Chemically Induced Multiplet Electron−Nuclear Polarization in Zero and Low Magnetic Fields

We report the first observation of chemically induced electronic−nuclear multiplet spin polarization of transient radicals at very low and zero magnetic fields. Dimethoxyphosphonyl radicals, which have a hyperfine coupling constant of ∼70 mT, were produced in solution by photolysis of 2,4,6-trimethylbenzoyl phosphonic acid dimethyl ester. The radicals were detected using a modified L-band time-resolved electron paramagnetic resonance (TREPR) setup. The polarization is very strong, and signals are easily detected with custom-built resonators that match or nearly match each electronic−nuclear transition. A theoretical description for the formation of this large polarization has been proposed and is found to be in good agreement with experimental data. TREPR spectra and time-resolved kinetics at low and high magnetic fields have been measured. Signals detected at high magnetic field decay an order of magnitude faster than do those at low field, which can be explained by field-dependent HFI-induced spin relax...