Tuning Color, Contrast, and Redox Stability in High Gap Cathodically

Article pubs.acs.org/Macromolecules

Tuning Color, Contrast, and Redox Stability in High Gap Cathodically Coloring Electrochromic Polymers Kangli Cao,†,‡ D. Eric Shen,† Anna M. Ö sterholm,† Justin A. Kerszulis,† and John R. Reynolds*,† †

School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics (COPE), Georgia Tech Polymer Network (GTPN), Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Shanghai Institute of Satellite Equipment, Shanghai 200240, China S Supporting Information *

ABSTRACT: The design of high bandgap electrochromic polymers (ECPs) that switch from a high energy absorbing colored state to a near-IR absorbing colorless state requires a challenging balance to be struck between achieving large changes in the absorption profile while maintaining sufficiently low oxidation potentials for use in full-color electrochromic devices. Previous studies on high bandgap ECPs have investigated structure− property relationships in dioxythiophenes copolymerized with various arylenes. Here, we expand this understanding by looking more closely at the effect of the dioxythiophene moiety as well as by varying the substituents on the arylene moiety. Three ECPs have been synthesized to perform this study with repeat units composed of electron-rich dimethoxyphenylene in alternation with dimers of 3,4-dialkoxy- and 3,4-propylenedioxythiophenes, yielding high gap polymers that are vibrant yellow or orange in the charge neutral state. Comparing these newly synthesized polymers to a structurally similar set previously reported, we elucidate the subtle steric and electronic effects that govern bandgap and redox properties in dioxythiophene copolymers.

■

INTRODUCTION Conjugated polymers have been widely explored for electrochromic applications, owing to the ease with which the coloration and redox properties can be tuned through synthetic means.1−6 Cathodically coloring cyan, magenta, and yellow electrochromic polymers (ECPs), as well as numerous secondary colors, that all switch from a vibrantly colored state to a colorless (bleached) state upon electrochemical oxidation have been synthesized, allowing for a broad palette of vibrant colors, as well as browns and blacks, to be accessible through blending of ECP solutions. 7−11 The use of dioxythiophene (DOT)-based polymers enables the design of ECPs that undergo this colored-to-clear transition at low oxidation potentials. Mechanistically, oxidation of a neutral polymer leads to the formation of cation radicals (polarons) and dications (bipolarons), which give rise to conformational changes of the polymer backbone and a change in the absorption profile.12 Spectroscopically, this is seen as a decrease of the neutral π−π* absorption band in the visible range and the appearance of new charge carrier absorption bands at longer wavelengths. For the majority of DOT-based ECPs, these charge carrier absorbances are shifted into the NIR, with minimal tailing into the visible region resulting in films that are highly transmissive and nearly colorless in their fully oxidized state. When considering high gap ECPs that are yellow or orange in their charge neutral states, and hence absorbing in the 350−500 nm range (i.e., above 2.5 eV), the charge carrier © XXXX American Chemical Society

transitions need to be transferred across the entire visible spectrum into the NIR, as illustrated in Figure 1, for these materials to become colorless upon oxidation. Attaining this spectral shift is challenging to accomplish, and as a result most cathodically coloring yellow EC materials absorb in the long wavelength range of the visible in their oxidized state, most often giving rise to a residual blue tint.13−21 In 2010, the homopolymer of bis(ethylhexyloxy)thiophene (PAcDOT), an orange-to-clear ECP, was reported.22 The branched alkoxy groups attached directly onto the thiophene backbone not only afforded high solubility but also induced enough steric distortion to reduce the extent of conjugation relative to a fully planar polymer, shifting the λmax to 480 nm while maintaining a low oxidation potential (ca. 0.5 V vs Ag/ Ag+) as a result of the electron donating oxygens. While the color and redox properties are in line with the structure− property relationships we have developed for DOT-based ECPs,7 the main challenge with this particular high gap polymer is the relatively long switching time that exceeds 5 s to reach 95% of full optical contrast. In the following year, the first cathodically coloring yellow-tocolorless ECP, an alternating 3,4-propylenedioxythiophene-altphenylene copolymer (PProDOT-Ph), was reported, completReceived: August 12, 2016 Revised: October 24, 2016

A

DOI: 10.1021/acs.macromol.6b01763 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. An L*a*b* color wheel with the high bandgap color space demarcated as well as representative UV−vis−NIR spectra of a high gap polymer in its charge neutral (ECP0, black line) and oxidized (ECP+, dashed blue line) states.

ing the subtractive color palette.23 Compared to other PXDOTbased electrochromes, PProDOT-Ph is able to maintain a subsecond switching speed but requires a relatively high oxidation potential (ca. 1.1 V vs Ag/Ag+) to reach its fully bleached state, which is attributed to the aromaticity of the phenylene unit.23 As a result, while PProDOT-Ph could potentially be used in blends with other ECPs to obtain various hues,10 or a broadly absorbing black or brown color,9,11 the relatively large potential required to bleach the PProDOTPh component can be beyond the stable limits of other more easily oxidizable ECPs, which decreases the redox switching stability of the blend films.24,25 In a quest to further tune the redox and color properties of high gap ECPs, a series of XDOT−arylene copolymers were designed and synthesized to extract structure−property relationships between color and oxidation potential with the goal of yielding a second generation of yellow-to-colorless switching polymer electrochrome.26 This study yielded seven new yellow and orange ProDOT-based ECPs and showed that increasing the electron richness of the arylene, by incorporating e.g. carbazole (Cbz) or pyrene, or by using dimers of DOTs in the repeat unit, led to a decrease of the oxidation potential while maintaining a high bandgap due to steric interactions between the DOT and the ortho C−H on the arylene. Utilizing this design logic, the ProDOT was then replaced with a dimer of the acyclic bis(ethylhexyloxy)thiophene (biAcDOT) to afford the polymer PAcDOT2-Ph. This balance between sterics and electron richness gave rise to a polymer with an oxidation potential 0.3 V lower than PProDOT-Ph, but with an absorbance profile that is blue-shifted relative to the PAcDOT homopolymer and with significantly faster switching speeds (70%), with number-average molecular weights (Mn) ranging from 24.1 to 60.2 kDa and dispersities (Đ) ranging from 1.7 to 2.3 after Soxhlet extraction. The monomer synthesis and characterization are given in the Supporting Information (Scheme S1). All three polymers are highly soluble (>10 mg/mL) in common organic solvents including chloroform, THF, and toluene owing to the ethylhexyloxy chains on the DOT units. The thermal stability of the polymers was studied by TGA as shown in Figure S1, and all of the polymers are found to be stable up to 320 °C. The polymerizations were initially attempted by a more intuitive DA polymerization approach than the one ultimately selected, utilizing biAcDOT or biProDOT with 1,4-dibromo2,5-dimethoxybenzene, but these conditions were not successful in obtaining high molecular weight polymers (see Scheme S2 in the Supporting Information). We attribute this to sterics that could hinder oxidative addition of the Pd, leading to low molecular weights. The dibromides should be on the least sterically hindered site to stabilize oxidative addition and the proposed transitions states. Oxidative polymerization was also carried out using AcDOT2-Ph(OMe)2 with excess FeCl3 in ethyl acetate at room temperature and resulted in PAcDOT2Ph(OMe)2 with a Mn of 11.7 kDa and a dispersity of 3.9, but led to low yields around 20%, likely due to a large fraction of low molecular weight oligomers, as detailed in the Supporting Information (Scheme S3).

■

RESULTS AND DISCUSSION Scheme 1 shows examples of previously studied yellow/orange polymers (top row) as well as the repeat unit structures of the new family of high gap copolymers of 2,5-methoxy-1,4phenylene polymerized with a variety of DOTs (bottom row). We will show that the introduction of the bulky and electron-rich methoxy units in place of the smaller hydrogen atoms on the phenylene ring raises the HOMO level and lowers the oxidation potential regardless of the nature of the DOT unit. For the AcDOT-based polymers, the introduction of the methoxy units increases the steric hindrance in the backbone and pushes the bandgap to higher energy. In contrast, for the ProDOT-based polymers, we show that the incorporation of methoxy units led to a decrease in the bandgap and a subsequent red-shift in the neutral state film spectra. Furthermore, we show that the nature of the DOT unit has a significant impact on the position of the charge carriers bands, with the ProDOT-based polymers exhibiting more redshifted bipolaron bands and, as a result, a higher electrochromic contrast. By combining the two DOTs into a copolymer (bottom-right structure in Scheme 1), we designed a material that exhibits the beneficial properties of the orange PProDOT2Ph(OMe)2 (low oxidation potential, high electrochromic contrast, fast switching speed) but with a higher bandgap similar to PAcDOT2-based polymers. In addition to the differences observed in the optical and electrochemical D


Article

Macromolecules Electrochemical and Optical Properties of High Gap ECPs. The polymers in Scheme 1 possess various structural motifs that readily allow for fine-tuning of the optical and electronic properties. Through the variation of the arylene and DOT units, the onset of oxidation (Eox) can be readily controlled (as shown in Table 1 and in the differential pulse Table 1. Optical and Electrochemical Properties of the ECPs Studied polymer PProDOT-Ph PProDOT-Ph(OMe)2d PProDOT2-Ph PProDOT2-Ph(OMe)2 PAcDOT2-Ph PAcDOT2-Ph(OMe)2 PAcDOT2/ProDOT2Ph(OMe)2

Eoxa (mV vs Fc/Fc+)

λmaxb (nm)

Egb,c (eV)

451 206 191 126 438 330 220

448 471 488 500 464 446 476

2.44 2.26 2.23 2.18 2.30 2.38 2.25

Figure 2. Neutral state spectra of PAcDOT2-Ph(OMe)2 (green), PAcDOT2-Ph (blue), PAcDOT2/ProDOT2-Ph(OMe)2 (cyan), ProDOT2-Ph (red), and ProDOT2-Ph(OMe)2 (magenta) on ITO-coated glass in 0.5 M TBAPF6/PC electrolyte solution. Films were sprayed to an absorbance of 1.00 ± 0.03.

a

As determined by DPV as the onset of the current for oxidation. bFor films cast onto ITO-coated glass. cBandgap determined by onset of light absorption. dValues from ref 26.

spectra, with the −Ph(OMe)2-substituted polymers exhibiting the largest shifts (ca. 40 nm). These new absorbance peaks are likely the result of aggregation-induced absorbance, which has been well documented in other thiophene-based polymers.34 One possible explanation is that AcDOT-based polymers contain a larger degree of twisting because of their bulky side chains that are located closer to the conjugated backbone and extend along its plane. As a result, this twisting likely persists both in solution and in the solid state, leading to comparable effective conjugation lengths and similar spectra in terms of λmax and broadness. In contrast, ProDOT analogues have bulky side chains located further from the conjugated backbone and, due to the tetrahedral geometry at the central carbon of the propylene bridge, extend out of the conjugated backbone plane. This allows the polymer to adopt a more planar geometry in the solid state, giving rise to the red-shifted absorbance seen in the films. Turning to the PAcDOT2/ProDOT2-Ph(OMe)2 copolymer, it is interesting to note that its solution spectrum nearly overlaps with that of PProDOT2-Ph(OMe)2; however, in thin film it exhibits a more modest red-shift than PProDOT2Ph(OMe)2. The PAcDOT2/ProDOT2-Ph(OMe)2 copolymer adopts some of the relaxation afforded by the ProDOT groups; however, twisting along the backbone is still conferred by the AcDOT moieties. When comparing the optical properties of methoxysubstituted phenylene polymers with the unsubstituted analogues, the trend is more complicated: PAcDOT2-Ph(OMe)2 has a higher bandgap than PAcDOT2-Ph, while PProDOT-Ph(OMe)2 and PProDOT2-Ph(OMe)2, on the other hand, have lower bandgaps than the unsubstituted analogues. With the AcDOT systems, the introduction of relatively bulky and electron-rich methoxy units in place of the smaller hydrogen atom on the phenylene ring has a significant effect on the backbone planarity/sterics because the side chains on the AcDOTs extend along the plane of the backbone, causing them to sterically interact with the methoxy groups. This leads to greater torsional strain along the backbone, and a decrease in conjugation, with an accompanying increase in the bandgap giving rise to a brighter yellow color. In the case of the ProDOT polymers, the addition of the methoxy groups plays a smaller role in steric interactions, since the side chains on the ProDOTs are located further from the methoxy groups and

voltammetry (DPV) results in Figure S2) by tuning the electron-richness and/or the steric strain along the polymer backbone. By increasing the number of electron-rich DOTs in the repeat unit, the Eox is substantially lowered. For example, the addition of a second ProDOT unit in PProDOT2-Ph lowers the Eox by 260 mV compared to PProDOT-Ph. Similarly, by replacing the unsubstituted phenylene with the electron-rich dimethoxyphenylene in PAcDOT2-Ph(OMe)2, PProDOT-Ph(OMe)2, and PProDOT2-Ph(OMe)2, the Eox is lowered by 75− 250 mV compared to the unsubstituted analogues. Looking more closely at the DOT moiety, comparatively, ProDOT units have less steric hindrance than AcDOT units, leading homopolymers of the former to have a more relaxed backbone,6 a longer effective conjugation length, and a lower Eox. This trend is preserved here, as can be observed when comparing the oxidation potential and optical bandgaps in films of PProDOT2-Ph(OMe)2 and PProDOT2-Ph with PAcDOT2Ph(OMe)2 and PAcDOT2-Ph, respectively. By combining these trends, the lowest Eox of the series is found for PProDOT2Ph(OMe)2. To better understand whether fine control of redox properties and spectra could be obtained, an alternating copolymer (PAcDOT2/ProDOT2-Ph(OMe)2) was also synthesized. This copolymer has an Eox and absorption maximum in between PAcDOT2-Ph(OMe)2 and PProDOT2-Ph(OMe)2. Interestingly, the redox and absorbance profiles do not appear to be simply an overlap from those of the parent polymers, indicating that the film properties are derived from the novel AcDOT2-Ph(OMe)2/ProDOT2-Ph(OMe)2 repeat unit. These results suggest that redox and optical properties can be readily modified by adjusting the ratio of AcDOT and ProDOT along the copolymer backbone. The optical properties of polymer solutions (shown in Figure S3) and spray-cast thin films (shown in Figure 2 and Figure S4) were analyzed and compared for more insight into the structural differences between the AcDOT and ProDOT polymers. It is interesting to note that for PAcDOT2Ph(OMe)2 and PAcDOT2-Ph the solution spectra and the film spectra have nearly identical λmax values. For polymers containing ProDOT, however, a red-shifted absorbance shoulder is observed in film spectra compared to the solution E


Article

Macromolecules

Figure 3. Spectroelectrochemistry and photographs of (a) PAcDOT2-Ph(OMe)2, (b) PProDOT-Ph(OMe)2, (c) PProDOT2-Ph(OMe)2, and (d) PAcDOT2/ProDOT2-Ph(OMe)2. The films were spray-cast onto ITO-coated glass to an absorbance of 1.00 ± 0.03. The applied potential was increased in 10−50 mV steps between the fully colored and bleached states in 0.5 M TBAPF6/PC. The arrows show the progression of the peak evolution as a function of increasing potential.

S5a,b). Interestingly, the energy of the polaron band is not particularly affected by the choice of DOT unit. Additionally, substitution of the phenylene ring with methoxy groups leads to a red-shifting of the polaron band for both the AcDOT and ProDOT copolymers, which is different from the trend that phenyl substitution had on the neutral absorbance. This is likely because in the oxidized state quinoidalization acts as a greater driving force to planarization of the polymer backbone than the side chains have on twisting the polymer out of plane. With a more planar backbone in all the polymer systems, the electronic effects of the electron-donating methoxy unit dominate energetics of the polymer by stabilizing the polaron better than the unsubstituted phenyl ring, leading to a red-shifting of the polaron band. The copolymer (Figure 3d) is interesting in that its neutral state more closely resembles PAcDOT2Ph(OMe)2, whereas the oxidized state resembles PProDOT2Ph(OMe)2, again demonstrating how a copolymerization approach can be used to readily fine-tune the optical properties by adjusting the ratio of AcDOT and ProDOT along the backbone. The maximum transmittance in the oxidized state is found for the ProDOT-containing polymers. The two PAcDOT polymers (red and green trace in Figure 4) have a small drop in transmissivity at ca. 480 nm in their oxidized states, suggesting that the neutral state is not completely bleached. In addition, the spectra show some tailing from the NIR at higher wavelengths from the charge carrier bands. PProDOT-Ph (black trace) also exhibits a drop in the transmittance plot with a minimum around 620 nm. Unlike the two PAcDOT-based polymers, this residual absorbance appears to originate from residual polarons rather than an incomplete bleaching of the

should not interact with them to a high degree. Instead, the electron richness of the methoxy groups has a stronger effect on the HOMO as it lowers the bandgap for PProDOT-Ph(OMe)2 and PProDOT2-Ph(OMe)2. This demonstrates that the steric and electronic effects observed here are intertwined and depend upon the neighboring groups present and not just on the substituents themselves. Spectroelectrochemistry and Color Properties of High Gap ECPs. The spectroelectrochemical behavior of the polymers was evaluated by monitoring the absorption changes upon a simultaneous change of the applied external bias across the films. The spectra of the Ph(OMe)2-based polymers are shown in Figure 3, whereas the phenyl-substituted analogues are shown in Figure S5. As the potential is increased, the π−π* absorption bands (350−550 nm) are depleted, while absorption bands originating from polaronic and bipolaronic charge carriers appear. The bipolaron absorption band for the PAcDOT-based polymers (Figure 3a and Figure S5c) is significantly blue-shifted (λmax: 1400−1500 nm) compared to the more planar ProDOT analogues (λmax > 1700 nm; Figure 3b,c and Figure S5a,b). The blue-shifted bipolaron band results in some tailing into the visible range, slightly compromising the long-wavelength transmissivityand by extension the color neutralityof the fully oxidized state. When comparing polymers containing phenyl groups with those containing −Ph(OMe)2 groups, the bipolaron band is further red-shifted as a result of the incorporation of the electron-rich substituent on the phenylenes. This is particularly evident when comparing the PAcDOT-based polymers (Figure 3a and Figure S5c) but also observable when comparing the bipolaron absorption onsets for the PProDOT analogues (Figure 3b,c and Figure F


Article

Macromolecules

Δ%T) and only slightly lower than PProDOT2-Ph(OMe)2 (70 Δ%T). This is likely due to the position of its bipolaron band, which is red-shifted relative to the AcDOT polymer and blueshifted relative to the ProDOT polymer, but also due to the more complete bleaching of the neutral state that is observed for the copolymer. To summarize, the contrast values at λmax increase with the incorporation of ProDOT units as the steric twist is reduced and the electron richness increased. Similarly, incorporating the electron-rich −Ph(OMe)2 moiety leads to an increase in the Δ%T compared to the unsubstituted phenylene polymers, due in part to the red-shifting these charge carrier bands exhibit as well as the presence of favorable S−O interactions26,35,36 between neighboring XDOTs and the methoxy groups on phenylene, facilitating a higher degree of planarity when fully oxidized. The colors of the polymers can be quantified and more readily compared by converting the spectra into L*a*b* color coordinates where the L* represents the lightness−darkness (100−0, white−black) of a given color, a* the red/green balance, and the b* the yellow/blue balance. The L*a*b* coordinates for the neutral and oxidized states are summarized in Table 2. Figure S6 shows how the a*b* values of the respective polymers change upon electrochemical oxidation from their fully colored to their fully colorless states. PProDOT-Ph, PAcDOT2-Ph, and PAcDOT2-Ph(OMe)2 all have a vibrant yellow color as confirmed by their low a* values and high b* values. Of the three, PAcDOT2-Ph(OMe)2 is the purest yellow of the family, having the highest b* and lowest a*. In addition to being the most vibrant yellow, it also has the lowest oxidation potential of the three, 250 mV lower than PProDOT-Ph, which is otherwise very similar in color. By replacing the AcDOTs with ProDOTs, the steric hindrance between chains is reduced, leading to a more relaxed backbone. This in turn causes the neutral state color of PProDOT2Ph(OMe)2 and PProDOT2-Ph both to have a slightly redshifted absorption profile translating into the colors having a substantial a* component, placing them in the orange color space. These two polymers offer a new strategy for obtaining orange-to-colorless ECPs that have higher electrochromic contrast, more colorless oxidized states, and (as will be shown) faster switching speeds than the orange PAcDOT homopolymer (albeit at a slightly higher potential).22 The color of the copolymer PAcDOT2/ProDOT2-Ph(OMe)2 falls right in between the parent compounds, giving it a warm golden-yellow hue. Upon oxidation, all polymers reach a color neutral transmissive state as demonstrated by the low a*b* values in Table 2. In comparing the effects of the different heterocycles, it was also observed that polymers containing ProDOT underwent a red-shifting of their spectra during repeated cycling to more red-orange hues. For example, the a*/b* values of PProDOT2Ph(OMe)2 shift from an initial 26/55 as sprayed to 47/36 after 10 switching cycles, and drifting further to 47/21 after 1000 switches, as shown spectroscopically in Figure S7. In contrast, the PAcDOT2/ProDOT2-Ph(OMe)2 copolymer spectrum only undergoes a slight red-shift, while the spectra of PAcDOT2Ph(OMe)2 before and after repeated cycling are nearly identical. This trend reflects the tendency of ProDOTcontaining polymers to relax their torsional strain and achieve a more planar backbone after repeated redox switching, as discussed above, while AcDOT-containing polymers remain highly twisted due to the sterics afforded by the branched alkyl

Figure 4. Comparison of the transmittance spectra (%) of the oxidized forms of PProDOT-Ph (black trace), PAcDOT2-Ph (red trace), PProDOT2-Ph (blue trace), PAcDOT2-Ph(OMe)2 (green trace), PProDOT-Ph(OMe)2 (orange trace), PProDOT2-Ph(OMe)2 (magenta trace), and PAcDOT2/ProDOT2-Ph(OMe)2 (cyan trace).

neutral state. These trends offer a framework for deriving structure−property relationships related to the color neutrality of the fully oxidized state and by extension the optical contrast (as shown in Table 2) achievable. Of the evaluated polymers, Table 2. L*a*b* Color Coordinates for All Polymers in the Neutral and Transmissive States and Total Change in Contrast upon Switching polymer PProDOT-Ph ProDOTPh(OMe)2 PProDOT2-Ph PProDOT2Ph(OMe)2 PAcDOT2-Ph PAcDOT2Ph(OMe)2 PAcDOT2/ ProDOT2Ph(OMe)2

Δ%Ta (at λmax)

neutral state L*, a*, b* color coordinatesb

oxidized state L*, a*, b* color coordinatesb

67 62

96, −8, 78 86, 25, 74

86, −1, −8 87, −2, −1

62 70

84, 35, 50 80, 47, 36

89, 2, −3 91, −1, −4

51 59

87, 9, 76 89−4, 85

87, 2, 4 81, −1, −1

66

87, 21, 73

88, −1, −3

a Difference between steady-state transmittance measured at fully oxidized and fully neutral states (all films sprayed to 10%T at λmax). b For a film cast onto ITO-coated glass.

the orange PProDOT2-Ph(OMe)2 exhibited the highest contrast (70 Δ%T at 500 nm) and the most transmissive oxidized state followed by the copolymer PAcDOT2 / ProDOT2-Ph(OMe)2. PProDOT-Ph reaches a similar contrast as the copolymer if measured at λmax; however, the color neutrality is compromised by the residual polaron band. The polymers containing ProDOT units (Figure 3b,c and Figure S5a,b) exhibit higher optical contrast than their AcDOT analogues. This is in part the result of their red-shifted charge carrier bands as discussed above, where the bipolaron bands of the AcDOT polymers tail into the visible region. Additionally, the lower contrast of the AcDOT-based polymers can be ascribed to the high twist (lower degree of planarity) due to steric hindrance which could make it energetically more difficult to fully planarize upon electrochemical oxidation. The contrast of the copolymer PAcDOT2/ProDOT2-Ph(OMe)2 (66 Δ%T) is higher than PAcDOT2-Ph(OMe)2 (59 G


Article

Macromolecules

Figure 5. Chronoabsorptometry of polymers in 0.5 M TBAPF6/PC electrolyte solution. (a) PAcDOT2-Ph(OMe)2 measured from −0.5 to 0.8 V vs Ag/Ag+, (b) PProDOT2-Ph(OMe)2 measured from −0.5 to 0.7 V vs Ag/Ag+, and (c) PAcDOT2/ProDOT2-Ph(OMe)2 measured from −0.5 to 0.75 V vs Ag/Ag+. (d) Percent transmittance and time to reach 95% of the full optical contrast for PAcDOT2-Ph(OMe)2, PProDOT2-Ph(OMe)2, and PAcDOT2/ProDOT2-Ph(OMe)2.

ProDOT moieties is sufficient to induce a noticeable enhancement. To evaluate the effect that DOTs, as well as substituents on the phenyl units, have on cross-linking and/or nucleophilic attack at the open positions of the phenyl ring,28−31 films were monitored with repeated square-wave potential steps of 10 s, switching between the fully reduced and fully oxidized states for 100 cycles under ambient laboratory conditions (i.e., in the prescence of light, O2, and humidity). From this, several trends are observed. First, as shown in Figure 6 and Figure S9b,f, when comparing PProDOT-Ph and PProDOT2-Ph, as well as PProDOT-Ph(OMe)2 and PProDOT2-Ph(OMe)2, increasing the number of DOTs in the repeat unit leads to an improvement in maintaining contrast over 100 switches. With a higher density of electron-rich units, the cation radical and dication states are better stabilized, leading to a decrease in reactivity of these species. Similarly, in examining the effects of phenyl ring substitution, when comparing PAcDOT2-Ph(OMe)2 with PAcDOT2-Ph, the absence of the methoxy units leads to a nearly 6% decrease in electrochromic contrast over the course of 100 cycles as a result of the colored state losing vibrancy, while the methoxy-substituted version loses 4%. This trend is also evident for the ProDOT-containing polymers, where PProDOT2-Ph loses 3% contrast over the course of 100 cycles whereas PProDOT2-Ph(OMe)2 only loses 0.3%. This improvement upon methoxy substitution can be attributed to unsubstituted phenylene rings being more susceptible to irreversible cross-linking or nucleophilic attack during repeated electrochemical switching.28−30 Finally,

chains. These results demonstrate how, through understanding the subtle relationship between sterics and electron-richness, redox potentials can be adjusted while the desired coloration can be retained across two structurally different materials. Switching Kinetics of High Gap ECPs. To evaluate the rate at which the bleaching/coloration processes occur in spraycast films, the transmittance change at λmax was monitored as a function of time by applying square-wave potential steps for periods of 60, 30, 10, 5, 3, 1, and 0.5 s, as shown in Figure 5 for the new polymers (PAcDOT2-Ph(OMe)2, ProDOT2-Ph(OMe)2, and PAcDOT2/ProDOT2-Ph(OMe)2) and in Figure S8 for the previously published polymers (PProDOT-Ph, PProDOT2-Ph, PProDOT-Ph(OMe)2, and PAcDOT2-Ph). The two AcDOT-containing polymers complete 95% of a full switch between 2 and 3 s, while the ProDOT-containing polymers exhibit faster switching kinetics, all undergoing a full switch in a second or less. The slower switching speeds of the AcDOT-based polymers compared with their ProDOT-based analogues are more clearly delineated in Figure 5d, which shows an overlap of the chronoabsorptometry curves during a 10 s switch. This is in line with what was previously observed when comparing PAcDOT and PProDOT homopolymers.22,37 Interestingly for the PAcDOT2/ProDOT2-Ph(OMe)2 copolymer, the switching speeds more closely resemble those of PProDOT2-Ph(OMe)2 than the AcDOT analogue. This is in contrast to its optical and redox properties, which were almost in the middle of its parent polymers’ properties. While the exact structure−property relationships affecting redox switching speeds are not fully understood, evidently the presence of the H


Article

Macromolecules

level, allowing the polymer to be fully oxidized at a lower potential, while also providing improved redox stability and higher electrochromic contrast, when copolymerized with DOTs. However, with the AcDOT systems, the presence of the methoxy units increases the steric hindrance in the backbone and pushes the optical bandgap toward higher energy, in contrast to the trend observed in the ProDOT analogues. This complexity ultimately allows for fine-tuning with regards to the electronic, steric, and optical effects of the resulting polymer films. Incorporating the two DOTs into a PAcDOT2/ProDOT2-Ph(OMe)2 copolymer combines the low oxidation potential and high EC contrast of PProDOT2Ph(OMe)2 with the higher bandgap of PAcDOT2-based polymers, thus giving rise to a yellow copolymer with fast switching kinetics, high contrast (>65%), and improved electrochemical stability. From this understanding, ECPs were obtained with yellow and orange neutral states that switch to colorless oxidized states. The incorporation of the protecting methoxy groups on the phenylene rings improves upon previous designs of high bandgap ECPs, bringing the field closer to having a full color palette of stable, high-performance ECPs.

Figure 6. Contrast loss measured at λmax versus number of switching cycles in 0.5 M TBAPF6/PC electrolyte solution with repeated squarewave potential steps of 10 s for PProDOT-Ph measured from −0.5 to 1.1 V, PProDOT-Ph(OMe)2 measured from −0.5 to 1.0 V, PAcDOT2Ph measured from −0.5 to 0.9 V, PAcDOT2-Ph(OMe)2 measured from −0.5 to 0.7 V, PProDOT2-Ph measured from −0.5 to 0.8 V, PProDOT2-Ph(OMe)2 measured from −0.5 to 0.8 V, and PAcDOT2/ ProDOT2-Ph(OMe)2 measured from −0.5 to 0.75 V.

■

ASSOCIATED CONTENT

S Supporting Information *

ProDOT-based polymers appear to be more robust than the AcDOT analogues, which can be observed when comparing PAcDOT2-Ph with PProDOT2-Ph or PAcDOT2-Ph(OMe)2 with PProDOT2-Ph(OMe)2. Computational studies on DOTs have previously shown that AcDOT moieties are twisted far out of plane from one another due to torsional strain arising from their side chains extending along the plane of the conjugated backbone.6 On the other hand, ProDOT moieties are less twisted as the side chains extend out of the plane of the conjugated backbone. This large twist between AcDOT and the phenyl moiety can leave the phenyl ring more exposed to attack, while the more coplanar ProDOT can sterically shield the phenyl ring from side reactions. Interestingly, in both PProDOT-Ph(OMe)2 and PProDOT2Ph(OMe)2 the decrease in contrast over 100 switches occurs in a different fashion from the AcDOT polymers. As was exemplified in the discussion of the color coordinates in Table 2, the absorbance profile of the orange PProDOT2Ph(OMe)2 undergoes a red-shift during prolonged cycling (1000+ switches). In addition to this shift, we observed an unexpected decrease in switching speed, with the film requiring approximately 1 min to switch to the fully colored state, as shown in Figure S10; nevertheless, when afforded enough time to switch, the colored and colorless states could be fully recovered even after several thousand redox cycles. While this phenomenon is not yet fully understood, it seems to suggest that the ion and/or electron diffusion in the films becomes slower over repeated cycling, rather than the polymer undergoing chemical side reactions which would not otherwise be reversible.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01763. Experimental details; Schemes S1−S3 and Figures S1− S10 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.R.R.). Author Contributions

K.C. and D.E.S. contributed equally. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Funding from the Air Force Office of Scientific Research (FA9550-14-1-0271) is greatly appreciated. The authors gratefully acknowledge Mr. James Ponder Jr. for his input during the synthesis as well as for gathering the GPC data of the new polymers.

■

REFERENCES

(1) Beaujuge, P. M.; Reynolds, J. R. Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices. Chem. Rev. 2010, 110, 268−320. (2) Iċ ļ i, M.; Pamuk, M.; Algi, F.; Ö nal, A. M.; Cihaner, A. DonorAcceptor Polymer Electrochromes with Tunable Colors and Performance. Chem. Mater. 2010, 22, 4034−4044. (3) Yen, H.-J.; Liou, G.-S. Solution-Processable Triarylamine-Based Electroactive High Performance Polymers for Anodically Electrochromic Applications. Polym. Chem. 2012, 3, 255−264. (4) Sonmez, G.; Sonmez, H. B.; Shen, C. K. F.; Jost, R. W.; Rubin, Y.; Wudl, F. A Processable Green Polymeric Electrochromic. Macromolecules 2005, 38, 669−675. (5) Beaujuge, P. M.; Ellinger, S.; Reynolds, J. R. The Donor-Acceptor Approach Allows a Black-to-Transmissive Switching Polymeric Electrochrome. Nat. Mater. 2008, 7, 795−799.

■

CONCLUSION Here, we have reported on the synthesis, characterization, and structure−property relationships of high bandgap polymers based on repeat units of electron-rich dimethoxyphenylene in alternation with AcDOT dimers or ProDOT dimers. We show that the introduction of electron-rich methoxy units in place of the smaller hydrogen atom on the phenylene raises the HOMO I


Article

Macromolecules (6) Kerszulis, J. A.; Johnson, K. E.; Kuepfert, M.; Khoshabo, D.; Dyer, A. L.; Reynolds, J. R. Tuning the Painter’s Palette: Subtle Steric Effects on Spectra and Colour in Conjugated Electrochromic Polymers. J. Mater. Chem. C 2015, 3, 3211−3218. (7) Dyer, A. L.; Thompson, E. J.; Reynolds, J. R. Completing the Color Palette with Spray-Processable Polymer Electrochromics. ACS Appl. Mater. Interfaces 2011, 3, 1787−1795. (8) Ö zkut, I.̇ M.; Algi, M. P.; Ö ztas, Z.; Algi, F.; Ö nal, A. M.; Cihaner, A. Members of CMY Color Space: Cyan and Magenta Colored Polymers Based on Oxadiazole Acceptor Unit. Macromolecules 2012, 45, 729−734. (9) Ö sterholm, A. M.; Shen, D. E.; Kerszulis, J. A.; Bulloch, R. H.; Kuepfert, M.; Dyer, A. L.; Reynolds, J. R. Four Shades of Brown: Tuning of Electrochromic Polymer Blends Toward High-Contrast Eyewear. ACS Appl. Mater. Interfaces 2015, 7, 1413−1421. (10) Bulloch, R. H.; Kerszulis, J. A.; Dyer, A. L.; Reynolds, J. R. An Electrochromic Painter’s Palette: Color Mixing via Solution CoProcessing. ACS Appl. Mater. Interfaces 2015, 7, 1406−1412. (11) Ö sterholm, A. M.; Shen, D. E.; Gottfried, D. S.; Reynolds, J. R. Full Color Control and High-Resolution Patterning from Inkjet Printable Cyan/Magenta/Yellow Colored-to-Colorless Electrochromic Polymer Inks. Adv. Mater. Technol. 2016, 1, 1600063. (12) Bredas, J. L.; Street, G. B. Polarons, Bipolarons, and Solitons in Conducting Polymers. Acc. Chem. Res. 1985, 18, 309−315. (13) Oguzhan, E.; Bilgili, H.; Koyuncu, F. B.; Ozdemir, E.; Koyuncu, S. A New Processable Donor−Acceptor Polymer Displaying Neutral State Yellow Electrochromism. Polymer 2013, 54, 6283−6292. (14) Camurlu, P.; Cirpan, A.; Toppare, L. Conducting Polymers of Octanoic Acid 2-Thiophen-3-yl-ethylester and Their Electrochromic Properties. Mater. Chem. Phys. 2005, 92 (2), 413−418. (15) Mert, O.; Sahin, E.; Ertas, E.; Ozturk, T.; Aydin, E. A.; Toppare, L. Electrochromic Properties of Poly(diphenyldithieno [3,2-b; 2′, 3′-d] thiophene). J. Electroanal. Chem. 2006, 591, 53−58. (16) Xu, L.; Zhao, J.; Cui, C.; Liu, R.; Liu, J.; Wang, H. Electrosynthesis and Characterization of an Electrochromic Material from Poly(1,4-bis (2-thienyl)-benzene) and its Application in Electrochromic Devices. Electrochim. Acta 2011, 56, 2815−2822. (17) Cihaner, A.; Algı, F. An Electrochromic and Fluorescent Polymer Based on 1-(1-naphthyl)-2, 5-di-2-thienyl-1H-pyrrole. J. Electroanal. Chem. 2008, 614, 101−106. (18) Içli-Ö zkut, M.; Ö ztas, Z.; Algi, F.; Cihaner, A. A Neutral State Yellow to Navy Polymer Electrochrome with Pyrene Scaffold. Org. Electron. 2011, 12, 1505−1511. (19) Xu, C.; Zhao, J.; Yu, J.; Cui, C. Ethylenedioxythiophene Derivatized Polynapthalenes as Active Materials for Electrochromic Devices. Electrochim. Acta 2013, 96, 82−89. (20) Dey, T.; Invernale, M. A.; Ding, Y.; Buyukmumcu, Z.; Sotzing, G. A. Poly(3, 4-propylenedioxythiophene)s as a Single Platform for Full Color Realization. Macromolecules 2011, 44, 2415−2417. (21) Liou, G. S.; Lin, H. Y. Synthesis and Electrochemical Properties of Novel Aromatic Poly (amine−amide)s with Anodically Highly Stable Yellow and Blue Electrochromic Behaviors. Macromolecules 2009, 42, 125−134. (22) Dyer, A. L.; Craig, M. R.; Babiarz, J. E.; Kiyak, K.; Reynolds, J. R. Orange and Red to Transmissive Electrochromic Polymers Based on Electron-Rich Dioxythiophenes. Macromolecules 2010, 43, 4460−4467. (23) Amb, C. M.; Kerszulis, J. A.; Thompson, E. J.; Dyer, A. L.; Reynolds, J. R. Propylenedioxythiophene (ProDOT)−Phenylene Copolymers Allow a Yellow-to-Transmissive Electrochrome. Polym. Chem. 2011, 2, 812−814. (24) Padilla, J.; Seshadri, V.; Otero, T. F.; Sotzing, G. A. High Contrast Solid-State Electrochromic Devices from Substituted 3,4Propylenedioxythiophenes Using the Dual Conjugated Polymer Approach. J. Electroanal. Chem. 2007, 609, 75−84. (25) Guan, S.; Elmezayyen, A. S.; Zhang, F.; Zheng, J.; Xu, C. Deterioration Mechanism of Electrochromic Poly(3, 4-(2, 2dimethylpropylenedioxy)thiophene) Thin Films. J. Mater. Chem. C 2016, 4, 4584−4591.

(26) Kerszulis, J. A.; Amb, C. M.; Dyer, A. L.; Reynolds, J. R. Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-toTransmissive Electrochromic Conjugated Polymers. Macromolecules 2014, 47, 5462−5469. (27) Remmele, J.; Shen, D. E.; Mustonen, T.; Fruehauf, N. High Performance and Long-Term Stability in Ambiently Fabricated Segmented Solid-State Polymer Electrochromic Displays. ACS Appl. Mater. Interfaces 2015, 7, 12001−12008. (28) Meerholz, K.; Heinze, J. Solid State Electrochemical Experiments on Defined Oligomers of the Poly-p-phenylene Series as Models of Conducting Polymers. Synth. Met. 1991, 43, 2871−2876. (29) Meerholz, K.; Heinze, J. Electrochemical Solution and SolidState Investigations on Conjugated Oligomers and Polymers of the αThiophene and the p-Phenylene Series. Electrochim. Acta 1996, 41, 1839−1854. (30) Goldenberg, L. M.; Lacaze, P. C. Anodic Synthesis of Poly(pphenylene). Synth. Met. 1993, 58, 271−293. (31) Tsai, E. W.; Basak, S.; Ruiz, J. P.; Reynolds, J. R.; Rajeshwar, K. Electrochemistry of Some β-Substituted Polythiophenes. J. Electrochem. Soc. 1989, 136, 3683−3689. (32) Padilla, J.; Ö sterholm, A. M.; Dyer, A. L.; Reynolds, J. R. Process Controlled Performance for Soluble Electrochromic Polymers. Sol. Energy Mater. Sol. Cells 2015, 140, 54−60. (33) Estrada, L. A.; Deininger, J. J.; Kamenov, G. D.; Reynolds, J. R. Direct(hetero)arylation Polymerization: An Effective Route to 3, 4Propylenedioxythiophene-based Polymers with Low Residual Metal Content. ACS Macro Lett. 2013, 2, 869−873. (34) Scharsich, C.; Lohwasser, R. H.; Sommer, M.; Asawapirom, U.; Scherf, U.; Thelakkat, M.; Neher, D.; Köhler, A. Control of Aggregate Formation in Poly(3-hexylthiophene) by Solvent, Molecular Weight, and Synthetic Method. J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 442−453. (35) Lin, C.; Endo, T.; Takase, M.; Iyoda, M.; Nishinaga, T. Structural, Optical, and Electronic Properties of a Series of 3,4Propylenedioxythiophene Oligomers in Neutral and Various Oxidation States. J. Am. Chem. Soc. 2011, 133, 11339−11350. (36) Spencer, H. J.; Skabara, P. J.; Giles, M.; McCulloch, I.; Coles, S. J.; Hursthouse, M. B. The First Direct Experimental Comparison Between the Hugely Contrasting Properties of PEDOT and the AllSulfur Analogue PEDTT by Analogy with Well-Defined EDTT− EDOT Copolymers. J. Mater. Chem. 2005, 15, 4783−4792. (37) Reeves, B. D.; Grenier, C. R. G.; Argun, A. A.; Cirpan, A.; McCarley, T. D.; Reynolds, J. R. Spray Coatable Electrochromic Dioxythiophene Polymers with High Coloration Efficiencies. Macromolecules 2004, 37, 7559−7569.

J


Tuning Color, Contrast, and Redox Stability in High Gap Cathodically

Recommend Documents