Dithienylpyrrole- and Tris[4-(2-thienyl)phenyl]amine-Containing Copolymers as Promising Anodic Layers in High-Contrast Electrochromic Devices

Abstract

Three dithienylpyrrole- and tris[4-(2-thienyl)phenyl]amine-containing copolymers (P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA)) were deposited on indium tin oxide (ITO) surfaces using electrochemical polymerization. Spectroelectrochemical characterizations of polymer films revealed that P(MPS-co-TTPA) film was light olive green, greyish-green, bluish grey, and grey in neutral state, intermediate state, oxidized state, and highly oxidized state, respectively, whereas P(MPO-co-TTPA) film was green moss, foliage green, dark greyish-green, and bluish-grey in neutral state, intermediate state, oxidized state, and highly oxidized state, respectively. The ΔT_max of P(MPS-co-TTPA) film at 964 nm, P(MPO-co-TTPA) film at 914 nm, and P(ANIL-co-TTPA) film at 960 nm were 67.2%, 60.7%, and 67.1%, respectively, and the coloration efficiency (η) of P(MPS-co-TTPA) film at 964 nm, P(MPO-co-TTPA) film at 914 nm, and P(ANIL-co-TTPA) film at 960 nm were calculated to be 260.3, 176.6, and 230.8 cm² C⁻¹, respectively. Dual type complementary colored electrochromic devices (ECDs) were constructed using P(MPS-co-TTPA), P(MPO-co-TTPA), or P(ANIL-co-TTPA) as anodic copolymer layer and PProDOT-Et₂ as cathodic polymer layer. P(MPO-co-TTPA)/PProDOT-Et₂ ECD revealed high ΔT (55.1%) and high η (766.5 cm² C⁻¹) at 580 nm. Moreover, P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs showed satisfactory long-term cycling stability and optical memory.

1. Introduction

Electrochromic materials have increasingly garnered attention due to their potential applications in low-energy consumption displays, antiglare mirrors, smart windows, and adaptive camouflages [1]. Electrochromic materials are mainly classified as inorganic metal oxides, conjugated polymers, Prussian blue, metal metallohexacyanates, metal phthalocyanines, and viologens [2]. Among these materials, conjugated polymers have been widely investigated due to their satisfactory optical contrast, high coloration efficiency, rapid electrochromic switching speed, multiple color exhibitions, and tunable optical band gap with chemical structure modifications. Recently, conjugated polymers such as polytriphenylamines [3], polycarbazoles [4], polyanilines [5,6], polybenzothiadiazoles [7], polyindoles [8], polythiophenes [9], and polybenzotriazoles [10] have been reported for particular electrochromic behaviors. Among these conjugated polymers, polytriphenylamines and polycarbazoles have hole transport properties and can be oxidized to form stable aminium radical cations, and the polymer films exhibit distinct color variations during the redox process. Reynolds et al. reported that the optical contrast of multifunctional triphenylamine polymers was up to 45% [11] and Hsiao et al. reported that electroactive polyamides and polyimides containing trityl-triphenylamine units revealed good thermal stability and a green-blue or blue coloring variation upon applying potentials, with the ∆T between neutral and oxidized states of polymer film up to 84% at 828 nm [12]. Polythiophenes and their derivatives have garnered attention for use in organic electrochromic materials due to their special optical and electrochemical characteristics such as low band gap and reversibility of the redox processes. Moreover, each repeat unit of poly(2,5-dithienylpyrrole)s (PSNS) incorporated a pyrrole ring between two thiophene units, which gave rise to decreases in the onset potential of polymer films. The incorporation of alkyl and alkoxy groups to the nitrogen atom of central pyrrole unit in PSNS backbone gave rise to good solubility of PSNS in general solvents and tunable band gap of PSNS [13]. Koyuncu et al. reported a new low band gap electrochromic polymer (poly(SNSC-BSe)) consisting of 2,5-dithienylpyrrole, carbazole, and 2,1,3-benzoselenadiazole units; the poly(SNSC-BSe) film showed a high contrast ratio (51%) in the near-infrared region, a high coloration efficiency (274 cm² C⁻¹), and retained 94.6% of its electrochemical activity after 1000 cycles [14]. Soganci et al. synthesized two amide-substituted poly(dithienylpyrrole)s (P(PBA) and P(PBA-co-EDOT)) and reported their multichromic behaviors: P(PBA-co-EDOT) displayed low band gap (1.67 eV), high optical contrast (77% at 1000 nm), and high coloration efficiency (697.01 cm² C⁻¹) [15]. Furthermore, copolymers supply effectual ways for modulating the electrochromic properties of conjugated polymers, thereby combining or changing the electrochromic behaviors of individual homopolymers. Camurlu et al. [16] reported the optoelectronic performances of poly(2,5-dithienylpyrrole)s with fluorophore units, which were namely PSNS-Carb and PSNS-Flo. PSNS-Carb showed yellow to blue upon doping and displayed an optical contrast of 27.78%. Two copolymers (P(SNS-Carb-co-EDOT) and P(SNS-Flo-co-EDOT)) were also synthesized electrochemically: P(SNS-Carb-co-EDOT) displayed a wide range of colors upon oxidation (ruby, orange, yellow, green, blue, and gray), whereas P(SNS-Flo-co-EDOT)) showed several colors from boysenberry to orange, yellow, green, blue, and gray. Moreover, the optical contrast and coloration efficiency of P(SNS-Carb-co-EDOT) and P(SNS-Flo-co-EDOT) were higher than those of PSNS-Carb and PSNS-Flo, respectively.

In this study, three 2,5-dithienylpyrrole- and tris(4-(thiophen-2-yl)phenyl)amine-based copolymers (P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA)) were copolymerized electrochemically. The chemical structure of 1-(4-(methylthio)phenyl)-2,5-di(thiophen-2-yl)-pyrrole (MPS) is similar to 1-(4-methoxyphenyl)-2,5-di(thiophen-2-yl)-pyrrole (MPO) and 4-(2,5-di(thiophen-2-yl)-pyrrol-1-yl)benzonitrile (ANIL). Methylthio-phenyl, methyloxy-phenyl, and cyano-phenyl were incorporated to the central pyrrole rings of MPS, MPO, and ANIL units, respectively. The methylthio-phenyl and methyloxy-phenyl groups are electron-donating units in MPS and MPO, respectively, whereas the cyano-phenyl group is an electron-withdrawing unit in ANIL. The highest occupied molecular orbital (HOMO) level, lowest unoccupied molecular orbital (LUMO) level, and band gap of PSNS can be modulated using various electron-donating and electron-withdrawing units. Moreover, tris(4-(thiophen-2-yl)phenyl)amine (TTPA) with a triphenylamine unit as the core and three thiophene units as the arms has attracted increasing attention due to its high stability to electrochemical redox reaction and electrochromic switching [17]. The electron-donating triphenylamine unit decreases the oxidation potential of the entire TTPA. Accordingly, poly(TTPA) shows lower E_onset than that of polythiophenes. It was interesting to combine the TTPA unit and SNS derivatives in copolymer backbones and investigate the electrochromic characteristics of copolymer films. Moreover, the electrochromic devices (ECDs) were fabricated using P(MPS-co-TTPA), P(MPO-co-TTPA), or P(ANIL-co-TTPA) as the anodic coloring electrode, PProDOT-Et₂ as the complementary counter electrode, and an ionic liquid/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) composite film as the electrochromic electrolyte. The percent transmittance variations, coloration efficiency, long-term cycling stability, and open-circuit memory of ECDs were comprehensively explored.

2. Materials and Methods

2.1. Materials and Electrochemical Preparation of Polymer Electrodes

Dithienylpyrrole derivatives, TTPA, ProDOT-Et₂, and 1-ethyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide ([EPI⁺][TFSI^–]) were synthesized based on previously published procedures [18,19,20,21]. P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were electrosynthesized potentiostatically at 1.0 V on ITO surfaces with a charge density of 30 mC cm⁻². The reference electrode used in the preparation of the polymers is an Ag/AgCl electrode. The active area of copolymer films on ITO surfaces was 1.8 cm². P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were electrosynthesized using feed molar ratios of MPS/TTPA, MPO/TTPA, and ANIL/TTPA, respectively, at 1/1. PProDOT-Et₂ film was coated onto ITO surfaces at +1.4 V.

2.2. Fabrication of ECDs

Electrochromic electrolyte was prepared using a mixture containing ionic liquid ([EPI⁺][TFSI^–]), poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP), propylene carbonate, and dimethylformamide organic solvent [22]. Dual-type ECDs were framed using P(MPS-co-TTPA), P(MPO-co-TTPA), or P(ANIL-co-TTPA) film as the anodic electrode material and PProDOT-Et₂ as the cathodic electrode material. The anodic and cathodic polymer films were positioned to face each other and were separated by an electrochromic electrolyte, as shown in Figure 1.

2.3. Spectroelectrochemical Characterizations of Copolymer Films and ECDs

Spectroelectrochemical properties of copolymer films and ECDs were measured using a CHI660a electrochemical analyzer (CH Instruments, Austin, TX, USA) and a V-630 JASCO UV-Visible spectrophotometer (JASCO International Co., Ltd., Tokyo, Japan). Copolymer films were characterized in a three-component system, copolymer films coated on ITO glass plate were used as working electrodes, a platinum wire and an Ag/AgCl electrode were used as counter and reference electrodes, respectively.

3. Results and Discussion

3.1. Preparation of Copolymer Films

The CV curves of neat TTPA, MPS, MPO, and ANIL monomers, and the mixtures of two monomers (TTPA + MPS, TTPA + MPO, and TTPA + ANIL) in 0.1 M LiClO₄/ACN solution are presented in Figure 2. The homopolymer and copolymer films can be clearly seen with continuously increasing cycles, indicating the formation of homopolymer and copolymer films on the ITO glass surface during the electrocoating process [23].

As presented in Figure 2, the E_onset of TTPA, MPS, MPO, ANIL, TTPA + MPS, TTPA + MPO, and TTPA + ANIL are 0.79, 0.70, 0.69, 0.81, 0.72, 0.70, and 0.78 V, respectively. The onset potential disparities of TTPA vs. MPS, TTPA vs. MPO, and TTPA vs. ANIL are less than 0.1 V, indicating the feasibility of copolymerization using TTPA and dithienylpyrrole monomers [24]. The onset oxidation potential of P(ANIL-co-TTPA) is greater than those of P(MPS-co-TTPA) and P(MPO-co-TTPA); this can be attributed to an electron-donating methylthio substituent in a MPS unit and an electron-donating methyloxy substituent in a MPO unit decreasing the onset oxidation potential with respect to that of a cyano-containing ANIL unit. The oxidation peaks of PTTPA, PMPS, PMPO, PANIL, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were 1.16, 0.95, 0.9, 1.0, 1.12, 1.21, and 1.19 V, respectively, whereas the reduction peaks of PTTPA, PMPS, PMPO, PANIL, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were 0.66, 0.5, 0.55, 0.6, 0.34, 0.21, and 0.22 V, respectively, as shown in

Table 1. The onset potentials, oxidation potentials, and reduction potentials values of copolymer films.

Polymer Films	Onset Potentials/V	Oxidation Potentials/V	Reduction Potentials/V
P(MPS-co-TTPA)	0.72	1.12	0.34
P(MPO-co-TTPA)	0.70	1.21	0.21
P(ANIL-co-TTPA)	0.78	1.19	0.22

. The wave shapes and redox peaks of cyclic voltammograms (CVs) observed for P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films are different to those of PTTPA, PMPS, PMPO, and PANIL films, proving the formation of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films. The electrosynthetic schemes of P(MPS-co-TTPA) are presented in Figure 3.

The as-prepared P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were scanned at 25, 50, 100, 150, 200, and 250 mV s⁻¹ in 0.1 M LiClO₄/ACN solution. As shown in Figure 4a–c, CV curves of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films displayed distinct oxidation and reduction peaks; the anodic and cathodic peak current densities increased with increasing scan rates, implying that P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films adhered well to the ITO glass electrode. Moreover, Figure 4d–f shows linear relationships between peak current densities and scan rates, illustrating that the reduction-oxidation processes of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were not restricted by diffusion control [25].

3.2. Spectroelectrochemical Studies of Copolymer Films

The spectroelectrochemical investigations of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films coated on ITO surfaces were carried out in [EPI⁺][TFSI^–] solution. As displayed in Figure 5, the absorption peaks of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were found at 417, 369, and 385 nm, respectively, in their neutral state, which indicated the π–π* transition of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) in [EPI⁺][TFSI^–] solution. Upon applying potentials more than 1.2 V, the peaks of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films at 417, 369, and 385 nm, respectively, decreased little by little and their charge carrier absorption bands appeared at 964, 914, and 960 nm, respectively, implying the formation of polaron and bipolaron of copolymer films [26]. The band gap energy values (E_g) of PMPS, PMPO, PANIL, PTTPA, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) can be calculated according to the following Planck equation [27,28],

E_g = 1241/λ_onset

(1)

where λ_onset is the wavelength at which the onset of absorption occurs. The E_g of PMPS, PMPO, PANIL, PTTPA, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) were 2.25, 2.17, 2.21, 2.31, 2.27, 2.22, and 2.28 eV, respectively. P(MPO-co-TTPA) showed a narrower band gap than those of P(MPS-co-TTPA) and P(ANIL-co-TTPA); this can be attributed to PMPO showing a narrower band gap than those of PMPS and PANIL.

Table 2. Electrochromic behaviors of copolymer films at various voltages.

Polymer Films	0 V	0.6 V	1.2 V	1.6 V
P(MPS-co-TTPA)
P(MPO-co-TTPA)
P(ANIL-co-TTPA)

and Table S1 (in Supplementary Information) show the electrochromic photographs and colorimetric values, respectively, of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films at various potentials in solutions. P(MPS-co-TTPA) film was light olive green (0 V) in the neutral state, greyish-green (+0.6 V) in the intermediate state, bluish grey (+1.2 V) in oxidized state, and grey (+1.6 V) in highly oxidized state. P(MPO-co-TTPA) film was green moss at 0 V, foliage green at +0.6 V, dark greyish-green at +1.2 V, and bluish-grey at +1.6 V. However, P(ANIL-co-TTPA) film showed less color variations than those of P(MPS-co-TTPA) and P(MPO-co-TTPA) films; P(ANIL-co-TTPA) film was lawn green at 0, +0.6 V, and +1.2 V, and celandine green at +1.6 V—this may be attributed to P(ANIL-co-TTPA) film revealing higher onset potential of oxidation than those of (MPS-co-TTPA) and P(MPO-co-TTPA) films and an electron-withdrawing cyano unit deactivating the π–π* transitions of benzene ring on the pyrrole ring of P(ANIL-co-TTPA). The Commission Internationale de I'Eclairage (CIE) chromaticity diagrams of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films at various potentials are shown in Figure 6: the (x,y) values of P(MPO-co-TTPA) film showed significant variations before 0.6 V, whereas the (x,y) values of P(MPS-co-TTPA) and P(ANIL-co-TTPA) films showed significant variations after 0.6 V.

The switching kinetics of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were measured using a square-wave potential step technology [29]. P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were stepped by repeated potential between −0.2 and +0.9 V in an ionic liquid solution, with the time interval of 5 s. Figure 7 shows the time-dependent changes in the transmittance profiles of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films in solution state, and the transmittance changes (∆T) measured at 1st, 50th, and 100th cycles are listed in

Table 3. Color–bleach switching properties of copolymer films in solution state.

Polymer Films in [EPI+][TFSI–]	λmax/nm	Cycle No.	∆T/%	τc/s	τb/s
				T90%	T90%
P(MPS-co-TTPA)	570	1	39.47	2.05	1.85
		50	40.82	2.16	1.88
		100	39.35	2.21	2.02
	964	1	67.04	2.06	1.90
		50	66.68	2.16	1.82
		100	67.18	2.21	1.84
P(MPO-co-TTPA)	592	1	25.74	2.02	1.90
		50	26.09	2.04	1.95
		100	25.57	2.12	2.01
	914	1	60.69	1.99	1.93
		50	58.06	2.01	1.96
		100	56.23	1.97	1.91
P(ANIL-co-TTPA)	548	1	32.43	2.19	1.87
		50	31.45	2.15	1.97
		100	32.37	2.14	1.82
	960	1	66.38	2.17	1.96
		50	66.27	2.18	1.87
		100	67.08	2.11	1.85

. The ∆T_max of P(MPS-co-TTPA) film at 964 nm, P(MPO-co-TTPA) film at 914 nm, and P(ANIL-co-TTPA) film at 960 nm were 67.0%, 60.7%, and 66.4%, respectively, at the first cycle. Among these copolymer films, P(MPS-co-TTPA) film at 964 nm shows the highest ∆T. The coloring respond time (τ_c) and the bleaching respond time (τ_b) of copolymer films measured at 1st, 50th, and 100th cycles are also presented in Table 3. The τ_c and τ_b were determined at 90% of the full-transmittance changes and were in the range of 1.82–2.21 s for P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films.

ΔOD can be determined using the following equation [30]:

$$\Delta \mathrm{OD}=\log (\frac{T_{\mathrm{ox}}}{T_{\mathrm{neu}}})$$

(2)

where T_ox and T_neu are the transmittance of the oxidation state and neutral state, respectively. The ΔOD of P(MPS-co-TTPA) film at 964 nm, P(MPO-co-TTPA) film at 914 nm, and P(ANIL-co-TTPA) film at 960 nm in solution states are 66.5%, 69.7%, and 55.9%, respectively.

The coloration efficiency (η) of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films can be calculated using the following equation [31]:

$$\eta =\frac{\Delta \mathrm{OD}}{q/A}$$

(3)

where q and A are the consumed charge and electrode area, respectively. The η of P(MPS-co-TTPA) film at 964 nm, P(MPO-co-TTPA) film at 914 nm, and P(ANIL-co-TTPA) film at 960 nm were 260.3, 176.6, and 230.8 cm² C⁻¹, respectively.

Table 4. Comparisons of transmittance changes and coloration efficiencies of polymer films.

Polymer Films	Electrolyte	λ/nm	ΔTmax/%	∆ODmax/%	ηmax/cm2 C−1	Ref.
P(TPVB-co-EDOT)	0.1 M TBP6/DCM	555	44	–	–	[32]
P(TPVB-co-EDOT)	0.1 M TBP6/DCM	1000	75	–	–	[32]
P(SNS-PN-co-EDOT)	0.1 M NaClO4/LiClO4/ACN	480	8	–	–	[33]
P(SNS-PN-co-ProDOT)	0.1 M LiClO4/ACN	850	42	–	256.0	[34]
P(MPS-co-TTPA)	[EPI+][TFSI–]	964	67.2	66.5	260.3	This work
P(MPO-co-TTPA)	[EPI+][TFSI–]	914	60.7	69.7	176.6	This work
P(ANIL-co-TTPA)	[EPI+][TFSI–]	960	67.1	55.9	230.8	This work

summarizes the comparisons of ∆T and η for several polymer films. The ∆T of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films in solution state were higher than those reported for P(TPVB-co-EDOT) film [32] at 555 nm, P(SNS-PN-co-EDOT) film [33] at 480 nm, and P(SNS-PN-co-ProDOT) film [34] at 850 nm. In another aspect, the η of P(MPS-co-TTPA) film in solution state was higher than those reported for P(SNS-PN-co-ProDOT) film [34] at 850 nm.

3.3. Spectroelectrochemistry of Dual Type ECDs

Figure 8 shows the spectroelectrochemical spectra of ECDs at various potentials. P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs displayed absorption peaks at ca. 430 nm at −0.4 V, which is in keeping with the spectra of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films at −0.4 V. In the aforementioned circumstances, the cathodic PProDOT-Et₂ layer is in oxidized state and it does not reveal conspicuous absorption peak in UV-Vis spectra. Upon increasing the voltage progressively, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films start to oxidize and the PProDOT-Et₂ film starts to reduce, new absorption bands at ca. 580 nm emerge, and the ECDs are blue at +1.4 V, as shown in

Table 5. Electrochromic behaviors of ECDs at various voltages.

ECDs	0 V	0.6 V	1.0 V	1.4 V
P(MPS-co-TTPA)/PProDOT-Et2
P(MPO-co-TTPA)/PProDOT-Et2
P(ANIL-co-TTPA)/PProDOT-Et2

. The colorimetric values of ECDs at various potentials are summarized in Table S2 (in Supplementary Information), and the CIE chromaticity diagrams of P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs at various potentials are shown in Figure 9.

The time-dependent changes in the transmittance profiles of P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs are presented in Figure 10, which were switched square wave potentials between −0.4 and +1.4 V with a time interval of 5 s, and the ΔT and switching time estimated at various cycles are listed in

Table 6. Color–bleach switching properties of ECDs.

ECDs	λmax/nm	Cycle No.	ΔT/%	τc/s	τb/s
				T95%	T95%
P(MPS-co-TTPA)/PProDOT-Et2	576	1	49.21	1.01	0.93
		50	49.53	1.01	0.96
		100	48.89	1.14	0.97
P(MPO-co-TTPA)/PProDOT-Et2	580	1	55.12	0.98	0.99
		50	54.12	1.01	0.99
		100	53.60	1.05	1.13
P(ANIL-co-TTPA)/PProDOT-Et2	582	1	46.34	1.17	1.34
		50	45.76	0.98	1.25
		100	44.91	1.17	1.32

. The transmittance changes of P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs are 49.21% at 576 nm, 55.12% at 580 nm, and 46.34% at 582 nm at the first cycle, respectively, indicating that P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films are promising electrochromic materials to increase the ΔT when we use P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films as electrode materials in ECDs. For the τ_c and τ_b of ECDs, the switching time of ECDs were shorter than those of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films in solution state, implying that the ECDs changed color faster at various potentials than the copolymer films in solution state.

Table 7. The transmittance changes and coloration efficiency of ECDs.

ECDs	λ/nm	∆Tmax/%	∆ODmax/%	η/cm2 C–1	Ref.
P(NTP-co-EDOT)/PEDOT	650	23	–	–	[35]
P(SNS-An-Fc-co-EDOT)/PEDOT	601	22	–	484	[36]
P(SNBS-co-EDOT)/PEDOT	485	15	–	–	[37]
P(PTP-co-EDOT)/PEDOT	545	15	–	–	[33]
P(Cz4-co-CIn1)/PProDOT-Me2	575	32	24.6	372.7	[38]
P(MPS-co-TTPA)/PProDOT-Et2	576	49.53	60.94	691.2	This work
P(MPO-co-TTPA)/PProDOT-Et2	580	55.12	67.58	766.5	This work
P(ANIL-co-TTPA)/PProDOT-Et2	582	46.34	45.03	625.4	This work

shows the comparisons of ΔT and η for P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs and reported dual-type ECDs. P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs show higher ΔT than those reported for P(NTP-co-EDOT)/PEDOT [35], P(SNS-An-Fc-co-EDOT)/PEDOT [36], P(SNBS-co-EDOT)/PEDOT [37], P(PTP-co-EDOT)/PEDOT [33], and P(Cz4-co-CIn1)/PProDOT-Me₂ [38] ECDs. On the other hand, P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs show higher η than those of P(SNS-An-Fc-co-EDOT)/PEDOT [36] and P(Cz4-co-CIn1)/PProDOT-Me₂ [38] ECDs.

3.4. Optical Memory Effect of ECDs

The optical memory experiment of P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs were carried out by imposing potential for 1 s for each 200 s time interval. The monitor potentials for P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs were −0.4 and 1.0 V in bleached and colored states, respectively. As shown in Figure S1 (in Supplementary Information), P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs showed less than 1.5% transmittance change in bleached state (−0.4 V) and less than 4% transmittance change in colored state (1.0 V), thus inferring that ECDs that employed P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) as anodic layers displayed satisfactory optical memory effects.

3.5. Redox Stability of ECDs

Long-term redox stabilities of P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs were estimated by CV measurements for one thousand cycles at a scan rate of 100 mV s⁻¹. As presented in Figure 11, 96%, 93%, and 95%, respectively, of electrochemical activity was maintained for P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs after 500 cycles, and 90%, 90%, and 92%, respectively, of electrochemical activity was retained after 1000 cycles, indicating the ECDs revealed satisfactory long-term redox stability after scanning for 1000 cycles.

4. Conclusions

P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films were synthesized electrochemically and characterized in [EPI⁺][TFSI^–] solution. Spectroelectrochemical investigations revealed that P(MPS-co-TTPA) film was light olive green, greyish-green, bluish grey, and grey at 0, 0.6, 1.2, and 1.6 V, respectively. P(MPS-co-TTPA) and P(MPO-co-TTPA) films showed four kinds of color variations from neutral state to highly oxidized state. Color-bleach switching characterizations of polymer films revealed that P(MPS-co-TTPA) film has high ΔT_max (67.2% at 964 nm) and high η (260.3 cm² C^–1 at 964 nm). The ∆T of P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films in an ionic liquid solution were higher than those reported for P(TPVB-co-EDOT), P(SNS-PN-co-EDOT), and P(SNS-PN-co-ProDOT) films. Dual type complementary colored ECDs that employed polymer films as anodic and cathodic layers were constructed; the switching time of ECDs were shorter than those of corresponding copolymer films in [EPI⁺][TFSI^–] solution. P(MPS-co-TTPA)/PProDOT-Et₂ ECDs revealed high ΔT (49.5%) and high η (691.2 cm² C⁻¹) at 576 nm, whereas P(MPO-co-TTPA)/PProDOT-Et₂ ECDs displayed reasonable switching time and satisfactory electrochromic memory. P(MPS-co-TTPA)/PProDOT-Et₂, P(MPO-co-TTPA)/PProDOT-Et₂, and P(ANIL-co-TTPA)/PProDOT-Et₂ ECDs showed higher ΔT than those reported for P(NTP-co-EDOT)/PEDOT, P(SNS-An-Fc-co-EDOT)/PEDOT, P(SNBS-co-EDOT)/PEDOT, P(PTP-co-EDOT)/PEDOT, and P(Cz4-co-CIn1)/PProDOT-Me₂ ECDs. In view of the above electrochromic properties, P(MPS-co-TTPA), P(MPO-co-TTPA), and P(ANIL-co-TTPA) films could be employed as the anodic layers in ECDs.