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

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.