The photoredox activity of well-known Ru^II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr^III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr^III complex is very similar to that for a Ru^II polypyridine, and thereby validate the concept of estimating the redox potential of d³ spin-flip excited states in analogous manner as for the MLCT states of d⁶ compounds. Building on this insight, we use our Cr^III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

中文翻译：

---

揭示 CrIII 多吡啶配合物中自旋翻转激发态的氧化性质及其在光氧化还原催化中的应用

众所周知的 Ru ^II配合物的光氧化还原活性源于金属到配体电荷转移 (MLCT) 激发态，其中基于配体的电子可以引发化学还原，而以金属为中心的空穴可以引发氧化。Cr ^III多吡啶显示出相似的光氧化还原特性，尽管它们具有根本不同的电子结构。它们的光活性激发态具有自旋翻转性质，与电子基态的区别仅在于一个电子自旋的变化，但在其他方面具有相同的 d 轨道占有率。我们发现从 10 个不同的供体到 Cr ^{III配合物的自旋翻转激发态的光诱导电子转移的驱动力依赖性与 Ru II}的驱动力依赖性非常相似聚吡啶，从而验证以与d ⁶化合物的MLCT状态类似的方式估计d ^{3自旋-翻转激发态的氧化还原电位的概念。}基于这一认识，我们将我们的 Cr ^III配合物用于以前未使用该化合物类别探索过的光催化反应，包括甲氧基芳基的需氧溴化、1,1,2,2-四苯基乙烯的氧化、芳基硼酸的需氧羟基化和乙烯基化N-苯基吡咯烷。与已充分探索的贵金属基光催化剂相比，这项工作有助于理解第一行过渡金属配合物的基本光化学性质。