Scientists develop new method for reversible control of material properties in fluids

Researchers have developed a reversible, vapor-controlled system capable of toggling the physical and optical traits of advanced fluids on demand. Led by Nagoya University and Kyoto University in Japan, the study demonstrates a method to control the optical and physical properties of materials from the molecular level to the macroscopic scale. Illustration of vapor-controlled reversible host–guest chemistry inducing optical and phase switching. Credit: Yoko Sasayama Host–guest liquid system At the core of the development is host–guest chemistry. It is a process where two distinct molecules lock together purely through physical forces, completely avoiding permanent chemical bonds. The researchers combined two key ingredients: a tiny, cyclic, tube-shaped host molecule and a guest material known as a Functional Molecular Liquid (FML), which features long, thread-like carbon chains. Compared to ordinary liquids, these non-evaporating fluids pack advanced optical, electronic, and catalytic capabilities. When mixed, the FML’s carbon chains spontaneously slide inside the host’s central cavity like a thread through a needle. The resulting dumbbell-shaped complex instantly changes color from bright yellow to deep red while hardening from a fluid into a rigid solid. Simultaneously, its room-temperature phosphorescence — its ability to emit a soft, continuous glow in the dark — is completely shut off. The transformation is fully reversible. When the red solid is exposed to hexane vapor, the six-carbon gas molecules act as competitors. The molecules crowd into the host cavities and kick the FML out. Freed from its cage, the FML instantly liquefies, turns back to yellow, and resumes glowing in the dark. “We had some surprising discoveries. At first, the immediate color change to red when we mixed the FML with the host molecule was concerning. We didn’t expect such a distinct color change let alone the phosphorescence being quenched so I thought it decomposed. Conversely and fortunately, the hexane vapors released the FML as expected,” said Associate Professor Yosuke Tani. Whiff of vapor flips the switch Applying a vacuum draws the hexane away, causing the system to lock back into its solid, red state. The team captured these microscopic transformations in real time and verified the precise 3D architecture of the complex using microcrystal electron diffraction (MicroED). In chemistry, bridging the gap between how individual molecules behave and how a visible object looks or acts is hard. At the molecular level, atoms interact using tiny, invisible forces like weak electrostatic pulls or geometric fits. At the macroscopic scale, materials have bulk properties like being a liquid or solid, having a specific color, or glowing in the dark. The main challenge in supramolecular chemistry is amplification. For instance, getting trillions of tiny, reversible molecular movements to coordinate so effectively that they cause a dramatic, instant transformation you can see with the naked eye. Normally, small tweaks to a molecule’s structure get drowned out or lost when scaled up to a bulk material. Achieving this level of precise control means scientists can design smart materials that physically transform, changing color, state, or brightness, in response to simple environmental triggers like a waft of gas. This new host–guest framework provides a blueprint for next-generation smart materials by using gas vapors as an environmental trigger. Future applications could include re-writable optical memory, gas-sensing security tags, or adaptive soft robotics that alter their structural stiffness on demand.The findings were published in the journal Chemical Science.Recommended ArticlesGet the latest in engineering, tech, space & science - delivered daily to your inbox.Mrigakshi is a science journalist who enjoys writing about space exploration, biology, and technological innovations. Her work has been featured in well-known publications including Nature India, Supercluster, The Weather Channel and Astronomy magazine. If you have pitches in mind, please do not hesitate to email her.