For 20 years, researchers have experimented with artificially created, tree-like molecules that act like antennae and might someday play a key role in a number of processes, from releasing a drug inside the body to converting light to energy more efficiently than Mother Nature. Now researchers at the University of Rochester have computed what seems to be the most efficient configuration for one class of these molecules, and have published their results in the July 10 issue of Physical Review Letters.
"These are giant, light-harvesting molecules," explains Shaul Mukamel, professor of chemistry. "Each one is designed like a net that captures light and funnels its energy into its core where we can make it do almost anything we want." The fractal-shaped molecules, called dendrimers, are currently used in laboratories to control chemical reactions, since researchers can use them to release a chemical at the perfect moment, but only recently have scientists realized some of their potential. The organic molecules look like snowflakes, with tiny limbs branching out in all directions to capture a ray of light and channel its energy into the molecule's heart. There the light energy can be harnessed to generate power like a super-efficient photoelectric cell or to release a chemotherapeutic drug inside a tumor after it's been carried through the bloodstream. Dendrimers might even one day act as the initial necessary step in photosynthesis, improving the light-harvesting task performed naturally within any green leaf, to yield crops that can grow where none grew before.
To determine the most efficient configuration, graduate student Subhadip Raychaudhuri and chemistry research associate Vladimir Chernyak, worked with Mukamel and Yonathan Shapir, professor of physics, to run thousands of computer simulations that incorporated everything known about the way atoms interact, to see whether there was a certain shape or size of dendrimer that caught and funneled light's energy best. "What surprised us is that there is a point of diminishing returns," says Shapir.
Many scientists have envisioned building bigger and bigger molecules, since a dendrimer needs as many light-catching limbs as possible to capture light, much as a big tree snares more light than a smaller one. But the researchers found a problem: the larger the molecule, the harder time it had funneling that light energy toward its center. Like a squirrel running along a tree branch, when the light encountered a fork in the limb it could either run out toward a twig, or in toward the trunk. If the tree were too big, the squirrel might not know which way was twigward or trunkward, and the hapless creature would run around lost until it died. In the same way, the researchers watched as their simulations showed that dendrimers lost the light they captured if they were too big. The light would simply dissipate long before it reached the center, where a drug or other chemical was waiting to be activated.
In research funded by the National Science Foundation, the team of chemists and theoretical physicists ran the simulations over and over until they found the magic number for one particular family of dendrimers: nine. The dendrimers could branch out up to nine times before their ability to funnel light became compromised. The team expects the findings to set the guidelines for the synthesis of large fractal-like dendrimers that could be used in many chemical reactions, from making magnetic resonance imaging (MRI) scans more accurate, to converting sunlight to energy for weakened plants.