Molecular switches edging toward computers

Research from Penn State University and Rice University indicates that future computers may contain components that function on the action of single molecules.

According researchers at Penn State and Rice University, specially designed single molecules can switch between ‘off’ and ‘on’ states. In addition, conformational changes, which happen when molecules alter their arrangement by rotation of their atoms around a single bond, determine how and when that conductance switching occurs in those molecules.

The researchers determined that limiting conformational changes reduces switching between the ‘on’ and ‘off’ states. So, just as squeezing a lot of people into a small room limits their ability to move freely, researchers determined the same thing was happening at a much smaller scale with molecules. Conformational changes do not occur as frequently when the molecules have less room to move in their host environment, or matrix.

Because switching provides the basis of logic and memory in computer systems, the discovery of what causes such switching in single molecules may help researchers move closer to making molecular computers a reality.

‘We essentially tightened the noose around the molecule and showed that once its motion was reduced switching went way down,’ said Paul Weiss, associate professor of chemistry at Penn State.

‘Our next step is figuring out how to control the molecules’ movement between ‘on’ and ‘off.’ In bundles of thousands of molecules, our collaborator, Mark Reed, in electrical engineering at Yale University, and his group, have been able accomplish movement between the states.’

According to the research, a dense, well-ordered matrix inhibits the rate at which conductance switching occurs among single molecules within that matrix.

In a loose, poorly ordered matrix, those same molecules switch between ‘on’ and ‘off’ much more frequently. Researchers tracked the molecules’ movement between ‘on’ and ‘off’ using scanning tunnelling microscopy in matrices of alkanethiolate monolayers.

The molecules, known as phenylene ethynylene oligomers and comprised of alternating benzene rings and two carbon atoms with triple bonds between them and a functional group on the central of three rings, were the first single molecules to have their switching documented.

‘It had been predicted that single molecules did not switch, but we proved they did and we identified at least part of the mechanism,’ Weiss said. ‘Two important advances are determining the limit at one molecule and establishing that its persistence time – the length of time information can be held in a switch at room temperature – can be hours.’

Researchers found the molecules that underwent conformational changes remained anchored in the same spot on the matrices and that the molecules’ apparent size in images changed when they switched.

They appeared to stand higher in the matrix when they were ‘on,’ lower when they were ‘off,’ and the respective states lasted as long as 26 hours, which indicated changes in conductance.

Also, while the research on 10,000 molecules at once showed that groups of molecules could be switched at will, the researchers focusing on single molecules proved they could turn the molecules off, but turning them on was more problematic.

‘Clearly, we have an indication it can be done,’ Weiss said. ‘It’s just a matter of setting up the experiment in the most efficient manner.’