What the research is about
Have you ever used a chemical glow stick at a concert? When you bend it, it suddenly begins to glow in the dark. The light appears because bending the stick allows solutions inside to mix, triggering a chemical reaction.
If we focus on the idea that force can trigger a chemical reaction, it becomes easier to understand the concept of a "mechanophore." A mechanophore is a molecule that responds to mechanical force. When force is directly applied to the molecule, specific chemical bonds break or transform, producing visible signals such as color or light. In other words, mechanophores can visualize invisible forces.
Over the past 20 years, this field has advanced significantly. Traditionally, researchers designed mechanophores by incorporating relatively weak bonds into molecules so that they would react easily when force was applied. However, this approach had a drawback: those weak bonds could also react to heat or ultraviolet (UV) light.
What scientists have long sought is a molecule that remains stable under heat and light, yet react selectively only when strong mechanical force is applied.
To address this challenge, a research team led by Professor Hideyuki Otsuka at Institute of Science Tokyo (Science Tokyo) focused on a molecule called DAANAC. DAANAC is a highly stable organic molecule built on robust carbon-carbon bonding.
Why this matters
Experiments by the research team showed that DAANAC hardly decomposes even when exposed to temperatures above 200°C or strong UV light. Yet when strong mechanical force is applied, its chemical bonds break and the molecule emits fluorescence.
Theoretical calculations revealed that DAANAC is among the most mechanically resilient mechanophores reported to date.
This addresses a long-standing trade-off. Many previously reported mechanophores were deliberately designed to be fragile so they could react easily. As a result, incorporating them into materials sometimes weakened the materials themselves.
Professor Otsuka's team prioritized maintaining the strength of the host material while still enabling a force-selective response. They incorporated DAANAC into a material and conducted stretching experiments. They found that the material retained its strength, while only the areas where stress was concentrated emitted light.
This makes it possible to detect invisible stress concentrations inside a material as visible light-without weakening the material itself.
What's next
This achievement opens the door to creating new materials that combine strength with built-in sensing capability. For example, materials used in bridges, airplanes, and automobiles could one day signal-through light-where mechanical stress is being applied.
Because DAANAC is resistant to heat and strong light, such systems could function reliably even in harsh environments. This could lead to safer, longer-lasting infrastructure and vehicles.
Moreover, this technology allows scientists to directly observe how molecular connections change inside materials. Detecting these invisible, microscopic changes is essential for developing safer and more durable materials in the future.
Comment from the researcher
I have always wanted to observe how materials change in the most natural way possible. To do that, it was essential not to compromise the material's strength. Designing a molecule that responds only to mechanical force was not easy, but we can now capture invisible microscopic changes as light. I hope this research will contribute to the development of materials that people can use with greater confidence.
(Hideyuki Otsuka, Professor, Department of Materials Science and Engineering, School of Materials and Chemical Technology, Institute of Science Tokyo)
