To understand the charge transport mechanism of oligo(p-phenylene vinylene) (OPV) molecular wires

twelve OPV molecular wires were synthesized

whose anchoring groups were thioacetyl (SAc) and amine (NH2)

respectively.Conductive probe-atomic force microscopy and scanning tunneling microscopy-break junction (STM-BJ) methods were employed for the characterization of the electron transport properties of these OPV wires.A detailed analysis of the single-molecular resistance as a function of molecular length

temperature as well as electric field

revealed a clear transition of charge transport mechanism from tunneling to hopping at a critical length of 2 nm.The effect of anchoring groups on the charge transport properties was investigated by comparing the single-molecular conductance of OPV wires terminated with dithioacetyl and diamine groups.The results showed that the anchoring groups indeed affect the contact resistance

but they do not change the intrinsic charge transport mechanism.Moreover

the electronic properties of the OPV wires were studied theoretically using a combination of density functional theory and non-equilibrium Green's functions method.The calculated results demonstrated that the energy difference between the frontier molecular orbitals and the metal Fermi level played an important role in controlling the conduction mechanism of the OPV wires.