Slow Electron Spin Relaxation at Ambient Temperatures with Copper Coordinated by a Rigid Macrocyclic Ligand

Creators: Espinosa, Matthew R.¹; Guerrero, Fernando¹; Kazmierczak, Nathanael P.¹; Oyala, Paul H.¹; Hong, Alexandria¹; Hadt, Ryan G. (Corresponding)¹; Agapie, Theodor (Corresponding)¹

1. California Institute of Technology

Abstract

Paramagnetic transition metal complexes can serve as quantum bits, storing phase information through unpaired electrons. Despite their promise, these systems often require low temperatures and tend to rapidly decohere. Recent efforts have sought to improve longitudinal relaxation (T₁), which provides an upper limit for phase coherence (T_m), by investigating existing literature compounds with reduced vibrational coupling and orbital angular momentum. However, synthetic strategies for improving T₁ through novel ligand design have remained scant. Here, we disclose the synthesis of a new modular macrocyclic ligand framework with four nitrogen donors (N₄) derived from phenanthroline that supports room-temperature coherent Cu(II) spin centers. The optimized complex more than doubles the T₁ over the next best Cu(II)-N₄ compound and exhibits a room temperature coherence time (T_m) of 0.28 μs, close to previously reported values. This performance enhancement arises from a tight binding site with short Cu–N distances, resulting in a stronger ligand field and reduced thermal accessibility of symmetric vibrational modes. This work demonstrates a practical approach to enabling spin coherence at room temperature, a factor critical to accessing relevant quantum bits and biological sensors, through a designer macrocyclic ligand platform.

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Acknowledgement

This work is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Condensed Phase and Interfacial Molecular Science (DE-SC0022089). M.R.E. thanks the National Science Foundation MPS-Ascend Postdoctoral Research Fellowship for support under Grant No. 2316741. F.G. acknowledges support by the Resnick Sustainability Institute, the National Academy of Sciences Ford Fellowship, and the National Science foundation Graduate Research Fellowship under Grant No. 2139433. N.P.K. acknowledges support by the Hertz Fellowship and the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1745301. We thank the Beckman Institute and the Dow Next Generation Grant for instrumentation support. Michael Takase is thanked for assistance with crystallography. The computations presented here were conducted in the Resnick High Performance Computing Center, a facility supported by Resnick Sustainability Institute at the California Institute of Technology.

Contributions

M.R.E., F.G., and N.P.K. contributed equally to this study. All authors contributed to the formulation of this project and have given approval to the final version of the manuscript.

Supplemental Material

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c00803.

Experimental details, synthesis and characterization of compounds, crystallographic characterization, continuous wave and pulsed electron paramagnetic resonance data, computational work, nuclear magnetic resonance data, and UV–vis data (PDF)

Data Availability

Deposition Numbers 2414749–2414753 contain 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.

Attached Files

Article: espinosa-et-al-2025-slow-electron-spin-relaxation-at-ambient-temperatures-with-copper-coordinated-by-a-rigid.pdf
Supporting information: ja5c00803_si_001.pdf

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