The photochemical reactions of conjugated organic molecules can theoretically follow several paths. In practice, however, selection rules rarely allow this in solution because of unfavourable geometries.

Carry out the reaction on a surface and catalysts can direct the reactants towards alternative and allowed products.
Researchers at the University of California - Los Angeles and the University of Washington have used an alkanethiolate monolayer on a gold surface as the stage on which to manipulate two isolated molecules under ultraviolet light [Kim et al., Science (2011) 331, 1312]. Defect sites created in the self-assembled monolayer act as directors prompting the chemical players to perform otherwise geometrically unfavorable photochemical reactions. The defects are shaped so that the two molecules of interest can fit in the defect together in a particular geometrical arrangement. The team has demonstrated proof of principle with a reaction involving the photodimerization of naphthalene groups; a reaction studied in solution in the 1970s by Chandross at Bell Labs.
"This is one step in measuring and understanding the interactions between light and molecules, which we hope will eventually lead to more efficient conversion of sunlight to electrical and other usable forms of energy," explains team leader Paul Weiss. "Here, we used the energy from the light to induce a chemical reaction in a way that would not happen for molecules free to move in solution; they were held in place by their attachment to a surface and by the unreactive matrix of molecules around them."
"The standard procedure for this type of chemistry is to combine a bunch of molecules in solution and let them react together, but through random combinations, only 3 % of molecules might react in this way," UCLA's Houk explains. "Our method is much more targeted. Instead of doing one measurement on thousands of molecules, we are doing a range of measurements on just two molecules."
This new approach to chemical regioselectivity emulates the way in which biological catalysts, enzymes, effectively entrap substrates and make them react in a way that would not occur quickly in solution. Team member Kendell Houk of UCLA told Materials Today that the next step in reaction control might focus on the surface. "Methods to direct self-assembly would be needed to enable more complex reactions," he says.
The team has used a specialized scanning tunneling microscope (STM) to measure the absorption of light and charge separation in molecules on the surface and to verify the proper alignment and the reaction of the molecules. In particular they have investigated molecules designed for use in solar cells with this STM. This, Weiss says, is allowing them to optimize the molecules, in collaboration with their synthetic chemist colleagues at Washington in the group of Alex Jen.

David Bradley