Department of Chemistry, University of Rochester, Rochester, New York 14627, USA.
J Am Chem Soc. 2011 Apr 6;133(13):4791-801. doi: 10.1021/ja104536j. Epub 2011 Mar 8.
Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(•+) + B ⇌ A + B(•+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)) by neutral donors (P(+) + D → P(•) + D(•+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(•)/D(•+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(•-) + D(•+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(•-)/D(•+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(•-)/D(•+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(•-)/D(•+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A(•-)/D(•+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.
对于在乙腈中平衡的自由基阳离子（A(•+) + B ⇌ A + B(•+)）的放能和吸能电子转移反应的速率常数（k），可以通过简单的桑德罗斯-玻尔兹曼（SB）函数很好地拟合，该函数是反应自由能（ΔG）的函数，在放能区域具有一个限制速率常数 k(lim)的平台，随后，在接近热中性点附近，log k 与 ΔG 的斜率为 1/RT。另一种电荷转移反应，即激发吡喃阳离子（P(+)）的电子转移猝灭中性供体（P(+) + D → P(•) + D(•+)），也观察到了类似的行为。在这种情况下，当猝灭常数（log k(q)）的对数与 ΔG + s 作图时，观察到 SB 依赖性，其中位移项 s 等于+0.08 eV，ΔG 是净反应的自由能变化（E(redox) - E(excit)）。该位移项归因于产物遭遇对（P(•)/D(•+)）中自由基阳离子的部分去溶剂化，这使其自由能相对于游离物种升高。值得注意的是，使用激态氰基芳烃受体和芳烃供体对中性反应物（A* + D → A(•-) + D(•+)）的电子转移猝灭也被发现遵循 SB 依赖性，log k(q)与 ΔG 呈线性关系，s 为正，为+0.06 eV。与长期以来接受的预测相反，即 A(•-)/D(•+)遭遇对相对于自由基离子的自由能为负值，-0.06 eV。该预测仅包含相对于自由基离子的 A(•-)/D(•+)遭遇对的库仑稳定化。相比之下，这里呈现的结果表明，s 的正值表明 A(•-)/D(•+)遭遇对的溶剂稳定化降低，这超过了乙腈中的库仑稳定化。这些猝灭反应被提议通过快速互变的遭遇对进行，其中激基复合物为中间体，A*/D ⇌ 激基复合物 ⇌ A(•-)/D(•+)。在每种情况下都观察到弱激基复合物荧光。对于在吸能区域的几个反应，使用时间相关单光子计数法确定了激基复合物可逆形成和衰减的速率常数。从瞬态动力学得出的猝灭常数与从传统的 Stern-Volmer 图得出的猝灭常数吻合良好。对于激发态电子转移过程，当 ΔG 超过约+0.1 eV 时，在将猝灭常数与反应自由能相关联时需要谨慎。在这一点之后，可能会出现额外的激基复合物失活途径-荧光、系间窜越和非辐射衰减-从而导致机制发生变化。
