Vibrational Excitation in Single-Molecule Transistors: Deviation from the Simple Franck−Condon Prediction

We investigated the inner-sphere reorganization of ferrocene ((Cp) 2Fe ( n+ )) and tris(2,2'-bipyridine) iron ((bpy) 3Fe ( n+ )) in a single-molecule-transistor geometry. In (Cp) 2Fe ( n+ ) ( n = 0 and 1), almost no vibrations were excited during single-electron transport, whereas in (bpy) 3Fe ( n+ ) ( n = 1, 2, and 3), many distinct vibrations appeared, consistent with its larger reorganization energy. The observed excitation intensities varied significantly across devices, however, and could not be accounted for by "Franck-Condon" factors. This observation indicates that a quantitative account of electron-vibration coupling in single-electron tunneling requires further investigation.