One of the goals in molecular-scale electronics is to understand the influence of structural characteristics of a molecule on its electronic conduction behavior (1). Studies (2) by Reed and Tour have shown that oligo(phenyleneethynylene)s containing nitro groups exhibit NDR (negative differential resistance) effect and thus have potential applications as electronic switching and memory devices. The physical mechanism is not well understood. It was hypothesized that either the redox properties (3) or the conformational change of the molecules under external electric field could be responsible (4). To isolate the effect of redox properties from that of conformational change in studying electron transport property, we prepared two oligo(phenyleneethynylene) molecules (1 and 2, Figure) that are similar in conformation but different in electron density distribution. To further examine the charge effect, a third analog, the positively charged molecule 3, was prepared from 2 by methylating the nitrogen atom on the pyridine ring. These molecules were synthesized by repetitive palladium-catalyzed Sonagashira coupling reaction of the respective benzene and pyridine building blocks. The rod-like structures and the terminal thiol groups allowed the molecules to form self-assembled monolayers (SAM) on gold electrodes. The electron transport property was studied by I/V characterization and the density of states by scanning tunneling microscopy.