Discrete complexes comprising stacks of up to nine aromatic molecules can be assembled in one step from a few simple building blocks
Aromatic molecules have a tendency to stack together in face-to-face arrangements that are stabilized by interactions between their conjugated π-electron systems. One of the best known examples is the stack formed by the base pairs in the DNA double helix. In synthetic systems, however, controlling the number of aromatic molecules that come together into a stack is not easy. They either contain just a few, such as in so-called 'tweezer'-type assemblies, or essentially an infinite amount, as found in some crystal structures.
Now, a team in Japan led by Michito Yoshizawa and Makoto Fujita at the University of Tokyo have shown how coordination cages can be used to assemble structures containing stacks of up to nine aromatic molecules1. Each individual cage is made up of three different types of building block — two large aromatic 'caps' are linked together with three 'pillar' ligands using a total of six metal ion 'hinges' to form a cylinder-like structure with a large central cavity. When these three components are mixed together in the presence of an additional aromatic compound such as triphenylene or pyrene, a unique self-assembled structure is formed. X-ray crystallography reveals that two coordination cages form an interpenetrated structure, and intercalated between each pair of caps is a pyrene molecule. This results in a structure containing a stack of seven aromatic molecules.
By increasing the length of the pillar ligands that join the caps of the coordination cages together, more room opens up inside the assembly and additional aromatic molecules can fit inside. Structures comprising stacks of either eight or nine aromatics were prepared by the Tokyo team, who suggest that these complexes may have interesting electro- and photochemical properties and could be used to fabricate nanodevices.
Yamauchi, Y., Yoshizawa, M. & Fujita, M. Engineering stacks of aromatic rings by the interpenetration of self-assembled coordination cages. J. Am. Chem. Soc. 130, 5832–5833 (2008).10.1021/ja077783+
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Cantrill, S. Aromatics stack up. Nature Chem (2008). https://doi.org/10.1038/nchem.6