New Insights on Radical Trapping in 12-Phosphatetraphene Uncovered!

Researchers utilize muon spin rotation (µSR) spectroscopy to uncover the unique behavior and structure of a phosphorus-containing organic radical

January 30, 2025

Muon spin rotation (μSR) spectroscopy is a powerful technique used to study the behavior of materials at the atomic level. In this study, researchers from the Institute of Science Tokyo employed μSR to examine phosphorus-containing 12-phosphatetraphene 1 molecule (muoniated radical). Their findings provide new insights into the radical's structure and behavior, advancing understanding of reactive species and radical behavior.

Unveiling Radical Stabilization in 12-Phosphatetraphene Using Muons

Muon spectroscopy of a 12-phosphatetraphene with extremely efficient radical trapping properties, Ito et al. (2025) | Scientific Reports | 10.1038/s41598-024-84611-w

Muon spectroscopy of a 12-phosphatetraphene with extremely efficient radical trapping properties
Ito et al. (2025) | Scientific Reports | 10.1038/s41598-024-84611-w

Muon spin rotation (μSR) spectroscopy is a powerful technique that helps to study the behavior of materials at the atomic level. It involves using muons—subatomic particles similar to protons but with a lighter mass. When introduced into a material, muons interact with local magnetic fields, providing unique insights into the material’s structure and dynamics, especially for highly reactive species such as radicals.

In a new study, a team of researchers led by Associate Professor Shigekazu Ito, from the School of Materials and Chemical Technology, Institute of Science Tokyo, Japan, utilized μSR spectroscopy to investigate the regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1. This compound is a phosphorus congener (a variant of a common chemical structure). The process of μSR spectroscopy initially involves the formation of a muonium (Mu), which is formed when a positively charged muon (μ⁺) captures an electron (e^–). This process continues as the reaction of a muonium (Mu = [μ⁺e^–]) with the phosphorus-containing compound, resulting in the formation of a muoniated radical at the phosphorus site. This regioselective addition is driven by the high reactivity of the phosphorus atom in the structure, which is a key feature of polyaromatic hydrocarbons. Their findings were published online in Scientific Reports on January 7, 2025

The study revealed that muon exclusively reacts with the phosphorus atom, forming a stable yet highly reactive muoniated radical at the phosphorus site, highlighting the molecule's high reactivity. Researchers observed this interaction in detail using transverse-field μSR (TF-μSR) spectroscopy, which allowed them to directly probe the magnetic environment surrounding the radical. TF-μSR measurements indicated that even at low concentrations (0.060 M in tetrahydrofuran), the muoniation reaction occurred efficiently, producing detectable signals.

“By utilizing μSR spectroscopy, we were able to observe the regioselective muoniation process in detail, providing direct evidence of the reactive nature of phosphorus in this structure,” explains Ito. “The ability to study this radical at low concentrations opens up new possibilities for investigating reactive species in various molecular systems.”

Researchers used density functional theory (DFT) to study the structure and stability of the muoniated radical. Hyperfine parameters A_μ and A_31P, derived from DFT, provided key insights into its electronic structure and stabilization. These calculations suggested that the structure of 12-phosphatetraphene 1 (muoniated radical) is stabilized in the flat, π-delocalized form due to the contribution of lowest possible (zero-point) energy. This stabilization prevents the formation of a thermodynamically favored saddle-type tetracyclic skeleton.

Another important observation from the study was the temperature dependencies of A_μ and A_31P. As the temperature increased, both A_μ and A_31P parameters decreased, suggesting a structural stabilization of the muoniated radical. These findings were supported by μSR and muon (avoided) level-crossing resonance experiments, which provided additional information on the dynamics of the muoniated radical and its structural characteristics.

“This study provides valuable insights on the dynamics and structural changes of the muoniated radical, which could influence future research into radical behavior and stabilization,” says Ito. Resolving strain in the molecular framework enhances stability and reactivity, optimizing the material for practical applications like electron-spin functional materials and nucleic acid regulation. This improvement increases reliability, opening new possibilities for advanced technologies and therapeutic uses.

The regioselective muoniation of peri-trifluoromethylated 12-phosphatetraphene 1 is expected to have implications in the fields of material science and biology by creating electron-spin functional materials and regulatory substances for nucleic acids, respectively. Overall, this study improves the understanding of phosphorus-containing radicals and highlights the versatility of μSR spectroscopy in investigating reactive species at the atomic level.

Reference

Authors:	Shigekazu Ito¹, Kohei Yasuda¹, Keisuke Ishihara¹, Victoria L. Karner², Kenji M. Kojima², and Iain McKenzie²
Title:	Muon spectroscopy of a 12-phosphatetraphene with extremely efficient radical trapping properties
Journal:	Scientific Reports
DOI:	10.1038/s41598-024-84611-w
Affiliations:	¹Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Institute of Science Tokyo, Japan ²Centre for Molecular and Materials Science, TRIUMF, Canada
Funding:	Japan society for the promotion of Science (19H02685, 22K19023)