Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX pubs.acs.org/cm All Donor Electrochromic Polymers Tunable across the Visible Spectrum via Random Copolymerization Dylan T. Christiansen,† Shunsuke Ohtani,‡ Yoshiki Chujo,‡ Aimeé L. Tomlinson,§ and John R. Reynolds*,† † School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, and Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡ Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan § Department of Chemistry/Biochemistry, University of North Georgia, Dahlonega, Georgia 30597, United States *S Supporting Information ABSTRACT: A series of conjugated, random terpolymers based on all donor repeat units are prepared via direct (hetero)arylation polymerization yielding cathodically colored electrochromic poly- mers that span the visible spectrum. The polymers are based on repeat units of a dialkylthiophene, a 3,4-propylenedioxythiophene, and dimers of 3,4-ethylenedioxythiophene. Using a tight feedback loop between computational and experimental chemistry, the colors of these polymers are controllably tuned to cover the visible spectrum by controlling the monomer ratios. Examinations via ultraviolet−visible−near-infrared spectroscopy, differential pulse voltammetry, spectroelectrochemistry, and colorimetry show that, while these systems can vary greatly in their spectral properties, their oxidation potentials are all low (<0.5 V vs Ag/AgCl). The color tunability allows for access to neutral state orange, red, pink, magenta, purple, and blue polymers of various hues with a* values ranging from 22 to 35 and b* values ranging from −44 to 45, while maintaining highly transmissive oxidized states. This approach allows access to a wide gamut of colors with only three monomers while affording materials with low oxidation potentials and high contrast. ■ INTRODUCTION regions of the color space (Figure 1a). All donor systems can The history of research of fully conjugated cathodically colored cover three of the four regions shown here (wide, mid, and low gap), as green materials, having a highly negative b*, require electrochromic polymers (ECPs) has yielded materials that 1−7 dual absorption. The current state of the art all donor polymers Downloaded via UNIV OF NORTH GEORGIA on August 17, 2019 at 16:55:48 (UTC). span the entire color palette. These polymers can be spray were examined to develop an approach to make a color tunable cast to form vividly colored films that upon oxidation become system (Figure 1b). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. highly transmissive in the visible region. Methods of creating Yellow ECPs, which utilize 1,4-phenylene linkages, have the wide range of possible colors involve incorporating historically suffered from high oxidation potentials and low numerous monomers in well-defined sequences. Random redox stability over 100 switches, which is attributed to the copolymerization has been used in many approaches as a open sites on the aryl ring leaving the materials susceptible to means of accessing neutral colors such as brown and black in − 13,14 8−12 nucleophilic attack or radical radical coupling. The electrochromic polymers. With random copolymerization incorporation of a dialkylthiophene (DAT) as a monomer of broad, low-energy (long wavelength) absorbances are often high redox stability in wide-gap electrochromic polymers has achieved by incorporating an electron-accepting moiety in the 15 − helped to overcome this challenge. For absorbing the middle polymer backbone. The drawback of donor acceptor systems of the spectrum, the homopolymer of 3,4-propylenedioxythio- is that they tend to have higher oxidation potentials caused by pohene (ProDOT) with 2-ethyl-hexyloxy side chains offers a the accepting moiety, which limits optical memory (i.e., high solubility and a low oxidation potential for switching as bistability) in the oxidized form, thus making their usefulness in devices limited. A new approach that circumvents the challenges caused by incorporating electron acceptors into the Special Issue: Jean-Luc Bredas Festschrift main chain is developed in this work. When considering how Received: April 1, 2019 to make color tunable systems without acceptors, it is Revised: May 28, 2019 important to understand the design of materials for specific © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.9b01293 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article Figure 1. (a) L*a*b* color space cross section and (b) all donor polymers used for inspiration that show tunability across the visible spectrum. Figure 2. Repeat unit structure of random copolymers and target polymer ratios that allow access to an array of colors that span the visible spectrum and oxidize to transmissive forms. well as a high contrast.16 This purple polymer also shows red, pink, magenta, purple, and blue, all of which can be exemplary contrast, redox stability, and switching speed. For oxidized to a highly transmissive form. Relative to the 1:1:1 narrow-gap polymers, typically donor−acceptor systems are ratio, an increased DAT content blue-shifts the absorbance of used. The drawbacks to these systems, as previously discussed, the polymer and an increased biEDOT content red-shifts the have been circumvented with the advent of a soluble poly(3,4- absorbance, while ProDOT brings a high solubility and a low ethylenedioxythiophene) (PEDOT) analogue that covers the oxidation potential. As detailed in Figure 2, the nomenclature narrow-gap portion of the visible spectrum.17 In this polymer, of the polymers discussed in this work will be determined by the biEDOT portion of the polymer brings electron richness the ratio of the monomers in the polymerization reaction. For and planarity, narrowing the optical gap and covering the low- example, a random copolymerization that is two parts DAT (x) energy portion of the visible region. and one part each ProDOT (y) and biEDOT (z) is xyz211. In this work, the combination of DAT, ProDOT, and biEDOT into random copolymers has been demonstrated to ■ RESULTS AND DISCUSSION give broadly tunable electrochromic materials that span the Quantum Chemical Calculations. To elucidate the visible spectrum (Figure 2). Using a tight feedback loop impact on the number and identity of aryl ring incorporation between theory and experiment, the ratios of these monomers on the color of these systems, density functional theory (DFT) are demonstrated to tune the absorbance of the resulting is utilized. We have found that pairing the mPW1PBE polymer, thus giving access to a wide range of colors in their functional with the cc-PVDZ basis set, while including neutral forms, while all being easily oxidized to highly dichloromethane through the conductor polarizable calculation transmissive states. Utilizing only these three monomers in model (CPCM), provides excellent correlation to experimental − different ratios provides access to polymers that are orange, data.18 21 A set of multiheterocycle oligomers is chosen to B DOI: 10.1021/acs.chemmater.9b01293 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article Table 1. Calculated First Excited State Gaps, Dihedral Angles, and Visible Excited State Wavelengths and Oscillator Strengths a for the Target Oligomers dihedral angles peak maximum − − − − − λ oligomer Eg (eV) Ar1 Ar2 Ar2 Ar3 Ar3 Ar4 Ar4 Ar5 Ar5 Ar6 (nm) f PEEP 2.62 179.7 178.1 179.7 473.3 1.60 EEPEE 2.38 176.4 178.7 178.7 176.4 521.1 2.03 PEEPEE 2.22 179.3 177.9 179.0 179.7 178.5 559.6 2.48 DEED 2.72 156.9 179.7 156.9 456.6 1.54 EEDEE 2.45 178.3 168.4 168.4 178.3 506.4 1.94 EEPPEE 2.23 178.4 179.2 175.9 179.2 178.4 556.7 2.44 DPPD 3.09 46.8 175.6 48.9 401.3 1.27 PDPDP 2.49 169.1 176.9 176.9 169.1 497.4 1.79 PDEEDP 2.35 163.7 163.0 179.6 163.0 163.7 528.6 2.25 DPDP 2.98 137.2 160.9 136.9 416.2 1.19 DPDPD 2.48 178.6 179.9 179.9 178.6 500.8 1.90 DPDPDP 2.53 166.2 179.1 178.2 169.9 128.3 489.9 2.51 aThe legend indicates how the structures and their corresponding dihedral angles are defined. model possible random copolymers with varying degrees of conjugation and ring strain and to assess their influence on the predicted color (a complete list of the structures studied is given in Figure S1, and their calculated frontier molecular orbitals are provided in Figure S2). For each system, the neutral geometry is optimized followed by a frequency calculation to ensure the most stable configuration is produced. Time-dependent density functional theory (TD-DFT) calcu- lations were then applied to these structures to simulate the ultraviolet−visible (UV−vis) spectra and predict the color of these oligomers. An examination of the calculated HOMO−LUMO gaps provides direction with respect to the potential spectral breath, and the absorption tunability limits, which may be attained by the copolymers. As the strain along the backbone is increased by adding DAT units, the HOMO−LUMO gap is widened. Oligomers possessing the most DAT are particularly influenced by distortion from planarity causing a gap increase. The most twisted species (those that most deviate from the defined 180° antiplanar geometry) are DPPD, DPDP, DEED, Figure 3. Calculated UV−vis−NIR absorbance of the model and DPDPDP with values as low as 128.3°, 156.9°, 136.9°, and chromophores showing the limits of spectral breadth accessible for 46.8° and first excited state gaps of 3.09, 2.98, 2.72, and 2.53 this approach. The side chains for each monomer have been reduced eV, respectively (see Table 1).