Kazmierczak Nathanael P, Xia Kay T, Sutcliffe Erica, Aalto Jonathan P, Hadt Ryan G
Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, United States.
J Am Chem Soc. 2025 Jan 22;147(3):2849-2859. doi: 10.1021/jacs.4c16571. Epub 2025 Jan 8.
Controlling the rate of electron spin relaxation in paramagnetic molecules is essential for contemporary applications in molecular magnetism and quantum information science. However, the physical mechanisms of spin relaxation remain incompletely understood, and new spectroscopic observables play an important role in evaluating spin dynamics mechanisms and structure-property relationships. Here, we use cryogenic magnetic circular dichroism (MCD) spectroscopy and pulse electron paramagnetic resonance (EPR) in tandem to examine the impact of ligand field (d-d) excited states on spin relaxation rates. We employ a broad scope of square-planar Cu(II) compounds with varying ligand field strength, including CuS, CuN, CuNO, and CuO first coordination spheres. An unexpectedly strong correlation exists between spin relaxation rates and the average d-d excitation energy ( = 0.97). The relaxation rate trends as the inverse 11th power of the excited-state energies, whereas simplified theoretical models predict only an inverse second power dependence. These experimental results directly implicate ligand field excited states as playing a critical role in the ground-state spin relaxation mechanism. Furthermore, ligand field strength is revealed to be a particularly powerful design principle for spin dynamics, enabling formation of a spectrochemical series for spin relaxation.
控制顺磁性分子中电子自旋弛豫速率对于分子磁学和量子信息科学的当代应用至关重要。然而，自旋弛豫的物理机制仍未完全理解，新的光谱可观测量在评估自旋动力学机制和结构 - 性质关系中起着重要作用。在这里，我们串联使用低温磁圆二色性（MCD）光谱和脉冲电子顺磁共振（EPR）来研究配体场（d-d）激发态对自旋弛豫速率的影响。我们采用了广泛的具有不同配体场强度的平面正方形Cu（II）化合物，包括CuS、CuN、CuNO和CuO第一配位球。自旋弛豫速率与平均d-d激发能（ = 0.97）之间存在意想不到的强相关性。弛豫速率趋势为激发态能量的倒数第十一次幂，而简化的理论模型仅预测倒数二次幂依赖性。这些实验结果直接表明配体场激发态在基态自旋弛豫机制中起着关键作用。此外，配体场强度被揭示为自旋动力学的一个特别强大的设计原则，能够形成自旋弛豫的光谱化学序列。
