Showing papers by "J. Fraser Stoddart published in 2004"

A Molecular Elevator

Abstract: We report the incrementally staged design, synthesis, characterization, and operation of a molecular machine that behaves like a nanoscale elevator. The operation of this device, which is made of a platformlike component interlocked with a trifurcated riglike component and is only 3.5 nanometers by 2.5 nanometers in size, relies on the integration of several structural and functional molecular subunits. This molecular elevator is considerably more complex and better organized than previously reported artificial molecular machines. It exhibits a clear-cut on-off reversible behavior, and it could develop forces up to around 200 piconewtons.

Molecular borromean rings.

Abstract: The realization of the Borromean link in a wholly synthetic molecular form is reported. The self-assembly of this link, which is topologically achiral, from 18 components by the template-directed formation of 12 imine and 30 dative bonds, associated with the coordination of three interlocked macrocycles, each tetranucleating and decadentate overall, to a total of six zinc(II) ions, is near quantitative. Three macrocycles present diagonally in pairs, six exo -bidentate bipyridyl and six endo -diiminopyridyl ligands to the six zinc(II) ions. The use, in concert, of coordination, supramolecular, and dynamic covalent chemistry allowed the highly efficient construction, by multiple cooperative self-assembly processes, of a nanoscale dodecacation with an approximate diameter of 2.5 nanometers and an inner chamber of volume 250 A3, lined with 12 oxygen atoms.

Whence Molecular Electronics

Abstract: Molecular electronics devices hold great promise for electronics applications. But how have the devices fared so far? In their Perspective, Flood et al ., review recent advances in this field. They conclude that several types of molecular electronics devices, such as molecular rectifiers and molecular switch tunnel junctions, have withstood scientific scrutiny: In these devices, the observed effects are indeed molecular in origin. Future practical devices will most likely consist of hybrid devices that combine molecular with existing electronics.

An Operational Supramolecular Nanovalve

Abstract: A functioning nanomachine in the form of a supramolecular nanovalve that opens and closes the orifices to molecular-sized pores and releases a small number of molecules on demand is reported. The nanovalve, which is used to open and close the nanocontainer, is a pseudorotaxane composed of two componentsa long thread containing a 1,5-dioxnaphthalene donor unit, which is attached to the solid support, and the moving part, the tetracationic cyclophane acceptor/receptor, cyclobis(paraquat-p-phenylene), which controls access to the interior of the nanopore. The nanocontainer is made out of mesoporous silica by using a dip-coating method. Operating the nanovalve involves three steps: (i) filling the container, (ii) closing the valve, and (iii) opening the valve to release the contents of the container on demand. The tubular pores, which are approximately 2 nm wide, are filled with stable luminescent Ir(ppy)3 molecules by allowing them to diffuse into the open pores. The orifices are then closed by pseudorotaxa...

Molecule-Independent Electrical Switching in Pt/Organic Monolayer/Ti Devices

Abstract: Electronic devices comprising a Langmuir−Blodgett molecular monolayer sandwiched between planar platinum and titanium metal electrodes functioned as switches and tunable resistors over a 102−105 Ω range under current or voltage control. Reversible hysteretic switching and resistance tuning was qualitatively similar for three very different molecular species, indicating a generic switching mechanism dominated by electrode properties or electrode/molecule interfaces, rather than molecule-specific behavior.

Meccano on the Nanoscale—A Blueprint for Making Some of the World's Tiniest Machines

Abstract: Molecular compounds—comprised of mechanically interlocked components—such as rotaxanes and catenanes can be designed to display readily controllable internal movements of one component with respect to the other. Since the weak noncovalent bonding interactions that contribute to the template-directed synthesis of such compounds live on between the components thereafter, they can be activated such that the components move in either a linear fashion (rotaxanes) or a rotary manner (catenanes). These molecules can be activated by switching the recognition elements off and on between components chemically, electrically, or optically, such that they perform motions reminiscent of the moving parts in macroscopic machines. This review will highlight how the emergence of the mechanical bond in chemistry during the last two decades has brought with it a real prospect of integrating a bottom-up approach, based on molecular design and micro- and nanofabrication, to construct molecular electronic devices that store information at very high densities using minimal power. Although most of the research reported in this review on switchable catenanes and rotaxanes has been carried out in the context of solution-phase mechanical processes, recent results demonstrate that relative mechanical movements between the components in interlocked molecules can be stimulated (a) chemically in Langmuir and Langmuir–Blodgett films, (b) electrochemically as self-assembled monolayers on gold, and (c) electronically within the settings of solid-state devices. Not only has reversible, electronically driven switching been observed in devices incorporating a bistable [2]catenane, but a crosspoint random access memory circuit has been fabricated using an amphiphilic, bistable [2]rotaxane. The experiments provide strong evidence that switchable catenanes and rotaxanes operate mechanically in a soft-matter environment and can withstand simple device-processing steps. Studies on single-walled carbon nanotubes used as one of the electrodes in molecular switch tunnel junctions have revealed that interfacial chemical interactions involving electrodes containing carbon, silicon, and oxygen are good choices when carrying out molecular electronics on the class of rotaxane- and catenane-based molecules reported in this review. This conclusion is supported by differential conductance measurements (at 4 K) made with single-molecule transistors using the break-junction method. It transpires that the electronic transport properties in such devices are more sensitive to the chemical nature of the molecule–electrode contacts than the details of the molecules' electronic structure away from the contacts. This result has profound implications for molecular electronics and highlights the importance of also considering the molecules and the electrodes as an integrated system. It all adds up to an integrated systems-oriented approach to nanotechnology that finds its inspiration in the transfer of concepts like molecular recognition from the life sciences into materials science and provides a model for how, in principle, to transfer elements of traditional chemistry to technology platforms that are being developed on the nanoscale. Before there can be any serious prospect of a technology, there has to be some good, sound science in the making. Molecular electronics is very much in its infancy and, as such, it can be expected to give rise to a great deal of intellectually stimulating science before it stands half a chance of becoming a viable companion to silicon-based technology.

Molecular‐Mechanical Switch‐Based Solid‐State Electrochromic Devices

Abstract: The dynamics of electrochemically driven, bistable molecular mechanical switches—such as certain nondegenerate, twostation, donor–acceptor [2]catenanes and [2]rotaxanes—have been the subject of numerous experimental investigations in the solution phase, in which the general mechanistic details of the redox-activated switching processes are becoming increasingly well understood. These molecular machines may have many technological applications, although few are likely to be liquid-solution-phase based. Thus, significant effort has been directed towards understanding and exploiting the bistability of [2]catenanes and [2]rotaxanes in other environments, including both Langmuir–Blodgett (LB) and self-assembled monolayers (SAMs), and in solid-state molecular-switch tunnel junctions (MSTJs). Herein we explore, at a fundamental level, the solid-state application of electrochromic devices by taking advantage of the colorimetric changes that accompany the electrochemically driven switching of certain bistable [2]catenanes and [2]rotaxanes. The molecular switches were immobilized within a solid-state polymer electrolyte, and a microfabricated, planar, three-terminal equivalent of a standard electrochemical cell was used for electrical addressing. The polymer environment significantly slows down certain steps within the molecular-mechanical switching cycle, but the overall mechanism remains unchanged from that observed in other environments. We also find that by varying the molecular structure of the switch, the colorimetric retentions times of these devices could be controlled over a dynamic range of 10 to 10 s. The fundamental properties of these devices were quantified through timeand temperature-dependent cyclic voltammetry (CV) measurements. In this way, the kinetic parameters (DG , DH , DS , and Ea) of the rate-limiting step in the switching cycle of the device could be evaluated for several different molecular switches. Four bistable molecular-mechanical systems—two [2]catenanes C1 and C2 and two [2]rotaxanes R1 and R2—along with appropriate control compounds, were investigated (Figure 1) for electrochromic device applica-

A nanomechanical device based on linear molecular motors

Abstract: An array of microcantilever beams, coated with a self-assembled monolayer of bistable, redox-controllable [3]rotaxane molecules, undergoes controllable and reversible bending when it is exposed to chemical oxidants and reductants Conversely, beams that are coated with a redox-active but mechanically inert control compound do not display the same bending A series of control experiments and rational assessments preclude the influence of heat, photothermal effects, and pH variation as potential mechanisms of beam bending Along with a simple calculation from a force balance diagram, these observations support the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform larger-scale mechanical work

Switchable Neutral Bistable Rotaxanes

Abstract: Two switchable neutral bistable [2]rotaxanes have been synthesized, and their chemically induced mechanical switching has been studied in solution by 1H NMR spectroscopy. One of the rotaxanes was prepared by a thermodynamically controlled slippage mechanism, while the other rotaxane was obtained by a dynamic covalent chemistry protocol involving the assembly of its dumbbell component by olefin metathesis. The recognition sites present in the rod section of the dumbbell component, namely, naphthodiimide (NpI) and pyromellitic diimide (PmI) residues, were chosen in the knowledge that the ring component, 1,5-dinaphtho[38]crown-10 (1/5DNP38C10), will bind preferentially to the NpI site. However, upon introduction of Li+ ions into the solution, a 1:2 complex is formed between the PmI site, encircled by the 1/5DNP38C10 ring and two Li+ ions. Since this complex is more stable overall than the binding between the 1/5DNP38C10 ring and the NpI site, the ring component moves from the NpI site to the PmI one. This me...

Controllable donor-acceptor neutral [2]rotaxanes

Abstract: In pursuit of a neutral bi- stable (2)rotaxane made up of two tetra- arylmethane stoppers—both carrying one isopropyl and two tert-butyl groups located at the para positions on each of three of the four aryl rings—known to permit the slippage of the p-electron- donating 1,5-dinaphtho(38)crown-10 (1/5DNP38C10) at the thermodynamic instigation of p-electron-accepting rec- ognition sites, in this case, pyromellitic diimide (PmI) and 1,4,5,8-naphthalene- tetracarboxylate diimide (NpI) units separated from each other along the rod section of the rotaxaneFs dumbbell component, and from the para posi- tions of the fourth aryl group of the two stoppers by pentamethylene chains, a modular approach was em- ployed in the synthesis of the dumb- bell-shaped compound NpPmD, as well as of its two degenerate counterparts, one (PmPmD) which contains two PmI units and the other (NpNpD) which contains two NpI units. The bistable (2)rotaxane NpPmR, as well as its two degenerate analogues PmPmR and NpNpR, were obtained from the corre- sponding dumbbell-shaped compounds NpPmD, PmPmD, and NpNpD and 1/5DNP38C10 by slippage. Dynamic 1 H NMR spectroscopy in CD 2Cl2 revealed that shuttling of the 1/5DNP38C10 ring occurs in NpNpR and PmPmR, with activation barriers of 277 K of 14.0 and 10.9 kcal mol 1 , re- spectively, reflecting a much more pro- nounced donor-acceptor stabilizing in- teraction involving the NpI units over the PmI ones. The photophysical and electrochemical properties of the three neutral (2)rotaxanes and their dumb- bell-shaped precursors have also been investigated in CH2Cl2. Interactions be- tween 1/5DNP38C10 and PmI and NpI units located within the rod section of the dumbbell components of the (2)ro- taxane give rise to the appearance of charge-transfer bands, the energies of which correlate with the electron-ac- cepting properties of the two diimide moieties. Comparison between the po- sitions of the visible absorption bands in the three (2)rotaxanes shows that, in NpPmR, the major translatio- nal isomer is the one in which 1/5DNP38C10 encircles the NpI unit. Correlations of the reduction potentials for all the compounds studied confirm that, in this non-degenerate (2)ro- taxane, one of the translational isomers predominates. Furthermore, after deac- tivation of the NpI unit by one-electron reduction, the 1/5DNP38C10 macrocy- cle moves to the PmI unit. Li + ions have been found to strengthen the in- teraction between the electron-donat- ing crown ether and the electron-ac- cepting diimide units, particularly the PmI one. Titration experiments show that two Li + ions are involved in the strengthening of the donor-acceptor in- teraction. Addition of Li + ions to NpPmR induces the 1/5DNP38C10 macrocycle to move from the NpI to the PmI unit. The Li + -ion-promoted switching of NpPmR in a 4:1 mixture of CD2Cl2 and CD3COCD3 has also been shown by 1 H NMR spectroscopy to involve the mechanical movement of the 1/5DNP38C10 macrocycle from the NpI to the PmI unit, a process that can be reversed by adding an excess of (12)crown-4 to sequester the Li + ions.

The Metastability of an Electrochemically Controlled Nanoscale Machine on Gold Surfaces

Abstract: The advent of supramolecular chemistry has provided chemists with the wherewithal to construct molecule-level machines 3] in an efficient manner using the protocol of template-direction. Synthetically accessible, linear motor molecules come in the guise of bistable [2]rotaxanes in which the ring component can be induced to move relative to the dumbbell-shaped one by altering the redox characteristics of the molecules. Such precisely controllable nanoscale molecular machines and switches have attracted a lot of attention 3] because of their potential to meet the expectations of a visionary and to act as some of the smallest components for the engineering of nanoelectromechanical systems (NEMs) and the fabrication of nanoelectronic devices. Although the redox-switching properties of numerous bistable [2]rotaxanes have been demonstrated in solution, the lack of coherence of the switches in this phase makes it difficult to harness the potential envisaged by Feynman. It is essential that we establish how to self-assemble these tiny switches in an orderly manner at surfaces and to investigate their switching properties in conjunction with their introduction into solid-state devices that have been shown to function as two-dimensional molecular electronic circuits. The fabrication of such devices required the design and synthesis of bistable [2]rotaxanes that are amphiphilic, so that they can be transferred 14±16] as molecular monolayers using the Langmuir ± Blodgett (LB) technique into a device setting. A molecular switch tunnel junction (MSTJ) has been fabricated by sandwiching such self-organized LB monolayers between a bottom Si electrode and a top Ti/Al electrode in a crossbar device architecture. The switch-on (high conductance) and switch-off (low conductance) states of each junction can be addressed respectively upon applying a 2 V or a 2 V bias. The proposed electromechanical switching mechanism (Figure 1) suggests that the ground state, where the cyclobis(paraquat-p-phenylene) (CBPQT , blue) ring initially encircles the tetrathiafulvalene (TTF, green) unit, represents the switch-off state. When a 2 V bias is applied, the CBPQT ring moves mechanically to the 1,5-dioxynaphthalene (DNP, red) ring system as a result of oxidation of the TTF unit to its radical cation. Although, when the bias is removed, neutrality is restored to the TTF unit, the CBPQT ring continues to reside on the DNP ring system, forming the metastable state. The observation of a switch-on state can be attributed to this slow-decaying metastable state that can be erased by applying a 2 V bias for a fleeting moment during the switching cycle. Since the mechanical motion associated with this decay is an activated process, these devices exhibit a hysteretic current ± voltage response. Herein, we describe how we have utilized a custom-designed variable temperature (VT) electrochemical apparatus to investigate the redox-switching behavior of an Au surface-confined linear motor-molecule, that is, a disulfide-tethered bistable [2]rotaxane SSR ¥ 4PF6, together with the corresponding dumbbell-shaped control compound SSD. In both cases, the appended disulfide function is used to immobilize the redox-active [2]rotaxane and dumbbell control onto gold surfaces as selfassembled monolayers (SAMs). The [2]rotaxane SSR ¥ 4PF6 was obtained (Figure 2) by a template-directed protocol wherein a CBPQT ring was clipped around the TTF unit of the dumbbellshaped precursor SSD. Here, we report i) the results of a semiquantitative electrochemical investigation carried out on the surface-confined SSR and the control (SSD) at room temperature in MeCN, leading to the identification of a [28] W. L. Jorgensen, J. Tirado-Rives, J. Am. Chem. Soc. 1988, 110, 1657. 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Profeta, P. Weiner, J. Am. Chem. Soc. 1984, 106, 765. [32] D. Fincham, D. Heyes, Adv. Chem. Phys. 1985, 63, 493. [33] M. Parrinello, A. Rahman, J. App. Phys. 1981, 52, 7182. [34] T. Darden, D. York, L. Pedersen, J. Chem. Phys. 1993, 98, 10089. [35] C. Zannoni, in The Molecular Physics of Liquid Crystals (Eds. : G. R. Luckhurst, G. W. Gray), Academic Press, London 1979, pp. 51 ± 83. [36] I. Haller, Prog. Solid State Chem. 1975, 10, 103. [37] K. Toyne, in Thermotropic Liquid Crystals (Ed. : G. W. Gray), Wiley, London, 28 ±63. [38] M. Hird, in Physical Properties of Liquid Crystals, Vol. 1: Nematics, (Eds. : D. A. Dunmur, A. Fukuda, G. R. Luckhurst), EMIS, IEE, London 2000, pp. 3 ± 16. [39] A. Ferrarini, P. L. Nordio, J. Chem. Soc. Perkin Trans. 1998, 2, 456. [40] M. E. Tuckerman, B. Berne, G. Martyna, J. Chem. Phys. 1992, 97, 1990.

A self-assembled multivalent pseudopolyrotaxane for binding galectin-1.

Abstract: A self-assembled pseudopolyrotaxane consisting of lactoside-displaying cyclodextrin (CD) "beads" threaded onto a linear polyviologen "string" was investigated for its ability to inhibit galectin-1-mediated T-cell agglutination. The CDs of the pseudopolyrotaxane are able to spin around the axis of the polymer chain as well as to move back and forth along its backbone to alter the presentation of its ligand. This supramolecular superstructure incorporates all the advantages of polymeric structures, such as the ability to span large distances, along with a distinctively dynamic presentation of its lactoside ligands to afford a neoglycoconjugate that can adjust to the relative stereochemistries of the lectin's binding sites. The pseudopolyrotaxane exhibited a valency-corrected 10-fold enhancement over native lactose in the agglutination assay, which was greater than the enhancements observed for lactoside-bearing trivalent glycoclusters and a lactoside-bearing chitosan polymer tested using the same assay. The experimental results indicate that supramolecular architectures, such as the pseudopolyrotaxane, provide tools for investigating protein-carbohydrate interactions.

The Role of Physical Environment on Molecular Electromechanical Switching

Abstract: The influences of different physical environments on the thermo- dynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bi- stable rotaxanes have been investigated systematically. The two bistable cate- nanes are comprised of a cyclobis(para- quat-p-phenylene) (CBPQT 4 + ) ring, or its diazapyrenium-containing analogue, that are interlocked with a macrocyclic polyether component that incorporates the strong tetrathiafulvalene (TTF) donor unit and the weaker 1,5-dioxy- naphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT 4 + ring, interlocked with a dumbbell component in which one in- corporates TTF and DNP units, where- as the other incorporates a monopyr- rolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic vol- tammogram (10-1500 mV s � 1 ) per- formed on all of the bistable switches (~ 1m m) in MeCN electrolyte solutions (0.1 m tetrabutylammonium hexafluoro- phosphate) across a range of tempera- tures (258-303 K) were recorded in a temperature-controlled electrochemical cell. The second cycle showed different intensities of the two features that were observed in the first cycle when the cyclic voltammetry was recorded at fast scan rates and low temperatures. The first oxidation peak increases in in- tensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systemati- cally with scan rate and temperature and has been assigned to the molecular mechanical movements within the cate- nanes and rotaxanes of the CBPQT 4 + ring from the DNP to the TTF unit. The intensities of each peak were as- signed to the populations of each co- conformation, and the scan-rate varia- tion of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT 4 + ring. The Gibbs free energy of activation at 298 K for the thermally activated movement was calculated to be 16.2 kcal mol � 1 for the rotaxane, and 16.7 and 19.2 kcal mol � 1 for the bipyri- dinium- and diazapyrenium-based bi- stable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational mo- tions of degenerate rotaxanes and cate- nanes, respectively, indicating that the detailed chemical structure influences the rates of movement. In all cases, when the same bistable compounds were characterized in an electrolyte gel, the molecular mechanical motion slowed down significantly, concomitant with an increase in the activation barri- ers by more than 2 kcal mol � 1 . Irrespec- tive of the environment—solution, self- assembled monolayer or solid-state polymer gel—and of the molecular structure—rotaxane or catenane—a single and generic switching mecha- nism is observed for all bistable mole- cules.

Redox-Controllable Amphiphilic [2]Rotaxanes

Abstract: With the fabrication of molecular electronic devices (MEDs) and the construction of nanoelectromechanical systems (NEMSs) as incentives, two constitutionally isomeric, redox-controllable [2]rotaxanes have been synthesizedand characterized in solution. Therein, they both behave as near-perfect molecular switches, that is, to all intents and purposes, these two rotaxanes can be switched precisely by applying appropriate redox stimuli between two distinct chemomechanical states. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by i) two π-electron rich recognition sites-a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety-with ii) a rigid terphenylene spacer placed between the two recognition sites, and then terminated by iii) a hydrophobic tetraarylmethane stopper at one end and a hydrophilic dendritic stopper at the other end of the dumbbells, thus conferring amphiphilicity upon these molecules. A template-directed protocol produces a means to introduce the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT 4 + ), which contains two π-electron accepting bipyridinium units, mechanically interlocked around the dumbbell-shaped components. Both the TTF unit and the DNP moiety are potential stations for CBPQT 4 + , since they can establish charge-transfer and hydrogen bonding interactions with the bipyridinium units of the cyclophane, thereby introducing bistability into the [2]rotaxanes. In both constitutional isomers, 'H NMR and absorption spectroscopies, together with electrochemical investigations, reveal that the CBPQT 4 + ring is predominantly located on the TTF unit, leading to the existence of a single translational isomer (co-conformation) in both cases. In addition, a model [2]rotaxane, incorporating hydrophobic tetraarylmethane stoppers at both ends of its dumbbell-shaped component, has also been synthesized as a point of reference. Molecular synthetic approaches were used to construct convergently the dumbbell-shaped compounds by assembling progressively smaller building blocks in the shape of the rigid spacer, the TTF unit and the DNP moiety, and the hydrophobic and hydrophilic stoppers. The two amphiphilic bistable [2]rotaxanes are constitutional isomers in the sense that, in one constitution, the TTF unit is adjacent to the hydrophobic stopper, whereas in the other, it is next to the hydrophilic stopper. All three bistable [2]rotaxanes have been isolated as green solids. Electrospray and fast atom bombardment mass spectra support the gross structural assignments given to all three of these mechanically interlocked compounds. Their photophysical and electrochemical properties have been investigated in acetonitrile. The results obtained from these investigations confirm that, in all three [2]rotaxanes, i) the CBPQT 4 + cyclophane encircles the TTF unit, ii) the CBPQT 4 + cyclophane shuttles between the TTF and DNP stations upon electrochemical or chemical oxidation/reduction of the TTF unit, and iii) folded conformations are present in which the CBPQT 4 + cyclophane, while encircling the TTF unit, interacts through its π-accepting bipyridinium exteriors with other π-donating components of the dumbbells, especially those located within the stoppers.

Thermally and electrochemically controllable self-complexing molecular switches

Abstract: Self-complexing molecular systems are obtained when an arm component (a π-donor) is covalently linked to a preformed macrocycle (a π-acceptor). The resulting self-complexing compounds are not only ...

Mechanical Shuttling of Linear Motor-Molecules in Condensed Phases on Solid Substrates

Abstract: From analyses of pressure-area isotherms and X-ray photoelectron spectra, we have demonstrated that redox-controllable molecular shuttles, in the shape of amphiphilic, bistable rotaxanes, are mechanically switchable in closely packed Langmuir films with chemical reagents. Additionally, mechanical switching has been proven to occur in closely packed Langmuir -Blodgett bilayers while mounted on solid substrates. The results not only constitute a proof of principle but they also provide the impetus to develop solid-state nanoelectromechanical systems that have the potential to reach up to the mesoscale.

Molecular shuttles based on tetrathiafulvalene units and 1,5-dioxynaphthalene ring systems.

Abstract: Six different degenerate [2]rotaxanes were synthesized and characterized. The rotaxanes contained either two tetrathiafulvalene (TTF) units or two 1,5-dioxynaphthalene (DNP) ring systems, both of which serve as recognition sites for a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. Three different spacer units were incorporated into the dumbbell components of the [2]rotaxanes between the recognition sites. They include a polyether chain, a terphenyl unit, and a diphenyl ether linker, all of which were investigated in order to probe the effect of the spacers on the rate of the shuttling process. Data from dynamic 1H NMR spectroscopy revealed a relatively small difference in the DeltaG++ values for the shuttling process in the [2]rotaxanes containing the three different spacers, in contrast to a large difference between the TTF-containing rotaxanes (18 kcal mol(-1)) and the DNP-containing rotaxanes (15 kcal mol(-1)). This 3 kcal mol(-1) difference is predominantly a result of a ground-state effect, reflecting the much stronger binding of TTF units to the CBPQT4+ ring in comparison with DNP ring systems. An examination of the enthalpic (DeltaH++) and entropic (DeltaS++) components for the shuttling process in the DNP-containing rotaxanes revealed significant differences between the three spacers, a property which could be important in designing new molecules for incorporation into molecular electronic and nanoelectromechanical (NEMs) devices.

A Mechanically Interlocked Bundle

Abstract: The prototype of an artificial molecular machine consisting of a tris- ammonium tricationic component in- terlocked with a tris(crown ether) com- ponent to form a molecular bundle with averaged C3v symmetry has been designed and synthesized.The system is based on noncovalent interactions, which include 1) N + H¥¥¥O hydrogen bonds; 2) CH¥¥¥O interactions be- tween the CH2NH2 + CH2 protons on three dibenzylammonium-ion-contain- ing arms, which are attached symmetri- cally to a benzenoid core, and three di- benzo(24)crown-8 macrorings fused onto a triphenylene core; and 3) p¥¥¥p stacking interactions between the aro- matic cores.The template-directed syn- thesis of the mechanically interlocked, triply threaded bundle involves post- assembly covalent modification, that is, the efficient conversion of three azide functions at the ends of the arms of the bound and threaded trication into bulky triazole stoppers, after 1,3-dipo- lar cycloaddition with di-tert-butylacet- ylenedicarboxylate to the extremely strong 1:1 adduct that is formed in di- chloromethane/acetonitrile (3:2), on account of a cluster effect associated with the paucivalent adduct.Evidence for the averaged C3v symmetry of the molecular bundle comes from absorp- tion and luminescence data, as well as from electrochemical experiments, 1 H NMR spectroscopy, and mass spec- trometry.The photophysical properties of the mechanically interlocked bundle are very similar to those of the super- bundle that precedes the formation of the bundle in the process of supra- molecular assistance to covalent syn- thesis.Although weak non-nucleophilic bases (e.g., nBu3N and iPr2NEt) fail to deprotonate the bundle, the strong tBuOK does, as indicated by both lumi- nescence and 1 H NMR spectroscopy. While deprotonation undoubtedly loos- ens up the interlocked structure of the molecular bundle by replacing relative- ly strong N + H¥¥¥O hydrogen bonds by much weaker NH¥¥¥O ones, the p¥¥¥p stacking interactions ensure that any structural changes are inconsequential, particularly when the temperature of the solution of the neutral molecular bundle in dichloromethane is cooled down to considerably below room tem- perature.

Can Multivalency Be Expressed Kinetically? The Answer Is Yes

Abstract: Inspired by the concept of multivalency and in pursuit of ever more intricate artificial molecular machines, we investigated the strict self-assembly of a triply threaded two-component superbundle, starting from a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core and a trifurcated trication wherein three bipyridinium units are linked 1,3,5 to a central benzenoid core. The result of the investigation was quite unexpected and surprising. It transpired that the rapid formation of a doubly threaded two-component complex was followed by an extremely slow conversion (a week at 253 K in CD3COCD3 to reach equilibrium) of this kinetically controlled product into a thermodynamically controlled one, namely a triply threaded two-component superbundle. This intriguing observation begs the question: are there instances in nature where multivalency is expressed as a kinetically controlled process, prior to an equilibrium state being reached, and if so, what are the biologic...

Threading/dethreading processes in pseudorotaxanes. A thermodynamic and kinetic study

Abstract: Pseudorotaxanes are host–guest complexes made of a thread-like component penetrating a bead-like component. We have investigated, by a stopped-flow technique, the threading/dethreading kinetics of pseudorotaxanes formed between a tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and thread-like compounds consisting of a naphthalene ring system with appended –(OCH2CH2)nOH groups (n from 0 to 3) on the 1 and 5 positions. The association and rate constants have been measured in MeCN (from 276 to 293 K) and H2O (from 276 to 313 K). In MeCN solution, the association constant increases with increasing length of the appended chain (e.g., Kass = 1.0 × 103 M−1 for n = 0 and Kass = 4.0 × 104 M−1 for n = 3, at 293 K); in H2O, the association constants are larger than in MeCN, but almost independent of the chain length. Both the threading (kt) and dethreading (kd) rate constants are larger in MeCN than in H2O and decrease with increasing chain length (e.g., kt > 108 and kt = 4 × 106 M−1 s−1 for n = 0 and n = 3, respectively, in H2O at 276 K). Thermodynamic and kinetic parameters have been obtained and the role played by the length of the chains appended to the naphthalene ring system is discussed.

Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes.

Abstract: A series of amphiphilic bistable [2]rotaxanesin which a ring-shaped component, the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), has been assembled around two recognition sites, a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system, situated apart at different strategic locations within the central polyether section of an amphiphilic dumbbell component that is terminated by a hydrophobic tetraarylmethane-based stopper (near the TTF unit) at one end and by a hydrophilic tetraarylmethane-based stopper (near the DNP ring system) at the other endhas been designed and synthesized. The effects of systematic changes in the constitutions of the three ethylene glycol tails (diethylene or tetraethylene glycol) and end groups (hydroxyl or methoxyl functions) attached to the hydrophilic stoppers on Langmuir film balance and surface rheology experiments at 20 °C were examined to determine the monolayer stabilities and co-conformations of the [2]rotaxanes and their free dumbbell count...

Polyvalent scaffolds. Counting the number of seats available for eosin guest molecules in viologen-based host dendrimers.

Abstract: We have prepared and investigated two dendrimers based on a 1,3,5-trisubstituted benzenoid-type core, containing 9 and 21 viologen units in their branches, respectively, and terminated with tetraarylmethane derivatives. We have shown that, in dichloromethane solution, such highly charged cationic species give rise to strong host−guest complexes with the dianionic form of the red dye eosin. Upon complexation, the absorption spectrum of eosin becomes broader and is slightly displaced toward lower energies, whereas the strong fluorescence of eosin is completely quenched. Titration experiments based on fluorescence measurements have shown that each viologen unit in the dendrimers becomes associated with an eosin molecule, so that the number of positions (“seats”) available for the guest molecules in the hosting dendrimer is clearly established, e.g., 21 for the larger of the two dendrimers. The host−guest interaction can be destroyed by addition of chloride ions, a procedure which permits eosin to escape from...

Powering a Supramolecular Machine with a Photoactive Molecular Triad

Abstract: ions across liposomal membranes by mimicking the photo-syntheticenergytransductionmechanism.Hereinthiscom-munication, we show, in a wholly artificial system, that aphotocurrent is gated by light, the wavelength of which isspecifically tuned to the maximum absorption of the light-absorbingporphyrinchromophoreofamoleculartriad(Fig-ure1a). This nanoscale power supply is used subsequentlytodriveanartificialsupramolecularmachineintheformofapseudorotaxane.Donor–chromophore–fullerene molecular triads, wherethe donor and chromophore have been widely varied, areone of the most successful approaches to mimicking

More on molecular electronics.

Abstract: Two recent news stories by Robert Service (“Next-generation technology hits an early midlife crisis,” 24 Oct., News Focus, p. 556; “Nanodevices make fresh strides toward reality,” 21 Nov., News of the Week, p. 1310) provide the impression that the field of molecular electronics stalled and then suddenly revived. The specific contributions described in the 21 Nov. news story are just two of a large number of breakthroughs that have been reported by many research groups over the past several years. Progress in the field has been continuous and is accelerating dramatically.

Complete Charge Pooling is Prevented in Viologen-Based Dendrimers by Self-Protection

Abstract: We have investigated the electrochemical behavior, and chemical and photosensitized reflection of two dendrimers based on a 1,3,5-trisubstituted benzenoid core which contain 9 and 21 4,4'-bipyridinium (usually called viologen) units, respectively, in their branches and are terminated with tetraarylmethane groups. For comparison purposes, the behavior of reference compounds that contain a single viologen unit have also been investigated. We has found that only part of the viologen units can be reduced in the dendrimer species. For the larger dendrim er, the number of reducible viologens (out of the 21 present) is 14 in electro-chemical experiments (in MeCN), 9 on reduction with bis(benzene)chromium (in MeCN), and 13 by photoinduced electron transfer with 9-methylanthracene as a photosensitizer and triethanolamine as a sacrificial reductant in CH 2 Cl 2 . The reduced viologen units undergo partial dimerization. The photo-chemical experiments have shown that only monomeric, one-electron-reduced viologen units arc formed at the beginning of the irradiation, followed by dimer formation, until a photostationary state is reached that contains 40% nonreduced, 33% monomeric reduced. and 27% reduced units associated in the dimeric form. The results suggest that, upon reduction of a fraction of the viologen units, the dendrimer structure shrinks, with the result that the bulky terminal groups protect other viologen units from being reduced.

Polyvalent interactions in unnatural recognition processes.

Abstract: The synthesis of two cluster compounds, one containing six secondary dialkylammonium ion centers and the other possessing six benzo-m-phenylene[25]crown-8 (BMP25C8) macrocycles, both appended to hexakis(thiophenyl)benzene cores, is described. The binding of these clusters with complementary mono- and divalent ligands is investigated with NMR spectroscopy to probe polyvalency in these unnatural recognition systems. The ability of the two different families of clusters to bind complementary monovalent ligands is compared with that of the monovalent receptor pair, namely the dibenzylammonium ion and BMP25C8. This comparison is made possible by determining an average association constant (KAVE) for the binding of each recognition site on the cluster with the corresponding monovalent ligand. We have found that the clustering of recognition sites together in one molecule is detrimental to their individual abilities to bind monovalent ligands. In the case of the polyvalent interaction between the hexakisBMP25C8 ...