Molecular wires, electron-conducting

The probability matrix plays an important role in many processes in chemical physics. For chemical reactions, the probability of reaction is often limited by tunnelling tlnough a barrier, or by the fonnation of metastable states (resonances) in an intennediate well. Equivalently, the conductivity of a molecular wire is related to the probability of transmission of conduction electrons tlttough the junction region between the wire and the electrodes to which the wire is attached. 

Many other opportunities exist due to the enormous flexibility of the preparative method, and the ability to incorporate many different species. Very recently, a great deal of work has been published concerning methods of producing these materials with specific physical forms, such as spheres, discs and fibres. Such possibilities will pave the way to new application areas such as molecular wires, where the silica fibre acts as an insulator, and the inside of the pore is filled with a metal or indeed a conducting polymer, such that nanoscale wires and electronic devices can be fabricated. Initial work on the production of highly porous electrodes has already been successfully carried out, and the extension to uni-directional bundles of wires will no doubt soon follow. 

Mujica V, Kemp M, Ratner MA (1994) Electron conduction in molecular wires. I. A scattering formalism. J Chem Phys 101 6849... 

Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4]. The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. 

It is useful to introduce a nomenclature for distinguishing between ET occurring by the conduction and superexchange mechanisms. The term electron transport is used in the context of molecular wire behavior, while electron transfer is used in the context of the superexchange mechanism. 

Molecular wires conduct an electrical signal (which could be just one electron) between two connected components over a long distance. This function can be brought about by linking a donor and acceptor by means of a rigid spacer. 

Before examining the electrochemical properties of this class of compounds (we will limit the discussion to homonuclear derivatives), it must be clear that the technological application of molecular wires belongs to solid-state chemistry. Nevertheless, since the main target of such new molecules is to conduct electricity, it seems useful to ascertain preliminarily their intrinsic ability towards intramolecular electron mobility by electrochemical investigations in solution, i.e. in the absence of intermolecular interactions. 

Mujica, V., Kemp, M., Ratner, M.A. Electron conduction in molecular wires. II. Application to scanning tunneling microscopy. J. Chem. Phys. 1994, 101, 6856. 

Polyaniline has been formed in the pores of Cu- or Fe-exchanged MCM-41 by adsorption of aniline vapour and subsequent oxidative polymerization (Figure 7.23), and these molecular wires demonstrate significant electronic conduction, although less than that of bulk polyaniline. Pyrolysis of polyacrylonitrile in the pores produces a graphitelike carbon chain, which exhibits microwave conductivity ten times that of bulk carbonized polyacrylonitrile. Such materials have potential for use in information processing as storage capacitors. 

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