4-(Trifluoromethoxy)phenyl-Containing Polymers as Promising Anodic Materials for Electrochromic Devices

Wen-Hsin Wang 1, Jui-Cheng Chang 1,2, Pei-Ying Lee 1, Yuan-Chung Lin 3 and Tzi-Yi Wu 1,* 1 Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan; winnie53035@yahoo.com.tw (W.-H.W.); d700215@gmail.com (J.-C.C.); leepeiying1018@gmail.com (P.-Y.L.) 2 Bachelor Program in Interdisciplinary Studies, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 3 Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan; yclin@faculty.nsysu.edu.tw * Correspondence: wuty@gemail.yuntech.edu.tw; Tel.: +886-5-534-2601 (ext. 4626)


Introduction
Electrochromic materials tune their colors reversibly upon applying various potentials or undergoing a redox process. To date, organic and inorganic electrochromic materials have received much interest owing to their probable utilizations in auto-dimming mirror, smart windows of architectures, energy storage devices, etc. [1][2][3]. Compared to inorganic electrochromic materials, organic electrochromic materials have the benefits of ease of electrochemical and chemical synthesis, large coloration efficiency, rapid electrochromic switching, low onset oxidation voltage, and satisfactory long-term cycling stability [4,5].
PEDOT are the most commonly used polymeric materials in electrochemical devices [6][7][8][9][10][11]. According to these conjugated polymers, PProDOT-Et2, a derivative of PEDOT, shows prominent performances as a cathodic polymer in ECDs [12]. Polypyrrole and polycarbazole were reported as anodic polymers in ECDs owing to their good hole transporting stability, attractive optical performances, and excellent electroactive characters. Polythiophenes are widely used as electrochromic electrodes due to their good reversible redox behaviors, large conductivity, and large color contrast between reduced and oxidized states [13]. However, the oxidized potentials of nonmodified polythiophenes are 1.5 V (vs. Ag/AgCl) [14]. PSNS consists of two thiophene rings and a pyrrole ring; the strong electron-donating pyrrole unit decreases the oxidized potentials of polymers to 0.7 V vs. Ag/AgCl [15].
Moreover, a trifluoromethoxy (CF3O-) substituent is an electron-withdrawing group, the incorporation of a trifluoromethoxy substituent on the side chain of PSNS decreases the LUMO/HOMO energy levels of PSNS slightly. The electrochromic properties will vary with the change of LUMO/HOMO energy levels.
This paper reports the electrosynthesis and electrochromic properties of three polydithienylpyrrole derivatives (PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP)). For a 3,6-di(2thienyl)carbazole unit, the incorporation of thiophene rings at 3,6-locations of carbazole unit leads to a red shift of the optical UV-Vis band and diminishes the band gap [16]. Dithieno [3,2-b:2′,3′-d]pyrrole (DTP) is an electron-donating group with a coplanar fused-heterocycle configuration, which assists to increase the polymer backbone planarity and narrow the band gap between conduction band and valence band [17]. The copolymerization of different monomers gives rise to desirable combinations of electrochromic behaviors presented in homopolymers. Therefore, we also synthesized DTC-and DTP-containing copolymers. In addition, dual-type polymer ECDs were prepared using PTTPP, P(TTPP-co-DTC), or P(TTPP-co-DTP) as the anodic material, PProDOT-Et2 as the cathodic material, and an ionic liquid-containing electrolyte as the separation layer. Optical and electrochemical characterizations of PTTPP film, P(TTPP-co-DTC) film, P(TTPP-co-DTP) film, PTTPP/PProDOT-Et2 ECD, P(TTPP-co-DTC)/PProDOT-Et2 ECD, and P(TTPP-co-DTP)/PProDOT-Et2 ECD were explored systematically using UV-Vis spectra and electrochromic switching techniques. The cycling stability between the oxidized and reduced states is a crucial factor for ECDs [18,19]. Optical memory responds to the energy depletion during the long-term manipulations of ECDs [20,21]. Accordingly, the dual-type polymer ECDs were further explored for optical memory and electrochemical cycling stability.

Fabrication of ECDs
The preparations of [EPI + ][TFSI -])/PVdF-HFP composite electrolytes were represented previously [25]. The anodic layers (PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP)) were prepared potentiostatically onto glass substrates at +0.9 V, respectively, whereas the cathodic layer (PProDOT-Et2) was deposited potentiostatically onto glass substrate at +1.4 V. The electrodes were isolated by the composite electrolyte. The electrode area of as-prepared ECDs was 1.8 cm 2 . Figure 2 displays the synthetic schemes of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) and Figure 3 shows the potentiodynamic measurements of neat TTPP, DTC, and DTP and their mixtures (TTPP+DTC and TTPP+DTP) in a solution. As the potentiodynamic measurements scanned continued, the anodic and cathodic peaks in Figure 3 increased stage by stage, implying PTTPP, PDTC, PDTP, P(TTPP-co-DTC), and P(TTPP-co-DTP) films were electrodeposited onto the ITO substrates. The onset oxidized potentials (Eonset) of TTPP, DTC, and DTP were 0.68, 0.82, and 0.80 V, respectively. The Eonset disparities of TTPP vs. DTC and TTPP vs. DTP were smaller than 0.15 V, inferring the copolymerization of TTPP vs. DTC and TTPP vs. DTP was workable [26]. Furthermore, the Eonset of TTPP was smaller than those of DTP and DTC, implying that a 4-(trifluoromethoxy)phenyl-based dithienylpyrrole was more susceptible to oxidation than either the DTP or DTC. The oxidized peaks of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP), as shown in Figure 3, locate at 0.77, 1.55, and 0.96 V, respectively, whereas the reduced peaks of PTTPP, P(TTPPco-DTC), and P(TTPP-co-DTP) situate at 0.17, 0.15, and 0.34 V, respectively.    Figure 4d-f displays their corresponding charts of peak current and sweep velocity. The peak current densities raise linearly with raising scan rate, inferring PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are well clung onto ITO surface and the redox reactions are non-diffusion limited processes [27].  Figure 4a-c displays the cyclic voltammograms of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) electrodes, respectively, at scan rates between 25 and 200 mV s −1 , while Figure 4d-f displays their corresponding charts of peak current and sweep velocity. The peak current densities raise linearly with raising scan rate, inferring PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are well clung onto ITO surface and the redox reactions are non-diffusion limited processes [27].

Electrochromic Characterizations of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP)
As shown in Figure 5, the UV-Vis band of PTTPP film in the reduction state situated at 364 nm. The absorption band of P(TTPP-co-DTC) and P(TTPP-co-DTP) in the reduction state shifted bathochromically relative to PTTPP film. It is worth noting that the absorption band red shift value of P(TTPP-co-DTP) film is larger than that of P(TTPP-co-DTC) film, which can be ascribed to the fused-heterocycle of DTP unit being able to increase the planarity of repeating units in P(TTPP-co-DTP) backbones and diminish polymeric bandgap. As the application of potential increases gradually, the π-π* transition absorbance peak of PTTPP film begins to diminish, and bipolaron and polaron bands generate at ca. 532 and 1050 nm. Under similar condition, the bipolaron and polaron bands generate at 916 nm for P(TTPP-co-DTC) film and 1302 nm for P(TTPP-co-DTP) film. Table 1 displays the electrochromic photoimages of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) in a solution at different redox potentials; the anodic polymer films display reversible electrochromic phenomena in their neutral and oxidized states. The PTTPP film was grayish-yellow at 0 V, grayish-blue at 1.0 V, and bluish-violet at 1.4 V. Under similar circumstances, P(TTPP-co-DTC) displays obvious color transition with three colors (grayish-green at 0 V, grayish-blue at 1.0 V, and bluish-purple at 1.2 V), while P(TTPP-co-DTP) film shows three kinds of colors from reduced to oxidized states (gray at 0 V, grayish-blue at 0.6 V, and blue at 1.2 V). The colorimetric values of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) estimated at different voltages are summarized in Table 1, and the chromaticity diagrams of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) in reduced and oxidized state are displayed in Figure 6. As shown in Figure 5, the UV-Vis band of PTTPP film in the reduction state situated at 364 nm. The absorption band of P(TTPP-co-DTC) and P(TTPP-co-DTP) in the reduction state shifted bathochromically relative to PTTPP film. It is worth noting that the absorption band red shift value of P(TTPP-co-DTP) film is larger than that of P(TTPP-co-DTC) film, which can be ascribed to the fused-heterocycle of DTP unit being able to increase the planarity of repeating units in P(TTPP-co-DTP) backbones and diminish polymeric bandgap. As the application of potential increases gradually, the π-π* transition absorbance peak of PTTPP film begins to diminish, and bipolaron and polaron bands generate at ca. 532 and 1050 nm. Under similar condition, the bipolaron and polaron bands generate at 916 nm for P(TTPP-co-DTC) film and 1302 nm for P(TTPP-co-DTP) film. Table 1 displays the electrochromic photoimages of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) in a solution at different redox potentials; the anodic polymer films display reversible electrochromic phenomena in their neutral and oxidized states. The PTTPP film was grayish-yellow at 0 V, grayishblue at 1.0 V, and bluish-violet at 1.4 V. Under similar circumstances, P(TTPP-co-DTC) displays obvious color transition with three colors (grayish-green at 0 V, grayish-blue at 1.0 V, and bluishpurple at 1.2 V), while P(TTPP-co-DTP) film shows three kinds of colors from reduced to oxidized states (gray at 0 V, grayish-blue at 0.6 V, and blue at 1.2 V). The colorimetric values of PTTPP, P(TTPPco-DTC), and P(TTPP-co-DTP) estimated at different voltages are summarized in Table 1, and the chromaticity diagrams of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) in reduced and oxidized state are displayed in Figure 6.  The band gap (Eg) of PTTPP film can be determined from the λonset of UV spectra by Planck equation (Eg = 1241/λonset) [28], and it is 2.34 eV ( Table 2). The HOMO/LUMO energy levels of PTTPP film were determined using the following equations [29]: where Eonset is corrected using ferrocene as internal standard. The EHOMO and ELUMO of PTTPP film are −4.91 and −2.52 eV, respectively. The band gap (E g ) of PTTPP film can be determined from the λ onset of UV spectra by Planck equation (E g = 1241/λ onset ) [28], and it is 2.34 eV ( Table 2). The HOMO/LUMO energy levels of PTTPP film were determined using the following equations [29]: where E onset is corrected using ferrocene as internal standard. The E HOMO and E LUMO of PTTPP film are −4.91 and −2.52 eV, respectively.  Double step chronoamperometry technique was used to estimate switching performances of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) [30]. Figure 7 exhibits the time-transmittance profiles of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) with an interval of 5 s between redox states. The coloration/bleaching time (τc and τb) of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are summarized in Table 3. The τc and τb are estimated at 90% of whole transmittance variation (T90%). The τc and τb of three polymer films at visible light and near infrared light regions were found to be The optical contrast ΔT (%) is a crucial parameter for electrochromic materials and devices [31]. The ΔTmax of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are 24.5% at 1050 nm, 49.0% at 916 nm, and 53.8% at 1302 nm, respectively, in [EPI + ][TFSI -] solution, indicating copolymers show higher ΔT than that of PTTPP homopolymer and the introduction of DTP group in the polymeric backbone leads to higher ΔTmax than that of DTC unit. For the three polymer films, P(TTPP-co-DTP) shows the highest ΔTmax (53.8%) at 1302 nm. As listed in Table 2, the ΔTmax of P(TTPP-co-DTP) film was larger than those reported for poly(1-co-EDOT) (ΔTmax = 32.9% at 500 nm) [32], PBCB (ΔTmax = 44% at 1000 nm) [33], and PBCP (ΔTmax = 39% at 1000 nm) [33].  Double step chronoamperometry technique was used to estimate switching performances of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) [30]. Figure 7 exhibits the time-transmittance profiles of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) with an interval of 5 s between redox states. The coloration/bleaching time (τc and τb) of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are summarized in Table 3. The τc and τb are estimated at 90% of whole transmittance variation (T90%). The τc and τb of three polymer films at visible light and near infrared light regions were found to be The optical contrast ΔT (%) is a crucial parameter for electrochromic materials and devices [31]. The ΔTmax of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are 24.5% at 1050 nm, 49.0% at 916 nm, and 53.8% at 1302 nm, respectively, in [EPI + ][TFSI -] solution, indicating copolymers show higher ΔT than that of PTTPP homopolymer and the introduction of DTP group in the polymeric backbone leads to higher ΔTmax than that of DTC unit. For the three polymer films, P(TTPP-co-DTP) shows the highest ΔTmax (53.8%) at 1302 nm. As listed in Table 2, the ΔTmax of P(TTPP-co-DTP) film was larger than those reported for poly(1-co-EDOT) (ΔTmax = 32.9% at 500 nm) [32], PBCB (ΔTmax = 44% at 1000 nm) [33], and PBCP (ΔTmax = 39% at 1000 nm) [33].  Double step chronoamperometry technique was used to estimate switching performances of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) [30]. Figure 7 exhibits the time-transmittance profiles of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) with an interval of 5 s between redox states. The coloration/bleaching time (τc and τb) of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are summarized in Table 3. The τc and τb are estimated at 90% of whole transmittance variation (T90%). The τc and τb of three polymer films at visible light and near infrared light regions were found to be The optical contrast ΔT (%) is a crucial parameter for electrochromic materials and devices [31]. The ΔTmax of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are 24.5% at 1050 nm, 49.0% at 916 nm, and 53.8% at 1302 nm, respectively, in [EPI + ][TFSI -] solution, indicating copolymers show higher ΔT than that of PTTPP homopolymer and the introduction of DTP group in the polymeric backbone leads to higher ΔTmax than that of DTC unit. For the three polymer films, P(TTPP-co-DTP) shows the highest ΔTmax (53.8%) at 1302 nm. As listed in Table 2, the ΔTmax of P(TTPP-co-DTP) film was larger than those reported for poly(1-co-EDOT) (ΔTmax = 32.9% at 500 nm) [32], PBCB (ΔTmax = 44% at 1000 nm) [33], and PBCP (ΔTmax = 39% at 1000 nm) [33].  Double step chronoamperometry technique was used to estimate switching performances of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) [30]. Figure 7 exhibits the time-transmittance profiles of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) with an interval of 5 s between redox states. The coloration/bleaching time (τc and τb) of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are summarized in Table 3. The τc and τb are estimated at 90% of whole transmittance variation (T90%). The τc and τb of three polymer films at visible light and near infrared light regions were found to be The optical contrast ΔT (%) is a crucial parameter for electrochromic materials and devices [31]. The ΔTmax of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are 24.5% at 1050 nm, 49.0% at 916 nm, and 53.8% at 1302 nm, respectively, in [EPI + ][TFSI -] solution, indicating copolymers show higher ΔT than that of PTTPP homopolymer and the introduction of DTP group in the polymeric backbone leads to higher ΔTmax than that of DTC unit. For the three polymer films, P(TTPP-co-DTP) shows the highest ΔTmax (53.8%) at 1302 nm. As listed in Table 2, the ΔTmax of P(TTPP-co-DTP) film was larger than those reported for poly(1-co-EDOT) (ΔTmax = 32.9% at 500 nm) [32], PBCB (ΔTmax = 44% at 1000 nm) [33], and PBCP (ΔTmax = 39% at 1000 nm) [33].  Double step chronoamperometry technique was used to estimate switching performances of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) [30]. Figure 7 exhibits the time-transmittance profiles of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) with an interval of 5 s between redox states. The coloration/bleaching time (τc and τb) of PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) are summarized in Table 3. The τc and τb are estimated at 90% of whole transmittance variation (T90%). The τc and τb of three polymer films at visible light and near infrared light regions were found to be 1.

Conclusions
TTPP was synthesized and its corresponding homopolymers (PTTPP) and copolymers (P(TTPPco-DTC) and P(TTPP-co-DTP)) were prepared. The anodic polymer films display reversible electrochromic phenomena in their neutral and oxidized states. The PTTPP film was grayish-yellow at 0 V, grayish-blue at 1.0 V, and bluish-violet at 1.4 V, whereas P(TTPP-co-DTP) film showed color changes from reduced to oxidized states (gray at 0 V, grayish-blue at 0.6 V, and blue at 1.2 V). Colorless-to-colorful switching investigations of anodic films show that P(TTPP-co-DTP) film has high ΔTmax (53.8% at 1302 nm) and PTTPP film has high η (379.64 cm 2 /C at 1050 nm). P(TTPP-co-DTP)/PProDOT-Et2 ECD exhibits high ΔTmax (48.1% at 592 nm) and a sufficient cycling stability, whereas PTTPP/PProDOT-Et2 ECD displays high η (890.96 cm 2 /C at 588 nm) and adequate optical memory at coloring and bleaching states, implying PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) films are promising candidates as anodic electrochromic materials for potential applications in motorcycle helmet-visors, electrochromic goggles, electrochromic display devices, and autodimming car mirror.

Conclusions
TTPP was synthesized and its corresponding homopolymers (PTTPP) and copolymers (P(TTPP-co-DTC) and P(TTPP-co-DTP)) were prepared. The anodic polymer films display reversible electrochromic phenomena in their neutral and oxidized states. The PTTPP film was grayish-yellow at 0 V, grayish-blue at 1.0 V, and bluish-violet at 1.4 V, whereas P(TTPP-co-DTP) film showed color changes from reduced to oxidized states (gray at 0 V, grayish-blue at 0.6 V, and blue at 1.2 V). Colorless-to-colorful switching investigations of anodic films show that P(TTPP-co-DTP) film has high ∆T max (53.8% at 1302 nm) and PTTPP film has high η (379.64 cm 2 /C at 1050 nm). P(TTPP-co-DTP)/PProDOT-Et 2 ECD exhibits high ∆T max (48.1% at 592 nm) and a sufficient cycling stability, whereas PTTPP/PProDOT-Et 2 ECD displays high η (890.96 cm 2 /C at 588 nm) and adequate optical memory at coloring and bleaching states, implying PTTPP, P(TTPP-co-DTC), and P(TTPP-co-DTP) films are promising candidates as anodic electrochromic materials for potential applications in motorcycle helmet-visors, electrochromic goggles, electrochromic display devices, and auto-dimming car mirror.