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 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.     

Increasing the persistency of stable free-radicals: synthesis and characterization of a nitroxide based [1]rotaxane

A persistent paramagnetic [1]rotaxane based on beta-cyclodextrin showing an increased persistency under reductive conditions has been prepared and characterized for the first time.     

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.     

Molecular switch based on a cucurbit[6]uril containing bistable [3]rotaxane

A bistable CB6-based [3]rotaxane with two recognition sites has been prepared very efficiently in a high yield synthesis through CB6 catalyzed 1,3-dipolar cycloaddition; this rotaxane behaves as a reversible molecular switch and exhibits conformational changes caused by the movement of rings under base, acid and heat stimuli from one location to the other.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Complexes of a [2]rotaxane ligand with terminal terpyridine groups

Permanently interlocked [2]rotaxane ligands can be created by capping a pyridine terminated [2]pseudorotaxane with terpyridine containing stoppers. The robust nature of the resulting [2]rotaxane ligand allows coordination to inert metals such as Ru(II) not possible under standard self-assembly conditions.     

Design of [2]rotaxane through image threshold segmentation of electrostatic potential image

An electrostatic potential (ESP)‐based image segmentation method has been used to estimate the ability of proton donation and acceptance involved in ring‐rod recognition. The relative binding strength of [2]rotaxane has also been further estimated from the difference of the characteristic image‐segmentation derived ESP between proton donor and proton acceptor. The size and electrostatic compatibility criteria are introduced to guide the design of interlocked [2]rotaxane. A library of 75 thermodynamically stable [2]rotaxane candidates has been generated, including 16 experimentally known systems. The theoretical results for 16 experimentally known [2]rotaxanes are in good agreement with both the experimental association constants and density functional theory‐calculated binding energies. Our ESP‐based image segmentation model is also applicable to the tristable [2]rotaxane molecular shuttle as well as [1]rotaxane with self‐inclusion function, indicating this simple method is generic in the field of constructing other supramolecular architectures formed with donor/acceptor molecular recognition. © 2016 Wiley Periodicals, Inc.     

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.     

Organogel formation by a cholesterol-stoppered bistable [2]rotaxane and its dumbbell precursor

The switching properties, gelation behavior, and self-organization of a cholesterol-stoppered bistable [2]rotaxane containing a cyclobis(paraquat-p-phenylene) ring and tetrathiafulvalene/1,5-dioxynaphthalene recognition units situated in the rod portion of the dumbbell component have been investigated by electrochemical, spectroscopic, and microscopic means. The cyclobis(paraquat-p-phenylene) ring in the [2]rotaxane can be switched between the tetrathiafulvalene and 1,5-dioxynaphthalene recognition units by addressing the redox properties of the tetrathiafulvalene unit. The organogels can be prepared by dissolving the [2]rotaxane and its dumbbell precursor in a CH2Cl2/MeOH (3:2) mixed solvent and liquified by adding the oxidant Fe(ClO4)3. Direct evidence for the self-organization was obtained from AFM investigations which have shown that both of the [2]rotaxane and its dumbbell precursor form linear superstructures which we propose are helical in nature.     

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.     

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]rotazane 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.     

Quantitative conformational study of redox-active [2]rotaxanes, part 1: Methodology and application to a model [2]rotaxane

This paper reports a novel methodology for the conformational analysis of [2]rotaxanes. It combines NMR spectroscopic (COSY, NOESY and the recently reported paramagnetic line-broadening and suppression technique) and electrochemical techniques to enable a quantitative analysis of the co-conformations of interlocked molecules and the conformations of their components. This methodology was used to study a model [2]rotaxane in solution. This [2]rotaxane consists of an axle that incorporates an electron-poor, doubly positively charged viologen that threads an electron-rich crown ether. It has been shown that the axle of the [2]rotaxane in its dicationic state adopts a folded conformation in solution and the crown ether is localised at the viologen moiety. Following a one-electron reduction of viologen, the paramagnetic radical cation of the [2]rotaxane retains its folded conformation in solution. The data also demonstrate that in the radical cation the crown ether remains localised at the viologen, despite its reduced affinity for the singly reduced viologen. The combined quantitative NMR spectroscopic and electrochemical characterisation of the electromechanical function of the model [2]rotaxane in solution provides an important reference point for the study of switching in structurally related bistable [2]rotaxanes, which is the subject of the second part of this work.     

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).     

Host-[2]rotaxane: advantage of converging functional groups for guest recognition

A host-[2]rotaxane was constructed by converting a diaminophenylcalix[4]arene into a [2]rotaxane using the DCC-rotaxane method (Zehnder, D.; Smithrud, D. B. Org. Lett. 2001, 16, 2485-2486). N-Ac-Arg groups were attached to the dibenzo-24-crown-8 ring of the rotaxane to provide a convergent functional group. To demonstrate the advantage provided by the rotaxane architecture for recognition of guests that contain a variety of functional groups, association constants (K(A)) for N-Ac-Trp, indole, N-Ac-Gly, fluorescein, 1-(dimethylamino)-5-naphthalenesulfonate, and pyrene bound to the [2]rotaxane were determined by performing (1)H NMR and fluorescence spectroscopic experiments. The host-[2]rotaxane had the highest affinity for fluorescein with a K(A) = 4.6 x 10(6) M(-)(1) in a 98/2 buffer (1 mM phosphate, pH 7)/DMSO solution. A comparison of K(A) values demonstrates that both the aromatic pocket and ring of the host-[2]rotaxane contribute binding free energy for complexation. Association constants were also derived for the same guests bound to the diaminophenylcalix[4]arene and to a diphenylcalix[4]arene that contained arginine residues displayed in a nonconvergent fashion. The host-[2]rotaxane provides higher affinity and specificity for most guests than the host with divergent N-Ac-Arg groups of the one that only has an aromatic pocket. For example, the K(A) for the complex of the host-[2]rotaxane and fluorescein in the DMSO/water mixture is more than 2 orders of magnitude greater than association constants derived for the other hosts.     

Investigation of the intracellular delivery of fluoresceinated peptides by a host-[2]rotaxane

The development of methods to transport peptides into cells via a passive mechanism would greatly aid in the development of therapeutic agents. We recently demonstrated that an impermeable fluoresceinated pentapeptide enters the cytoplasm and nucleus of COS 7 cells in the presence of a host-[2]rotaxane by a mechanism that does not depend on an active cell-mediated process. In this report, we further investigate the ability of the host-[2]rotaxane to deliver peptides possessing a wide range of polarities (negatively charged, positively charged, polar, and apolar side chains) into live cells. Only in the presence of the host-[2]rotaxane were the Fl-peptides taken up by COS 7 and ES2 cells. Flow cytometry experiments demonstrated that the level of delivery is largely temperature and adenosine 5'-triphosphate (ATP) independent, and the membranes remain intact. Although the level of transport does depend upon the nature of the side chains, it does not correlate with calculated LogD values, indicating that an additional interaction with the host-[2]rotaxane is modifying the permeability properties of the peptide. The amount of Fl-peptides transported from an aqueous phase into a chloroform phase in the presence of the host-[2]rotaxane correlates with the intensity of cellular fluorescence. Extraction and U-tube studies show that the Fl-peptide can be released from its complex with the host-[2]rotaxane into an aqueous phase, and the host-[2]rotaxane can transport a greater than a stoichiometric amount of an Fl-peptide through a CHCl3 layer. These studies demonstrate the utility of the host-[2]rotaxane in delivering peptides of all polarities across a cell membrane.     

Tetrathiafulvalene radical cation dimerization in a bistable tripodal [4]rotaxane

The template-directed synthesis of a bistable tripodal [4]rotaxane, which has cyclobis(paraquat-p-phenylene) (CBPQT4+) as the pi-electron-deficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of pi-electron-rich recognition sites located on all three legs of the tripodal dumbbell, is described. The chemical and electrochemical oxidation of the [4]rotaxane and its tripodal dumbbell have allowed us to unravel an unprecedented TTF.+ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF.+]2 and the mixed-valence one [(TTF)2].+, have been observed at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF.+]2 is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [4]rotaxane are encircled by CBPQT4+ rings, which renders the formation of the mixed-valence dimer [(TTF)2].+ highly unfavorable.     

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.     

Synthesis of a [2]rotaxane incorporating a "magic sulfur ring" by the thiol-ene click reaction

The mild and highly efficient thiol-ene click reaction has been used to construct a rotaxane incorporating dibenzo-24-crown-8 (DB24C8) and a dibenzylammonium-derived thread in high yield under the irradiation of UV light. A rotaxane containing a disulfide linkage in the macrocycle was also synthesized by the thiol-ene click reaction. It has been demonstrated that the formation of the [2]rotaxane with the disulfide bond in the macrocycle occurs by a mechanism that is different to the threading-followed-by-stoppering process. The successful construction of a rotaxane directly from its constituent components, the macrocycle containing a disulfide linkage and the dibenzylammonium hexafluorophosphate salt, suggests that the space within the macrocycle incorporating the disulfide linkage is smaller than the phenyl unit and a plausible reaction mechanism has been proposed as follows: A small amount of the initiator forms two radicals upon the absorption of UV irradiation; the radicals act as a "key" to "unlock" the disulfide bond in the macrocycle. The resulting crown ether like moiety in the macrocycle is clipped around the ammonium ion center in the dumb-bell-shaped compound. The [2]rotaxane is generated upon recombination of the disulfide linkage.     

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.