Color-changing materials change from solid to transparent

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Researchers have created materials that quickly change color, going from a perfectly light color to a range of vibrant hues, and vice versa.

The work could have applications in anything from skyscraper windows that control the amount of light and heat entering and exiting a building, to camouflage and switchable visors for military applications, and even to cosmetics and to clothes that change color. It also helps to fill a knowledge gap in a key area of ​​materials science and chemistry.

Electrochromic materials change color upon application of low electrical potential or low voltage. For the past 20 years, John R. Reynolds, a joint professor in the School of Chemistry and Biochemistry and the School of Materials Science and Engineering at Georgia Tech, has studied and developed electrochromic materials that can go from a wide range of vibrant colors to erasing.

But these materials, called polymers with cathodic coloring, have a drawback. Their transmissive state, or clear, is not completely clear. On the contrary, in this state the material has a light blue tint.

“This is good for many applications, including mirrors that cut glare from oncoming cars by darkening, but not for all potential uses,” says Reynolds.

For example, the Air Force is working on visors for its pilots that would automatically switch from black to light when an aircraft flies from sunlight to clouds. “And when they say clear, they want it to be crystal clear, not a light blue,” Reynolds says. “We would like to get rid of this tint.”

ACE practices

There is another family of electrochromic materials which can change color when exposed to oxidative voltage. These materials, known as anodically colored electrochromes (ACE), are colorless materials that stain 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 mechanism of absorption of these cations and therefore could not regulate the colors in a controllable way.

This is where Dylan T. Christiansen, a graduate student of the Reynolds Group, came in. 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 dramatically alter the absorption of the colored or radical cationic state.

Photograph of solutions of the colorless neutral states and the brightly colored radical cationic states of the four ACE molecules. Two of these materials combine to create a clear to black switching electrochromic mixture. (Credit: Reynolds Group / Georgia Tech)

The results have been spectacular. “I expected the color differences between the four molecules, but I thought they would be very minor,” Christiansen says. Instead, upon the application of oxidative voltage, 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 a neutral state with no tint.

Finally, just like mixing inks, the researchers found that a mixture of molecules that turn green and red makes a mixture that is clear and turns opaque black. Suddenly those Air Force visors turning from crystal to black seemed more achievable.

Dylan Christiansen studies the electrochromic properties of new materials
Dylan Christiansen studies the electrochromic properties of new materials in a transparent electrochemical cell that examines the change in color upon application of an oxidizing voltage. (Credit: Allison Carter / Georgia Tech)

“The beauty of this is that it’s so simple. These minor chemical changes – literally the difference of a few atoms – have a huge impact on color, ”says co-author Aimée L. Tomlinson, professor in the Department of Chemistry and Biochemistry at the University of North Georgia.

‘Better than expected’

How can such small changes have such an effect?

For the past five years, Tomlinson, a computational chemist, has analyzed Reynolds’ electrochromic materials with computer models that provide insight into what’s going on 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 researchers made could drastically alter the electronic structure of the radical cationic states of molecules and ultimately control the color.

The work continues to generate information about 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 stronger.

Tomlinson notes that because the work also helps shed light on how radical cations work – researchers still don’t fully understand them – it could help others manipulate them for future use in areas beyond electrochromism.

“I think what makes science really interesting is that [sometimes] you see something you really weren’t expecting, you chase it and end up with something better than what you expected when you started out, ”says Reynolds.

The search appears in the Journal of the American Chemical Society.

The Air Force Office of Scientific Research funded the research. Tomlinson also acknowledges the support of his university, while Reynolds acknowledges support for his NXN Licensing electrochromic polymer research program. All opinions or conclusions are those of the authors and do not necessarily represent the views of the sponsoring organizations.

Source: Georgia Tech


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