Unusually Large Dynamic Electron Polarization in an O₂(¹Δ_g)−2,2,6,6-Tetramethylpiperidine-1-oxyl Radical System

Abstract

Net absorptive (Abs) chemically induced dynamic electron polarization (CIDEP) is observed in the system of the lowest excited singlet molecular oxygen O2(1Δg) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in benzene by means of the time-resolved ESR (TR-ESR) spectroscopy. As a result of (1) the analysis of CIDEP time profiles by the modified Bloch and kinetic equations and (2) the quenching of CIDEP by triethylamine, it is confirmed that the Abs CIDEP is created through the interaction of O2(1Δg) with TEMPO. The creation of Abs CIDEP is reasonably interpreted in terms of the radical-triplet pair mechanism (RTPM) of the O2(3Σg-)−TEMPO pair with doublet precursor. The absolute magnitude of net Abs CIDEP created on TEMPO is experimentally estimated to be 340 ± 40 in the unit of Boltzmann polarization. To our knowledge, this is one of the largest magnitudes of CIDEP measured so far by the TR-ESR method. Based upon the RTPM theory, the origin of this unusually large CIDEP is attributed to the large spin relaxation effect via the electron spin dipole−dipole interaction of O2(3Σg-) formed in the quenching process of O2(1Δg) by TEMPO. As a result of the discussion about the large CIDEP in O2(1Δg) quenching by TEMPO, it is proposed that the quenching takes place through the enhanced intersystem crossing accompanied with strong electron exchange interaction between an oxygen molecule and a TEMPO radical.