Piezofluorochromism, the phenomenon of materials reversibly changing their fluorescent color when pressure is applied, is used to create the pressure sensors used in automotive and medical industries. By monitoring color changes, researchers can visually recognize phenomena, such as chemical changes, that actually take place. However, as devices get increasingly complicated, there is an increasing demand for ways to produce more sensitive sensors.
A research group led by Project Assistant Professor Takuya Ogaki, Associate Professor Yasunori Matsui, and Professor Hiroshi Ikeda at the Graduate School of Engineering, Osaka Metropolitan University, has identified a new way to produce fluorescence, by finding that an initially stacked benzene layer (cyclophane moiety) increased its fluorescent color change drastically when exposed to pressure.
Professor Ogaki explained the background to the research. "It is difficult to rationally design organic crystals that exhibit the desired color change," he said. "Even a slight change of the structure of organic molecules yields a completely different crystal structure."
The researchers focused on two closely related crystalline organoboron compounds containing a special structural unit called [2.2]paracyclophane (pCP). When exposed to very high pressure, such materials show a shift in fluorescence toward longer wavelengths, resulting in them glowing red. Using X-ray crystallography, they found that the reason for this color change differed between the two crystals.
In one crystal, called pCP-H, the electron clouds naturally form pairs in stacked layers known as π-stacked dimer layers. Pressure pushes these pairs even closer together, strengthening the weak electron forces between neighboring molecules and causing a pronounced change in the fluorescent color.
In the other crystal, pCP-iPr, molecules do not form these stacked layers, so the color change mainly comes from subtle changes within the individual molecules making up the crystal, resulting in a much smaller shift and a less intense color.
"Under ultra-high-pressure conditions, we discovered that cyclophanes, such as [2.2]paracyclophane, act like springs, expanding and contracting to alter the luminescent color through changes in molecular interactions," Professor Matsui explained.
Together, these results reveal that molecular pairing and the internal molecular structure affect how materials respond to pressure, providing valuable guidance for designing future pressure-sensitive materials.
Professor Ikeda concluded: "As materials function not only in molecular assemblies, like crystalline states, but also in monolayers, understanding both these processes is expected to become a new molecular design strategy."
The findings were published in Journal of Materials Chemistry C.
