Photoinduced electron transfer in porphyrin-oligothiophene-fullerene linked triads by excitation of a porphyrin moiety

Photoinduced charge-separation (CS) and recombination (CR) processes of porphyrin (H2P)-oligothiophene (nT)-fullerene (C-60) linked triads (H2P-nT-C-60, n = 4, 8, and 12), which were designed to reveal the function of nT as a molecular wire for the electron-transfer (ET) process, have been investigated by time-resolved fluorescence and absorption spectroscopic methods. After the excitation of the H2P moiety in toluene, (H2P)-H-1*-nT-C-60 showed predominantly the energy-transfer (EN) process generating H2P-nT-'C-60*, whereas the CS process was not observed. The EN rate constant depends on the length of nT. In benzonitrile (PhCN) and o-dichlorobenzene (o-DCB), H2P.+-nT-C-60- was produced from (H2P)-H-1*-nT-C-60. The most energetically stable final CS state was confirmed to be H2P-nT(.+)-C-60(.-), which was produced by the hole-shift process from (H2P)-H-1*-nT-C-60*_. The rate constant for the CS process from (H2P)-H-1*-nT-C-60 to H2P.+-nT-C-60(.-) decreased with the length of the nT moiety, indicating that the nT moiety acts as a molecular wire. The small damping factor showed a pronounced solvent polarity effect (0.03 Angstrom(-1) in PhCN and 0.11 Angstrom(-1) in o-DCB), indicating that a long-range ET process through the nT moiety is feasible in polar solvents. The final CS state (H2P-nT(.+)-C-60(.-)) returned to a neutral triad by the CR process between the vicinal electron (radical anion) and hole (radical cation). The lifetime of' the CS state showed the solvent polarity effect ranging from 1.5-2.4mus in PhCN to 14-27mus in o-DCB at room temperature. The longest lifetime (27,us) was observed with H2P-8T(.+)-C-60(.-) in o-DCB. From the temperature dependence of the CR rate constant, the reorganization energies were evaluated to be 0.8-1.1 eV for all triads in PhCN and o-DCB.