CBPQT

The position of the macrocycle on the thread of a rotaxane can be controlled through repulsive electrostatic interactions generated in a reversible way. Photoin-duced heterolysis of rotaxanes with diaryl-methoxy-cycloheptatriene- and aryl-alk-oxy-acridane-based molecular threads was used in the group of Abraham to demonstrate this concept. Abraham, Grubert, Grummt, and Buck [93] synthesized a rotaxane composed of the tetracationic macrocycle cyclobis(paraquat-p-phenylene) CBPQT (Fig. 9bl) and a thread that incorporated a diaryl-methoxy-cycloheptatriene unit (Fig. 9b2) the thread was folded. In the initial situation (Fig. 9b6), the macrocycle is located at the diary 1-methoxy-cycloheptatriene station (station 1), but due to folding, it interacts also with the aromatic station 2 (for the sake of convenience, the thread in... 

Vetter and Abraham [97] designed and synthesized rotaxanes where the axle has a 9-aryl-9-methoxy-acridane at each end, and where, consequently, the macrocycle CBPQT shuttles between one end and another. Where the axle has one aromatic ring in the central position, addition of acid produces acridinium cations at the ends of the axle, and thus forces the macrocycle to occupy the central position due to interactions with the uncharged central aromatic ring. Where the axle has two aromatic rings in the central position, addition of acid forces the macrocycle to shuttle between these two. The process can be reversed by addition of base. 

H Nuclear magnetic resonance (NMR) and UV-Vis spectroscopies were applied to studies on complexation between various TTF donors, among others, TTF 16 and the 7t-electron-accepting tetracationic cyclophane 28 (CBPQT ). The results obtained were of fundamental importance in designing interlocked molecular systems like molecular switches in which CBPQT and TTF units were incorporated <2001JOC3559>. H NMR investigations of similar systems are referred to at the end of this subsection. 

Fig. 13. Formation of the rotaxane 8-4PF6 by the stoppering approach. Bulky triisopropyl triflate is added to an equilibrating mixture of the cyclophane CBPQT-4PF6 and linear component 9 in order to put the stoppers in place, resulting in a 22% yield of the desired...

Figure 10 Relevant processes corresponding to the oxidation-induced dissociation of a pseudorotaxane that contains a TTF station as the main component of the thread and the tetracationic cyclophane CBPQT " as the macrocycle. The CVs corresponding to the free thread (green trace) and pseudorotaxanes (blue trace) are shown in the center of the figure. (Reproduced from Ref. 9. Wiley-VCH, 1997.)...

TTF c CBPQT Figure 13 A three-pole supramolecular switch. 

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 +/+. 

In this example, the speed of the translational motion was not addressed. However, the fact that the separation between anodic and cathodic peaks (ASp) was close to 60 mV for both oxidation processes suggests that there is unrestricted translational motion that is, the motions occur under thermodynamic control. Consistently, the NMR data needed to be acquired at 229 K (CD3CN) to freeze out rapid translational shuttling of the CBPQT" + ring between the BZD and BP stations.CV simulations performed under the conditions described in the paper and assumed from the analysis above can faithfully reproduce the CV of this rotaxane. 

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... 

CBPQT + ring. The major difference to the pseudorotaxane is the presence of a secondary electron-donor station in the form of 1,5-dioxynaphthalene (DNP, colored red). Thus, when the CBPQT + ring is repelled away from the TTF" " station, it has an alternative and favorable location to settle onto. The repulsion from TTF" " as well as the attraction to DNP in the catenane has led to its designation as a push-pull switch.- The added stability conferred when the CBPQT " " encircles the DNP station means that, once the oxidation stimulus is removed, the CBPQT " " takes some time to move back that is, the co-conformation with CBPQT " " at the DNP station is metastable. This feature is explored below when considering the effect of environment on the switching rates. 

Later, Stoddart et al. reported a nondegenerate molecular switch (Figure 15) driven by the change in the solvent polarity based on a [2]catenane comprising a cyclophane CBPQT + interlocked with a crown ether, which has a benzene unit and a naphthalene unit. The conformation of the [2]catenane can be changed due to the different interaction intensities between the cyclophane and the crown ether in polar and nonpolar solvents. 

On the basis of the above studies, Stoddart etal. synthesized a [2]catenane (Figure 39) comprising a CBPQT + ring interlocked with a 1/5DN38C10 derivative with a TTF unit substituting one of the two naphthalene (NP) units. The [2]catenane could be switched by electrochemical stimuli, inducing a relative motion of the two macrocycles. 

Recently, Stoddart et al. reported a novel push-button molecular switch (Figure 42) based on a single-station mechanically hetero[2]catenane. The [2]catenane could be easily switched by redox reactions between its two conformations, with a TTF unit inside or outside of the CBPQT" + cyclophane. This push-button molecular switch is an ideal candidate for introduction into solid-state electronic device settings owing to the unique combination of two discrete translational forms and the ability to toggle completely between these two electrochemically controllable states. ... 

Stoddart et al. developed an electrochemically driven molecular switch (Figure 49) based on the changes in the geometry of two parts of the same molecule. As demonstrated previously, the molecule preferred to exhibit a self-complexed conformation due to the n-ir charge-transfer interactions between the CBPQT + ring and the... 

Willner et al. developed a redox-active rotaxane (Figure 50) as a monolayer assembly on an Au electrode. The rotaxane comprises a CBPQT + cyclophane threaded onto a molecular string, which includes a rr-donor diiminobenzene unit stoppered by an adamantane unit. The cyclophane localizes on the diiminobenzene unit initially, and shuttling can be induced by the reduction or oxidation of the cyclophane, which can be characterized... 

Abraham et al. developed a photo-driven molecular shuttle (Figure 69) based on a [2]rotaxane with CBPQT +... 

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.)...

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