A meso,meso-linked porphyrin dimer [(ZnP)2] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C60) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)2-C60). Photoirradiation of Fc-(ZnP)2-C60 results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [1(ZnP)2*] to the C60 moiety to produce the porphyrin dimer radical cation-C60 radical anion pair, Fc-(ZnP)2·+C60·-. In competition with the back electron transfer from C60·- to (ZnP)2·+ to the ground state, an electron transfer from Fc to (ZnP)2·+ occurs to give the final charge-separated (CS) state, that is, Fc+-(ZnP)2-C60·-, which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 μs in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)2·+-C60·-) and the final CS state (Fc+-(ZnP)2-C60·-). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes, that is, a rate-limiting electron transfer from (ZnP)2 to Fc+ to give the initial CS state (Fc-(ZnP)2·+- C60·-), followed by a fast electron transfer from C60·- to (ZnP)2+ to regenerate the ground state, Fc-(ZnP)2-C60. This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.
ASJC Scopus subject areas
- Colloid and Surface Chemistry