Strange Crystals Melt and Change Color With Light

You may want to sit down for this one, because it's a real head-scratcher. A team of chemists from Osaka University in Japan have stumbled upon a crystal that can melt and change color when exposed to light! Here's what they found...

The solid organic material transforms into a liquid under certain conditions, and it undergoes an evolution in its luminescence as it melts, changing from green to yellow. "This is the first organic crystal we know of that exhibits a luminescent evolution during crystal melting, showing changes in intensity and color," says chemist Mao Komura. But what's happening at the molecular level to cause such a bizarre phenomenon? The crystal is a type of organic compound known as a heteroaromatic diketone, which the team has dubbed 'SO'. When first exposed to UV light, it emits a faint green light. But as the exposure continues, it melts and starts glowing yellow. The sharpness of the boundary between the states indicates that heating isn't responsible for the transition.

The team used theoretical calculations, X-ray analysis, and thermodynamic property analysis, along with data from previous research, to determine that diketone SO was actually switching from one molecular form (called "skew") to another ("planar"), which results in the change of color and state. "We found that the changes in luminescence arise from sequential processes of crystal loosening and conformational changes prior to melting," explains chemist Yosuke Tani. While this isn't the first substance to undergo photo-induced crystal-to-liquid transition (PCLT), it's the first time something like this has been seen in an organic crystal. And being able to study the process using light could lead to a range of potential uses in photonics, electronics, and drug delivery. "These visual indications of the steps of the PCLT process enabled us to advance the current understanding of crystal melting at the molecular level," says Tani.

The researchers, who published their study in Chemical Science, suggest that being able to control materials with light could have a variety of uses. This is especially because it's relatively affordable and simple to do, environmentally friendly, and non-invasive. One potential application is a reversible adhesive that can be modified through light exposure. The possibilities are endless. Who would have thought that shining a light on a crystal could make it melt and change color? Science truly never ceases to amaze us with its surprises.