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

摘要

A meso,meso-linked porphy-rin 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 ferro-cene-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~(centre dot+)-C_(60)~(centre dot-).Subsequent formation of the final charge-separated state Fc~+-(ZnP)_3-C_(60)~(centre dot-) 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)~(centre dot-) 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 porphy-rins.