  We report on the effect of sp3 hybridized carbon atoms in acene derivatives
adsorbed on metal surfaces, namely decoupling the molecules from the supporting
substrates. In particular, we have used a Ag(100) substrate and hydrogenated
heptacene molecules, in which the longest conjugated segment determining its
frontier molecular orbitals amounts to five consecutive rings. The
non-planarity that the sp3 atoms impose on the carbon backbone results in
electronically decoupled molecules, as demonstrated by the presence of charging
resonances in dI/dV tunneling spectra and the associated double tunneling
barriers, or in the Kondo peak that is due to a net spin S=1/2 of the molecule
as its LUMO becomes singly charged. The spatially dependent appearance of the
charging resonances as peaks or dips in the differential conductance spectra is
further understood in terms of the tunneling barrier variation upon molecular
charging, as well as of the different orbitals involved in the tunneling
process.
