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

• Published in: Journal of physical chemistry. C 2015-04, Vol.119 (15), p.8089-8094
• Main Authors: Pan, Haiping, Shen, Yan, Luan, Lin, Lu, Kai, Duan, Jiashun, Hu, Bin
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.5b01541

Two different electrogenerated chemiluminescence (ECL) systems, Ru­(bpy)3 2+/TPrA and Ru­(bpy)3 2+/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 (MFEECL). A positive MFEECL up to 210% is observed for the Ru­(bpy)3 2+/TPrA system, while a negative MFEECL of only −33% is observed based on the Ru­(bpy)3 2+/C2O4 2– system. The significant difference on MFEECL is ascribed to different signs of exchange interaction in radical pairs [Ru­(bpy)3 3+···TPrA•] and [Ru­(bpy)3 3+···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)3 3+···TPrA•] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru­(bpy)3 3+···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 MFEECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction.