Efficient Multistep Photoinitiated Electron Transfer in a Molecular Pentad

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Abstract
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 (P Zn ) and the other present as the free base (P). The metallated porphyrin bears a carotenoid polyene (C) and the other a diquinone species (Q A -Q B ). Excitation of the free-base porphyrin in a chloroform solution of the pentad yields an initial charge-separated state, C-P Zn -P ⋅+ -Q A ⋅- -Q B , with a quantum yield of 0.85. Subsequent electron transfer steps lead to a final charge-separated state, C ⋅+ -P Zn -P-Q A -Q B ⋅- , 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-Q A -Q B , gives a similar charge-separated state with a lower quantum yield (0.15 in dichloromethane), although the lifetime is increased to ∼340 microseconds. The artificial photosynthetic system preserves a significant fraction (∼1.0 electron volt) of the initial excitation energy (1.9 electron volts) in the long-lived, charge-separated state.