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 Dual‐Response [2]Rotaxane Based on a 1,2,3‐Triazole Ring as a Novel Recognition Station

Two novel multilevel switchable [2]rotaxanes containing an ammonium and a triazole station have been constructed by a Cu^I‐catalyzed azide–alkyne cycloaddition reaction. The macrocycle of [2]rotaxane containing a C6‐chain bridge between the two hydrogen bonding stations exhibits high selectivity for the ammonium cation in the protonated form. Interestingly, the macrocycle is able to interact with the two recognition stations when the bridge between them is shortened. Upon deprotonation of both [2]rotaxanes, the macrocycle moves towards the triazole recognition site due to the hydrogen‐bond interaction between the triazole nitrogen atoms and the amide groups in the macrocycle. Upon addition of chloride anion, the conformation of [2]rotaxane is changed because of the cooperative recognition of the chloride anion by a favorable hydrogen‐bond donor from both the macrocycle isophthalamide and thread triazole CH proton.     

Active‐Metal Template Synthesis of a Halogen‐Bonding Rotaxane for Anion Recognition

The synthesis of an all‐halogen‐bonding rotaxane for anion recognition is achieved by using active‐metal templation. A flexible bis‐iodotriazole‐containing macrocycle is exploited for the metal‐directed rotaxane synthesis. Endotopic binding of a Cu^I template facilitates an active‐metal CuAAC iodotriazole axle formation reaction that captures the interlocked rotaxane product. Following copper‐template removal, exotopic coordination of a more sterically demanding rhenium(I) complex induces an inversion in the conformation of the macrocycle component, directing the iodotriazole halogen‐bond donors into the rotaxane’s interlocked binding cavity to facilitate anion recognition.     

Calix[4]arene‐Based Rotaxane Host Systems for Anion Recognition

The synthesis, structure and anion binding properties of the first calix[4]arene‐based [2]rotaxane anion host systems are described. Rotaxanes 9? Cl and 12? Cl, consisting of a calix[4]arene functionalised macrocycle wheel and different pyridinium axle components, are prepared via adaption of an anion templated synthetic strategy to investigate the effect of preorganisation of the interlocked host’s binding cavity on anion binding. Rotaxane 12? Cl contains a conformationally flexible pyridinium axle, whereas rotaxane 9? Cl incorporates a more preorganised pyridinium axle component. The X‐ray crystal structure of 9? Cl and solution phase ¹H NMR spectroscopy demonstrate the successful interlocking of the calix[4]arene macrocycle and pyridinium axle components in the rotaxane structures. Following removal of the chloride anion template, anion binding studies on the resulting rotaxanes 9? PF₆ and 12? PF₆ reveal the importance of preorganisation of the host binding cavity on anion binding. The more preorganised rotaxane 9? PF₆ is the superior anion host system. The interlocked host cavity is selective for chloride in 1:1 CDCl₃/CD₃OD and remains selective for chloride and bromide in 10 % aqueous media over the more basic oxoanions. Rotaxane 12? PF₆ with a relatively conformationally flexible binding cavity is a less effective and discriminating anion host system although the rotaxane still binds halide anions in preference to oxoanions.     

Exploiting the 1,2,3‐Triazolium Motif in Anion‐Templated Formation of a Bromide‐Selective Rotaxane Host Assembly

Bromide is best : The first [2]rotaxane incorporating the triazolium anion‐binding motif is prepared using bromide anion templation. Preliminary anion‐binding investigations reveal that the rotaxane exhibits the rare selectivity preference for bromide over chloride ions.

     

A Conjugated Thiophene‐Based Rotaxane: Synthesis,Spectroscopy, and Modeling

A dithiophene rotaxane 1 ?β‐CD and its shape‐persistent corresponding dumbbell 1 were synthesized and fully characterized. 2D NOESY experiments, supported by molecular dynamics calculations, revealed a very mobile macrocycle (β‐CD). Steady‐state and time‐resolved photoluminescence experiments in solution were employed to elucidate the excited‐state dynamics for both systems and to explore the effect of cyclodextrin encapsulation. The photoluminescence (PL) spectrum of 1 ?β‐CD was found to be blueshifted with respect to the dumbbell 1 (2.81 and 2.78 eV, respectively). Additionally, in contrast to previous observations, neither PL spectra nor the decay kinetics of both threaded and unthreaded systems showed changes upon increasing the concentration or changing the polarity of the solutions, thereby providing evidence for a lack of tendency toward aggregation of the unthreaded backbone.     

Sulfate‐Selective Binding and Sensing of a Fluorescent [3]Rotaxane Host System

The chloride‐templated synthesis of a novel [3]rotaxane, capable of binding anionic guests, and incorporating a naphthalene group for fluorescence sensing is reported. Extensive ¹H NMR titration studies were used to probe the anion binding selectivity of the system. The rotaxane selectively recognises sulfate, undergoing an induced conformational change upon sulfate binding to form a 1:1 stoichiometric sandwich‐type complex, concomitant with significant quenching of the fluorescence. Binding of mono‐anionic guests results in the formation of a 2:1 stoichiometric guest–host complex, and a modest enhancement of the emission. Addition of an excess of sulfate in non‐competitive solvent also results in a 2:1 emissive complex.     

Supramolecular Control of Spin Exchange in a Spin‐Labelled [2]Rotaxane Incorporating a Tetrathiafulvalene Unit

The EPR properties of a novel triradical obtained by single‐electron oxidation of a nitroxide‐spin‐labelled rotaxane containing a tetrathiafulvalene unit and cyclobis(paraquat‐p‐phenylene) ring is reported. Rotaxanation is proved to have a dramatic effect on through‐space magnetic interactions between radical fragments. Analysis of the EPR spectra by a three‐jump model, allowed us to obtain structural information on the interlocked structure.     

Vernier‐Templated Synthesis,Crystal Structure,and Supramolecular Chemistry of a 12‐Porphyrin Nanoring

Vernier templating exploits a mismatch between the number of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, we present a detailed study of the Vernier‐templated synthesis of a 12‐porphyrin nanoring. NMR and small‐angle X‐ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo‐oligomerization reaction. UV/Vis/NIR titrations show that the three‐component assembly of the 12‐porphyrin nanoring figure‐of‐eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring–template 1:2 complex is among the largest synthetic molecules to have been characterized by single‐crystal analysis. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramolecular C?H???N contacts involving the tert‐butyl side chains. Scanning tunneling microscopy (STM) experiments show that molecules of the 12‐porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.     

Synthesis of a [2]rotaxane incorporating a "magic sulfur ring" by the thiol-ene click reaction

The mild and highly efficient thiol-ene click reaction has been used to construct a rotaxane incorporating dibenzo-24-crown-8 (DB24C8) and a dibenzylammonium-derived thread in high yield under the irradiation of UV light. A rotaxane containing a disulfide linkage in the macrocycle was also synthesized by the thiol-ene click reaction. It has been demonstrated that the formation of the [2]rotaxane with the disulfide bond in the macrocycle occurs by a mechanism that is different to the threading-followed-by-stoppering process. The successful construction of a rotaxane directly from its constituent components, the macrocycle containing a disulfide linkage and the dibenzylammonium hexafluorophosphate salt, suggests that the space within the macrocycle incorporating the disulfide linkage is smaller than the phenyl unit and a plausible reaction mechanism has been proposed as follows: A small amount of the initiator forms two radicals upon the absorption of UV irradiation; the radicals act as a "key" to "unlock" the disulfide bond in the macrocycle. The resulting crown ether like moiety in the macrocycle is clipped around the ammonium ion center in the dumb-bell-shaped compound. The [2]rotaxane is generated upon recombination of the disulfide linkage.     

Nitrate Anion Recognition in Organic–Aqueous Solvent Mixtures by a Bis(triazolium)acridine‐Containing [2]Rotaxane

The synthesis of a novel [2]rotaxane host system containing a bis(triazolium)acridine‐based axle component is reported. ¹H NMR anion‐binding titrations reveal that the rotaxane is able to recognise selectively the NO₃^? anion over a range of more basic oxoanions (AcO^?, HCO₃^? and H₂PO₄^?) in a competitive organic–aqueous solvent mixture.     

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.     

Rules of Macrocycle Topology: A [13]‐Macrodilactone Case Study

Shape is an inherent trait of a molecule that dictates how it interacts with other molecules, either in binding events or intermolecular reactions. Large‐ring macrocyclic compounds in particular leverage their shape when they are selectively bound by biomolecules and also when they exhibit macrocyclic diastereoselectivity. Nonetheless, rules that link structural parameters to the conformation of a macrocycle are still rudimentary. Here we use a structural investigation of a family of [13]‐macrodilactones as a case study to develop rules that can be applied generally to macrocycles of different sizes and with a variety of functionality. A characteristic “ribbon” shape is adopted by the [13]‐macrodilactones in the absence of stereogenic centres, which exhibits planar chirality. When one stereogenic centre at key positions on the backbone is incorporated into the structure, the planar chirality is dictated by the configuration of the centre. In cases where two stereogenic centres are present, their relationships can either reinforce the characteristic ribbon shape or induce alternative shapes to be adopted. The rules established in the case study are then applied to the analysis of a structure of the natural product migrastatin. They lay the groundwork for the development of models to understand macrocycle‐biomolecule interactions and for the preparation of macrocycles with designed properties and activities.     

β‐Substituted Terthiophene [2]Rotaxanes

Towards polythiophene polyrotaxanes : The β‐substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported.

     

Near‐Infrared Croconaine Rotaxanes and Doped Nanoparticles for Enhanced Aqueous Photothermal Heating

The photothermal effect is the generation of heat by molecules or particles upon high‐energy laser irradiation, and near‐infrared absorbers such as gold nanoparticles and organic dyes have a range of potential photothermal applications. The favourable photothermal properties of thiophene‐functionalised croconaine dyes were recently discovered. The synthesis and properties of novel croconaine rotaxane and pseudorotaxane architectures capable of efficient photothermal performance in both organic and aqueous environments are reported. The versatility of this dye‐encapsulation strategy was demonstrated by the preparation of two organic croconaine rotaxanes using different synthetic methods: the formation of an aqueous pseudorotaxane association complex, and the synthesis of water‐soluble, croconaine‐doped silicated micelle nanoparticles. All of these near‐infrared‐absorbing systems exhibit excellent photothermal behaviour, with pseudorotaxane and rotaxane formation vital for effective aqueous heat generation. Dye encapsulation provides steric protection to enhance the stability of a water‐sensitive croconaine dye, while rotaxane‐doped nanoparticles avoid detrimental band broadening caused by chromophore coupling.