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 (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-C ₆₀ 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-C ₆₀^.- was produced from ¹H ₂P*-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 H ₂P^.+-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^.+-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. 4 μs in PhCN to 14-27 μs in o-DCB at room temperature. The longest lifetime (27 μs) 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.