Discovery advances the field of color-changing materials

Research allows switch from crystal clear to opaque.

A new paradigm is established for the design of conjugated anodically coloring electrochromic molecules. It is shown that through crossconjugation the electronic energy levels of the radical cation state may be controllably tuned independent of the neutral state. It is shown how cross-conjugation can be used to tune the radical cation state independent of the neutral state. Manipulating the oscillator strengths of radical cation transitions allows for tuning of the color by shifting the λmax of the low-energy absorption by over 400 nm. The neutral states of these molecules are UV absorbing, providing solutions that are colorless with L*a*b* values of 100, 0, 0. They are oxidized to vibrantly colored radical cations with absorptions that span the visible spectrum, creating green, yellow, and red chromophores. These molecules are then mixed to create transmissive, colorless blends that switch to opaque black solutions.
A new paradigm is established for the design of conjugated anodically coloring electrochromic molecules. It is shown that through crossconjugation the electronic energy levels of the radical cation state may be controllably tuned independent of the neutral state. It is shown how cross-conjugation can be used to tune the radical cation state independent of the neutral state. Manipulating the oscillator strengths of radical cation transitions allows for tuning of the color by shifting the λmax of the low-energy absorption by over 400 nm. The neutral states of these molecules are UV absorbing, providing solutions that are colorless with L*a*b* values of 100, 0, 0. They are oxidized to vibrantly colored radical cations with absorptions that span the visible spectrum, creating green, yellow, and red chromophores. These molecules are then mixed to create transmissive, colorless blends that switch to opaque black solutions.

A graduate student at the Georgia Tech has made an accidental discover that allows materials to rapidly change color from completely clear to a range of vibrant hues — and back again.

This discovery could have an important role in applications like skyscraper windows that control the amount of light and heat coming in and out of a building, switchable camouflage and visors for military applications and color-changing cosmetics and clothing.

John R. Reynolds, a professor at the Georgia Institute of Technology said, “Electrochromic materials change color upon the application of a small electrical potential or voltage. Since the last 20 years, I have 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. They’re transmissive, or clear, the 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 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. We’d like to get rid of that tint.”

Anodically coloring electrochromes (ACEs) are from another family of electrochromic materials that can rapidly change color after the disclosure of oxidized voltage. They are colorless that only get colored upon oxidation.

But, due to the knowledge gap n the science behind the colored oxidized states, known as radical cations, scientists have not properly understood the absorption mechanism of these cations, and so the colors could not be controllably tuned.

Now, a graduate student named Dylan T. Christiansen has 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.

He said, “The results were spectacular. “I expected some color differences between the four molecules but thought they’d be very minor. 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.”

Aimée L. Tomlinson, a professor in the Department of Chemistry and Biochemistry at the University of North Georgia said, “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.”

Then using computational model, Tomlinson was able to show how the small chemical changes that were made can 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 — 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 did not expect, you pursue it, and you end up with something that is better than you expected when you started.”

A paper on the research was published in a recent issue of the Journal of the American Chemical Society (JACS).