Investigation of Organic Molecules Using Elementary Subatomic Particle: Muon Spin Spectroscopy for Reaction Analysis of the Open-Shell Singlet Heterocycle

The positive muon (µ+) is an elementary subatomic particle that is assigned as a member of lepton (spin I=1/2). Whereas muons are present in cosmic radiation, high-energy beam of proton from accelerators such as cyclotron and synchrotron produce almost fully polarized muons. The muon has one ninth of the proton mass (0.1126mp), and the polarized “ultralight proton” is applicable to similar spectroscopic analyses to electron-spin resonance (ESR) and nuclear magnetic resonance (NMR) without the alternative magnetic resonance techniques. In contact with matter, the positive muon captures an electron and become a muonium (Mu= [µ+e-]) that is a light isotope of hydrogen atom. The muoniums have been utilized for monitoring radical reactions of usual unsaturated organic molecules including benzene, alkene, alkyne, and ketone, and the corresponding radicals via muonium addition can be characterized by muon spin rotation/resonance/relaxation (µSR) spectroscopy. In this article, principles and usefulness of µSR for study of functional organic compounds are overviewed, and our recent findings on µSR measurements for the sterically encumbered 1,3-diphosphacyclobutane-2,4-diyl of singlet biradical are presented. A regioselective addition of muonium to the skeletal phosphorus atom in the 1,3-diphosphacyclobutane-2,4-diyl unit was characterized, which would be useful to understand the 4-membered heterocyclic molecular skeleton promoting the low-voltage p-type organic semiconductor behavior.