We analyze conductance switching properties during the stretching of Au-oligothiophene dimethylene dithiol-Au single-molecule junctions, by using first-principles calculations based on the density functional theory and the nonequilibrium Green's function approach. Our results of these sigma-pi hybrid molecular systems agree well with the experimental data and confirm that the longer molecular junction (four thiophene rings) has higher conductance than the shorter one (three thiophene rings). We attribute this behavior to their differently located molecular levels, with respect to the Fermi energy, at the point of the junction break. This breaking point, occurring at a longer stretching distance for the shorter molecule, affects the junction conductance significantly and can be determined by several characteristics of the junction.
