A Spectrochemical Series for Electron Spin Relaxation
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1.
California Institute of Technology
Abstract
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 CuS4, CuN4, CuN2O2, and CuO4 first coordination spheres. An unexpectedly strong correlation exists between spin relaxation rates and the average d–d excitation energy (R2 = 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.
Copyright and License
© 2025 The Authors. Published by American Chemical Society. This publication is licensed under CC-BY 4.0 .
Acknowledgement
The authors wish to thank Dr. Paul H. Oyala for assistance with EPR spectroscopy, Dr. Mike Takase for performing the (PPN)2[Cu(ox)2] X-ray crystallography, the Arnold and Mabel Beckman Foundation for support of the Beckman Institute Laser Resource Center, and Prof. Brendon J. McNicholas for helpful discussions about MCD spectroscopy. N.P.K. acknowledges support by the Hertz Fellowship and the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1745301. J.P.A. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant No. 2139433. Financial support from the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Atomic, Molecular, and Optical Sciences program (DE-SC0022920) is gratefully acknowledged.
Contributions
N.P.K. and K.T.X. contributed equally.
Supplemental Material
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c16571.
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Synthesis and characterization methods, powder X-ray diffraction, X-ray crystallography, UV–vis spectroscopy, MCD methods, MCD analysis, EPR methods, and computational methods (PDF)
Data Availability
Deposition Number 2383406 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.
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Additional details
- Hertz Foundation
- National Science Foundation
- National Science Foundation Graduate Research Fellowship Program DGE-1745301
- National Science Foundation
- National Science Foundation Graduate Research Fellowship Program 2139433
- Office of Basic Energy Sciences
- Atomic, Molecular, and Optical Sciences DESC0022920
- Accepted
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2024-12-26Accepted
- Available
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2025-01-08Published online
- Available
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2025-01-22Published in issue
- Publication Status
- Published