Using exquisite control at the nanoscale, researchers have demonstrated that they can foster an extremely unlikely chemical reaction—one that simply would not occur in the bulk. Exercising control over reactivity in this manner “has great potential for both understanding and measuring complex chemical reactions,” the researchers write in Science (DOI: 10.1126/science.1200830).
In the work, chemists led by Paul S. Weiss and Kendall N. Houk of the University of California, Los Angeles, and Alex K-Y. Jen of the University of Washington, Seattle, tied two anthracene analogs next to each other on a gold surface. This forced the molecules to react in a manner that, although theoretically possible in solution, rarely occurs there because of unfavorable geometry.
In principle, the mallet-shaped molecule 9-phenylethynylanthracene (PEA) should undergo a 4 + 4 photocycloaddition with another molecule of PEA. But because of geometric constraints, that reaction rarely happens. Instead, one PEA’s anthracene moiety tends to do Diels-Alder chemistry with the ethynyl unit on another PEA’s phenylethynyl handle.
To force the disfavored reaction, the researchers attach a thiol group to the end of PEA’s handle and tether two such molecules next to one another on a gold surface within the defect sites of a self-assembled alkanethiolate monolayer. The anthracene moieties are then poised in the correct orientation to do the photocycloaddition when photoexcited.
The team used scanning tunneling microscopy to follow the course of the reaction—a feat that took considerable modifications of the microscope. “The most important aspect of this work is to see the effect that photoexcitation has on single molecules in well-defined environments,” Weiss tells C&EN. “We now have a tool to do this.”
The researchers were able to “harness the full power of nanoscale science and engineering toward their ends,” comments Ted Sargent, a nanoscience expert at the University of Toronto.