Department of Chemistry, University of Rochester, Rochester, New York 14627, USA.
J Am Chem Soc. 2011 Aug 3;133(30):11580-7. doi: 10.1021/ja2024367. Epub 2011 Jul 7.
In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.
在 40 多年前的一项具有里程碑意义的出版物中，Rehm 和 Weller（RW）表明，在乙腈中激发态分子的电子转移猝灭常数可以与电子供体和受体的激发态能量和氧化还原电位相关联。该相关性是根据在遭遇对（A*/D ⇌ A(•-)/D(•+)，其中 A 为受体，D 为供体）中分子之间的电子转移来解释的，并通过一个半经验公式表示，该公式将猝灭常数 k(q)与反应自由能ΔG 相关联。我们已经重新研究了许多在吸能或弱放能区域的 Rehm 和 Weller 反应的机制。我们发现它们不是简单的电子转移过程。相反，它们涉及激基复合物作为主要的、动力学上和光谱上可观察到的中间产物。因此，Rehm-Weller 公式基于错误的机制。我们已经重新测量了许多这些反应的 k(q)，并且还使用准确确定的氧化还原电位和修订的激发能重新评估了ΔG 值。我们发现 ΔG 和 k(q)都存在显著差异，包括在高吸能性下的 A*/D 对，它们没有表现出任何猝灭。修订后的数据符合 Sandros-Boltzmann（SB）方程 k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]。这种行为归因于遭遇对和激基复合物（A*/D ⇌ 激基复合物 ⇌ A(•-)/D(•+)）之间的快速相互转化。k(lim) 表示大约是扩散限制的速率常数，而 s 表示自由基离子遭遇对和自由基离子（A(•-)/D(•+)与 A(•-) + D(•+)）之间的自由能差。相对于整个反应的ΔG，s 的值为正，s = 0.06 eV，而不是 RW 假设的负 0.06 eV。s 的正值涉及到 A(•-)/D(•+)相对于自由 A(•-) + D(•+)的较差的溶剂化，这与 A(•-)/D(•+)的库仑稳定化相反。SB 方程不涉及遭遇对和激基复合物之间相互转化的微观速率常数。符合该方程的数据除了它们比遭遇对的离解（重新形成相应的自由物种 A* + D 或 A(•-) + D(•+)）更快之外，不包含有关此类速率常数的任何信息。所有当前的结论都与我们最近对芳香供体猝灭激发态氰基芳烃受体的研究结果一致，这两个数据集显示出 k(q)对ΔG 的依赖关系不可区分。
