Doped state

Conductivities of polymers of technological interest such as polypyrrole and polythiophene are typically 1000 cm in the doped state, and the conductivity can be tuned by reversibly doping and undoping the polymer. Derivatives of these and other polymers have achieved even higher conductivities. 

Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

Synthesis Basically, two methods are available, which both start (evidently) from suitable monomers (1) chemical synthesis, followed by doping, and (2) electrochemical synthesis directly in a doped state. 

Polyacetylene in the doped state is sensitive to air and moisture. Other polymers (e.g., those of pyrrole, thiophene, and benzene) are stable in air and/or toward humidity in their doped and undoped states. Generally, when stored in the doped state, the polymers lose doping level by mechanisms not fully understood in most cases the loss is reversible. 

Some of the polymers can be processed like ordinary polymers even in the doped state, which is one of their virtues. Like ordinary polymers, blending of dilferent polymers (e.g., a conducting polymer and an ordinary polymer) is possible. The mutual compatibility of the two polymers can be improved by choosing in the conductive polymer a tenside-type dopant ion that has a tail having affinity to the non-conductive polymer. 

The self-discharge phenomena are revealed by the continuous decrease of the low energy bands characteristic of the doped states and by the corresponding increase of those characteristic of the undoped state. These phenomena are not easy to explain. One important fact is that the self-undoping processes do not induce irreversible degradation of the... 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4" -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

Upon connecting the anode and cathode in their doped state, a current is generated with Voc = 2.9 V, and Isc =1.9 mA. The electrodes become undoped by this discharge. The charging And discharging could be repeated many times. Poly(p-phenylene) also was used as electrode. The power density (kW/kg) of the battery was estimated to be more than ten times larger than that of a conventional Pb02 secondary battery. 

Further research on the substitution of the thiophene 3-position with phenyl groups containing electron-withdrawing or electron-donating groups (such as methyl, methoxy, fluoro, chloro, bromo, trifluoromethyl, sulfoxy) in the para position have lead to polymers with unique features [57]. The electron-withdraw-ing groups allow the formation of a radical anion and thus stabilize the n-doped state. As a result, such conducting polymers can be reversibly oxidized and reduced and electrochromic devices can be built with identical anode and cathode materials [58]. 

In poly[2,5-bis(3,4-ethylene-dioxy-2-thienyl)pyridine], a copolymer of thiophene and pyridine in a 2 1 ratio, the effective HOMO and LUMO energy levels of the ir-system are controlled in such a way that both p-doped and n-doped states are accessible (Scheme 9) [43]. The thin fdms display multicolor... 

Several such polymers have shown electrochromic behavior, among them poly(n-vinylcarbazole) [73] which switches from colorless in the neutral state to green in the doped state (Scheme 10) and poly(Ar-phenyl-2-(2 -thienyl)-5-(5"-vinyl-2"-thienyl)pyrrole) [74], which changes from yellow to reddish brown upon oxidation (Scheme 11). A study of the electrochromic properties of blends consist-... 

Polymethylacetylene can be doped by I2, but not by AsF5. Its conductivity in the doped state 36) is around 10-3 S cm 1. As might be expected, increasing content of mono- and di-substituted acetylenes in copolymers with acetylene gives increasing solubility at the expense of drastic reductions in the conductivity of both doped and undoped polymers. 

When doped, these polymers show conductivities in the range from 0.5 to50Scm 1, as illustrated in Table 1 (page 17) Kaeriyama and co-workers 264 show from optical spectroscopy that the band gaps increase with the size of the side group. In the doped states the spectra were all similar, but the conductivity drops with the size of the side group. 

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