Bright functional rotaxanes

Abstract

As a hot topic in research, various rotaxanes continue to be constructed. This tutorial review focuses mainly on bright rotaxanes with functional properties, which have been developed in recent years: fluorescent rotaxanes, rotaxanes as logic gates and information storage devices, gelation of rotaxanes, rotaxanes on solid surfaces, sensory rotaxanes, as well as rotaxanes related to biology and so on. Novel synthetic protocols towards several functional rotaxanes are also illustrated.

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Xiang Ma

Xiang Ma was born in 1980 and completed his BSc studies in Fine Chemicals (2003) at Tianjin University, China. In 2008, he received his PhD degree in Applied Chemistry from the East China University of Science and Technology (ECUST, Shanghai) under the supervision of Prof. He Tian. Now he works at the Institute of Fine Chemicals of ECUST. His research interests mainly focus on molecular-level devices and machines.

He Tian

Prof. He Tian received his PhD degree in 1989 from the East China University of Science & Technology (ECUST, Shanghai). In 1999, he was appointed Cheung Kong Distinguished Professor by the Education Ministry of China. His current research interests include the syntheses of novel functional organic dyes or copolymers and the development of interdisciplinary materials science that determines the electronic and optical properties of materials. Prof. Tian has published over 240 papers in international journals and five academic books (in Chinese). He has also been awarded 46 Chinese patents. Since 2006, he has been an Associate Editor of Dyes and Pigments.

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1. Introduction

Molecular devices and machines are assemblies of components designed to achieve a specific function. A rotaxane, as one typical species of molecular machine, consists of one or more macrocycles encircling the rod portion of a dumbbell-like component, where the dissociation of a ring from an axis is hindered by two bulky terminals (stoppers). A rotaxane containing n components is usually called an [n]rotaxane. Their counterparts that have the same structural characteristics except for an absence of bulky terminals are called pseudorotaxanes. However, some pseudorotaxanes can have some rotaxane-like character. Dethreading of the ring in such systems would be possible but requires an even higher activation barrier to be overcome so that, under appropriate conditions (e.g., mild temperature conditions), they behave as rotaxanes.

Employment of ‘template-directed’ synthetic methodology makes it facile to construct novel and imaginative kinds of rotaxanes, some of which are luminescent or provided with potential properties aimed at functional applications. We can call rotaxanes of these kinds ‘bright’.

Photo-excitation of the luminescent units included either in the dumbbell or in the macrocycle of a rotaxane engenders luminescent signals which can be utilized for conformational identification. Light input is such a ‘clean’ input mode that no waste product forms during the operation to power a rotaxane. Besides this, luminescent output (mainly fluorescent output; rotaxane examples using phosphorescent signals as the output are very rare) can respond very quickly, can be of low-cost for the detection of photons and for the non-contact readout (giving readouts without the need for direct contact with the rotaxane), and can be highly sensitive and easily carried out in a small space.

Molecular machines with potential applications are just what supramolecular scientists and engineers seek. More such rotaxanes have been constructed to realize specific functions via the relative motion of the different components in the system, or for various devices. Functional rotaxanes will continue to be important research objects.

The past several years have seen the publication of a book,¹ some reviews and special issues summarizing the topic of rotaxanes, not only on the overall concepts of molecular machines,^2,3 but also from particular perspectives including photo-driven molecular devices,⁴ porphyrin-containing catenanes and rotaxanes,⁵ anion and active metal templated assembly of mechanically interlocked structures,^6,7 switchable rotaxanes,⁸ light powered molecular machines,⁹ and so on. In this tutorial review, recent work on luminescent rotaxanes and rotaxanes with functional properties will be summarized.

2. Fluorescent rotaxanes

For switchable rotaxanes containing fluorophores, the switch of a rotaxane under external stimuli alters its internal conformation and the electron transfer process or the excited state and the luminescent properties of the fluorophores change subsequently. Since fluorescent signals provide many advantages mentioned above, many light-driven molecular machines employing fluorescent outputs have been built up in recent years. Either the enhancement¹⁰ or the quenching¹¹ of fluorescent signals could be utilized to indicate the shuttling conformations of rotaxanes. For example, a typical [2]rotaxane with dual fluorescence addresses¹²R1 contains an α-cyclodextrin (α-CD) macrocycle, an azobenzene unit, and two different fluorescent naphthalimide fluorophores, as shown in Fig. 1. The cis–trans photo-isomerization of the azobenzene unit resulted in the motion of the α-CD macrocycle on the thread. Because of the rigidity of the α-CD ring shifting close to the fluorophore, which hinders the vibration and rotation of the bonds in the methylene, the fluorescence intensities of the fluorophores nearer the CD rings increase, respectively. Because of the easy regulation and full reversibility of the fluorescence change of the two stopper units, this [2]rotaxane could be used as a molecular storage medium or switch with all-optical inputs and outputs.¹²

Fig. 1 Light-driven [2]rotaxane R1 molecular shuttle with dual fluorescence (F₅₂₀ and F₃₉₅) addresses.

Li et al.¹³ set up a molecular shuttle ([2]rotaxane R2 shown in Fig. 2) for driving a multi-level fluorescence switch. R2 comprises a macrocycle mechanically interlocked with a chemical ‘dumbbell’ having two different recognition sites, –NH₂⁺– and an amide, and a fluorophore anthracene moiety. The macrocycle exhibits selectivity for the NH₂⁺ and amide –CONH– groups under different acid–base conditions, and different conformations upon addition of Li⁺ or Zn.²⁺ These diverse processes are all accompanied by different fluorescent responses. As shown in Fig. 2, R2 enables multi-level expression of fluorescence in response to different triggers: (1) complete repression in the absence of any stimulus; (2) slight expression in response to base; (3) low-level expression following addition of Li⁺; (4) high-level expression in the presence of Zn²⁺.

Fig. 2 Movement processes of the multistable fluorescence [2]rotaxane shuttle R2.

When α-CD based rotaxane R3, shown in Fig. 3, was doped into a thermoreversible hydrosol–gel system with high flexibility and workability,¹⁴ clearly improved optical performance (fluorescence and induced circular dichroism) was observed. The fluorescence intensities of the rotaxane-doped hydrogel systems were enhanced, which should be attributed to the rigidity effect of the hydrogel system on this fluorescent rotaxane.

Fig. 3 α-CD based [2]rotaxane R3 doped in a hydrogel system.

3. Functional rotaxanes

3.1 Rotaxanes for logic gates and information storage

The encoding of information in electrical and optical signals in the form of binary digits is required for data processing and communication.¹⁵ Some switchable rotaxanes, with the expression of ‘0’ and ‘1’ states based on different conformations, have been constructed to perform Boolean logic operations on one or more inputs to produce an output, although fabricated logic gates based on rotaxane systems are few.^16,17

Qu and Tian et al. set up a lockable double INHIBIT logic gate based on a [2]rotaxane R3 that employs both configuration and fluorescence changes, as shown in Fig. 4.¹⁸R3 consists of an α-cyclodextrin (α-CD) macrocycle threaded onto a dumbbell with two different photo-switchable binding stations, an azobenzene station and a stilbene station. The interconversions among several different conformational states can be described using binary logic gates. The fluorescence response (intensity variance ΔF₅₂₀ at 520 nm) of R3²⁻ to the different combinations of UV irradiation (380 (I1) and 313 nm (I2)) and protons (I3) corresponds to a double INHIBIT logic gate. An INHIBIT logic gate is a two-input AND gate with one input carrying a NOT gate. In this double INHIBIT logic gate, one of the two inputs carrying NOT gates is 380 nm (I1) and the other is protons (I3), and the output (ΔF₅₂₀) is designated as ‘0’ or ‘1’. These outputs stand for five different supramolecular configurations, i.e., five different ‘0’ states described in one single molecular platform. Moreover, [2]rotaxane R3 can be described as a lockable INHIBIT gate, in which the molecule exists in a locked state under acidic conditions. It can also act as a normal INHIBIT gate when base is added to unlock the molecule. Because of the full reversibility of the photo-isomerization processes, the photo-induced shuttling motions of the α-CD ring on the dumbbell are repeatable and accompanied by a reversible fluorescent output signal.

Fig. 4 An INHIBIT logic gate based on a light-driven [2]rotaxane R3 and its truth table.

Reversible, erasable, and rewritable nanoscale recording on organic thin films is of practical importance in ultrahigh density information storage. The chemical modification of rotaxane molecules can be a promising route toward molecular memory for practical applications. Stoddart et al.¹⁹ fabricated a molecular memory circuit incorporating a bistable [2]rotaxane molecule as the data storage element. This 160 000-bit molecular electronic memory circuit was fabricated at a density of 10¹¹ bits cm⁻² (pitch 33 nm; memory cell size 0.0011 μm²). The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.

Gao et al.^20,21 have reported reversible, erasable, and rewritable nanorecording on rotaxane R4 (Fig. 5) Langmuir–Blodgett (LB) thin films, using a scanning tunneling microscope (STM) by inducing conductance transitions of this supramolecule. R4 consists of a dumbbell-shaped thread, containing two π-electron-rich recognition sites (tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP), see Fig. 5) and terminated by bulky stoppers, and a π-electron-deficient ring cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺). The ring can move back and forth between the two recognition sites in response to the external redox stimuli. The [2]rotaxane R4 thin films were prepared on highly oriented pyrolytic graphite (HOPG) substrates using the LB technique for STM recording. The repeatable and rewritable nanorecording was realized by applying voltage pulses onto the R4 thin films through the STM tip. Nanoscale dots can be written repeatedly with the voltage pulses. Interestingly, the marks written on the R4 thin films can be erased, re-recorded, and re-erased on the same site. The dots are so stable in air at room temperature that the R4 films are competent for reversible nanorecording due to the conductance transitions of R4 molecules.

Fig. 5 [2]Rotaxane R4 for reversible nanorecording by inducing different conductance transitions by scanning tunneling microscope (STM).

3.2 Gelation rotaxanes

Low-molecular-weight (LMW) gelators have attracted much attention in recent years. Introduction of a supramolecular system into a gelator is one of the promising approaches to investigate some of its special characteristics.¹⁴ To develop functional rotaxanes in non-solution media is just what supramolecular scientists and engineers seek. If a rotaxane system can be switchable in gel or solid state, then it will not be far to fabricate practicable molecular devices.

Fig. 6 shows two light-driven rotaxanes based on α-CD and cucurbit[7]uril (CB[7]).²² They were dispersed into a hydrogel system, which was confected using two aqueous surfactant solutions. These two dispersed hydrogel systems can be heated to form aqueous solutions and cooled reversibly back into a gel. Both the pseudo[2]rotaxane R5 hydrogel and pseudo[4]rotaxane R6 hydrogel display obvious Cotton effects similar to those of their aqueous solutions. Both rotaxanes R5 and R6 can be photo-irradiated by specific UV light to shuttle because of the photo-isomerization of the azobenzene units in the dumbbells, accompanied by corresponding UV and induced circular dichroism (ICD) signal changes. However, comparing the data for the hydrogel ICD spectra of pseudo[2]rotaxanes R5 and pseudo[4]rotaxanes R6, the ICD spectrum of α-CD-based R6 hydrogel changed much more drastically than that of the R5 hydrogel. These interesting phenomena are related to the molecular conglomeration state. In a gel, pseudorotaxane molecules exist in a much denser state and with more dimensional obstacles around than in solution. The two terminal inclusion parts between the CB[7] and viologen moieties of pseudo[4]rotaxane R6 exploit more dimensional space for the CD movement encircling the azobenzene unit in non-solution media.

Fig. 6 (a) α-CD and CB[7] based pseudo[2]rotaxane R5 and pseudo[4]rotaxane R6; (b) shuttling behavior of the light-driven cyclodextrin shuttles R5 and R6 in hydrogel.

Stoddart et al.²³ reported a cholesterol-stoppered bistable [2]rotaxane R7 containing a cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring and two recognition units, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP), in its dumbbell component (Fig. 7). Cholesterol is a very efficient initiator of organogels and liquid crystals. R7 and the precursory dumbbell both formed organogels. The sol–gel phase transition occurs when R7 (0.8 M) and dumbbell (0.5 M) in CH₂Cl₂–MeOH (3 : 2) solution are heated to 40 °C and cooled to 0 °C. When Fe(ClO₄)₃ (>0.2 M, CH₂Cl₂–MeOH (3 : 2)) is brought into contact with the gels to oxidize the samples, the gels dissolve in the solution. Interestingly, they found that the circular dichroism (CD) spectra of R7 and the dumbbell showed negative (λ_max = 282 nm) and positive (λ_max = 295 nm) Cotton effects, respectively, showing that R7 and the dumbbell have self-organized to form the possible right- and left-handed helical molecular stacks, respectively.

Fig. 7 Cholesterol-stoppered bistable [2]rotaxane R7.

3.3 Rotaxanes on the solid surface

A rotaxane shuttle can serve as a molecular device to switch on a surface. Many efforts have gone into developing such molecular devices. Two bistable [2]rotaxanes shown in Fig. 8, R8 and R9, were used to fabricate redox-controllable molecular nanovalves based on mesoporous silica nanoparticles.²⁴R8 and R9, with different spacer lengths between the recognition sites acting as their gatekeepers, were employed to attach covalently to the silica substrate. The nanovalves were efficient when the bistable [2]rotaxane-based gatekeepers were anchored deep within the pores. The shuttling of the macrocycle component of the bistable [2]rotaxane closer to and away from the pores’ orifices could close and open the silica nanopores, respectively, which controlled luminescent probe molecules’ (such as coumarins, tris(2-phenylpyridine)iridium, and rhodamine B) loading into or release from the mesoporous silica substrate. The lengths of the linkers between the surface and the rotaxane determined the effectiveness of the nanovalves. The shorter the linkers, the less leaky were the nanovalves. The controlled release of the probe molecules was investigated by measuring their luminescence intensities in response to ascorbic acid, which induces the rings’ movement away from the pores’ orifices, and consequently opens the nanovalves.

Fig. 8 Redox-controllable [2]rotaxanes R8 and R9 which can act as valves to control the release of trapped luminescent molecules.

A typical [2]rotaxane featuring succinamide and naphthalene diimide hydrogen-bonding stations for a benzylic amide macrocycle is shown in Fig. 9 (R10).²⁵ It was reported that R10 could shuttle and switch its net position both in solution and in a monolayer. The pyridine units of the macrocycle allowed the attachment of the rotaxanes to an acid-terminated self-assembled monolayer of alkane thiols on gold. Redox-switched shuttling of the rotaxane R10 could be maintained in the molecular monolayer environment, occurring on the millisecond time scale, and which could be characterized by X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and so on.

Fig. 9 Redox-switched [2]rotaxane R10 monolayer attached on a gold surface.

Beer et al.²⁶ reported a redox-active sensory rotaxane, prepared via anion templated assembly, and grafted it on a gold electrode surface, as shown in Fig. 10 (R11). The unique interlocked binding cavity of R11 self-assembled monolayers (SAM) show an amplified and selective response to chloride. Larger oxoanions, such as phosphate, are unable to access the interlocked cavity and R11 is not highly responsive to these.

Fig. 10 Rotaxane R11 self-assembled monolayers (SAMs) on gold electrode surfaces using ferrocene functionalized wheel and axle components.

More recently, a mechanical actuator driven electrochemically by artificial molecular muscles, based on a monolayer of the redox-controllable and bistable [3]rotaxane R12 (Fig. 11), was reported.²⁷R12 was utilized to make a microcantilever undergo reversible deflections when subjected to altering oxidizing and reducing electrochemical potentials. The microcantilever devices were prepared by precoating one surface with a gold film and allowing the palindromic [3]rotaxane molecules to adsorb selectively onto one side of the microcantilevers, utilizing thiol–gold chemistry. The artificial molecular muscles R12 are responsible for the deflections, which can be repeated over many cycles. The microcantilevers deflect in one direction following oxidation and in the opposite direction upon reduction. This result is a significant advance towards functional nano-electromechanical systems (NEMS).

Fig. 11 Redox-controllable and bistable [3]rotaxane R12.

The nanoscale movements within another surface-bound molecular [2]rotaxane have been used as the active components in a plasmonic device.²⁸ Redox-controllable rotaxane molecules were coated on a gold nanodisk array, to allow its plasmonic properties to be switched reversibly when it was exposed to chemical oxidants and reductants.

3.4 Rotaxanes with sensing function

The unique topological cavities of rotaxanes make them of great interest in the molecular sensing field.²⁹ As mentioned above, R11 SAMs can behave as a sensor responding to chloride. More recently, a β-cyclodextrin (β-CD) polypseudorotaxane with a light-active rotor graft (Fig. 12, R13) was employed as a photo-lockable ratiometric viscosity sensor.³⁰ For the light-active fluorescent rotor a cyanostilbene and a tetramethyljulolidine unit were combined with the β-CD. The rotor is so viscosity-sensitive that the change of the fluorophore’s nonradiative decay induced by environmental viscosity could be influenced by the photo-isomerization process. Control (locking and activating) of the microscopic viscosity response at the molecular level could be realized by UV and visible light.

Fig. 12 Pseudorotaxane R13 with a light-active rotor graft.

Highly selective alkaline metal ion sensors based on rotaxanes bearing plural metal-binding sites have been reported by Hiratani et al.³¹ They constructed a racemic [2]rotaxane able to form a diastereomer with an amino acid derivative and show an optical response to the chiral recognition (Fig. 13, R14). The macrocycle is a 26-membered crownophane including naphthyl, polyether, hydroxyl and amide groups and the dumbbell is an asymmetric axle bearing two different fluorophores, anthracene and carbazole, at its two ends, respectively. The rotaxane is such a ‘bright’ fluorescence-sensing system that it is able to response to phenylalaninol via variation of its fluorescence signals. The fluorescence intensity of anthracene decreases with an increase in the concentration of L-phenylalaninol, whilst the fluorescence intensity of carbazole gradually increases, due to the selective diastereomer formation between R14 and phenylalaninol.

Fig. 13 Optical response of [2]rotaxane R14 on the chiral recognition of phenylalaninol.

Stoddart et al.³² fabricated a pyrene-modified β-cyclodextrin (pyrenecyclodextrin) decorated single-walled carbon nanotube (SWNT) field-effect (FET) device, which can serve as a tunable photosensor to sense a fluorescent adamantyl-modified Ru complex (ADA-Ru).

3.5 Rotaxanes related to biology

Research on functional biomolecular structures is a hot topic.^33,34 Some supramolecular rotaxanes are also used in the biological arena to realize specific functions.^35,36 Smithrud et al. investigated the ability of a host-[2]rotaxane R15 (Fig. 14) to deliver peptides of all polarities across a live cell membrane.³⁶ It was found that only in the presence of R15 were F1-peptides taken up by COS 7 and ES2 cells; an additional role of the host-[2]rotaxane was modifying the permeability properties of the peptide. Fig. 14 also shows another fluorescent β-CD polypseudorotaxane R16 with many anthryl grafts, which could act as promising DNA concentrators, and with good binding ability for calf thymus DNA.³⁷ The polypseudorotaxanes are expected to have many exciting applications as a sensitive analytical tool in DNA chemistry with promising potential for controlling gene expression and delivery.

Fig. 14 Crown-like [2]rotaxane R15 and β-CD polypseudorotaxane R16.

Moreover, Smith et al. reported that squaraine-rotaxane dyes could be readily converted into extremely bright and highly stable NIR fluorescent probes for in vitro and in vivo optical imaging of live cells.³⁸ It can be foreseen that rotaxanes related to biology will continue to be a promising and hot research area.

3.6 Other functional rotaxanes

A supramolecular pseudorotaxane R17 (Fig. 15) based on α-cyclodextrin (α-CD) and an azobenzene-containing surfactant (1-[10-(4-phenylazophenoxy)decyl]pyridinium bromide, termed AzoC10) was fabricated.³⁹ The photocontrolled inclusion and exclusion reaction of the azobenzene can make R17 undergo reversible assembly and disassembly. Upon respective irradiation by UV light at 365 nm and visible light, R17 can undergo trans–cis photo-isomerization reversibly. The reversible photo-isomerization process can be carried out many times.

Fig. 15 Photocontrolled reversible assembly and disassembly of α-CD based pseudorotaxane R17.

Stoddart et al.⁴⁰ reported a liquid-crystalline bistable [2]rotaxane R18 functionalized with dendritic mesogenic stoppers, as shown in Fig. 16, and showed that it exhibits a smectic A (S_A) phase over a wide temperature range, including at room temperature. The formation of smectic A or C phases would align the bistable rotaxane moieties in layers in their dynamic states. The incorporation of the three rodlike cores with fluoro substituents into both the stoppers of LC bistable[2]rotaxane R18 and the control LC dumbbell compound induced and stabilized a S_A LC phase from ambient temperature to over 100 °C.

Fig. 16 Liquid-crystalline bistable [2]rotaxane R18.

4. Novel synthetic protocol to functional rotaxanes

‘Template-directed’ synthetic methodology^6,7 has been widely employed to effectively construct novel rotaxanes. In this section, several functional rotaxanes synthesized recently in novel ways, focusing mainly on [1]rotaxane synthesis, ‘click’ chemistry and so on, are simply illustrated.

4.1 [1]Rotaxane synthesis

When the host macrocycle and the axle in a [2]rotaxane are bonded covalently, it becomes a [1]rotaxane, which might be expected to exhibit properties different from other rotaxanes. A fully optical CD-based [1]rotaxane formed via self-complementary and Suzuki-coupling capping in a direct way is shown in Fig. 17 (R19).⁴¹ An azobenzene-modified β-CD derivative was firstly employed to self-include in aqueous solution and then capped with a fluorophore stopper to form [1]rotaxane. The unthreaded reference compound Ref-R19 was obtained in a similar way but in a DMSO medium. R19 can be photo-induced to shift reversibly using different irradiation, although not on a large scale; the process is accompanied by obvious absorption and induced circular dichroism (ICD) signal variances. Notably, this direct method, utilizing the interesting self-inclusion complexation, can be used to construct intriguing [1]rotaxanes simply and conveniently. Another CD-based [1]rotaxane isomeric to R19 was also reported via the self-complementation of the azobenzene modified β-CD on the secondary hydrogen position and consequent Suzuki-coupling capping in aqueous solution.⁴²

Fig. 17 [1]Rotaxane R19 synthesized via self-complementary and Suzuki-coupling capping.

Another [1]rotaxane R20 prepared via covalent bond formation, based on a crown-like macrocycle, has been reported, as shown in Fig. 18.⁴³ In the presence of potassium tert-butoxide, the equimolar reaction of 4 with 5 gave bicyclic compound 3 (mixture of two isomers). Then, without separation of the isomers, 3 was used in an aminolysis with 9-(3-(aminopropyl)aminocarbonyl)anthracene (2). Treatment of mixed isomers of 3 with 2 in DMF at room temperature gave [1]rotaxane R20 and a macrocycle with a long tail Ref-R20. [1]Rotaxane R20 has a three-dimensional small cavity formed by the ring and the chain connected with the macrocycle. The inside of the cavity can catch only the lithium ion, with a drastic enhancement of the fluorescence intensity of the R20 solution, which might make it a candidate for a lithium ion sensor.

Fig. 18 Synthesis of [1]rotaxane R20 based on a crown-like macrocycle via covalent bond formation.

A very interesting way to construct complex and functional rotaxane architectures is self-sorting, by which two crown ether based rotaxane systems have been organized recently. Huang et al.⁴⁴ used self-organization of two AB-type heteroditopic monomers to form supramolecular alternating copolymers. Another cascade-stoppered hetero[3]rotaxane was constructed via integrative self-sorting as well.⁴⁵

4.2 Click to construct rotaxanes

The ‘click’ reaction is widely used for convenient covalent bond formations. It has been used to prepare many rotaxanes effectively and conveniently.⁴⁶ For example, Stoddart et al.⁴⁷ reported a redox-switchable α-cyclodextrin-based [2]rotaxane R21 (Fig. 19) formed via a cycloaddition click reaction, wherein an α-CD firstly incorporates a tetrathiafulvalene (TTF) unit of the dumbbell to form a pseudorotaxane, and then a threading-followed-by-stopping approach is employed via a copper(I)-catalyzed azide–alkyne cycloaddition. The host α-CD ring preferred to rest on the TTF station. After the oxidation of TTF to its radical cation or to its dication, α-CD would move onto the 1,2,3-triazole ring unit. The redox-switchable process of rotaxane R21 is reversible. This is the first example of a redox-switchable cyclodextrin-based [2]rotaxane.

Fig. 19 Redox-switchable α-cyclodextrin-based [2]rotaxane R21.

Another mechanically interlocked light-harvesting system [3]rotaxane R22 (Fig. 20) has been synthesized in high yield through Cu(I)-catalyzed azide–alkyne cycloaddition.⁴⁸R22 contains two hexyl-substituted truxene units as the antenna groups and an oligo(para-phenylenevinylene) (OPV) moiety unit as the energy-collecting core. As shown in Fig. 20, the synthesis of R22 was essentially a one-pot synthesis employing the two crucial intermediates containing terminal alkyne groups and triazo groups, respectively, in which the threading and stopping processes were performed in one step. This method is a very valuable tool in the synthesis of functional rotaxanes.

Fig. 20 Crown-like [3]rotaxane R22 synthesized by ‘click’ reaction.

5. Conclusion and future prospects

Recent progress towards rotaxanes with fluorescent and functional properties for logic and information storage devices, ion sensing, gelation, surface devices, novel synthetic protocols and so on, has been described above. The various functions and potential applications of rotaxanes extend beyond those outlined here. For example, a synthetic molecular rotaxane can operate by an information ratchet mechanism, in which knowledge of a particle’s position is used to control its transport away from equilibrium.⁴⁹ However, there is still a long way to go for us to fabricate mature devices employing functional supramolecular systems applicable in people’s daily life. More effort needs to be put into this research field.

Several light-driven rotaxanes, in which photo- or electron transfer between or among different chromophore moieties could be altered or controlled by photo-induced motion, have been constructed recently. A recent review⁹ has cited and introduced several rotaxanes with this function. In addition, charge transfer reactions along a supramolecular redox gradient based on a [2]rotaxane triad have also been investigated more recently.⁵⁰ Some interesting cucurbit[8]uril (CB[8])-stabilized supramolecular charge-transfer (CT) complexes incorporating an electron-deficient unit and an electron-rich moiety in the cavity of CB[8] have also been built.⁵¹

Besides the rotaxanes reviewed here, increasing and outstanding research results in the field of functional rotaxane systems will be achieved beyond all doubt. People will benefit greatly from applicable devices, and even macroscopic products, based on functional supramolecular ‘bright’ systems in the not too distant future.

Acknowledgements

We gratefully acknowledge financial support from the NSFC/China (50673025 & 20802019), National Basic Research 973 Program (2006CB806200) and the Scientific Committee of Shanghai.

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