Good chemists are passive-aggressive—they manipulate
molecules without actually touching them.
In a feat of manipulating substances at the nanoscale, UCLA
researchers and colleagues demonstrated a method for isolating two molecules
together on a substrate and controlling how those two molecules react when
excited with ultraviolet light, making detailed observations both before and
after the reaction.
Their research is published in Science.
“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,” said lead study author Paul S. Weiss, a distinguished professor
of chemistry and biochemistry who holds UCLA’s Fred Kavli Chair in Nanosystems
Sciences. “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.”
Controlling how molecules combine in order to study the
resulting reactions is called regioselectivity. It is important because there
are a variety of ways that molecules can combine, with varying chemical
products. One way to direct a reaction is to isolate molecules and to hold them
together to get regioselective reactions; this is the strategy used by enzymes
in many biochemical reactions.
“The specialized scanning tunneling microscope used for
these studies can also measure the absorption of light and charge separation in
molecules designed for solar cells,” Weiss said. “This gives us a new
way to optimize these molecules, in collaboration with synthetic chemists. This
is what first brought us together with our collaborators at the Univ. of Washington, led by Prof. Alex Jen.”
Alex K-Y. Jen holds the Boeing-Johnson Chair at the Univ. of Washington, where he is a professor of
materials science and engineering and of chemistry. The theoretical aspects of
the study were led by Kendall Houk, a UCLA professor of chemistry and
The study’s first author, Moonhee Kim, a graduate student in
Weiss’ lab, managed to isolate and control the reactions of pairs of molecules
by creating nanostructures tailored to allow only two molecules fit in place.
The molecules used in the study are photosensitive and are used in organic solar
cells; similar techniques could be used to study a wide variety of molecules.
Manipulating the way molecules in organic solar cells come together may also
ultimately lead to greater efficiency.
To isolate the two molecules and align them in the desired—but
unnatural—way, Kim utilized a concept similar to that of toddler’s toys that
feature cutouts in which only certain shapes will fit.
Two molecules are placed in proximity in “cutouts” in self-assembled monolayers. When excited with ultraviolet light, they are constrained to react along a pathway different than they would if they could reorient in solution. Credit: UCLA
She created a defect, or cutout, in a self-assembled
monolayer, or SAM, a single layer of molecules on a flat surface—in this case,
gold. The defect in the SAM was sized so that only two organic reactant
molecules would fit and would only attach with the desired alignment. As a
guide to attach the molecules to the SAM in the correct orientation, sulfur was
attached to the bottoms of the molecules, as sulfur binds readily to gold.
“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 said. “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.”
After the molecules were isolated and trapped on the substrate,
they still needed to be excited with light to react. In this case, the energy
was supplied by ultraviolet light, which triggered the reaction. The
researchers were able to verify the proper alignment and the reaction of the
molecules using the special microscope developed by Kim and Weiss.