Bistable molecular machines

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

So far, several example of the chemically and electrochemically controlled switching of bistable linear molecular machines have been presented. The final section of this chapter will be dedicated to illustrating how such molecular switches and motors, when designed ingenuously, can also be powered by nature s most abundant and powerful energy source - light. 

Would not it be amazing if the tiny nanomotions of molecular machines could be cooperatively scaled up to do work on macroscopic objects Toward that end, it has been demonstrated [226, 227] that a monolayer of molecular muscles - palindromic bistable [3]rotaxanes (Fig. 31a) immobilized on the surface of a thin... 

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... 

Another example of the application to molecular machines is a molecular shuttle that functions as a chiropti-cal switch (Figure 14a). The bistable rotaxane 24 consists of a chromophoric macrocycle and a UV-weak thread with a photoisomerizable olefin on one side and a chiral center on... 

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. 

Bistable rotaxanes have been designed through this methodology as promising powerful molecular machines, where the fullerene is used to probe the motion of the macrocycle and, more importantly, to induce shuttling. 

For both rotaxane and catenane-based molecular machines, it is desirable to have recognition sites such that they can be easily controlled externally. Hence, it is preferable to build sites that are either redox-active or photo-active [144]. Catenanes can also be self-assembled [157]. An example of catenane assembled molecular motors is the electronically controllable bistable switch [158]. An intuitive way of looking at catenanes is to think of them as molecular equivalents of ball and socket and universal joints [153,159,160]. 

Mechanically interlocked molecules, such as bistable catenanes [13] and [2]rotax-anes [14], constitute some of the most appropriate candidates to serve as nanoscale switches and machines in the rapidly developing fields of nanoelectronics [15] and nanoelectromechanical systems (NEMS) [16]. The advantages of using mechanically interlocked molecules in the fields of molecular electronics and... 

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