Coordination-driven self-organization of switchable [2]rotaxane

A bistable porphyrin-containing [2]rotaxane is synthesized with a shuttling benzylic-amide macrocycle mechanically locked onto the thread subunit by formations of H-bonds with two potential stations. This macrocycle comprises two pyridine groups, which would be easily coordinated with zinc porphyrin. The Zn(II) coordination of porphyrin moiety on the thread subunit, immediately followed by the coordination with pyridine groups on the macrocycle, leads to an intermolecular axle-macrocycle-type nanostructure. Moreover, the self-assembly way shows great difference from the two states of the rotaxane monomer: The coordination-driven self-organization of the trans-state E2 leads to a network structure, whereas the cis-state Z2 gives birth to an irregular assembly.     

An Anionic Ring Locked into an Anionic Axle: A Metastable Rotaxane with Chemically Activated Electrostatic Stoppers

The use of the electrostatic stoppers concept in the field of mechanically interlocked molecules is reported; these stoppers are chemically sensitive end groups on a linear guest molecule that allows for the conversion of a pseudo-rotaxane species into a rotaxane complex by a change in the medium acidity. The chemical stimulus causes the appearance of negative charges on both ends of the linear component, passing from cationic to anionic, and causing a significant ring-to-axle electrostatic repulsion. This phenomenon has two different and simultaneous effects: 1) destabilizes the complex as a consequence of confining an anionic ring into an anionic axle, and 2) increases the dissociation energy barrier, thus impeding ring extrusion. This newly formed metastable rotaxane species is resistant to solvent and temperature effects and performs as a two-state degenerated molecular shuttle in solution.     

En route to a molecular sheaf: active metal template synthesis of a [3]rotaxane with two axles threaded through one ring

We report that a 2,2':6',2″-terpyridylmacrocycle-Ni complex can efficiently mediate the threading of two alkyl chains with bulky end groups in an active metal template sp(3)-carbon-to-sp(3)-carbon homocoupling reaction, resulting in a rare example of a doubly threaded [3]rotaxane in up to 51% yield. The unusual architecture is confirmed by X-ray crystallography (the first time that a one-ring-two-thread [3]rotaxane has been characterized in the solid state) and is found to be stable with respect to dethreading despite the large ring size of the macrocycle. Through such active template reactions, in principle, a macrocycle should be able to assemble as many axles in its cavity as the size of the ring and the stoppers will allow. A general method for threading multiple axles through a macrocycle adds significantly to the tools available for the synthesis of different types of rotaxane architectures.     

Triggering a [2]rotaxane molecular shuttle through hydrogen sulfide

A novel chemically-controlled [2]rotaxane molecular shuttle was successfully designed and synthesized. A H₂S-responsive bulk barrier was introduced between the two identical recognition stations of the [2]rotaxane to prevent dynamic shuttling of the macrocycle. Upon addition of H₂S, the complete intramolecular cascade reaction occurs in a controllable manner, resulting in removal of the bulk barrier and the shuttling motion of the macrocycle between the two stations recovers.     

Dynamic Control of Chiral Space Through Local Symmetry Breaking in a Rotaxane Organocatalyst

We report on a switchable rotaxane molecular shuttle that features a pseudo‐meso 2,5‐disubstituted pyrrolidine catalytic unit on the axle whose local symmetry is broken according to the position of a threaded benzylic amide macrocycle. The macrocycle can be selectively switched (with light in one direction; with catalytic acid in the other) with high fidelity between binding sites located to either side of the pyrrolidine unit. The position of the macrocycle dictates the facial bias of the rotaxane‐catalyzed conjugate addition of aldehydes to vinyl sulfones. The pseudo‐meso non‐interlocked thread does not afford significant selectivity as a catalyst (2–14 % ee), whereas the rotaxane affords selectivities of up to 40 % ee with switching of the position of the macrocycle changing the handedness of the product formed (up to 60 % Δee).     

A [3]rotaxane with two porphyrinic plates acting as an adaptable receptor

Following a multistep procedure, the copper(I)-templated strategy allowed preparation of a multifunctional [3]rotaxane. The dumbbell consists of a central two-bidentate chelate unit and two terminal stoppers. The two rings threaded on the rotaxane axis consist each of a 1,10-phenanthroline-incorporating macrocycle, rigidly connected to an appended zinc-complexed porphyrin. The copper(I) template can be removed, affording a free rotaxane whose two rings can glide freely along the axis and spin around it. The dumbbell being very long (approximately 85 A in its extended conformation from one stopper to the other), the porphyrin-porphyrin distance can be varied over a wide range. The two porphyrinic plates constitute the key elements of a receptor able to complex various guests between the plates. The ability of the threaded rings to move freely makes the host perfectly adjustable, allowing capture of geometrically very different guests. The copper(I)-complexed rotaxane also acts as an efficient receptor, although its adaptability is obviously more limited than that of its free rotaxane counterpart.     

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.     

A photochemically driven molecular-level abacus

A molecular-level abacus-like system driven by light inputs has been designed in the form of a [2]rotaxane, comprising the pi-electron-donating macrocyclic polyether bis-p-phenylene-34-crown-10 (BPP34C10) and a dumbbell-shaped component that contains 1) a Ru(II) polypyridine complex as one of its stoppers in the form of a photoactive unit, 2) a p-terphenyl-type ring system as a rigid spacer, 3) a 4,4'-bipyridinium unit and a 3,3'-dimethyl-4,4'-bipyridinium unit as pi-electron-accepting stations, and 4) a tetraarylmethane group as the second stopper. The synthesis of the [2]rotaxane was accomplished in four successive stages. First of all, the dumbbell-shaped component of the [2]rotaxane was constructed by using conventional synthetic methodology to make 1) the so-called "west-side" comprised of the Ru(II) polypyridine complex linked by a bismethylene spacer to the p-terphenyl-type ring system terminated by a benzylic bromomethyl function and 2) the so-called "east-side" comprised of the tetraarylmethane group, attached by a polyether linkage to the bipyridinium unit, itself joined in turn by a trismethylene spacer to an incipient 3,3'-dimethyl-4,4'-bipyridinium unit. Next, 3) the "west-side" and "east-side" were fused together by means of an alkylation to give the dumbbell-shaped compound, which was 4) finally subjected to a thermodynamically driven slippage reaction, with BPP34C10 as the ring, to afford the [2]rotaxane. The structure of this interlocked molecular compound was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, the co-conformational behavior of the molecular shuttle. The stable translational isomer is the one in which the BPP34C10 component encircles the 4,4'-bipyridinium unit, in keeping with the fact that this station is a better pi-electron acceptor than the other station. This observation raises the question- can the BPP34C10 macrocycle be made to shuttle between the two stations by a sequence of photoinduced electron transfer processes? In order to find an answer to this question, the electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2]rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units have been investigated. In an attempt to obtain the photoinduced abacus-like movement of the BPP34C10 macrocycle between the two stations, two strategies have been employed-one was based fully on processes that involved only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). Both mechanisms imply a sequence of four steps (destabilization of the stable translational isomer, macrocyclic ring displacement, electronic reset, and nuclear reset) that have to compete with energy-wasteful steps. The results have demonstrated that photochemically driven switching can be performed successfully by the sacrificial mechanism, whereas, in the case of the intramolecular mechanism, it would appear that the electronic reset of the system is faster than the ring displacement.     

Structural Changes of a Doubly Spin‐Labeled Chemically Driven Molecular Shuttle Probed by PELDOR Spectroscopy

Gaining detailed information on the structural rearrangements associated with stimuli‐induced molecular movements is of utmost importance for understanding the operation of molecular machines. Pulsed electron–electron double resonance (PELDOR) was employed to monitor the geometrical changes arising upon chemical switching of a [2]rotaxane that behaves as an acid–base‐controlled molecular shuttle. To this aim, the rotaxane was endowed with stable nitroxide radical units in both the ring and axle components. The combination of PELDOR data and molecular dynamic calculations indicates that in the investigated rotaxane, the ring displacement along the axle, caused by the addition of a base, does not alter significantly the distance between the nitroxide labels, but it is accompanied by a profound change in the geometry adopted by the macrocycle.     

Rotaxanes with fluorocarbon blocking groups

A novel [2]-rotaxane has been prepared in which perfluoropropyl stoppers have been utilized as electronic blocking groups. To further investigate this capability, a linear compound that was precapped with perfluoropropyl groups was also prepared. This compound did not demonstrate rotaxane formation when mixed with dibenzo-24-crown-8, providing further evidence that the perfluoropropyl groups were effective electronic stoppers for the macrocycle. [structure: see text]     

Halogen-Bonding Strapped Porphyrin BODIPY Rotaxanes for Dual Optical and Electrochemical Anion Sensing

Anion receptors employing two distinct sensory mechanisms are rare. Herein, we report the first examples of halogen-bonding porphyrin BODIPY [2]rotaxanes capable of both fluorescent and redox electrochemical sensing of anions. ¹H NMR, UV/visible and electrochemical studies revealed rotaxane axle triazole group coordination to the zinc(II) metalloporphyrin-containing macrocycle component, serves to preorganise the rotaxane binding cavity and dramatically enhances anion binding affinities. Mechanically bonded, integrated-axle BODIPY and macrocycle strapped metalloporphyrin motifs enable the anion recognition event to be sensed by the significant quenching of the BODIPY fluorophore and cathodic perturbations of the metalloporphyrin P/P^+. redox couple.     

Synthesis of a [2]rotaxane operated in basic environment

A tight [2]rotaxane with two chromophores as stoppers is described, in which the macrocycle is able to reversibly move by tuning of base. This moving process can result in intramolecular photo-induced electron transfer (PET), changing the photo-physical properties.     

Size Complementarity of Macrocyclic Cavities and Stoppers in Amide-Rotaxanes

New [2]rotaxanes were prepared by the threading and the slipping procedure, the latter having the advantage of not needing templating interactions. As a consequence, the first [2]rotaxane consisting of a tetraamide macrocycle and a pure hydrocarbon thread was synthesized (see 12a in Scheme 2). Sterically matching wheels and axles being the basic requirement of a successful slipping approach to rotaxanes, mono- and bishomologous wheels 5b , c with larger diameters than the parent 5a were synthesized and mechanically connected to amide axles 10a – c which were stoppered with blocking groups of different spatial demand (Scheme 1). The deslipping kinetics of the resulting rotaxanes 8a – c and 9a , b were measured and compared; it emerges that even slight increases in the wheel size require larger stoppers to stabilize the mechanical bond. Moreover, when the deslipping rate of 8a (amide wheel and amide axle) was determined in either DMF or THF, a strong dependence on the solvent polarity, which is caused by a differing extent of intramolecular H-bonds between the wheel and the axle, was observed. As expected, no such dependence was detected for rotaxane 12a (amide wheel and hydrocarbon axle) whose components cannot interact via H-bonds. The comparison of the sterically matching pairs of macrocycles and blocking groups, found by a systematic fitting based on the results of slipping and deslipping experiments, with other rotaxane types bearing similar stoppers allows conclusions concerning the relative cavity size of wheels of various structure.     

A Flexible Copper(I)‐Complexed [4]Rotaxane Containing Two Face‐to‐Face Porphyrinic Plates that Behaves as a Distensible Receptor

A new cyclic [4]rotaxane composed of two flexible bis‐macrocycles and two rigid axles is described. Each bis‐macrocycle consists of two rings attached to antipodal meso positions of a central Zn porphyrin through single C? C bonds. Each ring incorporates a 2,9‐diphenyl‐1,10‐phenanthroline chelation site. The axles contain two coplanar bidentate sites derived from the 2,2′‐bipyridine motif. The building blocks were assembled by using a one‐pot threading‐and‐stoppering reaction, which afforded the [4]rotaxane in 50 % yield. The “gathering‐and‐threading” effect of copper(I) was utilised in the formation of a [4]pseudorotaxane, which was immediately converted to the corresponding [4]rotaxane by a quadruple CuAAC stoppering reaction. The rotaxane contains two face‐to‐face zinc porphyrins, which allowed the coordination of ditopic guest substrates. The rotaxane host showed remarkable flexibility and was able to adjust its conformation to the guest size. It can be distended and accommodate rod‐like guests of 2.6 to 15.8 Å in length.     

A Fullerene‐Substituted Pillar[5]arene for the Construction of a Photoactive Rotaxane

Transformation of a methylene group of the pillar[5]arene scaffold into a ketone has been achieved by treatment with N‐bromosuccinimide followed by hydrolysis of the bromide intermediate and oxidation of the resulting secondary benzylic alcohol with BaMnO₄. Condensation of the resulting macrocycle including a ketone function with p‐toluenesulfonyl hydrazide followed by reaction of the corresponding tosylhydrazone with C₆₀ under modified Bamford–Stevens conditions gave a fulleropillar[5]arene derivative. This building block has been used to prepare a rotaxane. The resulting molecule combining the fullerene‐functionalized macrocycle with an axle bearing a porphyrin stopper is a photoactive molecular device in which the porphyrin emission is efficiently quenched by the fullerene moiety.     

Logic Gating by Macrocycle Displacement Using a Double‐Stranded DNA [3]Rotaxane Shuttle

Molecular interlocked systems with mechanically trapped components can serve as versatile building blocks for dynamic nanostructures. Here we report the synthesis of unprecedented double‐stranded (ds) DNA [2]‐ and [3]rotaxanes with two distinct stations for the hybridization of the macrocycles on the axle. In the [3]rotaxane, the release and migration of the “shuttle ring” mobilizes a second macrocycle in a highly controlled fashion. Different oligodeoxynucleotides (ODNs) employed as inputs induce structural changes in the system that can be detected as diverse logically gated output signals. We also designed nonsymmetrical [2]rotaxanes which allow unambiguous localization of the position of the macrocycle by use of atomic force microscopy (AFM). Either light irradiation or the use of fuel ODNs can drive the threaded macrocycle to the desired station in these shuttle systems. The DNA nanostructures introduced here constitute promising prototypes for logically gated cargo delivery and release shuttles.     

Effects of Axle‐Core,Macrocycle, and Side‐Station Structures on the Threading and Hydrolysis Processes of Imine‐Bridged Rotaxanes

Imine‐bridged rotaxanes are a new type of rotaxane in which the axle and macrocyclic ring are connected by imine bonds. We have previously reported that in imine‐bridged rotaxane 5 , the shuttling motion of the macrocycle could be controlled by changing the temperature. In this study, we investigated how the axle and macrocycle structures affect the construction of the imine‐bridged rotaxane as well as the dynamic equilibrium between imine‐bridged rotaxane 5 and [2]rotaxane 7 by using various combinations of axles ( 1 A , B ), macrocycles ( 2 a – e ), and side‐stations (XYL and TEG). In the threading process, the flexibility of the macrocycle and the substituent groups at the para position of the aniline moieties affect the preparation of the threaded imines. The size of the imine‐bridging station and the macrocyclic tether affects the hydrolysis of the imine bonds under acidic conditions.     

Formation of copper(I)-templated [2]rotaxanes using ��click�� methodology: influence of the base, the thread and the catalyst

Three new copper(I)-assembled [2]rotaxanes incorporating the same macrocycle and different axes containing a bipy, a phen or a terpy have been synthesized thanks to CuAAC reaction for attaching the stoppers. The influence of the nature of the base used for the stoppering reaction was investigated on the formation of the bipy-containing rotaxane. The yield of the [2]rotaxane synthesis was increased when using a phen as a coordinating unit in the thread with [Cu(CH₃CN)₄](PF₆) as catalyst. The strong influence of the nature of the catalyst was clearly evidenced for the formation of the terpy rotaxane, increasing the yield of the stoppering reaction from 0 to 95% by just substituting the Cu(I) catalyst. Finally, the best conditions found for our systems are the use of Na₂CO₃ as a base and Cu(tren??)Br as a catalyst.     

Synthesis of a bistable[3]rotaxane and its pH-controlled intramolecular charge-transfer behavior

A[3]rotaxane 1 involving two naphtho-21-crown-7(N21C7) rings and a dumbbell-shaped component 4 was synthesized.The dumbbell-shape molecule 4 contains one viologen nucleus,two secondary alky] ammonium sites and two phenyl stoppers.After threading the N21C7 ring with the thread-like ammonium guest 3,the copper(1)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition(CuAAC "click" reaction),was performed to connect the pseudorotaxanes with viologen unit 2 and generate 1. Through treating the[3]rotaxane by the base and acid circularly,the two N21C7 rings can make shuttling motion along the axle.Meanwhile the distance between the electron-deficient viologen unit and the electron-rich naphthol group can be adjusted precisely along with a remarkable intramolecular charge-transfer (CT) behavior.     

Energy transfer switching in a bistable molecular machine

[reaction: see text] A novel [2]rotaxane, containing pyrene and perylene bisimide as both stoppers and photoactive units, has been prepared. The shuttling of the protonated macrocycle switched the energy transfer (EN) from a pyrene moiety to a perylene moiety, which resulted in changes of fluorescence of the perylene moiety.