Study of radical pairs generated by photoreduction of anthraquinone-2,6-disulfonic acid with thymine by Fourier transform electron paramagnetic resonance

Using laser photolysis at 308 nm and FT–EPR, the triplet sensitized electron transfer from thymine to 9,10-anthraquinone-2,6-disulfonate in aqueous solution was studied. The anthraquinone radical anion and the deprotonated thymine-1-yl radical are spin-polarized by the CIDEP triplet mechanism and radical pair mechanism. The structure of the anthraquinone radical anion is strongly influenced by the pH of the solution. In weak acidic solution the radical anion dominates whereas the pK of the radical protonation was determined to be 3.2. The deprotonated thymine-1-yl radical shows two different radical pair polarization patterns which are distinguished by the phase of the polarization. This unusual behavior can be attributed to two different states in the primary radical pair.