Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy
Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy
复制标题
质子从光酸转移到水线：第一原理模拟和快速荧光光谱
DOI：
10.1021/acs.jpcb.1c07254
复制
发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Martínez, Todd J.
中科院分区：
文献类型：
--
作者：
Walker, Alice R.;Wu, Boning;Meisner, Jan;Fayer, Michael D.;Martínez, Todd J.
质子转移反应在化学中普遍存在，尤其是在水溶液中。我们用快速荧光光谱和从头算分子动力学模拟方法研究了光酸8-羟基-1，3，6-三磺酸(HPTS)与水之间的光致质子转移。光激发导致HPTS羟基迅速释放质子。以前在HPTS/水上的实验用双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演化成一种“缔合”状态，在这种状态下，质子在HPTS羟基和最初的氢键水之间“共享”。较长的时间常数被解释为表明演化到“溶剂分离”状态，在这种状态下，共享的质子经历了长距离扩散。在这项工作中，我们用非常纯的HPTS改进了以前的实验结果。然后，我们用激发态从头算分子动力学解释了HPTS中水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周围形成了一个强大的氢键网络(“水线”)。然后，HPTS在≤3PS中去质子化，导致质子沿导线来回迁移，然后定位在单个水分子上。我们发现发射波长与质子-HPTS距离在模拟的时间尺度上呈近似线性关系，表明发射波长可以作为质子距离的标尺。我们的模拟表明，“缔合”态对应于一根带有可移动质子的水线，质子从这根水线扩散到广义的“溶剂分离”态对应于最长的实验时间常数。
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.