Catenanes spectroscopy

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. 

Catenane 10.76 displays some very interesting dynamic properties, which may be followed by 41 NMR spectroscopy. In particular, at 81°C the hydroquinone ring protons appear as a singlet signal... 

Dynamic NMR spectroscopy indicated that the macrocyclic crown component in the [2]-catenane 5 is revolving through the tetracationic cyclophane ring around 300 times per second at 25 °C while it is simultaneously pirouetting around it at about 2000 times per seeond. ... 

In the case of L2" + it was not possible to synthesize [2]catenanes because the cavity of this cyclophane is too large to give stable complexes with aromatic crown ethers in the templated synthetic approach. Starting from Li" +, however, it was possible to prepare the [2]catenane ligands and L4 + which were then used to prepare several mononuclear [2]catenane complexes (Figure 23). These compounds were characterized by NMR spectroscopy, mass spectrometry and, in some cases, X-ray crystallography. 

In the case of the previously discussed [2]catenane 18 + shown in Figure 30, switching from the more stable translational isomer (which contains a TTF unit inside the tetracationic cyclophane) to a less stable one can be obtained not only by oxidation of the TTF unit, but also by addition of o-chloroanil. H NMR spectroscopy shows that o-chloroanil gives an adduct, presumably CT in nature, with the TTF unit which locks this unit alongside the cavity of the cyclophane. On addition of Na2S205 the adduct is destroyed and the original isomer with the TTF unit inside the cavity of the cyclophane is restored [59, 66[. 

UV-Vis spectroscopy of the [2]catenane 214+ revealed - by virtue of the presence of a characteristic broad charge-transfer band with a maximum absorbance at 835 nm and the absence of a peak at 515 nm - that the sole co-conformer in a room temperature acetonitrile solution is the one in which the TTF unit resides inside the cavity of the tetracationic cyclophane. Upon either chemical or electrochemical oxidation of the TTF unit to its radical cationic (or dicationic) state, the Coulombic repulsion between the cyclophane and the positively charged TTF unit leads to its expulsion from the cavity of the cyclophane. As the 1,5-dioxynaphthalene site is left untouched by the TTF oxidation and has an intermediate affinity for residing within the cyclophane,... 

Aromatic templates, in conjunction with coordinative bonds, have been employed by Sanders et al. [42] to self-assemble a [2]catenane incorporating a chiral metallomacro-cycle. The 1,5-dioxynaphthalene-based macrocyclic polyether 60 threads onto the r-elec-tron-deficient compound 61 in MeCN. Thus, when both compounds and Zn(OS02CF3)2 are mixed in this solvent, threading of 60 onto 61 is followed by the [2 + 2] assembly of a helical metallomacrocycle as a result of the tetrahedral coordination of two Zn centers by the bipyridine ligands appended to the r-electron-deficient recognition sites. The resulting [2]catenane 62 was characterized by a combination of H-NMR spectroscopy and electrospray mass spectrometry. 

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