Department of Chemistry, Stanford University, Stanford, California 94305, United States.
The PULSE Institute, Stanford University, Stanford, California 94305, United States.
J Phys Chem B. 2021 Nov 18;125(45):12539-12551. doi: 10.1021/acs.jpcb.1c07254. Epub 2021 Nov 7.
Proton transfer reactions are ubiquitous in chemistry, especially in aqueous solutions. We investigate photoinduced proton transfer between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and water using fast fluorescence spectroscopy and ab initio molecular dynamics simulations. Photoexcitation causes rapid proton release from the HPTS hydroxyl. Previous experiments on HPTS/water described the progress from photoexcitation to proton diffusion using kinetic equations with two time constants. The shortest time constant has been interpreted as protonated and photoexcited HPTS evolving into an "associated" state, where the proton is "shared" between the HPTS hydroxyl and an originally hydrogen bonded water. The longer time constant has been interpreted as indicating evolution to a "solvent separated" state where the shared proton undergoes long distance diffusion. In this work, we refine the previous experimental results using very pure HPTS. We then use excited state ab initio molecular dynamics to elucidate the detailed molecular mechanism of aqueous excited state proton transfer in HPTS. We find that the initial excitation results in rapid rearrangement of water, forming a strong hydrogen bonded network (a "water wire") around HPTS. HPTS then deprotonates in ≤3 ps, resulting in a proton that migrates back and forth along the wire before localizing on a single water molecule. We find a near linear relationship between the emission wavelength and proton-HPTS distance over the simulated time scale, suggesting that the emission wavelength can be used as a ruler for the proton distance. Our simulations reveal that the "associated" state corresponds to a water wire with a mobile proton and that the diffusion of the proton away from this water wire (to a generalized "solvent-separated" state) corresponds to the longest experimental time constant.
质子转移反应在化学中无处不在，特别是在水溶液中。我们使用快速荧光光谱法和从头算分子动力学模拟研究了 8-羟基芘-1,3,6-三磺酸（HPTS）和水之间的光诱导质子转移。光激发导致 HPTS 羟基迅速释放质子。以前关于 HPTS/水的实验使用具有两个时间常数的动力学方程描述了从光激发到质子扩散的过程。最短的时间常数被解释为质子化和光激发的 HPTS 演变成“缔合”状态，其中质子在 HPTS 羟基和最初氢键合的水分子之间“共享”。较长的时间常数被解释为表明质子经历长距离扩散到“溶剂分离”状态。在这项工作中，我们使用非常纯的 HPTS 改进了以前的实验结果。然后，我们使用激发态从头算分子动力学来阐明 HPTS 中水溶液激发态质子转移的详细分子机制。我们发现初始激发导致水的快速重排，在 HPTS 周围形成强氢键网络（“水线”）。HPTS 随后在≤3 ps 内去质子化，导致质子在沿该线来回迁移之前在单个水分子上局部化。我们发现发射波长和质子-HPTS 距离之间存在近线性关系，表明发射波长可作为质子距离的标尺。我们的模拟表明，“缔合”状态对应于具有可移动质子的水线，并且质子从该水线（到广义的“溶剂分离”状态）的扩散对应于最长的实验时间常数。
