Square-planar Ni^II complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic Pt^II luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of ³MLCT states in square-planar Ni^II complexes by creating coordination environments that seemed particularly well adapted to the 3d⁸ valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the Ni^II center from nucleophilic attack in the axial directions. UV–Vis transient absorption spectroscopies reveal that upon excitation into ¹MLCT absorption bands and ultrafast intersystem crossing to the ³MLCT excited state, the latter relaxes onward into a metal-centered triplet state (³MC). A torsional motion of the tridentate ligand and a Ni^II-carbon bond elongation facilitate ³MLCT relaxation to the ³MC state. The ³MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar Ni^II complexes with enhanced photophysical properties. The longest ³MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5–10 compared to previously investigated square-planar Ni^II complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

中文翻译：

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延长方形平面镍 (II) 配合物的金属-配体电荷转移激发态寿命的分子设计原则

方形平面 Ni ^II配合物及其电子激发态在交叉偶联催化中起着关键作用，可以提供新的机会来补充众所周知的等电子 Pt ^II发光团。金属-配体电荷转移 (MLCT) 激发态及其失活途径在这些情况下特别相关。我们试图通过创建似乎特别适合 3d ⁸的协调环境来延长方形平面 Ni ^{II复合物中3 个}MLCT 状态的寿命价电子构型。使用刚性三齿螯合配体（其中中心环金属化苯基单元两侧是两个配位的 N-杂环卡宾）以及单齿异氰化物配体，可产生非常强的配体场。异氰化物主链上的大取代基进一步保护 Ni ^II中心免受轴向亲核攻击。UV-Vis 瞬态吸收光谱表明，在激发到¹ MLCT 吸收带和超快系统间交叉到³ MLCT 激发态时，后者会松弛到以金属为中心的三重态 ( ³ MC)。三齿配体的扭转运动和 Ni ^II -碳键伸长促进³ MLCT 松弛到³ MC 状态。^{随着配体场强的增加和空间保护的改善， 3} MLCT的寿命变得更长，从而揭示了具有增强的光物理特性的方形平面 Ni ^{II配合物的明确设计指南。}室温下在溶液中达到的最长^{3 MLCT 寿命为 48 ps，与之前研究的方形平面 Ni II}配合物相比，长了 5-10 倍。我们的研究有助于使具有部分填充 d 轨道的第一行过渡金属配合物更适合光物理学和光化学中的应用。