Turning on chemical reactivity using light enables on-demand trapping of noxious gases inside a porous coordination polymer.
For hundreds of years, porous materials such as charcoal have been used to purify liquids and gases by trapping guest molecules within their permeable structures. Recently, materials known as porous coordination polymers (PCPs) that combine tiny pore sizes with engineered chemical reactivity have emerged, with great promise for applications that involve trapping molecules, including storage, separation and catalysis. Ryotaro Matsuda and a team of researchers from Japan1 have now reported a new development in PCP materials: a nanoporous crystal in which the activity can be controlled with the flick of a switch thanks to a photosensitive internal framework.
PCPs are usually synthesized through self-assembly, where organic molecules (ligands) and metal ions spontaneously arrange in solution into an extended scaffold-like network. The ligands typically play the role of bridges between metal ‘hubs’ in the resulting metal–organic structure. By using ligands that contain groups with a particular reactivity, researchers can produce devices, such as custom gas sensors, that detect the presence of molecules trapped within the structure. However, if the ligands are too responsive, they can decompose during the self-assembly process, limiting the range of potential applications.
Matsuda and his co-workers solved this dilemma by employing ligands with photosensitive azide units that remain dormant until converted into reactive nitrogen radicals by light — an efficient ‘on-demand’ activation strategy. “We thought that if the PCPs were made of modules that can transform in response to external stimuli, it could be possible to create materials with highly reactive pore surfaces,” says Matsuda.
Mixing zinc ions with bipyridine and an azide-containing compound called 5-azidoisophthalic acid, the researchers obtained a PCP crystal with an interlocked, nanoporous framework stabilized by aromatic stacking forces (Fig. 1). After developing a technique that allowed in situ detection of gas molecule adsorption during photoirradiation — an experimental first — the team saw a pronounced change in their material after exposing it to ultraviolet (UV) light. In its dormant state, the PCP weakly absorbs gases like oxygen and carbon monoxide; but after activation under UV, the trapped gases become chemically bound to the PCP framework, setting off a dramatic increase in gas storage capability.
According to the researchers, this activation strategy could be used for a wide range of photosensitive PCP ligands, and should spark development of devices that are able to both sense and remove toxic gases from enclosed environments.
Sato, H., Matsuda, R., Sugimoto, K., Takata, M., & Kitagawa, S. Photoactivation of a nanoporous crystal for on-demand guest trapping and conversion. Nat. Mater. 9, 661 (2010).
This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.
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Nanoporous materials: Activity in a flash. NPG Asia Mater 3, 6 (2011). https://doi.org/10.1038/asiamat.2010.160