Selectivity of guest-host interactions in self-assembled hydrogen-bonded nanostructures observed by NMR.

We studied the incorporation of various small guest molecules into calix[4]hydroquinone nanotubes and nanoclusters using solid-state proton NMR spectroscopy in combination with quantum chemical calculations. While the molecules exhibit different types of hydrogen bonding and van der Waals interactions, they show different affinities to the nanotube host structures. As the guest molecules are located inside the complexes, they experience a shift in the NMR resonance line caused by screening effects from the aromatic electrons of the host superstructure. The abilities to fill the otherwise empty space within the tubes can hence be measured indirectly by the displacement of the NMR lines relative to the free molecules. In this way, we can probe which guest molecules are recognized by the calix[4]hydroquinones as suitable for filling their nanoporous superstructures. Selective guest-host interactions have been successfully achieved for the three component mixture of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes.