Porous materials such as zeolites and coordination polymers show promise for use in catalysis, separation and the purification of gases, and as desiccant substances. To realize these applications, however, it is essential to control the properties of the pores. In the case of porous coordination polymers, this control can be achieved by functionalization of the organic ligands and the use of building blocks that form a flexible and dynamic framework.

Susumu Kitagawa and colleagues at the Japan Science and Technology Agency and Kyoto University in Japan1 have now synthesized a coordination polymer made from components that allow movement of the framework, giving rise to the stepwise uptake and highly selective adsorption of guest molecules.

Fig. 1: Schematic illustration of the coordination polymer, with rotatable pillars and organic ligands, that act as a molecular gate. On the addition of water, the hydrogen bonding between ligands is lost and the gate becomes unlocked. On the further addition of water, the pillars rotate, opening up the structure and allowing more guest molecules to penetrate into the coordination polymer.

The cadmium-containing coordination polymer has rotatable pillars to which organic ligands are attached. These ligands — ethylene glycol chains — act as molecular gates that, as a result of lock/unlock interactions, cause the coordination polymer to transform reversibly between different single-crystal structures on the addition or removal of guest molecules. In the locked state, the ethylene glycol chain ends are linked by hydrogen bonds. On the addition of guest molecules, such as water, this interaction is lost and the gate is unlocked (Fig. 1). As the polymer is exposed to more guest molecules, the guests penetrate the structure, causing slippage of neighboring layers and resulting in further opening of the structure.

The molecular gate mechanism, the flexibility of the framework and the presence of guest-accessible metal centers within the coordination polymer result in the stepwise uptake of guest molecules such as water, methanol and carbon dioxide.

“Our compound does not adsorb carbon dioxide until reaching high pressure, and when the gate-opening pressure is reached, a large amount of carbon dioxide is suddenly adsorbed,” says Kitagawa. “If other gas molecules can be adsorbed at lower pressure, our compound could be used to separate carbon dioxide from a mixture of gases.”

Future research includes the production of smart porous materials in which the module–module interactions in the host framework that lock/unlock the molecular gate can be controlled by external stimuli, such as an electric field or light, leading to more precise control of the material properties.