Conduction intrachain

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

The results of Wegner and Riihe (1989) clearly show that electronic conduction in conducting polymers such as polythiophene and polypyrrole occurs via a hopping mechanism that is dominated by interchain rather than intrachain hopping. 

Metallic properties are also observed in columnar stacked complexes in which the intrachain separation is too long for conduction to be associated with the formation of a band from overlap of the metal dz2/pz orbitals. For these complexes the conduction process involves carriers present in the delocalized MOs extending over die whole complex. The conduction process in these compounds may be described by a hopping mechanism in which the on-site Coulomb repulsion energy has been reduced by the delocalization of the charge over the whole molecule. 

Ir(CO)3Br is isostructural with the chloro complex but with a slightly longer intrachain separation (2.854 A). The four-probe DC conductivity in the iridium atom chain direction is, however, only 3 x 10-4 Dr1 cm-1 and this, together with the variation of conductivity and thermopower with temperature, indicates that the bromo complex is a semiconductor.161 The structure of Ir(CO)3I shows more disorder and could not be refined accurately, but the Ir—Ir separation is 2.858 A. 

The compounds differ in their Pt—Pt intrachain separations, electrical conductivities, degree of partial oxidation of the platinum atom, and metallic color. These major differences occur even though two pairs of these four compounds contain the same anion, (FHF)-, and cation, Rb+ or Cs+. This provides the unique opportunity to explore only the effects of structural, not elemental, changes on the anisotropic electrical behavior of POTCP salts. 

The intrachain contact term, T[ J, is usually the most important and is related to the one-dimensional regime (i.e., conduction along a stack). The electrons traveling along the chain experience two kinds of scattering, q 0 and q = 2kF. 

Fig. 9. Energy bands of tetracyanoplatinates with different intrachain Pt-Pt distances R. The results are obtained from a two-band model in the limit of an one-electron calculation82. The upper and lower structures represent the conduction and the valence bands, respectively, k is the absolute value of the wavevector in the chain direction. P is the percentage of admixture of the orbitals of a2u parentage to the valence band orbitals...

As synthetic metals they are by no means unique in terms of high metallic conductivity since several salts of other systems have higher conductivity. This is in agreement with both experimental and theoretical results based on their intrachain electronic bandwidth 4tn, which is not particularly high, and certainly very small compared to ordinary metals. 

Hydrochloric acid as well as camphor sulfonic acid doped polyaniline prepared in chloroform often have [59] log a proportional to T as expected for quasi-one-dimensional variable range hopping, Equations (3) and (4). Generally, the higher conductivity samples have a weaker temperature dependence at low temperatures (Tq 700-1000 K for T<80 K), and lower conductivity samples a stronger temperature dependence (To 4000 K). The smaller Tq for the more highly conducting samples has been associated with weaker localization due to improved intrachain and interchain order. 

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