Long‐Lived Charge‐Separated State Generated in a Ferrocene–meso,meso‐Linked Porphyrin Trimer–Fullerene Pentad with a High Quantum Yield

Abstract

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‐C60. Photoirradiation of Fc‐(ZnP)3‐C60 results in photoinduced electron transfer from both the singlet and triplet excited states of the porphyrin trimer, 1(ZnP)${{\ast \hfill \atop 3\hfill}}$ and 3(ZnP)${{\ast \hfill \atop 3\hfill}}$, to the C 60 moiety to produce the porphyrin trimer radical cation–C 60 radical anion pair, Fc‐(ZnP)3.+‐C60.−. Subsequent formation of the final charge‐separated state Fc+‐(ZnP)3‐C60.− 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‐C60 because of exciton coupling in the porphyrin trimer as well as an increase in the number of porphyrins.