Efficient Multistep Photoinitiated Electron Transfer in a Molecular Pentad

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 metailated 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.