Weiss Emily A, Tauber Michael J, Kelley Richard F, Ahrens Michael J, Ratner Mark A, Wasielewski Michael R
Center for Nanofabrication and Molecular Self-Assembly and Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
J Am Chem Soc. 2005 Aug 24;127(33):11842-50. doi: 10.1021/ja052901j.
We observe well-defined regions of superexchange and thermally activated hopping in the temperature dependence of charge recombination (CR) in a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph(n)), n = 1-4, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI). A fit to the thermally activated CR rates of the n = 3 and n = 4 compounds yields activation barriers of 1290 and 2030 cm(-1), respectively, which match closely with theoretically predicted and experimentally observed barriers for the planarization of terphenyl and quaterphenyl. Negative activation of CR in the temperature regions dominated by superexchange charge transport is the result of a fast conformational equilibrium that increasingly depopulates the reactive state for CR as temperature is increased. The temperature dependence of the effective donor-acceptor superexchange coupling, V(DA), measured using magnetic field effects on the efficiency of the charge recombination process, shows that CR occurs out of the conformation with lower V(DA) via the energetically favored triplet pathway.
在一系列供体-桥-受体（D-B-A）体系中，我们观察到电荷复合（CR）的温度依赖性存在明确的超交换区域和热激活跳跃区域，其中D = 吩噻嗪（PTZ），B = 对亚苯基（Ph(n)），n = 1 - 4，且A = 苝-3,4:9,10-双（二甲酰亚胺）（PDI）。对n = 3和n = 4化合物的热激活CR速率进行拟合，分别得到1290和2030 cm⁻¹的活化能垒，这与理论预测以及实验观察到的三联苯和四联苯平面化的能垒紧密匹配。在由超交换电荷传输主导的温度区域中，CR的负活化是快速构象平衡的结果，随着温度升高，这种平衡使得CR的反应态分子数逐渐减少。利用磁场对电荷复合过程效率的影响测量得到的有效供体-受体超交换耦合V(DA)的温度依赖性表明，CR通过能量上有利的三重态途径发生于具有较低V(DA)的构象之外。
