Pernigraniline salt

Protonation of PB causes a violet-to-blue color change due to the formation of pernigraniline salt (PS). This color change is associated with the loss of the PB band at 530 nm and the appearance of a strong PS peak at ca. 700 nm. 

In situ polymerization of aniline is generally carried out in aqueous (NH4)2S208/HC1. The glass support is typically removed from the reaction mixture during the polymerization at the stage when a blue/violet film of pernigraniline salt has formed on its surface. This film is then reduced to the green ES product by reaction with a separate... 

The most electrically conductive state is the emeraldine salt (ES) that is between pernigraniline salt (Pas) at the high potential side and the leuco-emeraldine salt (LS) at the low potential side. The LS and the Pas are the most reduced and oxidised states, respectively, and have been found to be insulating. In an oxidation process from the LS to the ES, two electrons are withdrawn and two chloride ions are doped for every four benzene units. For the oxidation from the ES to the Pas, two electrons are withdrawn and two protons are released. The LS/ ES reaction is reversible, however, the hydrolysis occurs at the higher oxidised Pas states. 

G. Daprano, M. Leclerc, G. Zotti, Stabilization and characterization of pernigraniline salt - the acid-doped form of fully oxidized polyanilines, Macromolecules 1992, 25, 2145. 

Modern methods of manufacturing this probably oldest representative among synthetic organic pigments involve dissolving aniline in strong sulfuric acid. Oxidation is achieved with sodium dichromate in the presence of a copper salt or one of the above-mentioned oxidation catalysts. Oxidation with sodium chlorate initially affords an indamine polymer (pernigraniline) ... 

When 0 < y < 1 these structures are the poly(p-phenyleneamineimines), in which the oxidation state of the polymer increases with increasing content of the imine form. The fully reduced form (y = 1) is leucoemeraldine , the fully oxidized form (y = 0) is pernigraniline , and the 50% oxidized structure (y = 0.5) is emeraldine . Each structure can exist as die base or as its salt, formed by protonation, so that we can envisage four repeat units in the polymer chain, in amounts which depend on the extent of both oxidation and protonation of the structure (Fig. 5). 

PANI is unique in that its most oxidized state, the pernigraniline form (which can be accessed reversibly), is not conducting. In fact, it is the intermediately oxidized emeraldine base that exhibits the highest electrical conductivity. Protonic Acid Doping is the most general means by which to obtain this partially pro-tonated form of PANI [301]. Exposure of the emeraldine salt to alkali solutions reverses this process and brings a return to the insulating state. 

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. 

Figure 6.16 Four different redox forms of PANA (a) leucoemeraldine base (fully reduced form), (b) emeraldine base (half-oxidized form), (c) conducting emeraldine salt (half-oxidized and protonated form), and (d) pernigraniline base (fully oxidized form).

The emeraldine salt of polyaniline which is one of the three different forms of polyaniline—leucoemeraldine (the fully reduced state], emeraldine (the half oxidized state] and pernigraniline (the fully oxidized state] (Fig. 8.1]—is electrically conductive while the others are insulators. Conventional chemical synthesis of polyaniline is based on an oxidative polymerization of aniline by using an oxidant in the presence of a strong acid dopant in the reaction mediunL ... 

FIGURE 15.10 Polyaniline square scheme showing redox (horizontal) and proton exchange (vertical) reactions. The colors of the films are indicated in parentheses. Only the emeraldine salt form is conductive anions, not shown, are required for charge balance. Also not shown are the fully oxidized (pernigraniline) forms. 

The emeraldine (Figure 18.14) base is the most stable form of PANI. If acid is added, emeraldine salt is obtained in a form that is electrically conducting. Leucoemeraldine and pernigraniline, on the other hand, are poor conductors. None of the systems are planar. This may be taken as proof that planarity, with delocalized K orbitals, is not necessary for high conductivity. 

Polyanilines formed by both the chemical and electrochemical processes have been extensively studied to establish structure-property relationships. In this section, the structural studies of polyaniline are reviewed and the influence of structure on properties is considered in Chapter 5. The description of polyaniline structure is complicated by its complexity. As described above, polyaniline can exist in six different forms (the salt or base forms of leucoemeraldine, emeraldine and pernigraniline). In addition, the proto-nated forms of polyaniline also have counter-anions intimately associated with the positively charged polyaniline chains. Finally, it has also been observed that polyanilines may contain considerable amounts of solvent molecules. 

Fig. 1. (a) Leucoemeraldine base (b) emeraldine base (c) pernigraniline base (d) polyCparaphenylene sulfide) le) emeraldine hydrochloride salt polymer (f) poly(orthotoluidine) (emeraldine base form) (g) sulfonated polyaniline (self-doped salt form). 

PANI is commonly prepared by chemical polymerization of aniline using (NH4)2S20s in hydrogen chloride solution (187) and/or by electrochemical polymerization (188). PANI can exist in four different oxidation states leucoemeral-dine (neutral form), pernigraniline (quinoid-like form), emeraldine base (intermediate form), and emeraldine salt (conductive form), as shown in Figure 7. PANI is prepared as the emeraldine salt and then treated with base to yield the emeraldine base form, which is soluble in common organic solvents. After film casting or... 

---