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Photoinduced Electron Transfer in Porphyrin-Oligothiophene-Fullerene Linked Triads by Excitation of a Porphyrin Moiety

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journal contribution
posted on 29.07.2004, 00:00 by Takumi Nakamura, Mamoru Fujitsuka, Yasuyuki Araki, Osamu Ito, Junya Ikemoto, Kazuo Takimiya, Yoshio Aso, Tetsuo Otsubo
Photoinduced charge-separation (CS) and recombination (CR) processes of porphyrin (H2P)-oligothiophene (nT)-fullerene (C60) linked triads (H2P-nT-C60, 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, 1H2P*-nT-C60 showed predominantly the energy-transfer (EN) process generating H2P-nT-1C60*, 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-C60·- was produced from 1H2P*-nT-C60. The most energetically stable final CS state was confirmed to be H2P-nT·+-C60·-, which was produced by the hole-shift process from H2P·+-nT-C60·-. The rate constant for the CS process from 1H2P*-nT-C60 to H2P·+-nT-C60·- 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 Å-1 in PhCN and 0.11 Å-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·+-C60·-) 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 1.5−2.4 μs in PhCN to 14−27 μs in o-DCB at room temperature. The longest lifetime (27 μs) was observed with H2P-8T·+-C60·- 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.