Showing papers by "J. Fraser Stoddart published in 2012"

Large-Pore Apertures in a Series of Metal-Organic Frameworks

Abstract: We report a strategy to expand the pore aperture of metal-organic frameworks (MOFs) into a previously unattained size regime (>32 angstroms). Specifically, the systematic expansion of a well-known MOF structure, MOF-74, from its original link of one phenylene ring (I) to two, three, four, five, six, seven, nine, and eleven (II to XI, respectively), afforded an isoreticular series of MOF-74 structures (termed IRMOF-74-I to XI) with pore apertures ranging from 14 to 98 angstroms. All members of this series have noninterpenetrating structures and exhibit robust architectures, as evidenced by their permanent porosity and high thermal stability (up to 300°C). The pore apertures of an oligoethylene glycol–functionalized IRMOF-74-VII and IRMOF-74-IX are large enough for natural proteins to enter the pores.

Mesoporous silica nanoparticles in biomedical applications

Abstract: This tutorial review provides an outlook on nanomaterials that are currently being used for theranostic purposes, with a special focus on mesoporous silica nanoparticle (MSNP) based materials. MSNPs with large surface area and pore volume can serve as efficient carriers for various therapeutic agents. The functionalization of MSNPs with molecular, supramolecular or polymer moieties, provides the material with great versatility while performing drug delivery tasks, which makes the delivery process highly controllable. This emerging area at the interface of chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from sol–gel science, the fabrication of nanomaterials, supramolecular chemistry, controllable drug delivery and targeted theranostics in biology and medicine.

Dynamic imine chemistry.

Abstract: Formation of an imine—from an amine and an aldehyde—is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent ‘proof-reading’ and ‘error-checking’ associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form—sometimes days or even months—but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols—which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control—an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This tutorial review focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.

Great expectations: can artificial molecular machines deliver on their promise?

Abstract: The development and fabrication of mechanical devices powered by artificial molecular machines is one of the contemporary goals of nanoscience. Before this goal can be realized, however, we must learn how to control the coupling/uncoupling to the environment of individual switchable molecules, and also how to integrate these bistable molecules into organized, hierarchical assemblies that can perform significant work on their immediate environment at nano-, micro- and macroscopic levels. In this tutorial review, we seek to draw an all-important distinction between artificial molecular switches which are now ten a penny—or a dime a dozen—in the chemical literature and artificial molecular machines which are few and far between despite the ubiquitous presence of their naturally occurring counterparts in living systems. At the single molecule level, a prevailing perspective as to how machine-like characteristics may be achieved focuses on harnessing, rather than competing with, the ineluctable effects of thermal noise. At the macroscopic level, one of the major challenges inherent to the construction of machine-like assemblies lies in our ability to control the spatial ordering of switchable molecules—e.g., into linear chains and then into muscle-like bundles—and to influence the cross-talk between their switching kinetics. In this regard, situations where all the bistable molecules switch synchronously appear desirable for maximizing mechanical power generated. On the other hand, when the bistable molecules switch “out of phase,” the assemblies could develop intricate spatial or spatiotemporal patterns. Assembling and controlling synergistically artificial molecular machines housed in highly interactive and robust architectural domains heralds a game-changer for chemical synthesis and a defining moment for nanofabrication.

Synthesis, Structure, and Metalation of Two New Highly Porous Zirconium Metal-Organic Frameworks

Abstract: Three new metal–organic frameworks [MOF-525, Zr6O4(OH)4(TCPP-H2)3; MOF-535, Zr6O4(OH)4(XF)3; MOF-545, Zr6O8(H2O)8(TCPP-H2)2, where porphyrin H4-TCPP-H2 = (C48H24O8N4) and cruciform H4-XF = (C42O8H22)] based on two new topologies, ftw and csq, have been synthesized and structurally characterized. MOF-525 and -535 are composed of Zr6O4(OH)4 cuboctahedral units linked by either porphyrin (MOF-525) or cruciform (MOF-535). Another zirconium-containing unit, Zr6O8(H2O)8, is linked by porphyrin to give the MOF-545 structure. The structure of MOF-525 was obtained by analysis of powder X-ray diffraction data. The structures of MOF-535 and -545 were resolved from synchrotron single-crystal data. MOF-525, -535, and -545 have Brunauer–Emmett–Teller surface areas of 2620, 1120, and 2260 m2/g, respectively. In addition to their large surface areas, both porphyrin-containing MOFs are exceptionally chemically stable, maintaining their structures under aqueous and organic conditions. MOF-525 and -545 were metalated with i...

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Abstract: Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

High hopes: can molecular electronics realise its potential?

Abstract: Manipulating and controlling the self-organisation of small collections of molecules, as an alternative to investigating individual molecules, has motivated researchers bent on processing and storing information in molecular electronic devices (MEDs). Although numerous ingenious examples of single-molecule devices have provided fundamental insights into their molecular electronic properties, MEDs incorporating hundreds to thousands of molecules trapped between wires in two-dimensional arrays within crossbar architectures offer a glimmer of hope for molecular memory applications. In this critical review, we focus attention on the collective behaviour of switchable mechanically interlocked molecules (MIMs)—specifically, bistable rotaxanes and catenanes—which exhibit reset lifetimes between their ON and OFF states ranging from seconds in solution to hours in crossbar devices. When these switchable MIMs are introduced into high viscosity polymer matrices, or self-assembled as monolayers onto metal surfaces, both in the form of nanoparticles and flat electrodes, or organised as tightly packed islands of hundreds and thousands of molecules sandwiched between two electrodes, the thermodynamics which characterise their switching remain approximately constant while the kinetics associated with their reset follow an intuitively predictable trend—that is, fast when they are free in solution and sluggish when they are constrained within closely packed monolayers. The importance of seamless interactions and constant feedback between the makers, the measurers and the modellers in establishing the structure-property relationships in these integrated functioning systems cannot be stressed enough as rationalising the many different factors that impact device performance becomes more and more demanding. The choice of electrodes, as well as the self-organised superstructures of the monolayers of switchable MIMs employed in the molecular switch tunnel junctions (MSTJs) associated with the crossbars of these MEDs, have a profound influence on device operation and performance. It is now clear, after much investigation, that a distinction should be drawn between two types of switching that can be elicited from MSTJs. One affords small ON/OFF ratios and is a direct consequence of the switching in bistable MIMs that leads to a relatively small remnant molecular signature—an activated chemical process. The other leads to a very much larger signature and ON/OFF ratios resulting from physical or chemical changes in the electrodes themselves. Control experiments with various compounds, including degenerate catenanes and free dumbbells, which cannot and do not switch, are crucial in establishing the authenticity of the small ON/OFF ratios and remnant molecular signatures produced by bistable MIMs. Moreover, experiments conducted on monolayers in MSTJs of molecules designed to switch and molecules designed not to switch have been probed directly by spectroscopic and other means in support of MEDs that store information through switching collections of bistable MIMs contained in arrays of MSTJs. In the quest for the next generation of MEDs, it is likely that monolayers of bistable MIMs will be replaced by robust crystalline extended structures wherein the switchable components, derived from bistable MIMs, are organised precisely in a periodic manner.

Nanoporous Carbohydrate Metal–Organic Frameworks

Abstract: The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host–guest supramolecular chemistry in the 1970s and 1980s. The arrangement of −OCCO– and −OCCN– chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this −OCCO– bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal–organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbO...

Incorporation of an A1/A2-difunctionalized pillar[5]arene into a metal-organic framework

Abstract: An efficient synthetic route to an A1/A2-difunctionalized pillar[5]arene containing resolvable planar chirality has been developed and the arene employed as a strut in the synthesis of P5A-MOF-1, which has been demonstrated by X-ray powder diffraction analysis—supported by modeling—to be isoreticular with MOF-5. This metal–organic framework has an active domain that expresses good and selective uptake of neutral and positively charged electron-poor aromatic guests, which effect color changes of the cubic crystals from faint yellow to deep orange, arising from charge transfer between the guests and active domain of P5A-MOF-1.

Stimulated Release of Size-Selected Cargos in Succession from Mesoporous Silica Nanoparticles

Abstract: Combination drug therapy, a regimen in which multiple drugs with different therapeutic outcomes are used in parallel or in sequence, has become one of the dominant strategies in the clinical treatment of HIV/AIDS, diabetes, and cancer. In cancer therapy, for example, the U.S. Food and Drug Administration (FDA) approved the use in 2006 of Avastin in combination with Carboplatin and Paclitaxel for the initial systemic treatment of patients with lung cancer. Unlike monotherapy, combination therapy maximizes therapeutic efficacy against individual targets and is more likely to overcome drug resistance, while increasing the odds of a positive prognosis and reducing harmful side effects. Drug delivery systems, which administer medically active compounds to diseased cells in a targeted and controlled manner, have gained much attention in the past couple of decades. While polymers, dendrimers, micelles, vesicles, and nanoparticles have all been investigated for their use as possible drug delivery systems, most systems provide either delivery of a single drug or the simultaneous delivery of multiple drugs. Using these systems, however, it is difficult to control the administration order, timing, and dosage of each individual drug in a comprehensive manner. While it is possible to deliver a cocktail of drugs using several different co-administered drug delivery systems, this protocol has disadvantages. For example, it is not easy to expose several co-administered drug delivery systems to the same target at the right time, while also controlling the dosage rates and ratios of each individual drug. In order to administer chemotherapeutic combinations and produce synergistic actions, well-organized multidrug release systems, which can provide combination therapies by controlling the release behavior of each drug individually, need to be invented. Mesoporous silica nanoparticles (MSNs) have attracted widespread interest in the past decade for use in integrated functional systems. They have large surface exteriors and porous interiors that can be harnessed as reservoirs for smallmolecule-drug storage. These MSNs are nontoxic to cells and can undergo cellular uptake into acidic lysosomes by endocytosis when they are 100–200 nm in diameter, thus making them a popular candidate for drug delivery systems. In particular, MSNs can be functionalized with molecular, as well as supramolecular, switches in order to control the release of drug molecules in response to external stimuli. Oncommand release systems, which respond to a range of stimuli, including pH changes, light initiation, competitive binding, redox activation, biological triggers, and temperature changes, have been reported by us and others. To the best of our knowledge, however, all the on-command release systems reported to date cannot release multiple drugs in a step-by-step fashion. Cyclodextrins (CDs), because of their abilities to form inclusion complexes with guest molecules, have been the focus of much research. b-Cyclodextrin (b-CD), which comprises seven a-1,4-linked d-glucopyranosyl units with top and bottom cavities of 6.0 and 6.5 , respectively, has been employed as a gatekeeper in drug delivery systems. [*] Dr. C. Wang, D. Cao, Dr. Y.-L. Zhao, J. W. Gaines, Dr. O. A. Bozdemir, M. W. Ambrogio, Dr. M. Frasconi, Dr. Y. Y. Botros, Prof. J. F. Stoddart Center for the Chemistry of Integrated Systems Department of Chemistry and Department of Material Sciences Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: stoddart@northwestern.edu Z. Li, Prof. J. I. Zink Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095 (USA) E-mail: zink@chem.ucla.edu

Mesoporous Silica Nanoparticles in Biomedical Applications

Abstract: This tutorial review provides an outlook on nanomaterials that are currently being used for theranostic purposes, with a special focus on mesoporous silica nanoparticle (MSNP) based materials. MSNPs with large surface area and pore volume can serve as efficient carriers for various therapeutic agents. The functionalization of MSNPs with molecular, supramolecular or polymer moieties, provides the material with great versatility while performing drug delivery tasks, which makes the delivery process highly controllable. This emerging area at the interface of chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from sol–gel science, the fabrication of nanomaterials, supramolecular chemistry, controllable drug delivery and targeted theranostics in biology and medicine.

Ground-state thermodynamics of bistable redox-active donor-acceptor mechanically interlocked molecules

Abstract: Fashioned through billions of years of evolution, biological molecular machines, such as ATP synthase, myosin, and kinesin, use the intricate relative motions of their components to drive some of life’s most essential processes. Having control over the motions in molecules is imperative for life to function, and many chemists have designed, synthesized, and investigated artificial molecular systems that also express controllable motions within molecules. Using bistable mechanically interlocked molecules (MIMs), based on donor–acceptor recognition motifs, we have sought to imitate the sophisticated nanoscale machines present in living systems. In this Account, we analyze the thermodynamic characteristics of a series of redox-switchable [2]rotaxanes and [2]catenanes. Control and understanding of the relative intramolecular movements of components in MIMs have been vital in the development of a variety of applications of these compounds ranging from molecular electronic devices to drug delivery systems.These...

Solution-phase mechanistic study and solid-state structure of a tris(bipyridinium radical cation) inclusion complex.

Abstract: The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT2(•+)) ring to form inclusion complexes with 1,1′-dialkyl-4,4′-bipyridinium radical cationic (BIPY•+) guests has been investigated mechanistically and quantitatively. Two BIPY•+ radical cations, methyl viologen (MV•+) and a dibutynyl derivative (V•+), were investigated as guests for the CBPQT2(•+) ring. Both guests form trisradical complexes, namely, CBPQT2(•+)⊂MV•+ and CBPQT2(•+)⊂V•+, respectively. The structural details of the CBPQT2(•+)⊂MV•+ complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV•+ is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT2(•+) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT2(•+)⊂MV•+ and further suggest an electronic asymmetry in the SOMO caused by radica...

Photoinduced Memory Effect in a Redox Controllable Bistable Mechanical Molecular Switch

Abstract: Mechanically interlocked molecules (MIMs) in the form of multiand bistable rotaxanes in which the ring component can be switched between different co-conformations in response to external stimuli, constitute an artificial molecular switch. They are of importance when it comes to the development of integrated systems and devices, such as responsive surfaces, molecule-based muscles and actuators, 5] nanovalves for controlled drug delivery, and molecular electronic devices (MEDs). Although the operation of bistable molecular switches is based on classical switching processes between thermodynamically stable states, it has become clear that the fulfillment of useful functions will only become possible if the rates of the mechanical movement between such states can also be controlled. This approach was used recently to implement ratchet-type mechanisms which are essential ingredients for the construction of molecular motors, and is of considerable relevance for the development of sequential logic devices such as flip-flops and memories. For all these purposes, the ability to be able to adjust the shuttling kinetics by modulating the corresponding energy barriers through external stimuli in a convenient, efficient, and reversible manner is a goal which still poses a considerable challenge to chemists. Herein, we discuss the performance of a molecular switch in the form of a bistable [2]rotaxane (Scheme 1), which 1) undergoes relative mechanical movements of its ring and

Metal-Organic Frameworks Incorporating Copper-Complexed Rotaxanes

Abstract: MOFs on the move: A copper-coordinated [2]pseudorotaxanate which reacts with zinc nitrate to form threefold interpenetrated networks retains most of its solution-state chemistry, including its ability to undergo electronic switching of some of the copper(I) ions under redox control.

Cooperative self-assembly: producing synthetic polymers with precise and concise primary structures.

Abstract: The quest to construct mechanically interlocked polymers, which present precise monodisperse primary structures that are produced both consistently and with high efficiencies, has been a daunting goal for synthetic chemists for many years. Our ability to realise this goal has been limited, until recently, by the need to develop synthetic strategies that can direct the formation of the desired covalent bonds in a precise and concise fashion while avoiding the formation of unwanted kinetic by-products. The challenge, however, is a timely and welcome one, as a consequence of, primarily, the potential for mechanically interlocked polymers to act as dynamic (noncovalent) yet robust (covalent) new materials for a wide array of applications. One such strategy which has been employed widely in recent years to address this issue, known as Dynamic Covalent Chemistry (DCC), is a strategy in which reactions operate under equilibrium and so offer elements of “proof-reading” and “error-checking” to the bond forming and breaking processes such that the final product distribution always reflects the thermodynamically most favourable compound. By coupling DCC with template-directed protocols, which utilise multiple weak noncovalent interactions to pre-organise and self-assemble simpler small molecular precursors into their desired geometries prior to covalent bond formation, we are able to produce compounds with highly symmetric, robust and complex topologies that are otherwise simply unobtainable by more traditional methods. Harnessing these strategies in an iterative, step-wise fashion brings us ever so much closer towards perfecting the controlled synthesis of high order main-chain mechanically interlocked polymers. This tutorial review focuses (i) on the development of DCC—namely, the formation of dynamic imine bonds—used in conjunction with template-directed protocols to afford a variety of mechanically interlocked molecules (MIMs) and ultimately (ii) on the synthesis of highly ordered poly[n]rotaxanes with high conversion efficiencies.

Positive cooperativity in the template-directed synthesis of monodisperse macromolecules.

Abstract: Two series of oligorotaxanes R and R′ that contain −CH2NH2+CH2– recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the −NH2+– recognition sites are separated by trismethylene bridges, whereas in the R′ series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 A apart from one another so as to facilitate efficient [π···π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in th...

Mechanically Interlocked Molecules Assembled by π–π Recognition

Abstract: The beauty and utility of interlocked architectures have been making their way relentlessly into chemistry in the form of mechanically interlocked molecules (MIMs) for almost half a century now. Few would challenge the assertion that the emergence of template-directed synthetic protocols has aided and abetted the facile and efficient construction of MIMs compared with the statistical approaches employed while the field was still in its infancy. To date, a panoply of MIMs has been created in the wake of emerging recognition motifs and the template-directed synthetic protocols they have forged. Among these motifs, those dependent on π–π stacking in the form of donor–acceptor interactions have played an important role in the increasingly rapid development of the field. The few integrated systems that have so far emerged based on this class of MIMs demonstrate their ability to act as active components in many potential applications. This review focuses on the progress which has been accomplished during the past decade involving MIMs comprising aromatic π–π stacking interactions. While progress has been remarkable, opportunities still abound for MIMs assembled by π–π recognition.

Radically enhanced molecular switches

Abstract: The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4′-bipyridinium (BIPY2+) unit, is described. The BIPY2+ unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT4+ ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT4+ rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY2+ unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tr...

Efficient Long-Range Stereochemical Communication and Cooperative Effects in Self-Assembled Fe4L6 Cages

Abstract: A series of large, optically active Fe4L6 cages was prepared from linear 5,5′-bis(2-formylpyridines) incorporating varying numbers (n = 0–3) of oligo-p-xylene spacers, chiral amines, and FeII. When a cage was constructed from the ligand bridged by one p-xylene spacer (n = 1) and a bulky chiral amine, both a homochiral Fe2L3 helicate and Fe4L6 cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe4L6 cage was observed. In the case of larger cages (n = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands’ geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.

Controlling switching in bistable [2]catenanes by combining donor-acceptor and radical-radical interactions.

Abstract: Two redox-active bistable [2]catenanes composed of macrocyclic polyethers of different sizes incorporating both electron-rich 1,5-dioxynaphthalene (DNP) and electron-deficient 4,4′-bipyridinium (BIPY2+) units, interlocked mechanically with the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+), were obtained by donor–acceptor template-directed syntheses in a threading-followed-by-cyclization protocol employing Cu(I)-catalyzed azide–alkyne 1,3-dipolar cycloadditions in the final mechanical-bond forming steps. These bistable [2]catenanes exemplify a design strategy for achieving redox-active switching between two translational isomers, which are driven (i) by donor–acceptor interactions between the CBPQT4+ ring and DNP, or (ii) radical–radical interactions between CBPQT2(•+) and BIPY•+, respectively. The switching processes, as well as the nature of the donor–acceptor interactions in the ground states and the radical–radical interactions in the reduced states, were investigated by single-crys...

Mechanostereochemistry and the mechanical bond

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

Editorial: From supramolecular to systems chemistry: complexity emerging out of simplicity.

Abstract: On 1 March 1967, I took my leave of Edinburgh University with a PhD degree firmly in my grasp to take to the air and cross “The Pond” in a jet liner bound for Canada, where I was to spend the next three years at Queen s University, as a postdoctoral fellow in the laboratory of Professor (JKN) Jones. During this, my first ever flight, the parting words of my PhD examiner, Sir Edmund Hirst, were still ringing in my ears, “Whatever you do, Stoddart, make sure you tackle a big problem in chemistry”.

A Semiconducting Organic Radical Cationic Host–Guest Complex

Abstract: The self-assembly and solid-state semiconducting properties of single crystals of a trisradical tricationic complex composed of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT2(•+)) ring and methyl viologen radical cation (MV•+) are reported. An organic field effect transistor incorporating single crystals of the CBPQT2(•+)⊂MV•+ complex was constructed using lithographic techniques on a silicon substrate and shown to exhibit p-type semiconductivity with a mobility of 0.05 cm2 V–1 s–1. The morphology of the crystals on the silicon substrate was characterized using scanning electron microscopy which revealed that the complexes self-assemble into “molecular wires” observable by the naked-eye as millimeter long crystalline needles. The nature of the recognition processes driving this self-assembly, radical–radical interactions between bipyridinium radical cations (BIPY•+), was further investigated by resonance Raman spectroscopy in conjunction with theoretical investigations of the vibrational ...

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

Abstract: The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2]rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylene-bridged bipyridinium (P-BIPY2+) unit and a cyclobis (paraquat-p-phenylene) (CBPQT4+) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT4+ ring encircles the DNP unit. Reduction of the CBPQT4+ leads to the CBPQT2(•+) diradical dication while the P-BIPY2+ unit is reduced to its P-BIPY•+ radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT2(•+) ring along the dumbbell to surround the P-BIPY•+ unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY•+ unit, a property which is absent between these redox centers in the free dumbbell and in the 1∶1 complex formed between the CBPQT2(•+) ring and the radical cation of methyl-phenylene-viologen (MPV•+). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (kET = 1.33 × 107 s-1) and activation energy (ΔG‡ = 1.01 kcal mol-1) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus–Hush theory.

Tetrathiafulvalene hetero radical cation dimerization in a redox-active [2]catenane.

Abstract: The electronic properties of tetrathiafulvalene (TTF) can be tuned by attaching electron-donating or electron-withdrawing substituents. An electron-rich macrocyclic polyether containing two TTF units of different constitutions, namely 4,4'-bis(hydroxymethyl)tetrathiafulvalene (OTTFO) and 4,4'-bisthiotetrathiafulvalene (STTFS), has been synthesized. On two-electron oxidation, a hetero radical dimer is formed between OTTFO(•+) and STTFS(•+). The redox behavior of the macrocyclic polyether has been investigated by electrochemical techniques and UV-vis and electron paramagnetic resonance (EPR) spectroscopies. The [2]catenane in which the macrocyclic polyether is mechanically interlocked with the cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring has also been prepared using template-directed protocols. In the case of the [2]catenane, the formation of the TTF hetero radical dimer is prevented sterically by the CBPQT(4+) ring. After a one-electron oxidation, a 70:30 ratio of OTTFO(•+) to STTFS(•+) is present at equilibrium, and, as a result, two translational isomers of the [2]catenane associated with these electronically different isomeric states transpire. EPR titration spectroscopy and simulations reveal that the radical states of the two constitutionally different TTF units in the [2]catenane still experience long-range electronic intramolecular coupling interactions, despite the presence of the CBPQT(4+) ring, when one or both of them are oxidized to the radical cationic state. These findings in the case of both the free macrocyclic polyether and the [2]catenane have led to a deeper fundamental understanding of the mechanism of radical cation dimer formation between constitutionally different TTF units.

Dynamic Imine Chemistry

Abstract: Formation of an imine—from an amine and an aldehyde—is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent ‘proof-reading’ and ‘error-checking’ associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form—sometimes days or even months—but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols—which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control—an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This tutorial review focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.