Melting of a Novel Crystal Compound Triggered by Ultraviolet Light

While many materials melt when heated, researchers from Japan recently discovered a new material in which melting can occur by ultraviolet light rather than heat. What is most interesting is that this material exhibits changes in luminescent properties as it melts. This material is the first organic crystalline material found to show changes in luminescent color and intensity upon melting induced by ultraviolet light.

Investigators from Osaka University report their discovery of this new class of photo-responsive crystalline compounds, “heterodimers 1,2 diktone” in Chemical sciences On the 20th of April. Photoirradiation causes the crystals in these materials to melt, a phenomenon called crystal-to-liquid-induced phototransmission (PCLT).

This phenomenon can dramatically change material properties and enables a wide range of applications, for example, photo-responsive, reversible and light-controlled adhesives. Few substances have been shown to have the property of crystal melting; Hence, the discovery of a new class of PCLT materials is a major step forward in this field.

In characterizing a newly discovered class of PCLT materials, the researchers found that one member of this class, diketone SO, exhibits luminescence changes during the irradiation-induced melting process. “This is the first organic crystal that we know of to show a luminous evolution during crystal melting, showing changes in intensity and color, from green to yellow,” says lead author Mao Komura.

Real-time observation of crystal melting with luminescence evolution. Credit: Osaka University

These changes in luminescence, that is, changes in the way the material absorbs and emits light, indicates that SO was undergoing molecular-level changes in shape during the PCLT process. Building on previous research on luminescent molecules, the research team realized that they could further investigate these changes at the molecular level underlying PCLT to better understand the phenomenon of crystal melting.

“We found that changes in luminescence arise from sequential processes of crystal deconvolution and conformational changes before melting,” explains senior author Yosuke Tani. “These visual indications of the steps of the PCLT process enabled us to advance the current understanding of crystal melting at the molecular level.”

By applying single-crystal X-ray analysis, thermodynamic property analysis, and theoretical calculations to explore the mechanisms governing the behavior of this new PCLT material, the researchers show that disordered layering in a crystal is a key factor for PCLT in this class of materials.

This discovery of a new PCLT material, together with its properties, provides fundamental insights into the mechanism of crystal melting and will enable greater opportunities for designing PCLT materials with a variety of applications, including photolithography, thermal energy storage, and light-induced adhesion.