Electron Transfer Dynamics and Structural Effects in Benzonitrile Monolayers with Tuned Dipole Moments by Differently Positioned Fluorine Atoms

To
understand the influence of the molecular dipole moment on the
electron transfer (ET) dynamics across the molecular framework, two
series of differently fluorinated, benzonitrile-based self-assembled
monolayers (SAMs) bound to Au(111) by either thiolate or selenolate
anchoring groups were investigated. Within each series, the molecular
structures were the same with the exception of the positions of two
fluorine atoms affecting the dipole moment of the SAM-forming molecules.
The SAMs exhibited a homogeneous anchoring to the substrate, nearly
upright molecular orientations, and the outer interface comprised
of the terminal nitrile groups. The ET dynamics was studied by resonant
Auger electron spectroscopy in the framework of the core-hole clock
method. Resonance excitation of the nitrile group unequivocally ensured
an ET pathway from the tail group to the substrate. As only one of
the π* orbitals of this group is hybridized with the π*
system of the adjacent phenyl ring, two different ET times could be
determined depending on the primary excited orbital being either localized
at the nitrile group or delocalized over the entire benzonitrile moiety.
The latter pathway turned out to be much more efficient, with the
characteristic ET times being a factor 2.5–3 shorter than those
for the localized orbital. The dynamic ET properties of the analogous
thiolate- and selenolate-based adsorbates were found to be nearly
identical. Finally and most importantly, these properties were found
to be unaffected by the different patterns of the fluorine substitution
used in the present study, thus showing no influence of the molecular
dipole moment.