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In situ UV–vis spectroelectrochemical studies of the copolymerization of o-aminophenol and aniline
Synthetic Metals 156 (2006) 566–575
In situ UV–vis spectroelectrochemical studies of the copolymerization of o-aminophenol and aniline Anwar-ul-Haq Ali Shah, Rudolf Holze ∗ Institute f¨ur Chemie, AG Elektrochemie, Technische Universit¨at Chemnitz, 09107 Chemnitz, Germany Received 19 August 2005; received in revised form 15 February 2006; accepted 6 March 2006 Available online 2 May 2006
Abstract In situ spectroelectrochemical studies of the copolymerization of o-aminophenol (OAP) with aniline (ANI) were carried out. Electropolymerization at a constant potential was performed on indium tin oxide (ITO)-coated glass electrodes in aqueous sulfuric acid (0.5 M). Spectroelectrochemical results revealed the formation of an intermediate in the initial stage of copolymerization through the cross-reaction of OAP cation radicals and ANI cation radicals resulting in a head-to-tail dimer or oligomer. An absorption peak at 520 nm in the UV–vis spectra was assigned to this intermediate. The FTIR spectral analysis of the copolymer clearly demonstrates the incorporation of OAP units into the polymer backbone during polymerization. © 2006 Elsevier B.V. All rights reserved. Keywords: Spectroelectrochemistry; Copolymerization; Intermediates; Polyaniline; Poly(o-aminophenol)
1. Introduction Intrinsically conducting polymers have attracted the attention of researchers because of their numerous possibilities of commercial applications [1–3]. Polyaniline (PANI) is the most extensively studied conducting polymer [4,5], its use as an electrode material in the fabrication of secondary batteries [6], in microelectronics [7] and as an electrochromic display material [8] has been suggested because of its unique dopability, good redox reversibility, environmental stability and high electrical conductivity [9]. However, it has been observed that some of its properties still need further improvement. This can be effected by structural changes via the polymerization of substituted monomers or post-treatment [10–13]. Another method is copolymerization in which the polymerization solution contains a mixture of different monomers. Several reports on the copolymerization of aniline with substituted anilines are now available [14,15]. Chen et al. [14] have synthesized a copolymer from aniline and anthranilic acid (AA). They have reported an increase in the solubility and a decrease in conductivity of the copolymer with an increasing amount of AA in the copolymer. Manisankar et al. [16] prepared poly(aniline-co-4,4 -diaminodiphenyl sul∗
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fone) using cyclic voltammetry and showed that the polymer films could exhibit multicolor electrochromic behavior when the applied potential was changed. Electrochemical copolymerization of aniline with diphenylamine (DPA) using a pulsed potentiostatic method has been reported [17]. Li et al. [18] have obtained processable copolymer microparticles from the chemical oxidative copolymerization of chloroaniline and aniline. In situ UV–vis spectroscopy appears to be an effective and useful tool for studying the early stages of electropolymerization. Several reports are available on the use of in situ UV–vis spectroscopy to detect short-lived intermediates formed during electrooxidation of aniline and aniline derivatives [19]. Leger et al. [20] investigated the electropolymerization of o-toluidine by using UV–vis differential reflectance spectroscopy and identified possible intermediates as precursors in polymer formation. Zimmermann et al. [21] studied the initial states in the electropolymerization of aniline and p-aminodiphenylamine by in situ UV–vis and in situ FTIR spectroscopy. Santhosh et al. [22] reported copolymer formation between DPA and o-toluidine by potentiodynamic electropolymerization (i.e. during cyclic voltammetry of the feed solution) and in situ UV–vis spectroelectrochemical measurements. Malinauskas and Holze [23–25] have studied the early stages of electrooxidation of several ring substituted, N-alkysubstituted anilines and N-(sulfopropyl) aniline (NSPA) using ITO-coated glass as the working electrode.
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In all cases, stable intermediates were observed which react in the next following chemical step with neutral molecules to yield oligomers or polymers. They have also identified the interaction of the intermediate generated from NSPA with aniline to result in a copolymer. Electrochemical copolymerization of o-aminophenol (OAP) with aniline was reported by Mu [26]. Recently, we have reported the electrochemical copolymerization and bilayer structure formation of OAP with aniline and the characterization of the resulting copolymers with cyclic voltammetry and in situ conductivity [27]. It is of interest to investigate the copolymerization between these monomers with a spectroelectrochemical technique, which could provide separate data on the polymer growth and associated ion exchange processes. The present paper reports on in situ UV–vis spectroelectrochemical studies following the course of copolymerization of aniline with OAP in an attempt to identify conceivable stages of polymerization and also the subsequent copolymer formation. The incorporation of OAP units into the polymer backbone was identified by performing spectroelectrochemical studies with different feed ratios of OAP with a constant concentration of ANI. To verify the incorporation of OAP into the copolymer, PANI and copolymers were also deposited electrochemically on a gold electrode and characterized with FTIR spectroscopy. 2. Experimental All chemicals were of analytical grade. Aniline (Riedel-de H¨aen) was distilled under vacuum and stored under nitrogen in a refrigerator. o-Aminophenol (Fluka purum) was used as received. Ultrapure 18 M water (Seralpur pro 90C) was used for solution preparation. All solutions were made with 0.5 M sulfuric acid (Merck) as supporting electrolyte. UV–vis spectra were recorded with a PC-driven Shimadzu UV-2101 PC scanning spectrometer (resolution 0.1 nm). Spectroelectrochemical experiments were made in a quartz cuvette of 1 cm path length by inserting an indium-doped tin oxide (ITO)coated glass electrode (Merck) with a specific surface resistance of 10–20 cm−2 installed perpendicular to the light path. A platinum wire was used as counter electrode and a saturated calomel electrode (SCE) as reference connected to the cuvette with a salt bridge. Before each experiment, the ITO-coated glass sheets were degreased with acetone and rinsed with plenty of ultrapure water. In the reference channel of the spectrometer, a quartz cuvette containing an ITO-coated glass electrode without polymer coating was inserted. All the spectra recorded are background-corrected. By potentiostatic polymerization at ESCE = 0.9 V good adherent films of POAP could be grown on the ITO electrode. In our recent report [27], we have shown that reproducible films of POAP can also be grown on a gold electrode by cycling the potential between −0.2 < ESCE < 0.84 V at a scan rate of 50 mV/s; this procedure could be extended successfully to ITO electrodes as reported elsewhere [28]. The potentiostatic procedure was applied here for the sake of compatibility with literature data discussed below. For FTIR experiments PANI and copolymers were deposited potentiostatically on the surface of a gold electrode in a three-
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electrode setup with a custom built potentiostat. FTIR spectra were recorded with a Perkin-Elmer FTIR-1000 spectrophotometer and KBr pellets at 2 cm−1 resolution (eight scans each). 3. Results and discussion 3.1. Electrooxidation of o-aminophenol After switching the electrode potential of the ITO glass electrode in a solution of o-aminophenol to ESCE = 0.9 V, in initial spectra (after very few minutes) an absorption was found in the UV–vis spectrum at λ < 300 nm and a peak around λ = 470 nm associated with the early stages of electrooxidation (Fig. 1). The absorption at λ < 300 nm has been observed previously at λ = 258 nm during chemical polymerization of OAP and has been assigned to the → * transition in the aromatic benzene unit, whereas the absorption around λ = 470 corresponds to the cation radical of OAP [29]. A new band at λ = 410 nm soon developed and increased in intensity with the time of electrolysis. This peak is characteristic of the oxidized form of POAP [30]. A blue shift of the band at λ = 410 nm was observed with time of electrolysis. Both bands grew in intensity during continuous electrooxidation. Further interesting observations were noted on analyzing the behavior of these bands at various time intervals. Fig. 2 shows the growth of absorbance at λ = 410 and 470 nm in solutions of varying monomer concentrations, these wavelengths correspond to the two separate absorption bands seen in Fig. 1 in particular at low monomer concentrations. In the latter case, the band at λ = 410 nm becomes predominant in later stages of polymerization. With higher concentration of OAP a broad absorption of very low intensity was also observed in the red region of the visible spectrum especially after prolonged electrolysis. After interruption of electrolysis the band at λ = 470 nm quickly diminishes in intensity as compared to the intensity of the absorbance band at λ = 410 nm. Fig. 3 displays corresponding UV–vis spectra recorded after various time intervals after interruption of electrolysis. The tendencies observed (presented in Figs. 1–3) can be interpreted in the framework of a two-step mechanism of electrooxidation of OAP. After applying a sufficiently high positive potential, electrooxidation of OAP proceeds leading to a reaction intermediate absorbing at λ = 460–470 nm. In the following chemical step, the intermediate reacts with solution phase OAP molecules, yielding oligomers or polymers as the product of this consecutive process. The end product of electrolysis shows an absorbance at λ = 410 nm. After holding the ITO glass electrode for a certain time at a positive potential in a solution containing OAP, a thin polymer film was deposited on the electrode surface. This polymer film is bronze-brown and can be reduced electrochemically to its pale yellow (almost colorless) form at an appropriately negative potential. The redox response of POAP is attributed to oxidation–reduction processes of phenoxazine units in the polymer [31]. There is little spectroscopic evidence for the structure of the POAP. Nevertheless, the agreement of redox potential and spectroscopic data between 2-aminophenoxazin3-one (3APZ) and the polymer suggests that the main chain
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contains phenoxazine unit [32]. Fig. 4 shows UV–vis spectra of an ITO glass electrode covered with a POAP film obtained at different electrode potentials ranging from ESCE = −0.20 to 0.70 V. Three absorptions located at λ = 350, 410 and 610 nm are observable in the UV–vis spectra. These bands have been reported at λ = 340, 440 and 750 nm in the literature [33]. At ESCE = −0.20 V, the polymer is in the reduced state and the corresponding spectra show an absorption band located at ca. λ = 350 nm. Based on previous arguments [33], the band at λ = 350 nm can be attributed to the phenoxazine structure, which in turn corresponds to the totally reduced state of the polymer and disappears with an increase of the electrode potential. With increasing potential oxidation of the polymer takes place leading to the formation of radical cations in the polymer matrix. This process is associated with the development of a broad maximum at λ = 410 nm which corresponds to the oxidized phenoxazine units. Unexpected experimental results were observed in the red region of the spectra both during the electrooxidation of OAP with high concentration (see Fig. 1) and oxidation of POAP (Fig. 4). With higher concentration of OAP a broad absorption (shoulder) of very low intensity was observed in the red
region of the visible spectrum. This absorption at ca. λ = 610 nm has been reported at λ = 600 nm only in the reduced state of the polymer [34], but in our case this band (at λ = 610 nm) appears at 0.0 V, increases in intensity up to 0.4 V and then becomes nearly constant with further increase in potential. However, no absorption band was observed at 750 nm as reported elsewhere [31]. These experimental results might be explained by taking into account a recent approach towards the redox transition of POAP from its completely oxidized state to its completely reduced state proceeding through two consecutive reactions in which a charged intermediate species takes part [32]. According to this approach, during the oxidation of POAP, the incorporation of anions at less positive potentials and the expulsion of protons from the polymer at more positive potentials proceed simultaneously. The development of the two absorptions during the oxidation of POAP film in our experimental results can be matched with this suggestion. Since both oxidation processes take place simultaneously, therefore, two bands should be expected in the UV–vis spectra. This correlation may also provide help in the interpretation of the origin of the absorption of low intensity in the red region of the absorption spectra during the electrooxidation at high concentrations
Fig. 1. UV–vis spectra, obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing OAP in various concentrations (1, 2, 3 and 5 mM, as indicated).
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Fig. 2. Time dependence of the absorbance at λ = 410 nm (hollow circles) and λ = 470 nm (full circles) after stepping the electrode potential to ESCE = 0.90 V in solutions containing OAP in various concentrations (1, 2, 3 and 5 mM, as indicated).
Fig. 3. UV–vis spectra obtained at different time intervals (in minutes) after interruption of an electrolysis performed for 18 min. Dashed line shows a spectrum obtained at 18th min of electrolysis.
Fig. 4. UV–vis spectra of a POAP-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = −0.20 to 0.70 V at every 0.10 V. The POAP-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 5 mM OAP.
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of OAP (Fig. 1), which might arise from the second oxidation processes of oligomers or polymers of OAP. Although cyclic voltammograms of POAP show only one redox process the possibility of the existence of dicationic species cannot be ignored as reported by Ortega [35] on the basis of electron spin resonance (ESR) measurements. The author has suggested that the decrease and further absence of a detectable ESR signal at higher potentials might be due to the combination of polarons at positive potentials giving rise to bipolarons. Cintra and de Torresi [36] have reported very similar observations in the UV–vis spectra of a structurally closely related compound poly(5-amino-1-naphthol) during a change in potential from −0.2 to 0.7 V.
Fig. 5. UV–vis spectra obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing 10 mM (a) and 20 mM (b) ANI. Dashed lines show spectra obtained 5 min after interruption of electrolysis.
3.2. Electropolymerization of aniline In situ spectroelectrochemical measurements of homopolymerization of ANI were carried out with two different concentrations of aniline (10 and 20 mM) in aqueous solutions of 0.5 M H2 SO4 at an electrode potential of ESCE = 0.90 V. The spectra recorded during aniline oxidation depend on the concentration of aniline. At a low concentration of aniline (10 mM) an absorption band at λ = 350 nm and a broad band at λ = 700 nm were observed in the initial stages of polymerization (Fig. 5a). However, at a higher concentration (20 mM) two bands λ = 360 and 720 nm and a weak shoulder at λ = 445 nm were observed in the initial stages of polymerization (Fig. 5b). These bands have been reported at λ = 330, 440 and 720 nm elsewhere [37]. The absorption at λ = 445 nm was assigned to the aniline cation radical or oxidized benzidine dimer and the broad band at λ = 720 nm was assigned to the N-phenyl-paraphenylenediamine (PPD) dimer and its dication. The band at λ = 360 nm is assigned to the → * electronic transition of neutral species. Fig. 6a and b show UV–vis spectra of a PANI film on ITO glass electrode at different electrode potentials and the potential dependence of the absorbance at three selected wavelengths, respectively. The absorption around λ = 306 nm is caused by the → * transition of benzoid rings and is characteristic of the leucoemeraldine form of PANI. This band, as well as the decrease of its
Fig. 6. (a) UV–vis spectra of a PANI-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = 0.0 to 0.80 V at every 0.10 V. The PANI-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 20 mM ANI. (b) Absorbance vs. potential plot for three selected wavelengths, derived from spectra displayed above.
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Fig. 7. UV–vis spectra (a–d), obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing OAP and ANI at different concentrations (as indicated). (e) Long-wavelength part of spectrum (a).
intensity with electrode potential shifted to higher values have been reported at λ = 315 nm [38]. The main absorbance band in the red region of the spectra corresponds to the conducting emeraldine state of PANI. The growth of intensity of this
band, which proceeds with the potential shifted to higher values simultaneously with a decreasing intensity of the band at λ = 306 nm, shows a progressive oxidation of PANI film from its leucoemeraldine form into the emeraldine form. As reported
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Scheme 1. Copolymerization of aniline with o-aminophenol.
repeatedly [39–41], a blue shift of this band was observed during a potential shift to higher values. For the absorbance band located at λ = 420 nm an absorbance maximum as a function of electrode potential was observed around ESCE = 0.3 V. This band has been assigned to an intermediate state (polaron) formed during electrooxidation of the leucoemeraldine state of PANI [38]. 3.3. Electrooxidation of o-aminophenol and aniline A comparison of the in situ spectroelectrochemical results of the electrooxidation of mixtures of OAP and ANI with different molar concentrations of OAP in the feed and electrooxidation of OAP and ANI alone clearly shows distinct variations in the UV–vis spectra. Obviously, the incorporation of OAP units in the growing polymer during electrooxidation of the mixture of OAP and ANI changes the spectral characteristics. Fig. 7a–d shows spectra acquired during the electropolymerization of mixtures of OAP and ANI at various feed ratios of OAP (1, 2, 3
and 5 mM, higher concentrations were not employed because they impeded film formation) with a constant concentration of ANI (20 mM). An additional shoulder around λ = 520 nm absent in both cases of homopolymerizaion of OAP and ANI can be assigned as follows. At the applied potential (ESCE = 0.9 V) OAP and ANI can be oxidized to produce their respective cation radicals, which undergo cross-reaction (i.e. coupling of a OAP cation radical with ANI monomer and vice versa) to produce dimers/oligomers. The shoulder around λ = 520 nm is attributed to mixed dimer intermediate1 resulting from the cross-reaction between OAP and ANI cation radicals (see Scheme 1). The UV–vis absorption spectra recorded during the initial stages of electropolymerization of solutions containing both OAP and ANI show an absorption band at λ = 415 nm and a shoulder around λ = 520 nm. With low concentrations of OAP in the feed,
1 Semi-empirical calculations are currently pursued to support this assignment.
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however, an increase in the absorbance with low intensity can be observed in the red region of the visible spectrum especially after prolonged electrolysis. Fig. 7e shows an enlarged spectrum of the red region of Fig. 7a. The absorbance in the three characteristic regions of the spectra increases with polymerization time. The absorbance of the band at λ = 415 nm increases very rapidly as compared to the other bands. This band is also observed in the OAP electrooxidation alone at λ = 410 nm, but in that case, its development starts within a few minutes after commencement of electrolysis (Fig. 1) and has been assigned to POAP. In the present case (copolymerization), it develops from the very beginning of electrolysis and grows in intensity with the time of electrolysis. Also this band does not show any appreciable change in its peak position when increasing the feed ratio of OAP in the copolymerization mixture. On the other hand, the shoulder around λ = 520 nm assigned to the mixed dimer intermediate not only grows very slowly but also shows a blue shift and saturation in its intensity with increased concentration of OAP in the feed. The absorption in the red region can be assigned to the doped nature of the copolymer at ESCE = 0.90 V. The assignments of the peak to the mixed dimer intermediate is supported by the analysis of the spectral results after switching off the applied potential. After interruption of electrolysis the absorbance around λ = 520 nm shows a significant decrease as depicted in Figs. 8 and 9. The intermediates involved in the electropolymerization of N-alkyl-substituted anilines have also been reported to exhibit such a decrease of absorbance after switching off the applied potential [23]. Like with POAP and PANI, deposition of copolymer is observed on the ITO glass electrode after holding the electrode in a solution containing both OAP and ANI at positive potentials. The polymer film coated onto the electrode shows electrochromic properties and can be reduced and oxidized reversibly. Fig. 10a shows the UV–vis spectra of a copolymer film at different electrode potentials. Like with PANI three absorption bands are well defined. The band at λ = 300 nm
Fig. 8. UV–vis spectra obtained at different time intervals (in minutes) after interruption of electrolysis performed for 15 min in a solution containing 1 mM OAP and 20 mM ANI. Dashed line shows a spectrum obtained at 15th min of electrolysis.
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Fig. 9. Changes in absorbance for the peak (λ = 520 nm) corresponding to the intermediate formed as a result of cross-reaction between cation radicals of OAP and ANI after interruption of electrolysis. Previously electropolymerization was performed with ESCE = 0.90 V for solutions as described in Fig. 7: (a) , (b) , (c) , and (d) 䊉.
assigned to the → * transition of benzoid rings shows a decrease in its intensity with an increase in electrode potential. However, no red shift is observed in this band with progressive oxidation, whereas this shift was significant during PANI electrooxidation. At λ = 430 nm the band, which corresponds to the formation of the intermediate state during the electrooxidation of the leucoemeraldine state, is observed. For this band an absorbance maximum is attained at ESCE = 0.50 V rather than at ESCE = 0.3 V as for PANI. In addition this band does not exhibit any red shift as in the case of PANI. Differences were also observed for the main absorbance band in the red region of the spectra. Although the intensity of the band increases with progressive oxidation of the polymer its growth is roughly onethird of the growth in intensity of the corresponding band in PANI with progressive oxidation. Also this band attains maximal value at ESCE = 0.60 V which is not the case in PANI. In addition to this no conspicuous blue shift can be observed in this band with the increase of potential. This feature is much more prominent during PANI electrooxidation. Fig. 10b shows the absorbance at the three wavelengths as a function of applied potential. The spectroelectrochemical results obtained from the electrochemical polymerization of mixture of ANI and OAP clearly show the incorporation of OAP into the polymer during polymerization. This phenomenon is further supported by results obtained with FTIR spectroscopy. Fig. 11 shows the FTIR spectrum of PANI synthesized from an aqueous solution of 20 mM ANI in 0.5 M H2 SO4 . The peak at 3240 cm−1 is attributed to the N–H stretching vibrations. The characteristic bands at 1563–1565 cm−1 arise mainly from both C N and C C stretching vibrations of the quinoid diimine unit, whereas the band around 1475–1479 cm−1 is attributed to the C C ring stretching of the benzoid diamine unit. The bands around 1298 and
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Fig. 12. FTIR spectra of electrochemically synthesized poly(aniline-co-oaminophenol) with various concentrations of OAP in the comonomer feed (a) 1 mM, (b) 2 mM and (c) 3 mM. Fig. 10. (a) UV–vis spectra of a copolymer-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = 0.0 to 0.80 V at every 0.10 V. The copolymer-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 1 mM OAP and 20 mM ANI. (b) Absorbance vs. potential plot for three selected wavelengths, derived from spectra displayed above.
798 cm−1 can be assigned to C N stretching of the secondary aromatic amine and an aromatic C H out-of-plane bending modes, respectively [42]. The peak at 1120 cm−1 is attributed to the presence of SO2− 4 ions in the polymer matrix. Fig. 12
depicts the FTIR spectra of the copolymers synthesized with various concentrations of OAP (1, 2 and 3 mM) in the feed with a constant concentration of ANI (20 mM). Apparently, the FTIR spectra of copolymers present the same picture as that of PANI. However, in the FTIR spectra of copolymers an absorption peak appears at 1402 cm−1 which is not present in the spectrum of PANI. This peak at 1402 cm−1 , which becomes more prominent with the increase of OAP concentration in the feed, is indicative of the C–O–H deformation vibration of phenols [43]. The IR spectrum of OAP also shows a peak at about 1400 cm−1 [44]. Thus, the peak at 1402 cm−1 in Fig. 12 can be considered as evidence of the presence of OAP unit in the polymer backbone. The increase in its intensity with an increase of the concentration of OAP in the comonomer feed suggests the incorporation of more OAP units in the resulting copolymer. 4. Conclusions
Fig. 11. FTIR spectrum of electrochemically synthesized polyaniline.
UV–vis spectroelectrochemical studies of the homopolymerization of OAP and copolymerization of OAP with ANI provide evidence for the incorporation of OAP units into the copolymer. The generation of intermediate species having both OAP and ANI units through the cross-reaction between OAP cation radical and ANI cation radical, was concluded from the appearance of an absorbance band around λ = 520 nm. The decrease in absorbance for this band after switching off the potential supports this assignment. The band observed at 1402 cm−1 in the FTIR spectra supports the OAP incorporation into the copolymers.
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Acknowledgements One of us (A.A.S.) acknowledges financial support from the Higher Education Commission, Islamabad, Pakistan. Further financial support from the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft (Graduiertenkolleg GRK 829/1) is gratefully appreciated. References [1] L.A. Majewski, M. Grell, Synth. Met. 151 (2005) 175. [2] A.A. Argun, J.R. Reynolds, J. Mater. Chem. 15 (2005) 1793. [3] Frost, Sullivan, Conductive Polymers: Ease of Processing Spearheads Commercial Success, third ed., 2003. [4] B. Wessling, Synth. Met. 93 (1998) 143. [5] J.R. Santos Jr., J.A. Malmonge, A.J.G.C. Silva, A.J. Motheo, Y.P. Mascarenhas, L.H.C. Mattoso, Synth. Met. 69 (1995) 141. [6] K.S. Ryu, K.M. Kim, Y.S. Hong, Y.J. Park, S.H. Chang, Bull. Korean Chem. Soc. 23 (2002) 1144. [7] M. Angelopoulos, IBM. J. Res. Dev. 45 (2001) 57. [8] T. Kobayashi, H. Yoneyama, H. Tamura, J. Electroanal. Chem. 177 (1984) 281. [9] Y. Wei, R. Hariharan, S.A. Patel, Macromolecules 23 (1990) 758 (and references therein). [10] H. Tang, A. Kitani, S. Ito, Electrochim. Acta 42 (1997) 3421. [11] S. Ye, S. Besner, L.H. Dao, A.K. Vijh, J. Electroanal. Chem. 381 (1995) 71. [12] R. Holze, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 8, Academic Press, SanDiego, 2001, p. 209. [13] R. Holze, in: H.S. Nalwa (Ed.), Advanced Functional Molecules and Polymers, vol. 2, Gordon & Breach, Amsterdam, 2001, p. 171. [14] H.S.O. Chan, S.C. Ng, W.S. Sim, K.L. Tan, B.T.G. Tan, Macromolecules 25 (1992) 6029. [15] M.T. Nguyen, A.F. Diaz, Macromolecules 28 (1995) 3411. [16] P. Manisankar, C. Vehi, G. Selvanathan, R.M. Somasundaram, Chem. Mater. 17 (2005) 1722. [17] V. Rajendran, A. Gopalan, T. Vasudevan, T.C. Wen, J. Electrochem. Soc. 147 (2000) 3014.
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In situ UV–vis spectroelectrochemical studies of the copolymerization of o-aminophenol and aniline Anwar-ul-Haq Ali Shah, Rudolf Holze ∗ Institute f¨ur Chemie, AG Elektrochemie, Technische Universit¨at Chemnitz, 09107 Chemnitz, Germany Received 19 August 2005; received in revised form 15 February 2006; accepted 6 March 2006 Available online 2 May 2006
Abstract In situ spectroelectrochemical studies of the copolymerization of o-aminophenol (OAP) with aniline (ANI) were carried out. Electropolymerization at a constant potential was performed on indium tin oxide (ITO)-coated glass electrodes in aqueous sulfuric acid (0.5 M). Spectroelectrochemical results revealed the formation of an intermediate in the initial stage of copolymerization through the cross-reaction of OAP cation radicals and ANI cation radicals resulting in a head-to-tail dimer or oligomer. An absorption peak at 520 nm in the UV–vis spectra was assigned to this intermediate. The FTIR spectral analysis of the copolymer clearly demonstrates the incorporation of OAP units into the polymer backbone during polymerization. © 2006 Elsevier B.V. All rights reserved. Keywords: Spectroelectrochemistry; Copolymerization; Intermediates; Polyaniline; Poly(o-aminophenol)
1. Introduction Intrinsically conducting polymers have attracted the attention of researchers because of their numerous possibilities of commercial applications [1–3]. Polyaniline (PANI) is the most extensively studied conducting polymer [4,5], its use as an electrode material in the fabrication of secondary batteries [6], in microelectronics [7] and as an electrochromic display material [8] has been suggested because of its unique dopability, good redox reversibility, environmental stability and high electrical conductivity [9]. However, it has been observed that some of its properties still need further improvement. This can be effected by structural changes via the polymerization of substituted monomers or post-treatment [10–13]. Another method is copolymerization in which the polymerization solution contains a mixture of different monomers. Several reports on the copolymerization of aniline with substituted anilines are now available [14,15]. Chen et al. [14] have synthesized a copolymer from aniline and anthranilic acid (AA). They have reported an increase in the solubility and a decrease in conductivity of the copolymer with an increasing amount of AA in the copolymer. Manisankar et al. [16] prepared poly(aniline-co-4,4 -diaminodiphenyl sul∗
Corresponding author. Tel.: +49 371 5311 509; fax: +49 371 5311 832. E-mail address: [email protected] (R. Holze).
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.03.001
fone) using cyclic voltammetry and showed that the polymer films could exhibit multicolor electrochromic behavior when the applied potential was changed. Electrochemical copolymerization of aniline with diphenylamine (DPA) using a pulsed potentiostatic method has been reported [17]. Li et al. [18] have obtained processable copolymer microparticles from the chemical oxidative copolymerization of chloroaniline and aniline. In situ UV–vis spectroscopy appears to be an effective and useful tool for studying the early stages of electropolymerization. Several reports are available on the use of in situ UV–vis spectroscopy to detect short-lived intermediates formed during electrooxidation of aniline and aniline derivatives [19]. Leger et al. [20] investigated the electropolymerization of o-toluidine by using UV–vis differential reflectance spectroscopy and identified possible intermediates as precursors in polymer formation. Zimmermann et al. [21] studied the initial states in the electropolymerization of aniline and p-aminodiphenylamine by in situ UV–vis and in situ FTIR spectroscopy. Santhosh et al. [22] reported copolymer formation between DPA and o-toluidine by potentiodynamic electropolymerization (i.e. during cyclic voltammetry of the feed solution) and in situ UV–vis spectroelectrochemical measurements. Malinauskas and Holze [23–25] have studied the early stages of electrooxidation of several ring substituted, N-alkysubstituted anilines and N-(sulfopropyl) aniline (NSPA) using ITO-coated glass as the working electrode.
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In all cases, stable intermediates were observed which react in the next following chemical step with neutral molecules to yield oligomers or polymers. They have also identified the interaction of the intermediate generated from NSPA with aniline to result in a copolymer. Electrochemical copolymerization of o-aminophenol (OAP) with aniline was reported by Mu [26]. Recently, we have reported the electrochemical copolymerization and bilayer structure formation of OAP with aniline and the characterization of the resulting copolymers with cyclic voltammetry and in situ conductivity [27]. It is of interest to investigate the copolymerization between these monomers with a spectroelectrochemical technique, which could provide separate data on the polymer growth and associated ion exchange processes. The present paper reports on in situ UV–vis spectroelectrochemical studies following the course of copolymerization of aniline with OAP in an attempt to identify conceivable stages of polymerization and also the subsequent copolymer formation. The incorporation of OAP units into the polymer backbone was identified by performing spectroelectrochemical studies with different feed ratios of OAP with a constant concentration of ANI. To verify the incorporation of OAP into the copolymer, PANI and copolymers were also deposited electrochemically on a gold electrode and characterized with FTIR spectroscopy. 2. Experimental All chemicals were of analytical grade. Aniline (Riedel-de H¨aen) was distilled under vacuum and stored under nitrogen in a refrigerator. o-Aminophenol (Fluka purum) was used as received. Ultrapure 18 M water (Seralpur pro 90C) was used for solution preparation. All solutions were made with 0.5 M sulfuric acid (Merck) as supporting electrolyte. UV–vis spectra were recorded with a PC-driven Shimadzu UV-2101 PC scanning spectrometer (resolution 0.1 nm). Spectroelectrochemical experiments were made in a quartz cuvette of 1 cm path length by inserting an indium-doped tin oxide (ITO)coated glass electrode (Merck) with a specific surface resistance of 10–20 cm−2 installed perpendicular to the light path. A platinum wire was used as counter electrode and a saturated calomel electrode (SCE) as reference connected to the cuvette with a salt bridge. Before each experiment, the ITO-coated glass sheets were degreased with acetone and rinsed with plenty of ultrapure water. In the reference channel of the spectrometer, a quartz cuvette containing an ITO-coated glass electrode without polymer coating was inserted. All the spectra recorded are background-corrected. By potentiostatic polymerization at ESCE = 0.9 V good adherent films of POAP could be grown on the ITO electrode. In our recent report [27], we have shown that reproducible films of POAP can also be grown on a gold electrode by cycling the potential between −0.2 < ESCE < 0.84 V at a scan rate of 50 mV/s; this procedure could be extended successfully to ITO electrodes as reported elsewhere [28]. The potentiostatic procedure was applied here for the sake of compatibility with literature data discussed below. For FTIR experiments PANI and copolymers were deposited potentiostatically on the surface of a gold electrode in a three-
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electrode setup with a custom built potentiostat. FTIR spectra were recorded with a Perkin-Elmer FTIR-1000 spectrophotometer and KBr pellets at 2 cm−1 resolution (eight scans each). 3. Results and discussion 3.1. Electrooxidation of o-aminophenol After switching the electrode potential of the ITO glass electrode in a solution of o-aminophenol to ESCE = 0.9 V, in initial spectra (after very few minutes) an absorption was found in the UV–vis spectrum at λ < 300 nm and a peak around λ = 470 nm associated with the early stages of electrooxidation (Fig. 1). The absorption at λ < 300 nm has been observed previously at λ = 258 nm during chemical polymerization of OAP and has been assigned to the → * transition in the aromatic benzene unit, whereas the absorption around λ = 470 corresponds to the cation radical of OAP [29]. A new band at λ = 410 nm soon developed and increased in intensity with the time of electrolysis. This peak is characteristic of the oxidized form of POAP [30]. A blue shift of the band at λ = 410 nm was observed with time of electrolysis. Both bands grew in intensity during continuous electrooxidation. Further interesting observations were noted on analyzing the behavior of these bands at various time intervals. Fig. 2 shows the growth of absorbance at λ = 410 and 470 nm in solutions of varying monomer concentrations, these wavelengths correspond to the two separate absorption bands seen in Fig. 1 in particular at low monomer concentrations. In the latter case, the band at λ = 410 nm becomes predominant in later stages of polymerization. With higher concentration of OAP a broad absorption of very low intensity was also observed in the red region of the visible spectrum especially after prolonged electrolysis. After interruption of electrolysis the band at λ = 470 nm quickly diminishes in intensity as compared to the intensity of the absorbance band at λ = 410 nm. Fig. 3 displays corresponding UV–vis spectra recorded after various time intervals after interruption of electrolysis. The tendencies observed (presented in Figs. 1–3) can be interpreted in the framework of a two-step mechanism of electrooxidation of OAP. After applying a sufficiently high positive potential, electrooxidation of OAP proceeds leading to a reaction intermediate absorbing at λ = 460–470 nm. In the following chemical step, the intermediate reacts with solution phase OAP molecules, yielding oligomers or polymers as the product of this consecutive process. The end product of electrolysis shows an absorbance at λ = 410 nm. After holding the ITO glass electrode for a certain time at a positive potential in a solution containing OAP, a thin polymer film was deposited on the electrode surface. This polymer film is bronze-brown and can be reduced electrochemically to its pale yellow (almost colorless) form at an appropriately negative potential. The redox response of POAP is attributed to oxidation–reduction processes of phenoxazine units in the polymer [31]. There is little spectroscopic evidence for the structure of the POAP. Nevertheless, the agreement of redox potential and spectroscopic data between 2-aminophenoxazin3-one (3APZ) and the polymer suggests that the main chain
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contains phenoxazine unit [32]. Fig. 4 shows UV–vis spectra of an ITO glass electrode covered with a POAP film obtained at different electrode potentials ranging from ESCE = −0.20 to 0.70 V. Three absorptions located at λ = 350, 410 and 610 nm are observable in the UV–vis spectra. These bands have been reported at λ = 340, 440 and 750 nm in the literature [33]. At ESCE = −0.20 V, the polymer is in the reduced state and the corresponding spectra show an absorption band located at ca. λ = 350 nm. Based on previous arguments [33], the band at λ = 350 nm can be attributed to the phenoxazine structure, which in turn corresponds to the totally reduced state of the polymer and disappears with an increase of the electrode potential. With increasing potential oxidation of the polymer takes place leading to the formation of radical cations in the polymer matrix. This process is associated with the development of a broad maximum at λ = 410 nm which corresponds to the oxidized phenoxazine units. Unexpected experimental results were observed in the red region of the spectra both during the electrooxidation of OAP with high concentration (see Fig. 1) and oxidation of POAP (Fig. 4). With higher concentration of OAP a broad absorption (shoulder) of very low intensity was observed in the red
region of the visible spectrum. This absorption at ca. λ = 610 nm has been reported at λ = 600 nm only in the reduced state of the polymer [34], but in our case this band (at λ = 610 nm) appears at 0.0 V, increases in intensity up to 0.4 V and then becomes nearly constant with further increase in potential. However, no absorption band was observed at 750 nm as reported elsewhere [31]. These experimental results might be explained by taking into account a recent approach towards the redox transition of POAP from its completely oxidized state to its completely reduced state proceeding through two consecutive reactions in which a charged intermediate species takes part [32]. According to this approach, during the oxidation of POAP, the incorporation of anions at less positive potentials and the expulsion of protons from the polymer at more positive potentials proceed simultaneously. The development of the two absorptions during the oxidation of POAP film in our experimental results can be matched with this suggestion. Since both oxidation processes take place simultaneously, therefore, two bands should be expected in the UV–vis spectra. This correlation may also provide help in the interpretation of the origin of the absorption of low intensity in the red region of the absorption spectra during the electrooxidation at high concentrations
Fig. 1. UV–vis spectra, obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing OAP in various concentrations (1, 2, 3 and 5 mM, as indicated).
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Fig. 2. Time dependence of the absorbance at λ = 410 nm (hollow circles) and λ = 470 nm (full circles) after stepping the electrode potential to ESCE = 0.90 V in solutions containing OAP in various concentrations (1, 2, 3 and 5 mM, as indicated).
Fig. 3. UV–vis spectra obtained at different time intervals (in minutes) after interruption of an electrolysis performed for 18 min. Dashed line shows a spectrum obtained at 18th min of electrolysis.
Fig. 4. UV–vis spectra of a POAP-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = −0.20 to 0.70 V at every 0.10 V. The POAP-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 5 mM OAP.
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of OAP (Fig. 1), which might arise from the second oxidation processes of oligomers or polymers of OAP. Although cyclic voltammograms of POAP show only one redox process the possibility of the existence of dicationic species cannot be ignored as reported by Ortega [35] on the basis of electron spin resonance (ESR) measurements. The author has suggested that the decrease and further absence of a detectable ESR signal at higher potentials might be due to the combination of polarons at positive potentials giving rise to bipolarons. Cintra and de Torresi [36] have reported very similar observations in the UV–vis spectra of a structurally closely related compound poly(5-amino-1-naphthol) during a change in potential from −0.2 to 0.7 V.
Fig. 5. UV–vis spectra obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing 10 mM (a) and 20 mM (b) ANI. Dashed lines show spectra obtained 5 min after interruption of electrolysis.
3.2. Electropolymerization of aniline In situ spectroelectrochemical measurements of homopolymerization of ANI were carried out with two different concentrations of aniline (10 and 20 mM) in aqueous solutions of 0.5 M H2 SO4 at an electrode potential of ESCE = 0.90 V. The spectra recorded during aniline oxidation depend on the concentration of aniline. At a low concentration of aniline (10 mM) an absorption band at λ = 350 nm and a broad band at λ = 700 nm were observed in the initial stages of polymerization (Fig. 5a). However, at a higher concentration (20 mM) two bands λ = 360 and 720 nm and a weak shoulder at λ = 445 nm were observed in the initial stages of polymerization (Fig. 5b). These bands have been reported at λ = 330, 440 and 720 nm elsewhere [37]. The absorption at λ = 445 nm was assigned to the aniline cation radical or oxidized benzidine dimer and the broad band at λ = 720 nm was assigned to the N-phenyl-paraphenylenediamine (PPD) dimer and its dication. The band at λ = 360 nm is assigned to the → * electronic transition of neutral species. Fig. 6a and b show UV–vis spectra of a PANI film on ITO glass electrode at different electrode potentials and the potential dependence of the absorbance at three selected wavelengths, respectively. The absorption around λ = 306 nm is caused by the → * transition of benzoid rings and is characteristic of the leucoemeraldine form of PANI. This band, as well as the decrease of its
Fig. 6. (a) UV–vis spectra of a PANI-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = 0.0 to 0.80 V at every 0.10 V. The PANI-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 20 mM ANI. (b) Absorbance vs. potential plot for three selected wavelengths, derived from spectra displayed above.
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Fig. 7. UV–vis spectra (a–d), obtained at different time intervals (in minutes) after applying an electrode potential of ESCE = 0.90 V in solutions containing OAP and ANI at different concentrations (as indicated). (e) Long-wavelength part of spectrum (a).
intensity with electrode potential shifted to higher values have been reported at λ = 315 nm [38]. The main absorbance band in the red region of the spectra corresponds to the conducting emeraldine state of PANI. The growth of intensity of this
band, which proceeds with the potential shifted to higher values simultaneously with a decreasing intensity of the band at λ = 306 nm, shows a progressive oxidation of PANI film from its leucoemeraldine form into the emeraldine form. As reported
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Scheme 1. Copolymerization of aniline with o-aminophenol.
repeatedly [39–41], a blue shift of this band was observed during a potential shift to higher values. For the absorbance band located at λ = 420 nm an absorbance maximum as a function of electrode potential was observed around ESCE = 0.3 V. This band has been assigned to an intermediate state (polaron) formed during electrooxidation of the leucoemeraldine state of PANI [38]. 3.3. Electrooxidation of o-aminophenol and aniline A comparison of the in situ spectroelectrochemical results of the electrooxidation of mixtures of OAP and ANI with different molar concentrations of OAP in the feed and electrooxidation of OAP and ANI alone clearly shows distinct variations in the UV–vis spectra. Obviously, the incorporation of OAP units in the growing polymer during electrooxidation of the mixture of OAP and ANI changes the spectral characteristics. Fig. 7a–d shows spectra acquired during the electropolymerization of mixtures of OAP and ANI at various feed ratios of OAP (1, 2, 3
and 5 mM, higher concentrations were not employed because they impeded film formation) with a constant concentration of ANI (20 mM). An additional shoulder around λ = 520 nm absent in both cases of homopolymerizaion of OAP and ANI can be assigned as follows. At the applied potential (ESCE = 0.9 V) OAP and ANI can be oxidized to produce their respective cation radicals, which undergo cross-reaction (i.e. coupling of a OAP cation radical with ANI monomer and vice versa) to produce dimers/oligomers. The shoulder around λ = 520 nm is attributed to mixed dimer intermediate1 resulting from the cross-reaction between OAP and ANI cation radicals (see Scheme 1). The UV–vis absorption spectra recorded during the initial stages of electropolymerization of solutions containing both OAP and ANI show an absorption band at λ = 415 nm and a shoulder around λ = 520 nm. With low concentrations of OAP in the feed,
1 Semi-empirical calculations are currently pursued to support this assignment.
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however, an increase in the absorbance with low intensity can be observed in the red region of the visible spectrum especially after prolonged electrolysis. Fig. 7e shows an enlarged spectrum of the red region of Fig. 7a. The absorbance in the three characteristic regions of the spectra increases with polymerization time. The absorbance of the band at λ = 415 nm increases very rapidly as compared to the other bands. This band is also observed in the OAP electrooxidation alone at λ = 410 nm, but in that case, its development starts within a few minutes after commencement of electrolysis (Fig. 1) and has been assigned to POAP. In the present case (copolymerization), it develops from the very beginning of electrolysis and grows in intensity with the time of electrolysis. Also this band does not show any appreciable change in its peak position when increasing the feed ratio of OAP in the copolymerization mixture. On the other hand, the shoulder around λ = 520 nm assigned to the mixed dimer intermediate not only grows very slowly but also shows a blue shift and saturation in its intensity with increased concentration of OAP in the feed. The absorption in the red region can be assigned to the doped nature of the copolymer at ESCE = 0.90 V. The assignments of the peak to the mixed dimer intermediate is supported by the analysis of the spectral results after switching off the applied potential. After interruption of electrolysis the absorbance around λ = 520 nm shows a significant decrease as depicted in Figs. 8 and 9. The intermediates involved in the electropolymerization of N-alkyl-substituted anilines have also been reported to exhibit such a decrease of absorbance after switching off the applied potential [23]. Like with POAP and PANI, deposition of copolymer is observed on the ITO glass electrode after holding the electrode in a solution containing both OAP and ANI at positive potentials. The polymer film coated onto the electrode shows electrochromic properties and can be reduced and oxidized reversibly. Fig. 10a shows the UV–vis spectra of a copolymer film at different electrode potentials. Like with PANI three absorption bands are well defined. The band at λ = 300 nm
Fig. 8. UV–vis spectra obtained at different time intervals (in minutes) after interruption of electrolysis performed for 15 min in a solution containing 1 mM OAP and 20 mM ANI. Dashed line shows a spectrum obtained at 15th min of electrolysis.
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Fig. 9. Changes in absorbance for the peak (λ = 520 nm) corresponding to the intermediate formed as a result of cross-reaction between cation radicals of OAP and ANI after interruption of electrolysis. Previously electropolymerization was performed with ESCE = 0.90 V for solutions as described in Fig. 7: (a) , (b) , (c) , and (d) 䊉.
assigned to the → * transition of benzoid rings shows a decrease in its intensity with an increase in electrode potential. However, no red shift is observed in this band with progressive oxidation, whereas this shift was significant during PANI electrooxidation. At λ = 430 nm the band, which corresponds to the formation of the intermediate state during the electrooxidation of the leucoemeraldine state, is observed. For this band an absorbance maximum is attained at ESCE = 0.50 V rather than at ESCE = 0.3 V as for PANI. In addition this band does not exhibit any red shift as in the case of PANI. Differences were also observed for the main absorbance band in the red region of the spectra. Although the intensity of the band increases with progressive oxidation of the polymer its growth is roughly onethird of the growth in intensity of the corresponding band in PANI with progressive oxidation. Also this band attains maximal value at ESCE = 0.60 V which is not the case in PANI. In addition to this no conspicuous blue shift can be observed in this band with the increase of potential. This feature is much more prominent during PANI electrooxidation. Fig. 10b shows the absorbance at the three wavelengths as a function of applied potential. The spectroelectrochemical results obtained from the electrochemical polymerization of mixture of ANI and OAP clearly show the incorporation of OAP into the polymer during polymerization. This phenomenon is further supported by results obtained with FTIR spectroscopy. Fig. 11 shows the FTIR spectrum of PANI synthesized from an aqueous solution of 20 mM ANI in 0.5 M H2 SO4 . The peak at 3240 cm−1 is attributed to the N–H stretching vibrations. The characteristic bands at 1563–1565 cm−1 arise mainly from both C N and C C stretching vibrations of the quinoid diimine unit, whereas the band around 1475–1479 cm−1 is attributed to the C C ring stretching of the benzoid diamine unit. The bands around 1298 and
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Fig. 12. FTIR spectra of electrochemically synthesized poly(aniline-co-oaminophenol) with various concentrations of OAP in the comonomer feed (a) 1 mM, (b) 2 mM and (c) 3 mM. Fig. 10. (a) UV–vis spectra of a copolymer-coated ITO glass electrode, obtained at different electrode potential values, ranging from ESCE = 0.0 to 0.80 V at every 0.10 V. The copolymer-coated ITO glass electrode was prepared by electropolymerization at ESCE = 0.90 V in a solution containing 1 mM OAP and 20 mM ANI. (b) Absorbance vs. potential plot for three selected wavelengths, derived from spectra displayed above.
798 cm−1 can be assigned to C N stretching of the secondary aromatic amine and an aromatic C H out-of-plane bending modes, respectively [42]. The peak at 1120 cm−1 is attributed to the presence of SO2− 4 ions in the polymer matrix. Fig. 12
depicts the FTIR spectra of the copolymers synthesized with various concentrations of OAP (1, 2 and 3 mM) in the feed with a constant concentration of ANI (20 mM). Apparently, the FTIR spectra of copolymers present the same picture as that of PANI. However, in the FTIR spectra of copolymers an absorption peak appears at 1402 cm−1 which is not present in the spectrum of PANI. This peak at 1402 cm−1 , which becomes more prominent with the increase of OAP concentration in the feed, is indicative of the C–O–H deformation vibration of phenols [43]. The IR spectrum of OAP also shows a peak at about 1400 cm−1 [44]. Thus, the peak at 1402 cm−1 in Fig. 12 can be considered as evidence of the presence of OAP unit in the polymer backbone. The increase in its intensity with an increase of the concentration of OAP in the comonomer feed suggests the incorporation of more OAP units in the resulting copolymer. 4. Conclusions
Fig. 11. FTIR spectrum of electrochemically synthesized polyaniline.
UV–vis spectroelectrochemical studies of the homopolymerization of OAP and copolymerization of OAP with ANI provide evidence for the incorporation of OAP units into the copolymer. The generation of intermediate species having both OAP and ANI units through the cross-reaction between OAP cation radical and ANI cation radical, was concluded from the appearance of an absorbance band around λ = 520 nm. The decrease in absorbance for this band after switching off the potential supports this assignment. The band observed at 1402 cm−1 in the FTIR spectra supports the OAP incorporation into the copolymers.
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