The Influence of Linkers on Quantum Interference: A Linker Theorem

How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs.