Photoresponsive HostâGuest Functional Systems - American

Review pubs.acs.org/CR

Photoresponsive Host−Guest Functional Systems Da-Hui Qu, Qiao-Chun Wang, Qi-Wei Zhang, Xiang Ma, and He Tian* Key Laboratory for Advanced Materials & Institute of Fine Chemicals, East China University of Science & Technology, Meilong Road 130, Shanghai 200237, P. R. China 4.1.2. Covalent/Noncovalent Bond Transitions in Linear Supramolecular Polymers 4.2. Supramolecular Interactions in the Side Chain 4.3. Supramolecular Complexations as CrossLinkers between Branched Polymer Chains 4.4. Photoresponsive Supramolecular Micelles, Vesicles and Other Assemblies 5. Photoresponsive Host−Guest System on Surfaces 5.1. Photoresponsive Host−Guest Systems Immobilized on Nanoparticles 5.2. Photoresponsive Host−Guest Systems Organized at Planar Surfaces 5.3. Photoresponsive Host−Guest Systems Immobilized on Porous Materials 6. Summary Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Photocontrollable Capture and Release of Guest Molecules from Host Molecules 2.1. Single-Molecule Hosts 2.1.1. Single-Molecule Hosts Functionalized with Photoactive Units 2.1.2. Single-Molecule Hosts for Photoresponsive Guest Molecules 2.2. Multicomponent Supramolecular Hosts 2.2.1. Supramolecular Cages and Capsules Assembled by Photoactive Components 2.2.2. Supramolecular Cages or Capsules for Photoresponsive Guest Molecules 3. Photoresponsive Rotaxane, Catenanes, and Pseudorotaxanes as Light-Driven Molecular Machines 3.1. Light-Responsive Mechanical Motion from Photoinduced Electron Transfer 3.1.1. Photoinduced Electron Transfer in Pseudorotaxanes 3.1.2. Photoinduced Electron Transfer in Rotaxanes and Catenanes 3.2. Light-Responsive Mechanical Motion Powered by Photoisomerization 3.2.1. Photoinduced Trans−Cis Isomerization of Azobenzene and Stilbene 3.2.2. Photoinduced E/Z Isomerization of Fumaramide 3.2.3. Photoinduced Ring-Open/Close Isomerization of Spiropyran 3.3. Light-Responsive Mechanical Motion Powered by Excited-State Changes 3.4. Light-Responsive Mechanical Motion Powered by Photoheterolysis 4. Photoresponsive Supramolecular Polymers and Their Assemblies 4.1. Supramolecular Interactions in the Main Chain 4.1.1. Photoswitching between Monomer/ Oligomer and Supramolecular Polymers

© XXXX American Chemical Society

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1. INTRODUCTION Supramolecular chemistry mainly focuses on the chemical systems assembled by a discrete number of molecular components through weak and reversible noncovalent interactions. The importance and significance of supramolecular chemistry have manifested since the 1987 Nobel Prize for Chemistry was awarded to Pedersen,1 Cram,2 and Lehn,3 and afterward, much attention has been paid to this field, as it has urged and accelerated the development of many new concepts, such as molecular self-assembly, molecular recognition, host− guest chemistry, and dynamic covalent chemistry.4−7 In the domains of supramolecular chemistry, the development of host−guest systems, in particular, in which a host molecule can recognize and bind a certain guest molecule, was considered as an important contribution. A host−guest system refers to a chemical system that is made up of two or more molecular subunits self-assembled together to form a supramolecular complex in a controlled manner. Normally, the formation of a host−guest system involves more than one type of noncovalent interaction, for example, hydrophobic associa-

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Special Issue: 2015 Supramolecular Chemistry

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Received: November 4, 2014

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DOI: 10.1021/cr5006342 Chem. Rev. XXXX, XXX, XXX−XXX

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2. PHOTOCONTROLLABLE CAPTURE AND RELEASE OF GUEST MOLECULES FROM HOST MOLECULES Receptors or hosts in chemistry can be defined as molecules or supramolecular assemblies that can selectively recognize and bind specific guest molecules. Adjusting host−guest interactions is essential for the design and construction of molecular switches, molecular machines, and other complicated supramolecular assemblies. The binding affinity varies greatly between different hosts and guests. There are several factors that can influence the host−guest interactions, such as size and shape, polarity, charge, hydrophobicity, and hydrophilicity. An efficient strategy to achieve better control of the selectivity and binding affinity in host−guest complex is to introduce wellknown photoresponsive functional moieties into the host molecules. Pioneering works in this field were performed by Irie and Kato,32 Shinkai et al.,33 and Erlanger and co-workers.34 Inspired by these pioneering works, many photoresponsive receptors for anions35 and cations36 have been development and reviewed. However, a review for systems concerning controllable small-molecule capture and release is rare. In this section, we will discuss systems for photoswitchable capture and release in accordance with two different kinds of hosts, namely, single-molecule host molecules and multicomponent supramolecular hosts.

tion, hydrogen bonding, electrostatic interactions, metal coordination, van der Waals forces, and π−π stacking interactions.7 Recently, host−guest molecular recognition has played an important role in the development of advanced supramolecular systems or materials because of their good selectivity, high efficiency, and stimuli responsiveness. The well-established host−guest interaction has been utilized to construct various supramolecular systems, such as novel host systems that can capture and release guest molecules in a controlled manner8−12 and also a surge of mechanically interlocked molecular systems,13−21 supramolecular polymers and related assemblies,22 and surface-mounted host−guest systems.23,24 These above-mentioned systems that take advantage of host−guest molecular recognition belong to a hot topic of chemistry and materials sciences and will be emphasized in this review. On the other hand, stimuli responsiveness is an important factor that attracts more and more attention, mainly because of the requirement for controllable modulation in numerous potential applications. The known host−guest systems can respond to various stimuli, such as chemical, electrochemical, and photochemical stimuli.7 Compared with the other external stimuli used to induce structural or morphological transitions of the chemical systems, light is considered as an ideal external control element for in situ chemical manipulation.25−31 Besides its cleanness, light input has some other advantages: (i) it can be easily switched on and off, (ii) it provides a high level of spatiotemporal resolution with precisely regulated irradiation wavelength and intensity, and (iii) it can work in confined space and excitation with photons can lead to a fast response. These unique advantages have led to the extensive use of light for better control and modulation. Many strategies for the reversible light modulation of host− guest systems have been developed with a wide variety of photoswitches or photoremovable chromophore. The most commonly used photoswitches include azobenzene, stilbene, and fumaramide derivatives, which can switch between the trans and cis isomers (or E and Z isomers); spiropyran and diarylethene derivatives, which can interconvert between closed and open forms; anthracene and coumarin derivatives, which can perform reversible dimerization; and other irreversible photocage molecules. Alternatively, the photoresponsiveness can also be realized via a photoinduced electron transfer process and others. For an overview of the most commonly used photoswitches and their working mechanism, the reader is referred to recent reviews.25−30 It should be mentioned that these above-mentioned photoswitches employed in host−guest systems can separately or synergistically act as guest molecules and switching units. The combination of external light stimuli and host−guest systems has given birth to a large number of interesting photoresponsive systems. The aim of this review is to highlight a selection of important accomplishments of photoresponsive host−guest functional systems, in particular, to demonstrate how light is used to realize the controllable capture and release of guest molecules from host molecules, and then we will focus on the examples of how to construct a photoresponsive mechanically interlocked molecule to realize light-induced mechanical motion by taking advantage of the well-established host−guest interactions. Emphasis will also be given to photoresponsive supramolecular polymers and surfacemounted host−guest systems.

2.1. Single-Molecule Hosts

2.1.1. Single-Molecule Hosts Functionalized with Photoactive Units. Most photochromic moieties, such as azobenzene and diarylethene, are capable of reversibly modulating conformation changes upon external photochemical excitation.25−29 Thus, the introduction of these photoresponsive moieties into a known host molecule can lead to a photoswitchable host or receptor system with reversible binding affinity for a guest molecule, finally achieving release and uptake of the guest molecule in a controlled manner. In 1978, Ueno and co-workers37 reported an azobenzene-capped β-cyclodextrin (β-CD) that can regulate the 1:2 host−guest complexation as well as its binding ability in response to light stimulus. Compared to its precusor CD macrocycle, CD dimers can bind a large variety of guest molecules with relatively high binding affinity. Ueno and coworkers38 reported a photoresponsive CD dimer separated by an azobenzene linker. Unfortunately, no suitable guest molecule was found to selectively bind with one of the two configurations of the CD dimer. Reinhoudt and co-workers39 designed and synthesized a photoswitchable β-CD dimer 1 by tethering two β-CD macrocycles with a dithienylethene linker through the amide bond linkage (Figure 1). The photoswitching of the dithienylethene unit between the open and closed forms can be achieved upon irradiation with 460 or 313 nm light, respectively, which can lead to a change in the binding affinity of the dimer 1 for specific guest molecules, such as tetrakis(sulfonatophenyl)porphyrin 2 (TSPP). The dimer 1 binds TSPP 35 times more strongly in the open form (1o) than in the closed form (1c), which allows the photocontrolled release and uptake of guest molecules, indicating that the system is an interesting candidate for controlled drug delivery systems. On the basis of this work, several dithienylethenecontaining β-CD dimers were designed, and their binding affinity changes in repsonse to light stimuli were studied.40 These photoswitchable β-CD dimers, accompanied by other βB


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which shortens the distance between the two 1,3-phenylene rings of the azobenzene unit, resulting in the change of the relative orientation of the two cyclopentadienyl rings of the ferrocene unit. Accordingly, the relative orientation of the two porphyrin chelators is adjusted. The photoinduced transformations of the host molecule 4 can be utilized to regulate the conformation of a guest molecule, such as bidentate 4,4′biisoquinoline 5, in which the dihedral angle of the C−C bond that connects the two isoquinoline fragments is altered. The overall photochemical process is fully reversible, and the original state is regenerated upon visible-light irradiation. The same group also extended this concept to a three-component supramolecular system.45 In this system (Figure 3b), a pyridinecontaining dithienylethene derivative 6 can isomerize between the open and closed forms in response to UV- and visible-light irradiation, and the photochemical transformation change of compound 6 is explored to change the dihedral angle of the biphenyl core of building block 7 bearing four zinc porphyrin units, and as a result, the conformational change of scissoring component 8 is achieved. In the guanidinium receptor 947 shown in Figure 4, two photoisomerizable azobenzene moieties, each bearing a carboxylate recognition site, were connected through an isophthaloyl spacer via amide bonds. The photochemical properties of the azobenzene units were used to regulate host−guest recognition by hydrogen-bonding interactions. Only in the Z,Z isomer, in which both of the two azobenzene units are in cis configuration, the two carboxylate groups can make simultaneous hydrogen bonds and bind the guest. The other two isomers, namely, the E,Z mixed form or the E,E form, showed a lower affinity for guanidinium. Therefore, in these systems, light can be used to realize the fine control of the binding guest molecules or other biologically relevant molecules, providing the possibility to modulate the capture and release of such species in a reversible manner. Crown ether is an important class of hosts, and the host− guest interactions between crown ethers and guest molecules have played a significant role in the construction of mechanically interlocked systems. The incorporation of a photoresponsive unit into a crown ether derivative can also

Figure 1. Chemical structures of photoswitchable β-CD dimer 1 and porphyrin guest molecule 2.

CD dimers40−42 might find application in, for example, photodynamic cancer therapy. Irie and co-workers43 introduced two boronic acid functional groups into each side of a diarylethene photochromic unit and demonstrated the first example of photochromic saccharide tweezers. As shown in Figure 2, the two conformers of photochromic compound 3, namely, antiparallel and parallel, exchange rapidly at room temperature and only the antiparallel conformer can be isomerized to give the ring-closed form upon UV-light irradiation. Saccharides can form esters with boronic acids when the photochromic compound 3 is in its ring-opened form, while in the closed-ring form the distance between the two boronic acid groups is not suitable for the guest molecules; hence, the complex cannot form, thus achieving photoreversible switching of the tweezer function for saccharide molecules in this system. A very important and beautiful class of photocontrollable tweezers was developed by Aida, Kinbara and co-workers.44−46 As shown in Figure 3a, tweezer 4 employs an azobenzene unit as a photorepsonsive switch, a ferrocene unit as a pivot, and two porphyrin units as chelators.44 Irradiation with UV light interconverts the azobenzene switch from trans to cis forms,

Figure 2. Concept of a diarylethene-containing photoswitchable tweezer 3 for saccharide. C


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Figure 3. (a) Schematic representation of photoresponsive tweezer 4 for a rotary guest 5. (b) Schematic representation of a photoswitchable ternary supramolecular complex 6·7·8. The arrows represent the directions of interlocked movements in reponse to external light stimuli.

Figure 4. Photointerconversion of receptor 9 for modulating recognition of the guanidinium cation.

dissociate consequently. Yang and co-workers49 replaced the azobenzene unit with a stiff stilbene to synthesize a photoswitchable BMP32C10-based cryptand 12 (Figure 5b), which can also show the distinguishable binding affinity changes for guest molecule 11 in response to light irradiation. This kind of photoresponsive host−guest recognition motif can be used in the fabrication of more complicated photodriven molecular machines. Carcerands and hemicarcerands also belong to the family of container molecules. A carcerand can be defined as a host molecule that entraps its corresponding guest molecule without

realize the photocontrollable binding affinity with specific guest molecules. Huang and co-workers48 constructed a photodriven molecular switch by introducing an azobenzene moiety as the third arm of a BMP32C10-based cryptand (Figure 5a). In cryptand 10, the azobenzene moiety exhibits a trans conformation in the original state, and the cryptand does not bind the 2,7-diazapyrenium salt 11 because of its relatively small cavity size. However, UV irradiation resulted in the transto-cis isomerization of the azobenzene unit, and then cryptand 10 exhibits good binding affinity for guest 11. By irradiation with visible light or heating, the host−guest complex can D


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Figure 5. Structures and interconversions of photoresponsive cryptand hosts 10 (a) and 12 (b) containing an azobenzene and a stiff stilbene switch, respectively, and their corresponding guest molecule 11.

Figure 6. Schematic representation of the phototriggered guest release from a photoactive hemicarceplex 13.

It should be noted that the process of the photoinduced cleavage reaction to release the guest molecule is irreversible, while other photoresponsive units, such as anthracene groups, can be photochemically dimerized and hence could realize a reversible conversion. Houk and co-workers52 for the first time demonstrated the interconversion between hemicarcerands and carcerands using a photoswitchable gate. In the system shown in Figure 7, reversible photodimerization of anthracene groups was employed to switch the container molecule. In the original state, each half of the hemicarcerand was functionalized with an anthracene unit, and host 15 is in an opened state to allow a guest molecule, such as 1,4-dimethoxybenzene, to enter into its cavity. Irradiation at 350 nm can result in the intramolecular photodimerization of two authracene units situated at the two

decomplexation, even at high temperature. In contrast, a hemicarcerand allows a guest molecule to enter and form a stable complex, but it decomplexes at high temperature.50 The introduction of photoactive moieties into a carcerand or hemicarcerand system can lead to a photoresponsive host system with photoswitchable binding affinity for small molecules. Piatnitski and Deshayes51 demonstrated the photoinduced release of a guest molecule from a photoactive hemicarceplex 13. As shown in Figure 6, a 2-nitrobenzyl group was introduced as a photoremovable protecting group to link the two halves of the hemicarceplexes, and UV light triggered the cleavage of the substituent attached at the benzyl position to generate compound 14, which resulted in the release of the guest molecule. E


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Figure 7. Photochemically reversible interconversion between the open-form hemicarcerand 15 and closed-form carcerand 16.

Figure 8. Working mechanism of receptors 17a and 18a for photocontrollable capture and release of guest molecule 19: receptors 17a and 18a can bind 12 strongly in solution, and irradiation with UV light generates macrocycle 17c and 18c, respectively, and results in subsequent release of 19.

Molard et al.55 also use the reversible photodimerization of anthracene to control the binding affinity of a hydrogenbonding receptor toward a neutral organic molecule, as shown in Figure 8. Besides anthracene units, receptors 17a and 18a further bear two pyridine-2,6-diamide units as the receptor site, which can strongly bind barbital molecule 19 in 1:1 stoichiometry through sextuple hydrogen bonds. The wellknown photocycloaddition reaction of the two anthracene units occurred in an intramolecular pathway to change the structure of the receptor from acyclic to macrocyclic. As a result, the guest barbital 19 is effectively released from the receptor molecule. The photodimers 17c and 18c can be further converted to the original receptors 17a and 18a under thermal stimulus.

halves; thus, the container molecule 16 is in a closed state. The system can be reversed back to its opened state upon irradiation with short-wavelength UV light (254 nm) or thermally. The reversible photoreponsive properties of this system were studied by 1H NMR and fluorescence spectroscopy. Using the reversible photochemical gating to control the stability of the host−guest complex is important for capture and release systems in response to external light stimuli. In other research, the introduction of anthracene into a kind of resorc[4]arene molecule53,54 can also enable reversible cycloaddition in response to light and heat stimuli, and several molecular systems can switch between the closed form and the open form. However, no guest molecule was found to have distinguishable binding with one of the switchable isomers. F


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Figure 9. Chemical structure of the photoswitchable dendritic host 20 and its model structures in all-cis and all-trans conformations, respectively. Reproduced with permission from ref 56. Copyright 2011 American Chemical Society.

Recently, Müllen and co-workers made progress in the design of a photoswitchable dendritic host molecule.56 In the host molecule 20 (Figure 9), a perylene tetracarboxydiimide dye (PDI) was chosen to act as a central fluorescent chromophore, and an azobenzene unit is situated in the middle of each dendron to act as a photoswitchable unit that can change its configuration in response to light stimuli. Several basic pyridyl entities are introduced into the interior for the enhancement of host−guest interactions. The system belongs to a kind of rigid scaffold based on a third-generation polyphenylene dendrimer, which is responsible for site-specific placement of three types of functional groups, such as PDI, azobenzene, and a pyridyl unit. A simple, nonfluorescent molecule, p-nitrophenol, was chosen as the guest for uptake studies by means of NMR and UV/vis spectroscopy. Upon 365 nm irradiation, the dendritic host was switched from an opened state to a closed state, and as a result, p-nitrophenol guest molecules were encapsulated into the host. The guest molecules could be released in two ways, irradiation of the closed host with 450 nm light or the slow thermal process of cis-to-trans isomerization of the azobenzene units. The results clearly demonstrate that specific guest molecules can indeed been trapped into suitably designed artificial hosts via a photoswitching mechanism. 2.1.2. Single-Molecule Hosts for Photoresponsive Guest Molecules. It has been well-investigated that some natural host molecules can bind a photoresponsive guest. For example, β-CD has a good binding affinity with tranazobenzene to form a threaded structure; however, irradiation with UV light can result in the dissociation of the host−guest complex due to the size mismatch.57 Recently, pillararene,58 emerging as a new kind of host or macrocycle molecule, has attracted much attention. It has also been demonstrated that pillararenes have distinguishable binding affinity for some kinds of photorepsonsive guests. Huang and co-workers59 for the first time reported pillar[6]arene-based photoresponsive host−guest complexation. As shown in Figure 10, the trans form of azobenzenecontaining guest 21 can form a supramolecular complex with pillar[6]arene derivative 22, while it cannot form a complex with pillar[5]arenes because of the unsuitable cavity sizes. Upon irradiation with 365 nm UV light, the conformation of

Figure 10. (a) Chemical structures of azobenzene derivative 21 and pillar[6]arene 22 and (b) their photoswitchable complexing and decomplexing. Reprinted with permission from ref 59. Copyright 2012 American Chemical Society.

azobenzene derivative 21 was converted from trans form to cis form, and the threading of 21 through the cavity of pillar[6]arene 22 is prohibited due to the mismatch of the size between them; however, it can rethread through the cavity upon visible-light irradiation. This new pillar[6]arene-based G


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Figure 11. (a) Self-assembled M12L24 spherical nanocage 24 bearing 24 endohedral azobenzene groups (24a) and (b) its light-triggered capture and release of 1-pyrenecarboxaldehyde. Adapted with permission from ref 63. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA.

2.2.1. Supramolecular Cages and Capsules Assembled by Photoactive Components. As discussed in the previous part, a photoresponsive unit, such as azobenzene, diarylethene, and anthracene, has been introduced into many simple host structures, such as crown ethers, cyclodextrins, and dendrimers. With careful and rational design of the chemical systems, people can control the binding affinities of the host molecule for specific guest molecules by changing the geometry and inner space via photoinduced isomerization. In a multicomponent supramolecular cage or capsule system, it is also feasible to incorporate a photoactive unit in the interior or other part of the components that can self-assemble to form the target supramolecular complex for controlled capture and release of guest molecules in response to light stimuli. Fujita and co-workers have demonstrated that the photoswitching of azobenzene units situated in the interior of a coordination cage was utilized to modulate the cage’s binding affinity for a specific guest. As shown in Figure 11a, they have shown that 24-pyridine-containing bent bridging ligands (23) can spontaneously assemble into a spherical complex (24), upon addition of 12 metal ions, which possesses a very precise structure and uniform diameter.62,63 By introducing a photochromic azobenzene unit in the ligand,63 which can complex in the presence of metal ions, they have obtained a highly cationic spherical complex (24a) in which azobenzene groups face inward. This indicates that the

photoresponsive host−guest recognition motif can work in organic solvents and is a good supplement to the existing widely used cyclodextrin/azobenzene recognition motif. Ogoshi and co-workers60 also reported a photoresponsive host−guest system between a pillar[6]arene derivative and an azobenzene guest with two terminal quaternary ammonium salts in aqueous solution. Moreover, this photocontrollable threading/dethreading mechanism can be utilized to realize the photoreversible clear-to-turbid and turbid-to-clear transitions of the solution, which is induced by reversible switching of the clouding point. 2.2. Multicomponent Supramolecular Hosts

Learning from many naturally occurring systems, scientists have designed and constructed multicomponent supramolecular assemblies, especially supramolecular cages and capsules, that can capture and release guest molecules. Many supramolecular cages and capsules can be formed from two or more molecular components that assemble through noncovalent interactions,61 such as hydrogen bonding and metal−ligand coordination. Photochemical control of access to supramolecular hosts, such as cages or capsules, i.e., the transport of molecule in and out of the cavities, is important for designing photodriven switch and nanomachinary. Two main strategies can be chosen to construct these kinds of photoresponsive cages or capsules: (i) supramolecular cages and capsules assembled by photoactive components and (ii) supramolecular cages or capsules for photoresponsive guest molecules. H


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Figure 12. (a) Light-triggered trans−cis isomerization of azobenzene derivative 25. (b) Crystal structure of MOP-26 as an azobenzenefunctionalized cuboctahedral cage. (c) Schematic representation of the capture and release of MB guest molecule from stimuli-responsive MOP-26 in response to light stimuli. Adapted with permission from ref 64. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

molecule. Methylene blue (MB), which is too large to be captured in the cavity of the small cage, was selected as the guest molecule. The capture of MB into the cage of MOP-26 can be monitored by UV/vis spectroscopy. Upon UV irradiation, insoluble trans-MOP-26 isomerized to soluble cisMOP-26, and then the guest molecule was released (Figure 12c). Moreover, this process can reverse back upon irradiation with blue light; that is, the guest molecule could be encapsulated again. This study supplied a new direction in the ever-diversifying field of MOPs, representing a new class of photoresponsive materials. Incorporation of a light-switchable element into the host molecule can allow more applications of the principle of lightinduced capture and release of the guest species in a broader scope.65 It has been shown that covalently connected azobenzene derivatives were used for switching the interior hydrophobicity (but not structure) of a coordination cage, and the design and construction of light-switchable coordination cages are still desirable. Clever and co-workers have demonstrated a photochromic coordination cage that can achieve light-triggered uptake and release of a spherical guest, [B12F12]2−.66 As shown in Figure 13, the ligand 27 can be reversibly interconverted between a conformationally flexible open-ring form (o-27) and a rigid closed-ring form (c-27) by alternating irradiation with 365 nm UV light or white light, respectively (Figure 13a), in high efficiency, as evidenced by the reversible 1H NMR spectral changes. Upon the addition of stoichiometric amounts of Pd(II) ions into the open and close isomers, respectively, the corresponding coordination cages o28 and c-28 were formed quantitatively (Figure 13b), as confirmed by multiple characterizations. o-28 and c-28 can also

core of the spherical complex features a dense array of 24 azobenzene units. It can be imagined that the photoinduced configuration changes of azobenzene units can, to some extent, change the hydrophobicity in the complex 24a. When a hydrophobic guest, pyrene, was added to a CD3CN/D2O solution of complex 24a, the upfield-shifted signals of pyrene in the 1H NMR spectrum indicated its catpture into the core of 24a. It should be emphasized that hydrophobic accumulation of trans-azobenzene units in the sphere is the reason for the guest uptake. The hydrophobicity of the cavity of complex 24a can be switched by the reversible photo- and thermoisomerization of the azobenzene moieties. Another hydrophobic guest, namely, 1-pyrenecarboxaldehyde, was used as a probe. Upon irradiation with UV light, the trans-azobenzene moieties in complex 24a were converted to the cis isomers, which has around 20% conversion at PSS. cis-Azobenzene is more polar than the trans isomer, so the interior of 24a is expected to become less hydrophobic upon irradiation, which is evidenced by the fact that the formyl proton was less shielded. After heating at 50 °C for 12 h, all the azobenzene moieties in 24a returned to the trans forms. Thus, the guest molecule was again encapsulated by 24a (Figure 11b), as evidenced by the recovery of the 1H NMR spectrum to its original one. Zhou and co-workers64 introduced azobenzene moieties into metal−organic polyhedra (MOPs) and for the first time reported optically responsive MOPs derived from organic linkers containing azobenzene units, the structure of which is shown in Figure 12. In this system, an azobenzene precursor 25 was assembled to form MOP-26, which can undergo reversible trans−cis isomerization in response to light stimulus and, hence, realize the controlled capture and release of a guest I


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is very rare. The covalent attachment of azobenzene photoswitches to known molecular building blocks of hydrogenbonded capsules could provide an efficient way to integrate the photoisomerization and the assembly and disassembly of molecular capsules. Recently, Ballester and co-workers67,68 described the self-assembly of dimeric capsules based on tetraurea aryl-extended calix[4]pyrrole components (29) (Figure 14), and the dimeric capsules are prepared through a template-directed strategy with the encapsulation of guest 4,4′bipyridine N-oxide (30). Then they designed and constructed a tetraurea aryl-extended calix[4]pyrrole (29b)68 modified with four azobenzene groups at the upper rim. Similarly, it can selfassemble into a hydrogen-bonded dimeric capsule 30@29b· 29b in the presence of guest 30. The assembly can be formed only when all azobenzene units are in their trans configurations, as evidenced by 1H NMR spectroscopy. The photoinduced configuration changes of the azobenzene units in the 30@29b· 29b capsule are accompanied by the disintegration of the complex. The quantitative recovery of the capsule was achieved by the subsequent cis-to-trans isomerization of the azobenzene units in the dark. This study represents a unique example that can realize light-controlled reversible complexing and decomplexing of a tetraurea-based capsule by photoswitches situated on the container components. Previously discussed examples of supramolecular receptors or hosts can modulate the capture and release of small molecules in response to external light stimuli, however, the design and construction of systems that can realize the catch and release of nanometer-sized guest molecule remains challenging. Yoshizawa and co-workers69 reported a photoresponsive coordinated host system 31, which can catch and release large fullerene guests (Figure 15a). As shown in Figure 15b, a bispyridine ligand 32 with two incorporated anthracene panels was designed, and the addition of metal hinges to form a novel M2L2 molecular tube 31 capable of binding fullerene derivative 33. The host−guest complex 31·33a was formed as an orange solution in a quantitative yield via simple mixing the three components, including C60 molecules, ligand 32 and AgNO3. The removal of the metal hinges upon photoirradiation can generate free ligand 32, thus releasing C60 guest molecule. It should be noted that, unlike previous molecular cages and capsules, this open-ended tubular host can encapsulate functional C60 derivatives with large substituents. This will be

Figure 13. (a)Chemical structures of ligands o-27 and c-27, (b) the self-assembly of molecular cages o-28 and c-28, and (c) their interconversion upon light irradiation. Reproduced with permission from ref 66. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

be interconverted by the similar photochemical processes to the ligands. As shown in Figure 13c, both of the coordination cage isomers can encapsulate the spherical guest [B12F12]2−, but o-28 has a much higher association constant with guest molecule than c-28. Thus, irradiation of the host−guest complexes resulted in the reversible uptake and release of the guest due to the reversible switching processes. The fine modulation of the binding affinity for anionic guests might find applications in the fields of supramolecular catalysis, drug delivery, and functional materials. A variety of photoswitchable host−guest systems have been designed and constructed; however, the example of the lightinduced disassembly of the multicomponent supramolecular capsules that does not rely on photoswitchable guest molecules

Figure 14. Chemical structures of tetraurea aryl-extended calix[4]pyrroles 29a and 29b and the corresponding guest molecule 30 and the energyminimized structure of assembled supramolecular capsule 30@29b·29b. Reproduced with permission from ref 68. Copyright 2014 Royal Society of Chemistry. J


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isomerization of azobenzene 35, and the size of the generated cis-35 does not match the cavity of the capsule; as a result, cis35 was released and subsequent encapsulation of n-tridecane in the capsule occurred. Kinetic studies were performed to confirm that photoisomerization happened in the cavity of the capsule, which can break the capsule, thus allowing the entry of other guest species, as evidenced by 1H NMR spectroscopy. The reverse process is achieved by heating the mixture to 160 °C to convert cis-35 to its trans isomer, which rapidly replaces n-tridecane inside the capsule (Figure 16b). The reversible irradiation and heating processes can be repeated many times without degradation. In the following work, the photoinduced E/Z configuration changes of hemithioindigo (HTI),75 which can be readily isomerized at longer wavelengths (typically 410−430 nm) than azobenzene with the advantages of high efficiency and little photofatigue, are also used to effectively modulate the guest exchange processes in this kind of supramolecular capsule. On the basis of this well-established supramolecular system that can realize controllable guest exchanges in response to light stimuli, Rebek and co-workers76 had used this kind of reversible encapsulation to achieve supromolecular control of fluorescence (Figure 16c). A direct competition for the dimeric capsule 34·34 between 1 equiv of stilbene trans-36 and 2 equiv of trans-35 results in encapsulation of only the azobenzene compound, as confirmed by the 1 H NMR spectrum. Compound trans-36 shows high fluorescence at 388 nm upon excitation with 318 nm light due to the fact that no encapsulation happened. Irradiation with 365 nm UV light results in guest exchange: after irradiation for 50 min, trans-36 is completely encapsulated, and very weak fluorescence emission was observed. The reason for the fluorescence quenching is mainly due to the twisted shape with a 40° dihedral angle between the two aromatic planes of stilbene in the capsule 34·34. Heating to 160 °C for 2 min can recover the system to its starting state. It should be noted that this kind of supramolecular modulation of guest fluorescence can be repeated with good reversibility. The trans−cis configuration change of azobenzene derivatives was also used to realize a reversible light-driven capture and release processes into metal-coordination-based supramolecular cages77 or metal−organic frameworks.78 Kusukawa and Fujita79 have demonstrated that cis-azobenzene and cisstilbene derivatives can be encapsulated into a kind of nanosized coordination cage; however, the photoisomerization of the guest molecules was not studied. Shionoya and coworkers80 designed a molecular cage (37) formed by metal− ligand coordination of four rigid, bent bis-monodentate pyridyl ligands and two Pd(II) or Pt(II) ions, featuring a square-planar geometry that can encapsulate suitable bis-sulfonate guests through a specific anion recognition mechanism. In this lightswitchable system, a 4,4′-azobenzene bis-sulfonate (38) was used as a photoresponsive guest molecule to achieve a reversible light-driven encapsulation/release process.77 As shown in Figure 17, cis-4,4′-azobenzene bis-sulfonate (cis-38) has a suitable size for being completely encapsulated into metalcoordinated molecular cage 37, as evidenced by 1H NMR, DOSY-NMR, and ESI-TOF mass spectra. When the solution of the host−guest complex cis-38@37a was irradiated using white light for 3 h, guest compound cis-38 was quantitatively converted back to its isomer trans-38, whose size mismatch with the cage then results in the release of the guest from the supramolecular cage. Irradiation with 365 nm UV light resulted

Figure 15. (a) Schematic representation of a photoresponsive tubular host. The catch and release of the guest molecule can be triggered by the addition of metal ions and the removal of the hinges upon photoirradiation, respectively. (b) Chemical structures of coordinated host 31, ligand 32, and guest 33. Reprinted with permission from ref 69. Copyright 2013 American Chemical Society.

helpful for the design of novel photoresponsive molecular hosts in the future. 2.2.2. Supramolecular Cages or Capsules for Photoresponsive Guest Molecules. An alternative approach to realize photocontrolled binding affinity modulation involves the use of photoresponsive guests, such as an azobenzene unit. In this case, the multicomponent photoinactive host molecules have stronger binding affinity with one isomer of the photoactive guest than with the other. Then the binding affinity between the photoactive guest and the formed supramolecular host can be modulated by the photoinduced configuration change of the guest molecule. Conn and Rebek,70 as well as MacGillivray and Atwood,71 have devoted extensive studies toward the investigation of hydrogen-bonded selfassembled capsules. These capsules are formed exclusively when, and only when, suitable guests are present to fill the space inside. It has been determined that resorcinarene congener cavitand 34 (Figure 16a) can self-assemble into a dimeric supramolecular capsule 34·34 through octuplet hydrogen bonds, which can encapsulate various kinds of guest molecules, such as benzanilides72 and stilbenes.73 In their recent work, the cis− trans photoisomerization of azobenzenes have been applied to control the reversible encapsulation in a noncontact manner.74 trans-4,4′-Dimethylazobenzene 35 is proved to be a better guest molecule than n-tridecane for the capsule, and no ntridecane molecule was observed in the cavity of the capsule. Irradiation at 365 nm for 0.5 h results in the trans-to-cis K


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Figure 16. (a) Chemical structures of resorcinarene congener cavitand 34, trans-35, and trans-36. (b) Light-triggered guest exchange of trans-35 by n-tridecane in supramolecular capsule 34·34. (c) Fluorescence modulation of trans-36 through a light-induced guest exchange mechanism. Panel b reprinted with permission from ref 74. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA. Panel c reprinted with permission from ref 76. Copyright 2010 American Chemical Society.

clear, yellow solution of host−guest complex cis-38@37b was irradiated with white light, yellow crystals of [(37b)(trans38)2]n were produced immediately. In contrast, when a solution of cage 37b was mixed with 2 equiv of cis-38, immediate crystallization was observed with the formation of [(37b)(cis38)2]n. This principle of a light-induced phase change could open the way for developing new methods for nanoconstruction and for the spatially controlled lithographic deposition of supramolecular systems on surfaces, yielding a new potential application of this kind of photoresponsive host− guest complex.77 In this section, we have discussed how to use light to realize the controllable capture and release of guest molecule from a host or a cage molecule. Most of the single-molecule hosts we discussed here are based on the introduction of functional units into well-known host molecules, which show specific functions or enhance the binding ability for versatile guest molecules. The introduction of photoresponsive units into a single-molecule host system, achieving the reversible control of the binding stability, can pave the way for the design of more complicated supramolecular assemblies and systems. On the other hand, a multicomponent supramolecular host consists of several discrete functional components that noncovalently connect with each other to form a dynamic supramolecular assembly that shows adjustable recognition ability for an external guest molecule. Developing new single-molecule hosts or multicomponent assembled hosts is an extremely important issue in the field of supramolecular chemistry, because each newly found host or macrocycle can bring research growth in this field and thus speed up the development of supramolecular chemistry. Developing new photoswitches as guest molecules or introducing these new types of photoswitches into the skeletons of the multicomponent assembled host molecules can

Figure 17. (a) Efficient encapsulation of cis-38 by supramolecular cage 37a and reversible photoswitching host−guest complexation. (b) Quantitative encapsulation of cis-38 by supramolecular cage 37b. (c) Chemical structures of cage 37 and of guest 38 and its photoisomerization. Reprinted with permission from ref 77. Copyright 2010 American Chemical Society.

in a complete regeneration of the host−guest complex cis-38@ 37a, indicating a fully reversible process. Cage 37b, without the solubility-enhancing PEG chains, can also quantitatively form a soluble host−guest complex with cis-38. However, when the L


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donor−acceptor interaction. As a consequence, the pseudorotaxane structure collapses and the two components separated. This kind of dethreading movement can also be achieved by employing a porphyrin photoactive molecular triad as a nanoscale power supply.89,90 Besides the above-mentioned examples, intramolecular photosensitizations can also be used to conduct the dethreading/threading motion among pseudorotaxanes, which can be realized through introducing Ru− bipyridine or Re−CO−bipyridine photosensitizers into the linear rod91 or the macrocycle.92 In a pseudorotaxane without any big stoppers, the stimulusdriven separation/recombination motions are nonunidirectional, that is, the slippage of the ring onto or off of the rod would occur in both directions. In 2013, Stoddart and coworkers constructed a [2]pseudorotaxane system93 containing a CBPQT4+ and an asymmetric linear rod molecule (39), which consists of a central 1,5-dioxynaphthalene (DNP) donor linked at one side with a polyether chain and at the other side with a polyether−triazole segment and terminated on both ends with one 2-isopropylphenyl group and a 3,5-dimethylpyridinium unit, respectively, as shown in Figure 18. The CBPQT4+

be considered effective strategies to construct host−guest systems with controllable functions.

3. PHOTORESPONSIVE ROTAXANE, CATENANES, AND PSEUDOROTAXANES AS LIGHT-DRIVEN MOLECULAR MACHINES Mechanically interlocked molecules, such as rotaxanes and catenanes, along with their precursor, pseudorotaxanes, can also be referred to a kind of host−guest system. Even though rotaxanes and catenanes were sometimes called “supramolecules” in the early stage of this emerging field, they are molecules because the components among them cannot be separated until a covalent bond is broken. Nevertheless, we would like to include these two interlocked molecular systems in this section for the following two reasons: (1) the given rotaxane and catenane examples in this section were constructed through supramolecular synthesis, that is, using noncovalent bonding interactions to bring the bricks together in a rotaxane or a catenane precursor, and (2) the given examples are important types of molecular machines, and they are of different conformational and/or coconformational states, no matter whether stable or metastable ones, that are held through the supramolecular interactions among the rings and/ or the dumbbells. Introduction of photoactive units in rotaxanes, catenanes, and pseudorotaxanes can lead to the formation of photoresponsive molecular machines, which can perform photoresponsive threading/dethreading or conformational or coconformational motions in these systems. There are several reviews on this topic,81−86 but most of them are minireviews or summarize only the main work of the authors. In this section, we will show the overall view of the light-driven molecular machines, which are switchable pseudorotaxanes, rotaxanes, and catenanes powered by photoinduced electron transfer, the photoisomerization of photochromic units, excited-state changes, and reversible photolysis. 3.1. Light-Responsive Mechanical Motion from Photoinduced Electron Transfer

3.1.1. Photoinduced Electron Transfer in Pseudorotaxanes. When an electron-rich aromatic donor meets an aromatic electron-deficient acceptor, partial electron transfer from the donor to the acceptor can result in the formation of a charge-transfer (CT) complex. Photoexcitation at the CT absorption band would lead to a CT excited state, where an electron-transfer process from the donor to the acceptor occurs, generating a charge-separated state. Stoddart’s group87 designed a pseudorotaxane that can thus be constructed by threading a linear donor, for example, a naphthalene 1,5-diether derivative, into the center of an acceptor ring component, such as a tetracationic cyclobis(paraquat-p-phenylene)cyclophane (CBPQT4+), the typical ring used by Stoddart’s group. It has been demonstrated that direct photoexcitation could not result in the dissociation of the two parts, because the back-electrontransfer, which deactivated the charge-separated state to the stable ground state, would occur much faster than the mechanical dissociation of the two components. An alternative photoinduced dethreading movement can be successfully achieved by adding a photosensitizer and a sacrificial reductant.88 The photosensitizer, when excited by light, comes to a long-lived excited state and is then oxidized by losing an electron to the cyclophane acceptor. The backelectron-transfer process is precluded by the sacrificial reductant, and the reduced cyclophane deactivates the

Figure 18. A [2]pseudorotaxane system that realizes relative unidirectional translation in response to light irradiation. Reprinted with permission from ref 93. Copyright 2013 American Chemical Society.

cyclophane surrounds preliminarily the DNP recognition site; however, when photochemically reduced to the radical cation CBPQT(2+)(•+) by [Ru(bpy)3]2+ in the presence of phenothiazine, the back-electron-transfer from the phenothiazine radical cation (ptz•+) to the CBPQT(2+)(•+) is slower than the dethreading process, and as a result, the reduced ring would slip off from the positive pyridinium end because of the lower energy barrier. When the solution stayed in the dark, the charge recombines gradually and the free CBPQT4+ is regenerated, and the CBPQT4+ ring would pass over the 2-isopropylphenyl end rather than the pyridinium one, again for a lower energy, and the pseudorotaxane is restored. One can see that, in this elegant system, the ring moves relatively along the rodlike molecule unidirectionally, and this process could proceed M


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repeatedly with light as the sole energy supply, which is characterized by ease of generation, versatile manipulation, low cost, and excellent cleanness. On the other hand, it has been studied that bipyridinium radical cation (BIPY•+) would form a stable (K ∼ 2 × 107 M−1 for methyl viologen radical cation) 2:1 inclusion complex with cucurbit[8]uril (CB[8]) in aqueous solution.94 This phenomenon also provides a novel operating principle for constructing switchable pseudorotaxanes by the photochemical reduction of BIPY2+ to the radical cation in the presence of a photosensitizer. It has been demonstrated that a hexamethylenebridged bis-viologen can be folded into a CB[8] cavity to form a molecular loop by the [Ru(bpy)3]2+-sensitized photoreduction.95 Similar dimerization of the methyl viologen radical inside a CB[8] cavity by the photoinduced intramolecular electron transfer reduction, leading to the formation of a [5]rotaxane from a [3]pseudorotaxane, was also reported.96 Both of the dimer inclusion complexes can be reversibly restored to their original structures by allowing oxygen into the solution to quench the methyl viologen radical. 3.1.2. Photoinduced Electron Transfer in Rotaxanes and Catenanes. Exploiting the association/dissociation behaviors of a pseudorotaxane to serve as a molecular machine, on one hand, has the advantage of convenience, because a pseudorotaxane can be thermodynamically prepared by simply mixing the two free counterparts in a proper solvent; on the other hand, it has the drawback that the pseudorotaxane exists with the separated rodlike and ring components in the solution for the sake of the thermodynamic equilibrium; that is, it is a mixture but not a pure molecule. To push a pseudorotaxane system away from equilibrium, it can be converted to interlocked molecules: a rotaxane by introducing bulky stoppers at the rod ends and a catenane by “bending” the rod into another circle. These interlocked molecules turn into switchable molecular machines, when one or more additional stations are arranged in the rod (rotaxane) or in the same ring (catenane) and can conduct pirouetting and translation motions. Stoddart’s group had constructed a light-driven molecular shuttle (40) based on their photoinduced electron transfer pseudorotaxane systems. As can be seen from Figure 19, a ruthenium(II) tripyridine complex photosensitizer, a spacer, a 3,3′-dimethyl-4,4-bipyridinium (A2), and a 4,4-bipyridinium (A1) acceptor are in turn arranged along the rotaxane’s linear rod, and the big photosenstizer serves simultaneously as one of the stoppers. Like the conventional switching in pseudorotaxanes, the shuttling motion of the crown ether ring from A2 to A1 would occur (Figure 19b) upon the photoinduced oneelectron reduction of A1 by the Ru(II) complex in the presence of a sacrificial reductant.97,98 However, it should also be noted that, unlike traditional photochemically switching systems, where the back-electron-transfer is much faster such that no translational ring movement would be brought about without the aid of the sacrificer, the rate of the back-electron-transfer from A1 to the sensitizer (step 5) is slowed down to about 10 μs by arranging A1 at the far position from the sensitizer, and the shuttling of the ring from A1 to A2, which requires a time of about 50 μs, is found to operate with a quantum efficiency of 2% at 303 K (Figure 19a).99 The free-energy calculations also confirm this intramolecular light-driven mechanism.100 This system is a novel molecular machine that can act as a fourstroke engine powered exclusively by light at a single-molecule

Figure 19. Chemical structure of the light-driven molecular machine 40 and its working mechanism.

level and perform autonomous shuttling motions, while no waste product is produced. Leigh and co-workers101 reported a [2]rotaxane that is powered solely by light without the formation of byproduct. The system needs no sacrificer but an auxiliary electron donor, and it features a fast ring translocation and a much slower backelectron-transfer process. As shown in Figure 20, [2]rotaxane 41 is composed of a benzylic amide macrocycle interlocked onto a thread, which has two hydrogen-bonding stationsa succinamide (succ) station and a 3,6-di-tert-butyl-1,8-naphthalimide (ni) unitseparated by a C12 alkyl chain. At room temperature in acetonitrile, the macrocycle was held around the succ station by two sets of bifurcated hydrogen bonds. Laser excitation of the ni unit in the presence of an external donor led to the electron transfer process, and the ni unit was reduced to an anion radical, which binds more strongly with the macrocycle than the succ group; hence, the macrocycle moved to the second station. It is amazing that the movement took only about 1 μs, and the charge recombination is accomplished after 100 μs, and then the macrocycle shuttled is back to its original position and the oxidized donor recovered. The translational movement is found to be slowed down in poly(methacrylonitrile) solutions of different viscosities, but it was still active at high viscosities.102 It is interesting that strong repulsion exists between BIPY2+and CBPQT4+ while they have extremely strong binding affinity103 when every BIPY2+ unit is reduced to the radical cation (BIPY2+ to BIPY•+ and CBPQT4+ to CBPQT2(•+)). This radical pairing interaction would result in the formation a stable host−guest complex. Exploring the fact that a BIPY2+ can be N


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Figure 20. Photoinduced fast translational motion in the hydrogen-bonded molecular shuttle 41.

photochemically reduced to the BIPY•+ radical cation, a lightdriven bistable [2]rotaxane (42) was designed and synthesized by Stoddart’s group,104 as shown in Figure 21. The light

between the BIPY2+ group and the CBPQT4+ cyclophane, and the ring undergoes back-shuttling and resides again over the DNP unit. Besides the photoinduced electron transfer in the CBPQT4+based mechanically interlocked systems, a photoinduced mechanical movement through an electron transfer mechanism can also be applied in the metal-coordinated rotaxane or catenane systems. As we know, a Cu(I) ion adopts 4coordinating geometry and could bind with two 1,10phenanthroline units to form a stable compound, while Cu(II) favors a 5-coordinating geometry and would chelate with one 1,10-phenanthroline and one 2,2′:6′,2″-terpyridine group.105 When Cu(I) or Cu(II) was oxidized or reduced employing a photochemical means, the mechanical motion of the catenane106 or rotaxane107 can be realized. It is frank to say that the above light-powered molecular machines driven through the photoinduced electron transfer strategy, except the two examples (Balzani’s autonomous rotaxane99 and the Leigh’s fast responsive one101) that are powered solely by light, need additional oxidant and reductant to complete the working cycle and, thus, produce waste products, which are harmful in device fabrication and need to be removed, as for existing chemically fueled molecular machine systems. 3.2. Light-Responsive Mechanical Motion Powered by Photoisomerization

Although light-fueled molecular machines, through photoinduced electron transfer, do have the advantages of light stimulus, they need also chemicals and generate byproducts in the working process. In this section, we would like to introduce another method to power pseudorotaxanes, rotaxanes, and catenanes by light, namely, photoisomerization. Such lightfueled molecular machines through photoisomerization are powered exclusively by light, do not generate any waste products, and have consequently an additional advantage of cleanness. 3.2.1. Photoinduced Trans−Cis Isomerization of Azobenzene and Stilbene. 3.2.1.1. CBPQT4+- or Crown Ether-Based Systems. The photoinduced trans−cis isomerization of stilbenes and azobenzenes are the two typical series of photochromic systems that can provide useful strategies to

Figure 21. (a) Chemical structure of the light-driven bistable [2]rotaxane 42 and (b) its switching mechanism in response to light stimulus.

excitation of the [Ru(bpy)3]2+ could generate a two-electronreduced CBPQT2(•+) ring and a one-electron-reduced BIPY•+ recognition motif with the aid of a sacrificial electron donor. The reduced ring loses the donor−acceptor interaction with the DNP unit but has strong affinity with the reduced BIPY•+ recognition site. These effects result in the consequent ring shuttling from the DNP station to the BIPY•+ station. Once oxygen is introduced into the solution, the three BIPY•+ units are all oxidized to BIPY2+, on account of the restoration of the donor−acceptor interactions, as well as the steric repulsion O


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Figure 22. Operation of a light-gated stop−go molecular shuttle based on the photoresponsive degenerate [2]rotaxane 43, in which the rate of ringshuttling can be reversibly adjusted by light irradiation.

Figure 23. Schematic representation of a memory switch based on the multistate rotaxane 44.

generate light-driven “braking”, threading/dethreading, and shuttling motions of molecular machines. Vögtle’s group108 firstly reported the incorporation of an azobenzene unit into catenane to generate the photoswitchable brake motion induced by the trans−cis isomerization of the azobenzene unit. Pioneering works by Balzani and Stoddart109,110 also demonstrated that the photoisomerization of an azobenzene moiety can also be used in CBPQT4+-based pseudorotaxane systems to generate reversible threading/dethreading mechanical movements, because of the non-coplanar conformational change and the decrease in the conjugate capacity after the trans to cis isomerization. Alternatively, azobenzene can also be incorporated in the middle of the rod part of rotaxane systems to function as a

brake. Stoddart’s group also harnessed the photoisomerization of azobenzene to construct rotaxane 43 (Figure 22) to function as a light-gated stop−go molecular shuttle.111 On the symmetrical dumbbell was centered a tetrafluorinated azobenzene group, which was linked to two etherified DNP stations on each para side through a click reaction and ended with two 2,6-diisopropylphenyl stopper on both tails. In the initial trans state in acetonitrile solution, the CBPQT4+ ring shuttles between the two DNP recognitions sites on the 1H NMR time scale with a rate constant on the order of about 100 s−1. Upon the irradiation with UV light, more than 90% of the transazobenzene was converted to the cis isomer, whose configuration stops the shuttling of the CBPQT4+ ring, which can be confirmed by the new occurrence of well-resolved sets of P


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Figure 24. Photoswitching between a thermodynamically stable pseudorotaxane form and a kinetically inert rotaxane species by the photoinduced isomerization of azobenzene. Reproduced with permission from ref 114. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

isopropylphenyl-CHMe2 and triazole-CH2O proton signals in the 1H NMR spectrum. Irradiation of the cis-rotaxane with visible light results in 80% conversion to the trans configuration, where the slow shuttling motion reinstates. This stop−go gating process can be repeated by alternating UV- and visible-light irradiation. Using an azobenzene as a gate to modulate the rate of the shuttling movement can also be realized in a unsymmetic [2]rotaxane system employing CB[7] as a macrocycle.112 The fact that the cis-azobenzene would stop the shuttling of a suitable ring component can be utilized to set up a memory switch function.113 As illustrated in Figure 23, the thread component of rotaxane 44 contains two electron-donating groupsa tetrathiafulvalene (TTF) unit and a DNP unitand a photoisomerizable 3,5,3′,5′-tetramethylazobenzene unit located between the two recognition sites for the π-electrondeficient CBPQT4+ ring. The CBPQT4+ resides preferentially on the TTF station and can be switched to the DNP site by the oxidation of the TTF unit to its radical cation (TTF+•) form. It can shuttles back in two ways: (1) reducing TTF+• to TTF or (2) locking the ring at the DNP site by the photoinduced isomerization, reducing the TTF+• to TTF, and finally releasing the ring to the original state by the reverse cis-to-trans isomerization. The latter way provides the opportunities to write data on the rotaxane by an oxidation stimulus and locking it by UV-light irradiation. Even though the oxidized species has been reduced back to the original form, the written data can be

kept for a few hours in the dark until the thermal cis-to-trans isomerization of the azobenzene occurs. It can be seen from the above-mentioned examples that a trans-azobenzene could thread the macrocycle while the cis isomer could act as a stopper of the macrocycle in a rotaxane. In 2010, Credi’s group designed a novel pseudorotaxane system, with two azobenzene units standing at the two rod ends, which could be switched between the initial thermodynamically stable pseudorotaxane form and the kinetically inert rotaxane species by the photoinduced isomerization of azobenzene (Figure 24).114 In acetonitrile solution, the dibenzo[24]crown-8 ring was threaded by the bis-azobenzylammonium axle 45 (E,E isomer) and surrounded the ammonium center to form a pseudorotaxne species with a stability constant value of 820 M−1 at 298 K. The dethreading rate constant k(out)EE of the E,E pseudorotaxane was calculated to be ≥0.1 s−1. The consequent photoirradiation of the E,E solution at 365 nm led to >95% isomerization to the Z,Z isomer. Thanks to the fact that the Z-azobenzene unit imposed a stronger steric hindrance to the crown ring than the E isomer, the dethreading rate constant k(out)ZZ for the Z,Z isomer was then found to be 7.2 × 10−6 s−1, which indicates the formation of the kinetically inert rotaxane. In addition, a system that can realize light control of the stoichiometry and motion in CBbased pseudorotaxanes was also reported by the same group.115 Credi and co-workers made further efforts to demonstrate the directional transit of a nonsymmetric molecular axle through a crown ether by way of photoisomerization.116 As Q


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to the ammonium recognition site to afford a pseudorotaxane when they are mixed. The irradiation of the pseudorotaxane solution with UV light causes the isomerization of the transazobenzene to cis-azobenzene, and the crown ring would be pulled down from the cis-pseudorotaxane by adding K+ ion, which is a competitive guest for the ring. At this process, the ring slips exclusively over the pentyl group because the slippage from the cis-azobenzene end becomes more difficult than that from the cyclopentyl ending group because of the increased steric effects, and as a consequence, the directional transit of the axle through the ring is accomplished. The system can be reset by the thermal Z-to-E isomerization and the addition of 18crown-6 to extract the K+ ions. Very recently, the same group made the great accomplishment of achieving an autonomous light-powered directional threading/dethreading using a very similar system.117 Besides the crown ether-based pseudorotaxane system, the same group also realized the unidirectional transit of a nonsymmetric axle through a nonsymmetric heteroditopic tris(phenylureido)calix[6]arene wheel to achieve the tuning of the dethreading rate by light.118 It should be noted that using light to achieve repetitive directionally controlled molecular motion represents an important step for the construction of true molecular machines. 3.2.1.2. Cyclodextrin-Based Systems. The trans-azobenzene and stilbene units are both good guest candidates for the α- or β-cyclodextrin (CD) host. Each of the two slender axles, for the sake of the hydrophobic character and steric fitness, would form host−guest inclusion complexes with α- or β-CD in water, where the CD ring resides over the central NN or CC double bond. By contrast, the pudgy cis isomers would never fit in the CD cavity and the steric repulsion would exclude the CD ring outside from the double bond centers. These behaviors can be utilized to create photoinduced pseudorotaxanes and rotaxanes.119−121 Nakashima and co-workers first reported a light-driven CDbased molecular shuttle based on the photoisomerization of azobenzene.122 In the [2]rotaxane 47 shown in Figure 26, the α-CD surrounds the azobenzene unit in the initial trans state, giving a strong positive induced circular dichroism (ICD) band at 360 nm and a negative ICD band at 430 nm, which originates from the encapsulation of the asymmetric CD ring around the central NN bond. After irradiation with UV light, the macrocycle shuttles to the ethylene group, accompanied by the decreasing positive ICD signal at 360 nm, as well as the increasing positive ICD signal at 312 nm and the negative ICD signal at 430 nm. On the basis of this result, a more

shown in Figure 25, an asymmetric axle (46) was terminated by an azobenzene and a cyclopentyl pseudostopper at each end.

Figure 25. Photocontrol of the threading directionality of a crown ether macrocycle onto the azobenzene-containing nonsymmetric molecular axle 46. Reproduced with permission from ref 116. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

Owing to the fact that the rate constant of the ring to jump over the trans-azobenzene unit is 2 orders of magnitude larger than that to pierce the ring with the cyclopentyl group, the crown ring would pass over the azobenzene terminal exclusively

Figure 26. Chemical structures of molecular shuttles 47 and 48. R


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Figure 27. Chemical structure of the stilbene-containing rotaxane 49 and its photoinduced mechanical-shuttling motion.

complicated multimode driven molecular shuttle (48) was also constructed, whose mechanical movements can be driven thermally and photochemically.123 Anderson’s group for the first time reported a stilbene-based rotaxane molecular shuttle employing α-CD as a wheel (Figure 27).124 [2]Rotaxane 49 was prepared from the Pd-catalized Suzuki coupling of p-stilbene diboronic acid with iodoisophthalic acid in the presence of α-CD in alkaline aqueous solution and could undergo reversible trans−cis photoisomerization. In the original trans form, the 2D NMR spectrum revealed that the CD ring glided rapidly back and forth around the center stilbene, while in the cis form, the CD ring shuttled to the biphenyl end. It is interesting that, although the asymmetric CD has a narrow 6-rim and a broad 3-rim, there is only one coconformational cis isomer found where the broad rim stands near the isophthalic group. Perhaps inevitably, in the early stage of this emerging field, the mechanical motions of molecular machines were so fascinating that the researchers focused mainly on designing ingenious systems to obtain these motions, and relatively little attention was paid to using photons to monitor the operation of the molecular machines during that period. In fact, although the emission changes in a pseudorotaxane driven exclusively by light and inducing threading/dethreading motions have been mentioned by Balzani et al.,110 we first pointed out that the photonic inputs and outputs are the right matched couple and that their combination might lead to the construction of fulloptical molecular devices.125 In this study, the first full-optical [2]rotaxane (50), where the photoisomerization of stilbene was utilized to shuttle a CD macrocycle with the accompanying fluorescent output signal, was reported, as shown in Figure 28. The α-CD was interlocked into the phenylstilbene thread by two stoppers, a fluorescent 4-amino-3,6-disulfonic-1,8-naphthalimide disodium salt and an isophthalic acid group. In the alkaline solution, the CD ring encircles at the trans-stilbene station, and then irradiation with UV light results in the movement of the ring to the biphenyl station, increasing the fluorescent intensity at 530 nm by about 50%. The reversible back-shuttling motion can be obtained by the photoinduced cisto-trans isomerization with 280 nm irradiation. This [2]rotaxane is an optical molecular switch, where the two inputs and the output are 335 and 280 nm UV light and fluorescent emission at 530 nm, respectively. Many ingenious light-driven rotaxane systems, arranging photoisomerizable azobenzene and/or stilbene recongnition stations on the dumbbell molecules and mechanically locking CD rings on the stations by one or two nathphalimide fluorophores, have been designed and synthesized by our group after the first full-optical molecular shuttle, many of which are presented in our reviews.16,126 For example, as shown in Figure

Figure 28. Operation of a light-driven molecular shuttle based on [2]rotaxane 50.

29, rotaxane 51 containing two fluorophore stoppers, one isomerizable station, and one CD ring gives dual fluorescence addresses;127,128 [2]Rotaxanes 52 and 53, which consist of one or two fluorophore stoppers, two isomerizable stations, and one CD ring, can function as an all-optical half-adder129 or an INHIBIT logic gate,130 respectively, and [3]rotaxane 54, which is made up of two fluorophore stoppers, two isomerizable stations and two CD rings, could imitate the “plus 2” and “minus 2” calculations of a “Chinese abacus”.131 Upon the encapsulation of the chiral CD ring on an achiral guest chromophore, the guest becomes optically active, and ICD signals are observed. The ICD signal can also be used to monitor the operation of molecular machines. In fact, early in Nakashima’s research on [2]rotaxane,122,123 obvious ICD changes had been found during the photoisomerizationinduced shuttling motions. We also attempted to utilize ICD signals to serve as the output signals in molecular machine systems.132 Then we reported the direct synthesis the CDbased [1]rotaxane 55 via a self-complementary strategy.133 The photoisomerization of the azobenzene group powers the translational movements of the CD ring with the occurrence of distinctive ICD signals changes. The same situations were also found in its isomer with a different orientation of the CD ring, [1]rotaxane 56,134 and with [1]rotaxane 57,135 which contains two CD macrocycles without any stopper and S


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photoisomerization-induced shuttling motions in hydrogel indeed occur and induce distinctive ICD signal changes, however, with more difficulty with respect to those in solution. This result is consistent with the fact that the degree of the shuttling freedom in a hydrogel is the median between those in a solution and in a solid state. Compared with fluorescence output, room-temperature phosphorescence (RTP) has the merits of, besides the good sensitivity and selectivity as fluorescence, a long life and large Stokes shifts, which provides new means to improve the signalto-noise performance and achieve time-resolved techniques.143 We also employed RTP as the output signal to encode the different states of light-driven pseudorotaxane systems 58 and 59.144,145 As can be seen in Figure 31, utilizing the fact that αbromonaphthalene binds more strongly than the cis-azobenzene group but more weakly than the trans one with the β-CD host, the α-bromonaphthalene can be driven to move into or out from the cavity of β-CD by the alternating irradiation at 360 and 430 nm to induce the cis−trans isomerization of the azobenzene group. When the bromonaphthalene phosphor enters into the CD cavity, which prevents the quenching of the triplet state of the phosphor by the dissolved oxygen, it gives the RTP emission, and not vice versa. In this way, a phosphorescence molecular switch,144 or even molecular logic gates,145 can be achieved with high S/N ratio phosphorescence output signals. The shuttling motions in rotaxanes can be transformed to contraction/expansion behaviors by means of the dimerization of hermaphroditic CDs, where the guest unit is linked covalently to the host, to form the so-called Janus [2]rotaxane. These stimuli-responsive contraction/expansion behaviors resemble the essential properties of a skeletal muscle, and these kinds of Janus [2]rotaxane are called molecular muscles, one of which was first reported by Sauvage and co-workers.146 Easton and co-workers reported a light-driven molecular muscle based on the photoisomerization of stilbene (Figure 32).147 In this design, a stilbene rod with a propane amine tail was attached to the α-CD rim at the stilbene end. The resulting hermaphroditic compound would thread intermolecularly in aqueous solution to form a Janus complex, where the two CDs encircled the stilbene units. Two dimethoxytriazine stoppers were then introduced to the amino ends, and the Janus [2]rotaxane 60 was formed. When irradiated at 350 nm, the two trans-stilbene groups can be isomerized to the cis,cis state, driving the CD macrocycles to shuttle from the cis-stilbene moieties to the propyl ends, and the contraction process is accomplished. The stretching process can be achieved by the reversible back-isomerization upon irradiation at 254 nm. A similar strategy was also carried out by Harada’s group, who introduced an azobenzene photoswitchable unit in the Janus [2]rotaxanes.148,149 The introduction of an oligo(ethylene glycol) linker between the photoswitching group and the aliphatic station improves, to some extent, the size changes during the contraction/stretching cycle. Kaneda and co-workers even constructed a linear photochromically contractible and extendable oligomer with azobenzene-containing Janus [2]rotaxane repeating units, which has the potential to function as a molecular muscle.150 3.2.2. Photoinduced E/Z Isomerization of Fumaramide. The photoisomerization processes between fumaramide (E isomer) and maleamide (Z isomer) are another attractive means of inducing mechanical motions in amide-based rotaxanes and catenanes.151,152 Fumaramide groups bind

Figure 29. Chemical structures of a series of photodriven molecular shuttles.

performs the musclelike stretch/contraction motions in response to light stimuli, as shown in Figure 30.

Figure 30. Chemical structures of a series of photoresponsive [1]rotaxanes.

Besides the above-mentioned [2]rotaxane and [1]rotaxane systems, other CD-based supramolecular self-assembling systems136−140 were also designed and can respond to light stimuli. Several rotaxane systems141,142 were selected to be doped into hydrogel systems, which are more easily stored and transferred than in solutions and provide enough working space with respect to solid states. It has been proven that the T


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Figure 31. Light-driven pseudorotaxane systems 58 (a) and 59 (b) encoding a room-temperature phosphorescence response.

Figure 32. Chemical structure and schematic representation of a photodriven molecular muscle based on Janus [2]rotaxane 60.

strongly with benzylic amide macrocycles for the formation of two sets of bifurcated hydrogen bonds between the fumaramide carbonyl groups and the amide groups of the macrocycle, but after the photoisomerization to maleamide, the intercomponent hydrogen bonds are reduced from four to two, and the binding affinity between the two counterparts is greatly reduced. In an amide-based [2]rotaxane that contains only one fumaramide station, the rate of the internal macrocycle rotations is greatly accelerated because of the reduction of the energy barrier after the E station is photoisomerized to the Z one.153 Succinamide and N-acyl glycyl groups have a similar hydrogen-bonding mode as the fumaramide to the macrocycle and are of intermediate binding strengths to the macrocycle, being between those of the fumaramide and maleamide groups. As a result, a light-powered molecular shuttle can be made by arranging the macrocycle onto a two-station (one fumaramide and one succinamide or N-acyl glycyl stations) thread. Such a system was first reported by Leigh and co-workers; as shown in Figure 33a, the macrocycle of rotaxane 61 resides over the fumaramide station and moves to the succinamide station after photoisomerization, and it could be thermally switched back to the original fumaramide station in a more thermodynamically stable state.154

Figure 33. Reversible photoinduced-shuttling motion in rotaxanes 61 (a) and 62 (b).

The shuttling motions of the amide-based macrocycle can be utilized to create chiroptical changes by introducing a chiral Nacyl glycyl station,155 as shown in Figure 33b. When rotaxane 62 is in its E state, the macrocycle resides over the fumaramide recognition site and aromatic rings are far away from the chiral glycyl-L-leucine unit, and as a result, weak ICD signals can be detected. After photoisomerization of the fumaramide to U


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Figure 34. Chemical structures of light-driven amide-based [2]rotaxanes 63 (a), 64 (b), and 65 (c).

Figure 35. Shuttling motion of rotaxane 66 triggered by the photoisomerization of a spiropyran.

maleamide upon 254 nm irradiation, the macrocycle shuttles to the N-acyl glycyl station, and the aromatic rings of the macrocycle are very close to the chiral L-leucine center. Such a process can be evidenced by the strong and negative ICD responses at around 246 nm. The back-shuttling process can be achieved by the photoinduced Z-to-E isomerization (in or without the presence of a sensitizer) or by heating in C2H4Cl2 at 130 °C for several days. The light-driven positional changes of this kind of amidebased rotaxanes can also be addressed by fluorescence output signals. Very shortly after the first photodriven [2]rotaxane with a fluorescence address was reported by our group,125 Leigh and co-workers also created a light-activated [2]rotaxane (63) (Figure 34a) with a fluorescence response by making use of the E/Z photoisomerization of fumaramide, and it exhibited an ultrahigh on−off intensity ratio between the translational states.156 The [2]rotaxane 63 consists of a pyridinium-modified benzylic amide macrocycle trapped on a thread that contains two stationsa fumaramide station and a glycylglycine one separated by a C11 alkyl chain and terminated by the two stoppers, one anthracene fuorophore and one diphenylmethane unit. The pyridinium macrocycle can be switched between fumaramide and the glycylglycine stations, in response to irradiation at 312 nm, and the photostationary state emits ∼85 times more strongly than the Z-isomer at 417 nm. The Z isomer can also be restored to the E isomer by simply heating at 115 °C. The [2]rotaxane could function as a molecular

memory, which can be written by 312 nm and erased by heat, with a fluorescent output at 417 nm. Li and co-workers also reported an amide-based [2]rotaxane 64 using perylene bisimide and perylene moieties as the two stoppers (Figure 34b).157 The light-powered shuttling of the protonated macrocycle from the fumaramide station to the succinic amide ester station initiates the electron transfer from the perylene to the macrocycle, a process that reduces the energy transfer from the pyrene moiety to the perylene bisimide moiety and results in a fluorescence response at 556 nm, which is attributed to the emission of the perylene bisimide fluophore. Another bistable [2]rotaxane (65) with a modified amide macrocycle and two porphyrin and fluorine fluorophore stoppers (Figure 34c) was designed,158 and it seems that no energy or electron transfer process occurs during the reversible photo- and thermodriven switching cycle. 3.2.3. Photoinduced Ring-Open/Close Isomerization of Spiropyran. Spiropyran (SP) is a typical photoisomerizable compound that can be optically switched between the colorless SP and the deeply colored zwitterionic merocyanine (ME) form. The photoisomerization of SPs has been widely used to create optical molecular devices, but in contrast, it has rarely been used to power a switchable rotaxane. To the best of our knowledge, the only example of this kind is reported by Li and co-workers.159 As illustrated in Figure 35, the bistable [2]rotaxane 66 is composed of a benzylic amide macrocycle interlocked on a dumbbell containing a glycylglycine station and two tetraphenylmethane and SP stoppers on each end. V


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Figure 36. Subnanosecond photoinduced shuttling in the peptide-based [2]rotaxane 67.

Figure 37. Chemical structures of rotaxane 68 and catenanes 69 and 70 and their respective photoinduced mechanical movements.

to the macrocycles and are not to be utilized to generate the translocation of the ring components but to offer more switching states to these rotaxanes. The photoisomerization among these complicated rotaxanes can either endow functional switching behaviors161−163 or tune the energy transfer/ electron transfer interactions between the stoppers and the photoisomerizable units, giving fluorescent output signals.164,165

Upon the irradiation of rotaxane 66 at 365 nm, the closed SP was transformed to ME with a phenolic anion, which favors stronger hydrogen bonding with the macrocycle, attracting the macrocycle to move to the ME end. The back-shuttling can be induced by visible-light irradiation. This [2]rotaxane is a reversible molecular shuttle with an obvious color change, which can be detected by the naked eye. In addition, Credi and co-workers developed a simple ternary molecular machine system, in which the photoisomerizaiton of SP is explored as the driving force. In such a system, the threading/dethreading of a pH-switchable calix[6]arene bipyridinium pseudorotaxane was realized via photoinduced acid− base properties of the spiropyran photochrome. This work represents an ideal coupling between the two kinds of molecular switches (a pseudorotaxane and a spiropyran) that communicate with one another by intermolecular proton transfer, providing an additional means to the construction of novel functional molecular machines.160 There are also rotaxanes that incorporate photoisomerizable units that are usually not arranged on the threads but attached

3.3. Light-Responsive Mechanical Motion Powered by Excited-State Changes

Generally, the large amplitude mechanical shuttling motions in rotaxane take place over relatively long time scales, but in contrast, small displacements (several angstroms) of the ring components in a rotaxane might be unexpectedly fast. Figure 36 shows the subnanosecond photoinduced shuttling in a peptidebased [2]rotaxane reported by Leigh and co-workers.166 The traditional amide-based [2]rotaxane 67 was synthesized using a glycylglycine recognition motif, and the benzylic amide macrocycle was stoppered by anthracene-9-carboxyl and bistert-Bu phenyl bulky ends. X-ray crystallography shows that the macrocycle surrounds the glycylglycine part with four amide− W


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Figure 38. Light-responsive mechanical motions in rotaxane 71 powered by photoheterolysis.

would repel the tetracationic cyclophane. These phenomena offer the chance to create positional changes in rotaxane 71 (Figure 38).174 The axle molecule is composed of two recognition sitesone photoactive methoxyl diarylcycloheptatriene unit and one anisol groupand two bulky stoppers, and it folds at the polyglycol chain to bring both of the electron donors close to the CBPQT4+ ring, with the tetracationic ring staying on the cycloheptatriene station. The irradiation at 360 nm generates the tropylium with a methoxide counterion, and the CBPQT4+ ring moves to the anisol station. The restoration can be achieved by heating the tropylium rotaxane in methanol. Here, we have reviewed the most recent accomplishments in the field of photoresponsive rotaxanes and catenanes and their precursors, pesudorotaxanes, most of which can function as light-driven moleculular machines or a component of molecular machinery. The introduction of photoresponsive units into mechanically interlocked molecular systems can facilitate the realization of the intramolecular mechanical motion and control the rate and directionality of the molecular motions. With the well-understood photochemical processes of the known switches and the principles of various host−guest interactions, scientists are now good at designing various molecular machines based on the photoresponsive host−guest system. Although these machines exhibit beautiful and peculiar features, it should be note that there are still unsolved problems in this field. For example, in the E/Z photoisomerizaton systems, incomplete photoswitching and thermal back-isomerzation of Z isomers are often observed. These drawbacks limit their applications in switching or data storage. The development and exploration of new photoisomerizable units that have quantitative isomerization and long thermal half-lives to power the machines may be a solution. Furthermore, more work should be focused on finding what we can use light-driven molecular machines to do, i.e., their real practical applications, which is the key for the development of this field. Another challenge before we harness useful functions out of these photoresponsive molecular machines is to address them in “solid” space and to make them behave coherently. Although there have been several strategies, such as Langmuir−Blodgett (LB) monolayers, self-assembled monolayers (SAMs), and metal−organic frameworks (MOFs), to make ordered molecular machines, there is still a long way to go for the fabrication of solid-state molecular optical devices and circuits.

amide hydrogen bonds (not the normally favorable amide− carbonyl bonding), which may be due to the nonplanarity of the anthracene-9-carboxamide. The excitation of the anthracene carboxamide at 330 nm brings a near-planar conformation and allows the charge transfer from the excited state of the anthracene to the carbonyl group, making it a much better hydrogen-bond acceptor, thus inducing a ∼3 Å shuttling motion of the macrocycle to the N-carbonyl glycyl station on a time scale of subnanoseconds. In transition-metal (TM) complexes, the metal-centered (MC) ligand field excited state, which can be populated from the metal-to-ligand charge-transfer (3MLCT) state, is a deactivation pathway and sometimes leads to the dissociation of such complexes.167 It is obvious that the MC excited states are harmful for the applications of these complexes in the area of energy-transforming devices, such as DSSCs and OLEDs, because they reduce efficiency and shorten lifetime. However, such kinds of photoinduced ligand dissociation168 offer the opportunity to bring about coconformational changes in rotaxanes and catenanes. Ru(II) polypyridyl complexes are particularly well-adapted to this approach.169 From the MC ligand field state, which is significantly distorted relative to the ground state, the dissociation of a sterically hindered bipyridyl ligand could occur in high yield. Figure 37 shows the example of rotaxane 68.170 Two bisphenanthroline units were arranged on the thread, and a 6,6′-disubstitued-2,2′-bipyridine ligand was embedded on the 35-membered ring. The thread folds so that three chelates form with the Ru(II) to form a metal complex. The bipyridine ring serves as a sterically hindering chelate and can be shuttled from the Ru(II) center by visible-light irradiation. The reverse process can be achieved by heating the product in ethylene glycol. The examples of the catenanes 69 and 70 are also illustrated in Figure 37.171,172 The irradiation of these Ru(diimine)32+-based [2]catenanes in the presence of Et4NCl leads to decomplexation of the bipydyl chelates from the Ru(II) center, giving two new catenanes in which the metal is only coordinated to the two bis-phenanthroline moieties. Again, these coconformational changes can be reversed thermally. 3.4. Light-Responsive Mechanical Motion Powered by Photoheterolysis

Aryl cycloheptatrienes are electron donors and form chargetransfer complexes with the CBPQT4+ macrocycle.173 Moreover, these 7-membered rings can be converted into tropylium ions by photoheterolysis, and the resulting cationic tropylium X


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Figure 39. Formation of linear and cross-linked supramolecular polymers from the multifunctional molecule 72 and its photoinduced sol−gel switching.

ing.176−191 In this section, we will specifically focus, however, on the photoresponsive supramolecular polymers based on host−guest interactions and discuss their typical functions under light stimulus in terms of different architectures.

4. PHOTORESPONSIVE SUPRAMOLECULAR POLYMERS AND THEIR ASSEMBLIES Supramolecular polymers, generated by the marriage of supramolecular chemistry and traditional polymer science, are considered as one of the most promising candidates for smart materials.175 In supramolecular polymers, highly directional and ordered polymeric arrays of monomeric units are brought together by reversible noncovalent interactions, which provides them with not only some of the important properties of conventional polymers (for example viscosity) but also a wide range of novel and intriguing properties that are not known in conventional polymers under external stimuli, such as pH, chemical or electrochemical redox, photoirradiation, temperature/concentration changes, and enzyme modulation. Among those stimuli, photostimulus is especially attractive to realize the specific properties of supramolecular polymers, such as morphology adjustment, luminescence regulation, shape memory, and self-healing, on account of its convenient operation, low cost, sensitivity, noninvasiveness, and excellent penetration depth. Typically, photoresponsive supramolecular polymers can undergo light-controlled structural transformations of specific functional groups either along the polymer backbone or the side chains. Such kind of supramolecular polymers may find their potential applications in such fields as viscosity control, reversible optical storage, photochemical transduction/actuation, drug delivery/releasing, protein modulation, and tissue engineering. With the fast development of supramolecular polymerization,22 characterization,176 and nanotechnology,177,178 a great number of researchers have been focused on functional supramolecular polymers.179−191 In recent years, a number of excellent books and reviews have been published expositing progress in the area of functional supramolecular polymers based on the role of different noncovalent interactions, such as hydrogen bonding, host−guest recognition, donor−acceptor interaction, metal−ligand coordination, and π−π stack-

4.1. Supramolecular Interactions in the Main Chain

4.1.1. Photoswitching between Monomer/Oligomer and Supramolecular Polymers. Directional complementary/self-complementary ditopic monomers or blocks are widely used to construct linear homo- and copolymers, in which noncovalent interactions act as a dynamic linkage. Hence, reversibility is one of the essential characteristics for supramolecular polymers and the degradation/recombination processes can usually be realized on experimental time scales under appropriate external stimuli. Among those noncovalent interactions, host−guest recognition, which is ubiquitous in biological systems such as the enzyme−substrate structures, is one of the most important noncovalent interactions to hold supramolecular polymers together and endow them with stimuli responsiveness, because it combines relatively high strength with excellent reversibility. Crown ethers,186,192 cyclodextrins,188,193 calixarenes,183 cucurbiturils,194,195 and pillararenes196 are the main macrocyclic hosts that are extensively employed in supramolecular interlocked architectures and polymers.197,198 Although some photoswitchable crown ether-based small molecules have been synthesized and studied,199−201 examples of crown ether-based photoresponsive supramolecular polymers are relatively rare. Dong et al. have reported a photoresponsive linear and cross-linked supramolecular polymer based on host−guest interactions.202 As shown in Figure 39, they synthesized a triple-functional monomer (72), in which a dibenzo-24-crown-8 (DB24C8) moiety and a dibenzylammonium salt (DBA) segment are the host and guest groups, respectively, whereas the azobenzene moiety acts as a photoresponsive unit. This monomer is ready to selfassemble to form a linear supramolecular polymer by host− Y


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of the azobenzene moiety of the guest monomer may induce the polymerization/depolymerization processes under light input. On the basis of similar cyclodextrin−azobenzene host− guest recognition and further introducing the other orthogonal host−guest interaction between bis(p-sulfonatocalix[4]arene) (BSC4) and the methyl viologen moiety, a linear supramolecular polymer was formed (Figure 41).207 In the functional

guest interaction and can be further cross-linked through metal coordination with [PdCl2(PhCN)2]. They demonstrated that the decrease and increase of the binding strength between DB24C8 and DBA moieties, induced by the cis−trans photoisomerization of azobenzene groups, will obviously affect the molecular weight and viscosity of the complex, hence achieving sol−gel transitions (in cross-linked form). This work presented a new strategy to construct photoresponsive crown ether-based supramolecular polymers. Cyclodextrins (CDs) are well-known to form host−guest complexes with varieties of organic small molecules and polymers in aqueous solutions, which have also been widely used to construct supramolecular polymers.193 Due to their water solubility and low cytotoxicity, cyclodextrin-based stimuli-responsive supramolecular polymers may hold great potential for applications in biosystems, such as gene delivery, drug delivery, and tissue engineering. Harada and co-workers used a stilbene-bridged bis-β-CD dimer that can undergo trans−cis isomerization and a homoditopic guest to construct a stimulus-responsive supramolecular system that can be switched between a supramolecular host−guest dimer and a linear supramolecular polymer under photoirradiation.203 As seen in Figure 40, the

Figure 41. Formation of a photoresponsive linear helix supramolecular polymer using two orthogonal host−guest interactions from 73 and its reversible photoswitching processes.

guest compound 73, the binaphthyl unit was used for engendering the chirality, and the azobenzene group was a photoresponsive chromophore that could also thread into the cavity of α-CD, while the methyl viologen moiety not only improved the solubility of the guest in water but also acted as a binding group with BSC4. First, a pseudo[3]rotaxane was formed by the combination of monomer 73 and two α-CDs, which was then noncovalently connected by BSC4 to form a linear supramolecular polymer via host−guest and electrostatic interactions with the methyl viologen moiety. This supramolecular ploymer was provided with chirality because of the binaphthyl unit, which shows starlike micromorphology, as seen by scanning electron microscopy. It is interesting that when it was irradiated with 365 nm light, which induced the trans-to-cis isomerization of the azobenzene group, it can generate a helical fiber whose morphology was confirmed by cryogenic transmission electron microscopy (cryo-TEM) images. Cyclodextrin-based supramolecular polymers can also be used to construct supramolecular hydrogels. Jiang and coworkers developed a facile photocontrollable supramolecular approach to realize the reversible disassembly/reassembly process of a polypseudorotaxane (PPR) based hydrogel.208 In that work, they constructed a PPR hydrogel from the combination between poly(ethylene glycol) (PEG) and α-

Figure 40. Photoresponsive supramolecular system formed by a stilbene-bridged bis-β-CD dimer host and a diadamantyl-functionalized guest.

trans conformation of stilbene-bridged bis-β-CD dimer, formed dimers or oligomers with the diadamantyl-functionalized guest, whereas in the cis conformation of stilbene-bridged CD dimer, a high molecular weight supramolecular polymer was formed. Liu and co-workers employed the orthogonal recognition of porphyrin/β-CD derivative and azobenzene/α-CD to construct a light-controlled morphology-changeable nanoarchitecture.204 Another such kind of example was reported by Dong et al.205 In their work, an azobenzene dimer (A2) and a β-CD trimer (B3) was used to fabricate a novel class of A2−B3-type hyperbranched supramolecular polymer whose polymerization/ depolymerization processes could be reversibly controlled by alternating UV- and visible-light irradiation. These works may open the way to design water-soluble photoresponsive supramolecular polymeric materials. Our group also constructed a supramolecular polymer from low molecular weight monomers via dual orthogonal noncovalent interactions, namely, metal−ligand coordination and host−guest recognition.206 The trans−cis photoisomerization Z


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CDs. Then, an azobenzene derivative (74), which acted as a water-soluble competitive guest, was introduced into the hydrogel. After ultrasonication, the hydrogel transformed into a homogeneous solution in a few minutes (Figure 42), due to

Figure 43. A light-responsive bis(p-sulfonatocalix[4]arene)-based supramolecular polymer.

assemblies, but their applications in the field of photoresponsive supramolecular polymers are rarely explored. More recently, research on the host−guest interaction between CB[n]s and azobenzene compounds has been reported. Scherman and co-workers developed a new photocontrolled complexation/decomplexation process for the supramolecular complexes based on CB[8] and an azobenzene derivative guest both in solution and the solid state.212 As shown in Figure 44,

Figure 42. A photocontrollable PPR-based supramolecular hydrogel.

the stronger binding between α-CD and the trans-azobenzene guest and the dethreading of PEG chains from α-CDs. When this solution was irradiated by 365 nm UV light, the transazobenzene moieties were transformed to the corresponding cis-form which then dissociated from α-CDs on account of the decrease of the binding affinity. Hence, free PEG chains rethreaded into the CD cavity again and reformed the PPR hydrogel. Such sol−gel conversion was reversible when irradiated by visible light. Recently, a similar strategy was also employed in hyperbranched supramolecular polymers.209 p-Sulfonatocalixarenes are another important biocompatible hosts in aqueous solution. Liu and co-workers reported a linear photoresponsive supramolecular polymer based on host−guest recognition between BSC4 and homoditopic bis-4-(Nmethylpyridinium)azobenzene 75 with a rigid spacer.210 They demonstrated that the photoinduced isomerization of guest 75 could result in morphology conversion by self-assembly between linear fibers and spherical nanoparticles upon UVand visible-light irradiation, respectively (Figure 43). Our group reported a photochromic supramolecular polymer based on the host−guest interaction between BSC4 and a methylpyridine-functionalized homoditopic dithienylethene derivative.211 The linear supramolecular polymer was obtained by mixing BSC4 and the dithienylethene derivative in water through the so-called calixarene-induced aggregation. UV/vis absorbance spectroscopy results indicated that both the monomer and supramolecular polymer possessed good photochromic performance and reversibility. Furthermore, the micromorphology of the supramolecular self-assembly could also be controlled under light stimuli owing to the photochromic reaction of the dithienylethene moiety. Cucurbit[n]urils (CB[n]s) are hosts that have been widely used to fabricate different kinds of supramolecular self-

Figure 44. A photocontrolled supramolecular polymer based on the host−guest interaction between CB[8] and azobenzene-functionalized homoditopic bis-viologen derivative 76. Reprinted with permission from ref 212. Copyright 2013 American Chemical Society.

the 2:1 mixture of CB[8] and ditopic guest E,E-76 formed a linear supramolecular polymer on account of the host-stabilized charge-transfer interaction between the electron-sufficient azobenzene moiety and the doubly charged viologen unit. While upon irradiation with UV light, monomer E,E-76 was transformed to the Z,Z isomer, which is geometrically unfavorable to bind the viologen moiety inside the cavity of CB[8]; hence, the noncovalently linked supramolecular polymer was degraded to ternary complex CB[8]2·Z,Z-76. This kind of CB[8]-mediated supramolecular host−guest polymerization and E/Z-photoisomerization-induced complexation/decomplexation approach can be potentially employed in remotely switchable biological or industrial materials. Pillar[n]arenes are relatively new macrocycles which are becoming increasingly intriguing in the field of host−guest chemistry because of their symmetrical and rigid cavity and easy AA


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functionalization.213,214 Ogoshi and co-workers synthesized an azobenzene-linked pillar[5]arene dimer and a pyridineterminated ditopic guest.215 By taking advantage of the host− guest recognition between the pyridine units and pillar[5]arene moieties, they constructed a linear supramolecular polymer. The further reversible E/Z isomerization of the azobenzene moiety endowed this polymer with the ability of photoreversible switching between self-assembly and disassembly. Another example of the combination of pillar[5]arene with a stiff stilbene photoswitchable chromophore to construct photoresponsive supramolecular polymer was realized by Yang et al.216 They have also utilized dynamic covalent bonds, which supplied by reversible anthracene dimerization and combined with pillar[5]arene-imidazole host−guest recognition led to fabricated photo/thermal dual-stimuli responsive supramolecular polymers.217 As seen in Figure 45,

of the anthracene dimer units and hence formation of oligomers. The supramolecular polymers can be re-formed easily by cooling or irradiation again. Supramolecular polymers linked by dynamic bonds introduced in this work may provide us a new strategy to construct functional materials with responses to diverse external stimuli. 4.1.2. Covalent/Noncovalent Bond Transitions in Linear Supramolecular Polymers. Previously mentioned photoresponsive supramolecular polymers have been mainly focused on self-assembly/disassembly processes under light stimuli. On the other hand, covalent polymers with linear, hyperbranched, or dendroid covalent linkage are continuing to be dominant in the field of materials science because of their stability and other versatile properties. Actually, both covalent polymers and noncovalent polymers have their own dominant positions. As a result, establishing a bridge between these two kinds of polymers would make great sense. By taking advantage of the photoreversible dimerization of coumarins and their host−guest interactions with γ-CDs, our group have first demonstrated a simple but effective way to construct a covalent/noncovalent dual-modality switchable supramolecular polymer under external light stimuli.218 As shown in Figure 46a, the bis-branched monomer 77, an alkyl viologen derivative terminated by two coumarins at the ends, can form a noncovalently linked supramolecular polymer (NCP) with γ-CD in aqueous solution. In this supramolecular polymer, the cavity of γ-CD accommodates two coumarins due to its sufficiently large cavity volume. When irradiated by 365 nm UV light, NCP can be transformed to a covalent polymer (CP) on account of the γ-CD-facilitated highly efficient photoinduced cyclodimerization of coumarins. Reversibly, conversion of covalent polymer CP back to noncovalent polymer NCP can be achieved upon 254 nm UV-light irradiation, which led to the photochemical cleavage of the coumarin dimer units. On the contrary, without γ-CD, the photodimerization of monomer 77 can hardly precede under the same experimental condition. Hence, the γ-CD macrocycle

Figure 45. A photo/thermal double dynamic supramolecular polymer based on reversible photodimerization of anthracene moieties and pillar[5]arene−imidazole host−guest recognition. Reprinted with permission from ref 217. Copyright 2013 American Chemical Society.

such a supramolecular polymer has a double dynamic property, which means that heating this polymer for a short time (ca. 1 min) will result in depolymerization by dissociation of the host−guest complex, whereas heating the system for a long time (ca. 1 day) and then cooling can result in the degradation

Figure 46. Preparation of the supramolecular noncovalent linear polymer (NCP) and netlike polymer (NNP) by host−guest interactions between coumarin derivatives 77 (a) and 78 (b) and γ-CD, respectively, and the photoswitching between noncovalent polymers and their corresponding covalent polymers. Reproduced with permission from ref 219. Copyright 2013 The Royal Society of Chemistry. AB


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Figure 47. A photoresponsive supramolecular hydrogel based on host−guest complexation of a functionalized cyclodextrin derivative (79) and an azobenzene-containing copolymer (80). Reprinted with permission from ref 229. Copyright 2009 American Chemical Society.

alization. In the past few years, supramolecular functionalization strategies have acted as facile tools to construct photoresponsive supramolecular polymers that undergo efficient photoinduced mass transport. Clean and reversible photoisomerization of azobenzene derivatives can give rise to macroscopic mass transport of polymeric materials as well as to motions of single chromophores.221−223 For example, Vapaavuori et al. have reported a photoinduced surface-relief grafting in supramolecular polymer−azobenzene complex films.224 Later, some other examples of hydrogen-bonded azobenzene−polymer complexes with such properties as lightinduced softening225 and optical anisotropy226 were reported. Besides hydrogen bonding, halogen bonding, which possesses tunable interaction strength and high directionality, is another important noncovalent interaction that could be employed to construct supramolecular polymers for photoinduced surface patterning with high performance, as reported by Priimagi et al.227 Besides the above-mentioned noncovalent interactions, azobenzene-containing supramolecular polymers in which the host−guest interaction happened in the side chain have also been well-studied.228,229 Zhao and Stoddart have constructed a photoresponsive supramolecular hydrogel by taking advantage of the host−guest complexation between a deoxycholic acidmodified β-CD derivative (79) and an azobenzene-containing poly(acrylic acid) copolymer (80).229 As shown in Figure 47, in the initial state, the trans-azobenzene moieties of copolymer 80 bind strongly into the hydrophobic cavity of β-CD to form the branched polymer, which is accompanied by the formation of a supramolecular hydrogel. This hydrogel could be efficiently converted to a solution upon irradiation with 355 nm UV light, which resulted in the trans−cis isomerization of the azobenzene units. Furthermore, the reversible sol−gel transition could be achieved under irradiation by visible light. Such mild and clean control over gel−sol switching might find its application in light-controlled encapsulation and releasing fields.

played an important role as a microreactor for the photoreaction in this supramolecular system. Moreover, such a dualmodality photoswitchable supramolecular polymer with unique noncovalent/covalent transition can also be employed to construct a photo/thermal dual-stimuli-responsive supramolecular hydrogel with the help of cetyltrimethylammonium bromide (CTAB). Such kinds of modality-switchable supramolecular systems may supply an effective approach to increase further the molecular weight of supramolecular polymers and also potentially to combine the advantages and to overcome the shortcomings of both covalent polymers and noncovalent polymers in a single switchable platform, which is promising for the development of a new class of smart materials. On the basis of this photocontrolled reversible noncovalent/ covalent switching mentioned above, we further fabricated a netlike supramolecular polymer based on the host−guest interactions between three-arm coumarin-containing monomers and γ-CD in aqueous solution, whose linkages among monomers can be switched between covalent bonds and noncovalent bonds by external alternative UV-light irradiation.219 As shown in Figure 46b, we synthesized a watersoluble three-arm monomer (78), which possessed a coumarin unit in each arm. A netlike polymer (NNP) with noncovalent bond linkage was achieved in water. Upon irradiation with 365 nm UV light, which led to the photodimerization of coumarin units, NNP could be further converted into the corresponding covalently linked, reticulated polymer CNP. This process could be reversed by 254 nm UV light, which could lead to the photochemical cleavage of the coumarin dimer. Furthermore, such a photoswitching process can be used to construct a photochemically controllable supramolecular hydrogel without any additional gelators. Soon after, Zhang and co-workers also reported a covalently attached hyperbranched polymer from a supramolecular polymer by employing a similar noncovalent/ covalent transition strategy.220 4.2. Supramolecular Interactions in the Side Chain

4.3. Supramolecular Complexations as Cross-Linkers between Branched Polymer Chains

Photoresponsive supramolecular materials can also be achieved by grafting supramolecular functional groups as side chains to the covalent polymer backbone. It is a strategy that combines traditional polymer science with supramolecular chemistry, which provides us with not only the mature covalent polymerizations but also the versatile supramolecular function-

Supramolecular interactions can not only assemble monomers to polymers but also link covalently branched polymers to hyperbranched 3D networks, which can usually retain solvent molecules to form gels under appropriate conditions. In AC


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principle, there are two types of noncovalently cross-linked branched polymers. One approach to achieve a noncovalently cross-linked network is to introduce bifunctional cross-linkers (donor) to their complementary acceptor-branched polymers, as illustrated in Figure 48a.228 Sada and co-workers have used this method to

Figure 49. Chemical structure of the host−guest coexisting copolymer 81, which can form a supramolecular polymer hydrogel.

Figure 48. Schematic illustration of the formation of a noncovalently cross-linked network via two different approaches.

self-heal within only about 1 min in a natural atmosphere without any additive. Moreover, on the basis of the fact that the inclusion of a β-CD macrocycle with an α-BrNp moiety is able to induce RTP emission, this supramolecular polymer system can be excited to engender RTP signals. By introducing another guest azobenzene polymer (poly-Azo) into the binary system in aqueous solution, the RTP emission could be photostimulated to switch reversibly with an intensity change by alternative UV irradiation.

construct a supramolecular cross-linked polymer gel that is sensitive to thermo- and photostimuli.230 In this supramolecular system, thermally reversible cross-linking was generated by the supramolecular self-assembly of the polymer chain due to the complementary hydrogen bonding, which was thermally sensitive, between the polymer and the cross-linker. Additionally, incorporation of an azobenzene unit into the cross-linker provides the supramolecular network with a photoresponsive property, owing to the cis−trans isomerization. Up to now, supramolecular cross-linking of covalent polymers by functional cross-linkers has been shown to be promising in the design and construction of multistimuli-responsive systems. Recently, other examples have been reported;231 for instance, host− guest interactions between an azobenzene-branched polymer and CD dimers were used to fabricate a supramolecular linked network that could undergo a sol−gel transition232 because of the reversible supramolecular cross-linkage induced by the cis− trans photoisomerization of the azobenzene groups. Another way to construct a supramolecular cross-linked network is using two kinds of polymers with complementary functional branches (Figure 48b).233,234 Tribet et al. have studied the host−guest interaction between the complementary polycyclodextrin and polyazobenzene and found the photocontrollable viscosity transitions.235 Harada and co-workers236 prepared a light-responsive supramolecular polymer hydrogel using one polyacrylamide-based host polymer containing α-CD units in the side chains and another polyacrylamide-based guest polymer with azobenzene moieties in the side chains. These two polymers could associate and dissociate in response to light irradiation. They also prepared the host−guest coexisting polymer 81 to obtain a supramolecular hydrogel (Figure 49). Physical shaking of the macroscopic host and guest hydrogel pieces in water could engender the obvious and visible gel assembly. These supramolecular polymer hydrogels were photoresponsive and assembled in the macroscopic scale. Very recently, we reported a rapidly self-healing supramolecular polymer hydrogel with photostimulated roomtemperature phosphorescence (RTP) responsiveness.237 As shown in Figure 50, the supramolecular polymer hydrogel was constructed on the basis of the host−guest recognition between β-CD host polymer 82 and α-bromonaphthalene (α-BrNp) polymer 83. The hydrogel was effectively prepared via simply mixing aqueous solutions of the polymers together, and it could

4.4. Photoresponsive Supramolecular Micelles, Vesicles and Other Assemblies

Photoresponsive micelles and vesicles have become major topics of research in the fields of physical chemistry, polymer chemistry, and materials science due to their reversible responses to external light stimulus, which make them promising to be applied in controlled drug delivery system.238 For an in-depth overview of traditional photoresponsive micelles, the readers are referred to the review by Gohy and Zhao.239 In this section, we mainly discuss the photoresponsive micelles and vesicles that are constructed by supramolecular amphiphiles.240 Most micelles and vesicles are formed in aqueous solution; hence, water-soluble hosts like cyclodextrin derivatives have the intrinsic advantage in the fabrication of supramolecular amphiphiles with proper guest molecules.241−245 Ravoo’s group have developed many CD-functionalized vesicles and studied their photoresponsive behaviors.246−249 They have reported a photoinduced DNA capture and release system.250 As illustrated in Figure 51, CD containing vesicles were decorated with protonated trans-azobenzene-spermine (trans84) and hence exhibited high-affinity multivalent DNA binding. So when adding DNA into the trans-84-equipped vesicles, a ternary complex was obtained by multiple electrostatic interactions between protonated spermine units and negatively charged DNA. In addition, because the photoisomerization of azobenzene units can lead to the assembly/disassembly processes between compound 84 and CD macrocycles on the surface of vesicles and further switch the binding modes of 84− DNA complex (high-affinity multivalent binding in the transazobenzene state and low-affinity monovalent binding in the cis-azobenzene state), this ternary supramolecular complex can function to capture and release DNA. Later, a similar strategy was also utilized to develop a protein release system, such as the AD


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Figure 50. Rapidly self-healing supramolecular hydrogel constructed via host−guest recognition between host polymer 82 and guest polymer 83.

Figure 51. Photocontrolled capture and release of DNA in a ternary complex. Reproduced with permission from ref 250. Copyright 2011 WileyVCH Verlag GmbH & Co. KGaA.

capture and release of lectins.251 Besides those intervesicular complexes, they recently reported a photocontrolled intravesicular vesicle based on a host−guest interaction, whose permeability could be modulated under light stimuli.252 Yuan and co-workers constructed a pseudocopolymer from two covalent homopolymers through orthogonal self-assembly between a trans-azobenzene-end-capped poly(acrylic acid) and an α-CD-terminated poly(caprolactone).253 This pseudocopolymer can further assemble into a one-dimensional nanotube that possesses a reversible assembly/disassembly feature via the association/dissociation effect of the terminal supramolecular complexations, which is adjusted by the trans−cis photoisomerization of azobenzene moieties. Jin et al. demonstrated a large-scale vesicle aggregation between two kinds of complementary branched polymersome vesicles, by taking advantage of the azobenzene−CD host− guest interaction.254 As shown in Figure 52, two kinds of branched polymersome vesicles (85 and 86) were functionalized in the outer space with β-CDs and azobenzene moieties, respectively. Then concentration- and composition-dependent, large-scale macroscopic aggregates were obtained through multivalent host−guest interactions between the β-CD and

Figure 52. (a) The aggregation of two kinds of branched polymersome vesicles (85 and 86) based on the CD−azobenzene host−guest interaction and (b) the reversible aggregation/deaggregation processes in response to light stimuli. Reproduced with permission from ref 254. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

azobenzene groups at the vesicle surface (Figure 52a). Additionally, because of the photoisomerization of azobenzene moieties and its reversible host−guest interaction with β-CD AE


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amphiphiles. Huang and co-workers have constructed photoresponsive aggregations based on a water-soluble pillar[6]arene and an azobenzene-containing amphiphile.259 Recently, they reported the photoresponsive supramolecular self-assembly based on host−guest complexations between a water-soluble pillar[6]arene host (89) and a UV-degradable guest (90).260 As shown in Figure 54, compound 90 is able to form nanosheets

(Figure 52b), such aggregation/deaggregation processes were totally reversible upon external alternating UV- and vis-light irradiation. Later, they used such host−guest interactions again as supramolecular linkages to connect an azobenzenecontaining, hydrophobic, hyperbranched polymer and a βCD-cored, hydrophilic, hyperbranched polyglycerol to form amphiphilic supramolecular polymers. The resultant supramolecular amphiphile could further assemble into photoswitchable bilayer vesicles with a relatively narrow size distribution.255 Zhang and co-workers fabricated another kind of cyclodextrin-based supramolecular amphiphile. In that work, the initial hydrophobic copolymers were functionalized with the azobenzene units, and then host−guest complexes with βCDs in water are formed through host−guest interactions, which provided the molecule with an amphiphilic property.256 Recently, Liu and co-workers reported some unique photoresponsive organic nanoparticles in water whose fluorescence can be photomodulated owing to controllable aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) effects.257 As shown in Figure 53, the

Figure 54. Chemical structures of the water-soluble pillar[6]arene host 89 and the photoresponsive guest 90 and a schematic representation of photoresponsive aggregations. Reproduced with permission from ref 260. Copyright 2014 Royal Society of Chemistry. Figure 53. Fluorescence photomodulation of supramolecular selfassemblies formed by sulfonatocalixarenes and tetraphenylethenes.

that can finally self-assemble to nanorods after the addition of an equimolar amount of pillar[6]arene host 89 because of the strong host−guest interactions between 89 and 90. When irradiated with UV light, these nanorods transformed back to nanosheets again, perhaps owing to the π−π stacking between the photogenerated pyrene-1-butyric acid. Furthermore, the authors also utilized this supramolecular amphiphile to disperse multiwalled carbon nanotubes under photocontrol. The photoresponsive supramolecular polymers and assembles discussion in this section employ the driving force of macrocycle recognition or host−guest interactions. Most of these systems are working in an aqueous environment, which can enhance the possibilities of their potential applications, due to their better biocompatibility. For the future, novel concepts and strategies for the formation of photoresponsive supramolecular polymers need to be developed, and in particular, finding new and novel host−guest interaction types will absolutely speed up the development of this field. The

nanoparticles obtained from the calixarene-induced aggregation of the tetraphenylethene (TPE) guest 87 exhibit AIE fluorescence (λem = 480 nm) due to the inhibition of the intramolecular rotations of TPE 87. On the other hand, free TPE 87 could be photocyclized to the diphenylphenanthrene derivative 88, which exhibited emission at 385 nm and could be quenched in the nanoparticle state obtained by the addition of sulfonatocalix[4]arenes owing to the ACQ effect. Moreover, these two kinds of nanoparticles can further be switched to each other under light irradiation. In general, such kind of phototunable multiwavelength fluorescent supramolecular aggregation is superior to the single-mode stimuli-responsive system and, hence, might be more adaptable in practical applications, such as multicolor labeling for biological cells.258 Host−guest interactions of pillararenes with specific guests have also been employed to fabricate supramolecular AF


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adjustable mechanical properties of these functional polymeric materials should also be paid great attention, as they can be realized through rational design and utilization of versatile building blocks when designing these systems.185 Moreover, the related dynamic process during the supramolecular polymerization needs to be extensively investigated. Photoresponsive supramolecular polymers with fast response, biocompatibility, and functional diversification, such as self-healing materials, belong to an extremely charming trend for the future.193

5. PHOTORESPONSIVE HOST−GUEST SYSTEM ON SURFACES The host−guest systems discussed above that absorb light energy to generate specific functions all operate in solution, in which a large number of molecules behave independently from one another. It should be emphasized that studying these systems in solutions is of fundamental significance because it can provide more intrinsic information and help us to understand their working mechanisms. To realize more useful functions and applications, these photoresponsive host−guest systems are expected to operate in highly organized environments. Immobilizing these systems on surfaces is a superb method and is a crucial step to organize these molecules in a highly ordered way and then to achieve useful functions and successful operation, through enabling their interactions with the surrounding environment. The past decade has witnessed a surge of developments in this field,23,24,261−263 in which efficient strategies include deposition on curved surfaces, such as nanoparticles, organization at planar macroscopic surfaces, or immobilization into porous materials. In this section we will mainly focus on recent accomplishments in photoresponsive host−guest systems that are chemisorbed on surfaces. Chemisorbed monolayers or multilayers of host−guest systems on surfaces can be realized in such a way that host−guest assemblies are covalently bonded to the solid surfaces through a functional headgroup that is introduced into a host or a guest molecule.

Figure 55. Supramolecular host−guest complexes on AuNPs. The complexes were obtained by complexation between an α-CD host and a photoactive unit (91 or 92).

external light inputs. These features make the hybrid system a potential candidate for light-mediated experiments in biomedical research. Zhao et al.266 demonstrated that photoswitchable supramolecular catalysis of ester hydrolysis had been achieved using competitive host−guest recognition of a ZnII-coordinated β-CD dimer catalyst with azobenzene derivative 93 on the surfaces of AuNPs in the presence of reaction substrates. As shown in Figure 56, in the original state, β-CD rings are occupied by the trans-azobenzene units, and the β-CD dimer catalyst cannot recognize the substrate; therefore, its catalytic activity is passivated. UV irradiation resulted in the trans-to-cis isomerization of 93 grafted on AuNP surfaces, and as a result, the substrate can bind with the β-CD rings; thus, the catalyst becomes available for catalyzing the hydrolysis of the substrates. The catalytic process can be switched on and off repeatedly by alternating between UV- and visible-light irradiation. This might pave the way for the design of smart, switchable catalysts. The above-mentioned examples represent a strategy of immobilization where guest or thread-shaped molecules are chemically attached to nanoparticles. Alternatively, immobilization of a host−guest system on nanoparticles can also be achieved by covalent attachment of host molecules to nanoparticles, followed by complexation with guest or thread molecules. Luo et al.267 prepared α-CD-capped AuNPs, which can bind azobenzene-terminated poly(N-isopropylacrylamide) (PNIPAm) via host−guest molecular recognition. The attachment/detachment of azobenzene-terminated PNIPAm on αCD-capped AuNPs can be reversibly controlled due to the photoinduced conformation change of azobenzene. When PNIPAm-coated AuNPs in aqueous solution were irradiated with UV light, azobenzene-terminated PNIPAm was detached from the surface of AuNPs. After visible light (>420 nm) irradiation, trans-azobenzene-terminated PNIPAm in the supernatant solution can attach onto the surface of α-CD-capped AuNPs again. Tan and co-workers268 also used a similar strategy to attach a host−guest system on AuNPs and demonstrated the reversible phase transfer of AuNPs by the photoswitchable host−guest interaction between α-CD and azobenzene derivative 94 (Figure 57). This system was based on the reversible surface

5.1. Photoresponsive Host−Guest Systems Immobilized on Nanoparticles

The introduction of a photoresponsive host−guest system into various nanoparticles (NPs),23,24 such as Au, SiO2, or iron oxide nanoparticles, has been used to modulate reversibly a variety of nanoparticle properties and has attracted considerable interest. Sortino and his co-workers264 demonstrated a kind of highly photoresponsive gold nanoparticle that was achieved through a supramolecular host−guest complex between an αCD host and an azobenzene-terminated alkyl thiol (91) (Figure 55). The cavity of the host CD macrocycle was designed to promote the solubility of NPs in polar solvents and increase the interchain distance of the adsorbed photoactive azobenzene species, which was crucial for a high photoresponse to be obtained. As anticipated, the switching rate of the azobenzene unit was almost the same as that of the free azobenzene molecules observed in solution, which was confirmed by UV/ vis spectroscopy. The same group265 also moved their research on to the combination of host−guest systems and AuNPs by replacing the azobenzene unit with a photoactive nitric oxide donor (92), namely, a trifluoromethylnitroaniline derivative, to demonstrate the first example of AuNPs exhibiting photoregulated release of NO. The produced NO can be detected directly with an ultrasensitive NO electrode, and it has been found that the release process is strictly dependent on the AG


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Figure 56. Working principle of a photoswitchable supramolecular catalysis based on Au-surface-grafted host−guest complex between a β-CD dimer and an azobenzene derivative (93). Reproduced with permission from ref 266. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

by UV/vis absorption spectra. By alternating UV- and visiblelight irradiation, the same batch of α-CD-capped hydrophilic AuNPs can be recycled to catalyze multiple rounds of 4nitrophenol reduction. Furthermore, they managed to quench the catalytic reaction through removal of the catalytic AuNPs by using a phase-transfer strategy based on the host−guest interaction. The azobenzene−CD host−guest complex can also be immobilized on silica nanoparticles. Ritter and co-workers269 reported a multivalent system based on two different kinds of SiO2 NPs. One batch of silica NPs was functionalized with CD macrocycle, and the other one was functionalized with an azobenzene moiety. They also studied the photochemically controlled self-assembly of azobenzene- and CD-modified silica nanoparticles. Mixing the two kinds of NPs resulted in aggregation due to a multivalent host−guest interaction. Comparison of DLS data before irradiation at 365 nm (108 nm) and after irradiation at 365 nm (33 nm) confirmed UVtriggered agglomeration and disaggregation of the silica nanoparticles. Ravoo and co-workers270 also demonstrated that the aggregation of CD-capped silica NPs was fully reversible within the limits of the photostationary state. The hybrid supramolecular system with photoresponsiveness employs the host−guest interaction of β-CD-covered silica NPs and a ditopic azobenzene linker 95 in aqueous solution. As shown in Figure 58, these NPs show reversible aggregation and dispersion in response to light stimuli in dilute aqueous solution. The interaction of the CD-covered silica NPs and the azobenzene guest was investigated by versatile measurements, such as optical density measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM), and it should be noted that this supramolecular system is still stable after at least four cycles. The host−guest system can also be incorporated into hybrid materials to take action under photochemical stimuli. As shown in Figure 59, Ravoo and co-workers271 prepared a soft hybrid material composed of oleic acid-coated magnetic iron oxide

Figure 57. (a) Chemical structures of host molecule per-6-thio-α-CD and guest azobenzene ligand 94. (b) Photoreversible association and dissociation between azobenzene 94 and CD-coated AuNPs. (c) Photoinduced phase transfer of CD-capped AuNPs by 94 between the water and toluene phases. Reprinted with permission from ref 268. Copyright 2014 American Chemical Society.

modification of the α-CD-capped hydrophilic AuNPs with an azobenzene-containing surfactant ligand. Reversible phase transfer can be performed for multiple cycles, as evidenced AH


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Figure 58. Light-induced aggregation and dispersion of supramolecular host−guest system formed by the complexation of CD-covered silica NPs and the azobenzene guest 95. Reproduced with permission from ref 270. Copyright 2014 Royal Society of Chemistry.

application in drug delivery systems, contrast agent in MRI, and also hypothermia therapy. Scherman and co-workers272 fabricated a kind of photoresponsive hybrid raspberrylike colloid (HRC) through CB[8]based host−guest interactions. The system consists of the photoswitchable ternary host−guest complex between a CB[8] macrocycle, an azobenzene derivative, and a methyl viologen moiety (MV), as shown in Figure 60. First, 4-hydroxyazobenzene was immobilized onto the surface of silica microspheres and the formed azo−silica core was chosen as the core of the HRCs (Figure 60b). Next, MV-functionalized polymeric NPs (MV-NP corona) were selected as the corona NPs. Then the hybrid raspberrylike colloids were then prepared simply by mixing an aqueous dispersion of the Azo−silica core and an aqueous dispersion of the MV-NP corona precomplexed with CB[8]. Upon UV irradiation at 350 nm, the ternary complex was dissociated into MV@CB[8] binary complexes and free cisazobenzene, which results in the disassembly of the HRCs. The reverse process could be easily achieved by visible-light irradiation at 420 nm. The system may find potential application in a number of fields, including compartmentalized catalysis and drug delivery.

Figure 59. (a) Schematic representation of cyclodextrin-based hybrid vesicles MNP-CDV. (b) Chemical structure of the photoresponsive azobenzene cross-linker 96. Reproduced with permission from ref 271. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

5.2. Photoresponsive Host−Guest Systems Organized at Planar Surfaces

A Au electrode is the most commonly used planar surface on which photoresponsive host−guest systems are immobilized. Willner and co-workers273 reported the first example of lightinduced molecular motions of a rotaxane grafted on a gold electrode. As shown in Figure 61, rotaxane 97, which is immobilized onto the surface via a stepwise chemical reaction sequence, is composed of a ferrocene-modified β-CD macrocycle threaded onto an azobenzene-containing thread molecule. When the azobenzene moiety is the trans isomer, the β-CD macrocycle containing the redox-active ferrocene label is encircling the trans-azobenzene unit and the ferrocene unit is in close proximity to the Au electrode. As a result, a fast current response is observed with an electron transfer constant of K =

NPs absorbed in the bilayer of cyclodextrin-based vesicles (MNP-CDV), which can self-assemble in microscale linear aggregates in the presence of a magnetic field. A photoresponsive cross-linker (96) with two terminal azobenzene moieties can stabilize the formed linear aggregates through a host−guest interaction between azobenzene and CD. The system can be photoswitched between an adhesive and a nonadhesive configuration because of the photoisomerization of the azobenzene unit and the dissociation of the host−guest complex. This kind of magnetic hybrid may find potential AI


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Figure 60. (a) Stepwise formation of a ternary complex (MV/Azo)@CB[8] and its reversible disassembly and reassembly upon irradiation. (b) Formation of hybrid raspberrylike colloids and its light-induced disassembly. Reproduced with permission from ref 272. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.

using light stimuli. As shown in Figure 62, a mecapto group was situated at the end of the azobenzene derivative, and the two units are separated with a long alkyl chain. First, the azobenzene-containing molecule was preassembled with αCD to form a host−guest complex (98) in water. Next, the generated amphiphile 98 was self-assembled onto a gold substrate to form a mixed SAMs with n-butylthiol. The fabrication of the mixed SAMs is necessary because this can provide enough space for the photoinduced mechanical movement of α-CD. In the original state, α-CD encircled the azobenzene stations on the top of the surface, and the SAM has hydrophilic properties, as evidenced by its contact angle of 70°. UV-light irradiation makes the SAMs become hydrophobic, with a contact angle of 120°, due to the displacement of α-CD. The change in the wettability of the Au surface and the contact angle could be repeated for several cycles by alternating UVand visible-light irradiation, which benefits from the good reversibility of the host−guest interaction between CD and azobenzene. The system may pave the way to design a lightresponsive biocompatible biodevice for controllable cell adsorption. Another impressive work was reported by Leigh and coworker.275 In this work, they tranformed the submolecular light-driven shuttling motions of rotaxane into macroscopic mechanical transportation. The bistable [2]rotaxane 99, which consists of two stationsone tetrafluorosuccinamide station and one maleamideand a cationic amide macrocycle, was physisorbed as a monolayer on SAMs of 11-mercaptoundecanoic acid on Au(111) deposited on SiO2, with the long rotaxane axle lying parallel to the gold surface, as illustrated in Figure 63. The macrocycle of the rotaxane can be switched between the maleamide station (exposing the fluoroalkane segment) and the tetrafluorosuccinamide station (shielding the fluoroalkane segment) in response to the photoisomerization. The positional switching changes the surface wettability of the SAMs, giving obviously lower contact angles on the surfaces after the UV-induced E → Z isomerization. The photoresponsive surfaces show the amazing ability to do macroscopic

Figure 61. A surface-bound photoswitchable rotaxane 97 and its reversible switching processes on a Au electrode.

65 s−1, which corresponds to the oxidation of the ferrocene label. Upon irradiation with UV light, the β-CD macrocycle is displaced to the long alkyl chain part and the ferrocene unit is far from the Au electrode. At this state, the electron transfer constant decreased to 15 s−1. It should be noted that these switching process are fully reversible due to the good reversibility of azobenzene photoisomerization. This optoelectronic device takes advantage of light energy to modulate the distance between the redox-active ferrocene unit and the Au electrode, thus transducing optical input into an electrochemical signal. Zhang and his co-workers274 also grafted an azobenzene−CD supramolecular complex onto a rough Au substrate to form SAMs, which can be used to tune the surface wettability by AJ


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Figure 62. Switching process of a photoresponsive self-assembled monolayer decorated with CD-based host−guest complex 98.

Figure 63. Self-assembly of a photoswitchable rotaxane 99 with the fluoroalkane station exposed onto Au(111) via hydrogen-bonding interactions.

mechanical work, transporting a microdroplet of CH2I2 on a millimeter scale along a flat surface or even a 12° incline. Jia et al.276 assembled [2]rotaxane 100 onto a graphene surface to construct a molecular transistor, as shown in Figure 64. On the top of a Si/SiO2 substrate, a single-layer graphene sheet was placed and used as the conductive channel. Then a monolayer of 1-pyrenebutanoic acid was absorbed on the graphene surface by way of π−π stacking interactions. The [2]rotaxane 100 with a benzyl alcohol tail was linked covalently to the 1-pyrenebutanoic acid by means of esterification, and the formed rotaxane/pyrene ester monolayer acted as the light sensitizer. The ID of the hybrid transistor decreased about 12% when both light and sacrificial triethanolamine (TEOA) were simultaneously applied upon the device, while the current was restored to its original value after subsequent exposure of the devices to oxygen in the dark, and such a switching circle is well-reversible. It is interesting that the photoresponsive behavior of the rotaxane−graphene devices can be endowed with binary logic, such as a sequential logic circuit with a memory effect, an AND logic, a XOR logic, and an INH logic,

Figure 64. Chemical structure of rotaxane 100 and its photoinducedswitching process after its immobilization on a graphene surface. Reproduced with permission from ref 276. Copyright 2013 WileyVCH Verlag GmbH & Co. KGaA.

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(GOx), can control the oxidation of glucose with the help of GOx in the presence of a ferrocene-labeled redox-active polymer as mediator. Upon UV irradiation, the ferrocenelabeled redox polymers were released from the SAM surfaces, and as a result, glucose could not be oxidized to gluconic acid because of the lack of ferrocene molecules (Figure 65b). Additionally, the activation and deactivation of the host−guest biointerfaces are reversible and could be repeated for several cycles in response to light stimuli. The same group also demonstrated an orthogonal stimuli-responsive biointerface278 that is based on the host−guest interaction between β-CD grafted on pH-responsive polymers and azobenzene unit on photoresponsive SAMs. The integration of both light- and pHcontrolled biointerfaces has been used for reversible adsorption and release of electroactive cytochrome c, which can mimic the biocatalysis and energy transfer in biological processes. The advantage of an orthogonal responsive biointerface relies on its easier adaption to biological environments and its versatility in a range of potential applications. Scherman and coworkers achieved the fabrication of orthogonal stimuliresponsive biointerfaces based on CB[8]-mediated ternary host−guest complexes with azobenzene and viologen derivatives.279 In the design shown in Figure 66, an azobenzene derivative is covalently linked with gold surfaces, and the viologen derivative is functionalized with a green fluorescent dye molecule. Mixing the viologen derivative and CB[8] with the photoresponsive azobenzene-containing Au surface can result in the ternary complexation of the surface, as evidenced by the green fluorescent array that emerged. Upon UV irradiation, no green fluorescent array was observed (Figure 66b), indicating the dissociation of the heteroternary complexation. Meanwhile, the water contact angle of these surfaces increased, indicating a hydrophobic surface (Figure 66c). Moreover, the fluorescent array can also be rewritten onto the surfaces accompanied by the increase of the surface hydrophilicity in response to visible-light irradiation. This work, with increased complexity of the surfaces, demonstrated a straightforward host−guest process to achieve orthogonal stimuli responsiveness, which can find potential applications in the fabrication of complicated molecular devices.

and thus, addition and subtraction functions can also be mimicked. The immobilization of photoresponsive supramolecular host−guest assemblies onto surfaces, especially planar surfaces, for the construction of multifunctional biosurfaces and biointerfaces has shown many potential applications, such as bioelectrocatalysis, drug delivery, or reversible adsorption and resistance of small biomolecules and cells.263 Zhang and coworkers277 constructed a photoswitchable bioelectrocatalyzed surface using ferrocene-labeled redox-active polymers. As shown in Figure 65a, the ferrocene-labeled redox-active

Figure 65. (a, b) Reversible photoswitching activation and deactivation of the biointerface that can oxide glucose into gluconic acid. Reprinted with permission from ref 277. Copyright 2011 Royal Society of Chemistry.

polymers functionalized with α-CD (PAA-α-CD-Fc) were immobilized on gold surfaces through host−guest interactions between α-CD and azobenzene. The generated interfaces, when immersed into a solution of glucose and glucose oxidase

Figure 66. (a)Orthogonal dual-stimuli-responsive biointerfaces based on CB[8]-mediated ternary host−guest complex and their (b) fluorescence microscopy images and (c) water contact angle changes in different states. Reproduced with permission from ref 279. Copyright 2012 Nature Publishing Group. AL


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5.3. Photoresponsive Host−Guest Systems Immobilized on Porous Materials

molecules were entrapped inside the cavities. Upon UV irradiation, the trans-azobenzene units were converted to the cis form, and as a result, the pseudorotaxane systems dissociated, and the fluorescent dyes were released from the pores of the nanoparticles in relatively quick speed. Thus, sequential loading, blocking, and releasing procedures in a photocontrollable nanovalve system were successfully operated. Feng and co-workers286 designed a supramolecular nanogated ensemble that can respond to multiple external stimuli, such as light or addition of DTT or α-CD. As shown in Figure 68, the system employs MSNs as nanocontainers that are grafted with a kind of β-CD-containing polymer (poly-CDMS). The working mechanism of this nanovalve can be depicted as following: Calcein molecule was loaded by immersing modified MSNs in a phosphate-buffered saline solution of calcein. Then a certain amount of a diazo linker (102), which is a ditopic guest molecule bearing an azobenzene photoresponsive unit at each end, was added to switch off the nanovalves because of the formation of cross-linked polymeric networks based on the host−guest interaction between the trans-azobenzene and β-CD. Subsequent UV irradiation of the system resulted in the formation of cis-azobenzene, and the bound, cross-linked complex dissociated; as a result, calcein molecule was released from the pore of the MSNs using a photochemical strategy. It should be noted the release of the calcein molecule can also be realized by the addition of α-CD to form the complex with the diazo linker or the addition of a disulfide reducing agent, dithiothreitol, to cleave the sulfur bond and remove the β-CD from the polymer, and both of them have a faster releasing speed than the photochemical method. It is well-established that CB[7] possesses a suitable cavity and can form stable host−guest complexes with transcinnamamide derivatives that, upon irradiation of the complex solution of CB[7]-cinnamamide derivative by 300 nm UV light, can result in a trans-to-cis conformational change of cinnamamide, thus leading to dissociation of the host−guest complex due to the steric hindrance effect.287 Using this switchable host−guest interaction, Yang and co-workers288 reported a photoresponsive supramolecular nanovalve based on a CB[7] pseudorotaxane. In the system, the surfaces of MSNs were functionalized with cinnamamide-bearing stalks, which were surrounded by CB[7] rings to form pseudorotaxanes, acting as a photorepsonsive gated ensembles. They used light and pulsed light to realize remotely the controlled release of a cargo molecule, which relies on the disassembly of the host− guest complexes generated by 300 nm UV-light irradiation to convert trans-cinnamamide to the cis isomer. Recently, Yang and co-workers expanded their research on this topic and developed a near-infrared-light-responsive nanovalve based on mesoporous-silica-coated gold nanorods that can provide a high cargo molecule load.289 Importantly, the NIR light stimuli can produce plasmonic heating from the gold nanorod cores, which can decrease the binding affinity of the rings and the quaternary ammonium salt stalks and result in the dissociation of pseudorotaxane systems; thus, the nanovalves were opened and the cargo molecules were released from the MSN nanocontainers. It should be mentioned that the NIR light stimuli provide more effective and safer means to realize noninvasive controlled drug release. A more complicated MSN-based nanovalve has also been designed to respond to two different external stimuli at the same time. In such a system, MSNs are functionalized with

Photoresponsive host−guest systems were also immobilized on porous materials, in particular, on the surfaces of mesoporous silica nanoparticles (MSNs), to realize the capture and release of guest molecules,280−282 which represents a significant step for these systems toward future practical applications, especially as photoresponsive nanovalves for controlled drug release. In the design of such photoreponsive nanovalves, MSNs are employed to function as ideal nanocontainers due to their many advantages, such as optional pore sizes, low cytotoxicity, and easy modification. In most cases, MSNs are modified with bulky components, such as light-responsive, mechanically interlocked molecules.283,284 The concept of such kind of nanovalve depends on the ability to load cargo molecules in the pores of the MSNs and subsequently release the cargo molecules in response to a specific stimulus in a controlled manner. The redox- and pH-controllable molecular valves, in which electrochemically or chemically driven bistable rotaxane or pseudorotaxane systems are covalently attached to the MSNs, have been extensively studied in the past few years.280−284 Using light to trigger the nanovalve is a promising method because light can lead to a fast response and can function in a small space without producing any byproducts. Ferris et al.285 have developed a light-operated, mechanized MSN system that can function as a photocontrollable nanovalve that can capture dye molecules and then release them in response to light stimulus. The concept is based on the host−guest interaction between β-CD and trans-azobenzene, which possesses a relatively high binding affinity that is remakably reduced when the trans isomer was photochemically converted into cis isomer. The operation of this nanovalve is shown in Figure 67, MSNs were modified with an azobenzene compounent 101, and then a fluorescent molecule such as Rhodamine B was loaded into the cavity of the MSNs. Addition of β-CD resulted in the complexation of the trans-azobenzenes with β-CDs, and pseudorotaxanes were formed on the surface of MSNs, which can block the surface of the MSNs so that Rhodamine B

Figure 67. Schematic representation of a photocontrollable nanovalve. AM


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Figure 68. Schematic representation of a multiresponsive nanogated ensemble based on MSPs covered with a supramolecular polymeric network. Reprinted with permission from ref 286. Copyright 2009 American Chemical Society.

and surface-grafted host−guest molecular systems. In all the mentioned complex systems, light energy as a kind of clean and effective stimulus has shown the fascinating ability to control the manner of the host−guest interaction, thus realize some specific functions, such as photocontrolled release of guest molecules and photomodulation of mechanical motions. We hope that this review has pointed out the very substantial and important advances made in the field of photoresponsive host− guest systems and that the strides made so far will stimulate researchers to develop new host−guest systems and find their new applications. Extensive use of well-established host−guest interaction motifs and understanding the photoswitching processes have brought out many important progresses; however, it should be mentioned that the development of new host−guest interactions, especially the preparation of new host molecules, may play fundamental roles in the future research. On the other hand, incorporation of new types of photoswitches into host−guest system will also contribute to a large extent. Hereby, we foresee a challenging but bright future where photoreactions with higher conversion efficiency will be discovered and employed in host−guest interactions. On the other hand, photoinput accompanied by other external stimuli may one day synergistically control the function of the complicated multicomponent supramolecular systems. Finally, of course, the most exciting prospect is that light can find its way to control the supramolecular complex in a fast and easy way, so that real practical applications of host−guest systems will emerge.14,291

both azobenzene and a pseudorotaxane structure formed by bisalkylammonium threaded into a CB[6] ring. The azobenzene derivative can function as impellers inside the pores, and the pseudorotaxane acts as nanovalve components on the outer surface of the MSNs. This system can give a dual-controlled release, thus mimicking an AND logic function under the combination inputs of light and pH.290 In this section, we have summarized how to attach the different types of host−guest systems to various surfaces and, especially, how these systems can respond to external light stimuli, generate the configuration changes of chemical systems, and, as a result, change the properties of the surfaces or perform a specific function, such as controllable release of dye molecules. It should be noted that changing the working environment of these photoresponsive host−guest systems from solutions to surfaces represents a crucial and important step;23,24,263 however, there is still a long way to go toward practical application. More investigation can be focused on the development of new photoswitchable host−guest systems and biocompatible surfaces and interfaces.263

6. SUMMARY Roughly 50 years after the first host molecule, crown ether, was synthesized, it can be concluded that host−guest chemistry is an established but vibrant science. With a detailed understanding of host−guest recognition motifs, scientists have found different binding motifs and interaction mechanisms and developed many important concepts in the field of supramolecular chemistry. Tremendous progress has taken place, from understanding the principle of simple host−guest recognition motifs to knowing various molecular self-assembly mechanism and from working with simple host−guest inclusions to fabricating complexed host−guest systems, such as complicated supramolecular ploymers, highly ordered supramolecular assembles, and mechanically interlocked molecules. Moreover, the combination host−guest chemistry and surface science has stimulated remarkable progress toward understanding the working mechanism of how host−guest systems interact with their surroundings. In this review, we have exposited the advancements in host−guest systems on the basis of different architectures, such as molecular cages and capsules, mechanically interlocked molecules, supramolecular polymers,

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. AN


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Biographies

Qi-Wei Zhang was born in Zhejiang, China, in 1986. He obtained his B.S. in applied chemistry from ECUST in 2009 and received his Ph.D. in applied chemistry in 2014 from ECUST under the supervision of Prof. He Tian. He then joined Prof. Roeland Nolte’s group at Radboud University Nijmegen (Nijmegen, Netherlands) as a postdoctoral fellow. His current research focuses on controllable and functional supramolecular self-assemblies and polymers, molecularlevel devices and machines, and process catalysis through a supramolecular approach.

Da-Hui Qu was born in China in 1980. He obtained his B.S. from Qingdao University (Qingdao, P. R. China) in 2000 and received his Ph.D. in applied chemistry in 2006 from East China University of Science and Technology (ECUST) (Shanghai, P. R. China), under the supervision of Prof. He Tian. From July 2006 to January 2009, he worked with Prof. Ben L. Feringa as a postdoctoral fellow at the University of Groningen (Groningen, Netherlands). Since 2009, he has worked at ECUST, and he became a full professor in 2014. His current research focuses on controllable supramolecular systems and molecular-level devices and machines with adjustable optical properties.

Xiang Ma was born in Xinjiang, China, in 1980. He obtained his B.S. at Tianjin University (Tianjin, P. R. China) in 2003 and his Ph.D. in applied chemistry in 2008 from ECUST under the supervision of Prof. He Tian. From January 2011 to July 2012, he worked as a Research Associate at the Liquid Crystal Institute, Kent State University (Kent, OH). He is now an Associate Professor at ECUST, and his current research interests focus on controllable functional supramolecular machines, switches, and polymeric materials.

Qiao-Chun Wang received his B.S. in fine chemicals (1998) and Ph.D. in applied chemistry (2004), both from ECUST. He works at the Institute of Fine Chemicals, ECUST, where he is a Professor of Chemistry. His research interests include the synthesis of switchable rotaxanes and their applications. He Tian obtained his Ph.D. from ECUST, in 1989. Since 1999, he has been appointed Cheung Kong Distinguished Professor by the Education Ministry of China. His current research interests include the syntheses of novel functional organic dyes and polymers, as well as the development of interdisciplinary materials science that determines the electronic and optical properties of materials. Prof. Tian has already published over 330 papers in international journals and applied for 80 Chinese patents. Now Prof. Tian is Coeditor of Dyes & Pigments and an International Advisory Board Member for Chemical Science, Polymer Chemistry, ChemistryAn Asian Journal, and Advanced Optical Materials. He was selected as a member of the Chinese Academy of Science (2011) and a Fellow of the World Academy of Sciences (2013). AO


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ACKNOWLEDGMENTS We thank the NSFC/China (21272073, 21421004, 21372076, 21190033), National Basic Research 973 Program (2013CB733700, 2011CB808400), the Fok Ying Tong Education Foundation (121069), the Fundamental Research Funds for the Central Universities, and the Innovation Program of Shanghai Municipal Education Commission for financial support.

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