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线态的质子解离。苯胺中引入邻烷基对质子解离速率有显著影响，苯胺的解离速率为1.4 × 10^<10>， 2-甲苯胺的解离速率为4.1 × 10^9s^<-1>, 2,6-二甲苯胺的解离速率为1.7 × 10^9s^<-1>。烷基化作用下质子解离速率的显著降低部分归因于反应放热性的改变。烷基化苯胺增加了质子离解反应的激活势垒，表明烷基对氨基附近水结构的疏水作用影响了质子向溶剂转移的速率。本文研究了在飞秒激光脉冲激发下，取代基对水溶液中间、对氰苯胺和间、对甲氧基苯胺质子解离的影响。与苯胺（k_<dis> = 1.3 x 10^<10>s^<-1>）相比，氰基取代显著提高了反应速率（k_<dis> = 3.7 x 10^<11>s^<-1>），甲氧基取代显著降低反应速率。对位上的两个取代基对反应速率的影响都不大。在自由能变化的基础上，合理地解释了k_<dis>值对取代基和取代位置的强依赖性。

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.