Controlling interactions between molecules and their functions from the molecular level to the macroscopic scale is challenging in supramolecular chemistry and materials chemistry. To this end, a research team led by Associate Professor Yosuke Tani from WPI-ITbM at Nagoya University and Keisuke Wada from Kyoto University designed a vapor-controlled reversible host–guest chemistry system that controls the optical and physical properties of a functional molecular liquid (FML)—non-volatile fluids that can feature optical, electronic, and catalytic properties. Their research was published in Chemical Science on July 21, 2026.
In host–guest chemistry, a "host" molecule binds a "guest" molecule to form a complex without new chemical bonds. The researchers used a simple tube-like, cyclic-shaped molecule as the host and an FML with long thread-like carbon chains as the guest. Upon mixing, the FML's chains thread inside the host's cavity, forming a dumbbell-shaped host–guest complex.
Upon forming the host–guest complex, the FML's optical and physical properties immediately change. Originally, the FML could glow in the dark through phosphorescence, however, this is "turned off" in the FML–complex state. Additionally, the color changes from yellow to red and the phase changes from liquid to solid.
However, exposing the FML-complex to hexane vapors, a six-carbon chain molecule, liberates the FML from the complex with its original physical and optical properties. The hexane vapors behave as a more competitive guest, extruding the FML from the host molecules and forming a yellow-solid hexane-complex. Using a vacuum, the hexane vapors could be removed to reform the red FML-complex. The reversible process is visualized at the macroscopic scale. Additionally, they could obtain the 3D structure of the FML-complex by using microcrystal electron diffraction (MicroED).
Using the hexane vapors as a trigger, they successfully designed an optical switching and phase switching FML-based host–guest system. In the dark, the vapors behaved as an off/on switch for the FML's phosphorescence and for cycling between red and yellow in light. In tandem, the FML is phase changed between liquid and solid. Similarly, this also controls the interconversion between a single-phase solid state and a solid–liquid phase-separated state at the macro-scale.
"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. "Aside from these moments, using MicroED to solve the crystal structure and observing the transformations in real time under the microscope were unique and exciting experiences."
