A magenta polypyrrole derivatised with Methyl Red azo dye: synthesis and spectroelectrochemical characterisation

Electrochimica Acta 240 (2017) 239–249

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A magenta polypyrrole derivatised with Methyl Red azo dye: synthesis and spectroelectrochemical characterisation Andresa K.A. Almeidaa , Jéssica M.M. Diasb , Diego P. Santosb , Fred A.R. Nogueirac , Marcelo Navarrob,1, Josealdo Tonholoa , Dimas J.P. Limaa , Adriana S. Ribeiroa,1,* a b c

Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, Campus A. C. Simões, Tabuleiro do Martins 57072-970, Maceió-AL, Brazil Departamento de Química Fundamental, CCEN, Universidade Federal de Pernambuco 50670-901, Recife-PE, Brazil Instituto Federal de Alagoas, Campus Arapiraca, 57300-100, Arapiraca-AL, Brazil

A R T I C L E I N F O

Article history: Received 28 July 2016 Received in revised form 13 April 2017 Accepted 14 April 2017 Available online 18 April 2017 Keywords: Conjugated polymer Methyl Red Electropolymerisation Electrochromism BFEE

A B S T R A C T

A pyrrole derivative bearing 2-(4-dimethylaminophenylazo)benzoic acid, also known as Methyl Red (MR), was prepared by a simple synthetic route, and electropolymerised onto ITO/glass electrodes in (C4H9)4NBF4/CH3CN in presence of boron trifluoride diethyl etherate (BFEE). Films of polypyrrole (PPy) and PPy doped with MR (PPy/MR) were also deposited onto ITO/glass in order to compare their electrochromic properties with the films of PPy derivatised with MR. Cyclic voltammogram of the poly[3(N-pyrrolyl)propyl 2-(4-dimethylaminophenylazo)benzoate] (PMRPy) film displayed a redox pair with anodic peak potential (Epa) at ca. 0.53 V and cathodic peak potential (Epc) at 0.25 V vs. Ag/Ag+, corresponding to the polymer p-doping, whilst the PPy/MR film shows capacitive behaviour with a redox pair in the cathodic region (Epa = 0.36 V and Epc = 0.62 V), similar to the PPy film (Epa = 0.10 V, and Epc = 0.15 V), and an anodic wave in the same potential range of that for PMRPy film. The electrochromic properties of the PMRPy film, such as chromatic contrast (D%T = 34.2%), switching time (t = 10 s) and stability (D%T = 15% at the 100th cycle), were enhanced relative to the PPy/MR and PPy films. However, the colour of the PMRPy film changed from yellow (-0.8 V) to magenta (E = 1.0 V) in the first cycle and became light magenta at 0.8 V in the subsequent cycles. PMRPy films were also investigated in phosphate buffer solution (PBS, 2.0  pH  9.0) and after exposure to HCl vapour, in which the colour varied from magenta at pH = 2.0 to yellow at pH = 9.0. Such properties are interesting for application in pH sensors. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction One of the major challenges in the area of molecular electronics is focused toward the synthesis of materials based on conjugated polymers with novel optoelectronic properties, such as electroluminescence, electrochromism, third-order non-linear optics and chemical sensing [1,2]. Therefore, designing of hybrid systems based on oligomers and p-conjugated polymers having variable optical properties, striking chromic effects, enhanced performance, and narrow band gap energy remains important [3]. Organic dyes have been used in different areas of science, such as medicine, physics, and chemistry, to produce chromatic changes or to investigate the effect of a light absorber specimen in well-

* Corresponding author. Tel.+55-82-3214-1393. E-mail addresses: [email protected], [email protected] (A.S. Ribeiro). ISE member.

1 1

http://dx.doi.org/10.1016/j.electacta.2017.04.068 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

known systems. Therefore, the enhancement of optical contrast and colour modulation of p-conjugated polymers and their hybrid materials can be easily achieved by incorporating organic dyes (such as Indigo Carmine [4], Bromophenol Blue [5], Brilliant Yellow [6], Ponceau 4R [7], Remazol Black B and Dianix Red [8]) to the polymer film, or even by the modification of the monomer/ polymer structure by derivatisation with a dye. Using this approach, Cihaner and Algi [9] prepared a copolymer of a 2,5-di (2-thienyl)pyrrole (SNS) derivatised with an azo dye and 3,4ethylenedioxythiophene (EDOT), and Ajayaghosh et al. [3] synthesised soluble donor-acceptor type conjugated copolymers of N-alkylpyrroles and squaric acid, in order to achieve better optoelectronic properties in comparison with the non-modified polymers. Among p-conjugated polymers, polypyrrole (PPy) and its derivatives are particularly important due to their good electric conductivity and chemical stability in ambient atmosphere, besides their structural versatility, which allows tailoring of their

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electronic and electrochemical properties by the manipulation of the monomer structure [10–12]. The incorporation of azo-aromatic dyes, such as Methyl Red (2-[4-(dimethylamino)phenylazo]benzoic acid), into the polymer backbone would be quite interesting due to their reversible optoelectronic properties [9,13–16]. Methyl Red (MR) is a well-known pH indicator having dimethylamine-, azo- and carbonyl- groups in its chemical structure [17], which is promising to modify and/or interact with different materials. As MR presents different colours associated to pH variations, such as red (acid medium), orange and yellow (basic medium), we take advantage of this characteristic in order to obtain electroactive polymer films based on a pyrrole (Py) derivatised with MR deposited onto ITO/glass electrodes, in which the colour of the films varied from magenta (acid medium) to yellow (basic medium) according to the pH. In spite of the large number of recent papers and reviews describing the synthesis, spectroelectrochemical properties and applications of PPy derivatives, there are, to our knowledge, only a few reports concerning the preparation and properties of magenta films of PPy derivatives and/or their copolymers [9,11,12,18]. Electroactive polymer films that present magenta colour are important for development of non-emissive display technologies, since these electrochromic displays are assembled using materials that can exhibit three primary colours and can be employed to create full colour displays where the expression of any colour can be achieved through the control of the intensity of each of the primary colours [19,20]. Colour mixing in display technologies works using the principle of colour as a light and involves mixing RGB (red, green, blue) colours in varying intensities to create a multitude of colours. Similarly, subtractive primary colours CMY (cyan, yellow, magenta) can also be mixed to produce new colours. Mixing these three colours produces black, absence of light, which is also included in the colour mixing systems, and usually subtractive primary colours are represented as CMYK where “K” stands for “black” [21]. Therefore, when cyan and magenta mix, or overlap, they create blue; when yellow and magenta mix, they create red; when cyan and yellow mix, they create green; and finally, when cyan, magenta, and yellow mix, they create black [22]. So, it is possible successfully obtain multiple colours including intermediate red, green, and blue colours according to a subtractive colour-mixture process. On the basis of this approach, we expect that the magenta polymer based on Py derivatised with MR, so-called PMRPy, be useful as active layer in assemble of multicoloured optoelectronic devices based on CMYK system, as well as in pH sensors.

Furthermore, in order to investigate the electrochromic properties of PPy films modified with a dye according to the preparation method, we also electrodeposited films of PPy doped with MR.

2. Experimental 2.1. Materials and instrumentation All the chemical reagents for synthesis were purchased from Sigma-Aldrich or Acros and used as received. Anhydrous acetonitrile 99.8% (CH3CN <0.001% water, Sigma-Aldrich), tetrabutylammonium tetrafluoroborate ((C4H9)4NBF4, Aldrich), lithium perchlorate (LiClO4, Aldrich) and sodium dodecylsulphate (SDS, Sigma) were used as received, boron trifluoride diethyl etherate (BFEE, Sigma-Aldrich) was freshly distilled before use. Phosphate buffer solutions (PBS) were prepared with pH varying from 2.0 to 9.0. NMR spectra were recorded on a Bruker spectrometer operating at a frequency of 400 MHz. The FTIR spectrum was acquired on a Bruker IFS66 spectrophotometer. A Hewlett-Packard 8453 diode array spectrophotometer was used for the ultraviolet-visible-near infrared (UV-vis-NIR) spectra acquisition in the spectroelectrochemical experiments. 2.2. Synthesis of 3-(N-pyrrolyl)propyl 2-(4-dimethylaminophenylazo) benzoate (MRPy) 1-(3-Iodopropyl)pyrrole (0.50 g, 2.10 mmol), methyl red (0.56 g, 2.10 mmol) and triethylamine (0.58 mL, 4.20 mmol) were added to dry CH3CN (20 mL). The reaction mixture was stirred at 80  C for 3 h. After this period, the reaction mixture was extracted with 30 mL of H2O/CH3Cl (1:1, v/v) and the organic phase was evaporated in a rotatory evaporator. The crude product was purified by chromatography on silica gel (230-400 Mesh) using CH2Cl2 as eluent, to give 0.454 g (57% yield) of the MRPy as a red viscous liquid, Scheme 1. 1H NMR (400 MHz, CDCl3), d (ppm): 7.89 (dt, 2H), 7.79 (dd, 1H), 7.61 (m, 1H), 7.42 (m, 1H), 6.78 (d, 2H), 6.55 (t, 2H), 6.12 (t, 2H), 4.32 (t, 2H), 3.89 (t, 2H), 3.11 (s, 6H), 2.11(m, 2H). 13 C NMR (400 MHz, CDCl3), d(ppm): 168.25, 152.75, 152.65, 131.78, 129.66, 128.31, 127.89, 125.42, 120.54, 119.40, 111.58, 108.16, 62.40, 46.12, 40.422, 30.65. FTIR (ATR): 3099 (w, n (C-Ha pyrrole)), 2920 (m, nas (C-H)), 2808 (m, nas (C-H)), 1724 (s, n (C=O)), 1597 (m, nas (C=C)), 1364 (s, d (N-C)), 1244 (m, d (C-H)), 1141 (s, n (C-O)), 750 (s, dout-of-plane (C-H substituted benzene)), 723 (s, dout-of-plane (C-Ha pyrrole)) cm 1.

Scheme 1. Synthetic route to obtain MRPy.

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2.3. Electrochemistry Poly[3-(N-pyrrolyl)propyl 2-(4-dimethylaminophenylazo)benzoate] (PMRPy) films were deposited onto ITO/glass electrodes (1.0 cm2, Rs  10 V cm; Delta Technologies) in a single compartment cell using an Autolab PGSTAT30 galvanostat/potentiostat. In order to prevent the migration of water into the experimental system, a home-built non-aqueous Ag/Ag+ (0.1 mol L 1 in CH3CN) reference electrode (+ 0.298 V vs. normal hydrogen electrode; Analion), isolated from the working solution by a Vycor1 frit, was used in the experiments performed in CH3CN. An Ag/AgCl (KClsat.) reference electrode was used in the experiments performed in aqueous medium. A Pt foil was used as the counter electrode. Different deposition solution formulations were prepared aiming to obtain good quality polymer films (homogeneous and adherent to the ITO/glass surface). MRPy was employed at a concentration of 0.01 mol L 1 in supporting electrolytes consisting of: i) 0.10 mol L 1 LiClO4/0.05 mol L 1 SDS in H2O, ii) 0.10 mol L 1 LiClO4 in CH3CN, iii) 0.10 mol L 1 (C4H9)4NBF4 in CH3CN, iv) mixed electrolyte system of 0.10 mol L 1 LiClO4 in CH3CN with 20% BFEE (by volume), or v) 0.10 mol L 1 (C4H9)4NBF4 in CH3CN with 20% BFEE. PPy films were deposited onto ITO/glass electrodes from an aqueous solution of 0.05 mol L 1 Py and 0.1 mol L 1 LiClO4, and PPy doped with MR films were deposited from an aqueous solution of 0.05 mol L 1 Py, 0.005 mol L 1 MR and 0.05 mol L 1 SDS. LiClO4 was used as electrolyte instead (C4H9)4NBF4 in order to obtain better quality PPy films [23] and the surfactant (SDS) was used in order to facilitate solubilisation of MRPy and MR in water. The so-obtained films were washed several times with CH3CN to remove unreacted monomers and excess of the electrolyte. PMRPy films were immersed in 0.1 mol L 1 NaOH solution for 30 minutes to remove residual BFEE, washed with H2O and CH3CN. Electrochemical studies of the polymer films deposited on ITO/ glass were carried out in a solution of LiClO4 (0.10 mol L 1) in CH3CN, by varying the potential from 0.8 to 1.0 V vs. Ag/Ag+. Cyclic voltammograms of the polymer films were acquired using LiClO4 rather than the (C4H9)4NBF4 electrolyte, because the former generated better electrochemical response [24].

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Usually the methodologies described in the literature for preparation of pyrrole derivatives that generate polymers exhibiting magenta colour involves a greater number of synthetic steps, long period of reaction and harsh conditions. For example, Walczak and Reynolds [12] have synthesised a series of 3,4-alkylenedioxypyrroles derivatives, in which some of the so-obtained polymers are pink (same hue as magenta) at a certain oxidation state, by a somewhat undesirable synthetic route [28–30].

3.2. Electropolymerisation Initial attempts to electropolymerise MRPy onto ITO/glass surface were performed using either aqueous or organic media. Due to the low solubility of MRPy in H2O, even using SDS to facilitate the solubilisation, it was not possible to obtain a homogeneous solution of MRPy in H2O and, therefore, perform the electropolymerisation onto the ITO/glass surface. When (C4H9)4NBF4/CH3CN solution was employed as electrolyte, a series of cyclic voltammograms using the potential range of 0.0  El  1.0 V vs. Ag/Ag+, with El increasing in steps of 0.1 V, displayed an irreversible wave at 0.67 V, but the formation of an electroactive film deposited onto ITO was not evidenced. Furthermore, it was not possible to detect the nucleation loop [31] or any other reduction process in the reverse scan in the investigated potential range. Based on earlier results with other tertiary amine derivative [10], we attributed this irreversible wave (Fig. 1, full line) to the oxidation of the tertiary amine present in the MR moiety, since the cyclic voltammogram of MR in the same electrolyte revealed a similar behaviour (Fig. 1, dashed line). Cyclic voltammograms of MRPy registered in LiClO4/CH3CN also showed an irreversible wave at 0.70 V (see Fig. S1, Supplementary material) with no film formation onto the ITO/glass surface. When BFEE (20%) was added to the (C4H9)4NBF4/CH3CN electrolyte system, MRPy oxidation initiated at Eonset  0.84 V (Fig. 2), which was accompanied by polymerisation, as confirmed by the increase in the current densities of the redox pair at the 0.0– 0.6 V region, implying that the amount of PMRPy deposited on the

2.4. Spectroelectrochemistry PMRPy, PPy/MR and PPy films deposited onto ITO/glass were characterised by cyclic spectrovoltammetry and double step spectrochronoamperometry in 0.1 mol L 1 LiClO4/CH3CN solution as supporting electrolyte, using a platinum wire as the counter electrode and an Ag/Ag+ electrode as reference. Cyclic voltammograms were acquired within the potential scan range of 0.8  E  1.0 V, and chronoamperograms were obtained by application of pulses of E1 = 0.8 and E2 = 0.8 V for 60 s. The UV-vis-NIR spectra in the range of 300–1100 nm were recorded simultaneously with the electrochemical experiments.

3. Results and Discussion 3.1. Synthesis of MRPy The synthetic route to obtain MRPy was divided into two steps: i) the preparation of the 1-(3-iodopropyl)pyrrole according to the procedure described by Clauson-Kaas and Tyle [25–27], and ii) the esterification step by the nucleophilic substitution reaction of 1(3-iodopropyl)pyrrole and MR, in the presence of triethylamine/ CH3CN (Scheme 1). This method has the advantages of simplicity, mild conditions and good yields from readily available starting materials.

Fig. 1. Cyclic voltammograms on ITO/glass registered during the attempts to electrochemically polymerise MRPy using (C4H9)4NBF4/CH3CN solution (___) and MR solution in the same electrolyte ( ), n = 0.02 V s 1.

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Fig. 2. Cyclic voltammograms of MRPy in 0.1 mol L BFEE (by volume) mixed electrolyte, n = 0.02 V s 1.

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1

(C4H9)4NBF4/CH3CN with 20%

electrode is increasing. Layers of different thickness were obtained by varying the number of the performed voltammetric cycles, in order to produce films with deposition charge (Qdep) in the range of 10 and 60 mC cm 2. It is known from the literature that BFEE solution is a strong Lewis acid, widely used as a catalyst in electrochemical polymerisation of aromatic monomers, such as thiophene, furan and pyrrole [32–34]. BFEE interacts with the aromatic ring by the formation of p-complexes, thereby decreasing the resonance stabilisation of the aromatic ring and shifting the oxidation potential to a less anodic potential [35]. Moreover, the increased acidity of the solvent imparts greater stability to the cation radical, which can promote the electrooxidative polymerisation [36]. In presence of BFEE the irreversible wave at 0.67 V attributed to the tertiary amine oxidation was suppressed, suggesting that BFEE and the free electrons of the nitrogen in the tertiary amine of the MR moiety form a Lewis acid/base adduct, that allows the selective pyrrole ring activation toward polymerisation [10]. This assumption was based on an earlier result for a similar system and may be clarified by the analysis of theoretical calculations performed considering different possibilities of interactions between BFEE and MRPy (see Supplementary material, Fig. S2). Although the electrodeposition of PMRPy may occur in LiClO4/ CH3CN/BFEE system, the so-formed films are not homogeneously deposited onto the ITO/glass electrodes. In the cyclic voltammograms of MRPy registered during the electropolymerisation process in LiClO4/CH3CN/BFEE (Fig. S3, Supplementary material) it is possible to observe that, despite the increasing of the current density at  0.50 V during the cathodic sweep (attributed to the polymer reduction), the anodic wave corresponding to the polymer oxidation is absent. This behaviour suggests that the film formation has occurred, however, the so-formed polymer is soluble (or partially soluble) in CH3CN in its reduced form, and therefore it is not homogeneously deposited onto the ITO/glass electrode. In an earlier work [24], the role of the electrolyte system (LiClO4 and (C4H9)4NBF4) employed in the electrodeposition and

characterisation of a polypyrrole derivative was investigated. It was observed that both the anion and the cation present in the electrolyte used in the polymerisation process affect the structure and the electrochemical behaviour of the polymer films, since these ionic species are trapped in the polymer matrix during film formation [37]. In such a case, the deposition of the polypyrrolederivative films in the presence of (C4H9)4NBF4 and their subsequent characterisation in the presence of LiClO4 provided the best electrolytic conditions in terms of chromatic contrast (D% T) and stability for those films. This result could be attributed to the greater surface area obtained due to the formation of the polymer in presence of a large molecule [(C4H9)4NBF4] as electrolyte, thereby providing parallel ionic and electronic conduction pathways that facilitate the process of charge transfer and mass transport during the characterisation step when a smaller molecule (LiClO4) is employed as electrolyte. The same approach was used to choose the best electrolyte conditions in order to obtain PMRPy films with good D%T and stability. Cyclic voltammograms registered during the deposition of PPy using MR as dopant in aqueous/SDS medium (Py/MR, Fig. 3), showed similar behaviour of those ones acquired during deposition of PPy in aqueous LiClO4 (Fig. S4, Supplementary material), implying that in this case Py is preferably oxidised, instead the tertiary amine of the MR. Films of PPy/MR (Fig. S5, Supplementary material) and PPy were also prepared in CH3CN, however the films electrodeposited in aqueous medium were more homogeneous and adherent to the ITO/glass electrodes and therefore these were used in the characterisation step. The different behaviour observed during the MRPy and Py/MR electropolymerisation process may be attributed to the distinctive chemical structures (ester and carboxylic acid for MRPy and MR, respectively), way of doping and type of doping agent, besides their interaction with the solvent (organic for MRPy and aqueous for Py/ MR). In aqueous medium, Py is oxidised and polymerised to form a highly conjugated positively charged polymer backbone [38,39] and MR anions (MR ) from the carboxylate (COO ) dissociated in the solution are inserted into the film to provide charge

Fig. 3. Cyclic voltammograms of Py in 0.005 mol L n = 0.02 V s 1.

1

MR/0.05 SDS aqueous solution,

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Scheme 2. Schematic representation of the doping/undoping process for PMRPy and PPy/MR.

neutralisation (Scheme 2). Additionally, the conjugated system of MR can interact with the p-system of PPy leading to hybrid systems [5,38]. 3.3. Spectroelectrochemical characterisation The cyclic voltammogram of the PMRPy film displayed a bad defined redox pair with anodic peak potential (Epa) at ca. 0.53 V and cathodic peak potential (Epc) at 0.25 V vs. Ag/Ag+, corresponding to the polymer p-doping (Fig. 4). The difference (DEp) of  0.28 V between the anodic and cathodic peak potentials may be explained by kinetic limitations like ion diffusion or interfacial charge transfer processes, including slow heterogeneous electron transfer, effects of structural reorganisation processes within the polymer film, and electronic charging of a sum of two interfacial exchanges, namely the electrode/polymer and the polymer solution interfaces [40,41]. The p-doping process for PMRPy films may be considered as quasi reversible, since it is possible to notice the reverse peak, in which the oxidation charge (Qox) is 5.63 mC cm 2 while the reduction charge (Qred) is 1.57 mC cm 2, suggesting that the process has low coulombic efficiency (CE  30%) in the initial voltammetric cycles. On the other hand, the cyclic voltammogram of the PPy/MR film shows capacitive behaviour and a redox pair in the cathodic region, similar to the PPy film, and a bad defined anodic wave in the same potential range of that for PMRPy film. Furthermore, it is possible to observe that the redox pair attributed to the oxidation of PPy

Fig. 4. Cyclic voltammograms of the PMRPy (___), PPy/MR ( ) and PPy ( ) films deposited onto ITO/glass, recorded in LiClO4/CH3CN 0.10 mol L 1 with n = 0.02 V s 1.

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Table 1 Anodic peak potential (Epa), cathodic peak potential (Epc), lmax at Epc/V

lmax/nm

0.0 V

1.0 V

0.53

0.25

412

412

503a

1.87b

0.8 V PMRPy

0.8, 0.0 and 1.0 V, band gap energy (Eg) and colours of the PMRPy, PPy/MR and PPy films.

Epa/V

Eg/eV

Colour 0.8 V

PPy/MR

0.36

0.62

410

495a

515a

1.80

PPy

0.10

0.15

370

>800c

>800c

2.53

0.0 V

1.0 V

a

together with a broad band at l > 600 nm. considering the shoulder at 560 nm with the onset of p-p* transition at 662 nm. c broad band. Colours of PMRPy film as observed in the first cycle, corresponding to the polymer at fully dedoped state (after immersion in 0.1 mol L washing with H2O and CH3CN). Colours of PMRPy film in the subsequent cycles are shown in the Supplementary material (Fig. S6). b

was shifted to more cathodic region in the PPy/MR system (Table 1). This role of the dye on the stabilisation of the PPy cations (lowering the oxidation potential) was discussed by Ferreira et al. [5], wherein the charged species of the dye present into the PPy film during the oxidation process acts as counter-ions, stabilising the positively charged polymer chain [42,43]. The comparison between the Epa and Epc values found for PMRPy, PPy/MR and PPy films (Table 1) shows that the nature of the monomer and dopant agent is important in determining the switching potential of the polymer. Indeed, the derivatisation of pyrrole in the nitrogen atom usually increases the oxidation/ reduction potential of the polymer as discussed by Diaz et al. [44]. For example, whereas PPy oxidises at ca. 0.2 V vs. SCE, poly-Nalkylpyrroles oxidise in the range of 0.45–0.65 V, as observed for poly(N-methylpyrrole) and poly(N-phenylpyrrole), whose Epa are 0.45 and 0.65 V, respectively. Such behaviour may be attributed to the steric effects of the substituent, which could disturb the planarity along the polymer chain and thus destabilise the cationic form of the polymer. The changes in the absorption spectra of the PPy, PPy/MR and PMRPy films were plotted as a function of the potential applied to the electrode during cyclic voltammetry and are presented in Fig. 5. The absorption spectrum of the PMRPy film in the dedoped state (after immersion in NaOH solution and washing with H2O and CH3CN) exhibited a band with maximum (lmax) at 412 nm corresponding to p-p* transition, whilst the absorption spectra of the PPy/MR and PPy films presented bands with lmax at 410 nm and 370 nm, respectively. According to the literature, the absorption spectrum of the PPy in the dedoped state is dominated by a broad, single absorption band at around 370–400 nm [45,46], corroborating with our results. The bathochromic shift in the absorption spectra of PPy/MR when compared to the pristine PPy may be associated to the narrowing in the band gap energy (Eg) of PPy/MR due to the increase of the planarity of the polymer. Similar results have been reported for azo and anthraquinone dyes by Ferreira et al. [8]. The optical band gap energy (Eg) was calculated from the onset of the p–p* transition in the absorption spectra of each polymer film at the neutral state and are shown in Table 1. Usually, Eg values of the conjugated polymers are in the range of 1.5–3.0 eV [2,11,46]. Polymers with Eg greater than 3.0 eV are generally termed anodically-colouring because they are colourless in the neutral state, while absorbing (coloured) in the visible region in the oxidised state [47]. Whilst those with Eg less than 1.5 eV are

1

NaOH solution and

cathodically-colouring materials, and coloured in the neutral state. Polymers with intermediate values for Eg have distinct optical changes throughout the visible region and may exhibit several colours, such as PPy with Eg  2.7 eV [11,46,48]. The absorption spectra of the PMRPy film (Fig. 5a) show that with increasing potential the peak intensity of the band at 412 nm decreases and it is possible to observe the formation of a new band at 503 nm as well as a broad band above 600 nm and in the NIR region, assigned to the formation of bipolarons [49]. Whilst the absorption spectra of PPy/MR (Fig. 5b) and PPy (Fig. 5c) films show a decrease in the peak intensities at 410 nm and 370 nm, respectively, it can be observed the formation of a new absorption band with lmax at 515 nm for PPy/MR, assigned to the presence of MR into the PPy matrix, along with a broad band in the NIR region for both films, characteristic of the PPy structure [6]. Although it is possible to observe the increase of the absorption bands in the NIR region of the absorption spectra of the PMRPy film (Fig. 5a), such bands are less intense than those observed for PPy/MR and PPy films, suggesting a decreasing in the electronic conductivity of the oxidised (p-doped) PMRPy when compared to both PPy/MR and PPy. This effect can be explained by the steric hindrance caused by the voluminous groups present in the MR units in the polymer affecting its planarity or even due to the electronic effects of the MR substituents. As can be seen in Fig. 5b, the electrochromic features of the PPy/ MR system are related to the concerted colour changes of the doped/dedoped PPy (blue-gray/yellow) and the colours presented by the dye (red/yellow) during the PPy doping process. Indeed, the absorption band at 495 nm observed for the PPy/MR in the neutral state (0.0 V) may be attributed to the contribution of the dye in the colour of the polymer film (see Supplementary material for the absorption spectrum of MR in CH3CN, Fig. S7). At 0.8 V the main absorption (lmax = 410 nm) is commonly attributed to the PPy p-p* transition, whilst at 1.0 V the broad band in the NIR region corresponds to the formation of the PPy bipolaronic state. Therefore, it is possible to evidence characteristic absorption bands of both MR and PPy depending on the oxidation state of the film. Similar behaviour was reported by Tavoli and Alizadeh [50] in their study concerning the electrochromic properties of PPy doped with Eriochrome Cyanine R. Besides significant difference in the type of doping, PMRPy and PPy/MR are also distinct from each other in terms of their band structure. Keeping in mind the powerful effect of chemical structure on optoelectronic properties of conjugated polymers

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Fig. 5. Spectroelectrochemical characterisation of the PMRPy (a), PPy/MR (b) and PPy (c) films deposited onto ITO/glass, recorded in LiClO4/CH3CN 0.10 mol L 1 showing absorbance as a function of the applied potential (-0.8  E  1.0 V vs. Ag/ Ag+).

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[45], in this work, the inclusion of MR unit in the Py ring is shown to have a significant impact on PPy properties and its spectroelectrochemical behaviour in function of the solvent and/or pH variation. In LiClO4/CH3CN the polymer film previously treated with NaOH (yellow) becomes magenta when oxidised (doped with ClO4 ). In the subsequent cycles, the doping process of the PMRPy film takes place through the insertion/deinsertion of ClO4 ions in the structure of the polymer and the colour of the film changes from light magenta (transmissive) to magenta. On the other hand, in aqueous media (PBS) the film is yellow in presence of base and becomes magenta in acidic medium, indicating that the process of colour change involves the presence of protons in the medium and may occur from the protonation of the MR moiety. Therefore, as CH3CN is an aprotic solvent, the mechanism involved in the electrochromism process should be different for each media, as shown in Scheme 2. The electrochromic performance of the PMRPy, PPy/MR and PPy films, with respect to chromatic contrast (D%T), switching time (t) and stability to redox cycles, was investigated by double potential step spectrochronoamperometry. PMRPy film showed colour variation from yellow (E = 0.8 V) to magenta (E = 1.0 V), with D% T at 520 nm of 34.2% and t  10 s in its first cycle (Fig. 6a). In order to achieve the highest contrast for the PMRPy film, D%T was measured at 520 nm. In subsequent cycles, the colour of the PMRPy film varied from light magenta at 0.8 V to magenta in the oxidised state, instead of the initial yellow colour. Such behaviour may be related to the interaction between the PMRPy and the solvent (CH3CN), which is governed primarily by electrostatic effects dictated by the solvent properties, namely, its solvating ability, and dielectric constant [51], besides the influence of the doping agent. The results of the double step spectrochronoamperometry (charge/discharge cycles) show that the coulombic efficiency is low in the initial cycles with Qox = 11.57 mC cm 2 and Qred = 2.57 mC cm 2 (calculated for the 2nd cycle, since the 1st cycle has initiated at the fully dedoped state at 0.8 V), but it becomes stable after  20 cycles reaching ca. 79% at the 100th cycle (Qox = 2.62 mC cm 2 and Qred = 2.06 mC cm 2, see Fig. S8 in Supplementary Material). In addition, the chromatic contrast diminishes by ca. 50% over 70 cycles and then remains stable (Fig. S9, Supplementary Material). Despite the slight variation in the colour of the PMRPy film in CH3CN medium (D%T  15% for the 100th cycle), it is noteworthy that polypyrrole derivatives presenting magenta colour are still rare. Changing in colour from light-magenta (transmissive electrochrome) to magenta is a significant trait in CMY (cyan, magenta, yellow) colours, which makes the PMRPy film amenable for use as active layers in optoelectronic devices. The colour of the PPy/MR film changed from brownish-yellow (E = 0.8 V) to purple in the neutral state (E = 0.0 V) and to bluishpurple in the oxidised state. Such colours observed for PPy/MR may be associated to the concerted colour variation of the PPy according to its doping level (yellow to blue-gray) and MR (magenta). During the subsequent cycles it was noticed the leaching out of the MR to the solution, which became magenta (see Fig. S10 in Supplementary material), implying in a decrease of D%T at 520 nm from 19.0% in the first cycle to 6.8% in the 100th cycle (Fig. 6b). The change in colour of PPy film from pale yellow in the reduced/neutral state to blue-gray in the oxidised state is extensively reported in the literature [11,52–54], with D%T values at 700–800 nm varying from 10-25% depending on the experimental conditions [4,50]. PPy film obtained from the experimental conditions described in this work presented D%T = 17.3% (Fig. 6c), which is into the range of expected values for PPy films. PMRPy and PPy/MR films require  10 and 9 s, respectively, to achieve 95% of a full switch and present faster t when compared to the PPy film (17 s). Similar values of t have been reported in the

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literature for PPy films doped with different dyes, such as Remazol Black B (14 s), Dianix Red (9 s) and SDS-Indigo Carmine (8 s) [8,55]. Stability during long-term switching of PMRPy films may be improved after setting the optimum experimental conditions, i.e. finding a suitable potential range in order to avoid polymer overoxidation, adjusting the film thickness, etc. 3.4. Influence of the pH in the PMRPy chromatic changes

Fig. 6. Transmittance variation at 520 nm for PMRPy film (a), 520 nm for PPy/MR film (b) and 780 nm for PPy film (c) registered in the potential range from 0.8 to 0.8 V vs. Ag/Ag+ at the 1st cycle (____) and 100th cycle (——).

MR has different colours associated to pH variations, such as red in acidic medium and yellow in basic medium. This indicator has four possible basic centres to receive added protons with increasing of the acidity in aqueous solutions. These centres are the COO group, the a and b nitrogen of the azo linkage, and the nitrogen of the dimethylamino group. In acidic medium (4.5  pH  6.0), due to their unshared electron-pairs, the nitrogen atoms of the azo group are capable of binding protons, thus causing the formation of a quinonoid benzene ring resonance system. At this pH range, the acid form of MR, designated as HMR, is a zwitterion and has a resonance structure somewhere between the two extreme forms (structures A and B in Scheme 3, with R = COO ) [51]. The basic form is designated as MR (structure C in Scheme 3, with R = COO ). The acidity constant (pKa) for the equilibrium of the monoprotonated diazo group (Scheme 3) have been reported as 4.7–5.0 [51,56]. According to the literature [56,57], the absorption spectrum of the monoprotonated form of MR (HMR, structures A and B in Scheme 3) presents an absorption band at lmax = 525 nm. As the pH increases (> 4.9), the intensity of the band at 525 nm decreases and a new band centred at about 430 nm starts to develop, indicating the presence of the yellow basic form MR (structure C in Scheme 3). The neutral form of azo dyes generally has a weaker absorption band and hence less intense colour [58], where the shoulder at  560 nm decreases with increasing pH. Derivatisation of Py with MR will impact the acid-base chemistry of the latter since the carboxylic acid group of MR is converted to ester one (Scheme 3, with R = 3-(N-pyrrolyl)propyl). However, the UV-vis-NIR spectra of the PMRPy films in PBS showed reversible pH-dependent behaviour, similar to that described in the literature for MR solution [56,57], as illustrated in Fig. 7. Upon increasing pH from 2.0 to 9.0, the initial absorption at lmax = 515 nm (magenta colour) was gradually decreased, and a broad band at lmax = 460 nm with a shoulder at 565 nm appeared (yellow colour). Furthermore, the presence of new bands at around 425 and 900 nm at pH > 6.0 indicates that there are some structural changes to the polymer upon neutral and basic conditions, as observed by Ajayaghosh et al. [3] for a zwitterionic dye-based pyrrole-polysquaraine derivative in basic medium. Even though the structural changes associated with the pH-dependent reversible optical shifts of the PMRPy films are not yet very clear, they may be associated to the base- or acid-induced structural relaxations of the polymer chain between their deprotonated and protonated forms. When exposed to HCl vapours, the colour of the PMRPy film promptly changed from yellow to magenta in a reversible process, regaining its original colour (yellow) after the solvent evaporation, which is an interesting feature for application of this material for detection of acid vapours. Similar results were found for Polyaniline (PAni) [59,60] and PPy [60,61] prepared by different methodologies, such as PAni nanocomposites [62], PAni-polyvinyl alcohol composite membranes [63], PAni nanofibers [64] and PPybromophenol blue composites [65], for pH determination in aqueous solutions and/or acid vapours. In such cases the colour variation was from green at pH = 4 through blue at pH = 7 and purple at pH = 12 for PAni [64], and from yellow in acidic medium (pH = 2) to blue in basic medium (pH = 10) for PPy-bromophenol

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Scheme 3. Proposed structures for MR and MRPy in acidic (monoprotonated) and basic forms. R = COO or 3-(N-pyrrolyl)propyl.

polymerisation of MRPy was successfully achieved with the addition of BFEE in the electrolyte system. Cyclic voltammetry results shown that MRPy behaved differently in solutions without and with BFEE, with anodic peaks attributed to the oxidation of the tertiary amine of the MR moiety and to the pyrrole ring oxidation, respectively. Another well succeeded electropolymerisation route was tested, using MR as dopant of the PPy in aqueous medium (PPy/MR).

Fig. 7. Absorption spectra of the PMRPy film in PBS according to the pH variation from 2.0 to 9.0 (a-f, respectively).

blue composites [65]. The behaviour observed for PPy-bromophenol blue according to the pH variation suggest that the dye (dopant) plays an important role in the optical properties of the PPy films. Thus, thin films coated with conjugated polymers represent an attractive alternative to indicator-based pH sensor films due to their inherent optical response properties and wider dynamic range for pH measurement. This pH dependent changes have therefore been used to develop optical pH sensors for applications in chemistry, biochemistry, clinical chemistry and environmental sciences [64].

The polymer films exhibited electrochromic behaviour, with colours varying from yellow to magenta for PMRPy film during its first redox cycle. However, in subsequent cycles it was observed the decline in the colour contrast change, from light magenta to magenta, instead the initial yellow colour. Depletion of some electrochromic properties after consecutive switching may be consider normal in the electrochromism of conjugated polymers [66], although the stability during long-term switching of these films may be improved after setting the optimum experimental conditions. Furthermore, the colour of the PMRPy films changes reversibly according to the pH, from yellow in basic medium to magenta in acidic medium. The PPy/MR film shows colour variation from brownish-yellow to purple-magenta and purpleblue, although the MR trapped as dopant into the PPy undergoes leaching during the subsequent redox cycles showing a decrease in its electrochromic properties. The incorporation of an azo dye into the conducting polymer provided interesting electrochemical and optical properties, which makes the PMRPy films amenable for use as active layers in optoelectronic devices, such as displays based on CMYK system, and pH sensors. Interestingly, the colour change (yellow to magenta) is reversible according to the pH variation, but partially irreversible when changing the potential in LiClO4/CH3CN solution. This behaviour indicates the importance of acid-base equilibria in generating the colour changes, which suggests that PMRPy films may be more useful as a pH indicator. Further extension of this work in order to improve the colour contrast and stability of PMRPy film is currently in progress and the results will be reported in due course.

Acknowledgements 4. Conclusions The monomer MRPy was obtained from a simple synthetic route, in mild conditions and with a good yield (57%). The

The authors wish to thank the research funding agencies CNPq, CAPES, FINEP (FUNTEL and CTENERG Programs), and FAPEAL for financial support and for fellowships granted to AKAA (CAPES). The

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