Aqueous Electrolyte Compatible Electrochromic Polymers Processed

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Letter Cite This: ACS Macro Lett. 2018, 7, 1208−1214

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Aqueous Electrolyte Compatible Electrochromic Polymers Processed from an Environmentally Sustainable Solvent Graham S. Collier, Ian Pelse, 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

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ABSTRACT: Developing aqueous electrolyte compatible, redox-active polymers that can be processed from environmentally sustainable solvents is desirable because these traits will effectively reduce environmental impact and human health hazards during processing procedures and in the final device architecture. To achieve organic solvent solubility and aqueous compatibility, a poly(3,4-propylenedioxythiophene) containing four ester functionalities was synthesized via direct arylation polymerization. The resulting polymer was spraycast into a thin film from the environmentally sustainable solvent 2-methyltetrahydrofuran, and the presence of multiple polar functionalities rendered the film aqueous electrolyte compatible. The multiester-functionalized polymer exhibits a relatively low onset of oxidation (∼0.4 V vs Ag/AgCl) and electrochromic character by transitioning from a colored neutral state to a colorless oxidized state with increasing potential in 0.1 M NaCl aqueous electrolyte. Additionally, the esterfunctionalized polymer exhibits similar electrochromic properties in aqueous electrolytes when compared to traditional alkylsubstituted poly(3,4-propylenedioxythiophenes) in organic electrolytes, as evidenced by contrast values of ∼70% and switching speeds of ∼2 s. This work highlights the use of multipolar functionalities as a design strategy for synthesizing organic solvent processable, aqueous electrolyte compatible redox-active polymers without postpolymerization modifications or the sacrifice of electrochromic properties.

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necessary to facilitate aqueous compatibility and a solventresistant film.6,8,9,13,14 These hindrances have motivated researchers to design aqueous compatible polymers directly through synthetic methods. One design principle to generate aqueous electrolyte compatible conjugated polymers is the use of oligoether side chains that render films hydrophilic in nature due to the increased polarity compared to typical aliphatic solubilizing groups. Notably, this concept has been successful for the synthesis of thiophene-,15,16 benzodithiophene-,17,18 and naphthalenediimide-based19−21 conjugated polymers used as the active material in OECTs and other bioelectronic applications.4,16,17,22 Furthermore, oligoether-functionalized conjugated polymers have also been shown to improve solubility in polar media, such as ethanol, with improved solar cell and transistor properties of phenylene-based copolymers as well as improved conductivity of p-doped poly(thiophenes).15 While these results point to oligoether side chains being a promising route to aqueous electrolyte compatible organic electronics, these systems often exhibit poor solubility in traditional organic solvents, which inhibits

edox-active conjugated polymers have received considerable attention as materials for applications such as electrochromism,1 charge storage,2 and bioelectronics,3,4 due to the ability to readily manipulate redox properties through careful synthetic design.5 When considering bioelectronic applications, one main design criteria is for the conjugated polymers to be readily and reversibly doped and dedoped in aqueous media (i.e., aqueous electrolyte compatible) to ensure they are compatible with biological systems. Furthermore, polymers used in electrochromic and charge storage applications will benefit from aqueous compatiblility due to (1) reduced environmental impact through use of nontoxic supporting electrolytes, (2) increased ionic mobility in aqueous electrolytes compared to organic electrolytes, and (3) minimalized safety concerns during research and development as well as commercialization.6 In general, the most commonly studied aqueous compatible conjugated polymers are dispersions of commercially available poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or synthesized conjugated polyelectrolytes.6−9 PEDOT:PSS is not an ideal polymer for these applications because synthetic modification is difficult and it is an aqueous dispersion, which means that films can dissolve in aqueous media when cross-linking additives or creative deposition methods are not used.7,10−12 In regards to conjugated polyelectrolytes, several postpolymerization modifications are © XXXX American Chemical Society

Received: July 20, 2018 Accepted: September 5, 2018

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potential to design, synthesize, process, and test electrochromic polymers while minimalizing environmental impact. Synthesis of multiester ProDOT monomers (Scheme S1) was achieved following our previously developed synthetic protocols.14,23 Briefly, ProDOT-(CH2Br)2 was treated with suberic acid monomethyl ester in the presence of potassium carbonate (K2CO3) in N,N-dimethylformamide (DMF) at 100 °C to yield ProDOT-TetEster-H2 in a yield of 60%. Subsequent bromination of ProDOT-TetEster-H2 with Nbromosuccinimide (NBS) in tetrahydrofuran (THF) gave ProDOT-TetEster-Br2 in a yield of 71%. Palladium-catalyzed direct heteroarylation polymerization conditions commonly used for synthesizing electrochromic polymers were used to synthesize ProDOT-TetEster (Scheme 2).24,25 This protocol consists of heating a solution of ProDOT-TetEster-H2 and ProDOT-TetEster-Br2 in N,Ndimethylacetimide (DMAc) at 140 °C for 18 h using palladium(II) acetate (Pd(OAc)2) as the catalyst, K2CO3 as the base, and pivalic acid (PivOH) as the proton shuttle. The reaction mixture was subsequently cooled to room temperature, precipitated into methanol, and collected into a Soxhlet thimble. It is worth noting that there was a large fraction of fine, powder-like material passed through the filter while collecting the precipitate. This material was recovered and found to be insoluble in organic solvents, suggesting that higher molecular weight fractions of ProDOT-TetEster have limited solubility. Purification consisted of washing via Soxhlet purification with MeOH and subsequently recovering the polymer in the acetone fraction with a 50% yield. Molecular weight estimations via size-exclusion chromatography (SEC) using THF as the eluent and polystyrene standards revealed the polymerization yielded ProDOTTetEster with an Mn = 21.5 kg/mol with a dispersity (Đ = Mw/Mn) of 1.4 (see Figure S6 for the SEC elugram). Motivated by the Innovated Medicines (IMI)−CHM 21 solvent guide that promotes sustainable biological and chemical methodologies,28 and by our previous work with ester-functionalized polymers,23 we explored the solubility of ProDOT-TetEster in acetate-based solvents to determine the feasibility of processing films from these solvents. Surprisingly, this polymer was less soluble than ProDOT-BOE in ester solvents (∼2 mg/mL in propyl acetate for ProDOT-TetEster versus >50 mg/mL for ProDOT-BOE),23 which is attributed to decreased conformational entropy of the solubilizing side chains. However, ProDOT-TetEster was found to be readily soluble in the environmentally sustainable solvent 2-Me-THF, derived from furfural biomass, allowing it to be used as the processing solvent for this study. ProDOT-TetEster films were spray cast to an optical density of 1.1 (%T ≈ 8%) from 2-Me-THF with a solution concentration of 5 mg/mL. Contact angle measurements (Figure S7) revealed the films to exhibit a contact angle of 90° ± 4.6°, which is counterintuitive to our hypothesis that additional esters would render films hydrophilic and compatible with aqueous electrolytes. However, we suspect the high contact angle is due to a rough topology since we have shown in our previous studies that both the processing technique (spray casting) and the fast evaporation rate of the solvent (2-Me-THF) produce rough topologies and high contact angles.23,29 Indeed, investigation of film topology using optical microscopy and atomic force microscopy (AFM) (Figure S8) revealed films to exhibit heterogeneous features on length scales from 1 to 100 μm. Furthermore, AFM

thorough characterization and may make processing difficult. For example, molecular weight estimation via size-exclusion chromatography (SEC) is difficult for oligoether-functionalized polymers due to solubility constraints, which limits the understanding of molecular weight−property relationships.18,21 These solubility limitations set the precedent to explore other polar functionalities as alternative solubilizing side chains to oligoethers and investigate their viability as aqueous electrolyte compatible conjugated polymers. Recently, our group explored the feasibility of utilizing esterfunctionalized side chains to facilitate processing of 3,4propylenedioxythiophene (ProDOT) polymers from environmentally benign/sustainable solvents to reduce the environmental impact of manufacturing electrochromic polymers (ECPs) with high-throughput methods.23 We found that a ProDOT homopolymer with butyloctylester (BOE) side chains could be processed from benign solvents, such as ethyl acetate and propyl acetate, without postpolymerization modifications or sacrificing the polymer film’s redox properties. Additionally, this BOE polymer was synthesized via direct (hetero)arylation polymerization (DHAP), which is considered to be a green alternative to traditional cross-coupling reactions due to elimination of additional monomer preparatory steps and toxic byproducts.24−27 These encouraging results led us to hypothesize that we could introduce additional ester (polar) functionality to maintain solubility in environmentally benign solvents for high-throughput processing techniques and generate aqueous compatible films that allow for electrochemical switching in aqueous electrolytes. By combining environmentally benign processing, aqueous electrolyte compatibility, and DHAP protocols, we will effectively reduce the environmental impact from “cradle to grave” of these ProDOT-based ECPs, as illustrated in Scheme 1. With these considerations in mind, herein, we report the Scheme 1. Visual Representation of the Evolution of Environmentally Benign ProDOT Electrochromic Polymers: ProDOT-OEtHx, ProDOT-BOE, ProDOTTetEster

synthesis of a multiester ProDOT polymer that is processable from an environmentally sustainable organic solvent and is aqueous electrolyte compatible without requiring postpolymerization modifications. The polymer films, processed from an organic solvent and evaluated in an aqueous electrolyte, are electrochemically active and can be switch between a colored neutral state and a colorless oxidized state, highlighting the 1209

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ACS Macro Letters Scheme 2. Direct Arylation Polymerization for the Synthesis of ProDOT-TetEster

Figure 1. (a) Cyclic voltammetry of ProDOT-TetEster (black), ProDOT-OEtHx (red), and ProDOT-BOE (blue) films in 0.1 M aqueous NaCl from −0.3 to 1.0 V vs Ag/AgCl with a scan rate of 50 mV/s. (b) Cyclic voltammograms of ProDOT-TetEster films, after electrochemical conditioning with 10 CV cycles using the −0.3 to 1.0 V voltage window, at various scan rates while scanning from −0.3 to 1.0 V vs Ag/AgCl in 0.1 M aqueous NaCl electrolyte. The dotted lines correspond to the cathodic and anodic peak potentials for the slowest scan rate of 20 mV/s.

Figure 2. (a) UV−vis spectra of ProDOT-TetEster in 0.002 mg/mL of 2-Me-THF solution (black), as a pristine spray-cast film (red) and a spraycast film electrochemically conditioned (blue) by cycling 10 times from −0.3 to 1.0 V Ag/AgCl in 0.1 M aqueous NaCl electrolyte. (b) Absorbance as a function of potential of ProDOT-TetEster monitored at 0.05 V intervals from −0.3 to 1.0 V and pictures of a reduced (−0.3 V) and oxidized film (1.0 V) (inset).

measurements indicate that ProDOT-TetEster films have roughness values (Rq) of ∼24 nm, which are similar to the roughness values measured in our previous work with esterfunctionalized ProDOT polymers spray cast from 2-Me-THF. Nevertheless, conjugated polymer films are known to have

improved wettability in the presence of various electrolytes and at varying electrochemical potentials,30,31 which sets the precedent for studying the resulting redox properties of ProDOT-TetEster films in aqueous electrolytes. 1210

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Figure 3. (a) Color coordinates and (b) L* vs potential plot overlaid with a cyclic voltammetry trace of ProDOT-TetEster while scanning from −0.3 to 1.0 V vs Ag/AgCl in 0.1 M aqueous NaCl. The arrow in (a) indicates the scan direction from −0.3 to 1.0 V vs Ag/AgCl, beginning with the pristine film (circled) and progressing to the colorless oxidized film.

The redox properties of ProDOT-TetEster films in an aqueous electrolyte environment were evaluated by monitoring the current across a voltage window of −0.3 to 1.0 V vs Ag/ AgCl using cyclic voltammetry. Electrochemical measurements in organic electrolytes were attempted, but the polymer was partially soluble in propylene carbonate and acetonitrile, which led to leaching of the polymer film into solution, rendering the experiments unsuccessful. As shown in Figure 1(a), ProDOTTetEster shows an onset of oxidation of ∼0.4 V vs the Ag/ AgCl reference electrode in 0.1 M aqueous NaCl. Conversely, ProDOT polymers with ethylhexyl ether (ProDOT-OEtHx) and butyloctyl ester (ProDOT-BOE) solubilizing side chains do not show electrochemical activity in an aqueous environment in this voltage window. This is due to the hydrophobic nature of the alkyl-solubilizing side chains of ProDOT-OEtHx and ProDOT-BOE, while ProDOT-TetEster possesses more polar character due to the introduction of additional ester functionalities. Additionally, by monitoring the peak current density as a function of scan rate, we determined the electroactive kinetic limitations of ProDOT-TetEster films. As shown in Figure S8 and Figure 1(b), ProDOT-TetEster exhibits a linear dependence up to 100 mV/s. Scan rates above 100 mV/s were not able to fully resolve the oxidation peak within the −0.3 to 1.0 V window. The linear relationship indicates the redox activity is not a diffusion-controlled process for scan rates up to 100 mV/s. The optical absorbance properties of ProDOT-TetEster were investigated in 2-Me-THF solution and as a film spray cast from 2-Me-THF, as shown in Figure 2(a). In dilute solution, ProDOT-TetEster exhibits an absorbance maximum (λmax) at ∼540 nm and a second shoulder at ∼590 nm. When ProDOT-TetEster is spray cast to an optical density of 1.1 (%T ≈ 8%), the onset of absorbance is red-shifted from ∼630 nm to ∼700 nm due to increased π−π interactions between polymer chains. The absorbance spectrum of the pristine cast film is nearly featureless, with λmax = 561 nm. After electrochemical conditioning via 10 cycles between −0.3 and 1.0 V vs Ag/AgCl in 0.1 M aqueous NaCl, the onset of absorbance is slightly redshifted, and a clear λmax = 580 nm evolves along with a shoulder at ∼621 nm. This shift in the absorbance spectrum is attributed to increased ordering of polymer chains and a longer effective conjugation length as a result of the doping process.32−34

UV−vis−NIR spectroelectrochemistry (Figure 2(b)) was used to monitor the change in absorbance with increasing potential with 0.05 V intervals from −0.3 to 1.0 V vs Ag/AgCl in 0.1 M aqueous NaCl. With increasing potential, the absorbance profile in the visible region gradually decreases, which is accompanied by the formation of an absorbance band at ∼900 nm (polaron). As the potential is further increased, the polaron transition decreases, and another transition past 1400 nm forms (bipolaron). The absorbance profile in the visible region is nearly colorless. Thus, the film transitions from a purple colored neutral state to a colorless oxidized state (inset of Figure 2(b)). Colorimetric analysis based on the “Commission Internationale de l’Eclairage” 1976 L*a*b* color standards was used to evaluate the perceived color of ProDOT-TetEster films.9 As shown in Figure 3(a), the a* coordinates (∼35 for pristine films and ∼27 for electrochemically conditioned films) are smaller than typical ProDOT homopolymers, which is indicative of a more relaxed backbone. Conversely, the b* values (∼−50) for the ProDOT-TetEster films are consistent with previously reported ester-functionalized ProDOTs.23 As the potential increases, the a*b* values decrease, which is attributed to the polymer film going from a colored neutral film to a colorless oxidized film. This is also apparent by monitoring L* as a function of potential, as shown in Figure 3(b), where L* begins to increase from ∼40 near the onset of oxidation (∼0.4 V vs Ag/AgCl) and reaches a value of ∼90 by 1.0 V. In addition to small a*b* values (approximately −2 for both a* and b*) which indicate color neutrality, L* values of ∼90 indicate that the film reaches a highly transmissive oxidized state. The switching rate of the ProDOT-TetEster was studied by monitoring the change in transmittance (ΔT (%)) at λmax as a function of time by applying square-wave potential steps (−0.3 to 1.0 V vs Ag/AgCl in 0.1 M aqueous NaCl) to polymer films spray-cast from 2-Me-THF onto ITO electrodes (7 mm × 2.5 mm, Rs = 8−12 Ω/sq) for periods of 30, 10, 5, 2, 1, 0.5, and 0.25 s. Figure S10 shows that as the number of switching cycles increases for each time interval the ΔT (%) slightly decreases for ProDOT-TetEster. Furthermore, when measuring the time to reach 95% of a full-contrast switch (t95) in Figure S10, the polymer exhibits a bleaching time of ∼2 s and a coloration time of ∼3.5 s. The consistent decrease in ΔT (%) is attributed 1211

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Figure 4. (a) UV−vis absorbance spectra of ProDOT-TetEster after 10 cyclic voltammetry cycles (black trace), after 500 square-wave potential switches between −0.3 and 1.0 V vs Ag/AgCl (red trace), after 500 switches while holding at −0.3 V (blue trace), and a plot of current density versus time (inset) under an inert atmosphere. (b) Transmittance as a function of switching time from 30 to 0.25 s for a ProDOT-TetEster film spray-cast from 2-Me-THF.

M aqueous NaCl. Additionally, ProDOT-TetEster films exhibit electrochromic characteristics by reversibly transitioning from a colored neutral state to a colorless oxidized state with increasing electrochemical potential. Electrochromic properties, such as contrast and switching speed, of ProDOTTetEster films tested in aqueous electrolytes are comparable to well-studied ProDOT-based electrochromic polymers tested in organic electrolytes (ΔT (%) ≈ 70%, t95 ≈ 2 s). This work highlights the ability to exploit side-chain engineering as a means not only to allow processing from environmentally sustainable solvents but also to impose functionality that renders electrochromic films aqueous electrolyte compatible without postpolymerization modifications. Furthermore, this design strategy can be expanded to other conjugated polymer systems for color control of aqueous compatible ECPs or may find utility in redox applications such as supercapacitors or organic electrochemical transistors.

to the slow coloration time, which is brought on by a slower reduction rate of the film compared to oxidation rate. Due to this slow coloration time, the film cannot return to a fully colored state during the cycling experiments. Further investigation into the stability of the redox activity of ProDOT-TetEster films revealed the films to be oxidatively unstable in the presence of air while switching several hundred times, as illustrated in Figure S11. Measurements of the redox stability were therefore performed under an Ar atmosphere, which surprisingly led to distinct changes in the electrochemical behavior of the film as shown in Figure S12. We believe the changes in the redox activity is attributed to a surface polarization effect at the polymer:electrolyte interface and warrants further investigation.35 Figure 4(a) shows the UV−vis spectra after electrochemical conditioning (black trace) and switching the film 500 times (red trace) under an inert atmosphere and shows an absorbance decrease of ∼15%. This loss of absorbance is attributed to charges being trapped in the film while switching between the neutral and oxidized states, which is evident by the constant decrease in the current density over time as displayed in the inset of Figure 4(a). This is further exemplified in Figure 4(a) as the full absorbance is restored when the film is held at −0.3 V (blue trace). Finally, Figure 4(b) shows that ΔT (%) ≈ 70% for cycle times longer than 5 s, which is analogous to ester-functionalized ECPs studied in organic electrolytes.23 As the cycle time is decreased, ΔT (%) decreases, reaching a minimum of ∼15% at 0.25 s. Establishing robust design strategies to construct aqueous electrolyte compatible, redox-active, conjugated polymers is desirable due to the potential to minimalize hazards of application-driven devices by facilitating electrochemical reactions in nonhazardous solvent media such as water. This study provides a facile design strategy for synthesizing aqueous electrolyte compatible electrochromic polymers, which is accomplished by introducing multiple ester functionalities to the solubilizing side chains. ProDOT-TetEster was synthesized via direct arylation polymerization and could be processed into a thin film from the environmentally sustainable solvent 2-MeTHF. Cyclic voltammetry revealed ProDOT-TetEster to have a relatively low onset of oxidation (∼0.4 V vs Ag/AgCl) in 0.1

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00551.

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Materials and methods, synthetic procedures, 1H and 13 C of monomers and polymers, size-exclusion chromatography, contact angle data, film topology experiments, and results from stability studies (PDF)

AUTHOR INFORMATION

Corresponding Author

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

John R. Reynolds: 0000-0002-7417-4869 Notes

The authors declare the following competing financial interest(s): Electrochromic polymer technology developed at the Georgia Institute of Technology has been licensed to NxN Licensing. JRR serves as a consultant to NxN Licensing. 1212

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ACKNOWLEDGMENTS Funding from NxN Licensing Inc. for this work is acknowledged. Ian Pelse was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. Dr. Anna M. Ö sterholm is gratefully acknowledged for helpful discussions regarding the electrochemistry presented in this work.

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DOI: 10.1021/acsmacrolett.8b00551 ACS Macro Lett. 2018, 7, 1208−1214

Letter

ACS Macro Letters Lanzani, G.; Li, H.; Brédas, J.-L.; De Angelis, F. Surface Polarization Drives Photoinduced Charge Separation at the P3HT/Water Interface. ACS Energy Lett. 2016, 1, 454−463.

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DOI: 10.1021/acsmacrolett.8b00551 ACS Macro Lett. 2018, 7, 1208−1214