Combination of donor characters in a donor–acceptor–donor (DAD) type polymer containing benzothiadiazole as the acceptor unit

Organic Electronics 11 (2010) 1877–1885

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Combination of donor characters in a donor–acceptor–donor (DAD) type polymer containing benzothiadiazole as the acceptor unit Merve Sendur a, Abidin Balan a, Derya Baran a, Baris Karabay a, Levent Toppare a,b,c,⇑ a

Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey Department of Biotechnology, Middle East Technical University, 06531 Ankara, Turkey c Department of Polymer Science and Technology, Middle East Technical University, 06531 Ankara, Turkey b

a r t i c l e

i n f o

Article history: Received 3 August 2010 Received in revised form 2 September 2010 Accepted 2 September 2010 Available online 16 September 2010 Keywords: Conjugated polymers Electrochromism Donor–acceptor theory Copolymers p- and n-doping

a b s t r a c t A benzothiadiazole bearing donor–acceptor–donor (D–A–D) type monomer (M3) was synthesized using the combination of 3, 4-ethylenedioxythiophene (EDOT) and thiophene donor units to understand the effect of donor strength on the optoelectronic and electrochemical properties. The resulting monomer was polymerized electrochemically (P3) and compared with its symmetrical thiophene (P1) and EDOT (P2) bearing homologues whether there exists a combination of the electrochemical and optical characteristics. Also, copolymer studies were performed with symmetrical thiophene (M1) and EDOT (M2) containing monomers in order to compare the results with P3. Cyclic voltammetry (CV) and spectroelectrochemistry results revealed that P3 is a low band gap polymer (1.18 eV) having both p-and n-type property which is superior to the copolymers synthesized using M1 and M2. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Due to their favorable electronic, optical and mechanical properties conductive polymers (CPs) are used in a wide range of applications such as photovoltaics [1], light-emitting diodes [2], field-effect transistors [3], sensors [4], and electrochromic devices [5]. CPs as a class of electrochromic materials have attracted great interest due to several superior properties over their inorganic counterparts such as coloration efficiency [6], fast switching ability [7], multiple colors with the same material [8], and fine-tuning of the band gap with small structural modifications [5b,6–9]. Due to the fact that electrochromic and non emissive applications of CPs provide stimuli effect (viewing angle, light intensity or sunlight exposure) free, light-weight and flexible displays, their development is considered to be important for future display technologies [10]. ⇑ Corresponding author at: Middle East Technical University, Department of Chemistry, 06531 Ankara, Turkey. Tel.: +90 3122103251; fax: +90 3122103200. E-mail address: [email protected] (L. Toppare). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.09.001

In order to use CPs in electrochromic devices, there are two issues to be resolved; to have a low band gap to maximize the conductivity of the polymers and satisfy three additive colors in the neutral state to obtain full color space [11]. Approaches to achieve RGB (red, green, blue) colors with CPs have become the subject of many studies; however, only handful of them revealed true red [12], blue [13] or green [14] neutral state polymers. Up to date, donor–acceptor–donor (DAD) route has been considered as the most effective way to obtain low band gap polymers with desired neutral state colors. DA structured monomers are usually obtained with electropolymerizable heterocycles as the end group in order to achieve the necessary polymers electrochemically [15]. Electrochemical copolymerization is also a facile method to obtain desired neutral state colors for these types of materials. In copolymerization, electroactive monomers are polymerized from a bulk containing the co-monomers. Resulting polymer generally shows unique optical and electronic properties with enhanced mechanical and kinetic properties compared to those of the homo-polymers [16]. Fine tuning in the band gap and neutral state color

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can be achieved by tailoring the co-monomer feed ratio as well as the working potential of copolymerization [17]. However, exact structures of electrochemical copolymers are hard to determine due to the insolubility of the deposited films which hinders the solution characterizations. Recently, poly(4,7-di(2,3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5] thiadiazole) [14d] (P2) was characterized in terms of its electrochromic properties in our group. It was shown to be the first green to transmissive electrochromic material with two simultaneous maximum absorptions placed at around 400 and 750 nm. Its homologue with lower electron density, poly(4,7-dithien-2-yl2,1,3-benzothiadiazole) [18] (P1) has an absorption maximum at around 560 nm where its second transition is in UV region. At the first glance, it seems that a copolymer obtained from these two monomers should have an absorption covering the entire visible region, but the copolymers reported in this manuscript revealed neutral state colors in between the two, since absorption maximum of the copolymer usually lies between the two homo-polymers. Herein, we report our approach to explain the effect of different donor groups on the electronic and optical properties of DAD type polymers. An asymmetric monomer having both 3,4-ethylenedioxytiophene and thiophene as the donor groups was synthesized and electrochemically polymerized. Since the monomer contains two different donor units, we anticipated that its polymer may behave as a copolymer of the symmetric monomer having the same donor groups. Hence, the properties of copolymers achieved with the co-monomers having either thiophene or 3,4-ethylenedioxythiophene as the donor group were also reported. 2. Results and discussion 2.1. Synthesis Donor–acceptor–donor type materials 4,7-di(thiophen2-yl)benzo[c][1,2,5]thiadiazole (M1) [18,19], 4,7-di(2, 3-dihydro-thieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thia-

diazole (M2) [14d,20] and 4-(2,3-di hydrothieno[3,4b][1,4]dioxin-5-yl)-7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (M3) were synthesized as shown in Scheme 1 according to previously reported methods. In order to synthesize the asymmetric monomer (M3) 4,7 dibromobenzothiadiazole was first coupled with tributyl(2,3-dihydrothieno[3, 4-b][1,4]dioxin-5-yl)stannane and then the isolated compound was coupled with tributyl(thiophen-2-yl) stannane. 2.2. Cyclic voltammetry M3 was polymerized potentiodynamically between 0.3 V and 1.5 V vs. Ag wire pseudo reference electrode (0.3 V vs. Fc/Fc+) in a 0.1 M dichloromethane (DCM)/acetonitrile (AN) (5:95, v:v) / tetrabutylammonium hexafluorophosphate (TBAPF6) solution onto ITO-coated glass slides in order to observe the polymer characteristics. During polymerization, a characteristic oxidation peak of monomer was observed at 1.2 V accompanied by a reversible redox couple of the polymer. In Fig. 1, the increase in current density with the number of scans clearly shows the deposition of an electroactive film of P3 on ITO. The resultant polymer revealed reversible p-type doping properties where the p doping/de-doping was indicated by the peaks at 1.06 V and 0.85 V, respectively. The P3 film is blue in its neutral state and reveals light blue color when oxidized. Generally, EDOT bearing DAD type molecules have lower monomer oxidation potentials than thiophene bearing ones under identical experimental conditions owing to the electron rich ethylenedioxythiophene group. As reported M1, revealed a monomer oxidation at 0.85 V in 0.1 M DCM/AN (5:95, v:v)/TBAClO4 vs. the Ag/Ag+ reference electrode and its EDOT bearing homologue M2 has a monomer oxidation at 0.95 V in DCM/TBAPF6. In order to have a reliable comparison with M3, we performed all experiments in DCM/AN/TBAPF6 system vs. Ag wire pseudo reference electrode. As a result, M3 revealed a monomer oxidation at 1.2 V vs. Ag wire pseudo reference electrode which was lying between the two others. Since M3

Scheme 1. Synthesis scheme of the monomers; M1, M2 and M3.

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contains both ethylenedioxythiophene and thiophene groups, its oxidation potential value is between the values of the monomers containing only EDOT or thiophene as shown in Fig. 2. Since the polymer structure consists of electron withdrawing benzothiadiazole units, a rare property; n-type doping was observed for P3 with reversible peaks at 1.6 V and 0.9 V vs. the same reference electrode (Fig. 3). Oxidation and reduction onset values were used to determine the band gap energies for P1, P2 and P3 since

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all the polymers are both p- and n-type dopable. HOMO and LUMO energies were calculated as 5.75 and 4.65 eV, respectively for P3 (the value of NHE was used as 4.75 eV throughout this study) [1j]. As seen from Table 1, the LUMO levels of three polymers are same since benzothiadiazole was used as the acceptor unit. However, as the donor strength increases the HOMO energy levels decrease due to the electron rich EDOT units incorporated into the polymer structure. Calculated optical and electrochemical band gap values for P1 (1.65 eV), P2 (1.1 eV) and

Fig. 1. Electrochemical deposition of P3 on ITO-coated glass slide in a 0.1 M DCM/AN/TBAPF6 solvent-electrolyte couple.

Fig. 2. First cycles of M1, M2 and M3 in 0.1 M DCM/AN/TBAPF6 solvent-electrolyte couple during electrochemical polymerization.

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Fig. 3. p- and n-type doping of P3 in 0.1 M AN/TBAPF6.

Table 1 Monomer oxidation, corresponding polymer redox potentials, band energies (HOMO/LUMO values), optical and electronic band gap values of P1, P2 and P3. Polymer

Eox m (V)

Eox p (V)

Ered (V) p

P1 P2 P3

1.36 1.07 1.20

1.09 0.90 1.06

0.61 0.70 0.85

HOMO/LUMO energies (eV) 4.85/ 3.75 5.40/ 3.75 5.00/ 3.75

P3 (1.2 eV) are in good agreement confirming the identical electrochemical processes for both methods (CV and spectroelectrochemistry). The scan rate dependence of the P3 film was investigated using CV. A direct proportionality between the scan rate and the current intensity indicates a well adhered electroactive film on ITO and a non-diffusion controlled electrochemical process. The linear relationship can be seen even at very low and high scan rates which demonstrates that the number of electrons in the diffusion layer is always constant during the scans but increases as the scan rate increases [21] (Fig. 4). Copolymer studies were performed with M1 and M2 in order to observe whether the resulting copolymer will have the same properties with P3. Since M3 has both thiophene and EDOT units on its polymer backbone, its polymer P3 may reveal the similar electrochemical and optical characteristics with the copolymer (M1 and M2 as the co-monomers). A three-electrode cell with ITO working electrode was used to deposit polymers potentiodynamically. Oxidative polymerizations were achieved by repeated cycling between 0.3 V and +1.5 V with different feed ratios. For 1:1 (M1:M2) ratio 10 6 moles of each comonomers were used and the other ratios were determined according to the mole ratios. DCM/AN (5:95 v: v)/ TBAPF6 solvent-electrolyte system was used for polymeri-

Optical band gap (eV)

Electrochemical band gap (eV)

1.50 1.20 1.18

1.65 1.10 1.20

zation at a scan rate of 100 mV s 1 for 1:1, 1:2, 1:4, 1:10 and 2:1 (M1:M2) ratios. Table 2 summarizes the monomer and polymer redox potentials for electrochemically prepared copolymers. During potentiodynamic scans, two monomer oxidations were observed for M1 and M2 distinctively and reversible redox peaks were determined for the copolymers. As the amount of M1 was increased in the feed, the polymer redox peaks were slightly shifted to higher potentials due to the increased amount of thiophene units on the polymer backbone. Likewise, P3 showed similar electrochemical properties with those of the copolymers prepared from different feed ratios. 2.3. Optical studies In its neutral state, P3 showed maximum absorption at 408 nm and 642 nm in the visible region which resulted in a greenish-blue color. Likewise P2 also has two absorption maxima due to p–p* transition at around 428 nm and 755 nm corresponding to green color in its neutral state. In order to have a green color, there should be two transitions at around 400 nm and 700 nm in the visible region. P1 has only one transition at 560 nm whereas its second transition is not in the visible region. As we incorporated EDOT moieties into the P3 polymer structure the absorption in the UV region shifts to 408 nm which is

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Fig. 4. Scan rate dependence of P3 film for p-doping in 0.1 M AN/TBAPF6 solvent-electrolyte couple at 100, 150, 200, 250, 300, 350 and 400 mV s 1.

Table 2 Monomer oxidation and accompanied polymer redox potentials of copolymers prepared from co-monomers M1 and M2. Co-monomer composition

2M1:1M2

1M1:1M2

1M1:2M2

1M1:4M2

1M1:10M2

Eox poly (V)

1.34

1.30

1.28

1.28

1.26

Ered poly (V)

0.94

0.91

0.85

0.75

0.58

Fig. 5. Electronic absorption spectra of monomers M1, M2 and M3 (left) in CHCl3 and polymers P1, P2 and P3 in thin film form (right).

in the visible region but still does not correspond to green color since the second p–p* transition is not at around 700 nm. The normalized monomer and resulting polymer film absorptions of P1, P2 and P3 are shown in Fig. 5. It

can be seen that when thiophene units were replaced by EDOT units, monomer absorbance values were shifted to lower energies. This trend was also observed in the absorbance values of polymer films. Also, since the conjugation

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length was increased, all dominant wavelengths were shifted to right for polymers P1, P2 and P3 compare to the ones for their monomers. Fig. 6 displays the normalized absorbance of each copolymer film in neutral state prepared from different compositions. P1, P2 and P3 demonstrate a clear shift in the dominant wavelength as the co-monomer composition is varied from pure P1 to pure P2. As the concentration of M2 is increased in the copolymer chain, the p–p* absorptions of the copolymers were shifted to lower energies from 391 nm to 422 nm and 596 nm to724 nm. The observed red shift indicated a ‘‘tunable” electrochromic material can be synthesized by varying the amount of any monomer in the copolymer backbone. Here, it is noteworthy to emphasize that P3 showed the electrochemical and optical properties of the copolymer (1:4, M1:M2 ratio) hence P3 can be used like a copolymer. Most of the copolymer studies in the literature state that the copolymerization of two co-monomers often results in a random

copolymer. However, we achieved a controlled polymerization with similar properties to that of copolymers using M1 and M2 as the co-monomers. Table 3 summarizes kmax values and band gap energies for P1, P2, P3 and the copolymers obtained from M1 and M2. The band gap of P3 was calculated as 1.18 eV from the lowest p–p* transition which is very similar to the one reported for the copolymer obtained using 1:4 M1:M2 ratio. In order to probe the spectral response of P3 upon applied potential spectroelectrochemical studies were performed in 0.1 M AN/TBAPF6 solvent-electrolyte couple. During stepwise oxidation, the potential was gradually increased from 0.3 V to 1.2 V and the absorbance changes were observed for P3. In its neutral form P3 revealed only two transitions with no absorbance in NIR region. Upon p-type doping, as the peaks at 408 nm and 642 nm were decreased new bands in near-IR region intensified due to the charge carrier formation such as polarons and bipolarons (Fig. 7). Since P3 still absorbs after 600 nm in its doped

Fig. 6. Normalized absorbance spectra of P1, P2, P3 and the copolymer films prepared from different compositions (all in neutral state).

Table 3 Maximum wavelength values and band gap energies of P1, P2, P3 and resulting copolymers. Polymer

P1

2:1 (M1:M2)

1:1 (M1:M2)

1:2 (M1:M2)

1:4 (M1:M2)

P3

1:10 (M1:M2)

P2

560 1.5

384 574 1.41

391 596 1.36

395 613 1.06

404 619 1.18

408 642 1.18

422 724 1.2

428 755 1.2

Neutral state colors

kmax (nm) Optical band gap (eV)

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Fig. 7. Electronic absorption spectra of P3 in 0.1 M AN/TBAPF6 system upon p-type oxidation.

Fig. 8. Optical transmittance changes of P3 at 408 nm, 620 nm and 1050 nm in 0.1 M AN/TBAPF6 system while switching the potentials between its neutral and oxidized states.

state, the polymer was light blue. To have a transmissive state in the doped form the bands in the visible region should have depleted completely [13b]. Transmittance changes and switching abilities of P3 were determined while sweeping the potentials between its fully oxidized and reduced states within 5 s time intervals. The studies were performed in a monomer free 0.1 M AN/TBAPF6 solvent-electrolyte system. P3 revealed 22% transmittance change upon doping/de-doping process at

408 nm, 14% at 620 nm and 46% at 1050 nm (Fig. 8). Switching time is the duration for the polymer to switch between the two extreme states and it was calculated as 0.6 s, 0.75 s and 0.8 s at corresponding wavelengths. 3. Conclusion An EDOT, thiophene and benzothiadiazole bearing donor–acceptor type monomer was synthesized and

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polymerized electrochemically as an alternative to copolymer studies. The polymer was characterized by electrochemically and optically. The resulting polymer is both pand n-type dopable and showed the characteristics of a copolymer where two monomers are required. The results revealed that P3 can be synthesized and its polymerization can be controlled to avoid the formation of a random copolymer.

4. Experimental 4.1. General All chemicals were purchased from Aldrich except THF (tetrahydrofuran) which was purchased from Acros. Tributyl(thiophene-2-yl)stannane [22] and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane [23] were synthesized according to a previously described method. All reactions were carried out under argon atmosphere unless otherwise mentioned. All electrochemical studies were performed under ambient conditions using a Voltalab 50 potentiostat. Electropolymerizations were performed in a three-electrode cell consisting of an indium tin oxide doped glass slide (ITO) as the working electrode, platinum wire as the counter electrode, and a silver wire pseudo reference electrode. After each measurement the reference electrode was calibrated with ferrocene. The potential of quasi reference electrode was determined as 50 mV vs. the normal hydrogen electrode (NHE). 1 H and 13C NMR spectra were recorded in CDCl3 on a Bruker Spectrospin Avance DPX-400 Spectrometer. Chemical shifts are given in ppm downfield from tetramethylsilane. A Varian Cary 5000 UV–Vis spectrophotometer was used to perform the spectroelectrochemical studies of the polymer at a scan rate of 2000 nm/min. Column chromatography of all products was performed using Merck Silica Gel 60 (particle size: 0.040–0.063 mm, 230–400 mesh ASTM). Reactions were monitored by thin layer chromatography using silica gel coated aluminum sheets. Solvents used for spectroscopy experiments were spectrophotometric grade.

4.3. Synthesis of 4-bromo-7-(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole 4,7-Dibromobenzo[c] [1,2,5] thiadiazole (1.5 g, 5.1 mmol) and tributyl(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)stannane (3.3 g, 7.7 mmol) were dissolved in dry tetrahydrofuran (THF) (60 ml) and set for refluxing under argon atmosphere. The catalyst, dichlorobis(triphenylphosphine)-palladium(II) (60 mg, 0.085  10 3 mmol) was added and the mixture was stirred at 100 °C under argon atmosphere for 16 h while monitoring the reaction with TLC. At the end of 16 h, the mixture was cooled and THF was removed from the mixture by rotary evaporator. Then the residue was subjected to column chromatography (Chloroform: Hexane, 3:1) to afford a purple solid, 4-bromo-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5yl)benzo[c][1,2,5]thiadiazole. 1H NMR (400 MHz, CDCl3): d(ppm) 7.77 (d, 1H), 8.16 (d, 1H), 6.54 (s, 1H), 4.24 (m, 2H), 4.34 (m, 2H). 13C NMR (100 MHz, CDCl3): d(ppm) 61.95, 62.73, 100.63, 105.36, 115.65, 123.47, 124.02, 127.67, 127.84, 130.21, 139.34, 151.15. 4.4. Synthesis of 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)7-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (M3) 4-Bromo-7-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl) benzo[c][1,2,5]thiadiazole (1 g, 2.8 mmol), and tributyl(thiophen-2-yl)stannane (2.1 g, 5.6 mmol) were dissolved in THF (60 ml) and dichlorobis(triphenylphosphine)-palladium(II) (50 mg, 0.071  10 3 mmol) was added at room temperature. The mixture was refluxed for 12 h under argon atmosphere. Solvent was evaporated under vacuum and the crude product was purified by column chromatography (Cholorform: Hexane, 3:1) to obtain a dark red solid, 4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-7(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (M3) . 1H NMR (400 MHz, CDCl3): d(ppm) 6.5 (s, 1H), 7.8 (d, 1H), 6.9 (s, 2H), 8.3 (d, 1H), 8.0 (m, 1H), 7.1 (m, 1H), 7.4 (m, 1H), 4.3 (m, 2H), 4.2 (m, 2H); 13C NMR (100 MHz, CDCl3): d(ppm) 151.56, 151.38, 140.70, 140.66, 140.37, 139.21, 138.32, 126.64, 125.90, 125.11, 124.92, 124.85, 123.51, 101.13, 63.75, 63.04. 4.5. Electrochemical polymerization of M3

4.2. Synthesis of 4,7-dibromobenzo[c] [1,2,5] thiadiazole A mixture of hydrobromic acid (16 ml) and bromine (2 ml) was added slowly to a mixture containing hydrobromic acid (36 ml) and benzothiadiazole (2.0 g, 0.015 mol) via continuous stirring at 150 °C. The reaction was left for overnight reflux. Then, the mixture was kept in ice bath; orange crystals were produced in 3–4 h. The crystals were filtered, and dissolved in dichloromethane. The organic phase was extracted by two portions of sodium bisulfide solution, and then with saturated sodium chloride solution. After the washing process, mixture was dried over MgSO4 and the solvent DCM was evaporated by rotary evaporator. The yellowish solid was collected after drying in oven for overnight. 1H NMR (400 MHz, CDCl3): d(ppm) 7.65 (s, 2H). 13C NMR (100 MHz, CDCl3): d(ppm) 152.75, 132.13, 113.70.

For electrochemical polymerization, anodic electrodeposition of the polymer was performed from a dichloromethane (DCM) and acetonitrile (AN) mixture (5/95, v/v) with 0.1 M TBAPF6 (tetrabutylammonium hexafluorophosphate) solvent-electrolyte couple at a scan rate of 100 mV s 1. ITO-coated glass slides, Pt wire and Ag wire were used as working, counter and pseudo reference electrodes, respectively. The potentiodynamic scans were carried out between 0.3 and 1.5 V. After polymerization, the film was washed with ACN to remove the unreacted monomers and the supporting electrolyte. 4.6. Synthesis of copolymers A three-electrode cell containing ITO-coated glass slide as the working electrode was used to deposite copolymer films potentiodynamically. 4,7-Di(thiophen-2-yl)benzo[c]-

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[1,2,5]thiadiazole (M1) and 4,7-bis(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole (M2) were synthesized according to the previously reported method [14d,18,20b]. The oxidative electrochemical copolymerizations were achieved by repeated potential cycling in different solutions. M1 and M2 were used as the co-monomers in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)/acetonitrile (AN)/dichloromethane (DCM) (95:5, v: v) solvent-electrolyte couple at a scan rate of 100 mV s 1. All polymerizations were done under the same conditions for comparison purposes. For 1:1 (M1:M2) ratio 10 6 moles of each co-monomer were used and the other ratios; 1:2, 2:1, 1:4 and 1:10 (M1:M2) were determined accordingly to mimic M3. Potentiodynamic runs were between 0.3 and 1.5 V for all copolymer depositions with a scan rate of 100 mV s 1. After electrolysis, the films were washed with ACN to remove the supporting electrolyte and the unreacted monomers.

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