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

Two different electrogenerated chemiluminescence (ECL) systems, Ru­(bpy)₃²⁺/TPrA and Ru­(bpy)₃²⁺/C₂O₄^2–, 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_ECL). A positive MFE_ECL up to 210% is observed for the Ru­(bpy)₃²⁺/TPrA system, while a negative MFE_ECL of only −33% is observed based on the Ru­(bpy)₃²⁺/C₂O₄^2– system. The significant difference on MFE_ECL 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)₃^2+*. As a consequence, we can tune MFE_ECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction.