Molecular machines construction

In particular, rotaxane dendrimers capable of reversible binding of ring and rod components, such as Type II, pseudorotaxane-terminated dendrimers, can be reversibly controlled by external stimuli, such as the solvent composition, temperature, and pH, to change their structure and properties. This has profound implications in diverse applications, for instance in the controlled drug release. A trapped guest molecule within a closed dendrimeric host system can be unleashed in a controlled manner by manipulating these external factors. In the type III-B rotaxane dendrimers, external stimuli can result in perturbations of the interlocked mechanical bonds. This behavior can be gainfully exploited to construct controlled molecular machines. 

To conclude this brief note (for details see texts of Biochemistry) we stress that the thermodynamic efficiency of molecular motors can be quite high, approaching 100% but never reaching this thermodynamic limit see Everett and also Neilson and Crawford in References to Appendix. We must always be aware of the heat losses in any real process and this is true all the way from the simplest molecular machines to multi-molecular constructs to man-made machines. 

There may not be any alternative way for life to begin or to develop than through the reduction and then condensation reactions of organic molecules aided by inorganic ions which require energy and the use of a mutatable reproductive code. Certainly, none is known on Earth. If we look at the molecular machines involved, the principles of their construction and activity appears to have remained virtually unchanged, to this day (see Section 4.7 and Appendix 4C). 

Product 129 containing a cyclopentene-based photochromic fragment was proposed as an axis of pseudorotaxane structures, which has potential for use in the construction of a prototype of molecular machines active as a functional stopper (08DP294). 

Multielectron storage devices can be used as (i) redox catalysts, also called electron mediators, for multielectron processes, (ii) electrochemical sensors with signal amplification, and (iii) molecular batteries that can be foreseen to power molecular machines in the future or that can be used to construct flexible rechargeable batteries.10... 

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

The future targets of supramolecular photochemistry in CD chemistry will contain photoresponsive molecular machines, emission-based sensors, and energy transport systems. For construction of such systems, the design of three-dimen-sionally correct arrangement of component units will become important. The molecular modeling computation approach will be helpful for designing the systems and deeper understanding of structural features of chromophore-modified CDs and their complexes. 

Applying this new exciting transition metal dtc-based catenane high yielding synthetic procedure to the construction of novel redox-controlled molecular machines and switches is the subject of ongoing research within the group. 

In the last few years, several examples of molecular machines and motors have been designed and constructed. It should be noted, however, that the molecular-level machines described in this chapter operate almost in solution. Although the solution studies of chemical systems as complex as molecular machines are of fundamental importance, it seems reasonable that, before functional supramolec-ular assemblies can find applications as machines at the molecular level, they have to be interfaced with the macroscopic world by ordering them in some way. 

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 Kim group has capitalized on the ability of CB[8] to simultaneously bind two aromatic rings in the construction of a variety of molecular machines [26,27]. A... 

In the previous section we have described pseudorotaxane systems in which electron transfer inputs govern dethreading/threading processes, opening the way to the control of nuclear movements (molecular machines). In this section, we will see that, in their turn, nuclear movements induced by an appropriate stimulation (e.g., an acid/base reaction) can govern the occurrence of electron transfer processes or CT interactions. This aspect of pseudorotaxane chemistry can be exploited for the construction of electronic devices for information processing at the molecular level. 

The unique architecture of rotaxanes and catenanes (as well as pseudorotaxanes see Volume III, Part 2, Chapter 6) lend themselves to the occurrence of large-amplitude motions by their component parts—a property reminiscent of the movements displayed by the working parts of machines in the macroscopic world. The concept of machine at the molecular level is not a new one. Our body can be looked upon as an extremely complex ensemble of molecular-level machines that power our movements, repair damage, and orchestrate our inner worlds of thought, sense, and emotion [69]. The challenge of constructing artificial molecular machines was posed for the first time by Feynman [70] in his famous address, There is Plenty of Room at the Bottom, to the American Physical Society in 1959. In his address, he raised a number of interesting issues, such as ... 

Chemists are now venturing to exploit their knowledge about molecular interactions to design molecules that can be controlled to self-assemble into novel structures. Some scientists hope to create a set of molecular building blocks to construct "molecular machines," while others are designing nanostructures with electrical properties useful for creating the next generation of computer chips. A few scientists even dare to attempt to create "artificial life."... 

---