Length Dependence of Electron Conduction for Oligo(1,4-phenylene ethynylene)s:  A Conductive Probe-Atomic Force Microscopy Investigation

The dependence of electron conduction of oligo(1,4-phenylene ethynylene)s (OPEs) on length, terminal group, and main chain structure was examined by conductive probe-atomic force microscopy (CP-AFM) via a metal substrate−molecular wire monolayer−conductive probe junction. The electron transport in the molecular junction was a highest occupied molecule orbital (HOMO)-mediated process following a coherent, non-resonant tunneling mechanism represented by the Simmons equation. The length of OPEs was the dominant factor in determining electron conduction across the metal−molecular wires−metal junction, where the resistances of OPEs scaled exponentially against molecular length in a structure-dependent attenuation factor of 0.21 ± 0.01 Å^-1.