Shuttling dynamics in an acid-base-switchable [2]rotaxane.

Molecular shuttles are an intriguing class of rotaxanes which constitute prototypes of mechanical molecular machines and motors. By using stopped-flow spectroscopic techniques in acetonitrile solution, we investigated the kinetics of the shuttling process of a dibenzo[24]crown-8 ether (DB24C8) macrocycle between two recognition sites or "stations"--a secondary ammonium (-NH2+-)/amine (-NH-) center and a 4,4'-bipyridinium (bipy2+) unit--located on the dumbbell component in a [2]rotaxane. The affinity for DB24C8 decreases in the order -NH2+- > bipy2+ > -NH-. Hence, shuttling of the DB24C8 macrocycle can be obtained by deprotonation and reprotonation of the ammonium station, reactions which are easily accomplished by addition of base and acid to the solution. The rate constants were measured as a function of temperature in the range 277-303 K, and activation parameters for the shuttling motion in both directions were determined. The effect of different counterions on the shuttling rates was examined. The shuttling from the -NH2+- to the bipy2+ station, induced by the deprotonation of the ammonium site, is considerably slower than the shuttling in the reverse direction, which is, in turn, activated by reprotonation of the amine site. The results show that the dynamics of the shuttling processes are related to the change in the intercomponent interactions and structural features of the two mutually interlocked molecular components. Our observations also indicate that the counterions of the cationic rotaxane constitute an important contribution to the activation barrier for shuttling.     

A Novel Supramolecular System: Combination of Two Switchable Processes in a [2]Rotaxane

A novel supramolecular system, which is made up of a dibenzo[24]crown‐8 (DB24C8) ring component linked with a calix[4]arene derivative, a dumbbell component, containing a secondary ammonium center (‐NR₂H₂⁺‐) and a 4,4′‐bipyridinium (BIPY²⁺) unit, and stoppered with two 3,5‐di‐tert‐butylphenyl groups on the two termini of the dumbbell component, has been synthesized. The system displays a combination of two processes: the pH‐induced shuttling of a DB24C8 ring and the complexation/decomplexation of K⁺ ions. The switching process of this supramolecular system was investigated in detail by ¹H NMR spectroscopy. The results showed that the supramolecular system can only switch smoothly in CD₃CN. The two separated switchable processes can run together smoothly in this supramolecular system.     

Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V₁ and V₂, interconnected by a rigid terphenylene bridge. In their parent dication states, V₁²⁺ and V₂²⁺ can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side‐to‐side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO₂ nanoparticle, viologen V₂²⁺ is reduced at significantly more negative potentials (?0.3 V) than in flexible analogues that contain aliphatic bridges between V₁ and V₂. These overpotentials are analysed in terms of electron‐transfer rates and a donor–bridge–acceptor (D–B–A) formalism, in which D is the doubly reduced viologen, V₁⁰, adjacent to the TiO₂ surface (TiO₂–V₁⁰), B is the terphenylene bridge and A is viologen V₂²⁺. We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V₂²⁺ by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution‐based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO₂ surface.     

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.     

A 1,2,3,4,5-pentaphenylferrocene-stoppered rotaxane capable of electrochemical anion recognition

The chloride anion templated synthesis of an electrochemical anion sensory interlocked host system, prepared by the integration of redox-active 1,2,3,4,5-pentaphenylferrocene stopper groups into the structure of a rotaxane capable of binding anionic guests is described. Extensive (1)H NMR and electrochemical titration investigations were used to probe the anion recognition and sensing properties of the rotaxane, compared to the axle and model system components. A characteristic electrochemical response was observed for chloride binding by the rotaxane, which was attributed to the topologically constrained cavity of the interlocked host molecule.     

Introducing negative charges into bis-p-phenylene crown ethers: a study of bipyridinium-based [2]pseudorotaxanes and [2]rotaxanes

This paper describes novel host-guest systems comprising viologen cations (guests) and the derivatives of bis-para-phenylene-34-crown-10 (hosts) with anionic groups COO(-) or SO(3)(-). The structure of the resulting charge-compensated host-guest complexes, their association constants and their electrochemical behaviour have been studied. In the solid state, the viologen cations thread the negatively charged crown ethers forming electroneutral zwitterion-like [2]pseudorotaxane salts; in solution this threaded geometry is preserved. The association constants of [2]pseudorotaxane salts incorporating the 1,1'-diethylviologen moiety in solution are significantly higher than those of previously reported analogues. The extrapolated association free energies in non-aqueous media exceed -40 kJ mol(-1) at 25 degrees C. This significant increase of the interaction free energy makes these compounds stable even in aqueous solutions. The association constants of [2]pseudorotaxane salts incorporating sterically more hindered 1,1'-diethyl-3,3'-dimethylviologen moieties are significantly lower. Structurally related [2]rotaxane salts, in which the oppositely charged ionic components are mechanically interlocked, have been prepared in good yields. It has been shown that [2]rotaxane salts incorporating anti-isomers of bisfunctionalised crown ethers are cycloenantiomeric. In both [2]pseudorotaxane and [2]rotaxane salts, the electrostatic interactions between the viologen moieties and the negatively charged crown ethers lead to very significant negative shifts of viologen reduction potentials up to 450 mV. The findings of the present study are valuable for the design of nanoscale molecular electronic devices.     

A shuttling molecular machine with reversible brake function

Design, synthesis, and demonstration of a prototype of a shuttling molecular machine with a reversible brake function are reported. It is a photochemically and thermally reactive rotaxane composed of a dianthrylethane-based macrocycle as the ring component and a dumbbell shaped molecular unit with two, secondary ammonium stations separated by a phenylene spacer as the axle component. The rate of shuttling motion was shown to be reduced to less than 1 % (from 340 to <2.5 s(-1)) by reducing the size of the ring component from 30-crown-8 to 24-crown-8 macrocycles upon photoirradiation. The ring component was turned back to 30-crown-8 by thermal ring opening, thus establishing a reversible brake function that works in response to photochemical and thermal stimuli.     

Towards Controlling the Threading Direction of a Calix[6]arene Wheel by Using Nonsymmetric Axles

Traffic control : By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure).

     

Highly effective and reversible control of the rocking rates of rotaxanes by changes to the size of stimulus-responsive ring components

We have designed and synthesized rotaxanes whose rates of rocking motion (pendular motion) were switched reversibly through changes to the size of the ring component in response to external stimuli. The ring molecules of the rotaxanes incorporate a metaphenylene unit, which swings like a pendulum, and a dianthrylethane unit, which undergoes reversible isomerization in response to photo- and thermal stimuli and changes the size of the ring component. The rocking rates were estimated quantitatively by variable-temperature (VT) NMR spectroscopy and saturation transfer experiments, which revealed substantial changes in the rates between the open and closed forms, particularly in the case of rotaxanes with an isopropoxy group attached to a phenylene unit.     

Dynamic covalent approach to [2]- and [3]roxtanes by utilizing a reversible thiol-disulfide interchange reaction

A dynamic covalent approach to disulfide-containing [2]- and [3]rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of rotaxanes. The rotaxanes were synthesized from the dumbbell-shaped compounds and dibenzo-[24]crown-8 (DB24C8) with catalysis by benzenethiol. The yields of isolated rotaxanes reached about 90 % under optimized conditions. A kinetic study on the reaction forming [2]rotaxane 2 a and [3]rotaxane 3 a suggested a plausible reaction mechanism comprising several steps, including 1) initiation, 2) [2]rotaxane formation, and 3) [3]rotaxane formation. The whole reaction was found to be reversible in the presence of thiols, and thermodynamic control over product distribution was thus possible by varying the temperature, solvent, initial ratio of substrates, and concentration. The steric bulk of the end-capping groups had almost no influence on rotaxane yields, but the structure of the thiol was crucial for reaction rates. Amines and phosphines were also effective as catalysts. The structural characterization of the rotaxanes included an X-ray crystallographic study on [3]rotaxane 3 a.     

Light- and pH-regulated Water-soluble Pseudorotaxanes Comprising a Cucurbit[7]uril and a Flavylium-based Axle

A linear double pyridinium-terminated thread comprising a central chalcone moiety is shown to provide two independent binding sites with similar affinity for cucurbit[7]uril (CB7) macrocycles in water as judged from NMR, UV-Visible and fluorescence spectroscopies. Association results in [2] and [3]pseudorotaxanes, which are both pH and photosensitive. Switching from the neutral chalcone to the cationic flavylium form upon irradiation at 365 nm under acidic conditions provided an enhanced CB7 association (K_1:1 increases from 1.2×10⁵ M⁻¹ to 1.5×10⁸ M⁻¹), limiting spontaneous on-thread cucurbituril shuttling. This co-conformational change in the [2]pseudorotaxane is reversible in the dark with k_obs=4.1×10⁻⁴ s⁻¹. Threading the flavylium moiety into CB7 leads to a dramatic increase in the fluorescence quantum yield, from 0.29 in the free axle to 0.97 in the [2]pseudorotaxane and 1.0 in the [3]pseudorotaxane.     

A pH‐Dependent,Mechanically Interlocked Switch: Organometallic [2]Rotaxane vs. Organic [3]Rotaxane

We present the first [2]rotaxane featuring a functional organometallic host. In contrast to the known organic scaffolds, this assembly shows a high post‐synthetic modifiability. The reactivity of the Ag₈ pillarplex host is fully retained, as is exemplified by the first transmetalation in a rotaxane framework to provide the respective Au₈ analogue. Additionally, a transformation under acidic conditions to give a purely organic [3]rotaxane is demonstrated which is reversible upon addition of a suitable base, rendering the assembly a pH‐dependent switch. Hereby, it is shown that the mechanically interlocked nature of the system enhances the kinetic stability of the NHC host complex by a factor of >1000 and corresponds to the first observation of a stabilizing “rotaxand effect”.     

A molecular shuttle for driving a multilevel fluorescence switch

A [2]rotaxane-based molecular shuttle comprised a macrocycle mechanically interlocked to a chemical "dumbbell" has been prepared in high yields by a thermodynamically controlled, template-induced clipping procedure. This molecular shuttle has two different recognition sites, namely, -NH2 +- and amide, separated by a phenyl unit. The macrocycle exhibits high selectivity for the -NH2+- recognition sites in the protonated form through noncovalent interactions, which include 1) N+-H...O hydrogen bonds; 2) C-H...O interactions between the CH2NH2+CH2 protons on the thread and the oligo(ethylene glycol) unit in the macrocycle; 3) pi...pi stacking interaction between macrocycle and aromatic unit. Upon deprotonation of the [2]rotaxane the macrocycle glides to the amide recognition site due to the hydrogen bonds between the -CONH- group and the oligo(ethylene glycol) unit in the macrocycle. The deprotonation process requires about 10 equivalents of base (iPr2NEt) in polar acetone, while the amount of base is only 1.2 equivalents in apolar tetrachloroethane. Upon addition of Li+, the conformation of the [2]rotaxane was altered as a result of the collective interactions of 1) hydrogen bonds between pyridine nitrogen and amide hydrogen atoms; 2) coordination between the oligo(ethylene glycol) unit, amide oxygen atom and Li+ cation. Then, when Zn2+ ions are added, the macrocycle returns to the deprotonated -NH- recognition site owing to coordination of the macrocycle and -NH- from the axle with the Zn2+ ion. All the above-mentioned movement processes are reversible through the alternate addition of TFA/iPr2NEt, Li/[12]-crown-4 and Zn2+/ethylenediaminetetraacetate (EDTA), by virtue of hydrogen bonding and metal-ion complexation. Significantly, the three independent movement processes are all accompanied by fluorescent responses: 1) complete repression in the protonated form; 2) low-level expression in the deprotonated form; 3) medium-level expression following addition of Li+; 4) high-level expression on complexation with Zn2+.     

Electrochemical and photochemical control of host-guest complexation at surfaces

The application of electrochemistry or photochemistry to modulate supramolecular interactions between host-guest systems in solution is a burgeoning field. In particular, the speed and reversibility associated with electrochemically or photochemically actuated supramolecular interactions has allowed the creation and modulation of novel solution-based devices. In recent years, great advances have been made in transferring these systems from the solution to the solid state, to facilitate the development of molecular-electronics components that can operate in unison under the influence of an externally applied stimulus. These studies pave the way for the creation of responsive surfaces with advanced materials and nanotechnology applications.     

Light‐Switchable Janus [2]Rotaxanes Based on α‐Cyclodextrin Derivatives Bearing Two Recognition Sites Linked with Oligo(ethylene glycol)

Two Janus [2]rotaxanes, 5a and 5b , with α‐cyclodextrin (α‐CD) derivatives substituted on the 6‐position with two recognition sites (azobenzene and heptamethylene (C7)) that were linked with linkers of different lengths (oligo(ethylene glycol) with a degree of polymerization equal to 2 or approximately 21) were synthesized and characterized. 2D ROESY NMR spectroscopy and circular dichroism (cd) spectra demonstrated that the recognition site of the α‐CD moiety was switched by photoisomerization of the azobenzene moiety in 5a and 5b . The different size changes of 5a and 5b in hydrodynamic radius (R_H) owing to the different length of linker between two recognition sites were observed by pulse‐field‐gradient spin‐echo NMR spectroscopy. The kinetic results indicated that the different length of linker had no or a weak effect for the photoisomerization process of 5a and 5b .     

Quantitative conformational study of redox-active [2]rotaxanes, part 1: Methodology and application to a model [2]rotaxane

This paper reports a novel methodology for the conformational analysis of [2]rotaxanes. It combines NMR spectroscopic (COSY, NOESY and the recently reported paramagnetic line-broadening and suppression technique) and electrochemical techniques to enable a quantitative analysis of the co-conformations of interlocked molecules and the conformations of their components. This methodology was used to study a model [2]rotaxane in solution. This [2]rotaxane consists of an axle that incorporates an electron-poor, doubly positively charged viologen that threads an electron-rich crown ether. It has been shown that the axle of the [2]rotaxane in its dicationic state adopts a folded conformation in solution and the crown ether is localised at the viologen moiety. Following a one-electron reduction of viologen, the paramagnetic radical cation of the [2]rotaxane retains its folded conformation in solution. The data also demonstrate that in the radical cation the crown ether remains localised at the viologen, despite its reduced affinity for the singly reduced viologen. The combined quantitative NMR spectroscopic and electrochemical characterisation of the electromechanical function of the model [2]rotaxane in solution provides an important reference point for the study of switching in structurally related bistable [2]rotaxanes, which is the subject of the second part of this work.