Using the basic chemistry of photosynthesis to harvest solar energy for use by humanity has long been a dream of photochemists. There are many approaches to achieving this goal. In our group, we use organic chromophores and electron and energy donors and acceptors which may be related to those found in natural reaction centers. These moieties are linked by covalent bonds that replace one role of the protein matrix by controlling donor-acceptor electronic coupling. Although such artificial reaction centers do not reproduce all aspects of the much more sophisticated natural reaction centers, they can approach the performance of the natural ones in terms of the quantum yield of charge separation, the fraction of photon energy conserved, and the lifetime of photoinduced charge separation.
The molecular pentad shown above is an example of an artificial photosynthetic reaction center. It consists of a porphyrin dyad (PA-PB) covalently linked to a carotenoid polyene (C) and a diquinone moiety (QA-QB). Excitation of the pentad with visible light is followed by photoinduced electron transfer from the C-P-1P-QA-QB singlet state to yield a charge-separated state C-P-P· + -QA· - -QB. This state preserves some of the light energy as chemical potential. Competing with charge recombination of this species are additional electron transfer reactions operating in series and in parallel which converge on a final C· + -PA-PB-QA-QB· - state. This final state preserves over one half of the excitation energy as chemical energy, is formed with a quantum yield of 0.83, and has a lifetime of hundreds of microseconds. The pentad is a molecular-scale photovoltaic that mimics the process by which photosynthetic organisms harvest sunlight and convert it to electrochemical potential.
For more information about pentads, see:
"Molecular Mimicry of Photosynthetic Energy and Electron Transfer," D. Gust, T. A. Moore and A. L. Moore, Accounts of Chemical Research, 26, 198 - 205 (1993).
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