Showing papers by "J. Fraser Stoddart published in 2005"
TL;DR: Two switchable, palindromically constituted bistable [3]rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell, supporting the hypothesis that the cumulative nanoscale movements within surface-bound molecular muscles can be harnessed to perform larger-scale mechanical work.
Abstract: Two switchable, palindromically constituted bistable (3)rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell. These designs are reminiscent of a "molecular muscle" for the purposes of amplifying and harnessing molecular mechanical motions. The location of the two cyclobis(paraquat-p-phenylene) (CBPQT 4+ ) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically ( 1 H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale. Fast scan-rate cyclic voltammetry at low temperatures reveals stepwise oxidations and movements of one-half of the (3)rotaxane and then of the other, a process that appears to be concerted at room temperature. The active form of the bistable (3)rotaxane bears disulfide tethers attached covalently to both of the CBPQT4+ ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each coated on one side with a monolayer of 6 billion of the active bistable (3)rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants. The beam bending is correlated with flexing of the surface- bound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound "molecular muscles" can be harnessed to perform larger-scale mechanical work.
599 citations
TL;DR: This Account focuses on the application of multivalency to supramolecular chemistry in particular and the nanosciences in general.
Abstract: Multivalent interactions, which rely upon noncovalent bonds, are essential ingredients in the mediation of biological processes, as well as in the construction of complex (super)structures for materials applications. A fundamental understanding of multivalency in supramolecular chemistry is necessary not only to construct motors and devices on the nanoscale but also to synthesize model systems to provide insight into how biological processes work. This Account focuses on the application of multivalency to supramolecular chemistry in particular and the nanosciences in general.
563 citations
TL;DR: A reversibly operating nanovalve that can be turned on and off by redox chemistry is demonstrated and traps and releases molecules from a maze of nanoscopic passageways in silica by controlling the operation of redox-activated bistable [2]rotaxane molecules tethered to the openings of nanopores leading out of a nanoscale reservoir.
Abstract: In everyday life, a macroscopic valve is a device with a movable control element that regulates the flow of gases or liquids by blocking and opening passageways. Construction of such a device on the nanoscale level requires (i) suitably proportioned movable control elements, (ii) a method for operating them on demand, and (iii) appropriately sized passageways. These three conditions can be fulfilled by attaching organic, mechanically interlocked, linear motor molecules that can be operated under chemical, electrical, or optical stimuli to stable inorganic porous frameworks (i.e., by self-assembling organic machinery on top of an inorganic chassis). In this article, we demonstrate a reversibly operating nanovalve that can be turned on and off by redox chemistry. It traps and releases molecules from a maze of nanoscopic passageways in silica by controlling the operation of redox-activated bistable [2]rotaxane molecules tethered to the openings of nanopores leading out of a nanoscale reservoir.
417 citations
TL;DR: This Account delineates different strategies that can be applied to the formation of molecules possessing this distinctive topology, culminating with two successful syntheses of such compounds, thus cutting the Gordian knot of topological chemistry.
Abstract: The molecular expression of topologically interesting structures represents a formidable challenge for synthetic chemists. The nontrivial link known as the Borromean rings has long been regarded as one of the most ambitious targets in this field. Of ancient provenance, this symbol comprises three interlocked rings in an inseparable union, but cut any one of the rings and the whole assembly unravels into three separate pieces. This Account delineates different strategies that can be applied to the formation of molecules possessing this distinctive topology, culminating with two successful syntheses of such compounds, thus cutting the Gordian knot of topological chemistry.
199 citations
TL;DR: The structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics studies with a force field optimized from DFT calculations are reported and several experiments that validate the predictions are reported.
Abstract: Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT4+) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several...
180 citations
TL;DR: Recent advances in understanding of the nature of the mechanical bond has led to the construction of molecular machines with controllable motions that have, in one instance, been introduced into molecular electronic devices.
Abstract: Mechanically interlocked molecular compounds can be synthesized in high yields by using template-directed assistance to covalent synthesis. Catenanes and rotaxanes are two classes of mechanically interlocked molecules that have been prepared using a variety of methods such as “clipping”,“slipping”, and “threading-followed-by-stoppering”, under both kinetic and thermodynamic regimes. These different methods have utilized a range of templates such as transistion metals, p-donor/p-acceptors, and hydrogen-bonding motifs. Multivalency has emerged as another tool to aid and abet the supramolecularly assisted synthesis of mechanically interlocked molecules. Recent advances in our understanding of the nature of the mechanical bond has led to the construction of molecular machines with controllable motions that have, in one instance, been introduced into molecular electronic devices.
151 citations
TL;DR: Olefin metathesis allows for a magic ring synthesis to occur wherein two free macrocycles can be employed as the stationary materials, leading to the formation of the same [2]catenane.
127 citations
TL;DR: The inherent modularity of the overall process should allow for the rapid and straightforward access to many other analogous mechanically interlocked systems in which either the branched core or the dendritic periphery can be modified to suit the needs of any given application of these molecules.
Abstract: The versatility and efficiency of dynamic covalent chemistry (DCC) has been exploited in the convergent synthesis of mechanically interlocked dendrimers that are based upon the mutual recognition expressed between secondary dialkylammonium ions and crown ether-like macrocycles. Reversible imine bond formation is employed to clip two acyclic fragments, one of them a diformylpyridine unit bearing a dendritic side chain, and the other a complementary dianiline in the shape of the di(o-aminophenyl)ether of tetraethylene glycol, around each arm of a tritopic trisammonium ion core, thereby affording a branched [4]rotaxane. This template-directed strategy has been demonstrated to work in very high yields (>90%) with successive generations (G0−G2) of a modified Frechet-type dendritic wedge attached to the 4-position of the diformylpyridine unit. Reduction of these dynamic dendritic systems is achieved upon treatment with borane·THF and results in kinetically stable compounds. The inherent modularity of the overal...
110 citations
TL;DR: A gradual increase in the fluorescence intensity of the BHEEN unit concomitant with the photocurrent generation, even at a potential (0 V) much lower than that required (-300 mV) for the direct reduction of the CBPQT(4+) unit, confirms that the dethreading process is driven byThe photocurrent generated by the triad-SAM.
Abstract: A tetrathiafulvalene-porphy- rin-fullerene (TTF-P-C60)molecular triad, which generates electrical current by harnessing light energy when self- assembled onto gold electrodes, has been developed. The triad, composed of three unique electroactive compo- nents, namely, 1)an electron-donating TTF unit, 2)a chromophoric porphyrin unit, and 3)an electron-accepting C 60 unit, has been synthesized in a modular fashion. A disulfide-based anchoring group was tagged to the TTF end of the molecule in order to allow its self- assembly on gold surfaces. The surface coverage by the triad in a self-assem- bled monolayer (SAM)was estimated to be 1.4 nm 2 per molecule, a density which is consistent with hexagonal close-packing of the spherical C60 com- ponent (diameter ~ 1 nm). In a closed electronic circuit, a triad-SAM func- tionalized working-electrode generates a switchable photocurrent of ~ 1.5 m Ac m 2 when irradiated with a 413 nm Kr-ion laser, a wavelength which is close to the porphyrin chro- mophore7s absorption maximum peak at 420 nm. The electrical energy gener- ated by the triad at the expense of the light energy is ultimately exploited to drive a supramolecular machine in the form of a (2)pseudorotaxane comprised of a p-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT 4 + )cyclophane and a p-elec- tron-rich 1,5-bis((2-hydroxyethoxy) ethoxy)naphthalene (BHEEN)thread. The redox-induced dethreading of the CBPQT 4 + cyclophane from the BHEEN thread can be monitored by measuring the increase in the fluores- cence intensity of the BHEEN unit. A gradual increase in the fluorescence in- tensity of the BHEEN unit concomi- tant with the photocurrent generation, even at a potential (0 V)much lower than that required (300 mV)for the direct reduction of the CBPQT 4 + unit, confirms that the dethreading process is driven by the photocurrent generat- ed by the triad-SAM.
104 citations
TL;DR: The synthesis and characterization of a [2]rotaxane, 2) a [3]rotxane, 3) a branched [4]rotAXane, 4) a bis [2?]rotaxanes, and 5) a novel cyclic [4 ]rotaxan are presented, demonstrating, in incrementally more complex systems, the efficacy of this one-pot strategy for the construction of interlocked molecules.
Abstract: The template-directed construction of crown-ether-like macrocycles around secondary dialkylammonium ions (R2NH2+) has been utilized for the expedient (one-pot) and high-yielding synthesis of a diverse range of mechanically interlocked molecules. The clipping together of appropriately designed dialdehyde and diamine compounds around R2NH2+-containing dumbbell-shaped components proceeds through the formation, under thermodynamic control, of imine bonds. The reversible nature of this particular reaction confers the benefits of "error-checking" and "proof-reading", which one usually associates with supramolecular chemistry and strict self-assembly processes, upon these wholly molecular systems. Furthermore, these dynamic covalent syntheses exploit the efficient templating effects that the R2NH2+ ions exert on the macrocyclization of the matched dialdehyde and diamine fragments, resulting not only in rapid rates of reaction, but also affording near-quantitative conversion of starting materials into the desired interlocked products. Once assembled, these "dynamic" interlocked compounds can be "fixed" upon reduction of the reversible imine bonds (by using BH3.THF) to give kinetically stable species, a procedure that can be performed in the same reaction vessel as the inital thermodynamically controlled assembly. Isolation and purification of the mechanically interlocked products formed by using this protocol is relatively facile, as no column chromatography is required. Herein, we present the synthesis and characterization of 1) a [2]rotaxane, 2) a [3]rotaxane, 3) a branched [4]rotaxane, 4) a bis [2]rotaxane, and 5) a novel cyclic [4]rotaxane, demonstrating, in incrementally more complex systems, the efficacy of this one-pot strategy for the construction of interlocked molecules.
100 citations
TL;DR: 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.
Abstract: 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.
TL;DR: The necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM, which shows good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems.
Abstract: Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT4+), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT4+@mpTTF) is that in which the CBPQT4+ ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT4+ ring surrounding the DNP (CBPQT4+@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure−area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxa...
TL;DR: The tristable [2]catenane should consist of a CBPQT4+ ring interlocked with a polyether macrocyle containing DNP, TTF, and FBZD units as the tunable RGB color-generating donors.
Abstract: We propose a design for an electrochemically driven RGB dye based on a tristable [2]catenane, in which the color of the molecule can be switched between Red, Green, and Blue by merely changing voltage. Based on DFT calculations, we conclude that the tristable [2]catenane should consist of a CBPQT4+ ring interlocked with a polyether macrocyle containing DNP (red), TTF (green), and FBZD (blue) units as the tunable RGB color-generating donors. Thus, at controllable voltages 0, V1, and V2, the [2]catenane is expected to display green, blue, and red colors, respectively. The advent of these RGB tristable molecules may have potential applications in low cost paperlike electronic displays.
TL;DR: A dynamic coordinative-directed solubilization of single-walled carbon nanotubes (SWNTs) in aqueous solutions has been achieved through a combination of a Zn(II) metalloporphyrin complex and a cis-protected Pd( II) complex, which are believed to form charged acyclic and/or cyclic adducts on or around the side walls of SWNTs.
Abstract: A dynamic coordinative-directed solubilization of single-walled carbon nanotubes (SWNTs) in aqueous solutions has been achieved through a combination of a Zn(II) metalloporphyrin complex and a cis-protected Pd(II) complex, which are believed to form charged acyclic and/or cyclic adducts on or around the side walls of SWNTs. The solubilization of SWNTs in aqueous solution only occurs when these acyclic and/or cyclic complexes are allowed to enter simultaneously into a self-assembly process with SWNTs under mild conditions. The aqueous solubility properties that these dynamic complexes confer upon SWNTs are believed to involve noncovalent bonding interactions between the two entities. They have been probed in solution using ultraviolet and visible absorption spectroscopy and in thin films using high-resolution transmission electron microscopy. The supramolecular electronic effects that the individual components of their acyclic and/or cyclic complexes impart upon a single semiconducting SWNT have been probed within a nanotube field-effect transistor device.
TL;DR: In this article, a molecular shuttle in the shape of an amphiphilic bistable [2]rotaxane has been designed, synthesized, and characterized, which can act as recognition sites for the tetracationic cyclophane cyclobis(paraquat-p-phenylene), to reside around.
TL;DR: A range of covalently linked donor-acceptor compounds which contain a hydroquinone unit, a 1,5-dioxynaphthalene ring system, or a tetrathiafulvalene unit as thepi-donor and cyclobis(paraquat-p-phenylene) (CBPQT(4+)) as the pi-accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines.
Abstract: A range of covalently linked donor-acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5-dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the pi-donor, and 4) cyclobis(paraquat-p-phenylene) (CBPQT(4+)) as the pi-accepting tetracationic cyclophane were prepared and shown to operate as simple molecular machines. The pi-donating arms can be included inside the cyclophane in an intramolecular fashion by virtue of stabilizing noncovalent bonding interactions. What amounts to self-complexing/decomplexing equilibria were shown to be highly temperature dependent when the pi-donating arm contains either an HQ or DNP moiety. The thermodynamic parameters associated with the equilibria have been unraveled by using variable-temperature (1)H NMR spectroscopy. The negative DeltaH degrees and DeltaS degrees values account for the fact that the "uncomplexed" conformation becomes the dominant species, since the entropy gain associated with the decomplexation process overcomes the enthalpy loss resulting from the breaking of the donor-acceptor interactions. The arm's in-and-out movements with respect to the linked cyclophanes can be arrested by installing a bulky substituent at the end of the arm. In the case of compounds carrying a DNP ring system in their side arm, two diastereoisomeric, self-complexing conformations are observed below 272 K in hexadeuterioacetone. By contrast, control over the TTF-containing arm's movement is more or less ineffective through the thermally sensitive equilibrium although it can be realized by chemical and electrochemical ways as a result of TTF's excellent redox properties. Such self-complexing compounds could find applications as thermo- and electroswitches. In addition, the thermochromism associated with the arm's movement could lead to some of the compounds finding uses as imaging and sensing materials.
TL;DR: Borohydride reduction of a Borromean Ring (BR) complex containing six zinc(II) ions and 12 imine bonds has resulted in its demetallation, producing a neutral BR compound and also its free macrocycle, following cleavage of at least one of theImine bonds in the ethanolic reaction mixture.
TL;DR: A template containing two secondary dialkylammonium ion recognition sites for encirclement by olefin-bearing dibenzo[24]crown-8 derivatives has been used to promote Olefin cross metatheses with ruthenium-alkylidene catalysts.
TL;DR: An overview of the field with regard to the solution dynamics and stereochemistry of mechanically-interlocked compounds, as well as to some related structural types, can be found in this paper.
Abstract: The advent of self-assembly and template-directed synthetic protocols has led to a tremendous surge in the number of mechanically-interlocked compounds being prepared and studied. As these investigations are being carriedout, it is becoming increasingly apparent that many of these compounds, known as catenanes and rotaxanes, possess unique dynamic and stereochemical properties. In addition, the drive to create molecular switches and machines for nanotechnological applications has generated a need to understand how to control those properties in condensed phases. Here, we present an overview of the field with regard to the solution dynamics and stereochemistry of mechanically-interlocked compounds - as well as to some related structural types - and review the recent results from our own research in some detail. 1 H NMR spectroscopy has proven to be a powerful tool for probing both degenerate and nondegenerate dynamic processes in these compounds, as well as for identifying stereoisomers if they are present in solution. The results of several variable temperature NMR investigations on the effects of structural changes upon the dynamic processes and stimulated relative motions of components in catenanes and rotaxanes, as well as in some self-complexes and pretzelanes, are discussed. A review with 90 references.
TL;DR: Employing halogen atom labels on one of the ligand precursors, the lability of at least some of the 30 dative and 12 imine bonds stabilizing and constituting the three rings of a metallo-Borromean linked compound are scrambled in acidic methanolic solution.
TL;DR: In this paper, the authors focus on the application of multivalency to supramolecular chemistry in particular and the nanosciences in general, and present an account of how multivalent interactions are essential ingredients in the mediation of biological processes and in the construction of complex (super)structures for materials applications.
Abstract: Multivalent interactions, which rely upon noncovalent bonds, are essential ingredients in the mediation of biological processes, as well as in the construction of complex (super)structures for materials applications. A fundamental understanding of multivalency in supramolecular chemistry is necessary not only to construct motors and devices on the nanoscale but also to synthesize model systems to provide insight into how biological processes work. This Account focuses on the application of multivalency to supramolecular chemistry in particular and the nanosciences in general.
TL;DR: The [2]rotaxane monolayers were found to adopt highly hydrated tilted and/or folded conformations on the water surface largely due to the hydrophilic nature of their tetracationic ring component.
Abstract: Surface sensitive X-ray techniques have been used to elucidate the structures of amphiphilic [2]rotaxane and dumbbell monolayers at the air/water interface. The [2]rotaxanes were found to adopt highly hydrated tilted and/or folded conformations on the water surface largely due to the hydrophilic nature of their tetracationic ring component. This conformation was less pronounced in monolayers of the dumbbell precursors. Increasing the surface pressure resulted in an expansion of [2]rotaxane monolayers in the vertical direction and decreased hydration.
TL;DR: The introduction of a stereogenic center by a stereospecific synthesis into an optically active, donor-acceptor pretzelane, that exhibits helicity as well as fixed chirality, leads to a marked preference for one conformational diastereoisomer over the other in both acetone and dimethylsulfoxide that can be understood from computational models.
TL;DR: The life of a scientific nomad is traced through thick and thin from the Athens of the North to the City of Angels with brief and not so brief interludes on the edge of the Canadian Shield, in the Socialist Republic of South Yorkshire, on the Plains of Cheshire beside the Wirral, and in the Midlands in the heartland of Albion as discussed by the authors.
Abstract: The hurly-burly life of a scientific nomad is traced through thick and thin from the Athens of the North to the City of Angels with brief and not so brief interludes on the edge of the Canadian Shield, in the Socialist Republic of South Yorkshire, on the Plains of Cheshire beside the Wirral, and in the Midlands in the heartland of Albion.
14 Nov 2005
TL;DR: In this paper, the free energies of binding and activation associated with complexation/dissocia- tion processes are deter- mined by the combined strengths of the noncovalent bonds formed by, and by the size complementarity between, the constituents of a complex, as well as by the differential solvation between the complexed and dissociated states.
Abstract: Kinetically stable superarchitectures can be constructed efficiently by relying upon the assistance that mechanical constraints can offer to noncovalent bonding interactions. Thus, the free energies of binding and activation associated with complexation/dissocia- tion processes are deter- mined by the combined strengths of the noncovalent bonds formed by, and by the size complementarity between, the constituents of a complex, as well as by the differential solvation between the complexed and dissociated states. Using this information, kinetically stable complexes can be constructed through the careful design of stereoelectronically-
TL;DR: Two self-complexing compounds based on donor-acceptor interactions, one comprised of a π-electron-deficient cyclo-bis(paraquat-p-phenylene) (CBPQT 4 + ) ring attached to a side-arm component containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) units, have been synthesized as discussed by the authors.
Abstract: Two self-complexing compounds based on donor-acceptor interactions, one comprised of a π-electron-deficient cyclo-bis(paraquat-p-phenylene) (CBPQT 4 + ) ring attached to a side-arm component containing π-electron-rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) units, and the other comprised of a CBPQT 4 + ring carrying two side-arms, one containing a TTF unit and the other a DNP unit have been synthesized. 'H NMR spectroscopy and UV/Vis spectroelectrochemistry have revealed that, while the latter compound behaves as a reconfigurable redox-active molecular switch involving only the side-arm containing the TTF unit in a self-complexing role with the CBPQT 4 + ring (the side-arm containing the DNP unit is essentially a 'spectator'), the former compound behaves as a molecular switch that is reversible and reconfigurable when its starting self-complexing conformation is retained, but becomes only configurable and irreversible when the self-complexing conformation is partially transformed into 'uncomplexed' conformation on oxidation of the TTF unit.
TL;DR: The Borromean topology has been studied extensively in the field of topological chemistry and two successful synthesizations of such compounds have been reported in the last few decades as discussed by the authors.
Abstract: The molecular expression of topologically interesting structures represents a formidable challenge for synthetic chemists. The nontrivial link known as the Borromean rings has long been regarded as one of the most ambitious targets in this field. Of ancient provenance, this symbol comprises three interlocked rings in an inseparable union, but cut any one of the rings and the whole assembly unravels into three separate pieces. This Account delineates different strategies that can be applied to the formation of molecules possessing this distinctive topology, culminating with two successful syntheses of such compounds, thus cutting the Gordian knot of topological chemistry.