Rotaxane self-assembly

A potential for large ring and cavity formation extends to many P-containing organic molecules, in addition to the inorganic phosphates discussed above. These compounds include phosphate derivatives of fashionable structures such as crown ethers, calixarenes, carceplexes, catenanes, rotaxanes, self-assembly molecules and so on, and varieties which are discussed in Chapter 10, such as cyclo-dextrins, phosphosaccharides, phosphoproteins and nucleic acids. Phosphorus analogues of cyclo-polypyridyls and porphyrins are discussed in Chapters 7 and 8. 

Rotaxanes (see Rotaxanes—Self-Assembled Links, Self-Processes) are interlocked structures in which one or more (polyrotaxanes) cyclic components are threaded by a dumbbell-shaped molecule (the axle). In pseudorotaxanes, the two components can dissociate but with a considerable kinetic barrier. The formation of pseudorotaxanes generally... 

A classic example of the formation of a macrocycle by a neutral template is that of the versatile host compound and component of molecular machines, the so-called blue box, or cyclobis paraquat-para-phenylene. Reaction of the horseshoe precursor with dibromo-para-xylene leads to the formation of a tricationic intermediate that is capable of binding the template molecule (Scheme 3), which closes the macrocycle to form the tetracationic cyclophane. The jT-ir interactions of the charge-transfer variety (the complex of the product and template is colored, whereas the components are not) assisted by the charge on ihe product are a major driving force in the process, as revealed in X-ray structures of complexes. It should be noted that the interaction is of the jr-n type assisted by the complementary positive charge on the bipyridinium residues and r-electron-rich nature of the template. This supramolecu-lar synthon can be used for other cyclophanes, catenanes, and rotaxanes (see Self-Assembly of Macromolecular Threaded Systems, Self-Assembled Links Catenanes, and Rotaxanes—Self-Assembled Links, Self-Processes). 

Chemisorbed Self-Assembled Monolayers Zeolitelike Metal-Organic Frameworks (ZMOFs) Design, Structure, and Properties Molecular Devices Molecular Machinery Physisorption for Self-Assembly of Supramolecular Systems A Scanning Ttmneling Microscopy Perspective Rotaxanes—Self-Assembled Links Template Strategies in Self-Assembly... 

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Self-assembly in construction of pseudorotaxanes, rotaxanes, and catenanes with crown ether fragments 98PAC419. 

Keywords. Rotaxane dendrimers. Host-guest interaction, Recognition, Self-assembly, Supra-molecular chemistry... 

Fig. 15. A branched [4]rotaxane from self-assembly of the triply-branched trisfbipyridini-um)derivative and bis-p-phenylene-34-crown-lO...

A useful summary of the various and numerous types of rotaxanes, catenanes, and knots can be found in a review of template routes to interlocked molecular structures 468). Inorganic chemistry is centrally involved in the templating involved in self-assembly and in controlled synthesis of such species. 

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

F. M. Raymo, J. F. Stoddart, Slippage-A Simple and Efficient Way to Self-Assemble Ini Rotaxanes , Pure Appl Chem. 1997, 69,1987-1997, and references dted therein. 

Figure B. Pictorial representation of the self-assembly of pseudorotaxa-nes based on (a) charge-transfer and C-H—O hydrogen-bonding interactions between 1,1 -diben-zyl-4,4 -bipyridinium dication and 1,5-dinaphtho[38] crown-10 (1/5DN38C10), and (b) hydrogen-bonding interactions between dibenzyl ammonium ion and dibenzo[24]crown-8 (DB24C8). A possible route towards the synthesis of rotaxanes and catenanes is also schematized.

The dynamic nature of the system offers the crown ether access to the ammonium centre allowing self-assembly of the corresponding dynamic [2]ro-taxanes 31 to occur. Fixing of the interlocked assembly can be achieved by reducing the imine groups in 31 to the corresponding amine so that a kinetically inert [2]rotaxane forms. 

Stoddart and co-workers have made use of Frechet-type dendrons as dendritic stoppers for self-assembled [n]rotaxanes [65]. The solubility enhancement that results from incorporating dendritic wedges at the termini facilitated the purification of these materials by column chromatography despite the polycationic nature of their bipyridinium backbone. Again, the dendritic wedges did not alter the electrochemical characteristics of the viologen subunits. Flowever, the enhanced solubility resulting from the presence of the dendritic components en-... 

The impact of self-assembly on the geometry and visible absorption spectrum of a rotaxane formed by squaraine 17a and anthracene-based tetralactam macrocycle 16a was assessed using quantum-chemical DFT, TD-DFT, and QM/MM approaches [57]. 

An admixture of 16b and 17b self-assembles quantitatively at millimolar concentration in chloroform solution to produce an inclusion complex whose absorption and emission maxima are red-shifted by 40 nm [56]. Clicking both ends of this pseudorotaxane with two molar equivalents of stopper S2 produces the squaraine rotaxane 16b D 19b in near-quantitative yield [58]. Pseudorotaxane 16b D 17b partially dissociates at micromolar concentrations in chloroform and produces two emission peaks, one at 638 nm which corresponds to free squaraine 17b and one at 694 nm, corresponding to the pseudorotaxane. In contrast, the squaraine rotaxane 16b D 19b does not dissociate under these conditions or in more polar solvents such as pure methanol. 

Asakawa M, Ashton PR, Ballardini R, Balzani V, Belohradsky M, Gandolfi MT, Kocian O, Prodi L, Raymo EM, Stoddart JF, Venturi M (1997) The slipping approach to self-assembling [n]rotaxanes. J Am Chem Soc 119 302-310... 

Fujimoto and coworkers used a self-assembled Janus [2]-pseudorotaxane406 to obtain a Janus[2]rotaxane 407 (Figure 8.2.4) [33]. A quadruply stranded alkaline earth metal containing helical catenate, a charge-neutral heterotopic homodinuclear [2]catenane, was reported by Castro et al. [34]. 

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

An interesting example of a system whose components become mechanically bound upon surface immobilization is rotaxane trans-21 (Fig. 13.24), consisting of a ferrocene-functionalized /3-cyclodextrin (fi-CD) macrocycle threaded on a molecule containing a photoisomerizable azobenzene unity and a long alkyl chain.33 A monolayer of trans-21 was self-assembled on a gold electrode. Therefore, the ring... 

Like the currently popular area, called nanoscience , the field of supramolecular chemistry has rather hazy boundaries. Indeed, both areas now share much common ground in terms of the types of systems that are considered. From the beginning, electrochemistry, which provides a powerful complement to spectroscopic techniques, has played an important role in characterizing such systems and this very useful book goes considerably beyond the volume on this same topic by Kaifer and Gomez-Kaifer that was published about 10 years ago. Some of the classic supramolecular chemistry topics such as rotaxanes, catenanes, host-guest interactions, dendrimers, and self-assembled monolayers remain, but now with important extensions into the realms of fullerenes, carbon nanotubes, and biomolecules, like DNA. 

---