Oxygen Reduction Intermediates‐Mediated Electron Transport in Single‐Molecule Junctions

Abstract Single‐molecule electrical measurements have recently emerged as a unique platform for exploring single‐molecule physical chemistry and reaction processes. Herein, the electrochemical technique has been first for exploring oxygen reduction reaction (ORR) in single‐molecule junctions of iron porphyrins modified with pyridine groups (Fe‐TPyP). Single‐molecule conductance measurements in O₂‐saturated solutions clearly show that the FeTPyP binds to O₂ to form the ferric‐superoxide porphyrin complex ((Fe‐O₂^{• −})‐TPyP) and form the intermediate molecular junctions. This is further supported by the observed molecular evidence of Fe─O and FeO─O stretching vibrations of (Fe‐O₂^{• −})‐TPyP during in situ Raman and ex situ electron paramagnetic resonance (EPR) experiments. DFT calculations also reveal that the potential determining step in ORR is the protonation of (Fe‐O₂^{• −})‐TPyP, resulting in the highest probability for forming Au‐(Fe‐O₂^{• −})‐TPyP‐Au junctions during the ORR process. This work reveals the impact of ORR on electron transport in single‐molecule junctions and provides a new way to explore the electrocatalytic processes of molecular catalysts at a single‐molecule level by using the break junction method.