Controlling interactions between molecules and their functions from the molecular level to the macroscopic scale is challenging in supramolecular 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)—nonvolatile fluids that can feature optical, electronic and catalytic properties.