Probing Ultrafast Structural Dynamics in Proton Transfer Reactions in Solution with Time-Resolved Infrared and X-Ray Spectroscopies

光酸是一种分子系统，在电子激发下表现出明显的酸度增加，使质子转移动力学能够使用时间分辨光谱进行实时研究。然而，光酸性的潜在机制仍然知之甚少。该项目的目标是结合时间分辨红外光谱和x射线光谱的结构分辨电位，确定液体溶液中光酸-碱配合物的氢键几何形状，以及这些氢键几何形状在光酸发色团的电子激发下的变化。利用飞秒红外光谱，我们将研究萘酚的OH振动和氨基萘以及氨基芘的NH拉伸振动的响应。光酸碱氢键和溶剂对电子基态和激发态的频移的贡献将与量子化学计算进行比较。利用瞬态二维红外光子回波光谱研究了光酸碱配合物在两种电子态下的氢键动力学。光酸行为归因于分子内电荷转移。分子内电荷转移的关键作用将通过研究具有不同取代基的萘酚分子家族来研究，这些取代基被理解为影响分子内电荷转移能力。时间分辨x射线光谱将用于研究质子给体氨基的阳离子光酸的N - k边，我们将直接获得质子给体氨基在紫外激发下的电荷分布变化。用时间分辨x射线吸收得到的结果将与阳离子光酸的NH拉伸模式的瞬态红外吸收相结合，从而将两种结构分辨技术联系起来。

Photoacids are molecular systems that exhibit a pronounced increase in acidity upon electronic excitation, enabling proton transfer dynamics to be studied in real time using time-resolved spectroscopy. The underlying mechanism for photoacidity is, however, still poorly understood. The goal of the project is to combine the structure resolving potential of time-resolved IR and x-ray spectroscopies to determine hydrogen bond geometries of photoacid-base complexes in liquid solution, and the changes of these hydrogen bond geometries upon electronic excitation of the photoacid chromophores. With femtosecond IR spectroscopy we will investigate the response of OH vibrations of naphthols and NH stretching vibrations of aminonaphthalenes, as well as aminopyrenes. Contributions of the photoacid-base hydrogen bond and of the solvent to the observed frequency shifts in both the electronic ground and excited states will be compared with quantum chemical calculations. Dynamics of hydrogen bonds of the photoacid-base complexes in both electronic states will be studied with transient two-dimensional IR photon echo spectroscopy. Photoacid behaviour has been ascribed to intramolecular charge transfer. The key role played by the intramolecular charge transfer will be studied by investigating a family of naphthol molecules with different substituents, understood to affect the intramolecular charge transfer capability. Time-resolved x-ray spectroscopy will applied to study the N K-edge of a cationic photoacids of the proton donating amino-group, with which we will have direct access to the electronic charge distribution changes of the proton donating group upon UV excitation. The results obtained with time-resolved x-ray absorption will be combined with transient IR absorbance of NH stretching modes of the cationic photoacid, thereby linking the two structurally resolving techniques.