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PubMed2006

Sequential O- and N-acylation protocol for high-yield preparation and modification of rotaxanes: synthesis, functionalization, structure, and intercomponent interaction of rotaxanes.

A pseudorotaxane consisting of a 24-membered crown ether and secondary ammonium salt with the hydroxy group at the terminus was quantitatively acylated by bulky acid anhydride in the presence of tributylphosphane as catalyst to afford the corresponding rotaxane in high yield. Large-scale synthesis without chromatographic separation was easily achieved. The ammonium group in the resulting rotaxane was quantitatively acylated with excess electrophile in the presence of excess trialkylamine. Various N-functionalized rotaxanes were prepared by this sequential double-acylation protocol. 1H NMR spectra and X-ray crystallographic analyses of the rotaxanes showed that the crown ether component was captured on the ammonium group in ammonium-type rotaxane by strong hydrogen-bonding intercomponent interaction. The conformation around the ammonium group was fixed by the hydrogen-bonding interaction. Meanwhile, the conformation of the amide-type rotaxane was determined by the weak CH/pi interaction between the methylene group in crown ether and the benzene ring of the axle component. The N-acylation of ammonium-type rotaxane is useful for the preparation of both functionalized rotaxanes and weak intercomponent interaction-based rotaxanes.

Acylation↗

PubMed2004

Asymmetric benzoin condensation catalyzed by chiral rotaxanes tethering a thiazolium salt moiety via the cooperation of the component: can rotaxane be an effective reaction field?

Although some reactions on rotaxanes have been reported, the characteristic features of the rotaxanes providing unique reaction fields have hardly been studied, especially as catalyst. In our continuous studies on interlocked molecules such as rotaxanes and catenanes, we have noticed the importance of such interlocked structures with high freedom in functionalized materials such as molecular catalyst. For catalytic asymmetric benzoin condensations, two optically active rotaxanes possessing thiazolium salt moieties were prepared using the binaphthyl group as the chiral auxiliary. The benzoin condensations of aromatic aldehydes catalyzed by the chiral rotaxanes as catalysts gave optically active benzoins with ca. 30% ee in moderate to high chemical yields depending upon the structure of rotaxane and the reaction conditions employed. From the results, two intrarotaxane chirality transfers are confirmed: (i) through-space chirality transfer from wheel to axle and (ii) through-bond chirality transfer controlled with an achiral wheel. Because these asymmetric reaction fields are specific to the rotaxane structure, the importance and possibility of the "rotaxane field" as a particular reaction field is demonstrated in this work.

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PubMed2001

Facile synthesis of rotaxanes through condensation reactions of DCC-[2]rotaxanes.

[reaction: see text] In this Letter, we present an easy method for the synthesis of rotaxanes using a novel DCC-[2]rotaxane. The DCC-[2]rotaxane is composed of dibenzo-24-crown-8 ether, an amino acid tether, and di-tert-butyl phenyl rings as blocking groups. It is relatively stable and can be purified by column chromatography. A series of model rotaxanes were obtained in good yields by condensing the DCC-[2]rotaxanes with N-(2-aminoethyl)-3,5-di-tert-butylbenzylamide in acetonitrile and chloroform.

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PubMed2006

Template synthesis of [2]rotaxanes with large ring components and tris(biphenyl)methyl group as the blocking group. The relationship between the ring size and the stability of the rotaxanes.

We synthesized a series of macrocyclic phenanthrolines 3a-e and a tris(biphenyl)methyl derivative 4. [2]Rotaxanes with large ring components (10a,b) were synthesized by the template method, and the stability of the rotaxanes was examined. The study revealed that the tris(biphenyl)methyl group is an effective blocking group for the rotaxanes with up to a 33-membered ring. Even a rotaxane with a 37-membered macrocyclic phenanthroline (10b) could be isolated. The dissociation of 10b occurred at 60 degrees C.

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PubMed2005

A [2]rotaxane capped by a cyclodextrin and a guest: formation of supramolecular [2]rotaxane polymer.

A [2]rotaxane capped by a beta-cyclodextrin and a 2,4,6-trinitrophenyl group has been prepared by dissolving 6-aminocinnamoyl beta-cyclodextrin in water with 1-adamantane carboxylic acid and complexation with alpha-cyclodextrin followed by the reaction with 2,4,6-trinitrobenzene sulfonic acid sodium salt. The [2]rotaxane has been found to form supramolecular polymers by host-guest interactions.

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PubMed2004

Redox behavior of ferrocene-containing rotaxane: transposition of the rotaxane wheel by redox reaction of a ferrocene moiety tethered at the end of the axle.

A rotaxane with a ferrocene moiety at the axle terminus was prepared. The redox potential of the ferrocene moiety decreased by ca. 80 mV when the rotaxane had a crown ether wheel capable of moving on the axle. Thus, the stabilization of the oxidized state of the ferrocene moiety is assumed to accompany the transposition of the wheel component on the axle toward the ferrocene moiety. [reaction--see text]

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PubMed2006

Determining the intracellular transport mechanism of a cleft-[2]rotaxane.

Rotaxanes are a class of interlocked compounds that have been extensively investigated for their potential utility as switches or sensors. We recently demonstrated that rotaxanes have further application as agents that transport material into cells. This novel finding prompted our investigation into the mechanism by which rotaxanes are involved in transmembrane transport. Two-dimensional NMR analysis showed that a cleft-containing rotaxane exists in two dominant conformations ("closed" and "open"). To determine the importance of conformational flexibility on the ability of the rotaxanes to bind guests and transport material into cells, the rotaxane was chemically modified to lock it in the closed conformation. Charged guests interact less favorably with the locked rotaxane, as compared to the unmodified rotaxane, both in an aqueous solution and in DMSO. In a chloroform solution, both rotaxanes bind the guests with similar affinities. The locked rotaxane exhibited a reduced capacity to transport a fluoresceinated peptide into cells, whereas the unmodified rotaxane efficiently delivers the peptide. Flow cytometry experiments demonstrated that a high percentage of the cells contained the delivered peptide (89-98%), the level of delivery is concentration dependent, and the rotaxanes and peptide have low toxicity. Cellular uptake of the peptide was largely temperature and ATP independent, suggesting that the rotaxane-peptide complex passes through the cellular membrane without requiring active cell-mediated processes. The results show that the sliding motion of the wheel is necessary for the delivery of materials into cells and can enhance the association of guests. These studies demonstrate the potential for rotaxanes as a new class of mechanical devices that deliver a variety of therapeutic agents into targeted cell populations.

Animals↗

PubMed2004

A photoswitchable rotaxane with a folded molecular thread.

Novel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraquat-4,4-biphenylene) and a dumbbell-shaped molecular thread incorporating a photoactive diarylcycloheptatriene station as well as a photoinactive anisol station have been synthesized with yields of nearly 50 % by the alkylative endcapping method. The rotaxane was transformed into the related rotaxane incorporating a diaryl tropylium unit by electrochemical oxidation. The precursor of the cycloheptatrienyl rotaxane, the related pseudorotaxane, and the rotaxanes incorporating the diarylcycloheptatriene and the corresponding tropylium unit were characterized by (1)HNMR spectroscopy and UV/Vis spectroscopy. According to the NMR spectra, both the cycloheptatriene and the tropylium rotaxane possess a folded conformation enabling the tetracationic cyclophane to interact with two stations. The diarylcycloheptatriene station is incorporated inside the cavity of the cyclophane and the anisol station resides alongside the bipyridinium unit of the cyclophane. In contrast, the anisol station is inside the cyclophane in the tropylium rotaxane. The exchange between both conformations can be achieved by introducing the methoxy leaving group into the cycloheptatriene ring; the tropylium rotaxane is generated by photoheterolysis of this methoxy-substituted rotaxane, which reacts thermally back to the cycloheptatriene rotaxane, thus closing the switching cycle. These induced conformational changes achieve a so-called molecular machine.

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PubMed2004

Synthesis of [1]rotaxane via covalent bond formation and its unique fluorescent response by energy transfer in the presence of lithium ion.

Although there have been a lot of reports on the synthesis and properties of [n]rotaxanes (mainly n = 2), only a few reports on the synthesis of [1]rotaxane has been published by Vögtle's group and others (see ref 5). Generally speaking, [1]rotaxane might be expected to exhibit properties different from other rotaxanes, because the rotor and the axle in the [1]rotaxane is bound covalently and closely. We report on a novel method to make [1]rotaxanes via covalent bond formation from a macrocyclic compound. That is, we first prepared a bicyclic compound from macrocycle and then proceeded to [1]rotaxane by aminolysis. This is the first synthetic example of preparation of [1]rotaxane via covalent bond formation, not utilizing weak interactions such as hydrogen bonding, charge transfer, via metal complexation, etc. This method might provide a powerful and new tool for construction of [1]rotaxane as a new supramolecular system. In addition, we investigated energy transfer from rotor to axle using [1]rotaxane that we prepared. Energy transfer occurred perfectly from the naphthalene ring of the rotor to the anthracene ring of the axle. We found also that only lithium ion among alkali ions can drastically enhance the fluorescence intensity. This finding could be applicable to ion-sensing systems, switching devices, and so on.

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PubMed2003

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

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

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PubMed2000

[1]rotaxanes and pretzelanes: synthesis, chirality, and absolute configuration [In Process Citation]

The synthesis of aliphatically bridged [1](n)rotaxanes and (n)pretzelanes in preparative yields and the dependency of their chiroptical properties on the length (n) of their bridge are reported. A cycloenantiomeric bis(sulphonamide)[2]rotaxane with a sulphonamide group in its axle and its wheel was intramolecularly dialkylated by homologous bifunctional oligomethylene reagents to form chiral [1](n)rotaxanes bearing bridges of different lengths (n) between the axle and the wheel. Intramolecular dialkylation by 1,omega-dibromoalkanes of a topologically chiral bis(sulphonamide)[2]catenane with a sulphonamide group in both of the macrolactam rings leads to pretzel shaped molecules ((n)pretzelanes) with homologous bridges between the two macrocycles. Their yields decrease with decreasing length of the bridge. The shortest bridge isolated so far in reasonable amounts consists of six methylene groups ((6)pretzelane). Remarkably, a covalent connection of axle and wheel in a [2]rotaxane was successful even with much shorter bridges-down to only three methylene groups ([1](3)rotaxane). The structural changes of the [1](n)rotaxanes with decreasing bridge length is expressed by an increasing high-field shift in the 1H NMR spectra. Enantiomeric resolution of the racemates of both series was achieved in seven cases for the [1](n)rotaxanes and two for the (n)pretzelanes by use of chiral HPLC columns. The circular dichrograms of both compound families show a strong dependency on the length of the bridge. However, the shortest bridges displayed some additional unexpected deviations. A new specification of the absolute configuration of supramolecules, such as [n]catenanes, [n]rotaxanes and (n)pretzelanes is introduced together with some nomenclature additions.

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PubMed2001

Self-assembly and dynamics of [2]- and [3]rotaxanes with a dinuclear macrocycle containing reversible Os-N coordinate bonds.

With a dinuclear macrocycle 2 that contains weak reversible OsVI-N coordinate bonds, self-assembly and equilibrium dynamics of [2]- and [3]rotaxanes have been investigated. When the macrocycle 2 was mixed together with threads 4a-e, which all contain an adipamide station but different sizes of end groups, [2]pseudorotaxane- and rotaxane-like complexes were immediately formed with large association constants of >7 x 103M(-1) in CDCl3 at 298 K. Exchange dynamics, explored by 2D-EXSY experiments, suggest that assembly and disassembly of complexes occur through two distinct pathways, slipping or clipping, and this depends on the size of the end groups. The slipping pathway is predominant with smaller end groups that give pseudorotaxane-like complexes, while the clipping pathway is observed with larger end groups that yield rotaxane-like complexes. Under the same conditions, exchange barriers (deltaG++) were 14.3 kcalmol(-1) for 4a and 16.7 kcalmol(-1) for 4d, and indicate that the slipping process is at least one order of magnitude faster than the clipping process. Using threads 13a and 13b that contain two adipamide groups, more complicated systems have been investigated in which [2]rotaxane, [3]rotaxane, and free components are in equilibrium. Concentration- and temperature-dependent 1H NMR spectroscopic studies allowed the identification of all possible elements and the determination of their relative distributions in solution. For example, the relative distribution of the free components, [2]rotaxane, and [3]rotaxane are 30, 45, and 25 %, respectively, in a mixture of 2 (2mM) and 13a (2mM) in CDCl3 at 10 degrees C. However, [3]rotaxane exists nearly quantitatively in a mixture of 2 (4 mM) and 13 a (2 mM) in CDCl3 at a low temperature - 10 degrees C.

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PubMed2007

Preparation and properties of rotaxanes formed by dimethyl-beta-cyclodextrin and oligo(thiophene)s with beta-cyclodextrin stoppers.

Novel cyclodextrin rotaxanes with oligothiophene as an axis molecule have been prepared by the Suzuki coupling reaction of 6-O-(4-iodophenyl)-beta-CD (6-I-Ph-beta-CD) with di(1,3,2-dioxaborolan-2-yl)-oligothiophene (oligothiophene diboric ethylene glycol esters) in aqueous solutions of dimethyl-beta-cyclodextrin (DM-beta-CD). These reactions gave [2]rotaxanes and [3]rotaxanes, which were isolated by reversed phase chromatography. The fluorescence intensities of rotaxanes are higher than those of dumbbell-shaped molecules (without DM-beta-CD) in aqueous solutions. The inclusion ratio and chain length of rotaxanes have been found to relate to the emission properties and emission intensities of oligothiophene. In aqueous solutions, fluorescence quantum yields of rotaxanes are higher than those of dumbbell-shaped molecules. The increase in the fluorescence efficiency of rotaxane is caused by suppression of intermolecular interactions, indicating the effect of insulated oligothiophene with DM-beta-CD. beta-CD at the both ends of rotaxanes functions not only as bulky stoppers but also as the recognition site for guest molecules, as verified by fluorescence quenching experiments.

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PubMed2002

Post-assembly processing of [2]rotaxanes.

The concept of using [2]rotaxanes that carry one or more surrogate stoppers which can subsequently be converted chemically into other structural units, resulting in the formation of new interlocked molecular compounds, is introduced and exemplified. Starting from simple NH2(+)-centered/crown-ether-based [2]rotaxanes, containing either one or two benzylic triphenylphosphonium stoppers, the well-known Wittig reaction has been employed to make, 1) other [2]rotaxanes, 2) higher order rotaxanes, 3) branched rotaxanes, and 4) molecular shuttles--all isolated as pure compounds, following catalytic hydrogenations of their carbon-carbon double bonds, obtained when aromatic aldehydes react with the ylides produced when the benzylic triphenylphosphonium derivatives are treated with strong base. The two starting [2]rotaxanes were characterized fully in solution and also in the solid state by X-ray crystallography. The new interlocked molecular compounds that result from carrying out post-assembly Wittig reactions on two [2]rotaxanes were characterized by (dynamic) 1H NMR spectroscopy. In the case of a molecular shuttle in which the crown ether component is dibenzo[24]-crown-8 (DB24C8), shuttling is slow on the 1H NMR timescale, even at high temperatures. However, when DB24C8 is replaced by benzometaphenylene[25]-crown-8 as the ring component in the molecular shuttle, the frequency of the shuttling is observed to be around 100 Hz in [D4]methanol at 63 degrees C.

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PubMed2004

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.

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PubMed2003

Quantitative comparison of kinetic stabilities of metallomacrocycle-based rotaxanes.

Four mononuclear metallomacrocycles with identical cavities but different transition metals (Os(VI), Pd(II), Pt(II), and Re(I)) were prepared. With these metallomacrocycles, the corresponding rotaxanes 2-Os, 2-Pd, 2-Pt, and 2-Re were self-assembled by hydrogen-bonding interactions. The kinetic stabilities of the rotaxanes were determined quantitatively and compared with each other by (1)H NMR spectroscopic techniques, including two-dimensional exchange spectroscopy (2D-EXSY) experiments. The activation free energies (DeltaG( not equal )) for the exchange between the rotaxanes 2-Os, 2-Pd and 2-Pt and their free components were determined to be 15.5, 16.0, and 16.4 kcal mol(-1), respectively. These magnitudes imply that the rotaxanes 2-Os, 2-Pd and 2-Pt are kinetically labile at room temperature and exist only as equilibrium mixtures with free components in solution. In contrast, the rotaxane 2-Re is kinetically stable enough to be isolated in pure form by silica gel chromatography under ordinary laboratory conditions. However, at higher temperatures (>60 degrees C) 2-Re was slowly disassembled into its components until the equilibrium was established. The rate constants were measured at three different temperatures, and the Eyring plot yielded the activation enthalpy DeltaH(not equal)=35 kcal mol(-1) and the activation entropy DeltaS(not equal)=27 eu for the disassembly of the rotaxane 2-Re in Cl(2)CDCDCl(2). These thermodynamic parameters gave the activation free energy DeltaG(not equal)(off)=27.1 kcal mol(-1) at 25 degrees C. Consequently, 2-Re is one example of a novel metallomacrocycle-based rotaxane that contains a coordination bond with enough strength to allow both for isolation in pure form around room temperature and for self-assembly at higher temperatures.

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PubMed2004

Controllable donor-acceptor neutral [2]rotaxanes.

In pursuit of a neutral bistable [2]rotaxane made up of two tetraarylmethane 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 pi-electron-donating 1,5-dinaphtho[38]crown-10 (1/5DNP38C10) at the thermodynamic instigation of pi-electron-accepting recognition sites, in this case, pyromellitic diimide (PmI) and 1,4,5,8-naphthalenetetracarboxylate diimide (NpI) units separated from each other along the rod section of the rotaxane's dumbbell component, and from the para positions of the fourth aryl group of the two stoppers by pentamethylene chains, a modular approach was employed in the synthesis of the dumbbell-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 corresponding dumbbell-shaped compounds NpPmD, PmPmD, and NpNpD and 1/5DNP38C10 by slippage. Dynamic 1H NMR spectroscopy in CD2Cl2 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), respectively, reflecting a much more pronounced donor-acceptor stabilizing interaction involving the NpI units over the PmI ones. The photophysical and electrochemical properties of the three neutral [2]rotaxanes and their dumbbell-shaped precursors have also been investigated in CH2Cl2. Interactions between 1/5DNP38C10 and PmI and NpI units located within the rod section of the dumbbell components of the [2]rotaxane give rise to the appearance of charge-transfer bands, the energies of which correlate with the electron-accepting properties of the two diimide moieties. Comparison between the positions of the visible absorption bands in the three [2]rotaxanes shows that, in NpPmR, the major translational 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]rotaxane, one of the translational isomers predominates. Furthermore, after deactivation of the NpI unit by one-electron reduction, the 1/5DNP38C10 macrocycle moves to the PmI unit. Li+ ions have been found to strengthen the interaction between the electron-donating crown ether and the electron-accepting diimide units, particularly the PmI one. Titration experiments show that two Li+ ions are involved in the strengthening of the donor-acceptor interaction. 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 1H 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.

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PubMed2005

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.

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