SELF-ASSEMBLED MOLECULAR RECTIFIERS

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 b y the scanning tunneling microscope whereas SAMs derived from decaneth iol, benzenethiol and 4-phenylethynylbenzenethiol do not. We suggest t hat rectification arises from the conjugated length of the molecular f ramework of 1 and an increase of the donor character of the sulfur ato m to this system upon adsorption to the metal. This is supported by su rface second-harmonic generation measurements which show a significant ly larger second order optical nonlinearity of SAMs of 1 compared to t he other systems. (C) 1997 American Institute of Physics.