A team of chemists from Osaka University in Japan have 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.
Additionally, 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 to undergo what is called the photo-induced liquid-crystal transition (PCLT). But being able to study the transition using light could help scientists better understand it, potentially opening up a whole range of potential uses in photonics, electronics and drug delivery.
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“This is the first organic crystal we know of that exhibits luminescent evolution upon crystal fusion, showing intensity and color changes from green to yellow,” said chemist Mao Komura.
The material is a type of organic compound known as a heteroaromatic diketone, one the team dubbed “SO” after the sulfur and oxygen in its two rings.
When first exposed to UV light, the SO crystal compound glows with a faint green light. As exposure continues, however, it turns yellow and slowly melts. Based on careful observations of the sharpness of the boundary between 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 analysis of thermodynamic properties), as well as data from Previous searchthe team determined that the diketone SO actually transitions from one molecular (“skew”) to another (“planar”) form.
Other information was obtained from other similar crystalline compounds, which did not melt or melt but did not change color. This tells the researchers something about the molecular changes that occur when these crystals change from solids to liquids.
“We found that luminescence changes result from sequential processes of crystal relaxation and conformational changes prior to melting,” said chemist Yosuke Tani, from Osaka University.
Working backwards, it shows that there is something special about the molecular arrangement of these materials, meaning they melt and switch between phases when exposed to certain types of light.
And being able to control materials with light could be very useful: it’s relatively affordable and simple to do, environmentally friendly and non-invasive. An example application suggested by the researchers is a reversible adhesive that can be changed by exposure to light.
Key to the progress described in this study is the way the diketone SO changed color, giving the researchers essential insight into what was happening at the smallest scales inside the crystalline compound.
“These visual indications of the steps in the PCLT process have allowed us to advance the current understanding of crystal melting at the molecular level,” said Tani.
The research has been published in chemical sciences.