Electrosynthesis of Copolymers Based on 1,3,5-Tris(N-Carbazolyl)Benzene and 2,2′-Bithiophene and Their Applications in Electrochromic Devices

Poly(1,3,5-tris(N-carbazolyl)benzene) (PtnCz) and three copolymers based on 1,3,5-tris(N-carbazolyl)benzene (tnCz) and 2,2′-bithiophene (bTp) were electrochemically synthesized. The anodic P(tnCz1-bTp2) film with a tnCz/bTp feed molar ratio of 1/2 showed four colors (light orange at 0.0 V, yellowish-orange at 0.7 V, yellowish-green at 0.8 V, and blue at 1.1 V) from the neutral state to oxidized states. The optical contrast (∆T%) and coloration efficiency (η) of the P(tnCz1-bTp2) film were measured as 48% and 112 cm2∙C−1, respectively, at 696 nm. Electrochromic devices (ECDs) based on PtnCz, P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films as anodic polymer layers and poly(3,4-dihydro-3,3-dimethyl-2H-thieno[3,4-b-1,4]dioxepin) (PProDOT-Me2) as cathodic polymer layers were assembled. P(tnCz1-bTp2)/PProDOT-Me2 ECD showed three various colors (saffron yellow, yellowish-blue, and dark blue) at potentials ranging from −0.3 to 1.5 V. In addition, P(tnCz1-bTp2)/PProDOT-Me2 ECD showed a high ∆T% value (40% at 630 nm) and a high coloration efficiency (519 cm2∙C−1 at 630 nm).


Spectroelectrochemical and Electrochemical Characterization
The electrochemical experiments were characterized using a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). A platinum wire, an Indium Tin Oxide (ITO) glass (1.5 cm 2 ), and an Ag/AgCl electrode were employed as the counter electrode, working electrode, and reference electrode, respectively. The spectroelectrochemical measurements were carried out using an Agilent Cary 60 UV-Visible spectrophotometer (Varian Inc., Walnut Creek, CA, USA) to monitor the UV-Visible spectra. Double potential chronoamperometry was performed with the three-electrode cell using an Agilent Cary 60 UV-Visible spectrophotometer and a CHI627D electrochemical analyzer.

Spectroelectrochemical and Electrochemical Characterization
The electrochemical experiments were characterized using a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA). A platinum wire, an Indium Tin Oxide (ITO) glass (1.5 cm 2 ), and an Ag/AgCl electrode were employed as the counter electrode, working electrode, and reference electrode, respectively. The spectroelectrochemical measurements were carried out using an Agilent Cary 60 UV-Visible spectrophotometer (Varian Inc., Walnut Creek, CA, USA) to monitor the UV-Visible spectra. Double potential chronoamperometry was performed with the three-electrode cell using an Agilent Cary 60 UV-Visible spectrophotometer and a CHI627D electrochemical analyzer.  The Eonset of PtnCz was comparable to that of PbTp, implying that PtnCz shows a similar electron-donating ability to that of PbTp. However, P(tnCz1-bTp1), P(tnCz1-bTp2) and P(tnCz1-bTp4) showed lower Eonset values than those of PtnCz and PbTp, indicating that the Eonset of The E onset of PtnCz was comparable to that of PbTp, implying that PtnCz shows a similar electron-donating ability to that of PbTp. However, P(tnCz1-bTp1), P(tnCz1-bTp2) and P(tnCz1-bTp4) showed lower E onset values than those of PtnCz and PbTp, indicating that the E onset of copolymers is lower than those of homopolymers. The onset potential of neat tnCz was close to that of neat bTp, demonstrating that the copolymerizations of tnCz and bTp are practicable. Figure 2a-c shows the electrosynthesis of neat tnCz, mixture (tnCz + bTp), and neat bTp in 0.2 M LiClO 4 /(ACN/DCM (1:2, by volume)) solution, respectively. The current density of cyclic voltammetry (CV) curves in Figure 2a-c increases with an increasing number of scanning cycles, implying the growth of PtnCz, P(tnCz1-bTp2), and PbTp on the ITO electrode [21]. As displayed in Figure 2a, the oxidation and reduction peaks of PtnCz, located at 1.31 and 0.98 V, respectively, are smaller than those of PbTp (the oxidation and reduction peaks of PbTp are situated at 1.68 and 1.04 V, respectively). The redox peaks of the P(tnCz1-bTp2) film shifted to lower potentials than those of PtnCz and PbTp, indicating that the copolymer gives rise to lower redox peaks than those of homopolymers. Moreover, the locations of the oxidation and reduction peaks and the waveshapes of the CV curves of the P(tnCz1-bTp2) film are different to those of the PtnCz and PbTp films, demonstrating the formation of the P(tnCz1-bTp2) film.

Electrochemical Polymerizations
Polymers 2017, 9,518 4 of 16 copolymers is lower than those of homopolymers. The onset potential of neat tnCz was close to that of neat bTp, demonstrating that the copolymerizations of tnCz and bTp are practicable. Figure 2a-c shows the electrosynthesis of neat tnCz, mixture (tnCz + bTp), and neat bTp in 0.2 M LiClO4/(ACN/DCM (1:2, by volume)) solution, respectively. The current density of cyclic voltammetry (CV) curves in Figure 2a-c increases with an increasing number of scanning cycles, implying the growth of PtnCz, P(tnCz1-bTp2), and PbTp on the ITO electrode [21]. As displayed in Figure 2a, the oxidation and reduction peaks of PtnCz, located at 1.31 and 0.98 V, respectively, are smaller than those of PbTp (the oxidation and reduction peaks of PbTp are situated at 1.68 and 1.04 V, respectively). The redox peaks of the P(tnCz1-bTp2) film shifted to lower potentials than those of PtnCz and PbTp, indicating that the copolymer gives rise to lower redox peaks than those of homopolymers. Moreover, the locations of the oxidation and reduction peaks and the waveshapes of the CV curves of the P(tnCz1-bTp2) film are different to those of the PtnCz and PbTp films, demonstrating the formation of the P(tnCz1-bTp2) film. The electrochemical polymerization routes of PtnCz and P(tnCz1-bTp2) are shown in Figure 3. Moreover, the homopolymers (PtnCz and PbTp) and copolymers (P(tnCz1-bTp1), P(tnCz1-bTp2), and P(tnCz1-bTp4)) were further studied using Fourier transform infrared (FT-IR). As shown in Figure 4, the C-S-C characteristic peaks of the P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films, located at 730, 735, 760, and 783 cm −1 , and the C-S-C characteristic peaks of P(tnCz1-bTp1), P(tnCz1-bTp2), and P(tnCz1-bTp4) are different to those of PbTp, indicating that copolymerization occurs during the electropolymerization of PtnCz and PbTp. The electrochemical polymerization routes of PtnCz and P(tnCz1-bTp2) are shown in Figure 3. Moreover, the homopolymers (PtnCz and PbTp) and copolymers (P(tnCz1-bTp1), P(tnCz1-bTp2), and P(tnCz1-bTp4)) were further studied using Fourier transform infrared (FT-IR). As shown in Figure 4, the C-S-C characteristic peaks of the P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films, located at 730, 735, 760, and 783 cm −1 , and the C-S-C characteristic peaks of P(tnCz1-bTp1), Polymers 2017, 9, 518 5 of 16 P(tnCz1-bTp2), and P(tnCz1-bTp4) are different to those of PbTp, indicating that copolymerization occurs during the electropolymerization of PtnCz and PbTp.  The as-prepared P(tnCz1-bTp2) film was also investigated at several scan rates between 10 and 200 mV•s −1 in LiClO4/(ACN + DCM) solution. As displayed in Figure 5, the P(tnCz1-bTp2) film showed distinct redox peaks, and the current density of the reduction and oxidation peaks showed a linear relationship with the scan rate, indicating that the P(tnCz1-bTp2) film was well-adhered onto indium tin oxide conductive glass and that the reduction and oxidation processes of P(tnCz1-bTp2) film were nondiffusional processes [33].   The as-prepared P(tnCz1-bTp2) film was also investigated at several scan rates between 10 and 200 mV•s −1 in LiClO4/(ACN + DCM) solution. As displayed in Figure 5, the P(tnCz1-bTp2) film showed distinct redox peaks, and the current density of the reduction and oxidation peaks showed a linear relationship with the scan rate, indicating that the P(tnCz1-bTp2) film was well-adhered onto indium tin oxide conductive glass and that the reduction and oxidation processes of P(tnCz1-bTp2) film were nondiffusional processes [33]. The as-prepared P(tnCz1-bTp2) film was also investigated at several scan rates between 10 and 200 mV·s −1 in LiClO 4 /(ACN + DCM) solution. As displayed in Figure 5, the P(tnCz1-bTp2) film showed distinct redox peaks, and the current density of the reduction and oxidation peaks showed a linear relationship with the scan rate, indicating that the P(tnCz1-bTp2) film was well-adhered onto indium tin oxide conductive glass and that the reduction and oxidation processes of P(tnCz1-bTp2) film were nondiffusional processes [33].

Spectroelectrochemical Characterizations of PtnCz, P(tnCz-bTp), and PbTp Films
Spectroelectrochemical characterizations of the PtnCz, P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films were carried out in 0.2 M LiClO4/(ACN + DCM) solution. Figure 6ac shows the UV-Vis spectra of the PtnCz, P(tnCz1-bTp2), and PbTp films, respectively, at various potentials. As shown in Figure 6a, the PtnCz film does not show a distinct absorption peak in its neutral state. Nevertheless, new absorption bands appear at 430 and 750 nm after a potential of more than 1.2 V was applied, which can be ascribed to the generation of polaron and bipolaron bands for the PtnCz film [34].

Spectroelectrochemical Characterizations of PtnCz, P(tnCz-bTp), and PbTp Films
Spectroelectrochemical characterizations of the PtnCz, P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films were carried out in 0.2 M LiClO 4 /(ACN + DCM) solution. Figure 6a-c shows the UV-Vis spectra of the PtnCz, P(tnCz1-bTp2), and PbTp films, respectively, at various potentials. As shown in Figure 6a, the PtnCz film does not show a distinct absorption peak in its neutral state. Nevertheless, new absorption bands appear at 430 and 750 nm after a potential of more than 1.2 V was applied, which can be ascribed to the generation of polaron and bipolaron bands for the PtnCz film [34].

Spectroelectrochemical Characterizations of PtnCz, P(tnCz-bTp), and PbTp Films
Spectroelectrochemical characterizations of the PtnCz, P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films were carried out in 0.2 M LiClO4/(ACN + DCM) solution. Figure 6ac shows the UV-Vis spectra of the PtnCz, P(tnCz1-bTp2), and PbTp films, respectively, at various potentials. As shown in Figure 6a, the PtnCz film does not show a distinct absorption peak in its neutral state. Nevertheless, new absorption bands appear at 430 and 750 nm after a potential of more than 1.2 V was applied, which can be ascribed to the generation of polaron and bipolaron bands for the PtnCz film [34].   The P(tnCz1-bTp2) and PbTp films show π-π* transition peaks of thiophene chains at 450 and 480 nm, respectively, in their neutral state, and the P(tnCz1-bTp2) and PbTp films show polaron and bipolaron bands at 700 and 900 nm, respectively. The PtnCz film in 0.  Table 2. The P(tnCz1-bTp2) and PbTp films show π-π* transition peaks of thiophene chains at 450 and 480 nm, respectively, in their neutral state, and the P(tnCz1-bTp2) and PbTp films show polaron and bipolaron bands at 700 and 900 nm, respectively. The PtnCz film in 0.  Table 2. Table 2. Colorimetric values (L*, a*, and b*), CIE (Commission Internationale de I'Eclairage) chromaticity values (x, y) and diagrams of the PtnCz, P(tnCz1-bTp2), and PbTp films at various applied potentials.
∆OD can be estimated by the following equation: The P(tnCz1-bTp2) and PbTp films show π-π* transition peaks of thiophene chains at 450 and 480 nm, respectively, in their neutral state, and the P(tnCz1-bTp2) and PbTp films show polaron and bipolaron bands at 700 and 900 nm, respectively. The PtnCz film in 0.  Table 2. Table 2. Colorimetric values (L*, a*, and b*), CIE (Commission Internationale de I'Eclairage) chromaticity values (x, y) and diagrams of the PtnCz, P(tnCz1-bTp2), and PbTp films at various applied potentials.
∆OD can be estimated by the following equation: The P(tnCz1-bTp2) and PbTp films show π-π* transition peaks of thiophene chains at 450 and 480 nm, respectively, in their neutral state, and the P(tnCz1-bTp2) and PbTp films show polaron and bipolaron bands at 700 and 900 nm, respectively. The PtnCz film in 0.  Table 2. Table 2. Colorimetric values (L*, a*, and b*), CIE (Commission Internationale de I'Eclairage) chromaticity values (x, y) and diagrams of the PtnCz, P(tnCz1-bTp2), and PbTp films at various applied potentials.
The coloration efficiency (η) can be calculated using the following equation: where Q d is the injected/ejected electronic charge of polymer films per active area and ∆OD is the discrepancy of optical density. As listed in Table 3, the η value of the PtnCz film at 766 nm, the P(tnCz1-bTp1) film at 680 nm, the P(tnCz1-bTp2) film at 696 nm, the P(tnCz1-bTp4) film at 689 nm, and the PbTp film at 950 nm were 103.7, 180.3, 112.0, 150.1 and 83.5 cm 2 ·C −1 , respectively. The coloration response time (τ c ) and bleaching response time (τ b ) of the PtnCz, P(tnCz1-bTp1), P(tnCz1-bTp2), P(tnCz1-bTp4), and PbTp films in 0.2 M LiClO 4 /(ACN + DCM) solution are also shown in Table 3; the τ c and τ b values were calculated at 90% of the full-transmittance change.
The coloration efficiency (η) can be calculated using the following equation: where Qd is the injected/ejected electronic charge of polymer films per active area and ∆OD is the discrepancy of optical density. As listed in Table 3, the η value of the PtnCz film at 766 nm, the P(tnCz1-bTp1) film at 680 nm, the P(tnCz1-bTp2) film at 696 nm, the P(tnCz1-bTp4) film at 689 nm, and the PbTp film at 950 nm were 103.  Table 3; the τc and τb values were calculated at 90% of the full-transmittance change.

Open Circuit Memory of Electrochromic Devices
The ability to maintain bleached and colored states in the open circuit of the PtnCz/PProDOT-Me 2 , P(tnCz1-bTp2)/PProDOT-Me 2 , and PbTp/PProDOT-Me 2 ECDs was monitored at a specific wavelength as a function of the time in the bleached and colored states by applying the voltage for 1 s for each 200 s time interval. As shown in Figure 10, the P(tnCz1-bTp2)/PProDOT-Me 2 ECD showed good optical memories in the neutral state of the P(tnCz1-bTp2) film; almost no transmittance change in the neutral state was observed. In the oxidation state of the P(tnCz1-bTp2) film, the P(tnCz1-bTp2)/PProDOT-Me 2 ECD was rather less stable than the P(tnCz1-bTp2) film in the neutral state, but the transmittance change was less than 3% in the oxidation state of the P(tnCz1-bTp2) film, implying that the P(tnCz1-bTp2)/PProDOT-Me 2 ECD showed a reasonable open-circuit memory.

Open Circuit Memory of Electrochromic Devices
The ability to maintain bleached and colored states in the open circuit of the PtnCz/PProDOT-Me2, P(tnCz1-bTp2)/PProDOT-Me2, and PbTp/PProDOT-Me2 ECDs was monitored at a specific wavelength as a function of the time in the bleached and colored states by applying the voltage for 1 s for each 200 s time interval. As shown in Figure 10, the P(tnCz1-bTp2)/PProDOT-Me2 ECD showed good optical memories in the neutral state of the P(tnCz1-bTp2) film; almost no transmittance change in the neutral state was observed. In the oxidation state of the P(tnCz1-bTp2) film, the P(tnCz1-bTp2)/PProDOT-Me2 ECD was rather less stable than the P(tnCz1-bTp2) film in the neutral state, but the transmittance change was less than 3% in the oxidation state of the P(tnCz1-bTp2) film, implying that the P(tnCz1-bTp2)/PProDOT-Me2 ECD showed a reasonable open-circuit memory.