Emeraldine base salt from protonation

Emeraldine Salt from Protonation of Emeraldine Base. 

FIG. 11.19 Oxidative doping (p-doping) of leuco-emeraldine base and protonic acid doping of emeraldine base, leading to the same final product, emeraldine salt. Reproduced from Fig. 34.3 in Mark (1996). Courtesy Springer Verlag. 

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]. 

Polyaniline provides the prototypical example of a chemically distinct doping mechanism [33,34], Protonation by acid-base chemistry leads to an internal redox reaction and the conversion from semiconductor (the emeraldine base) to metal (the emeraldine salt). The doping mechanism is shown schematically in Fig. II-2. The chemical structure of the semiconducting emeraldine base form of polyaniline is that of an alternating copolymer, denoted as [(1A)(2A)] , with... 

Polyaniline is a mixture of benzenoid and quinoid (p-phenyleneamineimine) entities. The neutral compound, emeraldine base is an equal mixture of amine and imine functionalities. Treatment with a non-oxidising acid results in protonation of the imine groups to give emeraldine salt. This causes positive charge to be transferred to the backbone and is the origin of the conductivity, which increases by 10 orders of magnitude from the base to the salt form. The polymer in the base and salt forms can be described as ... 

PolyanUine (PANI) and its derivates represent a very important class of conducting polymers, in which the electronic structure can be modulated by changing the oxidation and/or protonation levels [5]. The half-oxidized forms of PANI have more environmental stability than the others, and can be changed from the semiconductor state (called the emeraldine base, PANI-EB, see Scheme 8.2 for y = 0.5), to the highly conducting form (called the emeraldine salt, PANI-ES, see Scheme 8.1) just by changing the protonation degree of the polymeric backbone [5]. 

The terms leucoemeraldine , "emeraldine and "pernigraniline , used in the following discussion will refer to the different average oxidation states of the polymer where y = 1, 0.5 and 0 respectively, either in the base form, e.g. emeraldine base or in the protonated salt form, e.g. emeraldine hydrochloride.2/3 it seems highly likely that the true average emeraldine oxidation state/ where y is exactly equal to 0.5, may never have been synthesized from aniline. 

In addition to acids, polyaniline also responds well to bases. Figure 2b shows the response of the emeraldine salt form of polyaniline to anunonia. The resistance increases because polyaniline undergoes a transition from the emeraldine salt (conducting form) to the emeraldine base (insulating form). This conversion leads to a resistance increase of about two orders of magnitude for the nanofiber film. This response is much smaller and slower than the response to acid because of a different mechanism associated with each reaction. Upon exposure to base, a deprotonation step occurs removing the acid. This step takes longer than protonation so the response is slower. The deprotonation step is likely slower because, upon exposure of the emeraldine salt form to... 

Self-doping was eonfirmed by the similarity between absorption speetra of the sulfonated polyaniline and the emeraldine hydrochloride form (Figure 20.44). The effect of the sulfonate group on steric interactions between adjacent rings is evident from the blue shift in the absorption spectra of the sodium salt of the non-protonated sulfonated derivative compared to the emeraldine base (Figure 20.45). 

In this section, the reflectance of metallic PANI-CSA is compared with that of the semiconducting polyaniline (emeraldine base) and the conventional emeraldine salt, PANI protonated with H2SO4 (PANI-H2SO4), which is classified as a typical insulating-regime material in a doped polyaniline system [1161]. In particular, in order to better understand the role of disorder in the conducting emeraldine salt, the discussion focuses on a comparison of the data obtained from PANI-H2SO4 with those obtained from PANI-CSA [1161]. 

The chemical preparation of the emeraldine base form of polyaniline and its protonation to the salt form is described in MacDiarmid et al [20]. Materials were prepared both in the powder form and as films cast from solution. The magnetic susceptibility was measured via the Faraday technique [30] and electron spin resonance. Measurement of the temperature and protonation dependence of the magnetic susceptibility provides a direct probe of the development of a nonzero density of states at the Fermi energy and, hence, a metallic ground state [31]. The temperature dependent magnetic susceptibility, of selected samples of emeraldine as a function of... 

The emeraldine salt, (II-7), can also be obtained directly from leucoemeraldine by charge transfer doping, with reactions analogous to (II-1) and (II-3). Thus, the doping of polyaniline has been described as two-dimensional in a space where one axis describes the charge transfer chemistry and the second, orthogonal axis describes the acid/base (protonation) chemistry [37]. 

There are many different forms of polyaniline (PANi) that exist based on their oxidation states ranging from fully reduced to the fully oxidized. These oxidation states are affected by pH and are interchangeable from the protonation and/or oxidation reactions. It is reported that the most conductive form among various redox forms is the emeraldine salt as shown in Fig. 2 [67]. 

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