By Josh Perry, Editor
[email protected]

Researchers at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. developed specially-designed photoswitching molecules that absorbs heat as it melts and releases thermal energy as it solidifies but only when triggered by light.

Graduate student Ce?dric Viry, Professor Jeffrey Grossman, and postdoc Grace Han, along with their collaborators, are using specially designed “photoswitching” molecules to control the release of heat from materials used to store thermal energy in devices.
(Stuart Darsch/MIT)

According to a report from the school, MIT scientists have been studying phase-change materials (PCM) as a good source for storing thermal energy but had struggled to find a means for triggering the transition to release that heat. They had also been studying photoswitching molecules as a potential solution, but it was the combination of the two that led to this latest breakthrough.

“Shine a certain wavelength of light on a photoswitch, and its shape will change,” the article explained. “The same atoms are present, but their orientation relative to one another shifts. Moreover, they’ll stay in that shifted configuration until they’re exposed to another wavelength of light. Then they’ll snap back to their original shape, releasing thermal energy in the process.”

The scientists tuned the photoswitching material so that it matches the PCM in one form but not in another. When mixed with the PCM in mismatched form, the photswitch prevents the PCM from solidifying until a wavelength of light is applied. Then, it solidifies and releases the stored heat.

“To explore the viability of that approach,” the article continued, “the researchers used a conventional PCM called tridecanoic acid and prepared a special variation of the photoswitch molecule azobenzene, which consists of two linked rings of atoms that can be in different positions with respect to one another.”

In one form, the photoswitch material’s rings are flat, but in another the rings are tilted at 56 degrees. Shining ultraviolet (UV) light at the flat version and the rings twist, while shining visible light at the twisted version caused it to flatten out.

“When the PCM-azobenzene mixture, or composite, is solid with the azobenzene in its trans form, the two constituents pack together tightly,” the article said. “When heated, the composite absorbs thermal energy, and the PCM melts. Zapping it with UV light changes the azobenzene dopant from trans to cis. When that mixture cools, the cis azobenzene prevents solidification of the PCM, so the latent heat remains stored. Illumination with visible light switches the azobenzene back to its trans form. The mixture can now solidify, releasing its stored latent heat in the process.”

The concentration of photoswitch in the PCM and the exact combination of materials turned out to be critical for the process to work. While this is only at a proof of concept stage, researchers are excited about the potential applications for this development.

The research was recently published in Chemical Communications.