Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethyl benzene
Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethyl benzene
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DOI：
10.1021/ja031649h
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发表时间：
2004-06-23
影响因子：
15
通讯作者：
Suenobu, T
中科院分区：
文献类型：
--
作者：
Fukuzumi, S;Yuasa, J;Suenobu, T
在乙腈中加入三氟化钪[Sc(OTf)(3)]可显著加速烷基苯等多种电子给体的光致电子转移，而不存在Sc(OTf)(3)时，烷基苯光诱导电子转移不发生。在Sc(OTf)(3)与Sc(OTf)(3)形成络合物的条件下，Sc(OTf)(3)促进的吖啶自由基阴离子与Sc(OTf)(3)形成络合物的光致电子转移反应(k(ET))的速率常数保持不变。与吖啶不同，Sc(OTf)(3)促进的光致电子转移的k(Et)值随Sc(OTf)(3)浓度的增加而增大，在低浓度时对[Sc(OTf)(3)]表现为一级依赖关系，在高浓度时转变为二级依赖关系。K(Et)对[Sc(OTf)(3)]的一级和二级依赖性归因于Py自由基与Sc(OTf)(3)形成1：1和1：2的络合物。采用光致电子转移反应的自由能关系式，确定了在Sc(OTf)(3)存在下，与不存在Sc(OTf)(3)时相比，有Sc(OTf)(3)存在时，单重态激发态与基态阴离子之间的单电子氧化还原电势的正移。在Sc(OTf)(3)的存在下，Sc(OTf)(3)促进了六甲苯的光诱导电子转移，使六甲苯有效地氧化生成五甲基苯甲醇，在Sc(OTf)(3)的存在下，六甲苯的O-2饱和乙腈溶液与Sc(OTf)(3)一起作为光催化剂进一步被氧化。通过ESR和激光闪光光解对反应中间产物的直接检测、荧光猝灭和量子产率的研究，提出了光催化氧化的机理。
Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)(3). The rate constants of the Sc(OTf)(3)-promoted photoinduced electron-transfer reactions (k(et)) of acridine to afford the complex between acridine radical anion and Sc(OTf)(3) remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)(3). In contrast to the case of acridine, the k(et) value of the Sc(OTf)(3)-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)(3) to exhibit first-order dependence on [Sc(OTf)(3)] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k(et) on [Sc(OTf)(3)] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)(3). The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)(3) as compared to those in the absence of Sc(OTf)(3) have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)(3)-promoted photoinduced electron transfer from hexamethyl benzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O-2-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)(3). The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.