Muon spin rotation (µSR) spectroscopy is a significant method that allows for the investigation of material behaviors at the atomic scale. This technique utilizes muons, which are subatomic particles that are similar to protons but lighter in weight. When these muons are introduced into a material, they engage with surrounding magnetic fields, yielding valuable insights into the material’s constitution and behavior, particularly for highly reactive species, such as radicals.
In a recent investigation conducted by a team led by Associate Professor Shigekazu Ito from the School of Materials and Chemical Technology at the Institute of Science in Tokyo, Japan, µSR spectroscopy was employed to study the regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1. This compound belongs to a category of phosphorus-related variants of common chemical structures. The µSR process starts with the creation of muonium (Mu), which forms when a positively charged muon (µ+) captures an electron (e–). Following this, the muonium reacts with the phosphorus-containing compound, leading to the generation of a muoniated radical at the phosphorus site. This selective addition takes place due to the phosphorus atom’s significant reactivity, a characteristic feature of polyaromatic hydrocarbons. Their discoveries were published online in Scientific Reports on January 7, 2025.
The research indicated that the muon primarily reacts with the phosphorus atom, producing a stable yet very reactive muoniated radical at this site, demonstrating the molecule’s elevated reactivity. The researchers used transverse-field µSR (TF-µSR) spectroscopy to analyze this interaction in depth, allowing them to investigate the magnetic surroundings of the radical directly. TF-µSR measurements revealed that even at low concentrations (0.060 M in tetrahydrofuran), the muoniation process was efficient and generated signals that were easily detectable.
“Utilizing µSR spectroscopy, we successfully examined the regioselective muoniation process, offering direct evidence of how reactive phosphorus is within this structure,” Ito stated. “The chance to explore this radical at low concentrations opens up new avenues for examining reactive species across various molecular frameworks.”
To further analyze the structure and stability of the muoniated radical, the research team employed density functional theory (DFT). The hyperfine parameters Aμ and A31P, derived from the DFT calculations, provided crucial information regarding electron structure and stabilization. The calculations indicated that the muoniated radical of 12-phosphatetraphene 1 is stabilized in a planar, π-delocalized arrangement due to the influence of the lowest available (zero-point) energy. This stabilization inhibits the emergence of a more thermodynamically favorable saddle-type tetracyclic skeleton.
Another key observation from the research involved how temperature affects the Aµ and A31P parameters. As temperature rose, both parameters showed a decrease, implying that the muoniated radical experienced structural stabilization. These conclusions were corroborated by µSR and muon (avoided) level-crossing resonance experiments, providing further insights into the dynamics and structural properties of the muoniated radical.
“This research offers critical insights into the dynamics and structural adjustments of the muoniated radical, which may impact future studies regarding radical behaviors and stabilization techniques,” Ito remarked. Addressing strain in the molecular framework enhances both stability and reactivity, positioning the material for practical applications, including electron-spin functional materials and nucleic acid regulation. These advancements could lead to greater reliability and open new doors for cutting-edge technologies and therapeutic solutions.
The regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1 is anticipated to influence sectors like material science and biology by facilitating the development of electron-spin functional materials and regulatory agents for nucleic acids, respectively. Overall, this study deepens our understanding of phosphorus-based radicals and showcases the adaptability of µSR spectroscopy in exploring reactive species at the atomic level.
