self-healing property of the hydrogel could be controlled by redox reactions. If the cut surfaces of the hydrogel were coated with an aqueous solution of oxidant (NaClO, 7 mM, 20 μL), CD


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healing did not occur upon attaching the two pieces together. However, the healing was observed by spreading reductant GSH aq (20 mM, 20 μL) onto the oxidized cut surface and allowing the hydrogel to stand for 24 h. This elegant study represents a good example of self-healing supramolecular polymeric materials based on host−guest interactions and will further prompt the application of host−guest chemistry in the fabrication of advanced supramolecular polymeric materials. Using the same host−guest recognition motif, Yuan and coworkers prepared redox-responsive supramolecular diblock copolymer vesicles in aqueous solution by self-assembling two end-decorated homopolymers, polystyrene-β-cyclodextrin and poly(ethylene oxide)-ferrocene.613 Interestingly, the assembly and disassembly speeds of the supramolecular vesicles were controlled by changing the applied voltage. Furthermore, the supramolecular vesicles acted as nanocapsules carrying molecules within their hollow cavities, and the voltage regulated the drug release time. This valuable example represents the possibility of employing supramolecular nanocapsules in electrochemical therapeutics. Multiresponsive supramolecular polymeric materials give rise to more flexibility for the construction of intelligent materials.614−621 This can be achieved by consummate integration between the material structures and the environmental responsiveness of noncovalent interactions. Recently, Huang and co-workers reported a quadruple-responsive supramolecular polymer network gel constructed by orthogonal self-assembly, which showed good shape-persistent and elastic properties.615 Heteroditopic monomer 332 contained a 1,2,3triazole group between the B21C7 host unit and its complementary dialkylammonium salt guest moiety. Selfassembly of this monomer in solution formed a linear supramolecular polymer driven by the host−guest interactions between the B21C7 unit and dialkylammonium salt moiety. A supramolecular polymer network gel then was obtained by adding a cross-linker (Figure 41), [PdCl2(PhCN)2], to the linear supramolecular polymer solution because the 1,2,3triazole group acted as a ligand for coordination with palladium(II). The gel exhibited a reversible gel−sol transition in response to quadruple distinct stimuli (pH-, thermo-, cation-, and metallo-induced) (Figure 42). Generally, pH- and thermoinduced gel−sol phase transitions of supramolecular polymers are facile by adding base and acid or heating and cooling. Herein, the cation- and metallo-induced gel−sol phase transitions are unique by adding K+ to prevent the formation of supramolecular polymers or adding the competitive ligand PPh3 for the cross-linker to deconstruct the supramolecular polymer network. The net-like morphology of the supramolecular polymer gel was controlled by the amount of crosslinker added to the system, and the material could be molded into shape-persistent, free-standing objects with elastic behavior. These features were totally due to the dynamically reversible host−guest complexation and good mechanical properties of the cross-linked polymer network, making this supramolecular polymer gel an unprecedentedly intelligent soft material. Furthermore, the same research group has also used the same monomer 332 to prepare supramolecular polymer nanofibers via the electrospinning technique620 and obtain adjustable supramolecular polymer microstructures by a breath figure method.621

Figure 41. Cartoon representation of the formation of linear and cross-linked supramolecular polymers from monomer 332. Reproduced with permission from refs 40 and 615. Copyright 2012 Royal Society of Chemistry and 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 42. Reversible gel−sol transitions of the supramolecular polymer network gel 332 triggered by four different stimuli (pH-, thermo-, cation-, and metallo-induced). Reproduced with permission from refs 40 and 615. Copyright 2012 Royal Society of Chemistry and 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

5.3. Applications of Rotaxane-Based Molecular Machines

As described in the above section, component parts of rotaxanes can undergo controllable and reversible motions without damaging their mechanically interlocked structures. Therefore, rotaxanes have potential applications in the development of artificial molecular devices. Numerous examples of molecular machines based on pseudorotaxanes and rotaxanes in solution have been successfully reported. However, most useful molecular devices generally work in the solid state, at interfaces or on surfaces. To make these molecules useful for practical applications, the first important thing is to transfer molecular-machine technology from solution to solid substrates or surfaces. It is very challenging work because the behavior of rotaxane systems on solid supports must be different from that observed in solution. Several new questions have to be addressed: (i) Will these mechanically interlocked systems still move on solid supports just like what they do in solution? (ii) How does one analyze these new CE


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Figure 43. Chemical structures of rotaxanes 333 and 334 used in the molecular devices.

Figure 44. Chemical structures of [2]pseudorotaxane 335 and [2]rotaxane 336.

Figure 45. Chemical structure of the [2]rotaxane 337 used in the crossbar memory.

switches and gates have been already demonstrated in the solution phase. With a different operation mechanism from the above examples, Stoddart, Heath, and co-workers reported redox-active, degenerate, two-station, and V-shaped [2]rotaxane 333 and [3]rotaxane 334 (Figure 43) as a monolayer between electrodes made from titanium and aluminum oxide.622,623 These solid-state molecular electronic devices can be configured to generate AND and OR logic functions. On the basis of the Langmuir−Blodgett monolayer of the compounds, single-molecule-thick electrochemical junctions were fabricated and showed their transport properties. The strategy for the LB monolayer formation based on [2]pseudorotaxanes and [2]rotaxanes has also been utilized to develop solid-state molecular switch tunnel junction devi-

functional materials and their dynamics? (iii) Do the threaded molecules have preferential orientation on solid supports, which is required for large-amplitude motions in macroscopic world? To our delight, there are already some examples of switchable and detectable supramolecular machines that have been constructed into molecular devices through suitable engineering. 5.3.1. Molecular Electronic Devices Based on the Langmuir−Blodgett Monolayers. The Langmuir−Blodgett (LB) technique is an elegant approach to arranging molecules into well-organized monolayers and multilayered films, which can be widely applied to create ultrathin films with a specific architecture that can be used as chemical sensors, modified electrodes, or molecular electronic devices.622 Rotaxane-based CF


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ces.624−626 For example, one such device was developed on the basis of the [2]pseudorotaxane 335 and [2]rotaxane 336 incorporating both hydrophobic and hydrophilic regions to allow self-organization (Figure 44).624 The electrical properties were found to be highly dependent on the supramolecular structure, the presence of bistability within the (super)molecule, and the organization of the LB film. The device exhibited a (noncapacitive) hysteretic current−voltage response that switched the device between high- and low-conductivity states, although control devices exhibited no such response. This study represents a significant step toward elucidating molecular structure/device property relationships for active molecular electronic devices. Later, with a monolayer of bistable [2]rotaxane 337 containing both hydrophobic and hydrophilic regions (Figure 45) as the data storage elements, Stoddart, Heath, and coworkers developed a 160-kilobit molecular electronic memory circuit fabricated at a density of 1011 bits cm−1 (pitch 33 nm; memory cell size 0.0011 μm2).627 This memory circuit has achieved the dimensions of a dynamic random access memory circuit projected to be available by 2020. This proves that it is feasible and promising to use molecules as nanoscale components to create miniaturized electric circuits and develop molecular computing. 5.3.2. Rotaxane-Based Molecular Machines on Surfaces. Surface assembly of mechanically interlocked molecular systems628−631 affords the platform to utilize the motional characteristics of these molecules in electronics, data storage, and macroscale devices. Gokel, Kaifer, and co-workers reported a catenane that was attached to an electrode surface through a [2]pseudorotaxane intermediate.632 The bis-thiolated axle in the pseudorotaxane complex was assembled on a gold surface, which resulted in the formation of an interfacial structure containing two mechanically interlocked components. The preparation of layers of pseudorotaxanes or rotaxanes on a gold surface was reported in 2003.633,634 One end of the axle for the threaded molecules involving a disulfide group or a thioctic ester derivative was anchored to a gold surface, which played the role of a stopper for the system (Figure 46).633

consisting of a crown ether ring and a disulfide-containing axle attached on a gold surface.637 Recently, total internal reflection fluorescence microscopy (TIRFM) allowed the observation of rotary movement of a ring in a single rotaxane molecule immobilized on a glass substrate.165 These techniques are very useful for the identification of the mechanically anchored units on the surfaces. Further efforts have focused on achieving mobility of surface confined supramolecular architectures. Some related examples of rotaxane-modified surfaces with a machine-like function and practical applications will now be discussed. 5.3.2.1. Self-Assembled Monolayers on Gold Surfaces. A number of redox-active switchable rotaxanes on gold electrodes have been constructed, and their shuttling performances have been studied by chronoamperometric experiments.638−643 On the basis of this strategy, Willner and co-workers designed and prepared a [2]rotaxane electron relay 339 interlocked on a molecular wire connecting the enzyme to the electrode.641 It was used for the electrical contacting of a redox enzyme. In this system, the tetracationic cyclophane unit was used as an electron-transfer mediator for the wiring of the enzyme glucose oxidase (GOx). The adamantane was used as a stopper for the rotaxane, which was substituted by a bulky flavin adenine dinucleotide (FAD) unit that was recognized by apo-glucose oxidase (apo-GOx) (Figure 47). The shuttling motion of this system was triggered by the reduction and oxidation of the FAD unit. Because of the shuttling movement, the interlocked macrocycle transported electrons from the reduced FAD cofactor to the electrode surface, so the bioelectrocatalytic oxidation of glucose was achieved. An ion gate was reported by Kim and co-workers based on the pH-driven CB[6]-based systems.644 In this regard, SAMs were obtained by immersing a gold electrode in an aqueous solution containing pseudorotaxane 340. Complex 340 consisted of a CB[6] unit and a linear molecule with a 1,4butanediammonium station and a 1,2-dithiolane tethering unit (Figure 48). Dethreading/rethreading of CB[6] was achieved upon addition of NaOH and NH4Cl, respectively. When the supramolecular complex was in the threading coconformation, [Fe(CN)6]3− anions were prevented from reaching the electrode surface, while dethreading the macrocycle resulted in the accessibility of [Fe(CN)6]3− anions to the electrode surface, and thus the redox reaction took place. Functionalization of surfaces with pseudorotaxanes or rotaxanes has also found applications in the field of chemical sensing.645,646 In 2007, Beer and co-workers reported SAMs based on redox-active bisferrocene-functionalized [2]rotaxane 341, which was capable of selectively sensing chloride ions electrochemically (Figure 49). Using chloride anion as a template, the pseudorotaxane was initially self-assembled in solution from an anion binding macrocycle with an appended ferrocene redox-active center and an ion-paired thread with a bulky ferrocene redox-active stopper group at one terminus. The pseudorotaxane was subsequently grafted onto a gold surface by immersing a gold electrode into the solution. This system exhibited highly selective sensing for chloride as compared to other anions, such as H2PO4−, HSO4−, or Br−, which was demonstrated by monitoring the electrochemical response of the ferrocenyl-appended redox centers of the SAM. The attachment of stimuli-responsive molecular shuttles on surfaces can result in macroscopic switchable surfaces. For example, a monolayer of a light-switchable bistable [2]rotaxane provided a photoresponsive surface on which the millimeter-

Figure 46. Copper-complexed rotaxane 338 on a gold surface.

The early surface-attached rotaxanes have often been studied by cyclic voltammetry. Besides, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR) have also been used to determine whether rotaxanes have been attached onto the surfaces.635,636 In the work by Vance and co-workers, X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) were utilized to characterize a rotaxane CG


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Figure 47. Bioelectrocatalytic oxidation of glucose mediated by a GOx-FAD-reconstituted redox switchable rotaxane 339 on an Au electrode. Reproduced with permission from ref 641. Copyright 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 48. SAM of a pH-responsive CB[6]-based pseudorotaxane 340 on Au. Reproduced with permission from ref 644. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 50. Chemical structure of switchable fluorinated molecular shuttle 342 and graphical representation of a photoresponsive surface based on 342. Reproduced with permission from ref 647. Copyright 2005 Nature Publishing Group.

station to (Z)-maleamide. This bistable [2]rotaxane was physisorbed to the surface of a carboxylic acid-terminated SAM of 11-mercaptoundecanoic acid (MUA) on Au(111) deposited on glass or mica by hydrogen bonding between the pyridine nitrogens of the macrocycle and the carboxylic acid groups of the monolayer. The biased Brownian motion of the components of photoresponsive rotaxane 342 exposed or concealed fluoroalkane residues and modified the surface tension. The molecular shuttles organized over surfaces behaved coherently, and the collective nanoscale movements of their mechanically linked components could be translated into macroscopic motions.

Figure 49. Self-assembled monolayer of a redox-active ferrocenefunctionalized rotaxane 341 on Au.

scale directional transport of diiodomethane drops was achieved (Figure 50).647 At first, a tetraamide macrocycle containing pyridine units encircled a linear fragment including a tetrafluorosuccinamide station and a photoisomerizable fumaramide station to form rotaxane 342, which was photoresponsive by photoisomerization of the (E)-fumaramide CH


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Figure 51. Graphical representations of operation of nanovalves based on CBPQT4+. Reproduced with permission from ref 649. Copyright 2004 American Chemical Society.

5.3.2.2. Nanovalves on Silica Surfaces. The switching properties of molecular machines based on pseudorotaxanes make them excellent candidates for drug delivery. The release of drugs can be easily controlled by the change of the states of supramolecular systems, named nanovalves. It was shown by Stoddart, Zink, and co-workers that a pseudorotaxane-based machine successfully came into this function when it was trapped physically in a rigid nanoporous sol−gel silica framework or attached covalently to silica surfaces triggered by photochemical stimuli.648 On the basis of this strategy, the movement of the pseudorotaxane comprising of CBPQT4+ and BHEEN in mesostructured silica acts as a nanovalve to close the container with luminescent molecules trapped inside and release them on demand (Figure 51).649 Before operating the nanovalve, it is first essential to make gateposts for the nanocontainer with BHEEN attached to particles. The hexagonal cylindrical nanopores are all ca. 2.0 nm in diameter. The gate is closed by placing the film in an aqueous solution of CBPQT4+, resulting in encapsulating BHEEN derivatives, which block the pore openings in the macrocycles to form tethered pseudorotaxanes. After addition of NaBH3CN as a relatively mild reducing agent, the Ir(ppy)3 molecules escape from the containers because of the dissociation process caused by reduction of the CBPQT4+ ring. In another mode of operation, photosensitizers, such as 9-anthracenecarboxylic acid and tethered [Ru(bpy)2(bpy(CH2OH)2)]2+ (bpy = 2,2′bipyridine), are tethered and excited by light to transfer electrons to nearby CBPQT4+ rings and reduce them, leading to the dissociation.650 The method using light is less efficient at activating the release of the guests. To find more efficient methodologies for opening the gate, other supramolecular systems were functionalized to the nanovalves, such as a DB24C8/dialkylammonium ion pseudorotaxane and a CD/polyethylenimine (PEI) polypseudorotaxane.651−653 These nanosystems were controlled by pH stimulation and competitive binding. For the CD/PEI system, the guest molecules were entrapped in the pores of a mesoporous silica particle (Si-MP) blocked by threading of CDs onto the surface-grafted PEI chains at pH 11 (Figure 52).653 The CD/PEI polypseudorotaxane was dissociated when the pH was 5.5, leading to the release of guests. This approach

Figure 52. Schematic presentation of pH-responsive release of guest molecules from the nanovalves based on CD hosts. Reproduced with permission from ref 653. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

provided an effective route for CD-based polypseudorotaxanes to various useful applications, including molecular machines, stimuli-responsive nanocarriers, and sensors. One of the far-reaching applications for nanovalves involves delivering drugs and enzymes in biological environments. Therefore, achieving the function of nanovalves in aqueous and biological media has also been carried out successfully.654−656 For example, the recognition motif based on CB[6] and a tethered bisammonium unit was embedded into the chemically stable mesoporous silica supports.654 The pseudorotaxane system was associated by ion−dipole interactions and dissociated by simply changing the pH in water, which resulted in the operation of the nanovalves (Figure 53). Further investigation indicated that the gatepost in shorter linker length tightens up the nanovalves sufficiently to prevent leakage. Because high pH was not suitable for biological applications, the tethered linear molecules were subsequently changed to trisammonium molecules.655 Thus, novel mechanized nanoparticles based on the interaction of CB[6] with a trisammonium pseudorotaxane were reported, which operated CI


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Figure 53. Graphical representations of operating supramolecular nanovalves based on CB[6]. Reproduced with permission from ref 654. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 54. Graphical representations of pH-responsive nanovalves based on β-CD. Reproduced with permission from ref 657. Copyright 2010 American Chemical Society.

nanoparticles (MSNP) loaded with Hoechst dye and the anticancer drug doxorubicin were efficiently taken up into acidic endosomal compartments in human differentiated myeloid (THP-1) and squamous carcinoma (KB-31) cell lines. When the mechanized silica was combined with zincdoped iron oxide, a new generation of drug delivery system was created with heat-responsive activation.658 The nanovalve attached to this system remained closed in biological environments and opened on heating. These magnetic-core silica nanoparticles were effective in delivering anticancer drugs upon exposure to an oscillating magnetic field. Besides application in drug delivery, nanovalves consisting of pseudorotaxane systems and mesoporous silica nanoparticles, together with nanoimpellers attached in the supports, functioned as AND logic gates relying on light and pH inputs and provided sophisticated control of the contents of the pores

to encapsulate propidium iodide guest molecules at neutral pH and then release the contents under mildly acidic conditions. The very different basicity of the anilinium nitrogen atom from the other two alkyl nitrogen atoms, the different lengths of the oligomethylene spacers, and the functional group of terminal phenyl unit are important features for adjusting the operation of the nanovalves. Recently, the feasibility of mesoporous silica nanoparticles with nanovalves acting as drug delivery systems was investigated.657−659 To achieve the functions of gates in biological systems, the nanovalve was designed to be closed tightly at pH 7.4, but self-open in acidic endosomal compartments (Figure 54).657 Therefore, a series of aromatic amines were chosen as the stalk and β-CD as the cap. The pHsensitive nanovalves then were fabricated and functioned as drug delivery vehicles in cells. These mesoporous silica CJ


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(Figure 55).660 Azobenzene motifs tethered to the inner pore walls of the supports acted as nanoimpellers and could be

different linkers were employed to attach the bistable [2]rotaxane molecules covalently to the silica substrate.661 Nanovalves with shorter linkers were less leaky. These operational valves are true molecular machines consisting of a solid framework with movable parts to accomplish a specific task and may be used in the controlled release of drugs in the future. Later, the same group reported the design, synthesis, and operation of enzyme-responsive snap-top covered silica nanocontainers.662 The [2]rotaxanes had an α-CD wheel, a poly(ethylene glycol) thread, and a cleavable stopper (Scheme 130). The device contained guest molecules within the pores after formation of the rotaxanes; when cleavage of the stopper happened, the guests were released due to the dissociation of the pseudorotaxanes. It was indicated by luminescence spectroscopy that the ester-linked snap-top system was selectively activated by porcine liver esterase. However, the amide-linked system was left intact. This biocompatible controlled release system exploited enzymatic specificity. As the stoppering units can be changed in a wide range, activation of the stoppers can be easily chosen with various modes. 5.4. Metal−Organic Frameworks

Although a great number of investigations on the dynamic properties of rotaxanes have been derived from solution studies, there still exists a problem that the molecules are randomly dispersed and their motion incoherent in solution. To solve this problem, molecular machines on solid supports have been developed.34,35 On the other hand, efforts have focused on providing a platform for their strategic and precise placement in two- or three-dimensional space so that they can achieve a higher level of molecular organization. Metal−organic frameworks (MOFs) are a well-established class of crystalline materials with ultrahigh porosity and enormous internal surface areas consisting of organic linkers and inorganic metal nodes.663 To make the motion of rotaxanes coherent, the idea is that incorporating rotaxanes into the struts of MOFs will introduce dynamics onto their frameworks.36,664 It is an interesting and challenging topic that provides a new perspective for rotaxane chemistry. The initial study on rotaxane-modified MOFs is based on the preparation of 2D or 3D networks composed of organic [2]pseudorotaxane subunits and linking metal centers.665−669 Pioneering work by Kim and co-workers demonstrated that by threading CBs with N,N′-bis(4-pyridylmethyl)-1,4-diaminobutane dihydronitrate, followed by the reaction of the pseudorotaxane 344 with Ag(CH3C6H4SO3) and AgNO3, 1D and 2D coordination polymer networks were constructed (Scheme 131).665 The X-ray crystal structure of 346 revealed an unprecedented polyrotaxane in which cucurbituril beads were threaded on a 2D coordination polymer network. The 2D network consisted of large edge-sharing chair-shaped hexagons with an Ag(I) ion at each corner and a [2]pseudorotaxane subunit at each edge connecting two Ag(I) ions. The mean length of the edge was 20.9 Å, and the mean separation of the opposite corners was 38.0 Å. Subsequently, 3D coordination polymer networks containing binuclear lanthanide metal centers were obtained.666 The X-ray crystal structures of the coordination polymers revealed that the basic building unit of the framework consisted of a binuclear Tb3+ center and six [2]pseudorotaxane units, each of which contained 3-phenylcarboxylate groups at the terminals. The 3D polymer network had an inclined α-polonium topology with

Figure 55. Truth table for an AND gate based on dual-controlled nanoparticles. (a) Excitation with 448 nm light; (b) addition of NaOH; and (c) simultaneous excitation with 448 nm light and addition of NaOH. Reproduced with permission from ref 660. Copyright 2009 American Chemical Society.

interconverted between two configurations (trans and cis) upon absorption of light. Also, the CB[6]/bisalkylammonium [2]pseudorotaxanes tethered to the outer surfaces of the nanoparticle supports acted as nanovalves and could be associated and dissociated by pH stimuli. The movements were based on entirely different principles. Excitation with 448 nm light induced the dynamic wagging motion of the nanoimpellers, but the nanovalves remained shut and the contents were contained. Addition of NaOH opened the nanovalves, but the static nanoimpellers were able to keep the contents contained. Only when both controlled release mechanisms were activated simultaneously could the release of the guest molecules be observed. Nanovalves with pseudorotaxanes attached to the mesoporous silica particles were described above in detail. They achieve their gating behavior based on the association/ dissociation movement of the supramolecular complex. However, such nanovalves act like a cork in bottle; that is to say, the trapped molecules can only be released from the system but cannot be trapped again.650 Here, a reversible molecular valve (Figure 56) was demonstrated on the basis of a [2]rotaxane comprised of CBPQT4+ ring threaded onto an axle with a TTF unit and a DNP site.298 The CBPQT4+ ring moves back and forth on the axle by oxidation and reduction of the TTF unit, resulting in the release and encapsulation of the trapped molecules. Furthermore, by varying chain lengths, CK


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Figure 56. A reversible [2]rotaxane-based nanovalve. (a) The structural formula of the bistable [2]rotaxane; and (b) the proposed mechanism for the operation of the nanovalve. Reproduced with permission from ref 298. Copyright 2005 American National Academy of Sciences.

Scheme 130. Synthesis and Activation of Enzyme-Responsive Snap-Top Systemsa

a


of Co 71%, and Zn 92%, respectively (Figure 57). The use of CoII and ZnII ions in MeCN resulted in an octahedral coordination geometry comprising two [2]pseudorotaxane ligands, two MeCN molecules, and two water molecules all with trans orientations. Because of the fact that the linker could adopt only an anti conformation at the central ethylene unit when threaded through DB24C8, it produced a linear 1D MOF with a Co···Co distance of 22.1 Å. On the other hand, the reaction of 2 equiv of 347[BF4]2 with 4 equiv of DB24C8 and 1 equiv of [M(H2O)6][BF4]2 (M = Cu, Cd, Ni) in noncoordinating solvent MeNO2 produced X-ray quality crystalline materials (average yields ∼80%) of 2D MOFs with square nets and formula {[Cd(H2O)(BF4)(RL1)2][BF4]5· (MeNO2)15}x (Figure 57). This octahedral coordination

binuclear terbium centers behaving as six-connected nodes. The void space in the unit cell was filled with free [2]pseudorotaxanes, counteranions, and water molecules. Loeb and co-workers demonstrated that [2]pseudorotaxanes incorporating dipyridinium axles with DB24C8 wheels was used to produce [2]rotaxanes with metal complexes as stoppers in good yield.98 They further used the same supramolecular motif in metal−ligand self-assembly (crystal engineering) reactions and construction of rotaxane-modified MOFs (Figure 57).667,668 On one hand, when 1 equiv of 347[BF4]2 was mixed with 2 equiv of DB24C8 and 1 equiv of [M(H2O)6][BF4]2 (M = Co, Zn) in MeCN, slow evaporation of the reaction mixture gave crystalline material with formula {[Co(H2O)2(MeCN)2(347)][BF4]4·(MeCN)2(H2O)2}x in yields CL


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Scheme 131. (a) Formation of CB[6]-Based [2]Pseudorotaxane 344 and Its Self-Assembling 1D or 2D Coordination Polymers 345 and 346

Figure 57. (a) Formation of and (b) X-ray crystal structures of 1D, 2D, and 3D networks composed of DB24C8-based [2]pseudorotaxane subunits. Reproduced with permission from refs 667 and 668. Copyright 2003 and 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 58. (a) Chemical structure of [2]rotaxane 349 and (b) structure of MOFs by single-crystal X-ray diffraction. Reproduced with permission from ref 671. Copyright 2012 Nature Publishing Group.

geometry comprised four RL1 linkers in a square planar arrangement, along with one water molecule and one coordinated BF4 anion. To afford 3D rotaxane-modified MOFs based on this system, the dipyridinium ligand 348 containing bis(N-oxide) analogue was designed and larger lanthanide metal ions were employed (Figure 57).668 Thus, 3D MOFs with formula {[M(H2O)(OTf)(RL3)3][Cl][OTf]7·(2MeCN)}x, (M = Sm, Eu, Gd, Tb) were produced in moderate yield (∼50%). The use of Ln(III) ions as nodes resulted in an eight-coordinate metal center with a square anti-prismatic geometry comprised of six RL3 linkers, one water molecule, and one coordinated triflate anion. In this

3D structure (Figure 57), every linker is a [2]rotaxane, and the edges of the “cube” are defined by Sm···Sm distances of ∼23.5 Å. All of the rotaxane-modified MOF materials prepared above showed the same basic stability. Thermogravimetric analysis showed that all residual solvent was removed after heating to ∼100 °C. Each MOF studied then showed a stable phase until ∼225−250 °C, at which point decomposition of the metal− ligand framework was indicated by loss of DB24C8. Recently, a new approach to preparing three-dimensional rotaxane-modified MOFs was described by Sessler.670 It relied on the use of dicarboxylate anions threaded through a large, tetracationic molecular box through a one-pot self-assembly CM


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Figure 59. Synthesis of [2]pseudorotaxane [350·Cu]·PF6 and MOF-1040. Reproduced with permission from ref 672. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 60. (a) Single-crystal structure of MOF-1001 and the corresponding organic strut 351. (b) The cubic crystals of MOF-1001 and MOF-1001 pseudorotaxanes. Reproduced with permission from ref 673. Copyright 2009 American Association for the Advancement of Science.

crystals of MOF-1040 (Figure 59). Both oxidation and demetalation experiments were demonstrated to occur without disrupting the crystallinity of the frameworks, which indicated the presence of electronic switches that were presumably accompanied by geometrical changes involving shrinking and flattening in the coordination sphere of the copper ions in the MOFs. They mark a significant step toward transferring the solution-state chemistry of MIMs into the solid state in the form of a porous extended structure. A typical example for application of rotaxane-modified MOFs was presented by Stoddart, Yaghi, and co-workers.673 A πelectron-rich crown ether, capable of binding π-electrondeficient substrates, has already been introduced into MOFs. MOF-1001 was capable of docking paraquat dication (PQT2+) guests within the macrocycles in a stereoelectronically controlled fashion (Figure 60). This act of specific complexation yielded quantitatively the corresponding MOF-1001 pseudorotaxanes, as confirmed by X-ray diffraction and solidand solution-state NMR spectroscopic studies performed on MOF-1001, its pseudorotaxanes, and their molecular strut precursors. A control experiment involving the attempted inclusion of PQT2+ inside a framework (MOF-177) devoid of polyether struts showed negligible uptake of PQT2+, indicating the importance of the macrocyclic polyether in PQT2+ docking.

process involving Zn(II) cations. Such a system represents important progress toward controlling the features of 3D MOFs. In the above examples, rotaxane-modified MOFs were constructed, in which the dumbbell components of rotaxanes were used as struts in the framework and incorporated into MOFs. It is still a challenge to accomplish motion of components in the framework, or to locate switchable rotaxanes inside extended frameworks.102,671 Recently, Loeb and coworkers constructed a MOF material using a [2]rotaxane 349 as the organic linker and binuclear Cu(II) units as the nodes (Figure 58).671 Activation of the as-synthesized material created a void space inside the rigid framework that allowed the soft macrocyclic ring of the [2]rotaxane to rotate rapidly, unimpeded by neighboring molecular components. The linker was designed with two 3,5-benzenedicarboxylic acid groups as the linking struts, a benzylaniline recognition site as the crossbar, and a 24C6 macrocycle as the wheel. The Cu(II)based MOF [Cu2(349)(H2O)2]·3H2O was prepared by combining Cu(NO3)2·3H2O and the tetracarboxylate linker 349 in a 3:2:2 mixture of dimethylformamide (DMF)/EtOH/ H2O with trace HNO3. Variable-temperature 13C and 2H solidstate NMR experiments were used to characterize the nature and rate of the dynamic processes occurring inside this unique material. These results provide a blueprint for the future creation of solid-state molecular switches and molecular machines based on rotaxanes. Stoddart, Yaghi, and co-workers reported the successful incorporation of copper(I)-complexed [2]pseudorotaxanate struts into a MOF.672 Mixing [2]pseudorotaxane [350·Cu]· PF6 and Zn(NO3)2·6H2O in diethylformamide at 100 °C in a sealed tube for 48 h resulted in the formation of red cubic

6. CONCLUSIONS AND FUTURE PERSPECTIVES In this Review, the development of pseudorotaxanes and rotaxanes was introduced comprehensively in the order of their historical sequence. We discussed trends in detail from synthesis to stimuli-responsive movement in solution and then in the solid supports, and to their function and applications in molecular devices and materials science. In CN


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Biographies

their early stage of the development, investigations were focused on the noncovalent interactions as synthetic templates, mild reaction conditions for accomplishment of the template synthesis without disrupting their interactions, matched macrocyclic host and linear guest compounds as building blocks, and sometimes appropriate bulky moieties as stoppers.674,675 All in all, we provided a plentiful library for the construction of more complicated supramolecular architectures and the development of molecular machines. Given the unique structures and stimuli-responsive motional properties of pseudorotaxanes and rotaxanes, they are considered as ideal building blocks to obtain artificial nanomachines in solution. The association and dissociation movement of pseudorotaxanes makes them good candidates for achievement of the function of molecular locks and keys, molecular switches, and molecular logic gates. Furthermore, the shuttling or rotation of the wheel along the axle for rotaxanes provides good opportunities for them to constitute molecular machines, such as molecular shuttles, molecular switches, and molecular abacus. There are still more complicated artificial molecular machines that are being continuously explored.676−678 The efforts and studies of molecular machines based on pseudorotaxanes and rotaxanes in solution are the basis for understanding motional mechanisms and structure−function relationships for producing artificial molecular machines in the macroscopic world. Chemists have started their research on these molecular machines in solid supports, such as in the solid state, on surfaces, or in MOFs, which enable the functions of nanovalves, ion sensing, single-molecular imaging, and catalysis for the macroscopic systems. Therefore, the applications of rotaxane-based molecular machines have attracted much attention from biologists, materials and surface scientists, polymer scientists, and so on. Up to now, studies on the construction of drug delivery and release systems, stimuliresponsive supramolecular polymeric materials, and unidirectional charge-transfer cascades have been widely explored and developed rapidly. However, this is just the beginning of a long but exciting journey. For the purpose of constructing macroscopic molecular machines, a lot of difficulties still exist. For example, how does one control exactly the molecular motion of rings when these molecules are deposited on surfaces? It is a challenging but indispensable problem that has to be solved. Moreover, some new aspects have been involved for the development of rotaxanes. Small rotaxane-based molecular machines can be designed and operated to perform their tasks in synthesis, such as the preparation of sequencespecific peptide.679 Another intriguing prospect of preparing rotaxanes and the molecular switches is to amplify the molecular switch process into mechanical work within liquidcrystalline properties. Some pioneering studies based on liquidcrystalline rotaxanes have been already reported and demonstrated their mesomorphic features.680,681 It seems that a vast application field is ready for development using such linear interlocked molecules.

Min Xue was born in China in 1982. She obtained her B.S. in applied chemistry from Xi’an Jiaotong University in 2005. She then got her Ph.D. in chemistry from the Institute of Chemistry of the Chinese Academy of Sciences under the supervision of Prof. Chuan-Feng Chen in 2010. After that she moved to the group of Prof. Javier de Mendoza at the Institute of Chemical Research of Catalonia (ICIQ) in Spain as a postdoctor. In 2011, she joined the laboratory of Prof. Feihe Huang at Zhejiang University as a postdoctor. Her current research interests are the synthesis and assembly of pillararenes, molecular recognition, and host−guest chemistry.

Yong Yang was born in China in 1979. He obtained his B.S. in chemistry from Hunan University in 2002. He then joined Prof. Chuan-Feng Chen’s group at the Institute of Chemistry of the Chinese Academy of Sciences (ICCAS) to pursue his Ph.D. After obtaining his Ph.D. in 2007, he continued his research at ICCAS as a research assistant professor. In 2009 he moved to Prof. Javier de Mendoza’s group at the Institute of Chemical Research of Catalonia (ICIQ) in

AUTHOR INFORMATION

Tarragona (Spain) as a postdoctor. Since 2011 he works as an

Corresponding Author

*E-mail: [email protected].

independent PI at Zhejiang Sci-Tech University. His current interest is

Notes

the development and functionalization of hydrogen-bonding mediated

The authors declare no competing financial interest.

self-assembly systems. CO


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Feihe Huang was born in China in 1973. He obtained his degree of Doctor of Philosophy in Chemistry from Virginia Polytechnic Institute and State University (VT) under the guidance of Prof. Harry W. Gibson in March 2005. He then joined Prof. Peter J. Stang’s group at the University of Utah as a postdoctor. He became a Professor of Chemistry at Zhejiang University in December 2005. His current research interests are supramolecular polymers and pillararene supramolecular chemistry. Awards he has received include the 2004 Chinese Government Award for Outstanding Self-Financed Students Abroad, the Outstanding Ph.D. Dissertation Award from VT, the Thieme Chemistry Journals Award, Humboldt Fellowship for Experienced Researchers from the Humboldt Foundation, Fellow of the Royal Society of Chemistry, Chinese Chemical Society AkzoNobel Chemical Sciences Award, and Cram Lehn Pedersen Prize in Supramolecular Chemistry. He has published more than 160 supramolecular chemistry papers. His publications have been cited more than 8000 times with an h-index of 49. He has served as a guest editor for Chemical Society Reviews, Accounts of Chemical Research, Chemical Reviews, and Chemical Communications. He sits on the Advisory Boards of Chemical Society Reviews, Chemical Communications, Acta Chimica Sinica, Macromolecules, ACS Macro Letters, and Polymer Chemistry.

Xiaodong Chi was born in Taizhou, Zhejiang, China on July 5, 1987. He obtained his Bachelor’s degree in chemistry from Zhejiang SciTech University in June 2010. He then joined the group of Prof. Feihe Huang at Zhejiang University to pursue his Ph.D. degree in chemistry. His current research interests are the design and preparation of novel host−guest recognition systems based on pillar[n]arenes, self-assembly in aqueous media based on host−guest molecular recognition, and functional supramolecular materials constructed by the combination of traditional polymers and nonconvalent interactions.

supervision of Prof. Xuming Zheng in 2009. He then joined the

ACKNOWLEDGMENTS We greatly acknowledge the National Basic Research Program (2013CB834502), the National Natural Science Foundation of China (21125417, 21202145, 21434005), the Fundamental Research Funds for the Central Universities, the Key Laboratory of Supramolecular Structure and Materials, and the China Postdoctoral Science Foundation (2013M541767) for their generous financial support. F.H. is very grateful to Professor Harry W. Gibson for his enormous support and encouragement.

laboratory of Prof. Feihe Huang at Zhejiang University and obtained

REFERENCES

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Xuzhou Yan was born in Jiangsu, China in 1986. He received his B.S. degree in chemistry from Zhejiang Sci-Tech University under the

group at the University of Utah as a postdoctor. Awards he has received include the Chu Kochen Award of Zhejiang University (2013), and the Ministry of Education of the People’s Republic of China (MOE) National Scholarship (2009, 2012). His current research interests are focused on the construction of supramolecular polymeric materials, light-emitting supramolecular materials, and supramolecular coordination complexes. CP


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