PHOTOINDUCED STEPWISE CHARGE SEPARATION AND CHARGE RECOMBINATION IN PORPHYRIN-FULLERENE LINKED TRIADS AND TETRADS

摘要

A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C_(60) linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH_2-C_(60), ZnP-Im-C_(60) and ZnP-CH_2-Im-C_(60) as well as the reference dyads (ZnP-Im-CH_2-ref, ZnP-Im-ref and ZnP-CH_2-Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP~(.+))-imide radical anion (Im~(.-)) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP~(.+)-C_(60)~(.-) pair. The relatively strong electronic coupling without methylene linkage in ZnP-Im-C_(60) results in the preference of the tunneling superexchange ET over the sequential ET in the charge recombination (CR) process which requires the thermal activation to reach the higher energy state (i.e., ZnP~(.+)-Im~(.-)-C_(60)), whereas the sequential CR predominates in the triads with the methylene linkage. Meso-,meso-linked porphyrin dimer [(ZnP)_2] as a light-harvesting chromophore has been incorporated into a photosynthetic electron transfer model for the first time including ferrocene (Fc) as an electron donor and fullerene (C_(60)) as an electron acceptor to construct the ferrocene-meso, meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)~2-C_(60))- Photoirradiation of Fc-(ZnP)_2-C_(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [~1(ZnP)_2*] to the C_(60) moiety to produce the porphyrin dimer radical cation-C_(60) radical anion pair, Fc-(ZnP)_2~(.+)-C_(60)~(.-). In competition with the back electron transfer from C_(60)~(.-) to (ZnP)_2~(.+) to the ground state, an electron transfer from Fc to (ZnP)_2~(.+) occurs to give the final charge-separated (CS) state, i.e., Fc~+-(ZnP)2-C_(60)~(.-), which is detected as the transient absorption spectra by the laser flash photolysis. The back electron transfer to the ground state occurs via the reversed stepwise processes, i.e., a rate-limiting electron transfer from (ZnP)_2 to Fc~+ to give the initial CS state (Fc-(ZnP)_2~(.+)-C_(60)~(.-))> followed by a fast electron transfer from C_(60)~(.-) to (ZnP)_2~(.+) to regenerate the ground state, Fc-(ZnP)_2-C_(60). 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.