o-Phenylenediamine Electropolymerization by Cyclic Voltammetry

Aug 23, 2003 - Cyclic Voltammetry Combined with Electrospray. Ionization-Ion Trap Mass Spectrometry. Ilario Losito, Francesco Palmisano,* and Pier Gio...
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Anal. Chem. 2003, 75, 4988-4995

o-Phenylenediamine Electropolymerization by Cyclic Voltammetry Combined with Electrospray Ionization-Ion Trap Mass Spectrometry Ilario Losito, Francesco Palmisano,* and Pier Giorgio Zambonin

Dipartimento di Chimica, Universita’ degli Studi di Bari, Via E. Orabona 4, I-70126 Bari, Italy

The anodic oxidation of o-phenylenediamine at a platinum electrode in aqueous buffers at different pH (1-7), where growth of very thin (few nanometers thickness) and selfsealing polymeric films occurs, was investigated by cyclic voltammetry (CV) combined with off-line electrospray ionization-ion trap mass spectrometry (ESI-ITMS). Soluble oligomers (dimers, trimers, and tetramers in different oxidation states) formed in the course of the anodic oxidation were detected/identified by electrospray ionization-ion trap sequential mass spectrometry (ESI-ITMSn, n ) 1-5) while their relative abundance could be estimated from full-scan ESI-MS spectra of the quasi-molecular ions. In particular, oligomer abundances and the abundance ratios between different n-mers of the same class (e.g., structures corresponding to the same value of n but having different m/z ratios) were considered, and their variation with pH and potential range adopted for polymerization was studied. A mechanistic pathway for the growth of poly(o-phenylenediamine) film (PPD), relying on the integration of CV and ESI-ITMS, could be formulated for the first time. Chain propagation seems to compete with two different intramolecular oxidation processes, leading to the formation of phenazine or 1,4benzoquinonediimine units, whose relative importance is, in turn, dictated by polymerization conditions (pH and potential). The proposed mechanism can account for the different properties of PPD films prepared under different experimental conditions, as well as for the structural data obtained on the polymeric film itself by X-ray photoelectron spectroscopy. Among electrosynthesized polymers, the material obtained by electropolymerization of o-phenylenediamine, commonly known as PPD, has received a great deal of attention for more than two decades. Photovoltaic cells, anticorrosion coatings, pH measurements, and, in particular, enzyme entrapping permselective membranes in biosensor design represent its main fields of application.1 Recently, a renewed interest in PPD has been generated by the possibility of obtaining a biomimetic membrane by molecular imprinting of such a material during the electrosyn* Corresponding author. Tel +39-080-544-2016. Fax +39-080-544-2026. Email: [email protected]. (1) Losito, I.; De Giglio, E.; Cioffi, N.; Malitesta, C. J. Mater. Chem. 2001, 11, 1812 and references therein.

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thetic step, as first demonstrated in our laboratory2 and then further explored by other authors.3,4 Despite the remarkable research efforts devoted to PPD characterization,1,5 only limited structural information is available and can be summarized as follows. A ladder polymer possessing a phenazine-like structure, in which the amino groups of an o-phenylenediamine (o-PD) unit are condensed with the benzene ring of the adiacent unit along the polymer chain, seems to be more appropriate to describe PPD electrosynthesized at acid pH values (e1), although free primary amino groups are still present, at least at the polymer surface, as shown by X-ray photoelectron spectroscopy (XPS) analysis.1 When higher pH values are adopted for polymerization, the extent of conjugation is progressively decreased, as suggested by the increasing amount of free NH2 groups at the surface.1 This evidence is in agreement with some remarkable PPD properties, namely, the better electroactivity exhibited by the polymer obtained at low pH and the ability to interact with enzymes or (in the case of molecularly imprinted PPD) molecular templates, shown by PPD grown at higher pH (5-7). In the latter case, the presence of neutral or protonated NH2 groups could be responsible for the interactions with either enzymes or smaller molecules, used as molecular templates. A general mechanism for o-PD polymerization, able to explain changes in cyclic voltammetry (CV) behavior as well as in polymer structure observed with pH variations, is still lacking. A tentative model6 has been proposed for polymerization at low pH (e1) and is based on the electrochemical evidence of the presence of 2,3diaminophenazine (2,3-DAP), one of the possible dimers of o-PD, in the polymerization solution. Indeed, several soluble, colored species (2,3-DAP, for instance, gives red-orange solutions) are produced during PPD electrosynthesis on different substrates such as platinum or graphite. Recent investigations made in our laboratory,1,7 based on UV-visible spectroscopy and cyclic voltammetry, have shown that these soluble species have absorption maximums and voltammetric features similar to those of the polymer grown in the same pH conditions. These results indicate (2) Malitesta, C.; Losito, I.; Zambonin, P. G. Anal. Chem. 1999, 71, 1366. (3) Hong, S.; Wong, K.; Chan, A. S. C. Abstracts of Papers; 223rd ACS National Meeting, Orlando, FL, April 7-11, 2002. (4) Peng, H.; Zhang, J.; Nie, L.; Yao, S.; Zhang, Y.; Xie, Q. Analyst 2001, 126, 189. (5) Li, X. G.; Huang, M. R.; Duan, W.; Yang, Y. L. Chem. Rev. 2002, 102, 2925. (6) Dong-Hun, J.; Yong-Sup, Y.; Seung Mo, O. Bull. Korean Chem. Soc. 1995, 16, 392. (7) De Giglio, E.; Losito, I.; Torsi, L.; Sabbatini, L.; Zambonin, P. G. Ann. Chim. 2003, 93, 209. 10.1021/ac0342424 CCC: $25.00

© 2003 American Chemical Society Published on Web 08/23/2003

that such solution species are likely soluble oligomers of o-PD, involved in its polymerization mechanism. A knowledge of the molecular structure of such oligomers, complemented by structural information already available on the polymer, could be very helpful in understanding the evolution of o-PD polymerization and in postulating a mechanistic pathway. Recently, Deng and Van Berkel8 tackled a similar problem, i.e., the characterization of oligomers generated during the wellknown polyaniline electrosynthesis process, by coupling electrochemistry with electrospray ionization-tandem mass spectrometry (EC/ESI-MS/MS). A comparison with MS/MS spectra of available standards led them to hypothesise possible structures for electroproduced aniline dimers, whereas higher oligomers (from 3to 10-mers) were simply detected but not characterized from a structural point of view. In the case of poly(o-phenylenediamine), structural elucidation of oligomers is hampered by the much higher complexity of the system (coexistence, for a given n-mer, of different species in different oxidation states), the absence of reference standards even for the dimers of o-PD, and the substantial lack of knowledge about fragmentation mechanisms in ESI-MS. We have recently made a further step, making use of the power of sequential ion trap mass spectrometry (MSn).9 To this aim, the molecular weights of the soluble oligomers produced in the electrolytic solutions during o-PD polymerization were determined from ESI-MS full-scan spectra. Each of them was then isolated and sequentially fragmented in the ion trap, MSn (with n up to 5) spectra were acquired, and fragmentation patterns were obtained, from which possible candidate structures could be inferred. In the present work, the distributions of the oligomeric products detected and characterized by ESI-MSn in o-PD polymerization solutions have been studied under different polymerization conditions, namely, pH and potential. The results have been then elaborated, along with electrochemical data, to propose, for the first time, a general polymerization mechanism for o-PD. EXPERIMENTAL SECTION Materials. o-Phenylenediamine ∼99% (Aldrich Chemical Co.) was sublimated under vacuum at 80 °C before use. A 50 mM ammonium acetate solution (pH 7) was used as electrolytic solution; when necessary the pH was modified by small aliquots of trifluoroacetic acid, acetic acid, or ammonia. Acetonitrile (Aldrich Chemical Co.), mixed with electrolytic solutions before ESI-MS measurements, was of HPLC grade. Electrochemical Apparatus. All the electrochemical experiments were carried out with a 273 potenziostat/galvanostat controlled by the M270 software (EG&G, Princeton, NJ). A conventional three-electrode cell, with a Pt sheet (area ∼0.5 cm2) as the working electrode, an identical Pt sheet as the counter electrode, and an Ag/AgCl/saturated KCl electrode as the reference, was always adopted. The surface of the working electrodes was polished with 0.05µm alumina powder and then sonicated in triply distilled water before each experiment. Sample Preparation. Poly(-o-phenylenediamine) films were grown on the Pt working electrodes using cyclic voltammetry. (8) Deng, H.; Van Berkel, G. J. Anal. Chem. 1999, 71, 4284. (9) Losito, I.; Cioffi, N.; Vitale, M. P.; Palmisano, F. Rapid Commun. Mass Spectrom. 2003, 17, 1169.

The experiments were carried out in fresh electrolytic solutions (10 mL), buffered at different pH (1, 3, 5, 7) and containing 5 mM o-PD; unless otherwise stated, the potential was cycled in the range from 0 to +0.8 V versus Ag/AgCl/saturated KCl, at a scan rate of 50 mV/s, until a steady state was reached. Specific polymerization experiments were also performed by constantpotential (+0.8 V) electrolysis. At the end of the polymerization (20 cycles for CV electrosynthesis), an aliquot of the electrolytic solution (5 mL) was filtered through a 0.2-µm filter (Macherey Nagel), then mixed with an equal volume of HPLC-grade acetonitrile, and added with acetic acid (final concentration 1% v/v), to promote the protonation of molecular ions. ESI-MS Measurements. An LCQ ion trap (IT) mass spectrometer (Finnigan MAT), controlled by the Xcalibur software and equipped with a syringe pump for direct infusion of samples, was used for ESI-MS/MSn measurements, performed in positive ion mode. Before starting the analysis of o-PD polymerization solutions, an optimization of the ESI interface parameters was accomplished. An o-PD solution, at the same concentration adopted for electrosynthesis (5 mM), was mixed with acetonitrile (1:1), acidified with acetic acid (1% v/v), and infused into the spectrometer at a flow rate of 20 µL/min. The ESI-MS parameters were sequentially modified to optimize the signal relevant to the o-PD protonated molecular ion, having m/z 109, and the following values were obtained: sheath gas flow rate, 0.9 L/min; spray voltage, 5.5 kV; heated capillary temperature, 190 °C; capillary voltage, 30 V; tube lens voltage, 15 V; octapole 1 offset, -4.25 V; lens voltage, -22 V; octapole 2 offset, -10 V; octapole rf amplitude, 400 V. For each sample, a preliminary ESI-MS analysis was performed on a blank solution, i.e., a monomer-free electrolytic solution (having the same pH as the sample solution)/acetonitrile 1:1 mixture. A full-scan ESI-MS spectrum was first recorded on the blank solution in the range 50-700 m/z, then the sample mixture was introduced, and another full-scan spectrum was obtained. Both spectra were stored in the same acquisition file, so that the blank one could be subtracted from the sample one; by this approach, ions arising from solvent impurities, potentially interfering with those originating from o-PD polymerization, could be eliminated from the processed spectra. In the second stage of analysis, high-resolution spectra (acquired in the zoom scan mode, i.e., low scan rate acquisition in a narrow mass range) were obtained for those m/z ranges in which ions had been detected, then each ion was isolated in the ion trap, and sequential fragmentations were performed, thus obtaining MSn (with n up to 5) spectra. The details on this stage of analysis and its results are reported elsewhere.9 RESULTS AND DISCUSSION In the first stage of ESI-MS analysis, wide m/z range spectra (50-700) were recorded on o-PD polymerization solutions at different pH; those obtained from solutions at pH 1 and 7 are shown in Figure 1 for immediate comparison. The presence of a peak corresponding to protonated o-PD (m/z 109) and a fragment arising from ammonia loss through in-source collision-induced dissociation, [M - NH3 + H]+, at m/z 92, is apparent in both cases. An ion is also observed at m/z 133 in the pH 7 spectrum Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

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Figure 1. ESI-MS full-scan spectra (m/z range 50-450) of the electrolytic solutions obtained after polymerization of 5 mM o-PD on platinum by cyclic voltammetry between 0 and 0.8 V (vs Ag/AgCl/saturated KCl) at pH 1 and 7. In the insets, the zoom scans (higher resolution mass scans) for oligomer regions are shown.

and seems to be an adduct between the m/z 92 ion (more abundant in this case) and acetonitrile (M ) 41). More importantly, clusters of peaks can be observed at higher m/z ratios (around 213, 317, and 423) and were further investigated by recording high-resolution scans in narrow m/z ranges 4990 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

(zoom scan mode), as shown in the insets in Figure 1. Since the ESI source acts as a controlled current electrolytic cell, the presence of artifacts arising from monomer oxidation directly at the capillary tip needs to be excluded. This could be easily verified by direct electrospraying the monomer solution: no evidence for

Figure 2. Representative structures proposed for the oligomers detected by ESI-MS in the o-PD polymerization solutions.

the formation of either radical cations or oligomeric species was obtained. The most abundant ions detected in the zoom scans were isolated and fragmented sequentially (MSn) inside the mass spectrometer ion trap to get information on the corresponding structures. The interpretation of fragmentation patterns, discussed in detail in a previous work,9 led to correlate them to monoprotonated o-PD dimers, trimers, and tetramers in different oxidation states. Representative structures are reported in Figure 2. It is worth noting that the m/z 196 ion originates from the m/z 213 dimer through ammonia loss (-17). In many cases, two or more isomeric structures can be hypothesised to explain the ESI-MSn patterns. Moreover, the calculation of isotope distributions (represented by vertical segments in Figure 1 insets) for the main species showed that peaks at m/z 214 and 318, at least in mass spectra relevant to low polymerization pH values, are due not only to the M + 1 isotopes of those having m/z 213 and 317 but also to other structures, namely, oxygenated species (see Figure 2). The latter seem to arise from hydrolysis, occurring at low pH values, of oligomers (at m/z 213 and 317) containing iminic nitrogen.10 On the other hand, peaks at m/z 320-321 and 424-425 seem to be referred almost exclusively to the M + 1 and M + 2 isotopes of ions at m/z 319 and 423, respectively, whereas peaks at m/z 315 and 316, detected only in low-pH polymerization solutions, are related to other o-PD trimers, as shown in Figure 2. It is worth noting that the m/z 211 ion corresponds to protonated 2,3-DAP. Indeed, the fragmentation pattern observed for the m/z 211 ion is identical to that of synthetic 2,3-DAP,9 the only o-PD oligomer that can be chemically synthesized, purified, and used as a standard. To the best of our knowledge, this is the first direct confirmation of the presence of 2,3-DAP in o-PD polymerization solutions.

Starting from the Figure 2 scenario, the distribution of oligomeric species and the ratios between the amounts of the oxidized/reduced forms for a given n-mer (e.g., 211 vs 213, for n ) 2, or 317 vs 319, for n ) 3) were investigated under different polymerization conditions. To this aim, o-PD polymerization was performed, for each pH studied (1, 3, 5, 7): (i) by cyclic voltammetry, first in the typical potential window from 0 to 0.8 V (three replicates for each pH) and then in a restricted window reversing the potential scan around 0.5-0.6 V; (ii) by constantpotential (+ 0.8 V) electrolysis. The voltammograms obtained from experiments i are compared in Figure 3. Irreversible oxidation processes are always observed, and the relevant potentials are shifted toward less anodic values upon increasing the pH. This behavior is expected, since H+ ions are produced after coupling of o-PD units following their oxidation to radical cations; thus, the whole EC process is favored (i.e., the oxidation potentials are lower) in more basic polymerization solutions. A further consequence of the pH-dependent potential shift is the presence of peculiar oxidation processes in the first polymerization cycle at pH 5 and 7 that are not accessible in the 0-0.8-V potential range at pH 1 and 3. In any pH condition, oxidation is progressively hindered during prolonged cycling, as shown by the anodic shift of the peak potential and the decrease of peak current, although this phenomenon is more evident at pH 5 and 7. As already hypothesised,1 this behavior could arise from structural modifications of already formed PPD due to oxidation processes occurring during polymerization, similarly to the so-called “overoxidation” observed on polypyrrole.11 These processes seem to be located at high potential values and are able to affect significantly the PPD structure only at high polymerization pHs.

(10) Allinger, N. L.; Cava, M. P.; De Jongh, D. C.; Johnson, C. R.; Lebel, N. A.; Stevens, C. L. Organic Chemistry; Worth Publishers: New York, 1971.

(11) Palmisano, F.; Malitesta, C.; Centonze, D.; Zambonin, P. G. Anal. Chem. 1995, 67, 2207.

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Figure 3. Cyclic voltammograms obtained during polymerization of 5 mM o-PD at different pH and using two potential ranges. Scan rate, 50 mV/s.

Indeed, when a restricted potential range is adopted, thus excluding the high-potential oxidation processes even for polymerizations at pH 5 and 7, the current decrease during potential cycling becomes less pronounced and almost comparable between different pH values (see the right portion of Figure 3). These changes in the polymerization conditions have a remarkable effect on the distribution of oligomers found by ESI-MS after o-PD polymerization. Note that the distribution observed for the oligomers may reflect their relative rate of formation and consumption to form larger n-mers; then it can be expected that those species less likely to continue polymer growth tend to accumulate preferentially. The percent abundance for the monomer and the tetramers and the abundance ratios for the oxidized/reduced form of dimers (m/z 211 vs 213) and trimers (m/z 317 vs 319) identified by ESI-MSn are reported in Figure 4 as a function of the solution pH for each of the polymerization conditions (wide/short potential range CV and constant-potential electrolysis). It should be pointed out that spectral abundances (as percent values) of the main isotopes of each group of oligomers in the ESI-MS full-scan spectra might not strictly reflect the real (relative) amount of the oligomers in solution, due to possible differences in ionization efficiency (i.e., sensitivity) of the ESI source toward different oligomers. However, this drawback is only apparent, because data are used not for 4992 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

absolute quantitation but just for evaluating relative changes in the oligomer distributions related to different polymerization conditions; data remained internally consistent and proved to be extremely useful in understanding o-PD polymerization mechanism. As it can be seen from Figure 4, monomer consumption is higher the lower the pH adopted for polymerization by CV. Reducing the potential range does not significantly alter this behavior, which, on the contrary, is enhanced when potentiostatic conditions are used. These results are in agreement with the hypothesis made on overoxidation of already formed PPD during further polymerization cycles: the progressive passivation of the film surface would prevent further monomer oxidation at the Pt/ PPD-modified electrode, thus leading to a higher amount of residual o-PD in the electrolytic solution, especially at higher pH. As expected, this process is even more favored during potentiostatic polymerizations at pH 5 and 7, in which electrode potential is held at + 0.8 V. As far as the tetramers are concerned, an increase of their amount with polymerization pH is observed, especially for CV in a narrow range of potential, whereas a negligible amount is found at all pH values when polymerization is performed potentiostatically. The decrease of tetramer amount seems to occur when

Figure 4. pH and potential range dependence of the percent abundances of o-PD and its tetramers and of the ratios between the two types of dimers and trimers observed by ESI-MS in the electrolytic solutions. Squares, CV between 0 and 0.8 V; circles, CV in a shorter potential range (see Figure 3, right part); triangles, constant-potential electrolysis at +0.8 V.

potentials close to + 0.8 V are applied to the electrode; yet it is not clear whether it depends on an increased rate of consumption to form higher (insoluble) oligomers or on a decreased rate of production from shorter oligomers. This aspect will be further considered later, after the analysis of dimer and trimer ratio trends. In both cases, these ratios decrease (see Figure 4) with polymerization pH, despite the potential range adopted for polymerization; yet a remarkable difference in their values (up to almost 1 order of magnitude) is observed when the potential range is narrowed at low pH. These results suggest that oxidation processes requiring high anodic potentials (and high pH values) are responsible for consumption of dimers with m/z 211 and trimers with m/z 317. Information presented so far has been merged with data previously reported either for polyaniline8 or for PPD itself, at low pH,6 in order to propose a polymerization mechanism able to explain the trends of ESI-MS data on o-PD oligomers. In any pH condition, the first stage of polymerization is monomer oxidation to the corresponding radical cation, followed (see Figure 5) by radical coupling (head-to-head, head-to-tail, tail-to-tail) to give three possible different dimers at m/z 215 (MH+ ions) that can be further oxidized, by a two-electron, two-H+ process to a m/z 213 species, which, indeed, is the most abundant oligomer (see Figure 1). The slight deviation from the predicted M + 2 isotope abundance for the m/z 213 dimer (see the inset in Figure 1) might provide indirect evidence for the presence of very minute amounts of m/z 215 ions (at least at pH 1), but structures found for higher oligomers clearly indicate that dimers 215B,C (i.e., those arising

from tail-to-tail and head-to-head radical coupling, respectively) play a very minor role. The latter result is in good agreement with ESI-MS/MS data obtained by Deng and Van Berkel8 for the aniline dimer, which is essentially the product of head-to-tail coupling. The fully reduced dimer 215A may be involved in three different and competitive processes: (a) further radical coupling (i.e., chain propagation) to form a trimer with m/z 321; (b) oxidation to the 213 dimer; (c) internal coupling (intramolecular oxidation followed by cyclization) leading to the intermediate species 213B, which rapidly evolves into its fully oxidized form, i.e., 2,3-DAP (MH+ at m/z 211). This last possibility is strongly supported by CV experiments performed on a 2,3-DAP standard, which revealed that dimer 213B (i.e., the reduced form of 2,3DAP) is more easily oxidized than the o-PD monomer. The fully reduced trimer (m/z 321) plays a role similar to that of the m/z 215 dimer, leading either to partially (m/z 319) or to fully (m/z 317) oxidized trimers or to the fully reduced tetramer with m/z 427. The latter, like the trimer with m/z 321, is not found in the polymerization solutions, being the precursor of more oxidized tetramers, at m/z 423. The same path can be imagined also for higher oligomers, which are insoluble and remain undetected by ESI-MS. Then the key point in the polymerization mechanism seems to be the competition between oxidative coupling, leading to chain propagation, and intramolecular oxidation processes leading to the formation of (i) phenazine, (ii) 1,4-benzoquinonediimine units, (iii) or both, i.e., to oligomers in a different oxidation state. For instance oligomers 211 and 317 (the main forms, 317A and 317B) Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

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Figure 5. Schematic representation of the o-PD polymerization mechanism proposed from ESI-MS data on its oligomers.

arise from phenazine-forming processes, whereas oligomers 213 and 319 arise from 1,4-benzoquinonediimine-forming processes. The trends of the 211/213 and 317/319 ratios (see Figure 4) indicate that the latter are favored at higher potentials. It follows that when oxidation potentials are shifted toward lower values, e.g., by increasing the polymerization pH, type ii processes become more and more important in determining the final distribution of dimers and trimers (i.e., the amounts of 213 and 4994 Analytical Chemistry, Vol. 75, No. 19, October 1, 2003

319 are increased). The potential range restriction acts in the opposite sense: type i processes are favored and lead to an increase of species such as dimers 211 and trimers 317. The synergy between low pH and potential range leads to the remarkable ratios observed at pH 1. The findings reported so far are in agreement with our X-ray photoelectron spectroscopy data1 relevant to PPD synthesized at different pH values. Indeed, the increase of conjugation indirectly

observed by XPS (through quantitation of surface primary amino groups) on PPD synthesized at low pH can be related to the presence of higher amounts of conjugated o-PD oligomers in solution. As far as chain propagation processes are concerned, their ability to compete with intramolecular oxidations is related to the potentials at which generation of o-PD oligomer radical cations occurs. CV data indicate that, differently from other cases such as pyrrole,12 they are not lower than the monomer oxidation potential. Indeed, if this had been the case, a shift of the main oxidation peak toward lower potentials would have been observed upon cycling (i.e., after accumulation of oligomers in the electrolytic solution). Although it is not possible to predict the potentials at which oligomer radical cations are formed, the trend of abundance for tetramers (see Figure 4) suggests that processes located at the anodic extreme of the CV potential range are responsible for chain propagation beyond tetramers; i.e., the potential for radical cation generation from o-PD tetramers seems to be quite far from the monomer oxidation potential. As a consequence, tetramers are not observed (due to a higher consumption rate) in the potentiostatic polymerization at +0.8 V, whereas they tend to accumulate to a considerable amount after CV in a shorter potential range (where production rate is much higher than consumption rate). The 0-0.8-V polymerization appears as an intermediate case. The pH dependence of tetramer abundance is not explained by these considerations; yet the rate of their generation needs also to be taken into account. At low pH, processes such as phenazine-forming oxidations seem to be a more accessible path than elongation to trimers/tetramers. The latter are still formed and then converted into higher oligomers, but their concentration does not reach significant levels in solution, differently from dimers. Indeed, the percent abundance of the latter is almost 80% at pH 1 and falls to 30% at pH 7. Starting from these considerations, the possibility of coupling between radical cations of oligomers (same or different n-mers, such as a trimer with a tetramer) cannot be excluded at higher polymerization pHs. In any case, the generation of phenazine or 1,4-benzoquinonediimine units should occur after the base structure of the oligomers has been formed. On the other hand, it is unlikely that dimers such as 2,3-diaminophenazine can generate radical cations for further coupling with the monomer or other oligomer radicals. In fact, CV data (not shown) obtained on 2,3DAP solutions showed that its oxidation occurred at a potential at least 0.5 V higher than the monomer oxidation, and then it is not accessible in the range adopted for o-PD polymerization. (12) Diaz, A. F.; Bargon, J. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, p 83.

CONCLUSIONS The results shown above demonstrate that, despite the very high complexity of the system under study and of the often not univocal structure assignment, ESI-ITMS provides a highly informative tool in the study of the initial stages of the electropolymerization process for o-phenylenediamine. In the specific case, the characterization of soluble oligomers and their distribution under different potential and pH conditions can also shed light on the structure of the PPD film obtained under different conditions since they should likely represent the building blocks of the electrodeposited polymer. Electrochemical behavior as well as potential and pH dependence could be accounted for by a peculiar mechanistic pathway in which chain propagation by oxidative coupling competes with intramolecular oxidations, forming phenazine or 1,4-benzoquinonediimine units or both. All these processes (including initial monomer oxidation) are favored by an increase of polymerization pH, yet oligomer intramolecular oxidations, especially phenazine-forming ones, seem to occur at potentials lower than those of chain propagation processes. As a result, high amounts of phenazine-like dimers are found in the electrolytic solutions whenever lower potential oxidation processes are favored during polymerization, i.e., when low values of pH or potential are adopted. On the other hand, when higher potential processes are enabled to occur (higher values of pH or potential), higher oligomers, with a less conjugated structure (due to the predominance of 1,4-benzoquinonediimine-forming processes), accumulate in solution. This mechanism is able to explain XPS data on PPD (e.g., the increase in NH2 content on increasing pH, which means that the phenazine-like character of the film decreases and ring-opened structures enriched in free amino groups increase) as well as electrochemical data such as the electroactivity (related to the higher phenazine content) of films prepared under very acidic condions. ACKNOWLEDGMENT Work carried out with the financial support of MIUR and University of Bari. M.P. Vitale is gratefully acknowledged for ESIMS experiments performed during her undergraduate thesis work. Mr. S. Giacummo is gratefully acknowledged for his skilled technical support.

Received for review March 10, 2003. Accepted July 18, 2003. AC0342424

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