A team of chemists from Osaka University in Japan has identified a rare property in a crystal. When exposed to the cool glow of ultraviolet (UV) light, the solid organic material turns into a liquid.
In addition, this crystal undergoes an interesting series of changes in its luminescence as it melts, indicating changes in the structure of the crystal at the molecular level.
Although unusual, it is not the first substance found to undergo the so-called photoinduced crystal-to-liquid transition (PCLT). But if scientists can study the transition using light, they can better understand it, potentially opening up a whole host of potential applications in photonics, electronics and drug delivery.
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“This is the first organic crystal we know of that exhibits a luminescent evolution during crystal melting, with changes in intensity and color, from green to yellow,” say chemist Mao Komura.
The material is a type of organic compound known as a heteroaromatic diketone, one the team named “SO” after the sulfur and oxygen in its two rings.
Upon first exposure to UV light, the SO crystal compound glows in a faint green light. However, as exposure continues, it glows yellow and slowly melts. Based on close observations of the sharpness of the border between the states, it is clear that heating is not responsible for the transition.
Using theoretical calculations and a variety of study techniques (including X-ray analysis and thermodynamic property analysis), plus data from previous researchthe team determined that diketone SO actually switched from one molecular shape (“skew”) to another (“planar”).
Further insights were gained from other similar crystal compounds, which either did not melt or melted but did not change color. This tells the researchers something about the molecular changes that occur when these crystals shift from solids to liquids.
“We found that the changes in luminescence arise from sequential processes of crystal detachment and conformational changes prior to melting,” say chemist Yosuke Tani, of Osaka University.
Looking back, it shows that there is something special about the molecular arrangement of these materials, meaning that they melt and switch phases when exposed to certain types of light.
And it could be very useful to control materials with light: it’s relatively affordable and easy to do, environmentally friendly and non-invasive. An example application suggested by the researchers is a reversible adhesive that can be altered by exposure to light.
Key to the progress outlined in this study is the way diketone SO changed color, giving the researchers a vital insight into what was going on at the tiniest scales in the crystal compound.
“These visual indications of the steps of the PCLT process enabled us to advance the current understanding of crystal melting at the molecular level,” say Tani.
The research has been published in Chemical Science.