Chain reaction

Altering the chemical reactivity of molecules through self-assembly

Credit: © 2008 AAAS

The self-assembly of molecular building blocks is frequently used to create new structures. When self-assembly is carried out on a solid surface, the structures formed can exhibit properties that are absent in the isolated constituent molecules. For example, the electronic states of connected species can delocalize, leading to increased electron mobility. Now, John Yates and colleagues¹ at the University of Virginia, the University of Pittsburgh, and the National Energy Technology Laboratory, Pittsburgh, have shown that self-assembly can also alter chemical reactivity.

The researchers examined dimethyldisulphide (CH₃SSCH₃) molecules, which can organize themselves into linear chains of up to 15 molecules on gold surfaces. Using the tip of a scanning-tunnelling microscope (STM), electrons were injected into one end of a molecular chain, kick-starting a chemical reaction where successive sulphur–sulphur bonds dissociate and then reform to generate new dimethyldisulphide molecules (see figure, where unreacted chains are on the left and reacted chains are on the right).

Through a combination of STM imaging and theoretical calculations, Yates and co-workers found that the activation energy required to break the sulphur–sulphur bond in dimethyldisulphide was reduced by at least a factor of five by self-assembly of the molecules. As a result, the chain reaction could propagate through as many as ten neighbouring molecules.

The team speculate that this discovery could lead to 'designer' assemblies, where chain reactions yield the required products through low-energy and stereospecific pathways.