Pseudorotaxane Formation Research Articles

2]Pseudorotaxane formation between rigid Y-shaped 2,4,5-triphenylimidazolium axles and [24]crown-8 ether wheels.

A new templating motif for the formation of [2]pseudorotaxanes is described in which rigid, Y-shaped axles with an imidazolium core and aromatic substituents at the 2-, 4- and 5-positions interact with [24]crown-8 ether wheels ([24]crown-8 and dibenzo[24]crown-8). The Y-shape of the axle significantly enhances the association between axle and wheel when compared to simple imidazolium cations.

Self-recovering stimuli-responsive macrocycle-equipped supramolecular ionogels with unusual mechanical properties

A chiral, crown-ether-functionalized bisurea gelator forms supramolecular gels in ionic liquids. The resulting ionogels show a remarkably high thermal stability with gel-sol transition temperatures (T(gs)) reaching more than 100 °C. The mechanical strength of these ionogels is surprisingly high and even comparable to that of cross-linked protein fibres. Furthermore, the ionogels exhibit rapid self-recovery properties after structural damage caused by deformation. Pseudorotaxanes form from the gelators' benzo[21]crown-7 ethers as the wheels and secondary ammonium ions as the axles despite the competition between that cation and the imidazolium ions of the ionic liquid for crown ether binding. Pseudorotaxane formation as an external chemical stimulus triggers the gel-sol transition of the ionogels.

Molecular Switch Based on Very Weak Association between BPX26C6 and Two Recognition Units

A [2]rotaxane has been constructed through metal ion-templated pseudorotaxane formation from a weakly interacting host (bis-p-xylyl[26]crown-6)/guest (diphenylurea derivative) complex and post-assembly modification to form another weakly interacting tertiary ammonium ion recognition site for the host. The combination of this pair of weakly interacting components allowed bifunctional (pH, Na(+) ions) switching of the crown ether unit, highlighting the potential applicability of weakly associated components within molecular switches.

Molecular Interactions, Proton Exchange, and Photoinduced Processes Prompted by an Inclusion Process and a [2]Pseudorotaxane Formation

Appropriate design of the host and guest components allows formation of a novel [2]pseudorotaxane complex with an interrupted photoinduced electron transfer (PET)-coupled fluorescence resonance energy transfer (FRET) response. This is the first example of an inclusion complex with NO6-based azacrown ether as the host unit (H). Different guest molecules (G1, G2, G3, and G4) with varying stopper size are used for the studies. Unlike G1, G2, and G3, G4 with a relatively bulkier stopper fails to form a [2]pseudorotaxane complex. Isothermal titration microcalorimetry measurements reveal a systematic increase in the association constant for H·G1, H·G2, and H·G3 with a change in the stopper size. Thermodynamic data suggest that the formation of H·G1/H·G2/H·G3 is exclusively driven by a large positive entropic gain (TΔS = 19.69/26.80/21.81 kJ·mol(-1)), while the enthalpy change is slightly negative for H·G1/H·G3 (-2.61/-1.97 kJ·mol(-1)) and slightly positive for H·G2 (ΔH = 5.98 kJ·mol(-1)). For these three inclusion complexes, an interrupted PET-coupled FRET response is observed with varying efficiency, which is attributed to the subtle differences in acidity of the NH2(+) unit of the guest molecules and thus the proton exchange ability between the host and respective guest. This is substantiated by the results of the computational studies.

Pseudorotaxanes with Self‐Sorted Sequence and Stereochemical Orientation

Partner preferences in pseudorotaxane formation were exploited to establish an integrative self-sorting system able to discriminate simultaneously at the sequence and stereochemical level (see picture). It was found that calix[6]arenes were threaded selectively with a preferred orientation onto bisammonium axles, even when the structural differences between the possible building blocks were small and located remote from the binding sites.

Assembly of Gold Nanoparticles on Functionalized Si(100) Surfaces through Pseudorotaxane Formation

The assembly of gold nanoparticles (AuNPs) on a hydrogenated Si(100) surface, mediated by a series of hierarchical and reversible complexation processes, is reported. The proposed multi-step sequence involves a redox-active ditopic guest and suitable calix[n]arene-based hosts, used as functional organic monolayers of the two inorganic components. Surface reactions and controlled release of AuNPs have been monitored by application of XPS, atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM) and electrochemistry.

Programmable multilayers of nanometer-sized macrocycles on solid support and stimuli-controlled on-surface pseudorotaxane formation

Mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes are capable of mechanical motion on the nanoscale and are therefore promising prototypes for molecular machines in recent nanotechnology. However, most of the existing examples are isotropically distributed in solution, which prohibits concerted movement and with it the generation of macroscopic effects. Thus, arranging them in ordered arrays is of huge interest in recent research. We report the deposition of quite densely packed multilayers of tetralactam macrocycles on gold surfaces by metal-coordinated layer-by-layer self-assembly. Linear dichroism effects in angle-resolved NEXAFS spectra indicate a preferential orientation of the macrocycles. The sequence of the metal ions can be programmed by the use of different transition metal ions at each deposition step. Additionally, reversible on-surface pseudorotaxane formation was successfully realized by repeated uptake and release of axle molecules inside the macrocycles cavities.

Novel fully screen printed flexible electrochemical sensor for the investigation of electron transfer between thiol functionalized viologen and gold clusters

Abstract A novel electrochemical sensor was fabricated for the investigation of the effect of electron transfer between glutathione (GS) protected gold nanoparticles (AuNPs) and a new C 4 -thiol functionalized viologen axle molecule (V 2+ -SH) on pseudorotaxane formation. The sensor was fully screen printed with silver (Ag) ink on a flexible polyethylene terephthalate (PET) substrate. Square wave voltammetry (SQWV) based response of the sensor toward V 2+ -SH in the presence of Au 25 or 4 nm-Au demonstrated one-electron transfer processes, based on 59 mV changes in peak potentials to more negative values. The response demonstrated the capability of the novel electrochemical sensor to be used for studying complex systems in volumes as small as 5–10 μL, with less sample preparation time.

Conformational Regulation of Cyclophanes by Formation of Pseudorotaxane Based on a Charge-transfer Complex

Abstract Cyclophanes consisting of diethylene glycol-substituted oligothiophene units as a component have been synthesized. Formation of pseudorotaxane arising from complexation of the cyclophanes and the cyclobis(paraquat-p-phenylene) (CBPQT4+) has been confirmed. It has also been discovered that conformational properties of the cyclophanes are strongly affected by the formation of pseudorotaxane.

Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes

Homo- and heterodivalent crown-ammonium pseudorotaxanes with different spacers connecting the two axle ammonium binding sites have been synthesized and characterized by NMR spectroscopy and ESI mass spectrometry. The homodivalent pseudorotaxanes are investigated with respect to the thermodynamics of divalent binding and to chelate cooperativity. The shortest spacer exhibits a chelate cooperativity much stronger than that of the longer spacers. On the basis of crystal structure, this can be explained by a noninnocent spacer, which contributes to the binding strength in addition to the two binding sites. Already very subtle changes in the spacer length, i.e., the introduction of an additional methylene group, cause substantial changes in the magnitude of cooperative binding as expressed in the large differences in effective molarity. With a similar series of heterodivalent pseudorotaxanes, the spacer effects on the barrier for the intramolecular threading step has been examined with the result that the shortest spacer causes a strained transition structure and thus the second binding event occurs slower than that of the longer spacers. The activation enthalpies and entropies show clear trends. While the longer spacers reduce the enthalpic strain that is present in the transition state for the shortest member of the series, the longer spacers become entropically slightly more unfavorable because of conformational fixation of the spacer chain during the second binding event. These results clearly show the noninnocent spacers to complicate the analysis of multivalent binding. An approximate description which considers the binding sites to be connected just by a flexible chain turns out to be more a rough approximation than a good model. The second conclusion from the results presented here is that multivalency is expressed in both the thermodynamics and the kinetics in different ways. A spacer optimized for strong binding is suboptimal for fast pseudorotaxane formation.

Systems chemistry: logic gates based on the stimuli-responsive gel–sol transition of a crown ether-functionalized bis(urea) gelator

A quite simple, achiral benzo-21-crown-7-substituted bis(urea) low-molecular weight gelator hierarchically assembles into helical fibrils, which further develop into bundles and finally form a stable gel in acetonitrile. The gel–sol transition can be controlled by three different molecular recognition events: K+ binding to the crown ethers, pseudorotaxane formation with secondary ammonium ions and Cl− binding to the urea units. Addition of a cryptand that scavenges the K+ ions and Ag+ addition to remove the chloride and bases/acids, which mediate pseudorotaxane formation, can reverse this process. With the gelator, and these chemical stimuli, a number of different systems can be designed that behave as logic gates. Depending on the choice of components, OR, AND, XOR, NOT, NOR, XNOR and INHIBIT gates have been realized. Thus, the gel–sol transition as a property of the system as a whole is influenced in a complex manner. For some cases, the type of logic gate is defined by input signal concentration so that an even more complex reaction of the gel towards the two input signals is achieved.

Pseudorotaxane based on tetraazamacrocyclic copper complex and dibenzocrown ether

Host-guest interactions between two electron-rich dibenzocrown ethers and electron deficient macrocyclic copper(II) tetraimine complexes lead to the formation of pseudorotaxane in solution. The interactions are enhanced with the copper(III) complex compared to that of the copper(II) form as shown by the electrochemical studies. The larger--30-membered dibenzocrown donor interacted with the copper complex stronger than the smaller ones as revealed by NMR and electrochemical methods. The thiolated form of the copper(II) tetraimine complex was self-assembled at the gold electrode forming an electroactive monolayer able to interact with the crown ether in the solution. These donor-acceptor interactions lead to an increase of the barrier properties of the layer and decreased the electron transfer rate between the copper centre and the gold electrode surface as proved by the voltammetric data.

Pronounced pH effects on the kinetics of cucurbit[7]uril-based pseudorotaxane formation and dissociation

Deprotonation of the two terminal COOH groups on a 4,4'-bipyridinium linear derivative leads to a pronounced slow down on the kinetics of threading and unthreading by the cucurbit[7]uril host.

Polymer architectures assisted by dynamic covalent bonds: synthesis and properties of boronate-functionalized polyrotaxane and graft polyrotaxane

Poly[2]rotaxane and graft polyrotaxane were synthesized from a mixture of poly(crown ether) as a trunk polymer, boronic-acid-terminated secondary ammonium salt as an axle component and diol as an end-capping group by pseudorotaxane formation and subsequent catalyst-free dehydrative bondage between the boronic acid and the diol moieties. It turned out that the chemical stability of these supramolecular architectures primarily depends on the bulkiness of the diol group as the end-capping moiety and the inherence originating from the dynamic covalent bond of boronate.

Effects of solvent on weak halogen bonds: Density functional theory calculations

AbstractIn recent years, many applications of solution‐phase halogen bonding in anion recognition, catalysis, and pseudorotaxane formation have been reported. Moreover, a number of thermodynamic data of halogen bonding interactions in organic solution are now available. To obtain detailed information of the influence of the surrounding medium on weak halogen bonds, a series of dimeric complexes of halobenzene (PhX) with three electron donors (H2O, HCHO, and NH3) were investigated by means of DFT/PBE calculations in this work. The PCM implicit solvation approach was utilized to include the effects of three solvents (cyclohexane, chloroform, and water) as representatives for a wide range of dielectric constant. In some cases, halogen‐bond distances are shown to shorten in solution, accompanied by concomitant elongation of the CX bonds. For the remaining systems, the intermolecular distances tend to increase or remain almost unchanged under solvent effects. In general, the solvent has a slight destabilizing effect on weak halogen bonds; the strength order of halogen bonds observed in vacuum remains unchanged in liquid phases. Particularly, the interaction strength attenuates in the order I > Br > Cl in solution, consistent with the experimental measurements of weak halogen bond door abilities. The similarities between halogen and hydrogen bonding in solution were also elucidated. The results presented herein would be very useful in future applications of halogen bonding in molecular recognition and medicinal chemistry. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Electrostatic Barriers in Rotaxanes and Pseudorotaxanes

The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature (1)H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT(4+) ring over the central BIPY(2+) unit is in excess of 17 kcal mol(-1) at 343 K. Further information about the nature of the BIPY(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY(2+) moiety or a central DNP unit flanked by a BIPY(2+) moiety. The threading and dethreading processes of the CBPQT(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor-acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (ΔG(f)(++)) of the CBPQT(4+) for the case of the thread composed of a DNP flanked by two BIPY(2+) units was found to be as high as 21.7 kcal mol(-1) at room temperature. These results demonstrate that we can effectively employ the BIPY(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems.

Complexation Equilibria Involving Salts in Non‐Aqueous Solvents: Ion Pairing and Activity Considerations

Complexation of anions, cations and even ion pairs is now an active area of investigation in supramolecular chemistry; unfortunately it is an area fraught with complications when these processes are examined in low polarity organic media. Using a pseudorotaxane complex as an example, apparent K(a2) values (=[complex]/{[salt](o)-[complex]}{[host](o)-[complex]}) for pseudorotaxane formation from dibenzylammonium salts (2-X) and dibenzo-[24]crown-8 (1, DB24C8) in CDCl(3)/CD(3)CN 3:2 vary with concentration. This is attributable to the fact that the salt is ion paired, but the complex is not. We report an equilibrium model that explicitly includes ion pair dissociation and is based upon activities rather than molar concentrations for study of such processes in non-aqueous media. Proper analysis requires both a dissociation constant, K(ipd), for the salt and a binding constant for interaction of the free cation 2(+) with the host, K(a5); K(a5) for pseudorotaxane complexation is independent of the counterion (500 M(-1)), a result of the complex existing in solution as a free cation, but K(ipd) values for the salts vary by nearly two orders of magnitude from trifluoroacetate to tosylate to tetrafluoroborate to hexafluorophosphate anions. The activity coefficients depend on the nature of the predominant ions present, whether the pseudorotaxane or the ions from the salt, and also strongly on the molar concentrations; activity coefficients as low as 0.2 are observed, emphasizing the magnitude of their effect. Based on this type of analysis, a method for precise determination of relative binding constants, K(a5), for multiple hosts with a given guest is described. However, while the incorporation of activity coefficients is clearly necessary, it removes the ability to predict from the equilibrium constants the effects of concentration on the extent of binding, which can only be determined experimentally. This has serious implications for study of all such complexation processes in low polarity media.

Supramolecular vesicle: triggered by formation of pseudorotaxane between cucurbit[6]uril and surfactant

Supramolecular vesicles are triggered by the inclusion complex formed by CB[6] and surfactant. The combination of CB[6] with the surfactant plays a key role in the vesicle formation.

Cu(ii) assisted self-sorting towards pseudorotaxane formation

A new triamino macrocycle shows Cu(II) templated self-sorting of a pseudorotaxane out of sixteen such possibilities from the mixture of nine components of a tridentate, four bidentate ligands and four transition metal ions.

Studies on [3]pseudorotaxane formation from a bis-azacrown derivative as host and imidazolium ion-derivatives as guest

A new host molecule, having two azacrown derivatives bridged by luminescent naphthalene diimide functionality, is found to form a [3]pseudorotaxane derivative with imidazolim ion-based guest molecules in non-polar solvents through hydrogen-bonded adduct formation. Depending upon the length of the covalent linker that links the imidazolium ion and the luminescent naphthalene fragment in the guests, the [3]pseudorotaxane adducts adopt different conformation or orientation with varying π-π/donor-acceptor interaction. The mechanism for the naphthalene-based luminescence quenching by NDI fragment on adduct formation was found to be a combination of static, as well as dynamic with static quenching as the dominant one.