A synthetic molecular switch allows “painting” with natural light

Liquid crystals exist in a phase of their own. They can flow like liquids, but because their molecules are arranged in a somewhat ordered manner, they can be easily manipulated to reflect light. This flexibility has made liquid crystals the material of choice for energy-efficient phone, television, and computer displays.

In a new study in Natural chemistryresearchers from Dartmouth and Southern Methodist University hint at other applications of liquid crystals that might one day be possible, all powered by natural light. They include liquid crystal lasers, display screens that could be easily printed and erased, and microscopic labels that could be added to banknotes to deter counterfeiters.

At the heart of these fantastic gadgets is a synthetic molecular switch capable of triggering shape changes in liquid crystals, allowing them to reflect different colors. Designed in the lab of Dartmouth chemistry professor Ivan Aprahamian, the switch is composed of the organic molecule triptycene and a class of compounds called hydrazones that can turn on and off with a pulse of light.

In the study, Aprahamian and colleagues show that hydrazones can be attached to triptycene in such a way that the molecule’s symmetry breaks, making it chiral. Chiral molecules come in two mirror image forms that, like our hands, cannot be completely superimposed.

When the chiral triptycene interacts with a liquid crystal molecule, it sets off a chain of events that causes other liquid crystal molecules to align, rearranging themselves into twisted DNA-like helices.

In helical form, liquid crystals reflect ambient light at different wavelengths depending on their pitch or distance between coils in their helical structure; stretching and compressing the helix triggers color changes. In nature, chameleons and cephalopods also take advantage of structural features to instantly blend into their surroundings without any pigmentary changes to their skin.

“By increasing or decreasing the pitch of the helical structure, we can control the color it reflects,” explains Aprahamian. “Think of it like playing an accordion. Instead of compressing and expanding the instrument to control the sound you hear, we use light to control the pitch and color you see.”

The study provides as evidence vivid reproductions of Edvard Munch’s The Scream and Van Gogh’s Starry Night. The images were produced in Alexander Lippert’s lab at SMU with a microscope converted into a mini slide projector.

In a process reminiscent of multicolor screen printing, the researchers used the tiny projector to shine light through a series of stencils onto a makeshift screen made of liquid crystals doped with chiral triptycene. New colors were added, one by one, by shining light for varying lengths of time onto the portion of the screen left exposed by the stencil.

“Once the pattern is painted, it can stay there for days,” says Lippert, study co-author and associate professor at SMU. “You can also erase it and go back to a blank canvas.”

Aprahamian’s lab has designed hydrazone switches before, but this version is the first to be shown to reflect the visible color of a liquid crystal. It is also the first time that a stable, long-lasting multicolor image has been projected onto a liquid crystal display using a dopant that can turn on and off.

In previous experiments, Aprahamian had tried to make the switchable dopant with a chiral molecule called isosorbide. Although liquid crystals interact with isosorbide and form a helical structure, they do not reflect visible light. At a conference in Telluride in 2016, an MIT chemistry professor suggested Aprahamian try triptycene.

Chiral triptycene proved to be a major breakthrough because of its effectiveness in transferring chiral information to liquid crystals, Aprahamian says. Relatively few molecules are needed to bring together a large number of liquid crystals into a new configuration to change their properties.

“It’s called the sergeant and soldier effect,” says Aprahamian. “A few chiral ‘sergeant’ molecules control the properties of a large number of achiral ‘soldier’ ​​molecules.”

The study describes in detail what happens at the molecular level, which can help researchers further investigate liquid crystals for new applications.

“We can now build on this knowledge to create better liquid crystal dopants,” says the study’s first author, Indu Bala, an assistant professor at the Indian Institute of Technology Mandi, who worked on the project in as a postdoctoral researcher at Dartmouth.