Molecular self-assembly is the process through which molecular building blocks organize themselves into supramolecular structures. Controlling the principles of molecular self-assembly opens the door to new materials with special properties, for example, self-repairing coatings. The properties of these materials are strongly influenced by the way the building blocks are assembled; a small difference in their organization can lead to very different properties. Peter A. Korevaar and colleagues at Eindhoven University of Technology in The Netherlands have succeeded in monitoring and controlling a molecular self-assembly process through different pathways.

As reported in the January 18 advanced online issue of Nature (DOI: 10.1038/nature10720), the research team uses a molecular building block whose assembly can be studied with time using circular dichroism: S-chiral oligo(p-phenylenevinylene) or SOPV. Molecules of this kind are frequently used in organic electronic devices, in which small differences in the morphology of the material lead to large differences in their properties. At the start of the assembly process, SOPV first forms unstructured clusters, which ultimately grow into neatly organized left-handed “spiral staircase”-like helical structures. Detailed time-resolved investigation of this assembly process importantly showed that at early reaction times, aggregates with the opposite helicity were also present. These right-handed helices are metastable and convert to the ther modynamically favored left-handed hel-ices with time.

Aggregation of SOPV therefore involves two competing pathways which lead to assemblies with opposite helicity, one of which is favored kinetically and the other thermodynamically. Based on this understanding, the researchers demonstrated that the assembly process could be controlled to uniquely select the kinetic product. Addition of tartaric acid, a small molecule that attaches itself to the SOPV molecules, forces the assembly process toward the right-handed helices.

“This knowledge has significant impact on an optimal self-assembly process, and we can now use it for more applied supramolecular systems which are much more difficult to study,” said Korevaar.

Diagram showing the formation of helical S-chiral oligo(p-phenylenevinylene) polymer chains