Effects of Aromaticity and Connectivity on the Conductance of Five-Membered Rings

Even though five-membered rings, for example, thiophene, are ubiquitous in organic and molecular electronics, as a class of molecules, they resist a simple interpretation. Generally containing four sp(2)-hybridized carbon atoms, the fifth position can be filled by any number of substituents. This flexibility leads to a diverse range of electronic properties, but also presents a challenge for deriving a model description. Starting from a noninteracting Hamiltonian obtained from Kohn-Sham density functional theory calculations, we derive an effective four-site model that provides a unified description of these systems. We rationalize the zero-bias conductance of these molecules in terms of aromaticity and connectivity. The conductance was found to be highly sensitive to connectivity, as for benzene, but we also found the conductance to be sensitive to aromaticity. The model predicts the same relative conductance as reported in prior experiments in almost all cases and provides a link between chemical intuition and single-molecule conductance. The method used to develop the model is general and could be applied to other types of molecules.