Excited-state proton transfer (ESPT): Investigations on the single-molecule level for separating solvation from intrinsic reaction dynamics

质子转移反应是最重要的化学反应之一。一个理论上和成功的方法，其描述是基于马库斯理论，这是众所周知的，其相关性的电子转移反应。与后一种反应类型相反，激发态的质子转移反应（ESPT）是已知的，其与明亮的荧光发射相关。在这种特殊情况下，光酸和共辄碱表现出双重发射，即两者都是具有不同颜色的荧光。在发射之后，质子被洗牌回到共轭碱，并且系统因此被重置在其初始状态。这些特性使得能够通过单分子荧光光谱法多次观察单个反应。我们最近合成和描述了各种光稳定的光酸，甚至在固体基质中进行ESPT。通过记录的光酸和共轭碱的荧光强度，以及在一个温度依赖性的方式ESPT的动力学，我们试图确定的活化能以及一个单一的基元反应的自由能。由于我们期望满足光酸/质子受体对的不同几何形状，这些实验将能够解决质子转移反应的距离依赖性。初步的反聚束实验表明，在电子基态的逆反应并不反映质子扩散的动力学，但允许研究个别光循环的动力学。或者，可以分辨光酸/质子受体/环境系统中的不同组合物，由此可以将溶剂化对反应动力学的贡献与固有贡献分开。我们的长期目标是将这些方法联合收割机结合起来，仅通过实验手段粗略地勾画出一个简单反应的势能面。

Proton-transfer reactions are amongst the most important chemical transformations. One theoretical and successful approach for its description is based on Marcus-theory which is known for its relevance in electron-transfer reactions. In contrast to this latter reaction type, proton-transfer reactions in the excited-state (ESPT) are known which are associated with bright fluorescence emission. In this special case, the photoacid and the conjugated base exhibit dual emission, i.e. both are fluorescent with different colors. After emission, the proton is shuffled back to the conjugated base and the system is consequently reset in its initial state. These properties enable the observation of an individual reaction multiple times by means of single-molecule fluorescence spectroscopy. We recently synthesized and described a variety of photostable photoacids which undergo ESPT even in a solid matrix. By recording the fluorescence intensities of both the photoacid and the conjugated base as well as the kinetics of ESPT in a temperature-dependent manner, we seek to determine the activation energy as well the free energy of a single elementary reaction. As we expect to meet varying geometries for the photoacid/proton-acceptor pair, these experiments will enable to address the distance dependence of proton-transfer reactions. Preliminary antibunching experiments show that the backreaction in the electronic ground state does not reflect the kinetics of proton diffusion, but allows for studying the kinetics of individual photocycles. Alternatively, varying compositions in the system photoacid/proton-acceptor/surrounding can be resolved from which the solvation contribution to the reaction dynamics can be separated from intrinsic contributions. Our long-term goal is to combine these approaches and to roughly sketch a potential-energy surface of a simple reaction only by experimental means.