Applications of Copolymers Consisting of 2,6-di(9H-carbazol-9-yl)pyridine and 3,6-di(2-thienyl)carbazole Units as Electrodes in Electrochromic Devices

A series of carbazole-based polymers (PdCz, P(dCz2-co-dTC1), P(dCz2-co-dTC2), P(dCz1-co-dTC2), and PdTC) were deposited on indium tin oxide (ITO) conductive electrodes using electrochemical polymerization. The as-prepared P(dCz2-co-dTC2) displayed a high ΔT (57.0%) and multichromic behaviors ranging from yellowish green, greenish gray, gray to purplish gray in different redox states. Five organic electrochromic devices (ECDs) were built using dCz- and dTC-containing homopolymers and copolymers as anodic materials, and poly(3,4-(2,2-dimethylpropylenedioxy)thiophene) (PProdot-Me2) as the cathodic material. The P(dCz2-co-dTC2)/PProdot-Me2 ECD presented remarkable electrochromic behaviors from the bleached to colored states. Moreover, P(dCz2-co-dTC2)/PProdot-Me2 ECD displayed a high optical contrast (ΔT, 45.8%), short switching time (ca. 0.3 s), high coloration efficiency (528.8 cm2 C−1) at 580 nm, and high redox cycling stability.


Introduction
Over the past numerous years, several inorganic and organic electrochromic materials have been extensively studied for use in the rear-view mirrors of vehicles, displays, helmet visors, and windows of buildings [1]. In these electrochromic materials, reversible redox reactions lead to an important change in transmitted (or reflected) light. The promising inorganic electrochromic materials are transition metal oxides (e.g., WO 3 , Ta 2 O 5 , TiO 2 , Nb 2 O 5 , and MoO 3 ), whereas the potential organic electrochromic materials are π-conjugated polymers, viologen derivatives, metallophthalocyanines, and metallopolymers [2]. Among these organic electrochromic materials, recent research has concentrated interest on the applications of conjugated polymers as electrochromic (EC) electrodes due to their fast-electrochromic switching time [3], satisfactory coloration efficiency [4], and wide color availability through the chemical structures modification [5]. The organic EC Table 1. Feed species and molar ratio of anodic polymer electrodes (a)-(e).

Fabrication of Electrochromic Devices
ECDs were constructed using PdCz, P(dCz2-co-dTC1), P(dCz2-co-dTC2), P(dCz1-co-dTC2), or PdTC film as the anodic layer and PProdot-Me 2 film as the cathodic layer. The active areas of anode and cathode were 1.5 cm 2 . The ECDs were built by arranging the reduced and oxidized polymeric films to face each other, and they were isolated by a PMMA/PC/ACN/LiClO 4 composite electrolyte.

Characterizations of Polymer Films and ECDs
The as-prepared polymer films and ECDs were characterized using a CHI627D electrochemical analyzer (CH Instruments, Austin, TX, USA) and an Agilent Cary 60 UV-Visible spectrophotometer (Varian Inc., Walnut Creek, CA, USA). The system of electrochemical experiments was implemented in a three-constituent cell. The working electrode was an ITO coated glass plate, the counter electrode was a platinum wire, and an Ag/AgNO 3 electrode was used as the reference electrode. Spectroelectrochemical experiments were monitored using a spectrophotometer, the spectroelectrochemical characterizations of polymer films were performed in a UV quartz cuvette cell (Varian Inc., Walnut Creek, CA, USA), and the path length of cell was 1 cm. When the number of CV cycles increased, the current density of redox peaks increased with an increasing scanning number, implying the growth of polymer films on ITO surfaces [27]. As displayed in Figure 1a, the 1st and 2nd oxidation peaks of PdCz at ca. 0.87 and 1.30 V (vs. Ag/AgNO 3 ), respectively, can be ascribed to the presence of radical cation and dication in a dCz unit. The CV curves of PdTC show an oxidation and a reduction peaks at 1.05 and 0.37 V, respectively, as shown in Figure 1e. The electrochemical redox peaks of PdCz and PdTC were quasi-reversible. When the CV curves were swept in the mixture of dCz and dTC monomers, the oxidation peaks of P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1-co-dTC2) appeared at 1.25, 1.19 and 1.15 V, respectively, and the reduction peaks of P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1-co-dTC2) located at 0.50, 0.43 and 0.39 V, respectively, as shown in Figure 1b-d. The redox potentials and wave shapes of copolymers are different from those of homopolymers, and this is an evidence of the copolymerization of P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1-co-dTC2) films. The polymer films were further characterized using FT-IR, Figure 2 displayed the FT-IR spectra of electrochemically synthesized PdCz, P(dCz2-co-dTC1), P(dCz2-co-dTC2), P(dCz1-co-dTC2), and PdTC films. The characteristic peaks of PdCz are shown in Figure 2a. The characteristic band at 1095 cm −1 indicates the doping of PdCz film with the ClO 4 − . The characteristic peaks at around 1600 cm −1 represent the aromatic C=C stretching vibration. The band at 1220 cm −1 is related to C-C formation. The peak at around 1450 cm −1 can be ascribed to the C-N stretching of the carbazole unit [28].

Electrochemical Polymerization and FT-IR Characterization
Materials 2019, 12, x FOR PEER REVIEW 4 of 17 characteristic peaks at around 1600 cm −1 represent the aromatic C=C stretching vibration. The band at 1220 cm −1 is related to C-C formation. The peak at around 1450 cm −1 can be ascribed to the C-N stretching of the carbazole unit [28].  There was no conspicuous characteristic peak of PdCz at ca. 790 cm −1 as shown in Figure 2a. Figure 2b,c and d not only revealed the characteristic peaks of PdCz but also displayed the characteristic peak (-C-S-C-stretching) of PdTC, the formation of a new characteristic peak at 790 cm −1 could be attributed to the presence of dTC in P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1co-dTC2) films, implying dCz-and dTC-containing copolymer films were successfully synthesized. The polymerization schemes of PdCz, P(dCz-co-dTC), and PdTC are shown in Figure 3.  FT-IR spectra of (a) PdCz; (b) P(dCz2-co-dTC1); (c) P(dCz2-co-dTC2); (d) P(dCz1-co-dTC2) and (e) PdTC.
There was no conspicuous characteristic peak of PdCz at ca. 790 cm −1 as shown in Figure 2a. Figure 2b-d not only revealed the characteristic peaks of PdCz but also displayed the characteristic peak (-C-S-C-stretching) of PdTC, the formation of a new characteristic peak at 790 cm −1 could be attributed to the presence of dTC in P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1-co-dTC2) films, implying dCz-and dTC-containing copolymer films were successfully synthesized. The polymerization schemes of PdCz, P(dCz-co-dTC), and PdTC are shown in Figure 3. There was no conspicuous characteristic peak of PdCz at ca. 790 cm −1 as shown in Figure 2a. Figure 2b,c and d not only revealed the characteristic peaks of PdCz but also displayed the characteristic peak (-C-S-C-stretching) of PdTC, the formation of a new characteristic peak at 790 cm −1 could be attributed to the presence of dTC in P(dCz2-co-dTC1), P(dCz2-co-dTC2), and P(dCz1co-dTC2) films, implying dCz-and dTC-containing copolymer films were successfully synthesized. The polymerization schemes of PdCz, P(dCz-co-dTC), and PdTC are shown in Figure 3.

Electrochemical Properties of Polymer Films
The first rinse P(dCz2-co-dTC2) film was immersed into clean electrolyte solution, and then run in new CV curves. The as-prepared P(dCz2-co-dTC2) film was swept at 10, 50, 100, 150, and 200 mV s −1 in 0.2 M LiClO 4 /ACN/DCM using CV. As shown in Figure 4, the anodic and cathodic peaks of P(dCz2-co-dTC2) film showed a single quasi-reversible redox process and the anodic and cathodic peak current densities increased linearly with increasing scan rate (inset in Figure 4), implying the redox process of P(dCz2-co-dTC2) film was not a diffusion-controlled process [29].

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Electrochromic switching of ECDs
As displayed in Figure 8, PdCz/PProdot-Me2 and P(dCz2-co-dTC2)/PProdot-Me2 ECDs were monitored by potential stepping between bleached and colored states with a residence time of 10 s.

Open-Circuit Memory of ECDs
The open-circuit memory effect of PdCz/PProdot-Me 2 , P(dCz2-co-dTC2)/PProdot-Me 2 , and PdTC/PProdot-Me 2 ECDs was detected at bleached and colored states by applying the voltage for 1 s for each 100 s interval [40,41]. Figure 9 shows that the transmittance of the three ECDs was almost no change at the bleached state. At colored states, the three ECDs were less stable than those at bleached state. However, the loss in transmittance of ECDs at colored states was less than 4%. It is worth mentioning that P(dCz2-co-dTC2)/PProdot-Me 2 ECD showed less transmittance change at the colored state than those of PdCz/PProdot-Me 2 and PdTC/PProdot-Me 2 ECDs, disclosing that P(dCz2-co-dTC2)/PProdot-Me 2 ECD exhibited a satisfactory open-circuit memory effect.