1,4-Bis((9H-Carbazol-9-yl)Methyl)Benzene-Containing Electrochromic Polymers as Potential Electrodes for High-Contrast Electrochromic Devices

Four 1,4-bis((9H-carbazol-9-yl)methyl)benzene-containing polymers (PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF)) were electrosynthesized onto ITO transparent conductive glass and their spectral and electrochromic switching performances were characterized. The PbCmB film displayed four types of color variations (bright gray, dark gray, dark khaki, and dark olive green) from 0.0 to 1.2 V. P(bCmB-co-bTP) displayed a high transmittance variation (∆T = 39.56% at 685 nm) and a satisfactory coloration efficiency (η = 160.5 cm2∙C−1 at 685 nm). Dual-layer organic electrochromic devices (ECDs) were built using four bCmB-containing polycarbazoles and poly(3,4-ethylenedioxythiophene) (PEDOT) as anodes and a cathode, respectively. PbCmB/PEDOT ECD displayed gainsboro, dark gray, and bright slate gray colors at −0.4 V, 1.0 V, and 2.0 V, respectively. The P(bCmB-co-bTP)/PEDOT ECD showed a high ∆T (40.7% at 635 nm) and a high coloration efficiency (η = 428.4 cm2∙C−1 at 635 nm). The polycarbazole/PEDOT ECDs exhibited moderate open circuit memories and electrochemical redox stability. The characterized electrochromic properties depicted that the as-prepared polycarbazoles had a satisfactory application prospect as an electrode for the ECDs.


Introduction
Electrochromism refers to a phenomenon exhibited by certain electroactive species with reversible changes of color and absorption spectra in response to an applied voltage. Many classes of compounds and materials have been shown to exhibit electrochromism over the past four decades. Inorganic transition metal oxides (e.g., WO 3 , TiO 2 , V 2 O 5 , and NiO), inorganic coordination complexes (e.g., Prussian blue), π-conjugated polymers, and molecular dyes are the most frequently studied electrochromic materials [1][2][3][4][5]. π-conjugated polymers have recently attracted an increasing interest due to their superior benefits such as their ease of solution processing, rapid switching speeds, high coloration efficiency, and rich color palette during usage for electrochromic devices. π-conjugated polymers have a wide range of optical and electrochemical applications in field-effect transistors [6,7], capacitors [8,9], photovoltaic cells [10,11], light-emitting diodes [12,13], catalyst supports [14,15], and sensors [16,17].
2.3. Electrodepositions of PbCmB, PEDOT, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) Electrodes PbCmB, PEDOT, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) electrodes were potentiostatically electrodeposited at 1.0 V (vs. Ag/AgNO 3 ) on an ITO glass substrate with a charge density of 20 mC·cm −2 . The concentration of bCmB, bTP, dbBT, and TF monomers was 2 × 10 −3 M and the liquid electrolyte was 0.2 M LiClO 4 in ACN/DCM (1:1 by volume). The feed species and concentrations of the monomers are shown in Table 1. The reference and counter electrodes of the liquid electrolyte cell were an Ag/AgNO 3 electrode (calibrated with ferrocene/ferrocenium) and a Pt wire, respectively. The polymeric electrode area was 1.5 cm 2 .  Figure 2 shows the linear sweep voltammetry curves for the electrochemical oxidation of neat PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) in a 0.2 M LiClO 4 solution. The onset potentials in the cyclic voltammetry of neat bCmB, bTP, dbBT, and TF were 0.75, 0.81, 0.94, and 0.70 V (vs. Ag/AgNO 3 ), respectively. The incorporation of two electron-withdrawing bromide groups in the bithiophene unit increased the onset potentials. bCmB displayed a lower onset potential than the bithiophene unit; this could be attributed to the fact that biscarbazole-containing bCmB showed a stronger electrondonating ability than the bithiophene unit. The disparities of the onset potentials between neat bCmB and neat bithiophene derivatives were less than 0.2 V, implying the feasibility of copolymerizations using bCmB and bithiophene derivatives.

Electrochemical, Electrochromic, and Kinetic Characterizations
The electrochemical polymerization procedures and electrochemical properties of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) anodically coloring films were carried out using a CHI6277E (CH Instruments, Austin, TX, USA) electrochemical workstation with a scan rate of 100 mV s −1 . The absorption spectra and electrochromic switching performances of the single-layer electrodes in the solutions and the dual-layer ECDs were measured using a spectrophotometer (Jasco V-630 (JASCO International Co., Ltd., Tokyo, Japan)) and an electrochemical workstation (CHI6277E). The color-bleach kinetics of the polymers were switched between 0.0 and 1.2 V whereas the color-bleach kinetics of the ECDs were switched between −0.4 V and 1.8 V. Figure 2 shows the linear sweep voltammetry curves for the electrochemical oxidation of neat PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) in a 0.2 M LiClO4 solution. The onset potentials in the cyclic voltammetry of neat bCmB, bTP, dbBT, and TF were 0.75, 0.81, 0.94, and 0.70 V (vs. Ag/AgNO3), respectively. The incorporation of two electron-withdrawing bromide groups in the bithiophene unit increased the onset potentials. bCmB displayed a lower onset potential than the bithiophene unit; this could be attributed to the fact that biscarbazole-containing bCmB showed a stronger electrondonating ability than the bithiophene unit. The disparities of the onset potentials between neat bCmB and neat bithiophene derivatives were less than 0.2 V, implying the feasibility of copolymerizations using bCmB and bithiophene derivatives.  Figure 3 displays the electrogrowth of neat bCmB and mixtures of bCmB + bithiophene derivatives (or 2-(thiophen-2-yl)furan) in 0.2 M LiClO4/ACN/DCM). The potentiodynamic polymerization scans from the first to the tenth cycles revealed that the current densities of the redox peaks increased with the number of increasing cycles, implying the growth of polymers on the ITO substrate [32]. As shown in Figure 3a, PbCmB displayed two distinct oxidation peaks at 0.70 and 1.12 V as well as two evident reduction peaks at 0.31 and 0.65 V.  The potentiodynamic polymerization scans from the first to the tenth cycles revealed that the current densities of the redox peaks increased with the number of increasing cycles, implying the growth of polymers on the ITO substrate [32]. As shown in Figure 3a, PbCmB displayed two distinct oxidation peaks at 0.70 and 1.12 V as well as two evident reduction peaks at 0.31 and 0.65 V. The first oxidation and reduction peaks depicted the generation of radical cations in poly(1,4-bis((9H-carbazol-9-yl)methyl)benzene) and the second redox peaks represented the formation of dications. The incorporation of bithiophene, 3,3′-dibromo-2,2′-bithiophene, and 2-(thiophen-2-yl)furan into the polymeric chain slightly shifted the redox peaks. The first oxidation and reduction peaks of P(bCmB-co-bTP) were located at 0.78 and 0.33 V, respectively (Figure 3b). In a similar condition, two oxidation peaks of P(bCmB-co-dbBT) were situated at 0.70 and 1.09 V, respectively, and two reduction peaks of P(bCmB-co-dbBT) were located at 0.41 and 0.72 V, respectively ( Figure 3c). The first and second oxidation peaks of P(bCmB-co-TF) were located at 0.79 and 1.26 V, respectively, and the first and second reduction peaks of P(bCmB-co-TF) were situated at 0.44 and 0.71 V, respectively (Figure 3d). The peak potentials and CV wave shapes of PbCmB were diverse compared with those of P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF), proving the coating of P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) membranes onto the ITO glass substrate. The polymerization schemes of PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) are listed in Figure 4 [33].

Electrochemical Characterization of the Polymer Electrodes
The electrocoated P(bCmB-co-bTP) film was studied using various scan rate cyclic voltammetry measurements. As displayed in Figure 5, the P(bCmB-co-bTP) film showed two couples of well-defined redox peaks at various scan rates in 0.2 M LiClO4/ACN/DCM; the peak current densities of the P(bCmB-co-bTP) film were linearly proportional to the scan velocities, representing that P(bCmB-co-bTP) tightly adhered to the conductive glass and the redox behaviors of P(bCmB-co-bTP) were reversible and activation control [34]. The first oxidation and reduction peaks depicted the generation of radical cations in poly(1,4-bis((9H-carbazol-9-yl)methyl)benzene) and the second redox peaks represented the formation of dications. The incorporation of bithiophene, 3,3 -dibromo-2,2 -bithiophene, and 2-(thiophen-2-yl)furan into the polymeric chain slightly shifted the redox peaks. The first oxidation and reduction peaks of P(bCmB-co-bTP) were located at 0.78 and 0.33 V, respectively (Figure 3b). In a similar condition, two oxidation peaks of P(bCmB-co-dbBT) were situated at 0.70 and 1.09 V, respectively, and two reduction peaks of P(bCmB-co-dbBT) were located at 0.41 and 0.72 V, respectively ( Figure 3c). The first and second oxidation peaks of P(bCmB-co-TF) were located at 0.79 and 1.26 V, respectively, and the first and second reduction peaks of P(bCmB-co-TF) were situated at 0.44 and 0.71 V, respectively ( Figure 3d). The peak potentials and CV wave shapes of PbCmB were diverse compared with those of P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF), proving the coating of P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) membranes onto the ITO glass substrate. The polymerization schemes of PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) are listed in Figure 4 [33].
The electrocoated P(bCmB-co-bTP) film was studied using various scan rate cyclic voltammetry measurements. As displayed in Figure 5, the P(bCmB-co-bTP) film showed two couples of well-defined redox peaks at various scan rates in 0.2 M LiClO 4 /ACN/DCM; the peak current densities of the P(bCmB-co-bTP) film were linearly proportional to the scan velocities, representing that P(bCmB-co-bTP) tightly adhered to the conductive glass and the redox behaviors of P(bCmB-co-bTP) were reversible and activation control [34].   Figure 6a-d shows the absorption spectra of the PbCmB, P(bCmB-co-bTP), P(bCmBco-dbBT), and P(bCmB-co-TF) electrodes in 0.2 M LiClO4/ACN/DCM. There was no noticeable absorption peak of the PbCmB film between 370 and 1000 nm at 0.0 and 0.5 V, respectively. As the voltage increased stepwise from 0.0 V to 1.2 V, new charge carrier bands appeared at around 420 and 675 nm, signifying the existence of the generation of radical cations and dications [35] (Figure 6a). As displayed in Figure 6b, P(bCmB-co-bTP) showed a neutral absorption peak at around 420 nm; this could be attributed to the π-π*     Figure 6a-d shows the absorption spectra of the PbCmB, P(bCmB-co-bTP), P(bCmBco-dbBT), and P(bCmB-co-TF) electrodes in 0.2 M LiClO4/ACN/DCM. There was no noticeable absorption peak of the PbCmB film between 370 and 1000 nm at 0.0 and 0.5 V, respectively. As the voltage increased stepwise from 0.0 V to 1.2 V, new charge carrier bands appeared at around 420 and 675 nm, signifying the existence of the generation of radical cations and dications [35] (Figure 6a). As displayed in Figure 6b, P(bCmB-co-bTP) showed a neutral absorption peak at around 420 nm; this could be attributed to the π-π*  Figure 6a-d shows the absorption spectra of the PbCmB, P(bCmB-co-bTP), P(bCmBco-dbBT), and P(bCmB-co-TF) electrodes in 0.2 M LiClO 4 /ACN/DCM. There was no noticeable absorption peak of the PbCmB film between 370 and 1000 nm at 0.0 and 0.5 V, respectively. As the voltage increased stepwise from 0.0 V to 1.2 V, new charge carrier bands appeared at around 420 and 675 nm, signifying the existence of the generation of radical cations and dications [35] (Figure 6a). As displayed in Figure 6b, P(bCmB-co-bTP) showed a neutral absorption peak at around 420 nm; this could be attributed to the π-π* Polymers 2022, 14, 1175 7 of 17 (or n-π*) transition of the bithiophene heteroaromatic groups. The PbCmB film displayed four types of color variations from the neutral to the oxidation state, which were bright gray, dark gray, dark khaki, and dark olive green at 0.0, 0.7, 1.0, and 1.2 V, respectively. The L*, a*, and b* values of PbCmB are displayed in Table 2. Under identical situations, the P(bCmB-co-bTP) film was celadon, earth gray, iron gray, and navy blue at 0.0, 0.6, 0.8, and 1.2 V, respectively. Figure 6a-d shows the absorption spectra of the PbCmB, P(bCmB-co-bTP), P(bCmBco-dbBT), and P(bCmB-co-TF) electrodes in 0.2 M LiClO4/ACN/DCM. There was no noticeable absorption peak of the PbCmB film between 370 and 1000 nm at 0.0 and 0.5 V, respectively. As the voltage increased stepwise from 0.0 V to 1.2 V, new charge carrier bands appeared at around 420 and 675 nm, signifying the existence of the generation of radical cations and dications [35] (Figure 6a). As displayed in Figure 6b, P(bCmB-co-bTP) showed a neutral absorption peak at around 420 nm; this could be attributed to the π-π* (or n-π*) transition of the bithiophene heteroaromatic groups. The PbCmB film displayed four types of color variations from the neutral to the oxidation state, which were bright gray, dark gray, dark khaki, and dark olive green at 0.0, 0.7, 1.0, and 1.2 V, respectively. The L*, a*, and b* values of PbCmB are displayed in Table 2. Under identical situations, the P(bCmB-co-bTP) film was celadon, earth gray, iron gray, and navy blue at 0.0, 0.6, 0.8, and 1.2 V, respectively. The P(bCmB-co-dbBT) film was bright gray, slate gray, khaki, and dark greenish grey at 0.0, 0.7, 1.0, and 1.2 V, respectively. The P(bCmB-co-TF) film was light gray, slate gray, dark khaki, and dark greenish grey at −0.3, 0.6, 1.0, and 1.2 V, respectively. The incorporation of bithiophene, 3,3'-dibromo-2,2'-bithiophene, and 2-(thiophen-2-yl)furan in the polymer chain changed the color variations from the reduced state to the oxidized state.
The optical energy gap (Eg) of PbCmB calculated using the absorption onset wavelength (λonset) of the π-π* transition peak was 3.20 eV [36]. Table 3 shows the Eg of the reported polymers. PbCmB displayed a larger Eg than PBCPO [37], PMCP [38], and PDCP [39]. This could be attributed to two methylene groups interrupting the conjugated degree of the polymer chains. The Eonset of PbCmB (vs. Ag/AgNO3) was 0.80 V, the EFOC calculated from the potential of ferrocene/ferrocenium vs. Ag/AgNO3 was 0.69 V, and the Eonset (vs. EFOC) was evaluated as 0.11 V. The reference energy for ferrocene is 4.8 eV below the vacuum level [40]. EHOMO and ELUMO, corresponding with the energy levels of HOMO and LUMO, were calculated as −4.91 and −1.71 eV, respectively. Table 3. Transmittance changes and coloration efficiencies of polymers (or ECDs).
The optical energy gap (Eg) of PbCmB calculated using the absorption onset wavelength (λonset) of the π-π* transition peak was 3.20 eV [36]. Table 3 shows the Eg of the reported polymers. PbCmB displayed a larger Eg than PBCPO [37], PMCP [38], and PDCP [39]. This could be attributed to two methylene groups interrupting the conjugated degree of the polymer chains. The Eonset of PbCmB (vs. Ag/AgNO3) was 0.80 V, the EFOC calculated from the potential of ferrocene/ferrocenium vs. Ag/AgNO3 was 0.69 V, and the Eonset (vs. EFOC) was evaluated as 0.11 V. The reference energy for ferrocene is 4.8 eV below the vacuum level [40]. EHOMO and ELUMO, corresponding with the energy levels of HOMO and LUMO, were calculated as −4.91 and −1.71 eV, respectively. The P(bCmB-co-dbBT) film was bright gray, slate gray, khaki, and dark greenish grey at 0.0, 0.7, 1.0, and 1.2 V, respectively. The P(bCmB-co-TF) film was light gray, slate gray, dark khaki, and dark greenish grey at −0.3, 0.6, 1.0, and 1.2 V, respectively. The incorporation of bithiophene, 3,3 -dibromo-2,2 -bithiophene, and 2-(thiophen-2-yl)furan in the polymer chain changed the color variations from the reduced state to the oxidized state.
The optical energy gap (E g ) of PbCmB calculated using the absorption onset wavelength (λ onset ) of the π-π* transition peak was 3.20 eV [36]. Table 3 shows the E g of the reported polymers. PbCmB displayed a larger E g than PBCPO [37], PMCP [38], and PDCP [39]. This could be attributed to two methylene groups interrupting the conjugated degree of the polymer chains. The E onset of PbCmB (vs. Ag/AgNO 3 ) was 0.80 V, the E FOC calculated from the potential of ferrocene/ferrocenium vs. Ag/AgNO 3 was 0.69 V, and the E onset (vs. E FOC ) was evaluated as 0.11 V. The reference energy for ferrocene is 4.8 eV below the vacuum level [40]. E HOMO and E LUMO , corresponding with the energy levels of HOMO and LUMO, were calculated as −4.91 and −1.71 eV, respectively. Table 3. Transmittance changes and coloration efficiencies of polymers (or ECDs).
The coloration efficiency (η) could be obtained from the following equation [48]: where Qd represents the charge injection/extraction of the electrodes per active area. As listed in Table 4, the η of PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) was 80.3 cm 2 •C −1 at 685 nm, 160.5 cm 2 •C −1 at 685 nm, 86.5 cm 2 •C −1 at 685 nm, and 125.8 cm 2 •C −1 at 690 nm, respectively. The response time from the colored to the bleached state (τ b ) and from the bleached to the colored state (τ c ) of PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) in the solutions is listed in Table 4. The τ b and τ c were determined at 90% of the maximum ∆T. The τ c and τ b of the polymeric films were determined to be 1.46-3.51 s and 4.81-5.18 s, respectively. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state: The ∆ODs of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films in the solutions were 0.126 at 685 nm, 0.374 at 685 nm, 0.186 at 685 nm, and 0.221 at 690 nm, respectively. The P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films displayed a higher ∆OD than the PbCmB film.

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PEDOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PEDOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state: The ΔODs of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films in the solutions were 0.126 at 685 nm, 0.374 at 685 nm, 0.186 at 685 nm, and 0.221 at 690 nm, respectively. The P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films displayed a higher ΔOD than the PbCmB film.

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PE-DOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PE-DOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PbCmB/PEDOT, (b) P(bCmB-co-bTP)/PEDOT, (c) P(bCmB-co-dbBT)/PE-DOT, and (d) P(bCmB-co-TF)/PEDOT ECDs at different potentials. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state:

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PE-DOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PE-DOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state: The ΔODs of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films in the solutions were 0.126 at 685 nm, 0.374 at 685 nm, 0.186 at 685 nm, and 0.221 at 690 nm, respectively. The P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films displayed a higher ΔOD than the PbCmB film.

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PE-DOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PE-DOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PbCmB/PEDOT, (b) P(bCmB-co-bTP)/PEDOT, (c) P(bCmB-co-dbBT)/PE-DOT, and (d) P(bCmB-co-TF)/PEDOT ECDs at different potentials. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state:

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PE-DOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PE-DOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state: The ΔODs of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films in the solutions were 0.126 at 685 nm, 0.374 at 685 nm, 0.186 at 685 nm, and 0.221 at 690 nm, respectively. The P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films displayed a higher ΔOD than the PbCmB film.

Absorption Spectra and the Transmittance Changes of the ECDs
Dual-layer organic ECDs were constructed using the configurations of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs. The absorption spectra of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs are displayed in Figure 8a-d. At −0.4 V, the PbCmB/PEDOT ECD did not reveal a significant absorption peak in the UV-Vis region. The anodic PbCmB film was in its reduced state and the cathodic PEDOT film was in its oxidized state at −0.4 V, exhibiting a limpid color. The PbCmB/PE-DOT ECD showed a gainsboro color at −0.4 V. After progressively raising the potential to 2.0 V, the PbCmB film started to oxidize and the PEDOT film started to reduce. As a consequence, new absorption bands at 420 and 620 nm appeared stepwise. The PbCmB/PE-DOT ECD showed a dark gray color at 1.0 V and a bright slate gray color at 2.0 V. Under identical situations, the P(bCmB-co-bTP)/PEDOT ECD was light gray, dark gray, and slate gray at −0.6, 0.8, and 2.0 V, respectively. The P(bCmB-co-dbBT)/PEDOT ECD was gainsboro, dark gray, and bright slate gray at −0.4, 1.0, and 2.4 V, respectively. The P(bCmB-co-TF)/PEDOT ECD was gainsboro, light gray, and dark gray at −0.3, 1.6, and 2.4 V, respectively. The colorimetric results of the four ECDs are listed in Table 5. Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PbCmB/PEDOT, (b) P(bCmB-co-bTP)/PEDOT, (c) P(bCmB-co-dbBT)/PE-DOT, and (d) P(bCmB-co-TF)/PEDOT ECDs at different potentials. As shown in the following equation [48], the ∆OD could be determined by a logarithmic calculation of the transmission (%) at the oxidation state and the reduction state:

Open Circuit Memories of the ECDs
The open circuit memories of the dual-layer organic ECDs were monitored by applying potentials in colored and bleached states for 1 s for each 100 s interval. As displayed in Figure 10, the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs displayed sufficient open circuit memories with ≤ 1.1% transmittance variation in the bleached state. However, in an oxidized state of the PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) films and in a reduced state of the PEDOT film, the transmittance changes of the PbCmB/PEDOT, P(bCmB-co-bTP)/PE-DOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs in a colored state were less stable than the four ECDs in a bleached state. The P(bCmB-co-dbBT)/PEDOT ECD showed the largest transmittance change in a colored state. However, the transmittance change of the P(bCmB-co-dbBT)/PEDOT ECD in a colored state was less than 4.9%,  The response time of the PbCmB/PEDOT, P(bCmB-co-bTP)/PEDOT, P(bCmB-co-dbBT)/PEDOT, and P(bCmB-co-TF)/PEDOT ECDs was shorter than the PbCmB, P(bCmBco-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF) electrodes in the solutions, disclosing that the distances between the anode and the cathode in the ECDs were narrower than between the polymeric electrode and the Pt electrode in the solutions [49].

Conclusions
A series of redox-active polycarbazoles (PbCmB, P(bCmB-co-bTP), P(bCmB-co-dbBT), and P(bCmB-co-TF)) were electrochemically synthesized onto ITO glass surfaces. The obtained polycarbazoles showed a good redox reversibility. Compared with the homopolymer, the copolymers showed different color transitions from the neutral to the oxidized states. The P(bCmB-co-bTP) film was celadon, earth gray, iron gray, and navy blue at 0.0, 0.6, 0.8, and 1.2 V, respectively. Electrochromic switching studies of the four polycarbazoles exhibited that the ΔT of PbCmB and P(bCmB-co-TF) was 23.94% at 685 nm and 29.84% at 690 nm, respectively. Four ECDs were built using anodic polycarbazoles and a cathodic PEDOT layer. The ΔT of the PbCmB/PEDOT and P(bCmB-co-dbBT)/PEDOT ECDs was 35.3% at 640 nm and 36.4% at 635 nm at the second cycle, respectively. bCmBcontaining polycarbazoles and their corresponding ECDs showed high transmittance changes. The incorporation of bithiophene, 3,3'-dibromo-2,2'-bithiophene, and 2-(thiophen-2-yl)furan groups into the polycarbazoles showed different color transitions at various potentials and showed high transmittance changes. Moreover, five ECDs displayed a fast response time (≤ 2.8 s) and satisfactory open circuit memories in both the colored and bleached states. As a result, PbCmB and P(bCmB-co-bTP) may be good candidates for use as electrodes in ECDs.