Photochemically Controlled Ring Motions

The only example presently available of photochemically controllable ring motion through an electron transfer reaction in a catenane structure concerns a Cu+-based [2]catenate, which is discussed in Volume 111, Part 2, Chapter 8 [64], Examples of catenanes containing cis-trans photoisomerizable units and where ring motions can be photochemically controlled are also known [65]. 

Sauvage has demonstrated both electrochemical and photochemical control over ring motions in a catenate, 18 [57,58]. The observed behavior of the catenate is essentially similar to the analogous rotaxane, the only difference being that the 4-coordinate to 5-coordinate (dpp -> terpy) shuttling process is slower in the catenate and the reverse step is faster. Again, the issue of directionality is not addressed in this system. 

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). 

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