UV light patterns thermochromic crystals without damage, unlocking color-changing designs

Color-changing mood rings, forehead fever strips and car-shade indicators all change hues as they warm and cool, thanks to a phenomenon called thermochromism. On a smaller scale, thermochromism is used in nanotechnologies like sensors, electronics and computing. These applications require smart materials that can be patterned into designs without losing structural integrity, which can be difficult.

In a recent study, Yale researchers demonstrated that light-based patterning of materials can be achieved with a “connector” molecule called an anthracene heterodimer. “We made a new form of a ligand with the goal to make materials that you could transform without degrading them physically,” said Amymarie Bartholomew, corresponding author and assistant professor of chemistry.

Their approach could be used to pattern the magnetism or electrical conductivity of a material, enabling more advanced nanotechnologies, such as those found in energy storage and release, spintronics and quantum information science.

The study is published in the Journal of the American Chemical Society and was carried out in Bartholomew’s lab by Yale postdoctoral researcher Eric Schreiber.

The trouble with thermochromic materials

Imagine a “smart” Yale sweatshirt, where everything but the “Y” is a heat-activated technology that changes color with temperature changes. Sweating while walking up Science Hill might change the fabric from blue to green. In addition, your body heat activates cooling wristbands and health data tracking.

Instead of the sweatshirt being dyed with color, it’s “written” with light. As the ultraviolet (UV) light targets the material surrounding the Y logo, it essentially rewrites the chemical structure of that specific region of the material so it behaves differently with changing temperature.

That, in a nutshell, is what takes place with patternable thermochromism.

But the challenge in making these smart materials is that current photopatterning methods tend to damage the material.

The heterodimer and the lattice

Schreiber and Bartholomew set about creating a photopatternable material using a crystalline “scaffold”—or a metal-organic framework (MOF). It is an excellent building block because of its structural tunability, porosity and chemical responsiveness.

First, they needed to find a photoresponsive molecule that would change upon irradiation without degrading the material. Anthracene dimers met the criteria. The carbon-rich molecule, whose structure is like a stack of rings, can click together with another anthracene under light and later separate again.

However, in previous studies, anthracene-dimer MOFs often lost framework dimensionality upon light-triggered cleavage. The result was damaged crystals, reduced stability and general unreliability.

To overcome this, the researchers synthesized an anthracene heterodimer made of a combination of two different anthracenes bonded together. They used it as a linker to build a copper-connected lattice.

With this starting material, they set about their experiment.

As is customary in patterning studies, the researchers used their university letter—the Yale “Y”—as a photomask to demonstrate their science.

“We like to say in our group that we do a lot of really cool science as well as a healthy amount of arts and crafts,” Schreiber said. “This particular project involved cutting out tiny Ys and taping them over a surface.

“I took a razor and cut a piece of paper into the shape of a Y, laid this photomask over the top of the material, and shone a shortwave UV light on it,” he explained. “That kept everything underneath the Y in its pristine state, and everything around it converted to thermochromic material.”

The researchers found that when they used the dimeric form of the linker in the lattice, they obtained a pale blue solid that showed no temperature change. But if the dimer was split, the material became thermochromic, appearing brown at room temperature and pale green in the cold.

With that information, the group was able to spatially control the transformation so that some regions patterned and changed color with temperature, and others did not.

“It maintained the dimensionality and physical interconnectedness of the material while changing properties in a patterned way,” said Bartholomew.

Unlike typical photopatterning, the material didn’t need photoresist interventions that could cause damage. Now, manufacturers can “turn on” thermochromic behavior only where they want it, using UV exposure through a mask.

“More broadly, our findings represent a new strategy for achieving photopatternable properties in solids using a novel anthracene heterodimer ligand,” said Bartholomew.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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