SPring-8, the large synchrotron radiation facility

The research group of Kyoto University focused on PCPs, which consist of metal ions and organic substances connecting them and have nanosize flexible pores that allow gas molecules to be adsorbed. The porous structure of PCPs changes more flexibly than that of other porous materials. PCPs with such a structure respond to a slight difference in physical properties among gas molecules and allow gas molecules to be adsorbed while accompanying changes in the size and shape of their pores (Fig. 1). Many PCPs have this gate opening function and exhibit high gas adsorption selectivity, giving them high-efficiency separation and condensation functions that have attracted attention in various fields. In this study, the research group synthesized a composite from a host PCP with nanosize flexible pores, into which guest reporter molecules that can read out a structural change in the host were introduced (Fig. 2). This composite can also allow further gas molecules to be adsorbed as secondary guest molecules. When the structure of the PCP pores changes during the adsorption, the interaction between the PCP pores and reporter molecules changes, causing the structure of the reporter molecules to also change at the same time. Only gases that can cause structural changes in pores are selectively adsorbed onto such composites. Because the concentration at which a structural change occurs differs among gases, it is possible to determine the type and concentration of a gas using the output from the reporter molecules.

The research group used distyrylbenzene (DSB), a fluorescent oligomer,^*3 for reporter molecules. DSB can be introduced into PCP pores by sublimation, which causes DSB to form a twisted structure emitting weak green fluorescence because of the interaction with the PCP pores (Fig. 3). When various gas molecules (N₂, O₂, Ar, CO₂) were introduced, only CO₂ molecules were adsorbed onto the composite by the gate opening function. To clarify the mechanism underlying this adsorption, the scientists of the research group of Kyoto University carried out powder X-ray diffraction measurement using high-brilliance and high-resolution synchrotron radiation X-rays at the BL02B2 beamline for powder diffraction of SPring-8, in cooperation with a research team led by Yoshiki Kubota, an associate professor of Osaka Prefecture University, and Masaki Takata, Group Director of Quantum Order Research Group, RIKEN SPring-8 Center. They found that when CO₂ was adsorbed onto the PCP-DSB composite, the porous structure expanded and changed from a rhombic to a square structure (Fig. 3). Moreover, the research group of Kyoto University cooperated with the research group led by Motohiro Mizuno, a professor of Kanazawa University, in examining the relationship between the structure and mobility of DSB in the pores. It was clarified that the steric structure of DSB was changed from a twisted to a planar structure emitting strong blue fluorescence owing to the structural change in the PCP pores (Figs. 3 and 4). In other words, the degree of twist of the DSB structure depended on the concentration of adsorbed CO₂, enabling the DSB-PCP composite to serve as a fluorescent sensor that can accurately respond to CO₂ (Fig. 4). In addition, the composite shows different gate opening functions in the presence of CO₂ and acetylene (an explosive gas), which have similar physicochemical properties, enabling rigorous discrimination between these gases because of the differences in the degree of twist of the DSB structure and the fluorescence.

A change in fluorescence starts to occur a few seconds after the adsorption of a gas and is completed in a few minutes because the developed gas sensor uses the physical adsorption of a gas onto a porous material. The sensor can be reused simply by removing the gas. The composite used in this study is not only important in terms of environmental protection and industrial applications because it can easily detect CO₂ in air, but it also has academic significance because it uses a completely new gas detection mechanism based on synchronous changes in the structure and function of a host (PCP) and a guest (DSB).