Poly doped’ state

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. 

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). 

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. 

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-... 

Using physical organic chemical rationalizations, we were able to modify the electronic structure of a poly(heterocycle). The result was poly(isothianaphthene) a polymer which is already a semiconductor in the neutral (undoped) state. In the fully doped state, PITN is a transparent conducting polymer. 

As was mentioned above, conjugated organic polymers in their pristine state, are electrical insulators or, at best, semi-conductors. Their conductivity can be increased by orders of magnitude by a doping process. In the "doped" state, the backbone of a conducting polymer consists of a delocalised 7i-system. Early in the twenty-first century Schon et al. (2001) discovered super-conduction in solution cast doped regioregular poly(3-hexylthio-phene) at temperatures below 2.35 K The appearance of superconductivity seems to be... 

Regioregular poly(3-alkylthiophene)s have received a lot of attention, especially because of their high electrical conductivities in the doped state, and because of their unusual solvatochromic and thermochromic behavior . Hence, a lot of research has been focused on clarifying the structure of these materials, both in the solid state and in solution. Today, it is agreed that supramolecular aggregation of polythiophene chains plays an important role in their physical properties. 

A major reason for the failure of poly(acetylene)s in the above-mentioned applications is related to their inherent instability versus moisture and oxygen, and their high susceptibility to decomposition/rearrangement in the partially oxidized/doped state. Nevertheless, poly(ene)s stabilized by appropriate ligand systems and/or incorporated into cyclic structures are believed to exhibit similar stabilities to poly(thiophene)s, poly(pyrrole)s, poly(p-phenylene)s, PPV, and so on. In the following, we will outline the basic concepts of poly(ene)s as well as reviewing the structures that have been realized so far. 

Figure 4.8-2 Optical absorption of conjugated polymers with a degenerate ground state (trans-poly(acetylene)) (a), according to Suzuki et ah, 1980 and a non-degenerate ground state (poly(thiophene)) (b), according to Danno et ah, 1993 in various doping states. Doping concentrations are indicated in % in (a) and by the applied potential in (b).

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

In order to co-polymerize the IC unit, Suzuki and Stille polymerizations have been used. First, Blouin et al. [94] were able to obtain polyindolo[3,2-fr]-carbazole derivatives with bithiophene or biEDOT as co-monomers. Unfortunately, these studies demonstrated a relatively low oxidation potential for these polymers (especially for P35 and P37), limiting their applications in OFETs and PCs. However, for doped state applications, these polymers may exhibit interesting properties [35]. For instance, when copolymerized with bithiophene, the resulting copolymer shows a good electrical conductivity (as high as 0.7 Scm 1) but a low Seebeck coefficient of 4.3 iV K 1 [35]. Finally, the UV-Vis absorption maxima are similar for poly(2,8-indolocarbazole-a/f-bithiophene) and poly(2,8-indolocarbazole-a/f-bis(3,4-ethylenedioxythiophene)). A broad absorption band is centered at 430 nm whereas, for the 3- and 9-substituted copolymers, the broad band is centered around 490-500 nm [94],... 

There were however few exceptions—the most important being polythiophene and poly(p-phenylene vinylene). It was demonstrated [4,5] that in the case of polythiophene, the substitution of hydrogen in the 3 position of the thiophene ring, by an alkyl group longer than the propyl group, renders the polymer soluble in common solvents and the decrease of conductivity in the doped state, caused by the functionalization is rather minor. Since the initial work of Elsenbaumer [4,5] hundreds of papers devoted to soluble polythiophene derivatives have been published. 

The electronic resistance of poly aniline changes reversibly from the poorly conducting reduced (undoped) through the medium oxidized (doped) state to the strongly oxidized state and back without any significant difference between the initial and... 

From SECM studies of the electrochemical doping and undoping processes of poly-(didodecyl-terthiophene), the electron transfer between the polymer and a redox mediator in solution was investigated as a function of the doping state of the polymer [164], The electron transfer was found to occur at the polymer-electrolytic-solution interface and not inside the polymer film. The investigated polymer films are not permeable to redox species when placed in the neutral state and therefore behave as completely passivating films. 

Among these examples, copolymers of thiophene and acrylonitrile moieties can be electropolymer-ized, giving ECPs with electrochemical bandgap as low as 0.6 eV [221]. The same features were also observed in the case of copolymers of thiophene and thieno[3,4-h]thiophene separated by a cyanovi-nylene moiety [222], Various cyanoethylene thiophene polymers have been cycled between their n-doped and p-doped states to measure the stability and potential window for supercapacitor applications [223], The best compound, poly-(E)-alpha-[(2-thienyl)methylene]-2-thiophenacetonitrile, displays unfortunately 60% of electrochemical charge loss after 2000 cycles, for a 2 V potential range. 

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