New crystalline compound melts under ultraviolet light (with video)

May 16, 2023

(Nanowerk News) While many materials melt when heated, researchers from Japan recently discovered a new material in which melting can be induced by ultraviolet light, rather than just induced by heat. Even more interesting, this material exhibits a change in luminous properties upon melting. This material is the first organic crystalline material found to exhibit a change in color and luminescent intensity after melting caused by ultraviolet light. Figure 1: Photo-induced crystal melting with luminescence evolution. (Image: Yosuke Tani)

In the Chemistry (“Photo-Induced Crystal Melting by Luminescence Evolution Based on Conformational Isomerization”), researchers from Osaka University reported the discovery of a new class of photo-responsive crystalline compounds, ‘heteroaromatic 1,2-diketones’. Light irradiation causes the crystals in these materials to melt, a phenomenon called photo-induced crystal-to-liquid transition (PCLT).

This phenomenon can dramatically change material properties and allow for a wide range of applications, for example light-controlled reversible, light-responsive adhesives. Several materials have been shown to have this crystal-melting property; hence, the discovery of a new class of PCLT materials is a major step forward in this field. Autocatalytic conformational change Figure 2: Autocatalytic conformational changes. (Image: Yosuke Tani)

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

These changes in luminosity, that is, changes in the way the material absorbs and emits light, indicate that SO is undergoing a change of shape at the molecular level during the PCLT process. Building on previous research on luminescent molecules, the research team realized they could further investigate the changes at the molecular level underlying PCLT to better understand crystal melting phenomena.

“We found that the luminescence changes arise from a sequential process of crystal loosening and conformational changes prior to melting,” explained senior author Yosuke Tani. “This visual indication of the steps of the PCLT process allows us to advance the current understanding of crystal melting at the molecular level.”


All photo-induced crystal-to-liquid transition events under a fluorescence microscope, 10x speed.

By applying single crystal X-ray analysis, thermodynamic property analysis, and theoretical calculations to investigate the mechanisms governing the behavior of these new PCLT materials, the researchers demonstrated that disordered layers on crystals are a key factor for PCLT in this class. material.

The discovery of this new PCLT material, together with its characterization, provides fundamental insights into the mechanism of crystal melting and will enable greater opportunities to design PCLT materials with a wide range of applications, including photolithography, heat energy storage, and light-induced adhesion.

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