Photochemically Controlled Shuttling Processes

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. 

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

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