Rotaxanes electrochemically driven

The first example of electrochemically driven molecular shuttles is rotaxane 284+ (Fig. 13.25) constituted by the electron-deficient cyclophane 124+ and a dumbbellshaped component containing two different electron donors, namely, a benzidine and a biphenol moieties, that represent two possible stations for the cyclophane.10 Because benzidine is a better recognition site for 124+ than biphenol, the prevalent isomer is that having the former unit inside the cyclophane. The rotaxane... 

Rotaxane 316+ was specifically designed36 to achieve photoinduced ring shuttling in solution,37 but it also behaves as an electrochemically driven molecular shuttle. This compound has a modular structure its ring component is the electron donor macrocycle 2, whereas its dumbbell component is made of several covalently linked units. They are a Ru(II) polypyridine complex (P2+), ap-terpheny 1-type rigid spacer... 

Figure 13.38 The electrochemically driven ring shuttling of rotaxane 434+ incorporated into an Au-SAM.

Electrochemically Driven Molecular Machines Based on Transition-metal Complexed Catenanes and Rotaxanes... 

ELECTROCHEMICALLY DRIVEN MACHINES BASED ON PIROUETTING COPPER-COMPLEXED ROTAXANES... 

Fig. 5 Controlled molecular motion in rotaxanes light-driven shifting of the wheel along the rotaxane axle (top) and contraction of a molecular muscle stimulated hy electrochemical Cu(I)-Cu(II) interconversion (bottom).

Figure 40 An electrochemically driven molecular switch based on a rigid [2]rotaxane. ...

Figure 51 An electrochemically driven molecular shuttle based on an amide rotaxane...

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Stoddart and co-workers have developed molecular switch tunnel junctions [172] based on a [2]rotaxane, sandwiched between silicon and metallic electrodes. The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again driven by an electrochemical translation. They used electrochemical measurements at various temperatures [173] to quantify the switching process of molecules not only in solution, but also in self-assembled monolayers and in a polymer electrolyte gel. Independent of the environment (solution, self-assembled monolayer or solid-state polymer gel), but also of the molecular structure - rotaxane or catenane - a single and generic switching mechanism is observed for all bistable molecules [173]. 

In a recent full paper [32], a molecular switch 116+, which can be driven either photochemically or electrochemically, has been described. Synthesized in a 59% yield from the corresponding dumbbell compound using the slippage approach, this [2]rotaxane contains (Fig. 16) two chemically distinct recognition sites around which the BPP34C10 macrocycle can reside. Of the two sites, the unsubstituted bipyridinium unit is the better electron acceptor, and so it is around this site that the macrocycle encircles itself preferentially. However, upon reduction of the bipyridinium site to its radical cation - either... 

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. 

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