Sacrificial rotaxanes

Figure 6.8 Structural formula of the rotaxane 96+ and schematic representation of the intramolecular (below left) and sacrificial (below right) mechanisms for the photoinduced shuttling movement of macrocycle between the two stations A1 and A2.

R macrocycle between the two stations A and Ai, two strategies have been devised one was fully based on processes involving only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). 

The structure of the rotaxane was characterized by mass spectrometry and NMR spectroscopy, which also established, along with cyclic voltammetry, that the stable translational isomer is the one in which the R component encircles the Ai unit, in keeping with the fact that this station is a better electron acceptor than the other one. The electrochemical, photophysical, and photochemical (under continuous and pulsed excitation) properties of the [2] rotaxane, its dumbbell-shaped component, and some model compounds containing electro- and photoactive units (Fig. 13) were investigated. In an attempt to obtain the photoinduced abacus-like movement of the R macrocycle between the two stations A, and A2, two strategies were devised one was fully based on processes involving only the rotaxane components (intramolecular mechanism), while the other one required the help of external reactants (sacrificial mechanism). 

Photochemically controlled shuttling of the macrocycle component by a sacrificial mechanism has also been obtained in a rotaxane containing a phenanthroline and a terpyridine unit in its dumbbell-shaped component, a phenanthroline ligand in the macrocycle, and a Cu(I) center coordinated by... 

Fig. 15a-d. Sacrificial mechanism for the light-driven switching of the ring R between the two stations Ax and A2 in the [2]rotaxane represented in Fig. 13 [7f]. The dashed lines indicate the processes that are in competition with those needed to perform shuttling...

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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