Showing papers by "J. Fraser Stoddart published in 1996"

Self-Assembly in Natural and Unnatural Systems

Abstract: Although there are no fundamental factors hindering the development of nanoscale structures, there is a growing realization that “engineering down” approaches, in other words a reduction in the size of structures generated by lithographic techniques below the present lower limit of roughly 1 μm, may become impractical. It has, therefore, become increasingly clear that only by the development of a fundamental understanding of the self-assembly of large-scale biological structures, which exist and function at and beyond the nanoscale, downwards, and the extension of our knowledge regarding the chemical syntheses of small-scale structures upwards, can the gap between the promise and the reality of nanosystems be closed. This kind of construction of nanoscale structures and nanosystems represents the so-called “bottom up” or “engineering up” approach to device fabrication. Significant progress can be made in the development of nanoscience by transferring concepts found in the biological world into the chemical arena. Central to this mission is the development of simple chemical systems capable of instructing their own organization into large aggregates of molecules through their mutual recognition properties. The precise programming of these recognition events, and hence the correct assembly of the growing superstructure, relies on a fundamental understanding and the practical exploitation of non-covalent bonding interactions between and within molecules. The science of supramolecular chemistry—chemistry beyond the molecule in its very broadest sense—has started to bridge the yawning gap between molecular and macro-molecular structures. By utilizing inter-actions as diverse as aromatic π–π stacking and metal–ligand coordination for the information source for assembly processes, chemists have, in the last decade, begun to use biological concepts such as self-assembly to construct nanoscale structures and superstructures with a variety of forms and functions. Here, we provide a flavor of how self-assembly operates in natural systems and can be harnessed in unnatural ones.

Selbstorganisation in natürlichen und in nichtnatürlichen Systemen

Abstract: Zwar steht der Entwicklung von nanometergrosen Strukturen prinzipiell nichts im Wege, doch setzt sich immer mehr die Auffassung durch, das sich Strukturminiaturisierungen unter die gegenwartig durch lithographische Techniken erreichbare 1-μ-Grenze als nicht mehr praktikabel erweisen werden. Es wurde daher deutlich, das nur durch ein grundlegendes Verstandnis der Selbstorganisation von funktionellen makroskopischen biologischen Strukturen mit Abmessungen im Nanometerbereich und sogar darunter (Verkleinerungsansatz) und durch die Erweiterung unseres Wissens uber die chemische Synthese von mikroskopischen Strukturen (Vergroserungsansatz) die Brucke zwischen Anspruch und Wirklichkeit bei Nanosystemen geschlagen werden kann. Die Konstruktion von Nanostrukturen und -systemen aus kleinen Molekulbausteinen ist das „engineering-up” zum Aufbau von molekularen Funktionseinheiten. Bedeutende Fortschritte konnen auf dem Gebiet der Nanowissenschaften erzielt werden, wenn die Konzepte, die in der Biologie gefunden wurden, auf die Chemie ubertragen werden. Im Zentrum dieser Aufgabe steht die Entwicklung von einfachen chemischen Systemen, die sich selbst durch gegenseitige Erkennung zu groseren Molekulaggregaten organisieren konnen. Die genaue Programmierung derartiger Erkennungsprozesse und somit auch der korrekte Aufbau der Uberstrukturen setzen ein fundamentales Verstandnis und die Nutzung inter- sowie intramolekularer nichtkovalenter bindender Wechselwirkungen voraus. Die supramolekulare Chemie – eine Chemie, die in jeder Hinsicht uber die Chemie der Molekule hinausgeht – hat begonnen, den grosen Graben zwischen molekularen und makromolekularen Strukturen zu schliesen. Durch Nutzung von so unterschiedlichen Wechselwirkungen wie aromatischen π-Stapel- und Metall-Ligand-Koordinationswechselwirkungen als Informationsquellen der Aufbauprozesse haben Chemiker in den letzten zehn Jahren biologische Konzepte wie die Selbstorganisation zur Konstruktion von Nanostrukturen und Uberstrukturen mit einer Vielzahl von Formen und Funktionen herangezogen. Wir wollen hier einen Eindruck davon vermitteln, wie die Selbstorganisation in naturlichen Systemen funktioniert und wie diese Prinzipien nutzbringend auf nichtnaturliche Systeme angewendet werden konnen.

Amino Acid Derivatives of β-Cyclodextrin

Abstract: The syntheses of the heptaamino acid-substituted β-cyclodextrins per-6-[(phenylalanyl)amino]-β-cyclodextrin (6), per-6-cysteinyl-β-cyclodextrin (7), as well as the per-2,3-dimethyl-per-6-cysteinyl-β-cyclodextrin (12) are described. The amino acids were coupled to the primary face of the β-cyclodextrin torus using the backbone carboxylic acid functionality of phenylalanine and the side chain thiol group of cysteine. In the case of the heptacysteinyl derivatives, polyzwitterionic compounds were obtained and shown to be highly water soluble.

A Switchable Hybrid [2]-Catenane Based on Transition Metal Complexation and π-Electron Donor-Acceptor Interactions.

Abstract: A bimodal [2]-catenane has been synthesized via a copper(I) templated synthesis. The compound contains both a transition metal coordination site and a set of π-electron rich and π-electron deficient aromatic units suitable for the formation of acceptor-donor complexes. Each constituent ring is thus different from the other, and the organic backbone can adopt two favored contrasting orientations by circumrotation of one ring within the other: (i) in the metal complex mode, each dpp unit (dpp = 2,9-diphenyl-1,10-phenanthroline) is entwined about the other, while a cationic species is complexed in the coordination site thus created; (ii) in the organic π-electron acceptor-donor complex mode, the dpp fragments are remote from one another, and the π-electron rich and π-electron deficient units stack to form a complex. The conversion of one binding mode to the other implies complete topographical rearrangement of the molecule. It can be triggered by adding or removing the cation center (Cu+, Li+, or H+), bonde...

Self-Assembly of (n)Rotaxanes Bearing Dendritic Stoppers ┴

Abstract: We have developed a synthetic approach to the self-assembly of (2)-, (3)-, and (4)rotaxanes, incorporating bis-p-phenylene-34-crown-10 as the ring component(s) surrounding bipyridinium-based dumbbell-shaped components bearing dendritic stoppers at both ends. As a result of the hydrophobic dendritic framework, these ( n)rotaxanes are soluble in a wide range of organic solvents, despite the polycationic natures of their bipyridinium-based backbones. In all cases, they could be purified by means of column chromatography employing relatively low polar eluants. The molecular shuttlingaction of the (2)rotaxane containing two bipyridinium units on the rod portion of the dumbbell- shaped component has been investigated by variable-temperature 1 H-NMR spectroscopy in a range of solvents (CDCl3, CD2Cl2, THF-d8, and (CD3)2CO) for the first time. This investigation reveals a marked dependence of the rate of the shuttling process upon the polarity of the media. On going from CDCl3 to (CD3)2CO, the rate constant increases from ca. 200 to 33000 times per second. Molecular dynamics simulations, performed in CHCl3 and Me2CO on the (2)rotaxane, suggest that significant conformational changes occur upon changing the polarity of the medium resulting in both steric and electronic hindrance of the shuttling process in CHCl3. Three-dimensional representations, as well as the approximate sizessi.e. overall lengths and molecular volumes which range from 3 to 6 nm and from 4 to 6n m 3 , respectivelysof these molecular compounds, were obtained by means of molecular modeling studies. Thus, these nanometer-scale dendritic rotaxanes resemble naturally-occurring chemical systems incorporating an active component, in so far as the rotaxane-like core with its distinctive recognition features is surrounded by a molecular shell in the form of the dendritic framework.

The art and science of self-assembling molecular machines

Abstract: In this review, we show how noncovalent bonding interactions between -electron rich aromatic ring systems (e.g. hydroquinone) and the -electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) can be used to self-assemble novel molecular architectures which are not only interesting to us, because of their fascinating topologies, but also because they have the potential to be developed into molecular structures with switchable properties on the nanometre scale. The high efficiency observed in the self-assembly of a [2]catenane, and its dynamic properties in solution, represent the first step in the design and self-assembly of other molecular assemblies better suited for the study of molecular switching processes. Therefore, a series of [2]rotaxanes, mechanically-interlocked molecular compounds, consisting of a linear -electron rich dumbbell-shaped component and the -electron deficient tetracationic cyclophane as the cyclic component, have been self-assembled and evaluated. All of the so-called molecular shuttles show translational isomerism and one of them, comprising benzidine and biphenol recognition sites as the non-degenerate -electron rich sites, shows molecular switching properties when it is perturbed by external stimuli, such as electrons and protons. The versatility of our approach to nanoscale molecular switches is proven by the description of a series of molecular assemblies and supramolecular arrays, consisting of -electron rich and -electron deficient components, which display molecular switching properties when they are influenced by external stimuli that are photochemical, electrochemical and/or chemical in nature. However, the molecular switching phenomena take place in the solution state. Therefore, finally we describe how simple molecular structures can be ordered on to a solid support at the macroscopic level using Langmuir - Blodgett techniques. This is a necessary condition which must be fulfilled if we wish to construct supramolecular structures with device-like properties at the macroscopic level.

A Convergent Synthesis of Carbohydrate-Containing Dendrimers

Abstract: The synthesis of carbohydratecontaining dendrimers has been achieved by a convergent growth approach. The synthetic strategy involves: 1) the synthesis of the triglucosylated derivative of tris(hydroxymethyl)methylamine (TRIS), 2) the introduction of a glycine-derived spacer and 3,3′-iminodipropionic acid derived branching units on to the TRIS derivative by amide bond formation, 3) condensation of the above saccharidecontaining dendrons with a trifunctional 1,3,5-benzenetricarbonyl derivative, used as the core, by formation of amide bonds, and 4) deprotection of the saccharide units. A 9-mer and an 18-mer, carrying nine and eighteen saccharide units at the periphery, respectively, have been synthesized, in high yields at each step, by this synthetic strategy. By a variety of chromatographic and spectroscopic techniques, the dendrimers were shown to be structurally homogeneous, monodisperse, and error-free at all steps in their growth. These investigations were complemented by molecular modeling studies on the dendrimers. The presence of slightly distorted C3 symmetry was noted in both the 9-mer and the 18-mer.

Cyclobis(Paraquat-4,4′-Biphenylene)–an Organic Molecular Square

Abstract: Template-directed syntheses of cyclobis(paraquat-4,4′-biphenylene) (1)– a Molecular square–have been achieved by use of π-electron-rich macrocyclic hydroquinone-based and acyclic ferrocene-based templates. In particular, the use of a polyether-disubstituted ferrocene derivative as a template permits synthesis of 1 (which is accessible only in very low yields without templates) on a preparative scale. Furthermore, the use of a macrocyclic hydroquinone-based polyether template in corporating an ester function in one polyether chain–an (oriented) macrocycle–affords a 1 : 1 mixture of two topologically stereoisomeric [3]catenanes. Ester hydrolysis of the π-electron-rich macrocyclic components mechanically interlocked with 1 within the catenated structures releases the tetracationic cyclophane in quantitative yield as a result of the degradation of the [3]catenanes. The molecular square has been characterized by X-ray crystallography, FAB mass spectrometry, 1H NMR and 13C NMR spectroscopies, and elemental analysis. The binding properties of 1 and of the smaller cyclophane cyclobis(paraquat-p-phenylene) toward a series of π-electronrich guests have also been investigated with the above techniques and UV/VIS spectroscopy. The self-assembly of the resulting supramolecular complexes in solution and in the solid state is driven mainly by π–π stacking interactions and hydrogen-bonding interactions, as well as by edge-to-face T-type interactions. In particular, the complexation of ferrocene or a ferrocene-based derivative within the cavity of 1 suggests the possibility of constructing functioning ferrocene-based molecular and supramolecular devices that can be controlled electrochemically in the form of catenanes, rotaxanes, and pseudorotaxanes.

Bis[2]catenanes and a bis[2]rotaxane–Model Compounds for Polymers with Mechanically Interlocked Components†

Abstract: The self-assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis-para-phenylene[34]crown-10 (BPP34C10) and 1,5-naphtho-para-phenylene[36]-crown-10 (1/5NPP36C10) containing a fused five-membered ring with a secondary amine function is described. These intermediate N-allylimido macrocyclic polyethers undergo template-directed reactions with 1,1′-[1,4-phenylenebis-(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) and 1,4-bis(bromo-methyl)benzene to produce [2]catenanes containing an N-allyl functionality. The N-allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′-biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self-assembly reactions at both recognition sites to yield bis[2]catenanes. The self-assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′-[1,4-phenylenebis(methylene)]bis-4,4′-bipyridinium bis-(hexafluorophosphate) with a compound containing two linked 1,4-bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell-shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self-assembly of a bis[2]rotaxane. The X-ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.

Second-Sphere Coordination

Abstract: The design and synthesis of artifical receptors, able to recognize and bind transition metal complexes, is an area of growing interest in supramolecular chemistry. Macrocyclic hosts such as crown ethers, cyclodextrins, and cyclophanes have been employed to generate second-sphere adducts with numerous transition metal complexes as a result of nonconvalent bonding interactions, such as hydrogen bonding, π-π stacking, and hydrophobic interactions. The effect of second-sphere coordination on the chemical, electrochemical, and photochemical properties of the adducts, as well as on their geometries in solution and in the solid state, has been investigated; these investigations have demonstrated the potential of second-sphere coordination in modulating the behavior of transition metal complexes by designed external intervention.

Synthetic Cyclic Oligosaccharides-Syntheses and Structural Properties of a Cyclo[(1 → 4)-α-L-rhamnopyranosyl-(1 → 4)-α-D-mannopyranosyl]trioside and -tetraoside.

Abstract: An efficient polycondensation-cyclization approach to the synthesis of cyclodextrin analogues is demonstrated by the preparation of cyclohexaoside 1 and cyclooctaoside 2. The key intermediate, disaccharide 3, bearing the cyanoethylidene group as a glycosyl donor function and the trityloxy group as a glycosyl acceptor function was prepared in 15 steps starting from L-rhamnose and D-mannose. The crucial cyclooligomerization of the disaccharide monomer 3 was carried out in the presence of TrClO4 as a promoter with the use of ultra-dry conditions at normal concentrations. This reaction led to formation of the cyclic oligosaccharides 28 and 29 (in 34 and 31% yield, respectively), which were deprotected to afford 1 and 2, respectively. The X-ray crystal structural analysis of the cyclooctaoside 2 reveals a cylindrical shape for the cyclic oligosaccharide with C4 symmetry. Individual molecules of 2 are arranged perfectly in stacks that form nanotubes in the solid state.

Template-Directed Syntheses of Rotaxanes

Abstract: Rotaxanes are molecules comprised of a dumbbell-shaped component encircled by one or more macrocyclic components. The early syntheses of rotaxanes were mainly based upon statistical threading or upon directed methodologies involving chemical conversion. However, with the advent of supramolecular chemistry, a series of host-guest and template-directed approaches to rotaxanes have been developed and employed successfully. We have devised a template-directed approach to rotaxanes incorporating π-electron deficient bipyridinium-based components and π-electron rich hydroquinone-based polyether components. The noncovalent bonding interactions responsible for the self-assembly of these molecular compounds are (i) π-π stacking interactions between the complementary aromatic units, (ii) hydrogen bonding interactions between the acidic hydrogen atoms in the α-positions with respect to the nitrogen atoms on the bipyridinium units and some of the polyether oxygen atoms, as well as, (iii) edge-to-face T-type interactions between some of the hydrogen atoms on the hydroquinone rings and the π-clouds of the aromatic spacers separating the bipyridinium units. By employing these methodologies, we have synthesised a range of [2]rotaxanes, [3]rotaxanes, and [4]rotaxanes. Furthermore, the dynamic processes involving the shuttling of the cyclic components along the dumbbell-shaped components associated with some of these rotaxanes have been investigated in some detail. The reversible control of the process via external stimuli - such as chemical and electrochemical - has been achieved in the case of a [2]rotaxane incorporating benzidine and biphenol recognition sites. These results suggest the possibility of generating, on the nanoscopic level, molecular devices in the shape of rotaxanes able to store and process informations, thus, affording molecular machines.

Self-assembling wholly synthetic systems

Abstract: The idea of synthesizing, efficiently and precisely, large and complex artificial molecular assemblies and supramolecular arrays from simple modular components has resulted in some chemical scientists using self-assembly in chemical synthesis, thus mimicking the many and beautiful examples of this paradigm in nature. Coordination of organic ligands around metal centres, hydrogen bonding interactions, and donor/acceptor π-π-stacking interactions have been employed to self-assemble numerous structures and superstructures, such as double and triple helices, grids, cages, and square-like networks, two- and three-dimensional aggregates, tubular ensembles, as well as a large number of mechanically interlocked molecular compounds, such as catenanes, rotaxanes, and knots.

Self-assembly in chemical systems

Abstract: Molecular and supramolecular assemblies, some of which should be amenable to electrochemical control, have been assembled from molecular components composed of simple building blocks Self-assembly in chemical systems appears to take place under very precise constitutional control and, in some cases, it is easier to construct the molecular and supramolecular assemblies than it is to make some of the molecular components on their own