  Electrostatic potentials strongly affect molecular energy levels and charge
states, providing the fascinating opportunity of molecular gating. Their
influence on molecular vibrations remains less explored. Here, we investigate
Ethyl-Diaminodicyanoquinone molecules on a monolayer of MoS$_2$ on Au(111)
using scanning tunneling and atomic force microscopy and spectroscopy. These
molecules exhibit a large dipole moment in gas phase, which we find to
(partially) persist on the MoS$_2$ monolayer. The self-assembled structures
consist of chains, where the dipoles of neighboring molecules are aligned
anti-parallel. Thanks to the decoupling efficiency of the molecular states from
the metal by the MoS$_2$ interlayer, we resolve vibronic states of the
molecules, which vary in intensity depending on the molecular surrounding. We
suggest that the vibrations are strongly damped by electrostatic interactions
with the environment.
