PARENT SESSION

Symposium S8D Artificial Photosynthesis and Biological Hydrogen Production
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 510A
Chair: Tom Moore
Co-Chair: Leif Hammarström

Porphyrins and chlorins tailored for self-assembled artificial antennas. Teodor Silviu Balaban^{*,1, 2}, ¹ Forschungszentrum Karlsruhe, Karlsruhe, Germany² Center for Functional Nanostructures, Karlsruhe, Germany

ABSTRACT- Light-harvesting occurs in nature with stunning efficiency, even under low light illumination conditions. Implementation of the architectural principles of natural antenna systems to synthetic chromophores is a goal of the still distant artificial photosynthesis, which might help improve the rather modest performance of hybrid solar cells. These are organic-inorganic hybrids which convert directly light into current, while in natural photosynthesis light is used to synthesize ATP and reductant and thus store its energy as a biochemical potential. All photosynthetic organisms use very similar reactions centers which after receiving the excitation energy from an antenna, suffer charge separation which is stabilized by several subsequent electron transfer steps. This fact has been carefully conserved by evolution. On the contrary, there is great diversity in the architecture of antenna systems which has allowed adaptation of photosynthetic organisms to different habitats. There are two different organization principles for natural antennas: chromophore-protein complexes and self-assembled chromophores. While the protein complexes are much harder to engineer synthetically, we have been successful in synthesizing chromophores with extended pi-systems such as porphyrins and chlorins which have the same groups that are responsible for self-assembly as in green photosynthetic bacteria which form chlorosomes (green sacs). As in the natural system, there is efficient energy transfer within the supramolecular assemblies, which also show strong fluorescence, a fact which is quite untypical for aggregated chromophores. This property, which lends hope for possible applications, is due to the very orderly manner in which the pigment molecules are organized. We will present new and detailed structural data on these supramolecular assemblies which include a synchrotron single crystal structure determination, scanning probe and transmission electron microscopy images as well as low angle X-ray scattering spectra. From these studies, we could resolve the way in which self-assembly in these artificial antennas occurs. Because the same functional groups are involved, this finding is relevant also for the long standing problem of the structure of the chlorosomal antennas.

KEY WORDS: chlorosome, self-assembly, artificial antenna, bacteriochlorophyll