Spinless conductivity

To account for this phenomenon of spinless conductivity, physicists have introduced the concept of transport via structural defects in the polymer chain. In a conventional semiconductor, an electron can be removed from the valence band and placed in the conduction band, and the structure can be assumed to remain rigid. In contrast, an electronic excitation in polymeric materials is accompanied by a distortion or relaxation of the lattice around the excitation, which minimizes the local lattice strain energy. The combined... 

Similar to Kivelson s model. Chance and co-workers proposed interchain hopping for spinless conductivity in doped polyacetylene, doped poly(p-phenylene) and other doped polymers [102]. The mechanism accounts for the observed dopant concentration dependence of the conductivity in rans-polyacetylene and the observation of anomalously... 

The mechanisms by which these polymers conduct electricity have been a source of controversy ever since conducting polymers were hrst discovered. At first, doping was assumed to remove electrons from the top of the valence band, a form of oxidation, or to add electrons to the bottom of the conduction band, a form of reduction. This model associates charge carriers with free spins, unpaired electrons. This results in theoretical calculations of conduction that are much too small (59). To account for spinless conductivity, the concept of transport via structural defects in the polymer chain was introduced. From a chemical viewpoint, defects of this nature include a radical cation for oxidation effects, or radical anion for the case of reduction. This is referred to as a polaron. Further oxidation or reduction results in the formation of a bipo-laron. This can take place by the reaction of two polarons on the same chain to produce the bipolaron, a reaction calculated to be exothermic see Figure 14.17 (55). In the bulk doped polymer, both intrachain and intrachain electronic transport are important. 

One attractive aspect of the soliton theory of charge transport is that the carriers (cations or anions) carry no spin, i,e, the conducting compositions do not contain unpaired electrons, ESR experiments on the doping of PA(49) show that in certain intermediate doping regimes the spin concentration is much lower than expected from the observed conductivity values this phenomenon is referred to as spinless conductivity. If the conduction involved a normal process of defect-induced hole or electron transport, there would be a direct correlation between ESR determined spin concentration and conductivity. The same conclusion of spinless conduction is obtained from ESR experiments on doped PPP(61) however the soliton theory is not applicable to the PPP system(25). In Section VII, we present an alternate transport mechanism based on bipolarons (dications or dianions) which is applicable to all conducting polymer systems(26),... 

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

Importantly, deep oxidation of polyaniline leads to a material that becomes insulating and spinless. This phenomenon was demonstrated in case of poly(fV-methylaniline) by monitoring ESR signal and electric conductivity of the sample (Wei et al. 2007). Deep oxidation results in the formation of the so-called polaron pairs that are evidenced by optical spectra. Because the hopping probability of two polarons on a single chain is too small, polaron pairs do not contribute to electric conductivity and ESR signal. 

Bipolaron — Bipolarons are double-charged, spinless quasiparticles introduced in solid state physics [i]. A bipolaron is formed from two -> polarons (charged defects in the solid). For chemists the double-charged states mean dications or dianions, however, bipolarons are not localized sites, they alter and move together with their environment. By the help of the polaron-bipolaron model the high conductivity of -> conducting polymers can be explained. 

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