Conversion of Polyaniline from Insulating to Conducting State in

5618

J. Phys. Chem. B 2001, 105, 5618-5625

Conversion of Polyaniline from Insulating to Conducting State in Aqueous Viologen Solutions S. W. Ng, K. G. Neoh,* J. T. Sampanthar, and E. T. Kang Department of Chemical and EnVironmental Engineering, National UniVersity of Singapore, Kent Ridge, Singapore 119260

K. L. Tan Department of Physics, National UniVersity of Singapore, Kent Ridge, Singapore 119260 ReceiVed: July 26, 2000; In Final Form: March 26, 2001

The interactions of polyaniline coatings on low-density polyethylene films with aqueous viologen solutions were investigated. The coatings were characterized by X-ray photoelectron spectroscopy, electron spin spectroscopy, UV-visible absorption spectroscopy and sheet resistance measurements. The results showed that the polyaniline coating can be converted from the insulating base form to the electrically conducting salt form after reaction with the viologen solutions. The reaction rate is dependent on the intrinsic oxidation state of the polyaniline, the type and concentration of the viologen, and the reaction temperature. It is proposed that the conversion of the polyaniline to a conductive state occurs via an oxidative doping mechanism whereby electrons are transferred to the viologen dications. In the proposed mechanism, dissolved oxygen is deemed to play an important role, and this was verified experimentally.

Introduction 1862,1

polyaniline (PANI) is among the Reported first in oldest synthetic polymers. Interest in PANI increased in the mid 1980s, when studies revealed unusual chemical, electrical, and optical properties associated with the polymer in both the insulating and conducting forms. The attractive properties of PANI include controllable electrical conductivity, comparatively lower density than metals, good environmental stability,and ease of preparation from common chemicals.2 Doping of PANI has, until now, relied strongly upon the protonic acid doping of its 50% intrinsically oxidized form, the emeraldine base (EB). The exposure of EB to a protonic acid, such as HCl, causes its transformation to the electrically conductive emeraldine salt (ES). This process is highly pH dependent and in order to achieve a substantial increase in conductivity, an acid solution of pH less than 4 is necessary.3 This doping condition in turn presents a harsh and corrosive environment for industrial applications. Increase in electrical conductivity of PANI through electron transfer to organic electron acceptors offer an alternative route for doping. The interactions of PANI with organic electron acceptors, such as tetrachloro-o-benzoquinone and 2,3-dichloro-5,6-dicyano-p-benzoquinone, have been explored.4 These reactions, however, require the use of organic solvents, which, again may not be favorable for processes where PANI coatings are formed on substrates susceptible to such solvents. In hope of finding a milder environment for doping, we explored the use of viologen, a well-known redox agent,5 for converting PANI from an insulating to a conducting state. In this article, the interactions of thin PANI coatings with aqueous solutions of viologens are studied using UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy * To whom correspondence should be addressed. Tel: +65 8742186. Fax: +65 7791936. E-mail: [email protected]

(XPS), electron spin resonance (ESR), and sheet resistance (Rs) measurements. The experimental results show that a conversion of PANI from the insulating to the conducting state can be achieved in aqueous viologen solutions in the near-neutral pH range. Experimental Section Sample Preparation. Commercial low-density polyethylene (LDPE) films (from Goodfellow Inc., UK) of 0.125 mm in thickness were washed in acetone in an ultrasonic bath to remove surface impurities. The cleaned sheets were dried under reduced pressure and cut into 2 × 5 cm strips. These films were treated with oxygen plasma on both sides in an Anatech SP100 plasma system, at an oxygen pressure of 0.6 Torr for 60s. The plasma-treated films were further exposed to air for approximately 5 min to facilitate the formation of surface oxide and peroxide groups.6 These groups enhance the attachment of the PANI onto the surface of the LDPE. The plasma-pretreated films were immersed in a polymerizing mixture containing 0.025 M (NH4)2S2O8 and 0.1 M aniline in 1 M HClO4 for 2 h. The PANI coated LDPE films were then undoped in 0.5 M NaOH for 1 h to obtain the EB thin films. PANI of intrinsic oxidation state higher than 50% was prepared by reacting the EB thin films in 0.01 M (NH4)2S2O8 for 10 min. These samples will be denoted as nigraniline (NA) because the XPS analysis shows that about 70% of the nitrogen exists as imine nitrogen (see Results and Discussions). On the other hand, the fully reduced form of PANI, the leucoemeraldine base (LB), was prepared by treating EB with 50 vol % hydrazine in deionized water for 24 h. Benzyl viologen dichloride (BV2+ 2Cl-) was synthesized, according to the method reported in the literature,7 by reacting 1.58 g (0.01 mol) of 4,4′-bipyridine (from Aldrich) and 3 mL (0.024 mol) of benzyl chloride (from BDH) in 30 mL of ethanol

10.1021/jp002669y CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001

Conversion of Polyaniline for 7 h, at 65 °C. About 30 mL of ether was added to the reaction mixture after cooling the mixture to room temperature. The solution was then cooled to below 5 °C to induce the precipitation of viologen. The precipitate was filtered, washed with ethanol, and dried under reduced pressure. Elemental analysis of the product gave a C/N ratio of 11.6 (theoretical value ) 12) and a C/Cl ratio of 11.8 (theoretical value ) 12). Poly(butyl viologen dibromide) was synthesized using butyl dibromide and 4,4′-bipyridine as previously reported in the literature,8 whereas ethyl viologen dibromide was obtained from Aldrich. Aqueous viologen solutions of concentrations ranging from 0.01 to 0.15 M were prepared by dissolving appropriate amounts of salt in deionized water. The interactions of the viologens with thin PANI coatings were carried out at different solution temperatures, concentrations, as well as in the presence and absence of degassing. Degassing was achieved by bubbling argon vigorously into the viologen solution in a test tube for a minimum period of 30 min. PANI films were then placed into the solutions before the tubes were stoppered and sealed. Sample Characterization. UV-visible absorption spectra measurements were carried out on a Shimadzu UV-3101 PC scanning spectrophotometer, with pristine LDPE films as reference. Sheet resistances were monitored using the standard two-probe method. XPS measurements were carried out on a VG ESCALAB MkII spectrometer with a Mg KR X-ray source (1253.6 eV photons). The X-ray power supply was at 12 kV and 10 mA. Pressure in the analysis chamber was maintained at 10-8 mbar or lower during each measurement. All core-level spectra were referenced to the C 1s hydrocarbon peak at 284.6 eV. In peak synthesis, the line width (full width at halfmaximum, or fwhm) was maintained constant for all components in a particular spectrum. The peak area ratios for the various elements were corrected using the experimentally determined instrumental sensitivity factors and is accurate to within (10%. Electron spin resonance (ESR) measurements were carried out on a Skokie 8400 spectrometer, using a modulation amplitude of 500mG, a power rating of 10dB and a signal phase of 0 degree. Elemental (C, H, N) analysis was done on a Perkin-Elmer PE2400 CHN Elemental analyzer, whereas halogen determination was done using the Scho¨niger Combustion Method. Results and Discussion Interaction of Emeraldine Base with BV2+ 2Cl-. The UVvisible absorption spectra of a thin EB coating on LDPE and the corresponding 0.15 M HCl-doped salt (ES) are shown in Figure 1a. The spectrum of EB shows absorption peaks at around 320 nm and 620 nm, attributed to the π-π* transition and the exciton-like transition from the benzenoid rings to the quinoid rings respectively.9,10 Upon protonation of EB, the peak at 620 nm disappears and new absorption bands at 430 nm and in the near-IR region arise due to the formation of a new electronic structure with positive charges on the polymer chain.11 The very substantial decrease in the intensity of the π-π* transition is also consistent with the metallic behavior of ES.9 When an EB coating is placed in an aqueous solution of BV2+ 2Cl- (0.02 M) in air, a change in the color of the coating from blue to green is clearly visible. The UV-visible spectra of the EB coating after different periods of immersion in the BV2+ 2Cl- solution are shown in Figure 1b. A comparison of Figure 1, parts a and b, shows that the treatment of EB with BV2+ 2Cl- results in rather similar spectral changes as those observed in acid doping of EB. In both cases, the presence of the long

J. Phys. Chem. B, Vol. 105, No. 24, 2001 5619

Figure 1. UV-visible absorption spectra of (a) EB and ES (treated with 0.15 M HCl for 30 min), (b) EB treated with 0.02 M BV2+ 2Clat 25 °C in air, for various periods of time.

absorption tail extending into the near-IR region indicates a predominance of intrachain free carrier excitations.9 Sheet resistance measurements further confirm the conversion of the EB film from the insulating state (Rs on the order of 1010Ω/sq) to the conducting state (Rs on the order of 106Ω/sq) within the first hour of reaction. Prolonged treatment (48 h) resulted in a Rs on the order of 105Ω/sq. In comparison, the Rs of EB after 30 min of treatment with 0.15 M HCl is about 103Ω/sq. The lower conductivity of the EB film after treatment with 0.02 M BV2+ 2Cl- as compared to the film protonated by 0.15 M HCl is consistent with the observation that the π-π* transition band (at 320 nm) of the viologen-treated samples remains prominent in Figure 1b, whereas it is much reduced in Figure 1a. The effect of viologen concentration on the conductivity of the treated films will be discussed in a later section. In the protonation of EB by acids, it has been shown that the imine nitrogens are preferentially protonated.12 XPS analysis of the viologen-treated sample in air shows that preferential doping of the imine nitrogens also occurs in this case. A comparison of the N 1s core-level spectra in Figure 2, parts a and b, shows a decrease in the proportion of imine nitrogen (peak component at the binding energy of about 398.2 eV13), and a simultaneous increase in the positively charged nitrogens (at binding energies greater than 400 eV13) after the exposure of the EB coating to 0.02 M BV2+ 2Cl- for 30 min. The proportions of the different nitrogen species as determined by XPS are given in Table 1. The results given in Table 1 were obtained by averaging the results from different samples prepared under the same conditions, and the reproducibility is within 25%. The presence of Cl species in the viologen-treated EB film is readily ascertained from the presence of the Cl 2p core-level spectrum which consists of a spin-orbit split doublet (Cl 2p3/2 and Cl 2p1/2 components) with peaks at 197 eV and 198.6 eV,14 attributable to Cl- anions (Figure 3).

5620 J. Phys. Chem. B, Vol. 105, No. 24, 2001

Ng et al.

TABLE 1: Surface Composition of PANI in Different Oxidation States before and after Treatment with BV2+ 2Cl- at 25 °C sample EB V-EB1 V-EB2 LB V-LB NA V-NA

treatment conditions EB in non-degassed 0.02M BV2+ 2Cl- for 30 min EB in degassed 0.10M BV2+ 2Cl- for 5 min LB in non-degassed 0.02M BV2+ 2Cl- for 72 h NA in non-degassed 0.02M BV2+ 2Cl- for 120 h

sNd /N

sNHs/N

sN+s/N

Cl-/N+

0.48 0.15 0.17 0.05 0.24 0.69 0.27

0.40 0.46 0.46 0.76 0.42 0.13 0.36

0.12 0.39 0.37 0.19 0.34 0.18 0.37

0.32 0.89 0.29 0.22

Figure 4. ESR spectrum of EB treated with 0.15 M BV2+ 2Cl- at 25 °C in air, for various periods of time. Figure 2. XPS N 1s core-level spectra of (a) pristine EB, (b) EB treated with 0.02 M BV2+ 2Cl- at 25 °C for 30 min, (c) pristine LB, (d) LB treated with 0.02 M BV2+ 2Cl- at 25 °C for 72 h, (e) pristine NA, and (f) NA treated with 0.02 M BV2+ 2Cl- at 25 °C for 120 h. The experiments were carried out in air.

Figure 3. Cl 2p core-level spectrum of EB treated with 0.02 M BV2+ 2Cl- at 25 °C in air for 1 h.

As the pH of the BV2+ 2Cl- solutions used in this study varies from 5 to 6, it is unlikely that protonation of EB, as in acid doping,3 can occur to a significant extent in the viologen solutions to account for the large change in conductivity of the PANI base. Our proposed mechanism therefore centers on the oxidative doping of EB. Emeraldine can be expected to be an effective electron donor due to the pair of lone electrons on the nitrogen,15 whereas viologens have been known to be excellent

electron acceptors.16 The mixing of these two organic molecules may then result in electron transfer from emeraldine to viologen. From the XPS and UV-visible absorption spectroscopy results, it appears that the imine nitrogens in EB are oxidized by the transfer of an electron to the viologen dications, BV2+, forming positive polarons. The viologen in turn, changes to the monocation radical, BV+•, and the Cl- anion initially associated with the viologen dication is transferred onto the EB backbone, serving as counterion to the positively charged nitrogen. The possible mechanism proposed is shown in Scheme 1 (not in stoichiometric proportions). Because the reaction of BV+• with molecular oxygen is particularly fast,17 this reaction (Step 2 in Scheme 1) can convert the radical cation back to the dication, thus regenerating the dopant molecule: BV+• + O2 f BV2+ + O2-. Incidentally, viologen radical cations have also been known to react with O2- to produce a further reduced product of oxygen.17 It can be seen from Table 1 that the Cl-/N+ ratio of the viologen-treated EB is substantially less than 1. Therefore, in order for charge neutrality to be maintained, the oxygencontaining anion must have also contributed to the pool of counterions. In Scheme 1, X- denotes Cl- initially, but will also represent O2- and other oxide anions as the reaction progresses. The importance of the role of O2 is discussed in a later section. ESR studies of the thin EB coatings exposed to 0.15 M BV2+ 2Cl- solution in air show an initial increase in spin intensity and narrowing of line width (Figure 4). These two features correspond to the formation of paramagnetic centers and their increasing mobility,18 thus confirming the formation of polarons as charge carriers in viologen-treated EB. The signal intensity reaches a maximum at 20 min, after which further doping results in the decrease in spin intensity. This phenomenon can be

Conversion of Polyaniline


SCHEME 1

Figure 5. Sheet resistance (Rs) of EB after treatment with different concentrations of BV2+ 2Cl- at 25 °C in air.

explained in terms of the relative stability of polarons and bipolarons.19,20 At low levels of doping, the spins or polarons do not interact. The charge carriers are pinned via strong Coulombic interaction with their associated anions. At higher doping levels, the concentration of charge defects is high. The polarons are no longer associated with specific counterions but are subjected to the mean field of a number of surrounding counterions. The interactions between polarons result, possibly, in the formation of a more favorable, spinless bipolaron lattice and, hence, the decrease in spin intensity. This decrease in intensity is not accompanied by a decrease in conductivity of the film. Effect of Reaction Conditions. The rate of doping of EB in BV2+ 2Cl- solution was found to vary with viologen concentration and temperature. Figure 5 shows the change in Rs with time of EB immersed in different concentrations of BV2+ 2Clat 25 °C in air. All curves show the same trend of an initially

steep gradient after which the curves gradually approach an asymptotic value. The steep initial gradient indicates rapid doping, and the slowing down of the doping process with time can be explained in terms of the decrease in imine and viologen concentrations as the reaction proceeds. Alternatively, the electrostatic repulsion between the viologen dication and the increasingly charged PANI may also contribute to the decrease in doping rates. The observed increase in the reaction rate with increase in BV2+ 2Cl- concentration is consistent with the rate law observed in most chemical reactions. XPS analysis of the viologen-treated EB samples shows that the more conductive samples possess a higher N+/N ratio. The N+/N ratio of the EB film treated with 0.15 M BV2+ 2Cl- for 10 min is 0.44, and the Rs is 6 × 104Ω/sq, which is 1 order of magnitude higher than that observed with EB doped in 0.15 M HCl. For a BV2+ 2Cl- solution of 0.02 M, the reaction rates, as indicated by the decrease in Rs of the EB films, were seen to increase significantly when the solution temperature was increased from 0 °C to 25 °C, but between 25 °C and 70 °C, the differences in rates were minimal (Figure 6). In all cases, except at 0 °C, the largest drop in Rs occurs within the first 5 to 10 min of treatment, after which the rates decrease. On the other hand, at 0 °C, a sharp drop in Rs is observed only after 30 min of exposure to BV2+ 2Cl-. After 1h, the Rs of the EB film treated with BV2+ 2Cl- at 0 °C remains at 108Ω/sq, whereas at other temperatures, Rs has decreased to between 2 × 106Ω/sq to 6 × 106Ω/sq. In addition to BV2+ 2Cl-, two other types of viologen, poly(butyl viologen dibromide) and ethyl viologen dibromide were tested. At a concentration of 0.05 M and a treatment time of 30 min, the decrease in Rs of PANI treated with the latter is similar to that observed with BV2+ 2Cl- under similar conditions, whereas in the case of poly(butyl viologen dibromide), the corresponding Rs is higher by a factor of 2. Effect of PANI Oxidation State. BV2+ 2Cl- was found to react with the fully reduced LB as well as NA, in addition to the 50% oxidized EB. A thin coating of LB changed from colorless to green when placed in an aqueous solution of BV2+ 2Cl-, whereas that of NA changed from purple to green.


Ng et al. which give rise to the increase in absorbance at 620 nm. Likewise, the XPS N 1s spectrum of LB after viologen treatment shows an increase in the proportions of imine as well as positively charged nitrogens, with a corresponding decrease in amine nitrogens as compared to those of pristine LB (compare Figure 2, parts c and d, and values in Table 1). The Cl 2p corelevel spectrum of the viologen-treated LB can be deconvoluted in a similar manner as Figure 3 and the Cl-/N+ ratio is also rather similar to that of viologen-treated EB (Table 1). Although small amounts of imine could have resulted from the often-observed auto oxidation of LB in air,9,21 it is highly likely that the reaction of LB with viologen dications may also have contributed to imine formation. A possible mechanism may involve the one electron oxidation of an amine nitrogen by BV2+, resulting in a positively charged amine, which in turn deprotonates in the nonacidic viologen solution to form an imine nitrogen as shown below:

Figure 6. Sheet resistance (Rs) of EB after treatment with 0.02 M BV2+ 2Cl- at different temperatures in air.

Figure 7. UV-visible absorption spectra of LB treated with 0.02 M BV2+ 2Cl- at 25 °C in air, for various periods of time.

However, for the same concentration of BV2+ 2Cl- and similar reaction conditions, the doping rates of LB and NA were observed to be significantly slower than that of EB. (a) Leucoemeraldine. The UV-visible absorption spectrum of LB consists of a single absorption peak at approximately 320 nm, consistent with that expected for a fully reduced PANI9 (Figure 7). The fully reduced nature of the LB samples is supported by the XPS N 1s core-level spectrum which shows the presence of only a very small amount of surface imine units (Figure 2c). When the LB coating was treated with 0.02 M BV2+ 2Cl- in air, an increase in the absorption of the 620 nm band is observed in the UV spectrum after 6 h (Figure 7). This increase is accompanied by the emergence of the absorption tail beyond 800 nm and the 430 nm peak. After 72 h, the absorption spectrum becomes similar to that of EB after treatment with BV2+ 2Cl-, with a well-defined peak at 430 nm and the absorption tail extending toward the IR region. These changes in the absorption spectrum of LB exposed to BV2+ 2Cl- thus indicate not only the creation of positive polarons, but also the initial formation of imine groups and quinoid rings

These imine groups can subsequently react, again with BV2+ to form the positive polarons responsible for the increase in conductivity of viologen-treated EB films as described previously. Because LB was not observed to convert completely to the EB structure prior to oxidative doping, the formation of imine groups and polarons is believed to have occurred concurrently, as indicated by the simultaneous increase in absorbance at 620 nm and the tail beyond 800 nm. As there can be a mix of positively charged amine nitrogens, imine nitrogens and the oxidized imine nitrogen at any one time, the colorless LB film was observed to turn green directly when placed in BV2+ 2Cl-, without going through a clear transition from colorless to blue (indicating EB), then green. The possible mechanism proposed for the doping of LB in BV2+ 2Cl- is summarized in Scheme 2 (not in stoichiometric proportions). As in Scheme 1, X- may denote Cl-, O2- or other oxide anions. The Rs of LB treated with 0.02 M BV2+ 2Cl- decreased from 1010Ω/sq to 9 × 105Ω/sq after 72 h of exposure. The final Rs of the viologen-treated LB film is approximately 1 order of magnitude higher than that of EB film similarly treated with 0.02 M BV2+ 2Cl- solution for the same period of time. (b) Nigraniline. The doping of PANI having intrinsic oxidation states higher than 50% has not been investigated extensively although theoretical analyses of the protonation and redox doping of pernigraniline were explored by Santos and Bredas.22,23 They predicted that polarons are remarkably stable upon the addition or removal of a single electron from the pernigraniline backbone. The redox doping of “pernigraniline” (it is unclear whether the PANI is actually fully oxidized because pernigraniline has been shown to be unstable in the dry state24) by I2 and FeCl3 were documented by Cao,25 whereas experimental protonation of the polymer in dilute aqueous acids were observed to yield the protonated emeraldine structure.25,26 This phenomenon may be explained in terms of hydrolysis reactions resulting in a reduction in the polymer chain length and formation of benzoquinones.24,27 The N 1s core-level spectrum of the NA film obtained by treating EB with (NH4)2S2O8 confirms that an oxidation state substantially higher than 50% has been achieved (Figure 2e and Table 1). The UV-visible absorption spectrum of this sample is also similar to that reported earlier for “pernigraniline” with an absorption peak at approximately 560 nm due to transitions from the valence to the conduction band, a second peak at 280

Conversion of Polyaniline


SCHEME 2

Figure 8. UV-visible absorption spectra of NA treated with 0.02 M BV2+ 2Cl- at 25 °C in air, for various periods of time.

nm attributed to π-π* transitions on the quinoid rings, and a third peak at 330 nm due to π-π* transitions on the benzenoid rings.25 The treatment of the NA film with 0.02 M BV2+ 2Clresults in a decrease in absorbance of the 280 nm and 560 nm peaks, and an increase in absorbance of the 430 nm peak and IR tail (Figure 8). These changes are similar to those observed when “pernigraniline” is doped with I2.25 As in the previous cases, the 430 nm absorbance and the IR-tail are attributed to the gap state arising from oxidative doping of PANI. After 4 days of treatment, the Rs of NA decreases from 8 × 109Ω/sq to 2 × 106Ω/sq, which is 1 order of magnitude higher than that of EB treated with viologen under the same conditions. The treatment of pernigraniline with HCl also results in an order of magnitude lower conductivity than that of similarly treated EB.26 The doping mechanism of NA is proposed to be similar to that

of EB, where the one-electron oxidation of imine groups leads to the formation of polarons. Interestingly, XPS analysis of NA treated with viologen for 5 days (Figure 2f) indicates not only an increase in positively charged nitrogens, but also the formation of amine groups at (peak at 399.4 eV13). Thus, it is likely that the doping of NA in BV2+ 2Cl- solution, like the protonation of NA in dilute aqueous acids, is accompanied by the reduction of some imine groups to amine groups. While the actual reaction is not clear, the reduction of imine to amine nitrogens may have involved the viologen radical cations produced during oxidation of NA, and water molecules associated with the imine nitrogens through hydrogen bonding. As PANI in the three oxidation states studied has different proportions of amine and imine groups, it is possible that the three reactions proposed so far (the oxidation of imine to form polarons, oxidation of amine to imine and reduction of imine to amine) may actually occur in all 3 oxidation states, but to different extents, depending on the stability of each starting polymer and the end product. The XPS data seems to point toward a conversion of the less stable LB and NA to the emeraldine structure upon doping. The final forms of the viologen-treated polymers are similar in composition regardless of the intrinsic oxidation state of the starting polymer. Effect of Oxygen. The results discussed in the earlier sections were obtained with nondegassed viologen solutions in air. The rates of doping of PANI in all three oxidation states (EB, LB, and NA) were observed to decrease when interactions of the films were carried out in degassed viologen solutions of low concentrations. An EB coated LDPE film placed in 0.01 M BV2+ 2Cl- exposed to air reached a Rs value of 106Ω/sq in half an hour, whereas the Rs of an EB coating of similar thickness remained above 108Ω/sq when placed in a degassed solution of 0.01 M BV2+ 2Cl- for the same period of time. The difference in rates is visually observable and is further indicated by the slower rise of the IR tail in the absorption spectra of EB placed in the degassed solution (Figure 9a).


Ng et al. to a doped state.29 Thus, the importance of O2 in the doping of LB is further confirmed. From the proposed mechanisms in Schemes 1 and 2, the conversion of dissolved oxygen to O2and possibly other oxygen-containing anions could supplement the Cl- counterions. This is supported by the XPS data of the viologen-treated EB, LB and NA in Table 1 (Samples V-EB1, V-LB, V-NA) which indicates that in all three cases the Clto N+ ratio is significantly less than 1. Hence, to maintain charge neutrality, other anion species must also serve as counterions to the N+ sites of the polymer backbone. It should be noted that while a diminished oxygen concentration retards the reaction between BV2+ 2Cl- and PANI at low viologen concentrations (BV2+ 2Cl- to PANI ratio of approximately less than 2), the reaction can still take place rapidly at sufficiently high viologen concentrations (BV2+ 2Cl- to PANI ratio of greater than 6) since the lack of “recycled” BV2+ no longer has much effect due to the high BV2+ concentration in the solution. The XPS analysis of an EB coating placed in a degassed solution of 0.1 M BV2+ 2Cl- for 5 min (Sample V-EB2 in Table 1) shows the formation of positively charged nitrogens and the decrease in imine proportions similar to the case of EB placed in a more dilute nondegassed viologen solution (Sample V-EB1 of Table 1). However, the Cl-/N+ ratio of the former, at 0.89, is much closer to 1, thus confirming that charge neutrality is maintained mainly by the Cl- anions from BV2+ 2Cl- and oxygen does not play a major role in the doping process in this case. Conclusion

Figure 9. UV-visible absorption spectra of (a) EB after treatment with 0.01 M BV2+ 2Cl- at 25 °C, for 30 min, (b) NA after treatment with 0.02 M BV2+ 2Cl- at 25 °C, for 24 h, and (c) LB after treatment with 0.02 M BV2+ 2Cl- at 25 °C, for 72 h, under degassed and nondegassed conditions.

In the degassed solution, the amount of dissolved oxygen would be much reduced, although it may not be entirely eliminated. In the second step of our proposed mechanisms in Schemes 1 and 2, the viologen radical cation, BV+•, formed through the one-electron oxidation of the PANI film, requires an oxygen molecule to revert back to the dication structure. This dication can then be cycled again through another redox reaction with the PANI film. When the dissolved oxygen level is reduced, the cycling of viologen may proceed to a limited extent while depleting the remaining oxygen. However, this reaction would soon terminate and the BV+• may then dimerize to form a more stable dimer species in water.28 Thus, the doping of PANI will be retarded. The decrease in doping rate of NA when placed in degassed solutions of BV2+ 2Cl- is similar to that observed with EB (Figure 9b) but the absence of oxygen has the most pronounced effect in the doping of LB (Figure 9c). This is possibly due to the need for an extra step of BV2+ first reacting with the amine groups to form the imine groups which then undergo subsequent reaction with BV2+ to form positively charged nitrogen species, as can be seen in the reaction mechanism shown in Scheme 2. It is also interesting to note that when the LB-BV2+ system in the dry state is subjected to near UV-irradiation in the absence of O2, the reaction between LB and BV2+ is retarded and the 610 nm band attributable to BV+• (from the conversion of the viologen dications to the radical cations) can be clearly detected.29 On the other hand, when O2 is present, the irradiated dry LBBV2+ system shows a decrease in Rs and the LB is converted

The conversion of PANI from the insulating to the conducting state was successfully carried out by treatment of the base films in various oxidation states with aqueous BV2+ 2Cl- solutions, which are in the near-neutral pH range. Under such conditions, the conventional protonation of the PANI base forms is not known to result in a substantial increase in conductivity. The reaction mechanism is proposed to involve the transfer of electrons from PANI to the viologen dications resulting in the oxidative doping of PANI. Thus, the increase in conductivity results from a change in the total number of electrons associated with the PANI backbone, as opposed to the structural changes due to protonation. The rates of doping of the PANI depend on the temperature, concentration and type of viologen, the oxygen content in the solutions as well as the oxidation state of the PANI film. Acknowledgment. The authors wish to acknowledge the support provided by the National University of Singapore for this work and the assistance of Assoc. Professor N. Kocherginsky in the ESR measurements. References and Notes (1) Letheby, H. J. Chem. Soc. 1862, 15, 161. (2) Conjugated Polymers and Related Materials: The Interconnection of Chemical and Electronic Structure (Proceedings of the Eighty-first Nobel Symposium); Salaneck, W. R., Lunstrom, I., Ranby, B., Eds.; Oxford University Press: New York, 1993; Chapter 6. (3) Chiang, J. C.; MacDiarmid, A. G. Synt. Met. 1986, 13 , 193. (4) Khor, S. H.; Neoh, K. G.; Kang, E. T. J. Appl. Polym. Sci. 1990, 40, 2015. (5) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,4′-Bipyridine; John Wiley & Sons, Ltd.: Chichester, 1998. (6) Suzuki, M.; Kashida, A.; Iwata, H.; Ikada, Y. Macromoecules 1986, 19, 1804. (7) Malinauskas, A.; Holze, R. J. Electroanal. Chem. 1999, 461, 184. (8) Liu, F. T.; Yu, X. D.; Li, S. J. Polym. Sci.: Polym. Chem. Ed. 1994, 32, 1043.

Conversion of Polyaniline (9) Cao, Y.; Smith, P.; Heeger, A. J. Synt. Met. 1989, 32, 263. (10) McCall, R. P.; Ginder, J. M.; Leng, J. M.; Ye, H. J.; Manohar, S. K.; Masters, J. G.; Asturias, G. E.; MacDiarmid, A. G.; Epstein, A. J. Phys. ReV. B. 1990, 41, 5202. (11) Stafstrom, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.; Huang, W. S.; MacDiarmid, A. G. Phys. ReV. Lett. 1987, 59, 1464. (12) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F. Synt. Met. 1987, 18, 285. (13) Kang, E. T.; Neoh, K. G.; Khor, S. H.; Tan, K. L.; Tan, B. T. G. J. Chem. Soc., Chem. Commun. 1989, 695. (14) Moulder, J. F.; Stickle, W. F.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer Corporation: Minnesota, 1992; p 62-63. (15) Sein, L. T., Jr.; Wei, Y.; Jansen, S. A. Synt. Met. 2000, 108, 101. (16) White, B. G. Trans. Faraday Soc. 1969, 65, 2000. (17) Levey, G.; Ebbesen, T. W. J. Phys. Chem. 1983, 87, 829. (18) Assenheim, H. M. Introduction to E. S. R., Hilger & Watts Ltd.: London, 1966.

J. Phys. Chem. B, Vol. 105, No. 24, 2001 5625 (19) Shimoi, Y.; Abe, S. Phys. ReV. B: Cond. Matt. 1994, 50, 14781. (20) Focke, W. W.; Wnek, G. E. J. Electroanal. Chem. 1988, 256, 343. (21) Kang, E. T.; Neoh, K. G.; Tan, T. C.; Khor, S. H.; Tan, K. L. Macromolecules 1990, 23, 2918. (22) Santos, M. C.; Bredas, J. L. Phys. ReV. B 1989, 40, 11 997. (23) Santos, M. C.; Bredas, J. L. Phys. ReV. Lett. 1989, 62, 2499. (24) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 227. (25) Cao, Y. Synt. Met. 1990, 35, 319. (26) MacDiarmid, A. G.; Manohar, S. K.; Masters, J. G.; Sun, Y.; Weiss, H.; Epstein, A. J. Synt. Met. 1991, 41, 621. (27) Angelopoulous, M.; Asturia, G. E.; Ermer, S. P.; Ray, A.; Scherr, E. M.; MacDiarmid, A. G.; Akhtar, M.; Kiss, Z.; Epstein, A. J. Mol. Cryst. Liq. Cryst. 1988, 160, 151. (28) Rosseinsky, D. R.; Monk, P. M. S. J. Chem. Soc. Faraday Trans. 1990, 86, 3597. (29) Ng, S. W.; Neoh, K. G.; Sampanthar, J. T.; Kang, E. T., manuscript under preparation.

Conversion of Polyaniline from Insulating to Conducting State in

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