Colorless to Neutral Color Electrochromic Devices Based on

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Colorless to Neutral-Color Electrochromic Devices Based on Asymmetric Viologens Yolanda Alesanco, Ana I. Vinuales, German Cabañero, Javier Rodriguez, and Ramón Tena-Zaera ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11321 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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ACS Applied Materials & Interfaces

Colorless to Neutral-Color Electrochromic Devices Based on Asymmetric Viologens Yolanda Alesanco, Ana Viñuales*, Germán Cabañero, Javier Rodriguez and Ramón Tena-Zaera. Y. Alesanco, Dr. A. Viñuales, G. Cabañero, Dr. J. Rodriguez and Dr. R. Tena-Zaera. IK4-CIDETEC Research Center, Paseo Miramón 196, 20009 Donostia – San Sebastián (Spain) KEYWORDS. Gray-electrochromics, asymmetric-viologens, smart-windows, PVA gels, neutral-color.

ABSTRACT: Electrochromic materials have extensively been investigated owing to their potential fields of application, with a significant growing interest in expanding the provided colorations. However, among all palette of colors, colorless electrochromic devices (ECDs) which provide neutral-grayish colorations with a simple configuration remains a key challenge. The present study reports on the synthesis of asymmetrically 1-alkyl-1’-aryl substituted viologens and their incorporation in PVA-borax gel polyelectrolytes for ECDs which constitute the simplest device architecture (glass/TCO/EC gel/TCO/glass). We demonstrate herein that these EC gels based on single asymmetric viologens provide more neutralcolored state than their corresponding symmetric viologens (a* and b* ≤ |15|), while maintaining satisfactory colorless bleached state (%Tb > 70% in the whole visible range), transmittance changes (i.e., ~ 60 %) and cyclability (i.e., ~ 15 000 cycles). Additionally, the effect of the solvent on the observed coloration has also been investigated. This easy-to-make neutral-grayish color ECDs may significantly extend the potential of the electrochromic technology, since they adapt better aesthetically to the surrounding environment, being easier to implement in different applications.

1.

INTRODUCTION

Electrochromic (EC) materials, including some inorganic metal oxides, organic molecules such as viologens and conducting polymers, can vary their color reversibly, by means of redox reaction induced by an appropriate external voltage. Electrochromic devices (ECDs) have extensively been investigated owing to their potential fields of application, such as displays,1 rearview mirrors 2 or smart windows 3 for automotive and building industries. To expand the potential of the electrochromic technology, a large amount of inquiry has been conducted in the last years on generating full-color devices and diversifying the colors of the EC materials.4-6 However, among all palette of colors, ECDs which provide more neutral (i. e., grayish) colorations remain a field to be explored.7 Neutral-tones such as gray, adapt better aesthetically to the surrounding environment being easier to implement in different applications, and absorb in the most of the visible range, making them excellent candidates for effective light filtering. The reported strategies to achieve grayish-totransmissive electrochromic devices comprise different color chromophores through coupling their absorption profiles. These approaches include EC formulations based on 3 viologens,8 or more complex device configurations such as ECDs containing one of the electrodes doubly layered (i. e. blue and green viologen-modified nanostruc-

tured films successively deposited on the working electrode),9 devices with both electrodes bilayered (one of them by two cathodically colored polymers and the other by anodically colored polymers),10 or even devices including four electrodes (two working electrodes and two counter electrodes).11 An interesting approach to achieve neutral-color ECDs while avoiding the complexity in the device architecture comprises the design and synthesis of new molecules which incorporate two different chromophores in it.12 In this regard, some “black-to-transmissive” EC polymers have been synthesized through random copolymerization of two different monomers.13-14 The parallelism of this concept in the field of viologens might be the synthesis of asymmetric violgens. The so-called viologens, 1,1’disubstituted 4,4’-bipyridilium salts, exhibit different colors in their reduced state depending on the nature of the substituents bonded to the nitrogen atoms. Hence, 1,1’-alkyl substituted viologens provide blue/violet color,15 while 1,1’-aryl substituted viologens usually exhibit green colorations upon applying a suitable reduction potential.15-16 The most wide studied viologens are symmetrically 1,1’- alkyl or 1,1’-aryl substituted, thus provide pure colors in their reduced states. However, asymmetrically 1-alkyl-1’-aryl substituted viologens may show more neutral-colors about midway between alkyl and arylsubstituted viologens due to the contribution of both groups. However, up to our best knowledge, there is no

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report on asymmetric viologens specifically designed for neutral color electrochromics. The present study reports on ECDs comprising asymmetrically 1-alkyl-1’-aryl substituted viologens incorporated in a PVA-borax gel polyelectrolyte with a very simple configuration (glass/TCO/EC gel/TCO/glass). We demonstrate herein that these EC gels based on single asymmetric viologens provide more neutral-colored state than their corresponding symmetric viologens while maintaining satisfactory colorless bleached state and optical contrasts. Additionally, the effect of the solvent in the observed coloration has also been investigated. 2.

EXPERIMENTAL SECTION

Materials: Poly(vinyl alcohol) (PVA, Mw 61 000), sodium tetraborate (borax, 99.5%), potassium ferrocyanide (98.5%) potassium ferricyanide (99%), 4,4’-bipyridyl (98%), 2,4dinitrochlorobenzene (99%), p-cyanoaniline (98%), ethyl bromide (98%), benzyl bromide (98%), hydroquinone (≥ 99%) and 1-Butyl-3-methylimidazolium tetrafluoroborate (98%), were purchased from Sigma-Aldrich and used without further purification. Charcoal activated powder and required solvents such as acetone, ethanol, THF, acetonitrile and propylene carbonate were supplied by Scharlab and used as received. Fluorine-doped tin oxide and Indium tin oxide coated glass substrates ( Rs 6-8 and 25-35 Ωsq-1 respectively) were provided by Solems. Further details can be found in our previous report.6 UVcuring adhesive NOA-65 was provided by Norland Products. Synthesis: 1-(2,4-Dinitrophenyl)-4,4’-bipyridinium monochloride (intermediate [I]) was synthesized by Zincke reaction according to previously reported procedure.17-18 In brief, a mixture of 4,4’-bipyridyl (50 mmol) and 2,4dinitrochlorobenzene (50 mmol) was refluxed in ethanol (125 mL) for 16 hours. The ethanol was removed by rotary evaporation in vacuo, acetone was added (150 mL) and the solution was stirred for 2 hours. The resulting brownish appearance solid was filtered and recrystallized from ethanol and acetone followed by drying in vacuo. (Yield = 40%). 1H NMR (500 MHz, D2O, δ): 9.42 ppm (s, 1H), 9.28 ppm (d, 2H, J = 6.76 Hz), 8.97 and 8.95 ppm (d of d, 1H, J = 2.42 and 2.43 Hz), 8.87 ppm (d, 2H, J = 6.15 Hz), 8.72 ppm (d, 2H, J = 6.78 Hz), 8.30 ppm (d, 1H, J = 8.69 Hz), 8.06 ppm (d, 2H, J = 6.19 Hz). 1-(p-Cyanophenyl)-4,4’-bipyridinium monochloride (intermediate [II]) was synthesized as follows: a mixture of 1(2,4-dinitrophenyl)-4,4’-bipyridinium monochloride [I] (15 mmol) and p-cyanoaniline (30 mmol) was refluxed in deionized water (30 mL) for 48 hours. After cooling, the resulting solid was filtered. The solid residue was removed and the filtrate was decolorized with charcoal activated powder for 16h. The water of the filtrate was removed by rotary evaporation in vacuo, THF was added (150 mL) and the solid was filtered followed by drying under vacuum. (Yield = 79%). 1H NMR (500 MHz, D2O, δ): 9.32 ppm (d, 2H, J = 6.95 Hz), 8.86 ppm (d, 2H, J = 6.11

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Hz), 8.68 ppm (d, 2H, J = 6.93 Hz), 8.20 ppm (d, 2H, J = 8.75 Hz), 8.05 ppm (m, 4H). IR (bulk ATR): ν (cm-1) = 3104 (C-H olefin st), 2233 (-CN st), 1632 (C=N st), 1594, 1502 (C=C), 815 (o-phenylene H); IC: % Cl- calculated for C17H12N3Cl: 12.1%, found 11.8%. 1-Ethyl-1’-(p-cyanophenyl)-4,4’-bipyridinium dibromide (Et-pCNVio) was synthesized as follows: a mixture of 1-( p-cyanophenyl)-4,4’-bipyridinium monochloride [II] (3.5 mmol) and ethyl bromide (18 mmol) was refluxed in acetonitrile (20 mL) for 48 hours. The cooled solution was filtered and the resulting yellowish powder was washed with acetonitrile and acetone. (Yield = 87%). 1H NMR (500 MHz, DMSO-d6, δ): 9.71 ppm (d, 2H, J = 6.90 Hz), 9.45 ppm (d, 2H, J = 6.79 Hz), 9.00 ppm (d, 2H, J = 6.91 Hz), 8.92 ppm (d, 2H, J = 6.61 Hz), 8.33 ppm (d, 2H, J = 8.67 Hz), 8.19 ppm (d, 2H, J = 8.69 Hz), 4.75 ppm (q, 2H), 1.61 ppm (t, 3H). 13C NMR (500 MHz, D2O, δ ppm): 154.3, 152.1, 148.1, 147.6, 137.5, 129.8, 128.0, 120.4, 117.6, 60.5, 18.2 (1H and 13C NMR spectra provided as Supporting Information Figures S1 and S2 respectively). IR (bulk ATR): ν (cm-1) = 3098 (C-H olefin st), 2990 (C-H alkyl st), 2227 (-CN st), 1635 (C=N st), 1600, 1495 (C=C), 1435 (C-H alkyl δ), 824 cm−1 (o-phenylene H); IC: % Br- calculated for C19H17N3Br2: 35.7%, found 33.5% (Chromatogram available in the Supporting Information Figure S3). 1-Benzyl-1’-(p-cyanophenyl)-4,4’-bipyridinium dibromide (Bn-pCNVio) was synthesized following the same procedure described above for the (Et-pCNVio), using benzyl bromide as alkyl substituent instead (Yield = 89%). 1H NMR (500 MHz, DMSO-d6, δ): 9.47 ppm (d, 2H, J = 6.96 Hz), 9.25 ppm (d, 2H, J = 6.85 Hz), 8.80 ppm (d, 2H, J = 6.96 Hz), 8.68 ppm (d, 2H, J = 6.80 Hz), 8.22 ppm (d, 2H, J = 8.78 Hz), 8.07 ppm (d, 2H, J = 8.79 Hz), 7.58 ppm (s, 5H), 6.00 ppm (s, 2H). 13C NMR (500 MHz, D2O, δ ppm): 154.2, 152.6, 148.3, 148.2, 147.7, 137.6, 134.8, 132.9, 132.4, 132.0, 130.0, 129.9, 128.1, 120.4, 117.8, 67.6 (1H and 13C NMR spectra provided as Supporting Information Figures S4 and S5 respectively). IR (bulk ATR): ν (cm-1) = 3101, 3025 (C-H olefin st), 2987 (C-H alkyl st), 2227 (-CN st), 1635 (C=N st), 1495 (C=C), 1435 (C-H alkyl δ), 827 (o-phenylene H); IC: % Br- calculated for C24H19N3Br2: 31.4%, found 30.1%. (Chromatogram available in the Supporting Information Figure S6). Methods: UV-vis transmittance spectra were carried out with a Jasco V-570 spectrophotometer. Further details can be found elsewhere.6 The reduction of the electrochromic devices and the cyclic voltammetric studies (30 mV/s) were performed on a Biologic MPG potentiostat-galvanostat. Color of the electrochromic devices was examined by spectrophotometric method following the procedure developed by R. J. Mortimer and T. S. Varley,19-20 which provide xyY (1931) and L*a*b* (1976) color coordinates. Color interpretation of the registered color coordinates included to ease their interpretation were acquired from


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the L*a*b* color coordinates through a color converter software.

[I]

Photographs of the electrochromic devices were obtained with a Canon IXUS 105 camera with a 30 - 80 s-1 of exposition time, focal distance between 5 – 11 mm and maximum opening of 3 – 4, under fluorescent light (Phillips TL-D 80 18W/840) without the employment of flash.

[II]

Cyclability assays were performed with ECDs previously sealed with a colorless UV-curing adhesive using a TECHNIGRAF AKTIPRINT T/A 40-2 UV-curing tunnel. Fourier transform infrared spectra (FT-IR) of the pure solids were obtained by Attenuated Total Reflectance (ATR) technique with a 4100LE FTIR from Jasco. 1

H NMR and 13C spectra were measured on a Bruker 500 MHz spectrometer in D2O or DMSO-d6, using tetramethylsilane as an internal reference. Anion content was determined by Ion Chromatography system (IC) from Metrohm (2.850.2030 model), provided by 850 professional IC2 conductivity detector. The column (Metrosep A sup 7 250/4.0) was thermostated at 40 ºC and sodium carbonate anhydrous (0.382 g L-1; 0.7 mL min-1; 9.84 MPa) was employed as eluent. Detailed description of the device assembly can be found in the Supporting Information. 3.

RESULTS AND DISCUSSION

Two asymmetrically 1-alkyl-1’-aryl substituted viologens, specifically 1-ethyl-1’-(p-cyanophenyl)-4,4’-bipyridinium dibromide (Et-pCNVio) and 1-benzyl-1’-(p-cyanophenyl)4,4’-bipyridinium dibromide (Bn-pCNVio) were synthesized following the synthetic route shown in the Scheme 1. This procedure consists of three-step method wherein the aryl substituent was firstly attached by the wellknown two-step Zincke reaction, while the alkyl group was secondly bonded through the corresponding alkyl halide. It is worth to note that the attachment of the alkyl chain in the first step is also possible through the employment of an adequate non-polar solvent in which the mono-alkyl substituted intermediate precipitates.15 Regardless of the synthetic route, it is essential to ensure the formation of the pure mono-substituted intermediate, molecule [I] in the present case. The asymmetric viologens were proven by 1H and 13C NMR and Ion Chromatography (Figures S1-S6). Scheme 1. Synthetic route for asymmetric viologens Et-pCNVio and Bn-pCNVio.

[Bn-pCNVio]

[Et-pCNVio]

Corresponding symmetric viologens pCNVio, EtVio and BnVio were also synthesized for comparative purposes (Supporting Information). In this regard, pCNVio and EtVio symmetric viologens have been proven to provide green and violet colorations respectively at suitable applied potentials when similar PVA-borax electrolytic matrix is employed.6, 21

Spectroelectrochemical study of Et-pCNVio and BnpCNVio asymmetric viologens. The electrochemical behavior of Et-pCNVio and Bn-pCNVio asymmetric viologens was investigated using three-electrode ECD configuration provided with a pseudo-reference electrode which enables the in situ spectroelectrochemical study of these EC materials.6 To this end, EC formulations containing PVA-borax gel polyelectrolyte, Et-pCNVio and Bn-pCNVio as electrochromic materials (20 mmolL-1) and potassium ferrocyanide/ferricyanide salts as complementary redox species (5 mmolL-1) were formulated (Supporting Information) and the resulting Et-pCNVio and BnpCNVio gels were evaluated. Recently, this new concept of viscoelastic polyelectrolyte, very similar to the polyolbased gel known as “Slime” and released by Mattel Toy Corporation, has been demonstrated to provide very competitive and easily assembled ECDs while playing a crucial role in the observed colors.6, 21 Figure 1 shows the cyclic voltammetry (CV) registered for Et-pCNVio gel while using three-electrode configuration. Aside from the one which appears closer to 0V and assigned to the uncolored redox process of ferricyanide ion (Supporting Information Figure S7), two well-defined peaks were observed on the cathodic branche of the CV. The reduction peak observed at -0.7 V (vs Ag/Ag+) (1) can be related to the radical-cation (bipm+.) obtained through the first reduction of the most stable dicationic form (bipm2+), while the reduction peak registered at -1.2 V (vs Ag/Ag+) (2) can be correlated with the neutral state (bipm0), formed as a result of the second reduction of the dicationic form. The electrochemical behavior of Et-pCNVio asymmetric viologen, was compared to the ones exhibited by the corresponding symmetric viologens pCNVio, EtVio (Figure 1 inset), and it was found that the reduction potentials of the Et-pCNVio asymmetric viologen were about midway between the values registered for the corresponding symmetric viologens, showing intermediate



6 -1,4

-0,9

-0,4

0,1

0,6

1,1

4 2

Et-pCNVio gel

0 -2

-1,9

2 1

-1,3

-1

-0,7

-0,4

-8

Potential (V vs Ag/Ag +) -10

12 1

11

12

Et-pCNVio gel pCNVio gel EtVio gel

Potential (V vs Ag/Ag

-0.5 V -0.6 V -0.7 V -0.8 V -0.9 V -1.2 V

400

500

600

700

800

Wavelength (nm)

-2 -4

1 12

off

300

0

Arb. Unit

-4 -6

-1,6

1 1

90 80 70 60 50 40 30 20 10 0

+)

-6 -8

b)

-10

Figure 1. Three-electrode electrochromic device based on Et-pCNVio gel: cyclic voltammetry (CV) and the superposition of it (inset) with voltammograms of pCNVio and EtVio gels separately. Electrochemical studies of the symmetric viologens carried out in standard electrochemical cells can be 16, 22-23 found in the literature.

The electrochromic behavior of the Et-pCNVio and BnpCNVio gels was also studied using the same threeelectrode configuration. As shown by the UV-vis transmittance responses of Et-pCNVio and Bn-pCNVio gels under different applied potentials (Figure 2a and b respectively) both asymmetric viologens exhibited different transmittance profiles. Bn-pCNVio gel showed welldefined transmittance valleys at around 600 and 420 nm, meaning high absorption at these regions, while EtpCNVio gel, on the contrary, absorbed more equally along the most of the visible wavelength range. Accordingly, observed colorations upon applying the first reduction potentials in each case revealed more neutral colored state for Et-pCNVio gel (Figure 2c) than the one observed for Bn-pCNVio gel (Figure 2d). Color coordinates obtained by spectrophotometric method, also revealed more gray-EC behavior for the EtpCNVio gel (Table 1), as the chroma values were closer to a* = 0 and b* = 0 than the ones obtained for the BnpCNVio gel (Table S1). Indeed, the former exhibited more grayish coloration at low reduction potentials up to -0.9 V (bipm+˚) being a* and b* ≤ |15|, turning into more green color upon applying a reduction potential of -1.2 V (bipm0), since a* component became much more negative.

Transmittance (%)

-1,9

a)

Transmittance (%)

electrochemical character. Similar study was carried out with Bn-pCNVio asymmetric viologen and their corresponding symmetric viologens, Bn-pCNVio and pCNVio, leading to the same conclusion (Supporting Information Figure S8).

Current (mA)

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80

70

off

60

-0,4 V

50

-0,5 V

40

-0,6 V

30

-0,7 V

20

-0,8 V

10

-0,9 V -1,0 V

0 300

400

500

600

700

800

Wavelength (nm)

c)

d)

Figure 2. Three-electrode electrochromic devices containing Et-pCNVio and Bn-pCNVio gels. UV-vis transmittance response of Et-pCNVio (a) and Bn-pCNVio gels (b) at different applied potentials. Observed coloration for Et-pCNVio (c) and Bn-pCNVio gels (d) in their off state (left) and while an appropriate reduction potential (right) were being applied: -0.7 V (c) and -0.5 V (d).

In the case of Bn-pCNVio gel, b* components registered at different reduction potentials were negative up to -0.9 V showing more blue character, and acquired positive value upon applying -1.0 V, which corresponds to yellow color. It is especially noteworthy the grayish EC behavior achieved for the Et-pCNVio gel (a* and b* < |15|) while maintaining highly-transmissive and colorless bleached state (%Tb ~ 77 %), along with competitive transmittance changes (i.e., ~ 60 %).


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Table 1. % Transmittance, transmittance changes (∆%T) and color coordinates of three-electrode ECDs containing Et-pCNVio gel. Potential (V)

%Tb

%Tc

Δ %T(a)

x (b)

y (b)

Y (b)

L* (c)

a* (c)

b* (c)

off

-

-

0.309

0.326

78.4

91

0

-2

-0.5

59.7

17.9

0.307

0.337

64.0

84

-6

2

-0.6

41.5

36.0

0.317

0.353

44.1

72

-7

8

31.4

46.1

0.326

0.366

33.1

64

-7

11

-0.8

23.1

54.4

0.335

0.380

24.2

56

-8

15

-0.9

16.6

60.9

0.316

0.370

18.8

50

-10

9

-1.2

0.9

76.6

0.234

0.559

3.3

21

-38

19

-0.7

77,5

Color (d)

a)

Transmittance change at λ = 600 nm: (%Tb - %Tc) being Tb and Tc the percentage of transmittance at bleached (off) and colored states respectively. b), c)

Color coordinates (D65): xyY 1931 (x = blue / red; y = blue / green) (c) yellow (+)) .

(b)

and L*a*b* 1976 (a* = green (-) / red (+); b* = blue (-) /

d)

Color interpretation of the corresponding color coordinates (L*a*b) acquired through a color converter software (included to ease their interpretation).

Transmittance changes (∆%T) and %T registered at the maximum contrast wavelength for different applied potentials along with the corresponding color interpretation of the L*a*b color coordinates are also summarized in Table 1 (Et-pCNVio gel). Data for the Bn-pCNVio gel can be found in Table S1.

Comparative color performance study: asymmetric vs symmetric viologens. With the aim of assessing the more neutral color provided by the asymmetric viologens, their electrochromic behavior was compared to the one exhibited by symmetric viologens and the mixture of them. To this end, PVA-borax gel formulations comprising a mixture of the corresponding symmetric viologens (10 mmolL-1 of each), EtVio + pCNVio gel in the case of EtpCNVio and BnVio + pCNVio gel in the case of BnpCNVio, were also formulated (Supporting Information). Figure 3a shows the transmittance profiles of four different two-electrode ECDs containing Et-pCNVio gel, formulations comprising the corresponding symmetric viologens EtVio and pCNVio gels, as well as a composition including the mixture of them EtVio + pCNVio gel. In a similar way, figure 3b depicts transmittance spectra of Bn-pCNVio gel, along with BnVio, pCNVio and BnVio + pCNVio gels.

a)

Transmittance (%)

Thus, this system improves the performance of several polymer-based ECDs previously described in the literature which although they provided gray color in their neutral state,24-28 partially-doped state, 29-31 or full-doped oxidized state,32-36 did not exhibit a colorless bleached state.

80

Et-pCNVio gel

EtVio gel

70

pCNVio gel

EtVio + pCNVio gel

60 50 40

30 20 10

0 300

400

500

600

700

800

Wavelength (nm)

b)

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

Bn-pCNVio gel

BnVio gel

60

pCNVio gel

BenzVio + pCNVio gel

50 40 30

20 10 0 300

400

500

600

700

800

Wavelength (nm) Figure 3. UV-vis transmittance response of two-electrode ECDs containing Et-pCNVio (a) and Bn-pCNVio gels (b) vs corresponding symmetric viologens and the mixtures of them.



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The maximum absorption wavelength for the ECDs containing 1,1’-alkyl symmetric viologens, (EtVio and BnVio gels), was registered at around 550 nm, while the ones exhibited by pCNVio gel were found at 600 and 420 nm, as expected for violet and green materials respectively. Gels containing the mixture of the symmetric viologens, EtVio + pCNVio and BnVio + pCNVio gels, displayed also their maximum absorbance wavelengths at 600 and 420 nm, showing mainly the contribution of the 1,1’-aryl viologen. On the contrary, the ECDs comprising the asymmetric viologens, Et-pCNVio and Bn-pCNVio gels, exhibited a hypsochromic shift respect to pCNVio and mixed gels, but bathochromic shift respect to the EtVio and BnVio gels. This effect was less noticeable in the case of Et-pCNVio gel, being ascribed to the more equal absorption of this viologen along the most of the visible wavelength range, as mentioned above.

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viologens exhibit more neutral-color EC behavior than their corresponding symmetric ones. Besides that, the symmetric mixture formulations showed the presence of precipitates, which were also appreciable in the ECDs.

Switching times and EC properties. Switching responses of the two-electrode ECDs containing Et-pCNVio and Bn-pCNVio gels were also studied for two different applied potentials. As discussed before, less negative reduction potentials provided more neutral-grayish colorations, while more cathodic ones led to more green and blue colors for Et-pCNVio and Bn-pCNVio gels respectively.

The visual appearance (Figure 4a) and the CIE color space plots (Figure 4b) obtained for Et-pCNVio gel (1) at colored state, also showed more neutral-gray color than the ones exhibited by the corresponding symmetric systems EtVio (2) and pCNVio gels (3), and the mixture of them EtVio + pCNVio gel (4). Similar comparison performed for the Bn-pCNVio gel and their corresponding symmetric viologens and the mixture of them (Suporting Information Figure S9), also led to the same results.

a)

b)

1) Et-pCNVio

3 oo 4

2) y

1

o

2o

3)

4)

x

Figure 4. Two-electrode ECDs containing Et-pCNVio gel (1) vs corresponding symmetric viologens EtVio (2) and pCNVio (3), and the mixture of them (EtVio + pCNVio gel) (4). Photographs of each device at their colored states (a) and corresponding color coordinates represented in the chromacity diagram (b). Data represented in the L*a*b color space can be found in the Supporting Information Figure S10).

Color coordinates of each gel (Table 2), similarly revealed highly negative values of a* component for the pCNVio, EtVio + pCNVio and BnVio + pCNVio gels. Conversely, the asymmetric systems Et-pCNVio and BnpCNVio gels exhibited less negative values of a* than pCNVio gel, and less negative value of b* than EtVio and BnVio gels. These results confirmed that the studied ECgels containing Et-pCNVio and Bn-pCNVio asymmetric

Figure 5. Transmittance changes vs time (600 nm) of ECDs containing Et-pCNVio (a) and Bn-pCNVio gels (b) while square-wave potential-steps between bleached (0 V for 90 s) and colored state (-0.8 or -1.6 V for 30 s) were being applied; c) Evolution of the switching performance of an ECD containing Et-pCNVio gel after 500, 1 000, 2 000, 7 500, 10 000 and 15 000 cycles while square-wave potential-steps between bleached (0 V for 90 s) and colored state (-1.0 V for 30 s) were being applied..


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Table 2. % Transmittance, ∆%T and color coordinates of two-electrode ECDs containing different gels. EC-gel

Wavelength (nm)

%Tb

%Tc

Δ (a) %T

x (b)

y (b)

Y (b)

L*

(c)

a*

(c)

b*

(c)

Et-pCNVio

600

66.5

27.4

39.2

0.314

0.384

32.8

64

-17

14

Bn-pCNVio

600

55.7

13.9

41.8

0.255

0.335

23.4

55

-22

-5

EtVio

550

70.3

33.4

37.0

0.316

0.295

37.4

68

14

-9

BnVio

550

60.7

21.4

39.3

0.321

0.295

24.9

57

14

-8

pCNVio

600

48.6

3.2

45.4

0.244

0.545

15.7

47

-60

31

EtVio + pCNVio

600

54.7

7.3

47.4

0.278

0.531

20.7

53

-53

37

BnVio + pCNVio

600

48.8

9.1

39.7

0.291

0.501

20.7

53

-45

33

Color (d)

a)

Transmittance change: (%Tb - %Tc) being Tb and Tc the percentage of transmittance at bleached (off) and colored states respectively. b), c)

Color coordinates (D65): xyY 1931 (x = blue / red; y = blue / green) (c) yellow (+)) .

(b)

and L*a*b* 1976 (a* = green (-) / red (+); b* = blue (-) /

d)

Color interpretation of the corresponding color coordinates (L*a*b) acquired through a color converter software (included to ease their interpretation). Detailed information of each system individually for different applied potentials can be found in the Supporting Information. (Figures S11 - S13 and Tables S2 – S4). This comparative study was carried out choosing in each case the potential required to achieve ∆%T closer to 40% at the maximum contrast wavelength, since the cathodic potential required to form the radical+. cation (bipm ) differ from one viologen to another, not allowing to select one specific reduction potential for all the studied viologens. Please note that operational voltages of the two-electrode ECDs are higher (more cathodic) than the ones needed when three6, 37 electrode configuration is being used, as previously reported.

To evaluate both colorations, transmittance changes for ECDs containing Et-pCNVio and Bn-pCNVio gels were recorded at 600 nm vs time (Figure 5a) and b) respectively), while square-wave potential-steps between bleached (0 V for 90 s) and colored states (-0.8 or -1.6 V for 30 s) were being applied. At the two studied potentials, ECDs containing EtpCNVio gel exhibited faster coloring times and higher transmittance changes than the ones obtained for BnpCNVio gel (Table 3). This difference could be expected, due to the lower volume of the alkyl substituent the former presents, allowing faster mobility of the molecules within the PVA-gel matrix. It is worth to mention that switching times achieved by Et-pCNVio gel are similar to the ones obtained for PVA-borax gel-based formulations comprising pCNVio symmetric viologen,6 and also comparable to ECDs comprising solid or gel electrolytic matrix.38-39 In addition, switching responses acquired for EtpCNVio gel overrun those registered for some gray-black to colorless inorganic EC materials, such as nickel, molybdenum or iridium metal oxides,7 which habitually display long coloration and bleaching times even in the range of minutes.13, 40 Furthermore, it is worth pointing out that PVA-borax electrolytic matrix offers greater reversibility than other water-based electrolytes. In this context, it has been repeatedly published that redox process which occurred in

aqueous solution were not reversible while neither symmetrically substituted alkyl viologens 23 nor symmetric aryl viologens 41 were being employed. Additionally, some previously published studies affirmed that the simplest ECD configurations (substrate/TCO/EC formulation/TCO/ substrate) were unsuitable for electronic displays applications due to the dimerization, comproportionation and aging process present in this kind of devices.42 In order to assess the cycling performances of these systems, two-electrode ECDs containing Et-pCNVio and BnpCNVio gels were exposed to square-wave potential-steps between bleached and colored states. This cyclability study revealed that ECDs containing Et-pCNVio gel (Figure 5c) maintained their switching response and ∆%T up to 15 000 cycles (with a slight decrease of 3% of the %T at bleached state after 7 500 cycles, and 6% after 15 000) while preserving satisfactory neutral-color EC behavior (Supporting information Table S5). Conversely, in the case of Bn-pCNVio gel, although the ECDs also preserved their switching performances up to 15 000 cycles, they acquired more bluish EC character and more heterogeneous aspect during the first 500 cycles (Supporting Information Figure S14 and Table S6). Therefore, even though these systems comprise a water-based gel electrolyte and simple device architecture, they showed good reversibility, most notably in the case of Et-


ACS Applied Materials & Interfaces pCNVio gel. These results could be ascribed, firstly to the PVA-borax gel matrix which avoid the comproportionation,6 and secondly, the presence of ferrocyanide within it, as the latter may ease the dissociation of the dimer through stabilizing radical cation moieties avoiding the overlap of the parallel π-clouds of them.43

Table 3. Electrochromic properties and color coordinates (D65) of two-electrode ECDs containing Et-pCNVio and Bn-pCNVio gels. ECGel

Potential (V)

Δ %T(a)

tc (s)

tb (s) (b)

(b)

EtpCNVio BnpCNVio

ηc (cm2 C1 (c) )

ηb (cm2 C-1)

x (d)

y (d)

Y (d)

L*

(e)

a*

(e)

b*

(e)

Color

(f)

(c)

-0.8

42

14

7

118

188

0.313

0.390

29.9

62

-18

15

-1,6

56

11

15

157

243

0.248

0.479

14.7

45

-48

20

-0.8

20

22

24

180

255

0.293

0.338

37.3

67

-11

0

-1,6

30

19

8

152

341

0.255

0.335

23.4

55

-22

-5

a)

Transmittance change at λ = 600 nm: (%Tb - %Tc) being Tb and Tc the percentage of transmittance at bleached (off) and colored states respectively. b)

Switching times: time required for 90% of the total transmittance change to occur at the maximum contrast wavelength for the colored (tc) and bleached steps (tb). c)

Color efficiencies: ΔOD/(Q/A) being ΔOD = log (Tc/Tb), A = device area and Q = injected/ejected charge, for the colored (ηc) and bleached processes (ηb). d), e)

Color coordinates (D65): xyY 1931 (x = blue / red; y = blue / green) (e) / yellow (+)) .

(d)

and L*a*b* 1976 (a* = green (-) / red (+); b* = blue (-)

f)

Color interpretation of the corresponding color coordinates (L*a*b) acquired through a color converter software (included to ease their interpretation).

Other electrochromic properties such as coloration efficiencies (η), and color coordinates registered for EtpCNVio and Bn-pCNVio gels at the two studied potentials are also summarized in Table 3.

Dependence of electrochromic behavior on the solvent. The effect of the solvent on the electrochromic behavior of the Et-pCNVio and Bn-pCNVio asymmetric viologens has also been investigated, as the nature of the electrolyte has been proven to perform a crucial role in the observed coloration of the ECDs. Since the PVA-borax based gels employed so far in this work contained water as single solvent, alternative anhydrous electrolyte was used for comparative purposes. To this end, propylene carbonate (PC) based formulations comprising Et-pCNVio and Bn-pCNVio as electrochromic materials and hydroquinone as complementary redox species were formulated, and their electrochromic behavior was studied for different applied potentials (Supporting Information Figure S15 and Table S7). Interestingly, Et-pCNVio and Bn-pCNVio anhydrous solutions displayed mainly green coloration at the studied potentials showing their maximum absorbance wavelengths at around 650 and 420 nm. By comparing the UVvis spectra of these anhydrous solutions and the EtpCNVio and Bn-pCNVio gels (Figure 6 and Supporting Information Figure S16) a hypsochromic shift was ob-

served for ECDs containing gel electrolytes. It is worth to note that the spectroelectrochemical performance of anhydrous electrolyte in the absence of any viologen has also been studied in a three-electrode configuration to confirm that no coloration was observed due to the electrolyte by itself (Supporting Information Figure S17).

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 90 80 70 60 50 40 30 20 10 0 300


off_Gel

off_anhydrous

on_Gel

on_anhydrous

400

500

600

Wavelength (nm)

700

800

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Figure 6. Transmittance profiles of electrochromic devices containing Et-pCNVio gel vs anhydrous formulation in their bleached and colored states. Photographs of the device based on gel formulation (left) and anhydrous formulation (right) in their colored states (inset).

In order to gain further insights into the more neutralgray color observed in the case of Et-pCNVio and BnpCNVio gels, similar study was carried out employing ECDs based on symmetric alkyl viologens, (EtVio and BnVio) in the same anhydrous electrolyte (Supporting Information Figure S18 and Table S8). Curiously, analogous hypsochromic shift was observed in the EtVio and BnVio gels when comparing to anhydrous solutions (Supporting Information Figure S19), due to the more bluish character of these last (Supporting Information Table S9). In this regard, several studies based on symmetric 1-1’alkyl viologens demonstrated the formation of radicalcation as a dimer form ((bipm)22+) in ECDs containing aqueous electrolytes. In fact, it is well documented that the violet coloration observed when symmetric alkylsubstituted viologens are tested within a water-based electrolytes, is the result of both the blue monomer and the red dimer contributions.44 Previous studies related the formation of these dimers to a hypsochromic shift and the presence of new absorption bands at higher wavelengths in their absorption profiles.15, 45 Actually, the formation of radical-cation dimers have been demonstrated by ESR technique, for methyl and heptyl viologens within a PVA polymeric matrix, wherein the hypsochromic shift was also observed.23 In contrast, the dimer formation is hindered in some organic solvents such as propylene carbonate (PC).46 Hence, the reduced alkyl-symmetric viologen derivatives are blue in anhydrous solvents and purple in aqueous electrolytes due to the well-documented contribution of the both radical cation monomer and dimer formation.47 In a similar manner, the more neutral-gray colorations and the characteristic hypsochromic shift of the absorption profiles observed in ECDs containing EtpCNVio and Bn-pCNVio gels and not observed in anhydrous electrolytes can be ascribed to the presence of their radical-cation dimer. This fact explains the transmittance profile obtained for the Et-pCNVio gel, which absorbed quite equally along of the most visible wavelength, presumably due to the both monomer and dimer contributions by coupling their absorbance profiles. This is also in agreement with the mainly dark green coloration observed in the scarcely reported ECDs based on asymmetrically substituted 1-alkyl-1’-aryl viologens covalently bonded to a nanostructured electrode.48 In that case, the dimerization is impeded due to the poor mobility of the viologen molecules attached to the coated electrode, and it would only be allowed in concentrated systems wherein the viologen molecules are very close together showing more blackish color. Consequently, this kind of asymmetric viologens conveniently assembled within the PVA-borax gel polyelectrolytes provide colorless to neutral-gray electrochromic

behavior as a new color to the existing viologen-based electrochromics palette of colors, and providing the first proof of concept of using the dimerization phenomenon in asymmetric viologens, while allowing very competitive performances and cyclability (up to 15 000 cycles). This significant progress, in comparison to our previously reported study based on symmetrically substituted viologens and in complementarity with it,6 enhances the provided colorations including neutral-gray color. 4.

CONCLUSIONS

In summary, colorless to neutral-grayish color ECDs based on a single 1-Alkyl-1’-Aryl asymmetric viologen and the simplest device architecture (glass/TCO/EC gel/TCO/glass) were successfully developed. Reported 1ethyl-1’-(p-cyanophenyl)-4,4’-bipyridinium dibromide (EtpCNVio) and 1-benzyl-1’-(p-cyanophenyl)-4,4’bipyridinium dibromide (Bn-pCNVio) asymmetric viologens have been proven to provide more gray colored states than their corresponding symmetric viologens and the mixtures of them when tested within a PVA-borax gel electrolyte. Specifically, Et-pCNVio gel provided chromaticity coordinates very close to gray color (a* and b* ≤ |15|), while maintaining satisfactory colorless bleached state (%Tb ~ 77 %), transmittance changes (i.e., ~ 60 %), switching times (≤ 15s) and cyclability (i.e., ~ 15 000 cycles). These results offer many advantages over other neutral-color ECDs previously reported, such as an easier manufacturing process, competitive switching times and a colorless off state. Moreover, this type of ECDs may significantly extend the potential of the electrochromic technology since grayish colorations adapt better aesthetically to the surrounding environment, being easier to implement in different applications. Additionally, they absorb in the most of the visible range, making them excellent candidates for effective light filtering.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Synthesis of symmetric viologens (pCNVio, EtVio and BnVio); Preparation of formulations; Fabrication of Electrochromic Devices (ECDs); Characterization of 1-ethyl-1’-(pcyanophenyl)-4,4’-bipyridinium dibromide (Et-pCNVio) and of 1-benzyl-1’-(p-cyanophenyl)-4,4’-bipyridinium dibromide 1 13 (Bn-pCNVio) including H and C NMR spectra and anion chromatograms; Spectroelectrochemical studies of threeelectrode ECD containing Gel electrolyte and Anhydrous electrolyte for comparative purposes; Cyclic voltammetries of three-electrode ECDs containing Bn-pCNVio, pCNVio and BnVio gels; ∆%T and color coordinates of three-electrode ECDs containing Bn-pCNVio gel at different applied potentials; EC behavior of two-electrode ECDs containing BnpCNVio gel vs corresponding symmetric viologens BnVio and pCNVio, and the mixture of them (BnVio + pCNVio gel); CIE color coordinates obtained at colored states for twoelectrode ECDs containing Et-pCNVio gel vs corresponding symmetric viologens and the mixture of them, represented in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the L*a*b color space; EC-characterization of two-electrode ECDs containing different EC-gels (based on asymmetric, symmetric and mixture of symmetric viologens); Evolution of the electrochromic performance of two-electrode ECD containing Et-pCNVio gel after 500 – 15 000 cycles; Evolution of switching and electrochromic performances of two-electrode ECD containing Bn-pCNVio gel after 500 – 15 000 cycles; ECcharacterization of two-electrode ECDs containing different anhydrous EC-formulations (based on asymmetric, and symmetric viologens).

AUTHOR INFORMATION Corresponding Author * Ana Viñuales: [email protected]

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Table of Contents (TOC):


12

Colorless to Neutral Color Electrochromic Devices Based on

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