Catenanes ring movements

There are several examples of catenanes where ring movements can be induced by external stimulations like simple chemical reactions or homogeneous or heterogeneous electron transfer processes [91-93], but only very few cases are reported in which the stimulus employed is light. It has been shown that in azobenzene-containing [2]catenanes like 31 + (Fig. 29) it is possible to control the rate of thermally activated rotation of the macrocyclic components by photoisomerization of the azobenzene moiety [119, 120]. Such systems can be viewed as molecular-level brakes operated by light. 

There are several examples of catenanes where ring movement can be induced by homogeneous or heterogeneous electron-transfer reactions [3], However, there is only one case in which the electron-transfer process is photoinduced [32]. 

Solid-state properties of the catenane copolymer were determined using DSC and dynamic mechanical analysis (DMA) and were compared with pure PC. DSC revealed a glass transition temperature of ca. 150°C for all the catenane copolymers 22 to 24, which is essentially the same as the pure PC sample of the same molecular weight. This insensitivity of the Jg to the presence of the catenane repeat unit is possibly on account of the considerable flexibility/mobiUty of these catenanes. DMA of the copolymer containing 20% (w/w) catenane showed three transitions at —100, —6, and - -80°C. The first and third transitions are observed in PC films while the —6°C was linked to the catenane ring/chain movements. 

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

Another redox-driven intramolecular movement involved the half-turn of one ring of an asymmetric catenane.3 The Sauvage s catenane consisted of two intertwined rings, one containing a phen fragment (2) and the other containing both a phen and a terpy fragment (3). 

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.

Fig. 18 Mechanical movements of one ring relative to the other in a catenane, which from a macroscopic viewpoint are reminiscent of movements of a ball and socket joint (top) and of a universal joint (bottom)...

A catenane is a molecule composed of two or more interlocked macrocyclic components. From a macroscopic mechanical viewpoint the movement of one ring relative to the other in a catenane is reminiscent of a ball and socket joint (Fig. 18, top) [81]. Similarly, twisting of one ring around the main axis of the catenane forces the other ring to rotate in the same direction in a manner reminiscent of an universal joint (Fig. 18, bottom) [81]. 

As already pointed out in the case of rotaxanes, mechanical movements can also be induced in catenanes by chemical, electrochemical, and photochemical stimulation. Catenanes 164+ and 174+ (Fig. 19) are examples of systems in which the conformational motion can be controlled electrochemically [82, 83], They are made of a symmetric electron acceptor, tetracationic cyclophane, and a desymmetrized ring comprising two different electron donor units, namely a tetrathiafulvalene (TTF) and a dimethoxybenzene (DOB) (I64 1) or a dimethoxynaphthalene (DON) (174+) unit. Because the TTF moiety is a better electron donor than the dioxyarene units, as witnessed by the potentials values for their oxidation, the thermodynamically stable conformation of these catenanes is that in which the symmetric cyclophane encircles the TTF unit of the desymmetrized macrocycle (Fig. 19a, state 0). 

Figure 20. Pictorial representation of machine-like movements that can be obtained with pseudorotaxanes, rotaxanes, and catenanes (a) dethreading/rethreading of the molecular components in a [2]pseu-dorotaxane, (b) shuttling of the macrocyclic component along the axle in a [2]rotaxane, and (c) ring rotation in a [2]catenane.

The name of catenanes originates from latin catena which means a chain. Indeed these supramolecules are fundamentally made from interlocked macrocycles (Figure 1(a)) with, as already mentioned, ability of a relative movement of one macrocycle with respect to the another one(s) (pirouetting). The number of macrocycle is included in the used notation [n] catenanes denote n interlocked chains. Up to now supramolecules of up to 4 macrocycles were synthesized. Large catenanes (M = 10 ) are present in nature in DNA as intermediates during the replication, transcription, and recombination process. Since the first two-ring cate-nane was obtained in early sixties, smaller synthetic catenanes (M = 10 ) have attracted the interest of chemists and physicists. 

Figure 8. The electrochemically controlled movements of the ring components upon successive one-electron reduction/oxidation processes in the non symmetric catenane shown in Figure 6b [17].

In the case of supramolecular species [5, 6], spectroelectrochemical measurements are even more important. They, indeed, enable to establish the more stable isomer among the possible ones, and to evidence the occurrence of electro-chemically induced molecular movements or conformational rearrangements. In this regard, three studies performed on catenanes (supramolecular systems minimally formed by two interlocked rings) are illustrated in the following. 

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