Interplay of Collective Electrostatic Effects and Level Alignment Dictates the Tunneling Rates across Halogenated Aromatic Monolayer Junctions

Predictions about the electrical conductance across molecular junctions
based on self-assembled monolayers (SAMs) are often made from the SAM precursor
properties. Collective electrostatic effects, however, in a densely packed SAM can
override these predictions. We studied, experimentally and theoretically, molecular
tunneling junctions based on thiolate SAMs with an aromatic biphenyl backbone and
variable, highly polarizable halogen termini X (S-(C6H5)2X; X = H, F, Cl, Br, or I). We
found that the halogen-terminated systems show tunneling rates and dielectric behavior
that are independent of X despite the large change in the electronegativity of the terminal atom. Using density functional theory,
we show that collective electrostatic effects result in modulations of the electrostatic potential that are strongly confined spatially
along the direction of charge transport, thereby rendering the role of the halogen atoms insignificant for SAMs with conjugated
backbones.