Investigation of the electrochromic properties of tri-block polyaniline-polythiophene-polyaniline under visible light

Synthetic Metals 226 (2017) 80–88

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Investigation of the electrochromic properties of tri-block polyaniline-polythiophene-polyaniline under visible light Chuleekorn Chotsuwana,* , Udom Asawapiroma,1, Yukihiro Shimoib,1, Haruhisa Akiyamac,1, Aroonsri Ngamaroonchotea,1, Thanakorn Jiemsakula,1, Kanpitcha Jiramitmongkona,1 a

National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Pathumthani 12120, Thailand Research Center for Computational Design of Advanced Functional Materials (CD-FMat), Department of Materials and Chemistry, AIST, Tsukuba, Ibaraki 305-8561, Japan c Research Institute for Sustainable Chemistry (ISC), Department of Materials and Chemistry, AIST, Tsukuba, Ibaraki 305-8565, Japan b

A R T I C L E I N F O

Article history: Received 27 September 2016 Received in revised form 26 January 2017 Accepted 2 February 2017 Available online xxx Keywords: Tri-block copolymer Electrochromic polymer Polyaniline Polythiophene Visible light

A B S T R A C T

This paper describes the synthesis and electrochromic properties of a novel soluble polyanilinepolythiophene-polyaniline tri-block copolymer. The measurements of optical absorbance as a function of voltage and time on tri-block copolymer thin films demonstrated an improvement in stability at ambient conditions and visible light irradiation during the application of voltages. Switching experiments showed an improvement of stability of tri-block copolymer up to 3 times in comparison to the polythiophene block only. A comparison of optical absorbance characteristics for the tri-block copolymer relative to the polyhiophene block indicated that the polyaniline blocks substantially increased film stability and therefore has influence on the optical and electrochemical properties of tri-block polyanilinepolythiophene-polyaniline. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The synthesis of conjugated polymers for high performance electrochromic devices for controllable windows, sun glasses, and screens has been continually developed through different synthetic approaches such as electropolymerization and copolymerization [1–3]. Polythiophene and polyaniline are important representative classes of conjugated polymers used as active conducing layers in electrochromic devices [4–6]. Polyaniline is commonly used for the development of photovoltaic devices [7,8], supercapacitors [9,10], organic light emitting diodes (OLEDs) [11,12], photocatalysts [13,14], and electrochromic devices [15,16]. Polyaniline is very chemically and oxidatively stable under visible light irradiation [13,14] and has unique electrochemical properties through an oxidative or protonated doping process. Polyaniline is insoluble in common organic solvents but the solubility of polyaniline can be improved through the incorporation of alkyl groups into the main chain, copolymerization with

* Corresponding author. E-mail address: [email protected] (C. Chotsuwan). These authors contributed to this work equally.

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http://dx.doi.org/10.1016/j.synthmet.2017.02.001 0379-6779/© 2017 Elsevier B.V. All rights reserved.

alkylated polythiophene monomers [17–21], or water soluble functional groups such as sulfonic acid or carboxylic acid [22–24]. Polyaniline in doped states has very high conductivity compared with other conjugated polymers such as polypyrrole and polythiophene [25]. Polythiophene and its derivatives are known to be applicable for photovoltaics [26,27], OLEDs [28], and electrochromic devices [29–31] due to its high thermal stability, atmospheric stability, ease of chemical tuning for solubility, and good electrochemical stability. However, the ease of oxidation of polythiophene can lead to the over-oxidation and the degradation of the polymers under light irradiation [32]. To address some of the limitations of polythiophene and polyaniline, chemical and electrochemical polymerizations of polythiophene and polyaniline for novel copolymers with suitable properties (e.g. good conductivity, ease of solubility, and good stability with application of voltages) have been developed [33– 39]. The copolymerization of monomers and/or oligomers of thiophene and aniline units was shown to produce copolymers with intermediate properties between the parent polymers, polythiophene, and polyaniline. Various copolymerization approaches such as the polymerization of a thiophene-aniline monomer [33–36] the post-functionalization of oligomer aniline units have been reported [35,38,39]. However, with such

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approaches, the random copolymers that are produced limited the performance of the devices due to random distribution of the copolymer units and the side chains [35]. The problems of the random distribution of the copolymer were improved with the alternative design of copolymers with the tri-block (A-B-A) system. The use of tri-block copolymers has shown to have a better control of the polymer chain orientation which improved the performance of conjugated polymer based devices [40–42]. For example, the tri-block copolymers of polyaniline-polyfluorene-polyaniline used as an active layer in photovoltaic devices and OLEDs improved the performance of the devices due to the formation of good controlled structure on a substrate [41,42]. The copolymer made from two distinct polymer blocks should exhibit a unique set of optical and electrochemical properties [42]. By applying this strategic copolymerization of two polymer blocks to improve the properties of polythiophene and polyaniline, we reported a soluble A-B-A tri-block copolymer with polyaniline as the A block and alkyl functionalized polythiophene as the B block as a promising candidate for the electrochromic active material. We investigated the electrochromic properties of our newly synthesized tri-block copolymer via cyclic voltammetry, spectroelectrochemistry, and stability during repeated on-off switching cycles with applied voltage. The polyaniline blocks increased the stability of the tri-block copolymer up to 3 times during switching experiments under visible light irradiation and at ambient conditions.

refluxed for 16 h. After the completion of the reaction, it was cooled down to room temperature and quenched with 2 M HCl (20 mL). The organic layer was separated and then washed with water. The organic layer was then dried over Na2SO4 and concentrated under reduced pressure. Then, the yellow liquid was purified by column chromatography using silica gel with hexane as an eluent to obtain the product (1.5 g, 86%). 1H NMR (400 MHz, CDCl3, d): 6.86 (2H), 2.47 (4H), 1.59 (4H), 1.24 (36H), 0.86 (6H). 13C NMR (100 MHz, CDCl3, d): 142.12, 119.85, 31.92, 29.68, 29.36, 28.81, 22.69, 14.12. 2,5-Dibromo-3,4- didodecylthiophene (2): A mixture of (1) (1.738 g, 4.13 mmol) and NBS (2.205 g, 12.39 mmol) in THF (30 mL) was stirred at 50  C for 16 h in the dark. After the completion of the reaction, the mixture was cooled to room temperature and concentrated under reduced pressure. The reaction residue was dissolved in hexane and purified by column chromatography using silica gel and heptane as an eluent. After solvent evaporation, a yellow solid was obtained (3.35 g, 97%). 1H NMR (400 MHz, CDCl3, d): 2.47 (4H), 1.23 (40H), 0.85 (6H). 13C NMR (100 MHz, CDCl3, d): 141.46, 107.78, 31.92, 29.66, 29.53, 29.35, 28.95, 22.69, 14.12. 2,5-Bis(trimethylstannyl)thiophene (3): The monomer was prepared from the reaction of thiophene with n-buthylithium, followed by treatment with trimethyltin chloride according to the literature resulting in a white solid (3.30 g, 68%) [43]. 1H NMR (400 MHz, CDCl3, d): 7.35 (2H), 0.34 (18H). 13C NMR (100 MHz, CDCl3, d): 143.41,136.20, 7.70.

2. Experimental methods

2.3. Synthesis of polymers

2.1. General

4-Aminophenyl-terminated Poly(9,9-diocty)thiophene (P1): (2) (1.3748 g, 2.37 mmol), (3) (1.555 g, 2.82 mmol), 4-bromoaniline (77 mg, 0.45 mmol) and Pd(PPh3)4 (120.8 mg, 0.104 mmol) were refluxed in toluene (40 mL) under argon atmosphere for 3 days in the dark. After the reaction did not progress any further, it was allowed to cool down and aqueous 2 M HCl (20 mL) was added. The reaction mixture was then extracted with chloroform and the organic layer was washed with a saturated aqueous solution of EDTA, saturated aqueous solution of NaHCO3, and DI water, respectively. The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The remaining residue was dissolved in CHCl3 and precipitated out in a mixture of methanol/ 2 M HCl (10:1 v/v). The obtained solid was then purified by Soxhlet extraction with methanol, acetone, ethyl acetate, and chloroform, respectively. The chloroform phase was collected and evaporated under reduced pressure. The obtained solid was precipitated in methanol/2 M HCl (10:1 v/v) and the final dark red solid was filtered off and dried under vacuum. P1 (341.6 mg, 27%). 1H NMR (400 MHz, C2D2Cl4, d): 7.34 (4H), 7.05 (2H), 6.80 (4H), 6.09 (4H), 2.07 (4H), 1.18 (40H), 0.79 (6H). 13C NMR (100 MHz, C2D2Cl4 d): 151.9, 140.6, 140.0, 128.7, 127.1, 126.1, 121.7, 119.9, 55.3, 40.1, 31.7, 30.0, 29.0, 24.1, 22.5, 13.9. Poly(9,9-didodecylthiophene)-b-polyaniline (P2): P1 (140 mg) was dissolved in 10 mL of dry toluene. A solution of aniline (101.7 mg, 1.09 mmol) in 3.2 mL of THF, methanesulfonic acid (0.16 mL) in H2O (1.1 mL), and (NH4)2S2O8 (1244 mg, 5.45 mmol) in H2O (1.6 mL) were added by syringe into the solution of P1, respectively. The reaction was stirred for 3 days at room temperature. After the reaction was completed, toluene (40 mL) was added into the reaction mixture. The organic layer was collected and washed with brine, saturated Na2CO3, and then brine. The organic layer was dried over Na2SO4 and concentrated under reduced pressure. A mixture of methanol/2 M HCl (10:1 v/v) was used to precipitate out the solid polymer. The polymer was purified by Soxhlet extraction with acetone and toluene. The toluene layer was collected and concentrated. The polymer was re-precipitated in

All reactions were carried out under an argon atmosphere. The starting materials and reagents were purchased from commercial sources, which were listed in details. [1,3-Bis(diphenyl phosphino)propane]dichloronickel(II) (Ni(dppp)Cl2), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), N-Bromosuccinimide (NBS),ammonium persulfate ((NH4)2S2O8), 3,4-dibromothiophene, 4-bromoaniline, ethylenediaminetetraacetic acid (EDTA), aniline, tetrabutylammonium hexafluorophosphate (TBAPF6), acetonitrile were purchased from Sigma Aldrich. 1-bromododecane, magnesium turnings, diethyl ether (99.5%, extra dry over molecular sieve, stabilized), tetrahydrofuran; THF (99.5%, extra dry over molecular sieve, stabilized), toluene (99.85%, extra dry over molecular sieve), and methanesulfonic acid were purchased from Acros. 1 H NMR and 13C NMR spectra were obtained on Bruker Biospin DPX-300 and Bruker AVANCE 400 NMR spectrometers. CDCl3 and C2D2Cl4 were used as NMR solvents. NMR spectra were listed in Supporting information. The molecular weight of each polymer (P1 and tri-block P2) was determined on a Jasco gel permeation chromatographic (GPC) analyzer and was measured against polystyrene standard. The UV/Vis spectra were recorded on a Perkin Elmer Lambda 650 UV–vis spectrophotometer. Thermogravimetric analysis (TGA) was carried out on a Mettler Toledo TGA/SDTA 851 with a heating rate of 20  C min 1 up to 650  C using alumina crucibles under ambient conditions. 2.2. Synthesis of monomers 3,4-Didodecylthiophene (1): 1-Bromododecane (3.09 g, 12.4 mmol) was added dropwise to a suspension of magnesium turning (iodine etched) (0.3 g, 12.4 mmol) in diethyl ether (20 mL). The reaction mixture was refluxed for 1 h. With a syringe, the dodecylmagnesium bromide solution was slowly transferred to a solution of 3,4-dibromothiophene (1 g, 4.13 mmol) and Ni(dppp) Cl2 (13 mg, 0.24 mmol) in diethyl ether (30 mL). The mixture was

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methanol/2 M HCl (10:1 v/v) to obtain the dark red solid. The solid polymer was dried under vacuum. P2 (141.8 mg, 60%). 1H NMR (400 MHz, C2D2Cl4, d): 7.55 (4H), 7.29 (8H), 7.05 (2H), 6.80 (4H), 6.22 (4H), 5.06 (2H), 2.67 (4H), 1.39–1.18 (40H), 0.79 (6H). 13C NMR (100 MHz, C2D2Cl4 d): 147.23, 139.96, 135.42, 129.30, 125.82, 125.71, 125.57, 125.27, 123.67, 115.49, 114.01, 113.08, 31.63, 30.33, 29.83, 29.63, 29.44, 29.10, 13.95. 2.4. Cyclic voltammetry and UV–vis measurements of P1 and tri-block P2 The cyclic voltammetry measurements were carried out on an AUTOLAB potentiostat with Nova 1.7 software. A scan rate of 50 mV/s was set for all measurements. A solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile was used as a supporting electrolyte. The three-electrodes were assembled in a glass cell. Polymer film coated on a platinum rod disk electrode with an area of 0.5 cm2 electrode was used as a working electrode. A Pt rod as a counter electrode and a double junction reference electrode Ag/AgCl/sat. LiCl in ethanol (model 6.0726.110) were purchased from Metrohm company. The reference electrode Ag/AgCl is approximately 0.143 V above normal hydrogen electrode (NHE), which was reported by a Metrohm manufacture. The ferrocene/ferrocenium ions has a redox potential of +0.521 V vs. Ag/AgCl measured in acetonitrile. The HOMO energy value of the polymer were estimated by an onset of redox potentials measured against ferrocene/ferrocenium ion in acetonitrile and calculated with this equation EHOMOele = [Eonsetox + E1/2(Fc/Fc+)] 4.80 eV assuming that the known energy value of ferrocene being 4.80 eV below the vacuum level [44]. The UV–vis measurement of polymer in CHCl3 was performed in a quartz cuvette cell (a path length of 1 cm), and for the polymer film, a solution of polymer (5 mg/10 mL CHCl3) was spin coated on an ITO glass substrate (0.8 cm  1.5 cm) with 2000 rpm for 30 s. All ITO substrates (10 ohm
cm) were purchased from Lumtec. Each substrate was sonicated with deionized water and acetone, respectively, for 10 min each and dried with a stream of N2. After we obtained the UV–vis spectra of the polymers both in solution and as thin film, we normalized absorbance data in an excel

program by dividing each point with the maximum absorbance value. 2.5. Fabrication and spectroelectrochemical measurements of P1 and tri-block P2 films An ITO glass substrate was cut to a dimension of 0.8 cm  5 cm and cleaned with the same procedure as described in previous section. A solution of polymer (5 mg/10 mL CHCl3) was spin-coated on a substrate at 2000 rpm for 30 s with the thickness of 60–65 nm by AFM. Each sample was annealed at 170  C for 30 min under vacuum. The spectroelectrochemical measurements of polymer film were carried out in a quartz cuvette cell (a path length of 1 cm). A potentiostat instrument was connected to a UV–vis-NIR spectrophotometer (Cary 5000, Agilent) with an application of potential from 0 to 1.2 V vs. Ag(s) with an increment of 0.2 V for 10 s to ensure full color change at each applied potential. We used a silver wire as the pseudo-reference electrode, which was calibrated against the redox potential of ferrocene/ ferrocenium ion (+0.521 V vs. Ag/AgCl measured in acetonitrile). For doping process experiment, the absorbance of polymer film was monitored on Agilent 8435 spectrophotometer under an application of voltage from an AD8733 DC power supply. Polymer film coated on an ITO glass substrate was used as a working electrode, and a Pt wire was used as a counter electrode, with a separation distance of about 0.5 mm. Film stability experiments was performed by first the direct application of light (450 nm, 42.4 mWatt/cm2) for 5 min to the polymer film. Then, a voltage of +1.4 Vdc/-1.4 Vdc for 10 s was applied to the polymer film while monitoring the absorbance. Consecutively, the application of light was applied for 10 and then 15 min prior to the measurement of absorbance. 3. Results and discussions 3.1. Synthesis and characterization The synthesis of monomers is outlined in Scheme 1 3,4didodecyl thiophene (1) was synthesized via the Kumada coupling of 3,4-dibromothiophene with dodecylmagnesium bromide using

Scheme 1. Synthesis of monomers and polymers P1 and tri block P2.

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Ni(dppp)Cl2. The bromination of (1) with two equivalents of N-bromosuccinimide (NBS) afforded 2,5-dibromo-3,4-dodecylthiophene (2) in a high yield of 97%. The synthesis of tri-block P2 was carried out via a two-step procedure. First, P1 was generated in an aryl-aryl cross coupling of (2) and (3) via Stille coupling reaction with the presence of the 4-bromoaniline (8% used) to obtain a a,v-telechelic anilineterminated polymer (P1) [41,42]. The ratio of thiophene to 4-bromoaniline was used to control the molecular weight of P1, and then the polymer was characterized by 1H NMR to confirm the structure which showed a signal at 6.09 ppm whose integration indicates two 4-aminophenyl groups per P1 unit chain. Thus, the polythiophene blocks were quantitatively terminated with 4-aminophenyl units. The synthesis of the tri-block P2 was carried out by the oxidative coupling reaction of NH2-terminated P1 with aniline using (NH4)2S2O8 and methanesulfonic acid. The oligomers generated during the copolymerization were removed by acetone by Soxhlet extraction. The weight average (Mw) of P1 and tri-block P2 were found to be 14,700 and 16,000 g/mol, respectively by GPC performed using a UV–vis detector set for the different wavelengths (390 nm for P1 and 450 and 490 nm for tri-block P2) as shown in Table SI1. The Mw analysis of tri-block P2 corresponds to an average polyaniline degree of polymerization (DP) of 6 aniline repeating units for each side of polythiophene block. The DP of the polythiophene block is approximately 25 units determined by the calculation of GPC analysis as further evidence that the polyaniline blocks covalently grew from the di-functionalized polythiophene blocks. 3.2. Thermal properties The thermal stability of P1 and tri-block P2 was examined by thermal gravimetric analysis (TGA) to find the process window of the polymer during the annealing process of polymer film. The P1 and tri-block P2 showed an excellent thermal stability in air with an onset of decomposition temperature (Td) (5% weight loss) at 461  C and at 457  C (less than 5% weight loss), respectively as shown in Fig. SI1. These results indicated that both copolymers had good thermal stability even in the presence of oxygen. The DSC curves (Fig. SI2. and SI3.) for both P1 and triblock P2 showed a glass transition temperature (Tg) at 145  C and 146  C, respectively without an observation of a melting point suggesting that both polymers can form amorphous film on a substrate.

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3.3. Optical and cyclic voltammetry measurements The optical properties of P1 and tri-block P2 were determined by UV–vis spectroscopy. Fig. 1(a) shows the UV–vis spectra of tri-block P2 in comparison to P1 in chloroform and as a thin film on an ITO substrate. In chloroform, the UV–vis spectrum of tri-block P2 was slightly red-shifted when compared to P1. P2 has a lmax absorption at 456 nm assigned to be the p-p* transition of the polythiophene blocks. We hypothesized that the weak absorption of the P2 at 566 nm is assigned to be the charge transfer between the polyaniline and the polythiophene blocks. The details of this observation are still under investigation and not under scope of this study. The lmax absorption of P1 was at 448 nm from p-p* transition of the polythiophenes. The UV–vis spectrum of tri-block P2 is quite similar to P1 due to the dominant contribution of the polythiophene block (80%). As thin films, the absorption spectrum of each polymer is significantly red-shifted in comparison to the absorption spectrum of the polymer in solvent. In our case, both polymers P1 and P2 as thin films on an ITO glass substrate has higher degree of conjugation as comparison to the polymers in solution due to a higher degree of freedom movement of the polymer chain in solvents. It is known that polythiophene solid film on a substrate with a thermal annealing forming pi stacking structures, which results in a red shift of a UV–vis absorbance [45,46]. The UV–vis spectrum of tri-block P2 on an ITO substrate shows lmax absorption peak at 495 nm assigned to be the main polythiophene blocks with a shoulder at 575 nm assigned to be the charge delocalization of the polyaniline and the polythiophene blocks. As thin film, the lmax absorption of tri-block P2 film was slightly red shifted of about 5 nm as comparison to P1 film with lmax at 490 nm. The redox behavior and the HOMO and LUMO energy levels of both polymers were investigated by cyclic voltammetry (CV) as shown in Fig. 1(b). The potential window for P1 was set between 1.6 V to 1.5 V vs. Ag/Ag+ to prevent the over-oxidation of the polymer. The CV spectrum of P1 showed the oxidation peak at 1.1 V vs. Ag/AgCl and a reversible oxidation peak at 1.0 V vs. Ag/AgCl. The currents of the oxidation peak and the reversible oxidation peak were not symmetric with the ratio less than 1. This behavior was not fully reversible process. However, we observed the color change of the polymer film resulting from the doping process of the thiophene rings of the polymer chains. The reduction peak of P1 was shown to be irreversible process as shown in an inset Fig. 1(b). The main blocks of the polymer chain were mainly thiophene units and it was unlikely that the aniline end-groups

Fig. 1. (a) Normalized UV–vis absorption spectra of polymers P1 (blue line) and tri-block P2 (red line) in CHCl3 (solid line) and as thin films (dashed line). (b) CV of polymer thin films in 0.1 M TBAPF6 in acetonitrile electrolyte at a scan rate of 50mVs 1 vs. Ag/AgCl. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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showed any response in this potential region. For P1, the oxidation onset potential was at 0.92 V vs. Ag/AgCl. We found that P1 degraded with the application of potential higher than 1.5 V vs. Ag/AgCl in agreement with another report of similar polythiophenes [32]. We demonstrated in Fig. SI4 that P1 was stable with repeatedly 3 sweep cycles between 0 and 1.5 V vs. Ag/AgCl, but decomposed with an application of potential after 1.5 V vs. Ag/AgCl. With the polyaniline block end-capping to the main chain, the highest oxidation peak of P2 was shifted to 1.9 V vs. Ag/AgCl with a smaller oxidation peak at 0.95 V vs. Ag/AgCl. The lower oxidation peak at 0.95 V vs. Ag/AgCl showed a reversible process but the higher oxidation peak at 1.9 V vs. Ag/AgCl was not reversible. We assigned the lower oxidation peak to be the polythiophene blocks and the higher oxidation peak to be the polyaniline blocks. The oxidation onset potential of tri-block P2 was slightly shifted to a lower potential of 0.8 V. The reduction of P2 was also shown to be a reversible process as shown in an inset of Fig. 1(b). The polyaniline blocks of P2 slightly lowered the onset oxidation potential as comparison to the onset oxidation potential of P1, and the potential window of P2 was larger than that of P1. P2 film was also stable with repeatedly 3 sweep cycles between 0 and 1.5 V vs. Ag/AgCl (as shown in Fig. SI5). Therefore, the polyaniline blocks increased the potential window on the positive potential and showed two oxidation peaks with one reversible oxidation peak at a lower potential (0.95 V vs. Ag/AgCl) and with the color change for the tri-block P2 film. We hypothesized from this observation that the addition of the polyaniline blocks increased the stability the positive charges of P2 due to the electron donating properties of nitrogen of polyaniline. We then further used the redox potential obtained from CV measurements to estimate the HOMO and LUMO energy levels of P1 and tri-block P2, which are listed in Table 1. We used the Egopt to calculate the LUMO energy level of both polymers. Both HOMO and LUMO energy levels of P2 were slightly shifted to a higher value compared to P1. The polyaniline blocks have a small effect on the HOMO and LUMO energy level of tri-block P2, which maintained mainly polythiophene characteristics. We hypothesized that the polythiophene and polyaniline blocks were initially independent in their electrochemical response. After applying positive potentials, positive charges were formed in the polythiophene blocks, and then they were delocalized through the polyaniline units. As a result, P2 film stability has increased under an applied potential relative to P1 film. The increase in film stability of the tri-block polymer with the application of higher voltage is a good result for use of the polymer in electrochromic applications. 3.4. Spectroelectrochemical properties of P1 and tri-block P2 film The electrochromic properties of both polymer films was investigated with the spectroelectrochemical measurement illustrated in Fig. 2(a) and (b). An application of voltage from 0 to +1.2 V vs. Ag wire with increments of +0.2 V was performed on a Table 1 Summary of optical and electrochemical properties of polymers. Polymer

P1 Tri-block P2 a

Egb (eV)

absorbance (nm) CHCl3

Filma

448 456 566 (sh)

490 495, 575 (sh)

2.16 2.09

HOMOc (eV)

LUMOd (eV)

5.33 5.17

3.17 3.08

Film casted from polymer in CHCl3 on a ITO substrate. Calculated from the onset wavelength of optical absorption in solution. Calculated from the onset Eox value from CV using the equation EHOMOele = [Eonsetox + E1/2(Fc/Fc+)] 4.80 eV. d Calculated from the HOMO value and EOptg of optical absorption. b c

Fig. 2. Spectroelectrochemistry of (a) P1 and (b) tri-block P2 on an ITO glass substrate in 0.1 M TBAPF6 in acetonitrile with an application of voltage vs. Ag(s) with an increment step of 0.2 V.

potentiostat while measuring the absorbance change using a UV–vis-NIR spectrophotometer. The neutral form of the polymer P1 film exhibited an orange color with an absorption maximum in visible region at 490 nm assigned to be p-p* transition of the polymer P1, and in infrared region (NIR) at 1100 nm assigned to be a residual charge from electron donating of the terminal anilines. The exact explanation of this residual charge from a neutral form of P1 is still under investigation and out of the scope of this work. With an application of potential to the polymer film, the intensity of the absorption bands at 490 nm continually decreased and a new absorption band at 800 nm and a broad peak above 1200 nm were gradually intensified. Meanwhile, the absorption band at 1100 nm was merging with the broad peak above 1200 nm. The color of the P1 film went from orange to a light blue due to the decrease in absorption in the visible region while the two new absorption peaks at 800 nm and 1200 nm arose due to the formation of charge carriers polaron (radical cation) and biporalon (dication), respectively. This observation was consistent with the charging process of polythiophene and its derivatives [35,47,48]. The tri-block P2 showed similar optical behavior to P1 as shown in Fig. 2(b). With the application of voltage, the absorption peaks in the visible region at 495 nm and in NIR region at 1100 nm was merging with the broad absorption band above 1200 nm, while absorption peaks at 800 nm and above 1200 nm increased. However, the absorption peak above 1200 nm increased more rapidly in comparison to the absorbance change of P1. Interestingly, with the addition of polyaniline blocks, the formation of polarons and bipolarons of the tri-block P2 showed a different rate in a change of colors from orange to transparent in comparison to

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a) 490 nm

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c) 1020 nm

b) 800 nm

k = 2.14 s-1 R2 = 0.99

0.05 0.00

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Fig. 3. Curve fitting for absorbance vs. time (s) with the application of voltage at +1.4 V monitored at a) 490 nm, b) 800 nm, and c) 1020 nm for P1 film on an ITO glass substrate in 0.1 M TBAPF6 in acetonitrile. The curves were fitted to one exponential decay with graph pad software without any parameter constraints.

b) 800 nm

a) 495 nm

k fast = 0.23 s-1 k slow = 0.04 s -1

0.3 0.2 0.1 0

20

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Time (s)

Fig. 4. Curve fitting for absorbance vs. time (s) with the application of voltage at +1.4 V monitored at a) 495 nm, b) 800 nm, and c) 1020 nm for tri-block P2 film on an ITO glass substrate in 0.1 M TBAPF6 in acetonitrile. The curve a) was fitted to two exponential decay and the curves b) and c) were fitted to one exponential decay with graph pad software without any parameter constraints.

P1 (vide infra). The rate of the color change will be shown under Section 3.5 discussion on the doping process. 3.5. Doping process of P1 and tri-block P2 We further investigated the process of polaron and bipolaron formation in both P1 and tri-block P2 with the application of voltage (DC) from a power supply to the polymer film as a function of time. We applied a voltage from a power supply to the polymer film for a practical standpoint to imitate a device configuration instead of using a voltage from a potentiostat instrument. Fig. 3(a)– (c) show the absorbance change of the polymer films monitored in visible region at 490 nm and in NIR region at 800 nm at 1020 nm. To ensure that the color reached its full maximum change, we applied +1.4 V to each sample. With an application of +1.4 V from a power supply, the polymer film had a full color change after 10 s. The details of the color change as a function of applied voltage was carried out and shown in Fig. SI6. At 1.4 V, the color change of both polymer films P1 and P2 reached the saturation without decomposition. It is noted that an applied voltage from a power supply was higher than an applied voltage from a potentiostat instrument because of a potential drop across the electrodes. The curve fitting to calculate the rate of absorbance change vs. time of P1 and tri-block P2 was shown in details in Figs. 3 and 4. The curve fitting of Fig. 3 was done with one exponential decay with a Graphpad software without any parameter constraints. The curve fitting of Fig. 4 was done with two exponential decay for only Fig. 4a) and the other two curves were fitted with one exponential decay without any parameter constraints.

The decrease of absorption in a visible region corresponded to the decrease in the polymer film color while the increase of absorption at 800 nm and 1020 nm was the formation of polaron and bipolaron, respectively. For P1, the rate of the disappearance in color as indicated by the decreased in absorption was calculated to be first order with a value of 2.14 s 1 and the formation of bipolaron was also first order with a value of 0.47 s 1, respectively. Interestingly, the absorbance change at 800 nm was increased and then decreased to a constant absorbance value. We believed that this transient observation is a result of the conversion of the radical cation converting to dication. On the other hand, the decrease in absorption for tri-block P2 in visible region was second order with fast and slow rates with values of 0.23 s 1 (fast) and 0.04 s 1 (slow) while the formation of polaron and bipolaron was first order with a rate of 1.37 s 1 and 0.05 s 1, respectively. At 800 nm, we also observed the increase on absorbance and then decreased to a constant absorbance value. The change of the absorbance in tri-block P2 was much slower in comparison to P1. As mentioned previously, with the addition of the polyaniline blocks, the rate of bipolaron formation was much slower in tri-block P2. We initially hypothesized that due to the addition of the polyanilines to the tri-block P2, during the doping process, the formation of polaron delocalize through the polymer

Neutral

fast

Polaron

+

slow

Bipolaron

++

Scheme 2. Proposed conversion process of P2 converting from neutral form to polaron and bipolaron forms with fast and slow rates.

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Fig. 5. Absorbance vs. Time (s) monitored (a) at 490 nm for P1 not under visible light (b) at 490 nm for P1 under visible light irradiation (450 nm) at the time interval of 5, 10, and 15 min each. (c) at 495 nm for tri-block P2 not under visible light and (d) at 495 nm for tri-block P2 under visible light irradiation (450 nm) at the time interval of 5, 10, and 15 min each. Each film on an ITO glass substrate in 0.1 M TBAPF6 in acetonitrile with an application of +1.4 V and 1.4 V with a time duration of 10 s at ambient conditions.

chain more effectively, and then formed bipolaron which occurs in a slower process and trapped in the polyaniline blocks due to oxidative doping in comparison to P1 containing only polythiophene block. We showed the proposed the conversion of the charge species in Scheme 2. In our study, we proposed that P2 in a neutral form converted to polaron in a fast process and then to bipolaron in a slow process. The nature of these charge species for P2 is still under investigation. 3.6. Electrochromic switching properties The stability of the polymer film was investigated with an alternate application of voltage +1.4 and 1.4 V at ambient conditions under visible light (450 nm). A switching time of 10 s was chosen as being in excess of the time required for each film to reach 100% of the full change in absorbance after the switching of the potential. The electrochromic stability of both polymer films was determined by measuring the absorbance change as a function of the number of switching cycles. Switching data for the representative films of P1 and tri-block P2 are shown in Fig. 5. Both P1 and tri-block P2 film were switched alternatively between the colored neutral state (orange color) and transmissive oxidized state (transparent blue color). As comparison, we compared the switching of polymer films under the same applied voltage without the light irradiation in Fig. 5(a) for P1 and Fig. 5(c) for P2. Without light irradiation, the change of absorbance between the colored neutral state and transmissive oxidized state of both P1 and P2 polymer films slightly decreased. It is known that polythiophene and alkylated polythiophenes can undergo decomposition under light irradiation and oxygen [49–52]. As shown in

Fig. 5(b) and (d), after light irradiation, P2 still exhibited good stability in terms of maintaining the absorbance values without hysteresis after 15 min of light irradiation. However, the P1 polymer film showed a 65% decrease in absorbance in the neutral state after repeated cycling. With the application of voltage, the P2 film showed higher stability as a comparison to the P1 film. Polyaniline has been previously reported to be stable under both UV and visible light and increased the performance of other studied systems [13,14]. In our case, during doping process of the tri-block P2 at ambient under visible light irradiation, electrons in the polythiophene block were excited with both voltage and light irradiation generating radical cations in the valence band and then convert to either biradical cations or bipolarons. The polyaniline blocks assisted the delocalization and trapping of both positive charge (bipolarons) and biradical cations along the polymer chains, which increase the stability of the tri-block P2 with the application of voltage and visible light irradiation. 4. Conclusions We successfully synthesized and characterized soluble tri-block polymers based on polyaniline-polythiophene-polyaniline. This tri-block polymer provides an example of a p-conjugated polymer system that can be adapted to other p-conjugated polymers in order to increase their stability in electronic applications operated at ambient conditions and under visible light. We have observed that stability and absorbance change vs. time of the tri-block copolymer films was affected by the polyaniline block as comparison to polythiophene block only. The polyaniline block acts to increase charge delocalization of the tri-block copolymer and slows down the formation of positive charges along the

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polymer chain. As a result, it slows down the degradation of the polymer film at ambient with an application of voltage. This triblock copolymer structure represents an alternative candidate for electrochromic applications. Acknowledgements This work was supported by National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Thailand. The doping process and stability experiments of polymer films were supported by Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Japan. The authors would like to thank Soft Matter Modeling Group and Smart Materials Group for the assistant on optical and analytical measurements. The authors also would like to thank Dr. Takayuki Miyamae and Dr. Supapan Seraphin for fruitful discussions. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet.2017. 02.001. References [1] M.P. Beaujuge, J.R. Reynolds, Color control in pi-conjugated organic polymers for use in electrochromic devices, Chem. Rev. 110 (2010) 268–320. [2] A.A. Argun, P.H. Aubert, B.C. Thompson, I. Schwendeman, C.L. Gaupp, J. Hwang, N.J. Pinto, D.B. Tanner, A.G. MacDiarmid, J.R. Reynolds, Multicolored electrochromism in polymers: structures and devices, Chem. Mater. 16 (2004) 4401–4412. [3] A. Pron, Rannou, Possible conjugated polymers: from organic semiconductors to organic metals and superconductors, Prog. Polym. Sci. 27 (2002) 135–190. [4] L. Zhao, L. Zhao, Y.X. Xu, T.F. Qui, L.J. Zhi, G.Q. Shi, Polyaniline electrochromic devices with transparent graphene electrodes, Electrochim. Acta 55 (2009) 491–497. [5] D.M. DeLongchamp, P.T. Hammond, Multiple-color electrochromism from layer-by-layer-assembled polyaniline/Prussian blue nanocomposite thin films, Chem. Mater. 16 (2004) 4799–4805. [6] D.M. DeLongchamp, P.T. Hammond, Layer-by-layer assembly of PEDOT/ polyaniline electrochromic devise, Adv. Mater. 13 (2001) 1455–1459. [7] Z.P. Li, B.X. Ye, X.D. Hu, X.Y. Ma, X.P. Zhang, Y.Q. Deng, Facile electropolymerized-PANI as counter electrode for low cost dye-sensitized solar cell, Electrochem. Commun. 11 (2009) 1768–1771. [8] H.C. Sun, Y.H. Luo, Y.D. Zhang, D.M. Li, Z.X. Yu, K.X. Li, Q.B. Meng, In situ preparation of a flexible polyaniline/carbon composite counter electrode and its application in dye-sensitized solar cells, J. Phys. Chem. C 114 (2010) 11673– 11679. [9] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Graphene/polyaniline nanofiber composites as supercapacitor, Chem. Mater. 22 (2010) 1392–1401. [10] A. Snook, Kao Graeme, Best Pon, S. Adam, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1–12. [11] J. Jyongsik, H. Jungseok, K. Kyungho, Organic light-emitting diode with polyaniline-poly(styrene sulfonate) as a hole injection layer, Thin Solid Films 516 (2008) 3152–3156. [12] K. Fehse, G. Schwartz, K. Walzer, K. Leo, Combination of a polyaniline anode and doped charge transport layers for high-effciency organic light emitting diodes, J. Appl. Phys. 101 (2007) 124509. [13] H. Zhang, R. Zong, J. Zhao, Y. Zhu, Dramatic visible photocatalytic degradation performance due to synergetic effect of TiO2 with PANI, Environ. Sci. Technol. 42 (2008) 3803–3807. [14] M.A. Salem, A.F. Al-Ghonemiy, A.B. Zaki, Photocatalytic degradation of allura red and quinoline yellow with polyaniline/TiO2 nanocomposite, Appl. Catal. B: Environ. 91 (2009) 59–66. [15] T.-H. Chang, C.-W. Hu, R. Vittal, K.-C. Ho, Incorporation of plastic crystal and transparent UV-cured polymeric electrolyte in a complementary electrochromic device, Sol. Energy Mater. Sol. Cells 126 (2014) 213–218. [16] Z.-Q. Tong, H.-M. Lv, J.-P. Zhao, Y. Li, Near infrared and multicolor electrochromic device based on polyaniline derivative, Chin. J. Polym. Sci. 32 (2014) 1040–1051. [17] D. Kumar, Synthesis and characterizaiton of poly(aniline-co-o-toluidine) copolymer, Synth. Met. 114 (2009) 369–372. [18] S.R. Pawar Pritee, P.P. Sainkar, Patil Synthesis of poly(aniline-co-toluidine) coatings and their corrosion-protection performance on low-carbon steel, J. Appl. Polym. Sci 103 (2007) 1868–1878.

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