Photo- and chemically-produced phylloquinone biradicals: EPR and ENDOR study

Phylloquinone biradical triplet species were generated by 300 nm irradiation of frozen (77 K) solutions or by treatment with AlCl3. The shape of the (Δms=1) electron paramagnetic resonance (EPR) signal of the triplet is axially symmetric (E=0) with D=19±0.5 mT for photo-induced and D=11.2±0.5 mT for chemically induced radicals. A half-field signal (Δms=2) in the region of g≈4 was detected in both cases, confirming its assignment as a triplet. An additional line arising at the center of the (Δms=1) signal with g=2.0048±0.0002 was assigned to the phylloquinone radical anion (PhQ−). Electron nuclear double resonance (ENDOR) measurements of the triplet revealed the sign of the D parameter. For photo-generated radicals it appeared to be negative, which is the characteristic of radical dimers with well-separated partners (biradicals). Spin–spin distances of 5.3 and 6.3 Å, respectively, were estimated from the D parameter of photo-generated and chemically prepared phylloquinone biradicals.