Dynamics of porphyrin electron-transfer reactions at the electrode-electrolyte interface at the molecular level.

Electron transfer is an essential step in many important processes and has been the subject of intensive research, with relevance to areas such as molecular electronics, electrochemistry, biology, catalysis, information storage, and solarenergy conversion. Therefore, understanding interfacial electron transfer between molecules and electrodes, and their dynamics at the molecular level, is important for fundamental science as well as for technological applications. However, the present theoretical and experimental treatment of interfacial electron transfer between molecules and electrodes mainly relies on ensemble-averaged optical spectroscopic and electrochemical measurements. Thus, our understanding of the interfacial electron transfer dynamics at the molecular level is very limited. For example, we do not know how local defects or adsorption sites on a heterogeneous electrode surface affect the dynamics of redox reactions at the molecular level. Nevertheless, it is frequently assumed that reactions principally occur at steps or defects (“active sites”). On a homogeneous surface, is the redox reaction of adsorbed molecules homogeneous? How does the electrode potential affect the spatial distribution of an interfacial redox reaction at the molecular level? How is charge transferred laterally between molecules? Answers to these questions are critical for progress in our understanding. Therefore, there is a tremendous need to probe the dynamics of interfacial electron transfer at the molecular level. In the experiments reported here, a potential-pulse perturbation was employed to control the electrochemical oxidation of a simple porphyrin (5,10,15,20-tetra(4-pyridyl)21H,23H-porphine (TPyP); Scheme 1) at the Au(111)/0.1m H2SO4 interface, and scanning tunneling microscopy was employed to provide an insight into the electrochemical oxidation dynamics at the molecular level. TPyP on Au(111) was chosen as a model system for the following reasons: 1) The TPyP molecules can form an ordered monolayer at the Au(111)/0.1m H2SO4 interface, depending on the electrode potential. 2) The redox states of adsorbed porphyrins on an electrode surface can be distinguished by their different contrast in STM images. 3) The adsorption of TPyP onto the Au(111) electrode has a dramatic effect its electrochemical activity. For example, the reduction of pre-adsorbed oxidized TPyP can be very slow, taking as long as tens of minutes at 0.05 V. However, the oxidation of adsorbed TPyP at 0.2 V is much faster, occurring in seconds. (The potential for the onset of oxidation of adsorbed TPyP is about 0.1 V.)