Rotaxane-based molecular muscles

Accordion-Like Motion in Electrochemically Switchable Crown Ether/Ammonium Oligorotaxanes

Reversible oxidation reactions in electrochemically switchable oligorotaxanes with tetrathiafulvalene (TTF) decorated 24-crown-8 ether wheels generate intramolecular mixed-valence and radical-cation interactions between the wheels. This induces shuttling of the wheels and a contraction of inter-wheel distances. Further oxidation generates repulsive forces between the TTFs and maximizes the inter-wheel distances instead. These interactions and co-conformational changes were not observed for structurally similar controls in which acetyl groups along the axle prevent translational motion of the wheels. This operation mode of oligorotaxanes, which is reminiscent of an accordion-like motion, is promising for functional materials and nanodevices such as piston-type rotaxane motors.

Light triggers molecular shuttling in rotaxanes: control over proximity and charge recombination

The lifetime of a charge separated state is enhanced by the effects of solvent polarity and the coordination controlled shuttling of a dumbbell in a porphyrin/fullerene rotaxane.

We present the synthesis of novel rotaxanes based on mechanically interlocked porphyrins and fullerene and their advanced investigations by means of photophysical measurements. To this end, a fullerene-capped dumbbell-type axle containing a central triazole was threaded through strapped (metallo)porphyrins—either a free-base or a zinc porphyrin. Femtosecond-resolved transient absorption measurements revealed charge-separation between the porphyrin and fullerene upon light excitation. Solvent polarity and solvent coordination effects induced molecular motion of the rotaxanes upon charge separation and enabled, for the first time, subtle control over the charge recombination by enabling and controlling the directionality of shuttling.

Self-templated synthesis of amide catenanes and formation of a catenane coordination polymer

A self-templation strategy was used to synthesise isophthalamide [2]catenanes of various sizes in up to 51% yield without the need for metal ions as templates or mediators of covalent bond formation. Using this strategy a bis-monodentate catenane was prepared incorporating exohedral pyridine units. Upon complexation of this ligand with AgOTf a one-dimensional coordination polymer was obtained in the solid state in which both macrocycles of the catenane are involved in binding to the metal nodes, resulting in a rare example of a coordination assembly in which mechanical bonds are incorporated into the structure backbone.

Tetrathiafulvalene-calix[4]pyrrole: a versatile synthetic receptor for electron-deficient planar and spherical guests

The chemistry of tetrathiafulvalene-calix[4]pyrrole is reviewed with focus on conformational behavior, receptor properties and ionically controlled electron transfer processes.

Dynamic self-assembly of ions with variable size and charge in solution

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids. Here, we present evidence that the solution-based ionic self-assembly process leading to ionoids is a general phenomenon by characterizing new ionic building blocks which are capable of generating loosely bound globular and anisotropic structures similar to those in the established system. Using new cationic and anionic molecules, we show that variations in the size ratio between cationic and anionic component mainly affect size, shape and durability of the ionic clusters. Utilizing dynamic light scattering (DLS), continuously monitored phase-analysis light scattering (cmPALS) and continuous wave electron paramagnetic resonance (CW EPR) spectroscopy, we can thus define generalized ionic ratios, in which specific combinations of ionic compounds with certain size and charge densities are able to form these soft yet durable and long-lived ionic clusters. Furthermore, we characterize the temporal development of our dynamically self-assembled structures in solution from the level of the individual ionic building blocks to stable clusters with minimum lifetimes of months through previously established ionoid evolution diagrams (IEDs). The direct comparison of various cluster systems with respect to their shape, size and charges allows correlations of structural changes of the individual building blocks with the fate of self-assembled entities inside the crafted IEDs. This work generalizes the concept of ionoid formation to ions of specific sizes and charge densities, which may broaden the scope of this new type of highly dynamic and soft yet remarkably durable structures in the field of supramolecular chemistry.

Recently it was found that at ambient temperatures and in specific ternary solvents a cationic macrocyclic tetraimidazolium molecular box and small dianionic salts can self-assemble into highly defined, colloid-like ionic clusters, called ionoids.

Porphyrin cage compounds based on glycoluril – from enzyme mimics to functional molecular machines

This Feature Article gives an overview of the application of glycoluril-based porphyrin cage compounds in host–guest chemistry, allosterically controlled self-assembly, biomimetic catalysis, and polymer encoding.

Mechanical properties of supramolecular polymeric materials cross-linked by donor–acceptor interactions

We prepare a tough elastomer with aromatic donor and acceptor molecules introduced on the same polymer side chain.

Differences in solubilities, crystal structures, NMR spectra and fluorescence emissions induced by potassium cation/benzo-21-crown-7 molecular recognition

Changes of the fundamental properties of host–guest pairs induced by potassium cation complexation were investigated.

LCST behavior controlled by size-matching selectivity from low molecular weight monomer systems

LCST behavior was controlled by crown ether–cation recognition motifs via size-matching selectivity.

Fluorescent molecular spring that visualizes the extension and contraction motions of a double-stranded helicate bearing terminal pyrene units triggered by release and binding of alkali metal ions

A unique springlike motion of a fluorescent pyrene-terminated double-stranded helicate is visualized by the catch and release of alkali metal ions.

A [2]pseudorotaxane based on a pillar[6]arene and its application in the construction of a metallosupramolecular polymer

A novel metallosupramolecular polypseudorotaxane was constructed by pillar[6]arene-based host–guest recognition and metal coordination.

Towards a hexa-branched [7]rotaxane from a [3]rotaxane via a [2+2+2] alkyne cyclotrimerization process

Herein we report a facile synthetic route for the preparation of a hexa-branched [7]rotaxane by using Co-catalyzed [2+2+2] alkyne cyclotrimerization from a [3]rotaxane.

Gel-based soft actuators driven by light

Gels that display light-induced motile, life-like actions are reviewed and their potential applications as light-driven soft actuators are also discussed.

Heteroleptic copper phenanthroline complexes in motion: From stand-alone devices to multi-component machinery

Two and a half decades of copper phenanthroline-based switches, devices and machines have illustrated the rich dynamic nature of these metal complexes. With an emphasis on the metal-ligand dissociation as the rate-determining step the present review summarizes not only spectacular examples of machinery, but also highlights rate data collected during a variety of investigations. Copper-ligand exchange reactions are mostly triggered by redox processes, addition of metal ions or addition of ligands. While the rate data spread over >8 orders of magnitude, individual effects of solvent, steric bulk, flexibility, σ-basicity and the trajectory (intra- vs. intermolecular dissociation) have large impact. Unfortunately, in many cases the exact mechanism in the rate-determining step (nucleophile-induced vs. monomolecular metal-ligand dissociation) has not been determined, suggesting to invest further efforts in the physical (in)organic chemistry of such coordination-driven systems.

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.

Gold-Clip-Assisted Self-Assembly and Proton-Coupled Expansion-Contraction of a Cofacial FeIII -Porphyrin Cage

A molecular cage {Au₈ (μ-PAnP)₄ [Fe(H₂ O)₂ (TPyP)]₂ (OTf)₂ }(OTf)₈ (1) composed of two cofacial Fe^III -porphyrin can be self-assembled from the gold clip [Au₂ (PAnP)Cl₂ ] and Fe³⁺ (H₂ O)₂ (TPyP)⁺ (PAnP=9,10-bis(diphenylphosphino)anthracene, TPyP=meso-tetra(4-pyridyl)porphyrinato). The height of the cage is 8.579(3) Å. The addition of a base to a solution of the cage leads to a contracted and twisted cage {[Au₈ (μ-PAnP)₄ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (2), which has a height of ≈4.4 Å and porphyrin-porphyrin torsional angle of ≈20°. The contracted cage can be synthesized independently from the gold clip and Fe₂ (μ-O)(TPyP)₂ . The spectroscopy and crystal structure of an unclipped analog of the contracted cage, {[AuPPh₃ )₈ [Fe₂ (μ-O)(TPyP)₂ ]}(OTf)₈ (3), supports the DFT-calculated structure of 2. NMR and UV/Vis titrations show that the expansion-untwisting and contraction-twisting of the cage is reversible.

Naphthalene containing amino-ether macrocycle based Cu(ii) templated [2]pseudorotaxanes and OFF/ON fluorescence switching via axle substitution

A new naphthalene containing macrocycle, NaphMC, and a new fluorophoric bidentate linear axle derivative of 5,5'-dimethyl-2,2'-bipyridine (L3) along with two other ligands 1,10-phenanthroline (L1) and 5,5'-dimethyl-2,2'-bipyridine (L2) are explored towards the synthesis of Cu(ii) templated [2]pseudorotaxanes. All ternary complexes are well characterized by ESI-MS, UV/Vis, EPR spectroscopy, elemental analysis and emission spectroscopic studies. Single crystal X-ray diffraction studies confirm the geometry around the Cu(ii) center as a distorted trigonal bipyramid via the contribution of [3 + 2] orthogonal motifs of the wheel (NaphMC) and the bidentate chelating ligands L1 and L2 in the cases of pseudorotaxanes, CuPR1 and CuPR2, respectively. Furthermore, the fluorescence "OFF" state of the fluorophoric axle L3 is achieved via threading it to the Cu(ii) complex of NaphMC, whereas fluorescence switching "ON" is demonstrated by the substitution of L3 of CuPR3 with a stronger chelating ligand L1.

Dynamic Motion of Organic Ligands in Polar Layered Cobalt Phosphonates

By introducing the polar methoxy group into phenyl- or benzyl-phosphonate ligands, four cobalt phosphonates with layered structures are obtained, namely, [Co(4-mopp)(H₂ O)] (1), [Co(4-mobp)(H₂ O)] (2), [Co(3-mopp)(H₂ O)] (3), and [Co(3-mobp)(H₂ O)] (4), where 4- or 3-moppH₂ is (4- or 3-methoxyphenyl)phosphonic acid and 4- or 3-mobpH₂ is (4- or 3-methoxybenzyl)phosphonic acid. Compounds 1, 2, and 4 crystallize in the polar space groups Pmn2₁ or Pna2₁ , whereas compound 3 crystallizes in the centrosymmetric space group P2₁ /n. The layer topologies in the four structures are similar and can be viewed as perovskite type, where the edge-sharing [Co₄ O₄ ] rhombi are capped by the PO₃ C groups. The phenyl and MeO groups in compounds 1-3 are heavily disordered, whereas that in 4 is ordered. Structural comparison based on the data at 296 and 123 K reveals distinct dynamic motion of the organic groups in compounds 1 and 2. The fluctuation of the polar MeO groups in these two compounds is confirmed by dielectric relaxation measurements. In contrast, the fluctuation of polar groups in compounds 3 and 4 is not evident. Interestingly, the dehydrated samples of 3 and 4 (i.e., 3-de and 4-de) exhibit one-step and two-step phase transitions associated with the motion of polar organic groups, as proven by DSC and dielectric measurements. The magnetic properties of compounds 1-4 are investigated, and strong antiferromagnetic interactions are found to mediate between the magnetic centers through μ-O(P) and O-P-O bridges.

A pyridinium/anilinium [2]catenane that operates as an acid-base driven optical switch

A two-station [2]catenane containing a large macrocycle with two different recognition sites, one bis(pyridinium)ethane and one benzylanilinium, as well as a smaller DB24C8 ring was synthesized and characterized. ¹H NMR spectroscopy showed that the DB24C8 ring can shuttle between the two recognition sites depending on the protonation state of the larger macrocycle. When the aniline group is neutral, the DB24C8 ring resides solely at the bis(pyridinium)ethane site, while addition of acid forms a charged benzylanilinium site. The DB24C8 then shuttles between the two charged recognition sites with occupancy favoring the bis(pyridinium)ethane site by a ratio of 4:1. The unprotonated [2]catenane has a deep yellow/orange color when the DB24C8 ring resides solely at the bis(pyridinium)ethane site and changes to colorless when the crown ether is shuttling (i.e., circumrotating) back and forth between the two recognition sites thus optically signalling the onset of the shuttling dynamics.

Dual stimuli-responsive rotaxane-branched dendrimers with reversible dimension modulation

With the aim of mimicking biological machines, in which the delicate arrangement of nanomechanical units lead to the output of specific functions upon the external stimulus, the construction of dual stimuli-responsive rotaxane-branched dendrimers was realized in this study. Starting from a switchable organometallic [2]rotaxane precursor, the employment of a controllable divergent approach allowed for the successful synthesis of a family of rotaxane-branched dendrimers up to the third generation with 21 switchable rotaxane moieties located on each branch. More importantly, upon the addition and removal of dimethylsulfoxide (DMSO) molecule or acetate anion as the external stimulus, the amplified responsiveness of the switchable rotaxane units endowed the resultant rotaxane-branched dendrimers the solvent- or anion-controlled molecular motions, thus leading to the dimension modulation. Therefore, we successfully constructed a family of rotaxane-branched dendrimers with dual stimuli-responsiveness that will be a privileged platform for the construction of dynamic supramolecular materials.

Mechanically interlocked molecules are extensively applied as artificial molecular machines but rotaxane-branched dendrimers are rarely explored because of synthetic challenges. Here the authors present the construction of dual stimuli-responsive rotaxane-branched dendrimer which can be stimulated by DMSO or acetate ions.

A Nanosized Phenylene-Ethynylene-Butadiynylene [2]Catenane

In a convergent, template-directed synthesis, an efficient route to a phenylene-ethynylene-butadiynylene based [2]catenane is described. The key step is performed by the aminolysis of the corresponding precatenane, which is obtained by a sequence of metal-catalyzed cross-coupling and desilylation reactions. The cyclization reaction leads besides the [2]precatenane to a variety of larger precatenanes and offers an attractive approach to mechanically interlocked structures of different size.

Stimuli-Responsive Supramolecular Assemblies Constructed from Pillar[ n]arenes

Supramolecular assemblies are constructed from at least two molecules through various noncovalent bonding modes such as hydrogen bonding, cationic-anionic electrostatic interactions, aromatic interactions, metal-ligand bonding, hydrophobic-hydrophilic interactions, and charge-transfer interactions. Owing to the dynamic and reversible nature of these noncovalent bonds, the assembly and disassembly of these molecules are dynamic and reversible. Molecules self-assemble to form the most conformationally and thermally stable structures through these noncovalent interactions. The formation of these noncovalent interactions is affected by the properties of the environment such as its polarity, temperature, and pressure; thus, the structure of the assembled compounds is determined by the environment. The sizes and shapes of the supramolecular assemblies play an important role in determining their functions. Therefore, controlling their size and shape is important. Introducing stimuli-responsive groups into supramolecular assemblies is a useful way to control their size and shape. Controlling supramolecular structures and motions with external stimuli, i.e., periodic and rotational motions on the molecular scale, structures, and molecular weights at the nano- and micrometer scales, visible shrinking/expansion, and adhesive behavior at a macroscopic scale, is very useful. Macrocyclic host molecules are useful building blocks for the construction of stimuli-responsive supramolecular assemblies because their host ability can be tuned by changing the shape and electron density of the cavity. The size-dependent hosting ability of the cavity is similar to the lock-and-key model in biological systems. Stimuli-responsive supramolecular assemblies have been developed by using macrocyclic compounds such as cyclodextrins, cucurbit[ n]urils, calix[ n]arenes, crown ethers, and related macrocycles. We successfully developed new pillar-shaped macrocyclic hosts in 2008, which were coined pillar[ n]arenes. The unique structural features of pillar[ n]arenes allowed new properties. This year, 2018, marks one decade of research into pillar[ n]arene chemistry, and in that time the properties of pillar[ n]arenes have been widely investigated by various scientists. Thanks to their efforts, the characteristic properties of pillar[ n]arenes that result from their pillar-shaped structures have been elucidated. Their host ability, the chirality of their pillar-shaped structure, and their versatile functionality are unique features of pillar[ n]arenes not seen in other well-known hosts, and these properties are very useful for the creation of new stimuli-responsive supramolecular assemblies. In this Account, we describe photo-, pH- and redox-responsive supramolecular assemblies based on pillar[ n]arenes. First, we discuss molecular-scale stimuli-responsive supramolecular assemblies, i.e., pseudorotaxanes, pseudocatenanes, and supramolecular polymers. We also highlight subnanometer- and micrometer-scale stimuli-responsive supramolecular assembles such as particles and vesicles. Finally, we discuss the macroscopic stimuli-responsive structural changes of surfaces and gels. This Account will provide useful information for researchers working on not only pillar[ n]arene chemistry but also the chemistry of other macrocyclic hosts, and it will inspire new discoveries in the field of supramolecular assemblies and systems containing macrocyclic hosts.

Synthesis of diamido-bridged bis-pillar[5]arenes and tris-pillar[5]arenes for construction of unique [1]rotaxanes and bis-[1]rotaxanes

The pillar[5]arene mono- and di(oxyalkoxy)benzoic acids were successfully prepared in high yields by sequential alkylation of ω-bromoalkoxy-substituted pillar[5]arenes with methyl or ethyl p-hydroxybenzoate followed by a hydrolytic reaction under basic conditions. Under catalysis of HOBt/EDCl, the amidation reaction of pillar[5]arene mono(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes afforded diamido-bridged bis-pillar[5]arenes. 1H NMR and 2D NOESY spectra clearly indicated that [1]rotaxanes were formed by insertion of longer diaminoalkylene unit into the cavity of one pillar[5]arene with another pillar[5]arene acting as a stopper. The similar catalysed amidation reaction of pillar[5]arene di(oxybutoxy)benzoic acid with monoamido-functionalized pillar[5]arenes resulted in the diamido-bridged tris-pillar[5]arenes, which successfully form the unique bis-[1]rotaxanes bearing longer than diaminopropylene diamido bridges.

Spontaneous buckling of contractile poroelastic actomyosin sheets

Shape transitions in developing organisms can be driven by active stresses, notably, active contractility generated by myosin motors. The mechanisms generating tissue folding are typically studied in epithelia. There, the interaction between cells is also coupled to an elastic substrate, presenting a major difficulty for studying contraction induced folding. Here we study the contraction and buckling of active, initially homogeneous, thin elastic actomyosin networks isolated from bounding surfaces. The network behaves as a poroelastic material, where a flow of fluid is generated during contraction. Contraction starts at the system boundaries, proceeds into the bulk, and eventually leads to spontaneous buckling of the sheet at the periphery. The buckling instability resulted from system self-organization and from the spontaneous emergence of density gradients driven by the active contractility. The buckling wavelength increases linearly with sheet thickness. Our system offers a well-controlled way to study mechanically induced, spontaneous shape transitions in active matter.

Active matter composed of filaments and molecular motors can contract. Here, the authors report the spontaneous out-of-plane buckling of reconstituted contracting poroelastic actomyosin sheets in the absence of external cues.

Control of the threading ratio of cyclic molecules in polyrotaxanes consisting of poly(ethylene glycol) and α-cyclodextrins

We demonstrated a feasible method for providing polyrotaxanes (PRxs) with a controlled threading ratio of cyclic molecules and chain length of linear polymers by extending the linear polymers in the pseudo-PRx. This method gave PRxs with a lower threading ratio and a higher mobility of cyclic molecules compared to usual methods used previously with a high threading ratio. In addition, our PRx improved the thermal stability of the linear polymers in PRx despite the low threading ratio.

Viologen-Based Rotaxanes from Dibenzo-30-crown-10

Three [2]rotaxanes (4, 7, and 12) and one [3]rotaxane (8) were synthesized based on the dibenzo-30-crown-10/viologen binding motif. To the best of our knowledge, these are the first rotaxanes formed from dibenzo-30-crown-10 and viologens. The rotaxanes were all characterized by ¹H NMR, ¹³C NMR, and HRMS. An X-ray crystal structure of one of the [2]rotaxanes (7) was obtained. This work demonstrates for the first time that dibenzo-30-crown-10 does form pseudorotaxane complexes with viologens in solution.

Supramolecular Properties of a Monocarboxylic Acid-Functionalized "Texas-Sized" Molecular Box

A new carboxylic acid-functionalized "Texas-sized" molecular box TxSB-CO₂H has been prepared by combining two separate building blocks via an iodide-catalyzed macrocyclization reaction. A single-crystal X-ray diffraction analysis revealed a paired "clip-like" dimer in the solid state. Concentration-dependent behavior is seen for samples of TxSB-CO₂H as prepared, as inferred from ¹H NMR spectroscopic studies carried out in DMSO- d₆. However, in the presence of excess acid (1% by weight of deuterated trifluoracetic acid; TFA- d₁), little evidence of aggregation is seen in DMSO- d₆ except at the highest accessible concentrations. In contrast, the conjugate base form, TxSB-CO₂^-, produced in situ via the addition of excess triethylamine to DMSO- d₆ solutions of TxSB-CO₂H acts as a self-complementary monomer that undergoes self-assembly to stabilize a formal oligomer ([TxSB-CO₂^-] _n) with a degree of polymerization of approximately 5-6 at a concentration of 70 mM. Evidence in support of the proposed oligomerization of TxSB-CO₂^- in solution and in the solid state came from one- and two-dimensional ¹H NMR spectroscopy, X-ray crystallography, dynamic light scattering (DLS), and scanning electron microscopy (SEM). A series of solution-based analyses carried out in DMSO and DMSO- d₆ provide support for the notion that the self-assembled constructs produced from TxSB-CO₂^- are responsive to environmental stimuli, including exposure to the acetate anion (as its tetrabutylammonium, TBA⁺, salt), and changes in overall concentration, temperature, and protonation state. The resulting transformations are thought to reflect the reversible nature of the underlying noncovalent interactions. They also permit the stepwise interconversion between TxSB-CO₂H and [TxSB-CO₂^-] _n via the sequential addition of triethylamine and TFA- d₁. The present work thus serves to illustrate how appropriately functionalized molecular box-type macrocycles may be used to develop versatile stimuli-responsive materials. It also highlights how aggregated forms seen in the solid state are not necessarily retained under competitive solution-phase conditions.

Supramolecularly directed rotary motion in a photoresponsive receptor

Stimuli-controlled motion at the molecular level has fascinated chemists already for several decades. Taking inspiration from the myriad of dynamic and machine-like functions in nature, a number of strategies have been developed to control motion in purely synthetic systems. Unidirectional rotary motion, such as is observed in ATP synthase and other motor proteins, remains highly challenging to achieve. Current artificial molecular motor systems rely on intrinsic asymmetry or a specific sequence of chemical transformations. Here, we present an alternative design in which the rotation is directed by a chiral guest molecule, which is able to bind non-covalently to a light-responsive receptor. It is demonstrated that the rotary direction is governed by the guest chirality and hence, can be selected and changed at will. This feature offers unique control of directional rotation and will prove highly important in the further development of molecular machinery.

Unidirectional rotation in a synthetic molecular motor is typically driven by intrinsic asymmetry or sequences of chemical transformations. Here, the authors control the direction of a molecule’s rotation through supramolecular binding of a chiral guest and subsequent transfer of its chiral information.

Ring-through-ring molecular shuttling in a saturated [3]rotaxane

Mechanically interlocked molecules such as rotaxanes and catenanes comprise two or more components whose motion relative to each other can be controlled. A [2]rotaxane molecular shuttle, for example, consists of an axle bearing two recognition sites and a single macrocyclic wheel that can undergo a to-and-fro motion along the axle-shuttling between the recognition sites. The ability of mechanically interlocked molecules to undergo this type of large-amplitude change is the core mechanism behind almost every interlocked molecular switch or machine, including sophisticated mechanical systems such as a molecular elevator and a peptide synthesizer. Here, as a way to expand the scope of dynamics possible at the molecular level, we have developed a molecular shuttling mechanism involving the exchange of rings between two recognition sites in a saturated [3]rotaxane (one with no empty recognition sites). This was accomplished by passing a smaller ring through a larger one, thus achieving ring-through-ring molecular shuttling.