Catenane components

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. 

Template synthesis and chirality of catenanes, rotaxanes, and pretzelanes including N-macroheterocyclic lactams and related compounds as structure components 99PAC247. 

Other supramolecular structures such as catenanes and rotaxanes can be formed using zinc as a template ion for example, a benzylic imine catenate formed by Leigh et a/.288 The reversible five-component assembly of a [2]catenane from a chiral metallomacrocycle and a dinaphtho-crown ether has been achieved. Zinc is used as the metal component and drives assembly via the coordination to a bipyridyl unit 7r interactions between the aromatic components are also... 

Some racemates (Figure 3.23) are more efficiently resolved on the bonded-type CSP than the coated-type CSP by using chloroform as a component of the eluent. On the bonded-type CSP of 24n, topologically interesting catenanes and molecular knots are successfully resolved using a hexane-chloroform-2-propanol mixture.185 The first direct HPLC resolution of the smallest chiral... 

Alvaro, M., Ferrer, B., Fornes, V., Garcia, H. and Scaiano, J.C. (2002). Bipyridinium macroring encapsulated within zeolite Y supercages. Preparation and intrazeolitic photochemistry of a common electron acceptor component of rotaxanes and catenanes. J. Phys. Chem. B 106, 6815-6820... 

Figure 4. Schematic energy level diagram for a catenane based on charge-transfer (CT) interactions and for its separated components. The wavy lines indicate nonradiative decay paths of the electronic excited states.

Figure 8. Electrochemically controlled movements of the ring components upon one-electron oxidation/reduction in a catenane containing a non-symmetric ring.

Scheme 9 Hydrogen bonding interactions are responsible for the appropriate orientation of the components that lead to the formation of catenanes 12 and 14... 

Other applications of the polymer substrate technique include the synthesis of threaded macrocyclic systems (hooplanes, catenanes, knots), the retrieval of a minor component from a reaction system, and the trapping of reaction intermediates [Frechet, 1980a,b Hodge, 1988 Hodge and Sherrington, 1980 Mathur et al., 1980],... 

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

Interestingly, the dumbbell component of a molecular shuttle exerts on the ring motion the same type of directional restriction as imposed by the protein track for linear biomolecular motors (an actin filament for myosin and a microtubule for kinesin and dynein).4 It should also be noted that interlocked molecular architectures are largely present in natural systems—for instance, DNA catenanes and rotaxanes... 

It should also be recalled that a full electrochemical, as well as spectroscopic and photophysical, characterization of complex systems such as rotaxanes and catenanes requires the comparison with the behavior of the separated molecular components (ring and thread for rotaxanes and constituting rings in the case of catenanes), or suitable model compounds. As it will appear clearly from the examples reported in the following, this comparison is of fundamental importance to evidence how and to which extent the molecular and supramolecular architecture influences the electronic properties of the component units. An appropriate experimental and theoretical approach comprises the use of several techniques that, as far as electrochemistry is concerned, include cyclic voltammetry, steady-state voltammetry, chronoampero-metry, coulometry, impedance spectroscopy, and spectra- and photoelectrochemistry. 

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. 

Figure 13.21 The two coconformational isomers associated with a catenane incorporating two different recognition sites within one of its two macrocyclic components can be interchanged by appropriate stimuli (S, and S2).

Figure 13.22 The circumrotation of the tetracationic cyclophane component of catenane 254+ can be controlled reversibly by adding-protonating -hexylamine that forms a charge transfer adduct with the diazapyrenium unit of the catenane.

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... 

Figure 13.33 (a) The circumrotation of the tetracationic cyclophane component of catenane... 

Figure 13.34 Correlation between the half-wave reduction potentials of catenane 394+ and of its tetracationic ring component. Circles and squares correspond to the reduction of bipyridinium and /ra 1v-bis(pyridinium)ethylene units, respectively.

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