Huang Xiang, Domcke Wolfgang
Department of Chemistry, Technical University of Munich, Garching D-85747, Germany.
J Phys Chem A. 2021 Nov 18;125(45):9917-9931. doi: 10.1021/acs.jpca.1c08291. Epub 2021 Nov 8.
In the past decade, polymeric carbon nitrides consisting of heptazine (Hz) building blocks emerged as highly promising materials for photocatalytic hydrogen evolution from water or sacrificial electron donors with near-ultraviolet light. However, the complexity of these materials and their poor characterization at the atomic level are detrimental to the unraveling of the detailed mechanisms of the reactions leading to hydrogen evolution. Recently, it has been shown that a derivative of the Hz molecule, trianisole-heptazine (TAHz), is a potent photobase, which readily oxidizes various derivatives of phenol and even water by an excited-state proton-coupled electron-transfer (PCET) reaction. Energy profiles along minimum-energy reaction paths and relaxed PCET potential-energy surfaces, which previously were computed with ab initio electronic-structure methods for complexes of Hz and TAHz with protic substrates, led to qualitative insights. To obtain more quantitative insight, reaction dynamics simulations are required. In the present work, the time scales of the electron and proton transfer processes and the branching ratios of competing channels were explored with ab initio on-the-fly quasiclassical surface-hopping trajectory simulations for the hydrogen-bonded Hz-HO complex. By the analysis of representative trajectories, detailed insight into the interplay of various nonadiabatic electronic transitions, electron transfer, proton transfer, and vibrational energy relaxation is obtained. The HzH radicals which are formed by the photoreduction of Hz can disproportionate to Hz and HzH in an exothermic reaction with a low reaction barrier. The time scales and branching ratios of competing channels in H-atom photodetachment from the HzH molecule also were explored with ab initio surface-hopping simulations. These results delineate for the first time a quantitatively supported scenario of water oxidation and hydrogen evolution with a molecular carbon nitride photocatalyst.
在过去十年中，由七嗪（Hz）结构单元组成的聚合碳氮化物成为了极具前景的材料，可用于在近紫外光照射下从水或牺牲电子供体中进行光催化析氢。然而，这些材料的复杂性以及它们在原子水平上的表征不佳，不利于揭示导致析氢反应的详细机制。最近，研究表明Hz分子的一种衍生物，三苯甲醚基七嗪（TAHz），是一种强效光碱，它能通过激发态质子耦合电子转移（PCET）反应轻易地氧化各种酚类衍生物甚至水。此前用从头算电子结构方法计算的Hz和TAHz与质子底物络合物沿着最小能量反应路径和弛豫PCET势能面的能量分布，带来了定性的见解。为了获得更定量的见解，需要进行反应动力学模拟。在本工作中，通过对氢键结合的Hz-HO络合物进行从头算实时准经典表面跳跃轨迹模拟，探索了电子和质子转移过程的时间尺度以及竞争通道的分支比。通过对代表性轨迹的分析，深入了解了各种非绝热电子跃迁、电子转移、质子转移和振动能量弛豫之间的相互作用。Hz光还原形成的HzH自由基可以在一个具有低反应势垒的放热反应中歧化为Hz和HzH。还通过从头算表面跳跃模拟探索了HzH分子氢原子光解吸中竞争通道的时间尺度和分支比。这些结果首次描绘了分子碳氮化物光催化剂水氧化和析氢的定量支持情景。
