Cyclophane 4,4 -bipyridinium

Fig. 3-4. (A) Changes in chemical shift of protons of cyclophane -CH - groups between bipyridinium and phenyl in H NMR spectra of 3 as a function of (R)-DOPA concentration (a) 0, (b) 0.111, and (c) 0.272 mol (B) Change in chemical shift plotted against the analytical concentration of (R)- and (5)-DOPA. The solid line is calculated for 1 1 host - guest complexation. (Reprinted with permission from ref. [79]. Copyright 1998, American Chemical Society.)...

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-... 

Interestingly, the second reduction of the two bipyridinium units splits in the case of catenane 144+ (Fig. 13.15) obtained by interlocking cyclophane 124+ with a symmetric macrocycle containing two electron donor dimethoxynaphthalene (DMN)... 

For cyclophane 154+, in which the two bipyridinium units are connected by biphenylene spacers, the number of reduction processes, number of exchanged electrons, and potential values (Fig. 13.17) are very similar to those of cyclophane 124+ (Fig. 13.15). 

Macrocycle 16, containing three equivalent DMN electroactive units, shows three distinct oxidation processes (Fig. 13.17). Such a contrasting behavior between 16 and the tetracationic cyclophanes, in which the two incorporated bipyridinium units undergo simultaneous first and second reductions, can be interpreted considering that, in the cyclophanes, the rigidity of the structure prevents interaction between the two bipyridinium units, whereas the flexible structure of macrocycle 16 allows the three DMN units to approach one another. 

Starting with catenane 174+, obtained by interlocking macrocycle 16 with only one cyclophane 124+, it is found, in agreement with these expectations, that the two bipyridinium units of 124+ undergo their first reduction in separated processes that are cathodically shifted with respect to the free cyclophane (Fig. 13.17a). Comparison... 

For catenane 18s+—composed of 16 and two cyclophanes 124+—simultaneous reduction of the alongside units of the two cyclophanes is followed by simultaneous reduction of the inside units (Fig. 13.18). Both processes are displaced to less negative potentials compared with 174+ (Fig. 13.17a) because the ratio between electron donor and electron acceptor units decreases from 3/2 to 3/4. The splitting of the second reduction process of the bipyridinium units is consistent with the crowded structure of the catenane, which presumably prevents fast interchange between inside and alongside units. As far as oxidation is concerned, in catenane 18s+, only two processes are observed, a feature that is consistent with the presence of an outside and two equivalent inside DMN units (Fig. 13.17a). 

For instance, complex 246+ exchanges up to a total of nine electrons. On reduction, it shows two monoelectronic and one bielectronic processes involving the bipyridinium units, and three monoelectronic processes concerning the bpy moieties (Fig. 13.20). On oxidation, two monoelectronic processes are observed the first one, being reversible, is assigned to the oxidation of the metal center and the second one, not fully reversible, to the oxidation of the alongside DMN unit of the macrocycle interlocked with the cyclophane. 

In the major isomer, the bipyridinium unit is located inside the cavity of the macrocyclic poly ether and the /7Y//7,v-bis(pyridinium)ethylene unit is positioned alongside, as confirmed by the electrochemical analysis. The cyclic voltammo-gram of the catenane shows four monoelectronic processes that, by a comparison with the data obtained for the free cyclophane, can be attributed as follows the first and third processes to the first and second reductions of the bipyridinium unit, and the second and fourth ones to the first and second reductions of the trans-bis (pyridinium)ethylene unit. The comparison with the tetracationic cyclophane also evidences that all these reductions are shifted toward more negative potential values (Fig. 13.33b). 

The discussion can be restricted to the first and second reduction processes that are of particular interest in this context. The shift of the bipyridinium-based process is in agreement with the catenane coconformation in which the bipyridinium unit is located inside the cavity of the macrocyclic polyether (Fig. 13.33a) because of the CT interactions established with both the electron donor units of the macrocycle, its reduction is more difficult than in the free tetracationic cyclophane. The shift of the trans-1,2-bis(4-pyridinium)ethylene-based reduction indicates that, once the bipyridinium unit is reduced, the CT interaction that stabilize the initial coconformation are destroyed and, thereby, the tetracationic cyclophane circumrotates through the cavity of the macrocyclic polyether moving the tra ,v-bis(pyridinium)ethylene unit inside, as shown by comparison of its reduction potential with that of a catenane model compound.19 The original equilibrium between the two coconformations associated with catenane 384+ is restored upon oxidation of both units back to their dicationic states. 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 42H26+ (Fig. 13.37) is made up of two identical macrocycles 2 interlocked with a cyclophane containing two bipyridinium and two ammonium units.44 Because of the type of the macrocycles used, the stable coconformation of 42H26+ is that in which the two rings surround the bipyridinium units (Fig. 13.37a, state 0). Upon addition of one electron in each of the bipyridinium units, the two macrocycles move on the ammonium stations (Fig. 13.37b, state 1) and move back to the original position when the bipyridinium units are reoxidized. 

Figure 13.37 Redox controlled movements of the ring components in catenane 42H composed of three interlocked macrocycles. These motions are obtained upon reduction-oxidation of the bipyridinium units of the cyclophane.

This difference is a direct result of the close 7r-stacking interactions. Electrochemical reduction of the bis(bipyridinium) cyclophane occurs in three distinct stages ... 

This [2]catenane is composed of a jt-electron-deficient tetracationic cyclophane interlocked with a Jt-electron-rich macrocyclic polyether. In addition to a mechanical bond, [jt Jt] stacking interactions between the complementary aromatic units, [C-H---0] hydrogen bonds between the a-bipyridinium hydrogen atoms and the poly-ether oxygen atoms, and [C-H---Jt] interactions between the 1,4-dioxybenzene hydrogen atoms and the p-phenylene spacers in the tetracationic cyclophane hold the two macrocyclic components together and control their relative movements in solution. As a result of the asymmetry of the tetracationic cyclophane, two transla-... 

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... 

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