Ultrafast Electron Transfer Dynamics of Directly Linked Porphyrin-Electron Acceptor Dyads

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We have investigated the photoinduced electron transfer processes from S_2 excited states of directly linked Zinc porphyrin-imide (ZP-Im) dyads and its solvent effects by using femtosecond fluorescence up-conversion method with 〜120fs time resolution. The S_2 fluorescence lifetime changed from 〜130fs to 〜800fs with the species of imide. The energy gap dependence of the charge separation rate constants (k_<CS>) calculated form S_2 fluorescence lifetimes showed obvious "bell shapes" in low polar solvents (methylcyclohexane and toluene). S_2 fluorescence dynamics in polystyrene film were measured for the research of solvent viscosity effects. The energy gap dependence of k_<CS> was nearly the same with those in other solvents (like as toluene), which shows the ultrafast electron transfer from S_2 state can occurs even in the case that the freedom of the dynamic motion of the solvents around are restricted. In alcohol (ethanol and butanol) solutions, k_<CS> were larger than those in other solutions in the normal region, which caused by the hydrogen-bond interaction between ZIP-Im and the solvents. We have also measured the S_1 fluorescence dynamics with S_2 state excitation. In THF the lifetimes of S_1 state conversion (S_1 fluorescence rise) corresponded to the S_2 fluorescence decays, but in toluene and methylcyclohexane they were larger for 〜150fs than the S_2 fluorescence decays in top region. These results show that the S_1state conversion mainly caused by the back electron transfer reaction from the charge separation state and the rates of back electron transfer smaller than the charge separation rates from S_2 state. The S_2 fluorescence dynamics of directly linked ZP-quinone dyads were also investigated, and ultrafast electron transfer with 〜130fs lifetimes were also observed.