Complex nanoscale structures could be snapped together with a microscope
US researchers have come up with a chemical version of Lego with the development of ’protopolymers’, in which monomer compounds link together to create long chains without forming chemical bonds.
In a normal polymer, the monomer units are held together by covalent bonds, but in the protopolymers developed by Paul Weiss and Gregory McCarty at Pennsylvania State University, the units are simply held together by intermolecular interactions. The researchers discovered that, in certain instances, they could move these protopolymers around using a scanning tunnelling microscope (STM), which opens up the possibility that different protopolymers could be ’snapped together’ to form complex nanoscale structures.
The researchers created their protopolymer by exposing a copper surface to varying concentrations of p-diiodobenzene. They discovered that at fairly low concentrations the copper surface catalysed the formation of single molecules of phenylene, but that at higher concentrations the phenylene molecules would group together into long chains. These chains could cross small atomic perturbations, known as step boundaries, in the copper surface without breaking up. But the researchers were able to extract individual molecules from the chains using the STM, suggesting they were not covalently bound together.
’It amazed us that these extended structures could cross step boundaries,’ says Weiss. ’These monomers have not yet formed covalent chemical bonds, which would link them together as a large molecule, but they are aligned and their interaction is much stronger than any previously observed.’
At even greater concentrations of p-diiodobenzene, the copper surface would become covered in a layer of phenylene. Slightly shorter phenylene chains would then form on this layer, and the researchers discovered they could move these chains around with the STM. Furthermore, some of the longer chains naturally contained turns and joints, indicating that the chains are able to form more complex shapes. The researchers are currently investigating how to control the formation of these joints.
G S McCarty and P S Weiss, J. Am. Chem. Soc., 2004, 126, 16772