Dimeric complexes composed of d⁸ square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir₂ (dimen)₄]²⁺ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d⁸–d⁸ metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.

中文翻译：

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利用超快 X 射线溶液散射表征变形异构化势和互变动力学


由 d ⁸ 方形平面金属中心和刚性桥联配体组成的二聚体复合物提供了模型系统来了解吸引力色散力和空间应变之间的相互作用，以协助开发更广泛地模拟金属二聚体复合物的可靠方法。 [Ir ₂ (dimen) ₄ ] ²⁺ (dimen = para-diisocyanomenthane) 为此类现象提供了一个独特的案例研究，因为最佳结构的扭曲具有有限构象灵活性的配体抵消来自金属和配体的吸引力色散力，产生具有两种基态变形异构体的复合物。在这里，我们使用超快 X 射线溶液散射 (XSS) 和光学瞬态吸收光谱 (OTAS) 来揭示平衡分布的性质和变形异构体之间的交换率。这两种基态异构体具有光谱上不同的电子激发，可以使用可见光的飞秒持续时间脉冲选择性激发一种异构体或另一种异构体。然后，我们使用超快 XSS 和 OTAS 跟踪电子基态布居（通常称为基态空穴）非平衡耗尽的动态，揭示了基态平衡在 2.3 ps 内的恢复。这项实验和理论相结合的研究为描述桥联 d ⁸ –d ⁸ 金属配合物中的各种密度泛函近似提供了关键测试。结果表明，如果添加色散相互作用并使用包括精确交换的混合泛函，密度泛函理论计算可以重现主要实验观察结果。