Origin of an Absorptive Electron Spin Polarization Observed in Photochemical Hydrogen Abstraction Reaction by Benzophenone Derivatives. CW and Pulsed EPR Studies

Continuous wave time-resolved EPR (CW-TREPR) and pulsed Fourier transform EPR (FT-EPR) experiments were carried out to clarify the origin of an unusual electron spin polarization. We analyzed precisely the buildup and decay of the spin-polarized EPR spectra of the transient radicals generated by laser pulse irradiation of benzophenone derivatives in propan-2-ol. It was found that the reaction of the spin-equilibrated triplet state of benzophenone derivatives was responsible for the net absorptive polarization, because the contribution of absorptive polarization was (4/3) P eq, where P eq is the signal intensity at the Boltzmann population in the Zeeman splitting of a radical. Kinetic parameters were also determined from the dynamics of the spin polarization.