Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.
Institute of Experimental Physics, Faculty of Physics, University of Warsaw, ul. Pasteura 5, 02-093 Warsaw, Poland.
J Chem Phys. 2017 Jun 28;146(24):244505. doi: 10.1063/1.4990044.
The dynamics of unimolecular photo-triggered reactions can be strongly affected by the surrounding medium for which a large number of theoretical descriptions have been used in the past. An accurate description of these reactions requires knowing the potential energy surface and the friction felt by the reactants. Most of these theories start from the Langevin equation to derive the dynamics, but there are few examples comparing it with experiments. Here we explore the applicability of a Generalized Langevin Equation (GLE) with an arbitrary potential and a non-Markovian friction. To this end, we have performed broadband fluorescence measurements with sub-picosecond time resolution of a covalently linked organic electron donor-acceptor system in solvents of changing viscosity and dielectric permittivity. In order to establish the free energy surface (FES) of the reaction, we resort to stationary electronic spectroscopy. On the other hand, the dynamics of a non-reacting substance, Coumarin 153, provide the calibrating tool for the non-Markovian friction over the FES, which is assumed to be solute independent. A simpler and computationally faster approach uses the Generalized Smoluchowski Equation (GSE), which can be derived from the GLE for pure harmonic potentials. Both approaches reproduce the measurements in most of the solvents reasonably well. At long times, some differences arise from the errors inherited from the analysis of the stationary solvatochromism and at short times from the excess excitation energy. However, whenever the dynamics become slow, the GSE shows larger deviations than the GLE, the results of which always agree qualitatively with the measured dynamics, regardless of the solvent viscosity or dielectric properties. The method applied here can be used to predict the dynamics of any other reacting system, given the FES parameters and solvent dynamics are provided. Thus no fitting parameters enter the GLE simulations, within the applicability limits found for the model in this work.
单分子光引发反应的动力学可以受到周围介质的强烈影响，过去已经有大量的理论描述用于描述这种影响。准确描述这些反应需要知道反应的势能面和所受的摩擦力。这些理论大多从朗之万方程出发来推导动力学，但很少有例子将其与实验进行比较。在这里，我们探索了广义朗之万方程（GLE）在具有任意势和非马尔可夫摩擦的情况下的适用性。为此，我们在不同粘度和介电常数的溶剂中进行了具有皮秒级时间分辨率的共价连接有机电子给体-受体系统的宽带荧光测量。为了建立反应的自由能面（FES），我们求助于静态电子光谱。另一方面，非反应物质香豆素 153 的动力学为 FES 上的非马尔可夫摩擦提供了校准工具，假设其与溶质无关。一种更简单且计算速度更快的方法是使用广义斯莫卢霍夫斯基方程（GSE），它可以从纯调和势的 GLE 中推导出来。这两种方法在大多数溶剂中都能很好地重现测量结果。在长时间内，一些差异来自于对静态溶剂化变色的分析中的误差，而在短时间内则来自于多余的激发能。然而，只要动力学变得缓慢，GSE 就会显示出比 GLE 更大的偏差，而 GLE 的结果始终与测量的动力学定性一致，无论溶剂的粘度或介电性质如何。只要提供 FES 参数和溶剂动力学，这里应用的方法就可以用于预测任何其他反应系统的动力学。因此，在本工作中发现的模型的适用性范围内，GLE 模拟不包含拟合参数。
