Rotaxane System Research Articles

Acid/base responsive pseudo[3]rotaxanes from amine naphthotubes and bis-pyridinium/isoquinolinium guests.

A novel cooperative pseudo[3]rotaxane system was successfully constructed by the inclusion complexation of two identical amine naphthotubes with a bis-pyridinium/isoquinolinium guest. Single crystal structure analysis revealed that weak Csp3-H⋯O hydrogen bonds between the two hosts are responsible for the positive cooperativity during the formation of pseudo[3]rotaxanes. Moreover, intermolecular charge-transfer interactions between the electron-rich host and the electron-poor guests were observed. The pseudo[3]rotaxanes showed pH-controllable association/dissociation processes with naked-eye color changes in solution.

Construction of Mechanically Interlocked Fluorescence Photoswitchable [2]Rotaxane with Aggregation-Induced Emission and Molecular Shuttling Behaviors

Here, we report the design, synthesis, and optical behaviors of a multistimuli responsive [2]rotaxane system constructed from noncovalent interactions between diarylethene (DAE)-based axle and a tetraphenylethene (TPE)-based macrocycle using a snapping supramolecular assembly approach. The shuttling behavior of the macrocycle (Ring-TPE) between dialkylammonium and urea stations could be realized by the influence of acid-base stimuli using 1H NMR spectroscopy. Switching between the open-form (OF) [2]rotaxanes (DAE-R1-OF and DAE-R2-OF) is highly reversible using external chemical stimuli. These rotaxane systems exhibit enhanced blue fluorescence in their aggregation states despite being weak or nonemissive in solution. A significant increase in fluorescence emission intensity of typical TPE in DAE-R1-OF and DAE-R2-OF at ca. 467 nm was observed as the water content was increased to ≥70% in CH3CN/H2O solvent mixtures. However, the fluorescence emission of TPE at its maximum aggregation state (95% fw) could be rapidly quenched upon UV light irradiation due to a very efficient energy transfer from the excited TPE (donor) to the closed form of DAE (acceptor). In contrast, OF DAE does not affect the fluorescence of the TPE unit, which remains at high level. Furthermore, the [2]rotaxanes showed excellent photochromic and fluorescent properties in solution, making them suitable for information storage and reversible photo-patterning applications.

Chiroptical switching in the azobenzene-based self-locked [1]rotaxane by solvent and photoirradiation.

Because of its dynamic reversible nature and simple regulation properties, rotaxane systems provided a good route for the construction of responsive supramolecular chiral materials. Here, we covalently encapsulate the photo-responsive guest molecule azobenzene (Azo) in a chiral macrocycle β-cyclodextrin (β-CD) to prepare self-locked chiral [1]rotaxane [Azo-CD]. On this basis, the self-adaptive conformation of [Azo-CD] was manipulated by solvent and photoirradiation; meanwhile, dual orthogonal regulation of the [1]rotaxane chiroptical switching could also be realized.

Multi-responsive rotaxane with tunable fluorescence under azobenzene-based benzoxazine structure

Rotaxanes are known for the mechanically interlocked molecules for decades. The present work demonstrates a method to prepare multi-responsive rotaxane by conjugating with a multi-responsive supramolecule. Benzoxazine dimers, N, N’-bis(3,5-dimethyl-2-hydroxybenzyl) methylamine derivatives, are good models because their simple chemistry. An azobenzene containing benzoxazine with remaining hydroxyl group for further conjugation with rotaxane was designed. The ring opening of rotaxane using fluorescent phenol provides benzoxazine dimer with metal ion responsive and fluorescent properties. Based on this concept, light responsive benzoxazine conjugated with rotaxane system shows light, metal ion and rotaxane shuttling responsiveness which can be followed by fluorescent signals. The present work shows simple way to develop rotaxanes with multi-responsive functions using supramolecular chemistry of benzoxazine dimer prepared from light responsive phenol.

Optically probing molecular shuttling motion of [2]rotaxane by a conformation-adaptive fluorophore

A bistable [2]rotaxane with a conformation-adaptive macrocycle bearing a 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) unit was synthesized, which could be utilized to optical probe the molecular shuttling motion of the functionalized rotaxane system. The UV–vis, 1H NMR and PL spectroscopic data clearly demonstrated that the DPAC ring was interlocked onto the thread and the fluorescence intensity of the DPAC unit in the macrocycle was effectively regulated by the location change of the macrocycle along the thread under acid/base stimulation, which was attributed to the modulation of the intramolecular photo-induced electron transfer between the DPAC unit and the methyltriazole (MTA) unit. This bistable rotaxane system containing a conformation-adaptive fluorophore unit in the macrocycle moiety opens an alternative way to design functional bistable mechanically interlocked molecules.

Controllable FRET processes towards ratiometric Fe3+ ion sensor of pseudo [3]rotaxane containing naphthalimide-based macrocyclic host donor and multi-stimuli responsive rhodamine-modified guest acceptor

A multi-stimuli responsive pseudo [3]rotaxane NI-DBC/RB-DBA was composed of a green-emissive (λem = 530 nm) naphthalimide-based macrocyclic host and a turn-on red-emissive (λem = 595 nm) rhodamine-based guest towards Fe3+ ion, which illustrated interesting photo-physical properties in dioxane/H2O (4:6, v/v) solutions. Due to the chelation-induced ring unfolded form of red-emissive rhodamine unit upon the addition of Fe3+ ion, mono-fluorophoric pseudo [3]rotaxane NI-DBC/RB-DBA could be transformed into bi-fluorophoric pseudo [3]rotaxane NI-DBC/RB-DBA-Fe3+ to reveal the Förster resonance energy transfer (FRET) process, which could be further identified by time-resolved photoluminescence (TRPL) measurements to obtain a higher energy transfer efficiency of 45.4% than that (21.1%) of its analogous mixture without host-guest interactions. Regarding pH effects, both pseudo [3]rotaxane systems NI-DBC/RB-DBA and NI-DBC/RB-DBA-Fe3+ could also be employed as pH sensors in acidic (pH = 1–6) and basic (pH = 8–12) conditions, respectively. Moreover, the pH range of 6–8 could be appropriate to maintain excellent sensing capabilities of pseudo [3]rotaxane NI-DBC/RB-DBA to detect Fe3+ ion. Accordingly, pseudo [3]rotaxane NI-DBC/RB-DBA could show the FRET-ON sensing behaviour towards Fe3+ ion, which was attributed to the turn-on red-emissive rhodamine-functionalized RB-DBA acceptor. Owing to the best FRET process and ratiometric fluorescence of bi-fluorophoric pseudo [3]rotaxane NI-DBC/RB-DBA-Fe3+ after Fe3+ ion detection, pseudo [3]rotaxane NI-DBC/RB-DBA provided a prominent limit of detection (LOD) value of 39.01 nM, which was further proceeded as simple test strips towards Fe3+ ion recognition and applied to the detection of Fe3+ ion in various pristine water samples. Therefore, the developed pseudo [3]rotaxane with host-guest interactions has been well designed and constructed to utilize FRET-ON processes as a novel Fe3+ ion sensor.

Challenging the HSAB principle on molecular machines’ precursors

The Hard-and-Soft-Acids-and-Bases (HSAB) principle has been challenged for the first time on molecular machines’ precursors, while studying on calixarene-based rotaxane complexes containing tris(N-phenylureido)-calix[6]arene as wheel and a 4,4’-bipyridinium dication’s units as axle; The results clearly indicate the invariance of genuine vs. hard and soft acidic - bonding complex with the interlocked molecular system is present in terms of maintaining the chemical reactivity combined pattern of charge exchange, charge tunneling, opening and stabilizing reactivity tendency as provided within the orthogonal chemical space of chemical global indicators – in first (electronegativity and chemical hardness) and second (chemical power and electrophilicity) generations and of their principles, respectively. Moreover, we asses for the first time the molecular machine precursor system with the 3D total charge density isosuface confinement within two ends of 3D electrostatic potentials branches of the optimized interlocked rotaxane system. Further investigations are thus called for best generalizing the present findings.

A Track-Based Molecular Synthesizer that Builds a Single-Sequence Oligomer through Iterative Carbon-Carbon Bond Formation

Summary We report an artificial molecular machine that moves along a track, iteratively joining building blocks to form an oligomer of single sequence with a continuous backbone of carbon-carbon bonds. The rotaxane features a macrocycle bearing an aldehyde-terminated chain and an axle containing different phosphonium ylides separated by rigid spacers. Each ylide is large enough to block the passage of the macrocycle, trapping the ring between the stopper at the terminus of original threading and the next ylide along the track. Once a building block is reachable, it is removed from the track through a Wittig reaction that adds it to the terminus of the growing chain. Operation on a four-barrier tetra(phosphonium salt) track produces a tetra(diphenylpropane) of single sequence linked through alkene bonds. The prototype extends the principle for molecular machines that build polymers by moving along tracks to the synthesis of sequence-encoded chains with continuous carbon backbones.

Locked and Loaded: Ruthenium(II)-Capped Cucurbit[n]uril-Based Rotaxanes with Antimetastatic Properties.

We report here the first coupling of Ru(II) units with cucurbit[6/7]uril-based pseudorotaxane ligands meant for biological application. The resulting ruthenium-capped rotaxanes were fully characterized, and a structure of one supramolecular system was determined by X-ray diffraction. Because the biological properties of Ru-based metallodrugs are tightly linked to the ligand-exchange processes, the effect of salt concentration on the hydrolysis of chlorides from the Ru(II) center was monitored by using 1H NMR spectroscopy. The biological activity of Ru(II)-based rotaxanes was evaluated for three selected mammalian breast cell lines, HBL-100, MCF-7, and MDA-MB-231. The antimetastatic activity of the assembled cationic Ru(II)-rotaxane systems, evaluated in migration assays against MCF-7 and MDA-MB-231 cell lines, is notably enhanced compared to that of RAPTA-C, a reference that was used. The indicated synergistic effect of combining Ru(II) with a pseudorotaxane unit opens a new direction in searching for anticancer supramolecular metallodrugs.

Pillar[5]arene Based [1]rotaxane Systems With Redox-Responsive Host-Guest Property: Design, Synthesis and the Key Role of Chain Length.

Pillar[n]arenes are a new type of macrocyclic compounds, which were first reported in 2008 by Ogoshi. They not only have cylindrical, symmetrical, and rigid structures, but also have many advantages, including easy functionalization and rich host-guest properties. On the other hand, mechanically interlocked molecules (MIMs) exist extensively in nature which have been artificially synthesized and widely applied in the fields of nanotechnology and biology. Although pillar[5]arene-based MIMs have been investigated much over recent years, pillar[5]arene-based [1]rotaxanes are very limited. In this report, we synthesized a series of amide-linked pillar[5]arene-based [1]rotaxanes with ferrocene unit as the stopper. Under the catalysis of HOBT/EDCL, the mono-amido-functionalized pillar[5]arenes were amidated with ferrocene carboxylic acid to constructed ferrocene-based [1]rotaxanes, respectively. The structure of the [1]rotaxanes were characterized by 1H NMR, 13C NMR, 2D NMR, mass spectroscopy, and single-crystal X-ray structural determination. In the experiment, the monofunctionalized pillar[5]arene was synthesized with a self-inclusion property, which allows for forming a pseudo-rotaxane. The key role is the length of the imine chain in this process. The formation of a rotaxane was realized through amidation of ferrocene dicarboxylic acid, which acted as a plug. In addition, due to the ferrocene units, the pillar[5]arene-based [1]rotaxanes perform electrochemically reversible property. Based on this nature, we hope these pillar[5]arene-based [1]rotaxanes can be applied in battery devices in the future.

Pumping a Ring-Sliding Molecular Motion by a Light-Powered Molecular Motor.

Designing artificial molecular machines to execute complex mechanical tasks, like coupling rotation and translation to accomplish transmission of motion, continues to provide important challenges. Herein, we demonstrated a novel molecular machine comprising a second-generation light-driven molecular motor and a bistable [1]rotaxane unit. The molecular motor can rotate successfully even in an interlocked [1]rotaxane system through a photoinduced cis-to -trans isomerization and a thermal helix inversion, resulting in concomitant transitional motion of the [1]rotaxane. The transmission process was elucidated via 1H NMR, 1H-1H COSY, HMQC, HMBC, and 2D ROESY NMR spectroscopies, UV-visible absorption spectrum, and density functional theory calculations. This is the first demonstration of a molecular motor to rotate against the appreciably noncovalent interactions between dibenzo-24-crown-8 and N-methyltriazolium moieties comprising the rotaxane unit, showing operational capabilities of molecular motors to perform more complex tasks.

Heterorotaxanes.

Heterorotaxanes, in which at least two types of macrocycles were introduced as the wheel components in rotaxanes, have attracted more and more attention during the past few decades owing to their unique structural features and intriguing properties. The coexistence of varied macrocycles endows the resultant heterorotaxanes not only versatile shuttling and switching behaviors but also great potential for the construction of functional rotaxane systems for applications. In this feature article, a survey of the successful synthesis of heterorotaxanes will be provided based on the various strategies towards the synthesis of heterorotaxanes, i.e. orthogonal binding approach, self-sorting approach, cooperative capture approach, active metal template approach, etc.

Strong and Selective Halide Anion Binding by Neutral Halogen-Bonding [2]Rotaxanes in Wet Organic Solvents.

The design and construction of neutral interlocked host molecules for anion recognition are rare. Using an active-metal template approach, the preparation of a family of neutral halogen bonding (XB) rotaxanes containing two, three and four iodotriazole groups integrated into the macrocycle and axle components is achieved. In spite of the interlocked hosts' neutrality, such rotaxane systems are capable of binding halide anions strongly and selectively in wet organic solvent mixtures. Importantly, halide-binding strength and selectivity can be modulated by varying the number and position of the halogen bond donor iodotriazole groups within the interlocked cavity; the rotaxane containing the largest number of halogen bond donor groups exhibits the highest halide anion-binding affinities. By varying the percentage of water content in the solvent, neutral XB donor-mediated anion-binding strength is also demonstrated to be highly sensitive to solvent polarity.

Acid-Base Switchable [2]- and [3]Rotaxane Molecular Shuttles with Benzimidazolium and Bis(pyridinium) Recognition Sites.

For the purpose of developing higher level mechanically interlocked molecules (MIMs), such as molecular switches and machines, a new rotaxane system was designed in which both the 1,2-bis(pyridinium)ethane and benzimidazolium recognition templating motifs were combined. These two very different recognition sites were successfully incorporated into [2]rotaxane and [3]rotaxane molecular shuttles which were fully characterized by 1 H NMR, 2D EXSY, single-crystal X-ray diffraction and VT NMR analysis. By utilizing benzimidazolium as both a recognition site and stoppering group it was possible to create not only an acid/base switchable [2]rotaxane molecular shuttle (energy barrier 20.9 kcal⋅mol-1 ) but also a [3]rotaxane molecular shuttle that displays unique dynamic behavior involving the simultaneous motion of two macrocyclic wheels on a single dumbbell. This study provides new insights into the design of switchable molecular shuttles. Due to the unique properties of benzimidazoles, such as fluorescence and metal coordination, this new type of molecular shuttle may find further applications in developing functional molecular machines and materials.

Five-State Molecular Shuttling of a Pair of [2]Rotaxanes: Distinct Outputs in Response to Acid and Base Stimuli.

In this study we synthesized two acid-/base-controllable [2]rotaxanes featuring aminodiazobenzene and aminocoumarin units, respectively, as chromophores and dibenzo[24]crown-8 and dibenzo[25]crown-8 units, respectively, as their macrocyclic components. Each [2]rotaxane contained N-alkylarylamine (ammonium) and N,N-dialkylamine (ammonium) centers as binding sites for their crown ether components. The absorption patterns of the chromophores were dependent on the position of the encircling macrocyclic component and the degree of protonation, with three distinct states (under acidic, neutral, and basic conditions) evident for each [2]rotaxane. The mixed [2]rotaxane system displayed stepwise and independent molecular shuttling behavior based on the degree of protonation of the amino groups in response to both the amount and strength of added acids or bases; as such, the system provided five different absorption signals as outputs that could be read using UV/Vis spectroscopy.

Directional Molecular Transportation Based on a Catalytic Stopper-Leaving Rotaxane System.

Ratchet mechanism has proved to be a key principle in designing molecular motors and machines that exploit random thermal fluctuations for directional motion with energy input. To integrate ratchet mechanism into artificial systems, precise molecular design is a prerequisite to control the pathway of relative motion between their subcomponents, which is still a formidable challenge. Herein, we report a straightforward method to control the transportation barrier of a macrocycle by selectively detaching one of the two stoppers using a novel DBU-catalyzed stopper-leaving reaction in a rotaxane system. The macrocycle was first allowed to thread onto a semidumbbell axle from the open end and subsequently thermodynamically captured into a nonsymmetrical rotaxane. Then, it was driven energetically uphill until it reached a kinetically trapped state by destroying its interaction with ammonium site, and was finally quantitatively released from the other end when the corresponding stopper barrier was removed. Although the directional transportation at the present system was achieved by discrete chemical reactions for the sake of higher transportation efficiency, it represents a new molecular transportation model by the strategy of using stopper-leavable rotaxane.

ChemInform Abstract: High‐Generation Organometallic Rotaxane Dendrimer

AbstractReview: 2 refs.

A Protein Rotaxane Controls the Translocation of Proteins Across a ClyA Nanopore.

Rotaxanes, pseudorotaxanes, and catenanes are supramolecular complexes with potential use in nanomachinery, molecular computing, and single-molecule studies. Here we constructed a protein rotaxane in which a polypeptide thread is encircled by a Cytolysin A (ClyA) nanopore and capped by two protein stoppers. The rotaxane could be switched between two states. At low negative applied potentials (<-50 mV) one of the protein stoppers resided inside the nanopore indefinitely. Under this configuration the rotaxane prevents the diffusion of protein molecules across the lipid bilayer and provides a useful platform for single-molecule analysis. High negative applied potentials (-100 mV) dismantled the interlocked rotaxane system by the forceful translocation of the protein stopper, allowing new proteins to be trapped inside or transported across the nanopore. The observed voltage threshold for the translocation of the protein stopper through the nanopore related well to the biphasic voltage dependence of the residence time measured for the freely diffusing protein stopper. We propose a model in which molecules translocate through a nanopore when the average dwell time decreases with the applied potential.

High-generation organometallic rotaxane dendrimer

Rotaxane dendrimers are a recent and novel family of mechanically interlocked dendrimers. Merging the unique characteristics of both rotaxanes and dendrimers endows the resultant rotaxane dendrimers with appealing topologic structures as well as promising properties. Thus, the design and construction of novel rotaxane dendrimers has attracted considerable attention during the past two decades [1]. However, only a very few successful examples of discrete, well-defined rotaxane dendrimers equipped with mechanically interlocked moieties on branches have been realized. Because in-depth investigations of the potential applications of rotaxane dendrimers requires the facile and efficient preparation of rotaxane dendrimers with complex and diverse structures, the development of a highly efficient synthetic strategy for producing high-generation rotaxane branched dendrimers is extremely desirable. Very recently, a novel approach for the facile construction of high-generation branched rotaxane dendrimers has been developed by Prof. Hai-Bo Yang and coworkers from East China Normal University as reported in Proc Natl Acad Sci USA [2]. In their report, the host-guest complexes of the pillar[5]arene and neutral alkyl chain components were selected as the rotaxane moiety. Moreover, inspired by their sustaining investigations on platinum-acetylide complexes, the platinum-acetylide moiety was selected for use as the connecting links for dendrimer growth. According to this design strategy, a neutral organometallic rotaxane was designed and synthesized as the key building block. Starting from such precursor, a novel family of rotaxane branched dendrimers with three different types of end-groups (including triisopropylsilyl-protected and unprotected alkyne, as well as a ferrocene group) up to the fourth generation have been successfully prepared via the sequential couplingdeprotection divergent growth processes. Remarkably, in the case of the resultant fourth generation rotaxane dendrimer G4, 45 rotaxane moieties were mono-dispersedly distributed in the dendrimer skeleton. This is the most complex discrete multiple rotaxane system ever reported (Scheme 1). Moreover, an interesting “rotaxane effect” has been observed in such systems. The existence of the pillar[5]arene wheels could enhance the rigidity of branches as well as the resultant rotaxane dendrimers. This was proven by detailed investigation of the rotaxane dendrimers and the corresponding model dendrimers without pillar[5] arene wheels by atomic force microscopy (AFM), transmission electron microscopy (TEM), diffusion ordered spectroscopy (DOSY), and dynamic light scattering (DLS) analysis. According to this inspiring study, the preparation of novel rotaxane dendrimers with well-defined topology structure should be feasible, thus laying the foundation for in-depth investigation of rotaxane dendrimers in dynamic supramolecular systems and materials science. Moreover, the facile surface modification may allow for the construction of functional dynamic supramolecular materials with potential uses in molecular devices, supramolecular cargos, and nanoelectronics.

Multistimuli Sensitive Behavior of Novel Bodipy-Involved Pillar[5]arene-Based Fluorescent [2]Rotaxane and Its Supramolecular Gel.

Fluorescent [2]rotaxane BC12P5 is successfully constructed with 1,4‐diethoxypillar[5]arene as wheel over a long alkyl axle with Bodipy chromophore as one stopper for the first time. NMR spectra clearly reveal its molecular shuttle nature triggered by multiple external stimuli including solvent polarity and temperature. In particular, the fluorescence nature introduced into [2]rotaxane BC12P5 renders it a good sensor for the external stimuli. Nevertheless, the supramolecular gel successfully fabricated from this novel rotaxane system via self‐assembly in dimethyl sulfoxide (DMSO) also shows reversible gel–sol phase transition upon multiple external stimuli such as heating/cooling, shaking/resting, or the addition of different anions. Interestingly, exposure of the supramolecular gel film to HCl or ammonia vapor induces the change in the film fluorescence intensity, endowing this system with a potential application in gas detecting.