Showing papers by "J. Fraser Stoddart published in 1995"

The self-assembly of branched [n]rotaxanes—the first step towards dendritic rotaxanes

Abstract: The self-assembly, by means of a slippage procedure, of three novel rotaxanes incorporating, respectively, one, two and three bisparaphenylene-34-crown-10 macrocyclic components and a single branched component, consisting of three bipyridinium units attached covalently to a 1,3,5-trisubstituted benzene central core and each bearing at its other end a substituted tetraarylmethane blocking group, is described.

The controlled self-assembly of a [3]rotaxane incorporating three constitutionally different components

Abstract: The self-assembly, by means of a slippage procedure, of a [3]rotaxane comprised of three constitutionally different sub-units—a dumbbell-shaped component, incorporating two 4,4′-bipyridinium units encircled by one bisparaphenylene-34-crown-10 macrocycle and one 1,5-dinaphtho-38-crown-10 macrocycie—is described.

Self-assembly in chemical synthesis

Abstract: More and more chemists are beginning to realise that the conventional synthetic methodology of making compounds group-by-group or molecule-by-molecule, employing reagents or catalysts to make or break covalent bonds and so manipulate functional groups and transform molecular structures, is insufficient by itself to construct the materials they would like to make in the future. Along its many synthetic trails, nature does not rely upon the inefficient use of protecting groups and the complexity of reagents according to the usual manner and practice of the synthetic organic chemist in a traditional sense. Indeed, one of the keys to the efficient operation of biological systems is their ability to self-assemble,1 self-organise2 and self-replicate.3

Towards mechanically‐linked polymers

Abstract: This short review describes the progress which is being made towards the self-assembly of mechanically-linked polymers. A new concept in polymer synthesis - self-assembly - is demonstrated to have the potential to create novel high molecular weight polymers which possess repeat units that do not just consist of a main chain backbone built up of entirely covalent bonds, but are constructed of mechanical linkages comprised of catenane and rotaxane motifs.

Advantages of the Rotaxane Framework for the Construction of Switchable Molecular Devices

Abstract: One of the main goals of research in supramolecular chemistry is the fabrication of molecular devices exhibiting properties that can be controlled on demand by applying an external stimulus [1]. Thus, external switching or control of certain properties is a crucial step in order to attain information storage in molecular systems [2]. Stoddart and coworkers have proposed the use of rotaxanes in which the relative positions of the cyclic (bead) and linear (thread)components can be controlled by a suitable switching scheme [3]. In this approach, the versatile π-acceptor host cyclobis(paraquat-p-phenylene) 1 4+ (see structure in the next page) is commonly utilized as the bead component. If a thread containing two or more different π-donor stations is used, the cyclic host will distribute itself among the various π-donor sites according to its relative affinity for each one of them. Selective oxidation or protonation of one of the stations should create a large barrier for the movement of the tetracationic bead over the positively charged station. This electrostatic barrier effectively switches the average structure of the rotaxane system by forcing the bead to other regions along the thread.

Macrocyclic polyethers incorporating resorcinol residues as templates for cyclobis(paraquat-p-phenylene) in the self-assembly of [2]catenanes

Abstract: The self-assembly of [2]catenanes incorporating cyclobis(paraquat-p-phenylene) and either bis(metaphenylene)-32-crown-10 or tris(metaphenylene)-48-crown-15 has been achieved. The dynamic processes associated with the relative motions of the two rings have been studied by variable temperature 1H NMR spectroscopy. Both [2]catenanes display rapid relative movements of the two components, associated with free energies of activation in the region 12–14 kcal mol−1 for different processes. The resorcinol residues of the macrocyclic polyethers are bound in the cavity of the cyclophane by π-π stacking, electrostatic interactions, and T-type hydrogen bonding.

Cyclophanes with self-recognising components

Abstract: High dilution techniques have been employed to construct a series of charged flexible cyclophanes comprised of π-electron-rich hydroquinone and π-electron-deficient bipyridinium residues linked by polyether spacer units of varying length: significant conformational changes, leading to intramolecular and intermolecular interactions between the π-donors and π-acceptors, have been observed—as evidenced by X-ray crystallographic and variable temperature 1H NMR and 2D ROESY spectroscopic investigations—within the series of the cyclophanes both in the solid and solution states as a result of increasing the lengths of the polyether spacer units.

The Self-Assembly of Redox-Active and Photo-Active Catenanes and Rotaxanes

Abstract: The science of supramolecular chemistry1 — the chemistry of the noncovalent bond — has developed rapidly in the last twenty five years, since Charles Pederson2 revealed that the formation of macrocyclic polyethers could be templated by the coordination of the initially acyclic polyether precursor to metal cations, which preorganise the ligand such that cyclisation becomes a favourable and competitive process alongside polymerisation. The previous two decades have revealed that there are three concepts that underpin supramolecular science at a fundamental level. They are: (i) self-assembly,3 (ii) self-organisation,4 and (iii) self-replication.5

Towards Molecular and Supramolecular Devices

Abstract: Ever since Watson and Crick1 unravelled the structure of DNA, much interest has been directed towards molecular recognition in natural systems. It has become apparent that many biological processes and chemical reactions are guided by noncovalent bonding interactions, which generate and control the orientation of interacting centres. These features enable biochemical processes to occur with great speed and efficiency. It is only recently that the scientific community has started to take advantage of the principles of Nature and construct unnatural molecular and supramolecular systems designed to exploit the use of noncovalent bonding interactions. The development of this science has given rise to a new field of chemistry — namely supramolecular chemistry.2 One of the main driving forces behind the development of supramolecular chemistry has been the urge to understand how natural supramolecular systems operate, and then to extend this understanding to wholly synthetic molecular and supramolecular architectures with novel functions, such as chemical sensing and information storage.2 Catenanes, rotaxanes and pseudorotaxanes are three classes of molecular and supramolecular structures that have received a great deal of attention recently from the Birmingham group.3 The order inherent within these self-assembled structures makes them ideal for nanometre-scale information storage devices. Many studies have been carried out on the self-assembly, decomplexation and reorganisation of these molecular and supramolecular structures in the solution state. By the use of external stimuli, such as light,4 current,3 pH,5, and the dielectric constant of the solvent,6 precise and delicate control of molecular and supramolecular structure is possible in solution.

The synthesis and structural mapping of unsymmetrical chemically modified α-cyclodextrins by high-field nuclear magnetic resonance spectroscopy

Abstract: Hexakis(2,3-di-O-methyl-6-deoxy-6-iodo)-α-cyclodextrin (I-DMαCD) was prepared from hexakis(2,3-di-O-methyl)-αCD (DMαCD) using iodine and triphenylphosphine. The subsequent attempted substitution of I-DMαCD with the phenoxide anions of either 4-hydroxybenzyl alcohol or 4-benzyloxyphenol produced unexpected products. These products were found to be a result of an elimination reaction involving one of the iodomethyl groups to produce one 6-deoxyhex-5-eno-D-glucopyranoside residue within the DMαCD structure in addition to the substitution of the other five iodomethyl groups either by (4-hydroxymethyl)phenyl or by (4-benzyloxy)phenyl ether functions, respectively. The products have been characterised fully using high resolution positive-ion FABMS and high resolution 1H and 13C NMR spectroscopies.

Self-Assembly: Whither and Thither Molecular Machines

Abstract: The production of molecular machines1,2 that might be able to function as information processing systems presents3,4 a considerable challenge to the chemical community. The so-called bottom-up approach5 to device manufacture has intrigued physical scientists6 and electronic engineers7 for many years. Only recently are chemists8–13 beginning finally to learn how to self-assemble molecular4 and supramolecular systems such that information might ultimately be written into them, stored in them, processed in them, and eventually read back out of them.

Molecular recognition by catenated structures

Abstract: Catenanes and rotaxanes have provided considerable challenges to the ‘artists’ that practise chemical synthesis. In certain instances, where the mutual recognition between the components of these interlocked molecules is strong, their self-assembly becomes easy — at least, once the components have been constructed! The challenge that faces the supramolecular chemist is to take advantage of existing knowledge and seek inspiration from the information processing systems of the natural world, so that the molecular recognition present in wholly synthetic compounds can be used and the information contained in them can be relayed in a manner similar to that taking place in biological systems.