Charge transfer chromophore-stopped [2]rotaxane through [2 + 2] cycloaddition

Three charge-transfer chromophore-terminated [2]rotaxanes were synthesized, using a high-yield [2 + 2]cycloaddition reaction in apolar solvent at room temperature. Two solvent-driving molecular shuttles were constructed, which exhibit distinct conformations in different solvent as a result of the shuttling movement of the macrocycle.     

2]Pseudorotaxane and [2]rotaxane molecular shuttles: self-assembly through second-sphere coordination of thiocyanate ligands

A Pd(Py)(2)(SCN)(2) complex is shown to form a [2]pseudorotaxane complex with a macrocyclic tetralactam, which binds in the solid state through an unexpected complexation geometry. The intermolecular complexation is further applied to template the formation of a [2]rotaxane. The interlocked product acts as a degenerate molecular shuttle in solution, which is consistent with the co-conformation observed in the solid state.     

Synthesis of a [2]rotaxane through first- and second-sphere coordination

In an effort to expand the application of a new template from interpenetrated to interlocked molecular species, we report the synthesis of a new [2]rotaxane by means of both first- and second-sphere coordination of a palladium(II) dichloride subunit.     

Anion recognition and cation-induced molecular motion in a heteroditopic [2]rotaxane

A heteroditopic [2]rotaxane consisting of a calix[4]diquinone–isophthalamide macrocycle and 3,5‐bis‐amide pyridinium axle components with the capability of switching between two positional isomers in response to barium cation recognition is synthesised. The anion binding properties of the rotaxane’s interlocked cavity together with Na⁺, K⁺, NH₄⁺ and Ba²⁺ cation recognition capabilities are elucidated by ¹H NMR and UV‐visible spectroscopic titration experiments. Upon binding of Ba²⁺, molecular displacement of the axle’s positively charged pyridinium group from the rotaxane’s macrocyclic cavity occurs, whereas the monovalent cations Na⁺, K⁺ and NH₄⁺ are bound without causing significant co‐conformational change. The barium cation induced shuttling motion can be reversed on addition of tetrabutylammonium sulfate.     

Blue-colored donor-acceptor [2]rotaxane

[reaction: see text] A guest molecule-a bis-N-tetraethyleneglycol-substituted 3,3'-difluorobenzidine derivative-has been synthesized, and its complexation with the host, cyclobis(paraquat-p-phenylene), has been investigated. This host-guest complex was then employed in the template-directed synthesis of a blue-colored [2]rotaxane. The color of this [2]rotaxane arises from the charge-transfer absorption band between the HOMO of the guest and the LUMO of the host. This host-guest complex, and the derived [2]rotaxane, completes the donor-acceptor-based RGB (red/green/blue) color complex set.     

A [2]rotaxane capped by a cyclodextrin and a guest: formation of supramolecular [2]rotaxane polymer

A [2]rotaxane capped by a beta-cyclodextrin and a 2,4,6-trinitrophenyl group has been prepared by dissolving 6-aminocinnamoyl beta-cyclodextrin in water with 1-adamantane carboxylic acid and complexation with alpha-cyclodextrin followed by the reaction with 2,4,6-trinitrobenzene sulfonic acid sodium salt. The [2]rotaxane has been found to form supramolecular polymers by host-guest interactions.     

Optically probing molecular shuttling motion of [2]rotaxane by a conformation-adaptive fluorophore

A bistable [2]rotaxane with a conformation-adaptive macrocycle bearing a 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) unit was synthesized, which could be utilized to optical probe the molecular shuttling motion of the functionalized rotaxane system. The UV–vis, ¹H NMR and PL spectroscopic data clearly demonstrated that the DPAC ring was interlocked onto the thread and the fluorescence intensity of the DPAC unit in the macrocycle was effectively regulated by the location change of the macrocycle along the thread under acid/base stimulation, which was attributed to the modulation of the intramolecular photo-induced electron transfer between the DPAC unit and the methyltriazole (MTA) unit. This bistable rotaxane system containing a conformation-adaptive fluorophore unit in the macrocycle moiety opens an alternative way to design functional bistable mechanically interlocked molecules.     

The electrochemical polymerisation of a [2]rotaxane

We report the synthesis of a dithienylpyrrole-stoppered rotaxane and its subsequent electrochemical polymerisation onto a platinum working electrode surface. We have shown that the tetracationic cyclophane moiety of the rotaxane does not impair electropolymerisation of this derivative. Indeed, functionalised films can be conveniently prepared by oxidative polymerisation of the dithienylpyrrole stopper units, to yield a network of rotaxane units interconnected by a conducting polymer backbone.     

An acid-base switchable [2]rotaxane

A chemically addressable, bistable [2]rotaxane, which incorporates a dumbbell-shaped component containing both secondary dialkylammonium and 1,2-bis(pyridinium)ethane recognition sites for its ring component, dibenzo[24]crown-8 (DB24C8), has been assembled. (1)H NMR spectroscopy has demonstrated that deprotonation (and reprotonation) of the secondary dialkylammonium (dialkylamine) recognition site induces the DB24C8 ring to move away from this site to the 1,2-bis(pyridinium)ethane one (and back again) in a discrete manner, particularly when the experiment is performed in CDCl(3) solution.     

Protecting a squaraine near-IR dye through its incorporation in a slippage-derived [2]rotaxane

The photophysical properties and chemical stability of a squaraine derivative were enhanced after using Na+ ions to template a high-yield slippage synthesis of a [2]rotaxane from the dye and a molecular cage.     

Co-conformational mechanoisomerism in a calix[6]arene-based [2]rotaxane

The synthesis of a [2]rotaxane, comprising a calix[6]arene-wheel and a dibenzyl-ammonium axle, is here reported. By virtue of its inherent directionality, the calix-wheel makes non-degenerate two equivalent stations of the symmetrical axle. In this way, the neutral rotaxane shows two co-conformations, named endo-alkyl and endo-benzyl, in which an alkyl or benzyl moiety of the axle are included inside the calix-cavity, respectively. NMR and DFT studies showed that the co-conformation preferred by the neutral mechanomolecule is the ‘endo-alkyl’ one, which is more stabilized by C-H···π interactions between the included alkyl chain and the aromatic wall of the calix-cavity.     

A redox-switchable alpha-cyclodextrin-based [2]rotaxane

A bistable [2]rotaxane comprising an alpha-cyclodextrin (alpha-CD) ring and a dumbbell component containing a redox-active tetrathiafulvalene (TTF) ring system within its rod section has been synthesized using the Cu(I)-catalyzed azide-alkyne cycloaddition, and the redox-driven movements of the alpha-CD ring between the TTF and newly formed triazole ring systems have been elucidated. Microcalorimetric titrations on model complexes suggested that the alpha-CD ring prefers to reside on the TTF rather than on the triazole ring system by at least an order of magnitude. The fact that this situation does pertain in the bistable [2]rotaxane has not only been established quantitatively by electrochemical experiments and backed up by spectroscopic and chiroptical measurements but also been confirmed semiquantitatively by the recording of numerous cyclic voltammograms which point, along with the use of redox-active chemical reagents, to a mechanism of switching that involves the oxidation of the neutral TTF ring system to either its radical cationic (TTF*+) or dicationic (TTF2+) counterparts, whereupon the alpha-CD ring, moves along the dumbbell to encircle the triazole ring system. Since redox control by both chemical and electrochemical means is reversible, the switching by the bistable [2]rotaxane can be reversed on reduction of the TTF*+ or TTF2+ back to being a neutral TTF.     

A neutral redox-switchable [2]rotaxane

A limited range of redox-active, rotaxane-based, molecular switches exist, despite numerous potential applications for them as components of nanoscale devices. We have designed and synthesised a neutral, redox-active [2]rotaxane, which incorporates an electron-deficient pyromellitic diimide (PmI)-containing ring encircling two electron-rich recognition sites in the form of dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units positioned along the rod section of its dumbbell component. Molecular modeling using MacroModel guided the design of the mechanically interlocked molecular switch. The binding affinities in CH(2)Cl(2) at 298 K between the free ring and two electron-rich guests--one (K(a) = 5.8 × 10(2) M(-1)) containing a DNP unit and the other (K(a) = 6.3 × 10(3) M(-1)) containing a TTF unit--are strong: the one order of magnitude difference in their affinities favouring the TTF unit suggested to us the feasibility of integrating these three building blocks into a bistable [2]rotaxane switch. The [2]rotaxane was obtained in 34% yield by relying on neutral donor-acceptor templation and a double copper-catalysed azide-alkyne cycloaddition (CuAAC). Cyclic voltammetry (CV) and spectroelectrochemistry (SEC) were employed to stimulate and observe switching by this neutral bistable rotaxane in solution at 298 K, while (1)H NMR spectroscopy was enlisted to investigate switching upon chemical oxidation. The neutral [2]rotaxane is a chemically robust and functional switch with potential for applications in device settings.     

A clicked bistable [2]rotaxane

[structure: see text]. A universal diazide-terminated polyether, incorporating tetrathiafulvalene (TTF, green) and 1,5-dioxynaphthalene (DNP, red) units, was prepared and subsequently employed in the template-directed synthesis of a switchable donor/acceptor [2]rotaxane. The triazole rings (magenta), which are introduced into the rotaxane during requisite click reactions, do not present themselves as competing recognition sites for the tetracationic cyclophane (blue) as it is induced to switch between the TTF unit, when it becomes dicationic (green adorned with yellow extremities), and the DNP unit.     

Infrared spectroscopic characterization of [2]rotaxane molecular switch tunnel junction devices

Langmuir-Blodgett monolayers of a bistable [2]rotaxane were prepared at packing densities of 118, 73, and 54 A(2)/molecule. The monolayers were both characterized via infrared spectroscopy before and after evaporation of a 2 nm film of titanium and incorporated into molecular switch tunnel junction devices. The study suggests that the evaporation process primarily affects portions of the molecule exposed to the metal atom source. Thus, in tightly packed monolayers (73 and 54 A(2)/molecule), only the portions of the [2]rotaxane that are present at the molecule/air interface are clearly affected, leaving key functionality necessary for switching intact. Monolayers transferred at a lower pressure (118 A(2)/molecule) exhibit nonspecific damage and poor switching behavior following Ti deposition. These results indicate that tightly packed monolayers and sacrificial functionality displayed at the molecule/air interface are important design principles for molecular electronic devices.     

A light-driven rotaxane molecular shuttle with dual fluorescence addresses

[reaction: see text] A molecular shuttle containing an alpha-CD macrocycle, an azobenzene unit, and two different fluorescent naphthalimide units was synthesized. The cis-trans photoisomerization of the azobenzene unit resulted in the motion of the CD macrocycle on the track. Because of the easy regulation and full reversibility of the fluorescence change of the two stopper units, the molecular shuttle could be used as a molecular storage medium or switch with all-optical inputs and outputs.     

Synthesis of a mechanically linked oligo[2]rotaxane

A bifunctional [2]rotaxane, bearing two free functional groups each in the ring and axial parts, was synthesized, followed by its polycondensation with methylene diphenyl diisocyanate leading to a mechanically linked oligo[2]rotaxane.     

Counterion-induced translational isomerism in a bistable [2]rotaxane

Translational isomerization can be induced by changing the anions associated with a bistable rotaxane in which the tetracationic cyclophane (blue box), cyclobis(paraquat-p-phenylene), encircles a dumbbell component containing bispyrrolotetrathiafulvalene (green) and a dioxynaphthalene (red) recognition sites. The rotaxane was isolated as both its hexafluorophosphate and tris(tetrachlorobenzenediolato)phosphate(v) (TRISPHAT(-)) salts. Photophysical measurements and NMR spectroscopy carried out in acetone (CD(3)COCD(3)) and acetonitrile (CD(3)CN) solutions reveal that the much larger TRISPHAT(-) anion favors predominantly the encirclement of the green site by the blue box.     

An interdigitated functionally rigid [2]rotaxane

The synthesis of a functionally rigid [2]rotaxane incorporating pi-electron rich 1,5-disubstituted naphthalene (NP) ring systems, encircled by the pi-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene), is described; in the solid state, the molecules of this donor-acceptor [2]rotaxane line themselves up in parallel pi-pi stacks of alternating NP ring systems and bipyridinium units, affording an interdigitated superstructure.