Fine-Tuning the Color Hue of π-Conjugated Black-to-Clear

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Fine-Tuning the Color Hue of π‑Conjugated Black-to-Clear Electrochromic Random Copolymers Chi Kin Lo, D. Eric Shen, and John R. Reynolds* School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

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S Supporting Information *

ABSTRACT: A family of dioxythiophene (DOT)-based conjugated random copolymers, capable of reversibly switching between a neutral black and an oxidized transmissive state, are reported for electrochromic (EC) applications. By replacing 3,4-ethylenedioxythiophene (EDOT) in the previous generation of a core donor−acceptor−donor (D−A−D) terheterocycle with a methyl-alkylated 3,4-propylenedioxythiophene (ProDOT), we enhance the solubility of precursors and monomers in this new generation of broadly absorbing electrochromic polymers (ECPs). By adjusting the newly designed terheterocycle monomer feed ratio, we obtain three polymers with varying hues of color neutrality (a* and b* < ±10) throughout the entire switching voltage range and with integrated contrasts 45−49% throughout the visible region (380−700 nm). The a* values of the polymers ranged between −8 (green hue) and +5 (red hue), with their b* values ranging between −4 and −7 (blue hue) in their neutral states; additionally, the a* and b* values are all below ±8 in their oxidized states. In electrochromic devices (ECDs), we exploit this variation in color hue to achieve color balance with the counter electrode materiala minimal color-changing polymer (MCCP) that is colorless in its fully neutral and oxidized states but light red at intermediate potentials. This red hue was balanced by the green hue of ECP−Random40, leading to an ECD that was color-neutral (a* ranging from −0.8 to −11 and b* ranging from −0.5 to 1.7) throughout the switching voltage range. This family of high-contrast and color-neutral ECPs are scalable with high yielding syntheses for all monomers due to the enhancement of monomer solubility and the usage of commonly available DOT moieties, which will aid in advancing the development of large-scale polymer-based EC window and display applications. This method provides materials that switch from black to transmissive through only gray intermediates as needed for switchable color tinting applications.

1. INTRODUCTION Electrochromism, which describes the change in a material’s color upon application of a current or voltage, has been explored for use in a wide variety of applications including displays,1 windows,2 electronic inks/papers,3 electronic fabrics,4 and so on. Electrochromic devices are prevalent in the automotive industry as self-dimming rear-view mirrors; they have been employed for heat/light regulation in office buildings and on commercial aircraft as on-demand tinted windows. Many types of materials demonstrate electrochromism, ranging from inorganic oxides,5−7 to organic discrete molecules, 8−10 to triphenylamine-based polymers.11−13 Cathodically coloring electrochromic conjugated polymers (ECPs) are unique among these materials for the wide palette of colors that are readily accessible through structural design, which is enhanced by straightforward synthetic modifications of the polymer structure.14 Additionally, there are many practical advantages related to the scalability and processability of conjugated polymers. Polymerization of 3,4-dioxythiophene (DOT)-based ECPs is readily performed using direct arylation polymerization, a technique that is relatively insensitive to oxygen and moisture and has high atom efficiency, making it advantageous for industrial scale synthesis.15,16 Additionally, the polymer structure can be © XXXX American Chemical Society

designed to be soluble in a range of environmentally benign solvents including water, allowing them to be solutionprocessed using a range of coating and printing techniques such as spray coating, bar/blade coating, slot-die coating, and inkjet printing.17−19 An ongoing focus of electrochromic (EC) research is the goal of obtaining true color-neutral (black) switching to clear materials. A popular strategy in color neutrality is color mixing or blending of multiple electrochromes to extend the absorption window throughout the entire visible region.20−23 Random copolymerization is another promising approach to construct polymer backbones with various lengths of chromophore to extend the absorption across the entire visible spectrum.24−31 In earlier studies,24,32,33 the random-copolymerization approach was successful in synthesizing such broadly absorbing materials. Specifically, a family of polymers was developed to contain an electron-rich propylene dioxythiophene (ProDOT), acyclic dioxythiophene (AcDOT) moieties, and a donor−acceptor−donor (D−A− D) unit.33 Importantly, 3,4-ethylenedioxythiophene (EDOT) Received: July 10, 2019 Revised: August 16, 2019

A

DOI: 10.1021/acs.macromol.9b01443 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route and Repeat Unit Structures of the Random Copolymersa

The organic-soluble, ProDOT−BTD−ProDOT “D−A−D” terheterocycle is highlighted in blue.

a

Figure 1. (a) Normalized absorption spectra of the ECP−RandomXX copolymers with various D−A−D terheterocycle feed ratios. The bathochromic shift of the low-energy absorption and the hypsochromic shift of the high-energy absorption as the D−A−D terheterocycle ratio increases are indicated by the red and blue arrows, respectively. (b) Transmittance spectra of the polymers in their oxidized (solid lines) and neutral (dotted lines) states. The shaded area (380−700 nm) represents the visible region over which the human eye is sensitive.

intermediate hues (and lack thereof), and the overall performance of these novel polymers in a more applied framework. By strategically utilizing the slight difference in color hues within our polymer family, we can balance the red hue in the counter electrode material and successfully achieve color-neutral ECDs with their a* and b* below ±10 throughout the entire switching voltage range.

was strategically selected as the donor in the D−A−D unit so that the steric hindrance of the polymer backbone was relaxed, which was believed to allow for greater backbone planarity in the oxidized state, providing higher degrees of visible light transmittance and thus higher electrochromic contrast. However, the EDOT−acceptor−EDOT unit has poor solubility in organic solvents; thus, its purification requires multiple sublimation steps done in small batches. This greatly reduces the yield as well as the prospects of scaling the polymer synthesis. In this paper, we enhance the electrochromic properties of the relaxed family of random copolymers by replacing the EDOT moiety on the D−A−D unit with dimethyl-ProDOT, a replacement which strikes a critical balance by imparting greater monomer solubility (i.e., 40 mg/mL in dichloromethane)and by extension scalability and processability without compromising the electronic and optical properties of the polymers incorporating this core subunit. The D−A−D terheterocycle is oxidatively stable both in the solid state and in solution. Furthermore, we show how tuning the monomer feed ratio of this subunit allows for a family of broadly absorbing polymers with a readily tunable color hue, while also having greater color neutrality (a* and b* < ±10) than previously synthesized D−A polymers throughout the entire color switch. This yields materials that switch from black to transmissive through only gray intermediates as needed for switchable color tinting applications. Finally, we construct dual polymer electrochromic devices (ECDs) to illustrate the contrasts, the

2. RESULTS AND DISCUSSION The polymer design and synthetic route are shown in Scheme 1. The backbone consists of two random copolymerized segments: one is a made of a ProDOT and an AcDOT unit; the other consists of a ProDOT and a ProDOT−BTD− ProDOT D−A−D terheterocycle. The donor−acceptor nature of the D−A−D terheterocycle reduces the energy gap and controls the absorption in the red to near-infrared.34 As a result, the materials with the highest D−A−D terheterocycle composition afford the most broadened of the absorption spectra spanning from 400 to 750 nm. On the other hand, because of the sterically bulky ethylhexyl substituents of AcDOT being close to the conjugated backbone, its presence induces twisting and torsional strain with adjacent ProDOT moieties, affording a mechanism that allows for high energy visible light absorption (blue). Previous studies have demonstrated that the AcDOT subunit is more twisted than the ProDOT subunit. When compared to the purple-colored ProDOT homopolymer,36 the absorption profile of the B

DOI: 10.1021/acs.macromol.9b01443 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Spectral and electrochemical results for ECP−Random25. (a) Spectroelectrochemical results from −0.5 to 0.8 V (dashed line represents spectrum of the as-cast polymer; gray box represents spectral range sensitive to human eyes between 380 and 700 nm). (b) First 10 cyclic voltammetric scans of the polymer film. The colored dots correspond to the spectra of the same colors in (a).

AcDOT homopolymer35 is hypochromically shifted, leading to an orange-colored material. The detailed synthesis of the ProDOT−BTD−ProDOT D− A−D terheterocycle is presented in the Supporting Information. Because of the high reactivity of ProDOT to a second C− H activated coupling and to prevent the formation of oligomers, a 12-to-1 ProDOT:BTD feed ratio was used to ensure the formation of the terheterocycle,37 with the excess ProDOT being recovered during purification. After purification by silica column chromatography, the pure terheterocycle was obtained in 62% yield, translating to a 50% improvement over the previous synthesis,32 where the D−A−D terheterocycle could only be purified by multiple sublimation steps due to its lack of solubility. Similar to the terheterocycle synthesis, the polymers were synthesized by direct arylation reaction (Supporting Information). All polymers have elemental accuracy within ±0.4% as well as number-average molecular weight (Mn) above 24000 g/mol (PS standards) and dispersity (ĐM) of ∼2. Gel permeation chromatograms (Figure S1) for all polymers display monomodal traces, confirming they are sufficiently soluble in the eluent solvent (chloroform). The monomer feed ratio (between 20% and 40% of the D− A−D terheterocycle) has a noticeable and predictable effect on the polymer absorption spectra as the donor−acceptor interaction strategy is used to lower the absorption energy and thus the energy gap.34 As expected, an enhancement of the long wavelength absorption appears as the D−A−D terheterocycle content increases (red arrow, Figure 1a). This increasing donor−acceptor character also leads to a bathochromic shift in the lower energy absorption peak and the onset of absorption. Conversely, the higher energy absorption peak is hypsochromically shifted (∼50 nm between ECP−Random20 and ECP− Random40) as we increase the terheterocycle content (blue arrow, Figure 1a). As Amb et al. previously show,38 the higher energy transitions are localized on the electron-rich moiety in the backbone. By increasing the D−A−D content, we effectively decrease the electron-rich character resulting from the AcDOT−ProDOT subunit of the polymer and blue-shift the higher energy absorptions. The overall effect can be seen in the spectrum of ECP−Random40, which has the broadest absorption in the family, spanning the entire visible region. The change in comonomer feed ratio also affects the redox properties of the polymers in the solid state. Polymer solutions were spray-coated onto ITO substrates, with the films then electrochemically conditioned with 10 cyclic voltammetry

(CV) scans to allow the ions and solvent penetrating through the films to equilibrate. After the packing and morphology of the films were reorganized, differential pulse voltammetry (DPV) results were collected to estimate the oxidation potential of each polymer. The DPV traces (all CV and DPV results can be found in Figures S2−S5) indicate there is a lowering of the oxidation potential from 0 to −0.3 V vs Ag/Ag+ as the terheterocycle feed ratio increases from 20% to 40%. While increasing the terheterocycle feed ratio increases the amount of acceptor in the polymer, this simultaneously increases the amount of the electron-rich and sterically unencumbered methyl-alkylated ProDOT moiety, making ECP−Random40 the easiest to be oxidized. Spectroelectrochemistry was performed to examine how the absorption changes as a function of the applied potential, with the properties of ECP−Random25 shown in Figure 2 as an example (see Figures S2−S5 for the other polymers). Starting at −0.5 V, the polymer is in its neutral state, with the corresponding spectra shown as a bold black line in Figure 2a. The first spectral change occurs at 0 V (red curve), with the low-energy absorbance beginning to decrease and the absorbance in the near-IR beginning to increase. This corresponds to the onset of oxidation in the CV, as indicated by the red dot, shown in Figure 2b. Upon further oxidation, the absorbance in the visible continues to decrease until 0.4 V (cyan curve) when the polymer is nearly fully bleached, with barely any additional change in the visible between 0.4 V and the fully oxidized state at 0.8 V. These trends are similarly observed across all the random copolymers, with nearly all of the spectral change occurring in a narrow potential window between ca. 0 and 0.4 V. The electrochemical break-in effect described above can be observed in both the spectral and electrochemical properties of ECP−Random25, ECP−Random30, and ECP−Random40 (Figure 2 and Figures S2−S5). For ECP−Random25, the spectrum of the unswitched film is shown as a dashed line in Figure 2a, while the first CV of the freshly sprayed film is shown as a thinner black line in Figure 2b. Looking first at the redox properties, there is an immediate change between the first and second CVs, with the onset of oxidation being lowered by ca. 0.3 V, and the oxidation peak broadening. This is characteristic of dioxythiophene-based conjugated polymers, as the structural reorganization that occurs upon oxidation/ reduction, as well as the swelling/deswelling as ions and solvent move through the film, leads to better organization of C

DOI: 10.1021/acs.macromol.9b01443 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. (a) Photograph series of the polymers as cast and under potentiostatic control in a three-electrode setup with 100 mV steps between images. (b) Colorimetry plot (a*b*) for the copolymers. The shaded area represents minimal color change in the color-neutral range (a* and b* = ±10).

Table 1. Electrochemical and Spectral Contrast Properties of the ECP−Random Family of Copolymers integrated contrast (%) DPV oxidation onset vs Ag/ Ag+ (V) ECP−Random20 ECP−Random25 ECP−Random30 ECP−Random40

0.02 −0.06 −0.20 −0.28

absorption peaks (nm) 512, 504, 488, 465,

630 645 656 669

Eonset abs (eV)

380−780 nm

380−700 nm

coloration efficiency at high-energy absorption peaks (cm2/C)

1.61 1.59 1.58 1.55

42 41 39 41

54 49 46 45

169 124 117 96

the film. Turning to the spectral properties, after the first 10 CV cycles, both the low-energy absorption peaks and the onsets of absorption are bathochromically shifted by ∼10, ∼20, and ∼30 nm for ECP−Random25, ECP−Random30, and ECP−Random40, respectively, compared to the pristine ascast films. Because these spectral changes only occur beyond 700 nm, there are no observable changes to the color of the polymer films (Figure 3a). This phenomenon is supported by the L*a*b* values, which are similar between the as-cast and electrochemically conditioned films. The only exception is ECP−Random20, which has an absorption shift within the observable visible region. Thus, we see a slight change in its pink hue (Figure 3a), indicated by a greater shift in the a* values between the as-cast film and the electrochemically conditioned films compared to the other three polymers. The spectroelectrochemical results, summarized in Table 1, allow for quantitative evaluation of the switching contrast between the colored (neutral) and the transmissive (oxidized) states. Note that while some references indicate that the visible region of the electromagnetic spectrum ranges between 380

and 780 nm, the human eye is quite insensitive past 700 nm.19 The red cone cells in human eyes can sense light of 450−780 nm, but the sensitivity drops off significantly past 700 nm.39 Table 1 reports the film integrated contrasts of our polymers both from 380 to 780 nm and from 380 to 700 nm. The integrated contrasts of all polymers between 380 and 780 nm are ∼40%. Because this wavelength range covers the neutral state absorption of the majority of chromophores in these polymers, we conclude that all polymers have similar overall switching contrasts. On the other hand, between 380 and 700 nm, ECP−Random20 has the highest, and ECP−Random40 has the lowest, integrated contrasts. Compared to ECP− Random20, ECP−Random40 contains more chromophores that absorb between 700 and 780 nm. Therefore, when considering switching contrast only within the wavelength range sensitive to human eyes (380−700 nm), ECP− Random40 has the lowest value among all polymers. The coloration efficiency (CE) for the polymers was also calculated, ranging from 96 to 169 cm2/C. These values are comparable to a previous version of black EC polymer.40 D

DOI: 10.1021/acs.macromol.9b01443 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. Photographs of electrochromic devices with ECP−Random30 and ECP−Random40 matched to MCCP under potentiostatic control along with their L*, a*, and b* color values.

oxidized states but has a slight pink coloration in its partially oxidized state.44 Because of their green hues, i.e., their −a* values, ECP−Random30 and ECP−Random40 were selected to be used in EC device fabrication, which should strategically cancel out the MCCP’s +a* reddish hue at its intermediate state to afford a truly color-neutral device at all switching states. As shown by the photographs and colorimetry results in Figure 4, in the intermediate switching voltages (0−0.4 V), the red hue of MCCP is better balanced by ECP−Random40, thus leading to an a* value not exceeding ±2 while the device with ECP−Random30 clearly appears with a stronger red hue and an a* value as high as 6. Both devices display color neutrality throughout the entire switching ranges and can be fully switched from colored to transmissive between −0.2 and 0.6 V. The subtle color variations resulting from the D−A−D terheterocycle content are advantageous in tuning color hues of the ECDs. Specifically, we can exploit the difference in color hues of our neutral ECPs to better engineer an ECD that achieves improved color neutrality and color switching contrast. For example, if color neutrality is the most important criteria, ECP−Random30 can deliver a device with the lowest a* and b* values (