Time-resolved FT-EPR study of photoreduction of duroquinone by triethylamine in methanol

Using time resolved Fourier transform EPR spectroscopy the photoreduction o f duroquinone by triethylamine in methanol solution was investigated. It is found that the spin-polarized (CIDEP) duroquinone triplet deactivates by e lectron transfer from triethylamine generating duroquinone radical anion an d amine radical cation, and by hydrogen transfer from the solvent generatin g durosemiquinone radical and hydroxymethyl radical, respectively. All radi cals are observed at different conditions and are spin-polarized by triplet mechanism and partially by STo radical pair mechanism. The time dependence of FT-EPR intensities of radical cation and radical anion on the amine con centration is investigated in the range of 1 to 100 mM triethylamine. The c ontribution of the triplet mechanism to the spin polarization of radicals c hanges with different triethylamine concentrations. The durosemiquinone rad ical is found to be transformed into duroquinone radical anion in the prese nce of triethylamine in the solution. CIDNP experiments indicate that the h ydrogen back transfer between the durosemiquinone radical and hydroxymethyl radical pair has a significant influence on the time behaviour of duroquin one radical anion. The intensity of triethylamine radical cation is found t o be decreased with the increase of triethylamine concentration, which is i nterpreted that the triethylamine radical cation is deprotonated by the ami ne, Based on the FT-EPR results, a new complete mechanism is proposed.