  A scanning tunnelling microscope has been used to determine the conductance
of single molecular wires with the configuration X-bridge-X, X-bridge-Y and
Y-bridge-Y (X = thiol terminus and Y = COOH). We find that for molecular wires
with mixed functional groups (X-bridge-Y) the single molecule conductance
decreases with respect to the comparable symmetric molecules. These differences
are confirmed by theoretical computations based on a combination of density
functional theory and the non-equilibrium Green functions formalism. This study
demonstrates that the apparent contact resistance, as well as being highly
sensitive to the type of the anchoring group is also strongly influenced by
contact-asymmetry of the single molecular junction which in this case decreases
the transmission. This highlights that contact asymmetry is a significant
factor to be considered when evaluating nano-electrical junctions incorporating
single molecules.
