标题： 利用飞秒时间分辨光谱研究氢键七嗪-水复合物的光化学成像：一项计算研究 显示英文标题

标题： Imaging the Photochemistry of the Hydrogen-Bonded Heptazine-Water Complex with Femtosecond Time-Resolved Spectroscopy: A Computational Study

摘要： 石墨相氮化碳（$g$-CN）作为一种有前景的廉价非金属光催化剂，在利用太阳光分解水方面引起了广泛关注。七嗪（Hz）分子是石墨相氮化碳的基本结构单元。在质子环境中，Hz 分子及其衍生物的光化学性质一直是近期若干实验和计算研究的主题。 在本工作中，采用氢键结合的 Hz$\cdots$H$_2$O 复合物作为模型体系，通过准经典非绝热轨迹模拟，探索该复合物中的光诱导电子和质子转移过程。所用方法包括$ab$ $initio$ ADC(2) 电子结构方法和一种计算效率高的表面跳跃算法。 Hz发色团光学激发的亮态$^1\pi\pi^*$通过三个$^1n\pi^*$态和一个低能电荷转移态弛豫，该电荷转移态驱动质子从 H$_2$O 转移到 Hz，进入 Hz 的长寿命光学暗态 S$_1$($\pi\pi^*$)。使用飞秒时间分辨瞬态吸收(TA)泵浦-探测(PP)光谱以及二维(2D)电子光谱(ES)，以准经典门控窗口近似框架计算探索了这一超快且复杂的动力学过程的成像。通过比较 Hz$\cdots$H$_2$O 复合物与游离 Hz 分子的光谱，可以在光谱信号中识别出氢键对超快内部转换动力学的影响。尽管 TA PP 和 2D ES 光谱主要对电子激发态动力学敏感，而对质子转移动力学敏感性较低，但它们仍可以提供机制上的见解，有助于加速水分解光催化剂的优化。 显示英文摘要

摘要： Graphitic carbon nitride ($g$-CN) has attracted vast interest as a promising inexpensive metal-free photocatalyst for water splitting with solar photons. The heptazine (Hz) molecule is the building block of graphitic carbon nitride. The photochemistry of the Hz molecule and derivatives thereof in protic environments has been the subject of several recent experimental and computational studies. In the present work, the hydrogen-bonded Hz$\cdots$H$_2$O complex was adopted as a model system for the exploration of photoinduced electron and proton transfer processes in this complex with quasi-classical nonadiabatic trajectory simulations, using the $ab$ $initio$ ADC(2) electronic-structure method and a computationally efficient surface-hopping algorithm. The population of the optically excited bright $^1\pi\pi^*$ state of the Hz chromophore relaxes through three $^1n\pi^*$ states and a low-lying charge-transfer state, which drives proton transfer from H$_2$O to Hz, to the long-lived optically dark S$_1$($\pi\pi^*$) state of Hz. The imaging of this ultrafast and complex dynamics with femtosecond time-resolved transient absorption (TA) pump-probe (PP) spectroscopy and two-dimensional (2D) electronic spectroscopy (ES) was computationally explored in the framework of the quasi-classical doorway-window approximation. By comparison of the spectra of the Hz$\cdots$H$_2$O complex with those of the free Hz molecule, the effects of the hydrogen bond on the ultrafast internal conversion dynamics can be identified in the spectroscopic signals. Albeit the TA PP and 2D ES spectroscopies are primarily sensitive to electronic excited-state dynamics and less so to proton transfer dynamics, they nevertheless can provide mechanistic insights which can contribute to the acceleration of the optimization of photocatalysts for water splitting.