Dopant anion

PPQs possess a stepladder stmcture that combines good thermal stabiUty, electrical insulation, and chemical resistance with good processing characteristics (81). These properties allow unique appHcations in the aerospace and electronics industries (82,83). PPQ can be made conductive by the use of an electrochemical oxidation method (84). The conductivities of these films vary from 10 to 10 S/cm depending on the dopant anions, thus finding appHcations in electronics industry. Similarly, some thermally stable PQs with low dielectric constants have been produced for microelectronic appHcations (85). Thin films of PQs have been used in nonlinear optical appHcations (86,87). 

Indole differs from the more simple heterocycles in that when doped, the structure appears to contain one dopant anion to two monomer units, and its conductivity is considerably lower (by four orders of magnitude) when doped to a similar level [39]. From IR spectra, and the fact that /V-substituted indoles do not polymerise, the... 

Tourillon and Gamier [428] found using SEM that the morphology of the material is affected by the nature of the dopant anion and that there is considerable swelling of the polymer on doping (with increases in thickness of the order of 40% for... 

XPS studies of other S-containing heterocycles, such as PTH, poly-thieno(2,3-h)thiophene (PTH23TH) and polydithienothiophene (PDTT), [55] also indicated that, in the oxidized state, the CIs and S2p core-level spectra appear at a higher BE than the neutral polymer and are characteristic of positively polarized atoms. This result appears to be inconsistent with the fact that, in the oxidized polymer, only one out of every 3 to 4 monomer units is associated with a dopant anion and the Cls and S2p core-level spectra should reveal the simultaneous presence of neutral and charged (or polarized) species (see also Sect. 3.2.1 below). 

For polypyrroles and polythiophenes, n is usually ca. 3 for optimal conductivity, ie. there is a positive charge on every third or fourth pyrrole or thiophene along the polymer chain, near which the dopant anion A is electrostatically attached. For polyanilines, the ratio of reduced (amine) and oxidised (imine) units in the polymer is given by the y/( 1 - y) ratio. The conducting emeraldine salt form of polyaniline has y = 0.5, i.e. there are equal numbers of imine and amine rings present. 

A counterion (A ) is incorporated during synthesis to balance the charge on the polymer backbone. Common chemical oxidants are FeClj and (NH4)2S20g, which provide Cl and HS04 /S04, respectively, as the dopant anions. Electrochemical oxidation provides greater flexibility in terms of the anion that can be incorporated from the electrolyte (MA salt or HA acid) added to the polymerisation medium. 

If the dopant anion (A ) is small and mobile (eg. Cl ) and the polymer has a high surface area to volume ratio, then upon reduction the anion will be efficiently ejected from the polymer. However, extensive studies with polypyrroles have shown [4] that if the dopant is large and immobile (eg. if A" is a polyelectrolyte such as polystyrene sulfonate) then an electrically induced cation exchange process occurs, according to Eqn. 3,... 

A characteristic feature of the parent polypyrroles, polythiophenes and polyanilines is their insolubility in water and common organic solvents (although the EB form of polyaniline is soluble in NMP, DMSO and several other solvents). This intractability and consequent difficulties in processing have until recently limited their exploitation. However, the introduction of substituents onto the aromatic rings of the polymers, the use of surfactant-like dopant anions and the generation of colloidal dispersions have markedly enhanced the processability of ICPs (see Section 8 below). 

Hi. Use of dopant anions containing molecular recognition groups... 

An alternative and often facile route to appropriately functionalised ICPs, that avoids the synthetic problems outlined in (ii) above, is the use of sulfonated species containing the desired molecular recognition/receptor site as the dopant anion for the conducting polymer chains. For example, calixarene-containing polypyrroles [34] and polyanilines [35] for selective metal ion detection have recently been prepared via the use of sulfonated calixarenes as dopant anions. We have similarly found that the incorporation of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants in polypyrroles provides a simple route to metal ion-selective ICPs [36]. 

This alternative approach has also been successfully employed to produce optically active polyanilines. The use of optically active dopant anions such as (H-) - or ( - ) - camphorsulfonate (CSA ) [37-39], (-i-)-or( —) - tartrate [40] and related chiral anions induces macroasymmetry in to the polyaniline chains. We [41] and others [42] have recently shown that films of optically active polyaniline salts such as PAn( -1- )-HCSA, or the optically active emeraldine base (EB) derived from them, exhibit chiral discrimination towards chiral chemicals such as the enantiomers of CSA and amino acids. 

Of course the reduction process also results in exclusion of small dopant anions from the polymer backbone. The dopants can be chosen so that the release process has the desired effect on the chemical composition of the immediate environment. For example. Miller described the triggered release of glutamate [59] and salicylate [60] amongst other compounds. In our own laboratories we have demonstrated the ability to release quinones [61] and metal complexing agents, dithiocarbamates (VII shown below) [62]. 

For organic solvent solubility, an alternative approach to solubilising polyanilines and polypyrroles, without sacrificing high electrical conductivity, is the use of surfactant-like dopant anions. With polypyrrole this has recently been achieved via oxidation of the pyrrole monomer with ammonium persulfate in the presence of dodecylbenzene sulfonate [128,129]. Similarly, the conducting emeraldine salt form of PAn.HA can be readily solubilised in a range of organic solvents via the use of camphorsulfonic acid or dodecylbenzenesulfonic acid as the dopant, HA [130,131]. 

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. 

Figure 6. The structures of emeraldine base and emeraldine salt forms of PANI. A represents the dopant anion.

The phase diagrams and ionic conductivities of anion-deficient ZrO2 compounds have been widely investigated, to probe the influence of both dopant size and concentration (e.g.. Ref. [95]). Whilst Sc" + doped systems possess the highest values of ionic conductivity, factors such as cost and long-term stability have favored the use of (Zri j,Yj,)O2 x/2 as the current "best material, in which the m t and t c transitions are observed at x 0.05 and xw0.16, respectively [96]. On increasing dopant (anion vacancy) concentration, the ionic conductivity initially increases with X, but reaches a maximum close to the lower limit of stability of the c phase and then decreases rapidly [97]. Many theoretical studies have attempted to provide an explanation for this effect, and it is generally accepted that vacancies are trapped... 

An important breakthrough in PPy chemistry was the discovery by Lee and coworkers95 in 1995 of a chemical polymerization route to an unsubstituted PPy that was soluble in organic solvents. They exploited the surfactant-like qualities of added dodecylbenzenesulfonate (DBSA 9) as a dopant anion to solubilize PPy formed during oxidation of pyrrole by aqueous (Nn4)2S208. The PPy/DBSA product, isolated as a black powder in 42% yield after 40 h reaction at 0°C, was very soluble in m-cresol, and could be dissolved in weakly polar solvents such as chloroform and dichloro-methane by the addition of an equimolar amount of dodecylbenzenesulfonic acid. A film cast from chloroform solution exhibited an electrical conductivity of 5 S cm-1, and its UV-visible spectrum was similar to that of electrochemically deposited PPy. 

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