Self-assembled monolayers (SAMs) of C6H5-C=C-C6H4-C=C-C6H4SH (1) on Au(111) and Ag(111) exhibit electrical rectifying behavior when probed by the scanning tunneling microscope whereas SAMs derived from decanethiol, benzenethiol and 4-phenylethynylbenzenethiol do not. We suggest that rectification arises from the conjugated length of the molecular framework of 1 and an increase of the donor character of the sulfur atom to this system upon adsorption to the metal. This is supported by surface second-harmonic generation measurements which show a significantly larger second order optical nonlinearity of SAMs of 1 compared to the other systems.