Self-Assembled Monolayers with Distributed Dipole Moments Originating from Bipyrimidine Units

The concept of distributed dipoles in molecular self-assembly on solid substrates
was tested for the example of thiolate self-assembled monolayers (SAMs)
on Au(111) containing dipolar 2,5′-bipyrimidine units. These
were attached to a thiol anchoring group either directly or via a
phenylene–methylene spacer, with the spacer decoupling the
dipolar moiety from the substrate and promoting layer formation. As
expected, the SAMs containing the spacer groups exhibited a higher
quality, including a higher packing density and nearly upright molecular
orientation. The electrostatic effects of the dipolar bipyrimidine
moieties were tested through C 1s and N 1s photoemission spectra,
where electrostatic core-level shifts impact the shapes of the spectra.
Additionally, changing the orientation of the dipoles allows a variation
of the work function over a range of ∼1.35 eV. The experiments
were complemented by density-functional theory calculations. The work
function tuning range was reasonably high, but smaller than expected
considering that for SAMs with a single embedded pyrimidine group
per molecule work function changes already amounted to ∼1.0
eV. This behavior is rooted in an asymmetry of the studied SAMs: For
dipoles pointing away from the substrate, the expected doubling of
the work function change between monopyrimidine and bipyrimidine SAMs
essentially occurs. Conversely, for the downward-oriented pyrimidine
dipoles, the second polar ring has hardly any effect. Consistent observations
were made for the core-level shifts. We discuss several factors, which
are potentially responsible for this asymmetry, like disorder, depolarization,
or Fermi-level pinning. Of these, the most likely explanation is the
adsorption of airborne contaminants interacting with the nitrogen
atoms in the immediate vicinity of the outer surface. These are present
only in films with downward oriented dipoles. In spite of these complications,
some of the introduced distributed dipole SAMs serve as important
model systems for understanding electrostatic effects at interfaces.
They are also of interest for controlling carrier-injection barriers
in organic (opto) electronic devices.