Electron Transfer through Organic Molecules

Electron transfer through molecular frameworks is central to a wide range of chemical, physical, and biological processes. We demonstrate a means to measure electronically and to quantify electron transfer through organic molecules and films. We show quantitative agreement with universal values of electron transfer inferred from biological, electrochemical, photochemical, and related systems. Scanning tunneling microscopy was used to image adjacent chains and molecular terraces of different length alkanethiolates in an ordered self-assembled monolayer lattice on Au111. In electron transfer measurements using a scanning tunneling microscope, both the driving force and the electrode separation can be continuously varied. This allows independent electronic measurement of the molecular bridges through which electron transfer takes place. The differences between the measured topography in scanning tunneling microscopy and the physical heights of these molecules can be understood in terms of the transconductance through individual chains using a two-layer tunnel junction model.