Charge transport in conjugated aromatic molecular junctions: Molecular conjugation and molecule-electrode coupling

The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport. Utilizing aromatic systems with thiol end groups, we have calculated using density functional theory the expected conductance of junctions containing molecules with different levels of conjugation and of different lengths. The calculated variations in transport junction conductance are explained in terms of the continuity of the conjugation path between leads. Molecular conjugation describes this continuity within the molecule, and the interfacial terms (spectral densities or imaginary parts of the self-energy) describe its continuity at the molecule/metal interface. We compare the results from junction conductance calculations with isolated molecule electronic structure calculations These density functional theory calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold. The decay of the transport with length is found to be exponential for poly-Ph dithiol molecules. We compare the calculated conductance of conjugated aromatic molecules with their molecular orbital calculations and with the Green's function formulation and evaluate the relative significance of different factors (such as energetic alignment and spectral density) that control the conductance of molecules.