Department of Chemistry, Northwestern University, Evanston, IL, USA.
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL, USA.
Nature. 2023 Aug;620(7975):776-781. doi: 10.1038/s41586-023-06233-y. Epub 2023 Jul 19.
Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.
在分子和材料化学中，对具有不同自旋多重性的激发电子态之间的转变进行特定于设计的控制是至关重要的。先前的研究表明，自旋轨道和振子效应的组合，统称为自旋-振子效应，可以在非绝热交叉处加速量子力学上禁止的跃迁。然而，很难确定自旋-振子机制的确切实验表现。在这里，我们提出了相干光谱实验，揭示了驱动四个结构相关的双核 Pt(II) 金属-金属到配体电荷转移（MMLCT）配合物中有效单重态-三重态转换的自旋、电子和振动自由度之间的相互作用。光激发激活 Pt-Pt 键的形成，引发伸缩振动波包。这种分子结构依赖性的退相干和再相干动力学解析了自旋-振子机制。我们发现，沿着 Pt-Pt 伸缩坐标的矢量运动不可逆地调谐单重态和中间态的能隙，朝着锥形交叉点，并随后以棘轮式方式驱动最低稳定三重态的形成。这项工作证明了使用振子相干性作为探针来阐明自旋转换过程中自旋、电子和核动力学之间相互作用的可行性，这可能会激发新的模块化设计来调整激发态的性质。
