Picosecond Study on Proton-Dissociation Dynamics of Aromatic Amines in the Excited State

用皮秒时间分辨荧光法研究了水溶液中质子化苯胺衍生物最低激发单重态的质子解离。在苯胺中引入邻烷基团(S)对质子解离速率有显著影响，苯胺为1.4×10~(-1)，2-甲基苯胺为4.1×10~(-1)，2，6-二甲基苯胺为1.7×10~(-9)。烷基化反应质子解离速率的显著降低部分归因于反应放热的改变。烷基化苯胺中质子解离反应的活化势垒增大，表明烷基对氨基附近水结构的疏水作用影响质子转移到溶液中的速率。研究了取代基对间、对氰基苯胺、间、对甲氧基苯胺在水溶液中飞秒激光脉冲激发质子解离的影响。与苯胺(k_&lt；dis&gt；10&gt；S；11&gt；=3.7x10；-1&gt；)相比，间位氰基取代显著提高了产率(k_&lt；dis&gt；=3.7x10^；-1&gt；S)。对位上的两个取代基对反应速率的影响都很小。在自由能变化的基础上，合理地解释了取代基和取代位置对k&lt；dis&gt；值的强烈依赖性。

Proton dissociation from the lowest excited singlet state of protonated aniline derivatives in aqueous solution was studied by picosecond time-resolved fluorescence measurements. The proton dissociation rate was significantly influenced by introduction of an ortho-alkyl group(s) to aniline as 1.4 x 10^<10>s^<-1> for aniline, 4.1 x 10^9s^<-1> for 2-toluidine and 1.7 x 10^9s^<-1> for 2,6-xylidine. The remarkable decrease of the proton dissociation rate by alkylation was attributed partly to change in exothermicity of the reactions. The activation barrier for the proton-dissociation reaction was increased in the alkylated anilines, suggesting that the hydrophobic effects of the alkyl group on the water structure in the vicinity of the amino group influences the rate of proton transfer to solvent.Substituent effects on the proton dissociation of protonated aniline derivatives upon femtosecond laser pulse excitation were investigated for m-and p-cyanoanilines and m-and p-methoxyanilines in aqueous solution. The cyano substitution at the meta-position increased the rate significantly (k_<dis> = 3.7 x 10^<11>s^<-1>) compared to aniline (k_<dis> = 1.3 x 10^<10>s^<-1>), while the methoxy substitution reduced the rate remarkably. Either substituent at the para-position showed only slight influences on the rate. The strong dependences of the k_<dis> value on the substituent and the substituted position were reasonably explained on the basis of the free energy change.