Donor-Acceptor Oligorotaxanes Made to Order

Foldamer-Based Mechanoresponsive Materials: Molecular Nanoarchitectonics to Advanced Functions

Artificial molecules that respond to external stimuli such as light, heat, chemical signals, and mechanical force have garnered significant interest due to their tunable functions, variable optical properties, and mechanical responses. Particularly, mechanoresponsive materials featuring molecules that respond to mechanical stress or show force-induced optical changes have been intriguing due to their extraordinary functions. Despite the promising potential of many such materials reported in the past, practical applications have remained limited, primarily because their functions often depend on irreversible covalent bond rupture. Foldamers, oligomers that fold into well-defined secondary structures, offer an alternative class of mechanoactive motifs. These molecules can reversibly sustain mechanical stress and efficiently dissipate energy by transitioning between folded and unfolded states. This review focuses on the emerging properties of foldamer-based mechanoresponsive materials. We begin by highlighting the mechanical responses of foldamers in their molecular form, which have been primarily investigated using single-molecule force spectroscopy and other analytical methods. Following this, we provide a detailed survey of the current trends in foldamer-appended polymers, emphasizing their emerging mechanical and mechanochromic properties. Subsequently, we present an overview of the state-of-the-art advancements in foldamer-appended polymers, showcasing significant reports in this field. This review covers some of the most recent advances in this direction and draws a perspective for further development.

The Story of the Little Blue Box: A Tribute to Siegfried Hünig

The tetracationic cyclophane, cyclobis(paraquat-p-phenylene), also known as the little blue box, constitutes a modular receptor that has facilitated the discovery of many host-guest complexes and mechanically interlocked molecules during the past 35 years. Its versatility in binding small π-donors in its tetracationic state, as well as forming trisradical tricationic complexes with viologen radical cations in its doubly reduced bisradical dicationic state, renders it valuable for the construction of various stimuli-responsive materials. Since the first reports in 1988, the little blue box has been featured in over 500 publications in the literature. All this research activity would not have been possible without the seminal contributions carried out by Siegfried Hünig, who not only pioneered the syntheses of viologen-containing cyclophanes, but also revealed their rich redox chemistry in addition to their ability to undergo intramolecular π-dimerization. This Review describes how his pioneering research led to the design and synthesis of the little blue box, and how this redox-active host evolved into the key component of molecular shuttles, switches, and machines.

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

Threading of three rings on two stations: a convergent approach to [4]rotaxane

A novel approach to efficient and selective construction of [4]rotaxane was proposed to demonstrate the superiority of H-bonded azo-macrocycles in forging higher order rotaxanes. The single crystal structure reveals the importance of the interplay of multiple non-covalent bonding interactions, particularly π-stacking interactions, in stabilizing the host-guest complex. This may open an avenue to the synthesis of oligomeric mechanically interlocked molecules containing fewer stations but more rings.

Radical-Pairing Interactions in a Molecular Switch Evidenced by Ion Mobility Spectrometry and Infrared Ion Spectroscopy

The digital revolution sets a milestone in the progressive miniaturization of working devices and in the underlying advent of molecular machines. Foldamers involving mechanically entangled components with modular secondary structures are among the most promising designs for molecular switch‐based applications. Characterizing the nature and dynamics of their intramolecular network following the application of a stimulus is the key to their performance. Here, we use non‐dissociative electron transfer as a reductive stimulus in the gas phase and probe the consecutive co‐conformational transitions of a donor‐acceptor oligorotaxane foldamer using electrospray mass spectrometry interfaced with ion mobility and infrared ion spectroscopy. A comparison of collision cross section distributions for analogous closed‐shell and radical molecular ions sheds light on their respective formation energetics, while variations in their respective infrared absorption bands evidence changes in intramolecular organization as the foldamer becomes more compact. These differences are compatible with the advent of radical‐pairing interactions.

Single-site binding of pyrene to poly(ester-imide)s incorporating long spacer-units: prediction of NMR resonance-patterns from a fractal model

Co-polycondensation of the diimide-based diols N,N′-bis(2-hydroxyethyl)hexafluoroisopropylidene-diphthalimide, (HFDI), and N,N′-bis(2-hydroxy-ethyl)naphthalene-1,4,5,8-tetracarboxylic-diimide, (NDI), with aliphatic diacyl chlorides ClOC(CH₂)_xCOCl (x = 5 to 8) affords linear copoly(ester-imide)s. Such copolymers interact with pyrene via supramolecular binding of the polycyclic aromatic at NDI residues. This interaction results in upfield complexation shifts and sequence-related splittings of the NDI ¹H NMR resonances, but gives a very different final resonance-pattern from the copolymer where x = 2. Computational modelling of the polymer with x = 5 suggests that each pyrene molecule binds to just a single NDI residue rather than by intercalation between a pair of NDI's at a tight chain-fold, as was found for x = 2. The new single-site binding model enables the pattern of ¹H NMR resonances for copolymers with longer spacers (x = 5 to 8) to be reproduced and assigned by simulation from sequence-specific shielding factors based on a type of fractal known as the last-fraction Cantor set. As this type of fractal also enables an understanding of pairwise binding systems, it evidently provides a general numerical framework for supramolecular sequence-analysis in binary copolymers.

Nine ¹H NMR resonances assignable to specific copoly(ester-imide) sequences identified from a fractal model result from 1 : 1 supramolecular binding of pyrene to NDI residues.

Two-Parameter Power Formalism for Structural Screening of Ion Mobility Trends: Applied Study on Artificial Molecular Switches

Recent literature provides increasing samples of structural studies relying on ion mobility coupled to mass spectrometry in view of characterizing gas-phase conformation and energetics properties of biomolecular ions. A typical framework consists in experimentally monitoring the collisional cross sections for various experimental conditions and using them as references to select appropriate candidate structures issued from theoretical modeling. Although it has proved successful for structural assignment, this process is resource costly and lengthy, namely due to intricacies in the selection of appropriate input geometries. In the present work, we propose simplified methodologies dedicated to the systematic screening of ion mobility data acquired on systems built from repetitive subunits and detail their application to challenging artificial molecular switch systems. Capitalizing on coarse-grained design, we first demonstrate how the assimilation of subunits into adequately assembled building-blocks can be used for fast assignments of a system topology. Further focusing on topology-specific differential ion mobility trends, we show that the building-block assemblies can be fused into single fully convex solid figure models, i.e., sphere and cylinder, whose projected areas follow a two-parameter power formalism A × n^B. We show that the fitting parameters A and B were assigned as structural descriptors respectively associated with the dimensions of each constitutive subunit, i.e., size parameter, and with their assembled tridimensional arrangement, i.e., shape parameter. The present work provides a ready-to-use method for the screening of IM-MS data sets that is expected to facilitate the eventual design of input structures whenever advanced modeling calculations are required.

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.

A macrocyclic receptor containing two viologen species connected by conjugated terphenyl groups

A macrocyclic receptor molecule containing two viologen species connected by conjugated terphenyl groups has been designed and synthesised. The single-crystal X-ray structure shows that the two viologen residues have a transannular NN separation of ca. 7.4 Å. Thus, the internal cavity dimensions are suitable for the inclusion of π-electron-rich species. The macrocycle is redox active, and can accept electrons from suitable donor species including triethylamine, resulting in a dramatic colour change from pale yellow to dark green as a consequence of the formation of a paramagnetic bis(radical cationic) species. Cyclic voltammetry shows that the macrocycle can undergo two sequential and reversible reduction processes (E1/2 = -0.65 and -0.97 V vs. Fc/Fc+). DFT and TD-DFT studies accurately replicate the structure of the tetracationic macrocycle and the electronic absorption spectra of the three major redox states of the system. These calculations also showed that during electrochemical reduction, the unpaired electron density of the radical cations remained relatively localised within the heterocyclic rings. The ability of the macrocycle to form supramolecular complexes was confirmed by the formation of a pseudorotaxane with a guest molecule containing a π-electron-rich 1,5-dihydroxynaphthalene derivative. Threading and dethreading of the pseudorotaxane was fast on the NMR timescale, and the complex exhibited an association constant of 150 M-1 (±30 M-1) as calculated from 1H NMR titration studies.

Synthetic oligorotaxanes exert high forces when folding under mechanical load

Folding is a ubiquitous process that nature uses to control the conformations of its molecular machines, allowing them to perform chemical and mechanical tasks. Over the years, chemists have synthesized foldamers that adopt well-defined and stable folded architectures, mimicking the control expressed by natural systems ^1,2 . Mechanically interlocked molecules, such as rotaxanes and catenanes, are prototypical molecular machines that enable the controlled movement and positioning of their component parts ^3-5 . Recently, combining the exquisite complexity of these two classes of molecules, donor-acceptor oligorotaxane foldamers have been synthesized, in which interactions between the mechanically interlocked component parts dictate the single-molecule assembly into a folded secondary structure ^6-8 . Here we report on the mechanochemical properties of these molecules. We use atomic force microscopy-based single-molecule force spectroscopy to mechanically unfold oligorotaxanes, made of oligomeric dumbbells incorporating 1,5-dioxynaphthalene units encircled by cyclobis(paraquat-p-phenylene) rings. Real-time capture of fluctuations between unfolded and folded states reveals that the molecules exert forces of up to 50 pN against a mechanical load of up to 150 pN, and displays transition times of less than 10 μs. While the folding is at least as fast as that observed in proteins, it is remarkably more robust, thanks to the mechanically interlocked structure. Our results show that synthetic oligorotaxanes have the potential to exceed the performance of natural folding proteins.

Synthesis of rotaxanes and catenanes using an imine clipping reaction

Supramolecular chemistry and self-assembly provide a valuable chance to understand the complicated topological structures on a molecular level. Two types of classical mechanically interlocked molecules, rotaxanes and catenanes, possess non-covalent mechanical bonds and have attracted more attention not only in supramolecular chemistry but also in the fields of materials science, nanotechnology and bioscience. In the past decades, the template-directed clipping reaction based on imine chemistry has become one of the most efficient methods for the construction of functionalized rotaxanes and catenanes. In this review, we outlined the main progress of rotaxanes and catenanes using the template-directed clipping approach of imine chemistry. The review contains the novel topological structures of rotaxanes and catenanes, functions and applications.

Cooperative halogen bonding and polarized π-stacking in the formation of coloured charge-transfer co-crystals

Matched electron rich halogen bond acceptors and donor have been synthesized and the halogen bonded charge transfer cocrystals characterized.

Dynamic force spectroscopy of synthetic oligorotaxane foldamers

Wholly synthetic molecules involving both mechanical bonds and a folded secondary structure are one of the most promising architectures for the design of functional molecular machines with unprecedented properties. Here, we report dynamic single-molecule force spectroscopy experiments that explore the energetic details of donor-acceptor oligorotaxane foldamers, a class of molecular switches. The mechanical breaking of the donor-acceptor interactions responsible for the folded structure shows a high constant rupture force over a broad range of loading rates, covering three orders of magnitude. In comparison with dynamic force spectroscopy performed during the past 20 y on various (bio)molecules, the near-equilibrium regime of oligorotaxanes persists at much higher loading rates, at which biomolecules have reached their kinetic regime, illustrating the very fast dynamics and remarkable rebinding capabilities of the intramolecular donor-acceptor interactions. We focused on one single interaction at a time and probed the stochastic rupture and rebinding paths. Using the Crooks fluctuation theorem, we measured the mechanical work produced during the breaking and rebinding to determine a free-energy difference, ΔG, of 6 kcal·mol^-1 between the two local conformations around a single bond.

Where Ion Mobility and Molecular Dynamics Meet To Unravel the (Un)Folding Mechanisms of an Oligorotaxane Molecular Switch

At the interface between foldamers and mechanically interlocked molecules, oligorotaxanes exhibit a spring-like folded secondary structure with remarkable mechanical and physicochemical properties. Among these properties, the ability of oligorotaxanes to act as molecular switches through controlled modulations of their spatial extension over (un)folding dynamics is of particular interest. The present study aims to assess and further characterize this remarkable feature in the gas phase using mass spectrometry tools. In this context, we focused on the [4]5NPR⁺¹² oligorotaxane molecule complexed with PF₆^- counterion and probed its co-conformational states as a function of the in-source-generated charge states. Data were interpreted in light of electronic secondary structure computations at the PM6 and DFT levels. Our results highlight two major co-conformational groups associated either with folded compact structures, notably stabilized by intramolecular π-π interactions and predominant for low charge states or with fully stretched structures resulting from significant Coulombic repulsions at high charge states. Between, the oligorotaxane adopts intermediate folded co-conformations, suggesting a stepwise unfolding pathway under increasing repulsive Coulombic constraints. The reversibility of this superstructural transition was next interrogated under electron-driven (nondissociative electron transfer) and heat-driven (collision-induced unfolding) activation stimuli. The outcomes support the feasibility to either unfold or (partially) refold the oligorotaxane foldamer on purpose in the gas phase. Our results show that the balance between the stabilizing π-π interactions and the versatile Coulomb interactions dictates the elongation state of the foldamer in the gas phase and emphasizes the adequacy of mass spectrometry tools for the superstructural characterization of desolvated prototypical artificial molecular machines.

Reversible morphological changes of assembled supramolecular amphiphiles triggered by pH-modulated host–guest interactions

Acid–base modulated host–guest binding at the micellar–water interface triggers reversible oblate ellipsoid-to-lamellar morphological transitions revealing the relationship between and morphology.

Stacking Interactions Drive Selective Self-Assembly and Self-Sorting of Pyrene-Based M(II)4L6 Architectures

Subcomponent self-assembly of two isomeric bis(3-aminophenyl)pyrenes, 2-formylpyridine and the metal ions Fe(II), Co(II), and Zn(II) led to the formation of two previously unidentified structure types: a C2-symmetric M(II)4L6 assembly with meridionally coordinated metal centers, and a C3-symmetric self-included M(II)4L6 assembly with facially coordinated metal centers. In both structures the meta linkages within the ligands facilitate π-stacking between the pyrene panels of the ligands. A C2h-symmetric M(II)2L2 box was also obtained, which was observed to selectively bind electron-deficient aromatic guests between two parallel pyrene subunits. Similar donor-acceptor interactions drove the selective self-assembly of a singular M(II)4L4L'2 architecture incorporating both a pyrene-containing diamine and an electron-deficient NDI-based diamine. This heteroleptic architecture was shown to be thermodynamically favored over the corresponding homoleptic M(II)4L6 and M(II)4L'6 complexes, which were nonetheless stable in each others' absence. By contrast, an isomeric pyrene-based diamine was observed to undergo narcissistic self-sorting in the presence of the NDI-based diamine.

Donor–acceptor interaction-driven folding of linear naphthalene–glycol oligomers templated by a rigid bipyridinium rod

The construction of folded and helical supramolecular structures through the self-assembly of a series of flexible linear oligomers induced by a rigid rod-like template has been demonstrated.

Pseudorotaxane based on tetraazamacrocyclic copper complex and dibenzocrown ether

Host-guest interactions between two electron-rich dibenzocrown ethers and electron deficient macrocyclic copper(II) tetraimine complexes lead to the formation of pseudorotaxane in solution. The interactions are enhanced with the copper(III) complex compared to that of the copper(II) form as shown by the electrochemical studies. The larger--30-membered dibenzocrown donor interacted with the copper complex stronger than the smaller ones as revealed by NMR and electrochemical methods. The thiolated form of the copper(II) tetraimine complex was self-assembled at the gold electrode forming an electroactive monolayer able to interact with the crown ether in the solution. These donor-acceptor interactions lead to an increase of the barrier properties of the layer and decreased the electron transfer rate between the copper centre and the gold electrode surface as proved by the voltammetric data.

Mechanostereochemistry and the mechanical bond

The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by mechanical bonds, has led to the coining of the term mechanostereochemistry to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry.

Modular synthesis of bipyridinium oligomers and corresponding donor-acceptor oligorotaxanes with crown ethers

Donor-acceptor [4]- and [6]rotaxanes have been prepared from bipyridinium (BIPY(2+)) oligomers and 1,5-dinaphtho[38]crown-10 (DN38C10) by a threading-followed-by-stoppering protocol employing click chemistry. An efficient, straightforward route to the BIPY(2+) oligomers has been developed that requires little to no chromatographic purification. Unlike most donor-acceptor oligorotaxanes that have been reported to date, 100% of the recognition sites on the dumbbells are occupied by rings.

Measurement of the ground-state distributions in bistable mechanically interlocked molecules using slow scan rate cyclic voltammetry

In donor-acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers--present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor-acceptor vintage--can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes--one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites--the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor-acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.

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.