Emeraldine-base

The optical absorption of emeraldine base film, together with selected millisecond photoinduced absorption and long time photoinduced absorption results are shown in Fig. 7. The steady state (ms) spectrum of emeraldine base has features indicative of both the ring rotation polaron (at 0.9 eV and 2.9 eV) and the ring rotation polaron bound to imine groups (1.4 eV and 2.9 eV) as well as bleaching of the tt-tt band (3.7 eV) [43]. At long times only the trapped polaron is observed [17,27]. 

Recently picosecond photoinduced absorption studies of emeraldine base polymer in film and NMP solutions have been reported [48]. They show that for pumping at 2.0 eV and probing ant 2.9 eV a dispersive decay is obtained with more than 50% of the photoinduced signal for the thin film remaining after 5 ns. In contrast, only fast decay components ( 100 ps) were found in solution. These differences between solution and film dynamics are attributed to difTerences in the barriers to ring rotation and differences in interchain processes. In contrast to the luminescence observed from the exciton in LEB, no luminescence is observed for EB [24]. 

The optical absorbance of pernigraniline base. Fig. 8, though similar to that of EB is different in origin. In particular, the 2.3 eV absorption in PNB has been assigned to the excitation across a Peierls energy gap. This Peierls gap has been proposed to arise from two independent contributions, variation in bond length order parameter and variation in ring torsion 

Steady state photoinduced absorption experiments show the presence of both a %w energy (LE) pair of absorptions (at 1.1 eV) that is short lived [49,50], and a middle energy (ME) absorption (at 1.5 eV) that is long lived [40, 49, 50] Fig. 10, 11. Different photoinduced infrared active vibrations 

The emeraldine salt state can be obtained by equilibration of emeraldine base samples with protonic acid of appropriate pH or by p-doping (oxidation) of leucoemeraldine base [5]. When prepared in powder form, the resulting emeraldine salt is up to 50% crystalline [51, 52], though when prepared as films [6, 8] or as stretch oriented films [9,10] the resulting salt may be of lower crystallinity [11,52]. 

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Figure 5-19. N(ls) XPS core level spectra of emeraldine base adsorbed on ITO. The top most spectrum corresponds to ultra-thin Him (in the mono layer regime) while the bottom spectrum corresponds to thick film.

The experimental UPS spectra of the emeraldine base form of polyaniline is compared with VEH-derived DOVS in Figure 5-18 97. The DOVS were derived from the VEH band structure calculations shown at the bottom of Figure 5-18. 

The common form of polyaniline is a 1 1 combination of alternating reduced (A) and oxidized (C) units it is termed emeraldine (or emeraldine base). The emeraldine base is essentially non-conductive, but its conductivity increases by 9-10 orders of magnitude by treating with aqueous protonic acids. The conductive form of poly aniline can therefore be roughly depicted as a 1 1 combination of alternating A and D units. 

Assuming that the polymerisation does occur via a radical intermediate, then coupling is possible at all three positions on the ring (albeit with differing probabilities) and a number of alternative products to the emeraldine-based structure can be... 

The emeraldine base form of polyaniline may also react in non-aqueous electrolytes, such as a LiClOl -propylene carbonate solution, with the formation of the conductive emeraldine hydroperchlorate salt ... 

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. 

Results and Discussion. The 2-ethyl polyaniline concentration in the silica gel film was determined by constructing a Beer s law calibration curve from solutions of known concentration. Assuming an average molecular weight of 5000, the 2-Et PANi concentration in the silica gel was found to be 9.6 x 10 4 M. The refractive indices of CS2 and 2-Et PANi SiC>2 were estimated to be 1.6 and 1.4 at 1.06 im, respectively. The emeraldine base doped silica gel was found to have low losses due to scatter, and exhibited good transparency at 1.06 im. Spectrophotometric measurements at 1.06 fim yielded absorption coefficients of 0.1 cm-1 (> 99% T over 1 mm pathlength) for the CS2 reference and 4 cm 1 (96% T over 1 mm pathlength) for the 2-Et PANi doped silica film. 

Since the acid-base (precipitation) reaction takes place in non-aque-ous solution (isopropanol), a glass pH electrode could not be used to follow the titration. However, PANI is known to be pH sensitive as a result of the acid-base equilibrium between the emeraldine base (EB) and emeraldine salt (ES) forms of PANI [1-3]. Interestingly, the GC/ PANI electrode was found to give a reproducible response during the titrations despite the presence of the precipitate (trimeprazine tartrate) in the stirred solution. The same GC/PANI electrodes were used repeatedly for more than 2 months without any significant changes in the... 

Bis(diphenylphosphino)propane Differential scanning calorimetry Emeraldine base Enantiomeric excess... 

Figure 4.6 shows the schematic diagrams of ciclodextrins, polyaniline with emeraldine base, and inclusion complex formation of cyclodextrins and a conducting polymer chain insulated molecular wire. 

Scheme 1 Doping of PANI in its emeraldine base form with CSA...

FIG. 11.18 Conductivity of emeraldine base as a function of pH of the HCl dopant solution as it undergoes protonic acid doping ( ) and ( ) represent two independent series of experiments. From MacDiarmid, 2001. Courtesy John Wiley and Sons, Inc. 

Protonation by acid-base chemistry leads to an internal redox reaction (Fig. 11.19), without change of the number of electrons (Heeger, 2001 MacDiarmic, 2001). The semiconductor (emeraldine base, emeraldine salt, 100 S/cm). Complete protonation of the imine nitrogen atoms in emeraldine base by aqueous HC1 results in the formation of a delocalised polysemiquinone radical cation. This is accompanied by an increase in conductivity of more than 12 orders of magnitude. 

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. 

In the undoped state, PAni is a base. Three molecular structures are possible, one of them being the so-called emeraldine base (EB) shown in Fig. 1 of Chapter 11 [52]. There are several differences between it and the other CP chains discussed above Due to the presence of the N atoms, the chain has a zigzag shape and the benzene rings have either a benzene-like or a quinone-like bond pattern (see Chapter 11, Section IV.B.l) and may be twisted. In principle, the number of independent structural parameters is even larger than for the other CPs. However, quite a good (albeit partial) understanding of the structure has been achieved, as shown in Ref. 24, for instance. 

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 in the emeraldine base state doped with di(butoxyethoxyethyl) ester of sulphosuccinic acid had high film conductivity and an elongation at break of 195%. This high flexibility is particularly needed for elastomer coatings to impart elasticity on conductive materials. 

A mixture at 27°C consisting of freshly distilled aniline (0.1097 mol), 85 ml of 3M HCl, 95 ml of ethanol, and LiCl (16 g) was treated with ammonium persulphate (0.0274 mol), 60 ml of 2M HCl, and LiCl (8 g) also at 27°C. The mixture was reacted for roughly 2 hours while the potential of the reaction mixture was controlled by a standard calomel electrode. It was then treated with FeCL (0.0183 mol), LiCl (5 g), and 50 ml of 2M HCl. After an additional hour the reaction was terminated, and the polymer could be isolated by either filtration or by centrifuging. It was then washed with distilled water, dried, and converted to the emeraldine salt using 2M HCl. This salt was then converted to the emeraldine base by treatment with 2 liter of 0.3 M aqueous... 

TABLE 1. Effect of selected dopants on the film conductivity and elongation at break for polyaniline (emeraldine base). 

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... 

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. 

This material can also be formed upon "doping" the corresponding emeraldine base form of polyaniline with aqueous HCl, resulting in a large increase of the number of unpaired spins,probably as diaminobenzene radical cations.55 The similarity of the electronic properties of protonated phenyl... 

Alternative oxidants such as potassium iodate were also explored for the intrazeolite polymerization of aniline in NaY and acidic forms of Y zeolite. With peroxydisulfate, the polymerization proceeded only if a sufficient supply of intrazeolite protons was available. No polymer formed in either NaY or in acid zeolites with neutral iodate solution, but at low pH polyaniline was obtained in all hosts. The open nature of the zeolite host, even when partially filled with polymer, permits the introduction of base (such as ammonia). On admission of ammonia into the emeraldine salt-containing zeolite, the protonated polymer was converted into the neutral emeraldine base form. 

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