W-band time-resolved electron paramagnetic resonance spectroscopy on transient organic radicals in solution

Time-resolved electron paramagnetic resonance (TREPR) spectra of spin-polarized transient radicals in liquid solution, generated in a continuous flow-system of a W-band (95 GHz) high-field (3.4 T) EPR spectrometer, are reported. The organic free radicals are created by laser flash photolysis of ω,ω-dimethoxy-ω-phenylacetophenone (DMPA) and diphenyl-2,4,6-trimethylbenzoil phosphine oxide (TMDPO) inside the microwave cavity, and are observed at 10 ns to 20 μs delay times after the laser pulse. The analysis of the positions of the well-separated EPR signals at W-band yields the g-values of the observable transients with high accuracy. The chemically induced dynamic electron polarization (CIDEP) patterns are different from those in conventional X-band (9.5 GHz) EPR. This is due to different spin relaxation times at different magnetic fields, to field-dependent CIDEP mechanisms operating in the studied systems, and to the increased Boltzmann polarization at high fields.