  The structure of a physisorbed sub-monolayer of 1,2-bis(4-pyridyl)ethylene
(bpe) on epitaxial graphene is investigated by Low-Energy Electron Diffraction
and Scanning Tunneling Microscopy. Additionally, non-equilibrium heat-transfer
between bpe and the surface is studied by Ultrafast Low-Energy Electron
Diffraction. Bpe arranges in an oblique unit cell which is not commensurate
with the substrate. Six different rotational and/or mirror domains, in which
the molecular unit cell is rotated by 28{\pm}0.1{\deg} with respect to the
graphene surface, are identified. The molecules are weakly physisorbed, as
evidenced by the fact that they readily desorb at room temperature. At liquid
nitrogen temperature, however, the layers are stable and time-resolved
experiments can be performed. The temperature changes of the molecules and the
surface can be measured independently through the Debye-Waller factor of their
individual diffraction features. Thus, the heat flow between bpe and the
surface can be monitored on a picosecond timescale. The time-resolved
measurements, in combination with model simulations, show the existence of
three relevant thermal barriers between the different layers. The thermal
boundary resistance between the molecular layer and graphene was found to be
2{\pm}1{\cdot}10-8 K m2 W-1.
