By incorporating macrocyclic polyethers into the three-dimensional porous structure of metal-organic frameworks (MOFs), a team of chemists from UCLA and Northwestern University has enhanced the capabilities of MOFs to include specific binding of organic molecules (Science 2009, 325, 855). Thousands of MOFs made up of metal-based joints connected by linear organic struts have been prepared and shown to nonspecifically bind small guest molecules according to their size or shape—adsorbing H2 molecules within the pore structure, for example. But the team led by UCLA’s Qiaowei Li and Wenyu Zhang created a novel type of MOF by using zinc cluster joints with long phenylethynyl struts that contain large crown ethers at the midriffs. The researchers show that electron-deficient paraquat dications (1,1′-dimethyl-4,4′-bipyridinium) quantitatively and reversibly bind by threading through the middle of the crown ethers in a pseudorotaxane fashion. By comparison, frameworks lacking the polyethers exhibit negligible uptake of paraquat. The new MOFs are capable of binding incoming substrates in a manner akin to the docking of drug molecules within protein receptors, Li says, opening up possibilities for molecular-recognition applications.