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Electropolymerization of N-methylanthranilic acid and spectroelectrochemical characterization of the formed film
Synthetic Metals 159 (2009) 96–102
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electropolymerization of N-methylanthranilic acid and spectroelectrochemical characterization of the formed film Maija Blomquist a,b , Tom Lindfors a,∗ , Rose-Marie Latonen a , Johan Bobacka a a b
Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, 20500 Turku/Åbo, Finland The Finnish National Graduate School in Nanoscience (NGS-NANO), Nanoscience Center, P.O. Box 35, 40014 University of Jyväskylä, Finland
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 1 July 2008 Accepted 1 August 2008 Available online 13 September 2008 Keywords: Poly(N-methylanthranilic acid) Electropolymerization Cyclic voltammetry UV–vis Raman and FTIR spectroscopy
a b s t r a c t The electropolymerization of N-methylanthranilic acid (NMAA) is reported in this paper. The monomer is substituted both at ortho- and N-position and, to the best of our knowledge, it has not been previously electropolymerized. Electropolymerization of NMAA was done on glassy carbon and optically transparent (indium) tin oxide electrodes. The obtained films, which are probably of an oligomeric nature (oligoNMMA), were characterized with cyclic voltammetry (CV), in situ UV–vis and Raman spectroscopy, ex situ FTIR spectroscopy and scanning electron microscopy (SEM). Our results show that NMAA can be electropolymerized as thin films in 1.0 M HClO4 , but the oxidation and reduction peak currents in the CVs indicate that the formed oligoNMAA films are thinner than poly(N-methylaniline) or poly(N-butylaniline) films prepared under similar conditions. The CV and UV–vis measurements confirm that oligoNMAA have three oxidation states like suggested in the redox scheme of substituted polyanilines. The Raman spectra of oligoNMAA also verify that more quinoid units are formed at higher potentials in accordance with the redox scheme. The ex situ FTIR measurement proves that covalently attached carboxylic acid groups are present in the film structure and attached to the oligoNMAA backbone. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In this fundamental study, the electropolymerization of Nmethylanthranilic acid (NMAA) and characterization of the formed oligomeric film (oligoNMAA) films are reported. The chemical structure of NMAA is shown in Fig. 1 and reveals that in addition to the methyl group in N-position, also a carboxylic group is coupled to the ortho-position of the monomer ring. The scientific motivation of this research was to study how the combination of these two substituents influences the electropolymerization of NMAA, which was done in an aqueous solution of 1.0 M HClO4 . Poly(N-alkylanilines) (PNANIs) are interesting materials because substitution of the polymer backbone of polyaniline (PANI) lead to new properties, for example improved polymer solubility and processability [1,2]. On the other hand, the substitution decreases the electrical conductivity, which could explain the minor interest in substituted PANIs [1,3,4]. Poly(N-methylaniline) (PNMA) has so far gained most attention among PNANIs, which have also low or no pH sensitivity compared with PANI [5]. The reason for the lower pH sensitivity of PNANIs is that the characteristic
∗ Corresponding author. Tel.: +358 22154422; fax: +358 22154479. E-mail address: Tom.Lindfors@abo.fi (T. Lindfors). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.08.001
emeraldine base (EB) to salt transition (ES) (acid–base reaction) of PANI is blocked by the N-substituent. The pH sensitivity of PANI can be considered as an undesired side reaction limiting the use of PANI in many practical applications. For example, in potentiometric PANI based ion sensor applications where pH is not kept constant, the measured sensor response of the analyte ion can be erroneous because it may also contain a contribution of the pH response of PANI. It can therefore be important to be able to eliminate the pH sensitivity by N-substitution in certain applications. Also different types of ring substituted PANIs have been reported in the literature [6–19]. Among these, the ortho-substituted PANIs have been mostly studied and have been reported to give higher polymerization yields than the meta-substituted PANIs [8]. In the polymerization of substituted PANIs, oligomers are easily formed and thus the film growth is somewhat hampered compared to PANI [2,6]. Even though the ring substitution results in polymer films with lower conductivity than PANI, the major advantage of ring substitution is that the polymer film can be made accessible for further functionalization [20] or e.g. ion exchange applications [21]. The electrochemistry of PNANIs and poly(o-alkylanilines) is generally agreed to follow Scheme 1, where the oxidation state of the polymer can be varied between the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms [6,22]. The
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2.2. Electropolymerization of NMAA
Fig. 1. The chemical structure of NMAA.
emeraldine form is the only electrically conducting form. The chemistry is thus slightly more complicated than for most other conducting polymers (CPs) because of the three potential dependent oxidation states. It was also recently reported [23] that the emeraldine form of PNMA has a polaronic structure similar to PANI [24–26]. This paper continues our earlier work on PNANIs [27,28] and poly(o-alkylanilines) [7] and will focus on electropolymerization of NMAA. To the best of our knowledge, NMAA has not been electropolymerized earlier. The oligoNMAA films were characterized with cyclic voltammetry (CV), in situ UV–vis and Raman spectroscopy, ex situ FTIR spectroscopy and scanning electron microscopy (SEM). 2. Experimental 2.1. Chemicals NMAA was obtained from Aldrich and used as received. HClO4 was obtained from J.T. Baker. Double distilled water was used throughout this work.
The oligoNMAA films were prepared on glassy carbon (GC) disc working electrodes (WE; A = 0.07 cm2 ) with a polytetrafluoroethylene (PTFE) body. Prior to electropolymerization, the electrodes were polished with 15, 9, 3 m and 1 m diamond paste (in this order) and with 0.3 m Al2 O3 powder, rinsed with deionized water and dried. The electropolymerization of 0.1 M NMAA was done in 1.0 M HClO4 by cycling the potential between −0.2 V and 0.8 V (50–600 cycles) at a scan rate of 50 mV/s. After the polymerization, the oligoNMAA films were characterized in monomer-free solution of 1.0 M HClO4 (10 cycles; 50 mV/s). A GC rod served as the counter electrode (CE) and a Ag|AgCl| 3 M KCl as the reference electrode (RE). The potential was controlled with an Autolab (PGSTAT 20 or PGSTAT 100) potentiostat using the GPES software. Prior to all measurements, the solutions were purged with nitrogen and during measurements nitrogen was passed over the solutions. 2.3. UV–vis spectroscopy The oligoNMAA films were electropolymerized on 4 mm thick quartz glasses coated with a thin layer of tin oxide (TO). The same polymerization parameters were used as described for the oligoNMAA film preparation on GC (see Section 2.2). The potential was cycled 600 times between −0.2 V and 0.8 V with a scan rate of 50 mV/s, in order to obtain oligoNMAA films that were thick enough for the UV–vis measurements. The formed films were then conditioned overnight in 1.0 M HClO4 . The TO coated glass on which the oligoNMAA film was polymerized, was used as the WE, a platinum and a Ag|AgCl wire served as CE and pseudo RE, respectively. All three electrodes were placed in a plastic cuvette filled with 1.0 M HClO4 and the UV–vis transmission spectra were recorded with a
Scheme 1. The oxidation and reduction mechanism of substituted PANIs showing the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms. X and Y are the methyl and carboxyl substituents, respectively, and A− is a mobile anion.
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Hitachi U-2001 spectrophotometer. The spectra were recorded first at the open circuit potential, then within the potential interval of −0.2 V to 0.75 V and back to −0.2 V (vs. Ag|AgCl wire). The oligoNMAA films were first reduced at −0.2 V for 19 min before the UV–vis spectrum was recorded at this potential. Thereafter, the potential was always kept 9 min at the other applied potentials before the spectra were measured.
reflectance measurement equipped with a DTGS detector was used. 32 interferograms with a resolution of 4 cm−1 were recorded for the spectrum. 2.6. SEM measurements A LEO 1530 Gemini FEG-SEM instrument was used in the SEM measurements (acceleration voltage: 15 kV).
2.4. Raman spectroscopy 3. Results and discussion The oligoNMAA film was polymerized in a flow cell, which is described in our earlier work [29]. The same polymerization parameters were used as described for the oligoNMAA film prepared on GC (see Section 2.2). The potential was cycled 600 times between −0.2 V and 0.8 V (50 mV/s) in order to increase the film thickness. Both the polymerization and characterization solutions flowed continuously through the cell with a flow rate of approximately 0.3 ml/min. The solutions were transferred to the cell with a peristaltic pump. A Pt wire served as the CE and Ag|AgCl wire as the RE. The GC disc electrode (A = 0.07 cm2 ) was polished with 0.3 m Al2 O3 powder and rinsed with deionized water prior to the oligoNMAA film preparation. The potential was controlled with an Autolab (PGSTAT 20) potentiostat. After the polymerization and prior to the Raman measurements, the oligoNMAA films were equilibrated overnight in the flow cell filled with 1.0 M HClO4 . The flow cell, including the WE, was placed in a 90◦ angle relative to the incoming laser beam and the scattering was therefore collected at a 180◦ configuration. The Raman measurements were done with the 780 nm (Renishaw, NIR diode laser) excitation wavelength. The Raman spectra were measured with potential intervals of 0.1 V from −0.2 V to 0.7 V (vs. Ag|AgCl| 3 M KCl). The films were first reduced at −0.2 V for 10 min before the Raman spectrum was recorded at this potential. Thereafter, the potential was always kept 2 min at the other potentials before the spectra were measured. The laser light was switched on just prior to the Raman measurements in order to avoid unnecessary irradiation of the oligoNMAA films. All measurements were conducted with a Renishaw Raman imaging microscope (with WireTM v1.3 Raman software) connected to a Leica DMLM microscope. The spectrometer was always calibrated against a Si-standard (520.0 cm−1 ) before starting the Raman measurements. The Raman measurements were conducted with 50% of the maximum laser power in order to avoid degradation of the oligoNMAA film. The detector time used was 2× 30 s. 2.5. Ex situ FTIR spectroscopy In the ex situ FTIR reflection measurement, the oligoNMAA film was deposited on indium tin oxide (ITO) glass substrate (Delta Technologies, Limited, 4–8 /sq). The same polymerization parameters were used as described for the oligoNMAA film preparation on GC (see Section 2.2). The potential was cycled 300 times (50 mV/s) within the potential interval of −0.2 V to 0.8 V. After the polymerization, the film was conditioned at 0.45 V for 2× 15 s in order to convert the oligoNMAA film into the conducting emeraldine form. The film was then washed with deionized water, air-dried and left overnight in a dessicator. The ex situ FTIR measurement was performed using a SeagullTM variable angle reflectance accessory (Harric Scientific) with an angle of incidence of 45◦ . 800 interferograms with a resolution of 4 cm−1 were recorded for the spectrum. The spectrum was recorded on a Bruker IFS 66/S FTIR instrument equipped with an MCT detector. For comparison, the FTIR spectrum of the NMAA monomer was measured. The spectrum was recorded on NMAA ground with KBr and pressed into a pellet. The same FTIR instrument as for the
3.1. Polymerization and characterization of PNMAA Fig. 2a shows typical CVs recorded during the electropolymerization of 0.1 M NMAA in 1.0 M HClO4 on GC. OligoNMAA films prepared with 50, 100, 200, 300 and 600 potential cycles were studied. The oxidation and reduction peak currents in all CVs increased during the electropolymerization, which is an indication of a continuous growth of the oligomer/polymer film, even during the 600th cycle. It was calculated that the total charge consumed by the electropolymerization of NMAA (Qp ) was 4.55 mC (Fig. 2a; 200 cycles). The polymerization charge of each individual potential cycle was practically constant throughout the electropolymerization process consisting of 200 cycles. The oligoNMAA film formed on the electrode was almost transparent. A slight brownish color was observed only for the oligoNMAA film prepared with 600 cycles on ITO. The SEM measurements of NMAA films prepared with 200 potential cycles reveal that very thin oligoNMAA films were formed on the GC surface. However, the film thickness could not be measured accurately because scattered islands of oligoNMAA with more intense film formation were observed in the SEM images. As can be seen in Fig. 2a, the oligoNMAA films prepared with 20 and 50 potential cycles show two oxidation peaks, at Ep,a1 ≈ 505 mV and Ep,a2 ≈ 626 mV. The first oxidation peak can be assigned to the LE/E transition and the second peak to the E/PN transition. After approximately 100 scans, the two oxidation peaks merge into one peak and only one redox couple can be observed at the potentials of Ep,a ≈ 525 mV and Ep,c ≈ 424 mV. The reason why only one oxidation and reduction peak is seen in the CV is probably due to the overlapping of the LE/E and E/PN transitions, like previously observed with poly(N-butylaniline) films [5,27]. Depending on the alkyl chain length, the poly(o-alkylanilines) have also overlapping LE/E and E/PN transitions [6,7]. The overlapping oxidation peak of oligoNMAA became narrower with increasing number of cycles, which indicates that the chain length of the conjugated oligomeric segments is possibly becoming more uniform. The CV of the oligoNMAA film (10 cycles, 50 mV/s) in a monomer-free solution of 1.0 M HClO4 is shown in Fig. 2b. The total charge consumed by the reversible doping process of the oligoNMAA film (Qd ) was 0.31 mC. Knowing the polymerization charge (Qp = 4.55 mC) and the charge corresponding to the doping process (Qd = 0.31 mC), and by assuming a 100% yield of the polymerization, it is possible to estimate the doping level (y) of the film under the assumption that 2 electrons/monomer unit are consumed for the polymerization reaction, as follows (by iteration): y=
(2 + y)Qd Qp
(1)
Eq. (1) gives y = 0.146, i.e. the doping level is 14.6% (ca. one charge per 7 monomer units) for this film (Qp = 4.55 mC). According to LaCroix and Diaz [30], a 100% yield for the polymerization would give a film thickness of approximately 200 nm. The SEM measurements indicate that the film thickness of the oligoNMAA film is much less than 200 nm. It is therefore reasonable to assume that the doping level of the oligoNMAA film is higher than 14.6%.
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Fig. 2. (a) Polymerization of 0.1 M NMAA on GC in 1.0 M HClO4 . The 1st, 20th, 50th, 100th and 200th cycles are shown; v = 50 mV/s and (b) characterization of the oligoNMAA film (polymerized by 200 cycles) in 1.0 M HClO4 (10 cycles; v = 50 mV/s).
The oxidation peak current of the oligoNMAA film is low compared to the PNMA film [27], which was prepared with the same polymerization parameters as the oNMAA film: only 40 A for an oligoNMAA film prepared with 200 cycles and 400 A for a PNMA film polymerized with 50 cycles. In the case of oligoNMAA, it has to be considered that the carboxylic groups can act as steric hinders in the polymerization process of NMAA and thus retard the film formation. Electropolymerization of NMAA on GC was also performed in 1.0 M HCl and 1.0 M H2 SO4 (200 cycles, 50 mV/s). The film growth proceeded smoothly in both acids. In 1.0 M H2 SO4 , the redox currents were similar to films polymerized in 1.0 M HClO4 , but the film formation was greatly suppressed in 1.0 M HCl. 3.2. UV–vis spectroscopy CPs have usually potential dependent electrochromic properties, which can be observed with polymer films prepared on transparent TO or ITO substrates. Any visible color changes could not be observed with the oligoNMAA films, which indicates that thin films of oligoNMAA was formed on the TO substrates. The UV–vis spectra of stepwise oxidation of oligoNMAA are shown in Fig. 3a. According to the oligoNMAA spectra, the LE form can be distinguished at potentials between −0.2 V and 0.3 V. In this potential range a weak and broad absorbance maximum is observed at ∼600–620 nm, which was also observed in the UV–vis spectra of PNBA films in our earlier work [27]. The chemical interpretation of this maximum is still unclear. The LE to E transition takes place at E > 0.3 V. This is supported by the growing absorbance at >900 nm which is typical for the E form. It should be noted, that the typical absorbance maximum at ∼420 nm, which is related to the conducting E form of PNANIs, was not observed in the UV–vis spectra of oligoNMAA. This may indicate that the formed film is of an oligomeric nature and that the charge carriers generated in oligoNMAA are different from PNANIs. The CV of oligoNMAA in Fig. 2b confirms also that the film is converted to its conducting state at E > 0.3 V. At 0.5 V, the UV–vis spectrum indicates that the oligoNMAA film is in the mixed emeraldine and PN form. The absorbance of the emeraldine form at >900 nm can still be seen in the spectrum together with the absorbance maximum characteristic of the PN form at 745–750 nm. At 0.6–0.75 V, the entire oligoNMAA film has been transformed to the PN form [27]. It can be concluded from the UV–vis spectra that the oligoNMAA undergo the LE–E–PN transitions like the PNANIs in our earlier
studies [27,28]. As can be seen in Fig. 3b, a certain degree of hysteresis can be observed when the potential is decreased stepwise from 0.75 V to 0.2 V. 3.3. Raman spectroscopy The Raman spectra of the oligoNMAA film were measured with the 780 nm laser excitation wavelength. It is known from previous studies[28], that the 780 nm laser enhances vibrations of the quinoid structures of PNMA. The UV–vis spectra of oligoNMAA indicate that this is the case also with oligoNMAA. Unfortunately, the fluorescent background was quite pronounced with the 514 nm laser excitation wavelength, which would enhance more equally the vibrations of both quinoid and benzenoid vibrations. The 780 nm laser was therefore chosen for this study and the Raman spectra of oligoNMAA measured with this laser excitation wavelength are shown in Fig. 4. The spectra have been separated from each other in order to clarify the spectral changes. The peak originating from HClO4 is observed at ∼890 cm−1 and marked with an asterisk. All main vibrational Raman bands and the references are listed in Table 1. It should also be noted, that the wavenumbers of the vibrational bands are approximate, because the Raman spectra of oligoNMAA were noisy. At potentials of −0.2 V to 0.3 V, the oligoNMAA spectra show clear vibrational bands at 1555 cm−1 , 1296 cm−1 , ∼1065 cm−1 , ∼752 cm−1 , 591 cm−1 and 443 cm−1 . The oligoNMAA film is, according to the cyclic voltammograms and UV–vis spectra (Figs. 2b and 3), in its fully reduced LE form in this potential range. The bands at ∼1065 cm−1 and 591 cm−1 are related to ring deformation [31] and in-plane amine deformation of benzenoid units [32], respectively. The band at 1555 cm−1 is assigned to C C ring stretching of quinoid units [33–35]. Its intensity increases strongly at higher potentials and simultaneously this band is shifted to 1580 cm−1 . The band at 443 cm−1 is assigned to out-of-plane C H wag of quinoid units [32]. This band and the band at ∼1065 cm−1 can clearly been seen through the whole potential range. Vibrations of quinoid units are probably enhanced by the 780 nm laser, which can explain the presence of quinoid bands in the spectra of the LE form. At 0.4 V, the intensity of the band at 1579 cm−1 originating from C C ring stretching of quinoid units is increasing. Also a new band arising at ∼1160 cm−1 becomes visible in the Raman spectrum. This vibrational band is assigned to C H in-plane bending of quinoid units [33,35]. The changes observed in the Raman spectrum at 0.4 V is in good accordance with the CV and UV–vis measurements of
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Fig. 3. In situ UV–vis spectra of the oligoNMAA film. The spectra were first measured from (a) −0.2 V to 0.75 V and then from (b) 0.75 V back to −0.2 V. The applied potentials are indicated by the numbers given in the figures.
oligoNMAA (Figs. 2b and 3). At this potential, the reduced LE form is transformed to the conducting E form. The shift of the band at 1296 cm−1 to 1338 cm−1 is also characteristic of the LE to E transition [28]. This is accompanied with the appearance of a new band at 1283 cm−1 . These bands are assigned to CN stretching of the E form of oligoNMAA [33–35]. At the moment the origin of the 1296 cm−1 band is unclear. At potentials of 0.5–0.7 V, it can be clearly seen that several new bands appeared in the Raman spectra and they changed their shape. According to the UV–vis spectra in Fig. 3, the E to PN transition takes place in this potential interval. The non-conducting PN form consist of both quinoid and benzenoid units, whereas the laser is enhancing the vibrations of quinoid units. This is in good accordance with the new bands arising in the Raman spectra at E > 0.5 V, which are originating from vibrations of quinoid units. The bands at 1158 cm−1 and 1477 cm−1 are assigned to C H in-plane bending [33,35] and C N stretching of quinoid units [23–25], respectively. The out-of-plane C H bending of quinoid units [32] can be seen at 808 cm−1 and the ring in-plane deformation of quinoid units [31,33] at 739 cm−1 . It can be concluded that the Raman spectra of oligoNMAA shows characteristic bands similar to PNANIs. The potential dependent changes observed in the Raman spectra are also in good accordance with the cyclic voltammetric and UV–vis measurements of oligoNMAA.
3.4. Ex situ FTIR spectroscopy The ex situ FTIR spectrum of the oligoNMAA film is shown in Fig. 5a. Characteristic IR vibrations assigned to aromatic ring stretching of quinoid and benzenoid units in oligoNMAA can be seen at 1595 cm−1 and 1485 cm−1 , respectively [36–38]. The peak at 1720 cm−1 is a typical vibrational band of the C O stretching originating from carboxylic groups [39(a),40]. The weak vibrations at 1406 cm−1 and 1439 cm−1 originate from a combination of C O stretching and O H deformation vibrations in a dimer of NMAA [39(b)]. The C N stretching vibration in quinoid units [39(c)] can be found at 1672 cm−1 and the rather weak vibration bands at 1377 cm−1 and ∼1218 cm−1 are characteristic of C N stretching vibrations in benzenoid units [36,39(c)]. The peak originating from NH bending vibration [40] is visible at 1524 cm−1 . The broad band at 1086 cm−1 originates from the ClO− 4 counterions [37,39(d)]. It overlaps with two bands originating from the O C= stretching vibration [40] and from the C O vibration [39(d)], which appear at ∼1128 and at ∼1041 cm−1 , respectively. A characteristic peak of the C H ring out-of-plane bending vibration of para-substituted benzene rings [36] can be seen at 822 cm−1 . The out-of-plane C H deformation vibration originating from 1,2-ring substitution [41] appears at 760 cm−1 . The oligoNMAA film was conditioned at 0.45 V for 30 s before the FTIR measurement and was therefore assumed
Table 1 Assignments of the Raman bands of oligoNMAA
B, Q and E denote the benzenoid, quinoid and emeraldine form, respectively. Laser excitation wavelength: 780 nm.
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to be in the conducting emeraldine form. This is supported by our results due to the coexistence of both quinoid and benzenoid units in the IR spectrum. Most importantly, the FTIR measurement shows that the carboxyl groups still remain covalently attached to the polymer backbone after the electropolymerization process. The ex situ FTIR spectrum of NMAA monomer recorded from a KBr pellet is shown in Fig. 5b. The C C stretching vibrations in the aromatic ring of NMAA monomer can be found at 1436 cm−1 and 1253 cm−1 [42]. The vibration band at 1660 cm−1 is characteristic of the C O groups in aromatic carboxylic acids [39(a)]. The vibration band at 1581 cm−1 may originate from aromatic amine N H deformation vibration [39(e)] and the band at 1516 cm−1 from N H bending vibration in N-methylanthranilic acids [40]. The C N stretching vibration in aromatic amines can be found at 1330 cm−1 [39(c)]. The broad band at 1253 cm−1 includes, in addition to the C C stretching vibrations, O H deformation vibrations from monomeric carboxylic acids [39(b)]. The band at 1166 cm−1 is assigned to C O stretching vibration also in monomeric carboxylic acids [39(b)]. The medium strong rather broad band at 898 cm−1 (missing from the spectrum of oligoNMAA) is characteristic of O H out−of-plain deformation vibration in monomeric carboxylic acids [39(b)]. The C H ring out-of-plane bending vibration of para-substituted benzene rings cannot be found in the spectrum of NMAA monomer, instead, a strong vibration band from 1,2-coupled phenyl rings can be found at 746 cm−1 [42]. In conclusion, absence of the vibration bands from monomeric carboxylic acids (1253 cm−1 , 1166 cm−1 and 898 cm−1 ) and the strong band from 1,2-coupled phenyl rings from the spectrum of oligoNMAA give evidence of formation of a longer chain structure in the oligoNMAA product. In addition, presence of broader vibration bands in the spectrum of oligoNMAA compared to NMAA monomer also supports formation of oligoNMAA. 4. Conclusions Fig. 4. In situ Raman spectra of the oligoNMAA film at different applied potentials. The potentials applied were: (a) −0.2 V, (b) 0.4 V, (c) 0.5 V and (d) 0.7 V. The spectra were measured with 0.1 V intervals. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk.
It is shown that a novel electrically conducting substituted PANI can be electropolymerized as thin films on GC, TO and ITO electrodes in 1.0 M HClO4 . This fundamental study shows that the formed films are probably of an oligomeric nature, but grow continuously in the potential interval of −0.2 V to 0.8 V during electropolymerization by cyclic voltammetry. The results of the UV–vis and Raman measurements indicate that the oligoNMAA films have three oxidation states (LE, E and PN) like PNANIs. FTIR measurements confirm that the carboxylic groups remain covalently attached to the polymer backbone during the electropolymerization process. Acknowledgements This work is part of the activity of the Åbo Akademi Process Chemistry Centre appointed to National Centre of Excellence by the Academy of Finland for 2000–2011. Maija Blomquist gratefully acknowledges the Finnish National Graduate School of Nanoscience for the financial support. References
Fig. 5. Ex situ FTIR spectrum of (a) the oligoNMAA film and (b) the NMAA monomer.
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[30] J.C. LaCroix, A.F. Diaz, J. Electrochem. Soc. 135 (1988) 1457. [31] S. Quillard, G. Louarn, J.P. Buisson, S. Lefrant, J. Masters, A.G. MacDiarmid, Synth. Met. 49–50 (1992) 525. [32] M. Cochet, G. Louarn, S. Quillard, M.I. Boyer, J.P. Buisson, S. Lefrant, J. Raman Spectrosc. 31 (2000) 1029. [33] M. Bartonek, N.S. Sariciftci, H. Kuzmany, Synth. Met. 36 (1990) 83. [34] A. Hugot-Le Goff, M.C. Bernard, Synth. Met. 60 (1993) 115. [35] S. Quillard, K. Berrada, G. Louarn, S. Lefrant, M. Lapkowski, A. Pron, New J. Chem. 19 (1995) 365. [36] Z. Ping, G.E. Nauer, H. Neugebauer, J. Theiner, J. Electroanal. Chem. 420 (1997) 301. [37] N.S. Sariciftci, H. Kuzmany, H. Neugebauer, A. Neckel, J. Chem. Phys. 92 (1990) 4530. [38] F. Cataldo, P. Maltese, Eur. Pol. J. 58 (2002) 1791. [39] (a) G. Socrates, Infrared and Raman Characteristics Group Frequencies, Tables and Charts, 3rd ed., Wiley, Chichester, 2000, Table 10.8; (b) G. Socrates, Infrared and Raman Characteristics Group Frequencies, Tables and Charts, 3rd ed., Wiley, Chichester, 2000, Table 10.9; (c) G. Socrates, Infrared and Raman Characteristics Group Frequencies, Tables and Charts, 3rd ed., Wiley, Chichester, 2000, Table 9.3; (d) G. Socrates, Infrared and Raman Characteristics Group Frequencies, Tables and Charts, 3rd ed., Wiley, Chichester, 2000, Table 22.1; (e) G. Socrates, Infrared and Raman Characteristics Group Frequencies, Tables and Charts, 3rd ed., Wiley, Chichester, 2000, Table 9.2. [40] R.A. Nyquist, in: Interpreting Infrared, Raman, and Nuclear Magnetic Resonance Spectra, vol 1, Academic Press, 2001, Table 10.5. [41] K. Cory Schomburg, R.L. Carley, Langmuir 17 (2001) 1983. [42] C. Kvarnström, A. Ivaska, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, vol. 4, John Wiley & Sons Ltd., 1997, pp. 487–515 (Chapter 9).
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Electropolymerization of N-methylanthranilic acid and spectroelectrochemical characterization of the formed film Maija Blomquist a,b , Tom Lindfors a,∗ , Rose-Marie Latonen a , Johan Bobacka a a b
Process Chemistry Centre, c/o Laboratory of Analytical Chemistry, Åbo Akademi University, Biskopsgatan 8, 20500 Turku/Åbo, Finland The Finnish National Graduate School in Nanoscience (NGS-NANO), Nanoscience Center, P.O. Box 35, 40014 University of Jyväskylä, Finland
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 1 July 2008 Accepted 1 August 2008 Available online 13 September 2008 Keywords: Poly(N-methylanthranilic acid) Electropolymerization Cyclic voltammetry UV–vis Raman and FTIR spectroscopy
a b s t r a c t The electropolymerization of N-methylanthranilic acid (NMAA) is reported in this paper. The monomer is substituted both at ortho- and N-position and, to the best of our knowledge, it has not been previously electropolymerized. Electropolymerization of NMAA was done on glassy carbon and optically transparent (indium) tin oxide electrodes. The obtained films, which are probably of an oligomeric nature (oligoNMMA), were characterized with cyclic voltammetry (CV), in situ UV–vis and Raman spectroscopy, ex situ FTIR spectroscopy and scanning electron microscopy (SEM). Our results show that NMAA can be electropolymerized as thin films in 1.0 M HClO4 , but the oxidation and reduction peak currents in the CVs indicate that the formed oligoNMAA films are thinner than poly(N-methylaniline) or poly(N-butylaniline) films prepared under similar conditions. The CV and UV–vis measurements confirm that oligoNMAA have three oxidation states like suggested in the redox scheme of substituted polyanilines. The Raman spectra of oligoNMAA also verify that more quinoid units are formed at higher potentials in accordance with the redox scheme. The ex situ FTIR measurement proves that covalently attached carboxylic acid groups are present in the film structure and attached to the oligoNMAA backbone. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In this fundamental study, the electropolymerization of Nmethylanthranilic acid (NMAA) and characterization of the formed oligomeric film (oligoNMAA) films are reported. The chemical structure of NMAA is shown in Fig. 1 and reveals that in addition to the methyl group in N-position, also a carboxylic group is coupled to the ortho-position of the monomer ring. The scientific motivation of this research was to study how the combination of these two substituents influences the electropolymerization of NMAA, which was done in an aqueous solution of 1.0 M HClO4 . Poly(N-alkylanilines) (PNANIs) are interesting materials because substitution of the polymer backbone of polyaniline (PANI) lead to new properties, for example improved polymer solubility and processability [1,2]. On the other hand, the substitution decreases the electrical conductivity, which could explain the minor interest in substituted PANIs [1,3,4]. Poly(N-methylaniline) (PNMA) has so far gained most attention among PNANIs, which have also low or no pH sensitivity compared with PANI [5]. The reason for the lower pH sensitivity of PNANIs is that the characteristic
∗ Corresponding author. Tel.: +358 22154422; fax: +358 22154479. E-mail address: Tom.Lindfors@abo.fi (T. Lindfors). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.08.001
emeraldine base (EB) to salt transition (ES) (acid–base reaction) of PANI is blocked by the N-substituent. The pH sensitivity of PANI can be considered as an undesired side reaction limiting the use of PANI in many practical applications. For example, in potentiometric PANI based ion sensor applications where pH is not kept constant, the measured sensor response of the analyte ion can be erroneous because it may also contain a contribution of the pH response of PANI. It can therefore be important to be able to eliminate the pH sensitivity by N-substitution in certain applications. Also different types of ring substituted PANIs have been reported in the literature [6–19]. Among these, the ortho-substituted PANIs have been mostly studied and have been reported to give higher polymerization yields than the meta-substituted PANIs [8]. In the polymerization of substituted PANIs, oligomers are easily formed and thus the film growth is somewhat hampered compared to PANI [2,6]. Even though the ring substitution results in polymer films with lower conductivity than PANI, the major advantage of ring substitution is that the polymer film can be made accessible for further functionalization [20] or e.g. ion exchange applications [21]. The electrochemistry of PNANIs and poly(o-alkylanilines) is generally agreed to follow Scheme 1, where the oxidation state of the polymer can be varied between the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms [6,22]. The
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2.2. Electropolymerization of NMAA
Fig. 1. The chemical structure of NMAA.
emeraldine form is the only electrically conducting form. The chemistry is thus slightly more complicated than for most other conducting polymers (CPs) because of the three potential dependent oxidation states. It was also recently reported [23] that the emeraldine form of PNMA has a polaronic structure similar to PANI [24–26]. This paper continues our earlier work on PNANIs [27,28] and poly(o-alkylanilines) [7] and will focus on electropolymerization of NMAA. To the best of our knowledge, NMAA has not been electropolymerized earlier. The oligoNMAA films were characterized with cyclic voltammetry (CV), in situ UV–vis and Raman spectroscopy, ex situ FTIR spectroscopy and scanning electron microscopy (SEM). 2. Experimental 2.1. Chemicals NMAA was obtained from Aldrich and used as received. HClO4 was obtained from J.T. Baker. Double distilled water was used throughout this work.
The oligoNMAA films were prepared on glassy carbon (GC) disc working electrodes (WE; A = 0.07 cm2 ) with a polytetrafluoroethylene (PTFE) body. Prior to electropolymerization, the electrodes were polished with 15, 9, 3 m and 1 m diamond paste (in this order) and with 0.3 m Al2 O3 powder, rinsed with deionized water and dried. The electropolymerization of 0.1 M NMAA was done in 1.0 M HClO4 by cycling the potential between −0.2 V and 0.8 V (50–600 cycles) at a scan rate of 50 mV/s. After the polymerization, the oligoNMAA films were characterized in monomer-free solution of 1.0 M HClO4 (10 cycles; 50 mV/s). A GC rod served as the counter electrode (CE) and a Ag|AgCl| 3 M KCl as the reference electrode (RE). The potential was controlled with an Autolab (PGSTAT 20 or PGSTAT 100) potentiostat using the GPES software. Prior to all measurements, the solutions were purged with nitrogen and during measurements nitrogen was passed over the solutions. 2.3. UV–vis spectroscopy The oligoNMAA films were electropolymerized on 4 mm thick quartz glasses coated with a thin layer of tin oxide (TO). The same polymerization parameters were used as described for the oligoNMAA film preparation on GC (see Section 2.2). The potential was cycled 600 times between −0.2 V and 0.8 V with a scan rate of 50 mV/s, in order to obtain oligoNMAA films that were thick enough for the UV–vis measurements. The formed films were then conditioned overnight in 1.0 M HClO4 . The TO coated glass on which the oligoNMAA film was polymerized, was used as the WE, a platinum and a Ag|AgCl wire served as CE and pseudo RE, respectively. All three electrodes were placed in a plastic cuvette filled with 1.0 M HClO4 and the UV–vis transmission spectra were recorded with a
Scheme 1. The oxidation and reduction mechanism of substituted PANIs showing the leucoemeraldine (LE), emeraldine (E) and pernigraniline (PN) forms. X and Y are the methyl and carboxyl substituents, respectively, and A− is a mobile anion.
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Hitachi U-2001 spectrophotometer. The spectra were recorded first at the open circuit potential, then within the potential interval of −0.2 V to 0.75 V and back to −0.2 V (vs. Ag|AgCl wire). The oligoNMAA films were first reduced at −0.2 V for 19 min before the UV–vis spectrum was recorded at this potential. Thereafter, the potential was always kept 9 min at the other applied potentials before the spectra were measured.
reflectance measurement equipped with a DTGS detector was used. 32 interferograms with a resolution of 4 cm−1 were recorded for the spectrum. 2.6. SEM measurements A LEO 1530 Gemini FEG-SEM instrument was used in the SEM measurements (acceleration voltage: 15 kV).
2.4. Raman spectroscopy 3. Results and discussion The oligoNMAA film was polymerized in a flow cell, which is described in our earlier work [29]. The same polymerization parameters were used as described for the oligoNMAA film prepared on GC (see Section 2.2). The potential was cycled 600 times between −0.2 V and 0.8 V (50 mV/s) in order to increase the film thickness. Both the polymerization and characterization solutions flowed continuously through the cell with a flow rate of approximately 0.3 ml/min. The solutions were transferred to the cell with a peristaltic pump. A Pt wire served as the CE and Ag|AgCl wire as the RE. The GC disc electrode (A = 0.07 cm2 ) was polished with 0.3 m Al2 O3 powder and rinsed with deionized water prior to the oligoNMAA film preparation. The potential was controlled with an Autolab (PGSTAT 20) potentiostat. After the polymerization and prior to the Raman measurements, the oligoNMAA films were equilibrated overnight in the flow cell filled with 1.0 M HClO4 . The flow cell, including the WE, was placed in a 90◦ angle relative to the incoming laser beam and the scattering was therefore collected at a 180◦ configuration. The Raman measurements were done with the 780 nm (Renishaw, NIR diode laser) excitation wavelength. The Raman spectra were measured with potential intervals of 0.1 V from −0.2 V to 0.7 V (vs. Ag|AgCl| 3 M KCl). The films were first reduced at −0.2 V for 10 min before the Raman spectrum was recorded at this potential. Thereafter, the potential was always kept 2 min at the other potentials before the spectra were measured. The laser light was switched on just prior to the Raman measurements in order to avoid unnecessary irradiation of the oligoNMAA films. All measurements were conducted with a Renishaw Raman imaging microscope (with WireTM v1.3 Raman software) connected to a Leica DMLM microscope. The spectrometer was always calibrated against a Si-standard (520.0 cm−1 ) before starting the Raman measurements. The Raman measurements were conducted with 50% of the maximum laser power in order to avoid degradation of the oligoNMAA film. The detector time used was 2× 30 s. 2.5. Ex situ FTIR spectroscopy In the ex situ FTIR reflection measurement, the oligoNMAA film was deposited on indium tin oxide (ITO) glass substrate (Delta Technologies, Limited, 4–8 /sq). The same polymerization parameters were used as described for the oligoNMAA film preparation on GC (see Section 2.2). The potential was cycled 300 times (50 mV/s) within the potential interval of −0.2 V to 0.8 V. After the polymerization, the film was conditioned at 0.45 V for 2× 15 s in order to convert the oligoNMAA film into the conducting emeraldine form. The film was then washed with deionized water, air-dried and left overnight in a dessicator. The ex situ FTIR measurement was performed using a SeagullTM variable angle reflectance accessory (Harric Scientific) with an angle of incidence of 45◦ . 800 interferograms with a resolution of 4 cm−1 were recorded for the spectrum. The spectrum was recorded on a Bruker IFS 66/S FTIR instrument equipped with an MCT detector. For comparison, the FTIR spectrum of the NMAA monomer was measured. The spectrum was recorded on NMAA ground with KBr and pressed into a pellet. The same FTIR instrument as for the
3.1. Polymerization and characterization of PNMAA Fig. 2a shows typical CVs recorded during the electropolymerization of 0.1 M NMAA in 1.0 M HClO4 on GC. OligoNMAA films prepared with 50, 100, 200, 300 and 600 potential cycles were studied. The oxidation and reduction peak currents in all CVs increased during the electropolymerization, which is an indication of a continuous growth of the oligomer/polymer film, even during the 600th cycle. It was calculated that the total charge consumed by the electropolymerization of NMAA (Qp ) was 4.55 mC (Fig. 2a; 200 cycles). The polymerization charge of each individual potential cycle was practically constant throughout the electropolymerization process consisting of 200 cycles. The oligoNMAA film formed on the electrode was almost transparent. A slight brownish color was observed only for the oligoNMAA film prepared with 600 cycles on ITO. The SEM measurements of NMAA films prepared with 200 potential cycles reveal that very thin oligoNMAA films were formed on the GC surface. However, the film thickness could not be measured accurately because scattered islands of oligoNMAA with more intense film formation were observed in the SEM images. As can be seen in Fig. 2a, the oligoNMAA films prepared with 20 and 50 potential cycles show two oxidation peaks, at Ep,a1 ≈ 505 mV and Ep,a2 ≈ 626 mV. The first oxidation peak can be assigned to the LE/E transition and the second peak to the E/PN transition. After approximately 100 scans, the two oxidation peaks merge into one peak and only one redox couple can be observed at the potentials of Ep,a ≈ 525 mV and Ep,c ≈ 424 mV. The reason why only one oxidation and reduction peak is seen in the CV is probably due to the overlapping of the LE/E and E/PN transitions, like previously observed with poly(N-butylaniline) films [5,27]. Depending on the alkyl chain length, the poly(o-alkylanilines) have also overlapping LE/E and E/PN transitions [6,7]. The overlapping oxidation peak of oligoNMAA became narrower with increasing number of cycles, which indicates that the chain length of the conjugated oligomeric segments is possibly becoming more uniform. The CV of the oligoNMAA film (10 cycles, 50 mV/s) in a monomer-free solution of 1.0 M HClO4 is shown in Fig. 2b. The total charge consumed by the reversible doping process of the oligoNMAA film (Qd ) was 0.31 mC. Knowing the polymerization charge (Qp = 4.55 mC) and the charge corresponding to the doping process (Qd = 0.31 mC), and by assuming a 100% yield of the polymerization, it is possible to estimate the doping level (y) of the film under the assumption that 2 electrons/monomer unit are consumed for the polymerization reaction, as follows (by iteration): y=
(2 + y)Qd Qp
(1)
Eq. (1) gives y = 0.146, i.e. the doping level is 14.6% (ca. one charge per 7 monomer units) for this film (Qp = 4.55 mC). According to LaCroix and Diaz [30], a 100% yield for the polymerization would give a film thickness of approximately 200 nm. The SEM measurements indicate that the film thickness of the oligoNMAA film is much less than 200 nm. It is therefore reasonable to assume that the doping level of the oligoNMAA film is higher than 14.6%.
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Fig. 2. (a) Polymerization of 0.1 M NMAA on GC in 1.0 M HClO4 . The 1st, 20th, 50th, 100th and 200th cycles are shown; v = 50 mV/s and (b) characterization of the oligoNMAA film (polymerized by 200 cycles) in 1.0 M HClO4 (10 cycles; v = 50 mV/s).
The oxidation peak current of the oligoNMAA film is low compared to the PNMA film [27], which was prepared with the same polymerization parameters as the oNMAA film: only 40 A for an oligoNMAA film prepared with 200 cycles and 400 A for a PNMA film polymerized with 50 cycles. In the case of oligoNMAA, it has to be considered that the carboxylic groups can act as steric hinders in the polymerization process of NMAA and thus retard the film formation. Electropolymerization of NMAA on GC was also performed in 1.0 M HCl and 1.0 M H2 SO4 (200 cycles, 50 mV/s). The film growth proceeded smoothly in both acids. In 1.0 M H2 SO4 , the redox currents were similar to films polymerized in 1.0 M HClO4 , but the film formation was greatly suppressed in 1.0 M HCl. 3.2. UV–vis spectroscopy CPs have usually potential dependent electrochromic properties, which can be observed with polymer films prepared on transparent TO or ITO substrates. Any visible color changes could not be observed with the oligoNMAA films, which indicates that thin films of oligoNMAA was formed on the TO substrates. The UV–vis spectra of stepwise oxidation of oligoNMAA are shown in Fig. 3a. According to the oligoNMAA spectra, the LE form can be distinguished at potentials between −0.2 V and 0.3 V. In this potential range a weak and broad absorbance maximum is observed at ∼600–620 nm, which was also observed in the UV–vis spectra of PNBA films in our earlier work [27]. The chemical interpretation of this maximum is still unclear. The LE to E transition takes place at E > 0.3 V. This is supported by the growing absorbance at >900 nm which is typical for the E form. It should be noted, that the typical absorbance maximum at ∼420 nm, which is related to the conducting E form of PNANIs, was not observed in the UV–vis spectra of oligoNMAA. This may indicate that the formed film is of an oligomeric nature and that the charge carriers generated in oligoNMAA are different from PNANIs. The CV of oligoNMAA in Fig. 2b confirms also that the film is converted to its conducting state at E > 0.3 V. At 0.5 V, the UV–vis spectrum indicates that the oligoNMAA film is in the mixed emeraldine and PN form. The absorbance of the emeraldine form at >900 nm can still be seen in the spectrum together with the absorbance maximum characteristic of the PN form at 745–750 nm. At 0.6–0.75 V, the entire oligoNMAA film has been transformed to the PN form [27]. It can be concluded from the UV–vis spectra that the oligoNMAA undergo the LE–E–PN transitions like the PNANIs in our earlier
studies [27,28]. As can be seen in Fig. 3b, a certain degree of hysteresis can be observed when the potential is decreased stepwise from 0.75 V to 0.2 V. 3.3. Raman spectroscopy The Raman spectra of the oligoNMAA film were measured with the 780 nm laser excitation wavelength. It is known from previous studies[28], that the 780 nm laser enhances vibrations of the quinoid structures of PNMA. The UV–vis spectra of oligoNMAA indicate that this is the case also with oligoNMAA. Unfortunately, the fluorescent background was quite pronounced with the 514 nm laser excitation wavelength, which would enhance more equally the vibrations of both quinoid and benzenoid vibrations. The 780 nm laser was therefore chosen for this study and the Raman spectra of oligoNMAA measured with this laser excitation wavelength are shown in Fig. 4. The spectra have been separated from each other in order to clarify the spectral changes. The peak originating from HClO4 is observed at ∼890 cm−1 and marked with an asterisk. All main vibrational Raman bands and the references are listed in Table 1. It should also be noted, that the wavenumbers of the vibrational bands are approximate, because the Raman spectra of oligoNMAA were noisy. At potentials of −0.2 V to 0.3 V, the oligoNMAA spectra show clear vibrational bands at 1555 cm−1 , 1296 cm−1 , ∼1065 cm−1 , ∼752 cm−1 , 591 cm−1 and 443 cm−1 . The oligoNMAA film is, according to the cyclic voltammograms and UV–vis spectra (Figs. 2b and 3), in its fully reduced LE form in this potential range. The bands at ∼1065 cm−1 and 591 cm−1 are related to ring deformation [31] and in-plane amine deformation of benzenoid units [32], respectively. The band at 1555 cm−1 is assigned to C C ring stretching of quinoid units [33–35]. Its intensity increases strongly at higher potentials and simultaneously this band is shifted to 1580 cm−1 . The band at 443 cm−1 is assigned to out-of-plane C H wag of quinoid units [32]. This band and the band at ∼1065 cm−1 can clearly been seen through the whole potential range. Vibrations of quinoid units are probably enhanced by the 780 nm laser, which can explain the presence of quinoid bands in the spectra of the LE form. At 0.4 V, the intensity of the band at 1579 cm−1 originating from C C ring stretching of quinoid units is increasing. Also a new band arising at ∼1160 cm−1 becomes visible in the Raman spectrum. This vibrational band is assigned to C H in-plane bending of quinoid units [33,35]. The changes observed in the Raman spectrum at 0.4 V is in good accordance with the CV and UV–vis measurements of
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Fig. 3. In situ UV–vis spectra of the oligoNMAA film. The spectra were first measured from (a) −0.2 V to 0.75 V and then from (b) 0.75 V back to −0.2 V. The applied potentials are indicated by the numbers given in the figures.
oligoNMAA (Figs. 2b and 3). At this potential, the reduced LE form is transformed to the conducting E form. The shift of the band at 1296 cm−1 to 1338 cm−1 is also characteristic of the LE to E transition [28]. This is accompanied with the appearance of a new band at 1283 cm−1 . These bands are assigned to CN stretching of the E form of oligoNMAA [33–35]. At the moment the origin of the 1296 cm−1 band is unclear. At potentials of 0.5–0.7 V, it can be clearly seen that several new bands appeared in the Raman spectra and they changed their shape. According to the UV–vis spectra in Fig. 3, the E to PN transition takes place in this potential interval. The non-conducting PN form consist of both quinoid and benzenoid units, whereas the laser is enhancing the vibrations of quinoid units. This is in good accordance with the new bands arising in the Raman spectra at E > 0.5 V, which are originating from vibrations of quinoid units. The bands at 1158 cm−1 and 1477 cm−1 are assigned to C H in-plane bending [33,35] and C N stretching of quinoid units [23–25], respectively. The out-of-plane C H bending of quinoid units [32] can be seen at 808 cm−1 and the ring in-plane deformation of quinoid units [31,33] at 739 cm−1 . It can be concluded that the Raman spectra of oligoNMAA shows characteristic bands similar to PNANIs. The potential dependent changes observed in the Raman spectra are also in good accordance with the cyclic voltammetric and UV–vis measurements of oligoNMAA.
3.4. Ex situ FTIR spectroscopy The ex situ FTIR spectrum of the oligoNMAA film is shown in Fig. 5a. Characteristic IR vibrations assigned to aromatic ring stretching of quinoid and benzenoid units in oligoNMAA can be seen at 1595 cm−1 and 1485 cm−1 , respectively [36–38]. The peak at 1720 cm−1 is a typical vibrational band of the C O stretching originating from carboxylic groups [39(a),40]. The weak vibrations at 1406 cm−1 and 1439 cm−1 originate from a combination of C O stretching and O H deformation vibrations in a dimer of NMAA [39(b)]. The C N stretching vibration in quinoid units [39(c)] can be found at 1672 cm−1 and the rather weak vibration bands at 1377 cm−1 and ∼1218 cm−1 are characteristic of C N stretching vibrations in benzenoid units [36,39(c)]. The peak originating from NH bending vibration [40] is visible at 1524 cm−1 . The broad band at 1086 cm−1 originates from the ClO− 4 counterions [37,39(d)]. It overlaps with two bands originating from the O C= stretching vibration [40] and from the C O vibration [39(d)], which appear at ∼1128 and at ∼1041 cm−1 , respectively. A characteristic peak of the C H ring out-of-plane bending vibration of para-substituted benzene rings [36] can be seen at 822 cm−1 . The out-of-plane C H deformation vibration originating from 1,2-ring substitution [41] appears at 760 cm−1 . The oligoNMAA film was conditioned at 0.45 V for 30 s before the FTIR measurement and was therefore assumed
Table 1 Assignments of the Raman bands of oligoNMAA
B, Q and E denote the benzenoid, quinoid and emeraldine form, respectively. Laser excitation wavelength: 780 nm.
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to be in the conducting emeraldine form. This is supported by our results due to the coexistence of both quinoid and benzenoid units in the IR spectrum. Most importantly, the FTIR measurement shows that the carboxyl groups still remain covalently attached to the polymer backbone after the electropolymerization process. The ex situ FTIR spectrum of NMAA monomer recorded from a KBr pellet is shown in Fig. 5b. The C C stretching vibrations in the aromatic ring of NMAA monomer can be found at 1436 cm−1 and 1253 cm−1 [42]. The vibration band at 1660 cm−1 is characteristic of the C O groups in aromatic carboxylic acids [39(a)]. The vibration band at 1581 cm−1 may originate from aromatic amine N H deformation vibration [39(e)] and the band at 1516 cm−1 from N H bending vibration in N-methylanthranilic acids [40]. The C N stretching vibration in aromatic amines can be found at 1330 cm−1 [39(c)]. The broad band at 1253 cm−1 includes, in addition to the C C stretching vibrations, O H deformation vibrations from monomeric carboxylic acids [39(b)]. The band at 1166 cm−1 is assigned to C O stretching vibration also in monomeric carboxylic acids [39(b)]. The medium strong rather broad band at 898 cm−1 (missing from the spectrum of oligoNMAA) is characteristic of O H out−of-plain deformation vibration in monomeric carboxylic acids [39(b)]. The C H ring out-of-plane bending vibration of para-substituted benzene rings cannot be found in the spectrum of NMAA monomer, instead, a strong vibration band from 1,2-coupled phenyl rings can be found at 746 cm−1 [42]. In conclusion, absence of the vibration bands from monomeric carboxylic acids (1253 cm−1 , 1166 cm−1 and 898 cm−1 ) and the strong band from 1,2-coupled phenyl rings from the spectrum of oligoNMAA give evidence of formation of a longer chain structure in the oligoNMAA product. In addition, presence of broader vibration bands in the spectrum of oligoNMAA compared to NMAA monomer also supports formation of oligoNMAA. 4. Conclusions Fig. 4. In situ Raman spectra of the oligoNMAA film at different applied potentials. The potentials applied were: (a) −0.2 V, (b) 0.4 V, (c) 0.5 V and (d) 0.7 V. The spectra were measured with 0.1 V intervals. Excitation wavelength: 780 nm. The peak originating from HClO4 is marked with an asterisk.
It is shown that a novel electrically conducting substituted PANI can be electropolymerized as thin films on GC, TO and ITO electrodes in 1.0 M HClO4 . This fundamental study shows that the formed films are probably of an oligomeric nature, but grow continuously in the potential interval of −0.2 V to 0.8 V during electropolymerization by cyclic voltammetry. The results of the UV–vis and Raman measurements indicate that the oligoNMAA films have three oxidation states (LE, E and PN) like PNANIs. FTIR measurements confirm that the carboxylic groups remain covalently attached to the polymer backbone during the electropolymerization process. Acknowledgements This work is part of the activity of the Åbo Akademi Process Chemistry Centre appointed to National Centre of Excellence by the Academy of Finland for 2000–2011. Maija Blomquist gratefully acknowledges the Finnish National Graduate School of Nanoscience for the financial support. References
Fig. 5. Ex situ FTIR spectrum of (a) the oligoNMAA film and (b) the NMAA monomer.
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