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 (H 2 P)-oligothiophene ( n T)-fullerene (C 60 ) linked triads (H 2 P- n T-C 60, n = 4, 8, and 12), which were designed to reveal the function of n T 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 H 2 P moiety in toluene, 1 H 2 P*- n T-C 60 showed predominantly the energy-transfer (EN) process generating H 2 P- n T- 1 C 60 *, whereas the CS process was not observed. The EN rate constant depends on the length of n T. In benzonitrile (PhCN) and o -dichlorobenzene ( o -DCB), H 2 P ·+ - n T-C 60 ·- was produced from 1 H 2 P*- n T-C 60 . The most energetically stable final CS state was confirmed to be H 2 P- n T ·+ -C 60 ·-, which was produced by the hole-shift process from H 2 P ·+ - n T-C 60 ·- . The rate constant for the CS process from 1 H 2 P*- n T-C 60 to H 2 P ·+ - n T-C 60 ·- decreased with the length of the n T moiety, indicating that the n T 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 n T moiety is feasible in polar solvents. The final CS state (H 2 P- n T ·+ -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 H 2 P-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.