"We have been able to confirm that we now have the predictive power to design molecular switches that can be turned on or off at will, which was a critical test of our understanding of their function," said Paul Weiss of Pennsylvania State.
The team employed oligo(phenylene ethynylene) (OPE) molecules embedded in a self-assembled monolayer of alkanethiol molecules on a gold surface. Weiss and colleagues engineered their switches to turn either on or off under the influence of the same electric field. They achieved this by creating OPE molecules with a strong dipole and positioning them so that the positively charged end was either furthest from or next to the substrate.
"The OPEs that we engineered to have the strongest dipoles are the most reliable," said Weiss.
Moving a scanning tunnelling microscope (STM) tip over the surface caused molecules with a strong dipole and an exposed end opposite in charge to the STM tip to rise above the surrounding molecules. This was the "on" state of the switch. To turn the switch "off" the researchers used an STM tip with the same charge as the end of the molecule. This pushed the molecule down, making it lean sideways into the matrix.
"When the molecule is tilted over, electrons have a harder time going through this bond, so the switch is more resistive," said Weiss.
The team also tailored the chemistry of the OPE and alkanethiol molecules to increase switch stability. "We repositioned a nitro group attached to the side of one of the varieties of OPE switches so that it had a strong enough dipole and could interact with the amide groups on the surrounding matrix molecules through hydrogen bonding," explained Weiss.
The researchers reported their work in the Journal of the American Chemical Society.