Molecular systems, mechanically interlocked

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

Mechanically interlocked molecular compounds, including catenanes, rotax-anes, and carceplex, are constituted of molecules composed of two or more components that cannot be separated from each other [95-98]. The development of strategy for achieving controlled self-assembling systems by non-covalent interaction enables one to prepare such attractive compounds for applications in nanoscale molecular devices. The dithiafulvene derivatives are effective electron donors, which are good candidates to form those supramolecular systems with appropriate acceptors by virtue of intermolec-ular CT interactions. In this chapter, dithiafulvene polymers forming rotax-ane structures are especially described. 

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... 

Mechanically-Interlocked Molecular Systems Incorporating Cyclodextrins... 

Apart from the parent CDs, fully synthetic CD analogs may have some advantages in the construction of mechanically-interlocked systems. Thus, o/L-altemating cyclic oligosaccharides designed and synthesized in our laboratories have Sn symmetry and so behave as molecular cylinders rather than as molecular lampshades. 

Owing to the development of supramolecular chemistry and a series of efficient synthetic methods such as template synthesis and dynamic covalent synthesis, artificial molecular machines have exploded since the late 1980s. In this chapter, we mainly discuss mechanical molecular machines that is, the different components among the molecules or supramolecular aggregates, which are linked by noncova-lent interactions and are usually called mechanical bonds. Mechanical bonds, in which two or more molecular components become mechanically interlocked, one with another," play a dominant role in the development of artificial molecular machines. Threaded structures, such as pseudorotax-anes, rotaxanes, and catenanes, are excellent precursors in the construction of molecular machines, for the mechanical bonds in these supramolecular systems are easy to be altered by external stimuli, causing relative movements among different components. Molecular rotors and propellers are also discussed because of their great importance in the development of motorized machines. 

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. 

Other types of rotary motors can be constructed using catenanes, that is, species minimally composed of two mechanically interlocked macrocycles. In appropriately designed systems, the rotation of one ring around the other can be achieved by external stimulation. If suitable control elements are introduced in the structure, such a rotation can be performed unidirectionally. This concept was cleverly realized with benzylic amide catenanes by exploiting the isomerization of fumaramide to maleamide caused by UV light. Chiral C=N compounds such as imines also fulfill the criteria of two-step light-driven unidirectional molecular... 

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. 

We have evolved a synthetic strategy that has relied upon the formation of catenanes and rotaxanes in the beginning. We regard these compounds - with their mechanically interlocked molecular components - as prototypes for the construction of large and ordered molecular arrays. Once we have established for the chemical systems... 

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