Long-lived charge-separated state generated in a ferrocene-meso,meso-linked porphyrin trimer-fullerene pentad with a high quantum yield

A meso,meso-linked porphyrin trimer, (ZnP)(3), as a light-harvesting chromophore, has been incorporated for the first time into a photosynthetic multistep electron-transfer model including ferrocene (Fc) as an electron donor and fullerene (C-60) as an electron acceptor, to construct the ferrocene-meso,meso-linked porphyrin trimer-fullerene system Fc-(ZnP)(3)-C-60. Photoirradiation of Fc-(Znp)(3)-C-60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, (1)(ZnP)(3)* and (3)(ZnP)(3)*, to the C-60 moiety to produce the porphyrin trimer radical cation-C-60 radical anion pair, Fc-(ZnP)(3)(.+)-C-60(.-), Subsequent formation of the final charge-separated state Fc(+)(ZnP)(3)-C-60(.-) was confirmed by the transient absorption spectra observed by pico- and nanosecond time-resolved laser flash photolysis. The final charge-separated state decays, obeying first-order kinetics, with a long lifetime (0.53 s in DMF at 163 K) that is comparable with that of the natural bacterial photosynthetic reaction center. More importantly, the quantum yield of formation of the final charge-separated state (0.83 in benzonitrile) remains high, despite the large separation distance between the Fc(+) and C-60(.-) moieties. Such a high quantum yield results from efficient charge separation through the porphyrin trimer, whereas a slow charge recombination is associated with the localized porphyrin radical cation in the porphyrin trimer. The light-harvesting efficiency in the visible region has also been much improved in Fc-(ZnP)(3)-C-60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.