Researchers reproduced Edvard Munch’s “The Scream” on a display screen made of liquid crystals (right) controlled by a chiral molecular switch they developed that reflects light in different colors. [Image: Courtesy of Ivan Aprahamian]
Structural colors—responsible for the vivid hues of chameleons, butterflies and other animals—originate from micro- or nanostructures that control the transmission and reflection of visible light. Compared with pigments or dyes, structural colors are often brighter and more resistant to fading over time.
Now, researchers in the United States have created a stable, energy-efficient structural-color display powered solely by ambient light (Nat. Chem., doi: 10.1038/s41557-024-01648-0). At the heart of the technology is a synthetic molecular switch that triggers shape changes in liquid crystals with pulses of blue light.
“The system can produce stable structural colors—more stable than any other photoswitchable system can handle—helpful in developing energy-efficient reflective displays that do not require power input to operate and display adaptive information,” said study author Indu Bala, a former postdoctoral researcher at Dartmouth College. “We imagine such technology can be used in displays that could be easily ‘printed’ and erased, and microscopic tags that could be added to bank notes to deter counterfeiters.”
Light-activated molecular switch
Bala worked on the project while in the lab of Ivan Aprahamian at Dartmouth, which focuses on modular and tunable hydrazone-based building blocks for molecular switches, fluorophores, sensors and adaptive materials. Photochromic hydrazones are a class of organic compounds that rearrange their atoms and change shape in response to light. In the current study, Bala and her colleagues developed a synthetic molecular switch made up of photochromic hydrazones and the organic molecule triptycene.
These helices are capable of producing structural colors if the pitch length is on the order of the wavelength of visible light.
“This molecular switch can undergo a structural change, a 180-degree flip, when irradiated with suitable light,” said Bala. “By integrating the triptycene with bistable hydrazone photoswitches—a tool created in the Aprahamian lab—we aimed to lock the reflection properties of chiral liquid crystals for extended periods of time.”
The molecular switch acts as a photoswitchable chiral dopant for liquid crystals, which rearrange themselves into long helices when the switch flips. These helices are capable of producing structural colors if the pitch length is on the order of the wavelength of visible light. In other words, activation with UV or visible light causes a structural change in the dopant, which alters the pitch length and thus modifies the liquid crystal's optical properties, such as the reflection of specific wavelengths.
”Painting” with light
The researchers then teamed up with the lab of Alexander Lippert at Southern Methodist University to create dynamic and colored surfaces that change in response to light exposure with the help of digital light processing micropatterning techniques. By controlling the illumination times of patterned blue LED light (442 nm), they were able to “paint” complex patterns with a resolution of 76 μm—for example, reproductions of famous paintings such as Edvard Munch's “The Scream” and Vincent Van Gogh's “The Starry Night.”
“This breakthrough offers a high level of precision and flexibility, enabling the creation of detailed, stable and multicolored images, highlighting the system’s vast potential in the future of advanced display technologies,” said Bala. “The next step is to optimize the performance of the switch, for example by red shifting the switching wavelength to the red region, so we can use sunlight with filters to write the information.”