Carmieli Raanan, Mi Qixi, Ricks Annie Butler, Giacobbe Emilie M, Mickley Sarah M, Wasielewski Michael R
Department of Chemistry and Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Am Chem Soc. 2009 Jun 24;131(24):8372-3. doi: 10.1021/ja902864h.
The distance over which two photogenerated charges are separated in electron donor-acceptor systems for artificial photosynthesis depends on the structure of the system, while the lifetime of the charge separation and, ultimately, its ability to carry out useful redox chemistry depend on the electronic coupling between the oxidized donor and reduced acceptor. The radical ions produced by charge separation are frequently delocalized over the pi systems of the final oxidized donor and reduced acceptor, so that there is often significant uncertainty as to the average distance between the separated charges, especially in low dielectric constant media, where the Coulomb attraction of the ions may be significant and the charge distribution of the ions may be distorted, so that the average distance between them may be shorter than that implied by their chemical structures. The charge separation distances between photogenerated radical ions in three donor-acceptor molecules having different donor-acceptor distances were measured directly from their dipolar spin-spin interactions using out-of-phase electron spin echo envelope modulation (OOP-ESEEM). The measured distances in toluene at 85 K compare favorably to the calculated distances between the centroids of the spin distributions of the radical ions within the radical ion pairs. These results show that despite the intrinsically nonpolar nature of medium, the spin (and charge) distributions of the RPs are not significantly distorted by Coulomb attraction over these long distances. This study shows that OOP-ESEEM is well-suited for probing the detailed structural features of charge-separated intermediates that are essential to understanding how to design molecular structures that prolong and control charge separation for artificial photosynthesis.
在用于人工光合作用的电子供体-受体体系中，两个光生电荷分离的距离取决于体系结构，而电荷分离的寿命以及最终其进行有用氧化还原化学过程的能力则取决于氧化供体与还原受体之间的电子耦合。电荷分离产生的自由基离子常常离域于最终氧化供体和还原受体的π体系上，因此对于分离电荷之间的平均距离往往存在很大的不确定性，尤其是在低介电常数介质中，离子间的库仑吸引力可能很大，离子的电荷分布可能会发生畸变，使得它们之间的平均距离可能比其化学结构所暗示的距离更短。利用异相电子自旋回波包络调制（OOP-ESEEM），通过三种具有不同供体-受体距离的供体-受体分子中光生自由基离子之间的偶极自旋-自旋相互作用，直接测量了电荷分离距离。在85K的甲苯中测得的距离与自由基离子对中自由基离子自旋分布质心之间的计算距离相当。这些结果表明，尽管介质具有固有的非极性性质，但在这些长距离上，反应对的自旋（和电荷）分布并未因库仑吸引力而显著畸变。这项研究表明，OOP-ESEEM非常适合探测电荷分离中间体的详细结构特征，而这些特征对于理解如何设计分子结构以延长和控制人工光合作用中的电荷分离至关重要。
