Rotaxane and catenane host structures for sensing charged guest species

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.

A Versatile Axle for the Construction of Disassemblage Rotaxanes

Rotaxanes are unique mechanical devices that hold great promise as sensors. We report on two new rotaxanes that contain an acid or base sensitive trigger and readily disassemble in a wide range of environments. Disassemblage was observed under TLC and ¹H-NMR analysis. The axle is highly charged, which enhances solubility in aqueous environments, and can be readily derivatized with sensor components. The trigger was swapped in a one-pot method, which is promising for the rapid production of a series of sensors.

A room temperature phosphorescence encoding [2]rotaxane molecular shuttle

A novel bistable molecular shuttle, composed of a Pt(ii) porphyrin-containing dibenzo[24]crown-8 macrocycle threaded onto two different recognition sites (secondary dialkylammonium (NH2+) and 4,4'-bipyridinium (Bpym2+) units), and the anthracene (Anth) moiety as one terminal stopper, was synthesized by click chemistry. Its acid-base switchable shuttling could be addressed by both the room temperature phosphorescence (RTP) emission signals of the Pt(ii) porphyrin moiety and the fluorescence emission of the Anth unit, as well as their lifetime changes. When the macrocycle was switched to be located on the NH2+ site close to Anth, the Pt(ii) porphyrin moiety exhibited strong RTP emission, excited in the Anth band at 370 nm. This was due to the distance-dependent efficient singlet energy transfer between the Anth unit and the porphyrin moiety, followed by intersystem crossing from a singlet to a triplet state in Pt(ii) porphyrin, while its RTP emission dramatically decreased when located on the Bpym2+ site far from the Anth unit. When excited in the porphyrin band at 402 nm, the RTP emission lifetimes changed obviously. This is the first rotaxane-type molecular shuttle whose shuttling has been encoded by RTP signals.

Thermally Driven Dynamics of a Rotaxane Wheel about an Imidazolium Axle inside a Metal-Organic Framework

A new mechanically interlocked molecular linker was prepared by using ring-closing metathesis (Grubbs I) to clip a [24]crown-6 ether wheel around an axle containing both Y-shaped diphenylimidazole and isophthalic acid groups. A metal-organic framework (MOF) material was prepared using this linker and Zn^II ions. Single-crystal X-ray diffraction experiments showed that the MOF contains an imidazolium-based rotaxane linked by dimeric [Zn₂ (NO₃ )(DEF)] secondary building units (SBUs). Variable-temperature (VT), ² H solid-state NMR spectroscopy was used to characterize the motion of the "soft" wheel component around the rigid "hard" lattice of the framework. At higher temperatures (above 150 °C), it was demonstrated that the 24-membered, macrocyclic ring of the MOF undergoes rapid, thermally driven rotation about the axle inside the voids of the lattice.

Superior anion induced shuttling behaviour exhibited by a halogen bonding two station rotaxane

Two bistable halogen and hydrogen bonding-naphthalene diimide [2]rotaxanes have been prepared and the system incorporating a halogen bond donor anion recognition site is demonstrated to exhibit superior anion induced translational motion of the macrocyclic wheel component relative to the hydrogen bonding analogue.

Two bistable halogen and hydrogen bonding-naphthalene diimide [2]rotaxanes have been prepared and the system incorporating a halogen bond donor anion recognition site is demonstrated to exhibit superior anion induced translational motion of the macrocyclic wheel component relative to the hydrogen bonding analogue. Proton NMR spectroscopy is used to estimate the percentage occupancies of the macrocycle at the respective station and importantly indicates that the halogen bonding rotaxane displays superior positional integrity in competitive protic solvent media as a consequence of strong halogen bond–halide anion binding interactions.

Exploiting recognition-mediated assembly and reactivity in [2]rotaxane formation

A small molecular reaction network exploits recognition-mediated reactive processes in order to drive the assembly and formation of both a self-replicating linear template (thread) and a [2]rotaxane, in which the linear template is encircled by a diamide macrocycle. Complementary recognition sites, placed at strategic positions on the reactive building blocks, drive these assembly and replication processes. Template-instructed experiments show that the thread is capable of efficient self-replication and that no cross-catalytic relationships exist between the thread and the [2]rotaxane. The rate of [2]rotaxane formation is insensitive to the addition of a preformed template, however, [2]rotaxane formation does show enhanced diastereoselectivity, most likely originating from its recognition-mediated formation through a ternary reactive complex.

A bifunctional chiral [2]catenane based on 1,1'-binaphthyl-phosphates

A novel [2]catenane was synthesised by ring-closing metathesis from a Ca-bisphosphate template. The resulting interlocked structure features two chiral 1,1'-binaphthyl-phosphates, leading to a bifunctional catenane structure. Initial binding studies point at the applicability of such mechanically interlocked bisphosphates as artificial receptors for dicationic guest molecules.

Rigid, Bistable Molecular Shuttles Combining T-shaped Benzimidazolium and Y-shaped Imidazolium Recognition Sites

Two different recognition sites, one a T-shaped 2,4,7-triphenylbenzimidazolium and the other a Y-shaped 2,4,5-triphenylimidazolium are combined to construct a rigid bistable [2]rotaxane molecular shuttle on which the position of a 24-membered crown ether macrocycle can be controlled by acid-base chemistry. Molecular shuttling in both the neutral and dicationic versions were studied.

One-pot synthesis of a [c2]daisy-chain-containing hetero[4]rotaxane via a self-sorting strategy

The construction and efficient synthesis of hetero[n]rotaxanes with high structural complexity are always attractive challenges. Herein, we demonstrate a facile one-pot preparation of a hetero[4]rotaxane, by employing a self-sorting strategy, which contains an interpenetrated dibenzo-24-crown-8 (DB24C8) based [c2]daisy chain structure and is ended with a benzo-21-crown-7 (B21C7) based rotaxane at each side. The key to the design involved encoding the selective threading using a steric hindrance-related "language", where highly selective self-assemblies occurred in a three-component self-sorting process, which included the threading of a benzylalkylammonium into a B21C7 and interpenetrated dimerized formation of a DB24C8 based [c2]daisy chain simultaneously; the precise pre-assembled system resulted in the efficient synthesis of hetero[4]rotaxane with a high-level of structural complexity under the "CuAAC" reaction.

Iodide Recognition and Sensing in Water by a Halogen-Bonding Ruthenium(II)-Based Rotaxane

The synthesis and anion-recognition properties of the first halogen-bonding rotaxane host to sense anions in water is described. The rotaxane features a halogen-bonding axle component, which is stoppered with water-solubilizing permethylated β-cyclodextrin motifs, and a luminescent tris(bipyridine)ruthenium(II)-based macrocycle component. (1) H NMR anion-binding titrations in D2 O reveal the halogen-bonding rotaxane to bind iodide with high affinity and with selectively over the smaller halide anions and sulfate. The binding affinity trend was explained through molecular dynamics simulations and free-energy calculations. Photo-physical investigations demonstrate the ability of the interlocked halogen-bonding host to sense iodide in water, through enhancement of the macrocycle component's Ru(II) metal-ligand charge transfer (MLCT) emission.

Supercapacitors based on polyelectrolyte/ferrocenyl-surfactant complexes with high rate capability

The composite PFSC/CP was reported as a promising electrode material for efficient supercapacitors for the first time.

Novel hydrogen- and halogen-bonding anion receptors based on 3-iodopyridinium units

Iodopyridinium-based tripodal receptors strongly bind anionic species in their bowl-shaped cavity through the synergistic effect of hydrogen- and halogen-bonding interactions.

A high-yield synthesis and acid–base response of phosphate-templated [3]rotaxanes

We show a high-yielding synthesis (80%) of [3]rotaxanes using shape-persistent macrocycles, cyanostars, and anionic organo-phosphate templates catalyzed by low steric-demand click chemistry.

Ratiometric fluorophore for quantification of iodide under physiological conditions: applications in urine analysis and live cell imaging

The ratio of fluorescence intensity of fluorophoreI_{395 nm}/I_{475 nm}vs.log [I⁻] undergoes linear change over a broad iodide concentration range of 10⁻⁹to 10⁻⁵M and finds application in urine analysis and live cell imaging.

Halogen bonding anion recognition

The development of solution-based anion receptor molecules which exploit halogen bonding interactions is an emerging area of research. This Feature Article reviews recent advances which have been made in this rapidly developing field, surveying the use of iodoperfluoroarene, haloimidazolium and halotriazole/triazolium halogen-bond-donor motifs in anion receptor design and describing the application of mechanically interlocked rotaxane and catenane frameworks as halogen bonding anion host systems.

Electrochemical activation of a tetrathiafulvalene halogen bond donor in solution

Electrochemistry: a powerful tool for probing and controlling halogen bonding in solution.

Facile and efficient formation and dissociation of a pseudo[2]rotaxane by a slippage approach using pillar[5]arene-based cyclic host liquid and solvent

A pillar[5]arene-based [2]rotaxane was synthesized by heating an axle with valine ends in a cyclic host liquid.

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.

Smart molecules for imaging, sensing and health (SMITH)

This autobiographical review provides a personal account of the author's academic journey in supramolecular chemistry, including brief summaries of research efforts in membrane transport, molecular imaging, ion-pair receptors, rotaxane synthesis, squaraine rotaxanes, and synthtavidin technology. The article concludes with a short perspective of likely future directions in biomedical supramolecular chemistry.

Acid/Base and H2PO4(-) Controllable High-Contrast Optical Molecular Switches with a Novel BODIPY Functionalized [2]Rotaxane

A novel multifunctional mechanically interlocked switchable [2]rotaxane R4 containing two molecular stations and rotaxane arms terminated with boron-dipyrromethene (BODIPY) fluorophores and its derivatives were synthesized for the first time by CuAAC click reaction. The shuttling motion of macrocycle between the dibenzylammonium and triazolium recognition sites and the distance dependent photoinduced electron transfer process of R4 is demonstrated by utilizing external chemical stimuli (acid/base). Interestingly, the reversible self-assembly process of R4 was recognized by the acid-base molecular switch strategy. Notably, two symmetrical triazolium groups acted as molecular stations, H2PO4(-) receptors, and H-bonded donors. Both [2]rotaxane R4 and thread R2 demonstrated excellent optical responses and high selectivity toward H2PO4(-) ion. The specific motion and guest-host interactions of mechanically interlocked machines (MIMs) were also further explored by quantum mechanical calculations. The thread R2 also demonstrated to enable the detection of H2PO4(-) in RAW 264.7 cells successfully.

A pillar[6]arene-based [2]pseudorotaxane in solution and in the solid state and its photo-responsive self-assembly behavior in solution

A pillar[6]arene-based [2]pseudorotaxane was constructed in solution and studied in the solid state, and its photo-responsive self-assembly behavior in solution was investigated.

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.

Self-assembly of [3]catenanes and a [4]molecular necklace based on a cryptand/paraquat recognition motif

Hierarchical self-assembly centered on metallacyclic scaffolds greatly facilitates the construction of mechanically interlocked structures. The formation of two [3]catenanes and one [4]molecular necklace is presented by utilizing the orthogonality of coordination-driven self-assembly and crown ether-based cryptand/paraquat derivative complexation. The threaded [3]catenanes and [4]molecular necklace were fabricated by using ten and nine total molecular components, respectively, from four and three unique species in solution, respectively. In all cases single supramolecular ensembles were obtained, attesting to the high degree of structural complexity made possible via self-assembly approaches.

Catenanes: fifty years of molecular links

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Nitrate anion templated synthesis of a [2]catenane for nitrate recognition in organic-aqueous solvent media

The first example of a catenane synthesised using a nitrate anion template is demonstrated. Removal of the templating anion reveals a mechanically interlocked molecular host system which is capable of recognising nitrate selectively over a range of more basic mono-anionic oxoanions in a competitive organic-aqueous solvent mixture.