Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4′-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

中文翻译：

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通过时间分辨 X 射线吸收光谱法识别镍联吡啶光催化剂的激发态

使用镍配合物的光辅助催化是有机合成中交叉偶联反应的一个新兴领域。然而，光促进和增强这些复合物反应性的机制通常仍然难以捉摸。尽管光学技术已广泛用于研究光催化剂的基态和激发态，但它们缺乏询问特定原子的电子和结构变化的特异性。在此，我们报告了使用瞬态 Ni L 边缘和 K 边缘 X 射线吸收光谱对典型 Ni 光催化剂 (dtbbpy)Ni( o -tol)Cl (dtb = 4,4′-di- tert -丁基，bpy = 联吡啶，o -tol =邻甲苯基），在溶液中。我们通过直接实验证据明确证实，长寿命（∼5 ns）激发态是四面体金属中心三重态。这些结果证明了超快 X 射线光谱能够明确阐明重要的过渡金属光催化系统中激发态的性质。