Experimental 8 catenane

Houk, Stoddart and Williams have carried out a detailed investigation (involving experimental and theoretical studies) to quantify the strength of C-H- -O hydrogen bonds in the formation of catenanes [82]. While the formation of the [3]catenane 67 was successful, its conversion to the [4]catenane 68 could not be achieved (see Scheme 32). 

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. 

Three doubly spin-labelled [2]catenanes with different sizes were studied by 4-pulse DEER.52 The experimental distribution of interspin distances was compared with a theoretical pair-correlation function computed based on geometrical constraints. In chloroform solution the medium and large catenanes were close to fully expanded, but in glassy o-terphenyl they were partially collapsed. For the smaller catenane there was a higher population of shorter interspin distances, which was attributed to interactions between unsaturated sections of the molecule. 

Experimentally, sodium nitrate was added to an aqueous solution of molecular ring 16, and the solution was heated at 100 °C. Then, the equilibrium between 16 and its catenated dimer 15 is pushed by the polar media toward the catenane. After self-assembling in a high yield, catenane 15 was isolated as a CIO4 salt in a high yield (Eq. 3). It was confirmed that catenane 15 thus obtained did not dissociate into two rings in aqueous solution because its framework had been locked. 

The principle of the second synthetic approach to polycatenanes, i.e. stepwise polycondensation, has been proposed by Shaffer and Tsay, but not experimentally demonstrated [42, 43], This approach has the advantage over multifunctional polycondensation that a linear polymer is formed before cyclization (Scheme 7). However, the second step, which consists of the cyclization of n macrocycles along the polymer chain 19, is likely, again, to give rise to an undefined network, containing some rotaxane and catenane units 21, similar to the multifunctional polycondensation approach. 

In this section the unprecedented oligocatenanes, i.e. the [5]- and [7]catenanes 30 and 31 and the scarce experimental approaches to high molecular-weight linear polycatenane 9 have been presented. No synthetic Olympic network 32 has been reported to date, although their DNA analogs are known. The next section is dedicated to a new type of macromolecular architecture, structurally related to polycatenane 9, i.e. poly[2]catenanes. 

Scheme 9.17). The macrocycle (A) of Scheme 9.1 is the dpp-incorporating 30-membered ring 8 used earlier for making catenanes (see Protocol 5) it is sufficiently small to prevent release of the bis-porphyrin dumbbell. The preparation of 22+ takes place in seven steps.1819 One of the starting materials is 4-lithiotoluene 13, which may be obtained by direct interaction of an excess of freshly cut lithium with 4-bromotoluene 12 in ether under argon at reflux (Scheme 9.10).12 The resulting organolithium compound is titrated by the double titration method described by Gilman et alP The experimental procedure is very similar to that described in Protocol 1.

Also known are other types of topological isomers which draw the researchers attention, but they exist only on paper so far. Catenanes and rotaxanes are the only topological structure classes which have been investigated experimentally. 

Figure 20. Incidence angle dependence of SHG intensity for a thin film of benzylic amide [2] catenane. Full squares depict a least square fit to experimental data with fi.dij suceptibiUty tensor...

Fig. 9. Characterization of the coconformational distribution of a [2]catenane by DEER meas-nrements. a) Simphfied geometrical model of the [2]catenane consisting of two circular macrocycles to which spin labels are attached by a tether with length L = 0.93 nm. (b) Experimental four-pnlse DEER trace (dots) of the [2]catenane in frozen chloroform and fit by the simphfied geometrical model (solid hne) giving an effective radius Teir 1-50 nm for the macrocycle.

Fig. 10. Experimental (solid lines) and simulated (dashed lines) CW ESR spectra (=9.6 GHz) of (a) the singly labeled [2]catenane and (b) the doubly labeled [2]catenane.

Typical data from one of our molecular switches is shown in Figure 4 (we have incorporated additional data from various control molecules in this figure). For the structures shown in Figure 2, the controls include [2]catenanes with identical recognition sites (i.e., two DN groups), the dumbbell component of the [2]rotaxane structure, the TCP ring, and others. One always does control experiments, of course, but controls are critical here, because these devices are difficult to characterize fully, and one of the few experimental variables is mo-... 

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