Natural antennas inspire synthetic sun sponges
The evolutionary chemical machinery of nature – plus a dash of human ingenuity – has inspired synthetic light-harvesting antennas that convert sunlight into unprecedented amounts of usable energy.
Light-harvesting antennas are normally found in plants and photosynthesizing bacteria. The synthetic versions consist of carefully designed rings of proteins with attached pigment molecules that are in ideal positions to capture the sun’s energy, including wavelengths that plants naturally ignore.
A team affiliated with the Photosynthetic Antenna Research Center (PARC) at Washington University in St. Louis (WUSTL) – including WUSTL’s Dewey Holten and Christine Kirmaier; Paul Loach and Pamela Parkes-Loach of Northwestern University; Jonathan Lindsey of North Carolina State University; David Bocian of the University of California, Riverside; and Neil Hunter
of the University of Sheffield in England – has created prototypes of synthetic antennas.
The dyad modules assemble into rings that pack many pigments together for light harvesting. Because of the additional pigments attached to the dyads, the antennas absorb more of the sun’s light than natural systems. Photo courtesy of Hunter/Holten/PARC.
One of the two prototypes incorporates synthetic dyes called Oregon Green and Rhodamine Red; the other combines Oregon Green and a synthetic version of the bacterial pigment bacteriochlorophyll, which absorbs light in the near-IR. Both designs soak up more of the sun’s spectrum than the native antennas in purple bacteria that provided the inspiration and some of the components for the test bed.
Plants miss the middle of the visible spectrum as well as longer wavelengths such as the near-IR photons absorbed by the photosynthetic bacteria. Accessory pigments such as carotenoids fill some gaps, but large swaths of the solar spectrum pass through untouched. “Since plant pigments actually reject a lot of the light that falls on them, potentially there’s a lot of light you could gather that plants don’t bother with,” Hunter said.
To design and create pigments that can absorb at wavelengths that will fill some of the holes in the absorption of natural systems, the team relies on Lindsey.
“It can’t be done from first principles, but we have a large database of known absorbers, and so drawing on that and reasoning by analogy, we can design a large variety of pigments,” he said. “The effectiveness of the design depends not only on having extra pigments, but also pigments able to talk to one another, so that energy that lands on any one of them is able to hop onto the next pigment and then to the next one after that. They have to work together.”
“The energy cascades down like a waterfall,” Hunter said, “so you pour the energy at the top of the waterfall, and it hits one pigment and jumps to the next and the next and finally to the pigment at the bottom, which in terms of energy is the pigment that is reddest in color.”
Once assembled, the antennas are sent to the Holten-Kirmaier lab, where spectroscopy is used to excite each pigment molecule. The energy transfer is followed from one pigment to the next, all the way down to the bacteriochlorophyll target. With the correct pigments in the right locations, the transfer is extremely efficient with little energy lost on the way.
One day, a two-part system, consisting of an antenna and a second unit called a reaction center, may serve as miniature power outlets into which photochemical modules could be plugged. The sun’s energy could then be used directly to split water, generate electricity or build molecular-scale devices.
The results were published in Chemical Science (doi: 10.1039/C3SC51518D).
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