First observation of very long-lasting emissive polarization in EPR spectra of Fremy's salt when reduced by ascorbic acid. An unusual aspect of the radical pair mechanism

Emissive EPR spectral lines were observed in experiments dedicated to investigating the efficiency of ascorbic acid in reducing Fremy's salt. The emission was detected by CW spectroscopy and was exceptionally long lasting, being observed as long as 20 minutes. Fremy's salt emission lines were observed at the end of the redox reaction. The Fremy's salt absorption signal was totally quenched, and was replaced by an emission signal. This feature was observed under different experimental conditions. The inversion proceeded simultaneously for the three lines, which keep their relative intensities and hyperfine separation. The emission signal was also detected together with the normal absorption signal of ascorbyl free radical. No emission spectra were observed by using other nitroxides, or by using as reducing agent 1,4-dithio-DL-threitol instead of ascorbic acid. The observed spin polarization is accounted for on the basis of a Radical Pair Mechanism (RPM). The amount of polarization is related to the diffusion constant D, which decreases with increasing viscosity. By adding glycerol to the solution, enhanced polarisation was indeed observed, as expected, which confirms the proposed RPM polarisation mechanism. We report here for the first time, to our knowledge, the phenomenon of long- lasting emissive EPR lines.