Natural light: Prototyping light harvesters

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  • Published: Sep 1, 2013
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
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Picking pigments

Graduate student Michelle Harris in PARC’s Ultrafast Laser Facility. The laser setup allows them to measure energy transfer steps among pigments in light-harvesting antennas that take place in a trillionth of a second. ANGELES/WUSTL

A ring of protein and pigments, half synthetic and half natural, can be used to quickly prototype light-harvesting antennae that absorb more sunlight than their wholly natural counterparts. The system could be used to prototype ultraviolet and visible absorbing molecules for solar energy trapping.

Writing in the recently launched journal "Chemical Science", scientists from Washington University in St Louis, Northwestern University, North Carolina State University, the University of California, Riverside, and the University of Sheffield, UK, describe two prototype antennae built on the novel testbed. One incorporates the synthetic dyes Oregon Green and Rhodamine Red and the other unites Oregon Green and a synthetic version of the bacterial pigment bacteriochlorophyll to absorb energy in the near-infrared. The team explains that both designs soak up the sun more effectively than natural antennae found in purple bacteria and so vilifies the testbed approach to developing other light antennae. The researchers point out that the approach makes it far easier to assemble novel antennae than to start from scratch as would be the standard approach. They suggest that their methodology brings together synthetic ingenuity and the robust chemical machinery that has emerged through millions of years of evolution.

The light stuff

Nature has evolved many different systems to capture the sun’s energy, which usually exploit highly coloured pigments. The one with which many of us are most familiar, is the green pigment chlorophyll found in plants, which absorbs sunlight in the violet and red part of the visible spectrum and passes it on to the plant's sugar-making system in photosynthesis. Photosynthesis seems very efficient, but it does ignore the centre of the visible spectrum and beyond. "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," says Sheffield's Neil Hunter.

Team member Jonathan Lindsey of NCSU designed and synthesized pigments that can absorb at wavelengths that will fill some of the holes in the absorption of natural systems. "It can't be done from first principles," Lindsey explains, "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." The team is able to fix more than one synthetic or natural pigment at a time to the protein scaffolding. "The prototypes in the Chemical Science paper both have two but ultimately we'd like to add three or four or even more, he adds. One of our goals is to understand to what extent the protein can be derivatized with pigments."

From Hunter's perspective, "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," he says. "The energy cascades down like a waterfall. 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 colour.

Semi-synthetic

The researchers describe their approach to discovering and constructing novel light harvesters as semi-synthesis. They take naturally occurring materials and combine them with synthetic ones to make something that does not exist in nature. "By taking lots of material from nature we can make molecules that are architecturally more complex than those we can make from scratch," Lindsey says. Once assembled, the antenna molecules are tested using ultra-fast laser spectroscopy and other techniques to excite each pigment molecule and to trace the energy transfer from one pigment to the next and on to the target bacteriochlorophyll.

The next step would be to hook the optimal antennae to a second unit, a reaction centre, to act as a miniature power converter. Such a device would be modular and scalable and so could be used to tap the sun's energy and split water to release hydrogen gas for fuel, or to generate electricity directly.

Related Links

Chem Sci 2013, 4, 3924-3933: "Integration of multiple chromophores with native photosynthetic antennas to enhance solar energy capture and delivery"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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