Bistable CBPQT

Bistable rotaxane Rot-l" + shows a CV (Figure 14) that is a composite of rotaxane Rot-2 + and thread T-2. The potential for the first oxidation is shifted to more positive potentials by about 100 mV, consistent with the behavior seen for Rot-2" + after one-electron oxidation. The second oxidation, however, occurs at the same potential as the T-2 thread s second oxidation. These two observations justify the interpretation that the CBPQT + ring moves away from the singly oxidized BZD+ unit, presumably to the secondary BP unit, leaving the empty BZD+ monocation able to become oxidized to the BZD + dication at the same potential seen for the thread T-2 +/+. 

A significant number and variety of molecular machines have been created from the oxidation-driven movement of a TTF" " station out of the tetracationic CBPQT " " host, as discussed above in conjunction with Figure 10. A key example is the bistable [2]catenane Cat-2 + (Figure 19). The CV published in the original paper has a very similar form as the pseudorotaxane in Figure 10, indicating the movement of the TTF" " monocation out of the center of the... 

Early in 1994, Kaifer, Stoddart, and coworkers reported the first bistable molecular shuttle (Figure 70) based on a [2]rotaxane driven by chemical and electrochemical stimuli. The [2]rotaxane comprised a CBPQ U+ ring threaded by an axle consisting of two binding sites, benzidine and biphenol units. Under redox reactions and the addition of acid or base, the CBPQT + ring moves back and forth along the axle. This molecnlar shuttle can be switched by two different mechanisms and is a good candidate for the construction of complex molecular machines. 

Figure 29 Schematic structural representation of light-gated stop-go (CBPQT) bistable rotaxane. (Reproduced from Ref. 60. American Chemical Society, 2009.)...

This method has been applied in other studies to obtain a bistable fullerene-rotaxane, where the CBPQT + macrocycle was able to switch from two different electron-rich stations, a 1,5-dihy-droxynaphthalene (DNP), and a TTF. The macrocycle closes at the TTF station after the clipping reaction with p-xylylene dibromide, because the TTF template unit is more electron-rich than DNP. The further oxidation of TTF to the cation caused an electrostatic repulsion between the station and the positively charged macrocycle that switches to the other electron-rich DNP station (Figure 2.19). 

FIGURE 2.19 Bistable rotaxane assembled via it-it interactions. Electron-deficient CBPQT macrocycle can switch from two different electron-rich stations. 

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