Supramolecular chemistry, in which molecules and molecular complexes are held together by noncovalent bonds, is just beginning to come into its own with the emergence of nanotechnology. Metal–organic frameworks (MOFs) are commanding much of the attention because of their appetite for greenhouse gases, but a new player has joined the field—supramolecular organic frameworks (SOFs). Researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) have unveiled the first known two-dimensional (2D) SOFs that self-assemble in solution, an important breakthrough that holds implications for sensing and separation technologies, energy sciences, and, perhaps most importantly, biomimetics. The researchers report their work in the September 30, 2013 online edition of the Journal of the American Chemical Society (DOI: 10.1021/ja4086935).
Traditional molecular chemistry involves strong covalent bonds formed by the sharing or exchange of electrons between the atoms comprising a molecular system. Supramolecular chemistry involves systems that are held together by weaker, noncovalent connections, such as hydrogen bonds, electrostatic, and van der Waals forces. While nature uses supramolecular chemistry to form the double-helix of DNA or to fold proteins, this research team believes that these ideas could also be translated to nanotechnology, where single layers of 2D structurally ordered materials—such as graphene—could fulfill many requirements. The key is that they should be processed in solution.
“Solution-based processing allows for mass production and reduced manufacturing costs, and is an important step for transferring materials to a dry state without losing their structural integrity,” said Yi Liu, who oversees the supramolecular electronics research group at Berkeley Lab’s Molecular Foundry. “Solution-based processing also allows for bio-related applications such as biomimetic sensing, where the framework structure is advantageous for the capturing of guest molecules and the amplification of chemical signals.”
Supramolecular organic frameworks feature a porous framework with honeycomb periodicity similar to a metal–organic framework.
However, the self-assembly of well-defined 2D supramolecular systems polymers in solution has been a challenge because such polymers tend to precipitate out of solution, making them difficult to manipulate and characterize. To meet this challenge, co-researcher Zhan-Ting Li of Shanghai Institute of Organic Chemistry and Fudan University, China, and his collaborators used a combination of self-assembling tripods and macrocycle rings to form a porous framework with honeycomb periodicity, similar to that of a MOF, but which remains rigid in solution. Equipping the tripods with bulky hydrophilic groups that resist stacking preserved the solubility and single-layer 2D architecture of the framework.
“That our framework is held together by reversible, noncovalent supramolecular interactions ensures good solubility in water,” Li said. “The precise dimensional control of our solution-based processing facilitates the structural and chemical customization of our frameworks.”
The tripods of these SOFs were made from aromatic bipyridine molecules whose trio of struts or arms was interlocked with the struts of their neighboring molecules through the macrocycles, which were made from cucurbituril molecules. The molecules used in this study were proof-of-principle starters. Other molecules for the struts could be employed in the future for the design of similar or more complex architectures.
Liu and his collaborators at the Molecular Foundry and in Shanghai are now working to create soluble SOFs in three dimensions.
