Novel crystal compound melts under UV light

While many materials melt when heated, researchers from Osaka University have discovered a novel material in which melting can be induced by ultraviolet light instead of being induced only by heat. Even more intriguing, this material exhibits changes in its luminescent properties while it melts. It is the first known organic crystalline material found to show changes in luminescent colour and intensity upon ultraviolet light-induced melting.

The researchers reported their discovery of a new class of photo-responsive crystal compounds, ‘heteroaromatic 1,2-diketones’, in the journal Chemical Science. Light irradiation causes the crystals in these materials to melt, a phenomenon termed photo-induced crystal-to-liquid transition (PCLT). This phenomenon can dramatically change a material’s properties and makes possible a broad range of applications — for example, photo-responsive, reversible adhesives that could be controlled by light — but few materials have been shown to have this crystal-melting property.

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

These changes in luminescence, ie, changes in the way the material absorbs and emits light, indicated that SO was undergoing molecular-level changes in shape during the PCLT process. Building upon past research on luminescent molecules, the research team realised they could further investigate these molecular-level changes underlying PCLT to better understand the crystal-melting phenomenon.

“We found that the changes in luminescence arise from sequential processes of crystal loosening and conformational changes prior to melting,” said 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.”

Video: The whole event of the photo-induced crystal-to-liquid transition under fluorescence microscope, 10x speed. Credit: 2023, Yosuke Tani, ‘Photoinduced Crystal Melting with Luminescence Evolution Based on Conformational Isomerisation’, Chemical Science.

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

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

Image courtesy of the researchers (modified from the original) and shared under CC BY 3.0