  Acquiring quantitative information on charge transfer (CT) dynamics at the
nanoscale remains an important scientific challenge. In particular, CT
processes in single molecules at surfaces needs to be investigated to be
properly controlled in various devices. To address this issue, the dynamics of
switching molecules can be exploited. Here, a Nickel-tetraphenylporphyrin
adsorbed on the Si(100) surface is used to study the CT process ruling the
reversible activation of two chiral molecular conformations. Via the electrons
of a scanning tunneling microscope (STM), a statistical study of the molecular
switching reveals two specific locations of the molecule for which their
efficiency is optimized. The CT mechanism is shown to propagate from two
lateral aryls groups towards the porphyrin macrocycle inducing an
intramolecular movement of two symmetric pyrroles. The measured switching
efficiencies can thus be related with a Markus-Jordner model to estimate
relevant parameters that describe the dynamics of the CT process. Numerical
simulations provide a precise description of the molecular conformations and
unveil the molecular energy levels that are involved in the CT process. This
quantitative method opens a completely original approach to study CT at the
nanoscale.
