A recent article published in Nature Communications revealed rapid changes in the nuclear structure and crystal field of a single-molecule magnet (SMM) post-photoexcitation.
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The researchers used time-resolved K-edge X-Ray absorption spectroscopy (TR-XAS) to directly track the changes in bond length around the metal ions within a manganese (Mn) III-based trinuclear SMM.
They also demonstrated the potential of TR-XAS for probing the interplay between electronic, vibrational, and spin degrees of freedom in molecular magnets.
Background
SMMs represent a unique class of magnetic materials characterized by their magnetic bistability and hysteresis at the molecular level. These properties arise from their large spin and uniaxial anisotropy, making them promising candidates for various applications, including high-density data storage, quantum computing, and spintronics.
However, conventional methods of manipulating magnetization in SMMs often rely on external magnetic fields or electric currents, which are slow and energy-consuming.
Recently, there has been a growing interest in exploring the feasibility of using light to regulate magnetization in SMMs. This interest arises from the ability of light to induce rapid and reversible changes in the electronic and nuclear structure of molecules.
About the Research
In this paper, the authors focused on the Mn(III)-based trinuclear SMM having the chemical formula [Mn(III)3O(Et-sao)3(β-pic)3(ClO4)], where β-pic and saoH2 are 3-methylpyridine and salicylaldoxime, respectively, and denoted as Mn3. This complex comprises three Mn(III) ions arranged in a triangular configuration, resulting in a ground state with a total spin (S) of six.
Due to the high-spin d4 electron configuration of the Mn(III) ions, the system exhibits a Jahn-Teller (JT) distortion, which induces a geometrical deformation, lowers symmetry, and stabilizes the structure. This JT distortion also determines magnetic anisotropy, representing the preference of the spin alignment along a specific axis.
In Mn3, the JT distortion is axially elongated, leading to a uniaxial anisotropy along the z-axis. The researchers hypothesized that by exciting an electron from the antibonding dz2 orbital to the dx2-y2 orbital, they could transiently switch the JT distortion from axial elongation to axial compression, thereby changing the magnetic anisotropy from an easy-axis to an easy-plane type.
To investigate this hypothesis, the study employed TR-XAS, a cutting-edge technique that utilizes ultrafast X-Ray pulses to probe changes in the electronic and nuclear structure following photoexcitation. TR-XAS offers elemental specificity, direct structural insight, and femtosecond time resolution, making it an ideal tool for studying dynamic processes in molecular systems.
The experimental setup involved using a 400 nm pump laser to excite the highest energy crystal-field transition in Mn3, coupled with a femtosecond X-Ray free-electron laser (XFEL) to measure the K-edge X-Ray absorption spectra of Mn3 before and after excitation.
These spectra capture transitions from the 1s core level to the unoccupied 3d and 4p orbitals of the metal ions, providing sensitivity to the coordination symmetry and ligand field.
Research Findings
After photoexcitation, the authors observed an increase in the intensity of the pre-edge peak at 6539 eV, corresponding to the 1s to 3d transition, and a slight redshift of the main edge peak at 6549 eV, corresponding to the 1s to 4p transition.
These changes indicated a decrease in coordination symmetry and a modification in the crystal field surrounding the Mn(III) ions. Coherent vibrational motion with a frequency of approximately 180 cm-1 was also detected, attributed to an in-phase oscillation of the three Mn(III) ions along their JT axes.
By comparing experimental data with simulations based on density functional theory and time-dependent density functional theory, the researchers deduced changes in bond lengths within the inner coordination sphere of Mn3 following photoexcitation.
They determined that axial Mn-oxygen (Mn-O) bond lengths increased by approximately 0.05 Å, while Mn-nitrogen (Mn-N) bond lengths decreased by 0.03 Å, indicating a shift of the Mn(III) ions towards the β-pic ligands along the JT axis.
However, the equatorial bonds exhibited minimal change, less than 0.01 Å, suggesting a restriction of in-plane motion. Ultimately, the authors concluded that the photoinduced dynamics of Mn3 are predominantly governed by a single JT mode that modulates nuclear structure and magnetic anisotropy.
Conclusion
This study showcased the effectiveness of TR-XAS in tracking nuclear motion within SMMs, providing valuable insights into their magnetic properties by revealing the complex interplay between electronic, vibrational, and spin degrees of freedom. It also emphasized the potential of utilizing light to manipulate magnetization in SMMs through transient changes in the crystal field.
The researchers suggested that enhancing the flexibility of SMMs to accommodate significant alterations in nuclear coordinates could pave the way for ultrafast photomagnetic switching, thus presenting exciting prospects for next-generation magnetic materials design.
Overall, TR-XAS analysis of Mn(III)-based trinuclear SMM post-photoexcitation revealed rapid structural and magnetic anisotropy changes, predominantly governed by a dominant JT mode, offering promising avenues for the development of adaptable SMMs for accelerated photomagnetic switching.
Journal Reference
Barlow, K., Phelps, R., Eng, J. et al. (2024). Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy. Nat Commun. doi.org/10.1038/s41467-024-48411-0
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