Redox-active cyclophane

Two efficient syntheses of strained cyclophanes indicate the synthetic potential of allyl or benzyl sulfide intermediates, in which the combined nucleophilicity and redox activity of the sulfur atom can be used. The dibenzylic sulfides from xylylene dihalides and -dithiols can be methylated with dimethoxycarbenium tetrafiuoroborate (H. Meerwein, 1960 R.F. Borch, 1968, 1969 from trimethyl orthoformate and BFj, 3 4). The sulfonium salts are deprotonated and rearrange to methyl sulfides (Stevens rearrangement). Repeated methylation and Hofmann elimination yields double bonds (R.H. Mitchell, 1974). 

Willner and coworkers demonstrated three-dimensional networks of Au, Ag, and mixed composites of Au and Ag nanoparticles assembled on a conductive (indium-doped tin oxide) glass support by stepwise LbL assembly with A,A -bis(2-aminoethyl)-4,4 -bipyridinium as a redox-active cross-linker.8 37 The electrostatic attraction between the amino-bifunctional cross-linker and the citrate-protected metal particles led to the assembly of a multilayered composite nanoparticle network. The surface coverage of the metal nanoparticles and bipyridinium units associated with the Au nanoparticle assembly increased almost linearly upon the formation of the three-dimensional (3D) network. A coulometric analysis indicated an electroactive 3D nanoparticle array, implying that electron transport through the nanoparticles is feasible. A similar multilayered nanoparticle network was later used in a study on a sensor application by using bis-bipyridinium cyclophane as a cross-linker for Au nanoparticles and as a molecular receptor for rr-donor substrates.8... 

Tetrathiafulvalene-containing systems have also been reported. One such pseudorotaxane represents a redox-active system in which a similar molecular motion can be controlled by two different inputs. Reversible dethreading-rethread-ing cycles involving this product can be performed by either oxidation and consecutive reduction of the linear component or reduction and consecutive oxidation of the electron-accepting cyclophane tetracation. As such, the system corresponds to a further level of sophistication in systems of this type. Formally, the input (elec-trochemical)/output (absorption spectrum) characteristics of this assembly correspond to those of an XNOR logic gate. 

In other work, the new redox-active [2]-rotaxane 39, incorporating a 1,4-phenylenediamine subunit in its linear component, has been synthesised in high yield (Figure 4.17). ° The redox behaviour of this product indicates considerable (increased) resistance to oxidation of the phenylenediamine subunit. This subunit appears even more promising with respect to its ability to repel the cationic cyclophane upon oxidation. 

Figure 8. Examples of pseudorotaxane components containing units capable of exhibiting specific properties, (a) A wire with an ammonium function and a photoactive anthracene as a stopper [17b] (b) a bipyridinium-type wire with a photo- and redox-active Ru(bpy)j + moiety [18] (c) a crown ether incorporating a binaphthyl unit [19] (d) a cyclophane containing a Re complex and two bi-pyridinium units (20. ...

Figure 5. Some pseudorotaxane components containing units capable of exhibiting specific properties. The wirelike molecule 11H+, with an ammonium center and a photoactive anthracene unit as a stopper [40] the bipyridinium-containing wire 12 +, with a photo- and redox-active [Ru(bpy)3]2+ moiety [41] crown ether 13, incorporating a binaphthyl unit [42] cyclophane 14 +, which contains a [Ru(bpy)3]2+ complex and...

Willner et al. developed a redox-active rotaxane (Figure 50) as a monolayer assembly on an Au electrode. The rotaxane comprises a CBPQT + cyclophane threaded onto a molecular string, which includes a rr-donor diiminobenzene unit stoppered by an adamantane unit. The cyclophane localizes on the diiminobenzene unit initially, and shuttling can be induced by the reduction or oxidation of the cyclophane, which can be characterized... 

The controlled movement in this system is controlled by the interaction among redox-active units at the redox-active tetrathiafulvalene (TTF) units. In the neutral, unperturbed state, the tetracationic cyclophane units (cyclobis(paraquat-para-phenylene), CBPQ F ) are most stable coordinated to the TTF moieties, due to electron donation and Jt-stacking interactions. However, upon oxidation of the TTF units, the CBPQT " rings become electrostatically repelled, migrating to the... 

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