Mechanically Interlocked Molecules Assembled by π-π Recognition

Supramolecular and molecular capsules, cages and containers

Stemming from early seminal notions of molecular recognition and encapsulation, three-dimensional, cavity-containing capsular compounds and assemblies have attracted intense interest due to the ability to modulate chemical and physical properties of species encapsulated within these confined spaces compared to bulk environments. With such a diverse range of covalent motifs and non-covalent (supramolecular) interactions available to assemble building blocks, an incredibly wide-range of capsular-type architectures have been developed. Furthermore, synthetic tunability of the internal environments gives chemists the opportunity to engineer systems for uses in sensing, sequestration, catalysis and transport of molecules, just to name a few. In this tutorial review, an overview is provided into the design principles, synthesis, characterisation, structural facets and properties of coordination cages, porous organic cages, supramolecular capsules, foldamers and mechanically interlocked molecules. Using seminal and recent examples, the advantages and limitations of each system are explored, highlighting their application in various tasks and functions.

Modular Synthesis of Improbable Rotaxanes with All-Benzene Scaffolds

"Improbable" rotaxanes consisting of interlocked conjugated components represent non-trivial synthetic targets, not to mention those with all-benzene scaffolds. Herein, a modular synthetic strategy has been established using an isolable azo-linked pre-rotaxane as the core module, in which the azo group functions as a tracelessly removable template to direct mechanical bond formations. Through versatile connections of the pre-rotaxane and other customizable modules, [2]- and [3]rotaxanes derived from all-benzene scaffolds have been accomplished, demonstrating the utility and potential of the synthetic design for all-benzene interlocked supramolecules.

Threading a Linear Molecule Through a Macrocycle Thanks to Boron: Optical Properties of the Threaded Species and Synthesis of a Rotaxane

Two BODIPYs and two boron β-diketonates were threaded through a macrocycle bearing a 2,2'-biphenol unit, showing thus the ability of boron to act as a gathering atom. The new threaded species were characterized by 1D and 2D NMR spectroscopy as well as by X-ray crystallography for one of them and their properties rationalized with quantum chemistry to unravel the vibronic contributions. The BODIPYs exhibited interesting fluorescence features with quantum yields up to 91 % and enhanced photostability compared to their non-threaded homologues. A rotaxane was synthesized using this threading strategy after stoppering and removing the boron with potassium hydroxide.

Assembly and Utility of a Drawstring-Mimetic Supramolecular Complex

Inspired by the drawstring structure in daily life, here we report the development of a drawstring-mimetic supramolecular complex at the molecular scale. This complex consists of a rigid figure-of-eight macrocyclic host molecule and a flexible linear guest molecule which could interact through three-point non-covalent binding to form a highly selective and efficient host-guest assembly. The complex not only resembles the drawstring structure, but also mimics the properties of a drawstring with regard to deformations under external forces. The supramolecular drawstring can be utilized as an interlocked crosslinker for poly(methyl acrylate), and the corresponding polymer samples exhibit comprehensive enhancement of macroscopic mechanical performance including stiffness, strength, and toughness.

Iminopyrrole-Based Self-Assembly: A Route to Intrinsically Flexible Molecular Links and Knots

The use of 2,5-diformylpyrrole in self-assembly reactions with diamines and Zn(II)/Cd(II) salts allowed the preparation of [2]catenane, trefoil knot, and Borromean rings. The intrinsically dynamic nature of the diiminopyrrole motif rendered all of the formed assemblies intramolecularly flexible. The presence of diiminopyrrole revealed new coordination motifs and influenced the host-guest chemistry of the systems, as illustrated by hexafluorophosphate encapsulation by Borromean rings.

From construction to application of a new generation of interlocked molecules composed of heteroditopic wheels

Over the last few decades, research on mechanically interlocked molecules has significantly evolved owing to their unique structural features and interesting properties. A substantial percentage of the reported works have focused on the synthetic strategies, leading to the preparation of functional MIMs for their applications in the chemical, materials, and biomedical sciences. Importantly, various macrocyclic wheels with specific heteroditopicity (including phenanthroline, amide, amine, oxy-ether, isophthalamide, calixarene and triazole) and threading axles (bipyridine, phenanthroline, pyridinium, triazolium, etc.) have been designed to synthesize targeted multifunctional mononuclear/multinuclear pseudorotaxanes, rotaxanes and catenanes. The structural uniqueness of these interlocked systems is advantageous owing to the presence of mechanical bonds with specific three-dimensional cavities. Furthermore, their multi-functionalities and preorganised structural entities exhibit a high potential for versatile applications, like switching, shuttling, dynamic properties, recognition and sensing. In this feature article, we describe some of the most recent advances in the construction and chemical behaviour of a new generation of interlocked molecules, primarily focusing on heteroditopic wheels and their applications in different directions of the modern research area. Furthermore, we outline the future prospects and significant perspectives of the new generation heteroditopic wheel based interlocked molecules in different emerging areas of science.

Active-metal template clipping synthesis of novel [2]rotaxanes

Mechanically interlocked molecules (MIMs) have been important synthetic targets in supramolecular chemistry due to their beautiful structures and intriguing properties. We present herein a new synthetic strategy to access [2]rotaxanes, namely active-metal template clipping. We discuss the design of the target [2]rotaxanes, synthesis and characterization of the axle, macrocycle precursors and macrocycles as well as preparation of the final [2]rotaxanes by active template copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) as key step of the synthesis. HRMS and NMR experiments have been performed to confirm the formation of the interlocked structures.

OPA, TPA and ECD spectra of π-conjugated interlocked chiral nanocarbons

This paper presents a theoretical investigation of the optical absorption and molecular chirality of π-conjugated mechanically interlocked nanocarbons, using one photon absorption (OPA) and two photon absorption (TPA) as well as electronic circular dichroism (ECD) spectra. Our findings reveal the optical excitation properties of mechanically interlocked molecules (MIMs) and chirality resulting from interlocked mechanical bonds. While OPA spectra are unable to distinguish interlocked molecules from non-interlocked molecules, we demonstrate that TPA and ECD can effectively discriminate between them, and can also differentiate [2]catenanes from [3]catenanes. Thus, we propose new methods to identify interlocked mechanical bonds. Our results provide physical insight into the optical properties and absolute configuration of π-conjugated interlocked chiral nanocarbons.

Active template strategy for the preparation of π-conjugated interlocked nanocarbons

Mechanically interlocked carbon nanostructures represent a relatively unexplored frontier in carbon nanoscience due to the difficulty in preparing these unusual topological materials. Here we illustrate an active-template method in which a [n]cycloparaphenylene precursor macrocycle is decorated with two convergent pyridine donors that coordinate to a metal ion. The metal ion catalyses alkyne-alkyne cross-coupling reactions within the central cavity of the macrocycle, and the resultant interlocked products can be converted into fully π-conjugated structures in subsequent synthetic steps. Specifically, we report the synthesis of a family of catenanes that comprise two or three mutually interpenetrating [n]cycloparaphenylene-derived macrocycles of various sizes. Additionally, a fully π-conjugated [3]rotaxane was synthesized by the same method. The development of synthetic methods to access mechanically interlocked carbon nanostructures of varying topology can help elucidate the implications of mechanical bonding for this emerging class of nanomaterials and allow structure-property relationships to be established.

Hydrogen bond templated synthesis of catenanes and rotaxanes from a single isophthalic acid derivative

Hydrogen bond templated [2]catenanes and [2]rotaxanes have been synthesized using azide precursors derived from a single isophthalic acid derivative precursor. The interlocked molecules were prepared using either stoichiometric or near stoichiometric amounts of macrocycle and CuAAC "click" precursors, with yields of up to 70% for the mechanical bond formation step. Successful preparation of the interlocked structures was confirmed by NMR spectroscopy and mass spectrometry, with detail of co-conformational behaviour being elucidated by a range of ¹H NMR spectroscopic experiments.

Emerging properties from mechanical tethering within a post-synthetically functionalised catenane scaffold

Maintaining close spatial proximity of functional moieties within molecular systems can result in fascinating emergent properties. Whilst much work has been done on covalent tethering of functional units for myriad applications, investigations into mechanically linked systems are relatively rare. Formation of the mechanical bond is usually the final step in the synthesis of interlocked molecules, placing limits on the throughput of functionalised architectures. Herein we present the synthesis of a bis-azide [2]catenane scaffold that can be post-synthetically modified using CuAAC 'click' chemistry. In this manner we have been able to access functionalised catenanes from a common precursor and study the properties of electrochemically active, emissive and photodimerisable units within the mechanically interlocked system in comparison to non-interlocked analogues. Our data demonstrates that the greater (co-)conformational flexibility that can be obtained with mechanically interlocked systems compared to traditional covalent tethers paves the way for developing new functional molecules with exciting properties.

BINOL as a chiral element in mechanically interlocked molecules

In this minireview we present the use of the axially chiral 1,1'-binaphthyl-2,2'-diol (BINOL) unit as a stereogenic element in mechanically interlocked molecules (MIMs). We describe the synthesis and properties of such BINOL-based chiral MIMs, together with their use in further diastereoselective modifications, their application in asymmetric catalysis, and their use in stereoselective chemosensing. Given the growing importance of mechanically interlocked molecules and the key advantages of the privileged chiral BINOL backbone, we believe that this research area will continue to grow and deliver many useful applications in the future.

A nitrogen-based chiral catenane for enantioenriching photocatalytic aerobic oxidation

Tröger's base's chirality merges catenanes’ photosensitizing characteristics to introduce the first nitrogen-based chiral hetero[2]catenane that proceeds enantioenriching photocatalytic aerobic oxidations.

Precision synthesis of linear oligorotaxanes and polyrotaxanes achieving well-defined positions and numbers of cyclic components on the axle

Interest in macromolecules has increased because of their functional properties, which can be tuned using precise organic synthetic methods. For example, desired functions have been imparted by controlling the nanoscale structures of such macromolecules. In particular, compounds with interlocked structures, including rotaxanes, have attracted attention because of their unique supramolecular structures. In such supramolecular structures, the mobility and freedom of the macrocycles are restricted by an axle and dependent on those of other macrocycles, which imparts unique functions to these threaded structures. Recently, methods for the ultrafine engineering and synthesis, as well as functions, of "defined" rotaxane structures that are not statistically dispersed on the axle (i.e., control over the number and position of cyclic molecules) have been reported. Various synthetic strategies allow access to such well-defined linear oligo- and polyrotaxanes, including [1]rotaxanes and [n]rotaxanes (mostly n > 3). These state-of-the-art synthetic methods have resulted in unique functions of these oligo-and polyrotaxane materials. Herein, we review the effective synthetic protocols and functions of precisely constructed one-dimensional oligomers and polymers bearing defined threaded structures, and discuss the latest reports and trends.

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.

[2]Catenane Synthesis via Covalent Templating

After earlier unsuccessful attempts, this work reports the application of covalent templating for the synthesis of mechanically interlocked molecules (MiMs) bearing no supramolecular recognition sites. Two linear strands were covalently connected in a perpendicular fashion by a central ketal linkage. After subsequent attachment of the first strand to a template via temporary benzylic linkages, the second was linked to the template in a backfolding macrocyclization. The resulting pseudo[1]rotaxane structure was successfully converted to a [2]catenane via a second macrocyclization and cleavage of the ketal and temporary linkages.

N-Arylimide Molecular Balances: A Comprehensive Platform for Studying Aromatic Interactions in Solution

Noncovalent interactions of aromatic surfaces play a key role in many biological processes and in determining the properties and utility of synthetic materials, sensors, and catalysts. However, the study of aromatic interactions has been challenging because these interactions are usually very weak and their trends are modulated by many factors such as structural, electronic, steric, and solvent effects. Recently, N-arylimide molecular balances have emerged as highly versatile and effective platforms for studying aromatic interactions in solution. These molecular balances can accurately measure weak noncovalent interactions in solution via their influence on the folded-unfolded conformational equilibrium. The structure (i.e., size, shape, π-conjugation, and substitution) and nature (i.e., element, charge, and polarity) of the π-surfaces and interacting groups can be readily varied, enabling the study of a wide range of aromatic interactions. These include aromatic stacking, heterocyclic aromatic stacking, and alkyl-π, chalcogen-π, silver-π, halogen-π, substituent-π, and solvent-π interactions. The ability to measure a diverse array of aromatic interactions within a single model system provides a unique perspective and insights as the interaction energies, stability trends, and solvent effects for different types of interactions can be directly compared. Some broad conclusions that have emerged from this comprehensive analysis include: (1) The strongest aromatic interactions involve groups with positive charges such as pyridinium and metal ions which interact with the electrostatically negative π-face of the aromatic surface via cation-π or metal-π interactions. Attractive electrostatic interactions can also form between aromatic surfaces and groups with partial positive charges. (2) Electrostatic interactions involving aromatic surfaces can be switched from repulsive to attractive using electron-withdrawing substituents or heterocycles. These electrostatic trends appear to span many types of aromatic interactions involving a polar group interacting with a π-surface such as halogen-π, chalcogen-π, and carbonyl-π. (3) Nonpolar groups form weak but measurable stabilizing interactions with aromatic surfaces in organic solvents due to favorable dispersion and/or solvophobic effects. A good predictor of the interaction strength is provided by the change in solvent-accessible surface area. (4) Solvent effects modulate the aromatic interactions in the forms of solvophobic effects and competitive solvation, which can be modeled using solvent cohesion density and specific solvent-solute interactions.

Synthetic strategies towards mechanically interlocked oligomers and polymers

Mechanically interlocked molecules have fascinated chemists for decades. Initially a tantalising synthetic challenge, interlocked molecules have continued to capture the imagination for their aesthetics and, increasingly, for their potential as molecular machines and use in materials applications. Whilst preliminary statistical attempts to prepare these molecules were exceedingly inefficient, a raft of template-directed strategies have now been realised, providing a vast toolbox from which chemists can access interlocked structures in excellent yields. For many envisaged applications it is desirable to move away from small, discrete interlocked molecules and turn to oligomers and polymers instead, either due to the need for multiple mechanical bonds within the desired material, or to exploit an extended scaffold for the organisation and arrangement of individual mechanically interlocked units. In this tutorial-style review we outline the synthetic strategies that have been employed for the synthesis of mechanically interlocked oligomers and polymers, including oligo-/polymerisation of (pseudo)interlocked precursors, metal-organic self-assembly, the use of orthogonal template motifs, iterative approaches and grafting onto polymer backbones.

Lanthanide-Containing Rotaxanes, Catenanes, and Knots

The valuable luminescence, magnetic, and catalytic properties of lanthanide cations are beginning to be exploited in conjunction with structurally exotic mechanically interlocked molecules (MIMs) such as rotaxanes, catenanes and knots. This Minireview provides an account of this rapidly developing research area commencing with the use of lanthanides in extended MIM-containing frameworks. Then, attention turns to discrete lanthanide-containing pseudorotaxanes, followed by fully interlocked rotaxanes, catenanes and knots - where lanthanides have not only been incorporated into MIM architectures but have also been used to template formation of the interlocked structure. Particular focus is paid to examples where the lanthanide MIMs have been put to useful applications, in what is still a relatively youthful avenue of research in both lanthanide coordination chemistry and the chemistry of mechanically interlocked molecules.

Mechanical-Bond-Induced Exciplex Fluorescence in an Anthracene-Based Homo[2]catenane

Collisional intermolecular interactions between excited states form short-lived dimers and complexes that lead to the emergence of excimer/exciplex emission of lower energy, a phenomenon which must be differentiated from the photoluminescence (PL) arising from the monomeric molecules. Although the utilization of noncovalent bonding interactions, leading to the generation of excimer/exciplex PL, has been investigated extensively, precise control of the aggregates and their persistence at very low concentrations remains a rare phenomenon. In the search for a fresh approach, we sought to obtain exciplex PL from permanent structures by incorporating anthracene moieties into pyridinium-containing mechanically interlocked molecules. Beyond the optical properties of the anthracene moieties, their π-extended nature enforces [π···π] stacking that can overcome the Coulombic repulsion between the pyridinium units, affording an efficient synthesis of an octacationic homo[2]catenane. Notably, upon increasing the ionic strength by adding tetrabutylammonium hexafluorophosphate, the catenane yield increases significantly as a result of the decrease in Coulombic repulsions between the pyridinium units. Although the ground-state photophysical properties of the free cyclophane and the catenane are similar and show a charge-transfer band at ∼455 nm, their PL characters are distinct, denoting different excited states. The cyclophane emits at ∼562 nm (quantum yield ϕ_F = 3.6%, emission lifetime τ_s = 3 ns in MeCN), which is characteristic of a disubstituted anthracene-pyridinium linker. By contrast, the catenane displays an exciplex PL at low concentration (10^-8 M) with an emission band centered on 650 nm (ϕ_F = 0.5%, τ_s = 14 ns) in MeCN and at 675 nm in aqueous solution. Live-cell imaging performed in MIAPaCa-2 prostate cancer cells confirmed that the catenane exciplex emission can be detected at micromolar concentrations.

Controlled binding of organic guests by stimuli-responsive macrocycles

Synthetic supramolecular chemistry pursues not only the construction of new matter, but also control over its inherently dynamic behaviour.

The influence of metal-complexing macrocycle size on intramolecular movement in rotaxanes

[2]rotaxanes composed of a dibenzo-24-crown-8 wheel (DB24C8) and an axle containing two metal-complexing tetraazamacrocyclic units linked with a p-xylylene bridge, coordinating the same or different metal ions (Ni and/or Cu) are described. A symmetric di-Ni rotaxane crystallizes in the monoclinic P21/c space group with one rotaxane cation and four PF6- counterions in the asymmetric part of the unit cell. In the crystal, the cations and anions of the investigated compound form an intertwined 3D-framework with C-HF and C-Hπ intermolecular interactions. The NMR ROESY spectrum of the diamagnetic di-Ni rotaxane confirms that at room temperature slow shuttling of the DB24C8 moiety takes place along the symmetric axle. The non-symmetric rotaxane, containing 14- and 16-membered nickel(ii)-complexing macrocycles, exists in solution as an equilibrium mixture of two conformers in the ratio of ca. 1 : 7. This rotaxane behaves as a potential-controlled molecular switch: in the major isomer, the DB24C8 moiety is localized in the vicinity of the 14-membered metal-complexing macrocycle, and moves towards the 16-membered unit upon the first oxidation. Reverse translocation occurs after the subsequent oxidation of the 14-membered complex unit. In contrast, no such movements of the DB24C8 moiety were observed upon investigation of the heteronuclear [2]rotaxane, coordinating copper(ii) and nickel(ii) ions.

[2]Rotaxane End-Capping Synthesis by Click Michael-Type Addition to the Vinyl Sulfonyl Group

We report the application of the click Michael-type addition reaction to vinyl sulfone or vinyl sulfonate groups in the synthesis of rotaxanes through the threading-and-capping method. This methodology has proven to be efficient and versatile as it allowed the preparation of rotaxanes using template approaches based on different noncovalent interactions (i.e., donor-acceptor π-π interactions or hydrogen bonding) in yields of generally 60-80 % and up to 91 % aided by the mild conditions required (room temperature or 0 °C and a mild base such as Et₃ N or 4-(N,N-dimethylamino)pyridine (DMAP)). Furthermore, the use of vinyl sulfonate moieties, which are suitable motifs for coupling-and-decoupling (CAD) chemistry, implies another advantage because it allows the controlled chemical disassembly of the rotaxanes into their components through nucleophilic substitution of the sulfonates resulting from the capping step with a thiol under mild conditions (Cs₂ CO₃ and room temperature).

Mechanically interlocked materials. Rotaxanes and catenanes beyond the small molecule

An overview of the progress in mechanically interlocked materials is presented. In particular, we focus on polycatenanes, polyrotaxanes, metal–organic rotaxane frameworks (MORFs), and mechanically interlocked derivatives of carbon nanotubes (MINTs).

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.

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.

Chiral Catenanes and Rotaxanes: Fundamentals and Emerging Applications

Molecular chirality provides a key challenge in host-guest recognition and other related chemical applications such as asymmetric catalysis. For a molecule to act as an efficient enantioselective receptor, it requires multi-point interactions between host and chiral guest, which may be achieved by an appropriate chiral 3D scaffold. As a consequence of their interlocked structure, catenanes and rotaxanes may present such a 3D scaffold, and can be chiral by inclusion of a classical chiral element and/or as a consequence of the mechanical bond. This Minireview presents illustrative examples of chiral [2]catenanes and [2]rotaxanes, and discusses where these molecules have been used in chemical applications such as chiral host-guest recognition and asymmetric catalysis.

Ion-pair electrostatic attraction-enhanced donor–acceptor interactions between the prototypic 1,4-dialkoxybenzene-viologen binding mode in water

Intermolecular ion-pair electrostatic attraction was demonstrated to remarkably enhance the donor–acceptor interaction between prototypic 1,4-dialkoxybenzene and viologen in water.

Introducing Stable Radicals into Molecular Machines

Ever since their discovery, stable organic radicals have received considerable attention from chemists because of their unique optical, electronic, and magnetic properties. Currently, one of the most appealing challenges for the chemical community is to develop sophisticated artificial molecular machines that can do work by consuming external energy, after the manner of motor proteins. In this context, radical-pairing interactions are important in addressing the challenge: they not only provide supramolecular assistance in the synthesis of molecular machines but also open the door to developing multifunctional systems relying on the various properties of the radical species. In this Outlook, by taking the radical cationic state of 1,1'-dialkyl-4,4'-bipyridinium (BIPY^•+) as an example, we highlight our research on the art and science of introducing radical-pairing interactions into functional systems, from prototypical molecular switches to complex molecular machines, followed by a discussion of the (i) limitations of the current systems and (ii) future research directions for designing BIPY^•+-based molecular machines with useful functions.

Surface-Assembled Mechanically Interlocked Architectures

Since the advent of supramolecular chemistry, there has been keen interest in the synthesis of interlocked molecules, given their unique potential to act as receptors, molecular machines and even motors. Despite advances in the complexity of molecular machines that can be synthesised and operated in solution, reports of the operation or even attachment of complex supramolecular systems on solid surfaces are less common. Synthetic challenges and a lack of adequate characterisation techniques to monitor the thermodynamic and kinetic influences governing assembly at the solution-surface interface has slowed progress in this area of research. This Review looks at the developments in the field of covalently assembled interlocked architectures on gold, silica and polymer surfaces, highlighting the differences observed between solution and surface assembly of these unique structures.

Stepwise, Protecting Group Free Synthesis of [4]Rotaxanes

Despite significant advances in the last three decades towards high yielding syntheses of rotaxanes, the preparation of systems constructed from more than two components remains a challenge. Herein we build upon our previous report of an active template copper-catalyzed azide-alkyne cycloaddition (CuAAC) rotaxane synthesis with a diyne in which, following the formation of the first mechanical bond, the steric bulk of the macrocycle tempers the reactivity of the second alkyne unit. We have now extended this approach to the use of 1,3,5-triethynylbenzene in order to successively prepare [2]-, [3]- and [4]rotaxanes without the need for protecting group chemistry. Whilst the first two iterations proceeded in good yield, the steric shielding that affords this selectivity also significantly reduces the efficacy of the active template (AT)-CuAAC reaction of the third alkyne towards the preparation of [4]rotaxanes, resulting in severely diminished yields.

Rotaxanes composed of dibenzo-24-crown-8 and macrocyclic transition metal complexing tetraimine units

New rotaxanes, composed of dibenzo-24-crown-8 and the axle containing two tetraazamacrocyclic Ni and/or Cu units, adopt either folded or extended conformations and di-Ni-rotaxanes exhibit a shuttling effect of DB24C8.

Rapidly accessible “click” rotaxanes utilizing a single amide hydrogen bond templating motif

The rapid synthesis (in yields of up to 47%) and co-conformational study of hydrogen bond templated rotaxanes are presented.