Mixed N‐heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth‐abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal‐to‐ligand charge transfer state (3MLCT), which typically decays via a triplet metal‐centered (3MC) state back to the ground state. We reveal by variable‐temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans‐ and cis‐[Fe(pdmi)2]2+with four NHC donors show 3MLCT→3MC population transfers with very different barriers and rationalize this by computational means. While trans‐[Fe(pdmi)2]2+possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm–1, which leads to a nanosecond 3MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi‐classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

中文翻译：

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由于三重态-三重态势垒而具有长寿命三重态金属-配体电荷转移态的四碳烯铁(II)络合物


N-杂环卡宾（NHC）/吡啶基铁（II）混合配合物最近引起了广泛的关注，因为它们具有作为由地球丰富的元素制成的光催化剂和光敏剂的潜力。其成功实现的最具决定性的挑战是最低三重态金属到配体电荷转移态（3MLCT）的寿命，它通常通过三重态金属中心（3MC）态衰减回到基态。我们通过变温超快瞬态吸收光谱揭示，具有四个NHC供体的三脚铁(II)双(吡啶)络合物异构体反式和顺式-[Fe(pdmi)2]2+显示出具有非常不同的3MLCT→3MC群体转移障碍并通过计算手段使其合理化。虽然反式-[Fe(pdmi)2]2+ 具有不可观察到的激活势垒，但顺式异构体表现出 492 cm-1 的势垒，这导致 77 K 时的纳秒 3MLCT 寿命。动力学和量子化学数据在半经典马库斯理论的背景揭示了两个三重态之间的高重组能和小电子耦合。这凸显了详细的结构控制和动力学知识对于合理设计来自第一行过渡金属（例如铁）的光敏剂的重要性。