Reactive & Functional Polymers 73 (2013) 1167–1174

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Reactive & Functional Polymers

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Azomethine coupled fluorene–thiophene–pyrrole based copolymers:Electrochromic applications

1381-5148/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.reactfunctpolym.2013.06.001

⇑ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33.E-mail address: mehmetyildirim@comu.edu.tr (M. Yıldırım).

Mehmet Yıldırım a,⇑, _Ismet Kaya b, Aysel Aydın b

a Çanakkale Onsekiz Mart University, Faculty of Engineering, Department of Materials Science & Engineering, 17020 Çanakkale, Turkeyb Çanakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Chemistry, 17020 Çanakkale, Turkey

a r t i c l e i n f o

Article history:Received 19 March 2013Received in revised form 4 June 2013Accepted 5 June 2013Available online 13 June 2013

Keywords:ElectrocopolymerizationElectrochromismCopolymerization ratio

a b s t r a c t

Two new copolymers were synthesized via the electrochemical copolymerization of 4,40-(9H-fluorene-9,9-diyl)bis(N-(thiophen-2-ylmethylene)aniline) (FTMA) with thiophene (Th) and pyrrole (Py). Accordingto the X-ray Photoelectron Spectroscopy (XPS) measurements, the polymers FTMA-co-Th and FTMA-co-Py possessed monomer ratios of nearly 1/5 (FTMA/Th) and 1/2 (FTMA/Py). Spectroelectrochemical inves-tigations showed that FTMA-co-Th was a red color at low potentials and a blue color at high potentials.FTMA-co-Py was purple at low potentials and dark gray at high potentials. Spectroelectrochemical mon-itoring showed good absorption recoveries over repeated potential scans. As a result, FTMA-co-Th andFTMA-co-Py may be good candidates for electrochromic devices (ECDs) and could be used as coloringagents in electrochromic layers in ECDs.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction Electrochemical copolymerizations of carbazole derivatives

Several types of electrochromic materials have been synthe-sized and characterized to date [1]. Electrochromic materials sus-tain reversible and persistent changes in their optical propertiesupon applied potential [2,3]. Due to their potential for structurallycontrollable HOMO–LUMO band gaps, simple tuning of colors, fastswitching times, high contrast ability and processibility, electro-chromic conducting polymers have received increasing attention[4]. As an example, polythiophenes are an important representa-tive class of conducting polymers which can be used as electricalconductors [5]. Synthesis of new polythiophene derivatives withimproved conductivity and processing properties is an importantresearch area in the field of conducting polymers [6]. Polythio-phenes, a versatile class of conjugated polymers, are still of grow-ing interest due to their potential applications in the developmentof new materials such as photovoltaics [7–9], electrochromic de-vices [10–12] and energy storage [13,14]. One of the most impor-tant characteristics exhibited by the polythiophene family is theirenhanced electrochromism at low potentials. A variety of conju-gated polymers are colored in both the oxidized and reduced statesbecause the band gap is in the visible region. After oxidation, theintensity of the p–p* transition decreases, and two lower energytransitions emerge to produce a second color. Therefore, thereare many absorption changes in the visible region of the spectrumthat make polymers useful in the construction of electrochromicdevices [15].

combined with thiophene moieties have been previously carriedout in the presence of thiophene, pyrrole and 3,4-ethylenedioxy-thiophene as the monomers [16]. The obtained polymers wereexamined as potential electrochromic devices. Schiff base-substi-tuted phenols and oligophenols, including electropolymerizablemoieties, have also been synthesized and copolymerized in thepresence of thiophene or pyrrole [17,18]. The obtained iminophe-nol-substituted polymers were investigated as alternative electro-chromic devices. However, the processibility of Schiff base-substituted electrochromic devices requires more research. Tomeet this need, we investigated the electrochromic application ofnewly synthesized imine-bounded 4,40-(9H-fluorene-9,9-diyl)-bis(N-(thiophen-2-ylmethylene)aniline) (FTMA) containing anelectropolymerizable thiophene moiety as the side group. Electroc-opolymerization was carried out in the presence of thiophene andpyrrole. The spectroelectrochemical and percent transmittancemeasurements of FTMA-co-Th and FTMA-co-Py were recorded.The obtained data showed that these new polymers could be usedin the construction of new electrochromic devices (ECDs) for vari-ous color changes.

2. Experimental

2.1. Chemicals

4,40-(9H-fluorene-9,9-di-yl)dianiline, thiophene-2-carbalde-hyde, thiophene (Th), and pyrrole (Py) were supplied from AldrichChemical Co., (St. Louis, Missouri, US). Methanol and MeCN were

1168 M. Yıldırım et al. / Reactive & Functional Polymers 73 (2013) 1167–1174

supplied from Merck Chemical Co., (Darmstadt, Germany). Borontrifluoride ethyl etherate (BF3�OEt2) was supplied from FlukaChemical Co., 4,40-(9H-fluorene-9,9-di-yl)dianiline and thio-phene-2-carbaldehyde were used as provided without furtherpurification.

2.2. Synthesis of 4,40-(9H-fluorene-9,9-diyl)bis(N-(thiophen-2-ylmethylene)aniline) (FTMA)

FTMA was prepared by the condensation of thiophene-2-carbal-dehyde (1.12 g, 0.01 mol) with 4,40-(9H-fluorene-9,9-diyl)dianiline(1.74 g, 0.005 mol) in methanol (30 mL) by stirring the mixture un-der reflux for 2 h (Scheme 1). The precipitated Schiff base was fil-tered, recrystallized from methanol and dried in a vacuumdesiccator at 80 �C [19].

FT-IR (cm�1): (CAH aromatic) 3072, 3023, (ACH@NA) 1616,(AC@CA phenyl) 1592, 1494, (CAS) 789. 1H NMR (DMSO-d6): dppm, 8.76 (s, 2H, ACH@NA), 7.96 (d, 2H, Ar–Hf), 7.81 (d, 2H, Ar–Hi), 7.67 (d, 2H, Ar–Hh), 7.49 (d, 2H, Ar–Hc), 7.44 (t, 2H, Ar–Ha),7.37 (t, 2H, Ar–Hb), 7.23 (t, 2H, Ar–Hg), 7.19 (d, 4H, Ar–Hd andHe). 13C NMR (DMSO-d6): d ppm, 153.76 (C5), 150.54 (C9),149.37 (C6), 143.29 (C4), 142.47 (C11), 139.53 (C16), 133.62 (C3),131.18 (C8), 128.50 (C12), 128.24 (C1), 127.98 (C13), 127.78 (C2),126.03 (C14), 121.12 (C7), 120.66 (C15), 64.33 (C10).

2.3. Electrochemical copolymerization of FTMA with Th and Py

Cyclic voltammetry (CV) measurements were carried out with aCHI 660 C Electrochemical Analyzer (CH Instruments, Texas, USA)at a potential scan rate of 0.2 V/s. CV was employed to assay theelectro activity of the compounds and determine the oxidation–reduction peak potentials. Electropolymerizations were separatelycarried out in the presence of Th and Py (Scheme 2). The systemconsisted of a CV cell containing an indium tin oxide (ITO)-coatedglass plate as the working electrode, platinum wire as the counterelectrode, and Ag wire as the reference electrode. The measure-ments were carried out in a LiClO4 (0.1 M)/MeCN:BF3�OEt2 (9:1,v/v) electrolyte solvent mixture at room temperature under an ar-gon atmosphere [20]. The electrocopolymerizations were carriedout as follows: FTMA (1 g/L) was dissolved in 10 mL of electrolytesolution and placed into a CV cell. 4 mg of Th or Py was added intothe cell. Electropolymerizations were run by repeated electro-chemical scanning of the solutions between 0 and 1.6 V forFTMA-co-Th and �0.4 and 1.6 V for FTMA-co-Py (scan rate: 0.2 V/s; cycles: 50). The resulting copolymer films were washed withMeCN to remove LiClO4 and BF3�OEt2. Polythiophene and polypyr-role films were also deposited onto ITO glass plates using the sameparameters (scan rate: 0.2 V/s, cycles: 50) for comparison.

NH2H2N

SO

2

MethanolReflux

H

Scheme 1. Synthesis of 4,40-(9H-fluorene-9,9-diyl)b

2.4. Spectroelectrochemistry

The electrochemically deposited polymeric films were used inspectroelectrochemical experiments. The measurements were per-formed in MeCN/LiClO4 using Ag wire as the reference electrodeand Pt wire as the auxiliary electrode. The data obtained fromthe CV measurements were used for spectroelectrochemical mea-surements [21]. The measurements were carried out to obtainthe absorption spectra of the copolymer films under applied poten-tials. The spectroelectrochemical cell included a quartz cuvette, aAg wire (RE), a Pt wire counter electrode (CE) and a polymerfilm-coated ITO/glass as transparent working electrode (WE). Themeasurements were carried out in 0.1 M LiClO4 as a supportingelectrolyte in MeCN.

2.5. Characterization techniques

Fourier transform infrared spectra (FT-IR) were recorded by aPerkin Elmer FT-IR Spectrum one using a universal ATR samplingaccessory (4000–550 cm�1). 1H and 13C NMR spectra (Bruker ACFT-NMR spectrometer operating at 400 and 100.6 MHz, respec-tively) were recorded using DMSO-d6 as a solvent at 25 �C. Tetra-methylsilane was used as an internal standard. The obtainedcopolymers were also structurally characterized by XPS measure-ments (PHI 5000 XPS), which were carried out using a monochro-matic Al anode at an energy of 1 eV. A CHI 660 C ElectrochemicalAnalyzer (CH Instruments, Texas, USA) was used to supply a con-stant potential during the electrochemical synthesis and CV exper-iments. An Analytikjena Specord S 600 single beamspectrophotometer was used to perform the spectroelectrochemi-cal studies of the polymers. The surface morphological propertiesand particle size of the copolymers were investigated using scan-ning electron microscopy (SEM) (Jeol JSM-7001F).

3. Results and discussion

3.1. Synthesis and characterization of the monomer (FTMA)

The synthesized Schiff base monomer, FTMA, is a yellow pow-der. The structure of the synthesized Schiff base is confirmed byFT-IR, 1H NMR and 13C NMR analyses. According to the FT-IR spec-trum of FTMA, C@N stretching of the imine bond appears at1616 cm�1. Additionally, the CAS stretching vibration of the thio-phene rings is observed at 789 cm�1 [22].

The 1H and 13C NMR spectra of FTMA are given in Fig. 1a and b,respectively. The peak assignments and the integrated area valuesare also shown. The characteristic imine proton (CH@N) signal at8.76 ppm confirms the Schiff base formation. In the 1H NMR spec-trum, the proton signals of the fluorene ring are found from 8.0 to7.6 ppm (except Hg). The peaks owing to the thiophene rings are

NNS

HC

SCH

is(N-(thiophen-2-yl-methylene)aniline) (FTMA).

NHCNC

H SS

FTMA

ElectrochemicalCopolymerization

S NH

or+NHCNC

H SS S

NHCNC

H SS NH

5n n

2m m

orFTMA-co-Th

FTMA-co-Py

Scheme 2. Electropolymerization reactions of FTMA in the presence of Th and Py.

Fig. 1. 1H NMR (a) and 13C NMR (b) spectra of FTMA.

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observed between 7.50 and 7.30 ppm. The expected pair of dou-blets of the p-disubstituted phenylene rings indicating the Hdand He protons are overlaid at 7.19 ppm (one doublet indicating

eight phenylene protons). Fig. 1b shows the 13C NMR spectrumof FTMA, in which the imine carbon (C5) signal is observed at153.76 ppm. Additionally, the C10 carbon in the fluorene ring is

Fig. 2. Potential sweep electropolymerization results of thiophene (a) pyrrole (b)FTMA in the presence of thiophene (c) and FTMA in the presence of pyrrole (d) (scanrate: 0.2 V/s, cycles: 50).

Fig. 3. XPS spectra of FTMA-co-

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observed at 64.33 ppm, as expected [23]. The other peaks related tothe Schiff base are shown in Fig. 1b. Consequently, the NMR dataconfirm the structure of FTMA. However, NMR spectra of the poly-mers could not be obtained due to their poor solubilities in alltested solvents. They are insoluble in methanol, DMSO, chloroform,DMF, THF and dioxane.

3.2. Electropolymerization

Thiophene derivatives readily undergo one-electron oxidationto form a radical cation (polaron) if the a-positions are unsubsti-tuted. Continuous oxidation subsequently affords higher orderedoligomers by a radical coupling mechanism that ultimately resultsin polymer formation [24]. Fig. 2 shows the potential sweep elec-tropolymerization results of FTMA in the presence of thiopheneand pyrrole as well as FTMA free thiophene and pyrrole solutions.Electropolymerization starts with the oxidation of thiophene orpyrrole and then proceeds by adding monomer units [17,18]. Dur-ing electropolymerization, as the number of cycles increases, thereis an increase in the current intensity. This is due to the increasingactive area of the working electrode as the electroactive polymercoating is built on the electrode [25]. As observed in Fig. 2, the elec-trocopolymerization results of FTMA clearly differ from the homo-polymerization results of thiophene and pyrrole. During therepeated potential scans, the FTMA solution with thiophene indi-cates oxidation and reduction bands at 0.80 and 0.42 V,

Py (a) and FTMA-co-Th (b).

Table 1XPS data: experimental and calculated atomic ratios in atom %.

Atomic ratios

Compounds C (%) N (%) S (%) C/N C/S N/S

FTMA-co-Tha 72.10 2.62 9.18 27.52 7.85 0.285FTMA-co-Thb 72.10 3.10 10.80 23.26 6.68 0.287FTMA-co-Pya 65.10 6.06 3.03 10.75 21.48 2.00FTMA-co-Pyb 65.10 11.40 5.50 5.72 11.84 2.07

a The theoretically calculated atomic percentages according to 1/5 (FTMA/Th)and 1/2 (FTMA/Py) monomer ratios.

b The results obtained from the XPS spectra.

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respectively. Similarly, the FTMA solution with pyrrole shows ananodic current increase at 0.5 V that reverses at �0.1 V.

3.3. FT-IR and XPS spectra of the polymers

The polymers coated on ITO by electropolymerization werescraped away for FT-IR and XPS analyses. According to the FT-IRspectra of the polymer powder, the imine C@N stretching appearsat 1634 and 1621 cm�1 for FTMA-co-Th and FTMA-co-Py, respec-tively [26]. The CAS stretching vibration of the thiophene ringsand the NAH stretching vibration of the pyrrole rings are observedas broad peaks at 756 and 1547 cm�1, as expected [27]. The broadpeak at approximately 1062 cm�1 is attributed to the incorporationof ClO�4 ions into the polymer film during the doping process [28].On the other hand, fluorene polymers synthesized by catalytic orchemical oxidative polymerization methods could exhibit fluore-none defects due to oxidation of the fluorene ring [23]. This defectwould be clearly visible by the C@O peak at approximately1700 cm�1. FTMA and its copolymers, however, show no such peakin the FT-IR spectra, indicating the absence of fluorenone defects.

To evaluate the surface composition of the synthesized copoly-mers, XPS investigations were carried out. The intensity scales re-lated to the binding energy levels of FTMA-co-Py and FTMA-co-Thare given in Fig. 3. As observed in Fig. 3a, the signals related to C1sare observed at 282 eV. The C1s component approximately 282 eVis mainly due to aromatic carbon groups of the polymeric conju-gated backbone [29]. The peak at 282 eV is attributed to CAC bond-ing irrespective of the type of hybridization. The N1s signals appearapproximately 398 eV, as expected. This peak related to N1s is as-signed to the nitrogen atoms of the imine (ACH@NA) units andpyrrole rings. The energy position of the nitrogen (398 eV) is indic-ative of CAN bonding [30]. The O1s signal is observed approxi-mately 530 eV. The 2s and 2p core levels of the S atom showsignals at 229 and 163 eV, respectively. This shows that the mono-

Fig. 4. CVs of FTMA-co-Th (a) and FTMA-co-Py (b) polymer films

mer units containing thiophene rings are present in the polymer, asexpected. The XPS spectrum of FTMA-co-Th is shown in Fig. 3b.There are signals related to C1s observed at 282 eV, mainly dueto the aromatic carbons of the polymeric conjugated backbone.The N1s signals appear approximately 398 eV, as expected. The2s and 2p core levels of the S atom have signals at 226 and162 eV, respectively. Additionally, the O1s signal is observed atapproximately 530 eV. This can arise from the oxidation of thetop layers of the polymer films. Moreover, the XPS spectra of thesynthesized polymers indicate that there are contaminants. Anumber of peaks assigned to Cl and O atoms could be identified.The signal related to Cl2p is observed at approximately 200 eV inthe spectra of the polymers [31]. This peak can be attributed tothe ClO�4 anion in LiClO4 which is used as a supporting electrolytein the electropolymerization medium. The presence of adsorbedLiClO4 on the polymer films is also confirmed by the peaks atapproximately 1090 cm�1 in the FT-IR spectra [32].

The approximate copolymerization ratios of the polymers arededuced from the XPS results. The atomic percentages of FTMA-co-Py and FTMA-co-Th as determined from the XPS analyses arelisted in Table 1. As observed in the structures of the monomers(FTMA, Th and Py), the copolymerization ratios are easily deter-mined using the N/S atomic ratio. FTMA contains two N and twoS atoms per molecule. Th and Py contain only one type of thesetwo atoms. According to the XPS results, the N/S ratio in FTMA-co-Th is 0.287 (2/7). This ratio indicates that five thiophenes areused per FTMA monomer. The N/S ratio of FTMA-co-Py is 2.07,which indicates two pyrroles per FTMA. As a result, the copolymer-ization ratios are 1/5 (FTMA/Th) and 1/2 (FTMA/Py). Theoreticallycalculated atomic percentages are determined using the obtainedcopolymerization ratios. Carbon percentages are adjusted to corre-spond with the XPS results, and the other atomic ratios are derived.All of the theoretically calculated values and the experimental XPSdata are summarized in Table 1.

3.4. CVs of the polymer films

The cyclic voltammograms of the synthesized copolymers(FTMA-co-Th and FTMA-co-Py) are shown in Fig. 4. The measure-ments were carried out in a 0.1 M concentrated LiClO4 electrolytesolution in MeCN. The CV of FTMA-co-Th shows one reversible oxi-dation peak at 0.80 V, which reverses at 0.42 V when the potentialrange between 0 and +1.6 V is scanned (Fig. 4a). Additionally, in thepotential scan range of �0.4–1.6 V, FTMA-co-Py has only onereversible oxidation peak at 0.5 V that reverses at �0.1 V (Fig. 4b)[33]. On the other hand, fluorene groups are known to oxidize at

(scan rate: 0.2 V/s, supporting electrolyte: LiClO4 in MeCN).

Fig. 5. Optoelectrochemical 2D (a and c) and 3D (b and d) spectra of FTMA-co-Py (a and b) and FTMA-co-Th (c and d) in the presence of 0.1 M LiClO4 in MeCN.

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high potentials (�1.9–2.0 V) [23]. The fluorene in FTMA is thus notoxidized at the working potential range.

Fig. 6. Electrochromical switching: Optical absorbance monitored for FTMA-co-That 474 and 800 nm (a) and for FTMA-co-Py at 707 nm (b) in the presence of 0.1 MLiClO4 in MeCN.

3.5. Electro-optical properties

Spectroelectrochemistry is the best method of examining thechanges in the optical properties of a polymer on ITO at appliedpotentials. Spectroelectrochemistry also provides information onthe properties of conjugated polymers, such as the band gap (Eg)and the intergap states that appear upon doping. Spectroelectro-chemical measurements were carried out at �0.4–1.6 V and 0–1.6 V potential scan ranges for FTMA-co-Py and FTMA-co-Th,respectively, in MeCN/LiClO4 (0.1 M) solution. The kmax for the p–p* transitions in the neutral states are observed at 487 nm forFTMA-co-Th (at 0 V), and 339 and 520 nm for FTMA-co-Py (at -0.4 V). The kmax for the p–p* transitions in the oxidized states (at1.6 V) are found to be 363 and 800 nm for FTMA-co-Th, and 352,456, and 700 nm for FTMA-co-Py. It can be said that as the appliedpotential increases up to 1.6 V, spectral changes occur in the com-pounds. Fig. 5a and c indicate the changes in the absorption spectraof FTMA-co-Th and FTMA-co-Py at various applied potentials. Asthe copolymer devices are oxidized, the intensities of the p–p*

transitions decrease while the intensities of the charge carrierbands at 800–900 nm increase due to polaron formation [16]. 3Dabsorption spectra of FTMA-co-Th and FTMA-co-Py are shown inFig. 5b and d, respectively. The spectral changes of the polymersin response to applied different potentials were clearly observed.

Fig. 7. SEM images of FTMA-co-Th (a and b) and FTMA-co-Py (c and d).

M. Yıldırım et al. / Reactive & Functional Polymers 73 (2013) 1167–1174 1173

3.6. Electrochromic switching and stability

To determine the electrochromic switching characteristics, a re-peated scan mode of CV coupled with optical spectroscopy is used.The potential scan range is chosen to be the same as in the CV mea-surements, and the scan rate is set to 0.2 V/s. The electrochromicswitching graphs are given in Figs. 6a and b. Measurements arecarried out using different wavelengths in which the absorptionintensities clearly change. During the switching experiments, thekmax values of FTMA-co-Th (474 and 800 nm) and FTMA-co-Py(707 nm) are used. The polymer film is deposited on ITO-coatedglass slides under constant potential conditions, as mentionedabove. The percent transmittances are then monitored at the kmax

while the polymer films are switched at different potential ranges.As observed in Fig. 6a, during the repeated potential scan, theabsorption intensities of FTMA-co-Th are quite stable at appliedpotentials for the two wavelengths (474 and 800 nm). Moreover,as observed in Fig. 6b, FTMAP-co-Py has good absorption recoveryat applied potentials. A percent transmittance change (DT%) of 18%was determined at 707 nm. The stability of the redox states overseveral scans is the most important property for an electroactivepolymer to be useful in the construction of new electrochromic de-vices [34]. The response times of FTMA-co-Th and FTMA-co-Pyfrom the reduced to the oxidized (1.6 V) states are found to be10 and 13 s, respectively. For electrochromic applications, the abil-ity of the material to exhibit a noteworthy color change is alsoimportant. FTMA-co-Th synthesized onto ITO-glass exhibits a redcolor in the neutral state and a blue color in the oxidized state,and FTMA-co-Py synthesized onto ITO-glass is dark gray in theneutral state and purple in the oxidized state.

3.7. Surface morphologies of the copolymer films

The surface morphological properties of FTMA-co-Th and FTMA-co-Py are obtained by scanning electron microscopy (SEM). SEMphotographs of the powder forms are shown in Fig. 7. After the

copolymers are electrochemically synthesized onto ITO, thecopolymeric materials are scraped from the ITO and the measure-ments are carried out. According to the SEM images, FTMA-co-Thhas a perforated surface with holes approximately 2–3 lm indiameter (Fig. 7a and b). This morphology is distinctly differentfrom that of the polythiophene homopolymers [35]. Fig. 7c and dshow the SEM images of FTMA-co-Py. According to these images,FTMA-co-Py has a brain-like plaited surface. The polymer surfaceis homogeneous (see Fig. 7c). The plaited structure of the brainsupplies a larger surface area in a small volume, which improvesthe performance. Similarly, this plaited morphology of FTMA-co-Py is expected to increase the functionality. The obtained surfaceis also clearly different from the previously synthesized polypyr-role homopolymers [36].

4. Conclusions

A fluorene Schiff base containing electropolymerizable thiopheneside groups was synthesized and structurally characterized via FT-IR,1H NMR and 13C NMR. The electrochemically synthesized copolymers,FTMA-co-Th and FTMA-co-Py, were investigated as electrochromicmaterials. The XPS results indicated 1/5 (FTMA/Th) and 1/2 (FTMA/Py) copolymerization ratios. The spectroelectrochemical resultsshowed that FTMA-co-Th was red at low potentials and blue at highpotentials, and FTMA-co-Py was purple at low potentials and dark grayat high potentials. The stable absorption recoveries of FTMA-co-Th andFTMA-co-Py could make these new materials appropriate for the con-struction of novel electrochromic devices. According to the SEMimages, FTMA-co-Th has a perforated surface with holes approximately2–3 lm in diameter, and FTMA-co-Py has a brain-like plaited surface,which can increase its functionality.

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