Cyclophanes self-assembly

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

Upon the first reduction, the voltammetric wave shifts by 20-30 mV, presumably due to donor-acceptor interactions resulting from the naphthalene and cyclophane moieties.39 The second reduction remains relatively unaffected, indicating that the complex disassembles prior to the second reduction of the cyclophane. Alternatively, the complex can disassemble through competitive binding interactions with a tetrathiafulvalene derivative. Thus, Cooke and coworkers demonstrate that self-assembly at exo-active surfaces can be reversibly controlled via an external electrochemical stimulus or competitive binding interactions. 

Controlled self-assembly allows exo-active surfaces to be viable supramolecular building blocks for constructing nanostructure assemblies. These nanostructure assemblies can be used to modify electrodes for sensing applications. Willner and coworkers have constructed nanostructure assemblies on electrodes through electrostatic cross-linking of citrate stabilized gold NPs and bipyridinium cyclophane (3,5-... 

Figure 13. The self-assembly of [2]pseudorotaxanes incorporating the tetracationic cyclophane 15-4PF6 and the acyclic dioxyarene-based polyethers 16-22.

They found that some cyclophanes 82 exchanged rapidly between the polymer chain ends and solution relative to the NMR time scale. However, some of the cyclics 82 were trapped in the middle of polymer chain on the NMR time scale and were thus excluded from the exchange process. These two papers demonstrated that this type of self-assembly provides a very strong driving force for threading. 

Keywords Crown compounds Liquid crystals Mesophases Metallomesogens Polymers Salt effects Self-assembly Substituent effects Supramolecular chemistry Phthalocyanines Cyclophanes... 

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

Fig. 30 Polymeric blue box cyclophane displays the emergent property of quantitative thermodynamic catenation by iodide-catalyzed self-assembly with an appropriate crown ether macrocycle. The analogous monomeric and dimeric species do not react quantitatively [207]...

Cyclophanes are cyclic hosts made by Unking aromatic rings. Several examples of cyclophanes are depicted in Fig. 2.21. While the cyclophane in (a) is a simple two-dimensional cyclophane, the cyclophane in (b) has four alkyl chains attached. Using a similar molecular design process, a cyclophane with eight alkyl chains can be synthesized and is called an octopus-type cyclophane. The alkyl chains self-assemble in an aqueous phase and form a three-dimensional cavity. Cyclophanes with rigid steroidal walls are called steroid... 

In an extension of the previous studies involving 13, it has been demonstrated that a number of other [2]-catenanes can be synthesised using self-assembly procedures in which the donor macrocyclic polyethers incorporate both hydroquinone and 1,5-dioxynaphthalene units, while the acceptor tetracationic cyclophanes contain bipyridinium and/or its extended analogue, bis(pyridinium)ethylene. Although the trans carbon-carbon double bond in the bis(pyridinium)ethylene units are... 

The above successful incorporation of the tetra-substituted benzene ring to produce the fused tetracationic cyclophane units, relies on being able to react the former in a para/para fashion rather than in an ortho/ortho or meta/meta one. The present product represents the successful interlinking of three major components. If one considers that a one-step synthetic procedure was employed, then it is a total of five molecular species that are, in fact, involved in the self-assembly process. 

The self-assembly of cyclophane-like square structures, comprised of cis bridging metal centres connected by rigid aromatic sides, represents the formation of another category of metallocycles which typically enclose cavities of nanometre dimensions. Although isolated structures of this type have been known for some time, it is only relatively recently that increasing attention has been given to the area. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

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