The chemical and electrochemical oxidative polymerization of 2-amino-4-tert-butylphenol

Document downloaded from: This paper must be cited as: The final publication is available at Copyright Additional Information http://hdl.handle.net/10251/77530 Abidi, M.; López-Bernabeu, S.; Huerta, F.; Montilla-Jiménez, F.; Besbes-Hentati, S.; Morallón, E. (2016). The chemical and electrochemical oxidative polymerization of 2-amino4-tert-butylphenol. Electrochimica Acta. 212:958-965. doi:10.1016/j.electacta.2016.07.060 http://dx.doi.org/10.1016/j.electacta.2016.07.060 Elsevier Accepted Manuscript Title: The chemical and electrochemical oxidative polymerization of 2-amino-4-tert-butylphenol Author: M. Abidi S. L´ opez-Bernabeu F. Huerta F. Montilla S. Besbes-Hentati E. Morall´ on PII: S0013-4686(16)31565-1 DOI: http://dx.doi.org/doi:10.1016/j.electacta.2016.07.060 Reference: EA 27675 To appear in: Electrochimica Acta Received date: 20-2-2016 Revised date: 26-4-2016 Accepted date: 11-7-2016 Please cite this article as: M.Abidi, S.L´ opez-Bernabeu, F.Huerta, F.Montilla, S.Besbes-Hentati, E.Morall´ on, The chemical and electrochemical oxidative polymerization of 2-amino-4-tert-butylphenol, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.07.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The chemical and electrochemical oxidative polymerization of 2-amino-4tert-butylphenol M. Abidi1,2, S. López-Bernabeu2, F. Huerta3, F. Montilla2, S. Besbes-Hentati1, E. Morallón2 1Laboratoire de Chimie des Matériaux, Faculté des Sciences de Bizerte. 7021, Zarzouna Université de Carthage, Tunisie. 2Dept. Química Física e Instituto Universitario de Materiales, Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain 3Dept. Ingenieria Textil y Papelera, Universitat Politecnica de Valencia, Plaza Ferrandiz y Carbonell, 1. E-03801, Alcoy, Spain Abstract Poly(2-amino-4-tert-butylphenol), poly(2A-4TBP), was synthesized from monomer aqueous solution using either electrochemical or chemical oxidation procedures. Several spectroscopic characterization techniques were employed to gain information on the chemical structure and redox behavior of the obtained materials. It was found that the chemical polymerization product could be described as an oligomer mixture containing up to 16 monomer units. In parallel to other polymers derived from o-aminophenol, phenoxazine rings constitute also the basic structure of poly(2A-4TBP). In addition, the occurrence of N-N couplings, which are favored by the presence of the voluminous tert-butyl substituent, seems also relevant. No significant structural differences were found between the chemically or electrochemically synthesized materials. Keywords: electrochemical polymerization; in situ FTIR; polyaminophenol 2 1. Introduction Aminophenols constitute an interesting class of compounds for electropolymerization owing to the presence of reactive amino and hydroxyl units. It has been reported that the relative position of these units at the aromatic ring plays a significant role in the electrochemical reactivity of the molecule and, consequently, the electrochemical behavior of the three positional aminophenol isomers, ortho-, metaand para-, is quite different [1–3] . Despite some early controversy on the formation, or not, of polymeric films from the electrochemical oxidation of p-aminophenol, it seems now proved that in aqueous media the monomer can be electropolymerized to yield complex oligomeric products whose chemical structure is strongly dependent on the pH of the working solution [4]. The electro-oxidation of m-aminophenol has been scarcely investigated because it yields an electroinactive polymer that blocks the electrode surface and shows a crosslinked structure similar to polyphenol [3,5]. On the contrary, it is well documented that o-aminophenol can be electrochemically polymerized in acidic medium to yield an electroactive polymeric material that shows phenoxazine ladder structures [3,6]. It was also suggested that the products formed upon oaminophenol electrochemical oxidation depend on the pH of the polymerization solution [7] although it is worth noting that similar structure and properties can be obtained regardless of the polymerization method employed, either chemical or electrochemical [1]. Comparisons have been also made in the literature between chemical and electrochemical polymerization products obtained from other aromatic anilines, with special attention paid to diamines [8]. It is usually found that the chemical polymerization methods result in higher polymer yields although oligomeric by-products are difficult to remove from the reaction mixture. On the other hand, electrochemical polymerization offers the advantage that the fine control of the anodic potential avoids polymer overoxidation and, consequently, materials with less defects can be isolated on the electrode surface. The formation of nonconducting films upon electrochemical oxidation could prevent polymer growth and such a possibility should be taken into account mostly, although not exclusively, in experiments conducted in neutral and alkaline media. Chemical and electrochemical polymerization of alkyl ring-substituted anilines has attracted interest because of the possibility to improve the poor solubility of unmodified polyaniline and, besides, to investigate the effect of blocking specific ring polymerization 3 sites with electron-donor substituents [9–11]. However, to the best of our knowledge, there is still no report dealing with the chemical or electrochemical polymerization of alkyl ringsubstituted aminophenols. In the present work, an attempt has been made to synthesize a polymer from both routes and to analyze the effect of the presence of a voluminous alkyl group on the well-studied polymerization process of aminophenols. It is known that the presence of bulky groups attached to the aromatic ring may avoid ππ-stacking between vicinal chains of the resulting polymer and this could lead to an improvement in solubility and processability of the resulting material. To achieve these goals, 2-amino-4-tert-butylphenol (2A-4TBP) has been selected as the monomer species and different spectroscopic techniques have been applied to the characterization of the oxidative polymerization products. 2. Experimental The solutions employed for polymerization were 1.0 M HClO4, prepared from Merck Suprapur concentrated acid and 18.2 M cm water obtained from an Elga Labwater Purelab system. 2-amino-4-tert-butylphenol monomer was purchased from Merck. Ammonium persulfate, from Merck, was used as the oxidant for chemical polymerization. Cyclic voltammetry experiments were carried out in a conventional three-electrode cell under N2 atmosphere. The working electrodes used were either platinum or ITO and a platinum wire was used as the counter electrode in all cases. All potentials were measured against the reversible hydrogen electrode (RHE) immersed in the working solution through a Luggin capillary. Cyclic voltammograms were recorded at a constant sweep rate of 50 mV s-1 and at room temperature. The polycrystalline platinum electrodes were thermally cleaned and subsequently protected from the laboratory atmosphere by a droplet of ultrapure water. ITO electrodes were cleaned with acetone and ultrapure water. Chemically produced polymers were deposited on the working electrodes by casting a small volume of suspension containing 1 mg mL-1 material in THF solvent. In situ UV–Vis spectra of polymers were recorded with a V-670 spectrometer from JASCO, which is equipped with a double monochromator system and a photomultiplier tube detector. A Nicolet 5700 spectrometer equipped with a nitrogen-cooled MCT detector was employed for the in situ FTIR experiments. The working Pt disc electrode was mounted on a 4 glass tube and its 1.0 cm2 surface was mirror-polished using alumina powder. The spectroelectrochemical cell was made of glass and was provided with a prismatic CaF2 window beveled at 60°. Spectra were collected at 8 cm-1 resolution in D2O solvent (from Aldrich with 99.9% D-atom purity) and are presented as R/R. XPS spectra were recorded with a VG-Microtech Multilab 3000 electron spectrometer using a non-monochromatized Mg-Ka (1253.6 eV) radiation source of 300 W and a hemispheric electron analyzer equipped with nine channeltron electron multipliers. The pressure of the analysis chamber during the scans was about 5×10-7 N m-2. After the survey spectra were obtained, higher resolution scans were performed at pass energy of 50 eV. The intensities of the different contributions were obtained by means of the calculation of the integral one of each peak, after having eliminated the baseline with S form and adjusting the experimental curves to a combination of Lorentz (30%) and Gaussian (70%) lines. All the binding energies were referred to the line of the C 1s to 284.4 eV, obtaining values with a precision of ±0.2 eV. The high-resolution mass experiments were performed in a MICROMASS Autospec spectrometer. 3. Results and discussion 3.1. Electrochemically polymerized 2A-4TBP Fig. 1 illustrates the cyclic voltammetry curves recorded for a polycrystalline platinum electrode immersed in 0.1 M HClO4 + 15 mM 2A-4TBP solution. The oxidation of the monomeric species starts at 0.76 V and, as deduced from the high slope of the voltammetric curve, is kinetically favored during the first voltammetric sweep. The oxidation peak is centered at 0.87 V and, through the first reverse scan, two cathodic peaks are recorded at 0.8 V and 0.7 V. The former is clearly associated with the reversible reduction of oxidized species generated during the forward sweep, whereas the 0.7 V broad wave seems related with the 5 reduction of the oligomer products formed at higher potentials. Such products cannot be reoxidized during successive forward sweeps, as manifested by the absence of anodic waves different from that of monomer oxidation. The peak current of the main oxidation feature decreases upon cycling and its initially fast kinetics decays gradually. Therefore, it is clearly deduced from the voltammetric profile that the anodic oxidation of aminophenol monomer yields electrochemically inactive oligomeric products that block the electrode hindering further oxidation. Cyclic voltammogram in Fig. 1b reveals that the blocking species are strongly adsorbed on platinum. This curve was recorded in a background electrolyte solution after 300 polymerization cycles, when the surface appeared covered with a purple film. The electrode was removed from the polymerization solution, washed with ultrapure water and transferred to the 0.1M HClO4 test solution with no monomer added. The voltammetric curve shows the characteristic profile of a platinum electrode covered with a thin, almost electroinactive oligomeric film. The featureless oxidation process undergone by poly(2A-4TBP) in Fig. 1b was monitored by in situ UV-vis spectroscopy in order to discern whether or not polaronic species are formed upon anodic polarization of this material. The polymer was deposited on an ITO coated glass electrode in a parallel experiment to that shown in Fig. 1a and then transferred to the spectroelectrochemical cell, where it was immersed at 0.05 V. In situ UV–Vis spectra were then recorded at increasing potentials and those obtained in the range from 0.05 to 0.65 V have been depicted in Fig.2. The first UV–Vis spectrum collected at 0.05 V shows one main absorption feature at 558 nm. The intensity of this peak remains almost constant at potentials below 0.45 V but rises sharply above that value. Such a behavior suggests a redox transformation of the oligomeric material, with the absorption probably associated with a benzenoid-to-quinoid transition related to polaronic moieties formed upon electrochemical doping [12]. Such a band is slightly blue-shifted with respect to polyaniline and closer to the polaronic transitions observed for other ring-substituted polyanilines, indicating the presence of polaronic domains with short conjugated segments. However, the formation of quinone-like moieties upon oligomer oxidation could also contribute significantly to this band [13]. The occurrence of a 6 redox polaronic transformation in poly(2A-4TPB) is also supported by other spectral features in Fig 2. Specifically, the spectrum acquired at 0.65V shows the appearance of three additional bands centered at 397, 416 and 447 nm. The former one can be assigned to a polaron-π* transition, where polarons are isolated one from another reflecting low electrical conductivity of the oligomeric material [14]. Regarding the other two features it is worth mentioning that, according to literature data on polyaminophenols, electronic absorptions in the 410-450 nm range can be attributed to the progressive formation of radical cations in phenoxazine rings or, alternatively, to the occurrence of N-N coupling to form azobenzene derivatives [15–18]. In the present case, both structures are probably promoted by the presence of the voluminous tertbutyl group in meta position relative to the nitrogen atom, which tends to hinder the C-N para-coupling and hydrazine-type dimers could be formed due to such steric hindrance [19]. Finally, the band at 742 nm, whose intensity remains almost constant at increasing potentials, is of uncertain nature but it has been assigned either to the bipolaronic transition in para-coupled polyaniline derivatives or to the polaron transition in poly(o-aminophenols) [15,16]. According to these results, poly(2A-4TBP) seems an oligomeric product originated from a variety of C-N, C-O and N-N monomer couplings. In situ FTIR spectroscopy has been used to confirm the existence of a redox transition in the electrochemically deposited poly(2A-4TBP). The filmed Pt electrode was transferred to the IR spectroelectrochemical cell, which contained a free of monomer test solution prepared with D2O to facilitate assignments in the 1500-1700 cm-1 spectral range. After some potential cycles, the Pt surface was carefully pressed against the prismatic CaF2 window, a reference spectrum was then collected at 0.1 V and, finally, the potential was stepped to higher values to collect sample spectra. By referring each sample to the unique reference, we can obtain information on the redox transformations undergone by the oligomeric material as a function of the applied potential. The computed in situ FTIR spectra are displayed in Fig. 3 within the frequency range 1100-2200 cm-1. There, the presence of several negative (downward) and positive (upward) bands reveals the activation of vibrational modes at increasing potentials, which is a characteristic behavior of reversible oxidation processes. In fact, since no spectral features appear in the spectra obtained at 0.3 V and 0.4 V, it can be inferred that poly(2A-4TBP) cannot be oxidized at so moderate potentials. This observation is consistent with the results obtained from the in situ UV-vis experiments in Fig. 2. On the contrary, several IR absorption bands appear in the spectra collected from 0.5 V. 7 The onset of oligomer oxidation at around this potential is testified by the occurrence of a clear negative band at 1613 cm-1, a group of less intense negative absorptions in the 13301400 cm-1 frequency range and, finally, two positive bands peaking at 1518 and 1445 cm-1. It can be observed that the integrated intensity of all these features, and therefore the oligomer oxidation level, increases at increasing potentials and, besides, new absorptions showing either negative or positive character are developing. Assignments proposed for the main bands are summarized in Table 1. Between the two main bands clearly related with the reduced form of poly(2A-4TBP), the feature appearing at 1518 cm-1 can be unambiguously assigned to the progressive vanishing of the aromatic C-C stretching due to oligomer oxidation, whereas the peak at 1312 cm-1 is related with the parallel transformation of secondary aromatic amines. The high intensity reached by the former feature is probably due to the presence of the electron-donor alkyl group in the aromatic ring [20]. The band at 1445 cm-1 has been sometimes attributed to the skeletal C-C stretching vibration of the aromatic ring, although the interpretation of this band is not unanimous. Apart from the C-C stretching, it has been also ascribed either to the existence of N-N coupling or to the formation of phenazine structures in polyaniline derivatives [21,22]. In our case, owing to the presence of adjacent alcohol and amino groups, it is very likely the formation of phenoxazine rings and/or N-N coupling. This hypothesis seems supported by the in situ UV-vis results presented in Fig. 2. With regard to the oxidized state, it is worth noting that the absorption bands appearing, roughly, below 1600 cm-1 support the existence of a reversible redox transformation of poly(2A-4TBP). On the contrary, most features appearing above that frequency strongly suggest an irreversible degradation process of the oligomeric material at high potentials. Indeed, the reversible formation of quinoid rings and intermediate-order C≈N bonds, which is common to most polyaniline derivatives, is confirmed respectively by the 1578 cm-1 feature and the group of overlapped bands in the surroundings of 1350 cm-1. Quinone imine centers are responsible for the feature at 1617 cm-1, while the couple of negative-going bands at 1750 cm-1 and 1292 cm-1 supports the formation of carboxylic acid terminations and confirms the occurrence of an overoxidation process at potentials beyond 0.9V. Furthermore, the negative band peaking at 1650 cm-1 reveals the formation of degraded quinone structures at 0.9V [3]. 14 for poly(o-aminophenol). In addition to the reversible redox transition, the formation of overoxidized structures containing quinone-like structures has been also detected by FTIR, while UV-vis suggests that these structures are more abundant in the electrochemically obtained material than in the chemical product. The conductivity of poly(2A-4TBP) is lower than that of poly(o-aminophenol) and three orders of magnitude inferior to poly(o-toluidine). According to this result, the key factor governing the poor poly(2A-4TBP) conductivity seems the formation of azo groups (which can break the extended conjugation) and, probably, not the eventual torsion angle between adjacent rings that relieves the steric strains promoted by the tert-butyl group. 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In situ UV–vis spectra recorded for a poly (2A-4TBP) film at different applied electrode potentials (from bottom to top: 0.05V, 0.25V, 0.45V, 0.55V, 0.65V). The poly(2A4TBP) film was prepared on ITO coated glass from a solution containing 15mM monomer in aqueous 0.1 M HClO4. 19 Fig. 3. Set of in situ FTIR spectra collected during the oxidation of an electrochemically obtained poly(2A-4TBP) film in 0.1M HClO4/D2O test solution. Reference potential 0.1 V. Sample potential labelled for each spectrum. 100 interferograms at each potential. 20 282 285 288 291 0.0 2.0x104 4.0x104 6.0x104 Intensity /a.u. Binding energy / eV C 1s 396 399 402 405 408 0.0 2.0x103 4.0x103 Intensity /a.u. Binding energy / eV N 1s 528 531 534 537 540 0.0 5.0x103 1.0x104 1.5x104 2.0x104 Intensity /a.u. Binding energy / eV O 1s Fig.4. High resolution XPS signals for C 1s, N 1s and O 1s obtained from a poly(2A-4TBP) film electrodeposited on a platinum substrate. 21 0.0 0.2 0.4 0.6 0.8 -100 -50 0 50 I / A E / V vs RHE Fig.5. Cyclic voltammogram recorded in 0.1M HClO4 electrolyte for poly(2A-4TBP) synthesized chemically and casted on a polycrystalline platinum electrode. v= 50 mV s-1. 400 480 560 640 720 800 0.00 0.04 0.08 0.12 0.16 Absorbance Wavelength / nm 385 445 562 739 Fig. 6. In situ UV-vis spectra recorded at different electrode potentials (from bottom to top: 0.05V, 0.25V, 0.45V, 0.55V, 0.65V) for a chemically synthesized poly (2A-4TBP) deposited on ITO. Test solution 0.1M HClO4. 22 Fig. 7. In situ FTIR spectra collected during the oxidation of a chemically synthesized poly(2A-4TBP) in 0.1M HClO4/D2O test solution. Reference potential 0.1 V. Sample potential labelled for each spectrum. 100 interferograms at each potential. 23 282 285 288 291 294 0 1x104 2x104 3x104 4x104 5x104 Intensity /a.u. Binding energy / eV C 1s 393 396 399 402 405 408 0 1x103 2x103 3x103 Intensity /a.u. Binding energy / eV N 1s 528 531 534 537 540 543 0.0 5.0x103 1.0x104 1.5x104 Intensity /a.u. Binding energy / eV O 1s Fig. 8. High-resolution XPS spectra of chemically synthesized poly (2A-4TBP) showing the curve-fitted signals for C 1s, N 1s and O 1s.