The Beauty of Chemistry in the Words of Writers and in the Hands of Scientists - Beauty in Chemistry - page 96

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 )
8.2 Ring Movements in Catenanes
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 reminis-
cent 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 stimula-
tion. Catenanes 16 4 þ and 17 4 þ (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) (16 4 þ ) or a dimethoxynaphthalene (DON)
(17 4 þ ) 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).
Monoelectronic oxidation of the TTF unit is accompanied by the circumrotation
of the desymmetrized ring through the cavity of the tetracationic cyclophane.
Indeed, after oxidation, the newly formed monocationic tetrathiafulvalene unit