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

光酸是一种分子体系，在电子激发下表现出显著的酸度增加，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，光酸度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能力来确定光酸-碱配合物在液体溶液中的氢键几何结构，以及这些氢键几何结构在光酸生色团电子激发下的变化。用飞秒红外光谱研究了萘酚的OH振动和氨基萘以及氨基比林的NH伸缩振动的响应。光酸-碱氢键和溶剂对观察到的电子基态和激发态频移的贡献将与量子化学计算进行比较。用瞬时二维红外光子回波光谱研究了这两种电子态的光酸碱配合物的氢键动力学。光酸行为被归因于分子内电荷转移。分子内电荷转移所起的关键作用将通过研究一类具有不同取代基的萘酚分子来研究，据悉这些取代基会影响分子内电荷转移能力。利用时间分辨X射线能谱研究了质子供体氨基的阳离子光酸的N K边，从而可以直接获得在紫外光激发下质子供体基团的电荷分布变化。时间分辨X射线吸收的结果将与阳离子光酸的NH伸缩模式的瞬时红外吸收相结合，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.