A chance discovery by a graduate student has led to materials that quickly change color from completely clear to a range of vibrant hues – and back again.
Graduate research assistant Dylan Christiansen investigates the electrochromic properties of new materials in a transparent electrochemical cell that allows examination of color change upon application of oxidative voltage. (Photo: Allison Carter, Georgia Tech)
A chance discovery by a graduate student has led to materials that quickly change color from completely clear to a range of vibrant hues – and back again. The work could have applications in everything from skyscraper windows that control the amount of light and heat entering and leaving a building, to switchable camouflage and visors for military applications, and even cosmetics and clothing. which change color. It also helps fill a knowledge gap in a key area of materials science and chemistry.
An article on the research was published in a recent issue of the Journal of the American Chemical Society (JACS).
Electrochromic materials change color upon application of a low electrical potential or voltage. Over the past 20 years, John R. Reynolds, a professor at the Georgia Institute of Technology, has researched and developed electrochromic materials capable of changing from a wide range of bright to light colors.
But these materials, called cathodic-dyed polymers, have a downside. Their transmissive, or clear, state is not completely clear. On the contrary, in this state, the material has a light blue tint. “It’s good for many applications – including mirrors that reduce glare from oncoming cars by turning dark – but not for all potential uses,” said Reynolds, who has joint appointments at the School of Chemistry and Biochemistry and School of Materials. Science and Engineering at Georgia Tech.
For example, the Air Force is working on visors for its pilots that would automatically change from dark to light when an aircraft flies from sunlight into clouds. “And when they say clear, they mean it to be crystal clear, not a light blue,” Reynolds said. “We would like to get rid of this tint.”
Towards a solution
There is another family of electrochromic materials that can change color when exposed to oxidative stress. These materials, known as anodic-colored electrochromes (ACE), are colorless materials that become colored upon oxidation. But there has been a lack of knowledge in the science behind the colored oxidized states, known as radical cations. The researchers did not understand the absorption mechanism of these cations and therefore the colors could not be tuned in a controlled manner.
Introducing Reynolds Group graduate student Dylan T. Christiansen. While tinkering with some ACE molecules, he experimented with a new approach to controlling the color of radical cations. Specifically, he created four different ACE molecules by making tiny changes to the molecular structures of ACEs that have little effect on the neutral and clear state, but significantly alter the absorption of the colored or radical cation state.
The results have been spectacular. “I expected color differences between the four molecules, but thought they would be very minor,” Christiansen said. Instead, upon application of oxidative stress, the four molecules produced four very different colors: two vibrant greens, one yellow, and one red. And unlike their cathodic counterparts, they are crystal clear in the neutral state, with no tint. Finally, like the mixture of inks, the researchers found that a mixture of the molecules that turn green and red form a clear mixture and turn opaque black. Suddenly, those Air Force visors that go from crystal clear to black seemed more approachable.
“The beauty of it all is that it’s so simple. These minor chemical changes — literally the difference of a few atoms — have a huge impact on color,” said Aimée L. Tomlinson, a professor in the Department of Chemistry and Biochemistry at the University of North Georgia and third author of the item. with Reynolds and Christiansen.
What is going on?
How can such small changes have such an effect? This is where Tomlinson, a computational chemist, comes in.
For the past five years, she has analyzed Reynolds electrochromic materials with computer models that provide information about what is happening at the sub-molecular level. Using these models, coupled with Christiansen’s data for the new ACE molecules, she was able to show how small chemical changes that were made can drastically alter the electronic structure of the molecules’ radical cation states and ultimately control color.
Work continues to generate insights into new ACE molecules through continuous feedback between Tomlinson’s models and experimental data. The models help guide laboratory efforts to create new ACE molecules, while experimental data from these molecules make the models even more robust.
Tomlinson notes that because the work also helps shed light on how radical cations work – they are still not well understood – it could help others manipulate them for future use in fields beyond electrochromism.
Reynolds commented on the serendipitous nature of the initial discovery. “I think what makes science really interesting is that [sometimes] you see something you really didn’t expect, you go after it, and you end up with something better than you expected when you started.
This work was funded by the Air Force Office of Scientific Research. Tomlinson also acknowledges the support of his university, while Reynolds acknowledges the support of his electrochromic polymer research program from NXN Licensing. All opinions or conclusions are those of the authors and do not necessarily represent the views of the sponsoring organizations.
QUOTE: Dylan T. Christiansen, Aimée L. Tomlinson, and John R. Reynolds, “A New Design Paradigm for Color Control in Anodic-Dye Electrochromic Molecules” (Journal of the American Chemical Society, February 22, 2019). https://pubsdc3.acs.org/doi/10.1021/jacs.9b01507
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