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

用飞秒荧光上转换法研究了直接连接的锌卟啉-酰亚胺(ZP-Im)二元化合物S_2激发态的光致电子转移过程及其溶剂效应。随着酰亚胺种类的增加，S_2的荧光寿命从~130fs变化到~800fs。由S_2荧光寿命计算的电荷分离速率常数(k&lt；CS&gt；)随能隙的变化在低极性溶剂(甲基环己烷和甲苯)中表现出明显的“钟形”。为了研究溶剂粘度效应，测量了S 2在聚苯乙烯薄膜中的荧光动力学。K&lt；CS&gt；与在其他溶剂(如甲苯)中的能隙关系基本相同，表明即使在限制溶剂的动态运动自由的情况下，也可以发生S_2态的超快电子转移。在乙醇(乙醇和正丁醇)溶液中，ZIP-Im与溶剂之间存在氢键作用，在正常范围内，k&lt；CS&gt；较大。我们还测量了S_1在S_2态激发下的荧光动力学。在四氢呋喃中，S_1态转换的寿命(S_1荧光上升)对应于S_2的荧光衰变，但在甲苯和甲基环己烷中，其寿命比顶区的S_2荧光衰变长约150fs。这些结果表明，S_1态的转换主要是由电荷分离态的反向电子转移反应引起的，且反向电子转移速率小于S_2态的电荷分离速率。本文还研究了直接连接的ZP-苯二酚二联体的S_2荧光动力学，并观察到寿命约为130fs的超快电子转移。

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.