Synthesis of a pH‐Sensitive Hetero[4]Rotaxane Molecular Machine that Combines [c2]Daisy and [2]Rotaxane Arrangements

The synthesis of a novel pH‐sensitive hetero[4]rotaxane molecular machine through a self‐sorting strategy is reported. The original tetra‐interlocked molecular architecture combines a [c2]daisy chain scaffold linked to two [2]rotaxane units. Actuation of the system through pH variation is possible thanks to the specific interactions of the dibenzo‐24‐crown‐8 (DB24C8) macrocycles for ammonium, anilinium, and triazolium molecular stations. Selective deprotonation of the anilinium moieties triggers shuttling of the unsubstituted DB24C8 along the [2]rotaxane units.     

A Strategy Utilizing a Recyclable Macrocycle Transporter for the Efficient Synthesis of a Triazolium‐Based [2]Rotaxane

A general synthesis of triazolium‐containing [2]rotaxanes, which could not be accessed by other methods, is reported. It is based on a sequential strategy starting from a well‐designed macrocycle transporter which contains a template for dibenzo‐24‐crown‐8 and a N‐hydroxysuccinimide (NHS) moiety. The sequence is: 1) synthesis by slippage of a [2]rotaxane building‐block; 2) its elongation at its NHS end; 3) the delivery of the macrocycle to the elongated part of the axle by an induced translational motion; 4) the contraction process to yield the targeted [2]rotaxane and recycle the initial transporter.     

Integrative Self‐Sorting: One‐Pot Synthesis of a Hetero[4]rotaxane from a Daisy‐Chain‐Containing Hetero[4]pseudorotaxane

The structural complexity of mechanically interlocked molecules are very attractive to chemists owing to the challenges they present. In this article, novel mechanically interlocked molecules with a daisy‐chain‐containing hetero[4]rotaxane motif were efficiently synthesized. In addition, a novel integrative self‐sorting strategy is demonstrated, involving an ABB‐type (A for host, dibenzo‐24‐crown‐8 (DB24C8), and B for guest, ammonium salt sites) monomer and a macrocycle host, benzo‐21‐crown‐7 (B21C7), in which the assembled species in hydrogen‐bonding‐supported solvent only includes a novel daisy‐chain‐containing hetero[4]pseudorotaxane. The found self‐sorting process involves the integrative recognition between B21C7 macrocycles and carefully designed components simultaneously containing two types of secondary ammonium ions and a host molecule, DB24C8 crown ether. The self‐sorting strategy is integrative to undertake self‐recognition behavior to form one single species of pseudorotaxane compared with the previous report. This self‐sorting system can be used for the efficient one‐pot synthesis of a daisy‐chain‐containing hetero[4]rotaxane in a good yield. The structure of hetero[4]rotaxane was confirmed by ¹H NMR spectroscopy and high‐resolution electrospray ionization (HR‐ESI) mass spectrometry.     

Efficient Intramolecular Energy Transfer between Two Fluorophores in a Bis‐Branched [3]Rotaxane

A bis‐branched [3]rotaxane, with two [2]rotaxane arms separated by an oligo(para‐phenylenevinylene) (OPV) fluorophore, was designed and investigated. Each [2]rotaxane arm employed a difluoroboradiaza‐s‐indacene (BODIPY) dye‐functionalized dibenzo[24]crown‐8 macrocycle interlocked onto a dibenzylammonium in the rod part. The chemical structure of the [3]rotaxane was confirmed and characterized by ¹H and ¹³C NMR spectroscopy and high‐resolution ESI mass spectrometry. The photophysical properties of [3]rotaxane and its reference systems were investigated through UV/Vis absorption, fluorescence, and time‐resolved fluorescence spectroscopy. An efficient energy‐transfer process in [3]rotaxane occurred from the OPV donor to the BODIPY acceptor because of the large overlap between the absorption spectrum of the BODIPY moiety and the emission spectrum of the OPV fluorophore; this shows the important potential of this system for designing functional molecular systems.     

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.     

Organometallic Rotaxanes with a Triazole Group in the Axle Component and Their Behavior as Ligands of PtII Complexes

Two ferrocenylmethyl ammonium salts were used as axle components of pseudorotaxanes with dibenzo[24]crown‐8. The pseudorotaxane with an alkyne terminal group in the axle component underwent a Cu‐catalyzed Huisgen coupling reaction (click reaction) with an alkyl azide to afford cationic [2]rotaxanes with a triazole group in the axle molecule. The rotaxane reacted with Ac₂O to produce neutral rotaxanes with an amide group in the axle component. Both cationic and neutral rotaxanes were treated with K[PtCl₃(CH₂?CH₂)] to form the Pt^II‐containing rotaxanes.     

Fluorescence Modulation in Tribranched Switchable [4]Rotaxanes

Two novel tribranched [4]rotaxanes with a 1,3,5‐triphenylene core and three rotaxane arms have been designed, synthesized, and characterized by ¹H and ¹³C NMR spectroscopies and HR‐ESI mass spectrometry. [4]Rotaxanes 1 and 2 each possess the same three‐armed skeleton. Each arm incorporates two distinguishable binding sites for a dibenzo[24]crown‐8 ring, namely a dibenzylammonium site and an N‐methyltriazolium site, and is terminated by a 4‐morpholino‐naphthalimide fluorophore as a stopper. [4]Rotaxane 1 has three di‐ferrocene‐functionalized dibenzo[24]crown‐8 rings whereas 2 has three simple dibenzo[24]crown‐8 rings interlocked with the thread component. Uniform shuttling motions of the three macrocycles in both 1 and 2 can be driven by external acid–base stimuli, which were confirmed by ¹H NMR spectroscopy. However, [4]rotaxanes 1 and 2 show distinct modes of fluorescence modulation in response to external acid–base stimuli. [4]Rotaxane 1 exhibits a remarkable fluorescence decrease in response to the addition of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) as a base, which can displace the ferrocene‐functionalized macrocycle from the dibenzylammonium station to the N‐methyltriazolium station. In contrast, the fluorescence intensity of [4]rotaxane 2 showed an enhancement with the addition of DBU. Time‐resolved fluorescence measurements have been performed. The different photoinduced electron‐transfer processes responsible for the fluorescence changes in the two molecular systems are discussed. Topological structures of this kind have significant potential for the design and construction of large and complex assemblies with controllable functions.     

An Interwoven Metal‐Organic Framework Combining Mechanically Interlocked Linkers and Interpenetrated Networks

New dibenzo[24]crown‐8 ether derivatives were prepared that contain appendages with thioether donors that can coordinate to a metal ion. These macrocycles were then combined with 1,2‐bis(pyridinium) ethane axles to create two types of [2]rotaxane ligands; those with the four thioether donors on the crown ether and those with six donor groups, four from the crown ether and two more attached to the stoppering groups of the dumbbell. The crown ethers and both types of [2]rotaxane ligands were allowed to react with Ag^I ions to form metal‐organic rotaxane framework (MORF) style coordination polymers. The interlocked hexadentate ligand forms the first example of a new type of lattice containing interwoven frameworks resulting from both interpenetration of frameworks due to the presence of an interlocked ligand and more classical interpenetration of independent frameworks.     

Self‐assembly of daisy chain oligomers from heteroditopic molecules containing secondary ammonium ion and crown ether moieties

MALDI‐TOF MS of the heteroditopic compound 2 ‐(benzylammoniomethyl)dibenzo‐24‐crown‐8 hexafluorophosphate ( 4 ) revealed oligomeric “daisy chain” species up to the hexamer. Similar results were obtained for 2‐(6′‐hydroxyhexylammoniomethyl)dibenzo‐24‐crown‐8 hexafluorophosphate ( 8 ). The complexations of two substituted dibenzylammonium salts, 2,2′‐dimethyldibenzylammonium hexaflurophosphate ( 9a ) and 2,2′,5‐trimethoxydibenzylammonium hexafluorophosphate ( 9b ), with dibenzo‐24‐crown‐8 were examined as models for slippage systems; association constants are reported for these systems. A crystal structure is reported for the new dimethylbenzylammonoium pseudorotaxane. The trimethoxy analog is shown to be capable of slippage formation of a rotaxane, albeit in low yield. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 975–985, 2010     

2]Rotaxanes containing pyridinium-phosphonium axles and 24-crown-8 ether wheels

A triethylphosphonium group attached to a pyridinium ethane moiety can be used as an axle for the self-assembly of [2]pseudorotaxanes and [2]rotaxanes. Although [2]pseudorotaxane formation is limited due to the bulk of the PR4+ group, [2]rotaxanes can be formed utilising 24-crown-8 ether, benzo-24-crown-8 ether and naphtho-24-crown-8 ether. The synthesis of these [2]rotaxanes and the X-ray structure of the [2]rotaxane containing a 24-crown-8 ether wheel are described. When the crown ether contains an aromatic group two possible conformational isomers exist; these are identified at low temperature by 1H and 31P NMR spectroscopy.     

Coordination-driven self-assembly of cavity-cored multiple crown ether derivatives and poly[2]pseudorotaxanes

The synthesis of a new 120 degree diplatinum(II) acceptor unit and the self-assembly of a series of two-dimensional metallacyclic polypseudorotaxanes that utilize both metal-ligand and crown ether-dialkylammonium noncovalent interactions are described. Judiciously combining complementary diplatinum(II) acceptors with bispyridyl donor building blocks, with an acceptor and/or donor possessing a pendant dibenzo[24]crown-8 (DB24C8) moiety, allows for the formation of three new rhomboidal bis-DB24C8, one new hexagonal tris-DB24C8, and four new hexakis-DB24C8 metallacyclic polygons in quantitative yields. The size and shape of each assembly, as well as the location and stoichiometry of the DB24C8 macrocycle, can be precisely controlled. Each polygon is able to complex two, three, or six dibenzylammonium ions without disrupting the underlying metallacyclic polygons, thus producing eight different poly[2]pseudorotaxanes and demonstrating the utility and scope of this orthogonal self-assembly technique. The assemblies are characterized with one-dimensional multinuclear ((1)H and (31)P) and two-dimensional ((1)H-(1)H COSY and NOESY) NMR spectroscopy as well as mass spectrometry (ESI-MS). Further analysis of the size and shape of each assembly is obtained through molecular force-field simulations. (1)H NMR titration experiments are used to establish thermodynamic binding constants and poly[2]pseudorotaxane/dibenzylammonium stoichiometries. Factors influencing the efficiency of poly[2]pseudorotaxane formation are discussed.     

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 pH‐Sensitive Lasso‐Based Rotaxane Molecular Switch

The synthesis of a pH‐sensitive two‐station [1]rotaxane molecular switch by self‐entanglement of a non‐interlocked hermaphrodite molecule, containing an anilinium and triazole moieties, is reported. The anilinium was chosen as the best template for the macrocycle benzometaphenylene[25]crown‐8 (BMP25C8) and allowed the self‐entanglement of the molecule. The equilibrium between the hermaphrodite molecule and the pseudo[1]rotaxane was studied by ¹H NMR spectroscopy: the best conditions of self‐entanglement were found in the less polar solvent CD₂Cl₂ and at high dilution. The triazole moiety was then benzylated to afford a benzyltriazolium moiety, which then played a dual role. On one hand, it acts as a bulky gate to trap the BMP25C8, thus to avoid any self‐disentanglement of the molecular architecture. On another hand, it acts as a second molecular station for the macrocycle. At acidic pH, the BMP25C8 resides around the best anilinium molecular station, displaying the lasso [1]rotaxane in a loosened conformation. The deprotonation of the anilinium molecular station triggers the shuttling of the BMP25C8 around the triazolium moiety, therefore tightening the lasso.     

Neutralization of a sec-ammonium group unusually stabilized by the "rotaxane effect": synthesis, structure, and dynamic nature of a "free" sec-amine/crown ether-type rotaxane

A fifteen-year riddle has been settled: neutralization, the most popular chemical event, of a crown ether/sec-ammonium salt-type rotaxane has been achieved and a completely nonionic crown ether/sec-amine-type rotaxane isolated. A [2]rotaxane was prepared as a typical substrate from a mixture of dibenzo[24]crown-8 ether (DB24C8) and sec-ammonium hexafluorophosphate (PF(6)) with a terminal hydroxy group through end-capping with 3,5-dimethylbenzoic anhydride in the presence of tributylphosphane as a catalyst in 90% yield. A couple of approaches to the neutralization of the ammonium rotaxane were investigated to isolate the free sec-amine-type rotaxane by decreasing the degree of thermodynamic and kinetic stabilities. One approach was the counteranion-exchange method in which the soft counterion PF(6)(-) was replaced with the fluoride anion by mixing with tetrabutylammonium fluoride, thus decreasing the cationic character of the ammonium moiety. Subsequent simple washing with a base allowed us to isolate the free sec-amine-type rotaxane in a quantitative yield. The other approach was a synthesis based on a protection/deprotection protocol. The acylation of the sec-ammonium moiety with 2,2,2-trichloroethyl chloroformate gave an N-carbamated rotaxane that could be deprotected by treating with zinc in acetic acid to afford the corresponding free sec-amine-type rotaxane in a quantitative yield. The structure of the free sec-amine-type rotaxane was fully confirmed by spectral and analytical data. The generality of the counteranion-exchange method was also confirmed through the neutralization of a bisammonium-type [3]rotaxane. The mechanism was studied from the proposed potential-energy diagram of the rotaxanes with special emphasis on the role of the PF(6)(-) counterion.     

From Binuclear Complexes to Molecular Necklaces: Incorporating Flexible Ligands into Rotaxanes

The conversion of binuclear complexes into larger molecular necklaces can be achieved through rigidifying flexible ligands by threading them through a crown ether to form either an interpenetrated [2]pseudorotaxane or a permanently interlocked [2]rotaxane. The resulting complexes and assemblies are characterized by ¹H and DOSY NMR in solution and single‐crystal X‐ray diffraction in the solid‐state.     

Energy Transfer and Concentration‐Dependent Conformational Modulation: A Porphyrin‐Containing [3]Rotaxane

A zinc porphyrin‐containing [3]rotaxane A was synthesized through a copper(I)‐catalyzed azide–alkyne cycloaddition (CuAAC) reaction. Energy donors and acceptor porphyrin were introduced to dibenzo[24]crown‐8 (DB24C8) and dibenzyl ammonium (DBA) units of [3]rotaxane A to understand the intramolecular energy transfer process. Investigations of the photophysical properties of [3]rotaxane A demonstrated that the intramolecular efficient energy transfer readily occurred from the donors on the wheels to the porphyrin center on the axis. The fluorescence of energy donors in the region of 400 to 450 nm was efficiently absorbed by the porphyrin acceptor under irradiation at 345 nm, and finally a red light emission at about 600 nm was achieved. Further investigation indicated that the conformation of [3]rotaxane A was self‐modulated by changing its concentration in CH₂Cl₂. The triazole groups on the wheel coordinated or uncoordinated to Zn²⁺ through intramolecular self‐coordination with the change in the concentration of [3]rotaxane A in CH₂Cl₂. Therefore, this conformational change was reversible in a non‐coordinating solvent such as CH₂Cl₂ but inhibited in a coordinating solvent such as THF. Such interesting behaviors were rarely observed in porphyrin derivatives. This self‐modulation feature opens up the possibility of controlling molecular conformation by varying concentration.     

Dendritic Pseudorotaxanes

Self-organization is the key . A series of dendritic pseudorotaxanes were efficiently constructed from complementary building blocks—namely, a three-armed, triply charged ammonium salt and the first, second, and third generations of benzyl ether dendrons bearing the dibenzo[24]crown-8 moiety. The pseudorotaxane arising from the third-generation dendron is shown in the picture.     

Coordination polymers containing rotaxane linkers

A class of coordination polymers in which the linking ligands are mechanically interlocked rotaxane molecules is reviewed. To date, four different, axle - wheel templating motifs have been used to create the [2]pseudorotaxane linkers for these unique solid-state materials; (1) protonated diaminoalkane axles with cucurbit[6]uril wheels, (2) 1,2-bis(4,4'-bipyridinio)ethane axles with dibenzo[24]crown-8 wheels, (3) 2,6-naphthalene dicarboxylate axles with tetra-imidazolium macrocycle wheels and (4) a Cu(i) complex of a 1,10-phenanthroline containing dicarboxylate axle with a 1,10-phenanthroline containing crown ether wheel. The synthesis and solid state structure of each coordination polymer is described. The future directions of this area of research and some designs for the next generation of these compounds are discussed.     

Translational isomerism in a [3]catenane and a [3]rotaxane

[structure: see text] Post-assembly covalent modification using Wittig chemistry of [2]rotaxane ylides, wherein NH(2)(+) centers in the dumbbell-shaped components are recognized by dibenzo[24]crown-8 (DB24C8) rings, has afforded a [3]catenane and a [3]rotaxane with a precise and synthetically prescribed shortage of DB24C8 rings. The nondegenerate pairs of translational isomers present in both of these interlocked molecular compounds provide the fundamental platform on which to construct sensory devices and nanochemomechanical systems.     

Toward High‐Generation Rotaxane Dendrimers That Incorporate a Ring Component on Every Branch: Noncovalent Synthesis of a Dendritic [10]Pseudorotaxane with 13 Molecular Components

By taking advantage of the fact that cucurbit[6]uril (CB[6]) forms exceptionally stable host–guest complexes with protonated amines, and that its homologue CB[8] can encapsulate a pair of electron‐rich and electron‐deficient guest molecules to form a stable 1:1:1 complex, we synthesized a novel dendritic [10]pseudorotaxane, or second‐generation rotaxane dendrimer (from a topological point of view), in which 13 molecular components are held together by noncovalent interactions. A triply branched molecule containing an electron‐deficient bipyridinium unit on each branch formed a branched [4]pseudorotaxane with 3 equivalents of CB[8]. Addition of 3 equivalents of 2,6‐dihydroxynaphthalene produced a first‐generation rotaxane dendrimer, which was characterized by NMR spectroscopy and CSI‐MS. The reaction of the branched [4]pseudorotaxane with 3 equivalents of a triply branched molecule that has an electron‐donor unit at one arm and CB[6]‐containing units at the other two gave the dendritic [10]pseudorotaxane, the structure of which was confirmed by NMR spectroscopy, UV/Vis titration experiments, and CSI‐MS.