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.

Functionalised cyclodextrin-based metal-organic frameworks.

CD-MOF, an extended framework incorporating the C8 symmetric cyclic oligosaccharide, γ-cyclodextrin (γ-CD), is based on the coordination of alkali metal cations to alternating α-1,4-linked d-glucopyranosyl units on the primary and secondary faces of the γ-CD tori. Here, we report the synthesis and characterisation of an amino-functionalised CD-MOF that is isostructural with pristine CD-MOF and adsorbs CO2 permanently.

Heterosupramolecular chemistry: programmed pseudorotaxane assembly at the surface of a nanocrystal.

Gold nanocrystals, stabilized by thiols covalently bound to a dibenzo[24]crown-8 moiety, have been programmed to recognize and selectively bind dibenzylammonium cations in solution. This results in a self-organization process at the surface of a nanocrystal with the assembly of a pseudorotaxane (see picture).

An efficient synthesis of cyclodextrin-based carbohydrate cluster compounds.

[formula: see text] The photoaddition of the thiol 2,3,4,6-tetra-O-acetyl-beta-D-1-thioglucopyranose to the allyl ether functions of per-2-allyl-, per-6-allyl-, and per-2,6-diallyl-beta-cyclodextrin derivatives provides a remarkably simple and efficient way for attaching glucopyranose units onto (1) the secondary face, as well as (2) the primary face, of beta-cyclodextrin--not to mention (3) both the primary and secondary faces, simultaneously--in yields of up to 70%.

Dynamic hemicarcerands and hemicarceplexes.

The reversible nature of the imine bond formation in CDCl(3) solution has been exploited to exchange substituted for unsubstituted m-phenylenediamine (MPD) units in hemicarcerand octaimines. Moreover, acid-catalyzed imine exchange has been shown to provide a novel mechanism whereby ferrocene (Fc) can be released as an entrapped guest from the hemicarceplex C(2)B(4)&crcldt;Fc dissolved in CDCl(3) to give the hemicarcerand C(2)B(4) when excess of both MPD and trifluoroacetic acid are present.

Toward interlocked molecules beyond catenanes and rotaxanes.

A linear bis secondary dialkylammonium ion-containing scaffold-based upon an anthracenyl core-has been synthesized. It has been demonstrated that it is possible to dock either one or two dibenzo[24]crown-8 (DB24C8) macrocycles onto this scaffold to afford either a [2]- or [3]pseudorotaxane, respectively. In solution, the association constants for the formation of each of these species has been quantified by employing (1)H NMR spectroscopy, and both species survive in the "gas phase" as evidenced by FAB mass spectrometry. Additionally, the X-ray crystal superstructure of the [3]pseudorotaxane has been determined.

A rotaxane-like complex with controlled-release characteristics.

A rotaxane-like complex, based on a dumbbell-shaped component containing an NH(2)(+) recognition site for a [25]crown-8 ring component and a slippage stopper in the form of a p-(tert-butyl)phenyl group, has been synthesized by a "threading-followed-by-stoppering" approach. The half-life for dissociation of this complex, which is very sensitive to its environment, can be varied from minutes to months by changing the temperature and the polarity of the solvent.

Ammonium ion binding with pyridine-containing crown ethers.

Dipyrido[24]crown-8 (DP24C8) has been synthesized and shown to form [2]pseudorotaxanes spontaneously with dibenzylammonium ions. These complexes, which have been demonstrated by (1)H NMR spectroscopy to form faster in solution than when the macrocyclic polyether is dibenzo[24]crown-8 (DB24C8), are also stronger than their DB24C8 counterparts. One of the [2]pseudorotaxanes has been used to construct a [2]rotaxane (see above) comprising a dumbbell-shaped component based on a dibenzylammonium ion which is encircled by a DP24C8 macrocycle and terminated by (triphenylphosphonium)methyl stoppers.

Artificial molecular-level machines. Dethreading-rethreading of a pseudorotaxane powered exclusively by light energy.

The reversible light-driven dethreading-rethreading of a pseudorotaxane is obtained in solution by exploiting the (E)-(Z) photoisomerisation of azobenzene, and monitored through fluorescence signals.

Dynamic chirality in donor-acceptor pretzelanes.

[Figure: see text]. A series of donor-acceptor pretzelanes has been synthesized, using self-assembly and template-directed protocols, and the dynamic processes that these pretzelanes undergo have been investigated in solution. These compounds exist as libraries of diastereoisomers as a result of their multiple stereoelements, which are dynamically interconverted by several different, in some cases competing, processes. Altering the structure of the pretzelanes changes the rates and mechanisms by which these diastereoisomers equilibrate. Additionally, inserting an element of fixed chirality allows the equilibrium to be biased, while maintaining the barrier to the equilibration processes. These results bode well for the future construction of molecular devices based on switchable diastereoisomerism involving metastable states.

Molecular self-assembly processes.

A logical and stepwise approach to the establishment of the concept of self-assembly in the synthesis of wholly unnatural products is proposed. The approach is based on the use of irreversibly interlocked molecular systems in the shape of catenanes and rotaxanes as the vehicles through which to transfer from host-guest chemistry the knowledge and experience gained on relatively small molecules to much larger molecules including polymers in which the molecular components are reversibly intertwined. The proposal presents a manifesto for making the transformation from supramolecular to polymolecular chemistry. A number of recent template-directed syntheses of catenanes and rotaxanes are presented as examples of structure-directed synthesis, to illustrate that there are inherently simple ways of making apparently complex unnatural products from appropriate substrates without the need for reagent control or catalysis.

Synthesis of cyclodextrin-based carbohydrate clusters by photoaddition reactions.

The syntheses of homogeneous cyclodextrin-based carbohydrate clusters, persubstituted with beta-D-thioglucosyl or D-thiolactosyl residues on either (a) the primary face, (b) the secondary face, or (c) both the primary and the secondary faces of their cyclodextrin tori, are described. The key step in the synthetic methodology, namely the attachment of the carbohydrate residues to the cyclodextrin torus, proceeds in moderate-good yields (42-70%) by the photoaddition of thiol groups, positioned at the anomeric centers of the carbohydrate residues, to allyl ether functions on the cyclodextrins. Facile removal of protecting groups then affords the free cluster compounds. Extensive 1-D and 2-D NMR spectroscopic investigations were performed on these compounds to determine their structures and establish their homogeneities, and a brief computer molecular modeling study allowed estimates of the dimensions of the clusters to be determined.

The magnitude of [C-H...O] hydrogen bonding in molecular and supramolecular assemblies.

Ab initio calculations at the MP2/6-311++G** level on model systems (N-methylpyridinium complexes of dimethyl ether and dimethyl phosphate anion) provide quantitative measures of the large stabilization energies that arise from [C-H...O] contacts in charged systems. These attractive interactions control (i) the self-assembly of bipyridinium-based catenanes and rotaxanes in solution, (ii) the self-organization of left-handed Z-DNA with alternating [dC-dG] sequences in the solid state, and (iii) the binding of pyridinium derivatives with single- and double-stranded DNA. Slightly attractive interactions occur between the donor ether and phosphate moieties and a neutral pyridine molecule in the gas phase. Electrostatic potential and solvation calculations demonstrate that [C-H...O] interactions which involve a cationic [C-H] donor are dominated by electrostatic terms.

Supramolecular daisy chains.

Two series of self-complementary daisy chain monomers, in which a secondary ammonium ion-containing arm is grafted onto a macrocycle with either a [24]- or [25]crown-8 constitution, have been synthesized. In the solid- and 'gas'-phases, the parent [24]crown-8-based monomer forms dimeric superstructures, as revealed by X-ray crystallography and mass spectrometry, respectively. Elucidation of the complicated solution-phase behavior of this compound was facilitated by the synthesis and study of both deuterated, and fluorinated, analogues. These investigations revealed that the cyclic dimeric superstructure also dominates in solution, except when extremes of either concentration (low), temperature (high), or solvent polarity (highly polar, e.g., dimethyl sulfoxide) are employed. Whereas, upon aggregation, the [24]crown-8-based daisy chain monomers have the capacity to form stereoisomeric superstructures further complicating the study of this series of compounds. The assembly of [25]crown-8-based monomers gives only achiral superstructures. The weaker association exhibited between secondary dialkylammonium ions and crown ethers with a [25]crown-8 constitution, however, resulted in limited oligomerization--only dimeric and trimeric superstructures were formed at experimentally attainable concentrations--of [25]crown-8-based daisy chain monomers.

Chromatography of mechanically interlocked molecular compounds.

Catenanes and rotaxanes are molecules composed of mechanically interlocked components which are not linked to each other by covalent bonds. These molecular assemblies behave as discrete molecules with defined properties significantly different from those of the parent "free" components. High-performance liquid chromatography has been employed successfully to characterize some tetracationic catenanes and rotaxanes incorporating either cyclobis(paraquat-p-phenylene) or cyclobis(paraquat-4,4'-biphenylene) as the charged components and either hydroquinone-containing macrocycles or dumbbell-shaped entities as the neutral components. In each case, significant differences in the retention times of the mechanically interlocked molecular compounds, in comparison with those of their components as their "free" forms, were observed.

Switching devices based on interlocked molecules.

An architectural rationale and an experimental program aimed at the development of molecular electronics switching devices for memory and computing applications are discussed. Two-terminal molecular switch tunnel junctions are identified as the critical device components of molecular electronics-based circuitry. They can be tiled in two dimensions and are tolerant of manufacturing defects. Singly and multiply configurable solid-state switching devices that are based upon electrochemically switchable molecular and supramolecular systems are discussed in terms of both the synthesis of the molecular components and the fabrication and performance of the devices.

Dual-mode "co-conformational" switching in catenanes incorporating bipyridinium and dialkylammonium recognition sites.

Three [2]catenanes and three [3]catenanes incorporating one or two pi-electron-rich macrocyclic polyethers and one pi-electron-deficient polycationic cyclophane have been synthesized in yields ranging from 4 to 38%. The pi-electron-rich macrocyclic components possess either two 1,4-dioxybenzene or two 1.5-dioxynaphthalene recognition sites. The pi-electron-deficient cyclophane components incorporate two bipyridinium and either one or two dialkylammonium recognition sites. The template-directed syntheses of these catenanes rely on i) pi...pi stacking interactions between the dioxyarene and bipyridinium recognition sites, ii) C-H...O hydrogen bonds between some of the bipyridinium hydrogen atoms and some of the polyether oxygen atoms, and iii) C-H...pi interactions between some of the dioxyarene hydrogen atoms and the aromatic spacers separating the bipyridinium units. The six catenanes were characterized by mass spectrometry and by both 1H and 13C NMR spectroscopy. The absorption spectra and the electrochemical properties of the catenanes have been investigated and compared with those exhibited by the component macrocycles and by related known catenanes. Broad and weak absorption bands in the visible region, originating from charge-transfer (CT) interactions between electron-donor and electron-acceptor units, have been observed. Such charge-transfer interactions are responsible for the quenching of the potentially fluorescent excited states of the aromatic units of the macrocyclic polyether components. The redox behavior of these novel compounds has been investigated and correlations among the observed redox potentials are illustrated and discussed. The catenanes undergo co-conformational switching upon one-electron reduction of the two bipyridinium units. One of them--in its reduced form--can be also switched by acid/base inputs and exhibits AND logic behavior. The co-conformational rearrangements induced by the redox and acid/base stimulations lend themselves to exploitation in the development of molecular-level machines and logic gates.

Molecular-based electronically switchable tunnel junction devices.

Solid-state tunnel junction devices were fabricated from Langmuir Blodgett molecular monolayers of a bistable [2]catenane, a bistable [2]pseudorotaxane, and a single-station [2]rotaxane. All devices exhibited a (noncapacitive) hysteretic current-voltage response that switched the device between high- and low-conductivity states, although control devices exhibited no such response. Correlations between the structure and solution-phase dynamics of the molecular and supramolecular systems, the crystallographic domain structure of the monolayer film, and the room-temperature device performance characteristics are reported.

Binding studies between tetrathiafulvalene derivatives and cyclobis(paraquat-p-phenylene).

The complexation between a number of different pi-electron donating TTF derivatives and the pi-electron accepting tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) has been studied by (1)H NMR and UV-vis spectroscopy. The results demonstrate that the strength of association between the donors (TTF derivatives) and acceptor (CBPQT(4+)) is strongly dependent on the pi-electron donating properties (measured by the first redox potential ) of the TTF derivatives. However, the first redox potential () is not the only factor of importance. The extended pi-surface of the TTF derivatives also exerts a stabilizing influence upon complexation. The kinetics for the complexation-decomplexation were studied using (1)H NMR spectroscopy and are related to the bulkiness of the TTF derivatives. These effects may serve to improve the design of interlocked molecular systems, especially (bistable) molecular switches, in which CBPQT(4+) and a derivatized TTF unit are incorporated.