A serendipitous discovery by a grad student at the Georgia Institute of Technology has led to materials that quickly change color from completely clear to a range of vibrant hues and then 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 the military, and even color-changing cosmetics and clothing. It also fills a gap in our knowledge of materials science and chemistry.
Electrochromic materials change color upon application of small voltages. 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. For example, the Air Force is working toward visors for pilots that 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.
There is another family of electrochromic materials that changes color when exposed to oxidizing voltages. These materials, known as anodically coloring electrochromes (ACEs), are colorless materials that turn color 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.
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. (Courtesy: Image: Reynolds Group, Georgia Tech)
While tinkering with some ACE molecules, Georgia Tech grad student Dylan T. Christiansen tried a new approach to controlling color in radical cations. Specifically, he created four different ACE molecules by making tiny changes to their 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 vastly 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, 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. Air Force visors that switch from crystal clear to black suddenly looked more attainable.
But how could such tiny changes in a few atoms have such an effect? That’s where computational chemistry comes in.
Using models of ACEs at the submolecular level together with Christiansen’s data for the new ACE molecules, researchers could show that the small chemical changes they made drastically altered 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. The work is also helping to illuminate how radical cations work and could help others manipulate them for future use in fields beyond electrochromism.
