Nonexponential Length Dependence of Molecular Conductance in Acene-Based Molecular Wires

In the nonresonant regime, molecular conductance decays exponentially with distance, limiting the fabrication of efficient molecular semiconductors at the nanoscale. In this work, we calculate the conductance of a series of acene derivatives connected to gold electrodes using density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) formalism. We show that these systems have near length-independent conductance and can exhibit a conductance increase with molecular length depending on the connection to the electrodes. The analysis of the molecular orbital energies and transmission functions attribute this behavior to the dramatic decrease of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap with length, which shifts the transmission peaks near the Fermi level. These results demonstrate that the anchoring configuration determines the conductance behavior of acene derivatives, which are optimal building blocks to fabricate single-molecule devices with tunable charge transport properties.