Molecular Design of Heptazine-Based Photocatalysts: Effect of Substituents on Photocatalytic Efficiency and Photostability
Molecular Design of Heptazine-Based Photocatalysts: Effect of Substituents on Photocatalytic Efficiency and Photostability
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DOI：
10.1021/acs.jpca.0c00488
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发表时间：
2020-05-14
影响因子：
2.9
通讯作者：
Domcke, Wolfgang
中科院分区：
文献类型：
--
作者：
Ehrmaier, Johannes;Huang, Xiang;Domcke, Wolfgang
最近，一种七嗪(三三嗪)分子的衍生物被合成，并被证明在365 nm的LED光下催化水的氧化为羟基自由基(E.J.Rabe等，J.Phys)。化学。让我们来吧。2018，9,6257-6261)。利用精确定义的分子催化剂在纯净的溶剂中进行水光氧化的可能性为阐明水氧化光催化的基本反应机理开辟了新的视角。本文采用基于波函数的从头算电子结构计算方法，研究了水分子与水分子形成的氢键络合物及其衍生物(TAHZ、三氯HZ和三氰基HZ)的光催化活性，探讨了HH3个CH位上的化学取代基对光催化活性的影响。苯甲醚是给电子取代基，氯是弱吸电子取代基，CN是强吸电子取代基。结果表明，给电子(吸电子)取代基能提高(降低)水分子中H原子的光致吸收势垒。水氧化产生的高流动性和高活性的羟基自由基可以与还原的生色团自由基重新结合，生成光水合物。计算了取代基对光水化反应热力学的影响。在所研究的四个生色团中，TAHZ由于其光水合物的亚稳性而脱颖而出，这表明该光催化剂在被OH自由基氧化后能够自我修复。此外，还研究了取代基对H原子从还原生色团中的光分离反应的影响，从而结束了催化循环。给电子(拉电子)取代基降低(提高)了驱动光剥离反应的斥力(2)pi sigma*态的能量。四个生色团均呈现S-1/T-1间隙倒置现象。这一特征消除了长寿命的三重态，从而避免了分子氧活化为高活性的单线态氧。
Recently, a derivative of the heptazine (tris-triazine) molecule, trianisole-heptazine (TAHz), was synthesized and was shown to catalyze the oxidation of water to hydroxyl radicals under 365 nm LED light in a homogeneous reaction (E. J. Rabe et al., J. Phys. Chem. Lett. 2018, 9, 6257-6261). The possibility of water photo-oxidation with a precisely defined molecular catalyst in neat solvents opens new perspectives for clarifying the fundamental reaction mechanisms involved in water oxidation photocatalysis. In the present work, the effects of chemical substituents on the three CH positions of Hz on the photocatalytic reactivity were explored with wave function-based ab initio electronic-structure calculations for hydrogen-bonded complexes of Hz and three selected Hz derivatives (TAHz, trichloro-Hz, and tricyano-Hz) with a water molecule. While anisole is an electron-donating substituent, Cl is a weakly electron-withdrawing substituent and CN is a strongly electron-withdrawing substituent. It is shown that the barrier for the photoinduced abstraction of an H atom from the water molecule is raised (lowered) by electron-donating (electron-withdrawing) substituents. The highly mobile and reactive hydroxyl radicals generated by water oxidation can recombine with the reduced chromophore radicals to yield photohydrates. The effect of substituents on the thermodynamics of the photohydration reaction was computed. Among the four chromophores studied, TAHz stands out on account of the metastability of its photohydrate, which suggests self-healing of the photocatalyst after oxidation of TAHzH radicals by OH radicals. In addition, the effect of substituents on the H atom photodetachment reaction from the reduced chromophores, which closes the catalytic cycle, has been investigated. The energy of the repulsive (2)pi sigma* state, which drives the photodetachment reaction is lowered (raised) by electron-donating (electron withdrawing) substituents. All four chromophores exhibit inverted S-1/T-1 gaps. This feature eliminates long-lived triplet states and thus avoids the activation of molecular oxygen to highly reactive singlet oxygen.