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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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    Two different electrogenerated chemiluminescence (ECL) systems, Ru­(bpy)<sub>3</sub><sup>2+</sup>/TPrA and Ru­(bpy)<sub>3</sub><sup>2+</sup>/C<sub>2</sub>O<sub>4</sub><sup>2–</sup>, are chosen to study the relationship between the sign of exchange interaction in radical pairs and magnetic field effects (MFEs) on electrogenerated chemiluminescence intensity (MFE<sub>ECL</sub>). A positive MFE<sub>ECL</sub> up to 210% is observed for the Ru­(bpy)<sub>3</sub><sup>2+</sup>/TPrA system, while a negative MFE<sub>ECL</sub> of only −33% is observed based on the Ru­(bpy)<sub>3</sub><sup>2+</sup>/C<sub>2</sub>O<sub>4</sub><sup>2–</sup> system. The significant difference on MFE<sub>ECL</sub> is ascribed to different signs of exchange interaction in radical pairs [Ru­(bpy)<sub>3</sub><sup>3+</sup>···TPrA<sup>•</sup>] and [Ru­(bpy)<sub>3</sub><sup>3+</sup>···CO<sub>2</sub><sup>–•</sup>] 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)<sub>3</sub><sup>3+</sup>···TPrA<sup>•</sup>] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru­(bpy)<sub>3</sub><sup>3+</sup>···CO<sub>2</sub><sup>–•</sup>]. 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)<sub>3</sub><sup>2+*</sup>. As a consequence, we can tune MFE<sub>ECL</sub> between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction
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