A multi-mode-driven molecular shuttle: photochemically and thermally reactive azobenzene rotaxanes

The shuttling process of alpha-CyD in three rotaxanes (1-3) containing alpha-cyclodextrin (alpha-CyD) as a ring, azobenzene as a photoactive group, viologen as an energy barrier for slipping of the ring, and 2,4-dinitrobenzene as a stopper was investigated. The trans-cis photoisomerization of 1 by UV light irradiation occurred in both DMSO and water due to the movement of alpha-CyD toward the ethylene group, while the photoisomerization of 2 occurred in DMSO, but not in water. No photoisomerization was observed for 3 in both water and DMSO. The activation parameters of 1 and 1-ref in DMSO are subject to a compensation relation between deltaS(double dagger) and deltaH(double dagger); however, in water, the deltaS(double dagger) terms are not compensated by the deltaH(double dagger) terms. Alternating irradiation of the UV and visible lights resulted in a reversible change in the induced circular dichroism (ICD) bands of trans-1 and cis-1. In contrast, after the UV light irradiation, the ICD band of trans-2 decreased without the appearance of any bands of cis-2. The NMR spectra of 2 in DMSO showed coalescence of the split signals for the methylene and for the viologen protons due to the shuttling of alpha-CyD. Both the NOE differential spectra for cis-1 in water after UV light irradiation and 2 in DMSO after heating to 120 degrees C showed the negative NOE peaks assigned to interior protons of alpha-CyD, suggesting that alpha-CyD in cis-1 exists at the one ethylene moiety, and alpha-CyDs in cis-2 and 2 heated in DMSO exist at the propylene moieties.     

A [3]rotaxane with three stable states that responds to multiple-inputs and displays dual fluorescence addresses

A [3]rotaxane molecular shuttle containing two alpha-cyclodextrin (alpha-CD) macrocycles, an azobenzene unit, a stilbene unit, and two different fluorescent naphthalimide units has been investigated. The azobenzene unit and the stilbene unit can be E/Z-photoisomerized separately by light excited at different wavelengths. Irradiation at 380 nm resulted in the photoisomerization of the azobenzene unit, leading to the formation of one stable state of the [3]rotaxane (Z1-NNAS-2CD); irradiation at 313 nm resulted in the photoisomerization of the stilbene unit, leading to the formation of another stable state of the [3]rotaxane (Z2-NNAS-2CD). The reversible conversion of the Z1 and Z2 isomers back to the E isomer by irradiation at 450 nm and 280 nm, respectively, is accompanied by recovery of the absorption and fluorescence spectra of the [3]rotaxane. The E isomer and the two Z isomers have been characterized by 1H NMR spectroscopy and by two-dimensional NMR spectroscopy. The light stimuli can induce shuttling motions of the two alpha-CD macrocycles on the molecular thread; concomitantly, the absorption and fluorescence spectra of the [3]rotaxane change in a regular way. When the alpha-CD macrocycle stays close to the fluorescent moiety, the fluorescence of the moiety become stronger due to the rigidity of the alpha-CD ring. As the photoisomerization processes are fully reversible, the photo-induced shuttling motions of the alpha-CD rings can be repeated, accompanied by dual reversible fluorescence signal outputs. The potential application of such light-induced mechanical motions at the molecular level could provide some insight into the workings of a molecular machine with entirely optical signals, and could provide a cheap, convenient interface for communication between micro- and macroworlds.     

Effective enhancement of fluorescence signals in rotaxane-doped reversible hydrosol-gel systems

Rotaxane-based nanoscale architectures have a huge potential to be processed into widely applied devices. In this work, light-driven rotaxanes with fluorescent chromophores based on alpha-cyclodextrin (alpha-CyD) have been doped into amphoteric thermoreversible hydrosol-gels to form a new type of disperse system with reversible optical signals. The photoisomerizations with alpha-CyD shuttling have been studied by induced circular dichroism (ICD). Compared with their corresponding solutions, the rotaxane-doped hydrosol-gel systems produce much more obvious fluorescent binary signals.     

Modular Synthesis and Dynamics of a Variety of Donor–Acceptor Interlocked Compounds Prepared by Click Chemistry

A series of donor–acceptor [2]‐, [3]‐, and [4]rotaxanes and self‐complexes ([1]rotaxanes) have been synthesized by a threading‐followed‐by‐stoppering approach, in which the precursor pseudorotaxanes are fixed by using Cu^I‐catalyzed Huisgen 1,3‐dipolar cycloaddition to attach the required stoppers. This alternative approach to forming rotaxanes of the donor–acceptor type, in which the donor is a 1,5‐dioxynaphthalene unit and the acceptor is the tetracationic cyclophane cyclobis(paraquat‐p‐phenylene), proceeds with enhanced yields relative to the tried and tested synthetic strategies, which involve the clipping of the cyclophane around a preformed dumbbell containing π‐electron‐donating recognition sites. The new synthetic approach is amenable to application to highly convergent sequences. To extend the scope of this reaction, we constructed [2]rotaxanes in which one of the phenylene rings of the tetracationic cyclophane is perfluorinated, a feature which significantly weakens its association with π‐electron‐rich guests. The activation barrier for the shuttling of the cyclophane over a spacer containing two triazole rings was determined to be (15.5±0.1) kcal mol^?1 for a degenerate two‐station [2]rotaxane, a value similar to that previously measured for analogous degenerate compounds containing aromatic or ethylene glycol spacers. The triazole rings do not seem to perturb the shuttling process significantly; this property bodes well for their future incorporation into bistable molecular switches.     

Fluorescence Modulation in Tribranched Switchable [4]Rotaxanes

Two novel tribranched [4]rotaxanes with a 1,3,5‐triphenylene core and three rotaxane arms have been designed, synthesized, and characterized by ¹H and ¹³C NMR spectroscopies and HR‐ESI mass spectrometry. [4]Rotaxanes 1 and 2 each possess the same three‐armed skeleton. Each arm incorporates two distinguishable binding sites for a dibenzo[24]crown‐8 ring, namely a dibenzylammonium site and an N‐methyltriazolium site, and is terminated by a 4‐morpholino‐naphthalimide fluorophore as a stopper. [4]Rotaxane 1 has three di‐ferrocene‐functionalized dibenzo[24]crown‐8 rings whereas 2 has three simple dibenzo[24]crown‐8 rings interlocked with the thread component. Uniform shuttling motions of the three macrocycles in both 1 and 2 can be driven by external acid–base stimuli, which were confirmed by ¹H NMR spectroscopy. However, [4]rotaxanes 1 and 2 show distinct modes of fluorescence modulation in response to external acid–base stimuli. [4]Rotaxane 1 exhibits a remarkable fluorescence decrease in response to the addition of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) as a base, which can displace the ferrocene‐functionalized macrocycle from the dibenzylammonium station to the N‐methyltriazolium station. In contrast, the fluorescence intensity of [4]rotaxane 2 showed an enhancement with the addition of DBU. Time‐resolved fluorescence measurements have been performed. The different photoinduced electron‐transfer processes responsible for the fluorescence changes in the two molecular systems are discussed. Topological structures of this kind have significant potential for the design and construction of large and complex assemblies with controllable functions.     

Electrostatic barriers in rotaxanes and pseudorotaxanes

The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY²⁺) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5‐dioxynaphthalene (DNP) units flanking a central BIPY²⁺ unit in the dumbbell component and encircled by the cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) tetracationic cyclophane, has been synthesized employing a threading‐followed‐by‐stoppering approach. Variable‐temperature ¹H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT⁴⁺ ring over the central BIPY²⁺ unit is in excess of 17 kcal mol^?1 at 343 K. Further information about the nature of the BIPY²⁺ unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY²⁺ moiety or a central DNP unit flanked by a BIPY²⁺ moiety. The threading and dethreading processes of the CBPQT⁴⁺ ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY²⁺ unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self‐folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δ${G{{{\ne}\hfill \atop {\rm f}\hfill}}}The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H?NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems.     

Isomeric Squaraine‐Based [2]Pseudorotaxanes and [2]Rotaxanes: Synthesis,Optical Properties,and Their Tubular Structures in the Solid State

On the basis of formation of [2]pseudorotaxane complexes between triptycene‐derived tetralactam macrocycles 1 a and 1 b and squaraine dyes, construction of squaraine‐based [2]rotaxanes through clipping reactions were studied in detail. As a result, when two symmetrical squaraines 2 d and 2 e were utilized as templates, two pairs of isomeric [2]rotaxanes 3 a – b and 4 a – b as diastereomers were obtained, owing to the two possible linking modes of triptycene derivatives. It was also found, interestingly, that when a nonsymmetrical dye 2 g was involved, there existed simultaneously three isomers of [2]rotaxanes in one reaction due to the different directions of the guest threading. The ¹H NMR and 2D NOESY NMR spectra were used to distinguish the isomers, and the yield of [2]rotaxane 5 a with the benzyl group in the wider rim of the host 1 a was found to be higher than that of another isomer 5 b with an opposite direction of the guest, which indicated the partial selection of the threading direction. The X‐ray structures of 3 b and 4 a showed that, except for the standard hydrogen bonds between the amide protons of the hosts and the carbonyl oxygen atoms of the guests, multiple π???π stacking and C? H???π interactions between triptycene subunits and aromatic rings of the guests also participated in the complexation. Crystallographic studies also revealed that the [2]rotaxane molecules 3 b and 4 a further self‐assembled into tubular structures in the solid state with the squaraine dyes inside the channels. In the case of 4 a , all the nonsymmetrical macrocyclic molecules pointed in one direction, which suggests the formation of oriented tubular structures. Moreover, it was also found that the squaraines encapsulated in the triptycene‐derived macrocycles were protected from chemical attack, and subsequently have potential applications in imaging probes and other biomedical areas.     

Self-assembly of novel [3]- and [2]rotaxanes with two different ring components: donor-acceptor and hydrogen bonding interactions and molecular-shuttling behavior

Three of the first kind of hetero[3]rotaxanes, which comprise one linear component and one neutral and one tetracationic ring component, have been assembled by using the intermolecular hydrogen bonding and donor-acceptor interactions. Three neutral [2]rotaxanes and three tetracationic [2]rotaxanes have also been synthesized as intermediate products or for the sake of property comparison. The linear molecules are incorporated with two glycine subunits, for templating the formation of the neutral tetraamide cyclophane, and one or two hydroquinone subunits, for inducing the formation of the tetracationic cyclophane. Variable-temperature (1)H NMR investigation reveals that the shuttling behavior of the tetracationic ring component along the linear component is substantially influenced by the existence of the neutral ring component. The spatial repelling interaction of the neutral ring on the electron-deficient tetracationic ring simultaneously weakens the latter's "positioning" tendency at both electron-rich hydroquinone sites of the linear component. As a result, the activation energy associated with the shuttling process of the tetracationic ring between the two hydroquinone sites is remarkably reduced in comparison to that of the shuttling process of the corresponding neutral ring-free [2]rotaxanes. For the first time, the rotation of the dipyridinium subunit around the axis formed by the two methylene groups connecting them within the tetracationic cyclophane has been investigated by variable-temperature (1)H NMR spectroscopy and the associated kinetic data have also been successfully obtained. Furthermore, the UV-vis and fluorescent properties of the new [2]- and [3]rotaxanes have been studied. The results demonstrate that [3]rotaxanes with different ring components possess unique kinetic features that are not available in [3]rotaxanes with identical ring components.     

Post-assembly processing of [2]rotaxanes

The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.     

Synthesis and characterization of a metallocycle-based molecular shuttle

Kinetically stable metallocycle-based molecular shuttles of [2]rotaxanes 4a and 4b, along with [3]rotaxanes 5a and 5b, have been prepared using the rhenium(I)-bridged metallocycle 2 and the dumbbell components containing two stations, 3a and 3b. The rotaxanes were self-assembled by hydrogen bonding interactions upon heating a Cl(2)CHCHCl(2) solution containing their components at 70 degrees C. Each rotaxane was isolated in pure form by silica gel chromatography under ordinary laboratory conditions and fully characterized by elemental analysis and various spectroscopic methods. The (1)H NMR signals for the amide NH and the methylene -(CH(2))(4)- of the station were considerably changed when occupied by the metallocycle. In [2]rotaxane 4b, which has a larger naphthyl spacer, the occupied and unoccupied stations gave widely separated signals in the (1)H NMR spectroscopy at room temperature, but averaged signals of two stations were observed in [2]rotaxane 4a, which has a smaller phenyl spacer. This is attributed to the shuttling of the metallocycle between two stations. The coalescence temperature experiment gave a shuttling rate of approximately 670 s(-)(1) at 19 degrees C in CDCl(3), corresponding to an activation free energy (DeltaG()) of 13.3 kcal/mol. With respect to the relative position of the chloride in the rhenium(I) center, two diastereomers are possible in the [2]rotaxane and three diastereomers are possible in the [3]rotaxane. In fact, the rotaxanes exist as diastereomeric mixtures in nearly equal amounts of all possible diastereomers on the basis of the amide NH signals of the station in the (1)H NMR spectroscopy.     

Four‐State Molecular Shuttling of [2]Rotaxanes in Response to Acid/Base and Alkali‐Metal Cation Stimuli

In this study, we synthesized [2]rotaxanes possessing three recognition sites—a dialkylammonium, an alkylarylamine, and a tetra(ethylene glycol) stations—in their dumbbell‐like axle component and dibenzo[24]crown‐8 (DB24C8) as their macrocyclic component. These [2]rotaxanes behaved as four‐state molecular shuttles: i) under acidic conditions, the DB24C8 unit encircled both the dialkylammonium and alkylarylammonium stations; ii) under neutral conditions, the dialkylammonium unit was the predominant station for the DB24C8 component; iii) under basic conditions, when both ammonium centers were deprotonated, the alkylarylamine unit became a suitable station for the DB24C8 component; and iv) under basic conditions in the presence of an alkali‐metal cation, the tetra(ethylene glycol) unit recognized the DB24C8 component through cooperative binding of the alkali‐metal ion. In addition, we observed that the [2]rotaxanes exhibited selective recognition for metal cations. These shuttling motions of the macrocyclic component proceeded reversibly.     

Molecular shuttles based on tetrathiafulvalene units and 1,5-dioxynaphthalene ring systems

Six different degenerate [2]rotaxanes were synthesized and characterized. The rotaxanes contained either two tetrathiafulvalene (TTF) units or two 1,5-dioxynaphthalene (DNP) ring systems, both of which serve as recognition sites for a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. Three different spacer units were incorporated into the dumbbell components of the [2]rotaxanes between the recognition sites. They include a polyether chain, a terphenyl unit, and a diphenyl ether linker, all of which were investigated in order to probe the effect of the spacers on the rate of the shuttling process. Data from dynamic 1H NMR spectroscopy revealed a relatively small difference in the DeltaG++ values for the shuttling process in the [2]rotaxanes containing the three different spacers, in contrast to a large difference between the TTF-containing rotaxanes (18 kcal mol(-1)) and the DNP-containing rotaxanes (15 kcal mol(-1)). This 3 kcal mol(-1) difference is predominantly a result of a ground-state effect, reflecting the much stronger binding of TTF units to the CBPQT4+ ring in comparison with DNP ring systems. An examination of the enthalpic (DeltaH++) and entropic (DeltaS++) components for the shuttling process in the DNP-containing rotaxanes revealed significant differences between the three spacers, a property which could be important in designing new molecules for incorporation into molecular electronic and nanoelectromechanical (NEMs) devices.     

Amphiphilic Bistable Rotaxanes

Two molecular shuttles/switches—a slow one and a fast one—in the shape of amphiphilic, bistable [2]rotaxanes have been synthesized and characterized. Both [2]rotaxanes contain a hydrophobic, tetraarylmethane and a hydrophilic, dendritic stopper. They are comprised of two π‐electron‐rich stations—a monopyrrolotetrathiafulvalene unit and a 1,5‐dioxynaphthalene moiety—which can act as recognition sites for the tetracationic cyclophane, cyclobis(paraquat‐p‐phenylene), to reside around. In addition, a model [2]rotaxane, incorporating only a monopyrrolotetrathiafulvalene unit in the rod section of the amphiphilic dumbbell component and cyclobis(paraquat‐p‐phenylene) as the ring component, has been investigated. The dumbbell‐shaped components were constructed using conventional synthetic methodologies to assemble 1) the hydrophobic, tetraarylmethane stopper and 2) the hydrophilic, dendritic stopper. Next, 3) the hydrophobic stopper was fused to the 1,5‐dioxynaphthalene moiety and/or the monopyrrolotetrathiafulvalene unit by appropriate alkylations, followed by 4) attachment of the hydrophilic stopper, once again by alkylation to give the dumbbell‐shaped compounds. Finally, 5) the [2]rotaxanes were self‐assembled by using the dumbbells as templates for the formation of the encircling cyclobis(paraquat‐p‐phenylene) tetracations. The two [2]rotaxanes differ in their arrangement of the π‐electron‐rich units, one in which the SMe group of the monopyrrolotetrathiafulvalene unit points toward the 1,5‐dioxynaphthalene moiety ( 2 ?4 PF₆) and another in which it points away from the 1,5‐dioxynaphthalene moiety ( 3 ?4 PF₆). This seemingly small difference in the orientation of the monopyrrolotetrathiafulvalene unit leads to profound changes in the physical properties of these rotaxanes. The bistable [2]rotaxanes were both isolated as brown solids. ¹H NMR and UV‐visible spectroscopy, and electrochemical investigations, reveal the presence of both possible translational isomers at ambient temperature. As a consequence of the existence of both possible translational isomers in these bistable [2]rotaxanes, they exhibit a complex electrochemical behavior, which is further complicated by the presence of folded conformations wherein the monopyrrolotetrathiafulvalene unit is involved in an “alongside” interaction with the tetracationic cyclophane. In the molecular shuttle/switch 2 ?4 PF₆ a “knob”, in the shape of the SMe group, is situated between the monopyrrolotetrathiafulvalene and the 1,5‐dioxynaphthalene recognition sites, making it possible to isolate both translational isomers ( 2 ?4 PF₆?GREEN and 2 ?4 PF₆?RED) and to investigate the kinetics of the shuttling of the cyclobis(paraquat‐p‐phenylene) tetracation between the two recognition sites. The shuttling processes, which are accompanied by clearly detectable color changes, can be followed by ¹H NMR and UV‐visible spectroscopy, allowing the rate constants and energies of activation for the translation of the cyclobis(paraquat‐p‐phenylene) tetracations between the two recognition sites to be determined. In the molecular shuttle/switch 3 ?4 PF₆, there is no “knob” situated between the 1,5‐dioxynaphthalene and the monopyrrolotetrathiafulvalene recognition sites, resulting in a considerably faster shuttling of the cyclobis(paraquat‐p‐phenylene) tetracation between these two sites, making the separation of the two possible translational isomers of 3 ?4 PF₆ impractical. However, the shuttling of the cyclobis(paraquat‐p‐phenylene) tetracation can be followed by dynamic ¹H NMR spectroscopy. At low temperatures, the major translational isomer is 3 ?4 PF₆?RED, while 3 ?4 PF₆?GREEN is the major isomer at higher temperature. In the bistable [2]rotaxanes shuttling of the cyclobis(paraquat‐p‐phenylene) tetracations can be driven by electrochemical oxidation of the monopyrrolotetrathiafulvalene unit. In complexes in which one of the two dumbbell stoppers is missing, electrochemical oxidation causes dethreading.     

Guest-dependent complexation of triptycene-derived macrotricyclic host containing one anthracene moiety with paraquat derivatives: construction of [2]rotaxanes

Complexation between the triptycene-derived macrotricyclic polyether containing an anthracene unit and paraquat derivatives in both solution and solid state was investigated. It was found that the macrotricyclic host with multi-cavity structure could form a series of [2]pseudorotaxanes with different terminal functionalised paraquat derivatives in different threading modes, which subsequently resulted in the construction of two novel [2]rotaxanes.     

Degenerate [2]rotaxanes with electrostatic barriers

A synthetic approach to the preparation of [2]rotaxanes (1-5·6PF(6)) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT(4+)) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5·6PF(6), there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT(4+) ring in the [2]rotaxanes. Thus, the CBPQT(4+) rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the (1)H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature (1)H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF(6) and 2·6PF(6) indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT(2(˙+)) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF(6) and 2·6PF(6) can shuttle along the recognition sites--two DNP units and one-electron reduced bipyridinium radical cation--under redox control.     

The Effect of Incorporating Fréchet Dendrons into Rotaxanes and Molecular Shuttles Containing the 1,2‐Bis(pyridinium)ethane⊂[24]Crown‐8 Templating Motif

Fréchet‐type dendrons (G0–G3) were added as both axle stoppering units and cyclic wheel appendages in a series of [2]rotaxanes, [3]rotaxanes, and molecular shuttles that employ 1,2‐bis(pyridinium)ethane axles and 24‐membered crown ethers wheels. The addition of dendrimer wedges as stoppering units dramatically increased the solubility of simple [2]rotaxanes in nonpolar solvents. The X‐ray structure of a G1‐stoppered [2]rotaxane shows how the dendritic units affect the structure of the interlocked components. Increased solubility allows observation of how the interaction of dendritic units on separate components in interlocked molecules influences switching properties and molecular size. In a series of [2]rotaxane molecular shuttles incorporating two recognition sites, it was demonstrated that an increase in generation on either the stoppering unit or cyclic wheel could influence both the rate of shuttling and the site preference of the wheel on the axle.     

Pillar[5]arene-based [3]rotaxanes: Convenient construction via multicomponent reaction and pH responsive self-assembly in water

Four pillar[5]arene based [3]rotaxanes(1-4) involving two 1,4-diethoxypillar[5]arene(DEP5) rings and a dumbbell-shaped component were successfully synthesized.The dumbbell-shape molecules contain one longer bridge,two triazole sites and two multicomponent stoppers.After threading DEP5 rings with linear guests(G1-G4) which contain two benzaldehyde units,the base catalyzed three-component reaction of dimedone,malononitrile and benzaldehyde was performed to construct the stoppers and connected the pseudorotaxanes with stoppers to generate 1-4.The structures of [3]rotaxanes and their self-assembly behaviors were characterized by ~1 H NMR,~(13)C NMR,NOESY,HR-ESI-MS,DLS and TEM technologies.We hope that pillar[5]arene based [3]rotaxanes may have potential applications in drug delivery systems and molecular devices.     

Functionally rigid bistable [2]rotaxanes

Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two pi-electron-rich stations-two NP moieties or a MPTTF unit and a NP moiety-with (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed (> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for much better control over the position of the ring component in the ground state but also for control over the location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically (1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ ring from one station to another as compared with previous systems-a finding that is thought to be a result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses of structural conformations that have characterized the first generations of molecular switches based on the donor-acceptor recognition motif.     

Light‐Switchable Janus [2]Rotaxanes Based on α‐Cyclodextrin Derivatives Bearing Two Recognition Sites Linked with Oligo(ethylene glycol)

Two Janus [2]rotaxanes, 5a and 5b , with α‐cyclodextrin (α‐CD) derivatives substituted on the 6‐position with two recognition sites (azobenzene and heptamethylene (C7)) that were linked with linkers of different lengths (oligo(ethylene glycol) with a degree of polymerization equal to 2 or approximately 21) were synthesized and characterized. 2D ROESY NMR spectroscopy and circular dichroism (cd) spectra demonstrated that the recognition site of the α‐CD moiety was switched by photoisomerization of the azobenzene moiety in 5a and 5b . The different size changes of 5a and 5b in hydrodynamic radius (R_H) owing to the different length of linker between two recognition sites were observed by pulse‐field‐gradient spin‐echo NMR spectroscopy. The kinetic results indicated that the different length of linker had no or a weak effect for the photoisomerization process of 5a and 5b .     

Efficient End‐Capping Synthesis of Neutral Donor–Acceptor [2]Rotaxanes Under Additive‐Free and Mild Conditions

Efficient end‐capping synthesis of neutral donor–acceptor (D –A) [2]rotaxanes without loading any catalysts or activating agents was achieved by utilizing high reactivity of a pentacoordinated hydrosilane toward salicylic acid derivatives. As components of [2]rotaxanes, an electron‐deficient naphthalenediimide‐containing axle with a salicylic acid terminus and several electron‐rich bis(naphthocrown) ether macrocycles were employed. End‐capping reactions with the pentacoordinated hydrosilane underwent smoothly even at low temperature to afford the corresponding [2]rotaxanes in good yields. A [2]rotaxane containing bis‐1,5‐(dinaphtho)‐38‐crown‐10 ether as a wheel molecule was synthesized and isolated in 84 % yield by the end‐capping at ?10 °C, presenting the highest yield ever reported for the end‐capping synthesis of a neutral D –A [2]rotaxane. It was found that the yields of the [2]rotaxanes in the end‐capping reactions were almost parallel to the formation ratios of the corresponding pseudo[2]rotaxanes estimated by utilizing model systems. These results indicate that the end‐capping reaction using the pentacoordinated hydrosilane proceeded without perturbing the threading process, and most of the pseudo[2]rotaxanes underwent efficient end‐capping reaction even at low temperature.