NIR and Raman studies of redox reaction of PDACz in protic and aprotic media

Electrochimica Acta 54 (2009) 4751–4759

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Electrochemical and spectroscopic characterization of poly(1,8-diaminocarbazole): Part II. Electrochemical, in situ vis/NIR and Raman studies of redox reaction of PDACz in protic and aprotic media Agata Tarajko a , Agnieszka Michota-Kaminska a , Michał J. Chmielewski b , Jolanta Bukowska a , Magdalena Skompska a,∗ a b

Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02 093 Warsaw, Poland Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01- 224 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 24 November 2008 Received in revised form 10 February 2009 Accepted 14 February 2009 Available online 9 March 2009 Keywords: Poly(1,8-diaminocarbazole) Vis/NIR in situ spectroscopy In situ Raman spectroscopy Doping–undoping in protic and aprotic media

a b s t r a c t Electrochemical redox reactions of poly(1,8-diaminocarbazole) (PDACz) films in aqueous (0.1 M HClO4 ) and nonaqueous (0.1 M LiClO4 in acetonitrile) solutions were studied by cyclic voltammetry, in situ vis/NIR and Raman spectroscopy. It has been demonstrated that spectroelectrochemical behavior of the polymer is strongly dependent on the nature of the solution used for doping–undoping but not on the medium used for electropolymerization. A redox couples Fe2+ /Fe3+ , Fe(CN)6 4−/3− and tertrathiafulvalene were used as the probes for the studies of electroactivity of the oxidized polymer films. The results were discussed in terms of different mechanism of deprotonation process of the polymer in aqueous solution of 0.1 M HClO4 and in 0.1 M LiClO4 solution in aprotic acetonitrile and the reaction schemes in the two media are proposed. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Electrochemical polymerization of conducting polymers is a non-regiospecific process and usually more than one path of the monomer coupling is possible. This is a serious disadvantage of the electrochemical methods of synthesis since all structural defects strongly decrease the conjugation length along the polymer chain and, in consequence, the conductivity of the polymer film. One way of overcoming this problem is substitution of some reactive sites in the monomer molecule by non-reactive groups, as it is done in the case of 3,4-substituted thiophenes. On the other hand, monomer substitution with reactive groups may facilitate monomer coupling via these positions or direct the attack of other reagents on the selected carbon atoms, allowing for the structurally controlled electropolmerization. We have recently presented a new monomer belonging to this group, 1,8-diaminocarbazole (DACz), in which the amino groups are very active reaction sites in the molecule [1]. The FTIR spectra of the polymer films and results of quantum-chemical calculations of the unpaired-electron spin density in the radical cation pointed out that the most probable coupling is between nitrogen atom of amino group and carbon atom in para position (head-to-tail coupling) [2]. A strong similarity between the IR spec-

∗ Corresponding author. Tel.: +48 22 822 02 11; fax: +48 22 822 59 96. E-mail address: [email protected] (M. Skompska). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.02.083

tra of the polymer films obtained by electrochemical synthesis from aqueous solution of HClO4 and from 0.1 M LiClO4 in acetonitrile suggested that the type of linkages between the monomer units is practically the same. On the other hand, the electropolymerization profiles obtained in these two media are significantly different suggesting also different polymer electroactivity [1,2]. Therefore, the aim of this paper is to study in detail the redox processes of poly(1,8-diaminocarbazole) (PDACz) obtained in protic and aprotic solutions to determine the crucial parameters governing the polymer electroactivity and disclose the mechanism of the polymer doping–undoping in different media. The results were achieved by means of cyclic voltammetry, in situ vis/NIR and Raman spectroscopy. 2. Experimental All electrochemical experiments were carried out using PGSTAT 30 potentiostat (Autolab, Ecochemie, Netherlands), in conventional, one compartment cell with a Pt disc working electrode (geometric surface area 0.02 cm2 ) and Pt wire counter electrode. The reference electrode used in acetonitrile solutions was Ag/Ag+ (0.1 M AgNO3 , in CH3 CN) electrode, whereas Ag/AgCl/Cl− (sat., aq.) was used in aqueous solutions. All potentials below are referred to Ag/Ag+ reference electrode. Its potential is about 0.31 V vs. Ag/AgCl/Cl− (sat. aq.) electrode. The potential difference between Ag/Ag+ (AN) and Ag/AgCl/Cl− (aq.) reference electrodes was determined from the

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shift of the formal potential of the ferrocene/ferrocenium redox couple. Electropolymerization of DACz was carried out in 5 mM solution of 1,8-diaminocarbazole in acetonitrile containing 0.1 M LiClO4 or in aqueous solution of 0.1 M HClO4 . EQCM studies were carried out using a QCM unit (Type M3, UELKO, Poland) combined with potentiostat PGSTAT 30. All measurements were made at room temperature (20 ± 2 ◦ C). The working electrode was Au, deposited on 10 MHz AT-cut quartz crystals (International Crystal Manufacturing Co. Ltd., Oklahoma City, OK), with piezoelectrically and electrochemically active areas of 0.21 and 0.23 cm2 , respectively. In situ spectroelectrochemical experiments were done using double-beam UV/Vis spectrometer (Lambda 12, PerkinElmer) combined with potentiostat PGSTAT 30. The electrochemical cell containing the ITO/PDACz working electrode, Pt gauze counter electrode and a miniature Ag/Ag+ reference electrode in the supporting electrolyte was mounted in the spectrophotometer sample compartment. The surface area of ITO working electrode was about 1.2 cm2 . The reference cell contained an identical uncoated ITO electrode in the same electrolyte. Raman spectra were recorded with a Jobin Yvon Spex T64000 Raman microscope, equipped with a Kaiser holographic notch filter, a 600 groove mm−1 , holographic grating, and a 1024 × 256 pixel liquid nitrogen cooled CCD detector. The microscope attachment was based on an Olympus BX system with a 50× long-distance objective. The excitation source was a Laser-Tech model LJ800 mixed argon–krypton ion laser operating at wavelength of 647.1 nm. A band-pass filter (Coherent model 35-8663), necessary for elimination of weak plasma line having intensities comparable with the Raman scattering, was placed in the laser beam prior to the sample. The sample was irradiated with a laser beam of ca. 10 mW average power at the sample. Electrode potential during in situ Raman measurements was controlled by means of Microlab (Ecochemie, Netherlands). 3. Results and discussion 3.1. Electrochemical studies of the polymer activity in aqueous and acetonitrile solutions As it has been reported by us previously [2], the shape of cyclic voltammograms obtained during electropolymerization of 1,8-diaminocarbazole suggested that the resistance of the polymer markedly increased after complete oxidation. This type of

behavior has been also found for polyaniline and corresponds to the removal of 0.5 electron per repeat unit at high doping level [3]. Fully oxidated polyaniline undergoes deprotonation and transforms into insulating form of pernigraniline [4–6]. High charging state has been also found for other polymers, for example 0.6 for PEDOT [7] and alkylated PEDOT [8] or even 1 for poly(4,4 bimethoxybithiophene) [9]. However, for most of the polymers the oxidation potential is relatively high (0.7–0.9 V) and so high doping level cannot be obtained due to overoxidation of the polymers. We have also observed stronger decrease of the Faradaic current after the oxidation peak of PDACz in acidic aqueous solution than in acetonitrile, which suggests higher resistance of the film [2]. Since the polymer conductivity is a crucial parameter influencing the polymer growth and it is also important for further practical applications, we performed a detailed studies of the PDACz electroactivity in a broad potential range in the two media. The PDACz films were deposited on Pt electrodes by cyclic voltammetry from aqueous and acetonitrile solutions, according to the procedures described by us previously [1,2]. Then, the films were placed in the monomer-free solutions to obtain reproducible redox behavior and finally the polymers were studied in solutions containing redox couples of the formal potential significantly higher than that of the polymer. The PDACz film obtained from aqueous solution of HClO4 was investigated in the 6 mM solution of FeSO4 in 0.1 M HClO4 . The formal potential of Fe2+ /Fe3+ couple is ca. 0.35 V (vs. Ag/Ag+ , AN), whereas the formal potential of PDACz in 0.1 M HClO4 , estimated as (Epa + Epc )/2, is about 0.06 V (Fig. 1a). Thus, if the polymer at the high doping level is conducting, the oxidation wave of the polymer should be followed by the peak corresponding to oxidation of Fe2+ at the polymer/solution interface. However, it is not the case (Fig. 1a). The oxidation wave of Fe2+ (curve 1) is observed only at a bare Pt electrode, whereas no peak in this range is formed at Pt covered with PDACz film (curve 3). The cyclic voltammogram is quite the same as that obtained in the absence of redox couple in the solution. This confirms that the polymer becomes resistive when fully oxidized in HClO4 (aq.) and no charge is transferred across the film. In order to check if the observed effect is not influenced by modification of the potential distribution across the metal/polymer/solution interfaces due to the presence of the redox couple in the solution, the similar experiment has been done in the solution of 0.1 M HClO4 containing 5 mM K4 [Fe(CN)]6 . The formal potential of the Fe(CN)6 4−/3− redox couple is located just after the potential of oxidation peak of the polymer, i.e. still in the range of high polymer electroactivity. As visible in Fig. 1b, the

Fig. 1. (a) Comparison of the cyclic voltammograms at a bare Pt (curve 1) and Pt/PDACz electrode (curve 2) in the solution of 6 mM FeSO4 in 0.1 M HClO4 . Curve 3 corresponds to the voltammogram obtained at Pt/PDACz electrode in 0.1 M HClO4 . Scan rate 0.1 V s−1 . PDACz film was deposited from HClO4 . (b) Comparison of the cyclic voltammograms at a bare Pt (curve 1) and Pt/PDACz electrode (curve 2) in the solution of 5 mM K4 Fe(CN)6 in 0.1 M HClO4 . Curve 3 corresponds to the voltammogram obtained at Pt/PDACz electrode in 0.1 M HClO4 . Scan rate 0.1 V s−1 . PDACz film was deposited from HClO4 .

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Fig. 2. (a) Comparison of the cyclic voltammograms at a bare Pt (curve 1) and Pt/PDACz electrode (curve 2) in the solution of 5 mM tetrathiafulvalene (TTF) in 0.1 M LiClO4 /AN. Curve 3 corresponds to the voltammogram at Pt/PDACz electrode in 0.1 M LiClO4 /AN. PDACz film was deposited from AN. Inset: comparison of the voltammogram obtained by subtraction of the curve (3) from the curve (2) (line A) with voltammogram of TTF on the bare Pt electrode (line B). (b) Comparison of the cyclic voltammograms at a bare Pt (curve 1) and Pt/PDACz electrode (curve 2) in the solution of 5 mM TTF in 0.1 M LiClO4 /AN. Curve 3 corresponds to the voltammogram at Pt/PDACz electrode in 0.1 M LiClO4 /AN. PDACz film was deposited from aqueous solution of 0.1 M HClO4 .

redox peaks for the process taking place at the polymer/solution interface are well developed and the position of oxidation peak of Fe(CN)6 4− is located at the same potential as that at the bare Pt electrode. A comparison of the results obtained for both electroactive species confirms that electroactivity of PDACz films in aqueous solution of 0.1 M HClO4 is really limited from the side of positive potentials. The cyclic voltammograms obtained for the system Pt/PDACz/Fe2+/3+ presented in Fig. 1a also indicate that the PDACz film deposited from 0.1 M HClO4 is very compact because the redox couple does not penetrate to the Pt substrate through the polymer pores. Otherwise, a small peak due to oxidation of Fe2+ at the substrate would be observed. Similar experiments were performed in acetonitrile for PDACz deposited from AN solution using tetrathiafulvalene (TTF) as a redox probe. TTF oxidizes in two one-electron reversible steps, firstly to radical cation and then to dication. The oxidation and reduction peaks of TTF/TTF•+ couple at a bare Pt electrode are located in the region of PDACz electroactivity, namely at 0.019 and −0.065 V vs. Ag/Ag+ (curve 1 in Fig. 2a). The oxidation peak of TTF•+ to TTF2+ appears at 0.39 V, i.e. after oxidation peak of PDACz. As visible in the inset in Fig. 2a, the first oxidation peak of TTF at Pt/PDACz electrode was nearly as high as that at the bare Pt electrode, whereas the second one was strongly diminished. The presence of the second peak may result from TTF•+ → TTF2+ reaction at the polymer/solution interface, depressed in comparison to that at Pt by a sluggish charge

transfer through the polymer film of increased resistance. Nevertheless, it would mean that the conductive window of PDACz is broader in acetonitrile than that in aqueous solution of HClO4 . Another possibility is that TTF•+ penetrates through the polymer pores and the oxidation takes place at the Pt substrate. In order to check if electroactivity of highly oxidized PDACz is different in the two media, the compact polymer film deposited from aqueous solution of HClO4 was studied in acetonitrile solution containing of 5 mM TTF. Appearance of the second pair of redox peaks corresponding to transformation TTF•+ /TTF2+ in the region of high resistance detected in HClO4 , as shown in Fig. 2b, indicate clearly that electroactivity of the highly oxidized PDACz depends strongly on the type of the solution and it is higher in aprotic acetonitrile than in aqueous solution of 0.1 M HClO4 . In order to gain more knowledge on electroactivity of PDACz in protic and aprotic media, the spectroelectrochemical vis/NIR experiments were performed. The results are presented below. 3.2. Spectroelectrochemical in situ studies of doping process of PDACz films Spectroelectrochemical studies were carried out for two polymer films deposited on ITO electrodes – one was obtained from acetonitrile solution containing 0.1 M LiClO4 and the second one from aqueous 0.1 M HClO4 . The polymer-modified electrodes were placed in the monomer-free solutions of the supporting electrolytes

Fig. 3. Evolution of absorption spectra (a) and changes in absorbance (b) upon oxidation of PDACz/0.1 M LiClO4 , AN (film electrodeposited on ITO from AN) by potential steps within the range from −0.6 to 0.6 V (vs. Ag/Ag+ ). Inset: cyclic voltammogram of the film in the same solution. The numbers at the spectra in Fig. 2a correspond to the potentials indicated on the cyclic voltammogram. The plots (A − A0 ) vs.  were obtained by subtraction of the initial spectrum A0 (of the neutral film at −0.6 V) from the spectra in Fig. 3a recorded at −0.5 ≤ E/V ≤ 0.6

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Fig. 4. Evolution of absorption spectra (a) and the changes in absorbance (A − A0 ) (b) upon oxidation of PDACz (electrodeposited on ITO from HClO4 ) from −0.6 to 0.5 V (vs. Ag/Ag+ ) by the potential steps in aqueous solution of 0.1 M HClO4 . The initial absorbance A0 corresponds to the electrode potential of −0.6 V. Inset: cyclic voltammogram of the film in the same solution.

and gradually oxidized by steps of amplitude 100 or 50 mV from the neutral state at −0.6 V to the oxidized one at 0.6 V and the absorption spectra were recorded at each polarization potential. The spectra of the two polymers in the neutral states are very similar (Figs. 3a and 4a), with well developed absorbance peak at about 370 nm corresponding to ␲ → ␲* transition and a broad residual absorption band at about 600 nm (better visible in a differential plots presented in Figs. 3b and 4b). Gradual oxidation of the polymer obtained and studied in AN leads to evolution of a new absorption band at about 860 nm and a shoulder in the range between 400 and 550 nm with simultaneous decrease of the band at 370 nm. These changes are better visible in the differential spectra, i.e. after subtraction of the initial spectrum, A0 , (recorded for the neutral polymer at −0.6 V) from the spectrum obtained at the selected potential E (Fig. 3b). At the potential corresponding to the oxidation peak in the cyclic voltammogram (0 V, trace 3) the band at 860 nm becomes dominant. The presence of two strong absorption peaks, one in the range 780–860 nm and the second at 420 nm has been also reported for oxidized polyaniline PANI and ascribed to formation of localized polarons in the polymer chain [10–13]. The absorption band at 860 nm has been also observed in the spectra of oxidized polycarbazole due to the formation of radical cation on the “pyrrole” unit [14]. At higher potentials another absorption band, at about 700 nm develops and the two bands merge (trace 4, better distinguished in the normal spectra, Fig. 3a). Finally, in the spectrum of oxidized polymer, the band at 700 nm becomes dominant and gradually shifts to the lower wavelengths.

Evolution of absorption spectra during oxidation of the polymer deposited and studied in HClO4 is presented in Fig. 4a. Three broad bands, at about 450, 660 and 840 nm can be distinguished in the differential spectrum at the potential as low as −0.1 V, i.e. at the onset of the oxidation wave in the cyclic voltammogram (Fig. 4a, trace 1). Then, at the potential corresponding to the oxidation peak (0.1 V), the band at about 660 nm becomes dominant. Since our recent studies of PDACz by FTIR spectra indicated the same type of linkage between monomer units in the polymer films obtained from acetonitrile and aqueous acidic solutions, the observed differences in the absorption spectra in the two media has to be a result of differences in the polymer doping in acidic aqueous and neutral acetonitrile solutions. To verify this supposition, the polymer film deposited from 0.1 M HClO4 was studied consecutively in acetonitrile and in aqueous solution. As can be seen from Fig. 5a, the redox peaks of the polymer are markedly more intense in HClO4 (aq.) than in acetonitrile and the redox charges are about 107 and 47 ␮C, respectively. However, a decrease of the redox peaks in acetonitrile is not due to degradation of the polymer—the initial voltammograms were restored when the film was placed back in HClO4 . Similar behavior was observed when the film was obtained in AN and studied sequentially in acidic aqueous solution and in AN. Moreover, the spectra of the polymer deposited in AN and studied in aqueous solution of HClO4 are nearly the same as those for the polymer deposited and studied in HClO4 (cf. Figs. 4b and 5b). This confirms that the oxidation/reduction process of PDACz is independent of the polymerization bath but it depends strongly on the medium used for the polymer doping/undoping.

Fig. 5. (a) The cyclic voltammograms of PDACz film obtained on Pt disc from aqueous solution of 0.1 M HClO4 and then studied consecutively in: 0.1 M HClO4 (aq.) (curve 1), 0.1 M LiClO4 /AN (curve 2), 0.1 M HClO4 (aq.) (curve 3) and again in 0.1 M LiClO4 /AN (curve 4). (b) Evolution of differential absorption spectra of the PDACz obtained from AN and then oxidized in aqueous solution of 0.1 M HClO4 .

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Fig. 6. Plots of absorbance in a function of potential for oxidation of PDACz in the solution of 0.1 M LiClO4 in acetonitrile at 860 nm (curve 1) and at 660 nm (curve 2) and in the aqueous solution of 0.1 M HClO4 at 860 nm (curve 3) and at 660 nm (curve 4). Inset: the Nernst plots obtained from absorbance data for oxidation of PDACz in the solution of 0.1 M HClO4 at 660 nm.

The UV–vis spectroelectrochemical results were also used to estimate the doping level of the polymer, using “monomer unit model” [15]. It was done by quantitative analysis of absorption spectra using the Nernst equation in the form: RT [A − Amin ] E=E + ln nF [Amax − A] o



(1)

where Eo is the formal potential of redox reaction of the polymer, n is a number of electrons involved in oxidation/reduction of one monomer unit in the polymer chain, and the ratio (A − Amin )/(Amax − A) corresponds to the ratio of oxidized and reduced polymer sites, assuming that the absorption coefficients for oxidized and reduced forms are the same. The value of A corresponds to absorbance at applied constant potential after equilibration of the film, Amin is a minimum absorbance registered for the polymer in the neutral from, whereas Amax is the absorbance of fully oxidized polymer, corresponding to the value of plateau [15,16]. It must be emphasized that precise analysis of the spectroelectrochemical data by the Nernst equation is possible only under specific conditions: (i) there is no overlapping of the absorption

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bands at the wavelength measured, (ii) a simple stoichiometric reaction takes place [15]. The data extracted for two absorption bands, which may overlap, i.e. at 860 and 660 nm in a function of applied potential for polymer oxidation in acetonitrile solution and at in 0.1 M aqueous solution of HClO4 are presented in Fig. 6. As visible in Figs. 3a and 6 (curves 1 and 2) the overlapping of the bands at 860 and 660 nm in acetonitrile solution takes place in a very broad potential range and analysis of the data by Nernst equation is not reliable in this case. In aqueous solution of HClO4 the two bands merge at E < 0 V (curves 3 and 4), whereas at the higher potentials (0.1 V < E < 0.3 V) the absorbance at 860 nm is practically constant and data for this potential range were analyzed. The number of electrons involved in oxidation of one monomer unit (the doping level) calculated from the slope of the plot log[(A − Amin )/(Amax − A)] − E (inset in Fig. 6) is about 0.4, being close to that reported for polyaniline [3], and the formal potential obtained for (A − Amin )/(Amax − A) = 1 is 0.02 V. For comparison, the value of Eo obtained from mean value of the potentials of oxidation and reduction peaks is about 0.06 V. Since vis/NIR spectra and electrochemical methods do not provide information on the mechanism of the doping–undoping process of the polymer, the in situ Raman studies of PDACz were also undertaken. 3.3. In situ Raman studies of redox process of PDACz As it has been reported by us previously [2], the FTIR spectra of PDACz suggest the linkage of the monomer units via C–(NH)–C bonds, as in polyaniline. Thus, one can expect also similarities in the mechanism of oxidation of these two polymers. It is well known from the literature [17–23] that ex situ and in situ Raman spectra of various oxidation and protonation states of PANI are strongly dependent on the excitation line, because of resonance character of the spectrum. In the case of red excitation line (in our case 647.1 nm) the bands due to the oxidized segments of polyaniline are strongly enhanced [22,23] and dominate the whole spectrum. In Fig. 7 the set of Raman spectra of PDACz film (deposited from AN solution) in acidic aqueous solution (0.1 M HClO4 ), collected for the potentials changed from negative to positive values is presented. Based on the absorption spectra in visible region (Figs. 3 and 4) we may expect for 647.1 nm excitation, resonance or at least pre-resonance (for reduced form) enhancement of the Raman scattering. The spectra reveal several bands, some of which

Fig. 7. Evolution of Raman spectra of PDACz in acidic solution (0.1 M HClO4 ) upon polarization from −0.7 to 0.3 V vs. Ag/Ag+ /AN reference electrode recorded with excitation line exc = 647.1 nm (a) and cyclic voltammogram of PDACz in aqueous solution of 0.1 M HClO4 (b). The polymer film was deposited from acetonitrile.

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Fig. 8. Changes in the relative intensities of the Raman bands (normalized to 1070 cm−1 band) upon potentiostatic oxidation of PDACz film in aqueous solution of 0.1 M HClO4 .

have frequencies similar to these found in Raman spectra of polyaniline (PANI) [17–23], thus confirming similarities of the structural units in both polymers. The spectrum shows some dependence on the applied electrode potential. At negative potentials corresponding to the neutral form of the PDACz the band characteristic of protonated semiquinone radical cation, related to the C–N•+ stretching vibrations, is observed at 1357 cm−1 . This may be surprising but the presence of oxidized segments in reduced polymer has been also reported for polyaniline and explained by the reactive nature of leucoemeraldine, which could be locally oxidized even at negative potentials [19]. In our opinion the radical cations may be also trapped in the polymer structure due to short polymer chains. In such case a complete reduction of the polymer may be strongly hindered. The presence of the radical cations in the structure of the neutral polymer is also indicated by relatively high absorbance in the optical spectra at the wavelength corresponding to the presence of charged species (450–700 nm) (Figs. 3 and 4) and the presence of ClO4 − vibrations in the IR spectrum of the neutral polymer [2]. In the Raman spectrum of PDACz in its neutral form it appears also a band at 1610 cm−1 , which can be ascribed to the C–C stretching vibrations of the aromatic rings, with intensity comparable to the band diagnostic of semiquinone radical. However, one should remember that the intensity ratio of these two bands does not reproduce the relative amounts of both forms in the polymer matrix, for two reasons. First, the semiquinone species may also contribute to the rather broad and slightly asymmetric band at 1610 cm−1 . Secondly, intensity of the 1357 cm−1 band is considerably enhanced with excitation at 647.1 nm, being in resonance with electronic transition in these polymer segments, that contain semiquinone radical units (see Fig. 4). The spectral pattern remains practically unchanged up to −0.2 V. Above this potential, which corresponds to the onset of the oxidation wave of the polymer, the intensity of the band at 1357 cm−1 gradually decreases (Figs. 7a and 8). At E > 0.1 V, i.e. above the potential of the anodic peak, a new band grows up near 1480 cm−1 with concomitant intensity increase of the bands at 1260 and 1525 cm−1 (see Fig. 8). The 1480 cm−1 band is diagnostic of unprotonated quinoid segments of polyaniline (C N stretching vibrations) [19,20] and is resonantly enhanced with the red laser lines. Thus, the Raman spectra clearly indicate that oxidation of the polymer is accompanied with transformation of the semiquinone radicals to oxidized units in their basic form (quinone diimine). The band at 1260 cm−1 may be assigned to the single bond C–N stretching vibration in these units, while the 1525 cm−1 band,

which is also visible in the spectrum in the whole potential range, most probably corresponds to the C C stretching vibration of the quinone ring in protonated imine forms [22,23]. This last feature also contributes to the spectrum at negative potentials, but its intensity is enhanced at the potentials corresponding to the oxidized form of the polymer. On the other hand, the increase of the intensity of the band at 1260 cm−1 at the more positive potentials may be due to deprotonation of the free amine groups. To summarize, the in situ Raman spectra obtained during poly(1,8-diaminocarbazole) oxidation in 0.1 M HClO4 confirmed that substantial part of the polymer have the polyaniline-like structure. However, one can observe an important difference between redox behavior of the two polymers, reflected in the cyclic voltammograms. Two clearly separated anodic peaks can be distinguished in the course of PANI oxidation in acidic aqueous solutions [24,25]. The first oxidation process involves two consecutive, one-electron stages, corresponding to the formation two radical cations (polarons) centered on nitrogen atoms of amine units [26] and this step is pH-independent, i.e. does not involve the proton-transfer process [27]. In this state the C–N bond order is intermediate between those of amine C–(NH)•+ –C and imine C (NH)+ –C groups. The two radical cations may recombine to form dication (bipolaron). The second oxidation process also comprises two one-electron steps and leads to transformation of imine into diimine form [28]. This process is accompanied by deprotonation and its reversibility depends on the proton concentration in the solution [29]. Thus, the second step of PANI oxidation is irreversible in aprotic media due to absence of proton source [27]. The two overall reactions may be written as follows [30]: [–(C6 H4 )–N(H)–(C6 H4 )–N(H)–(C6 H4 )–N(H)–(C6 H4 )–N(H)–]n ⇔ [–(C6 H4 )–N(H)–(C6 H4 )–N(H)–(C6 H4 )–N+ (H) (C6 H4 ) N + (H)–]n + 2ne− ,

(2)

[–(C6 H4 )–N(H)–(C6 H4 )–N(H)–(C6 H4 )–N+ (H) (C6 H4 ) N+ (H)–]n ⇒ [–(C6 H4 )–N (C6 H4 ) N–(C6 H4 )–N (C6 H4 ) N–]n + (2n)e− + (4n)H+ .

(3)

In contrast, one predominant anodic peak is formed during oxidation of PDACz and its position in aqueous solution depends on pH [1]. In the range of more positive potentials the resistance of the polymer increases, as it was observed after the second peak of PANI. As it has been postulated in our recent paper [2], the monomer units in PDACz are mainly linked via C–(NH)–C4 bonds (head-to-tail coupling). An exemplary structure and its oxidation scheme is proposed in Fig. 9. In the first stage of oxidation the radical cations are created on nitrogen atoms of amino groups. This process is accompanied by the income of the charge compensating counter ions (ClO4 − ) from the supporting electrolyte into the polymer matrix. When the reaction takes place in aqueous acidic solution, the next step consists in reversible transformation of oxidized polymer into diimine structure, as in the case of PANI. In consequence of concomitant deprotonation, due to very poor basicity of imine nitrogens [31], the polymer becomes neutral and non-conducting. This process is also accompanied by the outcome of anions from the polymer matrix. In our opinion in AN solution the deprotonation process occurs according to different scheme. The H+ ions may be retained in the polymer structure due to protophobic nature of acetonitrile [32] and accommodated by basic neighbouring “free” amino groups, not involved in the coupling between the monomer units. Since the second amino group is involved in the linkage between the monomer units, the basicity of free amino group may be estimated from the

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Fig. 9. Possible schemes of oxidation of PDACz in aqueous HClO4 and in LiClO4 /AN solutions.

Fig. 10. (a) Current and resonant frequency responses to the potential steps from −0.6 to 0.37 V and back to −0.6 V applied to the Au/PDACz electrode in the solution of 0.1 M LiClO4 /AN. (b) Changes of the resonant frequency in a function of the charge passed during oxidation and reduction steps presented in Fig. 9a. (c) Comparison of the resonant frequency of Au/PDACz electrode before (curve 1) and after (curve 2) addition of TEA to the solution of 0.1 M LiClO4 /AN. An arrow indicates the moment of TEA addition. Before, the electrode was oxidized by the potential step from −0.6 to 0.37 V, as in Fig. 9a. Curve 3 corresponds to the current registered after the potential step. (d) Comparison of absorption spectra of oxidized PDACz (at 0.2 V vs. Ag/Ag+ ) in the solution of 0.1 M LiClO4 before (curve 1) and after (curve 2) addition TEA (solution in AN).

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values of pKa reported for aniline. According to the literature [33] it increases from 4.63 in aqueous solution to 10.62 in acetonitrile, supporting strong basic character of NH2 groups in this aprotic solvent. This process should be also reversible. However, the number of “free” amino groups which may accept protons is probably smaller than the number of liberated protons, due to for example the crosslinking between different polymer chains via the amino groups not involved in polymerization. Thus, part of protons is likely released from the polymer matrix due to the presence of traces of water or other scavengers for H+ ions. Since the positive charge is kept in the oxidized film, also the counter ions are not released from the polymer matrix and therefore, the fully doped PDACz film in AN reveals some conductivity even at high positive potentials, in contrast to the polymer deprotonated in acidic aqueous solution. One should expect that protons present in the polymer matrix oxidized in acetonitrile, and in consequence the counter ions, should be easily released by a strong basic agent, as triethylamine (TEA). In order to verify it, the electrochemical quartz crystal microbalance (EQCM) measurements were performed. 3.4. EQCM studies of deprotonation of the oxidized PDACz film in acetonitrile solution A thin PDACz film was deposited on Au electrode from AN solution. Then, the polymer-modified electrode was oxidized and reduced by potential steps from −0.6 to 0.37 V and back to −0.6 V in acetonitrile solution of 0.1 M LiClO4 and the current and resonant frequency responses were monitored. As visible in Fig. 10a, the changes of the current and resonant frequency are very reproducible in the consecutive potential steps. Decrease of the resonant frequency upon oxidation indicate that the polymer mass increases, due to accommodation of ionic and/or neutral species. Assuming that the polymer film is rigid, the changes of resonant frequency were recalculated into changes of mass, by means of Sauerbry equation [34], and plotted in a function of the charge passed during oxidation/reduction process (Fig. 10b). A small hysteresis of the plot suggests that the movement of anions is probably not the only process related with oxidation and reduction of the film but also exchange of some other species (for example solvent molecules) is involved. However, the molar mass of the species exchanged between polymer and the solution, obtained from the Faraday equation, 98.5 g mol−1 , matches well to the expected molar mass of ClO4 − (99.5 g mol−1 ). In the next experiment, the polymer was oxidized at 0.37 V and when the resonant frequency attained a stable value and the current dropped to zero, the solution of 0.1 M TEA in AN (50 ␮M) was added. In effect, the resonant frequency increased rapidly (mass decreased) by about 30%, probably due to expulsion of anions after withdrawing of protons from the polymer film by TEA (curve 2 in Fig. 10c). At the same time the current did not change because this process is a chemical step (curve 3). A blank experiment was also done to exclude the possibility that the change of the resonant frequency was due to the change of density of the solution in the vicinity of the electrode after addition of TEA. Deprotonation of PDACz in the presence of TEA should also result in the shift of the absorbance bands to the shorter wavelengths. The literature data obtained for PANI in the solutions of different pH have shown that deprotonation shifts the position of the absorption maximum from 840 nm in acidic medium to 600 nm in a basic solution [13,35]. As it is presented in Fig. 10d, addition of 100 ␮l of 0.1 M TEA to the solution of 0.1 M LiClO4 /AN, to the TEA concentration of 2 mM, results in the shift of the maximum of the spectrum of oxidized PDACz from 690 to 520 nm.

4. Conclusions Poly(1,8-diaminocarbazole) films obtained from acidic aqueous and LiClO4 /acetonitrile solutions are structurally very similar but their electrochemical and spectroelectrochemical properties are strongly dependent on the medium used for the polymer doping/undoping. Electroactivity of doped PDACz films in aqueous solution of 0.1 M HClO4 substantially decreases in the range of potentials higher by about 120–150 V than the potential of oxidation peak. This is related to differences in the scheme of oxidation and reduction in protic (aqueous solution of HClO4 ) and aprotic (AN) solutions. In both media, the oxidation of PDACz leads to formation of diamine structure but the following deprotonation step occurs probably in different way. In the case of aqueous HClO4 solution the protons are released from the oxidized polymer matrix and the polymer becomes isolating, as in the case of polyaniline. In contrast, in aprotic medium, the H+ ions are likely retained in the polymer matrix being accommodated by basic amino groups not involved in polymerization. In consequence, the counter ClO4 − ions are also not expulsed from the polymer matrix and the resistance of the oxidized polymer does not increase as strong as in HClO4 . Release of the protons and, in consequence, ClO4 − ions from the oxidized polymer matrix occurs after addition of a strong base (triethylamine) into AN solution. Deprotonation of the oxidized polymer results in the shift of the maximum of the absorption peak towards shorter wavelengths. The doping level of PDACz obtained from vis/NIR absorption spectra in aqueous solution of 0.1 M HClO4 is 0.4. Acknowledgement This work was partially supported by Polish Ministry of Science and Higher Education (grant no. N N204 0464 33). References [1] M. Skompska, M.J. Chmielewski, A. Tarajko, Electrochem. Commun. 9 (2007) 540. [2] A. Tarajko, H. Cybulski, M.J. Chmielewski, J. Bukowska, M. Skompska, Electrochim. Acta 54 (2009) 4743. [3] D. Ofer, R.M. Crooks, M.S. Wrighton, J. Am. Chem. Soc. 112 (1990) 7869. [4] W.-S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc., Faraday Trans. 1 (82) (1986) 2385. [5] L.W. Shacklette, J.F. Wolf, S. Gould, R.H. Baughman, J. Chem. Phys. 88 (1988) 3955. [6] L. Zhuang, Q. Zhou, J. Lu, J. Electroanal. Chem. 493 (2000) 135. [7] G. Zotti, G. Schiavon, S. Zecchin, L. Groenendaal, Chem. Mater. 11 (1999) 3624. [8] L. Groenendaal, G. Zotti, F. Jonas, Synth. Met. 118 (2001) 105. [9] M. Dietrich, J. Heinze, Synth. Met. 41–43 (1991) 503. [10] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 220 (1987) 67. [11] Z. Jin, Y. Su, Y. Duan, Sens. Actuator B 71 (2000) 118. [12] P.M. McManus, R.J. Cushman, S.C. Yang, J. Phys. Chem. 91 (1987) 744. [13] U.W. Grummt, A. Pron, M. Zagorska, S. Lefrant, Anal. Chim. Acta 357 (1997) 253. [14] P. Marrec, C. Dano, N. Gueguen-Simonet, J. Simonet, Synth. Met. 89 (1997) 171. [15] T. Amemiya, K. Hashimoto, A. Fujishima, K. Itoh, J. Electrochem. Soc. 138 (1991) 2845. [16] P. Marque, J. Roncali, J. Phys. Chem. 94 (1990) 8614. [17] Y. Furukawa, T. Hara, Y. Hyodo, I. Harada, Synth. Met. 16 (1986) 189. [18] Y. Furukawa, F. Ueda, Y. Hyodo, I. Harada, T. Nakajima, T. Kawagoe, Macromolecules 21 (1988) 1297. [19] M. Lapkowski, K. Berrada, S. Quillard, G. Louarn, S. Lefrant, A. Pron, Macromolecules 28 (1995) 1233. [20] G. Louarn, M. Lapkowski, S. Quillard, A. Pron, J.P. Buisson, S. Lefrant, J. Phys. Chem. 100 (1996) 6998. [21] M.C. Bernard, S. Cordoba de Torresi, A. Hugot-Le Goff, Electrochim. Acta 44 (1999) 1989. [22] M.C. Bernard, A. Hugot-Le Goff, Electrochim. Acta 52 (2006) 595. [23] M. Cochet, G. Louarn, S. Quillard, J.P. Buisson, S. Lefrant, J. Raman Spectrosc. 31 (2000) 1041. [24] A.F. Diaz, J.A. Logan, J. Electroanal. Chem. 111 (1980) 111. [25] T. Kobayashi, H. Yoneyama, H. Tamura, J. Electroanal. Chem. 177 (1984) 293. [26] E.M. Genies, M. Lapkowski, Synth. Met. 24 (1988) 61. [27] A. Watanabe, K. Mori, M. Mikuni, Y. Nakamura, M. Matsuda, Macromolecules 22 (1989) 3323.

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