Single-molecule chemical reduction induced by low-temperature scanning tunneling microscopy: A case study of gold-porphyrin on Au(111)

Electron transfer in weakly coupled molecules typically occurs via a transient charging of the molecule by tunneling electrons. We report on the permanent electron injection into a single molecule in a controlled manner by employing low-temperature (LT) scanning tunneling microscopy (STM). Our model system is Au(III) 5,10,15,20-tetraphenylporphyrin (AuTPP) single molecules physisorbed on a single-crystal Au(111) surface. The results of our combined topographic STM images, tunnel conductance dI/dV spectra and two-dimensional spatial dI/dV mapping experiments support a switching of the molecular charge state of AuIIITPP on Au(111) induced by STM manipulation. We successfully realized STM tip-induced chemical reduction reactions in single AuTPP molecules, reducing AuIIITPP to AuIITPP, with the extra electron distributed either preferentially in a 5d state of the center gold ion or the aromatic (pi-conjugated) tetrapyrrole macrocycle of the porphyrin ligand. Details of the electron transfer process were identified and visualized with intra-molecular resolution by single-molecule tunneling spectroscopy and orbital mapping.