Biomimetic Antenna May Transform Solar Devices
ST. LOUIS, Dec. 5, 2011 — A newly developed light-harvesting antenna modeled on the chlorosome found in green bacteria could transform solar-powered devices and give silicon and printed dye photovoltaics a run for their money.
The invention of the solar cell in 1941 was inspired by a new understanding of semiconductors — materials that can use light energy to create mobile electrons and an electrical current. Although they both transform the energy from light, silicon solar cells have almost nothing to do with the biological photosystems in tree leaves and pond scum that use light energy to push electrons across a membrane — and ultimately create sugars and other organic molecules.
The photosystem in green bacteria consists of a light-harvesting antenna called a chlorosome and a reaction center. The energy of the light the pigments absorb is transferred to the reaction center (red) through a protein-pigment antenna complex called the baseplate (gold). The antenna (green) is made of rod-shaped aggregates of pigment molecules. (Images: Blakenship/WUSTL)
For roughly 70 years, no one has had the depth of understanding of these complex assemblages of proteins and pigments to exploit their secrets for the design of solar cells — until now.
At Washington University in St. Louis's Photosynthetic Antenna Research Center (PARC), scientists are exploring native biological photosystems, building hybrids that combine natural and synthetic parts, and building fully synthetic analogs of natural systems.
Chlorosomes are giant assemblies of pigment molecules. Perhaps nature's most spectacular light-harvesting antennae, they allow green bacteria to photosynthesize even in the dim light of the ocean depths.
Nature provides three starting points for the design of synthetic pigments: porphyrin, chlorin and bacteriochlorin. Each of these macrocyles has an alternating double-bond pathway (in blue) that gives the molecule its basic electronic properties, including the ability to absorb visible or near-infrared light. Hemoglobin is a porphyrin that lends blood its red color; chlorophyll, the pigment in green plants, is a chlorin; and the pigments in purple photosynthetic bacteria are bacteriochlorins. As the color-coded absorption spectra show, the three types of pigments absorb different colors of sunlight (brown).
Dewey Holten, professor of chemistry, and collaborator Christine Kirmaier, research professor of chemistry, are part of a team that is trying to make synthetic chlorosomes. Holten and Kirmaier use ultrafast laser spectroscopy and other analytic techniques to follow the rapid-fire energy transfers in photosynthesis.
The activity of an antenna consists of many pigment molecules absorbing photons and passing the excitation energy to the reaction centers.
In the reaction centers, the excitation energy sets off a chain of reactions that create ATP, a molecule often called the energy currency of the cell. Cellular organelles selectively break the bonds in ATP molecules when they need energy hits for cellular work.
Green bacteria, which live in the lower layers of ponds, lakes and marine environments, and in the surface layers of sediments, have evolved large and efficient light-harvesting antennae very different from those found in plants bathing in sunlight on Earth's surface.
The antennae consist of highly organized three-dimensional systems of as many as 250,000 pigment molecules that absorb light and funnel the light energy through a pigment/protein complex called a baseplate to a reaction center, where the energy triggers chemical reactions that ultimately produce ATP.
In plants and algae (and in the baseplate in the green bacteria), photo pigments are bound to protein scaffolds, which space and orient the pigment molecules in such a way that energy is efficiently transferred between them. But chlorosomes don't have a protein scaffold. Instead, the pigment molecules self-assemble into a structure that supports the rapid migration of excitation energy.
The absorption spectrum of a synthetic pigment in a polar solvent (magenta) that prevents the pigment molecules from forming assemblies differs substantially from the absorption spectrum of the pigment in a nonpolar solvent (blue). The difference shows that the pigments have the “hooks” they need to link up properly in solution.
This is intriguing because it suggests chlorosome mimics might be easier to incorporate into the design of solar devices than biomimetics that are made of proteins as well as pigments.
The PARC team’s goal was to see whether synthesized pigment molecules could be induced to self-assemble — even though the process by which the pigments align and bond is not well understood.
“The structure of the pigment assemblies in chlorosomes is the subject of intense debate, and there are several competing models for it,” said Holten. To design a pigment for a photosynthetic organism, a chemist first builds one of three molecular frameworks. All three are macrocycles, or giant rings: porphyrin, chlorin and bacteriochlorin. “One of the members of our team, Jon Lindsey, can synthesize analogs of all three pigment types from scratch,” Holten said.
The team wanted to study many variations of a pigment molecule to see what favored and what blocked assembly, and Lindsey had also developed the means to synthesize chlorins, the basis for the pigments found in the chlorosomes of green bacteria. The chlorins push the absorption to the red end of the visible spectrum, a regime where scientists would like to harvest for energy.
Doctoral student Olga Mass and co-workers in Lindsey’s lab synthesized 30 different chlorins, systematically adding or removing chemical groups thought to be important for self-assembly but also attaching peripheral chemical groups that take up space and might make it harder for the molecules to stack or that shift around the distributions of electrons so that the molecules might stack more easily.
The powdered pigments were shipped to Holten’s lab at Washington University and to David Bocian’s lab at the University of California, Riverside.
The two labs made up green-tinctured solutions of each of the 30 molecules in small testtubes and then poked and prodded the solutions by means of analytical techniques to see whether the pigment had aggregated and, if so, how much had formed the assemblies. Holten’s lab studied their absorption of light and their fluorescence — which indicated the presence of monomers, since assemblies don’t normally fluoresce; Bocian’s lab studied their vibrational properties, which are determined by the network of bonds in the molecule or pigment aggregate as a whole.
In one crucial test, Joseph Springer at Holten’s lab compared the absorption spectrum of a pigment in a polar solvent that would prevent it from self-assembling to the spectrum of the pigment in a nonpolar solvent that would allow the molecules to interact with one another and form assemblies.
“You can see them aggregate,” Springer said. “A pigment that is totally in solution is clear, but colored a brilliant green. When it aggregates, the solution becomes a duller green, and you can see tiny flecks in the liquid.”
The absorption spectra indicated that some pigments formed extensive assemblies and that the steric and electronic properties of the molecules predicted the degree to which they would assemble.
The PARC scientists have already taken the next step toward a practical solar device. Along with Pratim Biswas, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental and Chemical Engineering, the team has demonstrated getting the pigments to self-assemble on surfaces, which is the next step in using them to design solar devices, Holten said.
“We’re not trying to make a more efficient solar cell in the next six months. Our goal instead is to develop fundamental understanding so that we can enable the next generation of more efficient solar-powered devices,” Holten said.
The work was described in the New Journal of Chemistry.
For more information, visit: www.wustl.edu
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