The FT-EPR spectra of the anthraquinone-2,6-disulfonic acid radicals as probe for local proton concentrations in electron transfer reactions

The radical anion of anthraquinone-2,6-disulfonic acid (2,6-AQDS) and diffe rent donor radical cations are formed by electron transfer from various pyr imidines to the laser-induced triplet state of 2,6-AQDS. The electron param agnetic resonance (EPR) spectrum of the radical anion changes drastically w ith pH due to different protonated forms of this radical. These EPR spectra were used to define an effective pH value as a measure of the local proton (H3O+) concentration in the surroundings of the 2,6-AQDS radical anion in a unbuffered solution. In some cases this effective pH value was found to b e time-dependent reflecting the evolution of the protonation and deprotonat ion processes of acceptor and donor.