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

用皮秒时间分辨荧光测量方法研究了质子化苯胺衍生物在水溶液中的最低激发单重态质子解离。在苯胺上引入邻位烷基会显著影响质子解离速率，苯胺为1.4 x <10>10^9s ^<-1>-1，2-甲苯胺为4.1 x 10^9s^<-1>-1，<-1>2，6-二甲基苯胺为1.7 x 10^9s^-1。烷基化使质子解离速率显著降低的部分原因是反应热力学性质的改变。在烷基化苯胺中，质子解离反应的活化能垒增加，研究了飞秒激光脉冲激发下质子化苯胺衍生物质子解离的取代基效应，并对质子化苯胺衍生物的质子解离进行了研究。水溶液中的甲氧基苯胺。与苯胺（k_ = 1.3 × 10 ~ 4·s ~（-1））相比，间位氰基取代显著提高了反应速率（k_<dis>= 3.7 × <11><-1><dis>10 ~ 4·<10>s ~（-1<-1>）），而甲氧基取代显著降低了反应速率。对位取代基对反应速率的影响很小。根据<dis>自由能变化合理地解释了k值对取代基和取代位置的强烈依赖性。

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.