Tetrathiafulvalene radical cation dimerization in a bistable tripodal [4]rotaxane

The template-directed synthesis of a bistable tripodal [4]rotaxane, which has cyclobis(paraquat-p-phenylene) (CBPQT4+) as the pi-electron-deficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of pi-electron-rich recognition sites located on all three legs of the tripodal dumbbell, is described. The chemical and electrochemical oxidation of the [4]rotaxane and its tripodal dumbbell have allowed us to unravel an unprecedented TTF.+ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF.+]2 and the mixed-valence one [(TTF)2].+, have been observed at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF.+]2 is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [4]rotaxane are encircled by CBPQT4+ rings, which renders the formation of the mixed-valence dimer [(TTF)2].+ highly unfavorable.     

Poised on the brink between a bistable complex and a compound

[reaction: see text] An enmeshed supramolecular complex, based on a semi-dumbbell-shaped component containing an asymmetrically substituted tetrathiafulvalene site and a 1,5-dioxynaphthalene site for encirclement by a cyclobis(paraquat-p-phenylene) ring component and with a "speed bump" in the form of an thiomethyl group situated between the two recognition sites, has been self-assembled. This complex is a mixture in acetone solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature.     

In the twilight zone between [2]pseudorotaxanes and [2]rotaxanes

A [2]pseudorotaxane, based on a semi-dumbbell-shaped component containing asymmetrically substituted monopyrrolotetrathiafulvalene and 1,5-dioxynaphthalene recognition sites for encirclement by cyclobis(paraquat-p-phenylene) and with a "speed bump" in the form of a thiomethyl group situated between the two recognition sites, has been self-assembled. This supramolecular entity is a mixture in solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature, allowing it to be isolated by employing flash column chromatography. These two [2]pseudorotaxanes were both characterized in solution by UV/Vis and (1)H NMR spectroscopies (1D and 2D) and also by differential pulse voltammetry. The spectroscopic and electrochemical data reveal that one of the complexes behaves wholly as a [2]pseudorotaxane, while the other has some [2]rotaxane character to it. The kinetics of the shuttling of cyclobis(paraquat-p-phenylene) between the monopyrrolotetrathiafulvalene and the 1,5-dioxynaphthalene recognition sites have been investigated at different temperatures. The shuttling processes, which are accompanied by detectable color changes, can be monitored using UV/Vis and (1)H NMR spectroscopies; the spectroscopic data have been employed in the determination of the rate constants, free energies of activation, enthalpies of activation, and the entropies of activation for the translation of cyclobis(paraquat-p-phenylene) between the two recognition sites.     

The effects of conformation on the noncovalent bonding interactions in a bistable donor-acceptor [3]catenane

A switchable donor-acceptor bistable [3]catenane, composed of a crown ether containing a pair of alternating π-electron rich tetrathiafulvalene and 1,5-dioxynaphthalene units, encircled by two π-electron deficient cyclobis(paraquat-p-phenylene) rings, has been synthesised and the redox-activated switching it undergoes investigated.     

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.     

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.     

An interdigitated functionally rigid [2]rotaxane

The synthesis of a functionally rigid [2]rotaxane incorporating pi-electron rich 1,5-disubstituted naphthalene (NP) ring systems, encircled by the pi-electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene), is described; in the solid state, the molecules of this donor-acceptor [2]rotaxane line themselves up in parallel pi-pi stacks of alternating NP ring systems and bipyridinium units, affording an interdigitated superstructure.     

Cyclobis(paraquat-p-phenylene)-based [2]catenanes prepared by kinetically controlled reactions involving alkynes

[reaction: see text] Charged donor-acceptor [2]catenanes, in which the pi-accepting cyclobis(paraquat-p-phenylene) acts as a tetracationic template for the threading-followed-by-clipping of acyclic oligoethers, incorporating centrally a pi-donating 1,5-dioxynaphthalene ring system and terminated either by acetylene units or by acetylene and azide functions, are the products of copper-mediated Eglinton coupling and Huisgen 1,3-dipolar cycloaddition, respectively.     

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.     

Efficient templated synthesis of donor-acceptor rotaxanes using click chemistry

The mild reaction conditions, remarkable functional group compatibility, and complete regioselectivity of the Cu-catalyzed Huisgen 1,3-dipolar cycloaddition ("click chemistry") between organic azides and terminal alkynes have led to a threading-followed-by-stoppering approach to the synthesis of donor-acceptor rotaxanes incorporating cyclobis(paraquat-p-phenylene) (CBPQT4+) as the pi-accepting ring component. Rotaxane formation is initiated by reacting azide-functionalized pseudorotaxanes containing pi-donating 1,5-dioxynaphthalene (DNP) recognition units with appropriate alkyne-functionalized stoppers. The high yields obtained in this efficient, kinetically controlled post-assembly covalent modification, as well as the excellent convergence of the synthetic protocol, are demonstrated by the preparation of [2]-, [3]-, and [4]rotaxanes containing multiple DNP/CBPQT4+ donor-acceptor recognition motifs.     

Locking down the electronic structure of (monopyrrolo)tetrathiafulvalene in [2]rotaxanes

[reaction: see text] The redox potentials of a highly constrained [2]rotaxane have been measured and used to model the energy of the HOMO of tetrathiafulvalene-based bistable [2]rotaxanes in their two co-conformationally isomeric states. Restrained from co-conformational movements, the measured oxidation and reduction potentials provide insights into the orbital energies and electronic structure of a (monopyrrolo)tetrathiafulvalene unit when encircled by a tetracationic cyclobis(paraquat-p-phenylene) ring.     

Bispyrrolotetrathiafulvalene-containing [2]catenanes

Two [2]catenanes incorporating bispyrrolotetrathiafulvalene (BPTTF) and weaker aryl donors, hydroquinone (HQ) and 1,5-dioxynaphthalene (DNP), respectively, have been prepared and characterized. These [2]catenanes show a predominant amount (>95:5) of the co-conformation in which either the HQ or the DNP unit is encircled by a tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), contrary to what is observed in systems based on the parent tetrathiafulvalene (TTF). These new [2]catenanes act effectively as molecular switches which are always configured in the "on" state.     

Solid-state structures and superstructures of two charged donor-acceptor rotaxanes

Two [2]rotaxanes and a [2]pseudorotaxane containing 1,5-dioxynaphthalene recognition sites located in the middle of their dumbbell and thread components, respectively, and encircled by single cyclobis(paraquat-p-phenylene) rings have been synthesized under template control and their solid-state (super)structures have been solved. The investigations revealed that the stoppers on the dumbbell components, the solvents, and the counterions can affect the conformations adopted by the [2]rotaxanes and [2]pseudorotaxane in the solid state.     

Structures and properties of self-assembled monolayers of bistable [2]rotaxanes on Au (111) surfaces from molecular dynamics simulations validated with experiment

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+)) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 A(2)/molecule (one molecule per 4 x 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 A(2)/molecule, the 3 x 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is theta = 39 degrees and 61 degrees, respectively, while the tilt angle of the entire rotaxane is psi = 41 degrees and 46 degrees , respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 A(2)/molecule (the 4 x 5 grid/molecule) of 50 degrees and 51 degrees for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF(6)(-)) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 A at the optimum coverage for the GSCC and 40.0 A for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir-Blodgett monolayers of amphiphilic [2]rotaxanes.     

A clicked bistable [2]rotaxane

[structure: see text]. A universal diazide-terminated polyether, incorporating tetrathiafulvalene (TTF, green) and 1,5-dioxynaphthalene (DNP, red) units, was prepared and subsequently employed in the template-directed synthesis of a switchable donor/acceptor [2]rotaxane. The triazole rings (magenta), which are introduced into the rotaxane during requisite click reactions, do not present themselves as competing recognition sites for the tetracationic cyclophane (blue) as it is induced to switch between the TTF unit, when it becomes dicationic (green adorned with yellow extremities), and the DNP unit.     

Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes

A series of amphiphilic bistable [2]rotaxanes--in which a ring-shaped component, the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), has been assembled around two recognition sites, a tetrathia-fulvalene (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 end--has 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 degreesC were examined to determine the monolayer stabilities and co-conformations of the [2] rotaxanes and their free dumbbell counterparts. These experiments allow us to propose a model for the rotaxane's structures at different surface pressures. All the [2]rotaxanes form stable Langmuir films. These films typically pass from a liquid-expanded region to a liquid-condensed region. The transition between the two regions was either directly observed or ascertained using film stability experiments. Film balance and surface rheology experiments showed that the addition of the tetracationic cyclophane component and hydroxyl end groups markedly increased the stabilities and viscoelasticity of the films.     

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.     

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.     

Quantifying the working stroke of tetrathiafulvalene-based electrochemically-driven linear motor-molecules

A highly constrained [2]rotaxane, constructed in such a way that the tetracationic cyclobis(paraquat-p-phenylene) ring is restricted to reside on a monopyrrolotetrathiafulvalene unit, has been synthesised and characterised. This design allows the deslipping free energy barrier for the tetracationic ring in all three redox states of the rotaxane to be determined.     

Foldamers in pseudo[2]rotaxanes and [2]rotaxanes: tuning the switching kinetics and metastability

Hydrogen bonded arylamide foldamers have been introduced in switchable pseudo[2]rotaxanes and [2]rotaxanes, which also include a cyclobisparaquat(p-phenylene) (CBPQT⁴⁺) ring and a ‘dumbbell’ containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP, for rotaxanes). The foldamer size changes through folding and unfolding serve as a steric handle to modulate the mechanical movement of the CBPQT⁴⁺ ring along the dumbbell of the pseudo[2]rotaxanes and [2]rotaxanes. By varying the number of the repeating units in the foldamer, the kinetics of the solvent-dependent slippage/deslippage of pseudo[2]rotaxanes and the switching of the ring between TTF and DNP of the [2]rotoxanes can be tuned remarkably, with the time scope ranging from several minutes to several days, in twelve solvents of varying polarity, which have been confirmed by the ¹H NMR, UV–vis spectroscopy, and cyclic voltammogram experiments.