Conductance and transparence of long molecular wires

Electron tunneling through a molecular wire is studied as a function of the length and chemical structure of the molecule. The current intensity is calculated using the electron-scattering quantum-chemistry technique, the wire being connected at both ends to a planar metal-vacuum-metal nanojunction. The tunnel channels and the stepped $I(V)$ characteristics are discussed in detail for the oligo (thiophene ethynylene) molecular wire. At low bias voltage, the conductance G of a metal-molecular wire-metal junction follows a G=G₀e^-γL law with L the interelectrode separation. The inverse damping length γ depends on the internal wire electronic structure and the contact conductance G₀ on the electrode-wire end interactions. Both γ and G₀ can be optimized by changing the chemical structure of the wire, and are given for a large number of oligomers.