Electrodeposited Copolymers Based on 9,9′-(5-Bromo-1,3-phenylene)biscarbazole and Dithiophene Derivatives for High-Performance Electrochromic Devices

A 1,3-bis(carbazol-9-yl)benzene derivative (BPBC) was synthesized and its related homopolymer (PBPBC) and copolymers (P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK)) were prepared using electrochemical polymerization. Investigations of polymeric spectra showed that PBPBC film was grey, iron-grey, yellowish-grey, and greyish-green from the neutral to the oxidized state. P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films showed multicolor transitions from the reduced to the oxidized state. The transmittance change (ΔT) of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were 29.6% at 1040 nm, 44.4% at 1030 nm, 22.3% at 1050 nm, and 41.4% at 1070 nm. The coloration efficiency (η) of PBPBC and P(BPBC-co-CDTK) films were evaluated to be 140.3 cm2 C−1 at 1040 nm and 283.7 cm2 C−1 at 1070 nm, respectively. A P(BPBC-co-BT)/PEDOT electrochromic device (ECD) showed a large ΔT (36.2% at 625 nm) and a fast response time (less than 0.5 s), whereas a P(BPBC-co-CDTK)/PEDOT ECD revealed a large η (534.4 cm2 C–1 at 610 nm) and sufficient optical circuit memory.


Preparation of Electrolyte and Assembly of the ECDs
The electrolyte of the ECDs was prepared using 0.4 g of PMMA, 0.3 g of LiClO 4 , 1.1 g of PC, and 2.5 mL of ACN based on previous procedures [43]. PBPBC/PEDOT, P(BPBC-co-BT)/PEDOT, P(BPBC-co-CDT)/PEDOT, and P(BPBC-co-CDTK)/PEDOT dual type ECDs were assembled using the electrolyte as the separation layer between anodic and cathodic polymer layers ( Figure 2). The anodic polymer layers (PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films) were prepared potentiodynamically in a potential range from 0.0 to 1.6 V. The cathodic PEDOT layer was prepared potentiostatically at 1.4 V on indium tin oxide (ITO) glass. The active electrode area of the ECDs was 1.5 cm 2 .

Electrochemical, Optical, and Electrochromic Properties
Spectroelectrochemical properties of the polymers and ECDs were measured using an electrochemical analyzer (CHI627E, (CH Instruments, Austin, TX, USA)) and a Jasco V-630 absorption spectrometer (JASCO International Co., Ltd., Tokyo, Japan). The cyclic voltammetry (CV) experiments were also carried out using the CHI627E electrochemical workstation in a three-electrode cell with a working electrode (ITO glasses), a counter electrode (Pt wire), and a reference electrode (Ag/AgCl). The three electrodes were dipped in 0.2 M LiClO 4 /ACN/DCM, where the volume ratio of ACN/DCM = 4/1.

Electrochemical Characterization
As displayed in Figure 3, the E onset (vs. Ag/AgCl) values of BPBC, BT, CDT, and CDTK were 1.26, 1.22, 0.95, and 1.25 V, respectively. The E onset of BPBC was comparable to those of BT and CDTK, implying that the electron-withdrawing bromo substituted group increased the oxidized potential of 1,3-di(9H-carbazol-9-yl)benzene. The E onset of CDT Polymers 2021, 13, 1136 4 of 18 was smaller than that of CDTK, which can be attributed to the ketone group of CDTK increasing the E onset significantly.  Figure 4 shows the electro-synthesized curves of a neat BPBC monomer and the mixtures of two monomers (BPBC + BT, BPBC + CDT, and BPBC + CDTK) in 0.2 M LiClO 4 /ACN/DCM (ACN/DCM = 4:1, by volume). During the potential scan of CV in the anodic region, two semi-reversible oxidized waves were observed in Figure 4, which could be assigned to the formation of a cation radical and the quinoid-like dication of bicarbazole segments [44]. As the electrosynthesized curves increased with the increasing number of times, the PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films could be presented on the ITO substrates, manifesting that PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were coated on the ITO glasses [45]. of P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were dissimilar to those of PBPBC films, implying copolymer films were coated on the ITO surfaces ( Figure S1 Supplementary Materials). Figure 5 revealed the schemes of electrosynthesis of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK). The electrochemical properties of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were characterized by sweeping in monomer-free electrolyte at various scan rates. Figure 6 exhibited well-defined reversible oxidization and reduction processes for PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films. The anodic and cathodic peak currents exhibited a linear growth with increasing scanning rates, disclosing that the electroactive species transferred during the oxidation-reduction reactions were nondiffusion-limited and PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) were tightly attached to the ITO glasses [46].  Figure 7 displayed the absorption spectra of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) electrodes at various voltages. PBPBC film displayed a large absorption band at 350 nm and a small peak at around 430 nm, which could be attributed to the π-π* transition and n-π* transition of PBPBC, respectively. When the voltage was slowly increased, the absorption band of the π-π* transition faded little by little, and the charged carrier bands ascended at ca. 380, 430, 750, and 1100 nm [47]. As the potential was increased to 1.3 V, the absorption band at 750 to 1100 nm began to drop slightly, implying the appearance of a bipolaron band caused by further oxidations of the 1,3-bis(carbazol-9-yl)benzene unit [48]. P(BPBC-co-BT) film displayed a π-π* transition shoulder of the BT ring at 420 nm, with the charged carrier bands of P(BPBC-co-BT) located at 680 and 1050 nm. The oxidation of polythiophene segments occurred at the beginning, and then, at a slightly higher potential, bicarbazole segments were oxidized in two steps.
The bandgap (E g ) of the PBPBC film determined using the absorption edge values of the UV spectrum was 2.64 eV [49]. The E onset(ox) of PBPBC (vs. Ag/AgCl) was 1.25 V, and the E FOC value obtained from Fc/Fc + was 0.81 V. The E onset(ox) (vs. E FOC ) was evaluated as 0.44 V. The HOMO energy level of PBPBC was estimated using the E onset and the energy level of the Fc/Fc + redox couple (-4.8 eV below the vacuum level) [50,51]. The lowest unoccupied molecular orbital (LUMO) energy level of PBPBC (vs. vacuum level) was calculated by the addition of E g from the HOMO level (-5.24 eV), and the E LUMO of PBPBC was -2.60 eV. Table 2. Colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PBPBC, (b) P(BPBC-co-BT), (c) P(BPBC-co-CDT), and (d) P(BPBC-co-CDTK) at several potentials.

Films
0.0 the UV spectrum was 2.64 eV [49]. The Eonset(ox) of PBPBC (vs. Ag/AgCl) was 1.25 V, and the EFOC value obtained from Fc/Fc + was 0.81 V. The Eonset(ox) (vs. EFOC) was evaluated as 0.44 V. The HOMO energy level of PBPBC was estimated using the Eonset and the energy level of the Fc/Fc + redox couple (-4.8 eV below the vacuum level) [50,51]. The lowest unoccupied molecular orbital (LUMO) energy level of PBPBC (vs. vacuum level) was calculated by the addition of Eg from the HOMO level (-5.24 eV), and the ELUMO of PBPBC was -2.60 eV. Table 2. Colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PBPBC, (b) P(BPBC-co-BT), (c) P(BPBC-co-CDT), and (d) P(BPBC-co-CDTK) at several potentials. the UV spectrum was 2.64 eV [49]. The Eonset(ox) of PBPBC (vs. Ag/AgCl) was 1.25 V, and the EFOC value obtained from Fc/Fc + was 0.81 V. The Eonset(ox) (vs. EFOC) was evaluated as 0.44 V. The HOMO energy level of PBPBC was estimated using the Eonset and the energy level of the Fc/Fc + redox couple (-4.8 eV below the vacuum level) [50,51]. The lowest unoccupied molecular orbital (LUMO) energy level of PBPBC (vs. vacuum level) was calculated by the addition of Eg from the HOMO level (-5.24 eV), and the ELUMO of PBPBC was -2.60 eV. Table 2. Colorimetric values (L*, a*, and b*), CIE chromaticity values (x, y), and diagrams of (a) PBPBC, (b) P(BPBC-co-BT), (c) P(BPBC-co-CDT), and (d) P(BPBC-co-CDTK) at several potentials.  − , being more likely to get trapped onto the polymeric chains [52]. However, P(BPBC-co-CDT) had a slower coloration time (2.5 s at 780 nm) compared to its bleaching time (1.5 s at 780 nm), inferring that a high planar CDT ring in the polymer backbone gave rise to a slower coloring velocity.  The transmittance changes between the bleached and colored states of PBPBC, P(BPBCco-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were 29.6% at 1040 nm, 44.4% at 1030 nm, 22.3% at 1050 nm, and 41.4% at 1070 nm at the second cycle. The ∆T of P(BPBCco-BT) and P(BPBC-co-CDTK) films in near-infrared spectral zone were larger than that of PBPBC, inferring BT-and CDTK-containing copolymers presented higher ∆T in the near-infrared region than that of PBPBC homopolymer. The ∆T of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) in the near infrared (NIR) region was larger than those in the visible region, which could be ascribed to the emergence of a significant polaron and bipolaron upon oxidizing. Table 4 lists the comparison of ∆T with the reported polymers in solutions. P(BPBC-co-BT) displayed a higher ∆T than those reported for P(DiCP-co-CPTK2) at 890 nm [53], PITD-2 at 675 nm [54], and P2 at 779 nm [55]. However, the ∆T of P(BPBC-co-BT) was lower than those of DPPA-2SNS at 900 nm [56] and PI-6A at 573 nm [57].

Kinetics Studies of Polymeric Coloring and Bleaching
The optical density (∆OD) of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBCco-CDTK) at visible and NIR regions can be estimated using the following equation [58]: where T red and T ox indicate the transmittance of electrodes in the reduced and the oxidized state, respectively. Similar to the trend of ∆T, the PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) showed higher ∆OD in near-infrared spectral regions than those in visible regions. Another crucial criterion of electrochromism is the coloration efficiency (η), which can be calculated from the following equation [61]: where Q d stands for the injected/ejected charge as a function of electrode active area. As presented in Table 3, the η of PBPBC, P(BPBC-co-BT), P(BPBC-co-CDT), and P(BPBC-co-CDTK) films were evaluated to be 140.3 cm 2 C −1 at 1040 nm, 130.0 cm 2 C −1 at 1030 nm, 121.7 cm 2 C −1 at 1050 nm, and 283.7 cm 2 C −1 at 1070 nm, respectively. As shown in Table 4, P(BPBC-co-BT) displayed a larger η value than those reported for P(DiCP-co-CPTK2) at 890 nm [53], PITD-2 at 675 nm [54], P2 at 779 nm [55], and PI-6A at 573 nm [57], whereas the η of P(BPBC-co-BT) was comparable to that reported for DPPA-2SNS at 900 nm [56]. Figure 9 shows the photoluminescence (PL) spectra of as-prepared polymeric films. The emission peaks of P(BPBC-co-BT) and P(BPBC-co-CDT) exhibited larger PL maxima than that of PBPBC, implying the incorporation of dithiophene derivatives (BT and CDT) in the polymer backbone gave rise to a bathochromic shift in PL spectra. However, P(BPBCco-CDTK) displayed a hypsochromic shift in the PL maximum with respect to those of P(BPBC-co-BT) and P(BPBC-co-CDT), which may be ascribed to the electron-withdrawing ketone groups of P(BPBC-co-CDTK) changing the electronic distribution and decreasing π-π stacking or the planarity of the polymer backbone.
When the applied voltage was increased gradually, the anodic layers began to oxidize, and the cathodic layers began to reduce. Therefore, absorption bands of ECDs began to turn up at 610-630 nm, and the noticeable color of the four ECDs was blue at high potentials as shown in Table 5. Table 5 also shows the CIE diagrams of the four ECDs at bleached and colored states. Table 5. Electrochromic photographs, colorimetric values (L*, a*, and b*), and CIE chromaticity values (x, y) of PBPBC/PEDOT, P(BPBC-co-BTP)/PEDOT, P(BPBC-co-CDT)/PEDOT, and P(BPBC-co-CDTK)/PEDOT ECDs at several potentials.