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.     

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 new thermo- and photo-driven [2]rotaxane

A new rotaxane with functional chromophores porphyrin and fluorene as stoppers has been synthesized. It displays dual fluorescent character. The macrocycle ring can shuttle between the fumaramide part and the succinimide part on the dumbbell. Heating and UV light irradiating on the rotaxane lead to the reversible E/Z conversion, driving the ring to shuttle between the two stations. The optical properties of porphyrin and fluorene stoppers of the thread do not change so much while the ring shuttles because the macrocycle does not interact with the two stoppers. However, the active group, propargyloxyl, is introduced onto the two sides of the ring so that the rotaxane has the potential to be functionalized by further decoration.     

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.     

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.     

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.     

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.     

A tripodal [2]rotaxane on the surface of gold

Tripodal [2]rotaxane, 3, and the structurally related axle, 2, incorporating a viologen moiety, a crown ether, and three thiol anchoring groups have been synthesized. Analogous monopodal derivatives, 1, have also been prepared. Self-assembled monolayers of the above tripodal and monopodal systems on gold have been studied by cyclic voltammetry. It has been shown that a thiol anchoring group is required to attach the monopodal viologen 1 to the surface of gold and that the maximum surface coverage of 1 corresponds to 2.7 x 10(-10) mol.cm(-2). The adsorbed monopodal viologen 1 does not thread bis-p-phenylene-34-crown-10 ether, 6. However, the tripodal axle 2 adsorbed on the surface of gold threads the crown ether 6 to form a hetero [2]rotaxane. In the case of the tripodal axle 2, the surface coverage is 7 x 10(-11) mol.cm(-2), while for the tripodal [2]rotaxane 3 the surface coverage reaches 1.1 x 10(-10) mol.cm(-2).     

Coordination-driven self-organization of switchable [2]rotaxane

A bistable porphyrin-containing [2]rotaxane is synthesized with a shuttling benzylic-amide macrocycle mechanically locked onto the thread subunit by formations of H-bonds with two potential stations. This macrocycle comprises two pyridine groups, which would be easily coordinated with zinc porphyrin. The Zn(II) coordination of porphyrin moiety on the thread subunit, immediately followed by the coordination with pyridine groups on the macrocycle, leads to an intermolecular axle-macrocycle-type nanostructure. Moreover, the self-assembly way shows great difference from the two states of the rotaxane monomer: The coordination-driven self-organization of the trans-state E2 leads to a network structure, whereas the cis-state Z2 gives birth to an irregular assembly.     

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 very efficient synthesis of a mannosyl orthoester [2]rotaxane and mannosidic [2]rotaxanes

The direct preparation of mannosyl[2]rotaxane derivatives by O-glycosylation from tetra-O-acetyl-alpha-D-mannosyltrichloroacetimidate and a tert-butylanilinium alcohol in the presence of dibenzo-24-crown-8 is described. The method appears to be very efficient and allows for the preparation of either orthoester or mannosyl rotaxane derivatives, depending on reaction conditions.     

Self-assembly of Novel Hetero[3]rotaxane,[2]Rotaxanes and [2]Catenane

One linear template 13 and one cyclophane template 15, both incorporating two electron rich 1,4-dialkoxybenzene units and one diamide unit, have been synthesized. By utilizing donor-acceptor interaction and/or intermolecular hydrogen bonding assembling principles, one novel hetero[3]rotaxane 22·4Cl, possessing one neutral and one tetracationic ring components, has been synthesized from 13, through neutral [2]rotaxane 21 as intermediate. With 15 as template, tetracationic [2]catenane 23·4PF6 was assembled by using donor-acceptor interaction, but no neutral [2]rotaxane could be obtained under the typical conditions of hydrogen bonding assembling principle. The interlocked supramolecular compounds have been characterized and their spectral properties are investigated.     

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.     

Trifluoromethyl acting as stopper in [2]rotaxane

A modified dumbbell obtained by replacing one of the phenyl groups of the dibenzylammonium with a strong electron-withdrawing trifluoromethyl group templated the synthesis of the smallest [2]rotaxane reported so far. The trifluoromethyl group not only enhances the templating effect of the dumbbell but also acts as the stopper to prevent dethreading of a [20]crown ether macrocycle.     

Shuttling dynamics in an acid-base-switchable [2]rotaxane.

Molecular shuttles are an intriguing class of rotaxanes which constitute prototypes of mechanical molecular machines and motors. By using stopped-flow spectroscopic techniques in acetonitrile solution, we investigated the kinetics of the shuttling process of a dibenzo[24]crown-8 ether (DB24C8) macrocycle between two recognition sites or "stations"--a secondary ammonium (-NH2+-)/amine (-NH-) center and a 4,4'-bipyridinium (bipy2+) unit--located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order -NH2+- > bipy2+ > -NH-. Hence, shuttling of the DB24C8 macrocycle can be obtained by deprotonation and reprotonation of the ammonium station, reactions which are easily accomplished by addition of base and acid to the solution. The rate constants were measured as a function of temperature in the range 277-303 K, and activation parameters for the shuttling motion in both directions were determined. The effect of different counterions on the shuttling rates was examined. The shuttling from the -NH2+- to the bipy2+ station, induced by the deprotonation of the ammonium site, is considerably slower than the shuttling in the reverse direction, which is, in turn, activated by reprotonation of the amine site. The results show that the dynamics of the shuttling processes are related to the change in the intercomponent interactions and structural features of the two mutually interlocked molecular components. Our observations also indicate that the counterions of the cationic rotaxane constitute an important contribution to the activation barrier for shuttling.     

A clipped [3]rotaxane derived from bis-nor-seco-cucurbit[10]uril

This paper describes the synthesis of host 1 by the double bridging reaction of bis-ns-CB[10] with 2 under acidic conditions. Host 1 functions as a double cavity host for aliphatic and aromatic ammonium ions (3-17) in water. Conducting the bridging reaction in the presence of guest 4 delivers [3]rotaxane 1·4(2) by a clipping process.