Chemistry welcomes a new bond: The mechanical bond has endowed molecules with component parts whose movements can be controlled and monitored. In his Nobel Lecture, J. F. Stoddart describes how being ...able to template the formation of mechanically interlocked molecules has led to the design and synthesis of shuttles, switches, and machines at the nanoscale.
Constructing multicolor photoluminescent materials with tunable properties is an attractive research objective on account of their abundant applications in materials science and biomedical ...engineering. By comparison with covalent synthesis, supramolecular chemistry has provided a more competitive and promising strategy for the production of organic materials and the regulation of their photophysical properties. By taking advantage of dynamic and reversible noncovalent bonding interactions, supramolecular strategies can, not only simplify the design and fabrication of organic materials, but can also endow them with dynamic reversibility and stimuli responsiveness, making it much easier to adjust the superstructures and properties of the materials. Occasionally, it is possible to introduce emergent properties into these materials, which are absent in their precursor compounds, broadening their potential applications. In an attempt to highlight the state‐of‐the‐art noncovalent strategies available for the construction of smart luminescent materials, an overview of color‐tunable materials is presented in this Review, with the emphasis being placed on the examples drawn from host–guest complexes, supramolecular assemblies and crystalline materials. The noncovalent synthesis of room‐temperature phosphorescent materials and the modulation of their luminescent properties are also described. Finally, future directions and scientific challenges in the emergent field of color‐tunable supramolecular emissive materials are discussed.
Supramolecular chemistry has provided a platform for the rational design of smart optical materials. Noncovalent bonding interactions, not only afford convenient and efficient approaches to fabricate emissive materials, but also endow them with dynamic reversibility and stimuli responsiveness. This review highlights the state‐of‐the‐art noncovalent strategies available for the construction of color‐tunable luminescent materials.
Rotaxane-Based Molecular Muscles Bruns, Carson J; Stoddart, J. Fraser
Accounts of chemical research,
07/2014, Volume:
47, Issue:
7
Journal Article
Peer reviewed
Conspectus More than two decades of investigating the chemistry of bistable mechanically interlocked molecules (MIMs), such as rotaxanes and catenanes, has led to the advent of numerous molecular ...switches that express controlled translational or circumrotational movement on the nanoscale. Directed motion at this scale is an essential feature of many biomolecular assemblies known as molecular machines, which carry out essential life-sustaining functions of the cell. It follows that the use of bistable MIMs as artificial molecular machines (AMMs) has been long anticipated. This objective is rarely achieved, however, because of challenges associated with coupling the directed motions of mechanical switches with other systems on which they can perform work. A natural source of inspiration for designing AMMs is muscle tissue, since it is a material that relies on the hierarchical organization of molecular machines (myosin) and filaments (actin) to produce the force and motion that underpin locomotion, circulation, digestion, and many other essential life processes in humans and other animals. Muscle is characterized at both microscopic and macroscopic length scales by its ability to generate forces that vary the distance between two points at the expense of chemical energy. Artificial muscles that mimic this ability are highly sought for applications involving the transduction of mechanical energy. Rotaxane-based molecular switches are excellent candidates for artificial muscles because their architectures intrinsically possess movable filamentous molecular components. In this Account, we describe (i) the different types of rotaxane “molecular muscle” architectures that express contractile and extensile motion, (ii) the molecular recognition motifs and corresponding stimuli that have been used to actuate them, and (iii) the progress made on integrating and scaling up these motions for potential applications. We identify three types of rotaxane muscles, namely, “daisy chain”, “press”, and “cage” rotaxanes, and discuss their mechanical actuation driven by ions, pH, light, solvents, and redox stimuli. Different applications of these rotaxane-based molecular muscles are possible at various length scales. On a molecular level, they have been harnessed to create adjustable receptors and to control electronic communication between chemical species. On the mesoscale, they have been incorporated into artificial muscle materials that amplify their concerted motions and forces, making future applications at macroscopic length scales look feasible. We emphasize how rotaxanes constitute a remarkably versatile platform for directing force and motion, owing to the wide range of stimuli that can be used to actuate them and their diverse modes of mechanical switching as dictated by the stereochemistry of their mechanical bonds, that is, their mechanostereochemistry. We hope that this Account will serve as an exposition that sets the stage for new applications and materials that exploit the capabilities of rotaxanes to transduce mechanical energy and help in paving the path going forward to genuine AMMs.
Currently, there is considerable interest in developing advanced rechargeable batteries that boast efficient distribution of electricity and economic feasibility for use in large-scale energy storage ...systems. Rechargeable aqueous zinc batteries are promising alternatives to lithium-ion batteries in terms of rate performance, cost, and safety. In this investigation, we employ Cu
(HHTP)
, a two-dimensional (2D) conductive metal-organic framework (MOF) with large one-dimensional channels, as a zinc battery cathode. Owing to its unique structure, hydrated Zn
ions which are inserted directly into the host structure, Cu
(HHTP)
, allow high diffusion rate and low interfacial resistance which enable the Cu
(HHTP)
cathode to follow the intercalation pseudocapacitance mechanism. Cu
(HHTP)
exhibits a high reversible capacity of 228 mAh g
at 50 mA g
. At a high current density of 4000 mA g
(~18 C), 75.0% of the initial capacity is maintained after 500 cycles. These results provide key insights into high-performance, 2D conductive MOF designs for battery electrodes.
Conspectus Cyclodextrin-based metal–organic frameworks (CD-MOFs), derived from γ-cyclodextrin (γ-CD) and alkali metal cations, constitute a class of porous, renewable, and edible MOFs that can be ...synthesized from a naturally occurring carbohydrate on a large scale. γ-CD is a C 8-symmetrical cyclic oligosaccharide composed of eight asymmetric α-1,4-linked d-glucopyranosyl residues that possesses a bucket-shaped cavity with an inner diameter of ∼1 nm and a depth of ∼0.8 nm. Upon combination of 1 equiv of γ-CD with 8 equiv of potassium hydroxide in an aqueous solution, followed by vapor diffusion of MeOH (or EtOH) into this solution during several days, CD-MOF-1 is obtained as cubic crystals. This carbohydrate-based MOF, which was discovered serendipitously in 2010, was the first highly crystalline CD-MOF to be obtained. X-ray crystallography of a single crystal reveals that it adopts the space group I432 with unit cell dimensions of approximately 31 × 31 × 31 Å3. Other CD-MOFs, namely, CD-MOF-2 and CD-MOF-3, can be obtained when potassium ions are replaced by rubidium and cesium ions, respectively. CD-MOFs comprise extended body-centered frameworks of (γ-CD)6 cubic units, which contain spherical pores that reside at the center of the cubes, interconnected by alkali metal cations, forming both cylindrical and triangular channels. During the past decade, CD-MOFs have emerged as a useful class of multifunctional materials based on porous frameworks with extended structures displaying robust crystallinity, permanent porosity, and excellent biocompatibility. The family of CD-MOFs has been joined by a growing collection of metal nodes involving alkali metal cations (Li+, Na+, K+, Rb+, Cs+) and γ-CD as well as its derivatives. As a result of the ability of their extended porous frameworks to absorb guest molecules, including gases, drugs, metal-based nanoclusters, and nanoparticles, CD-MOFs have potential applications in areas as disparate as templating syntheses of metal-based nanoparticles and gels, adsorption and separation, trapping highly reactive intermediates, catalyst supports, sensing, electrical memory, and drug delivery. In this Account, we tell the story of CD-MOFs, a scientific discovery made in our research laboratory at Northwestern University, and the opportunities to use these environmentally friendly porous materials across different fields of science and technology. The story includes representative synthetic protocols for the preparation of CD-MOFs, along with an overview of their structural features, functionalization, and chemical modification aimed at increasing their stabilities in aqueous environments, and finally, a summary of their applications. The examples we will discuss, however, are only illustrative, and there is a significant body of additional findings emanating from our laboratory and others, especially in the realm of developing new synthetic strategies, tuning the framework stabilities, and exploring the guest inclusion and emergent properties of CD-MOFs. We refer readers to the original communications, papers, and reviews cited herein. We hope that, in the telling of the story of CD-MOFs, this Account may promote new scientific discoveries and further development of CD-MOF-based technologies in the future.
The free primary hydroxyl groups in the metal–organic framework of CDMOF-2, an extended cubic structure containing units of six γ-cyclodextrin tori linked together in cube-like fashion by rubidium ...ions, has been shown to react with gaseous CO2 to form alkyl carbonate functions. The dynamic covalent carbon–oxygen bond, associated with this chemisorption process, releases CO2 at low activation energies. As a result of this dynamic covalent chemistry going on inside a metal–organic framework, CO2 can be detected selectively in the atmosphere by electrochemical impedance spectroscopy. The “as-synthesized” CDMOF-2 which exhibits high proton conductivity in pore-filling methanolic media, displays a ∼550-fold decrease in its ionic conductivity on binding CO2. This fundamental property has been exploited to create a sensor capable of measuring CO2 concentrations quantitatively even in the presence of ambient oxygen.
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.
The exploration of the chemistry associated with the little blue box has been a journey characterized by serendipity and fulfillment. We pay homage to Professor Siegfried Hünig, who forged the path to the modern frontiers of viologen chemistry on which we carry out research today.
We describe an example of “interpenetration isomerism” in three‐dimensional hydrogen‐bonded organic frameworks. By exploiting the crystallization conditions for a peripherally extended triptycene ...H6PET, we can modulate the interpenetration of the assembled frameworks, yielding a two‐fold interpenetrated structure PETHOF‐1 and a five‐fold interpenetrated structure PETHOF‐2 as interpenetration isomers. In PETHOF‐1, two individual nets are related by inversion symmetry and form an interwoven topology with a large guest‐accessible volume of about 80 %. In PETHOF‐2, five individual nets are related by translational symmetry and are stacked in an alternating fashion. The activated materials show permanent porosity with Brunauer‐Emmett‐Teller surface areas exceeding 1100 m2 g−1. Synthetic control over the framework interpenetration could serve as a new strategy to construct complex supramolecular architectures from simple organic building blocks.
On form: An example of “interpenetration isomerism” in three‐dimensional (3D) hydrogen‐bonded organic frameworks is reported. A trigonal prismatic triptycene building block assembles into either a two‐fold interpenetrated acs network (acs‐2 c) or a five‐fold interpenetrated acs network (acs‐5 c) depending upon the crystallization conditions.
The past quarter of a century has witnessed an increasing engagement on the part of physicists and chemists in the design and synthesis of molecular machines de novo. This minireview traces the ...development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuel, electrochemical potential and light activation become a minimum requirement for them to function away from equilibrium. The challenge facing this rapidly growing community of scientists and engineers today is one of putting wholly synthetic molecules to work, both individually and as collections. Here, we highlight some of the recent conceptual and practical advances relating to the operation of wholly synthetic rotary and linear motors.
A machine for every season: This minireview traces the development of artificial molecular machines from their prototypes in the form of shuttles and switches to their emergence as motors and pumps where supplies of energy in the form of chemical fuel, electrochemical potential and light activation become a minimum requirement for them to function away from equilibrium.
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