Stimuli-responsive host-guest systems based on the recognition of cryptands by organic guests

Construction of Supramolecular Polymers with Different Topologies by Orthogonal Self-Assembly of Cryptand-Paraquat Recognition and Metal Coordination

Recently, metal-coordinated orthogonal self-assembly has been used as a feasible and efficient method in the construction of polymeric materials, which can also provide supramolecular self-assembly complexes with different topologies. Herein, a cryptand with a rigid pyridyl group on the third arm derived from BMP32C10 was synthesized. Through coordination-driven self-assembly with a bidentate organoplatinum(II) acceptor or tetradentate Pd(BF4)2•4CH3CN, a di-cryptand complex and tetra-cryptand complex were prepared, respectively. Subsequently, through the addition of a di-paraquat guest, linear and cross-linked supramolecular polymers were constructed through orthogonal self-assembly, respectively. By comparing their proton nuclear magnetic resonance (1H NMR) and diffusion-ordered spectroscopy (DOSY) spectra, it was found that the degrees of polymerization were dependent not only on the concentrations of the monomers but also on the topologies of the supramolecular polymers.

Acid-Activated Motion Switching of DB24C8 between Two Discrete Platinum(II) Metallacycles

The precise operation of molecular motion for constructing complicated mechanically interlocked molecules has received considerable attention and is still an energetic field of supramolecular chemistry. Herein, we reported the construction of two tris[2]pseudorotaxanes metallacycles with acid-base controllable molecular motion through self-sorting strategy and host-guest interaction. Firstly, two hexagonal Pt(II) metallacycles M1 and M2 decorated with different host-guest recognition sites have been constructed via coordination-driven self-assembly strategy. The binding of metallacycles M1 and M2 with dibenzo-24-crown-8 (DB24C8) to form tris[2]pseudorotaxanes complexes TPRM1 and TPRM2 have been investigated. Furthermore, by taking advantage of the strong binding affinity between the protonated metallacycle M2 and DB24C8, the addition of trifluoroacetic acid (TFA) as a stimulus successfully induces an acid-activated motion switching of DB24C8 between the discrete metallacycles M1 and M2. This research not only affords a highly efficient way to construct stimuli-responsive smart supramolecular systems but also offers prospects for precisely control multicomponent cooperative motion.

Metal–organic cages for molecular separations

Separation technology is central to industries as diverse as petroleum, pharmaceuticals, mining and life sciences. Metal-organic cages, a class of molecular containers formed via coordination-driven self-assembly, show great promise as separation agents. Precise control of the shape, size and functionalization of cage cavities enables them to selectively bind and distinguish a wide scope of physicochemically similar substances in solution. Extensive research has, thus, been performed involving separations of high-value targets using coordination cages, ranging from gases and liquids to compounds dissolved in solution. Enantiopure capsules also show great potential for the separation of chiral molecules. The use of crystalline cages as absorbents, or the incorporation of cages into polymer membranes, could increase the selectivity and efficiency of separation processes. This Review covers recent progress in using metal-organic cages to achieve separations, with discussion of the many methods of using them in this context. Challenges and potential future developments are also discussed.

Repurposing Existing Molecular Machines through Accurate Regulation of Cooperative Motions

To understand how different external stimuli affect the cooperative motions in a molecular machine consisting of multiple components, we have investigated at the atomic level the effects of pH, solvent, and ionic strength on the mechanism underlying the ring-through-ring movement in a saturated [3]rotaxane. Our results indicate that different external stimuli regulate the stable states, the shuttling rate, and the mechanism that governs the ring-through-ring motion by controlling the cooperative movement of the components and triggering a gamut of responses, thereby opening to a vast number of potential applications, such as quaternary logical calculations. The present work cogently demonstrates that with existing nanomachines possessing a simple topology, but using different external stimuli-an approach coined multidimensional regulation-challenging tasks requiring precise control of the molecular motions at play can be achieved, and our methodology is particularly germane for de novo design of intelligent molecular machines.

A Cd(ii) and Zn(ii) selective naphthyl based [2]rotaxane acts as an exclusive Zn(ii) sensor upon further functionalization with pyrene

A new multi-functional [2]rotaxane, ROTX, has been synthesized via a Cu(i) catalysed azide-alkyne cycloaddition reaction between Ni(ii) templated azide terminated pseudorotaxane composed of a naphthalene based heteroditopic wheel, NaphMC, and an alkyne terminated stopper. Subsequently, ROTX has been functionalized with pyrene moieties to develop a bifluorophoric [2]rotaxane, PYROTX, having naphthalene and pyrene moieties. Detailed characterization of these two rotaxanes is performed by utilizing several techniques such as ESI-MS, (1D and 2D) NMR, UV/Vis and PL studies. Comparative metal ion sensing studies of NaphMC (a fluorophoric cyclic receptor), ROTX ([2]rotaxane with a naphthyl fluorophore) and PYROTX ([2]rotaxane having naphthyl and pyrene fluorophores) have been performed to determine the effect of dimensionality/functionalization on the metal ion selectivity. Although NaphMC fails to discriminate between metal ions, ROTX serves as a selective sensor for Zn(ii) and Cd(ii). Importantly, PYROTX shows exclusive selectivity towards Zn(ii) over various transition, alkali and alkaline earth metal ions including Cd(ii).

Efficient one-pot synthesis of [3]catenanes based on Pt(ii) metallacycles with a flexible building block

Three [3]catenanes were fabricated in high efficiency through the self-assembly of a 90° platinum(ii) receptor, a flexible bis(4,4′-bipyridinium) donor and a crown ether (DB24C8 or DB30C10).

A facile cucurbit[8]uril-based porous assembly: utilization in the adsorption of drugs and their controlled release

Cucurbit[n]urils (Q[n]s) are essential members of the supramolecular organic framework family owing to their distinct structure.

Pillar[5]arene-based ion-pair recognition for constructing a [2]pseudorotaxane with supramolecular interaction induced LCST behavior

Here, we report a novel [2]pseudorotaxane based on perbromoethylated pillar[5]arene/imidazolium iodide ionic liquid ion-pair recognition and this pseudorotaxane shows supramolecular interaction induced LCST behavior in solution.

A pillar[5]arene-based 3D polymer network for efficient iodine capture in aqueous solution

A pillar[5]arene-based 3D polymer network is constructed. It possesses good stability, recyclability and high efficiency in iodine capture in aqueous solution.

Multiple Stimuli-Responsive Conformational Exchanges of Biphen[3]arene Macrocycle

Conformational exchanges of synthetic macrocyclic acceptors are rather fast, which is rarely studied in the absence of guests. Here, we report multiple stimuli-responsive conformational exchanges between two preexisting conformations of 2,2',4,4'-tetramethoxyl biphen[3]arene (MeBP3) macrocycle. Structures of these two conformations are both observed in solid state, and characterized by ¹H NMR, ¹³C NMR and 2D NMR in solution. In particular, conformational exchanges can respond to solvents, temperatures, guest binding and acid/base addition. The current system may have a role to play in the construction of molecular switches and other stimuli-responsive systems.

pH-Responsive Host-Guest Complexations Between a Water-Soluble Pillar[6]arene Dodecyl-Ammonium Chloride and Aromatic Sulfonic Acids

In the present work, new host-guest binding motifs based on a water-soluble pillar[6]arene dodecyl-ammonium chloride (CP6) with two aromatic sulfonic acids in aqueous media were fabricated. In accordance with the integrated results of ¹H NMR, 2D NOESY, and florescence titration experiments, it was demonstrated that the host-guest binding of CP6 with the two aromatic sulfonic acids in aqueous solution not only has high binding constants but also has pH-responsiveness.

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Theoretical Prediction on a Novel Reduction-Responsive Nanoring Having a Disulfide Group for Facile Encapsulation and Release of Fullerenes C60 and C70

In this work, a novel reduction-responsive disulfide bond-containing cycloparaphenylene nanoring molecule (DSCPP) with a pyriform shape has been designed. In addition, the interactions between the designed nanoring (host) and fullerenes C₆₀ and C₇₀ (guests) were investigated theoretically at the M06-2X/6-31G(d,p) and M06-L/MIDI! levels of theory. By analyzing geometric characteristics and host-guest binding energies, it is revealed that the designed DSCPP is an ideal host molecule of guests C₆₀ and C₇₀. DSCPP presents excellent elastic deformation during the encapsulation of C₆₀ and C₇₀. The high binding energies suggest that both DSCPP⊃C₆₀ and DSCPP⊃C₇₀ (∼92 and 118 kJ·mol^-1 at the M06-2X/6-31G(d,p) level of theory) are stable host-guest complexes, and the guest C₇₀ is more strongly encapsulated than C₆₀ in the gas phase. The thermodynamic information indicates that the formation of the two host-guest complexes is thermodynamically spontaneous. In addition, the frontier molecular orbital (FMO) features and intermolecular weak interaction region between DSCPP and fullerenes gusts are discussed to further understand the structures and properties of the DSCPP⊃fullerene systems. Finally, the ring-opening mechanism of the DSCPP under reduction conditions is investigated.

Rational Design and Bulk Synthesis of Water-Containing Supramolecular Polymers

The utilization of structural water in chemical self-assembly has not only effectively eliminated the negative influences of solvents from solutions or gels but has also provided new insight into the fabrication of new materials in bulk. However, up to now, supramolecular polymerization triggered by structural water has been dominated more by serendipity than rational design. After carefully analyzing the chemical structures of artificial monomers and gaining a deep understanding of the water-triggered assembly process, we report herein the bulk formation of polymeric materials from water and low-molecular weight monomers by rational design instead of serendipity.

Synthesis, Structure and Supramolecular Properties of a Novel C3 Cryptand with Pyridine Units in the Bridges

The high-yield synthesis and the structural investigation of a new cryptand with C3 symmetry, exhibiting 2,4,6-triphenyl-1,3,5-triazine central units and pyridine-based bridges, are reported. The structure of the compound was investigated by single crystal X-ray diffractometry, NMR (nuclear magnetic resonance), HRMS (high resolution mass spectrometry) measurements, and theoretical calculations. The study of supramolecular behavior in solid state revealed the association of cryptand molecules by C-H---π and π---π contacts. Moreover, theoretical calculations indicated the high binding affinity of the cryptand for various organic molecules as guests.

Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Emerging from the origins of supramolecular chemistry and the development of selective chemical receptors that rely on lock-and-key binding, activity-based sensing (ABS)-which utilizes molecular reactivity rather than molecular recognition for analyte detection-has rapidly grown into a distinct field to investigate the production and regulation of chemical species that mediate biological signaling and stress pathways, particularly metal ions and small molecules. Chemical reactions exploit the diverse chemical reactivity of biological species to enable the development of selective and sensitive synthetic methods to decipher their contributions within complex living environments. The broad utility of this reaction-driven approach facilitates application to imaging platforms ranging from fluorescence, luminescence, photoacoustic, magnetic resonance, and positron emission tomography modalities. ABS methods are also being expanded to other fields, such as drug and materials discovery.

Host-Guest Complexations Between Pillar[6]arenes and Neutral Pentaerythritol Derivatives

It is demonstrated that three kinds of neutral pentaerythritol derivatives possess promising host-guest complexations with pillar[6]arenes both in solution and in the solid state. The inclusion structures were characterized by NMR spectroscopy and X-ray crystallography. The complexation properties in different solvents were also investigated.

Organic Cages as Building Blocks for Mechanically Interlocked Molecules: Towards Molecular Machines

The research on systems able to perform controllable motions under external stimuli arises great interest in the scientific community. Over the years, a library of innovative devices has been produced, classified in different categories according to the molecular or supramolecular level of motion. This minireview aims to highlight some representative studies, in which organic cages are used as building blocks for mechanically interlocked molecules, and in which intramolecular motions are triggered by external input. However, the application of organic cages in the construction of molecular machines is hardly achieved. A good compromise must actually be reached, between flexibility and rigidity of the cage's framework for an effective control of the intra- and/or intermolecular motion in the final mechanical device. Our final goal is to stimulate researchers' curiosity towards cage-like molecules, so that they take on the challenge of converting a cage into a molecular machine.

Constructing Adaptive Photosensitizers via Supramolecular Modification Based on Pillararene Host-Guest Interactions

In order to promote the development of photodynamic therapy (PDT), undesired side effects like low tumor specificity and the "always-on" phenomenon should be avoided. An effective solution is to construct an adaptive photosensitizer that can be activated to generate reactive oxygen species (ROS) in the tumor microenvironment. Herein, we design and synthesize a supramolecular switch based on a host-guest complex containing a water-soluble pillar[5]arene (WP5) and an AIEgen photosensitizer (G). The formation of the host-guest complex WP5⊃G quenches the fluorescence and inhibits ROS generation of G. Benefitting from the pH-responsiveness of WP5, the binding site between G and WP5 changes in an acidic environment through a shuttle movement. Consequently, fluorescence and ROS generation of the host-guest complex can be switched on at pH 5.0. This work offers a new paradigm for the construction of adaptive photosensitizers by using a supramolecular method.

Stimuli-Responsive Supramolecular Coaggregation and Disaggregation of Host-Guest Conjugates Having a Disulfide Linkage

Water-soluble cationic and anionic cyclophanes (1a and 2a, respectively) having a dabsyl group with a cleavable disulfide linkage were synthesized as a host-guest conjugate covalently bound with both host and guest components. Self-inclusion phenomena but not self-aggregation behaviors were observed for each cyclophane in aqueous media. Each cyclophane includes its own dabsyl moiety (guest component) in its macrocyclic cavity (host component) through hydrophobic interaction. When 1 equiv. of cationic 1a was added to an aqueous solution of anionic 2a, however, supramolecular coaggregates formed spontaneously through host-guest complexation. As regard the supramolecular coaggregates, the existence of larger particles was confirmed by DLS measurements and TEM observation. The hydrophobic interaction between the dabsyl moiety and macrocyclic cavity and electrostatic interactions between 1a and 2a play important roles in the supramolecular coaggregate formation. Each cyclophane having a cleavable disulfide linkage was easily transformed to the corresponding thiols by reducing reagents such as DTT, which was confirmed by MALDI-TOF MS. Disaggregation of the supramolecular coaggregates composed of 1a and 2a was successfully performed upon addition of DTT, with release of the thiol derivative of dabsyl. Such disaggregation of the coaggregates was also conducted by other external stimuli such as salts and competitive guests.

Supramolecular self-healing materials from non-covalent cross-linking host-guest interactions

The introduction of non-covalent bonds is effective for achieving self-healing properties because they can be controlled reversibly. One approach to introduce these bonds into supramolecular materials is use of host-guest interactions. This feature article summarizes the development of supramolecular materials constructed by non-covalent cross-linking through several approaches, such as host-guest interactions between host polymers and guest polymers, 1 : 2-type host-guest interactions, and host-guest interactions from the polymerization of host-guest inclusion complexes. Host-guest interactions show self-healing functions while also enabling stimuli-responsiveness (redox, pH, and temperature). The self-healing function of supramolecular materials is achieved by stress dispersion arising from host-guest interactions when stress is applied. Reversible bonds based on host-guest interactions have tremendous potential to expand the variety of functional materials.

Emerging functional materials based on chemically designed molecular recognition

The specific interactions responsible for molecular recognition play a crucial role in the fundamental functions of biological systems. Mimicking these interactions remains one of the overriding challenges for advances in both fundamental research in biochemistry and applications in material science. However, current molecular recognition systems based on host–guest supramolecular chemistry rely on familiar platforms (e.g., cyclodextrins, crown ethers, cucurbiturils, calixarenes, etc.) for orienting functionality. These platforms limit the opportunity for diversification of function, especially considering the vast demands in modern material science. Rational design of novel receptor-like systems for both biological and chemical recognition is important for the development of diverse functional materials. In this review, we focus on recent progress in chemically designed molecular recognition and their applications in material science. After a brief introduction to representative strategies, we describe selected advances in these emerging fields. The developed functional materials with dynamic properties including molecular assembly, enzyme-like and bio-recognition abilities are highlighted. We have also selected materials with dynamic properties in contract to traditional supramolecular host–guest systems. Finally, the current limitations and some future trends of these systems are discussed.

Supramolecular metallacyclic hydrogels with tunable strength switched by host–guest interactions

A new family of supramolecular hydrogels with tunable strength was successfully constructed through a combination of coordination-driven self-assembly, post-assembly polymerization and host–guest interactions.

Rotaxanes comprising cyclic phenylenedioxydiacetamides and secondary mono- and bis-dialkylammonium ions: effect of macrocyclic ring size on pseudorotaxane formation

[2]Rotaxanes, stabilized through multiple and cooperative hydrogen bonding system, were synthesized from dialkylammonium ions and macrocycle possessing two phenylenedioxydiacetamide units and appropriate spacers.

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.

Light-Activated Organic Molecular Motors and Their Applications

Molecular motors are at the heart of cellular machinery, and they are involved in converting chemical and light energy inputs into efficient mechanical work. From a synthetic perspective, the most advanced molecular motors are rotators that are activated by light wherein a molecular subcomponent rotates unidirectionally around an axis. The mechanical work produced by arrays of molecular motors can be used to induce a macroscopic effect. Light activation offers advantages over biological chemically activated molecular motors because one can direct precise spatiotemporal inputs while conducting reactions in the gas phase, in solution and in vacuum, while generating no chemical byproducts or waste. In this review, we describe the origins of the first light-activated rotary motors and their modes of function, the structural modifications that led to newer motor designs with optimized rotary properties at variable activation wavelengths. Presented are molecular motor attachments to surfaces, their insertion into supramolecular structures and photomodulating materials, their use in catalysis, and their action in biological environments to produce exciting new prospects for biomedicine.

Temperature Controls Guest Uptake and Release from Zn4L4 Tetrahedra

We report the preparation of triazatruxene-faced tetrahedral cage 1, which exhibits two diastereomeric configurations (T1 and T2) that differ in the handedness of the ligand faces relative to that of the octahedrally coordinated metal centers. At lower temperatures, T1 is favored, whereas T2 predominates at higher temperatures. Host–guest studies show that T1 binds small aliphatic guests, whereas T2 binds larger aromatic molecules, with these changes in binding preference resulting from differences in cavity size and degree of enclosure. Thus, by a change in temperature the cage system can be triggered to eject one bound guest and take up another.

Spontaneous Supramolecular Polymerization Driven by Discrete Platinum Metallacycle-Based Host-Guest Complexation

Considerable progress in platinum metallacycle-based supramolecular polymerization has promoted the fabrication and application of supramolecular materials. However, despite recent advances, supramolecular polymers constructed through platinum metallacycle-based host-guest complexation remain rare because of the dynamics of platinum metallacycles. Here, we achieve linear supramolecular polymerization via platinum metallacycle-based host-guest complexation by following the design rule of suppressing the dynamics of the metallacycles. The establishment of the platinum metallacycle-based host-guest system and the realization of this type of supramolecular polymerization are expected to open opportunities for platinum metallacycle-based functional materials.

Biological Macrocycle: Supramolecular Hydrophobic Guest Transport System Based on Nanodiscs with Photodynamic Activity

A biogenic macrocycle-based guest loading system has been developed by the self-assembly of membrane scaffold protein and phospholipids. The resulting 10 nm level transport system can increase the solubility of hydrophobic photodynamic agent hypocrellin B in aqueous medium and exhibited a cellular internalization capacity with substantial photodynamic activity.