Tethered Molecules Act As Light-Driven Reversible Nanoswitches

Tag : benzene ring

The ability to see is based on molecules in the eye that flip fromone conformation to another when exposed to visible light. Now, anew technique for attaching light-sensitive organic molecules tometal surfaces allows the molecules to be switched between twodifferent configurations in response to exposure to differentwavelengths of light. Because the configuration changes arereversible and can be controlled without direct contact, thistechnique could enable applications that can be controlled at themolecular scale.
The technology has been suggested as a possible basis for molecularmotors, artificial muscles, and molecular electronics. The researchresults, obtained by a team led by Paul S. Weiss, distinguishedprofessor of chemistry and physics at Penn State University andJames M. Tour, Chao professor of chemistry at Rice University, arereported in the June 2008 issue of the journal Nano Letters .
Until now, progress was impeded because, when such molecules wereattached to surfaces, they no longer could be switched back andforth, as they could be when they were in solution. The newtechnique uses a change in the shape of an azobenzene molecule inresponse to light to provide two different states. The azobenzenemolecule consists of a bridge of two nitrogen atoms attached to oneanother by a double bond, with each nitrogen atom also bound to abenzene ring. The two benzene rings can be on the same side of themolecule (cis configuration) or on opposite sides (transconfiguration). When the molecule absorbs energy, in the form oflight, it can change between cis and trans configurations in aprocess called photoisomerization. “This mechanism isessentially the same that we use in our eyes for vision," saidWeiss. “The molecule responds to light by making a changethat can be harnessed. In the eye, the change causes a neuralimpulse.”
The photoisomerization of azobenzene is understood well insolution, but the molecule must be attached to a surface in orderto provide a useful molecular switch or component of a motor.Previous attempts to accomplish the switching with attachedmolecules were unsuccessful, either due to interactions between themolecule and the surface to which it was attached or tointerferences between adjacent molecules. “To overcome thedifficulty of reversible photoisomerization of molecules onsurfaces, we used a carefully designed 'tether' to isolate thefunctional molecules from one another and from the metalsurface,” said Weiss. “We isolated the tetheredmolecules in the surrounding matrix on a self-assembled monolayerand confirmed this isolation using molecular-resolution scanningtunneling microscopy.”
When the tethered molecules were exposed to ultraviolet light in aspecially built scanning tunneling microscope, they switched fromthe trans to the more-compact cis state. This switch was confirmedby an apparent decrease in height of the molecule above thesurrounding surface. The researchers further found that exposure tovisible light caused a transition back to the more-extended transstate.
Weiss points out that this research advance is just the first stepin designing a device that can be driven or actuated by suchmolecular change. In order to perform useful work as a switch ornanoscale-drive motor, it will be necessary to coordinate themotion of multiple molecules and to build moving parts into somesort of assembly. According to Weiss, further research by the teamalready has found some surprises when the molecules are lined up towork in unison, like a chorus line.
This work was performed as part of the Penn State Center forNanoscale Science, with major funding from the National ScienceFoundation and additional funding from the United States Departmentof Energy and Visionarts, Inc.
SOURCE: Penn State University