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.