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Science 13 April 1990:
Vol. 248. no. 4952, pp. 199 - 201
DOI: 10.1126/science.248.4952.199
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Articles

Efficient Multistep Photoinitiated Electron Transfer in a Molecular Pentad

Devens Gust 1, Thomas A. Moore 1, Ana L. Moore 1, Seung-Joo Lee 1, Edith Bittersmann 1, David K. Luttrull 1, Aden A. Rehms 1, Janice M. DeGraziano 1, Xiaochun C. Ma 1, Feng Gao 1, Robert E. Belford 1, and Todd T. Trier 1

1 Department of Chemistry and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287

A synthetic five-part molecular device has been prepared that uses a multistep electron transfer strategy similar to that of photosynthetic organisms to capture light energy and convert it to chemical potential in the form of long-lived charge separation. It consists of two covalently linked porphyrin moieties, one containing a zinc ion (PZn) and the other present as the free base (P). The metailated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (QA-QB). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-PZn-P.+.-QA--QB, with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C.+-PZn-P-QA-QB.-, which is formed with an overall quantum yield of 0.83 and has a lifetime of 55 microseconds. Irradiation of the free-base form of the pentad, C-P-P-QA-QB, gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to sim340 microseconds. The artificial photosynthetic system preserves a significant fraction (sim1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state.


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