Spin-orbit coupling induced electron spin polarization: Influence of heavy atom position

Echo-detected FT-EPR spectra have been observed for the quinone anion radical generated from the photoexcitation of duroquinone (DQ) in the presence of N,N-dimethylaniline (DMA) and its monohalogen substituents in 1-propanol. When DMA and 4-Cl DMA were used as the electron donor, the net-emissive polarization due to a triplet mechanism was observed. In contrast, the electron donors of 4-BrDMA and 4-IDMA gave unusual net-absorptive (A) CIDEP spectra of the DQ anion radical at the delay time of 200 ns between the laser and first microwave pulses. It was found that the enhancement factor of the net-A polarization remarkably depends on the heavy atom position in the donor. The net-A polarization observed is attributable to a spin-orbit coupling mechanism (SOCM). Simple explicit expressions have been given for the electron spin polarization generation due to SOCM.