- Minor chemical changes have a huge impact on color of electrochromic materials
- How these changes occur is now better understood due to supercomputer modeling
- Applications range from skyscraper windows to automatically adjusting visors for pilots
A serendipitous discovery by Dylan T. Christiansen of Georgia Tech has led to materials that quickly change color from completely clear to a range of vibrant hues—and back again.
This 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.
The study also helps fill a knowledge gap in a key area of materials science and chemistry, according to the researchers. A recent Journal of the American Chemical Society article explained the research in detail, including the explanatory computational models that relied upon the Comet supercomputer at the San Diego Supercomputer Center (SDSC).
For twenty years, chemist John R. Reynolds of Georgia Tech has been studying and developing electrochromic materials that can change colors. Much of Reynolds’ work has focused on how a small electrical voltage changes electrochromic materials, called cathodically coloring polymers, from a wide range of vibrant colors to opaque but with a slight 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,” said Reynolds.
For example, the US 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 said. “We’d like to get rid of that tint.”
Tiny changes, huge impact
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 the colors could not be controllably tuned.
Enter Dylan T. Christiansen, a graduate student in the Reynolds group. 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. The results were spectacular.
“I expected some color differences between the four molecules, but thought they’d be very minor,” Christiansen said. 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.
“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,” said co-author Aimée L. Tomlinson of the University of North Georgia.
How could tiny changes have such an effect? That’s where Tomlinson, a computational chemist, and SDSC’s Comet supercomputer comes into play. For the last five years, Tomlinson has used Comet to analyze Reynolds’ electrochromic materials with computational models that provide insights into what’s happening at the sub-molecular level.
The Comet-generated models coupled with Christiansen’s data for the new ACE molecules showed how the small chemical changes can drastically alter the electronic structure of the molecules’ radical cation states, and ultimately control the color.
While the findings already provide significant insight into how molecule alterations change colors, the work continues to generate insights into new ACE molecules, thanks to continuous feedback between Tomlinson’s models and the experimental data. The models generated by Comet help guide efforts in the lab to create new ACE molecules, while the experimental data from those molecules makes the Comet models even stronger.
Tomlinson noted that the visualizations helped to illuminate how radical cations work. However, they are still not well understood. She said that this study could now 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,” said Reynolds in commenting on the serendipitous nature of the initial discovery.