Showing papers by "J. Fraser Stoddart published in 2021"
TL;DR: In this paper, the authors survey the use of non-covalent interactions in the construction of electronic devices and provide a supramolecular-level understanding of charge transport behaviour associated with each interaction, as well as demonstrate the theoretical background and experimental readiness of single-supermolecule electronics for potential applications, such as nucleic acid and peptide sequencing.
Abstract: Using individual molecules as conducting bridges for electrons offers opportunities when investigating quantum phenomena that are not readily accessible from experiments involving ensembles of molecules. The probing of single molecules has led, over the past few decades, to the rise of molecular electronics. Although single-supermolecule electronics is an emerging field, it is not yet a well-defined area of molecular electronics. There is little doubt, however, that single-supermolecule electronics is poised to have an impact on molecular electronics for the simple reason that non-covalent interactions between molecular components in complexes have a profound effect on electron conductivities. In this Review, we survey this emerging field from the standpoint of non-covalent interactions in mechanically interlocked molecules, as well as in supermolecules, and discuss the (super)structure–property relationship of four different interactions associated with (supra)molecular junctions. They are host–guest interactions, hydrogen bonding, π–π interactions, and non-covalent interactions present in mechanically interlocked molecules. We focus our attention on providing a supramolecular-level understanding of charge transport behaviour associated with each interaction, as well as demonstrating the theoretical background and experimental readiness of single-supermolecule electronics for potential applications, such as nucleic acid and peptide sequencing, and the design and production of quantum interference devices, random-access memories and integrated devices. Single-supermolecule electronics focuses not only on charge transport within individual supermolecules but also on the weak non-covalent interactions between components of supermolecules and mechanically interlocked molecules. This Review surveys the use of non-covalent interactions in the construction of electronic devices.
115 citations
TL;DR: In this article, the authors outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective, connecting biomolecular and artificial molecular machine is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium.
Abstract: Pumps and motors are essential components of the world as we know it. From the complex proteins that sustain our cells, to the mechanical marvels that power industries, much we take for granted is only possible because of pumps and motors. Although molecular pumps and motors have supported life for eons, it is only recently that chemists have made progress toward designing and building artificial forms of the microscopic machinery present in nature. The advent of artificial molecular machines has granted scientists an unprecedented level of control over the relative motion of components of molecules through the development of kinetically controlled, away-from-thermodynamic equilibrium chemistry. We outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective. A key insight connecting biomolecular and artificial molecular machines is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium at every instant. The operation of molecular motors and pumps can be described by trajectory thermodynamics, a theory based on the work of Onsager, which is grounded on the firm foundation of the principle of microscopic reversibility. Free energy derived from thermodynamically non-equilibrium reactions kinetically favors some reaction pathways over others. By designing molecules with kinetic asymmetry, one can engineer potential landscapes to harness external energy to drive the formation and maintenance of geometries of component parts of molecules away-from-equilibrium, that would be impossible to achieve by standard synthetic approaches.
98 citations
TL;DR: For example, CD-MOF-1 is a C8-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 ∼08 nm as mentioned in this paper.
Abstract: 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 C8-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 ∼08 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 A3 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 channelsDuring 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 deliveryIn 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
83 citations
TL;DR: This review highlights examples of molecular containers with preordained binding pockets and focuses on cavity-enabled functions, such as selective capture, controlled release, and efficient catalysis.
70 citations
TL;DR: Aromatic hydrocarbon belts (AHCBs) have fascinated scientists for over half a century because of their aesthetically appealing structures and potential applications in the field of carbon nanotechnology as discussed by the authors.
Abstract: Aromatic hydrocarbon belts (AHCBs) have fascinated scientists for over half a century because of their aesthetically appealing structures and potential applications in the field of carbon nanotechnology. One of the enduring challenges in synthesizing AHCBs is how do we cope with the build-up of energy in the highly strained structures during their synthesis? Successful preparations of AHCBs offer the prospect of providing well-defined templates for the growth of uniform single-walled carbon nanotubes-a long-standing interest in nanocarbon science. In this Review, we revisit the protracted historical background involving the rational design and synthesis of AHCBs and highlight some of the more recent breakthroughs, with emphasis being placed on the different strategies that have been used for building up curved and fused benzenoid rings into molecular belts. We also discuss the scientific challenges in this fledgling field and provide some pointers as to what could transpire in years to come.
67 citations
TL;DR: The van der Waals interactions associated with physisorption and electronic interactions have been investigated extensively in equilibrium systems with a focus on the van derWaals interactions as mentioned in this paper, with the focus on van derwaals interactions being associated with van der Wals interactions.
Abstract: Over the past century, adsorption has been investigated extensively in equilibrium systems with a focus on the van der Waals interactions associated with physisorption and electronic interactions i...
63 citations
TL;DR: This Account designs and synthesizes a new class of macrocycles, namely, molecular triangles, which have rigid structures with triangular geometries and highlights the potential applications of these molecular triangles and their complexes with a range of solvents and electron-rich molecules.
Abstract: ConspectusMacrocycles have had a profound influence on the establishment of supramolecular chemistry because of their abundant molecular recognition and self-assembly characteristics. The design of new macrocyclic receptors that can be tailored by synthesis to display new and exotic properties is an important research objective for chemists and materials scientists. Rigid macrocycles with π-conjugated aromatic units, in contrast with flexible ones, tend to possess large interior and exterior π-surfaces in addition to persistent shapes. These features not only endow these macrocycles with a wide range of host-guest properties but also render them ideal building blocks for constructing a diverse variety of supramolecular architectures. The incorporation of π-conjugated units into macrocycles also imbues them with a wealth of optical, electronic, and magnetic properties, resulting in their broad application in materials science and molecular nanotechnology.Recently, we have designed and synthesized a new class of macrocycles, namely, molecular triangles, which have rigid structures with triangular geometries. They consist of three chiral trans-1,2-cyclohexano apexes and three aromatic tetracarboxylic diimide linkers, such as pyromellitic diimide, naphthalene diimide, and perylene diimide. Benefiting from the availability of facile synthetic protocols, the geometries and properties of these rigid molecular triangles can be altered at will. By combining these tetracarboxylic diimide linkers, we have been able to synthesize both molecular equilateral and isosceles triangles. During the past few years, we have conducted research in a systematic manner on the structural features and self-assembly characteristics of these molecular triangles. The following points are worthy of note regarding these molecular triangles: (i) They possess shape-persistent inner cavities of a highly electron-deficient nature. These features endow them with the ability to complex with anions and electron-rich molecules, forming supramolecular nanotubes and two-dimensional tilings. (ii) Those with intrinsic chirality are able to self-assemble into solid-state nonhelical or single-handed helical superstructures, inducing selective chirality transfer from the macrocycles to their crystalline supramolecular assemblies. (iii) The triangular arrangement of aromatic tetracarboxylic diimide linkers contributes to through-space electron delocalization encompassing the entire macrocycle, conferring exotic electronic and spin properties. To date, the family of molecular triangles has exhibited a range of physicochemical properties, such as anion recognition, chiral assembly, supramolecular gelation, energy storage, solid-state luminescence, and nonlinear optical response.In this Account, we summarize our recent progress in research into these molecular triangles. We present an overview of their design and synthesis, as well as a general summary of their structural features. Thereafter, we discuss state-of-the-art developments in relation to their molecular recognition properties and their assembly characteristics. In addition, we highlight the potential applications of these molecular triangles and their complexes with a range of solvents and electron-rich molecules. Finally, we speculate on further structural modifications and application-oriented explorations based on this class of molecular triangles.
53 citations
10 Jul 2021
TL;DR: Molecular nanotopology as discussed by the authors is a rapidly developing field of research that is emerging out of the confluence of chemical topology with the mechanical bond, and it is a promising field for future research.
Abstract: Molecular nanotopology—a term we coined recently—is a rapidly developing field of research that is emerging out of the confluence of chemical topology with the mechanical bond. When perusing the in...
50 citations
TL;DR: The state-of-the-art non-covalent strategies available for the construction of smart luminescent materials can be found in this article, with the emphasis being placed on the examples drawn from hostguest complexes, supramolecular assemblies and crystalline materials.
Abstract: 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.
47 citations
01 Jul 2021
TL;DR: Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry as mentioned in this paper.
Abstract: Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry. The unique electronic, magnetic, optical and redox-responsive properties of the conjugated π-radicals render molecules designed with radical-pairing interactions useful for applications in various areas of chemistry and materials science. In particular, the ability to control formation of radical cationic or anionic species, by redox stimulation, provides a flexible trigger for directed assembly and controlled molecular motions, as well as a convenient means of inputting energy to fuel non-equilibrium processes. In this Review, we provide an overview of different examples of radical-pairing-based recognition processes and of their emerging use in (1) supramolecular assembly, (2) templation of mechanically interlocked molecules, (3) stimuli-controlled molecular switches and, by incorporation of kinetic asymmetry in the design, (4) the creation of unidirectional molecular transporters based on pumping cassettes powered by fuelled switching of radical-pairing interactions. We conclude the discussion with an outlook on future directions for the field. Radical-pairing interactions were, until recently, considered something of a chemical curiosity. But these weak interactions, on par with hydrogen bonds, are easily switched on and off and, as a result, have become widely exploited in supramolecular chemistry, particularly in the assembly of out-of-equilibrium structures.
35 citations
TL;DR: In this article, the Coulombic interaction between the gold electrodes and the positively charged pyridinium terminal groups was used as an electrostatic anchor to form robust gold-molecule-gold junctions.
Abstract: The charge transport in single-molecule junctions depends critically on the chemical identity of the anchor groups that are used to connect the molecular wires to the electrodes In this research, we report a new anchoring strategy, called the electrostatic anchor, formed through the efficient Coulombic interaction between the gold electrodes and the positively charged pyridinium terminal groups Our results show that these pyridinium groups serve as efficient electrostatic anchors forming robust gold-molecule-gold junctions We have also observed binary switching in dicationic viologen molecular junctions, demonstrating an electron injection-induced redox switching in single-molecule junctions We attribute the difference in low- and high-conductance states to a dicationic ground state and a radical cationic metastable state, respectively Overall, this anchoring strategy and redox-switching mechanism could constitute the basis for a new class of redox-activated single-molecule switches
TL;DR: In this article, a donor-acceptor complex is formed by the mechanical interlocking of cyclobis(paraquat-p-phenylene) and 1,5-dinaphtho[38]crown-10, in which the charge-transfer interactions between two 4,4'-bipyridinium and two 1.5-dioxynaphthalene units are enhanced by mechanical bonding, leading to increased absorption of visible light even at low concentrations in solution.
Abstract: Colored charge-transfer complexes can be formed by the association between electron-rich donor and electron-deficient acceptor molecules, bringing about the narrowing of HOMO-LUMO energy gaps so that they become capable of harnessing visible light. In an effort to facilitate the use of these widespread, but nonetheless weak, interactions for visible light photocatalysis, it is important to render the interactions strong and robust. Herein, we employ a well-known donor-acceptor [2]catenane-formed by the mechanical interlocking of cyclobis(paraquat-p-phenylene) and 1,5-dinaphtho[38]crown-10-in which the charge-transfer interactions between two 4,4'-bipyridinium and two 1,5-dioxynaphthalene units are enhanced by mechanical bonding, leading to increased absorption of visible light, even at low concentrations in solution. As a result, since this [2]catenane can generate persistent bipyridinium radical cations under continuous visible-light irradiation without the need for additional photosensitizers, it can display good catalytic activity in both photo-reductions and -oxidations, as demonstrated by hydrogen production-in the presence of platinum nanoparticles-and aerobic oxidation of organic sulfides, such as l-methionine, respectively. This research, which highlights the usefulness of nanoconfinement present in mechanically interlocked molecules for the reinforcement of weak interactions, can not only expand the potential of charge-transfer interactions in solar energy conversion and synthetic photocatalysis but also open up new possibilities for the development of active artificial molecular shuttles, switches, and machines.
TL;DR: In this paper, a pillared-paddlewheel metal-organic framework (MOF) was constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC).
Abstract: The interaction of low-energy light with matter that leads to the production of high-energy light is known as photon upconversion. This phenomenon is of importance because of its potential applications in optoelectronics, energy harvesting, and the biomedical arena. Herein, we report a pillared-paddlewheel metal-organic framework (MOF), constructed from a tetrakis(4-carboxyphenyl)porphyrin sensitizer and a dipyridyl thiazolothiazole annihilator, designed for efficient triplet-triplet annihilation upconversion (TTA-UC). Single-crystal X-ray diffraction studies reveal that the Zn-metalated sensitizers are coordinated to Zn2 nodes in a paddlewheel fashion, forming 2D sheets, to which are linked annihilators, such that each sensitizer is connected to five of them. The precise arrangements of sensitizers with respect to annihilators, and the high annihilator-to-sensitizer ratio, facilitate Dexter energy transfer. This level of organization in an extended structure leads to a high TTA-UC efficiency of 1.95% (theoretical maximum = 50%) at an excitation power density of 25 mW cm-2.
TL;DR: In this article, it is shown how to incorporate 1-anthracenecarboxylate (1-AC-) as a substrate into a γ-cyclodextrin-containing metal-organic framework (CD-MOF-1), where the metals are K+ cations, prior to carrying out [4+4] photodimerizations between pairs of substrate molecules, affording selectively one of four possible regioisomers.
Abstract: For the most part, enzymes contain one active site wherein they catalyze in a serial manner chemical reactions between substrates both efficiently and rapidly. Imagine if a situation could be created within a chiral porous crystal containing trillions of active sites where substrates can reside in vast numbers before being converted in parallel into products. Here, we report how it is possible to incorporate 1-anthracenecarboxylate (1-AC-) as a substrate into a γ-cyclodextrin-containing metal-organic framework (CD-MOF-1), where the metals are K+ cations, prior to carrying out [4+4] photodimerizations between pairs of substrate molecules, affording selectively one of four possible regioisomers. One of the high-yielding regioisomers exhibits optical activity as a result of the presence of an 8:1 ratio of the two enantiomers following separation by high-performance liquid chromatography. The solid-state superstructure of 1-anthracenecarboxylate potassium salt (1-ACK), which is co-crystallized with γ-cyclodextrin, reveals that pairs of substrate molecules are not only packed inside tunnels between spherical cavities present in CD-MOF-1, but also stabilized-in addition to hydrogen-bonding to the C-2 and C-3 hydroxyl groups on the d-glucopyranosyl residues present in the γ-cyclodextrin tori-by combinations of hydrophobic and electrostatic interactions between the carboxyl groups in 1-AC- and four K+ cations on the waistline between the two γ-cyclodextrin tori in the tunnels. These non-covalent bonding interactions result in preferred co-conformations that account for the highly regio- and enantioselective [4+4] cycloaddition during photoirradiation. Theoretical calculations, in conjunction with crystallography, support the regio- and stereochemical outcome of the photodimerization.
TL;DR: In this paper, the design and synthesis of two mechanically interlocked cyclic [2]daisy chains (c3]DCs employing radical and anionic templates were reported.
TL;DR: In this paper, the authors reported three uranium-based MOF structures, NU-135X (X = 0, 1, 2) with variable arm lengths, which were derived from trigonal planar uranyl nodes and triptycene-based hexacarboxylate ligands.
Abstract: The rational reticular design of metal—organic frameworks (MOFs) from building units of known geometries is essential for enriching the diversity of MOF structures. Unexpected and intriguing structures, however, can also arise from subtle changes in the rigidity/length of organic linkers and/or synthetic conditions. Herein, we report three uranium-based MOF structures—i.e., NU-135X (X = 0, 1, 2)—synthesized from trigonal planar uranyl nodes and triptycene-based hexacarboxylate ligands with variable arm lengths. A new chiral 3,6-connected nuc net was observed in NU-1350, while the extended versions of the ligand led to 3-fold catenated MOFs (NU-1351 and NU-1352) with rare 3,6-connected cml-c3 nets. The differences in the topology of NU-1350 and NU-1351/NU-1352 could be attributed to the slight distortions of the shorter linker in the former from the ideal trigonal prism geometry to an octahedral geometry when coordinated to the trigonal planar uranyl nodes.
TL;DR: In this article, it was shown that linear and cyclic molecules with a saturated bipyridinium-ethane backbone exhibit a similar conductance signature when compared to molecules with conjugated bipyrithinium -ethene backbones.
Abstract: Investigating how electrons propagate through a single molecule is one of the missions of molecular electronics. Electrons, however, are also efficient catalysts for conducting radical reactions, a property that is often overlooked by chemists. Special attention should be paid to electron catalysis when interpreting single-molecule conductance results for the simple reason that an unexpected reaction mediated or triggered by electrons might take place in the single-molecule junction. Here, we describe a counterintuitive structure-property relationship that molecules, both linear and cyclic, employing a saturated bipyridinium-ethane backbone, display a similar conductance signature when compared to junctions formed with molecules containing conjugated bipyridinium-ethene backbones. We describe an ethane-to-ethene transformation, which proceeds in the single-molecule junction by an electron-catalyzed dehydrogenation. Electrochemically based ensemble experiments and theoretical calculations have revealed that the electrons trigger the redox process, and the electric field promotes the dehydrogenation. This finding not only demonstrates the importance of electron catalysis when interpreting experimental results, but also charts a pathway to gaining more insight into the mechanism of electrocatalytic hydrogen production at the single-molecule level.
TL;DR: In this article, two parallel pyrene panels serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates.
Abstract: The development of synthetic receptors that recognize carbohydrates in water with high selectivity and specificity is challenging on account of their structural complexity and strong hydrophilicity. Here, we report on the design and synthesis of two pyrene-based, temple-shaped receptors for the recognition of a range of common sugars in water. These receptors rely on the use of two parallel pyrene panels, which serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates. In addition, eight polarized pyridinium C-H bonds, projecting from the roofs and floors of the temple receptors toward the binding cavities, form [C-H···O] hydrogen bonds, with the equatorially oriented OH groups on the sugars located inside the hydrophobic cavities. Four para-xylylene pillars play a crucial role in controlling the distance between the roof and floor. These temple receptors are highly selective for the binding of glucose and its derivatives. Furthermore, they show enhanced fluorescence upon binding with glucose in water, a property which is useful for glucose-sensing in aqueous solution.
TL;DR: In this paper, the authors reported the instantaneous co-crystallization and concomitant co-precipitation between hexachloroplatinate dianions and cucurbit[6]uril.
Abstract: Since anions are vital for many chemical, biological and environmental processes, their recognition and separation continue to attract attention from chemists, materials scientists and engineers. Employing exo-binding of artificial macrocycles to recognize selectively anions remains a challenge in supramolecular chemistry. Herein, we report the instantaneous co-crystallization and concomitant co-precipitation between hexachloroplatinate dianions and cucurbit[6]uril, a phenomenon which relies on the selective recognition of these dianions through the noncovalent bonding interactions on the outer surface of cucurbit[6]uril. The selective hexachloroplatinate dianion recognition is driven by the weak [Pt-Cl···H-C] hydrogen bonding and [Pt-Cl···C=O] ion-dipole interactions. The synthetic protocol is highly selective. It is not observed in combinations between cucurbit[6]uril and other Pt- and Pd- or Rh-based chloride anions. We have also demonstrated that cucurbit[6]uril is able to separate selectively hexachloroplatinate dianions from mixtures of hexachloroplatinate, tetrachloropalladate, and hexachlororhodate anions. This highly selective and fast co-crystallization process, in principle, could be exploited to recover platinum from the spent vehicular three-way catalytic converters and other platinum-bearing metal waste.
TL;DR: In this paper, the binding cavity of α-cyclodextrin tori was shown to be a good fit for metal-coordination complexes, such as Au(CN)2- and Ag(CN)-2- with linear geometries.
Abstract: We report the molecular recognition of the Au(CN)2- anion, a crucial intermediate in today's gold mining industry, by α-cyclodextrin. Three X-ray single-crystal superstructures-KAu(CN)2⊂α-cyclodextrin, KAu(CN)2⊂(α-cyclodextrin)2, and KAg(CN)2⊂(α-cyclodextrin)2-demonstrate that the binding cavity of α-cyclodextrin is a good fit for metal-coordination complexes, such as Au(CN)2- and Ag(CN)2- with linear geometries, while the K+ ions fulfill the role of linking α-cyclodextrin tori together as a result of [K+···O] ion-dipole interactions. A 1:1 binding stoichiometry between Au(CN)2- and α-cyclodextrin in aqueous solution, revealed by 1H NMR titrations, has produced binding constants in the order of 104 M-1. Isothermal calorimetry titrations indicate that this molecular recognition is driven by a favorable enthalpy change overcoming a small entropic penalty. The adduct formation of KAu(CN)2⊂α-cyclodextrin in aqueous solution is sustained by multiple [C-H···π] and [C-H···anion] interactions in addition to hydrophobic effects. The molecular recognition has also been investigated by DFT calculations, which suggest that the 2:1 binding stoichiometry between α-cyclodextrin and Au(CN)2- is favored in the presence of ethanol. We have demonstrated that this molecular recognition process between α-cyclodextrin and KAu(CN)2 can be applied to the stripping of gold from the surface of activated carbon at room temperature. Moreover, this stripping process is selective for Au(CN)2- in the presence of Ag(CN)2-, which has a lower binding affinity toward α-cyclodextrin. This molecular recognition process could, in principle, be integrated into commercial gold-mining protocols and lead to significantly reduced costs, energy consumption, and environmental impact.
TL;DR: In this paper, a substituted tetracationic cyclophane, cyclobis(paraquat-p-1,4-dimethoxyphenylene), which associates in its bisradical dicationic redox state with the methyl viologen radical cation (MV•+) to give a 1:1 inclusion complex was presented.
Abstract: The solid-state properties of organic radicals depend on radical-radical interactions that are influenced by the superstructure of the crystalline phase. Here, we report the synthesis and characterization of a substituted tetracationic cyclophane, cyclobis(paraquat-p-1,4-dimethoxyphenylene), which associates in its bisradical dicationic redox state with the methyl viologen radical cation (MV•+) to give a 1:1 inclusion complex. The (super)structures of the reduced cyclophane and this 1:1 complex in the solid state deviate from the analogous (super)structures observed for the reduced state of cyclobis(paraquat-p-phenylene) and that of its trisradical tricationic complex. Titration experiments reveal that the methoxy substituents on the p-phenylene linkers do not influence binding of the cyclophane toward small neutral guests-such as dimethoxybenzene and tetrathiafulvalene-whereas binding of larger radical cationic guests such as MV•+ by the reduced cyclophane decreases 10-fold. X-ray diffraction analysis reveals that the solid-state superstructure of the 1:1 complex constitutes a discrete entity with weak intermolecular orbital overlap between neighboring complexes. Transient nutation EPR experiments and DFT calculations confirm that the complex has a doublet spin configuration in the ground state as a result of the strong orbital overlap, while the quartet-state spin configuration is higher in energy and inaccessible at ambient temperature. Superconducting quantum interference device (SQUID) measurements reveal that the trisradical tricationic complexes interact antiferromagnetically and form a one-dimensional Heisenberg antiferromagnetic chain along the a-axis of the crystal. These results offer insights into the design and synthesis of organic magnetic materials based on host-guest complexes.
TL;DR: In this article, the authors reported the encapsulation of two methyl viologen radical cations (MV.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)2(.+) in MeCN at room temperature.
Abstract: Complexation between a viologen radical cation (V.+ ) and cyclobis(paraquat-p-phenylene) diradical dication (CBPQT2(.+) ) has been investigated and utilized extensively in the construction of mechanically interlocked molecules (MIMs) and artificial molecular machines (AMMs). The selective recognition of a pair of V.+ using radical-pairing interactions, however, remains a formidable challenge. Herein, we report the efficient encapsulation of two methyl viologen radical cations (MV.+ ) in a size-matched bisradical dicationic host - namely, cyclobis(paraquat-2,6-naphthalene)2(.+) , i.e., CBPQN2(.+) . Central to this dual recognition process was the choice of 2,6-bismethylenenaphthalene linkers for incorporation into the bisradical dicationic host. They provide the space between the two bipyridinium radical cations in CBPQN2(.+) suitable for binding two MV.+ with relatively short (3.05-3.25 A) radical-pairing distances. The size-matched bisradical dicationic host was found to exhibit highly selective and cooperative association with the two MV.+ in MeCN at room temperature. The formation of the tetrakisradical tetracationic inclusion complex - namely, [(MV)2 ⊂CBPQN]4(.+) - in MeCN was confirmed by VT 1 H NMR, as well as by EPR spectroscopy. The solid-state superstructure of [(MV)2 ⊂CBPQN]4(.+) reveals an uneven distribution of the binding distances (3.05, 3.24, 3.05 A) between the three different V.+ , suggesting that localization of the radical-pairing interactions has a strong influence on the packing of the two MV.+ inside the bisradical dicationic host. Our findings constitute a rare example of binding two radical guests with high affinity and cooperativity using host-guest radical-pairing interactions. Moreover, they open up possibilities of harnessing the tetrakisradical tetracationic inclusion complex as a new, orthogonal and redox-switchable recognition motif for the construction of MIMs and AMMs.
15 Sep 2021
TL;DR: In this paper, it was shown that electron transport in tetracationic cyclophane circuits is mediated by quantum interference between channels formed from two lowest unoccupied molecular orbitals (LUMOs), while their highest occupied molecular orbits (HOMOs) play no significant role.
Abstract: Summary Single-molecule electronics is a sub-field of nanoelectronics in which individual devices are formed from single molecules placed between source and drain electrodes. During the past few years, scientists have demonstrated that the flow of electricity through these devices is controlled by quantum interference (QI) between electrons passing from source to drain. Their future development, however, is hampered by difficulties in controlling interference effects. Herein, we demonstrate that electron transport in tetracationic cyclophane circuits is mediated by QI between channels formed from two lowest unoccupied molecular orbitals (LUMOs), while their highest occupied molecular orbitals (HOMOs) play no significant role. Energy differences between these two LUMO channels induce constructive interference, leading to high conductance. By contrast, phase differences between these LUMO channels result in destructive interference and a suppression in overall conductance. Such a design of single-molecule circuits enables the construction of single-molecule conductors and insulators based on a single cyclophane platform.
TL;DR: In this article, non-dissociative electron transfer is used 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.
Abstract: 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
TL;DR: In this paper, a trigonal prismatic hexacationic cage was designed and synthesized to serve as a receptor for contorted nanographenes derivatives with diameters of approximately 15'a and thickness of 7'a.
Abstract: Nanographenes have kindled considerable interest in the fields of materials science and supramolecular chemistry as a result of their unique self-assembling and optoelectronic properties. Encapsulating the contorted nanographenes inside artificial receptors, however, remains challenging. Herein, we report the design and synthesis of a trigonal prismatic hexacationic cage, which has a large cavity and adopts a relatively flexible conformation. It serves as a receptor, not only for planar coronene, but also for contorted nanographene derivatives with diameters of approximately 15 A and thicknesses of 7 A. A comprehensive investigation of the host-guest interactions in the solid, solution and gaseous states by experimentation and theoretical calculations reveals collectively an induced-fit binding mechanism with high binding affinities between the cage and the nanographenes. Notably, the photostability of the nanographenes is improved significantly by the ultrafast deactivation of their excited states within the cage. Encapsulating the contorted nanographenes inside the cage provides a noncovalent strategy for regulating their photoreactivity. Encapsulating large and contorted nanographenes inside artificial receptors remain challenging. This work reports the synthesis, characterization and binding properties of a trigonal prismatic cage compound that can serve as a receptor for contorted nanographene derivatives.
04 Aug 2021
TL;DR: A molecular weaving strategy, recently reported in Nature Chemistry and Nature by Leigh and co-workers, has outlined the precise synthesis of a molecular 74 knot and a two-dimensional molecular fabric, opening up a new route for incorporating predictable, well-defined topologies into molecular architectures.
Abstract: Although items exhibiting topology are ubiquitous in everyday life, the realization of topology at the nanoscale level relies on innovative synthetic strategies. A molecular weaving strategy, recently reported in Nature Chemistry and Nature by Leigh and co-workers, has outlined the precise synthesis of a molecular 74 knot and a two-dimensional molecular fabric. These breakthroughs hard on the heels of each other open up a new route for incorporating predictable, well-defined topologies into molecular architectures. They will fuel the exploitation of advanced materials with topology-related properties.
TL;DR: In this article, the pyridine-containing nanotube assemblies exhibited measurable proton conductivity by electrochemical impedance spectroscopy, with values as high as 10-3 S m-1 (90% R.H., 25 °C) that they attribute to differences in their internal pore sizes.
Abstract: Macrocycles that assemble into nanotubes exhibit emergent properties stemming from their low dimensionality, structural regularity, and distinct interior environments. We report a versatile strategy to synthesize diverse nanotube structures in a single, efficient reaction by using a conserved building block bearing a pyridine ring. Imine condensation of a 2,4,6-triphenylpyridine-based diamine with various aromatic dialdehydes yields chemically distinct pentagonal [5 + 5], hexagonal [3 + 3], and diamond-shaped [2 + 2] macrocycles depending on the substitution pattern of the aromatic dialdehyde monomer. Atomic force microscopy and in solvo X-ray diffraction demonstrate that protonation of the macrocycles under the mild conditions used for their synthesis drives assembly into high-aspect ratio nanotubes. Each of the pyridine-containing nanotube assemblies exhibited measurable proton conductivity by electrochemical impedance spectroscopy, with values as high as 10-3 S m-1 (90% R.H., 25 °C) that we attribute to differences in their internal pore sizes. This synthetic strategy represents a general method to access robust nanotube assemblies from a universal pyridine-containing monomer, which will enable systematic investigations of their emergent properties.
TL;DR: In this paper, the authors describe the coordination-driven self-assembly of organometallic half-sandwich iridium and rhodium-based building blocks with asymmetric ambidentate pyridyl-carboxylate ligands.
Abstract: Here we describe the coordination-driven self-assembly of organometallic half-sandwich iridium(III)- and rhodium(III)-based building blocks with asymmetric ambidentate pyridyl-carboxylate ligands. Despite the potential for obtaining a statistical mixture of multiple products, D 2 symmetric octanuclear cages were formed selectively by taking advantage of the electronic effects emanating from the two types of chelating sites - ( O , O' ) and ( N , N' ) - on the tetranuclear building blocks. The metal sources and the lengths of bridging ligands show influence on the selectivity of the self-assembly. Experimental observations, supported by computational studies, suggest that the D 2 symmetric cages are the thermodynamically favoured products. Overall, the results underline the importance of electronic effects on the selectivity of coordination-driven self-assembly, and demonstrate that asymmetric ambidentate ligands can be used to control the design of discrete supramolecular coordination complexes.