  In this contribution we demonstrate structural control over a transport
resonance in HS(CH$_{2}$)$_{n}$[1,4 - C$_{6}$H$_{4}$](CH$_{2}$)$_{n}$SH (n = 1,
3, 4, 6) metal - molecule - metal junctions, fabricated and tested using the
scanning tunnelling microscopy-based $I(z)$ method. The Breit-Wigner resonance
originates from one of the arene $\pi$-bonding orbitals, which sharpens and
moves closer to the contact Fermi energy as $n$ increases. Varying the number
of methylene groups thus leads to a very shallow decay of the conductance with
the length of the molecule. We demonstrate that the electrical behaviour
observed here can be straightforwardly rationalized by analyzing the effects
caused by the electrostatic balance created at the metal-molecule interface.
Such resonances offer future prospects in molecular electronics in terms of
controlling charge transport over longer distances, and also in single molecule
conductance switching if the resonances can be externally gated.
