Probing the conductance of the σ-system of bipyridine using destructive interference

Guidelines to predict trends in the electrical conductance of molecules have been developed for the π-system of conjugated systems. Little is known, however, about the conductance of the underlying σ-systems because the π-system usually dominates the transport. Here we study a family of bipyridine-based molecules using STM-break junction experiments and density functional theory transport calculations. We use different lengths and substitution patterns to probe the role of both the σ-system and the π-system in controlling conductance. By exploiting the destructive interference feature found in the π-system of the meta-coupled six-membered aromatic rings, we show that the conductance of the σ-system of a meta-coupled molecule can be probed directly and can even exceed that of its para-coupled analog. These results add to the understanding of the conductance through the chemically hidden σ-electrons.