Donor cyclophanes

The internal cavity of a cyclophane is endobasic if functional groups are present that are basic or electron donating the most obvious groups include the ethers, pyridines, amines, and phosphorus-based donors. Cyclophanes in this category would be expected to bind metal ions and also promote H-bonding interactions within the cavity. Hence, in this section, crown ethers and azamacrocycles could easily be included. Under this heading, we can also... 

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. 

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)... 

Electrochemical techniques can also give interesting information in the case of catenanes of higher complexity, as shown by the results obtained by investigating a series of catenanes made of up to seven interlocked rings.20 The three basic components of these catenanes are the tetracationic cyclophanes 124+ and 154+, and macrocycle 16 containing three electron donor DMN units (Fig. 13.16). For space reasons, only the electrochemical behavior of catenanes 174 1, 188+, 194+, and 204+ (Fig. 13.17), compared to those of their molecular components, is reported. 

As already seen for catenanes 134+ andl44+ (Fig. 13.15),ongoingffomseparated molecular components 16,124+, or 154+ to their catenanes substantial changes in the electrochemical behavior are expected because the electroactive units incorporated in the cyclophanes and macrocycle are engaged in donor-acceptor interactions and occupy spatially different sites. 

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). 

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

More recently, the second-generation molecular shuttle 374+ (Fig. 13.32) was designed and constructed.38 The system is composed of two devices a bistable redox-driven molecular shuttle and a module for photoinduced charge separation. In the stable translational isomer, the electron-accepting cyclophane 124+, which is confined in the region of the dumbbell delimited by the two stoppers Tj and T2, encircles the better electron donor tetrathiafulvalene (TTF) station. 

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. 

Figure 13.35 Redox controlled ring rotation in solution for catenane 404+, which contains the symmetric electron acceptor cyclophane 124+ and a nonsymmetric electron donor ring.

Ferguson, S. B., Sanford, E. M., Seward, E. M., Diederich, F., Cyclophane arene inclusion complexation in protic solvents - solvent effects versus electron-donor acceptor interactions. J. Am. Chem. Soc. 1991, 113, 5410-5419. 

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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