Two teams have combined organic monomer crystals to form large two-dimensional polymer single crystals and have determined their structures for the first time. Scientists see the work as a landmark development in research on synthetic 2-D polymer crystals, which might one day be used in separations and nonlinear optics, applications that could complement those of graphene.
A naturally derived 2-D network of carbon atoms, graphene has attracted considerable attention as a potential molecular electronics material. Recent successes in research on this natural material have sparked growing interest in the ability to develop synthetic polymer versions with distinct tailored properties.
Researchers have recently made several kinds of synthetic 2-D polymer crystals in different ways. But the scientists have not yet been able to obtain proof of their structures because they haven’t been able to synthesize large single-crystal 2-D polymers capable of being analyzed by X-ray diffraction. Now, two groups—those of A. Dieter Schlüter of the Swiss Federal Institute of Technology (ETH), Zurich, and Benjamin T. King of the University of Nevada, Reno—have achieved these goals almost simultaneously (Nat. Chem. 2014, DOI: 10.1038/nchem.2007 and 10.1038/nchem.2008).
The studies “are collectively a big step forward” for the field, says William Dichtel of Cornell University, an expert on another class of 2-D polymers called covalent organic frameworks. The X-ray diffraction structures “will aid a fundamental understanding of how 2-D polymerization works.” He notes that the orderly arrays of functional groups and small equal-sized pores in the new 2-D polymer crystals suggest promising applications.
The Schlüter and King groups had both made 2-D polymer crystals before. But structural changes that occurred in those crystals during polymerization made their X-ray diffraction structures inaccessible, and yields were only in the milligram range. The new syntheses start with monomer crystals that stay intact upon polymerization. Both groups are thus able to obtain X-ray diffraction structures of the products, and yields are on a gram scale.
The two teams begin with different monomer crystals, each containing three anthracene groups. During the syntheses, ultraviolet light induces photopolymerization reactions that dimerize anthracene, causing the monomer crystals to combine into 2-D polymer crystals. The resulting products have multiple layers, but solvent treatment causes them to exfoliate into smaller multilayer aggregates or nanometer-thin single-layer sheets.
Schlüter and coworkers show that the polymerization reaction is reversible. “Considering the enormous structural changes associated with the processes of both polymerization and depolymerization, this switchability … is truly remarkable and unprecedented,” the researchers write.
Nanostructured organic materials specialist Federico Rosei of the National Institute of Scientific Research, in Varennes, Quebec, says, “Future advances in the field will require learning how to maintain order in 2-D polymer crystals while varying their structure to fine-tune their properties.”
Schlüter and coworkers are interested in nonlinear optics applications for the crystal sheets, and King’s team is looking into using them for desalination and other separations.
The properties of 2-D polymer crystals make them “a tantalizing vision for chemists and materials scientists alike,” writes nanoscientist Neil R. Champness in a Nature Chemistry commentary. “The future of these materials is undoubtedly bright, and I have no doubt that this is a field that will grow and grow.”