Electrochromism is associated with electrochemically inducing a redox reaction that results in a reversible color change in transmittance and/or reflectance of optical properties. In the Reynolds group, conjugated redox active organic molecules are specifically designed and studied in order to access a broad range of colors as well as implementing targeted solid state properties for versatile application input. Our group has a long history of studying easily oxidized cathodically coloring organic polymeric electrochromes1-9 which are colored in their neutral state but become transmissive upon oxidation. However, we have also expanded to anodically coloring electrochromic molecules10-13 which are colorless in the neutral state and oxidize to a colored species. We work to bridge a knowledge gap by seeking to understand the fundamental redox and optical properties of vibrantly colored molecular electrochromes in order to switch from fully transparent and achieve high optical contrast. We work closely with Professor Aime’e Tomlinson (University of North Georgia) to create a tight feedback loop where we utilize density functional theory (DFT) and time dependent DFT (TDDFT) treatments in order to design and identify systems that provide promising utilization in high contrast electrochromes but also achieve a sound understanding on the structure-property relationship.
Anodically Electrochromic Molecules
We recently published a mini review13 which provides an overview of our recent efforts in developing a family of anodically coloring electrochromic (ACE) molecules. We developed a new paradigm for the design of conjugated ACE molecules where there is synthetic control on the absorption of the oxidized state. Through shortened cross conjugation, we can access the electronic energy levels of the radical cation state and fine-tune its oscillator strength and absorption in the visible light spectrum while keeping the neutral absorbance in the UV by changing the electronic effects through the addition of substituents that range from heavy electron withdrawing to electron donating.11
In addition, we further expanded our structure to 3-ring electrochromes which allows us to focus on building and expanding our knowledge by inducing steric interactions which can provide a range of structural conformations that range from planarity to torsional twists.12
Cathodically Electrochromic Molecules
A series of conjugated polymers displaying yellow and orange switching transmissive states was demonstrated by introducing dialkythiophenes in alternation with dioxythiophenes with varying side chains to induce differing levels of inter-ring strain. These subtle steric interactions generated tunable neutral colors ranging from yellow to deep red where these properties where further implored by subtractive color mixing to create black and brown blends that are redox stable.8 However, more recently, we have designed a random terpolymer based on all donor repeat units that yield cathodically colored electrochromic polymers that span the entire visible spectrum by control of the monomer ratio. This allowed for a large portion of the gap color space to be covered while also maintain ideal electrochromic properties such as achieving low oxidation potentials, high contrast, and long lasting optical memory.9
Solid State Applications
We recently investigated an ACE molecule capable of adsorbing onto a metal oxide surface through the attachment of a phosphonic acid anchoring group.12,13 We have proven to enhance the switching rates of the ACE molecules by anchoring on nanoITO and achieving 100 switches with 10 second intervals with a colorless state with 98% T at negative potential applied followed by 30% T and positive potential being applied. This alone demonstration proves the inherent stability that can be achieved as well as paving into varying color palettes.
In an additional applicational component, we demonstrate the ability to inkjet print color-neutral, low resistance electrodes on paper from PEDOT:PSS ink and develop a path towards high throughput fabrication of readouts for disposable electronics. The PEDOT:PSS/cellulose nanofiber paper supports the reversible oxidation of 3 electrochromic polymers with differing colors (cyan, magenta, and yellow) which generates the possibility for a fully printed color display.14
Color Mixing & Black to Transmissive Electrochromism
Lastly, we have demonstrated a straightforward strategy of accessing a wide variety of colors through synthetically tunable molecules which produces a full palette of vibrantly colored to highly transmissive states. However, fine color control is difficult to achieve and requires synthetic design strategies to optimize polymer structure and color relationship to access the full color palette. Color-mixing is a suitable approach to achieve and demonstrate the ability to co-process ECP mixtures to generate color properties of reasonable agreement to generate predicted color values.15 We have also expanded this concept to further demonstrate a set of various hues of brown electrochromic polymers which was acquired by mixing cyan and yellow primaries in combination ratios with orange and blue as secondary colors.16
Furthermore, we have demonstrated the properties of dioxythiophene based electrochromic polymers can be leveraged to generate high-contrast, black-to-transmissive materials through color mixing of 5 vibrant colored-to-colorless polymers. This work showcases subtle control over a range in process of coloration as well as enabling fine-tuned neutral optical transition.17 Lastly, we further expanded and showcase the capability of switching from black to transmissive through gray intermediates by substituting EDOTS with ProDOTs in the donor building block of D-A-D terheterocycle. This strategy involves controlling low-energy absorption at the edge of the visible light spectrum which also allows effectively varying slight color hue changes.18
- Reeves, B.D.; Thompson, B.C.; Abboud, K.A.; Smart, B.E.; Reynolds, J.R. Dual Cathodically and Anodically Coloring Electrochromic Polymer Based on a Spiro Bipropylenedioxythiophene [(Poly(spiroBiProDOT)]. Adv. Mater. 2002, 14 (10), 717-719.
- Schwendeman, I.; Hickman, R.; Sönmez, G.; Schottland, P.; Zong, K.; Welsh, D.M.; Reynolds, J.R. Enhanced Contrast Dual Polymer Electrochromic Devices. Chem. Mater. 2002, 14 (7), 3118-3122
- Walczak, R.M.; Reynolds, J.R. Poly(3,4-alkylenedioxypyrroles): The PXDOPs as Versatile Yet Underutilized Electroactive and Conducting Polymers. Adv. Mater., 2006, 18, 1121-1131.
- Dyer, A.; Thompson, E.J.; Reynolds, J.R. Completing the Color Palette with Spray-Processable Polymer Electrochromics. ACS Appl Mater Interfaces. 2011, 3(6): 1787-95.
- Kerszulis, J.A.; Amb, C.M.; Dyer, A.; Reynolds, J.R. Follow the Yellow Brick Road: Structural Optimization of Vibrant Yellow-to-Transmissive Electrochromic Conjugated Polymers. Macromolecules.2014, 47(16), 5462-5469.
- Kerszulis, J.A.; Johnson, K.E.; Kuepfert, M.; Khoshabo,D.; Dyer, A.L.; Reynolds, J.R. Tuning the Painter’s Palette: Suble Steric Effects on Spectra and Colour in Conjugated Electrochromic Polymers. J. Mater. Chem. C., 2015, 3, 3211-3218.
- Cao, K.; Shen, D.E.; Österholm, A.M.; Kerszulis, J.A.; Reynolds, J.R. Tuning Color, Contrast, and Redox Stability in High Gap Cathodically Coloring Electrochromic Polymers. Macromolecules, 2016, 49 (22), 8498-8507.
- Christiansen, D.T.; Reynolds, J.R. A Fruitful Usage of a Dialkylthiophene Comonomer for Redox Stable Wide-Gap Cathodically Coloring Electrochromic Polymers. Macromolecules, 2018, 51 (22), 9250-9258.
- Christiansen, D.T.; Ohtani, S.; Chujo, Y.; Tomlinson, A.L.; Reynolds, J.R. All Donor Electrochromic Polymers Tunable Across the Visible Spectrum via Random Copolymerization. Chemistry of Materials. 2019, 31 (17), 6841-6849.
- Christiansen, D.T.; Wheeler, D.L.; Tomlinson, A.L.; Reynolds, J.R. Electrochromism of Alkylene-Linked Discrete Chromophore Polymers with Broad radical Cation Light Absorption. Polymer Chemistry, 2018, 9 (22), 3055-3066.
- Christiansen, D.T.; Tomlinson, A.; Reynolds, J.R. New Design Paradigm for Color Control in Anodically Coloring Electrochromic Molecules. J. Am. Chem. Soc. 2019, 141 (9), 3859-3862.
- Nhon, L.; Wilkin, R.; Reynolds, J.R.; Tomlinson, A. Guiding Synthetic Targets of Anodically Coloring Electrochromes through Density Functional Theory. J. Chem. Phys. 2021, 154, 054110.
- Österholm, A.M.; Nhon, L.; Shen, D.E.; Dejneka, A.M.; Tomlinson, A.L.; Reynolds, J.R. Conquering Residual Light Absorption in the Transmissive States of Organic Electrochromic Materials. Mater. Horiz, 2022, 9, 252-260.
- Lang, A.W.; Österholm, A.M.; Reynolds, J.R. Paper-Based Electrochromic Devices Enabled by Nanocellulose-Coated Substrates. Adv. Funct. Mater. 2019, 29 (39): 1903487
- Bulloch, R.H.; Kerszulis, J.A.; Dyer, A.; Reynolds, J.R. An Electrochromic Painter’s Palette: Color Mixing via Solution Co-Processing. ACS Applied Materials & Interfaces, 2015, 7(3), 1406-1412
- Österholm, 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 Applied Materials & Interfaces, 2015, 7(3), 1413-1421.
- Savagian, L.; Österholm, A.; Shen, D.E.; Christiansen, D.T.; Kuepfert, M.; Reynolds, J.R. Conjugated Polymer Blends for High Contrast Black-to-Transmissive Electrochromism. Advanced Optical Materials, 2018, 6 (19), 1800594.
- Lo, C.K.; Shen, D.E.; Reynolds, J.R. Fine-Tuning the Color Hue of π-conjugated Black-to-Clear Electrochromic Random Copolymers. Macromolecules, 2019, 52 (17), 6773-5779.