The free space inside metal–organic frameworks (MOFs) can be used to store small molecules, with much research effort devoted to gases such as carbon dioxide (for sequestration) and hydrogen (for energy-related applications). The sizes of the pores inside a MOF depend on a number of factors, including the nature of the building blocks, the way they connect together into a network, and whether these networks interpenetrate.
One way to increase the pore size, and thereby enable MOFs to trap larger guest molecules, is to increase the length of the organic linkers between the metal-based nodes. This is not a guaranteed strategy for success, however, because making the organic linker longer can lead to interpenetrated frameworks or structures that are quite fragile and prone to collapse.
Now, Omar Yaghi at the University of California Berkeley and a team of co-workers from around the globe have succeeded in making a series of robust MOFs in which the length of the organic linker is systematically increased from 7 to 50 Å. The starting point for the team was a well-known MOF in which small metal clusters are linked into hexagonal sheets by single phenyl rings, each adorned with two carboxyl and two hydroxyl groups.
By increasing the number of phenyl rings in the linkers, MOFs with the same basic structure — but with much larger hexagonal channels — could be made. The longest linker comprised 11 phenyl groups connected in a row, and solubilizing hexyl chains were added at intervals along the backbone. The resulting MOF has a pore aperture measuring 98 Å (pictured) — the largest for any crystalline material reported so far.
The pore sizes found in this series of MOFs enable very large molecules to enter the void spaces found within. The MOF made from a linker with seven phenyl groups could accommodate myoglobin and the one built from a nine-phenyl linker could host green fluorescent protein.
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Cantrill, S. The big indoors. Nature Chem 4, 516 (2012). https://doi.org/10.1038/nchem.1398