jp7b10564_si_001.pdf (10.34 MB)
Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization
journal contribution
posted on 2017-11-27, 00:00 authored by Jens Kügel, Lucas Klein, Markus Leisegang, Matthias BodeWe 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.