Mechanically interlocked molecular switches

Beautiful Mechanically Interlocked Molecular Machines and Switches... 

Currently the main interest in template reactions lies in their key role in the controlled synthesis or the self-assembly of a variety of supramole-cular entities (449). One needs a combination of intuition, conjecture, and serendipity (450) a recent example of successfully combining serendipity and rational design is provided by the silver(I)-promoted assembly of one-dimensional stranded chains (451). One also needs an understanding of mechanism in order to optimize the selection and design of building blocks and templates for the generation of yet more sophisticated supramolecular structures references cited in this present review contain at least some kinetic or mechanistic information or speculation. Template routes to interlocked molecular structures have been reviewed (452), while a discussion of switching by transition metal contains a little about the kinetics and mechanisms of this aspect of template... 

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... 

A special issue devoted to molecular machines appeared in Accounts of Chemical Research in 2001. It reflects the current interest for this field in which ruthenium complexes act as important tools. Molecular machines are characterized by a mobile part and a stationary part. Photochemical and electrochemical inputs can make a machine work, offering the advantage of being switched on and off easily and rapidly. Mechanically interlocked molecules, such as rotaxanes and catenanes, are suitable candidates. Crown ethers, cyclophanes, and calixarenes are representative families of the cyclic... 

One of the potential applications of mechanically interlocked molecules is construction of molecular-scale devices such as molecular machines and switches. [2J(Pseudo)rotaxanes containing CB 6] were studied along... 

During the past 20 years, mechanically interlocked molecules, known as catenanes and rotaxanes, many of them redox-active, have become readily accessible using template-directed protocols that rely upon the precepts of molecular recognition and self-assembly and the tenets of supramolecular assistance to covalent synthesis. By incorporating different recognition units with dissimilar redox properties into appropriate components, these compounds can often be induced to switch hysteretically between ground and metastable co-con-... 

A similar solvent-driven molecular switch (Figure 19b) was also reported on the basis of chiral peptide rotaxanes, indicating for the first time that the chirality of a molecule could be switched on and ofF by controlling the interactions between mechanically interlocked submolecular components as evidenced by circular dichroism. ... 

Stoddart et al. also incorporated a bistable [2]catenane (Figure 39) into macromolecules to form a side-chain poly[2]catenane (Figure 82). This poly[2]catenane could also behave as a molecular switch that can be addressed in an on or off state electrochemically by the virtue of the redox properties of the TTF moiety. Furthermore, the switching properties remain even in spherical aggregates. This stndy provides a good mechanically interlocked switchable polymeric scaffold for the construction of solid-state molecular electronic devices. 

In the previous sections, our main focus was on mechanical molecular switches based on mechanically interlocked structures pseudorotaxanes, rotaxanes, and cate-nanes. Molecular motors, rotors, and propellers based on single rotor molecules, as important molecular machines, have also attracted great attention during the last two decades. Molecular motors can be defined as molecules that are able to convert any energy input into controlled motion. Inspired by the unidirectional rotary motion of Fi-ATPase, much effort has been focused on systems that allow controlled molecular rotation and translation. 

Mechanically interlocked molecules (MIMs), such as catenanes and rotaxanes, are molecules with at least two components that are not covalently bound, but interlocked in such a manner that they cannot be separated without the breaking of a covalent bond. Since this physical linkage is known as a mechanical bond [24], we refer to the stereochemistry of MIMs as mechanostereochemistry [25]. MIMs have been appreciated for their synthetic challenge and aesthetic value [26] as well as their potential applications. In particular, MIMs have garnered much interest as artificial molecular switches and machines [27-31] because their internal noncovalent bonding interactions can be modulated by external stimuli to control the relative translational and/or circumrotational motions of their interlocked... 

The continuing development of such systems exhibits great potential in the areas of sensing, molecular switches, and nanoscale machines capable of doing work. Realization of these ambitious goals will likely require new synthetic methods to construct more complex hierarchical ion pair networks containing multiple aligned mechanically interlocked assemblies for amplified functionality and macroscopic device incorporation. 

Like rotaxanes, catenanes are mechanically interlocked molecules. However, instead of interlocking one ring shaped macrocycle and a dumbbell shape, catenanes consist of interlocked macrocycles. The number of macrocycles contained in a catenane is indicated by the numeral that precedes it. Catenanes have bistable and multistable forms and a switchable, bistable [2]catenane is commonly exploited in nanotechnology and molecular electronics because its behavior can be controlled by electrochemical processes [89]. Collier et al. was the first to demonstrate the electroactivity of interlocked catenanes [90]. The authors affixed phospholipid counterions to a monolayer of [2]catenanes and then sandwiched this system between two electrodes. This work resulted in a molecular switching device that opened at a positive potential of 2 V and closed at a negative potential of 2 V. 

The versatility of this approach to interlocked molecules make it very attractive as a means towards nanoscale molecular machines in which the motions of the individual components are beginning to be translated into molecular logic operations and mechanical switches. We will return to this subject in the next chapter. 

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