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Changing the Sign of Exchange Interaction in Radical Pairs to Tune Magnetic Field Effect on Electrogenerated Chemiluminescence

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journal contribution
posted on 16.04.2015, 00:00 authored by Haiping Pan, Yan Shen, Lin Luan, Kai Lu, Jiashun Duan, Bin Hu
Two different electrogenerated chemiluminescence (ECL) systems, Ru­(bpy)32+/TPrA and Ru­(bpy)32+/C2O42–, are chosen to study the relationship between the sign of exchange interaction in radical pairs and magnetic field effects (MFEs) on electrogenerated chemiluminescence intensity (MFEECL). A positive MFEECL up to 210% is observed for the Ru­(bpy)32+/TPrA system, while a negative MFEECL of only −33% is observed based on the Ru­(bpy)32+/C2O42– system. The significant difference on MFEECL is ascribed to different signs of exchange interaction in radical pairs [Ru­(bpy)33+···TPrA] and [Ru­(bpy)33+···CO2–•] because they have a distant and proximate separation distance between two radicals of a pair, which result in different magnetic-field-induced intersystem crossing directions between singlet and triplet states. The experimental results suggest that an applied magnetic field can enhance the singlet → triplet conversion rate in radical pairs [Ru­(bpy)33+···TPrA] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru­(bpy)33+···CO2–•]. The increase/decrease of triplet density in radical pairs stimulated by an applied magnetic field leads to an increase/decrease on the density of light-emitting triplets of Ru­(bpy)32+*. As a consequence, we can tune MFEECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction.