  Although the dynamics of charge transfer (CT) processes can be probed with
ultimate lifetime resolution, the helplessness to control CT at the nanoscale
constitutes one of the most important road-blocks to revealing some of its deep
fundamental aspects. In this work, we present an investigation of CT dynamics
in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on
a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope
(STM) is used to create local ionic states in one fragment of the dyad. The CT
process is monitored by imaging subsequent changes in the neighbor acceptor
molecule and its efficiency is mapped revealing the influence of the initial
excited state in the donor molecule. In validation of the experiments,
simulations based on density functional theory show that holes have a higher
donor-acceptor CT rate compared to electrons and highlight a noticeable initial
state dependence on the CT process. We leverage the unprecedented spatial
resolution achieved in our experiments to show that the CT process in the dyad
is governed via molecule-molecule coherent tunneling with negligible
surface-mediated character.
