Conducting polymer, electron-conductive states

Conducting Polymers Electronically conducting polymers (such as polypyrrole, polythiophene, and polyaniline) have attracted considerable attention due to their ability to switch reversibly between the positively charged conductive state and a neutral, essentially insulating, form and to incorporate and expel anionic species (from and to the surrounding solution), upon oxidation or reduction ... 

The benefit of a hybrid phase for the intercalation-deintercalation of mobile species such as Li+ cations is well illustrated by the study of conductive polymers such as polyaniline or polypyrrole intercalated into a V2O5 framework as potential electrode materials in lithium batteries [34]. For PANI/V2O5, an oxidative post-treatment performed under an oxygen atmosphere allowed the authors to compare the conductivity attributed to the polymer, as in absence of reduced cations, there was no electronic hopping between ions, and the conductive state was due only to the... 

Lewis acid-induced polymerization is also a relatively convenient means to obtain conducting polymers. As with anodic electropolymerizations, monomers are oxidized by an external source, but in this instance electron transfer occurs from the monomer or growing chain to the chemical oxidant. Of the various Lewis acids that are readily available, FeCl3 has become the most popular choice given its effectiveness and low cost. Scheme 7 shows a typical polymerization where the resulting polymer is obtained in its conducting state with FeCl4" dopant ions. 

Following Refs. [61, 62], a two-band (valence and conduction band) configuration interaction model is introduced, using a basis of monoexcited configurations on the polymer chain. These correspond to electron-hole states n) = nen h) = ne)c <8> n h)v localized at sites n and n of the chain. Here,... 

The change that converts the polyacetylene molecule from a nonconductive to a conductive state involves the addition of some foreign material, a dopant, to the polymer. Two kinds of dopants are used those that attract electrons and remove them from the bonds that make up a polymer molecule, and those that donate electrons to the molecule. In either case, the normal electronic structure of the molecule is disrupted, and individual electrons within the molecule become more mobile. As their mobility increases, they tend to flow through a molecule and from one molecule to the next when an external electrical potential is applied to the polymer. 

Reprotonation of the DP-PPY and EM base by 1 moll-1 HC1 brings the polymers back to their positively charged and highly conductive state, with the respective N Is core-level spectra similar to those shown in Fig. 6(a) and Fig. 6(b). The EM oxidation state of PAN can also be completely transformed to the fully reduced LM upon treatment with phenylhydrazine (Fig. 6(f)). Similar treatment of DP-PPY also gives rise to a fully reduced polymer or PPY°, as evidenced from the disappearance of the low BE shoulder in the NIs core-level spectrum of Fig. 6(e). The reduction process, however, is more sluggish in this case. The PPY° in turn is susceptible to partial oxidation by electron acceptors, such as the halogens [48], Treatment of this re-oxidized PPY with a base again... 

Some conjugated polymers, such as polythiophene and polyaniline were synthesized already in the last century [8a,b], It is not surprising that, for example, polyaniline has played a major role in research directed toward synthetic metals because it possesses a relatively stable conducting state and it can be easily prepared by oxidation of aniline, even in laboratories without pronounced synthetic expertise (see section 2.6). It is often overlooked, however, that a representation of, for example, polypyrrole or polyaniline by the idealized structures 1 and 2 does not adequately describe reality, since various structural defects can occur (chart 1). Further, there is not just one polypyrrole, instead each sample made by electrochemical oxidation must be considered as a unique sample, the character of which depends intimately on the conditions of the experiment, such as the nature of the counterion or the current density applied (see section 2.5). Therefore, one would not at all argue against a practical synthesis, if the emphasis is on the active physical function and the commercial value of a material, even if this synthesis is quick and dirty . Care must be exercised, however, to reliably define the molecular structure before one proceeds to develop structure-property relationships and to define characteristic electronic features, such as effective conjugation length or polaron width. 

FTIR spectroscopy has been used to monitor the conducting states of a conducting polymer as well as to know if a doping experiment is successful [86, 87], The FTIR and UV-Vis spectra of unsubstituted PANI is similar to that of substituted PANI though with slight band shifts. Doped PANI and its derivatives exist in the emeraldine salt forms which are essentially delocalized polysemiquinone radical cations whose stability is maintained by the presence of dopant anions. The degree of electron delocalization in the polysemiquinone forms of the doped PANI manifests itself in the form of an electronic-like band at ca. 1100 cm 1 associated with polarons [86], The structures of emeraldine base and emeraldine salt form of PANI are presented in Figure 6. 

The filled circles indicate electrons, and the open circles represent holes. Process 1 indicates photoexcitation process 2 indicates decay of the electronic excited state the dark slanting lines with arrows indicate a hole or electron transfer process (left and right sides, respectively) and the thin lines connecting the conduction band of Ti02 with the hole level in PPV indicate an interfacial recombination process. The state levels are depicted as in this figure, with the holes placed at slightly lower energy than the polymer LUMO. 

In 2002, David Nilsson reported an electrochemical transistor using poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT PSS) as the active layer [14]. In this system, both the electronically conducting system (PEDOT) and the dopant are polymers and therefore nonvolatile as the oxidation state of the PEDOT is altered. PEDOT is pristinely doped and is therefore in its high conducting state. This allows us to define the drain, source, and gate as well as the transistor channel in PEDOT PSS solely, see Fig. 9.1. As the gate is addressed positively, vs. the source electrode, PEDOT in the channel is reduced to its neutral state. 

For polymers k is of the order of 0.2Wm K This is much smaller than the 50Wm K for steel, due to the lack of free conduction electrons, and the weak forces between polymer chains. Steady-state conduction occurs through the foam-insulated wall of a domestic refrigerator the temperature at any point in the foam remains constant, however ... 

M.S.A. Abdou and S. Holdcroft, Oxidation of jt-conjugated polymers with gold trichloride enhanced stability of the electronically conducting state and electroless deposition of Au°, Synth. Met., 60, 93 96 (1993). 

---