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.