Polyaniline electronic structures

J. M. Cinder, A. J. Epstein, Role of ring torsion angle in polyaniline-electronic structure and defect states, Physical Review B 1990, 41,10674. 

Polyaniline is structurally much more complicated than PA, even if we restrict our attention to the emeraldine base (EB) and salt (ES) forms. There are two classes of base forms, to which correspond two classes of salt forms ESI and II [28], and the EB - ES interconversion does not mix the classes. This interconversion corresponds to addition or removal of a proton onto the N atom in the chain without changing the total number of electrons this causes a conductivity change by more than 10 orders of magnitude, from 10-10 S/cm to > 1 S/cm [52]. 

Interest in polyaniline (PANl) sprung up only in the late 1980 s. The relevant structural studies were all carried out after the appearance of the Handbook of Conducting Polymers [1]. The material which has been known as polyaniline since the middle of the previous century is very intractable, and only recently have synthetic routes been developed that lead to a more well-defined polymer, which also exhibits appreciable crystallinity. The interesting variety of electronic structures that... 

The combination of the properties of nano-TiO and polyaniline enables to solve successfully the problems of the chemistry, physics and electronics. Specific electronic structures of the nano-TiO (as the n-type semiconductor) and polyaniline (as the electron s conductor in majority of the cases and as a p-type semiconductor under certain conditions) give the possibility to design the systems for different applications. For example, today such materials are equipped in the photocatalytic conversions of the different pollutants especially [7 4. The modification of the surface of TiO particles by polyanilines layers raises the catalytic activity of titanium (IV) oxide [5, 79]. Composite materials, which have integrated properties of 5-doped nano-TiO and polyaniline layers can be effective in the photo-catalytic processes especially. 

L. Dauginet-De Pra and S. Demoustier-Champagne, A comparative study of the electronic structure and spectroelectrochemical properties of electrosynthesized polyaniline films and nanotubes. Thin Solid Films, 479, 321-328 (2005). 

High Resolution XPS Study of the Electronic Structure of Polyaniline.383... 

In this paper, we report the result of an investigation on the relationship among the dopant concentration and the charge carriers as well as the associated molecular and electronic structure of the doped polyanilines. It is hoped that a study relating various data associated with the variations of molecular and electronic structures as well as the spin signals of the samples as functions of dopant concentration may provide a more comprehensive understanding of the doping effects in polyanilines. 

In this paper, we have investigated the electronic structure of different base forms of polyaniline by using X-ray photoelectron spectroscopy. The protonated emeraldine form has been also investigated to determine the charge delocalization along the chain as a function of the pH. 

In a first step, the electronic structure of the polyaniline base form has been followed versus the oxidation state. After that, we have proceeded further with the protonated emeraldine form. In all cases, the binding energy scale has been calibrated by setting the Cls core level main peak at 284.4 eV. 

In a second part, the electronic structure of the protonated emeraldine form of polyaniline have been analysed and in particular theNls core level. 

The solubility of this conducting polymer opens the way to processing pure, partially crystalline polyanilin or composites of polyanilin with the other commercial polymers into fibers and films, etc. In addition, this solubility enables extensive characterization of polyaniline as a macromolecular system (e.g. viscosity in solution as a probe of molecular weight, etc.) and of polyaniline as a conducting polymer (e.g. optical studies of spin-cast films as a probe of electronic structure of the salt or base forms). The latter is the subject of this paper. 

The available data provide the basis for an understanding of the electronic structure of the four principal forms of polyaniline the fully reduced leucoemeraldine, (lA)n the emeraldine base, [(lA)(2A)]n the oxidized and fully protonated emeraldine salt, [lS] (A )n and the fully oxidized bipolaron lattice, (-B-NH+=Q=NH+-)n. 

As noted above, because of the reduced interchain electron transfer interaction for conjugated chains in solution, the electronic structure is expected to be more nearly onedimensional. Thus, the possibility of a Peierls transition with the formation of an energy gap in the excitation spectrum might be anticipated. The susceptibility results shown in Figure 9 suggest that this is indeed the case for polyaniline in solution in sulfuric acid. Such a large decrease in % cannot be accounted for in the context of the half-filled band expected for [B-NH-B-NH-]+n- To reduce X by such a large factor would require an increase in the band width by the same factor i.e. to a band width of more than 200 eV ... 

The results of structural, spectroscopic and magnetic studies of the various forms of polyaniline carried out recently at UC-Santa Barbara have been briefly reviewed. Although polyaniline can be prepared in partially crystalline form, the molecular weight dependence of the [IS] (A")n absorption spectrum is particularly important, for it demonstrates the need for long, uninterrupted chains in order to obtain the intrinsic electronic structure. 

The other group of materials, which covers the electronic conductors, includes conjugated polymers whose electronic structure may be significantly modified by electrochemical processes, sometimes designated as doping processes, which involve the oxidation (removal of n electrons) or the reduction (addition of n electrons) of the polymer chain. Typical examples are the heterocyclic polymers, such as polypyrrole, polythiophene and their derivatives, and the polyanilines. 

P. Snauwaert, R. Lazzaroni, J. Riga, J. J. Verbist, Electronic-structure of polyanilines - an XPS study of electrochemically prepared compounds, Synthetic Metals 1986, 16, 245. 

Figure 8 shows UV-vis-NIR spectra of thin films of HCSA fully doped polyaniline emeraldine salt that were spin coated on quartz plates from solutions in chloroform and m-cresol, respectively. As discussed previously, different polymer conformations are responsible for these two totally different UV-vis-NlR spectra. Figure 8a indicates a random coil conformation for the polymer chains the three distinctive absorption peaks at 360,440, and 780 nm are consistent with an electronic structure... 

Let us suppose an infinite nondegenerate polymer chain (e.g., polythiophene) doped heavily with electron acceptors. At a high dopant content, the polymer-chain structure and electronic structure of the doped polymer are radically different from those of the intact polymer. As typical cases, we will describe two kinds of lattice structures of doped polythiophene (dopant content, 25 mole% per thiophene ring) a polaron lattice and a bipolaron lattice. They are the regular infinite arrays of polarons and bipolarons. The schematic polymer-chain structures are shown in Figure 4-16. Band-structure calculations have been performed for polaron and/or bipolaron lattices of poly(p-phenylene) [124], polypyrrole [124], polyaniline [125], polythiophene [124, 126], and poly( p-phenylenevinylene) [127], with the valence-effective Hamiltonian pseudopotential method on the basis of geometries obtained by MO methods. The schematic electronic band structures shown in Figure 4-17... 

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