Color-changing materials go from solid to see-through

Researchers have created 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 coming in and out of a building, to switchable camouflage and visors for military applications, and even color-changing cosmetics and clothing. It also helps fill a knowledge gap in a key area of materials science and chemistry.

Electrochromic materials change color upon the application of a small electrical potential or voltage. For the last 20 years John R. Reynolds, a professor with joint appointments in the School of Chemistry and Biochemistry and the School of Materials Science and Engineering at Georgia Tech, has been studying and developing electrochromic materials that can switch from a wide range of vibrant colors to clear.

But these materials, known as cathodically coloring polymers, have a drawback. Their transmissive, or clear, state is not completely clear. Rather, in this state the material has a light blue tint.

“That’s fine for many applications—including rear-view mirrors that cut the glare from oncoming cars by turning dark—but not for all potential uses,” says Reynolds.

For example, the Air Force is working toward visors for its pilots that would automatically switch from dark to clear when a plane flies from bright sunlight into clouds. “And when they say clear, they want it crystal clear, not a light blue,” Reynolds says. “We’d like to get rid of that tint.”

Handy ACEs

There is another family of electrochromic materials that can change color when exposed to an oxidizing voltage. These materials, known as anodically coloring electrochromes (ACEs), are colorless materials that turn colored upon oxidation. But there has been a knowledge gap in the science behind the colored oxidized states, known as radical cations. Researchers have not understood the absorption mechanism of these cations, and so they could not controllably tune the colors.

That’s where Dylan T. Christiansen, a graduate student in the Reynolds group, came in. While tinkering with some ACE molecules, he experimented with a new approach to controlling color in radical cations. Specifically, he created four different ACE molecules by making tiny changes to the ACEs’ molecular structures that have little effect on the neutral, clear state, but significantly change the absorption of the colored or radical cation state.

Photography of solutions of the colorless neutral states and vibrantly colored radical cation states of the four ACE molecules. Two of these materials combine to create a clear-to-black switching electrochromic blend. (Credit: Reynolds Group/Georgia Tech)

The results were spectacular. “I expected some color differences between the four molecules, but thought they’d be very minor,” Christiansen says. Instead, upon the application of an oxidizing voltage, the four molecules produced four very different colors: two vibrant greens, a yellow, and a red. And unlike their cathodic counterparts, they are crystal clear in the neutral state, with no tint.

Finally, just like mixing inks, the researchers found that a blend of the molecules that switch to green and red made a mixture that is clear and switches to an opaque black. Suddenly those Air Force visors that switch from crystal clear to black looked more attainable.

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

“The beauty of this is it’s so simple. These minor chemical changes—literally the difference of a few atoms—have such a huge impact on color,” says coauthor Aimée L. Tomlinson, a professor in the chemistry and biochemistry department at the University of North Georgia.

‘Better than expected’

How could such tiny changes have such an effect?

For the last five years Tomlinson, a computational chemist, has been analyzing Reynolds’ electrochromic materials with computational models that provide insights into what’s happening at the sub-molecular level. Using those models, coupled with Christiansen’s data for the new ACE molecules, she was able to show how the small chemical changes that researchers made could drastically alter the electronic structure of the molecules’ radical cation states, and ultimately control the color.

The work continues to generate insights into new ACE molecules thanks to continuous feedback between Tomlinson’s models and the experimental data. The models help guide efforts in the lab to create new ACE molecules, while the experimental data from those molecules makes the models ever stronger.

Tomlinson notes that because the work is also helping to illuminate how radical cations work—researchers still don’t understand them well—it could help others manipulate them for future use in fields beyond electrochromism.

“I think what makes science really interesting is that [sometimes] you see something you really did not expect, you pursue it, and you end up with something that is better than you expected when you started,” Reynolds says.

The research 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 her university, while Reynolds acknowledges support for his electrochromic polymer research program from NXN Licensing. Any opinions or conclusions are those of the authors and do not necessarily represent the views of the sponsoring organizations.

Source: Georgia Tech