Zhou Panwang, Han Keli
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics , Chinese Academy of Science , Dalian 116023 , Liaoning , China.
Acc Chem Res. 2018 Jul 17;51(7):1681-1690. doi: 10.1021/acs.accounts.8b00172. Epub 2018 Jun 15.
As one of the most fundamental processes, excited-state proton transfer (ESPT) plays a major role in both chemical and biological systems. In the past several decades, experimental and theoretical studies on ESPT systems have attracted considerable attention because of their tremendous potential in fluorescent probes, biological imaging, white-light-emitting materials, and organic optoelectronic materials. ESPT is related to fluorescence properties and usually occurs on an ultrafast time scale at or below 100 fs. Consequently, steady-state and femtosecond time-resolved absorption, fluorescence, and vibrational spectra have been used to explore the mechanism of ESPT. However, based on previous experimental studies, direct information, such as transition state geometries, energy barrier, and potential energy surface (PES) of the ESPT reaction, is difficult to obtain. These data are important for unravelling the detailed mechanism of ESPT reaction and can be obtained from state-of-the-art ab initio excited-state calculations. In recent years, an increasing number of experimental and theoretical studies on the detailed mechanism of ESPT systems have led to tremendous progress. This Account presents the recent advances in theoretical studies, mainly those from our group. We focus on the cases where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of excited-state double proton transfer (ESDPT), revealing the sensing mechanism of ESPT chemosensors, illustrating the effect of intermolecular hydrogen bonding on the excited-state intramolecular proton transfer (ESIPT) reaction, investigating the fluorescence quenching mechanism of ESPT systems by twisting process, and determining the size of the solute·(solvent) n cluster for the solvent-assisted ESPT reaction. Through calculation of vertical excitation energies, optimization of excited-state geometries, and construction of PES of the ESPT reactions, we provide modifications to experimentally proposed mechanisms or completely new mechanism. Our proposed new and inspirational mechanisms based on theoretical studies can successfully explain the previous experimental results; some of the mechanisms have been further confirmed by experimental studies and provided guidance for researchers to design new ESPT chemosensors. Determination of the energy barrier from an accurate PES is the key to explore the ESPT mechanism with theoretical methods. This approach becomes complicated when the charge transfer state is involved for time-dependent density functional theory (TDDFT) method and optimally tuned range-separated TDDFT provides an alternative way. To unveil the driving force of ESPT reaction, the excited-state molecular dynamics combined with the intrinsic reaction coordinate calculations can be employed. These advanced approaches should be used for further studies on ESPT systems.
作为最基本的过程之一，激发态质子转移（ESPT）在化学和生物系统中都起着重要作用。在过去几十年里，ESPT系统的实验和理论研究因其在荧光探针、生物成像、白色发光材料和有机光电子材料方面的巨大潜力而备受关注。ESPT与荧光性质相关，通常在100飞秒及以下的超快时间尺度上发生。因此，稳态和飞秒时间分辨的吸收、荧光和振动光谱已被用于探索ESPT的机制。然而，基于先前的实验研究，ESPT反应的直接信息，如过渡态几何结构、能垒和势能面（PES），很难获得。这些数据对于阐明ESPT反应的详细机制很重要，并且可以从最先进的从头算激发态计算中获得。近年来，对ESPT系统详细机制的实验和理论研究越来越多，取得了巨大进展。本综述介绍了理论研究的最新进展，主要是我们团队的研究成果。我们关注理论研究至关重要且不可或缺的情况，例如解决关于激发态双质子转移（ESDPT）的分步和协同机制的争论、揭示ESPT化学传感器的传感机制、说明分子间氢键对激发态分子内质子转移（ESIPT）反应的影响、研究通过扭曲过程使ESPT系统荧光猝灭的机制，以及确定溶剂辅助ESPT反应中溶质·(溶剂)n簇的大小。通过计算垂直激发能、优化激发态几何结构和构建ESPT反应的PES，我们对实验提出的机制进行了修正或提出了全新的机制。我们基于理论研究提出的新颖且富有启发性的机制能够成功解释先前的实验结果；其中一些机制已得到实验研究的进一步证实，并为研究人员设计新型ESPT化学传感器提供了指导。从精确的PES确定能垒是用理论方法探索ESPT机制的关键。当涉及电荷转移态时，对于含时密度泛函理论（TDDFT）方法来说，这种方法会变得复杂，而最优调谐范围分离TDDFT提供了一种替代方法。为了揭示ESPT反应的驱动力，可以采用激发态分子动力学结合内禀反应坐标计算的方法。这些先进方法应被用于对ESPT系统的进一步研究。
