materials

Article Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens

Qun Zhang 1, Li Yuan 1, Fanglan Guan 1, Xin Li 1,* , Rui Wang 1, Jian Xu 2, Yanyan Qin 3 and Guangming Chen 3,*

1 Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing Engineering Research Center of Textile Nanofiber, School of Materials design & Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China; [email protected] (Q.Z.); [email protected] (L.Y.); clygfl@bift.edu.cn (F.G.); [email protected] (R.W.) 2 College of Chemical and Environmental Engineering, Low-dimensional Materials Genome Initiative, Shenzhen University, Shenzhen 518055, China; [email protected] 3 College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China; [email protected] * Correspondence: [email protected] (X.L.); [email protected] (G.C.)

Abstract: As a promising electrochromic material, viologens have attracted increasing attention due to their high redox activity and adjustable electrochromic capability. In order to investigate the effect of alkyl substituents on electrochromic behavior, four alkyl-substituted viologens and a benzyl-substituted viologen were synthesized, namely 1,10-dioctyl-4,40-bipyridinium dibromide (OV), 1,10-didekyl-4,40-bipyridinium dibromide (DeV), 1,10-didodecyl-4,40-bipyridinium dibromide (DoV), 1,10-dihexadecyl-4,40-bipyridinium dibromide (HV), and 1,10-dibenzyl-4,40-bipyridinium dibromide (BV). The different photophysical and electrochemical properties of these viologens were attributed



 to their deviation in spatial structure caused by different substituents. Compared with benzyl- substituted BV, a slight blueshift occurred for the absorption peaks of alkyl-substituted viologens Citation: Zhang, Q.; Yuan, L.; Guan, F.; Li, X.; Wang, R.; Xu, J.; Qin, Y.; from 262 to 257 nm with the increase in alkyl chain length. Moreover, the first redox couple increased Chen, G. Substituent-Adjusted positively, and the dimerization of the compound decreased gradually, accompanied by the decrease Electrochromic Behavior of in optical contrast and distinct chromatic difference. A comparison of chromatic and optical contrasts Symmetric Viologens. Materials 2021, indicated that OV had the longest coloring response time (RTc), while it was shortest for HV. The 14, 1702. https://doi.org/10.3390/ bleaching response time (RTb) of viologen films gradually decreased with the alkyl chain length, and ma14071702 the OV film had the shortest RTb. Furthermore, when increasing the length of the alkyl chain, the cycling stabilities of alkyl viologens increased gradually. In addition, the OV film exhibited the best Academic Editor: Ya-Ju Lee contrast after 200 continuous cycles.

Received: 5 February 2021 Keywords: 4,40-bipyridine; symmetric viologens; electrochromic; substituent-adjusted Accepted: 25 March 2021 Published: 30 March 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Electrochromic (EC) materials have attracted growing interest for researchers due iations. to their extensive applications in displays [1–3], smart windows [4–8], anti-dazzling mir- rors [9–11], and optical sensors [12–14]. By applying certain voltages, the optical properties of these materials can be reversibly regulated in the visible and the near-infrared regions. Nowadays, research on electrochromic materials has focused on inorganic components such as traditional metal oxides [15–20]. Although different strategies have recently been Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. published to increase the coloring efficiency and duration [21,22], inorganic electrochromic This article is an open access article materials still have the challenges of slow switching time and monotonous color. Organic distributed under the terms and electrochromic materials [23–30] have also attracted great interest due to their excellent conditions of the Creative Commons properties, such as easy structure modification, rich redox states, wide voltage window, Attribution (CC BY) license (https:// and high energy density. creativecommons.org/licenses/by/ Viologen-based electrochromic materials, including small molecules, polymers, and 4.0/). composite materials, have been extensively studied [31–37]. As a type of quaternary

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Materials 2021, 14, 1702 2 of 13 pyridinium salt, the conjugated bi-pyridyl groups possess excellent electrochromic be- havior [38]. Viologens can undergo two steps of reversible reductions that exhibit three redox states [39,40], namely, di-cation (V2+), radical-cation (V+·), and neutral species (V0) pyridinium salt, the conjugatedunits. Divalent bi-pyridyl cation groupsV2+ is the possess most excellentstable form electrochromic and is colorless, be- while V0 is shallow havior [38]. Viologens can undergo two steps of reversible reductions that exhibit three yellow. Radical cations V+· are intensely colored with high molar absorption coefficients, redox states [39,40], namely, di-cation (V2+), radical-cation (V+·), and neutral species (V0) owing to the charge transfer between the +1 and 0 valent nitrogen. Moon’s study [41] units. Divalent cation V2+ is the most stable form and is colorless, while V0 is shallow showed a significantly different EC behavior between monoheptyl viologen (MHV+) and yellow. Radical cations V+· are intensely colored with high molar absorption coefficients, diheptyl viologen (DHV2+) in flexible electrochromic devices despite having similar owing to the charge transfer between the +1 and 0 valent nitrogen. Moon’s study [41] chemical structures. The color and spectral characteristics of small molecular viologens showed a significantly different EC behavior between monoheptyl viologen (MHV+) and can be modulated by structures such as the substituents on the nitrogen atoms. However, diheptyl viologen (DHV2+) in flexible electrochromic devices despite having similar chemi- the correlation between the substituent structures and EC characterization of viologens cal structures. The color and spectral characteristics of small molecular viologens can be has not been systematically investigated. modulated by structures such as the substituents on the nitrogen atoms. However, the Herein, four alkyl viologens have been designed and synthesized, namely correlation between the substituent structures and EC characterization of viologens has not 1,1′-dioctyl- 4,4′-bipyridinium dibromide (OV), 1,1′-didecyl-4,4′-bipyridinium dibromide been systematically investigated. (DeV), 1,1′-didohexadecyl-4,4′-bipyridinium dibromide (DoV), and Herein, four alkyl viologens have been designed and synthesized, namely 1,10-dioctyl- 4,40-bipyridinium dibromide1′-dihexadecyl-4,4 (OV), 1,10-didecyl-4,4′-bipyridinium0-bipyridinium dibromide dibromide(HV). The (DeV),effect of 1,1 the0- alkyl chain on their didohexadecyl-4,40-bipyridiniumelectrochromic dibromide behaviors (DoV), has and been 10-dihexadecyl-4,4 studied. For0 -bipyridiniumcomparison, a benzyl viologen, dibromide (HV). The effect1,1′-dibenzyl-4,4 of the alkyl′-bipyridinium chain on their dibromide electrochromic (BV), behaviors was also hassynthesized been to obtain further studied. For comparison,insight a benzyl into the viologen, different 1,1 influences0-dibenzyl-4,4 between0-bipyridinium alkyl and dibromidearomatic substituents. The pho- (BV), was also synthesizedtophysical, to obtain electrochemical, further insight spectroelectroch into the differentemical, influences and EC between properties of the viologens alkyl and aromatic substituents.have also been The photophysical,systematically investigated. electrochemical, spectroelectrochemi- cal, and EC properties of the viologens have also been systematically investigated. 2. Experimental 2. Experimental 2.1. Materials and Reagents 2.1. Materials and Reagents 4,4′-bipyridine, 1-bromooctane, 1-bromodecane, 1-bromododecane, 0 4,4 -bipyridine, 1-bromooctane,1-bromohexadecane, 1-bromodecane, and benzyl 1-bromododecane, bromide were 1-bromohexadecane, purchased from J&K Scientific Ltd (Bei- and benzyl bromide werejing, purchasedChina). Potassium from J&K chloride, Scientific DMF, Ltd (Beijing, acetonitrile, China). and Potassium ethanol were provided by Bei- chloride, DMF, acetonitrile,jing Chemical and ethanol Plant were (Beijing, provided China). by Beijing All reag Chemicalents were Plant of analytical (Beijing, grade and were used China). All reagents werewithout of analytical further grade purification. and were The used water without used further in the purification.experiments The was deionized. water used in the experiments was deionized. 2.2. Synthesis Procedure and Structure Characterizations 2.2. Synthesis Procedure and Structure Characterizations Scheme 1 showed the preparation route of substituted viologens. The pyridine Scheme1 showed the preparation route of substituted viologens. The pyridine pre- precursors with benzyl and different lengths of alkyl groups were synthesized by cou- cursors with benzyl and different lengths of alkyl groups were synthesized by coupling pling reactions. The obtained viologen derivatives were characterized by 1H NMR reactions. The obtained viologen derivatives were characterized by 1H NMR (Brucker (Brucker AVANCE II-500, Brucker, Germany) and FT-IR (Nicolet Nexus 670, Thermo AVANCE II-500, Brucker, Germany) and FT-IR (Nicolet Nexus 670, Thermo Nicolet Corpo- Nicolet Corporation, Madison, WI, USA), and were found to be consistent with the pro- ration, Madison, WI, USA), and were found to be consistent with the proposed structures. posed structures.

Compounds n OV 7 DeV 9 DoV 11 HV 15

SchemeScheme 1. Synthesis 1. Synthesis route route of the of theviologen viologen compounds compounds with with different different alkyl alkyl chains chains (n = (n 7, = 9, 7, 11, 9, 11,15). 15). 0 0 2.3. Synthesis of 1,1 -Dioctyl-4,42.3. Synthesis-Bipyridinium of 1,1′-Dioctyl-4,4 Dibromide′-Bipyridinium (OV) Dibromide (OV) 4,4’-bipyridine (1 mmol) and 1-bromooctane (4 mmol) were added into a 100 mL round-bottomed flask. Under a nitrogen atmosphere, 50 mL of DMF was injected into the flask. The reaction system was stirred at 60 ◦C for 72 h. After cooling to room temperature, the residue was filtered and washed with overdose ether, and it was then recrystallized in Materials 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/materials Materials 2021, 14, 1702 3 of 13

acetonitrile to give pure 1,10-dioctyl-4,40-bipyridinium dibromides (OV) in a 73% yield with 1 a yellowish brown color. H NMR (500 MHz, CH3OD) (ppm): 9.31 (d, 4H), 8.70(d, 4H), 4.76(t, 4H), 2.12 (d, 4H), 1.25-1.51 (m, 20H), 0.92 (t, 6H). IR (KBr): v 2918 cm−1, 2841 cm−1, 1663 cm−1, 1465 cm−1, 1220 cm−1, 719 cm−1.

2.4. Synthesis of 1,10-Didecyl-4,40-Bipyridinium Dibromide (DeV) Similarly to the procedure of OV, DeV was synthesized from 1-bromodecane, and a 1 yellow substance in 75% yield was obtained. H NMR (500 MHz, CH3OD) (ppm): 9.30 (d, 4H), 8.70 (d, 4H), 4.75 (t, 4H), 2.11 (d, 4H), 1.25-1.51 (m, 28H), 0.92 (t, 6H). IR (KBr): v 2918 cm−1, 2841 cm−1, 1663 cm−1, 1465 cm−1, 1224 cm−1, 719 cm−1.

2.5. Synthesis of 1,10-Didodecyl-4,40-Bipyridinium Dibromide (DoV) Similarly to the procedure of OV, DoV was synthesized from 1-bromododecane in 1 a 77% yield with a yellow color. H NMR (500 MHz, CH3OD) (ppm): 9.31 (d, 4H), 8.70 (d, 4H), 4.75 (t, 4H), 2.12 (d, 4H), 1.25–1.51 (m, 36H), 0.92 (t, 6H).IR (KBr): v 2919 cm−1, 2840 cm−1, 1663 cm−1, 1465 cm−1, 1224 cm−1, 719 cm−1.

2.6. Synthesis of 1,10-Hexadecyl-4,40-Bipyridinium Dibromide (HV) Similarly to the procedure of OV, HV with the yellow color was synthesized from 1 1-bromohexadecane in a 79% yield. H NMR (500 MHz, CH3OD) (ppm): 9.30 (d, 4H), 8.70(d, 4H), 4.78 (t, 4H), 2.10 (d, 4H), 1.25-1.50 (m, 52H), 0.91 (t, 6H). IR (KBr): v 2919 cm−1, 2841 cm−1, 1663 cm−1, 1465 cm−1, 1221 cm−1, 719 cm−1.

2.7. Synthesis of 1,10-Dibenzyl-4,40-Bipyridinium Dibromide (BV) Similarly to the procedure of OV, BV with a bright yellow color was synthesized from 1 benzyl bromide in a 77% yield. H NMR (500 MHz, CH3OD) (ppm): 9.30 (d, 4H), 8.70 (d, 4H), 7.61 (m, 4H), 7.52 (m, 6H), 4.78 (t, 4H). IR (KBr): v 2991 cm−1, 2941 cm−1, 2841 cm−1, 1663 cm−1, 1443 cm−1, 1219 cm−1, 743 cm−1, 696 cm−1.

2.8. Performance Measurements Photophysical measurements were recorded on a Perkin Elmer Lambda 750 UV- Visible spectrophotometer (Perkin Elmer, Akron, OH, America). The redox behavior of the viologens were examined with cyclic voltammetry (CV), which was performed with an electrochemical testing system (Zahner IM6, Zahner, Germany) at a sweep rate of 50 mV/s at voltages from 0 to −1.2 V, and under a square wave step potential from 0 to −0.7 V each for 15 s. The test system was characterized by the three-electrode system in the presence of 0.1 M of KCl solution. Although the pH of KCl is near-neutral, it is a strong electrolyte that has been used in many studies [42]. Platinum wire and saturated calomel were used as the counter and reference, respectively. The viologen-coated indium tin oxide (ITO) glass slide with a viologen film thickness of about 2 µm and an area of ca. 0.8 × 2 cm2 was used as the working electrode. The multipotential UV spectra were determined by a UV-visible spectrophotometer linked with a Zahner IM6 workstation. The color difference of the film was analyzed using the SC-80C automatic colorimeter of Beijing Kangguang Instrument Co., Ltd (Beijing, China). All data were directly obtained from the instrument, and the data error was basically the instrument error.

3. Results and Discussion 3.1. Photophysical Properties The photophysical properties of the substituted viologens were investigated in 0.05 mmol/L of KCl solution by UV-vis absorption spectroscopy, as shown in Figure1. All substituted viologens exhibited the same absorption bands at 195 nm, which were attributed to the n→δ* transition absorption of N atoms on the pyridine ring [43]. With the increase in alkyl chain length, the maximum absorption wavelength of four alkyl viologens blueshifted from 262 nm to 257 nm. OV exhibited a maximum absorption Materials 2021, 14, x FOR PEER REVIEW 4 of 13

Materials 2021, 14, 1702 4 of 13 tributed to the n→δ* transition absorption of N atoms on the pyridine ring [43]. With the increase in alkyl chain length, the maximum absorption wavelength of four alkyl vio- logens blueshifted from 262 to 257 nm. OV exhibited a maximum absorption wavelength wavelengthat 262 nm while at 262 HV nm showed while HVa minimum showed absorption a minimum wavelength absorption at wavelength 257 nm. These at 257 results nm. Theseindicate resultsd that indicatedthe absorption thatthe peak absorption of the B band peak in of the the π B-π* band transition in the πof–π the* transition pyridine ofring the wa pyridines slightly ring blueshifted was slightly due blueshiftedto the increase due in to alkyl the increasechain length in alkyl, which chain we length,akened whichthe effective weakened conjugation the effective because conjugation of the alkyl because chains’ of space the alkyl effect. chains’ On the space other effect. hand, Onthe theabsorption other hand, maximum the absorption of benzyl maximum viologens of was benzyl located viologens at 264 wasnm due located to the at 264conjugation nm due toof thethe conjugationaromatic ring. of the aromatic ring.

195 195

246

257

259 d ae

262

260 c Absorbance Absorbance ba

b Absorbancespectra 264 b Absorbance spectra 262

cf a

200 300 400 200 300 400 200 300 400 Wavelength(nm) 200 Wavelength(nm) 300 400 Wavelength / nm Wavelength / nm

0 0 FigureFigure 1.1. TheThe ultravioletultraviolet spectra spectra of of 4,4 4,4-pyridine′-pyridine and and substituted substituted viologens: viologens (a): ( OV,a) OV (b), ( DeV,b) DeV (c), DoV, (c) DoV (d), HV, (d) (e)HV 4,, 4 -pyridine, and (e) (f)4, 4 BV.′-pyridine, and (f) BV. 3.2. Electrochemical Properties 3.2. Electrochemical Properties To investigate the electrochemical properties of the target viologens, cyclic voltam- To investigate the electrochemical properties of the target viologens, cyclic voltam- metry (CV) measurements were carried out. As shown in Figure2A–E, these curves are similar,metry (CV) and measurements all substituted were viologens carrie underwentd out. As shown reversible in Figure two-electron 2A–E, these reduction curves and are singlesimilar, electron and all oxidation substituted [44]. viologens The two couplesunderwent of redox reversible peaks obtainedtwo-electron during reduction the cathode and scansingle can electron be attributed oxidation to the [44] two. The one-electron two couples redox of processes redox peaks of the obtained4,40-bipyridine during units, the correspondingcathode scan tocan the be generation attributed of to radical the cationictwo one and-electron neutral redox species processes [45]. The of redox the potentials4,4′-bipyridine of the units, five viologens corresponding are shown to the in Table generation1. As seen of radical from Table cationic1 and and Figure neutral2A, thespecies OV film[45]. exhibited The redox two potentials redox processes, of the five which viologens were theare anodicshown wavein Table potentials 1. As seen of − from0.34 andTable− 0.601 and V, Figure and the 2A cathodic, the OV wave film potentialsexhibited oftwo− 0.62redox and processes−0.98 V., whichThe first were redox the coupleanodic ofwave−0.34 potentials and −0.60 of − V0. was34 and correlated −0.60 V, withand the cathodic redox reaction wave potentials between the of divalent−0.62 and cation −0.98 OVV. T2+heand first radical redox cation couple OV of+• ,− respectively.0.34 and −0.6 The0 V second was correlated couple of with−0.62 the and redox−0.98 reaction V was assignedbetween the to thedivalent OV+• cationand di-reduced OV2+ and radical species cation OV0, respectively.OV+•, respectively. When The the octylsecond in cou- OV wasple of replaced −0.62 and by − a0.9 decyl8 V unit,was assigned the first redoxto the coupleOV+• and (− 0.27di-reduced V, −0.55 species V) of theOV0 HV, respec- film wastively. increased When the (Figure octyl2 inB), OV and wa thes replaced value of by the a firstdecyl redox unit, couple the first continued redox couple to grow (−0.2 in7 V a , positive−0.55 V) directionof the HV with film the was increase increased in the(Figure carbon 2B chain.), and the value of the first redox couple continuedAs the to alkyl grow chain in a positive lengthened, direction the oxidation with the increase potential in of the the carbon second chain redox. couple shifted negatively while the reduction potential had no obvious change. As shown in Figure2E and Table1, due to the aromatic benzyl substituent, the redox potential of BV was obviously different from those of alkyl viologens. The redox wave of the second redox couple was wide and not sharp, and the second oxidation potential was much higher than those of alkyl-substituted viologens, meaning that the reversibility of the second couple of redox peaks worsened a little more. Otherwise, the oxidation peaks of −0.82 V (Figure2A,C) and −0.78 V (Figure2B) were caused by the dimerization of viologens. However, with the increase in carbon chain length, the steric hindrance effect of the substituents increased in the same amount of time, which decreased the dimerization of the compounds, and the oxidation wave of about −0.82 V gradually decreased until it could be ignored (Figure2D). Figure2F shows the CVs of the HV film at different scanning potentials. With the increase in scanning rate, the oxidation potential increased while the reduction potential decreased, and the oxidation Materials 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/materials Materials 2021, 14, 1702 5 of 13

Materials 2021, 14, x FOR PEER REVIEWcurrent and reduction current showed a linear relationship with the scanning rate. These 5 of 13 results indicate that the redox events were controlled by the surface-confined electron transfer kinetics rather than the diffusion process [32,33].

2.0 A B 2.0 1.5

1.0 1.0

0.5 0.0 Current / mA / mA Current 0.0 / mA Current

-0.5 -1 -0.1 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential / V Potential / V

2.0 C 1.0 D

1.0 0.5

0.0 0.0 Current / mA / mA Current Current / mA / mA Current -1.0 -0.5

-2.0 -1.0

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential/V Potential/V 0.4 E 1 1.0 i pai pa 0 ipci a 0.0 Current/mA pc

0.5 / mACurrent -1 25 50 75 100 d ScanScan raterate/(mV/s) / (mV/s) -0.4 0.0 Current / mA / mA Current Current / mA / mA Current -0.5 -0.8 -1.0

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Potential / V Potential / V

FigureFigure 2. 2.CVsCVs for for (A (A) )OV, OV, ((BB)) DeV,DeV, (C ()C DoV,) DoV, (D) ( HV,D) HV, and (andE) BV (E scanned) BV scanned from 0 tofrom−1.2 0 V to in − 0.11.2 M V ofin KCl 0.1 solutionM of KCl at asolution scan at a scanrate rate of of 50 50 mV/s. mV/s. (F) ( CVF) CV of HV of HV films films at different at different scanning scanning velocities velocities (a: 100, b:(a: 75, 100, c: 50,b: 75, and c: d: 50, 25 and mV/s). d: 25 mV/s).

Table 1. Cyclic voltammograms data of substituent viologens.

First Oxidation Second Oxidation First Reduction Second Reduction Substituent Potential Potential Potential Potential Viologens Epa1(V) Epa2(V) Epc1(V) Epc2(V) OV −0.34 −0.60 −0.62 −0.98 DeV −0.27 −0.62 −0.55 −0.86 DoV −0.24 −0.62 −0.50 −0.94 HV −0.24 −0.70 −0.47 −0.95 BV −0.21 −0.87 −0.39 −0.90

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Table 1. Cyclic voltammograms data of substituent viologens.

Second Second First Oxidation First Reduction Substituent Oxidation Reduction Potential Potential Viologens Potential Potential Epa1(V) Epc1(V) Epa2(V) Epc2(V) OV −0.34 −0.60 −0.62 −0.98 DeV −0.27 −0.62 −0.55 −0.86 DoV −0.24 −0.62 −0.50 −0.94 HV −0.24 −0.70 −0.47 −0.95 BV −0.21 −0.87 −0.39 −0.90

3.3. EC Behaviors 3.3.1. Multipotential UV Absorption Spectroscopy The EC behaviors of the target viologens were further studied by measuring their spectro-electrochemical properties. The absorption spectra of viologens films were obtained with external potentials from 0 to −1.1 V in 0.1 M of KCl as the electrolyte solution. As shown in Figure3A–D, all the alkyl-substituted viologens exhibited similar absorption spectra. Taking Figure3A (OV) as an example, when voltages from 0 to −0.5 V were applied, the absorption spectra of the OV film exhibited no changes, indicating no electrochromic reaction taking place. When the potential stepped from −0.5 to −0.7 V, the absorbance band became clearer. Under −0.7 V, the maximum absorbance peak was characterized at 558 nm with the color change into purplish-red. Human eyes are especially sensitive to the wavelength of 558 nm where the largest optical attenuation for the OV film is also obtained. As the potential continued to increase negatively, the strong absorption bands around 558 nm gradually decreased, and the color changed from purplish-red to light yellow. These phenomena also corresponded to the conversion between OV2+ and OV+•, which indicated the occurrence of the second redox process, and a yellow-brown color of OV0 was generated [30]. Absorption spectra of the BV film (Figure3E) were obviously different from those mentioned above. The reduction of benzyl viologen was produced in the absorption spectra. Applying a negative potential on the BV film, the maximum absorption band was characterized at 402 nm. With the increase in the reductive potentials, new absorption bands around 563, 608, 664, and 736 nm continuously appeared in the visible region, leading to an obvious color change from blue to grayish yellow.

3.3.2. Color Analysis Detailed color parameters, where L* represents lightness, and a* and b* describe the chromaticity and saturability, respectively, are listed in Table2. The color of the OV film was revealed to turn from light yellow to purple when the voltage changed from −0.9 to −0.6 V, in which the chromatic difference (∆Eab*) was 61.61. As the length of the alkyl carbon number increased, the color changes were different and the ∆Eab* of DeV, DoV, and HV decreased to 45.88, 36.59, and 21.25, respectively. The BV film showed a lower ∆Eab* than that of OV but a higher ∆Eab* than those of other alkyl viologens, coupled with a color change from grayish yellow to blue due to the aromatic ring.

3.3.3. Contrast and Response time A square-wave potential step method for sample films was employed to investigate the optical contrast and response time. In this test, 0 and −0.7 V were used as the reduced and oxidized state voltage, respectively, in which each voltage lasted for 15 s. Figure4 and Table3 display the graphs and data sheets of the transmittance for the alkyl viologen films at 558 nm a test interval time of 15 s. Figure4a describes the optical transmittance of the OV film. The coloring response time (RTc) and bleaching response time (RTb) were defined as the time required to reach 90% of the total change in transmittance after switching the potential. The OV film bleached and colored when the voltage changed Materials 2021, 14, 1702 7 of 13

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from 0 V to −0.7 V, obtaining a maximum contrast (∆Tmax) of 33%. The RTb and RTc of the OV film were 0.95 s and 8.16 s (Table3), respectively.

1.0 1.5 A h 558nm B 558nm h

0.7 1.0

0.9 1.5 558nm Absorbance Absorbance l l 0.8 h 0.5 558nm 500 600 700 500 600 700 Wavelength(nm) Absorbance Absorbance Absorbance Absorbance h Wavelength(nm) 1.0 0.7

0.5 a 0.6 a

400 500 600 700 800 Wavelength / nm Wavelength / nm

0.8 C 2.4 D 2.0 558nm h 558nm 0.6 1.8 h 0.7

1.6

ce 2.0 558nm 558nm 558nm558nm l an 0.5 1.4 Absorbance spectra Absorbance Absorbance spectra Absorbance b h l h r 0.6 500 600 700 500 600 700

bso Wavelength(nm) Wavelength(nm) A

Absorbance Absorbance 1.6 0.5

a a 1.2 0.4 400 500 600 700 800 Wavelength / nm Wavelength / nm

402nm 1.2 E l

0.8

608nm 0.4 563nm 664nm

Absorbance Absorbance 736nm

0.0 a

400 500 600 700 800 Wavelength / nm

FigureFigure 3. Absorption3. Absorption spectra spectra of the of (A the) OV, (A ()B OV,) DeV, (B ()C DeV,) DoV, (C (D) )DoV, HV, and (D) ( EHV,) BV and upon (E applying) BV upon different applying potentials. different (a: 0,potentials b: −0.1, c: −0.2, d: −0.3, e: −0.4, f: −0.5, g: −0.6, h: −0.7, i: −0.8, j: −0.9, k: −1.0, and l: −1.1 V). (a: 0, b: −0.1, c: −0.2, d: −0.3, e: −0.4, f: −0.5, g: −0.6, h: −0.7, i: −0.8, j: −0.9, k: −1.0, and l: −1.1 V).

3.3.2. Color Analysis Detailed color parameters, where L* represents lightness, and a* and b* describe the chromaticity and saturability, respectively, are listed in Table 2. The color of the OV film was revealed to turn from light yellow to purple when the voltage changed from −0.9 to −0.6 V, in which the chromatic difference (ΔEab*) was 61.61. As the length of the alkyl carbon number increased, the color changes were different and the ΔEab* of DeV, DoV, and HV decreased to 45.88, 36.59, and 21.25, respectively. The BV film showed a lower ΔEab* than that of OV but a higher ΔEab* than those of other alkyl viologens, coupled with a color change from grayish yellow to blue due to the aromatic ring.

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Materials 2021, 14, x FOR PEER REVIEW 8 of 13 Table 2. Color variations and chroma of viologen films under different potentials. Materials 2021, 14, x FOR PEERTable REVIEW 2. Color variations and chroma of viologen films under different potentials. 8 of 13

MaterialsViologen 2021, 14 ,Films x FOR PEER REVIEW Color Variations Potential/V L* a* b* ΔEab*8 of 13 Table 2. Color variations and chroma of viologen films under different potentials. ViologenMaterials Films2021 , 14, 1702 Color Variations Potential/V L* a* b* ΔEab* 8 of 13

Viologen Films Table 2.− 0.9VColorColor variations Variations and−0.6V chroma ofPotential/V viologen−0.9 films under62.17L* different −a*6.43 potentials. 33.12b* ΔEab*

OV Table 2. Color variations and chroma of viologen films under different potentials. 61.61 −0.9V −0.6V −0.9 62.17 −6.43 33.12 Viologen Films Color Variations Potential/V−0.6 69.38L* a*35.22 b*−11.70 ΔEab* OV Table 2. Color variations and chroma of viologen films under different potentials. 61.61 Viologen Films −0.9VColor Variations −0.6V Potential/V−0.9 62.17L* −6.43a* 33.12b* ΔEab* −0.6 69.38 35.22 −11.70 OVViologen Films Color Variations Potential/V L * a * b * 61.61 ∆Eab * −0.9V −0.6V −−0.90.6 62.1769.38 −35.226.43 33.12−11.70 OV −0.9V −0.6V 61.61 −0.9V −0.6V −−0.90.9 62.1753.37 −31.526.43 33.12−49.34 −0.9−0.6 69.3862.17 35.22 −6.43−11.70 33.12 DeVOV OV 61.6145.88 61.61 −0.9V −0.6V −0.6−0.9 53.3769.38 31.52 35.22−49.34 −11.70 −−0.60.6 69.3856.27 35.223.69 −−11.7012.67 DeV 45.88 −0.9V −0.6V −0.9 53.37 31.52 −49.34 −0.6 56.27 3.69 −12.67 DeV −0.9V −0.6V 45.88 −−0.90.6 53.3756.27 31.523.69 −−49.3412.67 −0.9 53.37 31.52 −49.34 DeV DeV −0.9V −0.6V −0.9 53.37 31.52 −49.34 45.88 45.88 −0.9V −0.5V −0.6 −0.9 56.3856.27 20.45 3.69 −4.36 −12.67 DeV −0.6 56.27 3.69 −12.67 45.88 DoV −0.9V −0.5V −0.60.9 56.2756.38 20.453.69 −−12.674.36 36.59 −0.5 61.25 −2.86 −32.14 DoV −0.9V −0.5V 36.59 −0.9 56.38 20.45 −4.36 − −0.5 61.25 −2.86 −32.14 − DoV −0.9V −0.5V 0.9 56.38 20.45 4.3636.59 DoV − −0.9 56.38 20.45 − −4.36 − 36.59 0.5−0.5 61.2561.25 −2.86 2.86−32.14 32.14 DoV −0.9V −0.5V −0.9 56.38 20.45 −4.36 36.59 −0.9V - −0.5V DoV −−0.50.9 61.2559.34 −2.862.58 −32.14−41.21 36.59 HV 0.9V - 0.5V 21.25 − − −0.50.9 61.2559.34 −2.582.86 −−32.1441.21 HV −0.9 −0.5 63.7059.34 23.15 2.58 −43.47 −41.2121.25 HV −0.9V - −0.5V −0.9 59.34 2.58 −41.21 21.25 −0.5−0.5 63.7063.70 23.15 23.15−43.47 −43.47 HV −0.9V - −0.5V 21.25 −0.9 59.34 2.58 −41.21 HV −0.5 63.70 23.15 −43.47 21.25 −0.9V - −0.5V −0.9 59.34 2.58 −41.21 −0.5 63.70 23.15 −43.47 HV −0.9V - −0.4V −0.9 48.86 −12.53 10.68 21.25 BV −0.9 48.86 −12.53 10.68 53.44 BV −0.9V - −0.4V −0.50.9 63.7048.86 −23.1512.53 −10.6843.47 53.44 BV −0.4 −0.4 54.37 9.35 9.35 −37.76 −37.7653.44 −0.9V - −0.4V −0.9 48.86 −12.53 10.68 −0.4 54.37 9.35 −37.76 BV −0.9V - −0.4V −0.9 48.86 −12.53 10.68 53.44 BV −0.4 54.37 9.35 −37.76 53.44 3.3.3.−0.9V Contrast - and−0.4V Response time−−0.40.9 54.3748.86 −9.3512.53 −37.7610.68 BV Table 3. Electrochromic (EC) switching properties of vilogen films. 53.44 3.3.3.A Contrast square- waveand Response potential t imestep method for sample films was employed to investigate Viologen Films−0.4 R 54.37(s) 9.35 R−37.76(s) ∆T (%) the3.3.3. opticalA Contrast square contrast- waveand Response and potential response t imestep time. method In Tbthis for test,sample 0 and films −0. 7was V were Tcemployed used asto theinvestigate reducedmax andthe3.3.3. optical oxidizedA Contrast square contrast state- waveand voltage,Response and OVpotential response respectively, t imestep time. method In in this 0.95 for which test,sample each0 and films voltage −0. was7 V lastedwere 8.16employed used for 15 asto s theinvestigate. reduced 33 DeV 6.72 5.42 22 theand optical oxidizedFigure contrast 4 stateand Table voltage,and response 3 display respectively, time. the graphs In in this which test,and each0data and voltagesheets −0.7 V of werelasted the usedtransmittance for 15 as sthe. reduced for the 3.3.3.A Contrast square- waveand ResponseDoV potential tstepime method 5.38for sample films was employed 4.35 to investigate 20 alkylandthe opticaloxidizedFigure viologen contrast 4 stateand films Tablevoltage, and HVat 558 response3 display respectively,nm a time.the the test graphsIn in thisinterval 4.25which test,and each0timedata and voltageofsheets −0. 157 Vs . of werelastedFigure 3.23the usedtransmittance for 4a 15asdescribe sthe. reduceds forthe theop- 15 A square-wave potential step method for sample films was employed to investigate ticalalkyland oxidizedFigure transmittanceviologen 4 andstate films Table voltage, of BVat the558 3 display OVrespectively,nm a film. the the testThe graphs in interval color 5.94which anding eachtimedata response voltageofsheets 15 s time. of lastedFigure the 4.56 ( R transmittancefor Tc4)a 15 anddescribe s. bleacs forhingthe the op- 29 re- the optical contrast and response time. In this test, 0 and −0.7 V were used as the reduced sponsealkyltical Figure transmittanceviologen time 4 (R and filmsTb) Tablewe of atre the558 3defined display OVnm a film. theas the the test Thegraphs time interval color andrequireding timedata response sheetsofto 15reach s .oftime Figure the90% ( Rtransmittance Tcof4)a theanddescribe total bleac s changefor hingthe the op- re- in and oxidized state voltage, respectively, in which each voltage lasted for 15 s. transmittanceticalsponsealkyl transmittance viologen time (R afterfilmsTb) we of switchingatre the558 defined OVnm thea film. theas potential. the testThe time interval color requiredTheing time OV response offilmto 15reach bleachs .time Figure 90% ed(R Tcof4anda) thedand escribecolored total bleacs change thehingwhen op- re- thein Figure 4 andAs Table shown 3 display in Table the3, when graphs increasing and data the sheets length of the of the transmittance alkyl chain, for both the∆ Tmax and voltagesponsetransmittancetical transmittance timechanged (R afterTb) from we ofswitchingre the0 defined to OV −0.7 the film. V,as potential. obtainingthe The time color requiredThe aing maximum OVresponse filmto reach bleachcontrast time 90% ed(R Tc (ΔTofand) andthemax colored ) total bleacof 33% changehing when. T he re- the inR T b alkyl viologenRTc offilms the at alkyl 558 viologensnm a the test decreased. interval OVtime showed of 15 s. theFigure longest 4a d RescribeTc whiles the HV op- showed the andtransmittancevoltagesponse RTc timeofchanged theshortest (R afterOVTb )from wefilm Rswitchingre .0 weredefined Asto −0.7 the 0.95 the lengthV,as andpotential. obtainingthe 8. oftime16 the s requiredThe( alkylTa ablemaximum OV carbon3 )filmto, respectively. reach bleachcontrast chain 90%ed increased, of(ΔTand themax colored total) of the 33% change Rwhen. Tofhe in the R T b DeV, DoV, tical transmittance ofTc the OV film. The coloring response time (RTc) and bleacTbhing re- voltageandtransmittance RTc ofchanged theand OVafter HV from film filmsswitching 0were to gradually −0.7 0.95 the V, andpotential. obtaining decreased 8.16 s The( Ta toablemaximum OV 6.72 3 )film, s, respectively. 5.38 bleach contrast s, anded (ΔTand 4.25 max colored s,) respectively, of 33% when. The the exceptRT b the OV sponse time (RTb) were defined as the time required to reach 90% of the total change in andvoltage RTc ofchanged the OV from film 0were to −0.7 0.95 V, and obtaining 8.16 s ( aT ablemaximum 3), respectively. contrast (ΔT max) of 33%. The RTb film, which had the shortest RTb of 0.97 s. Both the RTb and RTc of the BV film were shorter transmittanceand RTc of the OVafter film switching were 0.95 the and potential. 8.16 s (T Theable OV 3), respectively.film bleached and colored when the than those of DeV in these samples, and the ∆Tmax was only lower than that of OV but voltage changed from 0 to −0.7 V, obtaining a maximum contrast (ΔTmax) of 33%. The RTb better than those of DeV, DoV, and HV. and RTc of the OV film were 0.95 and 8.16 s (Table 3), respectively. 3.3.4. Cyclic Stability The cyclic stability is considered an important requirement for further practical ap- plications. Therefore, the stability of the resulting viologen film was investigated by continuously cycling the devices between the colored and the bleached states, and the percentages of the retained transmittance after 200 cycles (∆T200) are given in Figure5f. Figure5a–e displays the transmittance of the viologen film over 200 continuous cycles. As shown in Figure5f, after 200 cycles, OV still showed the best transmittance contrast. The losses of transmittance contrast values for OV, DeV, DoV, and HV were 7.9%, 4.5%, 4.0%, Materials 2021, 14, x. https://doi.org/10.3390/xxxxx and 1.3%, respectively, indicating that the resultingwww.mdpi.com/journal/ viologens films becamematerials more stable Materials 2021, 14, x. https://doi.org/10.3390/xxxxx with the increase in the alkyl chain length in the visiblewww.mdpi.com/journal/ region for thematerials EC applications. Materials 2021, 14, x. https://doi.org/10.3390/xxxxx www.mdpi.com/journal/materials Materials 2021, 14, x. https://doi.org/10.3390/xxxxx OV still showed the best optical contrast, but HV showedwww.mdpi.com/journal/ the most stabilitymaterials in continuous

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Materials 2021, 14, x FOR PEER REVIEW 9 of 13

cycles due to the long carbon chain, which can minimize the effect of dimerization. The ∆T200 of BV was lower than that of OV, but better than those of the others.

80 Aa Bb

70 60

60 50 50 Transmittance / % / % Transmittance Transmittance / % / % Transmittance 40 40

0 50 100 150 0 50 100 150 Time / s Time / s

80 C c 90 Dd

70 85

80 60 Transmittance / % / % Transmittance Transmittance / % / % Transmittance 75 50 70 0 50 100 150 50 100 150 Time / s Time / s 70 Ee 60

50

40 Transmittance / % 30

0 50 100 150 Time / s

FigureFigure 4. The 4. Thesquare square wave wave transmittance transmittance curves curves of of different viologen viologen films: films: (a) ( OV,a) OV, (b) DeV,(b) DeV, (c) DoV, (c) DoV, (d) HV, (d and) HV, (e) and BV, in (e) BV, in 0.10.1 M Mof ofKCl KCl solution. solution.

Table 3. Electrochromic (EC) switching properties of vilogen films.

Viologen Films RTb (s) RTc (s) ΔTmax (%) OV 0.95 8.16 33 DeV 6.72 5.42 22 DoV 5.38 4.35 20 HV 4.25 3.23 15 BV 5.94 4.56 29

As shown in Table 3, when increasing the length of the alkyl chain, both ΔTmax and RTc of the alkyl viologens decreased. OV showed the longest RTc while HV showed the shortest RTc. As the length of the alkyl carbon chain increased, the RTb of the DeV, DoV, and HV films gradually decreased to 6.72, 5.38, and 4.25 s, respectively, except the OV film, which had the shortest RTb of 0.97 s. Both the RTb and RTc of the BV film were shorter than those of DeV in these samples, and the ΔTmax was only lower than that of OV but better than those of DeV, DoV, and HV.

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3.3.4. Cyclic Stability The cyclic stability is considered an important requirement for further practical ap- plications. Therefore, the stability of the resulting viologen film was investigated by con- Materials 2021, 14, 1702 tinuously cycling the devices between the colored and the bleached states, and10 of the 13 per- centages of the retained transmittance after 200 cycles (ΔT200) are given in Figure 5f.

80 70 Aa Tb Bb Tb 70 Tc Tc 60 60

50 50

40 Transmittance / %

Transmittance / % 40 30

20 30 0 50 100 150 200 0 50 100 150 200 Cycle number Cycle number

80 T T Cc b 90 Dd b Tc Tc % 70

80

60 Transmittance / % / % Transmittance Transmittance /

50 70

0 50 100 150 200 0 50 100 150 200 Cycle number Cycle number

70 Ee Tb f 35 T c 30 60 25 20 200 50 15 ΔT 10

Transmittance / % / % Transmittance 40 5 0 OV DeV DoV HV BV 30

0 50 100 150 200 Cycle number

Figure 5. The transmittance (a–e) and ∆T200 (f) of OV, DeV, DoV, HV, and BV films under continuous cycling between Figure 5. The transmittance (a–e) and ΔT200 (f) of OV, DeV, DoV, HV, and BV films under continuous cycling between colored and bleached states. colored and bleached states. 4. Conclusions Figure 5a–e displays the transmittance of the viologen film over 200 continuous cy- In this work, four alkyl-substituted and benzyl-substituted viologens have been cles.designed As shown and in successfully Figure 5f, synthesized.after 200 cycles The, influenceOV still showed of the different the best structures transmittance of the con- trast.substituents The losses on of the transmittance electrochromism contrast properties values has for been OV, investigated. DeV, DoV, Although and HV they were all 7.9%, 4.5%,showed 4.0%, a rapidand 1.3%, response respectively, time, good indicating optical contrast, that andthe stableresulting long-time viologens operation, films thebecame moredifference stable betweenwith the theincrease substituent in the structures alkyl ch reflectedain length different in the capabilityvisible region trends. for UV- the EC vis absorption spectroscopy showed that the absorption peak of the B band in the π-π*

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transition of the pyridine ring was slightly blueshifted due to the increase in the alkyl chain length, while the absorption maximum of benzyl viologens was located at 264 nm. The CV curves confirmed that the first redox couple was increased in a positive direction with the increase in alkyl chain length. In addition, the RTc of the alkyl substituent viologens was improved while the optical contrasts decreased gradually with the increase in the alkyl chain length. The OV film showed the maximum contrast even after 200 cycles, but the HV film showed the most stability because of the minimum dimerization. The BV film showed a better chromatic difference and ∆Tmax than those of DeV, DoV, and HV, but a lower performance than that of OV.

Author Contributions: Writing—original draft, data curation, formal analysis, writing—review & editing, Q.Z., experiment and writing—original draft, L.Y.; data treatment, analysis and figure- making, F.G.; conceptualization, funding acquisition, project administration, writing—review & editing, X.L.; conceptualization, funding acquisition and review, R.W.; substantial contributions to the conception and formal analysis, J.X.; revising it critically for important intellectual content, Y.Q.; conceptualization, writing—review & editing G.C. All authors have read and agreed to the published version of the manuscript. Funding: This project was financially supported by the National Natural Science Foundation of China (51873003), the Key and General Scientific and Technology Project of Beijing Educational Committee (KZ201910012015, KM202010012006), and the Beijing Scholar Program (RCQJ20303). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References 1. Moon, H.C.; Lodge, T.P.; Frisbie, C.D. Solution processable, electrochromic ion gels for sub-1 V, flexible displays on plastic. Chem. Mater. 2015, 27, 1420–1425. [CrossRef] 2. Christiansen, D.T.; Tomlinson, A.L.; Reynolds, J.R. New design paradigm for color control in anodically coloring electrochromic molecules. J. Am. Chem. Soc. 2019, 141, 3859–3862. [CrossRef][PubMed] 3. Wang, Y.Y.; Wang, S.; Wang, X.J.; Zhang, W.R.; Zheng, W.X.; Zhang, Y.M.; Zhang, S.X.A. A multicolour bistable electronic shelf label based on intramolecular proton-coupled electron transfer. Nat. Mater. 2019, 18, 1335–1342. [CrossRef] 4. Chang, M.J.; Chen, W.N.; Xue, H.D.; Liang, D.L.; Lu, X.F.; Zhou, G. Conjugation-extended viologens with thiophene derivative bridges: Near-infrared electrochromism, electrofluorochromism, and smart window applications. J. Mater. Chem. C 2020, 8, 16129–16143. [CrossRef] 5. Korgel, B.A. Materials science: Composite for smarter windows. Nature 2013, 50, 278–279. [CrossRef][PubMed] 6. Lee, E.S.; DiBartolomeo, D.L.; Selkowitz, S.E. Daylighting control performance of a thin-film ceramic electrochromic window: Field study results. Energy Build. 2006, 38, 30–44. [CrossRef] 7. Xu, J.; Zhang, Y.; Zhai, T.T.; Kuang, Z.; Li, J.; Wang, Y.; Gao, Z.; Song, Y.Y.; Xia, X.H. Electrochromic-tuned plasmonics for photothermal sterile window. ACS Nano 2018, 12, 6895–6903. [CrossRef] 8. Wang, M.; Xing, X.; Perepichka, I.F.; Shi, Y.H.; Zhou, D.Y.; Wu, P.H.; Meng, H. Electrochromic smart windows can achieve an absolute private state through thermochromically engineered electrolyte. Adv. Energy Mater. 2019, 9, 1900433. [CrossRef] 9. Wu, Q.T. High-tech anti-glare rearview mirror of Jingtai company. Automob. Parts 2007, 11, 47. 10. Xiao, X.F. Jingtai: The world’s leading supplier of anti-glare rearview mirrors and camera-based active safety systems. Commer. Veh. 2011, 3, 92–95. 11. Krebs, F.C. The new black. J. Nat. Mater. 2008, 7, 766–767. [CrossRef][PubMed] 12. Lee, M.H.; Yang, Z.; Lim, C.W.; Lee, Y.H.; Dongbang, S.; Kang, C.; Kim, J.S. Disulfide-cleavage-triggered chemosensors and their biological applications. Chem. Rev. 2013, 113, 5071–5109. [CrossRef] 13. Kim, H.N.; Ren, W.X.; Kim, J.S.; Yoon, J. Fluorescent and colorimetric sensors for detection of lead, cadmium, and mercury ions. J. Chem. Soc. Rev. 2012, 41, 3210–3244. [CrossRef] 14. Ma, Y.; Shen, L.; She, P.; Hou, Y.; Yu, Y.; Zhao, J.; Liu, S.; Zhao, Q. Constructing multi-stimuli-responsive luminescent materials through outer sphere electron transfer in ion pairs. Adv. Opt. Mater. 2019, 7, 1801657. [CrossRef] Materials 2021, 14, 1702 12 of 13

15. Wang, Y.; Song, Y.; Xia, Y. Electrochemical capacitors: Mechanism, materials, systems, characterization and applications. Chem. Soc. Rev. 2016, 45, 5925–5950. [CrossRef][PubMed] 16. Wang, F.; Wu, X.; Yuan, X.; Liu, Z.; Zhang, Y.; Fu, L.; Zhu, Y.; Zhou, Q.; Wu, Y.; Huang, W. Latest advances in supercapacitors: Fom new electrode materials to novel device designs. Chem. Soc. Rev. 2017, 46, 6816–6854. [CrossRef] 17. Dubal, D.P.; Chodankar, N.R.; Kim, D.H.; Gomez-Romero, P. Towards flexible solid-state supercapacitors for smart and wearable electronics. Chem. Soc. Rev. 2018, 47, 2065–2129. [CrossRef][PubMed] 18. Mouratis, K.; Tudose, I.V.; Bouranta, A.; Pachiu, C.; Romanitan, C.; Tutunaru, O.; Couris, S.; Koudoumas, E.; Suchea, M. Annealing effect on the properties of electrochromic V2O5 thin films grown by spray deposition technique. Nanomaterials 2020, 10, 2397–2409. [CrossRef][PubMed] 19. Garcia-Garcia, F.J.; Gil-Rostra, J.; Yubero, F.; González-Elipe, A.R. Electrochromism in WOx and WxSiyOz thin films prepared by magnetron sputtering at glancing angles. Nanosci. Nanotechnol. Lett. 2013, 5, 89–93. [CrossRef] 20. Marciel, A.; Graça, M.; Bastos, A.; Pereira, L.; Kumar, J.S.; Borges, J.; Vaz, F.; Peres, M.; Magalhães, S.; Lorenz, K.; et al. Molybdenum oxide thin films grown on flexible ITO-coated PET substrates. Materials 2021, 14, 821. [CrossRef][PubMed] 21. Garcia-Garcia, F.J.; Gil-Rostra, J.; Yubero, F.; Espinós, J.P.; Gonzalez-Elipe, A.R.; Chaboy, J. “In operando” X-ray absorption spectroscopy analysis of structural changes during electrochemical cycling of WO3 and WxSiyOz amorphous electrochromic thin film cathodes. J. Phys. Chem. C 2015, 119, 644–652. [CrossRef] 22. Wen, R.T.; Granqvist, C.; Niklasson, G. Eliminating degradation and uncovering ion-trapping dynamics in electrochromic WO3 thin films. Nat. Mater. 2015, 14, 996–1001. [CrossRef][PubMed] 23. Meng, J.Y.; Li, X.; Qin, M.M.; Pei, Y.F.; Yang, S.; Lan, Y.W.; Wang, R.; Chen, G.M. Effects of pore size of reverse opal structured PEDOT films on their electrochromic performances. Org. Electron. 2017, 50, 16–24. [CrossRef] 24. Liang, F.Y.; Li, X.; Zhao, N.; Xu, J. Preparation and elcetrochromic properties of SiO2@PANI photonic crystals. Chin. J. Polym. Sci. 2014, 6, 860–864. 25. Li, N.; Yu, L.N.; Wei, P.P.; Ji, J.Y.; Zhao, J.P.; Zhang, Q.; Li, Y.; Yin, Y.D. Electrochemical fabrication and sensing application of multicolored silver films. Adv. Mater. Interfaces 2018, 5, 1800277. [CrossRef] 26. Li, N.; Wei, P.P.; Yu, L.N.; Ji, J.Y.; Zhao, J.P.; Gao, C.B.; Li, Y.; Yin, Y.D. Dynamically switchable multicolor electrochromic films. Small 2019, 15, 1804974. [CrossRef][PubMed] 27. Qu, D.W.; Liu, L.W.; Li, X.; Chen, K.; Zheng, Y.P.; Xue, Y.M.; Chen, G.M. Fabrication of metal-supramolecular polymers of FeL/carbon nanomaterials with enhanced electrochromic properties. Compos. Sci. Technol. 2020, 198, 108252. [CrossRef] 28. Liang, F.Y.; Li, X.; Zhang, D.Q. Electrochromic properties of acid dye doped poly(3,4-ethyl-enedioxy-thiophene) by electropoly- merization. J. Nanosci. Nanotechnol 2015, 15, 3130–3134. [CrossRef] 29. Sonmez, G.; Meng, H.; Zhang, Q.; Wudl, F. A highly stable, new electrochromic polymer: Poly(1,4-bis(2-(30,40-ethylenedioxy) thienyl)-2-methoxy-5-2”-ethylhexyloxybenzene). Adv. Funct. Mater. 2003, 13, 726–731. [CrossRef] 30. Electrochromic Materials and Devices; Mortimer, R.J.; Rosseinsky, D.R.; Monk, P.M.S. (Eds.) Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; ISBN 9783527679850. 31. Shah, K.W.; Wang, S.X.; Soo, D.X.Y.; Xu, J. Viologen-based electrochromic materials: From small molecules, polymers and composites to their applications. Polymers 2019, 11, 1839. [CrossRef][PubMed] 32. Ding, J.J.; Zheng, C.N.; Wang, L.X.; Lu, C.B.; Zhang, B.; Chen, Y.; Li, M.Q.; Zhai, G.Q.; Zhuang, X.D. Viologen-inspired functional materials: Synthetic strategies and applications. J. Mater. Chem. A 2019, 7, 23337–23360. [CrossRef] 33. Hwang, E.; Seo, S.; Bak, S.; Lee, H.; Min, M.; Lee, H. An electrolyte-free flexible electrochromic device using electrostatically strong graphene quantum dot-viologen nanocomposites. Adv. Mater. 2014, 26, 5129–5136. [CrossRef][PubMed] 34. Giuseppina, A.C.; Sante, C.; Agostina, L.C.; Amerigo, B. Changes in fatigue recovery and muscle damage enzymes after deep-sea water thalassotherapy. Appl. Sci. 2020, 10, 8372–8383. 35. Yu, H.F.; Chen, K.I.; Yeh, M.H.; Ho, K.C. Effect of trifluoromethyl substituents in benzyl-based viologen on the electrochromic performance: Optical contrast and stability. Sol. Energy Mater Sol. Cells 2019, 200, 110020–110022. [CrossRef] 36. Dmitrieva, E.; Rosenkranz, M.; Alesanco, Y.; Viñuales, A. UiO66-NH2 as self-sacrificing template for Fe/N-doped hierarchically porous carbon with high electrochemical performance for oxygen reduction in microbial fuel cells. Electrochim. Acta. 2019, 323, 134792–134799. [CrossRef] 37. Marek, L.; Waclaw, G.; Carl, W.S. The electrochemical properties of some alkylviologens. J. Electroanal. 1998, 10, 486–491. 38. Cook, S.K.; Horrocks, B.R. Heterogeneous electron-transfer rates for the reduction of viologen derivatives at platinum and bismuth electrodes in acetonitrile. Chem. Electro. Chem. 2017, 4, 320–331. [CrossRef][PubMed] 39. Li, G.; Zhang, B.; Wang, J.; Zhao, H.; Ma, W.; Xu, L.; Zhang, W.; Zhou, K.; Du, Y.; He, G. Electrochromic poly(chalcogenoviologen)s as anode materials for high-performance organic radical lithium-ion batteries. Angew. Chem. Int. Ed. 2019, 58, 8468–8473. [CrossRef] 40. Gorkem, E.G.; Levent, T. A unique processable green polymer with a transmissive oxidized state for realization of potential rgb-based electrochromic device applications. J. Adv. Funct. Mater. 2008, 18, 2026–2030. 41. Oh, H.; Seo, D.G.; Yun, T.Y.; Kim, C.Y.; Moon, H.C. Voltage-Tunable Multicolor, Sub-1.5 V, Flexible electrochromic devices based on ion gels. Appl. Mater. Interfaces 2017, 9, 7658–7665. [CrossRef][PubMed] 42. Hsu, C.-Y.; Vasantha, V.S.; Ho, K.-C. A study of ion exchange at the poly(butyl viologen)-electrolyte interface by SECM. Electrochim. Acta 2008, 53, 6244–6251. [CrossRef] Materials 2021, 14, 1702 13 of 13

43. Yuan, L.; Li, X.; Li, C.J.J. Synthesis, characterization and electrochromic property research of symmetric viologen. J. Beijing Inst. Fashion. Tech. 2013, 33, 5–11. 44. Jiang, Q.L. The synthesis of viologen derivates and solid electrochromic devices. Master’s Thesis, Southwest University, Chong Qing, China, 2007. 45. Liang, F.Y.; Dong, Z.Y.; Li, X. Preparation and characterization of poly(3,4-dibromothiophene) and its polymer electrochromic device. J. Beijing Inst. Fashion Tech. 2012, 32, 8–13.