Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Phys Chem B. 2012 Feb 2;116(4):1370-84. doi: 10.1021/jp210892c. Epub 2012 Jan 23.
Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S(1) state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern-Volmer analyses of steady-state and lifetime data provide effective quenching rate constants k(q), which are often 10-100-fold larger than simple predictions for diffusion-limited rate constants k(D) in ionic liquids. Similar departures from k(D) are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of k(q) ≫ k(D). The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern-Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3-10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.
稳态和皮秒时间分辨发射光谱用于监测电子受体 9,10-二氰基蒽在其 S(1)态和给体 N,N-二甲基苯胺在各种离子液体和几种常规溶剂中的双分子电子转移反应。详细研究了这种猝灭反应，以便更好地理解为什么在离子液体中报道的类似扩散限制反应的速率有时比这些液体的粘性性质所预期的要高得多。与以前的研究一致，稳态和寿命数据的 Stern-Volmer 分析提供了有效的猝灭速率常数 k(q)，其通常比离子液体中扩散限制速率常数 k(D)的简单预测值大 10-100 倍。在具有相当高粘度的常规有机溶剂中也观察到类似的偏离 k(D)的情况，表明这种行为并非离子液体所特有。使用将球形对称扩散方程的近似解与电子转移的 Marcus 型描述相结合的模型对猝灭数据进行更完整的分析，揭示了频繁观察到 k(q)≫k(D)的原因。主要原因是离子液体的典型高粘度强调了扩散限制反应的瞬态成分，这使得从 Stern-Volmer 分析得出的速率常数的解释变得模糊。使用对猝灭过程的更合适描述，可以使用一组物理上合理的电子转移参数对离子液体和常规溶剂中的数据进行令人满意的拟合。这样做需要离子液体中的扩散系数通过显著因子超过流体力学预测，通常在 3-10 的范围内。溶质扩散的直接 NMR 测量证实了离子液体中的这种增强扩散。
