Molecular-scale machines

Figure 12.18 Schematic representation of a linear motor powered by light Adapted from V. Balzani, A. Credi and M. Venturi, Light-powered molecular-scale machines , Pure and Applied Chemistry Volume 75, No. 5,541-547 International Union of Pure and Applied Chemistry IUPAC 2003...

V. Balzani, A. Credi, M. Venturi (2003) Light-powered molecular-scale machines, Pure Appl. Chem., 75 541-547. 

In the same address, Feynman concluded his reflection on the construction of artificial molecular-scale machines as follows What would be the utility of such machines Who knows I cannot see exactly what would happen, but I can hardly doubt that when we have some control of the rearrangement of things on a molecular scale we will get an enormously greater range of possible properties... 

The past 15 years have witnessed an explosion of relatively inejqjensive ecjuipment and techniques for probing and manipulating materials on the nanometer4engfti scale. These capabilities have led to optimistic forecasts of futuristic nano- technologies including molecular-scale machines and robots that can manipulate matter with atomic precision. Many believe that such futuristic visions are mere hype, while others express the hope that they can be realized. 

Before returning to the polymeric aspects of DNA and proteins, it is interesting to note that synthetic versions of DNA are being considered for nano-based applications (92). Central to nanotechnology are molecular-scale machines. Nanoscopic tweezers have already been made, as well as nano-computational devices. Rotating shaft devices are being considered. 

Figure 11.10 Molecular structures of the components for a light-driven molecular scale machine. Adapted with permission from (Bolzani et al. 2006 Aust J Chem 59 193). Copyright (2006) CSIRO Publishing. 329...

Polyhedral heterocarborane clusters are promising materials for nanotechnology. This is evident from the interesting applications discussed in this chapter, which include molecular nanoparticles, nanomedicines, and molecular-scale machines and devices. The use of carboranes as building blocks in the computational design of nanostructured materials remains open to novel discoveries that will be possible only by pursuing further research in this fascinating area of molecular chemistry. 

Much of chemistry occurs at small scales. As Dalton and other scientists realized, atoms are the basic units of matter and combine chemically to form molecules. Although chemists usually work with substantially larger amounts of matter, researchers in the field of nanotechnology are beginning to develop the techniques to manipulate matter on the atomic and molecular scale. By finding the right conditions in which atoms and molecules will assemble into functional structures, or by constructing tiny machines that oscillate, researchers have entered the domain of the atom. 

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. 

An exciting development in the field of molecular machines has been the construction of a rudimentary molecular-scale muscle17-19,31 based on the topology of a rotaxane dimer, which can undergo contraction and stretching movements in solution. 

The feasibility and limitations of molecular machines can hardly be emphasized any better than by Feynman s mixed message [1], namely that An internal combustion engine of molecular scale is impossible. Other chemical reactions, liberating energy when cold, can be used instead. Nanoscale machines, like their macroscopic counterparts, require power supplies of appropriate kinds and magnitudes for their functions. While macroscopic machines enjoy the simplicity of distinct active (ON) and inactive (OFF) states in the presence and absence of power supplies, respectively, molecular machines are in perpetual Brow-... 

In the next chapter I will examine the art of self-defense—but, of course, on a molecular scale. Just as machine guns, battle cruisers, and nuclear bombs are necessarily sophisticated machines in our larger world, we will see that tiny cellular defense mechanisms are quite complex, too. Few things are simple in Darwin s black box. 

Tweezers, scissors and screwdrivers are primitive tools compared to computers, but their importance and wide applicability to daily life cannot be ignored. Similarly, molecular scale mechanical devices would be very useful in nanotechnology. For example, a molecular robot that fabricates molecular wires and molecular machines that could penetrate deep inside the body would provide huge contribution in the fields of molecular electronics and medicine, respectively. 

Photoisomerizations can often occur by several different mechanisms. Systems that isomerize via a controlled mechanism are potential candidates for molecular machines [184]. Energy in the form of light is absorbed and converted to controlled mechanical force on the molecular scale. Examples of a mono-directional rotor [185, 186], a switchable rotor [187], and a molecular shuttle [188] have been demonstrated. These systems are light-controlled, but there are also examples of systems which control molecular motion based on electro- and/or chemical modulation, such as the threading/imthreading of (pseudo)rotaxanes [189-196]. 

The ultimate aim of nanotechnology is the development of self-assembling molecular-scale devices that can themselves perform specific, precisely controlled operations at the molecular and atomic level. Current methods nsing natnral molecular machines — proteins, enzymes, antibodies, and the like — or synthetic molecnlar forms still rely to a large degree on bulk processes. They provide us with rudimentary devices that operate at the molecular and atomic level, but at present they lack the precision and positional control reqnired to develop more advanced nanotechnologies. 

The construction of molecular machines that mimic physical and biological systems has attracted the attention of many scientists over the past 30 years [1]. This interest stems in part from the ever-continuing effort to reduce the size of mechanical devices, because as one proceeds to smaller and smaller dimensions, one eventually arrives at the molecular scale. The miniaturization of motors has had a particular fascination. In 1959, the Nobel laureate physicist Richard Feynman offered a 1000 reward for the first operating... 

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