Supramolecular devices, definition

Molecular and supramolecular devices are by definition formed from covalently and non-covalently linked components, respectively. One might envisage that covalently built devices made up of distinct but interacting components, retaining at least in part their identity as if they were bound together in a non-covalent fashion, could also belong to the supramolecular domain. Such a case has been argued for supramolecular photochemistry [A. 10] and could be extended to other supramolecular functions. 

Such a definition of a supramolecular device does not exclude the traditional concept of host and guest or receptor and substrate. Molecular recognition events between host and guest may be an intrinsic part of the operation of a supramolecular device, which might, for example, be designed to bind and then signal the presence of a guest. 

Common components within supramolecular devices as they are studied today are photochemically or redox active molecules, i.e. molecules capable of absorbing and/or emitting light and molecules capable of losing or gaining an electron. The definition of a supramolecular device made up from these components is illustrated in Figure 11.1. If light excitation of a molecule ( o ) results in the formation... 

In the construction of molecular devices, however, the focus is on the functional interactions between the components rather than the chemical nature of their connectivity. This means that a supramolecular device can be an entirely covalent molecule if it possesses characteristics of a supramolecular nature. Thus, a supramolecular device may be defined as a complex system made up of molecular components with definite individual properties. 

One of the key goals in nanochemistry is the creation of devices that can function on the nanometre scale. The benefits that accrue with such miniaturisation include increased component density, lower costs and faster speeds, with longterm goals in molecular computing. A device can be described as an object that is invented and has a purpose. However, what is a device on the supramolecular level Thus far, we have considered the definition of supramolecular chemistry in terms of non-covalent interactions. However, we can consider a supramolecular device to be a system made up of linked molecular components with identifiable properties that are intrinsic to each component. The interaction energy between... 

As Figure 3 illustrates, redox active guests introduce PET processes almost by definition and luminescent on-off switching is the norm. However, the inhibitions outlined in Section 5 have not prevented the designers of switchable luminescent devices from exploring systems which bind redox active guests. The combined forces of inorganic coordination chemistry and supramolecular science have proved to be too attractive in many of these instances. It is to be hoped that some of this effort will filter across to the examination of more on-off systems like 17 and 18. 

These descriptions, while helpful, are by their nature noncomprehensive and there are many exceptions if such definitions are taken too literally. The problem may be linked to the definition of organometallic chemistry as the chemistry of compounds with metal-to-carbon bonds . This immediately rules out Wilkinson s compound, RhCl(PPh3)3, for example, which is one of the most important industrial catalysts for organometallic transformations known in the field. Indeed, it is often the objectives and thought processes of the chemist undertaking the work, as much as the work itself, which determine its field. Work in modern supramolecular chemistry encompasses not just host-guest systems but also molecular devices and machines, molecular recognition, so called self-processes ... 

Because of limited space, this chapter covers porphyrins and expanded porphyrins as receptors through mostly non-covalent or supramolecular interactions. As a general rule, examples wherein the porphyrin acts as a receptor via metal coordination are excluded. For a further reading on this latter topic, the reader is referred to a recent Chemistry Review article entitled Supramolecular Chemistry of Metallopor-phyrins. On the other hand, inspired by a recent review by Anslyn entitled Supramolecular Analytical Chemistry, we expand our definition of receptor-substrate interactions to include certain boronic acid binding motifs and several representative examples of metalated porphyrins that are applied to sensing applications, wherein the proposed mode involves metal complexation. Thus, included in this chapter is a brief summary of the seminal work carried out by Suslick and his group, where porphyrins are used to create the so-called artificial nose, that is, an array-based sensing device for various vapors. Also included are... 

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