Analyzing and Tuning the Energetic Landscape of H₂Pc Tautomerization

We present a detailed analysis of the energetic landscape of phthalocyanine (H₂Pc) tautomerization on Ag(111), i.e., the switching of protons between different sites in the molecular frame, which is induced and measured by a low-temperature scanning tunneling microscope (STM). We demonstrate that tautomerization of this molecule is preferentially triggered by the excitation of the N–H stretching mode. Interestingly, a step-like increase of the tautomerization rate is observed at a bias voltage that corresponds to the second harmonic of this vibrational mode, which we attribute to the crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. This hypothesis is supported by the analysis of four modified versions of H₂Pc, produced by single silver atom manipulation and/or deprotonation of the pristine H₂Pc molecule. Depending on the particular modification, the step-like increase varies strongly, spanning the entire range from further enhancement to almost complete disappearance. We interpret this behavior in terms of different tautomerization barrier heights which for most molecules lies between the first and second harmonics of the N–H stretching mode but which is strongly reduced well below the first harmonic for deprotonated H₂Pc.