Relax: A Sterically Relaxed Donor–Acceptor Approach for Color

Article pubs.acs.org/Macromolecules

Relax: A Sterically Relaxed Donor−Acceptor Approach for Color Tuning in Broadly Absorbing, High Contrast Electrochromic Polymers Justin A. Kerszulis, Rayford H. Bulloch, Natasha B. Teran, Rylan M. W. Wolfe, and John R. Reynolds* School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: A family of five electrochromic polymers (ECPs) based on 3,4-dioxythiophenes (XDOTs) (where X = ethylene (E) and propylene (Pro)) or a 3,4-diacyclic-substituted dioxythiophene (AcDOT, Ac) as main chain donors coupled in a random fashion with the sterically relaxed (i.e., low torsional energy barriers to planarity) donor−acceptor−donor unit EDOT−BTD− EDOT (EBE, BTD = 2,1,3-benzothiadiazole) were synthesized using direct heteroarylation polymerization conditions. Designed as broadly light absorbing materials to improve overall contrast, ECPs were solution spray-cast into thin films, and their electrochromic properties were measured and compared against the previously reported ECP-Black (random copolymer composed of ProDOT and BTD). These new polymers exhibit enhanced integrated contrasts across the visible region (380−780 nm, Δ%Tint) larger than 45%, the highest achieved being ∼52%, a substantial improvement over ECP-Black (Δ%Tint = ∼32%). Increasing torsional strain of the main chain donor units moves the short wavelength peak to higher energy, allowing hue control. Increasing the composition of the EBE monomer in random copolymers yields more level and uniform absorption across the visible, reducing hue saturation and giving more muted colors relative to the normally vibrantly colored ECPs with no observable loss in contrast. Pro-Ac0.65/EBE0.35 gave the highest integrated electrochromic contrast with color values (L*a*b* where L* indicates lightness, a* defines green and red, and b* defines blue and yellow hues, for negative and positive values, respectively), indicating improved color neutrality (45, 5, 3) when compared to ECP-Black (47, 3, −14). Two-component solution blends of Pro-Ac0.65/EBE0.35 with the previously reported all donor polymer ProDOT2-EDOT in differing weight ratios were prepared and cast into films giving more aesthetically pleasing black-to-transmissive electrochromes, while maintaining a high integrated contrast at ∼51%. The use of EBE demonstrates the synthetic capability to improve the contrast of broadly absorbing ECPs for black or dark-to-transmissive applications in electrochromic window and display-type devices.

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INTRODUCTION Electrochromism (EC) is a phenomenon described as the ability of a material to change its electromagnetic absorptive properties with the application of a current or voltage. The most practical color change is from an absorptive colored state to a transmissive, colorless one, enabling light to be continuously modulated. A range of well-studied materials (transition metal oxides, Prussian Blues, small organic molecules, and π-conjugated polymers) demonstrate this phenomenon, giving it the potential to revolutionize technologies spanning from displays, signage, windows, security, camouflage, and wearable fabrics.1−4 © XXXX American Chemical Society

In the quest to develop electrochromic windows or eyewear with black or dark colored states that switch to transmissive colorless forms, metal oxides have dominated the field with complementary device architectures. In a metal oxide-based electrochromic device (ECD) in the colored state, cathodically coloring tungsten oxide (absorbing long wavelength light when reduced) and anodically coloring nickel oxide (or vanadium oxide, both materials absorbing short and medium wavelength Received: May 25, 2016 Revised: August 5, 2016

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Macromolecules Scheme 1. General Polymerization Scheme To Produce Broadly Absorbing ECPs Using a Sterically Relaxed D−A−D Architecturea

a

Green and blue represent structurally different dioxythiophenes.

spectral broadness was first synthetically achieved in 2008 by Beaujuge et al.30 via the random oxidative polymerization of a ProDOT monomer with ter-heterocyclic ProDOT-BTDProDOT comonomer. Greater color neutrality was achieved by broadening of the absorption spectrum in the visible region as reported by Shi et al.,31 who employed a Stille polymerization, which allowed access to shorter segments of oligoProDOTs in a chain, absorbing more blue light, and thereby achieving a more black neutral state in a polymer referred to below as “ECP-Black”. Additional chemically polymerized black-to-transmissive ECPs have been reported since.32−34 The same broadening effect for black-to-transmissive electrochromism has also been achieved through the electropolymerization of monomers that utilize donor−acceptor− donor (D−A−D) interactions.35−38 Individual ECPs and blends with exceptional achromaticity have been published but unfortunately lack adequate transmission upon oxidation to be suitable in high contrast ECDs.39−42 Further, dark-totransmissive devices were achieved through the use of polymer blends43 or poly(3,4-ethylenedioxthiophene) (PEDOT) coupled with a yellow organic dye.44 The black-to-transmissive polymers detailed above possess a considerable degree of residual absorption in their transmissive states, leading to low visible light contrast during electrochromic switching. Many ophthalmic tints and coatings45,46 indicate a requirement for integrated contrast (Δ%Tint) across the visible of >50%, a value that ECP-Black is unable to meet. This low contrast may be a result of reduced delocalization of charge carriers across the acceptor moiety in the polymer backbone, as observed with D−A-based ECP systems.47,31 Carefully considering this, the reduced delocalization may be attributed to ortho C−H steric interactions between the hydrogens on BTD and the OCH2 units on the sevenmembered ring of neighboring ProDOTs. This is the likely steric interaction that would give a higher energy barrier to planarity, resulting in quinoidal oxidized states that do not adequately delocalize charge carriers. In the work reported here, we seek to achieve both an achromatic/black neutral state and increased charge delocalization in the oxidized state in a random copolymer, effectively red-shifting more of the residual red-light absorption of the most oxidized transmissive state into the near-infrared, absorbing less long wavelength visible light, and consequently enhancing contrast across the visible spectrum. Based on previous reports on colored-to-transmissive ECPs,48,49 high electrochromic contrast can be achieved at the D−A charge transfer (CT) peak by flanking an aromatic electron acceptor with sterically (torsionally) relaxed EDOTs.50

light when oxidized) electrochromes color-mix to achieve an aesthetically pleasing and highly color-neutral (black) hue.5,6 Transition metal oxide materials, however, typically take relatively long times to complete a full switch and tend to require elevated temperatures during preparation, making it difficult to employ them on flexible conducting plastic substrates.7 Recently, however, studies involving inkjet printing,8−10 sputtering,11 nanopaper transfer methods,12 and even electrochemical deposition with high film contrast and good cycling rates13 are showing great promise. While the speed of switching is a minor concern for EC windows for architectural uses in offices and homes, this slow switching is not as viable for applications such as eyeglasses, goggles, pilot/athletic visors, or automotive/aircraft windshields, along with reflective displays and signs, where subsecond color to colorless switches are desirable and, in some instances, practically necessary. Prussian blue (PB)14 and viologen15 species can exhibit high speed switching; however, the PB reduced colorless state is unstable to oxygen, making ambient device construction difficult at scale while having long lifetimes in mind. The “Gentex-like” technology utilizing viologen, where the electrochrome is a small molecule dissolved in an electrolyte, is self-clearing but due to diffusion has no EC memory. Throughout the recent decade, π-conjugated electrochromic polymers (ECPs) have demonstrated their facility in color tuning as a result of structural modification of the polymer’s repeat unit,16,17 color mixing via solution blending of primary colored ECPs,18,19 and device architectures which allow color control of EC films independently of one another.20,21 As ECPs are typically solution coated as films which can be imbibed with supporting electrolyte,22 they exhibit the ability to switch with high contrast more rapidly than metal oxides, with typical times on the order of a second for small (cm2) devices.23 Along with the ability to tune the optical properties, ECPs can be processed into thin films using roll-to-roll techniques,24,25 and modification of material solubility is achievable via postprocessing functionalization.26,27 Methods also exist to enable ECP aqueous processing with subsequent conversion to solventresistant forms, yielding fast electrochromic switching kinetics.28,29 To match the broadly absorbing black and neutral colors achieved by the metal oxides using a single ECP, a random backbone architecture has been employed where donor− acceptor (D−A) interactions are used to absorb long wavelength light, while varied lengths of donor segments absorb medium to short wavelength light. Using 3,4-propylenedioxythiophene (ProDOT) and 2,1,3-benzothiadiazole (BTD), B

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Macromolecules Scheme 2. Polymer Repeat Unit Structures with Sterically Relaxed EBE Units To Produce Broadly Absorbing ECPs

(27.9, 2.10, 40), Pro-Ac0.70/EBE0.30 (41.9, 3.38, 52), Pro-Ac0.65/ EBE0.35 (44.7, 4.48, 55), Pro-Ac0.75/EBE0.25 (50.9, 3.20, 64), and Pro-Pro/EBE (68.0, 3.17, 78). Ac-Ac/EBE has the lowest weight, a kinetic effect, which may be attributed to a steric barrier to oxidative addition of the bulkier aryl dibromide, resulting in reduced conversion (Xn) and lower Mn. As the steric barrier is reduced with addition of ProDOTs, the molecular weights increase. The molecular weights obtained were sufficient to cast thin solid films onto indium tin oxide (ITO)-coated glass for electrochromic characterization. In this report, two sets of polymerization approaches were used to achieve polymers with the same monomer components, but different primary structures, leading to different properties, and this is illustrated in Figure 1. In the first set, the sterically

We hypothesize that this method will enhance charge carrier delocalization, leading to a greater transmission of long wavelength visible light in the ECP’s oxidized state. Herein, we explore an EDOT flanked BTD acceptor (EDOT-BTDEDOT, EBE) and its incorporation into random copolymers, which results in improved contrast of black-to-transmissive ECPs. Films produced from coprocessing a pair of carefully selected polymers result in a flat spectral profile and an aesthetically pleasing indigo-black hue.

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RESULTS AND DISCUSSION

An approach for producing ECPs with random length chromophores and relaxed steric interactions between EBE’s and substituted 3,4-dioxythiophenes (XDOTs) is illustrated in Scheme 1, where x and y represent comonomer ratios. The blue or green spheres represent either ProDOT or AcDOT; the color representation in the scheme only indicates a difference. Our use of the EBE ter-heterocycle ensures that the BTD acceptor will always be flanked by the less-sterically demanding six-membered rings from the two EDOTs (as opposed to the seven-member ring in ProDOT) in random copolymerizations. We hypothesize that once fully oxidized, this will allow for greater planarization across the D−A−D segment, improving delocalization of charge carriers, red-shifting the charge carrier transitions, imparting higher transmittance to the oxidized forms, and thus increasing electrochromic contrast. Calculations of the torsional energy barrier to rotation between dimers of BTD and EDOT/ProDOT are shown in Figure S1a. Computational studies were performed at the B3LYP/6-31G +(d, p) and MP2/cc-pVTZ level of theory51,52 using the Gaussian 09 package.53 From their lowest relative energy conformation (0°, Figures S1c−f are taken from angles greater than this conformation) in the neutral state, the BTD-EDOT dimer exhibits a higher torsional energy barrier to twisting away from planarity than BTD-ProDOT. Thus, the calculations support improved “relaxation” or conjugation when EDOTs are flanking BTD vs ProDOTs. The EBE ter-heterocycle was synthesized using direct arylation coupling methods,54 followed by purification via sublimation, and washing the resulting red solid with methanol. All polymers in this study were synthesized via direct (hetero)arylation polymerization (DArP). The final structures are shown in Scheme 2 (specific synthetic methods used can be found in the Supporting Information). Polymer molecular weight was analyzed with gelpermeation chromatography (GPC) in chloroform (results shown in Figure S2), and the number weight-average, dispersity, and degree of polymerization (Mn (kDa), PDI, Xn) relative to polystyrene standards are as follows: Ac-Ac/EBE

Figure 1. Model to produce broadly absorbing ECPs with higher contrast by increasing steric strain, followed by increasing EBE content.

relaxed internal D−A−D monomer (EBE) was used in a molar ratio of 0.25, while the molar ratios for the external DOT units were varied, increasing external strain by raising AcDOT (Ac) content (vs ProDOT (Pro) content), in the polymers Pro-Pro/ EBE to Pro-Ac0.75/EBE0.25 to Ac-Ac/EBE (subscripts indicate molar ratios). In thin solid films, these polymers produce dualband absorptions where the position of the λmax of the D−A peak50 was essentially fixed, and the shorter wavelength π-to-π* peak was hypsochromically shifted with increasing backbone strain from AcDOT. In the second set, external strain was relatively fixed, and the ratio of internal D−A−D monomer EBE was increased from 0.25 to 0.35 equiv starting from ProAc0.75/EBE0.25 to Pro-Ac0.70/EBE0.30 to Pro-Ac0.65/EBE0.35. Here, because Pro-Ac0.75/EBE0.25 was able to absorb the most light across the visible spectrum with higher contrast than AcAc/EBE and similar contrast compared to Pro-Pro/EBE, this mixed-DOT Pro-Ac/EBE copolymer architecture was selected as the starting structure from which to measure the effect of changes in EBE content. Though Pro-Pro/EBE does indeed C

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results given in Table 2. The peaks are normalized to the longer wavelength D−A band to emphasize the effect on peak position

have the highest contrast of all materials, herein it is not the most broadly absorbing. Redox Behavior. To determine the effects that variation of backbone strain and EBE content have on the electrochemical oxidation onset, cyclic voltammetry (CV, Figure S3) and differential pulse voltammetry (DPV, Figure S4) were performed on drop-cast films on platinum button electrodes. Prior to measurements, films were cycled five times between −500 and 800 mV to condition the materials to the influx of electrolyte and subsequent change in optoelectronic propertiesso-called electrochemical break-in. The oxidation potential onsets via DPV presented in both Table 1 and Figure S3

Table 2. Spectral Properties of Each Polymer in the Series D−A peak (nm) polymer ECP-Black Pro-Pro/EBE Ac-Ac/EBE Pro-Ac0.75/ EBE0.25 Pro-Ac0.70/ EBE0.30 Pro-Ac0.65/ EBE0.35

Table 1. Electrochemical Potential Onsets Measured by Differential Pulse Voltammetry (DPV) polymer

oxidation onset (mV vs Ag/Ag+)

ECP-Black Pro-Pro/EBE Ac-Ac/EBE Pro-Ac0.75/EBE0.25 Pro-Ac0.70/EBE0.30 Pro-Ac0.65/EBE0.35

52 13 180 110 50 −45

absorption onset (eV)

π-to-π* peaka (nm)

pristinea

brokenina

pristinea

brokenina

−b 543 457 505

−b 652 670 660

−b 675 674 671

−c 1.59 1.47 1.54

1.61 1.50 1.45 1.50

495

666

677

1.50

1.49

487

668

680

1.49

1.48

For films sprayed to an optical density of ∼1.0 au (at D−A peak). 634 nm for ECP-Black λmax; peaks are not resolved in this spectrum (see Figure 2). cECP-Black exhibited no changes upon break-in.

a b

or relative peak intensity upon variation in monomer ratios. With a sterically relaxed EBE D−A−D architecture, lower energy onsets of light absorption were achieved that are ∼0.10 eV less than the previously reported ECP-Black. Slight changes in the spectra of the polymers during electrochemical break-in were observed, as shown in Figure S5. The shorter wavelength peak corresponding to the π-to-π* transition exhibits minimal change upon break-in, except for an increase in intensity seen with Pro-Pro/EBE. For the lower energy peak attributed to D− A CT interactions, however, there is a slight red-shift in both the λmax and the onset of absorption, allowing absorption of longer wavelength light and with a minor intensity increase. The changes between the pristine and broken-in spectra are likely due to backbone reorganization in the form of planarization from the added EDOTs flanking the acceptor. This is supported by the decrease in onset of electrochemical oxidation shown in CV cycles 1 and 2 in Figure S3, an observation which has been reported previously in πconjugated systems.47 In Figure 2a, as torsional strain is increased from Pro-Pro/ EBE to Pro-Ac0.75/EBE0.25 to Ac-Ac/EBE, the π-to-π* λmax

give an increasing trend of 13 to 110 to 180 mV for Pro-Pro/ EBE to Pro-Ac0.75/EBE0.25 to Ac-Ac/EBE, respectively, correlating with increasing torsional strain along the conjugated backbone. As the content of EBE was increased, there is a decrease in oxidation potential from 110 to 50 to −45 mV for Pro-Ac0.75/EBE0.25 to Pro-Ac0.70/EBE0.30 to Pro-Ac0.65/EBE0.35, respectively, which is attributed to increasing EDOT content. Thin Film Spectroscopy. Prior to optical characterization of films on ITO/glass, spray-cast films (deposited at the long wave peak to an absorbance of 1.0 ± 0.05) were subjected to repeated cyclic voltammetry (CV) cycling (25 “break-in” cycles) to observe any redox and optical changes between the pristine (as-cast) and broken-in states (post 25 cycles). All polymer films demonstrated reversible break-in cycling with electrochemical differences between the first and second scan, stabilizing over subsequent scans, as shown in Figure S3. The UV−vis spectra of the neutral states for the full family of polymers (after break-in) are shown in Figure 2 with the full

Figure 2. Spectra of electrochemically conditioned films (normalized at the long wavelength peak) showing variations of the π-to-π* transition with increasing strain (a) and EBE content (b). D

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Figure 3. Extreme state spectra for the ECPs where the π-to-π* transition was tuned through torsional strain (a). Extreme state spectra for polymers where EBE content was varied (b). Films sprayed to ∼1.0 au and then broken in. Visible range of 380−780 nm is bound by the black dashed lines.

A−D system substantially enhances the overall contrast across the visible relative to ECP-Black, as shown in Table 3. It is

blue-shifts from 543 to 505 to 457 nm, respectively, with a window of transmission gradually becoming more pronounced, until the window is positioned at 558 nm for Ac-Ac/EBE, leading to a green neutral state. This is effectively one structural “knob” with which to tune the hue of an ECP. A second “knob” that tunes saturation is shown in Figure 2b, where the steric strain is reduced/relaxed with increased EBE content from ProAc0.75/EBE0.25 to Pro-Ac0.70/EBE0.30 to Pro-Ac0.65/EBE0.35. The λmax for the high-energy π-to-π* peak in this series of polymers changes little from 505 to 495 to 487 nm, respectively. Concurrently, the ratio of the absorbance of the two peaks (πto-π*/D−A) decreases from 1.44 (for Pro-Ac0.75/EBE0.25) to 1.18 (for Pro-Ac0.70/EBE0.30), until becoming nearly equal at 1.04 (for Pro-Ac0.65/EBE0.35). Pro-Ac0.65/EBE0.35 exhibits dualband absorption with a shallow window at 586 nm, making it nearly achromatic. For all of these polymers, the D−A absorption peak remains relatively fixed at 671−680 nm. Spectroelectrochemistry. To examine the changes in absorption upon redox switching, and detail the electrochromic contrast from the neutral to the most oxidized states, spectroelectrochemistry of the polymer thin films was performed in 50 mV steps. The extreme states are presented in Figure 3 with the full series in Figure S6. The change in transmittance at the long wavelength peak is scrutinized as donor−acceptor systems typically exhibit poor transmission during cycling. In general for this material family, at the long wavelength peak, the polymers have fully oxidized states with a change in transmittance (Δ%T) > 50% with the exception of Ac-Ac/EBE, likely due to strain from the high AcDOT content preventing enhanced delocalization of the charged states. However, Ac-Ac/EBE exhibits a Δ%T at the long wavelength peak of 44%, still greater than the Δ%T of ECP-Black at 634 nm, which is 41%. Ac-Ac/EBE also has a higher contrast when compared to previously reported green-to-transmissive ECPs.55−58 In order to compare effects on contrast, the Δ%T for these broadly absorbing polymers have been quantified by integrating the neutral and oxidized state spectra across the visible (380− 780 nm). All the polymers with the exception for Ac-Ac/EBE have integrated contrast values (Δ%Tint) > 50%. Ac-Ac/EBE has the lowest integrated contrast due to its high degree of strain, preventing enhanced delocalization of charged states. With this, it is evident that the method of using a relaxed D−

Table 3. Integrated Contrast Values of Each Polymer in the Series (Δ%Tint across 380−780 nm) polymer

integrated contrast (%)

ECP-Black Pro-Pro/EBE Ac-Ac/EBE Pro-Ac0.75/EBE0.25 Pro-Ac0.70/EBE0.30 Pro-Ac0.65/EBE0.35

34.2 52.8 45.6 51.0 51.2 51.7

important to note that as the EBE content in Pro-Ac0.75/ EBE0.25, Pro-Ac0.70/EBE0.30, and Pro-Ac0.65/EBE0.35 is increased, there is little reduction in contrast with growing content of EBE. This means that a relaxed D−A−D approach could be used to tune a myriad of colors based on higher EBE content with minimal loss to contrast. Pro-Ac0.65/EBE0.35 has the most even absorption across the visible, appearing black-brown to the eye when neutral (see Figure S8). When fully oxidized, it exhibits a higher level of visible light transmission when compared to ECP-Black. The full spectroelectrochemical series is shown in Figure 4 where the general spectral properties and features are the same across the family of polymers (Figure S6). As the polymers are progressively oxidized, there is an uneven depletion in the intensity between the two peaks as demonstrated by the green spectrum corresponding to 150 mV in Figure 4, where the D− A CT peak appears to oxidize more rapidly than the π-to-π* peak. This is likely due to the EDOTs possessing a lower oxidation potential than other DOTs. This results in the color values forming a “loop” on colorimetry throughout the oxidation process (vide infra). Though this family of materials exhibits an elevated EC contrast relative to ECP-Black, in the oxidized state there is still a considerable degree of visible light absorption tailing from 600 to 780 nm when compared to all donor polymers.47 This tail absorbing red light, highlighted by the shaded triangle in Figure 4, though reduced in magnitude in this family of polymers relative to ECP-Black, continues to impart a pale blue hue to the oxidized states. E

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Figure 4. Spectroelectrochemistry of Pro-Ac0.65/EBE0.35 where neutral, intermediate, and oxidized states are represented by the blue, green, and red traces, respectively. The shaded triangle illustrates the persistent absorption tail in the oxidized state.

Colorimetry. L*a*b* color space results and photos for the polymers under study are presented in Figures 5 and 6 with examples as a function of thicknesses found in Figures S7 and S8. In this color visualization, a* represents green to red, and b* the blue to yellow, hues (negative to positive values respectively). L* depicts the lightness: a value of 0 would be black and 100 would be white. As the magnitudes of a* and b* increase, color becomes more saturated, and as one traverses between color points or values, the hue changes. As the ECPs reported here are broadly absorbing, they have a* and b* values closer to the origin than those for the more saturated and vibrantly colored ECPs. As detailed above, color has been controlled through two routes to enhance the transmittance of the fully oxidized states. The first route or “knob” to control neutral state color (a*, b*) is associated with the steric strain of the donors along the main chain from Pro-Pro/EBE (−31, 30) to Pro-Ac0.75/EBE0.25 (23, −4) to Ac-Ac/EBE (−14, 44) (shown in Figures 5a and 6). Pro-Pro/EBE has a broad, but uneven, absorption across the visible spectrum (Figure 2), allowing unequal amounts of blue and red light to pass, giving the film a deep purple appearance. Pro-Ac0.75/EBE0.25 absorbs much of the blue and green light from its short-wavelength π-to-π* peak, while transmitting a portion of red light, appearing as dark, brick-red in color. Both of these polymers absorb strongly where the y ̅ standard observer is most stimulated, giving them low L* values. However, due to their uneven absorption, they possess a degree of color saturation. Upon electrochemical oxidation, the a*b* values for the more color-neutral materials change minimally, while L* changes drastically, indicating an increase in lightness of the most oxidized transmissive state. Ac-Ac/EBE, with its highly blue-shifted π-to-π* band, absorbs more blue than red light; but with a deep window centered at 556 nm, it allows green light to pass and exhibits a lime-green color with a high b* value. Comparing this series, Pro-Ac0.75/EBE0.25 exhibited a longer wavelength peak intensity that more closely matched the absorbance of the shorter wavelength peak, thereby making it more achromatic or color neutral, while retaining a high EC contrast across the visible. As such, it was selected as the starting point to study the effect of increasing the content of the relaxed D−A−D EBE on color and contrast as the second route or “knob” to control color (a*, b*). Examining the results of

Figure 5. Colorimetry values (a*b*) for relaxed D−A polymers where the π-to-π* transition was tuned through strain (a) and EBE content (b). Lightness values for all polymers in this section (c).

Figures 5b and 6, as the content of EBE is raised from ProAc0.75/EBE0.25 (23, −4) to Pro-Ac0.70/EBE0.30 (13, 0) to ProAc0.65/EBE0.35 (5, 3), the saturation is progressively reduced and the colors become more muted. Because of the more even absorption that is present in these systems across the visible spectrum (Figure 2), more light is able to overlap with the y ̅ standard observer, raising the lightness. Pro-Ac0.70/EBE0.30 absorbs more red light than Pro-Ac0.75/EBE0.25, so color values decrease overall, making it brown. Subsequently, Pro-Ac0.65/ EBE0.35 absorbs even more red light, reducing the saturation further to appear black-brown. Though it has low a*, b* values of 5, 3, the positive magnitude is a result of residual red and F

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Figure 6. Photographs of the polymers in their neutral (N) and oxidized (Ox) forms as well as L*, a*, and b* color values.

Figure 7. Spectra of the neutral and oxidized states of two mixtures of Pro-Ac0.65/EBE0.35 and ProDOT2-EDOT (a). Dashed lines give the bounds of the visible spectrum from 380 to 780 nm. Colorimetry of mixtures and individual components (b). Photographs and color values for the two mixtures are inset. Squares represent neutral states, while circles represent oxidized states. 15% mixture (black), 20% mixture (red), Pro-Ac0.65/ EBE0.35 (blue), ProDOT2-EDOT (dark cyan).

Within this study, the most color neutral polymer developed, Pro-Ac0.65/EBE0.35, retains a red-brown hue as a result of the small disparity in the intensity in absorption between the two major bands, in combination with an absorption trough centered at 586 nm. To further reduce color saturation, with the ultimate goal of forming a true black electrochrome, we turned our attention to solution coprocessing of an additional high-contrast ECP, specifically ProDOT2-EDOT47 with a λmax at 590 nm, into thin films with Pro-Ac0.65/EBE0.35. This strategy was used to fill in the gap at the transmission window, smoothing the spectral profile of the films, while retaining a high EC contrast upon switching. Coprocessing an 85/15 w/w solution mixture of [Pro-Ac0.65/ EBE0.35]/[ProDOT2-EDOT] by spray-casting into films produces such an absorption with a particularly flat spectral profile throughout the range of 450−700 nm as shown in Figure 7 (and supported by overlaid spectra of the two polymers in Figure S9). While an overall increase in chromaticity is observed for this mixture, relative to the 80/ 20 composition, the most notable change in this is the production of an aesthetically pleasing indigo-black hue, seen in the lowering of the b* coordinate from 3 to −7. Additionally, an improvement to the integrated contrast is observed with this mixture, from 51.7% in the neat polymer to 53.0% in the blend. It should be noted that a mild asymmetry is still present in the

orange light transmitting due to the transmission window at 586 nm. A “loop” (vide supra) can be seen in all of the color plots in Figure 5a,b or Figure S7 and is a result of the longer wavelength D−A band bleaching faster upon oxidation than the π-to-π* band. This allows more red light to reach the eye with increasing voltage, so the a* becomes more positive and the polymers pass through reddened intermediate states. Ac-Ac/ EBE, on the other hand, passes through yellow-hued intermediate states because more red light along with green light is stimulating both the M and L cones, respectively, which the brain interprets as yellow. These new materials are more transmissive than previous D− A ECPs. Closely examining Figures 5 and 6, can the eye notice the difference between color points of the fully oxidized transmissive states? This is determined by taking a geometric distance of color points using the equation * = ΔEab

(ΔL*)2 + (Δa*)2 + (Δb*)2

where ΔEab * represents the difference between colors points, with values below 2.3 (or what the CIE determines as just noticeable dif ference (JND)) indicating differences that are not noticeable to the eye.59 Comparing the fully oxidized color states between ECP-Black and Pro-Ac0.65/EBE0.35, we calculate a ΔEab * of 4.8, confirming a noticeable difference, with ProAc0.65/EBE0.35 having the least color once fully oxidized. G

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spectral profile of the 15% ProDOT2-EDOT mixture. However, further incorporation of ProDOT2-EDOT by increasing the content to 20 wt % is not a viable strategy to address this asymmetry, producing a mixture with higher chromaticity and an observed contrast lower than that of the neat polymer (51.3%). In a similar pursuit to better color match for dark-totransmissive applications, Sassi et al.60 recently detailed Weitztype electrochromes utilizing polymerizable EDOT/ProDOT side chains which achieved absorption profiles, color values, and impressive contrasts comparable to the materials discussed herein. A film of PB was then added to absorb longer wavelength light, yielding the desired black state.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01114. All electrochemical and colorimetry details, photography settings, synthetic procedures, calculations, GPC analysis, optical and electrochemical characterizations, and 1H NMR spectra of all polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(J.R.R.) E-mail [email protected].

CONCLUSION Five new broadly absorbing polymers utilizing a relaxed donor−acceptor−donor architecture (where EDOTs flank BTD) were synthesized showing improved electrochromic contrast, where previously reported systems exhibited tailing absorption of long wavelength light, yielding pale blue oxidized transmissive state hues. Pro-Ac0.65/EBE0.35 (and all of the polymers and mixtures in this study with the exception for AcAc/EBE) achieved integrated contrast values Δ%Tint > 50%, a significant improvement over previous DOT-based black-totransmissive ECPs, making them suitable for potential application in eyewear and window type devices. To achieve more aesthetically pleasing indigo-black neutral state hues with high electrochromic contrast, mixtures of Pro-Ac0.65/EBE0.35 with the all donor polymer ProDOT2-EDOT were produced (15 and 20 wt % ProDOT2-EDOT) giving black-to-transmissive films, while maintaining a Δ%Tint > 50%. Though a persistent absorption tail from 650 to 800 nm remains in these new polymers, they demonstrate a method to improve overall contrast across the visible. Thus, the blue hue resulting from residual absorption due to the acceptor moiety in broadly absorbing D−A polymers can be mitigated through relaxing the charged state delocalization (“quinoidization”) by swapping the donor structure from the seven-member pendant ring ProDOT (higher steric strain) with six-member ring EDOT (lesser steric strain). Ultimately, this study leads us to ask the question, what is black when one considers varied hues in ECPs? Black neutral state colors are difficult to achieve as conjugated backbones that can capture light across the visible spectrum where steric interactions are used to absorb short and medium wavelength light while donor−acceptor interactions absorb long wavelengths are necessary. More challenging is achieving highly transmissive oxidized states when the varied steric hindrance and electron richness, or poorness, along a π system needed to attain the broad neutral absorption do not easily planarize or form stabilized oxidized states. From a device practicality standpoint, what is pleasing to our eye? Would it be better to use a single material with broad absorption giving L*a*b* color values as close to zero as possible while sacrificing contrast given the current state of the art? Should one compromise and use materials that are brown, deep purple, or burgundy but possess greater contrast? Or is it better to employ polymers with chromaticity and higher contrasts and then color match to achieve desired black hues? As there are considerable challenges in balancing the steric and electronic effects to achieve black neutral state colors, while stabilizing charges for highly transmissive states, these are questions to be considered in the exploration of new materials or architectures for electrochromic devices.

Funding

We appreciate funding of this work from BASF and the Air Force Office of Scientific Research FA9550-14-1-0271. Government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. We thank the Dow Chemical Company for GPC analysis of the studies polymers and the Partnership for an Advanced Computing Environment (PACE) at Georgia Tech for providing computational resources. Notes

The authors declare no competing financial interest.

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