Degenerate [2]rotaxanes with electrostatic barriers

A synthetic approach to the preparation of [2]rotaxanes (1-5·6PF(6)) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT(4+)) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5·6PF(6), there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT(4+) ring in the [2]rotaxanes. Thus, the CBPQT(4+) rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the (1)H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature (1)H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF(6) and 2·6PF(6) indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT(2(˙+)) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF(6) and 2·6PF(6) can shuttle along the recognition sites--two DNP units and one-electron reduced bipyridinium radical cation--under redox control.     

Linear artificial molecular muscles

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 CBPQT(4+) 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.     

A multistate switchable [3]rotacatenane

Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions-namely, the mixed-valence state (TTF(2) )(+.) and the radical-cation dimer state (TTF(+.) )(2) -under ambient conditions. A template-directed approach, based on donor-acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF(+.) )(2) has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF(+.) )(2) . The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.     

Ground-state equilibrium thermodynamics and switching kinetics of bistable [2]rotaxanes switched in solution, polymer gels, and molecular electronic devices

We report on the kinetics and ground-state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable [2]rotaxanes in acetonitrile solution, polymer electrolyte gels, and molecular-switch tunnel junctions (MSTJs). For all rotaxanes a pi-electron-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF4+ and RTTF4+) contain tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT4+ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF4+), the TTF unit is replaced by a pi-extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT4+ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT4+ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT4+ ring to the DNP site. Reduction back to TTF0 (BPTTF0) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical-switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground-state equilibrium properties of all three bistable [2]rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.     

Mechanism of oxidative shuttling for [2]rotaxane in a Stoddart-Heath molecular switch: density functional theory study with continuum-solvation model

The central component of the programmable molecular switch demonstrated recently by Stoddart and Heath is [2]rotaxane, which consists of a cyclobis-(paraquat-p-phenylene) ring-shaped shuttle [(CBPQT(4+))(PF(6)(-))(4)] encircling a finger and moving between two stations on the finger: tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). We report here a quantum mechanics (QM) study of the mechanism by which movement of the ring (and in turn the on-off switching) is controlled by the oxidation-reduction process. We use B3LYP density functional theory to describe how oxidation of the [2]rotaxane components (in using Poisson-Boltzmann continuum-solvation theory for acetonitrile solution) induces the motions associated with switching (translation of the ring). These calculations support the proposal that oxidation occurs on TTF, leading to repulsion between two positive charge centers (TTF(2+) and CBPQT(4+)) that drives the CBPQT(4+) ring from the TTF(2+) station toward the neutral DNP station. The theory also supports the experimental observation that the first and second oxidation potentials are nearly the same (separated by 0.09 eV in the QM). This excellent agreement between the QM and experiment suggests that QM can be useful in designing new systems.     

Room-temperature and solution-state observation of the mixed-valence cation radical dimer of tetrathiafulvalene, [(TTF)2]+*, within a self-assembled cage

A mixed-valence state of the cation radical tetrathiafulvalene dimer, [(TTF)2]+*, is generated by the electrochemical oxidation of a stacked TTF dimer accommodated within an organic-pillared coordination cage. This mixed-valence species is remarkably stable (t1/2 = approximately 1 day at room temperature in aqueous solution under air) and clearly characterized by cyclic voltammogram and electronic absorption spectroscopy.     

A redox-driven multicomponent molecular shuttle

A multicomponent [2]rotaxane designed to operate as a molecular shuttle driven by light energy has been constructed, and its properties have been investigated. The system is composed of (1) a light-fueled power station, capable of using the photon energy to create a charge-separated state, and (2) a mechanical switch, capable of utilizing such a photochemically generated driving force to bring about controllable molecular shuttling motions. The light-fueled power station is, in turn, a dyad comprising (i) a pi-electron-accepting fullerene (C60) component and (ii) a light-harvesting porphyrin (P) unit which acts as an electron donor in the excited state. The mechanical switch is a redox-active bistable [2]rotaxane moiety that consists of (i) a tetrathiafulvalene (TTF) unit as an efficient pi-electron-donor station, (ii) a dioxynaphthalene (DNP) unit as a second pi-electron-rich station, and (iii) a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) pi-electron-acceptor cyclophane, which encapsulates the better pi-electron-donating TTF station. Diethylene glycol spacers were conveniently introduced between the electroactive components in the dumbbell-shaped thread to facilitate the template-directed synthesis of the [2]rotaxane. A modular synthetic approach was undertaken for the overall synthesis of this multicomponent bistable [2]rotaxane, beginning with the syntheses of the P-C60 dyad unit and the two-station TTF-DNP-based [2]rotaxane separately, using conventional synthetic methodologies. These two components were finally stitched together by an esterification to afford the target rotaxane. Its structure was characterized by 1H NMR spectroscopy and mass spectrometry as well as by UV-vis-NIR absorption spectroscopy and voltammetry. The observations reflect remarkable electronic interactions between the various units, pointing to the existence of folded conformations in solution. The redox-driven shuttling process of the CBPQT4+ ring between the two competitive electron-rich recognition units, namely, TTF and DNP, was investigated by electrochemistry and spectroelectrochemistry as a means to verify its operational behavior prior to the photophysical studies related to light-driven operation. The oxidation process of the TTF unit is dramatically hampered in the rotaxane, thereby reducing the efficiency of the shuttling motion. These results confirm that, as the structural complexity increases, the overall function of the system no longer depends simply on its "primary" structure but also on higher-level effects which are reminiscent of the secondary and tertiary structures of biomolecules.     

Density functional theory studies of the [2]rotaxane component of the Stoddart-heath molecular switch

The central component of the programmable molecular switch recently demonstrated by Stoddart and Heath is [2]rotaxane, which consists of a cyclobis(paraquat-p-phenylene) shuttle (CBPQT(4+))(PF(6)(-))(4) (the ring) encircling a finger and moving between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). As a step toward understanding the mechanism of this switch, we report here its electronic structure using two flavors of density functional theory (DFT): B3LYP/6-31G and PBE/6-31G. We find that the electronic structure of composite [2]rotaxane can be constructed reasonably well from its parts by combining the states of separate stations (TTF and DNP) with or without the (CBPQT)(PF(6))(4) shuttle around them. That is, the "CBPQT@TTF" state, (TTF)(CBPQT)(PF(6))(4)-(DNP), is described well as a combination of the (TTF)(CBPQT)(PF(6))(4) complex and free DNP, and the "CBPQT@DNP" state, (TTF)-(DNP)(CBPQT)(PF(6))(4), is described well as a combination of free TTF and the (DNP)(CBPQT)(PF(6))(4) complex. This allows an aufbau or a "bottom-up" approach to predict the complicated [n]rotaxanes in terms of their components. This should be useful in designing new components to lead to improved properties of the switches. A critical function of the (CBPQT(4+))(PF(6)(-))(4) shuttle in switching is that it induces a downshift of the frontier orbital energy levels of the station it is on (TTF or DNP). This occurs because of the net positive electrostatic potential exerted by the CBPQT(4+) ring, which is located closer to the active station than the four PF(6)(-)'s. This downshift alters the relative position of energy levels between TTF and DNP, which in turn alters the electron tunneling rate between them, even when the shuttle is not involved directly in the actual tunneling process. Based on this switching mechanism, the "CBPQT@TTF" state is expected to be a better conductor since it has better aligned levels between the two stations. A second potential role of the (CBPQT(4+))(PF(6)(-))(4) shuttle in switching is to provide low-lying LUMO levels. If the shuttle is involved in the actual tunneling process, the reduced HOMO-LUMO gap (from 3.6 eV for the isolated finger to 1.1 eV for "CBPQT@TTF" or to 0.6 eV for "CBPQT@DNP" using B3LYP) would significantly facilitate the electron tunneling through the system. This might occur in a folded conformation where a direct contact between free station and the shuttle on the other station is possible. When this becomes the main switching mechanism, we expect the "CBPQT@DNP" state to become a better conductor because its HOMO-LUMO gap is smaller and because its HOMO and LUMO are localized at different stations (HOMO exclusively at TTF and LUMO at CBPQT encircling DNP) so that the HOMO-to-LUMO tunneling would be through the entire molecule of [2]rotaxane. Thus an essential element in designing these switches is to determine the configuration of the molecules (e.g., through self-assembled monolayers or incorporation of conformation stabilizing units).     

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.     

Redox-controllable amphiphilic [2]rotaxanes

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 synthesized and 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 pi-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 pi-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, (1)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 pi-accepting bipyridinium exteriors with other pi-donating components of the dumbbells, especially those located within the stoppers.     

Quantum Mechanical and Experimental Validation that Cyclobis(paraquat‐p‐phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of π‐electron‐rich guests by the π‐electron‐deficient host, cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge‐transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT⁴⁺, is an emerald‐green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas‐phase calculations that reinterpreted this CT band in terms of an intermolecular side‐on interaction of TTF with one of the bipyridinium (BIPY²⁺) units of CBPQT⁴⁺, rather than the encapsulation of TTF inside the cavity of CBPQT⁴⁺. We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT⁴⁺ arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY²⁺ units of a CBPQT⁴⁺ ring residing on a separate [2]rotaxane in a side‐on fashion. This [2]rotaxane has similar UV/Vis and ¹H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT⁴⁺. The [2]rotaxane exists as an equimolar mixture of cis‐ and trans‐isomers associated with the disubstituted TTF unit in its dumbbell component. Solid‐state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT⁴⁺.     

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.     

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

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.     

Weak ferromagnetism and strong spin-spin interaction mediated by the mixed-valence ethynyltetrathiafulvalene-type ligand

A new chromium complex with ethynyltetrathiafulvalene (TTF)-type ligands, [CrCyclam(C≡C-5-methyl-4'5'-ethylenedithio-TTF)(2)]OTf ([1]OTf), was synthesized. The cyclic voltammetry of the complex shows two reversible oxidation waves owing to the first and second oxidation of the TTF unit. The electrochemical oxidation of [1]OTf in a Bu(4)NClO(4) or Bu(4)NBF(4) solution of a 1:1 acetonitrile-chlorobenzene mixture gave isostructural crystals of [1][ClO(4)](2)(PhCl)(2)(MeCN) and [1][BF(4)](2)(PhCl)(2)(MeCN), where two mixed-valence TTF units of adjacent complexes form a dimer radical cation. The crystal structures are characterized by an alternating chain of S = 3/2 Cr(3+)Cyclam units and S = ? (TTF)(2)(+) dimers. These two paramagnetic components are connected directly by an ethynyl group, resulting in a strong intrachain spin-spin interaction of 2J/k(B) = -30 and -28 K for [ClO(4)](-) and [BF(4)](-) salts, respectively (H = -2J∑(i)S(i)·S(i+1)). Both salts show a weak ferromagnetic transition at 23 K thanks to interchain antiferromagnetic interaction between TTF dimers. The remanent magnetizations and coercive forces of nonoriented samples at 1.8 K are 0.016 μ(B) and 90 mT for the [ClO(4)](-) salt and 0.010 μ(B) and 50 mT Oe for the [BF(4)](-) salt, respectively. The weak ferromagnetism is attributed to the Dzyaloshinsky-Moriya interaction between adjacent TTF dimers and/or the single-ion anisotropy of [1](2+).     

Direct observation of mixed-valence and radical cation dimer states of tetrathiafulvalene in solution at room temperature: association and dissociation of molecular clip dimers under oxidative control

We have observed the mixed-valence and radical cation dimer states of a glycoluril-based molecular clip with tetrathiafulvalene (TTF) sidewalls at low concentration (1 mM) at room temperature. This molecular clip has four consecutive anodic steps in its cyclic voltammogram, which suggests a sequential oxidation of these TTF sidewalls to generate species existing in several distinct charge states: neutral monomers, mixed-valence dimers, radical cation dimers, and fully oxidized tetracationic monomers. The observation of characteristic NIR spectroscopic absorption bands at approximately 1650 and 830 nm in spectroelectrochemistry experiments supports the presence of intermediary mixed-valence and radical cation dimers, respectively, during the oxidation process. The stacking of four TTF radical cations in the dimer led to the appearance of a charge-transfer band at approximately 946 nm. Nanoelectrospray ionization mass spectrometry was used to verify the tricationic state and confirm the existence of other different charged dimers during the oxidation of the molecular clip.     

Molecular dynamics simulation of amphiphilic bistable [2]rotaxane langmuir monolayers at the air/water interface

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) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@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 rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that 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. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally.     

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.     

The nature of the [TTF]˙+···[TTF]˙+ interactions in the [TTF]2(2+) dimers embedded in charged [3]catenanes: room-temperature multicenter long bonds

The properties of tetrathiafulvalene dimers ([TTF](2)(2+)) and the functionalized ring-shaped bispropargyl (BPP)-functionalized TTF dimers, [BPP-TTF](2)(2+), found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are energetically unstable towards dissociation. When enclosed in the 4(+)-charged central cyclophane ring of charged [3]catenanes (CBPQT(4+)), [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are also energetically unstable with respect to leaving the CBPQT(4+) ring; since the barrier for the exiting process is only about 3 kcal mol(-1), that is, within the reach of thermal energies at room temperature (neutral [TTF](2)(0) dimers are stable within the CBPQT(4+) ring). However, the [BPP-TTF](2)(2+) dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP-TTF(+) macrocycle. Finally, it was shown that the [TTF](2)(2+), [BPP-TTF](2)(2+) dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation-anion interactions. In these environmental conditions at room temperature the TTF units of the [BPP-TTF](2)(2+) dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room-temperature multicenter long bond is formed, similar to those previously found in other [TTF](2)(2+) salts and their solutions.     

Functionally rigid bistable [2]rotaxanes

Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two pi-electron-rich stations-two NP moieties or a MPTTF unit and a NP moiety-with (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed (> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for much better control over the position of the ring component in the ground state but also for control over the location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically (1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ ring from one station to another as compared with previous systems-a finding that is thought to be a result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses of structural conformations that have characterized the first generations of molecular switches based on the donor-acceptor recognition motif.     

Two classes of alongside charge-transfer interactions defined in one [2]catenane

A [2]catenane, which incorporates hydroquinone (HQ) and a sterically bulky tetrathiafulvalene (TTF) into a bismacrocycle, has been designed to probe the alongside charge-transfer (CT) interactions taking place between a TTF unit and one of the bipyridinium moieties in the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+). A template-directed strategy employs the HQ unit as the primary template for formation of the tetracationic cyclophane CBPQT4+, affording the desired [2]catenane structure but as an uncharacteristic green solid. The X-ray crystal structure and detailed 13C NMR assignments have identified a stereoselective preference for catenation about the cis isomer. The 1H NMR spectroscopy, electrochemistry, and X-ray crystallography all confirm that the CBPQT4+ cyclophane encircles the HQ unit, thereby defining a structure which would normally determine a red color. The visible-NIR region of the absorption spectrum displays a band at approximately 740 nm that is unambiguously assigned to a TTF --> CBPQT4+ CT transition on the basis of resonance Raman spectroscopy using 785 nm excitation. The profile of the CT band changes depending on the ratio of the cis- to trans-TTF isomers in the [2]catenane for which the molar absorptivity of each isomer is estimated to be significantly different at epsilon max = 380 and 3690 M-1 cm-1, respectively. Molecular modeling studies confirmed that the observed difference in the absorption spectroscopic profile can be accounted for by both a better overlap of the HOMO(TTF) and LUMO+1(CBPQT4+) as well as a more stable face-to-face (pi...pi) conformation in the trans isomer compared to the edge-to-face cis isomer of the [2]catenane. The latter is arranged for pi-orbital overlap through the sulfur atoms of the TTF unit, thereby defining an [Spi...pi] interaction.