Switching/conductive properties

Incorporation of such dithienylethene units capable of reversibly interrupting conjugation into a polyene molecular wire permits reversible switching of conductive properties by photoirradiation.15 231 A typical example is shown below. 

The same argument could be made for design of molecular interconnects. Proposals have been made for molecular wires constructed from conducting polymers. But how would one isolate an individual molecular strand, much less attach it to a molecular switch Would such a one-dimensional wire exhibit conduction properties akin to bulk conduction These questions have yet to be answered. 

The fact that these unique polymer materials conduct electricity is fascinating enough, but the ability to switch their properties in situ using simple electrical stimuli is intriguing. Of course, this ability is dependent on possessing conductivity initially. 

Various different applications of the conductivity properties of the DCNQI salts have been suggested [13]. The very rapid change in microwave reflectivity with light-induced phase transformations in partially-deuterated [2,5-DMe-DCNQI]2 Cu alloys can be used for fast microwave switching [20]. Thin films of special DCNQI salts can be used as sensors for paramagnetic gases such as O2 and NO [21]. AppH-cations in electrophotography have also been discussed. 

Furthermore, porous CPs (e.g., polypyrrole, polyanUine) films have been used as host matrices for polyelectrolyte capsules developed from composite material, which can combine electric conductivity of the polymer with controlled permeability of polyelectrolyte shell to form controllable micro- and nanocontainers. A recent example was reported by D.G. Schchukin and his co-workers [21]. They introduced a novel application of polyelectrolyte microcapsules as microcontainers with a electrochemically reversible flux of redox-active materials into and out of the capsule volume. Incorporation of the capsules inside a polypyrrole (PPy) film resulted in a new composite electrode. This electrode combined the electrocatalytic and conducting properties of the PPy with the storage and release properties of the capsules, and if loaded with electrochemical fuels, this film possessed electrochemically controlled switching between open and closed states of the capsule shell. This approach could also be of practical interest for chemically rechargeable batteries or fuel cells operating on an absolutely new concept. However, in this case, PPy was just utilized as support for the polyelectrolyte microcapsules. 

This report describes some of the recent work on the electrochemical and electrode applications of polymers which are electroactive and can be switched to an electrically conductive state, as well as the inherently conductive (SN. The materials fall into two general categories. There are the polymer tllms which can be prepared in situ by the electrochemical polymerization of aromatic compounds, and there are the polyenes such as polyacetylene and polythiazyl. Many of the electrode applications being considered are based on the. electroactive/conductive properties of the films such as display devices, and storage batteries. Some applications make use of the conductive property of the materials such as protective coatings against corrosion, and other applications make use of the possibility for molecular selectivity such as chemically selective electrodes and sensors. 

A remarkable feature of phosphazene polymers of types (1) and (2) is that appropriate substituents (which are readily attached) can be used as toggle switches to turn several properties, such as hydrolytic stabiHty and electrical conductivity, on and off (1). 

Electrochemical polymeriza tion of heterocycles is useful in the preparation of conducting composite materials. One technique employed involves the electro-polymerization of pyrrole into a swollen polymer previously deposited on the electrode surface (148—153). This method allows variation of the physical properties of the material by control of the amount of conducting polymer incorporated into the matrix film. If the matrix polymer is an ionomer such as Nation (154—158) it contributes the dopant ion for the oxidized conducting polymer and acts as an effective medium for ion transport during electrochemical switching of the material. 

Electrical properties of liquids and solids are sometimes crucially influenced by H bonding. The ionic mobility and conductance of H30 and OH in aqueous solutions are substantially greater than those of other univalent ions due to a proton-switch mechanism in the H-bonded associated solvent, water. For example, at 25°C the conductance of H3O+ and OH are 350 and 192ohm cm mol , whereas for other (viscosity-controlled) ions the values fall... 

Sulfur vapor consists of a mixture of species that includes S8, S6, S4, and S2 (which like 02 is paramagnetic). Because the S8 molecule is nonpolar, it is soluble in liquids such as CS2 and C6H6. Selenium also consists of cyclic molecules that contain eight atoms, and tellurium is essentially metallic in character. In their vapors, several species are found that contain 2, 6, or 8 atoms. Both are useful as semiconductors, and selenium has been used in rectifiers. Because the electrical conductivity of selenium increases as the intensity of illumination increases, it has been used to operate electrical switches that open or close as a light beam is broken. Selenium was also used in light meters, but other types of meters are now available that are more sensitive. Table 15.1 gives a summary of the properties of the group VIA elements. 

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