Terahertz EPR spectroscopy using a 36-tesla high-homogeneity series-connected hybrid magnet

Thierry Dubroca; Xiaoling Wang; Frédéric Mentink-Vigier; Bianca Trociewitz; Matthieu Starck; David Parker; Mark S Sherwin; Stephen Hill; J Krzystek

doi:10.1016/j.jmr.2023.107480

Terahertz EPR spectroscopy using a 36-tesla high-homogeneity series-connected hybrid magnet

J Magn Reson. 2023 Aug:353:107480. doi: 10.1016/j.jmr.2023.107480. Epub 2023 May 19.

Authors

Thierry Dubroca¹, Xiaoling Wang², Frédéric Mentink-Vigier³, Bianca Trociewitz³, Matthieu Starck⁴, David Parker⁴, Mark S Sherwin⁵, Stephen Hill⁶, J Krzystek⁷

Affiliations

¹ National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA. Electronic address: dubroca@magnet.fsu.edu.
² National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA; Center for Molecular Magnetic Quantum Materials, University of Florida, Gainesville, FL 32611, USA.
³ National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA.
⁴ Department of Chemistry, University of Durham, Durham DH13LE, UK.
⁵ Department of Physics, University of California Santa Barbara, CA 93106, USA.
⁶ National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA; Center for Molecular Magnetic Quantum Materials, University of Florida, Gainesville, FL 32611, USA; Department of Physics, Florida State University, Tallahassee FL 32306, USA.
⁷ National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA. Electronic address: krzystek@magnet.fsu.edu.

PMID: 37331305
DOI: 10.1016/j.jmr.2023.107480

Abstract

Electron Paramagnetic Resonance (EPR) is a powerful technique to study materials and biological samples on an atomic scale. High-field EPR in particular enables extracting very small g-anisotropies in organic radicals and half-filled 3d and 4f metal ions such as Mn^II (3d⁵) or Gd^III (4f⁷), and resolving EPR signals from unpaired spins with very close g-values, both of which provide high-resolution details of the local atomic environment. Before the recent commissioning of the high-homogeneity Series Connected Hybrid magnet (SCH, superconducting + resistive) at the National High Magnetic Field Laboratory (NHMFL), the highest-field, high-resolution EPR spectrometer available was limited to 25 T using a purely resistive "Keck" magnet at the NHMFL. Herein, we report the first EPR experiments performed using the SCH magnet capable of reaching the field of 36 T, corresponding to an EPR frequency of 1 THz for g = 2. The magnet's intrinsic homogeneity (25 ppm, that is 0.9 mT at 36 T over 1 cm diameter, 1 cm length cylinder) was previously established by NMR. We characterized the magnet's temporal stability (5 ppm, which is 0.2 mT at 36 T over one-minute, the typical acquisition time) using 2,2-diphenyl-1-picrylhydrazyl (DPPH). This high resolution enables resolving the weak g-anisotropy of 1,3-bis(diphenylene)-2-phenylallyl (BDPA), Δg = 2.5 × 10^-4 obtained from measurements at 932 GHz and 33 T. Subsequently, we recorded EPR spectra at multiple frequencies for two Gd^III complexes with potential applications as spin labels. We demonstrated a significant reduction in line broadening in Gd[DTPA], attributed to second order zero field splitting, and a resolution enhancement of g-tensor anisotropy for Gd[sTPATCN]-SL.

Keywords: BDPA; EPR; Electron Paramagnetic Resonance; Gadolinium spin labels; High-field EPR; Organic radical; Terahertz EPR; g-tensor.