Applications of Three Dithienylpyrroles-Based Electrochromic Polymers in High-Contrast Electrochromic Devices

Three dithienylpyrroles (1-(4-(methylthio)phenyl)-2,5-di(thiophen-2-yl)-pyrrole (MPS), 1-(4-methoxyphenyl)-2,5-di(thiophen-2-yl)-pyrrole (MPO), and 4-(2,5-di(thiophen-2-yl)-pyrrol-1-yl)benzonitrile (ANIL)) were synthesized and their corresponding polydithienylpyrroles (PSNS) were electrosynthesized using electrochemical polymerization. Spectroelectrochemical studies indicated that poly(1-(4-(methylthio)phenyl)-2,5-di(thiophen-2-yl)-pyrrole) (PMPS) film was green, dark green, and brown in the neutral, oxidation, and highly oxidized state, respectively. The incorporation of a MPS unit into the PSNS backbone gave rise to a darker color than those of the MPO and ANIL units in the highly oxidized state. The PMPS film showed higher ΔTmax (54.47% at 940 nm) than those of the PMPO (43.87% at 890 nm) and PANIL (44.63% at 950 nm) films in an ionic liquid solution. Electrochromic devices (ECDs) employing PMPS, PMPO, and PANIL as anodic layers and poly(3,4-(2,2-diethypropylenedioxy)thiophene)(PProDOT-Et2) as a cathodic layer were constructed. PMPO/PProDOT-Et2 ECD showed the highest ΔTmax (41.13%) and coloration efficiency (674.67 cm2·C−1) at 626 nm, whereas PMPS/PProDOT-Et2 ECD displayed satisfactory ΔTmax (32.51%) and coloration efficiency (637.25 cm2·C−1) at 590 nm. Repeated cyclic voltammograms of PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs indicated that ECDs had satisfactory redox stability.

ECDs were built using PMPS, PMPO, or PANIL as the anodically coloring material, PProDOT-Et2 as the cathodically coloring material, and PVdF-HFP/ionic liquid composite membranes as electrolytes. PMPS, PMPO, and PANIL films were electrodeposited potentiostatically onto ITO-coated glasses at +0.9 V, and PProDOT-Et2 were electrodeposited onto ITO-coated glasses at +1.4 V. Film thicknesses of the deposited anodic and cathodic layers were obtained with an Alpha-Step profilometer (KLA Tencor D-120, KLA-Tencor, Milpitas, CA, USA). The approximate average thicknesses of anodic and cathodic layers are 100-105 nm. ECDs were assembled by anodic and cathodic polymers facing each other and were separated by PVdF-HFP/ionic liquid composite membranes.

Electrochemical Polymerizations of Anodic Polymer Films
The
ECDs were built using PMPS, PMPO, or PANIL as the anodically coloring material, PProDOT-Et 2 as the cathodically coloring material, and PVdF-HFP/ionic liquid composite membranes as electrolytes. PMPS, PMPO, and PANIL films were electrodeposited potentiostatically onto ITO-coated glasses at +0.9 V, and PProDOT-Et 2 were electrodeposited onto ITO-coated glasses at +1.4 V. Film thicknesses of the deposited anodic and cathodic layers were obtained with an Alpha-Step profilometer (KLA Tencor D-120, KLA-Tencor, Milpitas, CA, USA). The approximate average thicknesses of anodic and cathodic layers are 100-105 nm. ECDs were assembled by anodic and cathodic polymers facing each other and were separated by PVdF-HFP/ionic liquid composite membranes.

Electrochemical Polymerizations of Anodic Polymer Films
The cyclic voltammogram (CV) curves of MPS, MPO, and ANIL in EtOH/EA (1:1, by volume) solution containing 0.1 M LiClO 4 are shown in Figure 2, after scanning the potentials between −0.4 Polymers 2017, 9, 114 4 of 18 and 1.4 V at a scan rate of 100 mV·s −1 continuously for 20 cycles. PMPS, PMPO, and PANIL were electrodeposited onto the surface of the ITO working electrode, and the synthetic routes of PMPS, PMPO, and PANIL are displayed in Figure 1. The onset potentials of MPS, MPO, and ANIL are 0.7, 0.69, and 0.81 V, respectively. The onset potential of MPS is comparable to MPO, implying the incorporation of the methylthio-phenyl unit on the nitrogen atom of the pyrrole ring that shows a similar electron donating property to that of the methoxyphenyl unit. However, the incorporation of the benzonitrile unit on the nitrogen atom of the pyrrole ring shows a larger onset potential than those of the methylthio-phenyl and methoxyphenyl units, implying the incorporation of an electron withdrawing benzonitrile unit that increases the onset potential significantly. The oxidation peaks of PMPS, PMPO, and PANIL are located at 0.95, 0.9, and 1.0 V, respectively, whereas the reduction peaks of PMPS, PMPO, and PANIL appear at 0.5, 0.55, and 0.6 V, respectively.
Polymers 2017, 9,114 4 of 18 electrodeposited onto the surface of the ITO working electrode, and the synthetic routes of PMPS, PMPO, and PANIL are displayed in Figure 1. The onset potentials of MPS, MPO, and ANIL are 0.7, 0.69, and 0.81 V, respectively. The onset potential of MPS is comparable to MPO, implying the incorporation of the methylthio-phenyl unit on the nitrogen atom of the pyrrole ring that shows a similar electron donating property to that of the methoxyphenyl unit. However, the incorporation of the benzonitrile unit on the nitrogen atom of the pyrrole ring shows a larger onset potential than those of the methylthio-phenyl and methoxyphenyl units, implying the incorporation of an electron withdrawing benzonitrile unit that increases the onset potential significantly. The oxidation peaks of PMPS, PMPO, and PANIL are located at 0.95, 0.9, and 1.0 V, respectively, whereas the reduction peaks of PMPS, PMPO, and PANIL appear at 0.5, 0.55, and 0.6 V, respectively. Figure 3a-c shows the relationship of the peak current vs. scan rate of PMPS, PMPO, and PANIL films in a 0.1 M LiClO4/EtOH solution at scanning rates between 25 and 250 mV·s −1 . The scan rate dependence of the anodic and cathodic peak current densities shows a linear dependence on the scan rate as depicted in Figure 3d-f, indicating that the redox processes are not diffusion controlled and that the electroactive polymer films are well-adhered on the ITO-coated electrode surface [33].  The scan rate dependence of the anodic and cathodic peak current densities shows a linear dependence on the scan rate as depicted in Figure 3d-f, indicating that the redox processes are not diffusion controlled and that the electroactive polymer films are well-adhered on the ITO-coated electrode surface [33].

Electrochromic Properties of PMPS, PMPO, and PANIL Films
The absorption spectra of the PMPS, PMPO, and PANIL films coated on an ITO/glass electrode were investigated between −0.4 and +1.6 V in [EPI + ][TFSI − ] solution. As shown in Figure 4b, the PMPO film shows an evident π-π* transition peak at around 421 nm. However, the PMPS film shows a shoulder at about 440 nm ( Figure 4a); the incorporation of a methylthio group into the polymer backbone causes bathochromic shifts in the absorption band. On the other hand, the incorporation of an electron withdrawing benzonitrile unit into the PSNS backbone deactivates the phenyl unit on the pyrrole ring of PANIL, and the π-π* transition of the PANIL film in [EPI + ][TFSI − ] solution shifts hypsochromically to 360 nm.
Upon applying a potential of +0.8 V (vs. Ag/AgCl), the shoulder of the PMPS film at around 440 nm and the absorption peak of the PANIL film at around 360 nm decrease gradually, and charge carrier bands emerge at around 600-1000 nm. Table 1 shows the photos of PMPS, PMPO,

Electrochromic Properties of PMPS, PMPO, and PANIL Films
The absorption spectra of the PMPS, PMPO, and PANIL films coated on an ITO/glass electrode were investigated between −0.4 and +1.6 V in [EPI + ][TFSI − ] solution. As shown in Figure 4b, the PMPO film shows an evident π-π* transition peak at around 421 nm. However, the PMPS film shows a shoulder at about 440 nm ( Figure 4a); the incorporation of a methylthio group into the polymer backbone causes bathochromic shifts in the absorption band. On the other hand, the incorporation of an electron withdrawing benzonitrile unit into the PSNS backbone deactivates the phenyl unit on the pyrrole ring of PANIL, and the π-π* transition of the PANIL film in [EPI + ][TFSI − ] solution shifts hypsochromically to 360 nm.
Upon applying a potential of +0.8 V (vs. Ag/AgCl), the shoulder of the PMPS film at around 440 nm and the absorption peak of the PANIL film at around 360 nm decrease gradually, and charge carrier bands emerge at around 600-1000 nm. Table 1 shows the photos of PMPS, PMPO, and PANIL in the [EPI + ][TFSI − ] solution at various potentials. The PMPS film was green (0 V) in the neutral state, and PANIL in the [EPI + ][TFSI − ] solution at various potentials. The PMPS film was green (0 V) in the neutral state, dark green (1.2 V) in the oxidation state, and brown (1.6 V) in the highly oxidized state. The PMPO and PANIL films were light green (0 V) in their neutral state, whereas the PMPO and PANIL films were blue (1.6 V) and grey (1.6 V), respectively, in the highly oxidized state. The incorporation of an MPS unit into the PSNS backbone gives rise to darker color than those of the MPO and ANIL units.      The PMPO and PANIL films were light green (0 V) in their neutral state, whereas the PMPO and PANIL films were blue (1.6 V) and grey (1.6 V), respectively, in the highly oxidized state. The incorporation of an MPS unit into the PSNS backbone gives rise to darker color than those of the MPO and ANIL units.   The PMPO and PANIL films were light green (0 V) in their neutral state, whereas the PMPO and PANIL films were blue (1.6 V) and grey (1.6 V), respectively, in the highly oxidized state. The incorporation of an MPS unit into the PSNS backbone gives rise to darker color than those of the MPO and ANIL units.

Polymer Films and ECDs Reduction (0 V) Oxidation (+1.6 V)
PANIL/PProDOT-Et2 The CIE (Commission Internationale de I'Eclairage) chromaticity diagrams of the PMPS, PMPO, and PANIL films in neutral and oxidation states are shown in Figure 5, and the colorimetric values (L, a, b, L*, a*, and b*) and CIE chromaticity values (x, y) of the three polymer films at various potentials in the [EPI + ][TFSI − ] solution are summarized in Table 2. The b* of the PMPO film was negative between 1.2 and 1.8 V, demonstrating that the PMPO film was blue (1.6 V) in the highly oxidized state. The optical band gap (Eg) of PMPS, PMPO, and PANIL can be calculated according to the Planck equation [34], where λonset is the wavelength at which the onset of absorption occurs. The Eg of PMPS, PMPO, and PANIL were 2.25, 2.17, and 2.21 eV, respectively. The optical band gap (E g ) of PMPS, PMPO, and PANIL can be calculated according to the Planck equation [34], E g = 1241/λ onset (1) where λ onset is the wavelength at which the onset of absorption occurs. The E g of PMPS, PMPO, and PANIL were 2.25, 2.17, and 2.21 eV, respectively. The incorporation of methoxyphenyl into the PSNS backbone showed a lower E g than those of the methylthio-phenyl and benzonitrile units. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of PMPS, PMPO, and PANIL were determined using cyclic voltammetry. The E HOMO was calculated from E onset using the formula [35], where E onset is the onset potential of oxidation. E LUMO of the polymers was calculated using the formula, The HOMO energy level of PMPS, PMPO, and PANIL are −4.90, −4.88, and −5.00 eV, respectively, and the LUMO energy level of PMPS, PMPO, and PANIL are −2.65, −2.71, and −2.79 eV, respectively. The PANIL film shows a lower LUMO energy level than those of the PMPS and PMPO films, and this can be attributed to the incorporation of an electron withdrawing cyano group in the ANIL unit that decreases the LUMO energy level significantly.
A square-wave cyclic potential step method accompanied by UV-Vis spectroscopy was used to determine the optical contrast and switching time of the PMPS, PMPO, and PANIL films. The polymer films were immersed in [EPI + ][TFSI − ] solution and stepped by repeated potential between neutral and oxidation states with a time interval of 5 s. Figure 6 exhibits the transmittance-time profiles of the PMPS film at 600 and 940 nm, the PMPO film at 584 and 950 nm, and the PANIL film at 566 and 950 nm. The coloration switching time (τ c ) and bleaching switching time (τ b ) of the PMPS, PMPO, and PANIL films in the [EPI + ][TFSI − ] solution are summarized in Table 3. The optical switching time (T 95% ) of the PMPS film is 2.21 and 1.97 s at 600 and 940 nm, respectively, from the bleaching state to the coloring state at the 100th cycle, and 1.93 and 2.01 s at 600 and 940 nm, respectively, from the coloring state to the bleaching state at the 100th cycle. The coloration efficiency (CE) is also a useful parameter in electrochromic applications. CE can be calculated using the following equations at a specific wavelength [37]: where ∆OD represents the variation of the optical density at a specific wavelength.

Spectroelectrochemistry of ECDs
Dual-type ECDs composed of two electrochromic electrodes, one anodically coloring layer (PMPS, PMPO, or PANIL) and the other cathodically coloring material (PProDOT-Et 2 ), were facing each other and were separated by an electrolyte membrane. Figure 7a shows the spectroelectrochemical spectra of the PMPS/PProDOT-Et 2 ECD at potentials between −0.4 V and +1.6 V. PMPS/PProDOT-Et 2 ECD shows a peak at around 380 nm and a shoulder at around 430 nm at 0 V, and this can be attributed to the π-π* transition peak of the PMPS film in the neutral state. In this situation, PProDOT-Et 2 was light blue in its oxidation state, and the PMPS/PProDOT-Et 2 ECD was greyish-green at 0 V. However, the absorption of the π-π* transition peak for the PMPS film lessened and a new peak at 590 nm emerged at +1.6 V, and the PMPS/PProDOT-Et 2 ECD was cyan at +1.6 V. Under similar conditions, the PMPO/PProDOT-Et 2 ECD was light green at −0.4 V, bluish-grey at 0.6 V, light blue at 0.8 V, and blue at 1.6 V. The PANIL/PProDOT-Et 2 ECD was grey at −0.4 V, light blue at 0.8 V, and blue at 1.6 V. The CIE chromaticity values (x, y) and colorimetric values (L, a, b, L*, a*, b*) of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 dual type ECDs are summarized in Table 4. Moreover, the CIE chromaticity diagrams of the PMPS/PProDOT-Et 2 ECD at −0.6 and 1.6 V, PMPO/PProDOT-Et 2 ECD at −0.4 and 1.8 V, and PANIL/PProDOT-Et 2 ECD at −0.4 and 1.8 V are displayed in Figure 8.

Spectroelectrochemistry of ECDs
Dual-type ECDs composed of two electrochromic electrodes, one anodically coloring layer (PMPS, PMPO, or PANIL) and the other cathodically coloring material (PProDOT-Et2), were facing each other and were separated by an electrolyte membrane. Figure 7a shows the spectroelectrochemical spectra of the PMPS/PProDOT-Et2 ECD at potentials between −0.4 V and +1.6 V. PMPS/PProDOT-Et2 ECD shows a peak at around 380 nm and a shoulder at around 430 nm at 0 V, and this can be attributed to the π-π* transition peak of the PMPS film in the neutral state. In this situation, PProDOT-Et2 was light blue in its oxidation state, and the PMPS/PProDOT-Et2 ECD was greyish-green at 0 V. However, the absorption of the π-π* transition peak for the PMPS film lessened and a new peak at 590 nm emerged at +1.6 V, and the PMPS/PProDOT-Et2 ECD was cyan at +1.6 V. Under similar conditions, the PMPO/PProDOT-Et2 ECD was light green at −0.4 V, bluish-grey at 0.6 V, light blue at 0.8 V, and blue at 1.6 V. The PANIL/PProDOT-Et2 ECD was grey at −0.4 V, light blue at 0.8 V, and blue at 1.6 V. The CIE chromaticity values (x, y) and colorimetric values (L, a, b, L*, a*, b*) of the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 dual type ECDs are summarized in Table 4. Moreover, the CIE chromaticity diagrams of the PMPS/PProDOT-Et2 ECD at −0.6 and 1.6 V, PMPO/PProDOT-Et2 ECD at −0.4 and 1.8 V, and PANIL/PProDOT-Et2 ECD at −0.4 and 1.8 V are displayed in Figure 8.      The transmittance-time profiles of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs are shown in Figure 9. The ∆T max % of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs were 33% at 590 nm, 41% at 626 nm, and 25% at 628 nm, respectively. The η of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs, calculated from Equations (1) and (2), were found to be 637.25 cm 2 ·C −1 at 590 nm, 674.67 cm 2 ·C −1 at 626 nm, and 401.63 cm 2 ·C −1 at 628 nm, respectively. The PMPS/PProDOT-Et 2 and PMPO/PProDOT-Et 2 ECDs showed higher ∆T max % and η than those of the PANIL/PProDOT-Et 2 ECDs, indicating that the incorporations of the methoxyphenyl-and methylthiophenyl-substituted PSNS into the ECDs gave rise to higher ∆T max % and η than those of the benzonitrile-substituted PSNS. The ∆T max , ∆OD, and η max of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs are summarized in Table 5. The τ c and τ b estimated at various double-step potential cycles are listed in Table 3, and the T 95% of the PMPS/PProDOT-Et 2 ECD at 590 nm was estimated to be 0.99 s from the bleaching state to the coloring state and 1.01 s from the coloring state to the bleaching state at the 100th cycle. Under similar conditions, the T 95% of the PMPO/PProDOT-Et 2 ECD at 626 nm was estimated to be 1.42 s from the bleaching state to the coloring state and 1.12 s from the coloring state to the bleaching state at the 100th cycle, and the T 95% of the PANIL/PProDOT-Et 2 ECD at 628 nm was estimated to be 1.17 s from the bleaching state to the coloring state and 1.06 s from the coloring state to the bleaching state. The PMPS/PProDOT-Et 2 ECD shows shorter τ c than those of the PMPO/PProDOT-Et 2 and PANIL/PProDOT-Et 2 ECDs at the 100th cycle, implying that the PMPS/PProDOT-Et 2 ECD changes color faster from the bleaching state to the coloring state than those of the PMPO/PProDOT-Et 2 and PANIL/PProDOT-Et 2 ECDs. The long-term switching stability of the ECDs between the bleaching and coloring states is an important parameter in practical applications of ECDs [38,39]. The cycling stability of the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs were measured using CV at potentials between −0.4 and +1.4 V with a scan rate of 100 mV·s −1 . As shown in Figure 10, 94%, 91%, and 90% of the electrical activity was retained after 500 cycles for the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs, respectively, and 91%, 89%, and 87% of the electrical activity was retained after 1000 cycles for the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs, respectively, indicating that the PMPS/PProDOT-Et 2 , PMPO/PProDOT-Et 2 , and PANIL/PProDOT-Et 2 ECDs displayed reasonable long-term cycling stability.  The long-term switching stability of the ECDs between the bleaching and coloring states is an important parameter in practical applications of ECDs [38,39]. The cycling stability of the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs were measured using CV at potentials between −0.4 and +1.4 V with a scan rate of 100 mV·s −1 . As shown in Figure 10, 94%, 91%, and 90% of the electrical activity was retained after 500 cycles for the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs, respectively, and 91%, 89%, and 87% of the electrical activity was retained after 1000 cycles for the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs, respectively, indicating that the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs displayed reasonable long-term cycling stability.  The optical memory effect is also important for ECD applications [40]. The optical memory of the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs was evaluated at 590, 626, and 628 nm, respectively, with the function of time at −0.4 V and +1.0 V by applying a potential for 1 s for each 200 s time interval. As shown in Figure 11a-c, the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs showed good optical memories in a reduced state of the PMPS, PMPO, and PANIL films, and the transmittance change of the PMPS, PMPO, and PANIL films is less than 1% in their reduced states. However, in the oxidized state of the PMPS, PMPO, and PANIL films and in the reduced state of the PProDOT-Et2 film, the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs are less stable than the oxidized state of the PProDOT-Et2 film, but the transmittance change is less than 3% in the oxidized state of the PMPS, PMPO, and PANIL films, demonstrating that the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs show reasonable optical memory in the coloring and bleaching states. The optical memory effect is also important for ECD applications [40]. The optical memory of the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs was evaluated at 590, 626, and 628 nm, respectively, with the function of time at −0.4 V and +1.0 V by applying a potential for 1 s for each 200 s time interval. As shown in Figure 11a-c, the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs showed good optical memories in a reduced state of the PMPS, PMPO, and PANIL films, and the transmittance change of the PMPS, PMPO, and PANIL films is less than 1% in their reduced states. However, in the oxidized state of the PMPS, PMPO, and PANIL films and in the reduced state of the PProDOT-Et2 film, the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs are less stable than the oxidized state of the PProDOT-Et2 film, but the transmittance change is less than 3% in the oxidized state of the PMPS, PMPO, and PANIL films, demonstrating that the PMPS/PProDOT-Et2, PMPO/PProDOT-Et2, and PANIL/PProDOT-Et2 ECDs show reasonable optical memory in the coloring and bleaching states.