Pulsed EPR study on large dynamic electron polarisation created in the quenching of photo-excited xanthene dyes by nitroxide radicals in aqueous solutions

Abstract

Dynamic electron polarisation (DEP) produced by the quenching of dye molecules in the triplet excited states by nitroxide radicals was investigated in aqueous solutions by pulsed electron paramagnetic resonance and transient absorption spectroscopy. An analysis of the measured quenching rate constants suggests that quenching is promoted by either exchange or charge-transfer mechanisms for a triplet dye and a doublet radical pair. An unusually large DEP on the radical was found generated in the nitroxide and Eosin Y or Rose Bengal systems in aqueous solutions. Quantitative analysis indicates that the DEP values in aqueous solutions range from −40 to −150 in the unit of thermal spin polarisation, which is in contrast to previously reported small DEP values of less than −10 for organic triplet molecules in benzene solutions [22–27,29,31,32]. From the theoretical analysis of DEP, an origin of this large DEP was attributed to the notably slow diffusion motion of Eosin Y and Rose Bengal in water.